Performance evaluation of mixed metal oxide anodes for ...

© Daniela Rossetto de Menezes, 2021

Performance evaluation of mixed metal oxide anodes for zinc electrowinning

Mémoire

Daniela Rossetto de Menezes

Maîtrise en génie des matériaux et de la métallurgie - avec mémoire

Maître ès sciences (M. Sc.)

Québec, Canada

Performance evaluation of mixed metal oxide anodes for zinc electrowinning

Mémoire

Daniela Rossetto de Menezes

Sous la direction de :

Houshang Alamdari, directeur de recherche

iii

Abstract

The adoption of Mixed Metal Oxide (MMO)-coated anodes in zinc electrowinning cellhouses

would provide energy savings and resolve operational issues related to lead corrosion by-

products. But a major concern is that commercially available MMO anodes could deteriorate

prematurely in typical zinc electrolytes, due to intense MnO2 deposition. In this context, the

present study investigated the relationship that Mn2+ concentration in zinc electrolytes affects

the characteristics of MnO2 deposits and, consequently, the integrity of three types of IrO2-

bearing MMO anodes. For this purpose, firstly, an exploratory anode performance

assessment was conducted to monitor the anode potentials and the MnO2 formation rates

in the medium term, as a function of Mn2+ concentration. Then, scanning electron

microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to

characterize the anode samples after 72-hour galvanostatic polarization tests at different Mn2+

concentrations. The results have suggested that MnO2 deposits developed different

morphologies and induced different anode deterioration processes, depending on the Mn2+

concentration and the anode type. In particular, anodes type “D” were covered by MnO2 films

that would easily chip off after reaching a critical thickness, thus producing induced stresses.

According to SEM images, these MnO2 pieces detached take out MMO coating fragments

adhered to them. Meanwhile, MnO2 clusters of elongated crystallites developed over anodes

“E” and “F”, and they were found to induce ruptures throughout the MMO coatings.

Considering these results and specific criteria to define the Mn2+ tolerance levels of these

anodes, a financial analysis was proposed for screening the most suitable anode type for

industrial use, based on both the anodic potential demonstrated and the manganese control

strategy required for its satisfactory operation.

iv

Résumé

L'adoption d'anodes revêtues d'oxyde métallique mixte (MMO) dans les cuves d'extraction

électrolytique de zinc permettrait réduire la consommation énergétique et résoudre les

problèmes opérationnels liés à la corrosion des anodes de plomb. Néanmoins, c’est

préoccupant que tels anodes MMO disponibles commercialement pourraient être avariés

prématurément dans les électrolytes de zinc habituels, en raison d’être sujets à une

déposition intense de MnO2. Dans un tel contexte, ce projet a étudié la relation que la

concentration de Mn2+ dans l’électrolyte de zinc affecte les caractéristiques des dépôts de

MnO2 et, par conséquent, l'intégrité de trois types d'anodes MMO à base de IrO2. A cet effet,

une évaluation exploratoire a été réalisée pour suivre les potentiels anodiques et les taux

de formation de MnO2 à moyen terme, en fonction de la concentration de Mn2+. Ensuite, la

microscopie électronique à balayage (MEB) et la spectroscopie à rayons X à dispersion

d'énergie (EDS) ont été utilisées pour caractériser les anodes après des tests de polarisation

galvanostatique de 72 heures à différents concentrations de Mn2+. Les résultats ont suggéré

que les dépôts de MnO2 développent des morphologies différentes et induisent des

différents processus de détérioration des anodes, en fonction de la concentration de Mn2+

et du type d'anode. En particulier, les anodes de type «D» ont été recouverts de films de

MnO2 qui s'écaillaient facilement à partir d’une épaisseur critique, produisant ainsi des

contraintes induites. D’après des images par MEB, tels morceaux de MnO2 détachés ont

emporté des fragments du revêtement MMO adhérés en dessous. D’ailleurs, des

agglomérats de cristallites allongées de MnO2 se sont formés sur les anodes «E» et «F»,

ce qui a déclenché des ruptures dans leur revêtements MMO. À partir des résultats obtenus

et de certains critères pour la détermination des niveaux de tolérance à Mn2+ de chaque

type d’anode, une analyse financière a été proposée pour cribler le type d'anode le plus

approprié pour la production de zinc, en fonction de son potentiel et également de la

stratégie de contrôle de manganèse nécessaire à son fonctionnement adéquat.

v

Table of contents

Abstract ............................................................................................................................................... iii

Résumé ............................................................................................................................................... iv

Table of contents ................................................................................................................................. v

List of tables ....................................................................................................................................... vii

List of figures ..................................................................................................................................... viii

List of abbreviations, initials, acronyms .............................................................................................. xii

Acknowledgements ........................................................................................................................... xiv

Preface ............................................................................................................................................... xv

Introduction .......................................................................................................................................... 1

Chapter 1. Literature Review ......................................................................................................... 3

1.1. Hydrometallurgical production of zinc using lead-based anodes ........................................ 3

1.1.1. The Roasting-Leaching-Electrowinning (RLE) process ...................................................... 3

1.1.2. Lead-based anodes on zinc electrowinning ........................................................................ 8

1.1.3. The role of manganese in the conventional RLE process................................................. 10

1.2. MMO anodes for OER ....................................................................................................... 12

1.2.1. Development background ................................................................................................. 12

1.2.2. MMO anode constitution and properties ........................................................................... 14

1.2.3. Pathways of MMO anode deterioration ............................................................................. 17

1.2.3.1. Deterioration processes in ideal conditions ....................................................................... 17

1.2.3.2. Other processes that accelerate anode deterioration ....................................................... 19

1.3. The effect of Mn2+ ions to the performance of MMO anodes in zinc electrowinning ......... 20

1.3.1. Parameters that influence MnO2 deposition characteristics .............................................. 21

1.3.2. A manganese control strategy to implement MMO anodes in zinc cellhouses ................. 23

1.3.3. Anode cleaning options ..................................................................................................... 24

1.4. Other implications of MMO anode technology in zinc electrowinning ............................... 26

1.5. Objectives and hypotheses ............................................................................................... 27

Chapter 2. Methodology .............................................................................................................. 28

2.1. Outline of the experimental plan ........................................................................................ 28

2.2. Materials ............................................................................................................................ 29

2.2.1. Electrochemical equipment setup ..................................................................................... 29

2.2.2. Electrodes .......................................................................................................................... 30

2.2.3. Electrolytes ........................................................................................................................ 32

2.3. Experimental conditions .................................................................................................... 33

2.3.1. Part 1.a. Accelerated life tests (ALTs) ............................................................................... 33

vi

2.3.2. Part 1.b. Electrowinning tests with pseudo-stationary Mn2+ levels .................................... 34

2.3.3. Part 1.c. Electrowinning tests with step increase of Mn2+ levels ....................................... 35

2.3.4. Part 1.d. Testing anode cleaning methods ........................................................................ 35

2.3.5. Microscopic evaluation of MnO2 deposits and their effects ............................................... 37

Chapter 3. Results and Discussion: Exploratory Anode Performance Assessment ................... 38

3.1. Characterisation of the MMO anode types ........................................................................ 38

3.2. Long-term durability of MMO anodes in Mn-free conditions ............................................. 41

3.3. The relationship between Mn2+ levels, MnO2 deposition and MMO anode

potential increase .............................................................................................................. 43

3.3.1. Results from long-term galvanostatic polarizations ........................................................... 43

3.3.2. Complementary results from short-term galvanostatic polarizations ................................ 47

3.4. Definition of a MMO anode cleaning method .................................................................... 50

3.4.1. Chemical cleaning results ................................................................................................. 51

3.4.2. Mechanical cleaning results .............................................................................................. 54

3.5. Crystal phase characterization of MnO2 particles ............................................................. 57

Chapter 4. Results and Discussion: Identification of MnO2-induced deterioration

mechanisms and Mn2+ tolerance levels .................................................................... 59

4.1. Microscopy results of anodes type D ................................................................................ 59

4.2. Microscopy results of anodes type E ................................................................................. 68

4.3. Microscopy results of anodes type F ................................................................................. 74

4.4. Establishing Mn2+ tolerance levels for each anode type ................................................... 79

Chapter 5. Ranking the suitability of different anode types to industrial zinc production ............ 81

Conclusion ......................................................................................................................................... 83

Bibliography ....................................................................................................................................... 86

Appendix A. Potentiodynamic polarization analyses ..................................................................... 93

Appendix B. Cell and anode potentials of the galvanostatic polarization tests of Part 2 ............... 94

Appendix C. Research Paper ......................................................................................................... 97

vii

List of tables

Table 1-1. Effects of common zinc electrolyte impurities, adapted from [6, 7, 15]. .......................... 6

Table 1-2. Typical process conditions of zinc vs. copper electrowinning, based on [16, 55, 56,

57, 58]. ........................................................................................................................... 14

Table 2-1. Electrolytes compositions. ............................................................................................. 32

Table 2-2. MMO anode samples used in the cleaning tests, and the electrowinning conditions

applied to form their respective MnO2 deposits. ............................................................ 36

Table 3-1. Elemental composition of the cathode surface, according to XPS data. ....................... 42

Table 3-2. Outline of the evaluation of anode cleaning methods. ................................................... 51

Table 4-1. Summary of the results of Part 2 and definition of Mn2+ tolerance levels for the

anodes D, E and F. ........................................................................................................ 80

viii

List of figures

Figure 1-1. Schematic of RLE process steps. ................................................................................... 5

Figure 1-2. Breakdown of sources of ohmic drop in zinc cells. Based on [3]. .................................. 9

Figure 1-3. Rutile-type structure, where interstices are marked by black dots (left) and

oxygen diffusion through vacancy sites (right). 3D model from [86]. ........................... 18

Figure 2-1. Electrochemical setup for galvanostatic tests, with two independent cells. ................. 30

Figure 2-2. Electrode arrangement and dimensions. ...................................................................... 30

Figure 2-3. Samples of three MMO anode types, provided by De Nora Tech. ............................... 31

Figure 3-1. Element composition of MMO anodes, determined by XRF, with two acquisition

modes (film and default modes). .................................................................................. 38

Figure 3-2. X-ray diffractograms of anodes D, E and F (top), and patterns of the identified

crystallographic phases (bottom). ................................................................................ 39

Figure 3-3. SEM images of anode types D, E and F. Images obtained at 10 kV, with

magnifications of 400 x (top) and 2 kx (bottom). .......................................................... 40

Figure 3-4. Anode potentials of anode types D and E during the ALTs. ......................................... 41

Figure 3-5. SEM images of anode types D and E, after the ALTs of Part 1.a. Images

acquired at 10 kV, with magnifications of 400 x and 2 kx. ........................................... 41

Figure 3-6. XPS survey spectrum of a cathode from the ALT of anode D (top), where the

circled area corresponds to the high-resolution spectrum (bottom) presenting

convoluted peaks of S, Ga and Bi. ............................................................................... 42

Figure 3-7. Cell potentials of consecutive zinc electrowinning tests performed in Mn-free

conditions and at approximate concentrations of 50, 70 and 10 mg/L Mn2+. ............... 43

Figure 3-8. Mn2+ concentrations during the consecutive zinc electrowinning tests. ....................... 44

Figure 3-9. Anode potential values of the 46-day galvanostatic tests, in the first hours of

operation. The vertical dashed line represents the moment of Mn2+ addition. ............. 45

Figure 3-10. Anode potentials (top) and Mn2+ concentrations in the electrolyte (bottom)

during galvanostatic tests with step increases of Mn2+ (indicated by dashed

lines). ............................................................................................................................ 45

Figure 3-11. Cumulative Mn2+ mass depletion after each step addition of Mn2+ during

galvanostatic tests, for anodes D and E. Regression lines were added to

display the data trends more clearly. ........................................................................... 46

Figure 3-12. At the top, anode potentials of the beginning (left) and totality (right) of the

electrowinning tests at ~400 mg/L Mn2+, with anodes D and E. At the bottom,

cell potentials of the same tests. The dashed lines indicate the moments of

Mn2+ dosing. ................................................................................................................. 48

Figure 3-13. Mn2+ concentrations over time, measured from electrowinning tests with step

Mn2+ addition to reach ~400 mg/L at the test start. ...................................................... 48

Figure 3-14. Cell (left) and anode (right) potentials of zinc electrowinning tests performed

for 72 h at pseudo-stationary Mn2+ levels between 5 mg/L and 150 mg/L. ................. 49

Figure 3-15. Average cell potentials of electrowinning tests of Part 2, versus the

approximate Mn2+ levels of these respective cells. ...................................................... 49

Figure 3-16. SEM images of anodes D1, E1 and F1 after zinc electrowinning tests at 0 mg/L

Mn2+ and chemical cleaning. Magnifications of 400 x (top) and 2 kx (bottom). ........... 52

ix

Figure 3-17. SEM images of samples D2 and E2. Magnifications of 400 x and 2 kx. ...................... 52

Figure 3-18. On the left, spatial distribution of Ti signal over a region of the anode D2,

obtained by EDS. On the right, an SEM image of the same location, for reference. ...... 53

Figure 3-19. SEM images of the anode sample F2. .......................................................................... 53

Figure 3-20. Mn2+ deposits over the anode F2 before the cleaning. Magnifications of

250 x (left) and 2kx (right). ........................................................................................... 54

Figure 3-21. SEM images of anodes D1 and E1 after mechanical cleaning. Images

acquired at 10 kV in SE mode. Elemental maps obtained by EDS helped

distinguishing the regions that correspond to MnO2, MMO and the titanium

substrate. ...................................................................................................................... 55

Figure 3-22. EDS spectra of the anode samples D3, E3, D4 and E4, obtained after the

cleaning procedures. The X-ray signal of Mn is only absent in the samples

that were also cleaned chemically (D4 and E4). .......................................................... 56

Figure 3-23. SEM images of anodes D2 and E2, accompanied by respective EDS maps. Data

acquired at 10 kV in SE mode. Magnification of 400 x and 2 kx. ................................. 57

Figure 3-24. X-ray diffractograms of the particulate #1 and the anode type D. ................................ 58

Figure 3-25. X-ray diffractograms of the particulates #2 and #3, accompanied by the

reference patterns of α-MnO2 and MnOOH. ................................................................ 58

Figure 4-1. SEM images of the anode sample type D after the zinc electrowinning test at

5 mg/L Mn2+, before and after chemical cleaning. Magnifications of 400x to 5 kx. ....... 60

Figure 4-2. SEM images of the sample type D after the electrowinning test at 5 mg/L Mn2+,

before and after chemical cleaning. From left to right, magnifications of 22 kx,

17.7 kx, 20 kx and 22 kx. The pores of the MnO2 film are more visible in the

first image. ..................................................................................................................... 60

Figure 4-3. SEM images of the anode type D after the zinc electrowinning test at 10 mg/L

Mn2+. Images with magnification of 2 kx, in SE and BSE modes. ................................. 61

Figure 4-4. Comparison of the anode sample type D after the zinc electrowinning test at

10 mg/L Mn2+, before and after chemical cleaning. Magnifications of 400 x

to 2 kx. ......................................................................................................................... 61

Figure 4-5. SEM image of anode type D after the electrowinning test at 10 mg/L Mn2+.

Magnification of 26 kx. .................................................................................................. 62

Figure 4-6. SEM images of anode type D after the zinc electrowinning test at 10 mg/L Mn2+.

Loose MnO2 particle with unidentified fragments incorporated on it. ........................... 62


Images with magnification of 200x (left) and 400x (right). ........................................... 63


Zoomed out image with magnification of 400 x and close-ups with 2 kx. .................... 64

Figure 4-9. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+

and chemical cleaning. Images with magnification from 400 x and 2 kx. .................... 64

Figure 4-10. SEM images of anode type D after the first zinc electrowinning test at 50 mg/L

Mn2+. Images with magnification of 400 x and 2 kx. ..................................................... 65

Figure 4-11. SEM images of anode D after the second zinc electrowinning test at 50 mg/L

Mn2+. Magnifications of 400 x and 2 kx. Mud crack texture highlighted in the last

image. ........................................................................................................................... 65

x

Figure 4-12. SEM images of anode type D after the first test replicate at 50 mg/L Mn2+ and

chemical cleaning. Magnifications of 400 x and 2 kx. .................................................. 66

Figure 4-13. SEM images of anode type D after the second test replicate at 50 mg/L Mn2+

and chemical cleaning. Magnifications of 400 x and 2 kx. .......................................... 66

Figure 4-14. SEM images of anode type D after the first and second test replicates at

50 mg/L Mn2+, and after chemical cleaning. Magnification of 400 x. .......................... 66


Magnification of 200 x. ................................................................................................ 67

Figure 4-16. SEM images of anode D after the zinc electrowinning test at 75 mg/L Mn2+.

Magnifications of 200 x and 2 kx. The granular texture on MnO2 deposits is more

visible in the zoomed-in images. ................................................................................. 67

Figure 4-17. SEM images of anode type D after the test at 75 mg/L Mn2+ and chemical cleaning.

Magnifications of 400 x, 2 kx and 3.5 kx. .................................................................... 68

Figure 4-18. SEM images of anode type E after the zinc electrowinning test at 50 mg/L Mn2+.

Magnifications of 400 x and 1 kx. ................................................................................ 68


Examples of pits on the anode coating. ...................................................................... 69


Magnifications of 400 x and 2 kx. Note: the colour heterogeneities in the third

image are charging artifacts, due to the semi-conductive nature of the MMO

coating. ........................................................................................................................ 69




Magnifications of 1 kx (first pair) and 400 x (second and third ones).......................... 70


Magnifications of 2 kx and 14 kx. ................................................................................. 70


Magnifications of 2 kx and 6 kx. ................................................................................... 71

Figure 4-25. SEM images of anode type E after the testing at 125 mg/L Mn2+ and chemical

cleaning. Examples of ruptures observed on the smooth regions of the coating. ....... 71


Magnification of 100x (left) and 400x (right). ............................................................... 72

Figure 4-27. SEM images of anode type E after the test at 150 mg/L Mn2+. Spherical MnO2

agglomerates and loose, individual crystallites. Magnifications of 5 kx and 20 kx. ...... 72

Figure 4-28. SEM images of anode type E after the test at 150 mg/L Mn2+. MnO2 deposits

around coating cracks and star-shaped agglomerates. Magnifications of 5 kx

and 20 kx. ..................................................................................................................... 73

Figure 4-29. SEM images of anode type E after the electrowinning test at 150 mg/L Mn2+ and

chemical cleaning. Magnifications of 400 x and 2 kx. .................................................. 73

Figure 4-30. SEM images of anode type F after the electrowinning test at 5 mg/L Mn2+.



Magnifications of 1.5 kx and 3 kx. ............................................................................... 75

xi


Magnifications of 1 kx and 200 x. ................................................................................. 75


Magnifications of 12 kx, 10 kx and 12 kx, from left to right. ......................................... 75


Magnifications of 14 kx and 18 kx. ............................................................................... 76

Figure 4-35. SEM images of a sample F after the test at 100 mg/L Mn2+. Magnification: 400 x. ..... 76


Magnifications of 1 kx (left) and 12 kx (right). .............................................................. 76

Figure 4-37. SEM images of anode type F after the electrowinning test at 100 mg/L Mn2+

and chemical cleaning. Magnifications of 400 x and 2 kx. ........................................... 77


Magnification of 400 x. ................................................................................................. 77

Figure 4-39. SEM images of a sample F after the test at 125 mg/L Mn2+. Magnification: 5 kx. .......... 77


Magnifications of 1 kx, 5 kx and 25 kx. ........................................................................ 78

Figure 4-41. SEM images of anode type F after the test at 125 mg/L Mn2+ and chemical

cleaning. Magnifications of 400 x and 2 kx. ................................................................. 79

Figure 5-1. Hypothetical cost curves of activities relative to manganese control in a zinc

production plant, as a function of Mn2+ concentration. ................................................. 82

Figure A-1. Potentiodynamic polarisation curves of anodes D and E in three

electrolytes. ................................................................................................................. 93

Figure B-1 Anode and cell potential results of each electrowinning test of Part 2. ......................... 94

Figure B-2. Mn2+ concentration profile of anode type D, measured during the zinc

electrowinning tests of Part 2. ....................................................................................... 95

Figure B-3. Mn2+ concentration profile of anode type E, measured during the zinc


Figure B-4. Mn2+ concentration profile of anode type F, measured during the zinc


xii

List of abbreviations, initials, acronyms

[Mn2+] Concentration of Mn2+ ions in the electrolyte

at% Atomic fraction, in percentage

ALT Accelerated life test

BSE Backscattered electron

CER Chlorine evolution reaction

COD Crystallography Open Database

EDS Energy-dispersive X-ray spectroscopy

HER Hydrogen evolution reaction

MMO Mixed metal oxide

MP-AES Microwave-plasma atomic emission spectroscopy

MSE Mercury-mercurous sulphate electrode

NACE National Association of Corrosion Engineers, from the United States

OER Oxygen evolution reaction

R2 Coefficient of determination in regression lines

RLE Roasting-leaching-electrowinning

SE Secondary electron

SEM Scanning electron microscopy

SHE Standard hydrogen electrode

t Variable time in galvanostatic polarization tests

WDS Wavelength-dispersive spectroscopy

x Subscript of atomic ratios in chemical formulas (0 ≤ x ≤ 1)

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction spectroscopy

XRF X-ray fluorescence spectroscopy

xiii

To my beloved mom and dad,

and to all the amazing science professionals

who have inspired me in this journey.

xiv

Acknowledgements

First and foremost, I am profoundly thankful to Prof. Houshang Alamdari for having provided

me the opportunity of working in this project and for being a remarkable mentor throughout

my Master’s. His trust, support, resourcefulness and guidance were fundamental for the

achievement of the successful outcomes that resulted in this thesis. I am equally thankful to

Georges Houlachi for actively following the project and contributing to it with valuable

technical inputs.

I would also like to express my appreciation to Guillaume Gauvin for his assistance in

multiple experimental and analytical activities of this study. This recognition is well deserved

for his outstanding technical competences and positive work attitude. Plus, I would like to send

special thanks to several other professionals and students of the Faculty of Sciences and

Engineering who meaningfully collaborated to the success of my Master’s project, notably

Vicky Dodier, Hugues Ferland, Andrée Lord, Fariba Safizadeh, Nathalie Moisan, Geneviève

Bruneau and Shima Bekhrad. It has been great working with this highly-skilled team.

Finally, I want to acknowledge the financial and technical involvement of De Nora Tech and

Canadian Electrolytic Zinc Ltd. (CEZinc), industrial partners of this project, as well as the

financial support of the Natural Sciences and Engineering Research Council of Canada.

Special thanks to Carl Brown, Takashi Furusawa and Chuck Schultz for the meaningful

discussions and the provisions of MMO anode samples. Also, thanks to Elyse Benguerel,

Vincent Dufault-Bédard and Charles Desroches for contributing with information about the

zinc production process, while representing the perspectives and interests of CEZinc.

xv

Preface

The outcomes of this study have generated a research paper, whose copy is provided in

Appendix C. The first author of the article is Daniela Rossetto de Menezes, also author of

this thesis. The first author has been involved in the design and execution of experiments,

sample characterisations, data curation, results evaluation and writing. The co-authors are

Houshang Alamdari (research director of this Master’s project) and Carl Brown (representant

of the industrial partner De Nora Tech). The article will be submitted to Hydrometallurgy in

June 2021.

1

Introduction

The metallurgical industry of the XXI century acknowledges its economic challenges, its

social and environmental impacts, as well as the respective influences of such factors on

business risk forecasts [1]. Thus, more than ever before, base metal companies are fully

committed to improve their cost-curve positioning and secure their license to operate. This

brings technology development initiatives that target both sustainability and profitability

paybacks to a strategic level.

In particular, there is now a unique opportunity to improve the energy efficiency of zinc

cellhouses, while also mitigating health and environmental risks. It consists of adopting

mixed metal oxide (MMO)-coated anodes, in replacement of lead-based ones. Over 80% of

global zinc production involves a century-old electrowinning step, which employs lead-based

anodes to promote the oxygen evolution reaction (OER) [2] because they are relatively

inexpensive and simple to maintain. However, they correspond to the main cause of energy

losses in zinc cellhouses [3], while their lead corrosion products contaminate the zinc and

form a hazardous slime. The adoption of commercially available MMO anodes is expected

to decrease the cell potential by 300 mV, which corresponds to energy savings of 74 GWh/yr

(or 9%) in a zinc plant with a production capacity of 300 kt/a. MMO anodes, i.e. those

manufactured by De Nora Tech, display excellent durability and catalytic properties for OER

in sulphate electrolytes, and are safe for the cellhouse workplace.

Nevertheless, the implementation of MMO anodes in zinc production requires firstly a solid

understanding of the impact of zinc electrolyte impurities on the long-term anode

performance. In particular, a major concern is that MMO anodes would suffer premature

loss of performance due to MnO2 deposition [4]. The MnO2 anodic surface deposits are

formed from the oxidation of Mn2+ species contained within the commercial electrolyte. In

the current technology where Pb-Ag anodes are used, the concentration of Mn2+ ions in zinc

electrolytes is kept at (1-5) g/L, mainly to decrease the corrosion rate of these anodes. It is

generally agreed that the MnO2 deposition over MMO anodes should be at least limited to a

certain extent, so that IrO2-based catalytic sites continue to be accessible for OER species,

which is necessary to attain both minimal current efficiency loss and minimal anode potential

increase. But there is still no consensus regarding the best approach to control such MnO2

deposition on MMO anodes in zinc electrowinning.

2

In this context, the present study aimed to evaluate De Nora’s MMO anodes in zinc

electrowinning conditions correspondent to those of Canadian Electrolytic Zinc Ltd.

(CEZinc), at laboratory scale. Its initial goal was to provide a preliminary performance

assessment of two IrO2-bearing MMO anode types through medium-term electrowinning

experiments. This assessment was important to confirm the potential energy savings enabled

by such anode technology, as well as to observe the macroscale effects of MnO2 deposition.

Then, the next objectives were to determine the Mn2+ tolerance levels for three types of IrO2-

bearing MMO anodes and to investigate the MMO coating deterioration mechanisms

induced by MnO2. At this stage, it was attempted to progress the understanding of MnO2

deposition effects beyond what has ever been published.

Overall, the outcome of this research would be fundamental to enable the adoption of MMO

anodes in zinc cellhouses. Understanding the relationships between the Mn2+ levels and the

properties of MnO2 deposits formed over different kinds of MMO anode surfaces would help

to propose targets for Mn2+ control in zinc electrolytes. Moreover, preliminary experiments

included in the scope of this research would help to propose procedures for periodical MnO2

removal from the anodes. In turn, such Mn2+ targets and anode cleaning options would be

used to evaluate the technical and economic feasibilities of manganese control strategies

for zinc cellhouses, in order to select the most suitable MMO anode type(s) to proceed to

pilot-scale trials.

Having been studied and openly discussed for over 45 years [5], the properties and

applicability of IrO2-based anodes for OER are still vast research subjects. By further

investigating the long-standing issues related to MnO2 deposition on these anodes, we

aimed to contribute meaningfully to the advancement of engineering solutions that help the

zinc industry meet its green agenda.

3

Chapter 1. Literature Review

1.1. Hydrometallurgical production of zinc using lead-based

anodes

Industrial zinc metal production started in the 19th century, through pyrometallurgical routes.

In smelting furnaces, the calcination of zinc concentrates and reduction to zinc metal is

achieved with coke combustion. The Imperial Smelting Process (ISP), the main smelting

route still in operation nowadays, can process secondary zinc material sources, and obtain

both zinc and lead as products [2, 6, 7, 8]. But technological advancements have allowed

zinc electrowinning to progressively become more relevant than smelting. Today, more than

80% of zinc metal is produced by electrowinning worldwide [2]. A sulphate electrolyte is

generally employed – alternative recovery routes involving alkali or chloride-based solutions

are only used for specific sources of zinc and have a minor commercial contribution [7].

To be incorporated into electrolyte solutions, zinc is extracted from zinc blende, a mineral

concentrate composed mostly of sphalerite ((Zn,Fe)S) with about 50% Zn and 5% to 12%

Fe, and which has undergone flotation or other beneficiation techniques [6]. The extraction

is performed through pressure leaching or roasting-leaching. In pressure leaching, the

mineral concentrates are directly dissolved into sulphuric acid with oxygen gas added, and

elemental sulphur is formed [7]. The roasting-leaching zinc extraction is far more common,

composing the hydrometallurgical route so-called Roasting-Leaching-Electrowinning (RLE).

1.1.1. The Roasting-Leaching-Electrowinning (RLE) process

A typical RLE process starts with the oxidative roasting of zinc blende with air, producing

zinc oxide (also called calcine) and sulphur dioxide (Reaction 1). There is also the formation

of ferrite (ZnO.Fe2O3). The latter does not have a defined stoichiometry because ferrite and

other iron oxides available combine in the form of solid solutions [6].

ZnS + 3⁄2 O2 → ZnO + SO2 Reaction 1

The reaction commonly takes place in fluidized bed roasters, for refined control of sulphate

conversion and minimization of ferrite [6]. The temperature is set at (900-1000) °C and the

pressure is slightly below atmospheric [2, 9]. Heat from the output is recovered through a

waster heat boiler, then the calcine and the gases are separated through cyclones and

4

electrostatic precipitators [9]. The gas stream is sent to a sulphuric acid plant. The calcine

is cooled and ground prior to leaching [7, 10].

Mineral impurities that enter the roasting step form several sulphate and sulphide melts [11].

Thallium and indium can be extracted from flue dust for commercial production [7].

Meanwhile, SO2 gas is converted into sulphuric acid in a separated facility. Selenium and

mercury accompany the SO2 gas stream and are removed in the sulphuric acid plant [9, 11].

The leaching step is designed to maximize recovery of zinc from both calcine (Reaction 2)

and ferrite (Reaction 3), as well as the removal of dissolved Fe2+ (derived from ferrite

leaching). In some zinc plants, Fe2+ is precipitated from the electrolyte in the form of jarosite.

The process features reactors and decanters in series, and multiple recycle streams [12].

ZnO + 2H+ → Zn2+ + H2O Reaction 2

ZnO. Fe2O3 + 8H+ → Zn2+ + 2Fe3+ + 4H2O Reaction 3

Firstly, in the “neutral leach” reactors, roasted solids are combined with spent electrolyte and

raw acid, at a pH between 3 and 4, and a temperature of 70 °C. Then, the solids of the

output slurry are densified in a thickener and transferred to the “low acid” leaching. To

enhance zinc extraction, more acid is added to reach a pH of 2. The calcine reaction is

completed, and as ferrite leaching progresses, part of the Fe2+ dissolved forms ferric

hydroxide (Fe(OH)3). Then, the iron-rich solids are densified in a second thickener and enter

the jarosite circuit. At this point, the temperature (98 °C) and pH (1.0) are more severe, and

the ferrite leaching is completed. [12]

The jarosite conversion from Fe(OH)3 and Fe2+ starts upon the addition of Ca2+, Na+ or NH3+.

Reaction 4 is valid when using a monovalent cation M+ [12]. MnO2 or MnO4- may be added

to maintain the iron oxidation state at 3+ [7]. Also, extra calcine is added for pH control [12].

3Fe3+ + 2SO42− + M+ + 6H2O → M[Fe3(SO4)2(OH)6] + 6H+ Reaction 4

After the last jarosite precipitation tank, the slurry is transferred to a thickener, whose

overflow connects to the low acid leach tank, closing the loop. This overflow stream provides

jarosite seed to promote higher precipitation yield [12]. The underflow stream concentrates

the coarse jarosite crystals, which are washed through counter-current decanters, filtered

and treated according to the Jarofix process [13].

5

A simplified schematic of the RLE process is presented in Figure 1-1, to help visualizing that

the output of the neutral leach part is the zinc-rich electrolyte to be purified.

Figure 1-1. Schematic of RLE process steps.

Electrolyte purification refers to the removal of minor elements that are carried from the

leaching and which could impact the electrowinning performance. Impurities such as Pb, Ag,

Cu, As, Ge, Se, Te, Sb, Si, In, Hg, Ga and Sn exit the leaching circuit partially via co-

precipitation with jarosite, either as hydrous oxides or adsorbed species [6, 12, 14].

Nevertheless, the electrolyte is further treated through sequential cementation steps. The

general effects of impurities commonly associated with zinc electrolytes, as well as their

maximum allowable levels, are summarized in Table 1-1. Manganese is not listed because

it will be discussed in more detail in the next sections.

The cementation occurs through the mixing of impure electrolyte with metallic zinc particles

(zinc dust), which have a diameter of (70-300) μm [6]. The zinc particles oxidize and

dissolve, providing electrons to reduce more electropositive elements. But a large fraction

of such particles remains with unreacted cores because impurities precipitate on their

surface, which is why zinc dust is added at an excess of 5- to 10-fold [2].

6

In process layouts similar to that of CEZinc, the first cementation stage is operated at 70 °C

and leads to electrodeposition of mostly copper and cadmium [10]. The electrolyte is then

filtered, heated to 90 °C, and fed to a second cementation stage, for removal of nickel and

cobalt [10]. Copper sulphate (CuSO4) and antimony trioxide (Sb2O3) are commonly added

to assist the cobalt electrodeposition [6, 14]. The electrolyte is filtered again and is

concentrated by evaporation.

Table 1-1. Effects of common zinc electrolyte impurities, adapted from [6, 7, 15].

Elements Electrochemical

characteristics General effects Tolerance level range

K, Na, Ca,

Al, Mg

Reduction potential far

higher than HER*

These elements do not compete

with zinc for electrodeposition.

They are controlled to maintain

adequate electrolyte viscosity and

conductivity.

The total concentration of

these ions can reach up

to 60 g/L [1].

Ni, Co, Sn,

Fe

Reduction potential

close to zinc*

Electrodeposits

decrease the HER

overpotential

They electrodeposit and re-

dissolve, contaminating the zinc

and decreasing the current

efficiency. Also can cause pits in

the zinc deposited.

Tolerable in very low

concentrations

(0.001-0.1) mg/L, except

iron (up to 25 mg/L)

Cd, Pb,

Tl

Reduction potential

lower than zinc*,

Electrodeposits do not

decrease the HER

overpotential*

They contaminate the product, but

do not interfere with the zinc

electrodeposition or morphology

Tolerable in the range

(0.01-10) mg/L to meet

product quality

specifications

Cu, As,

Sb, Ge,

Se, Te

Reduction potential

lower than zinc*,

Electrodeposits

decrease HER

overpotential*

They electrodeposit and catalyse

the hydrogen evolution,

decreasing the zinc current

efficiency. They also lead to

porous zinc deposits

Tolerable in very low

concentrations

(0.001-0.1) mg/L

Cl, F Anionic species

Cl enhances corrosion of lead

anodes; F affects the cathode-

zinc adhesion

(10-100) mg/L for Cl,

(1-10) mg/L for F

* at typical zinc cellhouse conditions

7

In the electrowinning step, the zinc reduction (Reaction 5) is favoured over the hydrogen

evolution reaction (HER) (Reaction 6), even though zinc is more electropositive than

hydrogen gas in the electrochemical series of metals. This is possible because smooth

aluminum cathodes are employed, which add an overpotential to HER of about 0.7 V. The

zinc metal sheet is harvested from the cathodes about every two days [7].

Zn2+ + 2e− → Zn E°cathode = -0.763 V Reaction 5

H+ + e− → ½ H2 E° = 0.0 V Reaction 6

On the anode side, occurs the oxygen evolution reaction (OER) (Reaction 7):

H2O → 2H+ + ½ O2 + 2e− E°anode = -1.229 V Reaction 7

Thus, the net reaction is:

ZnSO4 + H2O → Zn + H2SO4 + ½ O2 E°cell = -1.992 V Reaction 8

Several process parameters are adjusted to better promote zinc electrodeposition over HER

and to increase productivity. For instance, the electrolyte is recirculated on cooling systems

to release part of the heat generated by the Joule effect so that the temperature remains at

about 40 °C. Higher temperatures accelerate mass transfer and reaction kinetic rates, but

drop the hydrogen overvoltage [7] and increase resistivity [2]. The electrolyte composition

is also controlled to optimize the zinc metal production. Feed electrolyte contains about

(55-70) g/L Zn2+ and (150-180) g/L H2SO4, while spent electrolyte has a Zn2+ drop of only

(5-10) g/L and a slight pH increase [7]. The current density applied is usually (480-600) A/m2

[2, 16]; values above 600 A/m2 significantly increase anode corrosion [17]. In general, the

current efficiency of zinc deposition reached is between 85% and 93% [18].

Multiple zinc plants share similarities in terms of the physical structure of cellhouses.

Cathodes and anodes are alternately placed, with a spacing of (65-90) mm [16]. Isolating

spacers may be used to maintain a uniform electrode distance [7]. The electrode area is up

to 1.8 m2 per face [16]. The cells are lined with acid-resistant polymers. Cellhouses also

present infrastructure for periodical electrolyte purging and removal of cell slime (solid

material decanted in the bottom of cells).

Several additives are used in zinc electrowinning cells. Glue is added to stimulate the

formation of smooth zinc deposits. That is because the colloids inhibit the growth of zinc

8

metal crystals by adsorbing on the nuclei so that the nucleation kinetics become dominant.

This leads to the formation of fine, well-compacted crystals and reduces susceptibility to

electrolyte entrapment [16]. Also, strontium salts (such as SrCO3) are dosed to the

electrolyte to mitigate the contamination of lead in the product. Sr2+ ions added to the cell

readily convert into the less soluble SrSO4, and precipitate incorporating PbSO4 [19].

Furthermore, surfactants are also dosed to modify the surface tension of the electrolyte,

decreasing the rise of acid droplets in the air due to the bursting of O2 bubbles. In

consequence, such additives minimize the formation of acid mist [16], enabling a safer

environment for cell operators and equipment exposed.

The conventional anode for zinc electrowinning is made of Pb-Ag or Pb-Ag-Ca alloys

(usually 0.5%-1% Ag [20]), which will be further discussed in the next sections.

1.1.2. Lead-based anodes on zinc electrowinning

Lead-based anodes have been applied to industrial base-metal electrowinning on sulphate

electrolytes since it started, over a century ago [16, 21]. Compared to other metals, lead

alloys are less expensive and stand fairly well in highly acidic conditions. Plus, after lasting

up to ten years [21], spent anodes can be re-melt to cast new ones within the electrowinning

plants [16]. These factors made lead anodes become the most common option in the industry.

Although the alloying elements of lead anodes help to reduce the anode potential or increase

corrosion resistance, the main electrochemically active component of the lead anode is

PbO2. The PbO2 passivation film forms on the anode surface upon immersion in acidic

electrolyte. It slows down the anode corrosion, and it regenerates whenever the metallic

substrate is uncovered. The good electrical conductivity and electrochemical activity of PbO2

are attributed to its oxygen deficiency [22]. In both polymorphic forms (α-PbO2 and β-PbO2),

it can be regarded as a n-type semiconductor doped with Pb [22]. The vacancy sites on the

crystalline structure are supposed to play an important role in the OER mechanism.

The OER in acidic electrolytes is a four electron-proton coupled reaction, whose mechanism

most commonly agreed on is provided as follows [23]. Firstly, an oxygen atom from a water

molecule associates to a catalytic site (represented as M) in the anode surface (Reaction

9). Intermediaries M-OH (Reaction 10) and M-O (Reaction 11) form, as the H+ ions and

electrons are transferred to the electrolyte. Next, the arrangement M-O can either associate

to a second water molecule, forming an M-OOH complex (Reaction 12), or combine to a -O

9

from another site, forming the O2 directly (Reaction 13). Using lead-based anodes, it is most

likely that the catalytic site M refers to oxygen vacancies of the PbO2 film.

M + H2O(l) → MOH + H+ + e− Reaction 9

MOH + OH− → MO + H2O(l) + e− Reaction 10

MO + H2O(l) → MOOH + H+ + e− Reaction 11

MOOH + H2O(l) → MOOH + H+ + e− Reaction 12

2MO → 2M + O2(g) Reaction 13

One major drawback of lead-based anodes is their high OER overpotential in zinc

electrowinning, of (600-800) mV [21]. Amongst typical sources of energy loss in zinc cells

reported [3, 24], the anodic overpotential has the most significant contribution (Figure 1-2).

For instance, with an average cell potential of 3.5 V [18], an anodic overpotential of 600 mV

corresponds to an energy loss of about 0.5 kWh per kilogram of zinc produced.

Figure 1-2. Breakdown of sources of ohmic drop in zinc cells. Based on [3].

The wear of lead-based anodes also induces operational issues. The α-PbO2 film formed

on a fresh anode surface tends to convert to β-PbO2, according to thermodynamic aspects

[22]. Plus, this transition is said to be more prominent when the anode potential is decreased,

for example in the advent of operational variability or interruptions [25]. The beta polymorph

is less adherent, and it flakes off from the anode surface over time [25]. The polymorphic

transition causes the formation of other lead corrosion products as well, such as PbO,

Pb(OH)2, and PbSO4 [26]. Suspended in the electrolyte, these PbO2 flakes can contaminate

10

the zinc electrodeposited. Also, the sedimented flakes form cell slime, which must be

periodically removed from the bottom of cells. Typically, the loose scales must be cleaned

off of anodes every about two weeks [27]. In any case, the management of lead-containing

residues demands special precautions against health and environmental risks.

The lead-based anodes are relatively soft, so the anode sheets tend to warp over time. This

phenomenon is due to residual stresses formed during the anode manufacture, as well as

heterogeneities of current distribution and corrosion processes at a microstructural level [28]

[17]. The anode bending worsens the uniformity of current distribution and can cause short

circuits, which damage the anodes [25]. Progress has been made to improve the mechanical

strength of lead anodes, in terms of selection of alloying elements and manufacturing

techniques (such as cold rolling rather than casting) [29]. Nevertheless, the recurrent anode

straightening services required in the long term still add up to the operational costs of

conventional zinc cellhouses.

1.1.3. The role of manganese in the conventional RLE process

Manganese can enter into the RLE process as a mineral impurity or an additive for ferrite

leaching (in the form of manganese dioxide, MnO2 [7], or potassium permanganate, KMnO4)

[30, 31]. In the electrowinning step, the Mn2+ ions oxidize, forming MnO2 (Reaction 15) [32]:

Mn2+ + 2H2O → MnO2 + 4H+ + 2e− E°MnO2 = 1.223 V Reaction 15

At higher anodic potentials, permanganate ions (MnO4-) are also formed, supposedly from

Mn2+ (Reaction 16) and MnO2 (Reaction 17) [32]:

Mn2+ + 4H2O → MnO4− + 8H+ + 5e− E°MnO4

- =1.512 V Reaction 16

MnO2 + 2H2O → MnO4− + 4H+ + 3e− E°MnO2- =1.705 V Reaction 17

There are a few reaction mechanisms proposed for the electrochemical formation of MnO2.

In acidic solutions, at relatively low anodic potentials, it has been suggested that Mn2+

species are firstly oxidized to Mn3+ (Reaction 18) [32]:

Mn2+ + Mn3+ → e− Reaction 18

The trivalent manganese ions may convert to Mn4+ by disproportionation (Reaction 19), and

then to MnO2 by hydrolysis (Reaction 20) [33]:

11

2Mn3+ → Mn2+ + Mn4+ Reaction 19

Mn4+ + 2H2O → MnO2 + 4H+ Reaction 20

Also, Mn3+ ions may form the intermediary manganite, MnOOH (Reaction 21) [34, 35, 36]:

Mn3+ + 2H2O → MnOOH + 3H+ Reaction 21

Then, further oxidation would occur (Reaction 22):

MnOOH → MnO2 + H+ + e− Reaction 22

In typical zinc cellhouses, the electrodeposition of smooth and thin MnO2 layers over lead-

based anodes is beneficial for the following reasons:

• Firstly, the MnO2 can decrease the corrosion rate of lead anodes. A compact

MnO2 film serves as an efficient barrier to oxygen and chlorine diffusion into the lead

substrate, which is the limiting step in the anode corrosion process [25].

• The MnO2 film can decrease the anode potential. Some forms of manganese

oxides [37, 38] and Pb-MnO2 anode composites [39, 40] are known to present

superior catalytic properties to OER than PbO2. In fact, fresh α-MnO2 deposits on

lead anodes decrease the overall anodic potential [31, 41]. Nevertheless, other non-

active manganese oxide compounds (such as γ-MnO2 [42] and the non-conductive

MnOOH [36]) are likely to accumulate on the anode surface over time.

• Additionally, MnO2 hinders the anodic formation of chlorine gas. MnO2 deposits

are not electrochemically active for chlorine evolution reaction (CER), so they

decrease the formation rate of Cl2 [43].

To obtain the benefits described above, the concentration of Mn2+ in zinc electrolyte is

controlled around 1 g/L, but spikes can reach up to 15 g/L [18]. Excess Mn2+ intensifies the

MnO2 deposition on lead anodes, causing extra ohmic resistance and anode flaking. As

previously mentioned, the lead anode flakes detach easily, contaminating the zinc product

and forming cell slime. On the other hand, when manganese inputs from the zinc ore and

leaching additives drop, anode flakes are intentionally re-added in the electrolyte upstream,

to convert them back to Mn2+.

12

Such operational aspects linked to the MnO2 formation on lead anodes are very particular

to zinc electrowinning. For comparison, copper electrolytes contain less than 1 g/L Mn2+ [44]

[21]. In copper electrowinning, the lead anode corrosion is controlled by dosing cobalt ions

in the electrolyte, so there is no need to maintain high manganese levels. Furthermore,

copper cellhouses can tolerate a certain level of Fe ions in the electrolyte which moderates

the MnO2 formation on the anodes [45]. Both practices are not applicable in zinc

electrowinning, because both cobalt and iron greatly impact the zinc electrodeposition

efficiency, as mentioned in Section 1.1.1.

1.2. MMO anodes for OER

Lead-based anodes have been applied to the electrowinning of base metals in sulphate

electrolytes since the first cellhouses were established. The industry has been unanimous

on preferring these anodes even with their operational drawbacks taken into account

because finding a better alternative to them has been a major challenge up to recent years.

A suitable anode to replace lead-based ones should combine high catalytic activity

specifically for OER, high electrical conductivity and low corrosion rates in strongly acidic

media. Other characteristics such as low acid mist production, long chemical stability and

high mechanical strength are also desirable for industrial operations. Furthermore, such

material should be safe, affordable and broadly available to reach commercial use [27].

Nowadays, the mixed metal oxide (MMO)-coated anodes have become the most promising

technology competing with conventional lead anodes. In particular, those with an IrO2-based

coating and titanium substrate are seen as the best candidates for the electrocatalysis of

OER in sulphate-based electrolytes [46].

1.2.1. Development background

Electrodes with a noble metal coating were firstly envisioned by two independent research

centers. In the 1950s, the Metals Division of the British company Imperial Chemical

Industries (ICI) started the studies on platinum-coated titanium electrodes. Meanwhile, a

rhodium-plated titanium electrode was created by Henri B. Beer from Magneto Chemie, in

The Netherlands [47]. Following patent disputes, both companies ended up involved in the

early developments to replace graphite anodes with titanium-based ones in the chlor-alkali

industry [47, 48]. These new anodes would provide lower overpotentials for chlorine

evolution reaction (CER) than conventional ones and could be operated at higher current

13

densities [47]. In the next decades, the durability, efficiency for CER, and manufacturing

costs of these anodes were improved by the optimization of manufacturing methods and

coating formulations [49].

It was later observed that anode coatings composed of oxides of the platinum group metals

(PGMs) also presented good electrocatalytic properties for CER and OER. The use of PGM

oxides was preferable over the metallic coatings because it demanded a lower content of

expensive noble metals. Improved stability of PGM oxide coatings over titanium substrate

was obtained by adding oxides of other valve metals [49]. This family of MMO-coated

anodes were registered by Diamond Shamrock Corporation as “Dimensionally Stable

Anodes” (DSATM), an allusion to their superior performance over conventional carbon ones,

which are degradable [50]. By this time, chlor-alkali plants were successfully implementing

titanium-based anodes with coatings of Pt-IrO2. Later, they shifted to RuO2-TiO2 due to

reduced costs [51, 52]. Overall, the use of MMO anodes for CER has enabled the transition

from mercury-type chlorine cells to membrane cells [49].

In the following decades, the applications of MMO-coated anodes continued to expand. The

first coating formulation combining tantalum and iridium oxides for OER was patented in

1967 [5], targeting electrowinning processes with sulphate or nitric-based electrolytes.

Meanwhile, anodes for CER evolution have been incorporated into electrowinning

processes of nickel [16], cobalt and copper, performed in chloride-based electrolytes. In

recent decades, these MMO anodes reached applications to water treatment,

electrorefining, electroplating, corrosion protection in buildings, metal recovery from dilute

solutions and other processes [4, 21].

From the zinc industry standpoint, a major milestone for the expansion of MMO anode uses

was achieved when copper tankhouses started to adopt this technology. That is because

both zinc and copper electrowinning processes are generally carried on sulphate-based

electrolytes, have similar layouts and follow process parameters of comparable value range,

as exemplified in Table 1-2. Thus, they are generally able to exchange and accommodate

new process technologies from each other with minimal modifications [16].

In the early 2000s, “Mesh on Lead” (MOL™) anodes were developed by ELTECH Systems

Corporation for sulphate-based electrowinning of base metals, especially copper. These

anodes were composed of a MMO-coated titanium mesh supported on a lead base. Having

a lower OER overpotential than typical lead anodes, they were expected to decrease the

14

specific energy consumption of tankhouses by (12-17)% [53]. A few years later, Freeport-

McMoRan Copper & Gold Inc. developed another type of MMO-coated titanium anode (so-

called “Alternative Anode”). This one was then implemented to a sulphate-based copper

electrowinning plant in Chino, New Mexico, which became the first of its kind to fully operate

with MMO anodes in 2008. Since the anode replacement, the plant has reported energy

savings in the order of 15% [16, 54].

Table 1-2. Typical process conditions of zinc vs. copper electrowinning, based on [16, 55, 56, 57, 58].

Parameter Zinc electrowinning Copper electrowinning

Current density (A/m2) 480 – 600 300 – 400

Temperature (°C) 38 – 40 45 – 50*

Metal ion concentration (g/l) 50 – 65 (Zn2+) 45 – 50 (Cu2+)

H2SO4 concentration (g/l) 150 – 200 150 – 200

Mn2+ concentration (mg/l) 1000 – 8000 10 – 300

Smoothing agents** glue, licorice glue, guar, polyacrylamides, saccharides

Pb anode alloying elements Ag, Ca, Sn, Al Sb, Ca, Sn, Co

Cathode type aluminum 316L stainless steel, copper

Electrode spacing (mm) 65 – 90 47.5 – 50

Cell potential (V) ≈ 3.5 ≈ 2.0

Anode potential (V) ≈ 2.1 ≈ 1.8

Cathode cycle (days) 1 – 2 5 – 7

* Chino has been operating at (30-34) °C after the implementation of MMO anodes [54].

** Copper electrolyte may also contain thiourea and traces of solvent extraction chemicals.

1.2.2. MMO anode constitution and properties

Generic MMO-coated anodes are generally composed of a substrate that contains one or

more valve metals alloyed (such as titanium, tantalum, zirconium, niobium, aluminum and

tungsten) [5]. The substrate is usually composed of titanium, even though improved

corrosion resistance may be attained through additional films of tantalum [59] or other metals.

15

The role of the valve metal substrate is to allow charge transfer only through the substrate-

coating interface, while isolating metal-electrolyte interfaces to prevent corrosion [60]. Valve

metals form dense and adherent oxide passivation films which provide effective corrosion

protection in acidic electrolytes, at anodic potentials commonly applied to electrowinning of

base metals (as is the case for titanium [61]) [60]. Meanwhile, the combination of valve and

noble metal oxides, in the coating-substrate interface, results in good electrical conductivity.

The electrocatalytic activity of the MMO anode coatings is provided by oxides of one or more

noble metals such as platinum, ruthenium, iridium, palladium, rhodium or osmium [5]. Valve

metal oxides are usually added to the coating to support and enhance the noble metal

catalyst [62]. The noble metal oxides can easily and reversibly transit to different oxidation

states with low energy barriers involved, even when incorporated within oxide structures.

Such oxidation state transitions are part of the OER or CER mechanisms proposed for MMO

anodes.

In particular, the OER mechanism on IrO2-bearing MMO anodes has been proposed as

follows [63, 64]. In the first three steps, iridium cations are oxidized: from Ir3+ to Ir4+ (Reaction

23), from Ir4+ to Ir5+ (Reaction 24), and then from Ir5+ to Ir6+ (Reaction 25). The resulting IrO3

species are unstable in acidic solutions and easily liberate O2 (Reaction 26).

Ir(OH)3 ↔ IrO(OH)2 + H+ + e− Reaction 23

IrO(OH)2 ↔ IrO2(OH) + H+ + e− Reaction 24

IrO2(OH) ↔ IrO3 + H+ + e− Reaction 25

IrO3 + H2O ↔ ½ O2 + IrO(OH)2 Reaction 26

Additionally, the hexavalent iridium oxide, IrO3, can also be converted into IrO42- and suffer

dissolution in acids (Reaction 27). This has been reported as one of the forms of deterioration

of MMO coatings [63, 65].

IrO3 + H2O ↔ IrO4−2 + 2H+ Reaction 27

The most common noble metal oxides used in MMO coatings are RuO2 and IrO2, for CER

and OER, respectively. Both electrocatalysts have been largely studied for water

electrolysis. RuO2 was shown to provide lower OER potentials than IrO2, and some ternary

16

mixtures containing both RuO2 and IrO2 have shown better catalytic performance than their

individual counterparts in laboratory trials [66, 67, 68, 69]. Both oxides form a rutile-type

structure, and the synergistic effect of combining IrO2 and RuO2 has been explained by the

formation of a solid solution when the content of RuO2 (in molar basis) is at least 30% [68].

Nevertheless, RuO2 is more susceptible to dissolution in acidic electrolytes than IrO2. In fact,

ruthenium achieves higher valence states than Ir (up to VIII) and forms the intermediate

RuO, which is even more unstable than IrO3 and other iridium species in acidic electrolytes.

The deterioration of Ru-bearing MMO coatings has been linked to RuO formation [65].

Several attempts have been made to improve the stability of RuO2-based MMO anodes for

OER electrocatalysis [66, 70], but they have not achieved commercial use so far. Therefore,

the most recommended MMO coatings for OER are based on IrO2.

Commercially available MMO anodes are generally manufactured by painting and thermal

decomposition [5, 47]. In this process, the valve metal substrate is firstly etched to enhance

the adhesion of the oxide coating. Then, the substrate is dipped into, rolled, electrostatically

sprayed or brushed with a paint, which is a solution containing organic solvents (such as

alcohols) and precursor salts of noble metals (such as chlorides). After this application, the

anode is subjected to a thermal treatment (generally at 300-550 °C), in which the solvents

are eliminated and the salts are converted into their respective oxides. The steps of painting

and thermal treatment are repeated multiple times. The final MMO coating can be composed

of dozens of layers [60], even though it has a total thickness of only a few microns [62]. This

technique usually yields MMO anode surfaces with pores and mud-crack features, which

form during drying and calcination steps [70, 71].

The properties of MMO coatings can be optimized by adjusting the crystal structure, the

crystal orientation and the morphology of the mixed metal oxide layers, which in turn, depend

on the paint composition, paint application method and calcination temperature, for example

[62, 70, 72, 73]. In fact, the catalytic activity of some IrO2-based coatings varies according

to the orientation of IrO2 nanocrystals formed on the MMO coating surface, and such

orientation changes depending on the calcination temperature [74, 75]. Added to that, lower

calcination temperatures (from 300 °C to 350 °C) provide lower degrees of crystallization,

which generally increase the OER overpotential but decrease the coating stability in the

electrolyte [73, 76].

17

IrO2-based coatings are often combined with Ta2O5, which provides effective protection

against anode passivation and assists with the stabilization of IrO2 [75]. The Ta2O5

component is mostly found in amorphous phase and it only forms solid solutions with IrO2

and TiO2 in very low percentages (as 2.5% in molar basis [77]). Even small additions of

Ta2O5 can decrease the overall crystallinity degree of the coating and modify its surface

morphology [77].

Besides, modern commercially available MMO anodes can also incorporate dopants and

oxides of elements other than valve metals, notably tin [73]. MMO coatings containing IrO2,

SnO2, and dopant elements such as Bi, Sb, Ta or Nb display comparable durability to other

benchmark MMO anodes, but relatively higher catalytic activity and higher tolerance against

electrolyte impurities such as fluorides and manganese ions [76] (which will be discussed in

the following sections). The enhanced catalytic activity has been explained by the fact that

such metal oxide combinations yield smaller crystallites of electrochemically active

components (with dimensions inferior to 5 nm, on average) [76, 78]. Furthermore, the

formation of a TiO2-SnO2 compound at the substrate-coating interface was shown as very

effective at protecting the anode against passivation, and the addition of Sb as a dopant to

SnO2-bearing coatings provided superior electrical conductivity [64, 79, 80].

Taking those aspects into consideration, MMO anodes can combine layers of different

properties and purposes. Outer MMO layers can be designed to maximize the

electrocatalytic activity, while inner layers can be optimized to better protect the substrate

from corrosion. For instance, the coating of a Ti/IrO2-Ta2O5 anode type developed by

Freeport-McMoRan Copper & Gold Inc. is composed of a crystalline under-layer and an

amorphous top-layer [55]. Other proposed MMO anode types contain interlayers of different

elemental compositions [81].

1.2.3. Pathways of MMO anode deterioration

1.2.3.1. Deterioration processes in ideal conditions

Understanding the processes that lead to the deterioration of MMO anodes is fundamental

to evaluate their applicability to industrial uses. Using accelerated life tests (ALTs) with

impurity-free synthetic electrolytes, three processes have been found to impair the

performance of MMO anodes containing IrO2 or RuO2 [72, 82, 83]:

18

• Erosion of the coating. Wear of coating outer layers can occur due to stress

promoted by bubbling of oxygen gas over time. MMO coatings can suffer a decrease

of roughness and porosity at the beginning of the service life [83].

• Dissolution of the electrocatalytic components. MMO anodes with IrO2 or RuO2

can suffer dissolution over time because they are supposedly converted into less

stable species in OER electrocatalytic cycles (IrO3, RuO), as previously discussed.

• Passivation of the valve metal substrate. Oxygen species can penetrate the

coating and reach the fresh substrate metal, where they form new oxide layers of

inferior electrical conductivity. The oxygen diffuses through discontinuations in oxide

layers (such as cracks [68], pores) and also through the crystal structure of the oxides

(either by crossing interstitial regions or by migration of oxygen vacancies [84])

(Figure 1-3). Once the passivation layers develop, the anode fails due to coating

detachment and/or interruption of current flow [82].

Figure 1-3. Rutile-type structure, where interstices are marked by black dots (left) and oxygen diffusion through vacancy sites (right). 3D model from [86].

It is important to note that the ALTs, generally used to study these deterioration processes,

employ current densities several times higher than industrial conditions do, in order to speed

up the reactions of interest. But such high currents cause the anodes to operate with

proportionally high potentials, which can end up triggering reactions that would not occur

otherwise at typical operational conditions, even after a long service time. As an example,

TiO2 oxide films are generally stable at low anodic potentials. However, they have been

shown to undergo a crystal phase transition (from amorphous to anatase) when titanium is

anodized to 6 V or higher, which turns the metal more vulnerable to corrosion and

subsequent passivation [85]. This relationship between service potential and passivation of

19

titanium-based anodes agrees with the results of NACE-standard ALTs: the higher the

corrosion resistance of the Ti alloy used as a substrate, the longer was the MMO anode

lifetime demonstrated [59].

1.2.3.2. Other processes that accelerate anode deterioration

The environmental conditions in which the MMO anodes are applied can contribute to

additional deterioration processes. For instance, short-circuiting, current reversal and certain

electrolyte impurities are known to compromise the anode service life.

Short circuits are problematic yet recurrent events in the electrowinning of base metals,

typically due to the growth of cathodic dendrites that contact the anode surfaces. (The

dendrites, in turn, are generally induced by suspended solid particles that reach the

cathodes [87]). In brief, the shorts produce intense heating and polarity reversal in the point

of contact. In cells operating with lead-based anodes, this weakens the PbO2 film locally,

which accelerates the lead corrosion and ultimately ends up carving holes through the anode

structure [25]. MMO anodes are also vulnerable to this issue: shorts can open holes on mesh-

type structures [88].

Partial cathodization of MMO anodes also occurs often in electrogalvanizing cells [89], which

is undesirable since it accelerates the anode deterioration [89]. In acidic electrolytes and at

reversed polarity, the noble metal oxides promote the hydrogen evolution reaction (HER).

Then, nascent hydrogen species attack the anode substrate, forming titanium hydride. This

process leads to detachment of the MMO coating [46]. Also, the current reversal is said to

enhance the MMO coating dissolution [90], by encouraging the formation of Ir2O3 [46] and/or

loss of Ta2O5 [89]. Plus, the higher the reversal current density, the worse the effect [89].

Only a few types of MMO coatings can withstand reverse polarization in specific low-current

applications for a reasonable duration, such as the CE-600 designed by Eltech Corporation

[91]. That is because polarity reversal is used as a procedure for cleaning MMO anodes in

water treatment or pool chlorination [90]. In these applications, reversal cycles, in the order

of minutes, are meant to prevent the overaccumulation of solid particles (such as hard water

scales) on the cathodic surfaces [90].

Added to those issues, several electrolyte impurities are known to potentially affect the

integrity of MMO anodes. For example, zinc electrolytes usually contain about 10 mg/L of

fluorides, which can enhance the dissolution of IrO2-Ta2O5 coatings. At fluoride levels of (25-

20

200) mg/L, IrO2 and Ta2O5 are supposedly converted into the soluble species IrF6 and TaO2,

respectively [92]. As the MMO coating is consumed in these reactions, the metallic substrate

becomes more vulnerable to passivation, which fluorides can also attack [46]. This results

in increased anode potential over time and shorter service life.

Certain organic compounds are also known to impair the MMO anode performance. These

organics can result from the ore, process additives or certain contaminants. The most

prominent electrowinning additives act on the improvement of cathode quality and

suppression of acid mist. It turns out that IrO2-bearing MMO anodes have shown higher

corrosion rates when operating in the presence of thiourea [93], which is a common grain

refiner used in copper electrowinning [57, 94]. But attention must be made to the fact that

some of these anodes had outdated formulations: they included platinum and were less

stable even in the absence of organics [93]. Platinized titanium anodes applied to chlor-alkali

operations have also shown vulnerability to naphthalene trisulfonic acid, and a few other

coating formulations have been affected by the presence of oils, sugar, concentrated

seaweed, and wetting additives [47]. Fortunately, so far, there has been no evidence that

typical zinc electrowinning additives (such as glue or licorice) could enhance the

deterioration of MMO anodes.

Nevertheless, amongst all the electrowinning conditions previously discussed, one major

threat to the performance of MMO anodes is the presence of manganese ions in the

electrolyte. This topic is covered in detail in the next section.

1.3. The effect of Mn2+ ions to the performance of MMO anodes in

zinc electrowinning

Since MMO anodes were first considered for use in zinc cellhouses, MnO2 deposition was

predicted to be problematic to their long-term performance [4]. Compared to other sulphate-

based electrolytes, typical zinc electrolytes contain higher Mn2+ concentrations, which tend

to intensify the degree of MnO2 deposition, and consequently, the anode potential increase.

This seems to be the main reason why MMO anodes were not yet introduced to zinc

cellhouses, even though they have already been successfully implemented in copper

electrowinning plants almost a decade ago [21].

To investigate this issue, several studies have characterized the MnO2 deposition on MMO

anodes using mostly short-term electrochemical techniques. The MnO2 deposits have been

21

described as compact and very adherent to the MMO surfaces, when compared to the easily

detachable MnO2 scales of lead anodes [27, 95]. In fact, the MnO2 deposits formed on MMO

anodes seem to have similar degrees of adhesion to those formed over sintered Pb-MnO2

anodes [41, 42]. As such, they may difficult the mobility of OER species in and out of catalytic

sites of the MMO coating. Thus, the anode potential increase could be attributed to such

surface coverage effect.

But if some crystalline forms of MnO2 are electrochemically active, could MnO2 deposits

assist with the OER catalysis, perhaps compensating for the IrO2-based catalytic sites

covered underneath them? Probably not. From one side, α-MnO2 deposits can effectively

promote the OER over Pb-Ag anodes [31], and this phase seems to form over

Ti/Ru1/3Sn2/3O2 anodes at 15 g/L Mn2+ [95]. But on the other side, this crystalline phase has

higher OER overpotentials than typical IrO2-bearing MMO coatings. Plus, the compact and

smooth MnO2 deposits decrease the electrochemically active surface area of the MMO

anodes [27]. Besides, MnO2 layers [27] (especially forms α and β [96]) and MnOOH [97] can

add ohmic resistance to the MMO surface. These aspects indicate that MnO2 deposits even

lead to non-uniform current distribution across the anode [46], ultimately resulting in current

efficiency loss.

1.3.1. Parameters that influence MnO2 deposition characteristics

Several studies indicate that the rate of MnO2 deposition on MMO anodes is increased at

higher Mn2+ concentrations in the electrolyte [27, 98, 99], similarly to what is observed on

typical lead anodes [97]. Also, at Mn2+ concentrations in the electrolyte as low as 10 mg/L,

it was shown that the formation of MnO2 deposits has very low rates, but still occurs [27].

From this perspective, the MnO2 deposition on MMO anodes seems unavoidable in zinc

cellhouses and decreasing the Mn2+ concentration of the electrolyte alone would only make

the issues of MnO2 deposition take longer to be revealed.

However, the presence of Mn2+ species in sulphate electrolytes does not preclude the use

of certain MMO anode types in certain electrowinning environments. In fact, some

commercially available Ti/IrO2-Ta2O5 anodes are able to operate well in copper electrolytes

purified by ion exchange, which have Mn2+ levels of (10-200) mg/L [58]. For instance,

alternative anodes developed by Freeport-McMoRan Copper & Gold Inc. have been

sustaining a stable anode potential while operating at 300 mg/L Mn2+ [55]. This stability has

been linked to the fact that MnO2 growth is inhibited by a copper levelling agent dosed to

22

these cells, named Cyquest 900 [55]. The effect of this additive was confirmed in further

laboratory tests and it was supposedly attributed to its polyacrylamide groups [100], but the

MnO2-inhibition mechanisms involved were not yet understood.

It is important to note that such results do not imply that organic additives can eliminate the

MnO2 deposition in absolute – they may simply modify its properties and/or limit the growth

to a certain extent. As long as the MMO anode with a MnO2 film of minimal thickness

demonstrates a low, stable anode potential over time, it could effectively provide energy

savings to the cellhouse. In other words: MnO2 deposits may be harmless to the MMO anode

performance if restricted to a certain quantity or controlled to a specific morphology.

The modification of MnO2 deposition characteristics may be achievable not only through the

use of additives, but also by improving the design of the MMO coating itself. Recently, De

Nora’s new dimensionally stable anodes (DSAs) have attained potential stability operating

at (100 ± 50) mg/L Mn2+ in long-term copper electrowinning tests [88]. Compared to previous

DSA models, these anodes are said to present differences in crystalline lattice and crystallite

size. Another MMO anode composed of amorphous IrO2, recently patented, was said to

present a highly catalytic surface that prevents the formation of MnO2 [101, 102]. However,

amorphous IrO2-based coatings are generally obtained through the use of low thermal

treatment temperatures (< 350 °C), and these are said to be less durable, for having an

accelerated release of soluble iridium species [73, 76]. All in all, there is little open

information about how such anode materials achieve this so-called “manganese tolerance”.

It is also unclear whether MnO2 deposits form with different morphologies or crystalline

phases depending on the anode surface characteristics.

The rate and extent of MnO2 deposition on MMO anodes also depend on operational

parameters of the electrowinning cell. Using cyclic voltammetry experiments and platinum

electrodes, it has been observed that lower temperatures decrease the MnO2 deposition

rate, while higher electrolyte acidity levels increase the potentials of intermediary steps of

MnO2 formation [27]. These relationships are reflected on process parameters selected for

industrial production of electrolytic manganese dioxide (EMD): temperatures of (80-98) °C and

H2SO4 concentration of (0.5-1.5) mol/L are applied to obtain high current efficiency [103].

The electrolyte pH and anodic potential during the electrowinning service also influence the

reaction paths of Mn2+ oxidation. In acidic electrolytes, at lower oxidation potentials, is

generally agreed that the MnO2 formation route involves manganic ions (Mn3+) and MnOOH,

23

while higher oxidation potentials promote the formation of permanganate ions (MnO4-) as

intermediate species [104]. But the temperature, current density and electrolyte acidity of

zinc cellhouses should not be modified to decrease the extent of MnO2 formation, because

they are already optimized for the production of zinc.

1.3.2. A manganese control strategy to implement MMO anodes in

zinc cellhouses

Considering the effects of MnO2 deposition, as previously discussed, the introduction of

MMO anodes to zinc cellhouses is expected to require a manganese control strategy. In

other words, one or more of the following process adaptations could accompany the anode

transition:

• The Mn2+ concentration may need to be decreased one or two orders of magnitude

relative to the current levels (dropping to ~10 g/L or ~100 mg/L Mn2+), to slow down

the MnO2 deposition rate.

• The MMO anodes may need to be cleaned periodically, for the removal of

accumulated MnO2 deposits over time.

• The use of additives may still be required to inhibit the MnO2 formation or at least to

modify its characteristics, for example yielding deposits more porous or less adherent

to the MMO coating.

• The prevention of Mn2+ buildup in the electrolyte loop may be carried by an

alternative MnO2 deposition process outside the cellhouse. (Such Mn2+ buildup is

more likely to occur using MMO anodes since they “produce” less MnO2 than current

Pb-Ag anodes).

• The current cell sludge cleaning routine may be replaced by a new procedure that

prevents the accumulation of MnO2 particles in the cells.

In consequence, the updated zinc RLE process could include:

• An accessory electrolyte purification system for Mn2+ removal, to avoid Mn2+ buildup

and/or to decrease the Mn2+ concentration in the cell;

• An anode cleaning system designed for MMO anodes;

• An additional system for additive dosing.

24

In the scope of an anode replacement project, once the MnO2 effects on a selected MMO

anode type are further investigated, the pertinent cellhouse adaptations can be specified.

Then, the manganese control strategy should be taken into consideration in the feasibility

evaluation of such project.

1.3.3. Anode cleaning options

In the long term, excessive accumulation of MnO2 layers on MMO anodes may be

unavoidable. If so, it would be necessary to apply anode cleaning procedures for a periodical

removal of MnO2. Based on the available literature, a few MMO anode cleaning options for

MnO2 removal are discussed in the following paragraphs. Favourable cleaning options

should be harmless to the anode integrity, efficient for MnO2 removal, cost-effective,

practical to implement in the industrial environment, and should involve minimum health or

environmental risks. Based on such criteria, the options involving highly concentrated

mineral acids and strong reducing agents would be less desirable, so they have not been

considered in this discussion.

Mechanical cleaning: stripping of MnO2 excess is relatively trivial to Pb-based anodes

because their surface is flat, MnO2 flakes are easily detachable, and the PbO2 layers are

regenerative. Although PbO2 and MnO2 covers formed during the anode conditioning have

superior corrosion protection [105], eventual scratches left by stripping do not compromise

the performance of lead anodes permanently. However, MMO anodes have high rugosity,

so that stripping could easily scratch surface peaks, while not completely removing MnO2

deposits accumulated in valleys, between mud-cracks or other deep features. Plus, overall,

forcing out strongly adhered MnO2 patches may cause the detachment of fragments of the

coating [46]. Still, the removal of excess MnO2 deposits by mechanical means may be

doable if designed accordingly.

Electrochemical cleaning: this option is based on the understanding that the MnO2

formation involves electrochemical steps and it is reversible. According to the literature, the

mechanism of MnO2 deposition on Ti/IrO2-Ta2O5 anodes (in acidic pH and at low anodic

potentials) is supposedly similar to the mechanism associated to lead anodes, presented in

Section 1.1.3. This mechanism would start with the oxidation reaction of Mn2+ to Mn3+,

leading to the formation of the species Mn4+ or MnOOH. Then, the MnO2 deposits would be

ultimately formed by the hydrolysis of Mn4+ ions or the decomposition of MnOOH [27]. Thus,

25

on an inverse sense, MnO2 reduction also supposedly involves the intermediary species

MnOOH and Mn3+ (Reactions 28 and 29) [27]:

MnO2 + H+ + e− → MnOOH (Reaction 28)

MnOOH + 3H+ → Mn3+ + 2H2O (Reaction 29)

Added to that, Mn2+ species may react with MnO2 deposits directly, and the resulting Mn3+

species would return back to the oxidation state 2+ (Reactions 30 and 31, respectively) [27]:

1

2MnO2 +

1

2Mn2+ + 2H+ → Mn3+ + H2O (Reaction 30)

Mn3+ + e− → Mn2+ (Reaction 31)

In laboratory tests, the reversibility of MnO2 deposition on MMO anodes has been explored

through quick electrochemical analyses. For example, linear voltammetry scans at 1 mV/s

[27, 100] and potentiostatic polarizations at 0.968 V vs. SHE for 10 min [98] have been used

for cleaning small amounts of MnO2 deposits. But these MnO2 reduction procedures take

about as long as the anode service time itself, in which MnO2 deposits were allowed to form.

While such low time efficiency is acceptable in short laboratory experiments, it becomes

unpractical in industrial operations: at any given moment, only 50% or less of the total MMO

anode capacity would be in actual use. The application of periodical pulses at 0.850 mV vs.

MSE was also attempted to prevent the accumulation of MnO2 deposits in long operations,

which would not only be inconvenient for zinc deposition but also seemed to deteriorate

Ti/IrO2-Ta2O5 anodes [27], according to experimental evidence. Overall, the question of how

to clean MMO anodes effectively by electrochemical methods remains open for additional

study and discussion.

Cleaning by FeSO4 solution: MMO anodes may be also cleaned by reducing agents such

as ferrous sulphate (FeSO4). In fact, the reduction of MnO2 is very favourable in the presence

of Fe2+ ions (Reaction 32) [27]:

MnO2 + 2Fe2+ + 4H+ → Mn2+ + 2Fe3+ + 2H2O (Reaction 32)

It is also because of this relationship that MnO2 is commonly added to the zinc leachate, in

the leaching step of RLE. This addition of MnO2 is aimed at maintaining all the dissolved

iron at the oxidation state 3+, to maximize its precipitation in the form of jarosite [7].

26

In copper electrowinning, iron is known to somewhat decrease the MnO2 deposition, either

on lead-based anodes [106] or Ti/IrO2-Ta2O5 anodes [99]. The Fe2+ species in solution are

constantly regenerated in the cells, due to the reduction of Fe3+ on the cathodic side.

Because of its effect, iron concentration in copper electrolytes is usually controlled in terms

of a Fe/Mn ratio, ranging at around (0.5 - 4.0) g/L [44]. But in zinc electrowinning, iron has a

higher impact on zinc deposition, so its concentration is usually kept at up to ~25 mg/L [20].

A solution containing FeSO4 has been used to remove MnO2 deposits from a De Nora’s

MMO anode after long-term copper electrowinning testing [88]. Surface composition

analyses by XRF have indicated that the anode was Mn-free after the washing. No further

details were provided about the quantity of MnO2 originally deposited, neither the washing

parameters. Since iron sulphate is largely available in the leaching step of the RLE process

and no anode damage has been attributed to iron ions in the literature, this cleaning option

holds potential to be explored at an industrial scale.

1.4. Other implications of MMO anode technology in zinc

electrowinning

Unusual studies on the use of MMO anodes in zinc electrowinning express concerns related

to the possible release of noble metals (such as iridium) to the electrolyte [107]. Noble metals

have the most positive reduction potentials in the electrochemical series. So, the reasoning

is: if the coating dissolution leads to the appearance of species chemically similar to

monoatomic ions of iridium or hexachloroiridates [108], these species could readily deposit

over the cathodes in zinc cells. This would be detrimental to cellhouses because iridium on

the cathodes would favour HER over zinc metal production. Monoatomic ions of tin, like Sn2+

and Sn4+, could also electrodeposit along with zinc and cause the same issue, from a

thermodynamic standpoint.

These concerns are pertinent, and as such, the impact of MMO anode dissolution products

should be further investigated. Although this study was centred on the evaluation of MMO

anodes in terms of Mn2+ ion effects, the experimental part also included surface

characterizations of cathode samples used in ALTs. Such analyses were meant to confirm

the absence of iridium deposits on these cathodes, thus indicating that the types of MMO

anodes tested in this study do not affect the zinc current efficiency. Apart from that,

addressing this topic in depth was considered beyond the scope of this research.

27

1.5. Objectives and hypotheses

The general objective of this project was to evaluate the performance of three types of MMO

anodes, in order to screen the most suitable anode type for application in a zinc

electrowinning plant. The project firstly focused on investigating the relationships between

Mn2+ concentration, MnO2 formation rate and anode potential, for each anode type. The

second focus was on studying the microscopic effects of MnO2 deposition, to elucidate the

mechanisms of MnO2-induced MMO anode deterioration. The results would help to estimate

the tolerance levels of Mn2+ ions in the electrolyte and would help to define manganese

control strategies for zinc cellhouses. Then, such factors could be accounted for in technical

and economic feasibility analyses of the anode transition.

By monitoring the anode potential in the medium term, firstly, we expected to validate the

expected energy economy to be provided by MMO anodes in zinc cellhouses. This is the

main outcome of an anode replacement project from an economic point of view. Moreover,

we also expected to observe the anode potentials increase proportionally to the Mn2+

concentration. That is because the higher availability of Mn2+ ions supposedly accelerates

the MnO2 formation rate, and higher amounts of MnO2 deposits on MMO anodes would

decrease the exposure of catalytic sites to the electrolyte.

For the second part of the anode performance evaluation, we expected that the MnO2

deposits would impair the anode performance for blocking catalytic sites of the anodes. It

was also expected that the MMO anodes could still operate with little to no potential increase

if the Mn2+ concentration was restricted under a certain threshold, so-called “Mn2+ tolerance”.

Based on the literature discussed, it was estimated that the Mn2+ tolerance levels for

selected De Nora’s MMO anode types would be around 100 mg/L. This means typical zinc

electrolytes should have their Mn2+ concentration decreased tenfold to accommodate such

MMO anodes. Yet, periodical anode cleaning could be required in the long term.

Finally, it is unclear why MnO2 deposits formed at low Mn2+ levels may not lead to a

substantial anode potential increase, and why the effects of MnO2 deposits vary depending

on the type of MMO anode. In this case, we supposed that MnO2 deposits could develop

different morphologies depending on the Mn2+ levels and selected MMO coating properties

(elemental composition, crystal structure and morphology). Then, each type of MnO2 deposit

morphology could affect the electrocatalytic performance of MMO anodes in a different way.

28

Chapter 2. Methodology

2.1. Outline of the experimental plan

The experiments that concern this study were divided in two parts. The first one (Part 1)

consisted of an exploratory anode performance assessment. In this module, we focused on

obtaining preliminary information about the anode potential and durability of two types of

MMO anodes in zinc electrowinning conditions. The tests were coordinated as follows:

• Part 1.a. Accelerated life tests (ALTs): Samples of two anode types were

subjected to long-term galvanostatic polarizations at a high current density, in

absence of Mn2+. The goal was to observe, via microscopic techniques, the

deterioration processes of these anodes when OER is the only anodic reaction

involved. This would serve as a baseline to compare with possible deterioration

processes driven by MnO2 deposition, in the next experiments.

• Part 1.b. Electrowinning tests with pseudo-stationary Mn2+ levels: Other

samples of two anode types were subjected to weekly zinc electrowinning tests with

electrolytes containing up to 70 mg/L Mn2+. Due to slow MnO2 deposition rates,

during each test, the Mn2+ concentrations decreased less than 10% on average, and

only up to 20% on a few occasions. Taking into account the errors of the Mn2+

concentration measurement and the relatively low sensitivity of the MnO2 formation

rates to this degree of Mn2+ concentration variation, these tests were considered

pseudo-stationery in the context of this evaluation. During the tests, we monitored

the anode and cell potentials as well as the Mn2+ depletion rates and observed

general aspects of the electrowinning (zinc morphology and adhesion, for example)

to check if the anodes had no side effect on the cathodic reactions. Here, the test

replicates were performed consecutively with the same anode samples, with no

specific routine for removal of MnO2 deposits, to observe whether the effect of the

MnO2 deposition on the MMO anodes was permanent.

• Part 1.c. Electrowinning tests with step increase of Mn2+ levels: Other samples

of two anode types were subjected to zinc-free galvanostatic polarization tests of

1100 h. In these experiments, Mn2+ ions were dosed cumulatively after every few

days to further observe the effects of Mn2+ concentrations on MnO2 deposition rates,

and consequently, on anode potentials. Thus, the parameters monitored in these

tests were anode potentials, cell potentials and daily Mn2+ concentrations.

29

• Part 1.d. Testing anode cleaning methods: Finally, a mechanical and a chemical

cleaning method were evaluated on samples of each anode type, which suffered

different degrees of MnO2 deposition. The cleaning methods were evaluated in terms

of compatibility with the MMO coating and effectiveness of MnO2 removal.

The results obtained through these tests served as a basis to design the second part of the

experimental plan, which consisted of an evaluation of MnO2 deposits and their effects on

MMO coatings at a microscopic level. In fact, contrary to what was expected, the

electrowinning tests from Part 1.b and 1.c evidenced that the initial stages of deterioration

of MMO coatings were not revealed promptly by the anode potential data. Thus, to improve

the investigations, the second experimental part consisted of combining zinc electrowinning

tests to a systematic anode surface characterization by microanalyses, especially SEM and

EDS. The anodes were characterized before each test, after the MnO2 deposition and also

after removal of the MnO2 deposits. This MnO2 removal was performed using a suitable

cleaning method, selected after the evaluation of Part 1.d. This systematic anode surface

characterization would be helpful to identify connexions between properties of MnO2

deposits and degrees of deterioration of MMO coatings. The experiments are described in

detail in the following sections.

2.2. Materials

2.2.1. Electrochemical equipment setup

Each electrochemical test was performed in cells that consisted of jacketed glass beakers

with a volume capacity of 2 L, according to Figure 2-1. The beakers were connected to a

water bath IsotempTM model 6200 H7 by FisherbrandTM, to control the electrolyte

temperature.

During the electrowinning tests and the ALTs, the current was provided by the power

supplies GPS 303DD (by GW Instek) and XKW (by Xantrex), respectively. The former had

higher current precision, while the latter could operate with currents of higher magnitude (up

to 30 A). In both cases, the anode and cell potentials were measured automatically by a

data logger Graphtec model GL240. The voltage was sampled every 20 s or less.

30

Figure 2-1. Electrochemical setup for galvanostatic tests, with two independent cells.

2.2.2. Electrodes

The electrodes were assembled vertically, on a cathode-anode-cathode arrangement (where

each cathode faced a side of the anode). Such arrangement, illustrated in Figure 2-2, enabled

cathode replacement without interruption of anodic reactions during galvanostatic tests. The

electrodes had exposed surface areas of 10 cm2 on each side, for the electrochemical

reactions of interest. The remaining surfaces were covered with two coats of an inert lacquer

(Miccrostop Stop-off Lacquer by Tolber), applied manually. These electrodes also had

central rods of 10 cm, to be affixed to specific openings on the cell caps, which were custom-

made with high-density polyethylene. The electrode spacing was 2.5 cm.

Figure 2-2. Electrode arrangement and dimensions.

31

The cathodes were manufactured from sheets of aluminum alloy series 1100 (which

contains 99% to 99.5% Al), with hardening treatment H14. Before each experiment, the

cathodes were wet polished manually with three types of silicon carbide (SiC) papers, of grit

numbers #320, #600 and #1200.

Three types of MMO anodes were provided by De Nora Tech for this investigation (Figure

2-3). They were equally manufactured with titanium substrates. However, the iridium-bearing

MMO coatings of each type had differences in terms of elemental composition, morphology

and crystalline structure, which were characterized using X-ray fluorescence spectrometry

(XRF), scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively.

The XRF analyses were performed using a spectrometer Rigaku model ZSX Primus II, with

two modes of X-ray penetration depth (film mode and default mode), scanning all elements

with atomic numbers between 9 (fluorine) and 92 (uranium). The SEM images were obtained

on a Tescan Vega 3 at a potential of 10 kV, on secondary electron mode. The XRD data were

acquired on a powder X-ray diffractometer model Aeris, by Pan Analytical. Anode samples

of 1 cm2 were directly scanned on default mode, at a 2θ range between 5° and 85°, scan

rate of 2.8 10-2 (°)/min and step size of 0.0110° (2θ). The radiation source was Cu Kα. Using

the software X'Pert HighScore Plus by Pan Analytical, the diffractograms were treated with

a default profile fit, to identify the crystal signatures. Then, a search and match routine was

run in connection with the Crystallography Open Database (COD) [109].

Figure 2-3. Samples of three MMO anode types, provided by De Nora Tech.

The anode potentials were measured using reference electrodes of silver/silver chloride with

saturated potassium chloride (Ag-AgCl/KClsat), with double junction, manufactured by

Sensorex. The reference potential was 0.204 V vs. SHE (standard hydrogen electrode).

32

2.2.3. Electrolytes

The electrolytes used in each experiment were prepared using deionized water (with

electrical resistivity of 18.2 MΩ.cm) and analytical-grade reactants. Sulphuric acid solutions,

zinc electrolytes and zinc-free electrolytes were employed in the different experiments

according to Table 2-1. In the tests involving the presence of manganese, the addition of

Mn2+ ions was made via injection of 50 mL of solutions containing the original, Mn-free

electrolyte components (H2SO4 and ZnSO4 or K2SO4) plus certain amounts of dissolved

MnSO4. This method was adopted to guarantee that the Mn2+ ions could easily disperse in

the cell immediately at the moment of injection, as a step input to the electrowinning system.

Table 2-1. Electrolytes compositions.

Electrolyte Application Composition Reactants

Acid solution Part 1.a 170 g/L H2SO4 (95-98)% H2SO4

Zinc electrolyte Part 1.b, Part 1.d,

Part 2

170 g/L H2SO4, 55 g/L

Zn2+ as ZnSO4

(95-98)% H2SO4,

99% ZnO

Zinc-free

electrolyte Part 1.c

170 g/L H2SO4, 65 g/L K+

as K2SO4

(95-98)% H2SO4,

99% K2SO4

The zinc-free electrolyte was meant to enable anodic reactions equivalent to those obtained

in zinc electrolyte, but without involving metallic electrodeposition on the cathode side (only

hydrogen evolution). Thus, this zinc-free electrolyte was used on the electrowinning tests of

Part 1.c with the intent of reproducing accordingly the OER and the MnO2 deposition on

MMO anodes, without requiring daily cathode replacement (which would make this

experiment more laborious).

During the experimental design, the replacement of ZnSO4 by K2SO4 (keeping the same

molar concentration of sulphates) was shown to be adequate for use in the zinc-free

electrolyte. This was evidenced by comparisons of OER overpotential profiles of two MMO

anode types, in both electrolyte types, through potentiodynamic polarization analyses. The

experimental details and results of these analyses are presented in Appendix A.

33

2.3. Experimental conditions

A few parameters are common to all the experimental parts described in the following

paragraphs:

• The galvanostatic polarization tests were performed at a temperature of (40 ± 1) °C.

• The SEM images and the EDS data were obtained on a Tescan Vega 3, with an EDAX

Element EDS Detector integrated, using a potential of 10 kV. The SEM analyses

were performed in both secondary electron (SE) and backscattered electron (BSE)

modes. Most images presented here are in SE mode unless otherwise stated.

Meanwhile, the EDS results (in the form of spectra or elemental mapping) were

acquired with an X-ray energy detection range of (0-7) eV and a resolution of 128.8 eV.

• The Mn2+ concentrations of electrolyte samples were obtained by microwave plasma

atomic emission spectrometry (MP-AES) using a spectrometer Agilent model 4100.

The electrolyte samples were filtered through wool glass (with a pore size of 0.2 μm)

to retain any eventual MnO2 particles in suspension, and then they were diluted

tenfold in HNO3 5%. The regression line for elemental quantification was defined

using the Mn absorption values centred at the wavelength of 403.076 nm.

2.3.1. Part 1.a. Accelerated life tests (ALTs)

The galvanostatic polarizations of Part 1.a (ALTs) were performed for 800 h at a current

density of 5 kA/m2, with samples of anodes D and E. During this test, the electrolyte acidity

was controlled daily by re-adding deionized water, according to the decrease of the

electrolyte level observed through the beaker. At the end of the test, the electrodes were

carefully rinsed with deionized water, with ethanol and then were dried at room temperature.

The anode samples were characterized by SEM, to observe the degree of deterioration of

the MMO coating.

Furthermore, the cathodes used in the ALTs of anode type D were analyzed by X-ray

photoelectron spectroscopy (XPS), to verify whether any elements from the MMO coating

had electrodeposited over its surface. Before the analyses, the cathodes were rinsed with

deionized water, with ethanol and then were dried at room temperature. The XPS data were

acquired on a spectrometer model Axis-Ultra by Kratos Analytical, with a monochromatic

source of Al Kα (1486.6 eV), using charge compensation and with electron detection at a

take-off angle of 90° relative to the sample.

34

2.3.2. Part 1.b. Electrowinning tests with pseudo-stationary Mn2+

levels

In Part 1.b, four zinc electrowinning tests were conducted in the form of galvanostatic

polarizations at a current density of 500 A/m2 and with durations of 5 to 10 days, to evaluate

the anode types D and E. The first test was performed with no manganese ions, and the

following ones were performed with single-step additions of Mn2+ after 1 h of testing. This

way, the Mn2+ concentrations of tests #2 to #4 “started” at 50 mg/L, 70 mg/L and 10 mg/L

(in this order). Electrolyte samples (with volumes of about 3 mL) were collected multiple

times in the first two days of experiments, then were collected every 24 h in the following

days. These samples were analyzed by MP-AES to monitor the Mn2+ levels.

The cathodes were replaced daily with no current interruption. The zinc deposits harvested

were rinsed with deionized water, with ethanol, then were dried at 70 °C for 1 h. After drying,

the zinc deposits were weighted to determine the amount of ZnO to be added to the cells,

in order to maintain the Zn2+ concentration. The ZnO make-ups were generally added

around 1.2 h after the cathode replacement.

At the end of the experiments, the anodes were gently rinsed with deionized water, with

ethanol, then were dried at room temperature. After drying, they were employed again in

each subsequent electrowinning test.

Also, after one of the tests (involving a sample of type D and zinc electrolyte with 50 mg/L

Mn2+), loose particulate was recovered from the anode surface for characterization by XRD.

The objective of this activity was to identify the crystalline phases of MnO2 deposits. The

particles were rinsed with deionized water, dried and analyzed by XRD using a powder X-

ray diffractometer model Aeris, by Pan Analytical. A zero-background holder was used due

to the low quantity of analyte available. Using a source of Cu Kα, the MnO2 sample was

scanned from 5° to 85° (2θ), at a scan rate of 2.7 10-2 (°)/min and a step size of 0.220° (2θ).

The diffractogram pattern was treated and analyzed with the software X'Pert HighScore

Plus. An automatic background correction was applied to the diffractogram, along with a

default profile fit. Finally, a search and match routine for XRD patterns was run connected

to the COD.

35

2.3.3. Part 1.c. Electrowinning tests with step increase of Mn2+

levels

In Part 1.c., two electrowinning tests were performed with samples of anode types D and E,

at a current density of 500 A/m2 for 1100 h (46 days). As previously mentioned, these tests

were performed on a zinc-free electrolyte, eliminating the need for cathode replacement.

Deionized water was added daily to re-adjust the electrolyte acidity, according to the

decrease of the electrolyte level. Electrolyte samples were collected daily to measure the

Mn2+ concentration values by MP-AES. By dosing manganese-rich solutions in the cells, the

Mn2+ levels of these electrowinning tests were increased in four steps. Firstly, the electrolyte

was set to about 15 mg/L Mn2+ after 1 h of testing, then it was raised to 100 mg/L after 6

days, then to 150 mg/L after 19 days, finally reaching 230 mg/L after 36 days. At the end of

the tests, the anodes were gently rinsed with deionized water, with ethanol, then were dried

at room temperature.

2.3.4. Part 1.d. Testing anode cleaning methods

The two cleaning methods were conducted as follows. Chemical cleaning was performed by

submerging the anodes in 80 mL of a solution containing 50 g/L H2SO4 and 50 g/L FeSO4.

This solution was prepared with deionized water, (95-98)% H2SO4 and laboratory-grade

FeSO4. The anodes remained in solution at room temperature for 30 min. During this time,

they were agitated occasionally to improve the mass transfer around loose MnO2 flakes that

would take longer to dissolve.

Meanwhile, the mechanical cleaning was performed on an ultrasonic cleaner Cole-Parmer

model 8892, with a wave frequency of 47 kHz. The water used for cleaning was firstly

degassed for 5 min before inserting the anodes. The sonication was performed for 60 min

at room temperature. After sonication, the anodes were rinsed with deionized water, with

ethanol, then were dried at room temperature.

Samples of three types of MMO anodes (D, E and F), with different degrees of MnO2

deposition, were subjected to the cleaning methods according to Table 2-2. To verify the

compatibility of the chemical cleaning to MMO anodes, a blank experiment was performed.

In this test, the samples D1, E1 and F1 were applied to zinc electrowinning at 500 A/m2 for

72 h in Mn-free conditions, and then were submerged in the cleaning solution for 2 h. After

that, they were rinsed, dried, and analyzed by SEM and EDS like the other anodes.

36

Table 2-2. MMO anode samples used in the cleaning tests, and the electrowinning conditions applied to form their respective MnO2 deposits.

Anode

samples

Electrowinning conditions Cleaning method

Mn2+ levels (mg/L) Duration (h)

D1, E1, F1 0 72 chemical cleaning

D2, E2 400 216 chemical cleaning

F2 400 72 chemical cleaning

D3, E3 10 – 200 630 mechanical cleaning

D4, E4 10 – 230 1100 mechanical and then chemical cleaning

The samples D2, E2 and F2 were subjected to galvanostatic polarizations of 72 h at 500 A/m2,

following the same methodology format of Part 1.b, but with zinc electrolytes containing

400 mg/L Mn2+. This would be the Mn2+ concentration expected in the electrolyte feed of a

hypothetical zinc cellhouse that had completed its anode technology transition, assuming

that the amount of manganese oxidized in (or after) the zinc electrowinning step equals the

amount of manganese entering the electrolyte loop as ore impurity. The idea was to verify if

the chemical cleaning would be effective for rapidly-developed MnO2 deposits, formed at

this Mn2+ level range. If so, perhaps the anode performance could be maintained in Mn-rich

electrolytes as long as the anodes were cleaned in short intervals.

A minor difference was proposed in the experimental plan of these three samples: D2 and

E2 were tested, cleaned, and tested again consecutively three times, while the sample F2

was tested and cleaned only once. This way, with a limited quantity of samples and

experiments, we were able to observe whether the surface of such anodes would deteriorate

progressively as the washing and cleaning cycles kept repeating.

Additionally, four samples were tested with the mechanical cleaning method. The samples

D3 and E3 had been previously used in the tests of Part 1.b, while D4 and E4 had been

used in the tests of Part 1.c and a few other galvanostatic polarizations at higher Mn2+ levels.

Hence the reason why the Mn2+ concentrations that formed the MnO2 deposits of D3, E3,

D4 and E4 are given as intervals in Table 2-2.

After each experiment, the anodes were characterized by SEM and EDS. We also took the

opportunity to characterize the crystalline phase of MnO2 particles dispersed in the cells of

37

anodes D2 and E2 during the electrowinning tests at 400 mg/L Mn2+. These were the only

experiments in which the quantity of suspended particulate of MnO2 formed was large

enough to be collected directly by decanting the electrolyte. Once separated, the particles

were rinsed in deionized water and were allowed to dry at room temperature. The XRD

analyses were performed on a powder X-ray diffractometer model Aeris, by Pan Analytical,

using a zero-background holder. The parameters of diffractogram acquisition and crystal

phase identification used in these analyses were the same as those of MnO2 particles

characterized in Part 1.b.

2.3.5. Microscopic evaluation of MnO2 deposits and their effects

In Part 2, samples of MMO anode types D, E and F were tested on multiple galvanostatic

polarization tests with durations of 72 h, at a current density of 500 A/m2. These tests were

performed exclusively in zinc electrolytes (containing 170 g/L H2SO4, 55 g/L Zn2+ as ZnSO4).

The experimental routine (cathode replacement, weighting of zinc metal, ZnO make-up, and

single Mn2+ step addition) was maintained identical to that of tests in Part 1.b. The electrolyte

was sampled daily for monitoring of Mn2+ levels.

The samples type D were tested in pseudo-stationary Mn2+ levels of 5 mg/L, 10 mg/L, 25 mg/L,

50 mg/L and 75 mg/L. The samples type E were tested at 50 mg/L, 75 mg/L, 100 mg/L,

125 mg/L and 150 mg/L. The electrowinning tests at 50 mg/L were repeated in duplicate

with both anode types D and E. In this set of experiments, it was also possible to investigate

the anode type F in more detail, testing its samples at Mn2+ levels of 5 mg/L, 10 mg/L, 25 mg/L,

100 mg/L and 125 mg/L. At the end of each test, the anodes were gently dipped into

deionized water and then in ethanol, as a way to clean electrolyte salts from the surface

while preserving fragile MnO2 morphology features that may be present. After this washing,

the anodes were dried at room temperature.

The dried anodes were then analyzed by SEM and EDS to characterize the MnO2 deposits

formed. Afterwards, the anodes were cleaned using the FeSO4 solution as described in Part

1.d, and were re-analyzed by SEM and EDS to characterize the anode surface conditions

after removal of MnO2. This second characterization also served to determine whether each

sample remained with a well-preserved surface; in this case, the anode could be reused in

the subsequent electrowinning test without the risk of compromising the experimental

replicability.

38

Chapter 3. Results and Discussion: Exploratory Anode Performance Assessment

3.1. Characterisation of the MMO anode types

Figure 3-1 presents the XRF results of the three types of MMO anodes studied, with a focus

on the metallic elements. To begin, it is worth mentioning that these elemental composition

analyses can be only considered semi-quantitative. That is because the depth of detection

fluorescence X-rays was longer than the coating but inferior to the total thickness of the

titanium substrate. As such, the XRF results serve the purpose of highlighting differences of

chemical formulations across the layers of the three MMO coatings. To enrich the elemental

comparison, the analyses were replicated on “film mode” and “default mode” – the former

had an X-ray detection range relatively more superficial than the latter one.

Figure 3-1. Element composition of MMO anodes, determined by XRF, with two acquisition modes (film and default modes).

Based on these results, the Sn/Ir ratios differ in each depth level and anode type. Tantalum

is only present in the anodes E, and it is more abundant at a lower depth. Also, anodes D

have relatively more iridium, and they are richer in bismuth at the outer MMO layers.

Meanwhile, the MMO coating of anodes type F presents a higher concentration of tin.

39

The XRD results of anode types D, E and F are presented in Figure 3-2. The sharp peak at

the 2θ position of 40.3° and the double peaks near 80°, present in each diffractogram, are

features of the characteristic pattern of titanium metal. Since the X-ray diffraction from the

substrate was detectable, the penetration depth of diffracted X-rays was longer than the

thickness of the MMO coatings.

Figure 3-2. X-ray diffractograms of anodes D, E and F (top), and patterns of the identified crystallographic phases (bottom).

Regarding the crystalline phases of the coatings, it is possible to observe broad peaks at

the positions of 27.0°, 34.1° and 52.4° for the three anode types. According to the phase

identification routine of the XRD software, these peaks match the pattern of the solid solution

(Snx,Ti(1-x))O2. As the increase of bond lengths and lattice parameters of this oxide is

proportional to the increase of the substitution of Ti by Sn [110], the estimation of x = 0.5

had optimal correspondence to the unit cell dimensions calculated from the diffractogram.

The identification of this rutile-like structure agrees with literature characterizations of SnO2

and IrO2 in electrocatalytic coatings. However, the diffraction pattern of IrO2 crystallites

almost overlaps with that of SnO2 (or (Sn,Ti)O2). As such, either iridium, titanium and tin

oxides integrate a single solid solution, or IrO2 crystallites are dispersed with (Sn,Ti)O4 ones

40

in the coating, and both share similar crystal parameters. Based on the literature [111], the

second option is somewhat more likely, because the peaks positioning of this crystal

structure are the same for all three anode types, even though they seem to have different

IrO2/SnO2 ratios according to the XRF data. In addition, bismuth seems to be incorporated

in this (these) crystal phase(s) as a dopant [76].

In particular, the X-ray diffractograms reveal certain differences in such rutile-like crystals of

each anode type. The peaks of this phase are broader for the anode E, suggesting that the

MMO crystallites of this anode type may be smaller or have a less uniform lattice. Also,

comparing the XRD data of types D and F, there is a difference in relative intensity between

the peaks 27.1° and 34.0°, which represent the plans (110) and (101), respectively. This

indicates a difference in the preferable orientation of these crystallites on the anodes surfaces.

Furthermore, the sharp peaks at 23.1°, 28.7° and 37.0° on the diffractogram of the anode E

agree with the diffraction pattern of Ta2O5. Interestingly, this information contrasts with the

literature, where tantalum oxide of MMO coatings is usually observed in amorphous form.

SEM images of the anodes D, E and F are presented in Figure 3-3. These images show that

the anodes have pronounced peaks and valleys at the scale of 100 μm due to substrate

treatments. Typical mud-crack features are also visible, at the scale of 10 μm, approximately.

Figure 3-3. SEM images of anode types D, E and F. Images obtained at 10 kV, with magnifications of 400 x (top) and 2 kx (bottom).

41

3.2. Long-term durability of MMO anodes in Mn-free conditions

The ALTs of Part 1.a. provided a glimpse on the long-term MMO anode durability when OER

is the only reaction present. The anode potentials recorded for samples of anode types D

and E were stable across the 800 h of testing, as is shown in Figure 3-4. The lack of a

progressive anode potential increase indicates that there was no significant electrocatalyst

dissolution nor substrate passivation during this period. These anodes were likely to

continue displaying this same performance for much longer than 800 hours, but this test

duration was enough to evaluate the anode durability in the scope of this study. (Note: the

slight decrease of cell potentials was due to corrosion of the aluminum cathodes).

Figure 3-4. Anode potentials of anode types D and E during the ALTs.

SEM micrographs of anode types D and E after the ALTs are provided in Figure 3-5. The

images evidence that the MMO coating of these anodes remained unaltered during the

experiments, except for a few fragments detached from the mud-cracks of anode D and pits

occasionally formed around mud-cracks of anode E.

Figure 3-5. SEM images of anode types D and E, after the ALTs of Part 1.a. Images acquired at 10 kV, with magnifications of 400 x and 2 kx.

42

Complementary, the XPS results of the cathodes used in the ALT of anode D have shown

no presence of any major components of the MMO anode (Ir, Ti, Sn or Ta). This suggests

that such elements would not deposit on the cathode, hence they do not pose a threat to the

zinc deposition in cellhouses. The XPS spectrum and surface elemental quantification (of

one of the analytes) are presented in Figure 3-6 and Table 3-1, respectively. The light

elements identified (C, S, N, O) seem to have originated from electrolyte contaminants and

corrosion products of the cathode, while Cu and Ga are likely minor components of the

aluminum sheet used in the cathode manufacture. The small quantity of bismuth present

may either belong to the original cathode composition or, more likely, was released from

MMO coating fragments before depositing over the cathode. Still, this may have been a

minor occurrence since the MMO coatings of these anode samples were well preserved,

according to the SEM images taken after the ALTs. The implications of bismuth deposition

on the cathodes for zinc metal production are yet to be investigated.

Figure 3-6. XPS survey spectrum of a cathode from the ALT of anode D (top), where the circled area corresponds to the high-resolution spectrum (bottom) presenting

convoluted peaks of S, Ga and Bi.

Table 3-1. Elemental composition of the cathode surface, according to XPS data.

Element O C Al S F Cu N Bi Ga

Quantity (at%) 47.56 37.10 11.07 2.25 0.75 0.47 0.36 0.36 0.07

43

3.3. The relationship between Mn2+ levels, MnO2 deposition and

MMO anode potential increase

3.3.1. Results from long-term galvanostatic polarizations

As previously described, Part 1.b of this project involved consecutive zinc electrowinning

tests in Mn-free conditions and electrolytes containing 50 mg/L, 70 mg/L and 10 mg/L Mn2+

approximately. Figure 3-7 presents the cell potentials recorded in these tests, for anodes D

and E. Here, cell potentials were chosen for presentation and discussion (instead of anode

potentials) because their measurements were more accurate during these experiments. The

results show that anode potential values remained approximately steady within a certain

variability range; they did not seem to be influenced by the Mn2+ concentration.

Figure 3-7. Cell potentials of consecutive zinc electrowinning tests performed in Mn-free conditions and at approximate concentrations of 50, 70 and 10 mg/L Mn2+.

Figure 3-8 presents the Mn2+ concentrations in the electrolytes during the consecutive

electrowinning tests, as measured by MP-AES. A slow but steady decline in the Mn2+ levels

was observed for both anode types, at ~50 mg/L and ~70 mg/L Mn2+. These results are

quantitative evidence that MnO2 was formed over the MMO anodes. Other (qualitative) signs

of MnO2 formation include the accumulation of thin, dark particles over the anode samples

(especially on type D) and the change of the electrolyte colour. The electrolytes, originally

transparent, would turn light pink after few hours of testing, indicating the appearance of

Mn3+ ions in the solution [104]. Electrolyte samples would become colourless again after a

few days of storage.

44

It is worth noting that the zigzags observed in the potential values do not have any link with

the MnO2 deposition. The downward trends were caused by the constant growth of zinc

deposits and depletion of Zn2+ ions (which lead to increases in electrolyte acidity). The peaks

occurred during the moments of cathode replacement and ZnO addition, since the

nucleation of new zinc metal deposits requires a higher potential to start over clean, polished

aluminum cathodes. In batch experiments like this one, the smaller the electrolyte volume

per cell, the higher the variation of electrolyte composition per cathode cycle.

Figure 3-8. Mn2+ concentrations during the consecutive zinc electrowinning tests.

The cell potential values varied in the range of ~100 mV across the electrowinning replicates,

as shown in Figure 3-7. Added to the daily potential fluctuations, there was a difficulty to

achieve a high degree of experimental replicability, maybe due to the electrical setup. These

aspects would limit the capacity of detecting small potential increments induced by MnO2

accumulation on the anodes.

Next, in Part 1.c, galvanostatic tests were performed with longer durations and progressive

step increases of Mn2+ levels, covering a broader range of magnitude (up to 230 mg/L). In

this experimental format, the use of a zinc-free electrolyte had not only the function of

simplifying the daily experimental routine, but also eliminating the daily potential zigzags

caused by zinc deposition. The anode potential values were measured in alternating days

to preserve the stability of the reference electrodes. Overall, with this configuration, we expected

to have a more sensitive detection of anode potential variations related to MnO2 deposition.

Figure 3-9 provides the anode potential values at the initial hours of these tests, for anode

types D and E. The full anode potential results of these experiments are provided in Figure

3-10, along with the corresponding Mn2+ concentration profiles over time, to facilitate the

observation of relationships between these variables. Also, in Figure 3-11, the MnO2

45

formation rates can be inferred from the quantities of Mn2+ ions depleted (consumed) over

time after each new Mn2+ step addition.

Figure 3-9. Anode potential values of the 46-day galvanostatic tests, in the first hours of operation. The vertical dashed line represents the moment of Mn2+ addition.

Figure 3-10. Anode potentials (top) and Mn2+ concentrations in the electrolyte (bottom) during galvanostatic tests with step increases of Mn2+ (indicated by dashed lines).

The graphs suggest that the addition of 15 mg/L Mn2+ at the time t = 1 h hardly caused any

modification in the anode potential trends. At this stage, the quantity of MnO2 deposits

formed must have been too little to interfere with the anode performance, since the apparent

consumption of Mn2+ ions for MnO2 formation (Figure 3-11) was very low: about 0.5 mg Mn2+

per day for either anode type.

46

Figure 3-11. Cumulative Mn2+ mass depletion after each step addition of Mn2+ during galvanostatic tests, for anodes D and E. Regression lines were added to display the

data trends more clearly.

Then, a faster increase of anode potentials occurred when the Mn2+ concentrations jumped

from ~15 mg/L to 100 mg/L. But after a few days at ~100 mg/L Mn2+, the slopes of anode

potentials became roughly steady; they no longer seemed to be affected by further additions

of manganese ions.

Two hypotheses were considered to explain why the anode potentials increased faster when

the Mn2+ concentrations changed from 15 mg/L to 100 mg/L. The first hypothesis was that

the MnO2 formation rate has slowed down at that stage. This was based on the information

that the current efficiency of MnO2 deposition can decrease over time [27]. However, in

Figure 3-11, the trends of Mn2+ depletion after the electrolyte reached 100 mg/L, 150 mg/L

and 230 mg/L Mn2+ were statistically similar for the anode D. Plus, for anode E, the highest

Mn2+ depletion rate was recorded after the electrolyte reached 230 mg/L Mn2+. Therefore,

these results did not indicate a decrease in the current efficiencies of MnO2 formation.

The other hypothesis (most reasonable) was that not all MnO2 formed would stay

necessarily attached to the anodes to contribute to the potential increase. In fact, MnO2

deposits start to detach over time. This was evidenced by the visual aspect of the

electrolytes: once transparent at the level of ~15 mg/L Mn2+, they turned into vivid pink after

reaching ~100 mg/L Mn2+ and then started to display a small number of fine particles in

suspension in the final days of the experiment, at ~230 mg/L Mn2+. These suspended solids

were very likely MnO2 particles that have detached from the anode surface or were formed

in the bulk solution.

47

But even in a situation where virtually all the MnO2 deposits formed in the experiment

continued covering the anode surfaces all throughout, their properties may have evolved in

a way that affected the anode potential differently depending on the stage of the test. In fact,

in a study involving six-day electrowinning tests, no clear relationship was found between

the quantity of MnO2 deposits formed on MMO anode samples and their anode potential

profiles over time [99]. Besides quantity or thickness, other properties of the MnO2 deposits

may have a crucial effect on the anode potentials.

Finally, it is worth noting that anode D has demonstrated lower potential values since the

test start, but they seem to increase at higher rates than those of anode E. The gap between

potential values was about 80 mV at the time t = 15 h, but it went down to 57 mV at t =

1000 h. This may have been a reflection of how the MnO2 deposition affected each anode

type differently.

3.3.2. Complementary results from short-term galvanostatic

polarizations

The anode and cell potentials were also recorded during the electrowinning tests performed

during Part 1.d. and Part 2. Originally, the main purpose of these tests was not to observe

the relationships between Mn2+ concentration, MnO2 deposition rate and anode potential

increase rate, as they were relatively shorter and the Mn2+ levels were measured only four

times per test. Still, the anode and cell potential data acquired were useful for complementing

the experimental observations discussed in Section 3.3.1.

In Part 1.d, samples of anodes D and E were subjected to galvanostatic polarizations with

zinc electrolytes for 72 hours, where Mn2+ ions were added at 1 h of operation, to raise the

Mn2+ levels to 400 mg/L. The anodes were tested, cleaned, and tested again at the same

conditions three times, as the goal was to evaluate the anode performance and integrity of

the MMO coatings after multiple cycles of MnO2 deposition and removal. The full

experimental description was provided in Chapter 2. Anode potentials, cell potentials and

Mn2+ concentrations relative to these tests are presented in Figure 3-12 and Figure 3-13.

The MnO2 formation rates observed in these tests were significantly higher than those of

Part 1.b or 1.c, because of the higher initial concentration of Mn2+ ions in the electrolyte.

Setting aside the significant variability of Mn2+ levels measured in individual test replicates,

the Mn2+ depletion was approximated to a linear profile and a regression line was applied to

48

the combined data set of the three replicates. This way, the Mn2+ depletion rate was (roughly)

estimated as 33.5 mg/d, with a coefficient of determination R2 equals 0.45.

Figure 3-12. At the top, anode potentials of the beginning (left) and totality (right) of the electrowinning tests at ~400 mg/L Mn2+, with anodes D and E. At the bottom, cell

potentials of the same tests. The dashed lines indicate the moments of Mn2+ dosing.

Figure 3-13. Mn2+ concentrations over time, measured from electrowinning tests with step Mn2+ addition to reach ~400 mg/L at the test start.

The potential profiles of each anode type were relatively stable after the first hours, even

though the MMO coatings may continue deteriorating due to intense MnO2 deposition (which

was revealed by microscopic results and is presented in the next section). This apparent

49

dissonance may be due to the low sensitivity of voltage measurements; small increments of

anode deterioration were camouflaged by voltage noise or variability from external

influences.

Meanwhile, in Part 2, samples of anode types D, E and F were also used for galvanostatic

polarization tests in zinc electrolytes with step Mn2+ additions. In this case, the pseudo-

stationary Mn2+ concentrations tested were between 5 mg/L and 150 mg/L, because the

Mn2+ tolerance levels were expected to be in this interval. These experiments provided the

anode and cell potential profiles of Figure 3-14, as well as the average cell potential values

expressed in function of Mn2+ levels in Figure 3-15. The potential data and Mn2+

concentration profiles measured over time are presented in more detail in Appendix B.

Figure 3-14. Cell (left) and anode (right) potentials of zinc electrowinning tests performed for 72 h at pseudo-stationary Mn2+ levels between 5 mg/L and 150 mg/L.

Figure 3-15. Average cell potentials of electrowinning tests of Part 2, versus the approximate Mn2+ levels of these respective cells.

The charts of Figure 3-14 and Figure 3-15 indicate that, at each given Mn2+ concentration,

the anodes type D tend to perform at the lowest potentials, and the anodes E, the highest.

50

This difference in performance between anodes D and E agrees with the data of Parts 1.c

and 1.d. Samples of anode type F had an intermediary potential range.

Moreover, the comparison of Figure 3-15 3-15 demonstrates a proportionality between the

average cell potential values and Mn2+ levels, for each anode type. This relationship is

clearer from these results than from those of Part 1.b or 1.c. All three regression lines have

approximately the same slope coefficient of 0.0002, meaning that the cell potential is raised

by 2 mV at each Mn2+ concentration increase of 10 mg/L.

However, this type of linear regression would be only adequate if the development of MnO2

deposits was proportional to the Mn2+ concentration in the range of (5-150) mg/L Mn2+ –

which turned out to not be true, according to SEM and EDS results of Part 2. For instance,

we would not expect differences in anode potential between the tests at 0 mg/L, 50 mg/L

and 75 mg/L with the anodes E, because no MnO2 deposition was observed from these tests

through SEM images.

3.4. Definition of a MMO anode cleaning method

In the previous section, it was shown that the MMO anode types operated at higher

potentials in the presence of Mn2+ ions, due to MnO2 deposition over their coatings. Such

deposits were relatively adherent to the coatings, in agreement with the literature [27,

95]. At the end of the electrowinning tests, MnO2 coverage was still visible over the anode

samples even after rinsing them thoroughly with deionized water. Considering these factors,

in a scenario where one of these anode types is applied to a zinc cellhouse, periodical

cleaning of the anode surfaces may be fundamental to restore their original performance.

Thus, it was important to identify an anode cleaning method that is effective for MnO2

removal and also harmless to the coating structure.

Samples of anode types D, E and F were subjected to cleaning routines and their MMO

coatings were evaluated by SEM and EDS. The overall results are presented in Table 3-2.

The tags used to identify the samples are the same ones of Table 2-2 in Section 2.3.4. For

simplification, “chemical cleaning” corresponds to the submersion of the anodes in an acidic

solution containing FeSO4 for 30 min, while “mechanical cleaning” corresponds to ultrasonic

cleaning in water for 60 min.

51

Table 3-2. Outline of the evaluation of anode cleaning methods.

Anode samples Cleaning method MnO2 removal MMO coating conditions

D1, E1, F1 chemical cleaning n/a preserved as original

D2, E2 chemical cleaning complete coating detachment

F2 chemical cleaning complete coating detachment

D3, E3 mechanical cleaning incomplete severe coating detachment

D4, E4 mechanical and chemical complete severe coating detachment

3.4.1. Chemical cleaning results

The chemical cleaning was found to be very effective at removing MnO2 deposits, with no

collateral damage to any of the three MMO coating types. According to EDS results, all the

samples subjected to the cleaning method became completely free of manganese. In

addition, the compatibility of this technique to the anode surfaces was confirmed through

SEM/EDS analyses of the anode samples D1, E1 and F1 after the extended cleaning

procedure (submersion in the chemical solution for 2 h). According to the SEM images

presented in Figure 3-16, these anodes were unaltered; their surfaces are identical to those

of the brand new samples from Figure 3-3.

However, significant damage was observed on all three samples applied to zinc

electrowinning tests at 400 mg/L Mn2+ and subsequently cleaned with the chemical method.

In the SEM images of Figure 3-17, it is possible to observe that most of the outer MMO

layer(s) is(are) missing, especially in the anode D2. Part of the mud-crack surfaces are still

available in the valleys, but they display significant signs of wear. The process that involves

the appearance of these irregular, multi-layered contours across the smoothed out regions

of the coating can be referred to as delamination. In minor locations, the titanium substrate

has been completely exposed, as illustrated by the EDS map in Figure 3-18. (Note: the

distinct texture of the substrate regions is probably due to etching treatment received during

the anode manufacture.)

52

Figure 3-16. SEM images of anodes D1, E1 and F1 after zinc electrowinning tests at 0 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x (top) and 2 kx (bottom).

Figure 3-17. SEM images of samples D2 and E2. Magnifications of 400 x and 2 kx.

53

Figure 3-18. On the left, spatial distribution of Ti signal over a region of the anode D2, obtained by EDS. On the right, an SEM image of the same location, for reference.

The main attention point of these results is that the MMO coating deterioration occurred

not due to the chemical cleaning, but due to the MnO2 deposits formed in the triplicate tests

at 400 mg/L Mn2+. Even the sample F2 has suffered significant surface degradation after

being tested at 400 mg/L Mn2+ only once, as illustrated by the SEM images of Figure 3-19.

In the first image on the left, the outer MMO layer is fractured and its individual patches

seem to be weakly bound to the anode. The titanium substrate is also visible in several

regions of this sample.

Figure 3-19. SEM images of the anode sample F2.

The high adhesion strength between MnO2 deposits and MMO coatings may explain the

damage observed on these anode surfaces. SEM images of anode F2 prior to chemical

cleaning (Figure 3-20) reveal a compact but fragmented MnO2 coating over the anode. The

image at higher magnification (2 kx) shows the presence of nodular agglomerates of needle-

like MnO2 crystallites, tightly aggregated. At lower magnification (250 x), these deposits

present multiple cracks, indicating that they may have undergone some level of stress during

the zinc electrowinning tests. The stress forces were probably caused by the growth of

individual MnO2 crystals and/or the pressure imposed by oxygen gas bubbles evolving and

54

accumulating underneath. In this context, it is important to note that MMO coatings

manufactured via a thermal composition can be composed of multiple layers of different

crystal characteristics. This means the interfaces between the layers and between the

titanium substrate may be relatively more vulnerable to rupture. Therefore, if the adhesion

strength of the MMO coating surface to the MnO2 is superior or comparable to the adhesion

strength of inner coating layers, the stress forces exerted over the MnO2 could induce

coating fractures and bonding failures in the MMO layer interfaces.

Figure 3-20. Mn2+ deposits over the anode F2 before the cleaning. Magnifications of 250 x (left) and 2kx (right).

Overall, this shows that even though the chemical cleaning was safe for the MMO coatings

and effective for MnO2 removal, periodical use of this technique with intervals of 72 h does

not guarantee the restoration of the original anode performance. Operating the MMO anodes

at 400 g/L Mn2+ for just a few hours was sufficient to compromise the coatings permanently.

3.4.2. Mechanical cleaning results

Although the chemical cleaning had already successfully met the requirements of a good

anode cleaning technique (MnO2 removal effectivity and compatibility with the MMO

coating), the mechanical method was also evaluated to complete the experimental program.

As this mechanical cleaning does not involve consumption, transport or manipulation of

chemical reactants, it could be seen as an option that is simpler to implement in the industry.

The results indicate that the mechanical cleaning was not suitable for the MMO anodes,

regardless of the coating type or the quantity of MnO2 deposits to be cleaned. According to

the SEM images of Figure 3-21, the surface of anode D3 is largely recovered by fragments

of MnO2 that were not effectively cleaned. Plus, in the regions without the MnO2 patches,

55

there are multiple signs of coating detachment. There, the anode surface is predominantly

smooth; just a few regions still display mud crack features. The results are similar for E3:

there is significant depletion of mud-crack features and even exposure of the substrate.

Figure 3-21. SEM images of anodes D1 and E1 after mechanical cleaning. Images acquired at 10 kV in SE mode. Elemental maps obtained by EDS helped distinguishing

the regions that correspond to MnO2, MMO and the titanium substrate.

In the case of samples D4 and E4 (which have suffered more intense MnO2 deposition than

D3 and E3), the manganese dioxide formations were still visibly covering most of the surface

after the mechanical cleaning. So, the chemical cleaning method was applied subsequently

to remove these MnO2 deposits, allowing the microscopic characterization of the surface

regions that were hidden underneath. In fact, the effectivity of the chemical cleaning at

eliminating MnO2 deposits is evidenced by the EDS results of Figure 3-22. Based on the

SEM images in Figure 3-23, the mud-crack features were more preserved in regions that

were being covered by MnO2 deposits until the end of the ultrasonic wash. Meanwhile, the

surface regions that were exposed during the mechanical cleaning had little coating

coverage. This difference suggests that, here, the coating deterioration was not only due to

MnO2 deposition: the ultrasonic procedure contributed to it.

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Figure 3-22. EDS spectra of the anode samples D3, E3, D4 and E4, obtained after the cleaning procedures. The X-ray signal of Mn is only absent in the samples that were

also cleaned chemically (D4 and E4).

Overall, a possible explanation for the failure of this cleaning technique is that the energy

dissipated by cavitation was generally too weak to chip off MnO2 deposits completely, but

intense enough to break apart the MMO coating, especially the outer layers. The mixed

oxides that constitute the coating are relatively brittle, and the pores and cracks that enhance

its electrocatalytic surface area end up making the overall structure more fragile.

57

Figure 3-23. SEM images of anodes D2 and E2, accompanied by respective EDS maps. Data acquired at 10 kV in SE mode. Magnification of 400 x and 2 kx.

3.5. Crystal phase characterization of MnO2 particles

As previously described in Chapter 2, samples of particulate material were collected on three

occasions, to characterize the crystalline phase of MnO2 deposits. The sample #1 was

obtained from the surface of anode type D after the electrowinning test of Part 1.b at 50 mg/L

Mn2+. Particulate samples #2 and #3 were obtained from the cells of anode types D and E,

respectively, after electrowinning tests of Part 1.d, at 400 mg/L Mn2+.

The X-ray diffractograms of these materials are presented below. Figure 3-24 shows that

the diffractogram of the particulate sample #1 matches that of the anode type D, presented in

Section 3.1. It turns out that the material collected consisted of loose fragments of the MMO

coating rather than MnO2 deposits. In fact, this evidences that the coating type D was already

going through a deterioration process during the first zinc electrowinning test of Part 1.b, at

50 mg/L Mn2+, even though there was no sensitive increase of the cell potential values

recorded during the electrowinning tests of this Part.

The XRD diffractograms of samples #2 and #3 are presented in Figure 3-25. Both of them

present a diffraction pattern that corresponds to α-MnO2 – a phase that is often referred to

58

as cryptomelane or hollandite and can incorporate alkaline ions in its tunnelled structure.

The intensity of the first two peaks is lower here than in the official pattern of α-MnO2 (due

to the preferential orientation of these particles on the sample holder), similarly to what was

observed in diffractograms of cryptomelane nanorods [112]. Moreover, the diffractogram of

sample #3 also displays the main diffraction peaks of manganite (MnOOH) in orthorhombic

form.

Figure 3-24. X-ray diffractograms of the particulate #1 and the anode type D.

Figure 3-25. X-ray diffractograms of the particulates #2 and #3, accompanied by the reference patterns of α-MnO2 and MnOOH.

This phase identification is consonant with the literature, since α-MnO2 formation has been

observed in electrowinning tests involving MMO anodes [95] or lead-based ones [31]. Also,

MnOOH species are commonly associated with MnO2 deposits and are supposedly involved

in intermediary reaction steps of MnO2 deposition [36].

59

Chapter 4. Results and Discussion: Identification of MnO2-induced deterioration mechanisms and Mn2+ tolerance levels

The results of Part 1 have taught us to that MnO2 deposits cause the deterioration of the

MMO coatings, which can be seen in just a few days of laboratory-scale zinc electrowinning.

Such deterioration process was better detected by microscopy techniques rather than

monitoring the anode potentials through multiple galvanostatic polarization replicates. Also,

it was shown that the anodes can be chemically cleaned before microscopic analyses to

reveal the morphological conditions of the MMO coating where it has been previously

covered by MnO2 deposits.

Combining all this information, the tests of Part 2 were designed as a series of short

galvanostatic tests in which the Mn2+ concentration of zinc electrolytes was varied in the

range of 5 mg/L to 150 mg/L. The MMO anode samples were analyzed by SEM and EDS

after each test, then they were chemically cleaned and analyzed again. The results are

presented in Sections 4.1 to 4.3, divided by MMO anode type.

Attention was made to describe the samples as representatively as possible, based on the

collection of multiple micrographs over wide scanning areas (only a fraction of the SEM

images acquired are presented in this chapter). Nevertheless, it is important to note that this

SEM characterization was not designed to be a proper statistical assessment. The

descriptions of MnO2 deposit morphologies and their impact on the MMO anode surfaces

could be refined through the execution of additional test replicates and evaluation of larger

sets of SEM images per sample.

4.1. Microscopy results of anodes type D

According to the microscopy results, MnO2 deposition occurred over anodes type D after

each zinc electrowinning test performed in the presence of Mn2+ ions. At the lowest Mn2+

concentration tested, 5 mg/L, the whole anode surface was covered by a thin MnO2 film.

SEM images of Figure 4-1 provide a comparison between the anode surface before and

after the chemical cleaning. The comparison shows that the MnO2 film is so thin that it only

slightly diffuses the electrons emitted from the anode surface underneath, which makes

mud-crack features appear with softer, blurrier edges in the SEM image. The film also has

round MnO2 agglomerates scattered on the top of it, with sizes of (2-4) μm.

60

Figure 4-1. SEM images of the anode sample type D after the zinc electrowinning test at

5 mg/L Mn2+, before and after chemical cleaning. Magnifications of 400x to 5 kx.

SEM images at higher magnification (Figure 4-2) show that the MnO2 film has open-ended

cracks with ramifications, evenly distributed throughout the surface. If the original MnO2

structure had significant amounts of hydrates, these cracks may be the result of shrinkage

during the drying process. The film thickness seems to be no larger than 0.5 μm. Faint,

irregular pores are shown, with diameters that range up to 0.2 μm. Overall, both the coating

and the film look intact; there is neither displacement nor absence of fragments.

Figure 4-2. SEM images of the sample type D after the electrowinning test at 5 mg/L

Mn2+, before and after chemical cleaning. From left to right, magnifications of 22 kx,

17.7 kx, 20 kx and 22 kx. The pores of the MnO2 film are more visible in the first image.

MnO2 deposits with different characteristics formed over anodes type D in zinc electrolytes

containing 10 mg/L Mn2+. SEM images of Figure 4-3 and Figure 4-4 show that this MnO2

61

film was still porous, uniform and thin enough to express the underlying anode surface

morphology, but it no longer had that translucid aspect of the MnO2 film formed at 5 mg/L

Mn2+. Based on Figure 4-5, the film thickness was estimated to be around 1 μm.

Figure 4-3. SEM images of the anode type D after the zinc electrowinning test at 10 mg/L Mn2+. Images with magnification of 2 kx, in SE and BSE modes.

Figure 4-4. Comparison of the anode sample type D after the zinc electrowinning test at

10 mg/L Mn2+, before and after chemical cleaning. Magnifications of 400 x to 2 kx.

62

Figure 4-5. SEM image of anode type D after the electrowinning test at 10 mg/L Mn 2+. Magnification of 26 kx.

Moreover, this MnO2 formation was completely fragmented. This could be due to shrinkage

or stresses related to crystal growth and/or movement of oxygen bubbles, since these

deposits were less porous and more compact than the previous ones. Interestingly, some

of the MnO2 pieces detached from the anode surface displayed smaller fragments of another

material incorporated in its structure, as exemplified in Figure 4-6. This material could not

be identified by EDS, but the contrast of SEM images in BSE mode suggests that it has an

elemental composition similar to the MMO coating. After seeing these fragments in multiple

samples with MnO2 deposition (which will be presented later), it was possible to notice that

their morphology reminds that of mud-crack regions in anodes D. It was then hypothesized

that the fragments originally belonged to the coating and they ended up adhered to MnO2

deposits during their growth. In the continuation of this work, this hypothesis could be verified

by performing elemental mappings with wavelength-dispersive spectroscopy (WDS), as this

technique has superior element detection capability than EDS.

Figure 4-6. SEM images of anode type D after the zinc electrowinning test at 10 mg/L Mn2+. Loose MnO2 particle with unidentified fragments incorporated on it.

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MnO2 deposits formed during electrowinning tests at 25 mg/L Mn2+ had a similar morphology

to those formed at 10 mg/L Mn2+. In the micrographs of Figure 4-7, they correspond to the

darker material that partially covers the anode sample. The sample was gently brushed with

a paper tissue before the SEM imaging to demonstrate how easily these MnO2 deposits

detach, and also to increase the chances of observing other MnO2 particles flipped over.

This is why intact MnO2 particles are concentrated mostly in the valleys of the surface.

Figure 4-7. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+. Images with magnification of 200x (left) and 400x (right).

SEM images at higher magnification, in Figure 4-8, exemplify two of several MnO2 pieces

observed in this anode sample that were supposedly flipped over. Similar to the case of

Figure 4-6, these particles have foreign fragments incorporated into their surface which

seem to belong to the MMO coating.

The microscopic analysis of this sample after chemical cleaning is presented in Figure 4-9.

Even if a few fragments may be missing, the overall conditions of the anode coating were

preserved, especially the mud cracks. The top right image evidences the depletion of a few

patches, which reminds of the wearing effect observed after the ALT of this anode type, in

Part 1.a. All in all, if the loss of such coating fragments was a single event in the

electrowinning cells, they would not pose a threat to the anode performance. But in the long-

term service, when MnO2 deposits can grow and detach multiple times, successive coating

fragmentation would become problematic.

64

Figure 4-8. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+. Zoomed out image with magnification of 400 x and close-ups with 2 kx.

Figure 4-9. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+ and chemical cleaning. Images with magnification from 400 x and 2 kx.

Using zinc electrolytes with 50 mg/L Mn2+, an anode sample type D was tested in duplicate

(tested, chemically cleaned and tested again). SEM results of this sample suggest that the

MMO coating detachment increased after the second replicate (Figure 4-10 and Figure 4-

11). In fact, larger “unknown” fragments were found attached to loose MnO2 pieces this time,

and they revealed more details about their surface morphology. For instance, in the third

image of Figure 4-11, the “unknown” fragment presents mud-crack features very similar to

those of the MMO coating, both in terms of size and shape. In the highlighted region of this

micrograph, it is even possible to observe this same mud-crack texture imprinted on the

MnO2 material itself, as if the MnO2 deposit acquired this shape for having grown in close

contact with the anode surface.

65

Figure 4-10. SEM images of anode type D after the first zinc electrowinning test at 50 mg/L Mn2+. Images with magnification of 400 x and 2 kx.

Figure 4-11. SEM images of anode D after the second zinc electrowinning test at 50 mg/L Mn2+. Magnifications of 400 x and 2 kx. Mud crack texture highlighted in the last

image.

Coherently, the SEM images of this sample obtained post-chemical cleaning suggest that

the deterioration of the coating conditions intensified after the second replicate. By

comparing the images of Figure 4-12 and Figure 4-13, one can observe that occurrences of

coating delamination were more frequent after the anode was re-tested at 50 mg/L Mn2+. In

the images in BSE mode in Figure 4-14, the dark regions correspond to the signal of titanium

from the substrate, which in turn indicates occurrences of complete coating detachment.

Thus, according to these images, the area of substrate exposure increased after the second

replicate. Overall, the results indicate that the coating deterioration process continues as the

anodes type D are applied to repeated cycles of operation and cleaning.

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Figure 4-12. SEM images of anode type D after the first test replicate at 50 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x and 2 kx.

Figure 4-13. SEM images of anode type D after the second test replicate at 50 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x and 2 kx.

Figure 4-14. SEM images of anode type D after the first and second test replicates at 50 mg/L Mn2+, and after chemical cleaning. Magnification of 400 x.

Finally, a sample of anode type D was tested to zinc electrolyte containing 75 mg/L Mn2+. In

this case, the characteristics of the MnO2 deposits formed and the effects on the anode

coating were similar to those of 50 mg/L Mn2+. SEM images of Figure 4-15 and Figure 4-16

present the MnO2 deposits at two levels of magnitude. Once again, the MnO2 deposits seem

easily detachable, and some of the loose MnO2 pieces have foreign fragments incorporated

on them, which supposedly correspond to fragments of the MMO coating. A minor difference

67

is that these MnO2 deposits have a rougher texture that resembles dispersed grains over a

smooth base.

Figure 4-15. SEM images of anode type D after the zinc electrowinning test at 75 mg/L Mn2+. Magnification of 200 x.

Figure 4-16. SEM images of anode D after the zinc electrowinning test at 75 mg/L Mn2+. Magnifications of 200 x and 2 kx. The granular texture on MnO2 deposits is more visible

in the zoomed-in images.

SEM images of the anode surface after the chemical cleaning are presented in Figure 4-17.

Coating delamination and missing mud-crack fragments were occasionally observed,

similarly to previous tests.

68

The fact that the MnO2 deposits morphology remained virtually unchanged in tests with

electrolytes containing between 25 mg/L and 75 mg/L Mn2+ suggests that they would also

continue as such in higher Mn2+ levels (at least in the range of ~100 mg/L). Likewise, the

coating detachment process associated to this type of MnO2 formation is expected to

continue occurring at higher Mn2+ levels.

Figure 4-17. SEM images of anode type D after the test at 75 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x, 2 kx and 3.5 kx.

4.2. Microscopy results of anodes type E

The anode type E started to display its first signs of MnO2 deposition after electrowinning

tests with electrolytes containing 100 mg/L Mn2+, but it could only be effectively confirmed

after tests at 125 mg/L Mn2+.

SEM images obtained from samples tested at 50 g/L Mn2+, presented in Figure 4-18, show

that the anode surfaces looked brand new, in general terms. Exceptionally, a few pits were

observed around mud-crack features in two instances (Figure 4-19). These pits have the

same aspect as those observed after the ALT in Part 1.a. As such, they are supposedly

related to the exit of oxygen bubbles from catalytic sites below the cracks.

Figure 4-18. SEM images of anode type E after the zinc electrowinning test at 50 mg/L Mn2+. Magnifications of 400 x and 1 kx.

69

Figure 4-19. SEM images of anode type E after the zinc electrowinning test at 50 mg/L Mn2+. Examples of pits on the anode coating.

After electrowinning tests at 75 mg/L and 100 mg/L Mn2+, the overall surface characteristics

of the coating continued well preserved and the presence of manganese could not be

detected. SEM images of the anode samples used in these tests are provided in Figure 4-20

and Figure 4-21. These images illustrate that small cracks were occasionally found on

smoother regions of the anode surface but there was no fragmentation of the MMO layers.

Also, the mud-crack regions were overall intact.

Figure 4-20. SEM images of anode type E after the zinc electrowinning test at 75 mg/L Mn2+. Magnifications of 400 x and 2 kx. Note: the colour heterogeneities in the third

image are charging artifacts, due to the semi-conductive nature of the MMO coating.

Figure 4-21. SEM images of anode type E after the zinc electrowinning test at 100 mg/L Mn2+. Magnifications of 400 x and 2 kx.

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However, the SEM results become remarkably different for a sample type E after the

electrowinning test at 125 mg/L Mn2+: SEM images of Figure 4-22 show the presence of

multiple clusters of MnO2 crystallites. Through the colour contrast of the images in BSE

mode, it becomes more evident that these MnO2 clusters have formed exclusively over

smooth areas of the anode, away from mud cracks. At higher magnifications (Figure 4-23

and Figure 4-24), it is possible to observe that individual MnO2 crystallites have the form of

needles, with diameters smaller than 0.2 μm.

Figure 4-22. SEM images of anode type E after the zinc electrowinning test at 125 mg/L Mn2+. Magnifications of 1 kx (first pair) and 400 x (second and third ones).

Figure 4-23. SEM images of anode type E after the zinc electrowinning test at 125 mg/L Mn2+. Magnifications of 2 kx and 14 kx.

71

Figure 4-24. SEM images of anode type E after the zinc electrowinning test at 125 mg/L Mn2+. Magnifications of 2 kx and 6 kx.

Moreover, the high magnification images show that the MnO2 deposits concentrate along

the contours of ruptures in the outer MMO layer. At this point, it was not possible to affirm if

the ruptures were already there when the MnO2 deposition began, or if they were induced

by the volume expansion of the growing crystallites. But the second proposition became

more plausible once SEM images were acquired after the chemical cleaning (Figure 4-25).

These images showed that this sample displayed coating ruptures more frequently than its

counterparts tested at 50 mg/L to 100 mg/L Mn2+. Then, this proposition led to the speculation

that rare ruptures seen on the samples tested at 75 mg/L and 100 mg/L Mn2+ could also be

linked to MnO2 deposits that were not yet developed enough to be detectable by EDS.

Figure 4-25. SEM images of anode type E after the testing at 125 mg/L Mn 2+ and chemical cleaning. Examples of ruptures observed on the smooth regions of the

coating.

The sample type E tested in an electrolyte containing 150 mg/L Mn2+ presented a much

higher accumulation of MnO2 deposits. This time, they were visible to the naked eye, in the

form of black spots that easily contrasted with the light-gray MMO coating. Interestingly,

these MnO2 clusters had blurry edges towards the direction of the ascension of oxygen

bubbles. This characteristic is also visible at a microscopic scale, especially with the colour

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contrast provided by SEM images in BSE mode, in Figure 4-26. For reference, the direction

of the yellow arrows inserted corresponds to the upward position of these anodes in

electrowinning cells. The images show that the MnO2 deposition still occurred preferentially

over the smooth regions of the anode surface, but the deposits formed were fragile so the

flow promoted by the oxygen evolution caused an erosion process on them.

Figure 4-26. SEM images of anode type E after the zinc electrowinning test at 150 mg/L Mn2+. Magnification of 100x (left) and 400x (right).

SEM images at higher magnifications (presented in Figure 4-27 and Figure 4-28) reveal that

the MnO2 deposits consisted of prismatic or bladed crystallites, about 0.4 μm long and no

more than 0.1 μm wide. The crystallites were concentrated on large agglomerates that

contoured ruptures of the smooth regions of the coating but were also dispersed across the

anode surface, in the form of loose individuals or agglomerates with radial symmetry.

Figure 4-27. SEM images of anode type E after the test at 150 mg/L Mn2+. Spherical MnO2 agglomerates and loose, individual crystallites. Magnifications of 5 kx and 20 kx.

Comparting these MnO2 deposits with those of the test at 125 mg/L Mn2+, no significant

morphological changes are seen. But the appearance of agglomerates with radial growth

could suggest a surge in secondary nucleation of MnO2. In other words, detached crystallites

(or their fragments) may have served as electrophoretically deposited seed sites onto the

73

substrate for the growth of more MnO2 material, independently of the MMO surface. This

agrees with the fact that the SEM images in BSE mode do not show any differences in

contrast between the cores of these MnO2 spheroids and the crystal tips growing outwards;

otherwise, hypothetical cores composed of iridium or tin oxides would have higher electron

backscattering intensity. In any case, the possible advent of secondary nucleation of MnO2

could be verified in definitive through the use of WDS, for example.

Figure 4-28. SEM images of anode type E after the test at 150 mg/L Mn2+. MnO2 deposits around coating cracks and star-shaped agglomerates. Magnifications of 5 kx and 20 kx.

The SEM images of Figure 4-29 were obtained after the sample was chemically cleaned.

These images evidence that the accumulation of MnO2 deposits promoted the detachment

between inner and outer layers of the coating, in some regions. The ruptures seem to

advance faster between MMO layers than on the surface. The top layer patches must

eventually break off as their structural integrity becomes compromised. Even though this

deterioration process induced by MnO2 is distinct to that observed on anodes type D, it also

occurs at a relatively short time interval relative to the expected anode service life.

Figure 4-29. SEM images of anode type E after the electrowinning test at 150 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x and 2 kx.

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Overall, based on these results, it cannot be said that anodes type E would be free of MnO2

deposition if operated in electrolytes with less than 100 mg/L Mn2+. As previously discussed,

the results of electrowinning tests in Part 1.b. have indicated the appearance of MnO2

deposits on anodes E operating at 50 mg/L Mn2+ for about a week. It turns out that, at low

Mn2+ levels, the MnO2 formation may not be detectable yet after 72-hour tests, but it could

still impact the anode performance in the long term.

4.3. Microscopy results of anodes type F

SEM images of a sample type F applied to zinc electrowinning at 5 mg/L Mn2+ are presented

in Figure 4-30. No signs of MnO2 deposition were detected in this sample during the

microscopic characterization. The anode had its original characteristics preserved.

Figure 4-30. SEM images of anode type F after the electrowinning test at 5 mg/L Mn2+. Magnifications of 400 x and 2 kx.

The MnO2 deposition on anodes F was first confirmed after electrowinning tests at 25 mg/L

Mn2+, but it may have actually initiated in tests at 10 mg/L Mn2+. The reason is that SEM

images obtained after tests at 10 mg/L Mn2+ (Figure 4-31) show the appearance of a few

small protrusions on smooth peaks of the coating, with dark contrast in BSE mode, which

turned out to be similar to MnO2 deposits observed on the sample tested at 25 mg/L Mn2+

(Figure 4-32).

In fact, the MnO2 deposition at 10 mg/L Mn2+ could not be confirmed because no manganese

was detected by EDS and because the protrusions seemed like a natural continuation of the

MMO coating itself, with no distinguishable boundaries. If early-stage MnO2 crystallites were

growing inside the pores of the coating, they would be hardly detectable through the

equipment employed in this characterization. However, the sample used in the

electrowinning test at 25 mg/L Mn2+ presented drusy MnO2 formations that were already

distinguishable by SEM imaging at a magnification of 1 kx. In Figure 4-33 and Figure 4-34,

75

close-up SEM images help to visualize that the MnO2 crystallites in such agglomerates

display the same prismatic morphology observed on the anode type E tested at 150 mg/L

Mn2+. They also concentrate around fractures of smooth regions of the coating. A minor

difference is that these crystallites are shorter, with lengths around 0.2 μm.

Figure 4-31. SEM images of anode type F after the electrowinning test at 10 mg/L Mn2+. Magnifications of 1.5 kx and 3 kx.

Figure 4-32. SEM images of anode type F after the electrowinning test at 25 mg/L Mn2+. Magnifications of 1 kx and 200 x.

Figure 4-33. SEM images of anode type F after the electrowinning test at 25 mg/L Mn2+. Magnifications of 12 kx, 10 kx and 12 kx, from left to right.

76

Figure 4-34. SEM images of anode type F after the electrowinning test at 25 mg/L Mn2+. Magnifications of 14 kx and 18 kx.

The next electrowinning test involving a sample of anode F was performed at a Mn2+

concentration of 100 mg/L. SEM images obtained after the test (Figure 4-35), reveal a

significant development of MnO2 deposits over smooth regions of the anode surface. At

higher magnifications (Figure 4-36), it is possible to observe that these MnO2 crystallites

kept the same morphology and association to coating fractures previously observed after

the test at 25 mg/L Mn2+. Fractures developed over this sample became more evident in

SEM images taken after the chemical cleaning (Figure 4-37).

Figure 4-35. SEM images of a sample F after the test at 100 mg/L Mn2+. Magnification: 400 x.

Figure 4-36. SEM images of anode type F after the electrowinning test at 100 mg/L Mn2+. Magnifications of 1 kx (left) and 12 kx (right).

77

Figure 4-37. SEM images of anode type F after the electrowinning test at 100 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x and 2 kx.

Finally, a sample of anode type F was also tested in zinc electrolyte containing 125 mg/L

Mn2+. SEM images at different degrees of magnification (Figure 4-38 and Figure 4-39) show

that the spatial distribution of the MnO2 deposition and the morphology of individual

crystallites continued very similar to those observed after the tests at 25 mg/L and 100 mg/L

Mn2+. Short, prismatic crystallites were found in large agglomerates and also contouring

fractures of the coating, which have been opened in regions that were previously smooth.

Figure 4-38. SEM images of anode type F after the electrowinning test at 125 mg/L Mn2+. Magnification of 400 x.

Figure 4-39. SEM images of a sample F after the test at 125 mg/L Mn2+. Magnification: 5 kx.

78

One interesting observation in this sample is the coexistence of two MnO2 deposit

morphologies right next to each other, as exemplified through the SEM images of Figure 4-40.

In the third image of the sequence, it is possible to observe acicular formations on the left

side, and radiating prismatic crystals on the right. The MnO2 deposit morphology was

expected to be uniform if the conditions of the reaction media are homogeneous. Possible

explanations for this divergence are:

• MnO2 depositions initiated in different layers of the MMO coating may have

developed different morphologies. Heterogeneities of chemical composition across

different layers or regions of the coating could influence the MnO2 growth.

• Each MnO2 deposit morphology may have formed separately during different stages

of the electrowinning test. For instance, variations on the current density, electrolyte

acidity or Mn2+ concentration throughout the test may have affected the MnO2

morphology.

Figure 4-40. SEM images of anode type F after the electrowinning test at 125 mg/L Mn2+. Magnifications of 1 kx, 5 kx and 25 kx.

SEM images obtained after chemically cleaning this sample are presented in Figure 4-41.

The images demonstrate that, in overall, the surface conditions of this anode were similar

to those observed after the test at 100 mg/L Mn2+. A few ruptures have appeared over the

smooth regions of the anode, specifically over the outer MMO layer or between the outer

and inner layers. Missing coating fragments and exposure of the titanium substrate were

eventually observed as well.

79

Figure 4-41. SEM images of anode type F after the test at 125 mg/L Mn2+ and chemical cleaning. Magnifications of 400 x and 2 kx.

4.4. Establishing Mn2+ tolerance levels for each anode type

The microscopic characterization of the MMO coating, combined with the chemical cleaning

by FeSO4 solution, was a sensitive and robust way to assess the anode integrity. This

method does not become susceptible to external influences like the anode potential

measurements from galvanostatic tests. Therefore, for the discussions here presented, we

can re-define “Mn2+ tolerance” as the Mn2+ concentration threshold that marks a surge of

both MnO2 formation rate and coating deterioration rate, estimated from galvanostatic tests

of 72 h and SEM/EDS data.

A summary of the results of Part 2 is provided in Table 4-1, along with indications of the Mn2+

tolerance levels of each anode type. This panoramic view help to visualize how the Mn2+

level increase in the electrolyte generally led to an increase in the amount of MnO2 deposits

observed as well as a worsening of MMO coating conditions. The respective Mn2+ tolerance

levels of anodes D, E and F are (5-10) mg/L, (100-125) mg/L and (5-25) mg/L Mn2+.

The anodes type D did not have any signs of deterioration due to the development of a thin

MnO2 film during the electrowinning test at 5 mg/L Mn2+. However, the detachment of small

MMO coating fragments starts to be observed after the test at 10 mg/L Mn2+. In the case of

the anodes type E, no MnO2 deposition was detected at up to 100 mg/L Mn2+, but well-

developed MnO2 deposits appear at 125 mg/L Mn2+, accompanied by a few ruptures of the

coating. The anodes type F were likely subjected to MnO2 deposition in tests involving Mn2+

levels of 10 mg/L and up, but negative effects on the coating were observed starting at

25 mg/L Mn2+. Interestingly, the tolerance levels generally agree with the electrolyte colour,

which reflects the formation of Mn3+ ions.

80

Table 4-1. Summary of the results of Part 2 and definition of Mn2+ tolerance levels for the anodes D, E and F.

81

Chapter 5. Ranking the suitability of different anode types to industrial zinc production

As different types of MMO anodes have demonstrated different anodic potentials and

different Mn2+ tolerance levels in zinc electrowinning tests, a question remains: which type

would be the most suitable for implementation in zinc cellhouses? The answer heavily

depends on financial aspects. Apart from trivial differences of purchase, re-coating and

replacement costs that integrate capital expenditure estimates, one should also consider the

comparison of operational costs related to the manganese control strategy to be adopted.

Regarding the hypothetical ranking of anode types D, E and F in terms of suitability to zinc

cellhouses, different scenarios for comparative cost analyses of manganese control are

presented below.

Scenario 1: Fixed anode cleaning periodicity of 72 h, no MnO2 deposition inhibition

Presupposing the implementation of periodical chemical cleaning of the MMO anodes, with

cycles of 72 h, the expected accumulations of MnO2 deposits on the anodes D, E and F

should be equivalent to those of the electrowinning tests of Part 2. As such, if other impurities

of industrial zinc electrolytes do not interfere with the MnO2 deposition, the Mn2+ tolerance

levels determined in Section 4.4 would apply. Then, the anodes type D are the most

advantageous in terms of energy savings, because they provided the lowest cell potentials

(~2.56 V) even under a certain accumulation of MnO2 deposits. On the other hand, anodes

type D have the lowest Mn2+ tolerance (~5 mg/L) and so they would require the highest

degree of electrolyte purification. Meanwhile, anodes E showed an inverse profile; they would

promote the lowest energy savings (with cell potentials of ~2.67 V) and require the least

amount of electrolyte purification (tolerance of ~100 mg/L Mn2+). The anodes Type F had

intermediary results in both aspects (tolerance of ~25 mg/L and cells at ~2.60 V).

Therefore, anodes D would be the most suitable in a scenario where its energy economy

compensates for the costs of intensive electrolyte purification. For instance, a scenario

where there are elevated electricity costs, energy supply restrictions, or the advent of low-

cost electrolyte purification technologies for Mn2+ removal. Contrarily, the type E would be

the most recommended in cases where it may be unfeasible to obtain zinc electrolytes with

Mn2+ content as low as 5 mg/L Mn2+, or where the energy economy it provides is a sufficient

payoff for its implementation.

82

Scenario 2: Optional anode cleaning or variable periodicity, solutions for MnO2

inhibition available

From an industrial point of view, cleaning the MMO anodes every 72 h may be feasible but

certainly not ideal. Decreasing the cleaning periodicity or even disregarding it altogether

could perhaps be achieved by finding ways to inhibit the MnO2 accumulation on anodes

surfaces in the long term.

In this scenario, the Mn2+ tolerance levels of Section 4.4 no longer apply to the anodes D, E

and F. Since the MnO2 deposition morphologies can evolve over weeks or months, the

coating deterioration results of three-day electrowinning tests would not necessarily help to

estimate anode performance losses after months or years of uninterrupted service. One

example of this divergence is the observation of MnO2 deposits on anodes E after a week

at 50 mg/L Mn2+ (Part 1.b), but not after 72 hours at 75 mg/L Mn2+ (Part 2).

Then, the optimal Mn2+ level for each anode type could be alternatively defined as the Mn2+

concentration that minimizes the combined costs of all activities that compose the

manganese control strategy of interest. Hypothetical cost curves of these activities are

provided in Figure 5-1, to illustrate this approach. For instance, operating with electrolytes

containing high Mn2+ concentrations would lead to higher MnO2 formation rates, so that the

anodes would require more frequent cleaning or perhaps higher doses of additives to inhibit

the MnO2 growth. On the other hand, electrolytes with lower Mn2+ levels would require a

higher degree of purification. In this case, the Mn2+ removal system would be more costly to

operate, requiring higher amounts of reactants or perhaps longer residence times.

Figure 5-1. Hypothetical cost curves of activities relative to manganese control in a zinc production plant, as a function of Mn2+ concentration.

83

Conclusion

The adoption of manganese control strategies is expected to be fundamental for the

achievement of satisfactory long-term performance of MMO anodes in zinc cellhouses, to

supersede conventional lead-based anodes. Thus, contributing to this anode transition

prospect, the present study has investigated the effects of MnO2 deposition on three types

of IrO2-bearing MMO anodes.

Firstly, this study provided an evaluation of anode potential, cell potential and Mn2+ depletion

profiles over time, mostly for anode types D and E. These data were obtained from

galvanostatic polarization tests in zinc or zinc-free electrolytes involving different Mn2+ levels,

at up to 400 mg/L. The results indicated that increasing the Mn2+ concentration led to higher

MnO2 formation rates and consequently higher potential increase. Moreover, after the

galvanostatic tests at 400 mg/L Mn2+, samples of anodes D, E and F were analyzed by SEM

and EDS. The microscopic results evidenced that intense deterioration of the MMO coatings

occurred due to MnO2 deposition in only a few days of testing (72-216) h.

In these electrowinning tests, measurements of Mn2+ concentrations and anode or cell

potentials were susceptible to significant variability due to external factors. In future

experiments, efforts could be put on improving the precision of these measurements, to

determine reaction coefficients of MnO2 formation as a function of Mn2+ levels. More precise

measurements would also help to confirm whether (and how much) the rates of MnO2

formation and anode potential increase vary according to the type of MMO coating.

Next, chemical and ultrasonic cleaning methods were applied on different anode types for

the removal of MnO2 deposits. According to SEM and EDS results, the chemical cleaning

method was very effective to reduce MnO2 deposits with no collateral effect to the MMO

coatings. On the other hand, the use of ultrasonic cleaning was ineffective for MnO2 removal

and worsened the anode surface conditions. After this evaluation, the chemical cleaning

was selected to incorporate the methodology of the second part of this study.

The chemical cleaning method holds potential to be applied on an industrial scale. Since it

consisted of immersing the anodes in an acidic solution containing FeSO4 at room

temperature, this method is not energy-intensive and the constituents of the cleaning

solution are available in the RLE process. Moreover, the MnO2 removal was observed to

occur in just a few minutes (the immersion time of 30 min was much longer than necessary).

84

Therefore, in future works, the parameters of the chemical cleaning could be optimized to

accelerate the procedure and decrease the amount of reactants required.

In the second part of this study, microscopic results revealed that the MnO2 deposits

developed different morphologies depending on the Mn2+ concentration and the anode type.

Moreover, each type of MnO2 deposit morphology was associated with a different coating

deterioration process. Such deterioration processes were irreversible, and their effects were

even more significant than those observed after 800 hours of ALTs at 5 kA/m2, in Part 1.

SEM and EDS results revealed that anodes type D have been covered with uniform MnO2

films even at Mn2+ levels of 5 mg/L. After further growth, at 10 mg/L Mn2+ or more, these

MnO2 films were prone to fragmentation and detachment from the coating. Evidence

suggests that these detached MnO2 pieces take out MMO coating fragments adhered to

them. Meanwhile, MnO2 clusters of elongated crystallites were found to develop over anodes

E and F. The growth of such MnO2 formations was found to induce ruptures throughout

MMO coating layers, in regions that did not present mud cracks. These ruptures would

weaken the structural integrity of the MMO coating locally, eventually leading to the

detachment of MMO fragments.

Finally, the tolerance levels of the anode types D, E and F were identified as (5-10) mg/L,

(100-125) mg/L and (5-25) mg/L Mn2+, respectively. Such tolerance levels apply in situations

where the anodes would operate with cycles of service and chemical cleaning of 72 h. In

this operational strategy, the periodical chemical cleaning would be fundamental to preserve

the MMO anode integrity in the long term, even when operating below the Mn2+ tolerance

levels. According to the results obtained, the chemical cleaning would completely remove

MnO2 deposits from anode surfaces at an early stage, before damage is caused to the

coatings. Alternatively, in hypothetical scenarios where the manganese control strategies

involve less frequent anode cleaning or includes the use of additives, for example, the

optimal Mn2+ levels should be determined considering also the operational costs of

electrolyte purification, MnO2 removal and consumption of additives.

Following this work, further investigations could be carried with the same MMO anode types

to understand how their properties induce the development of specific MnO2 deposit

morphologies. A more extensive characterization of the MMO coatings would be also

required to reveal differences of elemental and crystallographic compositions across the

MMO layers of each anode. This is especially relevant for the anode types E and F, which

85

displayed MnO2 crystallite clusters grown from and around fresh cracks opened on the

coating surface. It would be interesting to confirm whether the MnO2 deposition of these

anodes occurred preferentially in regions where Mn2+ ions could access the inner MMO

layers. Then, such MnO2 deposition preference could perhaps be associated with one or

more specific characteristics of these inner layers. Overall, this would help to identify the

most vulnerable compositional and structural features of these MMO coatings.

Added to that, future studies could focus on identifying strategies to inhibit or modify the

growth of MnO2 deposits. In this context, it would be pertinent to examine the long-term

progress of current efficiency, morphology and secondary nucleation rates of MnO2

deposition. These tests could be carried out in pilot-scale cells, running continuously for

several months.

Finally, it would be also important to confirm whether bismuth is released from the MMO

coatings in the industrial cells. For this purpose, ALTs could be replicated with each MMO

anode type, and a fourth replicate could be performed with a “Bi-free anode” (made of

platinum, for example) to serve as a control test. This way, the results of the MMO anodes

could be compared against the control, to elucidate if the bismuth detected on the cathodes

originated from other sources. Plus, the electrolyte of these ALTs could be analyzed by an

elemental quantification technique (such as ICP) to check whether any constituents of the

MMO coating have accumulated in the electrolyte (and how fast they have accumulated).

All things considered, the main experimental work to be done regarding this topic is to

investigate whether such MMO anode dissolution could cause detrimental effects to zinc

deposition. This could be answered, for example, through the monitoring of the current

efficiency of zinc deposition during pilot-scale tests.

86

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Appendix A. Potentiodynamic polarization analyses

This appendix presents the potentiodynamic polarization analyses concerning the anode

types D and E in three different types of electrolytes used in this study. This technique was

used to compare the OER overpotentials obtained with these MMO anodes in zinc

electrolyte, zinc-free electrolyte and acid solution.

The analyses were performed in jacketed glass beakers of 2 L, filled up with 1.8 L of

electrolyte. The beakers were connected to a thermostatic bath, to maintain the cell

temperature at (40 ± 1) °C through water recirculation. The stirring was performed by a

magnetic bar with a length of 3 cm, rotating at 7.33 rad/s. The composition and mode of

preparation of the three electrolyte types are presented in Section 2.2.2. The electrode

arrangement, cathodes and reference used are the same as those described in Section 2.2.3.

The potential scanning and data acquisition were performed by a potentiostat model

Interface 1010E, by Gamry. The scans covered a potential interval of (1-1.65) V vs. SHE,

with a sweep rate of 10 mV/s. The current was measured every 0.5 s. Each test was

repeated in triplicate to verify and confirm the replicability.

The results are presented and compared in the chart of Figure A-1 (using one replicate per

experiment, for simplification). The comparison indicates that both zinc and zinc-free

electrolytes provided a similar OER potential slope for either anode type D or E. In this case,

the OER potential starts at about 1.48 V. On the other hand, the curves obtained with acid

solution start ascending much earlier, at about 1.45 V. Overall, this comparison illustrates

how the zinc and zinc-free electrolytes could replace each other in long-term galvanostatic

tests without causing significant shifts of the anode potentials.

Figure A-1. Potentiodynamic polarisation curves of anodes D and E in three electrolytes.

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Appendix B. Cell and anode potentials of the galvanostatic polarization tests of Part 2

The charts of Figure B-1 present anode and cell potential profiles recorded during each

galvanostatic polarization experiment of Part 2. These profiles are identified in terms of the

nominal Mn2+ concentration (approximate concentration of Mn2+ ions at the moment they

were step-dosed). The methodology of these experiments is described in Section 2.3.5.

Figure B-1 Anode and cell potential results of each electrowinning test of Part 2.

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Occasionally, anode potential measurements suffered interference of reference electrodes

that presented malfunctioning. Examples of these interferences are found, for example, for

the anode type E during the second replicate test at 50 mg/L Mn2+ and the test at 100 mg/L

Mn2+, or for the anode type F during the tests at 0 mg/L, 5 mg/L, 25 mg/L and 100 mg/L

Mn2+. These potential profiles have unexpected drops of ~20 mV in the first hours of testing

or decrescent trends that do not agree with the cell potentials. Hence, in these cases, it is

best to refer to the cell potentials for indirect observation of the anode performance at

different Mn2+ levels. This is why the cell potentials were used for the average potential

comparison in Figure 3-15 of Section 3.3.

Complementarily, Figure B-2, Figure B-3 and Figure B-4 present the Mn2+ concentration

profiles of these same electrowinning tests, identified by their nominal Mn2+ levels. Overall,

little Mn2+ variation was detected throughout each test, regardless of the anode type. These

results illustrate that the processes of MnO2 growth and MMO coating deterioration were

studied in Mn2+ levels that can be considered pseudo-stationary in this context.

Figure B-2. Mn2+ concentration profile of anode type D, measured during the zinc electrowinning tests of Part 2.

Figure B-3. Mn2+ concentration profile of anode type E, measured during the zinc electrowinning tests of Part 2.

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Figure B-4. Mn2+ concentration profile of anode type F, measured during the zinc electrowinning tests of Part 2.

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Appendix C. Research Paper

Mn2+ tolerance levels and MnO2-induced deterioration mechanisms of mixed metal

oxide (MMO) anodes applied to zinc electrowinning

Daniela Rossetto de Menezes1, Houshang Alamdari1, Carl W. Brown, Jr.2

1 Department of Mining, Metallurgy and Material Engineering, Laval University, Quebec, Canada 2 De Nora Tech LLC, Ohio, United States

Abstract

The adoption of Mixed Metal Oxide (MMO)-coated anodes in zinc electrowinning cellhouses

would provide energy savings and a lead-free workplace. But a major concern is that commercially

available MMO anodes deteriorate prematurely in typical zinc electrolytes, due to intense MnO2

deposition. Thus, the present study investigated how the Mn2+ concentration in zinc electrolytes

affects the characteristics of MnO2 deposits and, consequently, the integrity of three types of IrO2-

bearing MMO anodes. Scanning electron microscopy (SEM) and energy-dispersive X-ray

spectroscopy (EDS) were used to characterize anode samples after galvanostatic polarization tests

performed at (5-150) mg/L Mn2+. The results have shown that MnO2 deposits developed different

morphologies and induced different anode deterioration processes, depending on the Mn2+

concentration and the anode type. In particular, anodes type “D” were covered by MnO2 films that

would easily chip off the anode after reaching a critical thickness. Evidence suggests that these MnO2

pieces detached take out MMO coating fragments adhered to them. Meanwhile, MnO2 clusters of

acicular and prismatic crystallites developed over anodes “E” and “F”, and they were found to induce

ruptures throughout the MMO coatings. Considering these results and specific criteria to define the

Mn2+ tolerance levels of these anodes, a financial analysis was proposed for screening the most

suitable anode type for industrial use, based on both the anodic potential demonstrated and the

manganese control strategy required for its satisfactory operation.

Keywords: mixed metal oxide, zinc electrowinning, manganese dioxide, manganese tolerance, coated

titanium anode.

1. Introduction

Over 80% of the global zinc production features a century-old electrowinning configuration,

which employs lead-based anodes for the promotion of the oxygen evolution reaction (OER) in

sulphate electrolytes [1]. These lead anodes cause elevated energy consumption and operational

issues related to lead corrosion products. A promising alternative recently proposed is their

replacement by mixed metal oxide (MMO)-coated anodes.

IrO2-bearing MMO anodes have demonstrated excellent energy efficiency and corrosion

resistance in sulphate electrolytes. Their OER overpotential is about 300 mV lower than those of

typical Pb-Ag anodes (0.5% - 1% Ag), at a current density of 500 A/m2 [2, 3]. Also, these anodes can

operate in sulphate-based electrolytes for over six years continuously, and for up to 20 years with re-

coating [4]. Contrary to lead anodes, they do not deform and do not release hazardous impurities.

This technology has been advancing over the last 40 years and has been operating successfully in

copper electrowinning plants for a decade [4].

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However, one important technical barrier to the immediate adoption of MMO anodes is the

high content of Mn2+ ions in zinc electrolytes. In parallel to OER, Mn2+ species generally oxidize and

form MnO2 deposits over the anodes, according to Reaction 1 [5].

Mn2+ + 2H2O → MnO2 + 4H+ + 2e− (1)

Some background MnO2 deposition is beneficial in conventional zinc cellhouses because it

attenuates the corrosion rate and chlorine evolution of lead-based anodes [6], which is why the Mn2+

concentration is generally maintained between (1-5) g/L [7]. But the degree of MnO2 deposition

promoted at this Mn2+ concentration range is considered harmful for typical MMO anodes. According

to the literature, accentuated MnO2 deposition blocks catalytic sites and adds extra ohmic resistance

to the MMO coating [8], leading to higher energy consumption and uneven current distribution [9].

However, the presence of Mn2+ species in sulphate electrolytes does not preclude the use of

MMO anodes. For instance, a type of Ti/IrO2-Ta2O5 anode developed by Freeport-McMoRan Copper

& Gold Inc. has maintained a stable potential for years, operating at 300 mg/L Mn2+ [4]. This stability

has been linked to the MnO2 inhibition effect caused by Cyquest 900, a copper levelling additive

employed with these anodes [4, 10]. Meanwhile, De Nora’s new dimensionally stable anodes (DSAs)

were shown to keep potential stability operating at (100 ± 50) mg/L Mn2+ in long-term copper

electrowinning tests [11], without involving additives.

In other studies, superior Mn2+ tolerance was said to be attained through the synthesis of more

amorphous [12, 13] or smaller [14, 15] crystallites of electrochemically active components of the

MMO coatings. However, amorphous IrO2-based coatings are generally obtained through the use of

low thermal treatment temperatures (< 350 °C), and these are said to be less durable, for having an

accelerated release of soluble iridium species [14, 16]. To avoid this issue, a patented MMO coating

composition combines IrO2, SnO2, and dopant elements such as Bi, Sb, Ta or Nb. This way, low

average crystallite sizes (below 5 nm) can be attained while maintaining moderate thermal treatment

temperatures, of (480-530)°C, to guarantee proper IrO2 durability [14].

Overall, this suggests that replacing lead-based anodes with MMO ones should be possible in

zinc cellhouses, as long as the MnO2 deposition is controlled to a certain extent. In this context, we

define “manganese control strategy” as the combination of process steps meant to restrict the Mn2+

buildup in the electrolyte and the accumulation of MnO2 deposits. Manganese control strategies in

zinc cellhouses could include, for example, electrolyte purification systems for the decrease of the

Mn2+ levels, anode cleaning systems for periodical removal of excess MnO2 deposits from the anodes,

and/or dosing of new additives that act as MnO2 deposition inhibitors.

Thus, among other aspects, the establishment of manganese control strategies require:

• Identifying the Mn2+ tolerance levels in zinc electrolytes, to enable the operation of MMO

anodes with neither potential increase nor coating deterioration;

• Identifying favourable MMO anode cleaning methods, which should be harmless to the anode

integrity, efficient for MnO2 removal, cost-effective, practical to implement in the industrial

environment, involving minimum health or environmental risks.

Thus, the present study focused on investigating the relationships between Mn2+ concentration,

MnO2 formation rate and coating deterioration processes, for three types of IrO2-bearing MMO

anodes. Plus, in this study, the methodology of the anode surface characterization involved the use of

99

a chemical cleaning method, which consisted of immersing the anodes in an acidic solution

containing FeSO4.

This cleaning method was based on the report that a FeSO4 solution has been effective for the

removal of MnO2 deposits from MMO anodes after long-term electrowinning tests [11]. In fact, the

reduction of manganese oxide is favourable in the presence of Fe2+, according to Reaction 2 [8]. MnO2

is added to zinc leachate because of this same reaction, to maintain the dissolved iron at the oxidation

state of 3+ in the jarosite precipitation circuit [5]. This also explains why the increase of Fe2+ ions

decreases the extent of MnO2 deposition on lead anodes in copper tankhouses [17, 18].

MnO2 + 2Fe2+ + 4H+ → Mn2+ + 2Fe3+ + 2H2O (2)

2. Materials and Methods

2.1. Materials

Laboratory-scale galvanostatic polarization tests were performed to mimic the zinc

electrowinning process in batch mode, with controlled conditions. These tests were run on jacketed

glass beakers with a volume capacity of 2 L (which were filled with 1.8 L of electrolyte). The beakers

were connected to a water bath model IsotempTM 6200 H7 by FisherbrandTM, for temperature control.

The current was provided by a power supply model GPS 303DD, by GW Instek. The anode and cell

potentials were measured automatically by a data logger Graphtec model GL240, where the voltage

was sampled every 20 s.

The electrodes were assembled vertically on a cathode-anode-cathode arrangement, to enable

cathode replacement without current interruption. The electrodes had exposed surface areas of 10 cm2

on each side; the remaining surfaces were covered by an inert lacquer (Miccrostop Stop-off Lacquer

by Tolber). These electrodes were affixed to custom-made cell caps of high-density polyethylene.

The electrode spacing was 2.5 cm.

The cathodes were manufactured from sheets of aluminum alloy series 1100 (99% to 99.5%

Al), with hardening treatment H14. Before each experiment, the cathodes were wet polished manually

with three types of silicon carbide papers, of grit numbers #320, #600 and #1200.

The anode potentials were measured using reference electrodes of silver-silver chloride with

saturated potassium chloride (Ag-AgCl/KClsat), with double junction, manufactured by Sensorex. The

reference potential was 0.204 V vs. SHE (standard hydrogen electrode).

The three anode types investigated in this work were provided by De Nora Tech, and are

referred to as anodes D, E and F. They were equally manufactured with titanium substrates. The

MMO coatings of each type had differences in terms of elemental composition, morphology and

crystalline structure, which were characterized using X-ray fluorescence spectrometry (XRF),

scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively.

Two types of electrolytes were used in this study. Zinc electrolytes were composed of 170 g/L

H2SO4 and 55 g/L Zn2+ (as ZnSO4); they were prepared using ZnO. Meanwhile, zinc-free electrolytes

were composed of 170 g/L H2SO4 and 65 g/L K+ (as K2SO4); they were prepared using K2SO4. Zinc

electrolytes allowed replicating cycles of zinc metal deposition and cathode harvesting similar to the

industrial zinc electrowinning process. But in galvanostatic tests that lasted for multiple weeks, the

use of the zinc-free electrolyte was preferred, to reproduce accordingly the anodic reactions of interest

without requiring daily cathode replacement (which would make this experiment more laborious).

100

Both electrolyte types were prepared using deionized water (with electrical resistivity of 18.2 MΩ.cm)

and analytical-grade reactants.

The addition of Mn2+ ions in the cells was made via injection of 50 mL of solutions containing

the original, Mn-free electrolyte components (H2SO4 and ZnSO4 or K2SO4) plus certain amounts of

dissolved MnSO4 (analytical grade). This method was adopted to guarantee that the Mn2+ ions could

easily disperse in the cell immediately at the moment of injection, as a step input to the electrowinning

system.

2.2. Analytical techniques

The Mn2+ concentrations of electrolyte samples were measured by microwave plasma atomic

emission spectrometry (MP-AES) using a spectrometer Agilent model 4100. The electrolyte samples

were filtered through wool glass (with a pore size of 0.2 μm) and then diluted tenfold in HNO3 5%.

The elemental quantification was calculated using the Mn absorption values centred at the wavelength

of 403.076 nm.

The SEM images and the EDS data were obtained on a Tescan Vega 3, with an EDAX Element

EDS Detector integrated, using a potential of 10 kV. The SEM analyses were performed in both

secondary electron (SE) and backscattered electron (BSE) modes. Meanwhile, the EDS results (in the

form of elemental mapping) were acquired with an X-ray energy detection range of (0-7) eV and a

resolution of 128.8 eV.

The XRF analyses were performed using a spectrometer Rigaku model ZSX Primus II, with

two modes of X-ray penetration depth (film mode and default mode), scanning all elements between

fluorine and uranium.

The XRD data were acquired on a powder X-ray diffractometer model Aeris, by Pan Analytical. The

samples were scanned at a 2θ range of 5° to 85°, scan rate of 2.8 10-2 (°)/min and step size of 0.0110°

(2θ). The radiation source was Cu Kα. Using the software X'Pert HighScore Plus by Pan Analytical,

the diffractograms were treated with a default profile fit. Then, a search and match routine was run in

connection with the Crystallography Open Database (COD) [19].

2.3. Chemical cleaning

The chemical cleaning, for removal of MnO2 deposits, was performed by submerging the

anodes in 80 mL of a solution containing 50 g/L H2SO4 and 50 g/L FeSO4. This solution was prepared

with deionized water, (95-98)% H2SO4 and laboratory-grade FeSO4. The anodes remained in solution

at room temperature for at least 30 min. During this time, they were agitated occasionally to improve

the mass transfer around loose MnO2 flakes that would take longer to dissolve.

2.4. Electrowinning tests with step increase of Mn2+ levels

Two electrowinning tests were performed with samples of anode types D and E, at a current

density of 500 A/m2, at a temperature of 40 °C and for a duration of 1100 h and. Zinc-free electrolyte

was employed. Deionized water was added daily to re-adjust the electrolyte acidity, according to the

decrease of the electrolyte level.

By dosing manganese-rich solutions in the cells, the Mn2+ levels of these electrowinning tests

were increased in four steps. Firstly, the electrolyte was set to about 15 mg/L Mn2+ after 1 h of testing,

then it was raised to 100 mg/L after 6 days, then to 150 mg/L after 19 days, finally reaching 230

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mg/L after 36 days. Electrolyte samples were collected daily to quantify the Mn2+ concentration

values by MP-AES.

2.5. Microscopic evaluation of MnO2 deposit morphologies and MMO coating deterioration

effects

In this part, samples of the anode types D, E and F were applied to electrowinning tests at a

current density of 500 A/m2, at a temperature of 40 °C and for a duration of 72 h. Since zinc electrolyte

was employed, these tests included daily cathode replacement and daily addition of ZnO to re-adjust

the electrolyte composition. The amount of ZnO dosed was calculated based on the mass of zinc

electrodeposited in the previous 24 h.

Except for blank tests (0 mg/L Mn2+), all the others were performed with single-step additions

of Mn2+ after 1 h of testing. This way, the samples type D were tested in the pseudo-stationary Mn2+

levels of (5, 10, 25, 50 and 75) mg/L. The samples type E were tested at (50, 75, 100, 125 and 150)

mg/L Mn2+. Meanwhile, the samples type F were evaluated at the Mn2+ levels of (5, 10, 25, 100 and

125) mg/L. The electrowinning tests at 50 mg/L of anode types D and E were repeated using the same

samples. The electrolyte was sampled daily to quantify the Mn2+ levels by MP-AES.

At the end of each test, the anodes were gently dipped into deionized water and then in ethanol,

to clean electrolyte salts from the surface while preserving fragile MnO2 morphology features that

may be present. Afterwards, the anodes were allowed to dry at room temperature and were analyzed

by SEM/EDS. Then, they were chemically cleaned, rinsed, dried and re-analysed by SEM/EDS.

2.6. Electrowinning tests in triplicate at 400 mg/L Mn2+

Following the same parameters of section 2.5, additional electrowinning tests were performed

with anodes D, E and F at 400 mg/L Mn2+. This could be about the Mn2+ concentration expected in

the electrolyte feed of a hypothetical zinc cellhouse that had completed its anode technology

transition, assuming that the quantity of manganese ions oxidized in (or after) the zinc electrowinning

step equals the quantity of ions entering the electrolyte loop as ore impurity.

Samples of anodes D and E were tested, chemically cleaned, and tested again consecutively

three times, while a sample type F was tested and cleaned once. The samples were then rinsed, dried

and analyzed by SEM/EDS. This difference in the number of test replicates was meant to demonstrate

whether the MnO2-induced anode deterioration processes would progressively worsen as the washing

and cleaning cycles kept repeating.

Also, MnO2 particles were collected from the cells where the anodes D and E were tested.

These MnO2 particles were characterized by XRD to identify their crystalline phases. Before the

analysis, the particles were thoroughly rinsed with deionized water and were allowed to dry at room

temperature.

3. Results and Discussion

3.1. Characterization of the MMO anode types

Figure 1 presents the XRF results of the three types of MMO anodes. To begin, it is worth

mentioning that these elemental composition analyses can be only considered semi-quantitative

because the depth of X-ray detection did not cover the thickness of the titanium substrate. As such,

the XRF results serve the purpose of highlighting differences of chemical formulations across the

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layers of the MMO coatings. To enrich the elemental comparison, the analyses were replicated on

“film mode” and “default mode” – the former had an X-ray detection range relatively more superficial

than the latter one.

Based on these results, the Sn/Ir ratios differ in each depth level and anode type. Tantalum is

only present in the anodes E, and it is more abundant at a lower depth. Also, anodes D have relatively

more iridium, and they are richer in bismuth at the outer MMO layers. Meanwhile, the MMO coating

of anodes type F presents a higher concentration of tin.

Figure 1. Elemental composition of MMO anodes, determined by XRF, with two acquisition modes

(film and default modes).

The XRD results of anode types D, E and F are presented in Figure 2. The sharp peak at the 2θ

position of 40.3° and the double peaks near 80°, present in each diffractogram, are features of the

characteristic pattern of titanium metal, from the substrate.

Also, the broad peaks at the positions of 27.0°, 34.1° and 52.4°, for the three anode types,

match the pattern of the solid solution (Snx,Ti(1-x))O2. As the increase of bond lengths and lattice

parameters of this oxide is proportional to the increase of the substitution of Ti by Sn [20], the

estimation of x = 0.5 had optimal correspondence to the unit cell dimensions calculated from the

diffractogram. The identification of this rutile-like structure agrees with literature characterizations

of SnO2 and IrO2 in electrocatalytic coatings. However, the diffraction pattern of IrO2 crystallites

almost overlaps with that of SnO2 (or (Sn,Ti)O2). As such, either iridium, titanium and tin oxides

integrated a single solid solution, or IrO2 crystallites were dispersed with (Sn,Ti)O4 ones in the

coating, and both phases share similar crystal parameters. Based on the literature [21], the second

option is somewhat more likely, because the peaks positioning of this crystal structure are the same

for all three anode types, even though they seem to have different IrO2/SnO2 ratios according to the

XRF data. In addition, bismuth seems to be incorporated in this (these) crystal phase(s) as a dopant [14].

In particular, the X-ray diffractograms reveal certain differences in such rutile-like crystals of

each anode type. The peaks of this phase are broader for the anode E, suggesting that the MMO

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crystallites of this anode type may be smaller or have more irregular lattices. Also, comparing the

XRD data of types D and F, there is a difference in relative intensity between the peaks 27.1° and

34.0°, which represent the plans (110) and (101), respectively. This indicates a difference in the

preferable orientation of these crystallites on the surfaces of the anodes.

Furthermore, the sharp peaks at 23.1°, 28.7° and 37.0° on the diffractogram of the anode E

agree with the diffraction pattern of Ta2O5. Interestingly, this information contrasts with the literature,

where tantalum oxide of MMO coatings is usually observed in amorphous form.

Figure 2. X-ray diffractograms of anodes D, E and F (top), and patterns of the identified

crystallographic phases (bottom).

SEM images of the anodes D, E and F are presented in Figure 3. These images show that the

anodes have pronounced peaks and valleys at the scale of 100 μm, and typical mud-crack features

with sizes of (10-20) μm.

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Figure 3. SEM images of anode types D, E and F. Images obtained at 10 kV, with magnifications

of 400 x (top) and 2 kx (bottom).

3.2. Crystal phase characterization of MnO2 particles

The X-ray diffractograms of the MnO2 particles are presented in Figure 4. Both of them present

a diffraction pattern that corresponds to α-MnO2 – a phase that is often referred to as cryptomelane

or hollandite and can incorporate alkaline ions in its tunnelled structure. The intensity of the first two

peaks is lower here than in the official pattern of α-MnO2 (due to the preferential orientation of these

particles on the sample holder), similarly to what was observed in diffractograms of cryptomelane

nanorods [22]. Moreover, the diffractogram of the sample obtained with the anode type E also

displays the main diffraction peaks of manganite (MnOOH) in orthorhombic form.

Figure 4. X-ray diffractograms of the particulates #2 and #3, accompanied by the reference patterns of

α-MnO2 and MnOOH.

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This phase identification is consonant with the literature, since α-MnO2 formation has been

observed in electrowinning tests involving MMO anodes [23] or lead-based ones [24]. Also, MnOOH

species are commonly associated with MnO2 deposits and are supposedly involved in intermediary

reaction steps of MnO2 deposition [25].

3.3. The relationship between Mn2+ levels, MnO2 deposition and MMO anode potential increase

Figure 5 provides the anode potential values at the initial hours of the electrowinning tests with

step additions of Mn2+. The full anode potential results of these experiments are provided in Figure

6, along with the corresponding Mn2+ concentration profiles. Also, in Figure 7, the MnO2 formation

rates can be inferred from the quantities Mn2+ ions depleted (consumed) over time after each new

Mn2+ step addition.

Figure 5. Anode potential values of the 46-day galvanostatic tests, in the first hours of operation. The

vertical dashed line represents the moment of Mn2+ addition.

Figure 6. Anode potentials (top) and Mn2+ concentrations in the electrolyte (bottom) during

galvanostatic tests with step increases of Mn2+ (indicated by dashed lines).

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Figure 7. Cumulative Mn2+ mass depletion after each step addition of Mn2+ during galvanostatic tests,

for anodes D and E. Regression lines were added to display the data trends more clearly.

The graphs suggest that the addition of 15 mg/L Mn2+ at the time t = 1 h hardly caused any

modification in the anode potential trends. At this stage, the quantity of MnO2 deposits formed must

have been too little to interfere with the anode performance, since the consumption of Mn2+ ions for

MnO2 formation (Figure 7) was very low: about 0.5 mg Mn2+ per day for either anode type. Then, a

faster increase of anode potentials occurred when the Mn2+ concentrations jumped from ~15 mg/L to

100 mg/L. But after a few days at ~100 mg/L Mn2+, the slopes of anode potentials became roughly

steady; they no longer seemed to be affected by further additions of Mn2+ ions.

Two hypotheses were considered to explain why the anode potentials increased faster when

the Mn2+ concentrations changed from 15 mg/L to 100 mg/L. The first hypothesis was that the MnO2

formation rate has slowed down at that stage. This was based on the information that the current

efficiency of MnO2 deposition has can decrease over time [26]. However, in Figure 7, the trends of

Mn2+ depletion after the electrolyte reached 100 mg/L, 150 mg/L and 230 mg/L Mn2+ were

statistically similar for the anode D. Plus, for anode E, the highest Mn2+ depletion rate was recorded

after the electrolyte reached 230 mg/L Mn2+. Therefore, these results did not indicate a decrease in

the current efficiencies of MnO2 formation.

The other hypothesis (most reasonable) was that not all MnO2 formed would stay necessarily

attached to the anodes to contribute to the potential increase. In fact, MnO2 deposits start to detach

over time. This was evidenced by the visual aspect of the electrolytes: once transparent at the level of

~15 mg/L Mn2+, they turned into vivid pink after reaching ~100 mg/L Mn2+ and then started to display

a small number of fine particles in suspension in the final days of the experiment, at ~230 mg/L Mn2+.

These suspended solids were very likely MnO2 particles that have detached from the anode surface

or were formed in the bulk solution.

Thus, after a certain Mn2+ threshold or after a certain degree of MnO2 deposition, the anode

rate of potential increase may no longer closely correspond to the rate of MnO2 formation because

part of the particles being formed is detaching from the surfaces of the anodes. Moreover, the

dissonance between the rates of MnO2 formation and anode potential increase may be related to

changes in the properties of the MnO2 deposits over time.

Cell and anode potentials of the electrowinning tests involving single-step Mn2+ dosing are

presented in Figure 8. Also, in Figure 9, the averages of cell potential values of these tests are provided

as a function of the Mn2+ concentration used. These results indicate two types of information. The

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first one was that the different anode types demonstrated different ranges of potential values. At a

given Mn2+ concentration, the anodes type D generally performed at the lowest potentials, and the

anodes E, the highest. This difference in performance was also noticeable in Figure 6, but the extra

data set of Figure 8 made it stand out more. Anode samples type F had an intermediary potential range.

Figure 8. Cell (left) and anode (right) potentials of zinc electrowinning tests performed for 72 h at

pseudo-stationary Mn2+ levels between 5 mg/L and 150 mg/L.

The second type of information evidenced in Figure 9 is the existence of proportionality

between the average cell potential values and Mn2+ levels, for each anode type. All three regression

lines have a coefficient of approximately 0.0002, meaning that the cell potential would be raised by

2 mV at each Mn2+ concentration increase of 10 mg/L. However, this type of linear regression would

be only adequate if the development of MnO2 deposits was proportional to the Mn2+ concentration in

the range of (5-150) mg/L Mn2+ – which turned out to not be true, according to SEM and EDS results

(presented in Section 4).

Figure 9. Average cell potentials of electrowinning tests of Part 2, versus the approximate Mn2+

levels of these respective cells.

Figure 10 and Figure 11 present the anode potentials, cell potentials and Mn2+ concentrations

from the electrowinning tests performed at ~400 mg/L Mn2+. The Mn2+ depletion rates and the potential

values measured here were significantly higher than those of previous tests. Combining the Mn2+

concentration values of the three replicates, the Mn2+ depletion was approximated to a linear profile

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and a regression line was applied, providing a slope of 33.5 mg/d, with a coefficient of determination

R2 equals 0.45.

Figure 10. At the top, anode potentials of the beginning (left) and totality (right) of the electrowinning

tests at ~400 mg/L Mn2+, with anodes D and E. At the bottom, cell potentials of the same tests. The

dashed lines indicate the moments of Mn2+ dosing.

Figure 11. Mn2+ concentrations over time, measured from electrowinning tests with step Mn 2+

addition to reach ~400 mg/L at the test start.

The potential profiles of each anode type were relatively stable after the first hours, even though

the MMO coatings may continue deteriorating over time due to intense MnO2 deposition (which was

revealed by microscopic results and is presented in the next section). This apparent dissonance is due

to the low sensitivity of voltage measurements; small increments of anode deterioration were

camouflaged by voltage noise or variability from external influences.

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3.4. Anode cleaning results

The chemical cleaning was found to be very effective at removing MnO2 deposits, with no

collateral damage to any of the three MMO coating types. According to EDS results, all the samples

subjected to the cleaning method became completely free of manganese. Plus, SEM images of the

control anode samples (tested in galvanostatic tests at 0 mg/L Mn2+ and chemically cleaned) show that

their MMO coatings continued looking brand new (Figure 12).

Figure 12. SEM images of anodes samples D, E and F after zinc electrowinning tests at 0 mg/L Mn2+

and chemical cleaning. Magnifications of 400 x (top) and 2 kx (bottom).

4. MnO2-induced anode deterioration mechanisms

4.1. Microscopy results of anode type D

According to the microscopy results, MnO2 deposition occurred over anodes type D after each

zinc electrowinning test performed in the presence of Mn2+ ions. At the lowest Mn2+ concentration

tested, 5 mg/L, the whole anode surface was covered by a thin MnO2 film. SEM images of Figure 13

provide a comparison between the anode surface before and after the chemical cleaning. The MnO2

film was so thin that it would only slightly diffuse the electrons emitted from the anode surface

underneath, making mud-crack features appear with softer, blurrier edges.

SEM images at higher magnification (Figure 14) show that the MnO2 film had open-ended

cracks with ramifications, evenly distributed throughout the surface. If the original MnO2 structure

had significant amounts of hydrates, these cracks may be the result of shrinkage during the drying

process. The film thickness seemed to be no larger than 0.5 μm. Faint, irregular pores were shown,

with diameters that range up to 0.2 μm. Overall, both the coating and the film looked intact; there was

neither displacement nor absence of fragments.

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Figure 13. SEM images of the anode sample type D after the zinc electrowinning test at 5 mg/L Mn2+,

before and after chemical cleaning. Magnifications of 400x to 5 kx.

Figure 14. SEM images of the sample type D after the electrowinning test at 5 mg/L Mn2+, before and

after chemical cleaning. From left to right, magnifications of 22 kx, 17.7 kx, 20 kx and 22 kx. The pores

of the MnO2 film are more visible in the first image.

Figure 15 and Figure 16 show MnO2 deposits formed over zinc electrolytes containing 10 mg/L

Mn2+. The MnO2 film was still porous, uniform and thin enough to express the underlying anode

surface morphology, but it no longer had that translucid aspect of the MnO2 film formed at 5 mg/L.

Based on Figure 17, the film thickness was estimated to be around 1 μm.

This MnO2 formation shares two main aspects in common with the subsequent ones, obtained

from electrolytes containing 25 mg/L and 50 mg/L Mn2+. First, these deposits were completely

fragmented. This could be due to shrinkage or stresses related to crystal growth and/or movement of

oxygen bubbles, since these deposits less porous and more compact than the previous ones. The

second aspect was that detached MnO2 pieces that were flipped over the surface often displayed the

presence of smaller fragments of another material.

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Figure 15. SEM images of the anode type D after the zinc electrowinning test at 10 mg/L Mn2+. Images

with magnification of 2 kx, in SE and BSE modes.

Figure 16. Comparison of the anode sample type D after the zinc electrowinning test at 10 mg/L Mn2+,

before and after chemical cleaning. Magnifications of 400 x to 2 kx.

Figure 17. SEM image of anode type D after the electrowinning test at 10 mg/L Mn2+. Magnification of

26 kx.

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The elemental composition of these smaller fragments could not be determined by the EDS

instrument employed. However, the colour contrast of SEM images in BSE mode suggests that their

elemental composition is equivalent to the one of the MMO coating. Examples of MnO2 deposit

patches with these fragments are provided in the SEM images of Figure 18, Figure 19 and Figure 20.

After observing these fragments in multiple samples type D with MnO2 deposition, it was possible to

notice that their morphology reminds that of mud-crack regions in anodes D (as, for example, in

Figure 20). It was then hypothesized that the fragments originally belonged to the coating and they

ended up adhered to MnO2 deposits during their growth. In the continuation of this work, this

hypothesis could be verified by performing elemental mappings with wavelength-dispersive

spectroscopy (WDS), as this technique has superior element detection capability than EDS.

Figure 18. SEM images of anode type D after the zinc electrowinning test at 10 mg/L Mn2+. Loose

MnO2 particle with unidentified fragments incorporated on it.

Figure 19. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+. Zoomed

out image with magnification of 400 x and close-ups with 2 kx.

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Figure 20. SEM images of anode D after the second zinc electrowinning test at 50 mg/L Mn2+.

Magnifications of 400 x and 2 kx. Mud crack texture imprinted on MnO2 is highlighted in the last

image.

One evidence that supported this hypothesis was that a higher degree of coating deterioration

was observed in the samples with a higher frequency of “foreign” fragments adhered to MnO2 pieces.

For instance, the sample type D with the most advanced degree of deterioration was the one tested in

duplicate at 50 mg/L Mn2+. SEM images of this sample, after chemical cleaning, are presented in

Figure 21 and Figure 22. For comparison, Figure 23 presents the sample tested at 25 mg/L Mn2+, also

after chemical cleaning.

Figure 21. SEM images of anode type D after the second test replicate at 50 mg/L Mn2+ and chemical

cleaning. Magnifications of 400 x and 2 kx.

Figure 22. SEM images of anode type D after the first and second test replicates at 50 mg/L Mn2+, and

after chemical cleaning. Magnification of 400 x.

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Figure 23. SEM images of anode type D after the zinc electrowinning test at 25 mg/L Mn2+ and

chemical cleaning. Images with magnification from 400 x and 2 kx.

Overall, at the range of (5-75) mg/L Mn2+, the thickness of the MnO2 deposits over the anodes

D was observed to increase somewhat proportionally to the Mn2+ levels. However, even though all

samples tested at 10 mg/L Mn2+ or more lost a few coating fragments, the sample tested twice at 50

mg/L Mn2+ had suffered more deterioration than the one tested once at 75 mg/L Mn2+. These

highlights indicate that a short-lasting overshoot of Mn2+ levels in zinc cellhouses would not pose a

threat to the anode integrity immediately. But in long-term operation at high Mn2+ levels, MnO2

deposits would likely grow and detach multiple times from the MMO anodes, causing successive

coating fragmentation which would shorten the anode service life.

4.2. Microscopy results of anodes type E

The anodes type E started to display the first signs of MnO2 deposition after electrowinning

tests with electrolytes containing 100 mg/L Mn2+, but it could only be effectively confirmed after tests

at 125 mg/L Mn2+. SEM images obtained from samples tested at 50 g/L, 75 mg/L and 100 mg/L Mn2+

(Figure 24, Figure 25 and Figure 26, respectively) show that the anode surfaces looked brand new, in

general terms. Small cracks were occasionally found on smooth regions of the anodes tested at 75

mg/L and 100 mg/L Mn2+, but mud-crack regions were overall intact. There was no fragmentation of

the MMO layers.

Figure 24. SEM images of anode type E after the zinc electrowinning test at 50 mg/L Mn2+.

Magnifications of 400 x and 1 kx.

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Magnifications of 400 x and 2 kx. Note: the colour heterogeneities in the third image are charging

artifacts.



However, SEM images of a sample type E at 125 mg/L Mn2+ show the presence of multiple

clusters of MnO2 crystallites. Through the colour contrast of the SEM images in BSE mode (Figure

27), it becomes more evident that these MnO2 clusters have formed exclusively over smooth areas of

the anode, away from mud cracks. At higher magnifications (Figure 28 and Figure 29), it is possible

to observe that the individual MnO2 crystallites had the form of needles, with diameters smaller than

0.2 μm.

Moreover, the high magnification images show that the MnO2 deposits concentrate around

ruptures in the outer MMO layer. These ruptures seem to have been induced by the volume expansion

of growing MnO2 crystallites. This is supported by the fact that the ruptures were more frequently

found over this sample than on its counterparts tested at 50 mg/L to 100 mg/L Mn2+ (as exemplified by

SEM images of Figure 30). Then, this proposition led to the speculation that rare ruptures seen on the

samples tested at 75 mg/L and 100 mg/L Mn2+ were perhaps linked to MnO2 deposits that were not

yet developed enough to be detectable by EDS.

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Magnifications of 1 kx (first pair) and 400 x (second and third ones).


Magnifications of 2 kx and 14 kx.



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Figure 30. SEM images of anode type E after the testing at 125 mg/L Mn2+ and chemical cleaning.

Examples of ruptures observed on the smooth regions of the coating.

The sample type E tested in an electrolyte containing 150 mg/L Mn2+ presented a much higher

accumulation of MnO2 deposits. The MnO2 clusters deposits were visible to the naked eye, and they

had blurry edges towards the direction of the ascension of oxygen bubbles. This characteristic was

also visible at the microscopic scale, especially with the colour contrast provided by SEM images in

BSE mode, in Figure 31. For reference, the direction of the yellow arrows inserted corresponds to the

upward position of these anodes in electrowinning cells. The images show that the MnO2 deposition

still occurred preferentially over the smooth regions of the anode surface, but the deposits formed

were fragile so the flow promoted by the oxygen evolution caused an erosion process on them.


Magnification of 100x (left) and 400x (right).

SEM images at higher magnifications (presented in Figure 32 and Figure 33) reveal that the

MnO2 deposits consisted of prismatic or bladed crystallites, about 0.4 μm long and no more than

0.1 μm wide. The crystallites were mostly found on large agglomerates that contoured coating

ruptures, but were also dispersed across the anode surface in the form of loose individuals or

agglomerates with radial symmetry.

Comparing these MnO2 deposits with those of the test at 125 mg/L Mn2+, no significant

morphological changes are seen. But the appearance of agglomerates with radial growth could

suggest a surge in secondary nucleation of MnO2. In other words, detached crystallites (or their

fragments) may have served as a substrate for the growth of more MnO2 material, independently of

the MMO surface. This agrees with the fact that the SEM images in BSE mode do not show any

differences in contrast between the cores of these MnO2 spheroids and the crystal tips growing

outwards; otherwise, hypothetical cores composed of iridium or tin oxides would have higher electron

backscattering intensity. In any case, the possible advent of secondary nucleation of MnO2 could be

verified in definitive through the use of WDS, for example.

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Figure 32. SEM images of anode type E after the test at 150 mg/L Mn2+. Spherical MnO2 agglomerates

and loose, individual crystallites. Magnifications of 5 kx and 20 kx.

Figure 33. SEM images of anode type E after the test at 150 mg/L Mn2+. MnO2 deposits around coating

cracks and star-shaped agglomerates. Magnifications of 5 kx and 20 kx.

The SEM images of this sample after the chemical cleaning (Figure 34) show multiple

occurrences of detachment between inner and outer layers of the coating. The ruptures seem to

advance faster between MMO layers than on the surface. The top layer patches must eventually break

off as their structural integrity becomes compromised. Even though this deterioration process induced

by MnO2 is distinct to that observed on anodes type D, it also occurs at a relatively short time interval

relative to the expected anode service life.

Figure 34. SEM images of anode type E after the electrowinning test at 150 mg/L Mn2+ and chemical


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Overall, based on these results, it cannot be said that anodes type E would be free of MnO2

deposition if operated in electrolytes with less than 100 mg/L Mn2+. Even though, the MnO2 formation

may not be detectable yet after 72-hour tests, but it could still impact the anode performance in the

long term.

4.3 Microscopy results of anodes type F

SEM images of a sample type F applied to zinc electrowinning at 5 mg/L Mn2+ are presented

in Figure 35. No signs of MnO2 deposition were detected in this sample during the microscopic

characterization. The anode had its original characteristics preserved.

Figure 35. SEM images of anode type F after the electrowinning test at 5 mg/L Mn2+. Magnifications of

400 x and 2 kx.

The MnO2 deposition on anodes F was first confirmed after electrowinning tests at 25 mg/L

Mn2+, but it may have initiated in tests at 10 mg/L Mn2+. The reason is that SEM images obtained

after tests at 10 mg/L Mn2+ (Figure 36) show the appearance of a few small protrusions on smooth

peaks of the coating, with dark contrast in BSE mode, which turned out to be similar to MnO2 deposits

observed on the sample tested at 25 mg/L Mn2+ (Figure 37).

In fact, the MnO2 deposition at 10 mg/L Mn2+ could not be confirmed because no manganese

was detected by EDS and because the protrusions seemed like a natural continuation of the MMO

coating itself, with no distinguishable boundaries. If early-stage MnO2 crystallites were growing

inside the pores of the coating, they would be hardly detectable through the equipment employed in

this characterization.

Figure 36. SEM images of anode type F after the electrowinning test at 10 mg/L Mn 2+.

Magnifications of 1.5 kx and 3 kx.

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Figure 37. SEM images of anode type F after the electrowinning test at 25 mg/L Mn2+. Magnifications

of 1 kx and 200 x.

However, the sample used in the electrowinning test at 25 mg/L Mn2+ presented drusy MnO2

formations that were already distinguishable by SEM imaging at a magnification of 1 kx. In Figure

38 and Figure 39, close-up SEM images help to visualize that individual MnO2 crystallites in such

agglomerates displayed the same prismatic morphology observed on the anode type E tested at 150

mg/L Mn2+. They also concentrated around fractures of smooth regions of the coating. A minor

difference is that these crystallites are shorter, with lengths around 0.2 μm.

Figure 38. SEM images of anode type F after the electrowinning test at 25 mg/L Mn2+.

Magnifications of 12 kx, 10 kx and 12 kx, from left to right.

Figure 39. SEM images of anode type F after the electrowinning test at 25 mg/L Mn 2+.


Next, the anode F tested at a Mn2+ concentration of 100 mg/L presented more MnO2 deposits

over smooth regions of the anode surface (Figure 40). These MnO2 crystallites kept the same

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morphology and association to coating fractures previously observed after the test at 25 mg/L Mn2+

(Figure 41). Fractures developed over this sample became more evident in SEM images taken after

the chemical cleaning (Figure 42).

Figure 40. SEM images of a sample F after the test at 100 mg/L Mn2+. Magnification: 400 x.

Figure 41. SEM images of anode type F after the electrowinning test at 100 mg/L Mn2+.

Magnifications of 1 kx (left) and 12 kx (right).

Figure 42. SEM images of anode type F after the electrowinning test at 100 mg/L Mn2+ and chemical


Finally, a sample type F tested at 125 mg/L Mn2+ (Figure 43 and Figure 44) had MnO2 deposits

with spatial distribution and crystallite morphology similar to those of the samples tested at 25

mg/L and 100 mg/L Mn2+. Short, prismatic crystallites were found in large agglomerates and also

contouring fractures of the coating, which have been opened in regions that were previously smooth.

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Figure 43. SEM images of anode type F after the electrowinning test at 125 mg/L Mn2+. Magnification of

400 x.

Figure 44. SEM images of a sample F after the test at 125 mg/L Mn2+. Magnification: 5 kx.

One interesting observation in this sample is the coexistence of two MnO2 deposit

morphologies right next to each other, as exemplified through the SEM images of Figure 45 In the

third image of the sequence, it is possible to observe acicular formations on the left side, and radiating

prismatic crystals on the right. The MnO2 deposit morphology was expected to be uniform if the

conditions of the reaction media were homogeneous. Possible explanations for this divergence are:

• MnO2 depositions initiated in different layers of the MMO coating could develop different

morphologies. Heterogeneities of chemical composition across different layers or regions of

the coating could influence the MnO2 growth.

• Each MnO2 deposit morphology may have formed separately during different stages of the

electrowinning test. For instance, variations on the current density, electrolyte acidity or Mn2+

concentration throughout test may have affected the MnO2 morphology.

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Figure 45. SEM images of anode type F after the electrowinning test at 125 mg/L Mn2+. Magnifications

of 1 kx, 5 kx and 25 kx.

SEM images of this sample, obtained after the chemical cleaning, also demonstrated ruptures

and cracks similar to those observed found on the sample tested at 100 mg/L Mn2+ (Figure 46).

Missing coating fragments and exposure of the titanium substrate were eventually observed as well.

Figure 46. SEM images of anode type F after the test at 125 mg/L Mn2+ and chemical cleaning.


4.4. Microscopy results with tests at 400 mg/L Mn2+

Significant damage was observed on all three samples applied to zinc electrowinning tests at

400 mg/L Mn2+. SEM images of the anodes D and E after the tests in triplicate (Figure 47) show that

outer layers of the MMO coatings have detached from multiple points. Also, the mud-cracks display

signs of intense wear. The titanium substrate has even been exposed in some regions, as illustrated

by an elemental map obtained by EDS (Figure 48).

In fact, even the sample type F has suffered significant surface degradation after being tested at 400

mg/L Mn2+ in only one replicate. According to SEM images (Figure 49), the outer MMO layer has

been fractured and a few of its patches seemed to be weakly bound to the anode. The titanium

substrate was also visible in several regions of this sample.

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Figure 47. SEM images of samples type D and E. Magnifications of 400 x and 2 kx.

Figure 48. On the left, spatial distribution of Ti signal over a region of the anode D, obtained by EDS.

On the right, an SEM image of the same region.

Figure 49. SEM images of the anode sample F2.

The high adhesion strength between MnO2 deposits and MMO coatings may explain the

damage observed on these anode surfaces. For instance, SEM images of anode F prior to chemical

cleaning (Figure 50) reveal a compact but fragmented MnO2 coating over the anode. The image at

125

higher magnification (2 kx) shows the presence of nodular agglomerates of needle-like MnO2

crystallites, tightly aggregated. At lower magnification (250 x), these deposits present multiple

cracks, indicating that they may have undergone some level of stress during the zinc electrowinning

tests. The stress forces were probably caused by the growth of individual MnO2 crystals and/or the

pressure imposed by oxygen gas bubbles evolving and accumulating underneath. In this context, it is

important to note that MMO coatings manufactured via a thermal composition can be composed of

multiple layers of different crystal characteristics. This means the interfaces between the layers and

between the titanium substrate may be relatively more vulnerable to rupture. Therefore, if the

adhesion strength of the MMO coating surface to the MnO2 is superior or comparable to the adhesion

strength of inner coating layers, the stress forces exerted over the MnO2 could induce coating fractures

and bonding failures in the MMO layer interfaces. Overall, this shows that operating the MMO anodes

at 400 g/L Mn2+ for just a few hours was sufficient to compromise the coatings permanently.

Figure 50. Mn2+ deposits over the anode F2 before the cleaning. Magnifications of 250 x (left) and 2kx

(right).

4.5. Establishing Mn2+ tolerance levels for each anode type

“Mn2+ tolerance” could be interpreted as the Mn2+ concentration threshold that marks a sudden

acceleration of both the MnO2 formation and the coating deterioration of MMO anodes. Overall, using

the results of 72-hour electrowinning tests as reference, the respective Mn2+ tolerance levels of anodes

D, E and F are considered to be as (5-10) mg/L, (100-125) mg/L and (5-25) mg/L Mn2+.

The anodes type D did not have any signs of deterioration due to the development of a thin

MnO2 film during the electrowinning test at 5 mg/L Mn2+. However, the detachment of small MMO

coating fragments started to be observed after the test at 10 mg/L Mn2+. In the case of the anodes type

E, no MnO2 deposition was detected at up to 100 mg/L Mn2+, but well-developed MnO2 deposits

appear at 125 mg/L Mn2+, accompanied by a few ruptures of the coating. The anodes type F were

likely subjected to MnO2 deposition in tests involving Mn2+ levels of 10 mg/L and up, but negative

effects on the coating were observed starting at 25 mg/L Mn2+.

4.6. Criteria for ranking the suitability of different anode types to industrial zinc production

As different types of MMO anodes have demonstrated different anodic potentials and different

Mn2+ tolerance levels in zinc electrowinning tests, a question remains: which type would be the most

suitable for implementation in zinc cellhouses? The answer heavily depends on financial aspects.

126

Apart from trivial differences of purchase, re-coating and replacement costs that integrate capital

expenditure estimates, one should also consider the comparison of operational costs related to the

manganese control strategy to be adopted. Regarding the hypothetical ranking of anode types D, E

and F in terms of suitability to zinc cellhouses, different scenarios for comparative cost analyses of

manganese control are presented below.

Scenario 1: Fixed anode cleaning periodicity of 72 h, no MnO2 deposition inhibition

Presupposing the implementation of periodical chemical cleaning of the MMO anodes, with

cycles of 72 h, the expected accumulations of MnO2 deposits on the anodes D, E and F should be

equivalent to those of the 72-hour electrowinning tests. As such, if other impurities of industrial zinc

electrolytes do not interfere with the MnO2 deposition, the Mn2+ tolerance levels determined in

Section 4.5. would apply. Then, the anodes type D are the most advantageous in terms of energy

savings, because they provided the lowest cell potentials (~2.56 V) even under a certain accumulation

of MnO2 deposits. On the other hand, anodes type D have the lowest Mn2+ tolerance (~5 mg/L) and

so they would require the highest degree of electrolyte purification. Meanwhile, anodes E showed an

inverse profile; they would promote the lowest energy savings (with cell potentials of ~2.67 V) and

require the least amount of electrolyte purification (tolerance of ~100 mg/L Mn2+). The anodes Type

F had intermediary results in both aspects (tolerance of ~25 mg/L and cells at ~2.60 V).

Therefore, anodes D would be the most suitable in a scenario where its energy economy

compensates for the costs of intensive electrolyte purification. For instance, a scenario where there

are elevated electricity costs, energy supply restrictions, or the advent of low-cost electrolyte

purification technologies for Mn2+ removal. Contrarily, the type E would be the most recommended

in cases where it may be unfeasible to obtain zinc electrolytes with Mn2+ content as low as 5 mg/L

Mn2+, or where the energy economy it provides is a sufficient payoff for its implementation.

Scenario 2: Optional anode cleaning or variable periodicity, solutions for MnO2 inhibition

available

From an industrial point of view, cleaning the MMO anodes every 72 h may be feasible but

certainly not ideal. Decreasing the cleaning periodicity or even disregarding it altogether could perhaps

be achieved by finding ways to inhibit the MnO2 accumulation on anodes surfaces in the long term.

In this scenario, the Mn2+ tolerance levels of Section 4.5 no longer apply to the anodes D, E

and F. Since the MnO2 deposition morphologies can evolve over weeks or months, the coating

deterioration results of three-day electrowinning tests would not necessarily help to estimate anode

performance losses after months or years of uninterrupted service.

Then, the optimal Mn2+ level for each anode type could be alternatively defined as the Mn2+

concentration that minimizes the combined costs of all activities that compose the manganese control

strategy of interest. Hypothetical cost curves of these activities are provided in Figure 51, to illustrate

this approach. For instance, operating with electrolytes containing high Mn2+ concentrations would lead

to higher MnO2 formation rates, so that the anodes would require more frequent cleaning or perhaps

higher doses of additives to inhibit the MnO2 growth. On the other hand, electrolytes with lower Mn2+

levels would require a higher degree of purification. In this case, the Mn2+ removal system would be

more costly to operate, requiring higher amounts of reactants or perhaps longer residence times.

127

Figure 51. Hypothetical cost curves of activities relative to manganese control in a zinc production

plant, as a function of Mn2+ concentration.

5. Conclusion

The adoption of manganese control strategies is expected to be fundamental for the

achievement of satisfactory long-term performance of MMO anodes in zinc cellhouses, to supersede

conventional lead-based anodes. To contribute to this anode transition prospect, the present study has

investigated the effects of MnO2 deposition on three types of IrO2-bearing MMO anodes.

Through short-term galvanostatic tests, with single and multiple Mn2+ dosing, it was observed

that increasing the Mn2+ concentration led to higher MnO2 formation rates and consequently higher

potential increase. Moreover, evidence was found that this anode potential increase can be (at least

partially) attributed to the coating deterioration processes induced by MnO2 deposits.

The MnO2 deposits developed different morphologies depending on the Mn2+ concentration

and the anode type. In fact, each type of MnO2 deposit morphology was associated with a different

coating deterioration process. According to SEM and EDS results, MnO2 films formed over the

anodes type D even at Mn2+ levels of 5 mg/L Mn2+. These films were prone to fragmentation after

reaching a critical thickness, and so MnO2 pieces would detach, “capturing” MMO coating fragments

with them. Meanwhile, anodes type E and F were subject to MnO2 deposits that consisted of clusters

of elongated crystallites. Evidence was found that the growth of such MnO2 formations induced

ruptures throughout MMO coating layers, in regions that did not present mud cracks. These ruptures

would weaken the structural integrity of the MMO coating locally, eventually leading to the

detachment of MMO fragments.

Based on these results, the tolerance levels of the anode types D, E and F were identified as

(5-10) mg/L, (100-125) mg/L and (5-25) mg/L Mn2+, respectively. Such tolerance levels apply in

situations where the anodes would operate industrially with cycles of service and chemical cleaning

of 72 h. In hypothetical scenarios where the manganese control strategies involve less frequent anode

cleaning or include the use of additives, for example, the optimal Mn2+ levels should be determined

considering also the operational costs of electrolyte purification, MnO2 removal and consumption of

additives.

It is also worth noting that the success of the anode cleaning method employed was fundamental

for studying the effects of MnO2 deposits on MMO anodes at a microscopic level. This chemical

cleaning involving the use of a FeSO4 solution potential to be applied on an industrial scale, since it

is not energy-intensive and the constituents of the cleaning solution are available in the RLE process.

128

Following this work, further investigations could be carried to investigate how the properties

of these MMO anode types induce the development of different MnO2 deposit morphologies. A more

extensive characterization of the MMO coatings would be required to understand which key

compositional and structural features make MMO coatings more vulnerable to damages induced by

MnO2 deposition.

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