Interactions of Clay Minerals and Their Effects on Copper-Gold Flotation Nestor Cruz Beng, Msc in Chemical Engineering

Interactions of Clay Minerals and their Effects on Copper-Gold Flotation Nestor Cruz BEng, MSc in Chemical Engineering

A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2015

Sustainable Minerals Institute Julius Kruttschnitt Mineral Research Centre

Abstract

The continuous depletion of mineral resources has resulted in the increased processing of low grade complex ores that in some cases contain clay minerals. These gangue minerals are often associated with ores containing copper and gold and can cause problems in mineral flotation. Some of the observations from industry when high clay ores are processed include the high flotation pulp viscosity, over-stable froth or absent froth, and lower grade and recovery.

Clay minerals are small flat particles that occur in colloidal size ranges with charges on their surfaces that may change with pH. These characteristics allow them to interact with water, reagents and other gangue minerals present in flotation pulp. Given these properties and interactions it is expected that clay minerals can cause high slurry viscosities and mechanical entrainment contributing to the decline of flotation kinetics and/or dilution of flotation concentrates. Some solutions to high viscosity and entrainment are the dilution of flotation pulp, desliming, and the blending of easy to process ores and problematic ores. However, the dilution of flotation pulp increases water consumption and reduces plant throughput, desliming removes fine valuable minerals as well, and blending ores is also impractical since at some point it is necessary to process the high clay content ores alone. The use of dispersants or depressants to reduce viscosity and entrainment could be an effective way to improve the flotation of this type of ores, but to make the best use of this alternative it is essential to understand at a fundamental level some of the interactions of clay minerals in flotation. This is the goal of this project, and the focus is on copper- gold flotation.

It is proposed that the high pulp viscosity and mechanical entrainment caused by clay minerals are associated with the type of network structures formed by these minerals. The main tools used in this thesis to demonstrate the effect of clay particle associations on the flotation of a copper-gold ore were rheology measurements, flotation tests and observation of clay aggregates in the flotation cell by using Cryo-SEM imaging. A non-swelling clay and a swelling clay, kaolinite and bentonite, were the two main clay minerals studied, and results confirmed that the detrimental effect on flotation was dependent on the type clay particle associations. The interaction of kaolinite and bentonite with some flotation reagents and gangue minerals was shown to modify these associations, and therefore, the flotation outcome. This is due to the structural differences between these two clay minerals.

Bentonite affected recovery through the formation of the house of cards network structure. Froth was almost absent when floating an ore containing this clay mineral. On the other hand, kaolinite caused a stable and abundant froth as supported by the mass-water recovery data from batch i flotation tests. Kaolinite particles did not form the same house of cards network structure and associated more in separated clusters which were easily entrained. The flotation outcome in the presence of these two clay minerals was totally different despite only minor differences in apparent viscosities or yield stress values in some cases. This finding supported the initial assumption that clay particle associations are the key to understanding flotation behaviour, and contributed to the conclusion that viscosity or yield stress values alone are not enough to predict flotation performance. A flotation cell has a wide distribution of shear rate values that can be very high close to the impeller, and very low or almost zero in the quiescent zone if dealing with high apparent viscosities.

Among the flotation reagents tested, the pH modifiers, lime and soda ash, had the greatest impact on the rheology of pure clay minerals, however, this effect was attenuated in the slurries containing mixtures of a copper-gold ore and clay minerals. It was also found that the gangue mineral, gypsum, had a strong interaction with bentonite.

The changes in apparent viscosities with the addition of lime and soda ash was caused by the 2+ + 2- increase of pH and the consequent addition of Ca from lime and Na and CO3 from soda ash. Results showed that these two pH modifiers did not change the type of particle associations in the slurries containing kaolinite or bentonite, but aggregates were stronger when lime was added with the exception of some high concentrations of bentonite in the slurry where soda ash caused a greater increase in viscosity than lime. Lime addition in flotation produced higher entrainment than soda ash specially when kaolinite was present, and this was the main difference in flotation between these two pH modifiers.

The effect of gypsum on rheology and flotation was more noticeable than the effect of pH modifiers and this is because gypsum released sufficient quantities of Ca2+ to modify double layers of the clay minerals and inhibit the swelling of bentonite, which was not the case when lime or soda ash was added to adjust pH. Cryo-SEM images showed that the presence of bentonite network structures affected true flotation, and in the absence of these structures entrainment was the primary factor affecting flotation. This was observed when kaolinite was present in the ore and when gypsum was added to the ore-bentonite mixture. Gypsum caused the bentonite to behave more like kaolinite in flotation.

Declaration by author

This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis.

I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award.

I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School.

I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis.

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Publications during candidature

Peer reviewed journals publications relevant to the thesis:

Cruz, N., Peng, Y., Farrokhpay, S. and Bradshaw, D. (2013). "Interactions of clay minerals in copper–gold flotation: Part 1 – Rheological properties of clay mineral suspensions in the presence of flotation reagents." Minerals Engineering 50-51: 30-37.

Cruz, N., Peng, Y. and Wightman, E. (2015). "Interactions of clay minerals in copper–gold flotation: Part 2 — Influence of some calcium bearing gangue minerals on the rheological behaviour." International Journal of Mineral Processing 141: 51-60.

Cruz, N., Peng, Y., Wightman, E. and Xu, N. (2015). "The interaction of clay minerals with gypsum and its effects on copper–gold flotation." Minerals Engineering 77: 121-130.

Cruz, N., Peng, Y., Wightman, E. and Xu, N. (2015). "The interaction of pH modifiers with kaolinite in copper–gold flotation." Minerals Engineering 84: 27-33.

Publications included in this thesis

Contributor Statement of contribution

Nestor Cruz (Candidate) Conception and design (60%)

Analysis and interpretation of data (70%)

Wrote the paper (70%)

Dr. Yongjun Peng Conception and design (30%)

Analysis and interpretation of data (20%)

Wrote the paper (30%)

Dr. Saeed Farrokhpay Conception and design (5%)

Analysis and interpretation of data (5%)

Prof. Dee Bradshaw Conception and design (5%)

Analysis and interpretation of data (5%)

Contributor Statement of contribution

Nestor Cruz (Candidate) Conception and design (80%)

Analysis and interpretation of data (70%)

Wrote the paper (70%)

Dr. Yongjun Peng Conception and design (20%)

Analysis and interpretation of data (20%)

Wrote the paper (30%)

Dr. Elaine Wightman Analysis and interpretation of data (10%)

Editing and revision of paper

Cruz, N., Peng, Y., Wightman, E. and Xu, N. (2015). "The interaction of clay minerals with gypsum and its effects on copper–gold flotation." Minerals Engineering 77: 121-130.

Contributor Statement of contribution

Nestor Cruz (Candidate) Conception and design (70%)

Analysis and interpretation of data (70%)

Wrote the paper (70%)

Dr. Yongjun Peng Conception and design (25%)

Analysis and interpretation of data (20%)

Wrote the paper (25%)

Dr. Ning Xu Conception and design (5%)

Analysis and interpretation of data (5%)

Wrote the paper (5%)

Dr. Elaine Wightman Analysis and interpretation of data (5%)

Editing and revision of paper

Cruz, N., Peng, Y., Wightman, E. and Xu, N. (2015). "The interaction of pH modifiers with kaolinite in copper–gold flotation." Minerals Engineering 84: 27-33.

Contributor Statement of contribution

Nestor Cruz (Candidate) Conception and design (70%)

Analysis and interpretation of data (70%)

Wrote the paper (70%)

Dr. Yongjun Peng Conception and design (25%)

Analysis and interpretation of data (20%)

Wrote the paper (25%)

Dr. Ning Xu Conception and design (5%)

Analysis and interpretation of data (5%)

Wrote the paper (5%)

Dr. Elaine Wightman Analysis and interpretation of data (5%)

Editing and revision of paper

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Contribution by others to the thesis

Dr. Yongjun Peng was responsible for setting up this thesis project as well as for seeking and organizing funding. He also made critical contributions to the interpretation of experimental data. Prof. Dee Bradshaw participated in establishing the initial goals and boundaries of the project which consisted in the understanding of the interactions of clay minerals in flotation with focus on pulp rheology.

Dr. Saeed Farrokhpay assisted in the initial planning of the research structure and the experimental design. He also helped with the interpretation of the first rheology results.

Dr. Yongjun Peng and Dr Elaine Wightman actively participated in preparing and reviewing the draft of the thesis and provided a prompt feedback.

Prof. Tim Nicholson assisted in conducting the first rheology tests that were critical to establish a standard experimental procedure.

Prof. Roger Smart and Dr. Ning Xu from the UniSA performed the Cryo-SEM imaging and assisted with the interpretation of the results.

Statement of parts of the thesis submitted to qualify for the award of another degree

None

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Acknowledgements

To Dr. Yongjun Peng, my principal supervisor, who gave me the opportunity to work in this interesting and challenging project. He was very supportive and open to new ideas, and his timely feedback and suggestions on the thesis and paper writing was crucial to achieve the goals set in this project.

To Prof. Dee Bradshaw, who was one of my associate supervisors for the first two years. She was one of my first contacts at The University of Queensland while I was living in Canada, and was an excellent guidance and support at the beginning of this project.

To Dr. Elaine Wightman for accepting being my associate supervisor in the last two years. She was extremely helpful with her prompt feedback and suggestions about paper and thesis writing.

To Newmont Mining Corporation, Newcrest Mining Limited and the Australian Research Council for the financial support of this project, and The University of Queensland for granting me the International Postgraduate Research Scholarship.

To Jon Worth, Douglas Brown and other Pilot Plant and Workshop stuff at the JKMRC for their permanent availability to assist with laboratory work.

To Prof. Tim Napier-Munn for his useful contributions to the direction of this project as a member of the review panel in three milestone assessments.

To all the students that I had the opportunity to meet at the SMI, and special thanks to the students at the JKMRC.

Finally, to my family and friends in Colombia and Canada. Despite the distance, they were always present to provide advice and support. Very special thanks to Sharis Lakfard for being patient and understanding the importance of this journey.

Keywords

Copper-gold flotation, clay minerals, rheology, clay network structures, flotation reagents and interactions

Australian and New Zealand Standard Research Classifications (ANZSRC)

ANZSRC code: 091404 Mineral Processing/Beneficiation, 100%

Fields of Research (FoR) Classification

FoR code: 0914, Resources Engineering and Extractive Metallurgy, 100%

Table of Contents

Chapter 1 Introduction ...... 1 1.1 Problem Statement ...... 1 1.2 Scope of this thesis and research objectives ...... 2 Chapter 2 Literature review ...... 3 2.1 Clay Science ...... 3 2.1.1 Types of clay minerals ...... 3 2.1.2 The properties of clay minerals ...... 6 2.1.2.1 Colloidal behaviour of clay minerals ...... 6 2.1.2.2 Clay mineral structure ...... 8 2.1.2.3 Clay mineral charge properties ...... 14 2.1.3 Aggregation of clay minerals and rheological behaviour ...... 18 2.1.4 Clay interactions ...... 24 2.1.4.1 Clay-water interactions ...... 25 2.1.4.2 Clay interactions with inorganic compounds ...... 27 2.1.4.3 Clay interactions with organic compounds ...... 32 2.2 Effect of clay minerals on flotation...... 37 2.2.1 Effect of pulp rheological behaviour on flotation ...... 38 2.2.2 Entrainment of clay minerals ...... 41 2.3 Literature review conclusions ...... 44 2.4 Research gaps and hypotheses ...... 46 2.5 Research plan ...... 47 2.6 Thesis outline ...... 50 Chapter 3 Experimental ...... 54 3.1 Introduction ...... 54 3.2 Mineral samples and reagents ...... 54 3.2.1 Single minerals ...... 54 3.2.2 Ore samples ...... 55 3.2.3 Reagents ...... 56 3.3 Pulp rheology measurement ...... 57 3.3.1 The reproducibility of rheology measurements ...... 62 3.4 Measurement of network structures ...... 63 3.4.1 Cryo-SEM imaging ...... 64 3.4.2 Settling and gel point tests ...... 64 3.5 Solubility tests ...... 66 3.6 Flotation experiments ...... 67 xi

3.6.1 The reproducibility of flotation tests ...... 68 Chapter 4 Rheological properties of kaolinite and bentonite suspensions in the presence of flotation reagents ...... 70 4.1 Introduction ...... 70 4.2 Results and discussion ...... 71 4.2.1 Kaolinite and bentonite interactions with water ...... 71 4.2.2 Kaolinite and bentonite suspensions in the presence of pH modifiers ...... 73 4.2.3 Kaolinite and bentonite suspensions in the presence of collector and frother ...... 79 4.2.4 Settling tests for bentonite and kaolinite suspensions...... 82 4.3 Conclusions ...... 83 Chapter 5 Influence of some calcium bearing gangue minerals on the rheological behaviour of clay minerals...... 85 5.1 Introduction ...... 85 5.2 Results and discussion ...... 86 5.2.1 Rheology comparison of the high and low clay content ores ...... 86 5.2.2 Rheology of calcium bearing minerals ...... 87 5.2.3 Interactions of calcium bearing minerals with clay minerals ...... 89 5.2.3.1 Kaolinite ...... 89 5.2.3.2 Illite ...... 92 5.2.3.3 Bentonite ...... 95 5.2.3.4 Solubility tests of calcium bearing minerals ...... 97 5.3 Conclusions ...... 99 Chapter 6 The interaction of clay minerals with gypsum and its effects on copper-gold flotation……...... 101 6.1 Introduction ...... 101 6.2 Results and discussion ...... 101 6.2.1 Flotation...... 101 6.2.2 Rheology measurements ...... 107 6.2.3 Cryo-SEM analysis ...... 109 6.2.4 Effect of clay aggregates on flotation ...... 119 6.3 Conclusions ...... 121 Chapter 7 The interaction of pH modifiers and clay minerals and its effect on copper-gold flotation...... 122 7.1 Introduction ...... 122 7.2 Results and discussion ...... 123 7.2.1 Interaction of lime and soda ash with kaolinite ...... 123 7.2.1.1 Flotation ...... 123 7.2.1.2 Pulp rheology ...... 129 7.2.1.3 Aggregates of kaolinite particles ...... 131 7.2.2 Effect of lime and soda ash in the ore-bentonite mixture ...... 133 xii

7.2.2.1 Flotation ...... 133 7.2.2.2 Pulp rheology and gel point ...... 138 7.2.2.3 Network structures of bentonite particles ...... 140 7.3 Conclusions ...... 142 Chapter 8 Conclusions and Future Work ...... 144 8.1 Summary ...... 144 8.2 Conclusions ...... 144 8.3 Recommendations for processing copper-gold ores with high clay contents in flotation 148 8.4 Recommendations for future work ...... 149

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List of Figures

Figure 2-1. Diagram showing the range of some particles, the wavelenghts of various forms of electromagnetic radiation, and appropriate particle separation processes for different size ranges (Gregory, 2006)...... 7

Figure 2-2. (a) Tetrahedron [TO4]; (b) tetrahedral sheet: Oa and Ob are the apical and basal oxygen atoms respectively; a and b are the unit-cell parameters (Brigatti et al., 2006)...... 9

Figure 2-3. (a) Ooct (OH, F, Cl) orientation in cis-octahedron and trans-octahedron, and (b) location of cis- and trans-sites in the octahedral sheet: Oa and Ob are apical and basal oxygen atoms respectively; a and b are unit cell parameters (Brigatti et al., 2006)...... 9

Figure 2-4. Detailed schematic of the (a) trans-octahedron and (b) cis-octahedron...... 10

Figure 2-5. Representation of a 1:1 and 2:1 layer structure: Oa, Ob, and Ooct are tetrahedral basal, tetrahedral apical, and octahedral anionic position respectively; M and T refer to octahedral and tetrahedral cation respectively (Brigatti et al., 2006) ...... 11

Figure 2-6. (a) trioctahedral sheet; (b) dioctahedral sheet: Oa refers to apical oxygen atoms shared with tetrahedra , and Ooct is the anionic site shared between adjacent octahedral; a and b are unit-cell parameters (Brigatti et al., 2006)...... 11

Figure 2-7. A sketch of montmorillonite particles (Lagaly, 2006)...... 12

Figure 2-8. High-resolution SEM images for two types of kaolinities, (A) Snobrite and (B) Q38: basal surfaces of Snobrite are relatively smooth while for the Q38 there are microislands and individual crystallites on the extensive (basal) surfaces (Du et al., 2010)...... 13

Figure 2-9. Development of patch-wise surface charge heterogeneity on montmorillonite particles dispersed in aqueous solutions (Tombácz and Szekeres, 2004)...... 17

Figure 2-10. Schematic representation of the dominant and hidden electric double layers forming around the montmorillonite lamellae under different solution conditions: the effect of indifferent electrolytes on particle charge heterogeneity besides the specific role of pH (Tombácz and Szekeres, 2004)...... 18

Figure 2-11. Aggregation of clay mineral layers in (a) a card-house (edge-face) and (b) band-type (face-face) networks (Lagaly, 2006)...... 20

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Figure 2-12. Shear stress (at a shear rate of 94.5 s-1) dependence on the pH value for the dispersion of Na+ -montmorillonite (•-) and (Na+, Ca2+) –bentonite (□--) (Lagaly, 2006)...... 21

Figure 2-13. Disarticulation of alkali smectite particles in aqueous dispersions (Lagaly, 2006)...... 21

Figure 2-14. Interpretation of the effect of pH the mode of paticles interaction and the Bingham yield stress of kaolinite suspensions (Rand and Melton, 1977; Ndlovu et al., 2011)...... 22

Figure 2-15. Yield stress-pH behaviour of bentonite clays dispersions over a reduction of pH from 10 to 2 (Goh et al., 2010)...... 24

Figure 2-16. Shear rate vs shear stress diagrams of Nevada smectite dispersed in deionized water at concentrations of 0.5, 1, 2, and 3% v/v (Vali and Bachmann, 1988)...... 26

Figure 2-17. Heterocaogulation between two oxides (TiO2 and Al2O3) indicated as surface charge density σ of the oxides and stability factor W as a function of pH: the mixtures of both oxides should coagulate spontaneously between pH = 6 and pH = 9 (-----) (Lagaly, 2006)...... 29

Figure 2-18. (A) Bingham yield value of a 9% (w/w) Na+ - kaolinite dispersion, (B) yield value when a synthetic ferrihydrite (1 g ferrihydrite per 10 g kaolinite) was added at pH =3 or at pH = 9.5 (C) (Lagaly, 2006)...... 31

Figure 2-19. Interlayer reactions of 1:1 and 2:1 clay minerals (Lagaly et al., 2006)...... 33

Figure 2-20. Yield value of the sodium bentonite slurries (6% w/w) versus the amount of cationic surfactant (DDAC) and anionic surfactant (LABS) (Gungor, 2000)...... 36

Figure 2-21. Rheograms for sodium bentonite without surfactant, with cationic surfactant (DDAC) and with anionic surfactant (LABS) (Gungor, 2000)...... 37

Figure 2-22. SEM micrograph of pulp sample (Patra et al., 2012a)...... 39

Figure 2-23. Contours of velocity magnitude showing the formation of a cavern in a 60 wt.% Bindura nickel slurry at 450 rpm (Bakker et al., 2010)...... 40

Figure 2-24. Schematic of Bateman 120 litre flotation cell showing the shear rate distribution when operating at 400 rpm (Re=1.5x105) (Anderson, 2008)...... 40

Figure 2-25. Schematic of an ascending bubble, its flow lines and attached wake representing the Boundary Layer Theory and Bubble Wake Theory (Smith and Warren, 1989a)...... 42

Figure 2-26. Schematic representing the Bubble Swarm Theory. Water and dispersed solids are trapped by rising layers of bubbles (Smith and Warren, 1989b)...... 43

Figure 2-27. Sketch of a Plateau border and it associated lamella (Ross and Van Deventer, 1988) . 44

Figure 2-28. Main literature review findings and research gaps...... 46

Figure 2-29. Research questions and research plan to demonstrate the hypotheses...... 47

Figure 2-30. Schematic of the scope of the thesis ...... 49

Figure 2-31. Schematic of the experimental plan...... 50

Figure 2-32. Structure of the thesis ...... 51

Figure 3-1. Flow curves showing the behaviour of flowing material...... 59

Figure 3-2. Anton Paar DSR 301 rheometer...... 61

Figure 3-3. Replicates of rheology tests for slurries with 10 wt.% bentonite using the Ares rheometer from TA instruments...... 62

Figure 3-4. Replicates of three rheolgy meausurements for the mixture made of 30 wt.% kaolinite and 70 wt.% ore using the Anton Paar DSR 301 rheometer. Slurry density 30 wt.%...... 63

Figure 3-5. Gel point calculation...... 66

Figure 3-6. Flotation cell used in the test work...... 68

Figure 3-7. Copper grade and recovery for some replicates for the flotation of the low clay content ore and the ore mixtures with 30 wt.% kaolinite and 15 wt.% bentonite...... 69

Figure 4-1. Rheograms of kaolinite and bentonite suspensions at different concentrations...... 71

Figure 4-2. Apparent viscosity of kaolinite and bentonite suspensions as a function of their concentrations at a shear rate of 100 s-1...... 72

Figure 4-3. Rheograms of 10 wt.% kaolinite suspensions in the absence and presence of pH modifiers...... 75

Figure 4-4. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of pH modifiers...... 75

Figure 4-5. Rheograms of 30 wt.% kaolinite suspensions in the presence of pH modifiers...... 76

Figure 4-6. Rheograms of 2 wt.% bentonite suspensions in the absence and presence of pH modifiers...... 77

Figure 4-7. Rheograms of 5 wt.% bentonite suspensions in the absence and presence of pH modifiers...... 77 xvi

Figure 4-8. Rheograms of 10 wt.% bentonite suspensions in the absence and presence of pH modifiers...... 78

Figure 4-9. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of PAX...... 79

Figure 4-10. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of PAX. ... 80

Figure 4-11. Rheograms of 8 wt.% bentonite suspensions in the absence and presence of PAX. .... 80

Figure 4-12. Rheograms of 8 wt.% bentonite suspensions in the absence and presence of Interfroth 6500...... 81

Figure 4-13. Settling rate of the 20 wt. % kaolinite suspensions at natural pH and pH 10 using lime and sodium carbonate as pH modifiers...... 82

Figure 5-1. Rheograms of the high and low clay content ores, and a mixture of low clay content ore and kaolinite (30 wt.%): slurries with solids density of 30 wt.% (14.6 vol.%) at natural pH...... 87

Figure 5-2. Rheograms to compare the flow behaviour of quartz, calcite, dolomite, gypsum and high clay content ore at 30 wt.% (14.6 vol.%) solid concentration and natural pH...... 88

Figure 5-3. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH...... 90

Figure 5-4. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by lime...... 91

Figure 5-5. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by sodium hydroxide...... 91

Figure 5-6. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH...... 92

Figure 5-7. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by lime...... 94

Figure 5-8. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by sodium hydroxide...... 94

Figure 5-9. Comparison of rheograms for 15 wt.% bentonite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH...... 96

Figure 5-10. Comparison of rheograms for 15 wt.% bentonite and its mixtures with 5 wt.% calcite, dolomite and quartz at pH 10 and with 5 wt.% gypsum at pH 8.4 adjusted by Na2CO3...... 96 xvii

Figure 6-1. Mass recovery as a function of water recovery from the flotation of the low clay content ore and its mixtures with clay minerals and gypsum...... 102

Figure 6-2. Copper grade as a function of copper recovery from the flotation of the ore and its mixtures with clay minerals and gypsum...... 103

Figure 6-3. Gold grade as a function of gold recovery from the flotation of the ore and its mixtures with clay minerals and gypsum...... 104

Figure 6-4. Copper recovery by true flotation from the flotation of the ore and its mixtures with clay minerals and gypsum...... 105

Figure 6-5. Copper recovery by entrainment from the flotation of the ore and its mixtures with clay minerals and gypsum...... 106

Figure 6-6. Rheograms for the ore and its mixtures with clay minerals and gypsum: total solids concentration 30 wt.% and pH adjusted to 10 with hydrated lime (Ca(OH)2)...... 107

Figure 6-7. Apparent viscosity as a function of shear stress (Pa) for the ore and its mixtures with clay minerals and gypsum: pH adjusted to 10 with hydrated lime (Ca(OH)2)...... 108

Figure 6-8. Calculated apparent yield stresses for the ore and its mixtures with clay minerals and gypsum using Herschel-Buckley model...... 109

Figure 6-9. Comparison of the sodium bentonite network structures and kaolinite aggregates in the clay-ore mixtures: sodium bentonite and kaolinite concentrations are 15 wt.% and 30 wt.% respectively; 1000x magnification...... 110

Figure 6-10. Comparison of the sodium bentonite network structures and kaolinite aggregates in the clay-ore mixtures: sodium bentonite and kaolinite concentrations are 15 wt.% and 30 wt.% respectively; 4000x magnification...... 111

Figure 6-11. Cryo-SEM images showing the network structure formed in bentonite-ore and bentonite-gypsum-ore mixtures: 1000x magnification...... 112

Figure 6-12. Cryo-SEM images showing the network structure formed in bentonite-ore and bentonite-gypsum-ore mixtures: 4000x magnification...... 113

Figure 6-13. Schematic showing the aggregation of the clay mineral layers with increasing attraction: (a) single layers, (b) band-type networks, (c) compact particles (Lagaly and Dékány, 2013)...... 114

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Figure 6-14. Cryo-SEM images illustrating the change in froth agglomerates with the addition of gypsum to the bentonite-ore mixture: 1000x magnification...... 115

Figure 6-15. Cryo-SEM images illustrating the change in froth agglomerates with the addition of gypsum to the bentonite-ore mixture: 4000x magnification...... 115

Figure 6-16. Cryo-SEM images showing the kaolinite particle aggregates in the mixtures with ore in the absence and presence of gypsum: 1000x magnification...... 116

Figure 6-17. Cryo-SEM images showing the kaolinite particle aggregates in the mixtures with ore in the absence and presence of gypsum: 8000x magnification...... 116

Figure 6-18. Cryo-SEM images showing the change in froth agglomerates with the addition of gypsum to the kaolinite-ore mixture. 8000x magnification...... 118

Figure 7-1. The mass recovery as a function of the water recovery from the flotation of the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration...... 125

Figure 7-2. Relationship between the water recovered (g) from the first two concentrates and the ionic strength (M) when lime and soda ash were used to adjust pH 10 for the slurries without and with the addition of 10 and 30 wt.% kaolnite...... 125

Figure 7-3. The copper grade as a function of the copper recovery from the flotation of the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration...... 126

Figure 7-4. The gold grade as a function of the gold recovery from the flotation of the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration...... 127

Figure 7-5. The copper recovery by true flotation from the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration...... 128

Figure 7-6. The copper recovery by entrainment from the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration...... 129

Figure 7-7. Rheograms for the ore and its mixtures with kaolinite: pH 10 was adjusted by lime or soda ash; 30 wt.% solids concentration...... 130

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Figure 7-8. Cryo-SEM images comparing the type of aggregates and network structures present in the pulp with 30 wt.% kaolinite with pH 10 adjusted with lime (a) and soda ash (b): 1000x magnification...... 131

Figure 7-9. Cryo-SEM images comparing the type of aggregates and network structures present in the pulp with 30 wt.% kaolinite with pH 10 adjusted with lime (a) and soda ash (b): 8000x magnification...... 132

Figure 7-10. Cryo-SEM images showing the first concentrate from the flotation of the ore mixed with 30 wt.% kaolinite with pH 10 adjusted with hydrated lime (a) and soda ash (b): 1000x magnification...... 132

Figure 7-11. Cryo-SEM images showing the first concentrate from the flotation of the ore mixed with 30 wt.% kaolinite with pH 10 adjusted with hydrated lime (a) and soda ash (b): 8000x magnification...... 133

Figure 7-12. Mass – water recovery when floating the mixtures ore and bentonite (5 wt.% and 15 wt.%). Flotation at pH 10 and 30 wt.% solids concentration...... 134

Figure 7-13. Copper grade as a function of copper recovery from the flotation of the ore and its mixtures with bentonite: pH adjusted to 10 with hydrated lime (Ca(OH)2) and soda ash (Na2CO3); 30 wt.% solids concentration...... 135

Figure 7-14. Gold grade as a function of gold recovery from the flotation of the ore and and its mixtures with bentonite: pH adjusted to 10 with hydrated lime (Ca(OH)2) and soda ash (Na2CO3); 30 wt.% solids concentration...... 136

Figure 7-15. Recovery of copper by true flotation for the ore and its mixtures with bentonite using hydrated lime (Ca(OH)2) and soda ash (Na2CO3) to adjust pH to 10 in the slurries with 30 wt.% solids concentration...... 137

Figure 7-16. Copper recovery by entrainment from the flotation of the ore and its mixtures with bentonite using hydrated lime (Ca(OH)2) and soda ash (Na2CO3) to adjust pH to 10 in the slurries with 30 wt.% solids concentration...... 138

Figure 7-17. Rheograms for the bentonite-ore mixtures used in flotation: pH adjusted to 10 with hydrated lime (Ca(OH)2) and soda ash (Na2CO3); total solid concentration, 30 wt.%; the rheograms for a slurry with 25 wt.% bentonite are also shown to demonstrate the change in rheological behaviour with gel point...... 139

Figure 7-18. The gel point of the ore and its mixtures with kaolinite and bentonite as well as the gel point of the mixture of 95% bentonite and 5% gypsum...... 140

Figure 7-19. Comparison of Cryo-SEM images for the flotation pulps with 15 wt.% bentonite when using hydrated lime (a) (Ca(OH)2) and soda ash (b) (Na2CO3) as pH modifiers to adjust pH to 10. Solids concentration 30 wt.%: Image magnification 1000x...... 141

Figure 7-20. Comparison of Cryo-SEM images for the mixture with 15 wt.% bentonite when using hydrated lime (a) (Ca(OH)2) and soda ash (b) (Na2CO3) as pH modifiers to adjust pH to 10. Solids concentration 30 wt.%. Image magnification 4000x...... 141

Figure 7-21. Cryo-SEM images comparing the first concentrate when using hydrate lime (a)

(Ca(OH)2) or (b) soda ash (Na2CO3) as pH modifiers to adjust pH to 10. Image magnification 2000x...... 142

xxi

List of tables

Table 2-1. Classification of planar hydrous phyllosilicates (Bergaya and Lagaly, 2006)...... 4

Table 2-2. Classification of non-planar hydrous phyllosilicates (Bergaya and Lagaly, 2006)...... 5

Table 2-3. Layer charge and idealized formulae (chemical composition) for some representative 1:1 and 2:1 clay minerals (Bergaya and Lagaly, 2006)...... 16

Table 2-4. Solubility of some of the gangue minerals present in a problematic ore (approximate values form different sources) ...... 32

Table 2-5. Characteristics of the clay minerals used in a study with three surfactants (non-ionic, anionic and cationic): OC% is the percentage of organic carbon, and d (001) is the distance between planes of atoms (Sanchez-Martin et al., 2008)...... 35

Table 3-1. Mineral compositions of the high and low clay content ores...... 56

Table 5-1. Miliequivalent per litre (meq/l) of Ca2+, Mg2+ and Na+ after dissolution of quartz, calcite, dolomite and gypsum...... 97

xxii

List of Abbreviations

PAX - Potassium Amyl Xanthate

SIPX - Sodium Isopropyl Xanthate

Cryo-SEM - Cryo Scanning Electron Microscopy

F-F - Face-Face

E-F - Edge-Face

E-E - Edge-Edge

IEP - Isoelectric Point

PZC - Point of zero charge

xxiii

Chapter 1 Introduction

1.1 Problem Statement

Ores that are easy to process are becoming scarce, and more complex low grade ores have to be processed to keep up with the demand for valuable minerals. Copper-gold ores with high clay contents are an example of these difficult to process ores. Clay minerals can interact with water, reagents, other minerals and ions, and the flotation of gold and copper ores provides the perfect environment for those interactions to occur and have a deleterious effect on grades and recoveries.

There are numerous examples of the effects of clay minerals in flotation concentrators. In Newcrest Telfer plant, it has been reported that when a high clay ore is floated at a normal solid density of about 30 wt.%, effective flotation is not possible and no froth can form on the top of the pulp phase. The pulp becomes a continuous gel-like substance with high viscosity. After diluting the pulp to a solid density of about 20 wt.%, froth can appear but becomes very stable and does not flow easily or break down quickly in the concentrate launders. In addition, the plant can experience spillage and high re-circulating load between the rougher and the cleaner circuits, which restrains capacity. A possible explanation for this problem is that high viscosity affects bubble dispersion inhibiting froth formation. Diluting the pulp decreases pulp viscosity, and flotation hydrodynamics may not be affected as much, but this brings a high entrainment problem. This is supported by the high concentrate mass pull reported of about 10% in that plant when a high clay ore is floated, which is significantly higher than the 1-5% normally achieved with non-clay ores. These changes in flotation can cause the copper and gold grades and recoveries to decrease significantly. Similar phenomena have been observed at many other copper-gold plants that process ores with a total clay content of about 30 wt.%, however, since not all the clay minerals have the same characteristics, it is expected that the flotation outcome will vary depending on the clay types present. To better understand this problem in copper and gold flotation an ARC Linkage Project was created with the involvement of two companies that had reported low recoveries and grades and other process constraints while processing copper-gold ores with high clay contents.

At present there are no reliable and effective ways to reduce the deleterious effects of clay minerals in copper and gold flotation. Remediation is largely by trial and error, and a common practice is to blend small proportions of the clay-bearing ores with “clean” ores to reduce their concentrations to 1 a point where the negative impacts are minimal. However, within a few years some companies will have to process a significantly greater amount of clay-bearing ores as the easy to process ores are depleted. Some plants are already processing high clay ores. As a result, there is an increasingly urgent need to understand the flotation behaviour of clay-bearing ores and to identify process solutions that will enable efficient flotation.

1.2 Scope of this thesis and research objectives

Currently there are no practical solutions to the problems encountered when floating copper-gold ores with high clay contents. It is reasonable that the first step to address this problem is to study the most relevant interactions that occur between clay minerals and the components in the flotation pulp. The most common observations reported by the industry are high flotation pulp viscosities that impede effective flotation or high entrainment that decreases the grade of flotation concentrates. Although entrainment was considered when analysing the flotation results, the mechanisms of entrainment have not been studied in detail since this thesis is focussed on pulp rheology and as such has the following objectives:

• To understand the interactions involving kaolinite (a non-swelling clay mineral) and bentonite (a swelling clay mineral) in the flotation of a copper-gold ore; • To determine the effect of these interactions on flotation performance.

Knowledge gained from the successful completion of these objectives could be used by other researchers as one of the inputs to develop a model that takes into account the most significant interactions of clay minerals to predict their effects on flotation. The first step to achieve the proposed objectives is to do a comprehensive review of clay mineral literature to find the existing gaps in mineral processing knowledge when dealing with copper-gold ores with high clay contents. Clay mineral science is a mature discipline different to mineral processing, and some of that information is applicable to this study. First, a general overview of clay science is presented, and this includes the types of clay minerals, their properties, and their interactions with water and other compounds. The second section discusses the possible effects of clay minerals in flotation. Results from this literature review revealed the research gaps for this specific problem in the flotation of copper-gold ores leading to the hypotheses posed in this thesis and the research plan to test them. The next chapter presents this literature review followed by the gaps, hypotheses, and the research objectives and plan proposed. 2

Chapter 2 Literature review

Clay minerals are unique in the sense that they have been studied extensively as a result of their wide application. The knowledge coming from these studies has been applied in many disciplines as well. In mineral flotation, the presence of clay minerals has been noted as deteriorating the flotation process, but limited studies have been conducted to understand the underlying mechanisms. It is believed that the deleterious effects of clay minerals in flotation can be explained with the information available from clay science. Therefore, this review will cover two bodies of knowledge, (1) clay science, and (2) the effects of clay minerals on flotation. The gaps between these two bodies of knowledge will then be bridged to understand the mechanisms governing the deleterious effects of clay minerals in flotation.

2.1 Clay Science

2.1.1 Types of clay minerals

Clay minerals are phyllosilicates that can be present in many ores as gangue minerals. Kaolinite, illite and smectite are the most common clay minerals, and can be present in tailing wastes from coal, heavy minerals and base metals processing (Zbik et al., 2008b; Zbik and Frost, 2010). Other examples of clay minerals that are found in other industrial ores are chrysotile in nickel sulphide ores, talc in base metal sulphide ores, muscovite and vermiculite (Ndlovu et al., 2011). It has been reported that illite, kaolinite and smectite can exist in ores containing gold and copper such as in the case of Newcrest’s Telfer ore and Newmont’s Carlin ore.

Table 2-1 shows the different types of clay minerals with a planar structure. Smectite and vermiculite fall into this category. In Table 2-2 the non-planar clay minerals are shown and kaolinite, talc and mica are in this group among others. The most important clay minerals are kaolinite, smectites, illites, and mixed-layer minerals due to their wide industrial applications.

Table 2-1. Classification of planar hydrous phyllosilicates (Bergaya and Lagaly, 2006).

Interlayer materiala Group Octahedral Species characterb

1:1 Clay minerals

None or H2O only Serpentine- Tri Amesite, berthierine, brindleyite, cronstedtite, ξ~0 kaolin fraipontite, kellyite, lizardite, nepouite Di Dickite, halloysite (planar), kaolinite, nacrite Di-tri Odinite

2:1 Clay minerals

None, ξ~0 Talc-pyrophyllite Tri Kerolite, pimelite, talc, willemsite Di Ferripyrophyllite, pyrophyllite

Hydrated Smectite Tri Hectorite, saponite, sauconite, stevensite, exchangeable cations, swinefordite ξ~0.2-0.6 Di Beidellite, montmorillonite, nontronite, volkonskoite

Hydrated Vermiculite Tri Trioctahedral vermiculite exchangeable cations, ξ~0.6-0.9 Di Dioctahedral vermiculite

Non-hydrated True (flexible) Tri Biotite, lepidolite, phlogopite, etc. monovalent cations, mica ξ~0.6-1.0 Di Celadonite, illite, glauconite, muscovite, paragonite, etc.

Non-hydrated divalent Brittle mica Tri Anandite, bityite, clintonite, kinoshitalite cations, ξ~1.8-2.0 Di Margarite

Hydroxide sheet, Chlorite Tri Baileychlore, chamosite, clinochlore, nimite, ξ variable pennantite Di Donbassite Di-tri Cookeite, sudoite

Regularly interstratified 2:1 clay minerals

ξ variable Tri Aliettite, corrensite, hydrobiotite, kulkeite Di Rectorite, tosudite

Table 2-2. Classification of non-planar hydrous phyllosilicates (Bergaya and Lagaly, 2006).

Modulated Linkage Unit layer c sin Traditional Species Component Configuration β value affiliation

1:1 Minerals with modulated structures

Tetrahedral Strip 0.7 nm Serpentine Antigorite, bemenitite, Sheet Islands 0.7 nm Serpentine caryopilite, ferropyrosmalite, friedelite, greenalite, manganpyrosmalite, mcgillite, nenenite Other None pyrosmalite, schallerite

2:1 Minerals with modulated structures

Tetrahedral Strip 0.95 nm Talc Minnesotaite sheet 1.25 nm Mica Eggletonite, ganphyllite Islands 0.96-1.25 mm Mica/Complex Ferristilpnomelane, ferrostilpnomelane, lennilenapeite, parsettensite, stilpnomelane, zussmanite Other 1.23 nm None Bannisterite 1.4 nm Chlorite Gonyerite

Octahedral sheet Strips 1.27-1.34 nm Pyribole Falcondoite, loughlinite, palygorskite, sepiolite, yofortierite

1:1 Minerals with rolled and spheroidal structures

None Trioctahedral Serpentine Chrysotile, percoraite Dioctrahedral Kaolin Halloysite (nonplanar)

Some of the general properties of clay minerals are (Bergaya and Lagaly, 2006):

1) Colloidal size; 2) The nanometer range of the thickness of the layer structure; 3) Anisotropy of the layer or particles; 4) Several types of surfaces: -external basal (planar) and edge surfaces; -internal (interlayer) surfaces; 5) The external and internal surfaces which can be modified easily by adsorption, ion exchange, or grafting; 6) Plasticity; 7) Hardening in drying or firing of most of the clays; 8) Swelling of some clays;

2.1.2 The properties of clay minerals

There are a great number of studies on the structures and properties of clay minerals (Sposito and Prost, 1982; Rao and Sridharan, 1985; Komadel et al., 1990; Abend and Lagaly, 2000; Lagaly and Ziesmer, 2003; Brigatti et al., 2006; Schoonheydt and Johnston, 2006; Karimi and Salem, 2011; Paineau et al., 2011a; Paineau et al., 2011b; Rao et al., 2011; Tiwari and Ajmera, 2011; Yan et al., 2011; Gridi-Bennadji et al., 2012). These studies are disseminated among many scientific and industrial areas of research. The properties of clay minerals which are relevant to mineral flotation will be reviewed in the following section.

2.1.2.1 Colloidal behaviour of clay minerals

Clay minerals can be considered colloidal materials, but there is no common agreement between different disciplines and professions about the upper limit for the size of clay particles (Bergaya and Lagaly, 2006). The proposed upper limit varies between 1 and 4 µm (Velde, 1995; Lagaly, 2005; Bergaya and Lagaly, 2006).

Figure 2-1 compares the size of clay minerals with the size of other particles (Gregory, 2006), and according to the graph, clay minerals can be easily found in the nanometer scale. Diffusion is an important transport mechanism for particles smaller than about 1 µm, and this tends to prevent particles from settling (Gregory, 2006). The surface area of particles begins to become significant relative to their volume at around the same size. In addition, the flat shape of clay minerals provides an even greater surface area when compared to other minerals of the same size that tend to occur as cubes or spheres. The ratio of length to thickness for sheet-like clay particles is typically near 20, making the surface area of a clay particle nearly three times that of a cube of the same volume (Velde, 1995). Also for smaller particles, colloid interactions become more significant relative to external forces such as gravity and fluid drag. These colloid interactions are important for aggregation and deposition of particles in the colloidal size range (Gregory, 2006).

This small size and very high surface area make clay minerals highly reactive and responsive to changes in processing environment (Hauser, 1945; Farrokhpay and Bradshaw, 2012). This can be illustrated by taking a cube of 1 mm edge length with a volume of 1 mm3, a total surface of 6 mm2, eight corners and twelve edges. If that cube is subdivided into cubes of 0.5 µm edge length, which can be easily the size of a clay particle, eight billion cubes will be produced. Their combined volume will still be 1 mm3, but the sum of the new surfaces will make a total of 12000 mm2, and there will be 64 billion corners and 96 billion edges. At this scale any deficiency in the molecular structure (i.e. a missing ion) results in high reactivity and certain properties which cannot be explained on the basis of chemical composition alone (Hauser, 1945).

The process of mineral flotation provides a perfect environment for the small flat clay particles to interact with water, reagents and other minerals. In addition to the colloidal properties of clay

minerals (their size and surface area), their chemical composition is a potential source of interaction through the charges found on the surface of most clay particles.

2.1.2.2 Clay mineral structure

It is generally accepted that the majority of clays are crystalline (Theng, 2012a), however, some authors propose that clay minerals are not crystals in the strict sense specially in the case of smectites (Lagaly, 2006) or some kaolinites with low crystallinity (Du et al., 2010). The basic building block in the phyllosilicate structure is an aluminosilicate layer containing a silica tetrahedral sheet and an alumina octahedral sheet, bonded in certain proportions (Theng, 2012a).

The tetrahedral sheets are composed of tetrahedrons having a cation coordinated to four oxygen atoms. Three of these oxygen atoms are called basal and they link the tetrahedrons forming an infinite two-dimensional hexagonal mesh pattern. The fourth oxygen atom is called apic (Figure 2-2). The octahedrons in the octahedral sheets are connected to each other by shared edges forming sheets of hexagonal or pseudo-hexagonal symmetry (Brigatti et al., 2006). A tetrahedral sheet can be bound to one or both sides of the octahedral sheet. The octahedrons have two different topologies related to OH position; the cis- and the trans- orientation which are shown in Figure 2-3 and Figure 2-4 (Brigatti, Galan et al. 2006).

Figure 2-2. (a) Tetrahedron [TO4]; (b) tetrahedral sheet: Oa and Ob are the apical and basal oxygen atoms respectively; a and b are the unit-cell parameters (Brigatti et al., 2006).

Figure 2-4. Detailed schematic of the (a) trans-octahedron and (b) cis-octahedron.

Common tetrahedral cations are Si4+, Al3+, and Fe3+, and octahedral cations are usually Al3+, Fe3+, Mg2+, and Fe2+. Other octahedral cations identified include Li+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, V3+, Cr3+, and Ti4+ (Brigatti et al., 2006).

The apical oxygen atoms of all tetrahedral have the same orientation and connect the tetrahedral and octahedral sheets forming a common plane with the octahedral anionic position Ooct (OH, F, Cl, O).

These Ooct are not shared with tetrahedra and are located near the centre of each tetrahedral ring.

The 1:1 layer structure consists of one tetrahedral and one octahedral sheet, and the 2:1 structure has one octahedral sheet sandwiched between two tetrahedral sheets (Figure 2-5). The unit cell of a 1:1 layer structure has six octahedral sites, four cis- and two trans- octahedrons, and four tetrahedral sites. The 2:1 layer unit cell is composed of six octahedral and eight octahedral sites. This 2:1 structure can be trioctahedral (all six octahedral sites occupied) or dioctahedral (four of the six octahedra are occupied), and the structural formula is often given on the basis of the half unit-cell content (Figure 2-6) (Brigatti et al., 2006).

The clay basal planes have permanent surface negative charge from isomorphic substitution of lattice elements, and this charge is pH independent (Luckham and Rossi, 1999). At the edges of the layers there are broken primary bonds in the tetrahedral and octahedral sheets, and pH dependent electrical charge is created from hydrolysis reaction from broken Al-O and Si-O bonds. These edges are positively charged in the neutral and acid pH ranges depending on the type of clay minerals (Peng and Zhao, 2011).

The previous description sees clay minerals as perfect crystals, however, clay particles are not considered crystals in the strict sense especially in the case of smectites which can be seen as an assemblage of silicate layers (Lagaly, 2006). When looking at montmorillonite particles using an electron microscope, the structure is like a paper torn into irregular pieces (Figure 2-7) (Lagaly, 2006). In smectites, particles have many points of weak contacts between the stacks of the layers, and at those points particles may break during interlayer reactions or due to mechanical forces that affect rheological behaviour.

Figure 2-7. A sketch of montmorillonite particles (Lagaly, 2006).

It is also reported that for kaolinite there are different morphologies, and the basal surfaces are not perfectly flat and in some cases poorly crystallized kaolinite has “high aspect ratios with ragged, stepped basal surfaces and a high proportion of attached nano-sized islands, forming cascade-like step complex basal surface structures with edge sites contributing up to 30% of the specific surface area” (Figure 2-8) (Du et al., 2010). It has been reported that for the poorly crystallized kaolinite the settling time of tailings increases due to the erratic shape of the particles and it is expected that the shape will influence rheological response as well.

Figure 2-8. High-resolution SEM images for two types of kaolinites, (A) Snobrite and (B) Q38: basal surfaces of Snobrite are relatively smooth while for the Q38 there are microislands and individual crystallites on the extensive (basal) surfaces (Du et al., 2010).

Kaolinite and sodium-bentonite are some of the most common clay minerals. A description of these two clay minerals is given below.

Kaolinite belongs to the kaolin group, which has dioctahedral 1:1 layer structure with a general 3+ composition of Al2Si2O5(OH)4 characterized by a predominance of Al in octahedral sites, with some isomorphous substitutions of Mg2+, Fe3+, Ti4+, and V3+ for Al3+ occurring (Velde, 1995; Benco et al., 2001; Brigatti et al., 2006). The layer thickness of kaolinite is approximately 0.7 nm, and consists of a silicate (Si2O5) sheet bonded to a gibbsite-type (Al(OH)3) layer (Benco et al., 2001). The kaolinite layer is essentially neutral and contiguous layers are strongly linked together by –Al–O–H•••O–Si– hydrogen bonding supplemented by dipole-dipole van der Waals interactions. Due to these characteristics, kaolinite in essence does not have interlayer material and does not show interlayer expansion or swelling in water (Theng, 2012a). 13

Poor structural order in kaolin minerals has been observed, and this may be explained in terms of a series of stacking faults or defects in the basal plane or the c-axis. This characteristic accounts for the well-known tendency of kaolin minerals to form a wide variety of ordered and disordered polytypes and twins (Brigatti et al., 2006).

Montmorillonite is a dioctahedral smectite and is the main component of bentonite. Montmorillonite particles can expand in water, making its extensive interlayer surface area (700- 800 m2/g) accessible to a broad range of guest molecules, including organic compounds (e.g. polymers). It has a 2:1 layer structure where the tetrahedral sheets are inverted and two thirds of the octahedral hydroxyl groups are replaced by tetrahedral apical oxygen atoms. Some characteristics of smectites are the total negative layer charge between 0.2 and 0.6 per half-cell unit, and the hydration of the interlayer cations. In the dioctahedral smectites the octahedral sheet may be mostly occupied by trivalent cations. In the case of trioctahedral smectites, that sheet is occupied by divalent cations.

The general composition of montmorillonite is:

+ 3+ 2+ 4+ (M y x nH2O)(Al 2-yMg y)Si 4O10(OH)2

In general swelling clays, such as montmorillonite, undergo isomorphous substitutions inducing a charge deficit compensated by interlayer exchangeable cations (Paineau et al., 2011b).

Interlayer hydration of smectites is mainly affected by the following factors (Brigatti et al., 2006):

• Hydration energy of the interlayer cation; • Polarization of water molecules by interlayer cations; • Variation of electrostatic surface potentials due to the differences in layer charge location; • Activity of water; • Size and morphology of smectite particles.

2.1.2.3 Clay mineral charge properties

It has been reported that most clay minerals have a basal permanent negative charge caused by isomorphous substitution, and in a few exceptions that charge is almost neutral (e.g. talc) (Schoonheydt and Johnston, 2006). However, recent work reported that the two basal plane surfaces of kaolinite can have positive or negative charge depending on pH (Gupta and Miller, 2010). In that

study a colloidal force measurement showed that the silica tetrahedral face is negatively charged at pH > 4 and the alumina octahedral face is negative at pH > 8 and positive at pH < 6.

The charge on the edges of clay minerals can be either positive or negative and this is dependent on the pH. This distribution of charges makes the value of the isoelectric point (iep) to be different to the point of zero charge (pzc). Also the anisotropic structure and charge properties lead to the aggregation of clay minerals increasing the viscosity of suspensions.

The charge properties of clay minerals may also contribute to the reactivity and interactions of clay minerals in flotation. For instance, the absence of layer charge in talc particles (the pyrophyllite group of clays, 2:1 type) makes their surface essentially hydrophobic, and interlayer swelling is non-existent. The behaviour of these clays in mineral flotation will be totally different to that of montmorillonite, another 2:1 type of clay.

As discussed previously, the charge on the basal plane of clay minerals is caused by isomorphic substitutions. Table 2-3 shows the layer charge for some 1:1 and 2:1 clay minerals. Montmorillonite can be used as an example to provide a more detailed explanation about this topic since this clay has a significant charge which may cause problems in mineral flotation.

Table 2-3. Layer charge and idealized formulae (chemical composition) for some representative 1:1 and 2:1 clay minerals (Bergaya and Lagaly, 2006).

Charge/ Dioctahedral species Trioctahedral species formula unit

Serpentine-kaolin group

~0 Kaolinite Serpentine IV VI IV VI (Si2) (Al2) O5(OH)4 (Si2) (Mg3) O5(OH)4

Talc-pyrophyllite group

~0 Pyrophyllite Talc IV VI IV VI (Si2) (Al2) O10(OH)2 (Si4) (Mg3) O10(OH)2

Smectite group

~0.2-0.6 Montmorillonite Hectorite (Si )IV(Al Mg )VIO (OH) , (Si )IV(Mg Li )VIO (OH) , 4+ 2-y y 10 2 4+ 3-y y 10 2 yM .nH2O yM .nH2O Beidellite Saponite (Si Al )IV(Al )VIO (OH) , (Si Al )IV(Mg )VIO (OH) , 4+-x x 2 10 2 4+-x x 3 10 2 yM .nH2O yM .nH2O

Vermiculite group

~0.6-0.9 Vermiculite Vermiculite (Si Al )IV(Al Mg )VIO (Si Al )IV(Mg M 3+)VIO 4-x x +2-y y 10 4-x x 32+-y y 10 (OH)2, (x+y)M (OH)2, (x-y)/2 M

True (flexible) mica group

~0.9-1.0 Celadonite Lepidolite (Si Al )IV(Fe Mg )VIO (Si Al )IV(Mg Li )VIO 4-x x +2-y y 10 4-x x + 3-y y 10 (OH)2, (x+y)K (OH)2, (x+y)K Muscovite Phlogopite IV VI + IV VI + (Si3 Al) (Al2) O10(OH)2, K (Si3 Al) (Mg3) O10(OH)2, K

Britlle mica group

2.0 Margarite Clintonite IV VI 2+ IV VI (Si2 Al2) (Al2) O10(OH)2, Ca (Si Al3) (Mg2Al) O10(OH)2, Ca2+

In montmorillonite, a negative charge associated with cation replacement in the tetrahedral sheet (i.e., Al3+ for Si4+) gives a localized charge distribution, while a much more diffuse negative charge comes from cation replacement in the octahedral sheet (i.e., Mg2+, for Al3+). This negative lattice charge is compensated by the dominant electric double layer on faces. The other polar sites, mainly octahedral Al-OH and tetrahedral Si-OH groups, are located at the broken edges. The charge in these amphoteric sites is not fixed, and either positive or negative charges can develop at the edges by direct H+ or OH- transfer from aqueous phase depending on the pH (Figure 2-9) (Tombácz and Szekeres, 2004; Tombácz and Szekeres, 2006). The variable edge charges are compensated by a cloud of counter ions in a hidden electrical double layer (EDL) that probably is covered by the dominant EDL extending from the particle faces (Figure 2-10) (Tombácz and Szekeres, 2004).Figure 2-9 and Figure 2-10 show in some detail the distribution of the surface charges, and the hidden and dominant double layers under different conditions of electrolytes and pH. σ0 is the

constant charge density of the dominant EDL, σd,f, and σd,e are the charge densities of the diffuse layers for faces and edges respectively, and ψ0,H is the constant potential of the hidden edl at constant pH (Tombácz and Szekeres, 2004).

Figure 2-9. Development of patch-wise surface charge heterogeneity on montmorillonite particles dispersed in aqueous solutions (Tombácz and Szekeres, 2004).

A more rigorous explanation of the charges on clay minerals is not the purpose of this literature review. The information given here is useful to emphasize that clay mineral surfaces can be very reactive not only due to their colloidal size, but also because of their electrical charge. This is crucial to understanding the interaction of clay minerals with reagents, and formation of structures that may increase viscosity in flotation slurries.

2.1.3 Aggregation of clay minerals and rheological behaviour

Clay minerals can have a great impact on the rheology of mineral suspensions due to their colloidal behaviour, structure and charge properties. Mueller et al. (2010) indicated that the rheology of slurries was a complex function of the physical properties and processes that occurred at the scale of the suspended particles. The most important factors determining the rheology are particle volume fractions, particle shapes, interactions between particles, the spatial arrangement of particles and the nature of the bulk flow field. Clay minerals have flat shape and this facilitates their interaction and spatial arrangements. Other factors such as the size and shape distribution of the particles and inter- particle forces can be important in some suspensions (Mueller et al., 2010). At low concentrations clay suspensions may be Newtonian with the viscosity independent of the shear rate. An increase in

the concentration makes the suspension more strongly non-Newtonian with a marked rise in viscosity as the shear rate decreases suggesting the possible appearance of a yield stress (He et al., 2004).

It has been also found that elongated particles or particles with large aspect ratios produce anisotropic behaviour as a preferred particle orientation results under the action of the shear field (Turian et al., 1997). The flow around non-spherical particles, such as clays, is different from that around a spherical particle, and the particle contribution to the suspension viscosity is also different. For instance, at the same volume fraction the degree of the interaction between non-spherical particles will be greater than that between spherical particles (Mueller et al., 2010).

According to some researchers the narrow size distribution of particles in the suspension produces yield stress and steady shear viscosity values higher than those of particles with the broader size distribution at the same volume fraction (Yang et al., 2001; He et al., 2004). It has been also reported that in silica-based suspensions the viscosity of the suspensions increases with the addition of fine particles (Olhero and Ferreira, 2004). In suspensions of colloidal particles (less than 1 µm) the interface separating the phases is very large and this causes the stability and rheology to be strongly affected by non-hydrodynamic electrical double-layer forces (Turian et al., 1997). The stability of this type of suspensions is dependent on the state of aggregation of the particles and the rheology is both chemistry and shear sensitive (Turian et al., 1997).

It is clear that clay minerals can have complex interactions that can significantly change the rheology of slurries because they are colloidal particles with anisotropic structure and charged surfaces. These unique properties lead to their aggregation increasing the viscosity of suspensions. There are three main modes of aggregation, edge-face (E-F), face-face (F-F) and edge-edge (E-E) (Gungor, 2000; Johnston and Premachandra, 2001). Figure 2-11 shows the aggregation of clay mineral layers in a house of cards (edge-face contacts) and band-type (face-face contacts) networks (Lagaly, 2006).

Figure 2-11. Aggregation of clay mineral layers in (a) a card-house (edge-face) and (b) band-type (face-face) networks (Lagaly, 2006).

The house of cards network only forms when the edges are positively charged, or in a slightly alkaline medium above the critical salt concentration, due to the electrostatic attraction between edges and faces (Lagaly, 2006). This network is characterized by non-Newtonian flow of the dispersions, and the development of yield stresses in acidic medium as shown in Figure 2-12. In the case of the Na+-montmorillonite dispersion, with increasing pH the house of card network breaks down due to the reduction of the positive charge on edges. Accordingly the shear stress (at a given shear rate) decreases to a sharp minimum at pH 4-5 (Figure 2-12). For a pH > 5 휏shear stress increases as a result of a higher degree of delamination, which results in a greater number of particles in the dispersion. Increasing the pH above 7 by adding NaOH, reduces the degree of delamination and the electroviscous effect, resulting again in a decrease in shear stress (Lagaly, 2006). This behaviour is probably unexpected since agglomeration of particles will increase shear stress, however, the electroviscous effect has to be taken into account in the delamination of the montmorillonite particles. The electroviscous effect is the interaction between the double layers of neighbouring particles affecting viscosity. This is more noticeable with high concentrations of suspensions (Schaller and Humphrey, 1966).

Figure 2-12. Shear stress (at a shear rate of 94.5 s-1) dependence on the pH value for the dispersion of Na+ -montmorillonite (•-) and (Na+, Ca2+) –bentonite (□--) (Lagaly, 2006).

Figure 2-13 illustrates the delamination of montmorillonite particles in suspensions into individual silicate layers or thin packets of layers when the exchangeable cations are alkali cations, preferentially Li+ and Na+ at low salt concentrations (< 0.2 mol/L for Na+ ions) (Lagaly, 2006).

Figure 2-13. Disarticulation of alkali smectite particles in aqueous dispersions (Lagaly, 2006).

The band-type network (face-face) may form three dimensional structures when the free energy of the system is at its lowest level (Rand and Melton, 1977), and this network in the Ca2+ -kaolinite has shown some elasticity in contrast to the more rigid house of cards structure (Lagaly, 2006). The edge-edge contact may occur under conditions of low ionic strength at the pH value of the isoelectric point of the edge surface of the kaolinite particle (Rand and Melton, 1977). When studying the aggregation of kaolinite particles, explanations of the modes of particle interaction can differ if the basal surfaces charges are considered negative or if they change with pH. By assuming that the silica and alumina basal planes have a fixed negative charge, the maximum yield stress for kaolinite suspensions is expected at the isoelectric point of pH < 3, but this yield stress value occurs for suspensions at pH 5 – 5.5. New findings about the dependence of the basal surface charges on pH (Gupta and Miller, 2010) give a better explanation for the correlation between the maximum shear-yield stress and the charges in the clay particles. It is reported that silica face-alumina face interaction is the main mode of aggregation at low pH, and this type of association increases the stacking of kaolinite layers promoting the edge-face and face-face associations with increasing pH. At pH 5-5.5 the maximum shear-yield stress value occurs and further increases in pH decrease the face-face and edge-face associations. When pH is high, all the surfaces of the kaolinite particles become negative and therefore dispersed in the suspension (Gupta et al., 2011).

For kaolinite it has been proposed that at low pH values when the edges of the particles are positively charged, electrostatic attraction is promoted between the edges and the negatively charges faces, forming a house of cards network with high apparent viscosity (Figure 2-14) (Rand and Melton, 1977; Ndlovu et al., 2011).

Figure 2-14. Interpretation of the effect of pH the mode of particles interaction and the Bingham yield stress of kaolinite suspensions (Rand and Melton, 1977; Ndlovu et al., 2011).

When charged particles are dispersed, ions with opposite charge to the particle surface accumulate closer to the particles to produce electro-neutrality, and this accumulation of ions is opposed by the tendency of ions to diffuse in the direction of decreasing concentration far from the particle. This results in a diffuse cloud of ions surrounding the particle or an electrical double layer. At low ion concentrations the diffuse double layers expand and at high ionic strengths the electrical double layer is compressed and coagulation is promoted due to reduction of the double layer repulsion. When the free energy of the system is at its lowest level, the particles are associated in a face-face structure producing card pack aggregates (Rand and Melton, 1977). Another proposed type of interaction is edge-edge coagulation of primary particles that may occur under conditions of low ionic strength at the pH value of the isoelectric point of the edge surface of the kaolinite particle (Rand and Melton, 1977).

Thixotropy, viscoelasticity and yield stress may be present in clay suspensions, and these properties differ with the chemical composition, size, and shape of the clay mineral in dispersion (Paineau et al., 2011b). For instance, the rheological behaviour of montmorillonite in water is highly pH and salt dependent (Brandenburg and Lagaly, 1988; Tombácz and Szekeres, 2004). Dispersions of bentonite can form a sol where particles form a stable colloidal dispersion, coagulated when destabilized by salts, flocculated (destabilized by polymers) or thickened forming a gel. When a stable structure is required gelation is desirable, but must be avoided when easy of flow is needed such as in the case of flotation (Abend and Lagaly, 2000).

Bentonite particles are quite basic in nature and that is reflected when preparing bentonite slurries that have pH values close to 10. Another well-known characteristic that swelling clay slurries display is time dependent flow behaviour, but it is unclear if the surface properties also display time dependent behaviour (Goh et al., 2010). Yield stress of pure bentonite slurries is shown at all pH values and at zeta potential of magnitude larger than 50 mV. In addition to this, it has been shown that bentonite dispersions exhibit higher yield stresses at low pH values with a minimum yield stress at around pH 7, and a gradual increase from pH 7 to 10 (Figure 2-15) (Goh et al., 2010). Bentonite suspensions display a significant yield stress at low concentrations due to the high swelling and particle aggregation of the fine clay particles producing a viscous gel-like structure, and that structure is a function of pH (Goh et al., 2010). In acid regime, the microstructure is formed by a face (-)/edge (+) interaction forming a house of cards network and in alkaline pH a band-like network structure is formed by cations mediated face (-)/face (-) interactions (Lagaly, 1989).

Figure 2-15. Yield stress-pH behaviour of bentonite clays dispersions over a reduction of pH from 10 to 2 (Goh et al., 2010).

The aggregation of clay mineral suspensions can be also related to sedimentation behaviour. For dispersed particles the sediment thickness increases with time to a maximum, and during settling of flocculated particles the sediment thickness decreases with time. These responses depend on electrolyte concentration and pH (Nasser and James, 2006). Some other factors affecting the sedimentation of a suspension are the density of the particles and medium, the gravity, the buoyancy, the network force by floc-floc contact, the drag acting at the interface between the wall of a container and the suspension, and the pressure difference between the top and the bottom of the suspension (Nakaishi et al., 2012).

2.1.4 Clay interactions

Clay minerals can interact with water, inorganic and organic compounds which in turn modify the rheology of the suspensions. Clay minerals with a 2:1 layer structure are more reactive than those with a 1:1 structure. Swelling clay minerals with a 2:1 layer structure, contain inorganic exchange cations such as Na+, Mg2+ and Ca2+ that are strongly hydrated in the presence of water, and when they are treated with some inorganic chemicals, different surfaces and rheological properties are developed (Paineau et al., 2011b); (Yildiz and Calimli, 2002). Kaolinite, a non-swelling clay mineral with a 1:1 structure, has low chemical reactivity and its anion exchange capacity is typically higher than its cation exchange capacity (Kau et al., 1998). The influence of organic compounds on clay mineral suspensions is more complex than inorganic compounds (Gungor, 2000). Organic

compounds may be adsorbed on the clay lattice by ion-dipole forces, van der Waals forces, or hydrogen bonding. They may also complex with counter ions of the clay, or if they are ionised, they may undergo cation or anion exchange with the original counter ions (Swartzen-Allen and Matijevic, 1974; Luckham and Rossi, 1999). On the surface of the particles organic compounds have an effect on the electrical double layer interactions and on the van der Waals interactions (Yalcin et al., 2002). Consequently, they alter the rheological behaviour of clay-water suspensions.

2.1.4.1 Clay-water interactions

The chemical and physical properties of clay minerals in water are linked to how water interacts with the clay surface. Common examples of these interactions are adsorptive, catalytic and cationic exchange reactions, and many of them are observable at the macroscopic level, including properties such as shrink-swell phenomena, water sorption, plasticity and catalysis (Schoonheydt and Johnston, 2006). Depending on the conditions of the aqueous environment, clay minerals can be present as single layers, particles or aggregates, and in an ideal dispersion individual layers are randomly oriented and constantly moving (Schoonheydt and Johnston, 2006). When clays are added to an aqueous solution, there is a gradual change in the structure of the water suspension as the clay becomes more abundant. The bulk properties of the suspension change as more of the water is associated with clays on surface layers, forming a slurry that becomes viscous in proportion to the amount of clay present. Rheograms in Figure 2-16 give an example of this situation by showing that at increasing concentrations of smectite, the shear stress or the viscosity increases (Vali and Bachmann, 1988; Velde, 1995). Small clay particles (< 2µm) stay in suspension for many hours due to thermal agitation (Brownian motion), and their flat shape, and tend to separate from the other minerals with the same grain size that do not have the same shape. The flat shape of clays keeps them from falling rapidly (Velde, 1995). This characteristic may be useful for the separation of clays before any further processing.

Figure 2-16. Shear rate vs. shear stress diagrams of Nevada smectite dispersed in deionized water at concentrations of 0.5, 1, 2, and 3% v/v (Vali and Bachmann, 1988).

The electrical charge and colloidal size of clay particles make them hydrate and interact, and kaolinite and smectites represent the two extremes of hydration and gel forming capacity potentials. Illites and chlorites are intermediate in these characteristics (Pusch, 2006). When considering clay- water interaction, smectites and vermiculates are the most important clay minerals because of their unique expansive nature (Schoonheydt and Johnston, 2006). The initial sorption in these clay minerals is influenced mostly by the hydration of exchangeable cations, and it has been shown that the properties of sorbed water are different from those of bulk water, particularly when less than three layer of water are present in the interlayer region. Adsorption of water starts with solvation of the exchangeable cations, followed by the occupancy of the remaining interlayer space. The degree of hydration changes with water vapour pressure, and water content (Schoonheydt and Johnston, 2006).

The basal oxygens, in the siloxane surface of the tetrahedral sheet, are weak electron donors (Lewis bases). As a result of this, 2:1 layer silicates that do not have isomorphous substitution in their structure, such as talc and pyrophyllite, are essentially hydrophobic. In these clay minerals the water molecules interact more with each other than with the surface. However, the basal alumina surface

of 1:1 phyllosilicates, such as kaolinite, can form hydrogen bonds with water molecules (Theng, 2012b). On the other hand, 2:1 phyllosilicates with a negative surface charge (i.e., montmorillonite) are hydrophilic due to the presence of charge-balancing inorganic cations (Theng, 2012b). These types of clay minerals have appreciable enthalpies of hydration.

The thickness of the particles is influenced by the type of exchangeable cations so divalent and polyvalent cations lead to large particles. For instance, Ca2+ -montmorillonite is often considered to consist of approximately 10 layers per particle compared to Na+ -montmorillonite with 3-5 layers or less. The interlayer distance in Ca2+ -montmorillonite is much smaller than for Na+ - montmorillonite implying that the Ca2+ clay has less interlayer water at low and medium bulk densities (Pusch, 2006).

Swelling after complete interlayer saturation is associated with double-layer interactions between external surfaces of particles, and it is an osmotic phenomenon. The extension of the double-layers is less in saline water than in fresh water conditions, meaning that the clay mineral expands to a larger volume in fresh water.

2.1.4.2 Clay interactions with inorganic compounds

In a flotation cell clay minerals may be in contact with some inorganic reagents, and other minerals. Ca2+ and Na+ cations from lime and soda ash, can modify the rheology of clays such as bentonite or kaolinite, and the same can happen with other minerals present in the ore (i.e., amorphous hydrated iron oxides). This section introduces some of these interactions.

2.1.4.2.1 Interactions with cations pH modifiers are probably the main source of inorganic cations in flotation, and some of these are lime (Ca(OH)2), sodium hydroxide (NaOH), and soda ash (Na2CO3), however, swelling clay minerals already contain some inorganic exchange cations such as Na+, Mg2+ and Ca2+ that are strongly hydrated in the presence of water, and when these clays are treated with some inorganic chemicals, different surface and rheological properties are developed (Yildiz and Calimli, 2002; Paineau et al., 2011b). One example of this is the interaction of bentonite with sodium carbonate

(Na2CO3), which is essentially an ion exchange reaction where the ions in the bentonite are replaced by alkali ions. This reaction creates an active bentonite with a high swelling capacity and

increased plasticity. If bentonite is treated with acid, the exchangeable ions are replaced by H- ions altering the structure by leaching Al3+, Fe2+ and Mg2+ ions increasing the specific surface area and porosity. Structural changes in the bentonite will depend on the exchangeable cations, clay type, and the ratio of the additive (Yildiz and Calimli, 2002). Exchangeable cations in clays are different to the ions added through reagents, but they are important when looking at the magnitude of clay swelling. The activity order for the exchangeable cations among all the monovalent and divalent series is (Gridi-Bennadji et al., 2012):

Mg2+>Ca2+>Sr2+Ba2+>Cs+>Rb+>Na+>Li+>K+

This means that smectites with divalent interlayer cations exhibit less swelling than those monovalent cations (Gridi-Bennadji et al., 2012).

If lime is added to a bentonite suspension, the Ca2+ cations cause a reduction or complete elimination of swelling potential. This is attributed to the substitution of the clay cations by Ca2+ and subsequent formation of calcium silicate and aluminate hydrates. The reduction in swelling results from the decreased affinity for water of the Ca-saturated clay (Abdi and Wild, 1993).

For kaolinite, a non-swelling clay, the response is different when adding sodium carbonate. This chemical acts as a dispersant by producing or increasing the repulsive potential between edges and faces. Alternatively, if Ca2+ cations are added to a kaolinite suspension, the dispersion is destabilized (Lagaly, 1989). The principal product of the interaction between the calcium cations and kaolinite is a calcium aluminate silicate hydrate gel and it is suggested that the clay plates are attacked particularly at the edges and the gel envelops the surface of the plates. This transforms the electrical double layer at the surface of the clay plates (Wild et al., 1993).

These two examples show that the response to different cations depends on the type of clays in suspension.

2.1.4.2.2 Clays interaction with other minerals

Clays can interact with other minerals and one good example of this is the interaction of clays and colloidal oxides. In general the colloidal behaviour of clay-oxide dispersions is complicated, and the dispersed oxides usually show a pzc (point of zero charge) at a particular pH. The particles are positively charged below this pH value and negatively charge above that value. This means that the

colloidal stability decreases with increasing pH to a minimum at the pzc., where spontaneous coagulation occurs, and increases again after that point (Figure 2-17) (Lagaly, 2006).

If aluminium and iron oxide particles are precipitated in the presence of kaolinite, alternating stabilisation and re-stabilisation are observed as a function of pH. Kaolinite and hydrated iron oxides can have isoelectrical points at pH >8. This means that there is a large range of pH values where heterocoagulation between positively charged iron oxides and negative faces of kaolinite particles can occur. The extent of these interactions depends mostly on the pH conditions at which the iron oxide and kaolinite are brought together (Lagaly, 1989). The coagulation process and the type of coagulate depends on the mass ratio between the oxide and the clay mineral (Lagaly, 2006).

The following are some effects that have to be considered in clay-oxide interactions (Lagaly, 2006): 29

1) Multivalent cations added into the solution not only act as coagulating species but also adsorb on the surface of clay mineral particles, changing the surface charge density (Lagaly, 2006). 2) In mixed oxide dispersion the more soluble oxide can cover the surface of other particles

forming core shell particles. A good example of this is for the mixture of alumina (Al2O3

pzc ~ 8.8) and titania (TiO2 pzc ~ 6) particles. These dispersions become unstable between the pzc values of the components (---- line in Figure 2-17). After coagulation, all particles of the mixed dispersion behaved like alumina particles because of the formation of alumina

shells around the TiO2 particles. Likewise, the surface structure of clay minerals could be changed by the deposition of aluminium oxide species (Lagaly, 2006). 3) The relative size of the different particles is an important factor to consider. The smaller particles can be coagulated or attached to the surface of the larger particles and change the colloidal behaviour of the larger particles. The adsorption of gibbsite particles on the edges of kaolinite and the basal surface of montmorillonite has been observed. Similar findings were reported for iron oxides and kaolinite, and bridging of iron oxide aggregates by montmorillonite particles (Lagaly, 2006). 4) When oxides are precipitated in the presence of colloidal particles, complexes between metal ions and OH- can result at the surface of the particles under conditions that do not cause metal hydroxide precipitation in homogenous solutions. Dissolution and surface precipitation in dispersions of clay minerals and oxide can therefore strongly change the surface structure of the clay particles (Lagaly, 2006). 5) When particles have surfaces with distinctly different charge densities with the same sign, the repulsion can change into attraction. This interesting effect promotes the mixing of differently charged particles during re-aggregation (Lagaly, 2006). 6) As mentioned previously, the properties of a clay-oxide dispersion can be strongly influenced by the way the particles were mixed together. A study found that for a 9% Na+ - kaolinite dispersion a high yield stress value was obtained at pH ~ 3 (Figure 2-18). When adding ferrihydrite at pH 3, the hydroxide was preferentially absorbed on the faces of the kaolinite particles and re-charge them (edge (+)/face (+)) causing the yield stress to disappear. The edges of the kaolinite particles become negative with increasing pH, and edge(-)/face(+) contacts formed a network of particles with a maximum of yield stress at pH ~ 7. The initial re-charging of the ferrihydrite reduced the positive face charge density, causing the network to disappear and the yield stress approached zero at pH ~ 10. On the

other hand, if the ferrihydrate was added at pH 9.5, all particles were negatively charged and a yield stress was not measured. By decreasing the pH the positive charge of the ferrihydrite increased causing the ferrihydrite particles to bridge the negative kaolinite particles increasing the yield stress to a sharp and high maximum. The very high positive charge density of the hydroxide at lower pH promoted adsorption of ferrihydrite on the basal plane surfaces of the kaolinite particles, and the kaolinite (-)/ferrihydrite(+)/kaolinite(-) network collapsed as shown by the strong decrease in yield stress (Lagaly, 2006).

7) Oxides, mainly in an amorphous form, can reduce the mechanical swelling of clay minerals. This may be attributed to cementation of the clay mineral particles by shells of oxides and penetration of poly-hydroxo metal ions (e.g. poly-hydroxo aluminium) and poly-hydroxo iron ions between the layers.

Some gangue minerals present in ores could interact with clay minerals, and this interaction could be based on the gangue solubility in water. For instance, calcite, dolomite, and gypsum can release some ions that interact with clay minerals. In general the solubility of these minerals in water is very low (Table 2-4), with gypsum having the highest solubility. Gypsum or calcium sulphate can occur in different forms depending on the level of hydration. It is expected that the gypsum present in ores is in the form of calcium sulphate dihydrate.

Table 2-4. Solubility of some of the gangue minerals present in a problematic ore (approximate values form different sources) Gangue mineral Solubility (g/l) at 20 °C (approximate values) Quartz 0.003 g/l

Calcite (CaCO3) 0.013 g/l

Dolomite (CaMg)(CO3)2 Poorly soluble. Some suggest less than pure calcite Gypsum (CaSO4)

Dihydrate (CaSO4.2H2O) 2.4 g/l Hemihydrate

(CaSO4.0.5H2O) 6.5 g/l

In mineral processing there is no information about the interaction of gypsum with clay minerals in flotation, however, this interaction is well known in the agricultural industry where gypsum is used to agglomerate clay particles with the Ca2+ cations released (Fisher, 2011).

Some previous research has been done regarding the high base consumption to increase the pH of flotation slurries with gypsum content (DiFeo et al., 2004). This is due to the activity of the cations released by the gypsum.

It seems that the study of the gypsum-clay mineral interaction in flotation is a novelty and for this reason it is included in this thesis.

2.1.4.3 Clay interactions with organic compounds

Collectors and frothers are organic compounds, but there is no specific information in the literature about the interaction of clay mineral with these reagents in flotation. However, there are many studies related to the interaction of clay with other organic compounds. This knowledge may be transferrable and this section presents some fundamental information about these type of interactions. 32

Collector molecules can be ionising or non-ionising compounds and it is reasonable to suspect that at least the anionic or cationic collectors can bond to some clay minerals. If this happens, clay minerals may consume some of the collector added to the slurry and could be transported to the froth. A similar situation may occur with frothers.

It is fairly well understood that uncharged polar organic molecules are largely adsorbed by interactions of their functional groups with the inorganic counterions at the silicate surface, and that the positively charged organic species are primarily taken up by cation exchange with the counterions (Theng, 2012b).

Figure 2-19 shows the different ways clay minerals can react with different types of organic compounds. Intercalation can occur in kaolin species (kaolinite, nacrite, and dickite), and in this way of interaction, the organic molecules penetrate into the interlayer space of clay minerals, and intercalated guest molecules can be displaced by other suitable molecules. By means of this interaction, kaolin species can adsorb particular types of neutral organic compounds (Lagaly et al., 2006).

Figure 2-19. Interlayer reactions of 1:1 and 2:1 clay minerals (Lagaly et al., 2006).

2:1 clay minerals have a broader diversity of reactions, and some examples are the displacement of interlayer water in smectites and vermiculites by many polar organic molecules, and the exchange of interlayer cations by various types of organic cations (Lagaly et al., 2006).

2.1.4.3.1 Intercalation reactions of kaolin species

Kaolin species are the only 1:1 clay minerals that intercalate various organic molecules. Serpentines are not reactive. Since kaolin particles are held together by hydrogen bonds, dipole-dipole interactions, and van der Waals forces, the organic compounds that are directly intercalated are divided into three groups (Lagaly et al., 2006):

• Compounds that form hydrogen bonds (e.g. hydrazine, urea, and formamide); • Compound with high-dipole moments like dimethyl sulphoxide, pyridine-N-oxide; • Potassium, rubidium, caesium, and ammonium salts of short-chain fatty acids (acetates, propionate, butyrates, and isovalerates).

Intercalation of kaolinite may not occur in a flotation environment since this reaction requires high concentration of the organic compound, and often temperature in the 60-80 oC range. In addition to this, intercalation is a slow process that often requires several days. The type of the kaolinite and the particles size also can affect the reaction rate (Lagaly et al., 2006).

2.1.4.3.2 Reactions of 2:1 clay minerals

Smectites can adsorb neutral molecules by various chemical interactions: hydrogen bonds, ion- dipole interaction, co-ordination bonds, acid-base reactions, charge transfer, and van der Waals forces. Polar molecules (i.e., alcohols, amines, amides, ketones, aldehydes and nitriles) form intercalation complexes with smectites, and even acid can intercalate. When intercalation occurs in solutions, solvent molecules are generally co-adsorbed in the interlayer space (Lagaly et al., 2006). Water molecules linked to hard cations such as Na+, Mg2+, and Ca2+ are displaced only by HO- or O= containing compounds, but not amines. Amines as soft bases can displace water molecules from soft interlayer cations such as Cu2+ and Zn2+ (Lagaly et al., 2006).

According to the literature many organic compounds can interact with clay minerals, especially those with 2:1 structure. Surfactants are the compounds of interest in this research. One study found that the clay mineral structure and surfactant nature influence the adsorption capacity of surfactants by clays (Sanchez-Martin et al., 2008). In that work the clay minerals montmorillonite, illite, kaolinite, muscovite, sepiolite, and palygorskite adsorbed three types of surfactants (non- ionic, anionic and cationic) to a greater or lesser extent. These surfactants adsorbed differently

depending on if they had charge or not, and the highest adsorption was obtained for the cationic surfactant, except in the case of kaolinite and sepiolite which have a lower CEC (Cation Exchange Capacity) (Table 2-5) (Sanchez-Martin et al., 2008).

Clay CEC OC (%) Specific surface d (001) mineral (cmol/kg) (m2/g) (Å)

Montmorillonite 82 0.06 750a 13.4 Illite 15 1.74 57 10.0 Kaolinite 6.1 0.10 12 7.16 Muscovite 21 0.10 105a 10.0 (12.6)b Sepiolite 5.0 0.08 189 12.3 Palygorskite 27 0.46 254 10.6

a Total superficial area b Impurities of montmorillonite

The greatest adsorption of non-ionic surfactants was observed on montmorillonite and illite, and the highest anionic surfactant adsorption was on kaolinite and sepiolite (Sanchez-Martin et al., 2008). This information may be useful when choosing a collector or frother for the flotation of an ore with a high clay content since interactions can occur in different clays systems.

2.1.4.3.3 Effect of organic compounds on rheology

As already discussed montmorillonite may have many interactions with organic compounds, and one study shows that cationic and anionic surfactants can modify the rheology of this type of clays (Gungor, 2000).

Flocculation with cationic polymers occurs by simple electrostatic attraction given by the adsorption on negatively charged surface of the Na-bentonite. For anionic polymers it has been proposed that adsorption and flocculation occur via hydrogen bonding between the solid surfaces and the hydroxyl groups on the polymer (Gungor, 2000).

Figure 2-20 shows the yield stress values for the Na-bentonite as a function of the concentration of distearly dimethyl ammonium chloride (DDAC) cationic surfactant, and linear alkyl benzene sulfonate (LABS) anionic surfactant. According to the author of this study, the DDAC surfactant

was adsorbed on the negative faces only at low pH values (pH ~2.4). This reduced the charge, and decreased the heteropolar (face-edge) interactions, which lowered the yield stress values (Gungor, 2000). The homopolar interaction, face-face, decreased at low DDAC concentrations; while at higher DDAC concentrations, the particles are held together by the interpenetrating alkyl chains and the shear stress increased.

Figure 2-20. Yield value of the sodium bentonite slurries (6% w/w) versus the amount of cationic surfactant (DDAC) and anionic surfactant (LABS) (Gungor, 2000).

At pH 9.5 the flow values change with the DDAC concentration, therefore the behaviour is -4 governed≈ by the electroviscous effect. At surfactant concentrations of more than 10 mole/L the particles settle as flocs because the exchange of the counterions by DDAC cations makes the particles less hydrophilic (Gungor, 2000).

The anions of the ionic surfactant (LABS) are not adsorbed by the clay particles in very dilute solutions and do not influence the flow behaviour of the acidic dispersions at concentrations less than 10-5 M (pH 2.4). At LABS concentration of more than 10-3 M, the anions break the edge- face contacts and ≈the yield stress values decrease. At pH 9.5, the stresses are constant up to about 5 X 10-4 M and then increase (Gungor, 2000).Figure 2-21 shows the rheograms for Na-bentonite suspensions (at pH 9.5) without surfactants, and with DDAC and LABS at 10-3 M concentration.

It is seen in the graph≈ that the suspension without surfactant and with the cationic surfactant follow the Bingham plastic model, but there is an increase in yield stress for the suspension with DDAC. The addition of the anionic surfactant causes the suspension to follow Newtonian flow (Gungor, 2000). 36

Figure 2-21. Rheograms for sodium bentonite without surfactant, with cationic surfactant (DDAC) and with anionic surfactant (LABS) (Gungor, 2000).

2.2 Effect of clay minerals on flotation

It has been demonstrated in some studies and in industrial practice that clay minerals can dramatically affect both grade and recovery in mineral flotation, with a number of different mechanisms for the interactions of the clay minerals being suggested. The most common mechanisms discussed are high viscosity, and slime coating (Jorjani et al., 2011; Wei et al., 2013; Forbes et al., 2014) (Arnold and Aplan, 1986; Wang et al., 2013), but it is still not clear which one of these mechanisms is more critical for influencing flotation performance. Patra et al. (2012a) used positively charged chrysotile particles and inert nylon fibers to differentiate between the effect of slime coating and pulp rheology on the flotation response of copper and suggested that slime coatings may not be a dominant factor when compared to pulp rheology. Tabatabaei et al. (2014) stated that the presence of non-sulphide gangue minerals such as carbonaceous material and clay minerals negatively affect the recovery of sulphides and the separation selectivity. These researchers concluded that there was no adequate evidence to show that these flotation problems were caused by the slime coating of gangue minerals on sulphides or reagent starvation (Connelly, 2011) by fine gangue particles. They concluded that it was more realistic to suggest that multiple processes contribute to the flotation performance of sulphides.

Given the main characteristics of clay minerals presented in this literature review, it is reasonable to suggest that high viscosity and high particle entrainment are among the main contributing factors affecting flotation performance, and this thesis focuses on these factors.

2.2.1 Effect of pulp rheological behaviour on flotation

The rheological behaviour of mineral slurries depends on the level of inter-particle interaction or aggregation (Farrokhpay, 2012), and as it has been discussed in this review, clay minerals can interact to form different structures or networks. A number of studies have been performed to understand mineral interactions when low grade and complex ores are processed. It has been found that in many cases non-Newtonian nature of the slurries becomes a common flow property during the processing of these ores while the slurries of typical ores (or the high grade ores) exhibit the Newtonian nature (Ferrini et al., 1979; Klein et al., 1995; Shi and Napier-Munn, 1996a; Prestidge, 1997a; Prestidge, 1997b; Boger, 2000; Akroyd and Nguyen, 2003; Klein and Pawlik, 2005; Boger, 2009; Das et al., 2011; Ndlovu et al., 2011; Nosrati et al., 2011; Nosrati et al., 2012).

It is clear that most of the clay minerals can increase the viscosity of flotation pulps producing non- Newtonian behaviour of the slurries, but until recently there have been no formal or peer reviewed studies correlating this with mineral flotation performance. A recent study by Zhang and Peng (2015) showed that bentonite increased the viscosity of a copper-gold ore slurry and decreased copper and gold flotation recovery, while kaolinite affected the viscosity slightly with simultaneous high gangue entrainment. The decrease in mineral flotation recovery by bentonite may be via the modification of hydrodynamic conditions inside flotation cells, and this is related to fluid flow which is mostly driven by the action of the impeller.

A study (Patra et al., 2012a) suggested that in the flotation of Ni ores containing high amounts of fibrous serpentines (chrysotile), pulp rheological behaviour negatively affected flotation. The yield stress value of the copper ore pulp without any fibers was 5 Pa, and adding either chrysotile or nylon fibers caused the yield stress values to increase above 50 Pa (Patra et al., 2012a). A decrease in copper recovery was also observed upon increasing the concentration of the fibers. Researches in this study proposed that high pulp viscosity resulted from the entanglement of fibrous minerals and network formation (Figure 2-22). This was independent of the surface chemistry of the minerals and was only related to the physical aspects of the minerals, in that case fibrous morphology. It was also suggested that the pulp rheology behaviour affected the flotation beneficiation by impacting the

efficacies of the various sub-processes in flotation (i.e., collision, attachment and detachment in the pulp) (Patra et al., 2012a).

Figure 2-22. SEM micrograph of pulp sample (Patra et al., 2012a).

It was also found that pulp viscosity influenced froth stability and an increase in the pulp yield stress caused the pulp height, texture and consistency to change. For the cases where fibers were added there was a patchy froth and absence of froth and this was likely due to the poor bubble dispersion, and hindered percolation of the bubbles through the pulp (Patra et al., 2012a).

The negative effect of high pulp viscosity on flotation is supported by other researchers (Wills and Napier-Munn, 2006; Ralston et al., 2007; Bakker et al., 2010). Viscosity affects the energy of dissipation profile through the cell and bubble rise velocity directly affecting the collision frequency and producing a low overall flotation rate (Ralston et al., 2007; Schubert, 2008; Xu et al., 2011), and changes gas dispersion parameters such as bubble size, gas hold-up, superficial gas velocity and bubble surface area flux (Shabalala et al., 2011). High viscosity could also reduce attachment efficiency in the flotation pulp phase (Wills and Napier-Munn, 2006; Schubert, 2008; Farrokhpay, 2012). At high yield stresses, a cavern of yielded fluid may form around the impeller and the fluid surrounding this cavern remains stagnant affecting the hydrodynamics in the flotation cell (Figure 2-23) (Bakker et al., 2010) decreasing the bubble size and gas hold-up. When a cavern exists, small bubbles are formed in the impeller zone, but the dispersion of these bubbles is poor in the cell (Shabalala et al., 2011). This implies that shear rates in a flotation cell vary from high to low values depending on the proximity of the slurry to the flotation cell impeller and the rheology of the slurry (Bakker et al., 2009a). This causes the low clay content ore mixtures which are non-Newtonian, to have different shear stresses and viscosities at different locations in the flotation cell. It has been demonstrated that there is a considerable variation of the shear rate distribution inside the flotation cell with the highest values closer to the impeller (Bakker et al., 2009b). The other areas in the

flotation cell may have relatively small shear rate values that can be even close to zero as show in Figure 2-24 (Anderson, 2008). This may not be the shear rate distribution of the flotation cell used in this project, but it gives an idea of how apparent viscosity values can be distributed in the flotation cell when working with non-Newtonian slurries.

Figure 2-23. Contours of velocity magnitude showing the formation of a cavern in a 60 wt.% Bindura nickel slurry at 450 rpm (Bakker et al., 2010).

Figure 2-24. Schematic of Bateman 120 litre flotation cell showing the shear rate distribution when operating at 400 rpm (Re=1.5x105) (Anderson, 2008).

The factors influencing hydrodynamics are flotation cell characteristics, impeller properties, and slurry properties such as density and rheology (Shabalala et al., 2011). In this thesis the only factor

that is changed is the slurry rheology by adding clay minerals. This increases pulp viscosity and may cause a high yield stress and apparent viscosities in the flotation cell.

2.2.2 Entrainment of clay minerals

It is known that entrainment increases with a decrease in particles size (Savassi et al., 1998; Wang and Peng, 2013) and mechanical entrainment is significant for particles under 30 µm (Trahar, 1981; Smith and Warren, 1989b; Liu and Peng, 2014). The size of clay minerals is much smaller than 30 µm, and it is usual to find clay particles of less than 2 µm in the flotation of coal, oil-sands and base metal minerals regardless of the grinding procedures (Tu et al., 2005; Liu and Peng, 2014). Given this small size of clay minerals in ores, it is highly likely that they will be transported to the froth via entrainment and have a negative effect on the concentrate grade.

Wang and Peng (2014) studied the negative effect of clay minerals in flotation through the transport of these particles to the concentrate. They found that the degree of entrainment of clay particles was high and also affected by the presence of electrolytes in the process water. The high particle entrainment in flotation in the presence of kaolinite was attributed to the formation of aggregates which entered flotation concentrates and then enhanced froth stability (Wang and Peng, 2014). Electrolytes in saline water contributed to the formation of aggregates that were entrained, but when the clay content was too high, entrainment and true flotation were affected due to high pulp viscosity which limited bubble and particle mobility. In that case, edge-edge (E-E) network structures were enhanced (Wang and Peng, 2014). Other factor affecting the entrainment of fine gangue minerals is the presence of electrolytes since ions can inhibit bubble coalescence (Craig et al., 1993; Henry and Craig, 2008) making a more stable froth (Liu and Peng, 2014). This was confirmed by Wang and Peng (2013).

The entrainment of clay minerals could be reduced by producing clay aggregates as it has been shown that the entrainment of fine and ultrafine particles decreases when using depressants or polymers to coagulate the fine gangue minerals (Cao and Liu, 2006; Liu et al., 2006). Given the properties of clay minerals, it would be expected that their entrainment in flotation would be dependent on the type of network structures or aggregates formed.

It is useful to highlight the entrainment mechanisms and their role in clay minerals transport to the concentrate. In entrainment there is not direct particle attachment to the bubbles since it is not a chemically selective process (Wang et al., 2015). Entrainment occurs in the inter-bubble water 41

(Smith and Warren, 1989b; Zheng et al., 2006; Wang and Peng, 2013; Wang et al., 2015) and the mechanisms are related to the state of the suspended solids in the water or the water film in the bubbles, and the drainage in the froth phase and water recovery (Wang et al., 2015).

Three mechanisms are proposed to explain the transport of minerals by entrainment across the pulp/froth interface; Boundary Layer Theory, Bubble Wake Theory and Bubble Swarm Theory (Wang et al., 2015). Figure 2-25 illustrates the first two mechanisms. In the Boundary Layer Theory mineral particles are entrained in the bubble lamella or the thin hydrodynamic layer of water surrounding the bubble (Gaudin, 1957; Wang et al., 2015), and in the Bubble Wake Theory water and mineral particles are transported in the wake of an ascending bubble (Smith, 1984; Wang et al., 2015). Clay minerals could be easily transported by these two mechanisms, but this should occur more easily for dispersed clay mineral particles. Clay agglomerates may be too big to be transported in the thin water layer surrounding the bubble or in the bubble wake.

Figure 2-25. Schematic of an ascending bubble, its flow lines and attached wake representing the Boundary Layer Theory and Bubble Wake Theory (Smith and Warren, 1989a).

The Bubble Swarm Theory (Figure 2-26) proposes that water and suspended mineral particles are transported to the froth by the congested arrangement of bubbles just below the forth/pulp interface (Figure 2-26b). There is some drainage of water and suspended solids through the bubble swarm, but other water and solids are squeezed upwards by the rising bubble swarm. This action pushes up the layers of bubbles on top the bubble swarm forming another layer of bubbles (Figure 2-26c). As result of this, more solids suspended in the water are pushed up to the froth (Wang et al., 2015). This mechanism seems to be more suitable for transportation of clay agglomerates given the lift provided by the buoyancy of the rising bubble swarm.

By looking at these three entrainment mechanisms, it would be reasonable to assume that entrainment of clay minerals depends on the dominant mechanism and the type of clay particle associations. That is, it would be expected that for dispersed clay particles entrainment would be significant at the bubble wake and lamella, but probably low by bubble swarm since dispersed clay particles could drain easily. On the other hand, if clay particles are agglomerated, most of the entrainment should occur by the bubble swarm mechanism.

Figure 2-26. Schematic representing the Bubble Swarm Theory. Water and dispersed solids are trapped by rising layers of bubbles (Smith and Warren, 1989b).

Some water including the suspended solids flows back from the froth phase to the pulp phase, and three mechanisms have been proposed for this transportation (Wang et al., 2015): the drainage through the Plateau borders, collapse of froth causing rapid flow of water and solids downwards in

the local area of froth and sedimentation produced by shear activity (Cutting, 1989; Wang et al., 2015).

The net flow of water including the suspended solids upwards gives the amount of material entrained to the concentrate, and the Plateau borders in the froth, formed by the thin water films or lamellae (Figure 2-27), are important for entrainment since they contain most of the liquid in the froth providing the drainage channels for entrained solids (Wang et al., 2015).

Similar to the entrainment of clay minerals, it is expected that the transfer of clay minerals back to the pulp depends on the way the clay particles are associated. For instance, dispersed clay particles will drain more easily through the Plateau borders than clay agglomerates.

Figure 2-27. Sketch of a Plateau border and it associated lamella (Ross and Van Deventer, 1988)

Since the main focus of this thesis is on the effect of pulp rheology on flotation performance, it was assumed that entrainment only occurred through bubble lamella.

2.3 Literature review conclusions

The literature review covers many aspects of clay minerals, and the overall conclusions relevant to mineral processing can be divided as follows:

Main characteristics of clay minerals:

• The small size of clay minerals, in the colloidal range, provides a great surface area for interactions with reagents, and other minerals. • In addition to their colloidal sizes, the surface charges on clay minerals contribute to their interactions. • The particle size and shape and electrostatic charge on clay minerals contribute to formation of different types of aggregates changing the rheological behaviour of clay slurries. • There are many types of clay minerals and it is likely that some may have a greater impact on flotation.

Interactions of clay minerals relevant to mineral flotation:

1) Inorganic reagents, and anionic and non-ionic organic compounds can react with clay

minerals. It is expected that pH modifiers, such as NaOH, Ca(OH)2 and Na2CO3, and collectors and frothers will have interactions with clay minerals in flotation, and these interactions could change rheology in the slurry.

2) It has been shown that gangue minerals such as fibrous serpentines have a negative impact on the flotation of copper by affecting pulp rheology through network structures. Clay minerals may have similar impact on flotation when network structures are formed in the slurry. In addition to this, clay minerals can affect flotation by entrainment and this may be highly dependent on the type of network or aggregates formed.

3) Some gangue minerals can interact with clay minerals. For instance, gypsum is a source of Ca2+ cations, and the amount of ions released by gypsum can be sufficient to cause changes in clay minerals network structures or aggregates affecting flotation outcome. Something similar may happen with ions from pH modifiers.

The next section shows the research gaps and hypotheses on which this thesis is focused.

2.4 Research gaps and hypotheses

This thesis focuses on the most significant clay interactions that can affect copper-gold flotation. The observations in industrial practice, presented in the introduction section, and the characteristics of the clay minerals suggest that clay mineral network structures or aggregates play an important role in flotation performance. This association of clay minerals can be influenced by presence of water, chemicals, ions and pH condition encountered in flotation. The next figure shows the literature review findings and the corresponding research gaps in the flotation of high clay content ores.

Figure 2-28. Main literature review findings and research gaps.

Based on the gaps identified, three hypotheses have been formulated

1) Kaolinite and bentonite affect the rheological behaviour of copper-gold flotation slurries, and the influence of bentonite, a swelling clay, is stronger. This is due to the swelling behaviour of bentonite and the formation of the house of cards network structure which can be modified by pH modifiers or other minerals.

2) Kaolinite and bentonite affect grade/recovery in the flotation of gold and copper, due to the formation of network structures in the flotation pulp and particle entrainment. This is dependent on clay type, and pH modifiers.

3) Gypsum can interact with clay minerals by releasing cations (i.e. Ca2+) that modify network structures or aggregates, changing viscosity and impacting flotation outcomes.

2.5 Research plan

Figure 2-29 shows the research questions and the research plan to test the hypotheses. The structure of the project was divided in four sections that complement each other. Single clay minerals and an ore with a low clay content, or easy to process ores, were used to carry out experimental work. A problematic ore with a high clay content, Carlin ore, was taken as a reference for rheology.

Figure 2-29. Research questions and research plan to demonstrate the hypotheses.

As shown in Figure 2-29, the aims of the four different sections were:

1) To study the rheological changes in slurries made of single clay minerals in the presence of some flotation reagents.

2) To investigate the effect of a selection of calcium bearing minerals, such as gypsum, on the rheological behaviour of clay mineral slurries.

3) To investigate the interaction of kaolinite and bentonite with gypsum and its effect on the flotation of a copper-gold ore.

4) To investigate the effect of pH modifiers, lime and soda ash, on the flotation of copper-gold mixtures with kaolinite and bentonite.

These research topics provide an understanding of some of the mechanisms by which clay minerals affect flotation of copper-gold ores, and should be transferable to the separation of other minerals from high clay content ores.

Rheology measurements were fundamental to all of these sections and Figure 2-30 shows some of the variables affecting rheology that were investigated. This figure also shows the ways clay minerals can affect flotation, the variables that were manipulated in this research, and the variables that might affect flotation outcome but were kept constant during experimentation. Similarly the possible interactions of clay minerals in flotation are shown, and the interactions were studied.

Figure 2-30. Schematic of the scope of the thesis

Figure 2-31 shows the experimental plan that was followed where rheology experiments were a key part complemented with the study of the network structures and flotation experiments. The mineralogical composition of a high clay content copper-gold ore gave important information about gangue minerals that can potentially interact with clay minerals. This ore showed high pulp viscosities and it was known to be problematic in flotation. The first step was to study the rheology of slurries made of single minerals (clay and calcium bearing minerals) to understand the behaviour of swelling and non-swelling clay minerals in the presence of some flotation reagents and calcium bearing minerals. Similar rheology measurements were done for a low clay content copper-gold ore and its mixtures with clay minerals and clay minerals-gypsum. Flotation results of these mixtures were used to find the effect of clay mineral network structures or aggregates. Cryo-SEM imaging was used to look at the clay particle associations and some settling and gel point tests complemented this.

Figure 2-31. Schematic of the experimental plan.

2.6 Thesis outline

Figure 2-32 shows the structure of this thesis that is divided into eight chapters. This is the result of the research gaps from the literature review. The sequence of the chapters follows a systematic approach to tackle those gaps and to understand some of the interactions of clay minerals in flotation.

Figure 2-32. Structure of the thesis

The goal of this chapter, Chapter 2, was to review the literature on clay minerals that is relevant to flotation. This led to the research questions and research plan as discussed previously.

Chapter 3 shows the way experiments were conducted. Rheology measurements were a central part in all the tests, and they were complemented with other useful measurements such as settling tests. Other important measurements were solubility tests, and Cryo-SEM imaging. Information from these tests was used to explain the outcome of flotation experiments.

Chapters 4 to 8 explain the main research outcomes of this thesis. Chapter 4 describes the rheology of single clay minerals, kaolinite and bentonite, at different pH values adjusted with lime, soda ash and sodium hydroxide. A collector and a frother were also studied. These pH modifiers were 51

selected because they add ions that not only increase pH such as OH- from lime, but also contribute to increase the viscosity of the slurries (Ca2+ and Na+). The collector tested was Potassium Amyl Xanthate (PAX), which is the strongest xanthate collector, and the frother was Cytec Interfroth 6500, an alkyl aril ester. It was expected that the effect of pH modifiers was stronger on the rheology of clay mineral slurries than the effect of the collector and frother due to the higher ionic strength of pH modifiers and their contribution to increasing pH. This was confirmed and the results were published in Minerals Engineering in 2013 (Cruz et al., 2013).

Chapter 5 is a consequence of the findings in Chapter 4 and the observations from an ore with a high clay content and difficult to process, Carlin ore. This ore contains some calcium bearing minerals such as gypsum, calcite and dolomite and its slurries can give very high viscosity values. This observation, along with literature review information and the findings from the first experiments on rheology in Chapter 4 hinted that some calcium bearing minerals such as gypsum could release Ca2+ cations in enough quantities to change rheological behaviour of clay mineral suspensions. For this reason it was decided to conduct rheology tests on baseline clay mineral slurries made of kaolinite, bentonite or illite and slurries with these same clay minerals mixed with calcium bearing minerals. Lime, soda ash and sodium hydroxide were used as pH modifiers to observe the effect of pH. Kaolinite has a 1:1 layer structure and it is a non-swelling clay while both the bentonite and illite have a 2:1 layer structure, but only bentonite swells in the presence of water. It was decided to include illite in this study because it is present in the problematic ore, and despite being a 2:1 clay mineral, it is not a swelling clay. The main goal was to prove that gypsum released enough quantities of Ca2+ cations to change rheological behaviour of the clay mineral slurries. The findings of this study are presented in a paper published in the International Journal of Mineral Processing in 2014 (Cruz et al., 2015a).

Chapter 6 is the result of the observations about gypsum in Chapter 5, and it includes some flotation experiments that were planned to determine the effect of gypsum on the flotation of a copper-gold ore mixed with either kaolinite or bentonite. This section comprises rheology measurements, Cryo- SEM imaging and flotation results. Cryo-SEM imaging was crucial to look at the clay particle associations in the flotation cell with and without the presence of gypsum. It was hypothesized that the flotation outcome was affected according to the type of clay particle associations, and that gypsum can change these associations affecting rheology and flotation performance. Entrainment was also calculated to find the recovery by entrainment and if true flotation was affected with the addition of clay minerals and gypsum to the copper-gold ore. The important conclusion from this

chapter is that the type of clay aggregates or network structures is key for flotation performance and apparent viscosity values at low shear rates can give a better indication of the presence of this network structures. In this research it was found that at low shear rates the apparent viscosity values were much higher when networks structures were present than in the case of loose aggregates. This means that more energy was required to start flow of the slurries with those structures, or more energy was needed to break the structures. It was also found that entrainment changed with gypsum addition. These findings are presented in a paper published in Minerals Engineering in 2015 (Cruz et al., 2015b).

Chapter 7 has the similar structure of Chapter 6, but the goal in this chapter was to study the effect of pH modifiers, lime and soda ash, on the flotation of a copper-gold ore mixed with either bentonite or kaolinite. The experiments in this chapter were also useful to find if the rheological behaviour of the clay minerals in the mixtures was similar to the behaviour of the slurries made of single clay minerals. Gel point measurements were conducted for some ore-clay mineral mixtures and clay mineral slurries to explain some unexpected rheology findings of the mixtures when adding lime or soda ash. These two pH modifiers were chosen because they showed the most significant effect on rheology. The effect of sodium hydroxide was less pronounced. The amount of Ca2+ and Na+ cations added by the pH modifier was calculated to compare it with the amount of Ca2+ released by gypsum. This gives an idea about the influence of ionic strength on rheology and flotation. According to the results, pH modifiers do not influence that much flotation outcome as gypsum does. The research shown in this chapter includes some results submitted for publication in Minerals Engineering in 2015.

Chapter 8 is the discussion of some recommendations and the conclusions from this thesis. Another group working on the same ARC Linkage is dealing with the manipulations of the clay mineral aggregates or network structures to improve true flotation and decrease entrainment. The findings in this thesis are fundamental for these manipulations using dispersants or chemicals.

Chapter 3 Experimental

3.1 Introduction

This chapter describes the experimental program that was used to understand the interaction of clay minerals in flotation by using a problematic ore (or the high clay content ore), single minerals, and a copper-gold ore (an easy to process ore with a low clay content, referred as to clean ore in this thesis) and its mixtures with clays and gypsum. The mineralogical composition of the high clay content ore was key in designing the experimental plan, and this ore was also used to provide a baseline for rheology investigations. The particle associations in the single mineral systems were investigated using a number of techniques including rheological measurements, settling and gel point tests. The copper-gold ore with a low clay content was mixed with clay minerals and other gangue minerals to prepare slurries. Flotation reagents were added to investigate aggregates, and flotation performance. Rheological behaviour, settling data, gel points, and high resolution images from Cryo-SEM were used to support clay interactions and flotation results.

3.2 Mineral samples and reagents

3.2.1 Single minerals

The sodium bentonite and kaolinite Q38 were supplied by the Sibelco Group, Australia. Quantitative XRD analysis indicated that the bentonite contained 63 wt.% montmorillonite, 25 wt.% albite and 12 wt.% quartz. The sample was labelled sodium bentonite, and most of the exchange cations are Na+, but some Ca2+ and/or Mg2+ cations may be present (Bleifuss, 1973). Kaolinite Q38 was a dry milled kaolinite with a surface area of 26 m2/g. Quantitative XRD analysis showed that the composition of the kaolinite sample was 85 wt.% kaolinite, 4 wt.% quartz and 11 wt.% muscovite. The relatively low-crystallinity of Q38 kaolinite with Hinckley crystallinity index (IH) of 0.5, derived from the XRD pattern (Plancon et al., 1988), was chosen in our experiments because it is close to the IH of the kaolinite in the clean ore (0.6). Illite was purchased from Ward’s Science with 5% muscovite.

Gypsum (calcium sulphate dihydrate) was purchased from Sigma – Aldrich and its purity was labelled as ≥ 99 %. Calcium sulphate can occur in different forms depending on the level of hydration. Gypsum is calcium sulphate dehydrate which is the one present in ores (Herrero et al.,

2009). This study used calcium sulphate dehydrate (Ca(SO4).2H20). Calcium sulphate hemihydrate

(Ca(SO4).0.5H20) was also examined in this thesis, but the addition of water to this mineral produced very high viscosities due to hydration and crystal formation (Lewry and Williamson, 1994). Calcite was purchased from GEO Discoveries with a purity of more than 98%. Dolomite was acquired from Mudgee Dolomite and Lime Pty Ltd with 56 wt.% calcium carbonate and 39 wt.% magnesium carbonate. Quartz was purchased from Sibelco Group, Australia and had a purity of more than 99 %.

Illite, dolomite, and calcite samples were crushed and pulverized to obtain a particle size distribution similar to that of the kaolinite and bentonite. After pulverizing the crushed samples, they were sieved using a 106 µm mesh. The undersize particles were pulverized again and sieved with a 75 µm mesh. This was repeated using 53 and 38 µm meshes. Quartz had an original P80 of about 52 µm, and it was pulverized and sieved following the same procedure, but starting with a 53 µm mesh. The particle size was measured using a Mastersizer Microplus from Malvern Instruments, and a standard procedure was followed to avoid particle agglomerates. This device uses a very dilute suspension (about 0.1 % solids) to measure the particle size distribution by laser diffraction. Kaolinite, illite and bentonite were found to have a P80 of about 14 µm, and the final P80 of the quartz, calcite, and dolomite was approximately 25 µm with a particle size distribution very close to that of the clay minerals. The P80 of gypsum was 77 µm and it was not possible to further reduce its size due to particle agglomeration after pulverization.

3.2.2 Ore samples

The high and low clay content ores (the problematic ore and clean ore) were used to establish the range of expected rheological behaviour. The mineral composition of each ore was obtained by using quantitative XRD analysis and the results are shown in Table 3-1. Head assays showed that the copper and gold content were 0.55 wt.% and 0.46 ppm, respectively, for the low clay ore. Flotation experiments were not conducted on the high clay content ore since its composition was different to the low clay ore, and the comparison of the flotation performance between these two ores was meaningless. The high clay content ore was used as a reference that produces high viscosities in flotation slurries, and as a guide to select gangue minerals used to interact with the 55

low clay content ore. Both the high and low clay content ores were crushed and ground to a P80 of 106 µm before rheology measurements.

Table 3-1. Mineral compositions of the high and low clay content ores. High clay content ore Low clay content ore

(wt.%) (wt.%) Calcite 16 Gypsum 4 Dolomite 2 Marcasite 2 Anatase 1 Illite 22 Kaolinite 8 3 Pyrite 3 1 Quartz 44 28 Muscovite 10 Albite 50 Clinochlore 8

3.2.3 Reagents

For the rheology study with single minerals, the three pH modifiers examined were sodium hydroxide (NaOH), hydrate lime (Ca(OH)2) and sodium carbonate (Na2CO3). They were AR grade and used to adjust the pH of clay and clay-gangue mineral suspensions. The natural pH of clay mineral suspensions varied from about 7.5 to about 8.5. These pH values are within the pH range typically used in copper-gold flotation. The study on the rheology of suspensions made of single clay minerals also included the testing of Potassium Amyl Xanthate (PAX), the strongest xanthate collector, and Cytec Interfroth 6500 (an alkyl aryl ester frother) which are widely used in copper- gold flotation.

For the flotation experiments sodium isopropyl xanthate (SIPX), and the promoter Aero 3894 were used as collectors. Both reagents were supplied by Cytec. The frother Polyfroth W22 was supplied by Huntsman Performance Products, and it is low molecular weight polyoxyalkylene alkyl ether

frother partially soluble in water. pH in flotation was adjusted using AR grade hydrated lime

(Ca(OH)2). These reagents are used in the industrial flotation plant where the low clay content ore was obtained.

3.3 Pulp rheology measurement

Since clay minerals greatly affect the viscosity of flotation slurries, it makes sense to use rheological measurements to understand how clay minerals change flow behaviour of slurries. Although the rheology of pure clay minerals has been widely studied, these studies are not conducted under a flotation context.

Unlike pure clay suspensions, real ore slurries are not stable for rheology measurements since particles settle very quickly without any agitation. Settling constantly changes the percentage of the solids before the instrument can display a steady reading (Kawatra et al., 1996) therefore making the measurement of rheology in settling slurries very challenging (Ferrini et al., 1979; Klein et al., 1995; Shi and Napier-Munn, 1996a; Shi and Napier-Munn, 1996b; Akroyd and Nguyen, 2003; Bazin and B-Chapleau, 2005). In flotation, the stirring of the pulp and air bubbles keep solids suspended, but when it comes to measure rheology, the main challenge is to keep particles suspended in the measuring device.

Many researchers have proposed methods for rheology measurement (Ferrini et al., 1979; Napier- Munn, 1984; KiljaŃSki, 1993; Shi, 1994; Klein et al., 1995; Kawatra et al., 1996; Shi and Napier- Munn, 1996a; Shi and Napier-Munn, 1996b; Boger, 2000; Bazin and B-Chapleau, 2005; Gustafsson et al., 2005; Amiri et al., 2010; Boudrias-Chapleau et al., 2010; Shabalala et al., 2011), but there is not agreement on a specific method. The capillary viscometer is a good option to measure rheology of fast settling suspensions (Sarmiento et al., 1979; Napier-Munn, 1984) since it uses the flow of the slurry to produce the required data, however, this type of viscometer has some disadvantages including the reduced control over shear rate which is essential for measuring the full rheology of non-Newtonian fluids, and the plugging of the capillary tube. Another proposed method is the recirculation of the slurry through the gap between the bob and cup in the Coutte geometry, however, this technique requires a significant amount of sample to recirculate in the system and special care to guarantee the laminar flow of the sample. Given the amount of rheology tests required in this work, this method is not practical. In addition to this, the purpose of the rheology measurements was to identify the behaviour of the different structures depending on the clay

mineral present, and by recirculating the sample, these structures may be broken before any measurement.

In this thesis, a conventional rotational rheometer with a Coutte geometry (bob and cup) was chosen for all the rheology measurement since fast settling only occurs for slurries with very low viscosities that are not a problem for flotation, and most of the slurries prepared for this work exhibited high viscosity values and consequently the particles did not settle quickly. The first rheology measurements on clay mineral slurries showed that these suspensions were non- Newtonian with pseudoplastic behaviour with the exception of slurries at low clay mineral concentrations. Rheograms were chosen as the most useful measurement since they showed the shear stress and viscosity response for a range of shear rate values (0.1 to 350 s-1), and it was possible to use a model (i.e., Bingham, Cason or Herschel-Bulkley) to calculate the yield stress of the slurries. Figure 3-1 shows the flow behaviour of slurries. For a Newtonian fluid the yield stress is zero. The calculated yield stress using the Bighman and Herschel-Bulkley models are also illustrated in that figure. The Bighman model is represented by the equation:

= + (1)

0 퐵 𝜎𝜎where𝜎𝜎 is𝜂𝜂 the𝜸𝜸̇ yield stress and is the intercept on the shear stress axis of the line fitting the constant slope region.𝜎𝜎0 is the Bingham viscosity and is the shear rate. 퐵 The Herschel-𝜂𝜂Bulkley model follows this equation:𝜸𝜸̇

= + (2) 푛 0 ̇ 𝜎𝜎The 𝜎𝜎 yield𝐾𝐾𝛾𝛾 stress in this equation is the intercept of the line fitting the yield stress region. This means𝜎𝜎0 that for a flow curve with yield value and pseudoplastic behaviour (Figure 3-1) the Bingham yield stress is higher, and in this case the calculated yield stress with the Herschel-Bulkley model is a better approximation to the actual value. is the consistency index, and n the flow index.

Yield stress can be calculated directly at very𝐾𝐾 low shear rates using the vane technique (Dzuy and Boger, 1985), but it may be cases where the suspension does not have a yield value. Additionally, this type of measurement does not provide the information that rheograms give in a range of shear rates. A positive aspect of the vane technique is the absence of wall slippage, and the localized structural disruption around the vane (Goh et al., 2010). Alternatively, an apparent viscosity at a specific shear rate value can be used to compare slurries. As shown in the literature review chapter, the shear rate distribution in flotation cells may vary from high values close to the impeller to very 58

low values in the quiescent zone, with the average shear rate in a flotation cell approximately 100 s- 1 (Ralston et al., 2007).

Figure 3-1. Flow curves showing the behaviour of flowing material.

The rheological behaviour of all pure gangue minerals, the low clay content ore and the problematic ore provided the baseline to analyse the interactions. Rheology measurements were conducted between natural pH and a pH of 10. These values cover the flotation range for the copper-gold ores used in this research. Brisbane tap water was used for the preparation of all the slurries in this research. One problem with the bentonite slurries is their strong time dependant behaviour as seen in the change of viscosity with time (Goh et al., 2010). This was observed during the rheology measurements and it was solved by making the measurements at fixed periods of time.

First, the rheological behaviour of pure clay minerals, swelling and non-swelling, were studied and these measurements were conducted using an Ares rheometer with a Couette geometry (bob and cup) from TA Instruments. Measurements were conducted within an hour after the preparation of the clay suspension at the ambient temperature of around 22 oC. There was no problem with settling for these clay mineral slurries. Each rheology measurement required a sample of 15 cm3 which was taken with a 20 cm3 syringe while the suspension was mixed. The rheometer automatically set the distance between the base of the cup and the tip of the bob which was 8.5 mm in this study. The

shear rate for the rheology measurement was between 0.1 and 350 s-1 and it took approximately 5 min to cover this range during each measurement.

The concentrations of bentonite suspensions tested in this study were 2, 5 and 10 wt.% equivalent to 0.8, 2.1 and 4.3 vol.%, respectively. The concentrations of kaolinite suspensions were 10, 20 and 30 wt.% equivalent to 4.3, 9.1, and 14.6 vol.%, respectively. These concentrations were on a wet basis and were chosen depending on the type of clay mineral. 30 wt.% kaolinite slurry gives similar apparent viscosity to 13 wt.% sodium bentonite slurry at a shear rate of 100 s-1. To prepare the suspension, kaolinite or bentonite was added to tap water while stirring. The samples were stirred continuously for 30 and 60 min for kaolinite and bentonite suspensions, respectively. This was because kaolinite particles were much easier to disperse than bentonite particles. The suspensions at natural pH, with added NaOH, Ca(OH)2 or Na2CO3 to adjust pH, and the suspensions conditioned with the collector or frother were subjected to rheology measurements.

For the rheology measurements on other gangue minerals and clay mineral-ore mixtures, an Anton Paar DSR 301 rheometer was used. This instrument also uses a Couette geometry (bob and cup) and it is possible to produce a rheogram in 35 seconds. This quick measurement is useful when dealing with the issue of particle settling, and the Ares rheometer took more than four minutes to produce a rheogram. The original set up of the Anton Paar equipment did not have the capability of using a Couette geometry, and an extra fixture was added to allow for this type of measurement (Figure 3-2).

For the rheology study of non-clay gangue minerals, quartz, calcite, dolomite and gypsum, mixtures were prepared with a proportion of 25 wt.% (11.8 vol.%) clay and 5 wt.% (2.1 vol.%) non-clay mineral. The concentrations of the kaolinite and sodium bentonite suspensions were 30 wt.% (14.6 vol.%) and 15 wt.% (6.6 vol.%), respectively. Pure quartz, calcite, dolomite and gypsum were tested at a 30 wt.% concentration. These concentrations were also on a wet basis. To prepare clay and non-clay gangue mineral mixtures, approximately 25 g non-clay gangue mineral was added to 350 g tap water in a 600 ml beaker while mixing with a magnetic stirrer. After 30 min, about 125 g clay mineral was added with the continuous mixing for 30 min for kaolinite mixtures and 1 hour for bentonite mixtures. Kaolinite particles were much easier to disperse than bentonite particles in general. The volume of prepared suspensions was about 400 ml and it was divided into four samples of 100 ml each. Slurries to make the rheograms were at natural pH, and pH 10 adjusted with NaOH, Ca(OH)2, or Na2CO3. The concentration of these three pH modifiers was high enough

to avoid the dilution of suspensions. This same procedure was followed with the single clay mineral slurries. The sample required here for rheology measurement was 20 cm3.

When flotation was performed, the 20 cm3 sample were drawn from the flotation cell after the addition of flotation reagents. This sample was immediately transferred to the cup of the Anton Paar DSR 301 rheometer. All the slurries used in flotation had 30 wt.% concentration to follow the solid density used in the industry. The content of gangue minerals in these mixtures with the clean low clay content ore were given in the flotation experiments section. It is important to mention that those gangue mineral concentrations are on a dry basis. As explained before, the solids concentration of the slurries made of single minerals was not fixed at 30 wt.% for all the suspensions. In many cases only one mineral was present in the slurry (e.g. kaolinite or bentonite), and using a wet basis was more appropriate. In this case the purpose of the rheology measurements was to study the behaviour of slurries at different clay mineral concentrations. On the other hand, for the flotation experiments the solids content in the slurries was fixed at 30 wt.%, and a dry basis was more reasonable to use and it followed the procedures in mineral processing.

Figure 3-2. Anton Paar DSR 301 rheometer.

3.3.1 The reproducibility of rheology measurements

For the rheology measurement a standard procedure was followed, but some sources of error could be present such as the weighting of water and samples, variations in ambient temperature, pH meter accuracy, grinding of the samples, time elapsed between the preparation of the samples and the rheology measurement and random and operator errors. When bentonite was present, it was found that the time period after the sample preparation was the biggest source of error if measurements were not taken at same time intervals. The contribution to the error by other factors should be minimal. Figure 3-3 shows the rheograms produced by the Ares rheometer of three replicates for a slurry made of 10 wt.% bentonite. It is clear that there was good reproducibility of the rheograms. The average Bingham yield stress of these three replicates was 1.29 Pa with a relative error of 4.9% and a confidence interval of 95%. When the Herschel-Bulkley model was used to calculate the yield stress, the mean value for the yield stress was 0.73 Pa and the relative error was 17.1% at 95% confidence interval. Bentonite slurries at 10 wt.% concentration gave the highest error since the rheology of bentonite was very time dependant, but the reproducibility was still good.

4.5

4.0

3.5

3.0

2.5

2.0

Shear stress (Pa) 1.5 10 wt.% bentonite - Natural pH 8.95 - Replicate 1 1.0 10 wt.% bentonite - Natural pH 8.95 - Replicate 2 0.5 10 wt.% bentonite - Natural pH 8.95 - Replicate 3

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 3-3. Replicates of rheology tests for slurries with 10 wt.% bentonite using the Ares rheometer from TA instruments.

Figure 3-4 show the rheograms obtained for replicates of the mixture containing 30 wt.% kaolinite and 70 wt.% ore. These rheograms were produced by the Anton Paar DSR 301 rheometer. The reproducibility of these duplicates was also good. The average Bingham yield stress was 0.92 Pa 62

and the relative error was 2.2% with 95% confidence interval. When the Herschel-Bulkley yield was calculated, the average was 0.55 Pa with the relative error of 6.7% for that same confidence interval.

30 wt.% kaolinite - 70 wt.% ore - pH 10 lime - Replicate 1 3.0 30 wt.% kaolinite - 70 wt.% ore - pH 10 lime - Replicate 2 30 wt.% kaolinite - 70 wt.% ore - pH 10 lime - Replicate 3 2.5

2.0

1.5

Shear stress (Pa) 1.0

0.5

0.0 0 50 100 150 200 250 300 350 400 Shear rate (s-1)

Figure 3-4. Replicates of three rheology measurements for the mixture made of 30 wt.% kaolinite and 70 wt.% ore using the Anton Paar DSR 301 rheometer. Slurry density 30 wt.%.

Both the Ares and Anton Paar rheometers gave good reproducibility for the rheology measurements, and in general the apparent viscosities of the slurries containing bentonite were very time dependant, but this was addressed by keeping fixed times during the measurements.

3.4 Measurement of network structures

The type of network structures or aggregates depends on the surface charge of clay minerals, specially the charge at the edges which varies depending on pH. It was mentioned in the literature review that different structures can be formed by face-face, edge-face, and edge-edge associations, and that rheological response depends on those aggregates. In this thesis these structures were studied by Cryo-SEM imaging, settling tests and gel point measurements.

3.4.1 Cryo-SEM imaging

Cryo-SEM provides detailed images of the network structures created as the result of clay minerals interactions. The Cryo-SEM technique has been used in many applications to avoid changes in structure induced by vacuum drying or freeze-drying since the water is vitrified without crystallisation as ice (Battersby et al., 1994). In Cryo-SEM the samples taken from the slurry are snap frozen preserving aggregate structures in vitrified water background. This technique has been developed to study the network structures of clays in mineral tailings (Peng, 2011), and uses Scanning Electron Microscope which provides a high resolution images of particles smaller than 1 µm, down to few nm thanks to the very short wave lengths of electrons (Gregory, 2006).

In this thesis, Cryo-SEM images were collected for mixtures of clean ores with different clays, and mixtures with clay minerals and gypsum. Two 6 mm long copper tubes with outer-diameter 4 mm and inner-diameter 3 mm were joined together by superglue to hold liquid samples. The samples collected from pulp and the froth of first concentrate were transferred into the copper tube using a wide-bore pipette. The copper tube with the sample inside was immediately sealed with dental wax, placed in a container and plunged into liquid nitrogen. Each sample was preserved in liquid nitrogen until transferred to the sample preparation chamber of the PHILIPS XL30 field emission gun scanning electron microscope (FESEM) equipped with Oxford cryo-transfer and fracture stage. The top tube of the two glue-jointed tubes was then knocked off using a metal knife on the fracture stage to expose a fresh sample surface inside the tube. The sample temperature was raised to 175 K to sublime vitrified water at the rate of 6 nm/s for 2 min with an estimated 720 nm depth of vitrified water sublimated. The sample was subsequently coated by platinum plasma for 3 min to form a 3 nm thick platinum coating to avoid charging during the imaging process. The FESEM was operated at 10 kV and coupled with Energy-dispersive X-ray spectroscopy (EDS) to identify the elemental composition of particles. The application of this technique to study kaolinite aggregation in pulp, froth and concentrate phases was published by Xu et al. (2014).

3.4.2 Settling and gel point tests

Other tools that can be useful in the analysis of the network structures formed in mineral slurries are settling and gel point tests. During experimentation it was observed that low viscosity slurries settled much faster than the slurries with high clay contents with high viscosities. This may be related to the different network structures in the slurries and their gel points as it is reported in some 64

studies with kaolinite (Nasser and James, 2006; Zbik et al., 2008b; Du et al., 2009; Nasser and James, 2009; Du, 2010).

For dispersed kaolinite particles the sediment thickness increases with time to a maximum, and during the settling of associated particles the sediment thickness decreases with time. These responses depend on electrolyte concentrations and pH (Nasser and James, 2006). Some other factors affecting the sedimentation of a suspension are the density of the particles and medium, the gravity, the buoyancy, the drag acting at the interface between the wall of a container and the suspension, and the pressure difference between the top and the bottom of the suspension (Nakaishi et al., 2012). In this thesis, settling tests were conducted on 5 and 20 wt. % suspensions of bentonite and kaolinite respectively. Bentonite suspensions were tested at natural pH, and pH 10 using lime and sodium carbonate as pH modifiers. Similar tests were conducted on kaolinite suspensions. Bentonite or kaolinite suspensions were drawn from the same slurry in such a way that each one of them was a representative sample from the original slurry. Three measuring cylinders (1000 ml) were used and the height of the suspension in each calibrated cylinder was 34 cm and suspensions were mixed by inverting the cylinders 10 times. After this, a photo was taken from the three cylinders at specific time intervals to measure the height and record any changes in the suspension.

The gel point is another option to look at the network structures and is defined as the lowest concentration of the slurry to form a self-supporting network. At this point all primary aggregates are interconnected across the container, and a network is just formed. It gives information about the nature of the compressibility and structure of sediments (Du, 2010). The settled particles have gradient concentration that varies from a maximum point at the bottom to the critical volume fraction which is the gel point, at the top of the sediment. For the measurement of this point, a set of 6 measuring cylinders was used. The same slurry was poured into each cylinder at different levels, approximately 20, 15, 10, 7, 5, and 3 cm in this case. The initial slurry height in each cylinder was recorded, and the samples were left to settle for approximately 100 hours. The gel point value was extrapolated from the graph with initial height of the slurry vs. final average solid fraction (Figure 3-5) which can be calculate from the final height of the sediment and the weight of solids.

Figure 3-5. Gel point calculation.

3.5 Solubility tests

Solubility tests were used to quantify the extent to which some gangue minerals, such as gypsum, release Ca2+. For these tests 5 wt.% quartz, calcite, dolomite and gypsum dihydrate suspensions were prepared with Brisbane tap water, and mixed for one hour using a magnetic stirrer. The total weight of each suspension was approximately 200 g. Following mixing, the suspensions were filtered using filter papers with an average retention capacity of 4 – 7 mµ, and the filtrates were collected and subjected to analysis by ICP-OES. These tests were conducted only at natural pH since the change in solubility is very small in the pH range of the experiments in this study between 7 and 10.

3.6 Flotation experiments

Flotation tests were conducted on clean ores, and clean ores with clays and/or gypsum added. These tests were carried out to determine the effect of kaolinite and bentonite on flotation performance, and the effect of the interaction of these minerals with gypsum and pH modifiers.

For the experiments with gypsum the low-clay copper-gold ore was used to prepare five different slurries by mixing it with known amounts of kaolinite (30 wt.%), sodium bentonite (15 wt.%), kaolinite-gypsum (30 wt.%-5 wt.%), sodium bentonite-gypsum (15 wt.%-5 wt.%), and quartz (30 wt.). The low clay content ore with no clay addition was used as a baseline. The mixture with quartz was a baseline with a similar particle size distribution to the mixture with 30 wt.% kaolinite. The purpose was to compare the difference in flotation outcome between these two slurries. A 30 wt.% quartz content was chosen since that was the highest clay content used. The solid concentration for all the slurries was maintained at 30 wt.% (about 14.6 vol.%) and the total weight of the mixtures was 1 kg for all cases. The slurries were prepared using Brisbane tap water.

The low clay content ore was wet ground to a P80 of 106 µm at a 60 wt.% solid concentration. Hydrated lime (800 g/t) and promoter Aero 3894 (8 g/t) were added to the mill to mimic the plant scale processing. After grinding, the slurry was transferred to a 3 L flotation cell where it was well mixed at 1500 RPM and a known volume of slurry was removed. The same weight of solids that was removed was replaced with clay mineral, quartz or a mixture of clay mineral and gypsum. Water was added to make up the slurry to 30 wt.% for flotation. The removed slurry was filtered, dried and weighted to confirm that a correct amount of solids was taken from the original slurry.

After adding the clay mineral or the mixture of clay mineral and gypsum to the flotation cell, stirring continued at 900 RPM for 30 min when adding kaolinite and for 45 min when adding bentonite. When adding bentonite to the ore, agglomerates were formed and more mixing time was required. It was also observed that after 30 or 45 min the rheology measurements became consistent for the mixtures with kaolinite or bentonite. Once the slurry was mixed, the pH was adjusted to 10 by the addition of hydrated lime.

These experiments were conducted in a 3 litre bottom-driven flotation cell (Figure 3-6). Four concentrates were collected after 1, 3, 7, and 17 min of flotation. During flotation the impeller speed was set at 800 RPM, and the air flow rate was 8 L/min. The pH was adjusted to 10 with hydrated lime or lime before collecting the concentrates and the collector SIPX and frother Polyfroth W22

were added prior to collecting the first and third concentrates. The collector dosage was 6 and 4 g/t before the first and third concentrates respectively. 15 g/t frother was added before the first and third concentrates. This flotation conditions were based on a standardized procedure used in a flotation plant treating this type of ore.

Figure 3-6. Flotation cell used in the test work.

A similar procedure was followed to test the effect of pH modifies on flotation, but there was no gypsum in the ore-clay mineral mixtures, and pH was adjusted with lime or soda ash. The concentrations of kaolinite tested in the mixtures were 30 and 10 wt.%, and the content of bentonite was 15 and 5 wt.%. All the given concentrations in these flotation tests were on a dry basis.

3.6.1 The reproducibility of flotation tests

It is expected that in the flotation tests more significant errors are present than in the rheology measurements since these tests are more dependent on the operator procedure. Apart from operator error, other possible sources are sampling, grinding, flotation, and random errors. Similar to rheology measurements, a standard procedure was followed.

To show the reproducibility of flotation tests, some data from Chapters 6 and 7 is presented in Figure 3-7. These results are explained in detail in those chapters. Figure 3-7 shows the copper grade and recovery graphs for the flotation of the replicates of the low clay content ore (100 wt.%), the ore mixed with 30 wt.% kaolinite and the mixture with 15 wt.% bentonite. The black lines show the average for grade and recovery for each one of these cases and the 95% confidence interval

error bars. The reproducibility was very good for the last concentrate especially for the flotation of the low clay content ore with no clay addition with the relative error of 1% for the grade and recovery and a confidence interval of 95%. The relative error for the mixture with 15 wt.% bentonite was 13% for grade and 3% for recovery. The 30 wt.% kaolinite-ore mixture had 14% and 1% relative errors for grade and recovery respectively. In general it was found that the error for the recovery was very low (1-3%).

15 Grade(%) 10

0 0 10 20 30 40 50 60 70 80 90 100 Recovery (%) 100 wt.% ore - Rep. 1 - Lime 100 wt.% ore - Rep. 2 - Lime 100 wt.% ore - Aver. - Lime 30 wt.% kaol. - Rep. 1 - Lime 30 wt.% kaol. - Rep. 2 - Lime 30 wt.% kaol. - Aver. - Lime 15 wt.% bent. - Rep. 1 - Soda ash 15 wt.% bent. - Rep. 2 - Soda ash 15 wt.% bent. - Aver. - Soda ash

Figure 3-7. Copper grade and recovery for some replicates for the flotation of the low clay content ore and the ore mixtures with 30 wt.% kaolinite and 15 wt.% bentonite.

Chapter 4 Rheological properties of kaolinite and bentonite suspensions in the presence of flotation reagents

4.1 Introduction

The rheological behaviour is a unique feature of clay minerals and this may result in a negative impact on mineral flotation. The goal of this chapter is to understand the rheological properties of kaolinite and bentonite suspensions in the context of copper-gold flotation in the absence and presence of pH modifiers, collector and frother surfactants. These two clay minerals are commonly found in ores, and can therefore represent a widespread problem in mineral flotation. There have been numerous observations of these clay minerals having a deleterious effect on mineral flotation with simultaneous observations of high flotation pulp viscosity. As discussed in the literature review, kaolinite is a non-swelling clay mineral with a 1:1 structure, and bentonite is a swelling clay mineral with a 2:1 structure, and this may make them behave differently in suspensions.

During flotation, pH modifiers (sodium hydroxide, lime or sodium carbonate) are typically used to adjust flotation pH to the alkaline range, while introducing cations. Collector and frother are added to modify surface hydrophobicity, reduce bubble sizes and form appropriate froth. The pH modifiers, collector and frother may interact with clay minerals and further alter the pulp rheology. The flotation reagents tested were the pH modifiers sodium hydroxide (NaOH), hydrated lime

(Ca(OH)2) and soda ash (Na2CO3), the collector PAX and the frother Cytec Interfroth 6500 that are typically used in copper-gold flotation.

4.2 Results and discussion

4.2.1 Kaolinite and bentonite interactions with water

The rheological behaviour of kaolinite and bentonite in the presence of water was investigated first. Figure 4-1 shows the rheograms of kaolinite and bentonite suspensions at different solids concentrations indicating the relationship between shear stress and shear rate. At a low kaolinite or bentonite concentration (e.g., 10 wt.% kaolinite, and 2 and 5 wt.% bentonite), the suspension behaved like a Newtonian fluid with the shear stress versus shear rate curve being linear and passing through the origin. The constant slope that is independent of the shear rate is known as the viscosity. However, at a higher kaolinite or bentonite concentration (e.g., 20 and 30 wt.% kaolinite, and 10 wt.% bentonite), the suspension became non-Newtonian with pseudoplastic characteristics. The viscosity of the pseudoplastic suspension decreased with an increase in the shear rate. Under shear, the large particle aggregates are separated into smaller units and, at very high shear, into individual particles. Such a process rheologically manifests itself as shear-thinning behaviour.

6 Kaolinite 10% - Natural pH 7.70 Kaolinite 20% - Natural pH 7.65 Kaolinite 30% - Natural pH 7.63 Bentonite 2% - Natural pH 8.24 5 Bentonite 5% - Natural pH 8.73 Bentonite 10% - Natural pH 8.85

2 Shear stress (Pa)

0 0 50 100 150 200 250 300 350 -1 Shear rate (s )

Figure 4-1. Rheograms of kaolinite and bentonite suspensions at different concentrations.

Figure 4-1 also indicates that slurries containing less than 10 wt.% kaolinite or 5 wt.% bentonite remained Newtonian in nature. However, in mineral flotation, interactions between clay minerals and other minerals present in the ore may also take place. It has been reported that aluminium and 71

iron oxide particles are precipitated in the presence of clay minerals alternating the stabilisation and re-stabilisation of suspensions depending on pH and the mass ratio between the oxide and the clay mineral (Lagaly, 1989; Lagaly, 2006). In addition to this, as discussed in the literature review, calcium bearing minerals could release sufficient quantities of cations to change rheological behaviour of clay mineral slurries. As a result, there are possibilities that a low concentration of clay minerals may promote the change of the fluid type in the presence of other minerals. A study of the interaction of clay minerals with calcium bearing minerals in mineral flotation is reported in the next chapter.

Figure 4-2 shows the apparent viscosity of kaolinite and bentonite suspensions as a function of their concentrations at a shear rate of 100 s-1. As discussed in the experimental chapter, this is a typical average shear rate in a flotation cell (Ralston et al., 2007). The trend of the calculated Bingham yield stress values was similar to the apparent viscosities at 100 s-1 and it was reasonable to use either apparent viscosities or Bingham yield stress values to compare rheology results. In line with the reports in literature, bentonite had a stronger effect on viscosity than kaolinite. For instance, at a 10 wt.% (4.3 vol. %) concentration, a bentonite suspension produced an apparent viscosity nearly 7 times higher than a kaolinite suspension. Compared to kaolinite suspensions, bentonite suspensions displayed a significant viscosity at low concentrations due to the high swelling and associations of the fine clay particles producing a viscous gel-like structure (Goh et al., 2010).

30 Kaolinite natual pH

Bentonite natual pH 25

Apparent viscosity (mPa) viscosity Apparent 5

0 0 5 10 15 20 25 30 Solid concentration (Mt.%) Figure 4-2. Apparent viscosity of kaolinite and bentonite suspensions as a function of their concentrations at a shear rate of 100 s-1.

4.2.2 Kaolinite and bentonite suspensions in the presence of pH modifiers

The natural pH of kaolinite suspensions was about 7.5. To examine the effect of pH modifiers on the rheology of kaolinite suspensions, NaOH, Ca(OH)2 and Na2CO3 were used to adjust the pH to 10. Rheograms of 10, 20 and 30 wt.% kaolinite suspensions in the absence and presence of pH modifiers are shown from Figure 4-3 to Figure 4-5 respectively.

Figure 4-3 shows that at 10 wt.% kaolinite, pH modifiers had little effect on the rheological behaviour of the suspension with Newtonian characteristics being observed. In the absence and presence of NaOH, Ca(OH)2 and Na2CO3, the shear stress versus shear rate behaviour was very similar. In contrast, lime and Na2CO3 had a significant effect on the rheological behaviour of kaolinite suspensions at 20 and 30 wt.% kaolinite as shown in Figure 4-4 and Figure 4-5. While lime increased the viscosity at the same shear rate compared to the baseline in the absence of any pH modifier, Na2CO3 decreased the viscosity. The effect of Ca(OH)2 and Na2CO3 on the rheological behaviour of kaolinite suspensions was more pronounced at 30 wt.% kaolinite. Again

NaOH had little effect at both 20 and 30 wt.% kaolinite. Ca(OH)2 exacerbated the aggregation of kaolinite particles corresponding to the increased viscosity of kaolinite suspensions at high kaolinite concentrations. This is in line with Lagaly (2006) who observed that multivalent cations including Ca2+ were more strongly attracted to the clay particles than monovalent cations including Na+ with a more important effect on the coagulation of clay mineral suspensions. Abdi and Wild (1993) and Wild (1993) also reported that Ca2+ cations from lime are attracted to the clay surfaces modifying their double layers resulting in the formation of gels.

This study also indicates that Na2CO3 may disperse kaolinite suspensions at a high kaolinite concentration resulting in reduced viscosity. In fact, Na2CO3 has been used as a dispersant. The dispersing effect of soda ash in the kaolinite slurry can be explained by previously reported work 2- showing that multivalent anions such as CO3 added negative charges to clay mineral surfaces by replacing anions with fewer charges at the broken edges of the particles (Rolfe et al., 1960; Penner and Lagaly, 2001) whereas monovalent anions did not add negative charges to clay particles (Rolfe et al., 1960). Only part of the negative charge associated with a multivalent anion was used by attraction to the exposed cation, while the increase in dispersion potential of a clay-water system through the adsorption of negatively charged ions on the clay particle was roughly proportional to the unused valency of the anions (Rolfe et al., 1960). Anion adsorption on clay mineral surfaces might seem unexpected particularly if the surfaces are negative. Cation exchange on clay surfaces is

far more common, but clay minerals also have some capacity for anion exchange. There are three general mechanisms for this (Bergaya et al., 2006):

1) Electrostatic interaction with particle edges when these are positively charged.

2) Exchange of structural OH groups at the edges and basal surfaces of kaolinite. This is known as “ligand exchange”.

3) Anions accompanying multivalent cations at exchange site positions.

2- In the experiments conducted in this thesis, the adsorption of CO3 on kaolinite edges through the first mechanism should not be possible because at the experimental pH values, more than 9, the 2- edges are negatively charged. Adsorption of the CO3 anions as counterions could be another way for these anions to add negative charge to the edges, but this should decrease with increasing pH (Bergaya et al., 2006). On the other hand, ligand exchange operates when the surface is uncharged or even negatively charged (Bergaya et al., 2006). This means that ligand exchange may be 2- predominant for the adsorption of CO3 anions on the kaolinite surfaces at the operating conditions in this thesis. It can be hypothesized that this mechanism is comparable to the one suggested by 2- Rolfe et al. (1960) if the CO3 anions are meant to replace structural OH groups. This assumption should apply to both edges and faces on the clay mineral.

It is also important to highlight that for multivalent anions the high concentration of a certain species in a solution does not imply that this species is the predominant on the clay surfaces (Bergaya et al., 2006). The concentration of anions in the solution depends on the disassociation constants, while the surface complexing constant also plays a role in the formation of surface species (Bergaya et al., 2006). For instance, it is reported by Bergaya et al. (2006) that the - 2- proportions of phosphate surface species s-H2PO4, s-HPO4 , and s-PO4 (s=surface) changes depending on pH with the s-PO4 predominant at alkaline pH. When Na2CO3 is used to raise pH, - 2- the species HCO3 and CO3 are formed (DiFeo et al., 2004) and they might attach to clay surfaces as in the case of the phosphate species.

Another reason for the dispersion effect of Na2CO3 in the kaolinite slurry could be the deactivation of Ca2+ ions, if present in the slurry, and replacement with Na+ ions. The Ca2+ ions can also form

CaCO3 as insoluble precipitates (Rolfe et al., 1960). In flotation, the formation of CaCO3 precipitate was highlighted by DiFeo et al. (2004) when using Na2CO3 as pH modifier. CaCO3 is 2+ 2- mostly insoluble in water and if there is more than a trace of both Ca and CO3 in water, they will

precipitate as CaCO3. This will take place in almost all natural waters if the pH is raised above 8.3 (Simon, 2002).

0.8

0.7 Kaolinite 10% - Natural pH 7.70 Kaolinite 10% - NaOH pH 10.00 0.6 Kaolinite 10% - LIME pH 10.00

Kaolinite 10% - Na2CO3 pH 10.00 0.5

0.4

0.3 Shear stress (Pa) 0.2

0.1

0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-3. Rheograms of 10 wt.% kaolinite suspensions in the absence and presence of pH modifiers.

2.5 Kaolinite 20% - Natural pH 7.65 Kaolinite 20% - NaOH pH 10.00 2.0 Kaolinite 20% - LIME pH 10.00

Kaolinite 20% - Na2CO3 pH 10.00

1.5

1.0 Shear stress (Pa)

0.5

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-4. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of pH modifiers.

Shear stress (Pa) Kaolinite 30% - Natural pH 7.63 2 Kaolinite 30% - NaOH pH 10.00 1 Kaolinite 30% - LIME pH 10.00 Kaolinite 30% - Na2CO3 pH 10.00 0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-5. Rheograms of 30 wt.% kaolinite suspensions in the presence of pH modifiers.

Similarly, the pH of bentonite suspensions was adjusted to 10.0 by NaOH, Ca(OH)2 and Na2CO3 and rheology measurements were performed. Figure 4-6 shows rheograms of 2 wt.% bentonite suspensions in the absence and presence of pH modifiers. In all cases, bentonite suspensions exhibited characteristics similar to those of Newtonian fluids, and it can be concluded that NaOH,

Ca(OH)2 and Na2CO3 did not affect the rheological behaviour of 2 wt.% bentonite suspensions. Figure 4-7 shows rheograms of 5 wt.% bentonite suspensions in the absence and presence of pH modifiers. In the absence of pH modifiers, the 5 wt.% bentonite suspension behaved like a Newtonian fluid. However, in the presence of NaOH, Ca(OH)2 and Na2CO3, the suspension becomes non-Newtonian and pseudoplastic. Figure 4-8 shows rheograms of 10 wt.% bentonite suspensions in the absence and presence of pH modifiers. In the absence of pH modifiers, the 10 wt.% bentonite suspension behaved like a non-Newtonian fluid. The addition of pH modifiers increased shear stress at the same shear rate corresponding to higher viscosity.

0.6

Bentonite 2% - Natural pH 8.24 0.5 Bentonite 2% - NaOH pH 10.00 Bentonite 2% - LIME pH 10.00 Bentonite 2% - Na2CO3 pH 10.00 0.4

0.3

0.2 Shear stress (Pa)

0.1

0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-6. Rheograms of 2 wt.% bentonite suspensions in the absence and presence of pH modifiers.

1.8 Bentonite 5% - Natural pH 8.73 1.6 Bentonite 5% - NaOH pH 10.00 Bentonite 5% - LIME pH 10.00 1.4 Bentonite 5% - Na2CO3 pH 10.00

1.2 1 0.8 0.6 Shear stress (Pa) 0.4 0.2 0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-7. Rheograms of 5 wt.% bentonite suspensions in the absence and presence of pH modifiers.

7 Bentonite 10% - Natural pH 8.85 6 Bentonite 10% - NaOH pH 10.00 Bentonite 10% - LIME pH 10.00

5 Bentonite 10% - Na2CO3 pH 10.00

Shear stress (Pa) 2

0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-8. Rheograms of 10 wt.% bentonite suspensions in the absence and presence of pH modifiers.

Unlike in kaolinite suspensions, the effect of lime on bentonite suspensions was not the strongest, and Na2CO3 promoted aggregation rather than acting as a dispersant in 5 and 10 wt.% bentonite suspensions, resulting in a higher viscosity. These phenomena may be associated with the activation of bentonite in the presence of different pH modifiers. Evangeline et al. (2009) and Raji and Sheela (2009) investigated the activation of bentonite treated by a number of sodium compounds. They found that Na2CO3 strongly activated bentonite increasing its swelling capacity and plasticity, followed by NaOH, which affects the rheology of bentonite suspensions more than in kaolinite slurries.

In general, soda ash acts as a dispersant in kaolinite suspensions, but activates the sodium bentonite for hydration of the interlayer cation (Na+) (Yildiz and Calimli, 2002; Karimi and Salem, 2011). The activation effect of soda ash is attributed to the replacement of Ca2+ by Na+. Clay minerals 2+ 2+ 2- selectively adsorb Ca and Mg ions, but the presence of the CO3 ions leads to the formation of precipitates in the form of Ca/Mg carbonates. This precipitation occurs outside of the interlayer making the Ca2+ and Mg2+ leave that space to indirectly promote the cation exchange (Kaufhold et al., 2013). Due to the strong affinity of monovalent cations to water, the activated bentonite is of high swelling capacity (Yildiz and Calimli, 2002). In contrast, lime may de-activate sodium bentonite and then reduce or eliminate its swelling potential. This is due to the substitution of the clay cations by calcium and subsequent formation of calcium silicate and aluminate hydrates (Abdi 78

and Wild, 1993). However, lime increases the viscosity of bentonite suspensions as well and therefore the aggregation of bentonite particles by lime as explained in kaolinite cases still plays a role in determining the rheological behaviour of bentonite suspensions.

4.2.3 Kaolinite and bentonite suspensions in the presence of collector and frother

In addition to pH modifiers, collector and frother are also essential in mineral flotation. In this study, PAX and Interfroth 6500 were selected to represent collectors and frothers typically used in copper-gold flotation and their effects on the rheological behaviour of kaolinite and bentonite suspensions were investigated at natural pH without the addition of any pH modifier. The purpose of these tests was to measure only the effect of collector and frother on rheology to be able to compare this effect with the one from pH modifiers.

Figure 4-9 shows rheograms of 20 wt.% kaolinite suspensions in the absence and presence of PAX. The addition of 50 g/t PAX to the kaolinite suspension possessed the pseudoplastic characteristics and slightly shifted the shear stress versus shear rate curve upward with higher shear stress and viscosity. Additions of 100 and 150 g/t PAX affected the rheology of the suspension similarly and further increased the shear stress and viscosity. A similar trend was observed when Interfroth 6500 was added to 20 wt.% kaolinite suspensions. As shown in Figure 4-10, the addition of 30 g/t Interfroth 6500 slightly increased the rheological properties of the suspension, while additions of 60 and 100 g/t Interfroth 6500 further increased these properties.

2.0

1.5

1.0

Kaolinite 20% - Natural pH 7.84

Shear stress (Pa) 0.5 Kaolinite 20% - Natural pH 7.84 - PAX 50 g/ton Kaolinite 20% - Natural pH 7.84 - PAX 100 g/ton Kaolinite 20% - Natural pH 7.84 - PAX 150 g/ton 0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-9. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of PAX.

2.0

1.5

1.0

Kaolinite 20% - Natural pH 7.84 Shear stress (Pa) 0.5 Kaolinite 20% - Natural pH 7.84 - Interfroth 6500 30 g/ton Kaolinite 20% - Natural pH 7.84 - Interfroth 6500 60 g/ton Kaolinite 20% - Natural pH 7.84 - Interfroth 6500 100 g/ton 0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-10. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence of PAX.

The effect of PAX and Interfroth 6500 on the rheological behaviour of 8 wt.% bentonite suspension was also investigated. The results are shown in Figure 4-11and Figure 4-12. PAX and Interfroth 6500 had a similar effect of slightly increasing the rheological properties regardless of the dosage.

2.5

2.0

1.5

1.0 Bentonite 8% - Natural pH 8.72

Shear stress (Pa) Bentonite 8% - Natural pH 8.72 PAX 50 g/ton 0.5 Bentonite 8% - Natural pH 8.72 PAX 100 g/ton Bentonite 8% - Natural pH 8.72 PAX 150 g/ton 0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-11. Rheograms of 8 wt.% bentonite suspensions in the absence and presence of PAX.

2.5

2.0

1.5

1.0

Shear stress (Pa) Bentonite 8% - Natural pH 8.72 0.5 Bentonite 8% - Natural pH 8.72 Interfroth 30 g/ton Bentonite 8% - Natural pH 8.72 Interfroth 60 g/ton Bentonite 8% - Natural pH 8.72 Interfroth 100 g/ton 0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 4-12. Rheograms of 8 wt.% bentonite suspensions in the absence and presence of Interfroth 6500.

In general, PAX and Interfroth 6500 did not modify the rheology of kaolinite and bentonite suspensions as much as the pH modifier. Schott (1968) found that surfactants such as polyethylene glycol, alkyl ethoxylated surfactants and nonylphenol ethoxylated surfactants reduced the viscosity of clay mineral suspensions by breaking up the flocculated clay particles into thinner flakes, while at higher concentrations, the viscosity increased due to the appearance of micelles acting as crosslinks between the particles. Yalçin et al. (2002) investigated the influence of the addition of anionic surfactants, sodium dodecyl sulfate and ammonium lauryl sulfate on the flow properties of bentonite–water systems. They found that both surfactants increased the viscosity of bentonite suspensions due to interactions between the tails of the surfactants and the edges of the clay particles, which resulted in the formation of more resistant structures against flowing. This study reveals that both PAX and Interfroth 6500 may cause the aggregation of kaolinite and bentonite particles. However, the mechanism for this is unknown. Whether the formation of PAX and Interfroth 6500 micelles or an interaction between their tails and the edges of the clay particles, or another mechanism responsible for this aggregation is out of the scope of this thesis due to their slight effect on the rheology of clay mineral suspensions.

4.2.4 Settling tests for bentonite and kaolinite suspensions

For bentonite suspensions with the pH modifiers there was no sediment formation after 48 hours, but for the suspension at natural pH, thin sediment was observed. This was most likely due to the swelling of bentonite, and the gel point of bentonite as explained in chapter 6. No conclusions can be drawn from this sedimentation test with bentonite since settling was not observed.

On the other hand, settling of the kaolinite dispersion was very clear and the behaviour was dependant on the pH, and pH modifier as shown in Figure 4-13.

35 33

31 Natural pH (7.80) 29 Lime (pH 10) 27 Na2CO3 (pH 10) 25 23 21 19 Sediment thickeness (cm) 17 15 0 10 20 30 40 50 60 70 Settling time (Hours)

Figure 4-13. Settling rate of the 20 wt. % kaolinite suspensions at natural pH and pH 10 using lime and sodium carbonate as pH modifiers.

Figure 4-13 indicates that the settling rate of the kaolinite suspension with Na2CO3 was the fastest in the first 6 hours of the test, and the suspension with lime showed the slowest settling rate. The kaolinite suspension at natural pH had a settling rate between those two suspensions. After 24 hours the thickness of the sediment for the suspension at natural pH and the one with sodium carbonate addition were similar. After 72 hours, the suspensions with pH modifiers had the same sediment thickness whereas the slurry at natural pH had a smaller sediment height.

Based on the study by Nasser and James (2006), the settling behaviour of these three kaolinite suspensions in this study corresponds to flocculated or aggregated particles. However, it is expected that at pH 10, the kaolinite particles should be dispersed since kaolinite surfaces are negative at high pH values. At natural pH (7.8) these negative charges may not be strong enough to keep particles dispersed. Zbik et al. (2008a) studied the Cryo-SEM images from sedimentation tests of a 4 wt.% kaolinite at pH 8 and 0.01 M NaCl and showed that at the earliest possible stage of dispersion, edge-edge (E-E) and edge-face (E-F) inter-particle associations already existed, but those associations were open, loose and easily disrupted. Those structures do not follow electrostatic DLVO theory since the kaolinite basal plane and edge sites are negatively charged at pH 8, but van der Waal’s forces in the near potential minimum can overcome the charge barrier and induce coagulation (Zbik et al., 2008a). It is suggested that in the settling of kaolinite suspensions initially there is a predominance of E-E structures and as particles settle sliding across each other, particle orientation changes from E-E to F-F and despite the formation of these structures, the settling without flocculants is very slow (Zbik et al., 2008a).

In this study the sedimentation tests results for the first 6 hours had some correlation to the rheology results. Rheology tests suggest that by adjusting the pH to 10 with lime, aggregation occurs increasing the apparent viscosity. Conversely, sodium carbonate may act as a dispersant in the kaolinite slurry. It is possible that the settling velocity is faster in the case of the suspension with aggregated particles, and that lower velocities are present in the suspension with more dispersed particles. However, aggregates of kaolinite particles in these tests can have many voids or spaces occupied by water, and the settling of this structure will be much slower if compared to the case where flocculants are added (Zbik et al., 2008a). It was also mentioned that the Ca2+ cations can have a greater contribution than Na+ cations to the formation of aggregates. The Ca2+ cations may contribute to different modes of interactions between faces and edges of kaolinite particles by bridging them. This may produce structures similar to house of cards aggregates with many voids inside. This kind of structures could give higher apparent viscosity values and at the same time their settling rate is inhibited.

4.3 Conclusions

Kaolinite and bentonite suspensions behaved as Newtonian fluids at low solid concentrations (10 wt.% for kaolinite and 2 and 5 wt.% for bentonite), and non-Newtonian fluids with pseudoplastic

characteristics at high solid concentrations (20 and 30 wt.% for kaolinite and 10 wt.% for bentonite). Bentonite had a stronger effect on the viscosity of suspensions than kaolinite.

The type of pH modifier had little effect on the rheological properties of kaolinite and bentonite suspensions at low solid concentrations (10 wt.% kaolinite and 2 wt.% bentonite in this study) but significantly altered the rheological properties of suspensions at high solid concentrations (20 and 30 wt.% kaolinite and 5 and 10 wt.% bentonite in this study). While lime exacerbated the aggregation in kaolinite suspensions, Na2CO3 dispersed kaolinite suspensions. In contrast, Na2CO3 acted as an aggregator instead of a dispersant in bentonite suspensions enhancing the aggregation more than lime. NaOH also induced the aggregation in bentonite suspensions, but had little effect in kaolinite suspensions.

Collector (PAX) and frother (Interfroth 6500) changed the rheological properties of kaolinite and bentonite suspensions at high solid concentrations (20 wt.% kaolinite and 8 wt.% bentonite in this study) to a similar extent. However, this effect was less pronounced compared to that of the pH modifiers.

Chapter 5 Influence of some calcium bearing gangue minerals on the rheological behaviour of clay minerals

5.1 Introduction

This study explores the rheological interaction between some clays and calcium bearing minerals which occur in copper-gold ores. Results in Chapter 4 showed that pH modifiers used in copper- gold flotation may have a strong effect on the rheology of clay mineral suspensions due to changes in pH and the contribution of ions from these flotation reagents. This suggests that ions released by minerals with some solubility in water can also affect the rheology of clay mineral suspensions as supported by some studies in soil science that show the interaction of clay minerals with calcium bearing minerals (Alther, 1986; Meer and Benson, 2007; Bradshaw et al., 2013). This has not been addressed from the perspective of flotation of copper-gold ores with a high clay content.

It is also known from literature that clay minerals interact with a number of oxide minerals such as iron oxide, aluminium oxide and titanium oxide (Lagaly, 2006). For example, Lagaly (2006) reported that a 9% kaolinite suspension displayed a high yield stress value at pH 3. When adding ferrihydrite at pH 3, the hydroxide was preferentially absorbed on the faces of kaolinite particles and re-charged them (edge (+)/face (+)) causing the yield stress to disappear. The edges of the kaolinite particles became negative with increasing pH, and edge(-)/face(+) contacts formed a network of particles with a maximum of yield stress at pH 7. The initial re-charging of the ferrihydrite reduced the positive face charge density, causing the network to disappear and the yield stress approached zero at pH 10. On the other hand, if the ferrihydrite was added at pH 9.5, all particles were negatively charged, and a yield stress was not measured. By decreasing the pH the positive charge of the ferrihydrite increased causing the ferrihydrite to bridge the negative kaolinite particles increasing the yield stress to a sharp and high maximum. The very high positive charge density of the hydroxide at lower pH promoted the adsorption of ferrihydrite on the basal plane surfaces of kaolinite particles, and the kaolinite (-)/ferrihydrite(+)/kaolinite(-) network collapsed as

shown by the strong decrease of yield stress (Lagaly, 2006). This is due to the changes in pH and surface charge of the clay and oxide minerals.

The interaction of clay minerals with calcium bearing minerals studied in this chapter is mainly caused by the Ca2+ cations released into the suspensions. This study follows the findings in Chapter 4 regarding the effect of ions on the rheology of clay mineral suspensions.

5.2 Results and discussion

5.2.1 Rheology comparison of the high and low clay content ores

The rheology of 30 wt.% high clay content ore and 30 wt.% low clay content ore is compared in Figure 5-1, and a difference in rheology is clear. The low clay content ore displayed the Newtonian nature of normal flotation slurries, whilst the high clay content ore displayed non- Newtonian behaviour with a yield stress value. This ore had a total clay content of 30 wt.%, and 16 wt.% calcite, 2 wt.% dolomite and 4 wt.% gypsum while the low clay content ore did not have calcium bearing minerals (Table 3-1). To have a better understanding of the differences in rheology between the two slurries in Figure 5-1, the Bingham yield stress values and the apparent viscosities at a shear rate of 100 s-1 were estimated. The high clay content ore had a Bingham yield stress of 1.1 Pa, and an apparent viscosity of 14 mPa.s. These two values for the low clay content ore were 0 Pa and 3.3 mPa.s. The high clay content ore has been problematic in recovering gold and copper minerals by flotation due to the high pulp viscosity. The low clay content ore has 3% kaolinite and 10% muscovite. Obviously, these clays did not have a significant effect on the rheology. To simulate the different rheological properties of the high clay content ore, kaolinite was added to the low clay content ore to make 30% kaolinite and the slurry of the mixture was then subjected to the rheology measurement. This rheogram is also shown in Figure 5-1 and located between the rheograms of the low and high clay content ores. The yield stress and the apparent viscosity for the mixture were approximately 0.4 Pa and 7.6 mPa.s, respectively. Although the rheological properties of the low clay content ore increased by adding kaolinite, they did not match those of the high clay content ore.

3.0 High clay content ore - 30 wt.% slurry - P80 106 µm - Natural pH 7.13 Low clay content ore - 30 wt.% slurry - P80 106 µm - Natural pH 7.82 Low clay content ore/30 wt.% Kaolinite mixture - 30 wt% slurry- Natural pH 7.55 2.5

2.0

1.5

1.0 Shear stress (Pa)

0.5

0.0 0 50 100 150 200 250 300 350 400 Shear rate (s-1)

Figure 5-1. Rheograms of the high and low clay content ores, and a mixture of low clay content ore and kaolinite (30 wt.%): slurries with solids density of 30 wt.% (14.6 vol.%) at natural pH.

In addition to the difference in clay concentration, the high clay content ore also has illite (a 2:1 layered non-expanding clay mineral) and calcium bearing minerals (calcite, gypsum and dolomite) present. The different rheograms in Figure 5-1 suggested that:

• Calcium bearing minerals cause changes in rheological properties, viscosity and yield stress. • Illite has a strong interaction with calcium bearing minerals. • Interactions between calcium bearing minerals and clay minerals cause changes in rheology of flotation slurries.

These were tested in the next sections.

5.2.2 Rheology of calcium bearing minerals

The rheology of dolomite, calcite and gypsum slurries at 30 wt.% was measured at natural pH. Their rheograms are shown in Figure 5-2. For comparison, the rheology of a quartz slurry was also measured. Its rheogram together with the rheogram of the high clay content ore is shown in Figure 5-2 as well. It should be noted that the size of these minerals and the ore is different. The P80 of the high clay content ore is 106 µm, the P80 of the quartz, calcite, and dolomite is 25 µm and the P80 of gypsum is 77 µm. Among these minerals, quartz showed Newtonian behaviour with no yield 87

stress and gypsum showed non-Newtonian behaviour with the highest yield stress. The rheograms for dolomite and calcite were identical and showed a transition from a Newtonian to a non- Newtonian fluid. The high clay content ore had a much higher yield stress than any mineral despite the greater P80 value. This suggests that the high viscosity of the high clay content ore is caused by factors other than the particle size and they might be related to the clay and calcium bearing minerals present in that ore.

The higher shear stresses from the gypsum, calcite and dolomite slurries when compared to the quartz slurry may be related to their surface charges at neutral pH. It is known that quartz has an isoelectric point (IEP) at very low pH and quartz particles are strongly negative and repulsive at neutral pH (Kilic et al., 2009). However, dolomite and calcite have an IEP at about 6.5 (Vdovic, 2001; Chen and Tao, 2004) resulting in moderate repulsion of particles at nature pH. The zeta potential of gypsum is slightly negative from pH 5 to 11 (Dávila-Pulido and Uribe-Salas, 2014) and therefore gypsum particles are weakly repulsive at neutral pH.

3.0 Quartz natural pH 7.75 Calcite natural pH 8.05 2.5 Dolomite natural pH 8.65

Gypsum natural pH 7.60 2.0 High clay content ore pH 7.13

1.5

1.0 Shear stress (Pa)

0.5

0.0 0 50 100 150 200 250 300 350 400 Shear rate (s-1)

Figure 5-2. Rheograms to compare the flow behaviour of quartz, calcite, dolomite, gypsum and high clay content ore at 30 wt.% (14.6 vol.%) solid concentration and natural pH.

5.2.3 Interactions of calcium bearing minerals with clay minerals

The rheological behaviour of kaolinite and illite, and their interactions with calcium bearing minerals in terms of modifying rheology were investigated. Bentonite, a typical 2:1 layered swelling clay mineral, was also included in this section for study since cations can interact differently with swelling clays. Lime, sodium hydroxide and soda ash were used as pH modifiers for all the rheology tests following the strategies used in the previous chapter.

5.2.3.1 Kaolinite

Figure 5-3 compares the rheograms of kaolinite slurries with and without mixing with calcium bearing minerals and quartz at natural pH. The addition of quartz, calcite, dolomite and gypsum increased the viscosity of the original kaolinite slurry by increasing the solid concentration from 25 wt.% for kaolinite to 30 wt.% for the mixtures. There are some rheological differences between the mixtures. For example, the addition of 5% calcium bearing minerals produced higher shear stress than the addition of 5% quartz as observed in Figure 5-2. However, the addition of 5% calcium bearing minerals produced similar rheograms and gypsum did not result in a higher viscosity than calcite and dolomite probably due to the low concentration of calcium bearing minerals in this measurement.

The increased rheological properties of the kaolinite slurry after mixing with quartz and calcium bearing minerals were attributed to the increase in volume fraction given by the addition of these minerals without a clear interaction with kaolinite at natural pH. It seems that at this pH the interaction between calcium cations and kaolinite is not significant.

Natural pH Quartz 5 wt.% - Kaolinite 25 wt.% natural pH 7.75 7.0 Calcite 5 wt.% - Kaolinite 25 wt.% natural pH 8.00 6.0 Dolomite 5 wt.% - Kaolinite 25 wt.% natural pH 8.12

5.0 Gypsum 5 wt.% - Kaolinite 25 wt.% natural pH 7.60 Kaolinite 25 wt.% natural pH 7.85 4.0

3.0

Shear stress (Pa) 2.0

1.0

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-3. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH.

Following the tests shown in Figure 5-3, the slurries of kaolinite and its mixtures were adjusted from natural pH to pH 10 by adding lime and sodium hydroxide and their rheograms are shown in Figure 5-4 and Figure 5-5, respectively. The gypsum – kaolinite slurry produced the highest shear stress and the difference between this slurry and the other ones was accentuated with sodium hydroxide. In addition to increasing the pH, sodium hydroxide supplies Na+ ions, but they do not contribute to increasing the rheological properties of kaolinite slurries as much as Ca2+ ions as demonstrated in the previous chapter. This is clearly reflected in the rheograms in Figure 5-4 and Figure 5-5 with lime producing higher shear stress than sodium hydroxide for kaolinite and its mixtures. It is interesting to note that when lime was replaced by sodium hydroxide to adjust the pH 10, the rheological properties of kaolinite-gypsum mixture only slightly decreased while the rheological properties of other mixtures decreased significantly. It appears that gypsum released sufficient Ca2+ ions to maintain the rheological properties even when sodium hydroxide was used to adjust the pH, while dolomite and calcite did not.

7.0

6.0

5.0

4.0

3.0

Shear stress (Pa) Quartz 5 wt.% - Kaolinite 25 wt.% 2.0 Calcite 5 wt.% - Kaolinite 25 wt.% Dolomite 5 wt.% - Kaolinite 25 wt.% 1.0 Gypsum 5 wt.% - Kaolinite 25 wt.% Kaolinite 25 wt.% 0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-4. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by lime.

Quartz 5 wt.% - Kaolinite 25 wt.% 7.0 Calcite 5 wt.% - Kaolinite 25 wt.% Dolomite 5 wt.% - Kaolinite 25 wt.% 6.0 Gypsum 5 wt.% - Kaolinite 25 wt.% Kaolinite 25 wt.% 5.0

4.0

3.0

Shear stress (Pa) 2.0

1.0

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-5. Comparison of rheograms for 25 wt.% kaolinite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by sodium hydroxide.

When soda ash was used as the pH modifier, pH 10 could not be achieved for the gypsum slurry even with large additions of soda ash. DiFeo et al. (2004) found that when soda ash was used to

adjust the pH of gypsum slurry, a number of reactants depleted OH- ions. In fact soda ash dispersed kaolinite and its mixtures as demonstrated in the previous chapter.

5.2.3.2 Illite

Illite is commonly associated with micas and smectites (Brigatti et al., 2006) (Ndlovu et al., 2014). The most significant difference between illite and montmorillonite is the presence of a non- exchangeable interlayer potassium ion. This cation holds the unit layers of the illite structure preventing polar ions to enter the interlayer (Wallace et al., 2004). This characteristic probably makes the illite more similar to kaolinite than to bentonite in terms of rheology.

The rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH are shown in Figure 5-6. Compared to the kaolinite case (Figure 5-3), the similarity was that calcium bearing minerals produced higher shear stress than quartz when mixing with illite, but the difference was that gypsum produced obviously higher shear stress than calcite and dolomite when mixing with illite but the three calcium bearing minerals produced similar shear stress when mixing with kaolinite. This suggests that the gypsum – illite interaction was more evident than the gypsum – kaolinite interaction.

3.0

Quartz 5 wt.% - Illite 25 wt.% natural pH 8.13 Calcite 5 wt.% -Illite 25 wt.% natural pH 8.10 Dolomite 5 wt.% - Illite 25 wt.% natural pH 8.26 2.0 Gypsum 5 wt.% - Illite 25 wt.% natural pH 7.81 Illite 25 wt.% natural pH 8.19

1.0 Shear stress (Pa)

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-6. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH.

The rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by lime and sodium hydroxide are shown in Figure 5-7 and Figure 5-8, respectively. One observation is that in contrast to kaolinite slurries with pH adjusted by lime and sodium hydroxide (Figure 5-4 and Figure 5-5), lime did not produce much higher rheological properties than sodium hydroxide for the illite mixtures with calcite, dolomite and quartz. This indicates that illite is less responsive to Ca2+ ions compared to kaolinite in this pH range. However, Figure 5-7 and Figure 5-8 indicated that gypsum interacted more strongly with illite than calcite, dolomite and quartz producing much higher shear stress.

The different rheology behaviour of kaolinite and illite in the presence of calcium bearing minerals may be attributed to variations in face and edge charges, surface areas, and chemical composition of the faces and edges. It is reported that illite has a greater degree of charge anisotropy, and its faces have a higher negative charge (Ndlovu et al., 2014). Illite also has a higher cation exchange capacity than kaolinite, and a greater specific surface area (Kahr and Madsen, 1995; House, 1998). With regard to the chemical composition of the faces and edges, kaolinite has siliceous and aluminous faces and edges, while illite has only siliceous (Konan et al., 2007). These two types of surfaces react differently with Ca2+ ions. It is suggested that the silica faces and the much larger specific area of illite particles allow for more intake of Ca2+ ions (Konan et al., 2007; Konan et al., 2008). This means that to saturate the illite surfaces more Ca2+ ions are needed to increase the viscosity of the illite slurries and among the three calcium bearing minerals, only gypsum is a source of a large amount of Ca2+ ions.

When soda ash was used as a pH modifier the target pH of 10 could not be achieved for the gypsum slurry as explained previously, and it dispersed illite and its mixtures as observed in the kaolinite case.

3.0 Quartz 5 wt.% - Illite 25 wt.% Calcite 5 wt.% - Illite 25 wt.% Dolomite 5 wt.% - Illite 25 wt.% Gypsum 5 wt.% - Illite 25 wt.%

2.0 Illite 25 wt.%

1.0 Shear stress (Pa)

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-7. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by lime.

3.0 Quartz 5 wt.% - Illite 25 wt.% Calcite 5 wt.% - Illite 25 wt.% Dolomite 5 wt.% - Illite 25 wt.%

Gypsum 5 wt.% - Illite 25 wt.% 2.0 Illite 25 wt.%

1.0 Shear stress (Pa)

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-8. Comparison of rheograms for 25 wt.% illite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at pH 10 adjusted by sodium hydroxide.

5.2.3.3 Bentonite

It is known that sodium bentonite has a high swelling capacity, and one reason for this is the hydrophilic nature of this type of clay mineral which contributes to the large intake of water in the interlayer space (Luckham and Rossi, 1999). In Chapter 4 it was found that the addition of sodium carbonate increased the rheological properties of bentonite slurries much more than lime. This indicated that the effect of lime on the rheology of bentonite slurry was twofold. On one hand, lime may aggregate the clay platelets and then increases the viscosity of bentonite slurries, but on the other hand, lime may de-activate sodium bentonite and then reduce or eliminate its swelling potential. Unlike lime, sodium carbonate activated bentonite increasing its swelling capacity.

Figure 5-9 shows the rheograms of 15 wt.% bentonite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH. In this case, a lower clay concentration was used due to the high viscosity of bentonite slurries resulting from its high swelling capacity. Figure 5-9 shows that the addition of 5% calcite, dolomite and quartz increased the shear stress of bentonite slurries as observed from the kaolinite and illite cases. However, the addition of 5% gypsum reduced the shear stress of the bentonite slurry suggesting that gypsum significantly de-activated bentonite more than lime would for bentonite slurries in the previous chapter.

Since soda ash activates bentonite, increasing its swelling capacity, it is interesting to know how it affects the rheology of bentonite slurries in the presence of calcium bearing minerals. Figure 5-10 shows the comparison of rheograms of 15 wt.% bentonite and its mixtures with 5 wt.% calcite, dolomite and quartz at pH 10 and with 5 wt.% gypsum at pH 8.4 adjusted by soda ash. The maximum pH soda ash could adjust the bentonite-gypsum mixture was 8.4. It was found that the gypsum - bentonite mixture still had the lowest shear stress with the addition of soda ash, and this pH modifier could not activate bentonite at all. In contrast, soda ash activated bentonite in the absence and presence of quartz, dolomite and calcite and increased the shear stress compared to their counterparts at natural pH. Clearly, calcite enhanced the ability of soda ash to activate bentonite more than dolomite.

50.0 Quartz 5 wt.% - Bentonite 15 wt.% natural pH 9.40 Calcite 5 wt.% - Bentonite 15 wt.% natural pH 9.15

40.0 Dolomite 5 wt.% - Bentonite 15 wt.% natural pH 9.40 Gypsum 5 wt.% - Bentonite 15 wt.% natural pH 8.01 30.0 Bentonite 15 wt.% natural pH 9.19

20.0 Shear Shear (Pa) stress 10.0

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-9. Comparison of rheograms for 15 wt.% bentonite and its mixtures with 5 wt.% gypsum, calcite, dolomite and quartz at natural pH.

50.0

40.0

30.0

20.0 Quartz 5 wt.% - Bentonite 15 wt.% Calcite 5 wt.% - Bentonite 15 wt.%

Shear Shear (Pa) stress Dolomite 5 wt.% - Bentonite 15 wt.% 10.0 Gypsum 5 wt.% - Bentonite 15 wt.% Bentonite 15 wt.%

0.0 0 50 100 150 200 250 300 350 Shear rate (s-1)

Figure 5-10. Comparison of rheograms for 15 wt.% bentonite and its mixtures with 5 wt.% calcite,

dolomite and quartz at pH 10 and with 5 wt.% gypsum at pH 8.4 adjusted by Na2CO3.

5.2.3.4 Solubility tests of calcium bearing minerals

Rheology measurements indicated that calcite, dolomite and gypsum interacted with kaolinite, illite and bentonite slurries differently. In all the slurries gypsum shows the strongest interaction by increasing the viscosity of kaolinite and illite slurries and deactivating bentonite. The effect of calcite is noticeable when mixed with bentonite at pH 10 adjusted with soda ash. This combination gives the highest viscosities of the bentonite mixtures. The influence of dolomite on the clay minerals is not that significant, but still surpasses the effect of quartz suggesting some cations released by this mineral.

Solubility tests were conducted to confirm that the dependence of the effect of calcium bearing minerals on the rheology of clay mineral slurries on cations released. Results are shown in Table 5-1 indicating the experimental values for the mili-equivalents per litre (meq/l) of some cations released by the calcium bearing minerals. The meq/l values provide a good indication of the amount of cations available to interact with the clay minerals. Values for quartz are also given since it was used as a reference in the rheology tests, and its solubility is virtually zero. The results agree with the solubilities reported in literature (Lier et al., 1960; Somasundaran et al., 1985; Lewry and Williamson, 1994; Warren, 2000; Kopittke et al., 2004; Azimi et al., 2007). Calculations indicate that the gypsum used in this work released approximately 59 times more meq of Ca2+ than calcite. Dolomite released a very small amount of Ca2+ and Mg2+ ions.

Table 5-1. Miliequivalent per litre (meq/l) of Ca2+, Mg2+ and Na+ after dissolution of quartz, calcite, dolomite and gypsum. Mineral Ca2+ Mg2+ Na+ Quartz 0.00 0.00 0.00

Calcite (CaCO3) 0.55 0.00 0.00

Dolomite (CaMg)(CO3)2 0.05 0.16 0.00

Gypsum dihydrate (CaSO4.2H2O) 32.24 0.00 0.00

Solubility tests confirm that the amount of Ca2+ ions from gypsum can have a key role in the interaction with kaolinite, illite and bentonite as observed in the rheology experiments. In addition, gypsum can certainly release a sufficient quantity of Ca2+ ions to interact with soda ash limiting the increase of pH in flotation slurries (DiFeo et al., 2004).

The main observation from the previous results is that kaolinite and illite have similar rheology behaviours in the presence of extra Ca2+ ions released by gypsum, but the sodium bentonite is deactivated for hydration with the presence of gypsum. This is the most important finding as it has a significant impact on copper-gold flotation as shown in the next chapter. The gypsum effect on bentonite has been previously studied (Alther, 1986), but not in the flotation context. Gypsum deactivates the sodium bentonite for hydration and swelling by providing the Ca2+ ions required to replace the Na+ ions present in the sodium bentonite. The exchange of the Na+ for bivalent cations is thermodynamically favourable, and complete replacement of Na+ by Ca2+ and Mg2+ may happen (Meer and Benson, 2007; Bradshaw et al., 2013).

It has been reported that an interlayer saturated with polyvalent cations does not expand as much as in the case of the Na+ saturated bentonite. This is because the repulsive effect of ion hydration is offset by the electrostatic attraction between the cation and silicate layers that make the interlayer space (Luckham and Rossi, 1999; Ferrage et al., 2005). In this case the length of the interlayer separation is not more than 10 Å, but when the interlayer is saturated with Na+ ions, the hydration of these cations may suddenly increase that separation up to 30-40 Å. This swelling continues to increase the separation of the interlayer to several hundred Angstroms (Luckham and Rossi, 1999), and this is called osmotic swelling. The other type of swelling is called crystalline swelling with the interlayer separation up to 20 Å (Luckham and Rossi, 1999). Swelling of bentonite is strongly influenced by the cation in the interlayer space of the montmorillonite. If Na+ is the dominant cation, both crystalline and osmotic swellings occur. When polyvalent cations such as Ca2+ and Mg2+ are predominant, only crystalline swelling happens (Bradshaw et al., 2013). As a result, the replacement of the Na+ by Ca2+ ions from the gypsum allows only crystalline swelling in the interlayer space. Hydration not only occurs in the interlayer space of the montmorillonites, but also on external surfaces and in both cases it can be continuous with unlimited adsorption of water. Other form of hydration is the capillary condensation of free water in micropores (Brigatti et al., 2006).

In this study, it was found that if gypsum was added to the bentonite slurry after one hour mixing, the slurry could not be dispersed and the viscosity remained very high. This suggests that the high viscosity values for the pure bentonite slurry were mostly given by the hydration of the bentonite, and that the Ca2+ ions do little to decrease viscosity once the clay has been hydrated. The dispersing effect of gypsum was only noticed if the bentonite and gypsum were mixed simultaneously or the bentonite was added to the water containing the gypsum. This study does not look at the details of

the clay hydration or formation of network structures and it is assumed that the main effect on rheology of bentonite slurries comes from the interlayer swelling.

Other significant observation is the high yield stress values for the mixture calcite – bentonite when the pH increased to 10 by using soda ash (Figure 5-10). This may indicate that the Ca2+ cations released by the calcite and combined with the Na+ ions from the soda ash contribute to an increase in viscosity. It had been reported that for montmorillonites in the presence of Na+ and Ca2+ ions in the exchangeable sites, a maximum viscosity is obtained when Ca2+ions occupy about 40 % of these sites (Bleifuss, 1973). This balance of cations in montmorillonite can be affected by the presence of extra cations in the surroundings as studied by Bradshaw et al. (2013). They explained that Na+ ions were exchanged by divalent cations present in the environment and that one of the sources was the dissolution of calcite. The exchange of the Na+ for bivalent cations is thermodynamically favourable, and there are examples that confirm complete replacement of Na+ by Ca2+ and Mg2+ (Meer and Benson, 2007; Bradshaw et al., 2013).

One explanation for how the correct balance of Na+ and Ca2+ ions gives the maximum hydration and viscosity is based on the level of hydration of Ca2+ and Na+ ions and the electrostatic attraction these cations have with the surfaces in the interlayer space. If this space is mostly saturated with Na+ ions, the attraction between the surfaces making this space is not that strong and allows the separation of the layers facilitating the osmotic swelling. A smaller proportion of Ca2+ ions in the interlayer space probably do not provide enough electrostatic attraction with the silicate layers of the interlayer space and osmotic swelling still occurs. This means that in osmotic swelling Na+ and Ca2+ could be present in the interlayer space. Since Ca2+ ions hydrate more than Na+ ions the hydration of the montmorillonite could be more extended in presence of both Ca2+ and Na+ ions than in the presence of only Na+ ions. The saturation of the interlayer space with Ca2+ ions does not allow osmotic swelling but crystalline swelling. This saturation happens in the gypsum – bentonite mixture as shown in the rheological behaviour of this slurry.

5.3 Conclusions

The high illite and kaolinite content and the presence of significant amounts of calcite, dolomite and gypsum had a synergistic effect on the rheological properties of the high clay content ore. These calcium bearing minerals released Ca2+ ions which had a strong interaction with kaolinite and illite and therefore enhanced rheological properties in a similar way to the pH modifier, lime. Gypsum

had the greatest effect on the rheology of kaolinite and illite and this was caused by the high solubility of gypsum in water releasing a large amount of Ca2+ ions when compared to calcite and dolomite.

Gypsum also had a greatest effect on the rheological properties of bentonite but produced the opposite response. Ca2+ ions released from gypsum deactivated sodium bentonite or prevented its swelling resulting in the lower rheological properties.

These findings demonstrate that when dealing with ores with high clay contents, it is important to consider the interaction of clay minerals with other gangue minerals and how this may contribute to the rheological properties.

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Chapter 6 The interaction of clay minerals with gypsum and its effects on copper-gold flotation

6.1 Introduction

The objective of this chapter is to investigate the effect of gypsum on the flotation of the copper- gold ore with either kaolinite or bentonite added. As discussed in Chapter 5, calcium bearing minerals can be present in ores and associated with clay minerals, and from the minerals tested, gypsum had the strongest rheological interaction with bentonite and kaolinite. This is because gypsum acts as a source for Ca2+ cations in slurries at an enough quantity to modify the kaolinite and bentonite particle associations. Results in Chapter 5 showed that Ca2+ cations from gypsum slightly increased the viscosity of kaolinite slurries, but decreased it to a great extent in bentonite suspensions. The bentonite-gypsum interaction was the most important finding in Chapter 5, and it led to this study. Gypsum inhibited the swelling of bentonite due to the high concentration of Ca2+ cations from gypsum in the slurry (32 meq/l). It is suggested that the presence of gypsum in ores with high kaolinite or bentonite content causes similar changes in rheology and that these changes affect flotation outcome. To confirm this, mixtures of a low clay content ore with either kaolinite or bentonite were used as baselines to measure rheology and flotation performance. Gypsum was then added to these mixtures to quantify its effect on the rheology and flotation of the baselines. Cryo- SEM imaging was used to identify particle associations and support rheology and flotation findings.

6.2 Results and discussion

6.2.1 Flotation

Six flotation tests were conducted using the copper-gold ore and its mixtures with clay minerals and gypsum. Figure 6-1 shows the mass recovery as a function of water recovery from these tests. As can be seen, flotation of the ore alone or baseline, resulted in a 5% mass recovery and 31% water

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recovery at the completion of flotation, and the mixture with 30 wt.% quartz had a mass and water recovery very similar to this baseline. The addition of 15% bentonite reduced the water recovery significantly to about 9%. This was because a froth phase was almost absent during the flotation of this mixture. The addition of 30% kaolinite increased water recovery and mass recovery significantly to 38% and 24%, respectively. These recoveries are different to the values for the mixture with quartz which had 34% water recovery and 6% mass recovery. This suggests that the entrainment of particles in these two slurries was the result of different mechanisms despite both having a similar particle size distribution. The addition of 5 wt.% gypsum to the bentonite-ore mixture produced much higher water and mass recoveries due to the formation of a more stable froth phase. The addition of 5 wt.% gypsum to the kaolinite-ore mixture also increased water and mass recoveries, but to a lower degree than the gypsum addition to the bentonite-ore mixture.

35 100 wt.% ore 30 wt.% kaolinite - 70 wt.% ore 30 wt.% kaolinite - 5 wt.% gypsum - 65 wt.% ore 30 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore 30 wt.% quartz - 70 wt.% ore

Mass recovery (%) 10

0 0 5 10 15 20 25 30 35 40 45 50 Water recovery (%)

Figure 6-1. Mass recovery as a function of water recovery from the flotation of the low clay content ore and its mixtures with clay minerals and gypsum.

Figure 6-2 and Figure 6-3 show copper grade as a function of copper recovery, and gold grade as a function of gold recovery, respectively from the flotation of the ore and its mixtures. The main observation was the poor flotation outcome that resulted from the mixture with bentonite and the change after gypsum addition. The baseline flotation of the ore resulted in a copper recovery of 92% at a grade of 10% copper (Figure 6-2), and 81% gold recovery at a grade of 7 ppm gold 102

(Figure 6-3). These copper and gold recoveries were almost the same as the recoveries obtained for the quartz mixture, but the addition of quartz caused a decrease in grade to 6% for copper and 4 ppm for gold. These recoveries might be an indication that in this work the decrease in feed grade for the prepared mixtures did not affect recovery.

Although copper and gold grades from the flotation of the bentonite-ore mixture were close to the baseline flotation, copper and gold recoveries were much lower (Figure 6-2 and Figure 6-3).The addition of gypsum to the bentonite-ore system changed the flotation response significantly by improving copper recovery from 83% to 93% and gold recovery from 64% to 85%. Meanwhile, the addition of gypsum to the bentonite-ore mixture decreased copper and gold grades throughout the flotation (Figure 6-2 and Figure 6-3). For example, copper grade went down from 8% to 2%, and the gold grade from about 5 ppm to 1 ppm at the end of flotation. When gypsum was added to the bentonite-ore mixture, it was observed that the froth became more abundant and stable as supported by the higher mass-water recovery curve in Figure 6-1.

30 100 wt.% ore 30 wt.% kaolinite - 70 wt.% ore 30 wt.% kaolinite - 5 wt.% gypsum - 65 wt.% ore 25 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore 30 wt.% quartz - 70 wt.% ore

10 Copper grade (%) grade Copper

0 0 10 20 30 40 50 60 70 80 90 100 Copper recovery (%)

Figure 6-2. Copper grade as a function of copper recovery from the flotation of the ore and its mixtures with clay minerals and gypsum.

The addition of kaolinite to the ore did not decrease copper and gold recoveries from flotation, but did decrease copper and gold grades to about 2%, and 1 ppm as shown in Figure 6-2 and Figure 6-3,

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respectively. The addition of gypsum to the kaolinite-ore mixture did not further affect copper and gold flotation significantly.

100 wt.% ore 30 wt.% kaolinite - 70 wt.% ore 30 wt.% kaolinite - 5 wt.% gypsum - 65 wt.% ore 10 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore 30 wt.% quartz - 70 wt.% ore Gold grade (ppm) Gold grade 5

0 0 10 20 30 40 50 60 70 80 90 100 Gold recovery (%)

Figure 6-3. Gold grade as a function of gold recovery from the flotation of the ore and its mixtures with clay minerals and gypsum.

The increase in recovery can be attributed to recovery by both entrainment and true flotation and it was assumed that the entrainment of clay aggregates was through bubble lamella similar to normal fine particles. The Savassi equation (Savassi et al., 1998; Bıçak et al., 2012) was used to calculate copper recovery by true flotation and this equation works on the assumption that valuable and gangue minerals are equally entrained. However, entrainment of valuable minerals or other gangue minerals might be affected by the shape of clay minerals. Wiese et al. (2015) explained that entrainment increases with the increase in particle aspect ratio, and this is for both hydrophobic and hydrophilic particles. These authors proposed that drag forces are affected by shape and consequently terminal velocities since these are related to the drag coefficient. Cylindrical particles will experience higher drag force than spheres and will move up through the liquid to a greater extent (Wiese et al., 2015). It is expected that clay minerals will create even higher drag forces than cylindrical particles or particles with more symmetrical shapes. This means that particle morphology can be one factor playing a role in entrainment selectivity, and it could be speculated that clay minerals might displace other minerals from the bubble wake and lamella affecting the expected proportions of entrained minerals.

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Recoveries calculated with the Savassi equation are shown in Figure 6-4, and it is seen that the overall copper recovery by true flotation from the flotation of the ore and the mixture with quartz was similar (about 92%). The true flotation recoveries for the mixtures with clay minerals and gypsum were close to those baselines (about 91%) except for the flotation of bentonite-ore mixture that produced much lower copper recovery by true flotation (82%). However, by examining copper recovery from the first concentrate in Figure 6-4, all the mixtures apart from the bentonite mixture, produced higher copper recovery than the baseline case. It seems that the addition of kaolinite to the ore increased copper recovery by true flotation marginally, but the addition of bentonite decreased copper recovery by true flotation significantly. The addition of gypsum with bentonite altered the deleterious effect of bentonite and improved copper flotation by true flotation.

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100 wt.% ore 40 30 wt.% kaolinite - 70 wt.% ore 30 wt% kaolinite - 5 wt.% gypsum - 65 wt.% ore 15 wt.% bentonite - 85 wt.% ore 20 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore Recovery by true flotation (%) flotation true by Recovery 30 wt.% quartz - 70 wt.% ore

0 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

Figure 6-4. Copper recovery by true flotation from the flotation of the ore and its mixtures with clay minerals and gypsum.

Figure 6-5 shows the copper recovery by entrainment. The flotation of the ore and the mixture with quartz produced about the same copper recovery by entrainment (only 0.5%). The flotation of the kaolinite-ore mixture produced a much higher overall copper recovery by entrainment (3.2%) than the flotation of the bentonite-ore mixture (1.2%). The addition of gypsum significantly changed this recovery especially in the case of the bentonite-ore mixture where the recovery by entrainment was

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increased to 3.0%. In the mixture with kaolinite, gypsum increased the recovery by entrainment to 3.8 %.

An in-depth study of entrainment was not one of the objectives of this thesis, but findings from this work can contribute to a systematic study of entrainment following this thesis. The degree of entrainment should be different for each mechanism depending on the clay particle associations. For instance, it would be expected that entrainment by the Boundary Layer Theory is more important than entrainment by the Bubble Swarm Theory when clay particles are dispersed.

100 wt.% ore 5 30 wt.% kaolinite - 70 wt.% ore 30 wt% kaolinite - 5 wt.% gypsum - 65 wt.% ore 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore 4 30 wt.% quartz - 70 wt.% ore

1 Recovery by entrainment (%) entrainment by Recovery

0 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

Figure 6-5. Copper recovery by entrainment from the flotation of the ore and its mixtures with clay minerals and gypsum.

From these flotation results it can be inferred that the presence of bentonite reduced true flotation, but the presence of gypsum caused bentonite to behave more like kaolinite. Kaolinite increased the recovery by entrainment as a result of the creation of a more stable froth which is supported by the high mass-water recovery curve (Figure 6-1). This entrainment increased the overall recovery, but diluted the concentrate grade (Figure 6-2 and Figure 6-3). The addition of gypsum to the kaolinite- ore system promoted additional entrainment, further increasing the recovery but decreasing the grade (Figure 6-2 to Figure 6-5). Addition of quartz with a particle size distribution similar to the clay minerals does not increase recovery by entrainment.

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6.2.2 Rheology measurements

The rheology measurements of the ore and its mixtures with clay minerals and gypsum at pH 10 are shown in Figure 6-6. The ore and quartz-ore mixture had the lowest shear stress values and lowest viscosities. The addition of 30 wt.% kaolinite or 15 wt.% sodium bentonite to the ore increased the viscosity as observed in Chapter 4. The addition of gypsum reduced the rheological properties of bentonite slightly, but had little effect on the rheological properties of kaolinite as shown in Figure 6-6. Due to the swelling property of bentonite, 15 wt.% bentonite increased the viscosity to the same degree as 30 wt.% kaolinite when mixed with the ore.

100 wt.% ore 30 wt % kaolinite - 70 wt.% ore 2.0 30 wt. % kaolinite - 5 wt.% gypsum - 65 wt.% ore 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore 30 wt.% quartz - 70 wt.% ore 1.5

1.0 Shear stress (Pa) 0.5

0.0 0 50 100 150 200 250 Shear rate (s-1)

Figure 6-6. Rheograms for the ore and its mixtures with clay minerals and gypsum: total solids concentration 30 wt.% and pH adjusted to 10 with hydrated lime (Ca(OH)2).

Adjusting the pH with hydrated lime added Ca2+ cations to the slurry, and gypsum contributed to additional Ca2+ cations that should cause kaolinite particles to form agglomerates increasing the viscosity. However, this increase was not observed when gypsum was added to the kaolinite-ore mixture. The unexpected rheology response was further examined by Cryo-SEM analyses in the next section. In contrast, gypsum decreased the viscosity of the bentonite-ore mixture by either inhibiting the formation of the bentonite network structures or reducing its swelling properties.

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At a first glance the rheograms in Figure 6-6 did not show any major difference between the ore and its mixtures with clay minerals and gypsum. The rheograms for ore-kaolinite and ore-bentonite mixtures were similar, but the flotation response for these two mixtures was very different. One difference between the bentonite-ore mixture and other mixtures was the higher shear stress values in the shear rate range between 0.1 and 50 s-1. (Figure 6-6). This suggests that for rheology it may be better to compare the yield stress regions of the rheograms or the shear stress values at low shear rates. Shear rates in a flotation cell vary from high to low values depending on the proximity of the slurry to the flotation cell impeller and the rheology of the slurry (Bakker et al., 2009a). This causes the ore-clay mixtures which are non-Newtonian, to have different shear stresses and viscosities at different locations in the flotation cell. It has been demonstrated that there is a considerable variation of the shear rate distribution inside the flotation cell with the highest values close to the impeller (Bakker et al., 2009a). The other areas in the flotation cell may have relatively small shear rate values that can be close to zero (Anderson, 2008).

Figure 6-7 shows the relationship between apparent viscosity and shear stress. The ore-bentonite mixture had much higher apparent viscosities values than the other mixtures in the shear stress range between 0.5 and 0.9 Pa. In this shear stress range the shear rate for this mixture is between 0.25 and 6 s-1 (Figure 6-6). This means that for the zones in the flotation cell with shear rates in that range, the apparent viscosity values are greater when a 15 wt.% bentonite is present in the mixture with the ore.

100 wt.% ore 2000 30 wt % kaolinite - 70 wt.% ore 30 wt. % kaolinite - 5 wt.% gypsum - 65 wt.% ore 15 wt.% bentonite - 85 wt.% ore 15 wt.% bentonite - 5 wt.% gypsum - 80 wt.% ore

1000 Apparent viscosity (mPa.s) viscosity Apparent

0 0 0.5 1 1.5 2 2.5 Shear stress (Pa)

Figure 6-7. Apparent viscosity as a function of shear stress (Pa) for the ore and its mixtures with clay

minerals and gypsum: pH adjusted to 10 with hydrated lime (Ca(OH)2).

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Figure 6-6 and Figure 6-7 suggest that the Herschel-Bulkley model may be more appropriate to calculate the yield stress for the slurries used in this work since this model fits the yield stress region in the rheograms well. Figure 6-8 shows the calculated apparent yield stresses for the ore and its mixtures with clay minerals and gypsum using the Herschel-Buckley model. This figure also illustrates the copper recoveries for the first two concentrates in the flotation of the clay-ore mixtures. Results show that there is a difference in yield stress between the bentonite-ore mixture (0.9 Pa) and the kaolinite-ore mixture (0.5 Pa). The trend in yield stress for the mixtures in Figure 6-8 has some correlation with copper recovery as it is seen that the mixture with the lowest yield stress had the highest recovery, and the mixture with the highest yield stress had the lowest recovery.

1.0 90

0.9 80

0.8

70 0.7 60 0.6 50 0.5 40 0.4 30 0.3 Copper recovery (%) 20 Apparent yield stress (Pa) stress yield Apparent 0.2

0.1 10

0.0 0 Low-clay ore and Kaolinite 30 wt. % Kaolinite 30 wt % Bentonite 15 wt.%Bentonite 15 wt.% Quartz 30 wt.% - - Gypsum 5 wt.% - - ore 70 wt.% - gypsum 5 wt.% - - ore 85 wt.% Ore 70 wt.% ore 65 wt.% oren 80 wt.% mixture

Figure 6-8. Calculated apparent yield stresses for the ore and its mixtures with clay minerals and gypsum using Herschel-Buckley model. Copper recovery for the first two concentrates is also shown.

6.2.3 Cryo-SEM analysis

The flotation results confirm that both kaolinite and bentonite have a detrimental effect on copper and gold flotation, and rheology tests suggest that the high viscosity could be one contributing

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factor. In this section Cryo-SEM was used to understand the types of aggregates and network structures formed by clay minerals and their interactions with gypsum.

Cryo-SEM images in Figure 6-9 and Figure 6-10 compare the type of aggregates and network structures present in the pulp of ore-bentonite and ore-kaolinite mixtures at different resolutions. It is clear that there are differences in the kaolinite and bentonite particle aggregates at the same pH. Figure 6-9a and Figure 6-10a show that the bentonite created a honeycomb like network structure. This structure appears to be made of many interconnected cells with pore size at about 5- 10 µm in diameter where E-E and E-F aggregates were predominant. Figure 6-9b and Figure 6-10b shows that the kaolinite platelets associate mainly in the mode of F-F and E-E as dense elongated strings and the resulting aggregates are joined to each other commonly in an E-F manner to form loose clumps. No honeycomb like network structure was observed in the pulp of ore-kaolinite mixture. This agrees with the findings from Morris and Zbik (2009) when preparing a 3 wt.% bentonite suspension. They explained that the bentonite sheets were larger and more flexible and their basal surfaces appeared to repel each other causing an expanded, spongy textural pattern or network structure that extended throughout the suspension via clay platelets networking with an E- E orientation. The cells in this structure retained water and had a porosity of 62%.

Since a greater proportion of kaolinite was added compared to sodium bentonite, 30 and 15 wt.% respectively, the kaolinite aggregates look more crowded with larger aggregates. Despite this, the 110

slurry containing sodium bentonite had higher shear stress and viscosity values at low shear rates as it was explained in the rheology section. This suggests that the network structure made by bentonite particles provided much more resistance to flow than the loose kaolinite aggregates.

Wang and Peng (2014) explained that the entrainment of clay particles in flotation depended on the type of clay particle association, and the recovery of clay minerals increased if they entered the froth as aggregates increasing froth stability. They stated that edge-edge (E-E) associations of clay platelets gave higher viscosities resulting in poor flotation. Face-face (F-F) aggregates are more compact with denser structures giving lower viscosities. In this current study, this difference in viscosity caused by the E-E and F-F aggregates was observed in the mixtures with bentonite (E-E predominant) and kaolinite (mostly F-F associations). In general it can be inferred that the bentonite particles were more strongly attached making a network structure that caused higher viscosities values and lower entrainment rates than kaolinite. The kaolinite aggregates were weaker and more dispersed, as supported by rheology tests and Cryo-SEM imaging, and were easily entrained by any of the three reported mechanisms (Boundary Layer Theory, Bubble Wake Theory and Bubble Swarm Theory), but as explained before, for calculations in this thesis it is assumed that entrainment occurred through bubble lamella or by the concept of Boundary Layer Theory.

When gypsum was added to the bentonite-ore mixture, the extensive E-E connected long strings were formed without any E-F associations and the bentonite aggregates were thicker due to more F- 111

F associations as compared to the bentonite-ore mixture (Figure 6-11b and Figure 6-12b). These observed changes in network structure agree with the findings from Morris and Zbik (2009) who added 0.05 M CaCl2 to a 3 wt.% bentonite suspension. They observed that the addition of calcium cations dramatically changed the arrangement of the bentonite particles, and large and relatively compact lettuce-like aggregates formed causing the fragmentation of the network structure. The larger aggregates formed due to the collapse of the electrical double layers after adsorption of Ca2+ cations (Morris and Żbik, 2009). Although the 5 wt.% gypsum in the slurries used in this work gave approximately a 0.016 M concentration of Ca2+ cations, the change in the network structures was still evident. The presence or absence of network structures was reflected in rheological and flotation performance as shown in the previous sections.

Figure 6-11. Cryo-SEM images showing the network structure formed in bentonite-ore and bentonite- gypsum-ore mixtures: 1000x magnification.

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Figure 6-12. Cryo-SEM images showing the network structure formed in bentonite-ore and bentonite- gypsum-ore mixtures: 4000x magnification.

The Ca2+ cations released by gypsum appear to be present in sufficient quantities to attach to the negative surfaces of the sodium bentonite particles and change their electrical double layers. The compression of these layers causes the particles to aggregate mostly in F-F orientations from original three dimensional honeycomb-like structure in the absence of Ca2+ cations. Ca2+ cations can also interfere with the interlayer swelling of the sodium bentonite, since the exchange of the Na+ cations in the interlayer for Ca2+ cations is thermodynamically favourable, and complete replacement of Na+ by Ca2+ may happen (Meer and Benson, 2007; Bradshaw et al., 2013). Ca2+ cations in the interlayer only allow for crystalline swelling with an interlayer separation of up to 20 Å. On the other hand, the hydration of Na+ cations in the interlayer can increase that separation to several hundred Angstroms (Luckham and Rossi, 1999), which is called osmotic swelling. In addition to this, even small amounts of Ca2+ cations can hold together the silicate layers, and build up band-type networks. Large amounts of Ca2+ cations contract these networks forming compact particle aggregates (Figure 6-13) (Lagaly and Dékány, 2013).

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Figure 6-13. Schematic showing the aggregation of the clay mineral layers with increasing attraction: (a) single layers, (b) band-type networks, (c) compact particles (Lagaly and Dékány, 2013).

Cryo-SEM images of the flotation froth also show some differences in clay particle aggregation as seen in Figure 6-14 and Figure 6-15 at two magnifications. In the slurry with no gypsum addition (Figure 6-14a and Figure 6-15a) the long strings of clay particles were formed from the aggregation at the borders of the bubbles before breakage during sampling, and the addition of gypsum made thinner strings with more compact aggregates that appeared to be made mostly of E-E and F-F associations. The concentration or packing of these aggregates was also reflected in the mass-water recovery and recovery by entrainment curves that showed the transport of more solids into the concentrate (Figure 6-1, and Figure 6-5). The aggregates in the bentonite-ore mixture formed less compacted strings in E-E, F-F and E-F association modes. It is interesting to note that a gel like material is present all over the particles in both the pulp and froth samples. It was found to contain 6 at.% Na, 18 at.% Al, 41 at.% Si, 19 at.% Ca and 16 at.% S using EDS. Some of this appears as threads between the bentonite strings while some present like separate films on the string surfaces or between the strings. However, no gypsum particles were found in both pulp and froth samples. It was assumed that the gypsum may react with bentonite to form the gel like material.

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Figure 6-14. Cryo-SEM images illustrating the change in froth agglomerates with the addition of gypsum to the bentonite-ore mixture: 1000x magnification.

Figure 6-15. Cryo-SEM images illustrating the change in froth agglomerates with the addition of gypsum to the bentonite-ore mixture: 4000x magnification.

Figure 6-16 and Figure 6-17 show Cryo-SEM images at two magnifications for the aggregation of kaolinite in the pulp of mixtures with and without gypsum. Figure 6-18 shows the aggregation of kaolinite in the froth for these two mixtures. Kaolinite aggregates associated in the E-E, E-F and F- F modes in both the pulp (Figure 6-16a) and froth (Figure 6-18a) of the mixture without gypsum with much smaller aggregate particles as compared to the sample with gypsum addition. The aggregates in the pulp (Figure 6-16b) and froth (Figure 6-18b) of the mixture with gypsum are

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much larger than samples without gypsum. The thick long strings are associated mostly in F-F and E-E modes with many inter-aggregate water sites all over the strings (Figure 6-17b, Figure 6-18b). Similar to the bentonite-gypsum-ore mixture, some threads were observed in the pulp sample of ore with kaolinite and gypsum addition (Figure 6-17b) again suggesting that gypsum may take part in the formation of these threads.

Figure 6-16. Cryo-SEM images showing the kaolinite particle aggregates in the mixtures with ore in the absence and presence of gypsum: 1000x magnification.

Figure 6-17. Cryo-SEM images showing the kaolinite particle aggregates in the mixtures with ore in the absence and presence of gypsum: 8000x magnification.

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The presence of these threads was clearer in the froth images (Figure 6-18). It seems that in the mixture containing gypsum, the threads attached to the kaolinite agglomerates increasing the entrainment of clay minerals as observed in the water-recovery graphs (Figure 6-1). This also resulted in a decrease in copper and gold grade (Figure 6-2 and Figure 6-3). EDS was used to determine the elemental composition of the threads being 0.3 at.% Na, 0.5 at.% Mg, 17 at.% Al, 26 at.% Si, 7 at.% S, 2 at.% K and 47 at.% Ca. However, the result is indicative only because the spot size of beam was relatively large (>2 µm) and the sampling volume of the beam includes that of the underlying kaolinite aggregates making the accurate measurement of individual gel-like strings difficult for this experiment set-up. The strings appear to be very high in Ca and it could be speculated that the Ca2+ cations from the gypsum interacted with Si, Al, and Na from the clay minerals forming these structures. Similar findings had been described by other researchers (Abdi and Wild, 1993; Wild et al., 1993) who explained the lime and gypsum can interact with kaolinite to produce gel structures.

According to flotation data, these gel-like structures and threads did not have a detrimental effect on recovery, but grade decreased. This was especially clear with the addition of gypsum to the mixture with bentonite. As discussed before, from the Cryo-SEM images it is observed that gypsum caused the formation of significant quantities of these gel-like structures and threads, but recovery was improved up to the values obtained for the flotation of the ore with no clay addition. Only grade was greatly affected. This effect on grade is predictable from the froth Cryo-SEM images (Figure 6-15b) that show a much higher proportion of those gels and threads present in the froth, implying that their entrainment caused the dilution of the concentrate. Similar behaviour was observed with kaolinite and gypsum, but in a lower proportion.

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Figure 6-18. Cryo-SEM images showing the change in froth agglomerates with the addition of gypsum to the kaolinite-ore mixture. 8000x magnification.

It is important to mention that gypsum crystals could be formed in the flotation process itself, and this may happen when recycled water is used (Deng, 2013; Deng et al., 2013; Deng et al., 2014). For instance, it is reported that recycled process water in some sulphide mineral flotation plants 2+ 2- contains high concentration of Ca and SO4 ions that exceed the solubility limit of gypsum. This would cause the precipitation of gypsum crystals, and the effects on clay minerals in this environment should be similar to the findings in this thesis with the difference that nucleation or precipitation of gypsum crystals could interfere on other particle surfaces. However, Deng et al. (2013) found that in gypsum supersaturated solutions the changes on surfaces of silica and sphalerite minerals were mainly attributed to the high concentration of Ca2+ cations. In a similar manner, in this thesis it is proposed that the effect of gypsum on clay mineral surfaces is mostly due to its contribution of high concentrations of Ca2+ cations. The presence of gypsum in an ore or formation of gypsum crystals in the flotation slurry implies that sufficient quantities of Ca2+ will be in equilibrium to interact with other minerals.

Adsorption of Ca2+ cations on other minerals different to clays is highly possible in the flotation experiments conducted in this thesis. It is known that the Ca2+ adsorption affects the surface charge of the minerals and the interaction mineral-flotation reagent and mineral-mineral interaction and this can be exacerbated at some specific pH values. This can have an adverse effect on the flotation performance of sulphide minerals (Deng, 2013). However, given the flotation results in Chapter 6, it seems that the possible detrimental effect of adsorption of Ca2+ cations on mineral surfaces was 118

greatly overcome by the effect of clay mineral network structures, and this is demonstrated when gypsum was added to the mixture with bentonite. The copper and gold recoveries for the mixture with bentonite were very low, but addition of gypsum, or Ca2+ cations, caused those recoveries to be at least equal to or better than the baseline cases. This suggests that for the flotation conditions used in this work the possible adsorption of Ca2+ cations on minerals have little effect on flotation outcome if any.

6.2.4 Effect of clay aggregates on flotation

The two clay minerals used in this work increased the shear stress values and viscosities in a similar manner when 15 wt.% bentonite or 30 wt.% kaolinite was mixed with the ore, but there were significant differences in the distribution of apparent viscosities at low shear rates. It seems that this difference in apparent viscosity is related to the different type of network structures formed by the clay minerals as shown by the Cryo-SEM images. A lower concentration of sodium bentonite particles (15 wt.%) gave higher shear stresses at low shear rates when compared to a higher concentration of kaolinite (30 wt.%) in the slurries. This indicates that the sodium bentonite aggregates are stronger or more difficult to break, and create more friction when the slurry starts flowing. At higher shear rates the mixtures with kaolinite show slightly larger shear stress values, and this may be related to the higher kaolinite concentration in the slurry (30 wt.%). It may be the case that once the network structures in the mixture with 15 wt.% bentonite were broken, there were less clay mineral particles than in the kaolinite mixture to create friction at high shear rates. The differences between bentonite and kaolinite occur as the result of differences in the clay mineral properties such as particle structure, swelling behaviour, surface charge density, and cation exchange capacity. This study indicates that bentonite aggregates have a greater impact on the hydrodynamics in the flotation cell, and are more likely to interfere with the transport of minerals to the froth.

At pH 10 kaolinite particles did not form the sponge-like structure as seen in the bentonite mixture. Kaolinite aggregates were more isolated compact agglomerates with predominant face-face (F-F) interactions. A higher concentration of these aggregates (30 wt.%) is needed to match the viscosity values of the network structures formed by bentonite at lower concentration (15 wt.%). However, similar viscosities did not translate to similar flotation performance. In this case it is important to look at the distribution of apparent viscosities especially at the low shear rate values since these

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values may be present at various locations in the high volume of the flotation cell when dealing with non-Newtonian slurries.

Patra et al. (2010; 2012a; 2012b) investigated the effect of network structures on flotation by using ores containing fibrous minerals and systems with nylon fibres. The network structures formed in their systems are different to the ones containing clay minerals, but there are some similarities in the way they alter the hydrodynamics in the flotation cell when sodium bentonite is present. These researchers found that network structures interfered with bubble dispersion, froth formation, and entrapment of aggregates. The pores in the networks were small (<20 µm) when compared with the bubble size in the flotation cell (1-2 mm), meaning that bubbles could not penetrate these pores in the network structures. This caused poor bubble dispersion, and hindered bubble percolation affecting froth formation (Patra et al., 2012a). Similar phenomenon may happen with the sponge- like structure formed by bentonite in Figure 6-10a and Figure 6-11a. The structures were not greater than 20 µm, and could affect bubble dispersion and movement through the pulp. There was little froth formed during flotation of the ore containing bentonite, and this could be attributed to the presence of this “porous” structure. It will be also reasonable to assume that some valuable minerals could be locked in the cells of the network structure.

The 30 wt.% kaolinite mixture with the ore did not affect froth formation. The froth was more stable than in the baseline case and there was significant entrainment of particles as confirmed by the mass-recovery data. In this case the overall recovery of gold and copper was slightly improved (Figure 6-3 and Figure 6-4) due to the entrainment of particles which at the same time diluted the concentrate grade. For this slurry the aggregates did not appear to be interconnected, and the hydrodynamics in the flotation cell were not badly affected allowing for entrainment of fine gangue minerals and the formation of abundant froth.

The addition of gypsum to the bentonite mixture shows that Ca2+ cations inhibited the formation of the sponge-like structure resulting in lower viscosities values, and froth formation could occur. The recovery was improved by both true flotation and entrainment, but grade decreased. This indicates that the type of network structure is important in flotation and may be manipulated by addition of electrolytes.

The detrimental effect of kaolinite is mostly due to entrainment. The highest entrainment occurred when gypsum was added to the kaolinite mixture with the ore. From the Cryo-SEM images aggregates were connected by long strings that appeared to be formed by the presence of gypsum.

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These aggregates did not increase viscosity (Figure 6-6), and their entrainment resulted in transport of more mass to the froth.

Kaolinite aggregates and the aggregates formed when gypsum was added to slurries with bentonite could have carried by entrapment some valuable and gangue minerals to the concentrate. Wang and Peng (2013) studied the entrapment of gangue minerals in the aggregates of coal particles and found that entrapment became more significant than entrainment for particles smaller than 38 µm. Although their approach was different, this information could apply to a possible entrapment in some of the flotation experiments in this thesis. Clay aggregates should be more complex in structure than coal aggregates and valuable and gangue minerals could be equally trapped within those aggregates. However, it was assumed that there was no entrapment and total recovery occurred only through true flotation and entrainment on bubble lamella as mentioned before.

6.3 Conclusions

The presence of bentonite and kaolinite resulted in the formation of different types of aggregates in the mixtures with the ore at pH 10 adjusted with lime. Bentonite aggregates had a porous or sponge- like structure with predominant edge-edge (E-E) interactions while kaolinite did not form a particular network structure and its aggregates mostly consisted of face-face (F-F) type associations. The structures in the mixture with bentonite had higher viscosities across a wider range of low shear stress values which might affect hydrodynamics in certain regions of the flotation cell and have a detrimental effect on flotation recovery. Recovery in the mixture of the ore with kaolinite was not affected, but grade decreased due to entrainment. The addition of gypsum to the slurry containing bentonite significantly changed the arrangement of aggregates by inhibiting the formation of interconnected network structures. This led to lower viscosity values with flotation behaviour similar to that of mixtures with kaolinite. There was an improvement in recovery, but grade decreased due to entrainment.

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Chapter 7 The interaction of pH modifiers and clay minerals and its effect on copper- gold flotation

7.1 Introduction

In Chapter 4 the effect of flotation reagents on the rheology of kaolinite and bentonite slurries was studied. From those reagents, lime and soda ash had the greatest impact on the rheology of these two clay mineral slurries, and their influence varied depending on the swelling or non-swelling behaviour of the clay mineral. In mineral flotation, lime is often used to adjust the pH, and while adjusting the pH, lime releases calcium cations which may have a negative or positive effect on mineral flotation. Lime is known to be an effective depressant for iron sulfides (in particular for pyrite), resulting in selective flotation in favor of other base metal sulfides (Gaudin, 1957; Klassen and Mokrousov, 1963). Recently, (Gibson and Kelebek, 2014) observed that lime also depressed pentlandite in the flotation of a nickel ore due to the adsorption of hydrophilic calcium species, whilst the flotation was improved when lime was replaced by soda ash as a pH modifier. In terms of high clay-content ores, calcium cations released from lime as the pH modifier may interact with clay minerals changing their flotation behaviour. However, lime as a pH modifier typically releases only a small amount of calcium cations into the flotation system, different to gypsum as presented in chapters 5 and 6 where gypsum released sufficient amounts of Ca2+ cations to change rheology and flotation outcomes. It is not known whether lime affects the flotation of high clay-content ores compared with other pH modifiers. This knowledge is important for any mineral process plant treating a high clay-content ore.

In this study, a low clay-content copper-gold ore was mixed with kaolinite or bentonite at varying proportions. Experiments were designed to understand the mechanisms by which pH modifiers (lime and soda ash) affect the flotation of the artificial clay ores through measurements of pulp rheology and the particle network structures formed.

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7.2 Results and discussion

It was demonstrated in Chapter 6 that the swelling behaviour of bentonite contributed to the formation of network structures that affected the recovery by diminishing flotation kinetics, while the detrimental effect of kaolinite was on the high entrainment that decreased gold and copper grades. To make this study simple, results are divided in two sections, one for kaolinite and one for bentonite.

7.2.1 Interaction of lime and soda ash with kaolinite

7.2.1.1 Flotation

Flotation of the ore (baseline) and the mixtures of the ore with kaolinite was conducted with pH adjusted by lime or soda ash. Figure 7-1 shows the mass recovered to the concentrate as a function of the water recovery. Some tests were repeated with error bars showing one standard deviation. The baseline flotation was slightly different when lime and soda ash were used to adjust the pH with the mass-water recovery curves almost overlapping. The mass recovery from these two baselines was the same (5%), but the water recovery was higher with lime than with soda ash being 31% and 24%, respectively. The difference in water recovery might be associated with the ionic strength induced by lime and soda ash. Corin (2014), Corin et al. (2011) and Manono et al.(2012; 2013) reported that the ratio of water and mass recoveries in flotation changed with the ionic concentration as a result of the modification of froth. In this study the ionic strength was calculated based on the amounts of lime and soda ash used to adjust pH 10. Figure 7-2 shows this relationship between the ionic strength and the water recovered from the first two concentrates from the slurries with 0, 10 and 20 wt.% kaolinite. These two concentrates were chosen because the pH modifiers were added before the beginning of flotation, and the flotation time for the first two concentrates was just 3 min and it would be expected that there was only a minor change in the calculated ionic strength. From Figure 7-2 the lowest ionic strength (0.013 M) was obtained when using soda ash to adjust the pH of the copper-gold ore without kaolinite addition, and the water recovered from the first two concentrates was about 92 g. On the other hand, the highest ionic strength (0.037 M) was calculated for the mixture with 30 wt.% kaolinite using lime, and the corresponding water recovered from these two concentrates was 572 g approximately. These results indicate that Ca2+ from lime

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changes the ionic strength more than Na+ from soda ash, possibly resulting in a more stable froth and higher water recovery.

The addition of kaolinite increases ionic strength and water recovery. One reason for the increase of ionic strength could be the extra quantities of lime and soda ash required to increase pH when kaolinite was added to the copper-gold ore. This is because some of the OH- from the dissociation 2- of lime and the ionization of the CO3 from soda ash react with the edges of kaolinite leaving less quantities of OH- to increase pH. In chapter 6, a mixture made of the same copper-gold ore, 30 wt.% kaolinite and 5 wt.% gypsum produced a higher water recovery (845 g) than the mixture with 30 wt.% kaolinite in this study. This is expected since gypsum contributes to the ionic strength.

The greatest difference in mass and water recovery between the two pH modifiers was observed when the kaolinite concentration was 30 wt.%. Flotation of this mixture produced an overall mass recovery of 25% and an overall water recovery of 38% when pH was adjusted with lime. Those recoveries were 13% and 29% respectively when soda ash was used. The addition of 10 wt.% kaolinite to the ore also shifted the mass-water recovery curve higher compared to the baseline flotation, but the mass-water recovery curves with the addition of 10 wt.% kaolinite for lime and soda ash were slightly separated. To better illustrate the difference in mass or water recovery as the result of the addition of lime or soda ash, the ratios of the overall mass or water recovery between lime and soda ash were calculated. The ratio of the overall mass recovery was 1.09 for the baseline, 1.3 for the mixture with 10 wt.% kaolinite, and 2.1 for the mixture with 30 wt.% kaolinite, while the ratio of the overall water recovery was 1.3 in all the cases. This suggests that as the kaolinite concentration increases in the slurry, the solid recovery with lime addition was more than that with soda ash, but the proportion of water recovered was the same with and without the addition of kaolinite. It is clear that the interaction of kaolinite with the pH modifier has an effect on mass recovery but not on water recovery.

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ORE BASELINE - LIME ORE BASELINE - SODA ASH 10 wt% kaolinite - Lime 10 wt% kaolinite - Soda ash 30 30 wt% kaolinite - Lime 30 wt% kaolinite - Soda ash

10 Mass recovery (%)

0 0 5 10 15 20 25 30 35 40 45 Water recovery (%)

The copper grade as a function of the copper recovery, and the gold grade as a function of the gold recovery from the flotation are shown in Figure 7-3 and Figure 7-4, respectively. In Figure 7-3, the copper grade-recovery curves from the two baselines were similar except for the first concentrate where the copper recovery was 40% and 75% with lime and soda ash, respectively. In the first concentrate there was a higher mass recovery and better froth formation with the addition of lime. The overall flotation outcome for the two baselines was similar with about 10% copper grade and 90% copper recovery. A similar trend was observed on the gold grade-recovery curves from the two baselines (Figure 7-4). The overall gold grade and recovery from the two baselines were about 8 ppm and 83%, respectively. These results indicate that for the baselines with lime and soda addition, the differences are mostly for the first concentrate. This might be related to the higher ionic strength produced by lime. After the first concentrate flotation outcome was very similar.

When adding kaolinite to the ore, the grade-recovery curves shifted downwards. The higher the kaolinite concentration in the mixture, the lower the grade-recovery curve. The grade-recovery curves from the flotation of these mixtures for lime and soda ash were well apart unlike the baseline cases. For the same mixture, lime produced a lower grade-recovery curve than soda ash. This is consistent with a higher mass recovery with lime as shown in Figure 7-1. It was also found that the addition of kaolinite increased the overall copper and gold recoveries.

COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 10 wt% kaolinite - Lime 10 wt% kaolinite - Soda ash 30 30 wt% kaolinite - Lime 30 wt% kaolinite - Soda ash

10 Copper grade (%) grade Copper 5

0 0 20 40 60 80 100 Copper recovery (%)

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GOLD BASELINE - LIME GOLD BASELINE - SODA ASH 10 wt% kaolinite - Lime 10 wt% kaolinite - Soda ash 20 30 wt% kaolinite - Lime 30 wt% kaolinite - Soda ash 18 16

14 12 10 8 6 Gold grade (ppm) Gold grade 4 2 0 0 20 40 60 80 100 Gold recovery (%)

To understand the mechanism by which the recovery was increased, the overall recovery was decoupled into recovery by entrainment and recovery by true flotation.The copper recoveries by true flotation and entrainment were calculated using Savassi equation (Savassi et al., 1998; Bıçak et al., 2012) and are shown in Figure 7-5 and Figure 7-6, respectively. As mentioned in Chapter 6, to use this equation it was assumed that entrainment occurred through bubble lamella and valuable and gangue minerals had the same probability of entrainment. The overall copper recoveries by true flotation from the flotation of the ore and its mixtures with kaolinite with lime and soda ash were similar, between 90 and 92% (Figure 7-5). The highest copper recovery by entrainment was about 3% from the flotation of the mixture with 30 wt.% kaolinite with lime (Figure 7-6) which was twice that of the same mixture with soda ash. This suggests that the main effect of presence of kaolinite was the dilution of flotation concentrates through entrainment, and this effect was exacerbated by the interaction with lime.

There was a clear difference in copper recovery by true flotation between lime and soda ash in the first concentrate. As shown in Figure 7-5, the true flotation from the baseline was significantly lower with soda ash than with lime. The addition of 10 wt.% kaolinite in the slurry improved the copper recovery by true flotation in the first concentrate with soda ash to the level of the baseline floated with lime. It was observed that by adding kaolinite, the froth was more stable which may 127

contribute to the improvement in copper recovery by true flotation. It is assumed that froth stability improved due to the increase in ionic strength and the presence of small kaolinite particles in the froth. Lime and soda ash additions resulted in a difference of about 35% in true flotation in the first concentrate from the baselines. That difference was reduced to about 10% in true flotation in the first concentrate when 10 wt.% kaolinite was mixed and to 2% when 30 wt.% kaolinite was mixed. It is also interesting to find that soda ash caused the true flotation in the first concentrate to improve by increasing the kaolinite concentration from 10 to 30 wt.%, but lime gave an opposite trend. One of the reasons for this phenomenon may be related to the pulp rheology which was studied in the next section.

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60 COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 10 wt% kaolinite - Lime 10 wt% kaolinite - Soda ash 50 30 wt% kaolinite - Lime 30 wt% kaolinite - Soda ash

40 Cu recovery by true flotation (%) by flotation true Cu recovery 30 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

Figure 7-5. The copper recovery by true flotation from the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration.

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COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 10Recovery wt% kaolinite - Lime by entrainment10 wt% kaolinite - Soda ash 4.0 30 wt% kaolinite - Lime 30 wt% kaolinite - Soda ash 3.5

3.0

2.5

2.0

1.5

1.0

0.5 Cu recovery by (%) entrainment 0.0 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

Figure 7-6. The copper recovery by entrainment from the ore and its mixtures with kaolinite: flotation conducted at pH 10 adjusted by lime or soda ash; 30 wt.% solids concentration.

7.2.1.2 Pulp rheology

Before the flotation was conducted, samples were taken for rheology measurements. Figure 7-7 shows the rheograms for the ore and its mixtures with kaolinite. The two baselines with lime and soda ash and the mixture with 10 wt.% kaolinite with soda ash showed similar Newtonian flows with a linear relationship between shear stress and shear rate passing the origin. The mixture with 10 wt.% kaolinite with lime and the mixtures with 30 wt.% kaolinite with soda ash and lime showed non- Newtonian flows and the corresponding yield stresses calculated by the Herschel-Bulkley model were 0.13 Pa, 0.15 Pa and 0.53 Pa, respectively. In the absence of kaolinite, the ore showed a similar rheological behaviour with lime and soda ash. However, with the addition of kaolinite, differences in the rheological behaviour were observed when pH was modified with lime compared to soda ash with lime addition resulting in a higher yield stress.

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ORE BASELINE - LIME ORE BASELINE - SODA ASH Kaolinite 10 wt % - ore 90 wt.% - LIME Kaolinite 10 wt % - ore 90 wt.% - Na2CO3 3.0 Kaolinite 30 wt % - ore 70 wt.% - LIME Kaolinite 30 wt % - ore 70 wt.% - Na2CO3

2.5

2.0

1.5

1.0 Shear stress (Pa) 0.5

0.0 0 50 100 150 200 250 300 350 400 Shear rate (s-1)

Figure 7-7. Rheograms for the ore and its mixtures with kaolinite: pH 10 was adjusted by lime or soda ash; 30 wt.% solids concentration.

There was no clear correlation of yield stresses with flotation performance, but in general an increase in kaolinite content corresponded to an increase in apparent viscosity, and an increase in mass and water recoveries causing the gold and copper grades in the concentrate to decrease. A slight improvement in the copper recovery was observed with a higher viscosity, but this may be related to entrainment as shown in Figure 7-6. The mixture with 30 wt.% kaolinite had the highest apparent viscosity with the highest recovery by entrainment. The trend of the recovery by entrainment of the different slurries (Figure 7-6) was very similar to the trend observed in the rheograms (Figure 7-7), and the mass-water recovery data (Figure 7-1). It is known that by increasing the clay mineral concentration in a mixture the apparent viscosity also increases (Blachier et al., 2014), and for the mixtures with kaolinite in this research, the entrainment of particles increased with the kaolinite concentration. It seems that kaolinite aggregates formed during flotation are easily entrained, and at a higher kaolinite concentration in the mixture more clay aggregates are available to be transported into the froth. Lime enhances the aggregation of kaolinite particles as indicated by the higher apparent viscosities and their recovery by entrainment. It was also noted that the addition of kaolinite produced an enhancement of true flotation for the first concentrate, and this could be due to the apparent increase in froth stability.

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7.2.1.3 Aggregates of kaolinite particles

Cryo-SEM images in Figure 7-8 and Figure 7-9 compare the type of aggregates and network structures present in the pulp with 30 wt.% kaolinite at different magnifications. Figure 7-8 shows that kaolinite aggregates did not form any particular network structure with the addition of either lime or soda ash, and there was not a visible difference in the arrangement of these aggregates between the two pH modifiers. A closer look at the aggregates in the pulp at a higher magnification (Figure 7-9) confirms that the aggregates were similar with predominant face-face (F-F) and edge- edge (E-E) associations between kaolinite particles.

Figure 7-8. Cryo-SEM images comparing the type of aggregates and network structures present in the pulp with 30 wt.% kaolinite with pH 10 adjusted with lime (a) and soda ash (b): 1000x magnification.

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Figure 7-9. Cryo-SEM images comparing the type of aggregates and network structures present in the pulp with 30 wt.% kaolinite with pH 10 adjusted with lime (a) and soda ash (b): 8000x magnification.

Similar observations were found in Figure 7-10 showing Cryo-SEM images of the first concentrates at 1000x magnification. The addition of lime or soda ash did not cause a difference in the arrangement of kaolinite aggregates in the flotation concentrates. This is also confirmed with the higher magnification Cryo-SEM images of those concentrates in Figure 7-11.

Figure 7-10. Cryo-SEM images showing the first concentrate from the flotation of the ore mixed with 30 wt.% kaolinite with pH 10 adjusted with hydrated lime (a) and soda ash (b): 1000x magnification.

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Figure 7-11. Cryo-SEM images showing the first concentrate from the flotation of the ore mixed with 30 wt.% kaolinite with pH 10 adjusted with hydrated lime (a) and soda ash (b): 8000x magnification.

Rheology measurements in this study suggest that the difference in kaolinite aggregates with lime and soda ash is given by the strength of the bonds between the clay platelets. Even if more Na+ cations are released by soda ash than Ca2+ cations by lime, the divalent Ca2+ cations have a stronger effect on rheology as confirmed by the rheology measurements. Stronger aggregates mean higher shear stresses to break them and to start or keep flow of the slurry. This breakage of aggregates is not the same at different locations in the flotation cell since shear rates have the highest values close to the impeller and much lower ones in the quiescent and froth zones. It is likely that stronger aggregates do not break easily during entrainment allowing for more mass to enter the concentrate. Bigger aggregates can also transport more water within their structures. This is reflected in the mass-recovery data, and at the same time affects concentrate grade, and recovery by entrainment.

7.2.2 Effect of lime and soda ash in the ore-bentonite mixture

7.2.2.1 Flotation

The mass-water recovery from the flotation of the ore-bentonite mixtures showed a different trend compared to that from the flotation of the ore-kaolinite when lime and soda ash were added to adjust the pH. The difference in mass-water recovery between lime and soda ash additions was not 133

as much as in the mixtures with kaolinite even for the highest concentration of bentonite (15 wt.%). Opposite to kaolinite, bentonite caused the water recovery to decrease for all the cases (Figure 7-12). The mixture with 15 wt.% bentonite showed the lowest water recovery at around 9% and 6% with lime and soda ash respectively. 5 wt.% bentonite in the ore increased mass recovery, but 15 wt.% bentonite decreased it. This could be related to the formation of strong network structures when the mixture contained 15 wt.% bentonite. Bentonite particles at 5 wt.% in the mixture were probably more dispersed facilitating their entrainment. In general the change in mass recovery with bentonite content was about 1%. This change was very small compared to the mixtures with kaolinite where the highest mass recovery was 24%. Other reason for this difference between the mixtures with kaolinite and bentonite could be the smaller amounts of lime and soda ash required to adjust pH to 10 in the slurries with bentonite. For instance, the natural pH of the mixture with 15 wt.% bentonite was about 8.4 and approximately 1.0 g of lime was needed to adjust pH to 10, while the mixture with 30 wt.% kaolinite had a natural pH of 7.8 and about 2.3 g of lime were required to achieve the same pH value. This was also observed with soda ash. This implies that less Ca2+ and Na+ cations were added by the pH modifiers to the mixture with bentonite.

ORE BASELINE - LIME ORE BASELINE - SODA ASH 5 wt% bentonite - Lime 5 wt% bentonite - Soda ash 7 15 wt% bentonite - Lime 15 wt% bentonite - Soda ash

Mass recovery (%) 2

0 0 5 10 15 20 25 30 35 Water recovery (%)

Figure 7-12. Mass – water recovery when floating the mixtures ore and bentonite (5 wt.% and 15 wt.%). Flotation at pH 10 and 30 wt.% solids concentration.

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Overall the recovery and grade of gold and copper decreased with increasing bentonite content (Figure 7-13 and Figure 7-14), but there were not clear differences between the addition of lime or soda ash. However, if only the first three concentrates are taking into account, it seems that for the mixture with 15 wt.% bentonite, the flotation with soda ash was slightly better than that with lime. This indicates that soda ash may not increase the viscosity of the mixtures with bentonite as it does in the pure bentonite slurries as presented in Chapter 4. Further discussion of this was presented in the rheology section. For this mixture with 15 wt.% bentonite, the overall copper recovery decreased from almost 92% for the lime and soda ash baselines to about 81%. The addition of 5 wt.% bentonite in the ore did not affect the overall copper recovery as much (91%) with both lime and soda ash. Copper grade for both bentonite concentrations and the two pH modifiers was almost the same (8%) and close to that obtained from the baselines (10%).

COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 5 wt% bentonite - Lime 5 wt% bentonite - Soda ash 30 15 wt% bentonite - Lime 15 wt% bentonite - Soda ash

15 Grade (%) Grade 10

0 0 10 20 30 40 50 60 70 80 90 100 Copper recovery (%)

Regarding gold recovery, Teague et al. (1999) stated that the flotation of free gold was limited by physical constraints such as size and shape of the gold particles, the degree of mass and water recovery, the stability of the froth, and the bubble loading of sulphide particles which prevented the bubble attachment of free gold. They also mentioned that pulp viscosity affected the flotation of native gold particles. This supports findings shown in Figure 7-14 where a 15 wt.% bentonite was 135

more detrimental on gold recovery by decreasing it from 82% for the baselines to about 65% and 58% with lime and soda ash respectively. At 5 wt.% bentonite concentration the overall gold recovery was about 79% with both lime and soda ash, close to the gold recovery achieved from the baselines (82%).

GOLD BASELINE - LIME GOLD BASELINE - SODA ASH 5 wt% bentonite - Lime 5 wt% bentonite - Soda ash 20 15 wt% bentonite - Lime 15 wt% bentonite - Soda ash 18 16 14

12 10 8 Grade (ppm) Grade 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100 Gold recovery (%)

Figure 7-14. Gold grade as a function of gold recovery from the flotation of the ore and its mixtures

with bentonite: pH adjusted to 10 with hydrated lime (Ca(OH)2) and soda ash (Na2CO3); 30 wt.% solids concentration.

Figure 7-15 and Figure 7-16 show the recoveries by true flotation and entrainment for the slurries with bentonite and with either lime or soda ash as the pH modifier. The maximum recovery by entrainment was observed at the highest bentonite content (15 wt.%) with lime, but that overall recovery was just 1.2%, which is much lower than 3.2% maximum recovery by entrainment when kaolinite was present in the ore mixture (Figure 7-6). A general observation from the mass-water recovery, grade-recovery, and true recovery figures is that when bentonite was absent, the initial flotation kinetics was much better with lime than with soda ash, but this difference decreased with bentonite addition since the lime-bentonite interaction greatly affected recovery. For instance, this interaction brought the copper recovery down to 8.5% from 75% when floating the ore with no clay addition. If soda ash was used, copper recovery decreased from 25% for the baseline to 7% (Figure 7-13). This might be related to the different interaction of lime and soda ash with bentonite.

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100

True recovery (%) True 30

20 COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 5 wt% bentonite - Lime 5 wt% bentonite - Soda ash 10 15 wt% bentonite - Lime 15 wt% bentonite - Soda ash 0 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

The copper recovery by entrainment with lime in Figure 7-16 was slightly higher than that with soda ash, and a similar trend was observed with kaolinite (Figure 7-6). However, when comparing Figure 7-6 and Figure 7-16, it was noticed that for the mixture with bentonite the difference in entrainment with lime and soda was not clear for the first two concentrates, but from the third concentrate this difference in entrainment became similar to the one in the mixture with kaolinite. Figure 7-6 and Figure 7-16 also show that in the slurry with kaolinite the entrainment increased with kaolinite content, but it remained the same in the slurries with 5 and 15 wt.% bentonite. These changes in entrainment with clay type may be associated with the different network structures formed by kaolinite and bentonite.

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COPPER BASELINE - LIME COPPER BASELINE - SODA ASH 5 wt% bentonite - Lime 5 wt% bentonite - Soda ash 1.4 15 wt% bentonite - Lime 15 wt% bentonite - Soda ash

1.2

1.0

0.8

0.6

0.4 Recovery by entrainment (%) entrainment by Recovery 0.2

0.0 0 2 4 6 8 10 12 14 16 18 Flotation time (min)

Figure 7-16. Copper recovery by entrainment from the flotation of the ore and its mixtures with

bentonite using hydrated lime (Ca(OH)2) and soda ash (Na2CO3) to adjust pH to 10 in the slurries with 30 wt.% solids concentration.

7.2.2.2 Pulp rheology and gel point

From the findings in Chapter 4 with pure clay minerals, it was expected that soda ash would increase the viscosity of flotation slurries in the presence of bentonite more than lime. Figure 7-17 shows rheograms of the ore and its mixtures with bentonite with pH adjusted by lime and soda ash. When the ore was mixed with 5% and 15% bentonite, soda ash did not produced higher pulp viscosity than lime. However, an ore mixture with 25 wt.% bentonite showed the expected rheological behaviour with lime and soda ash and soda ash indeed promoted the higher rheological properties than lime. This mixture was not used for flotation since the viscosity was too high, but it was good to confirm that this rheological response was linked to the gel point of the clay mixtures. Figure 7-18 shows the gel point of the ore and its mixtures with kaolinite and bentonite as well as the gel point of the mixture of 95% bentonite and 5% gypsum. The gel points of bentonite and bentonite-ore mixtures were much lower than those of kaolinite and kaolinite-ore mixture. This figure also shows that the gel points of the ore mixtures used in flotation were higher than 30 wt.%

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solid concentration used in flotation. The ore mixture with 25 wt.% bentonite had a gel point equal to the solids concentration of the flotation slurries, and in that case soda ash had a greater impact on viscosity than lime. It is interesting to note that gypsum caused the gel point of the bentonite to become similar to the gel point of kaolinite. This could be related to the change in flotation behaviour of the bentonite-ore mixture when gypsum was added as presented in Chapter 6. Gypsum caused bentonite to behave more like kaolinite in flotation. This is because gypsum inhibits the formation of bentonite house of cards network structure as explained in Chapter 6.

ORE BASELINE - LIME ORE BASELINE - SODA ASH Bentonite 5 wt.% - ore 95 wt.% - LIME Bentonite 5 wt.% - ore 95 wt.% - Na2CO3 7.5 Bentonite 15 wt.% - ore 85 wt.% - LIME Bentonite 15 wt.% - ore 85 wt.% - Na2CO3 Bentonite 25 wt.% - ore 75 wt.% - LIME Bentonite 25 wt.% - ore 75 wt.% - Na2CO3

6.0

4.5

3.0 Shear stress (Pa) 1.5

0.0 0 50 100 150 200 250 300 Shear rate (s-1)

Figure 7-17. Rheograms for the bentonite-ore mixtures used in flotation: pH adjusted to 10 with

hydrated lime (Ca(OH)2) and soda ash (Na2CO3); total solid concentration, 30 wt.%; the rheograms for a slurry with 25 wt.% bentonite are also shown to demonstrate the change in rheological behaviour with gel point.

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0.8

0.7 Kaolinite-ore

0.6 Bentonite-ore Bentonite 95 wt% - 5 wt% gypsum 0.5

0.4

0.3

0.2 Gel point (wt fraction) (wt point Gel Solids concentration of the flotation slurries 0.1

0 0 10 20 30 40 50 60 70 80 90 100 Clay concentration (wt%)

Figure 7-18. The gel point of the ore and its mixtures with kaolinite and bentonite as well as the gel point of the mixture of 95% bentonite and 5% gypsum.

7.2.2.3 Network structures of bentonite particles

Cryo-SEM imaging of the pulp of the mixtures with 15 wt.% bentonite with pH adjusted by lime and soda ash is shown in Figure 7-19 and Figure 7-20 at different magnifications. Some well- defined network structures were observed when the pH was adjusted with lime (Figure 7-19a and Figure 7-20a), but those structures looked less packed or dispersed if soda ash was used to adjust the pH (Figure 7-19b and Figure 7-20b). These images confirm that soda ash did not form more complex bentonite network structures than lime, and it seems that it did not activate bentonite for swelling as much as in the pure bentonite suspension. It appears that at this bentonite concentration (15 wt.%) soda ash still behaved more like a dispersant. An additional reason for this behaviour could be that the concentration of Na+ ions from soda ash was not high enough to enhance the swelling of the bentonite, or the formation of strong aggregates as it was observed with lime addition.

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Figure 7-19. Comparison of Cryo-SEM images for the flotation pulps with 15 wt.% bentonite when

using hydrated lime (a) (Ca(OH)2) and soda ash (b) (Na2CO3) as pH modifiers to adjust pH to 10. Solids concentration 30 wt.%: Image magnification 1000x.

Figure 7-20. Comparison of Cryo-SEM images for the mixture with 15 wt.% bentonite when using

hydrated lime (a) (Ca(OH)2) and soda ash (b) (Na2CO3) as pH modifiers to adjust pH to 10. Solids concentration 30 wt.%. Image magnification 4000x.

The Cryo-SEM images for the first concentrate collected during the flotation of the mixture with 15 wt.% bentonite (Figure 7-21) did not show any visible difference in clay aggregates.

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Figure 7-21. Cryo-SEM images comparing the first concentrate when using hydrate lime (a)

(Ca(OH)2) or (b) soda ash (Na2CO3) as pH modifiers to adjust pH to 10. Image magnification 2000x.

7.3 Conclusions

Flotation results show that there were no differences in overall copper or gold recovery from the flotation of the copper-gold ore mixed with kaolinite when lime and soda ash were used to adjust the pH. The kaolinite aggregates were similar and predominantly in the form of face-face (F-F) and edge-edge (E-E) associations with both lime and soda ash, but rheology suggests that the aggregates with lime were stronger. This is in agreement with the entrainment data from flotation as lime caused higher entrainment than soda ash. These strong aggregates may not break easily during the transport to the froth and further be recovered in the concentrate. It can be concluded that neither lime nor soda ash used as pH modifiers affected the overall copper or gold flotation recovery, but lime was more detrimental for flotation by increasing entrainment and diluting the concentrate grade.

The differences between lime and soda ash were even less pronounced for the mixtures with bentonite. The recovery by entrainment was slightly higher with lime, and the true recovery was almost the same when using these two pH modifiers. Rheology and Cryo-SEM images show that there were some differences in the network structures and their strength, but flotation results indicate that those differences were not enough to cause significant changes in flotation outcomes. The detrimental effect of bentonite on flotation was much stronger than the effect of kaolinite since

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bentonite formed more compact network structures, and the smaller amounts of lime and soda ash required to adjust the pH of the mixtures with bentonite did not contribute enough quantities of Ca2+ and Na+ cations to make a significant difference in flotation between these pH modifiers. An interesting finding is the apparent influence of gel point on the rheology of mixtures with bentonite and soda ash addition. It appears that for the mixtures with a gel point higher than the solids concentration of the flotation slurry, soda ash increases less the viscosity than lime. On the other hand, if the gel point of the mixture is less than the solids density of the mixture, soda ash increases more the viscosity than lime.

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Chapter 8 Conclusions and Future Work

8.1 Summary

The overall objective of this thesis was to understand some of the most important clay mineral interactions in copper-gold flotation. This understanding is becoming essential because the easy to process ores are getting scarcer and more complex ores such as the ones with high clay content have to be processed to supply the demand for valuable minerals. These problematic ores are difficult for flotation, and recovery and grade are affected as clay minerals may increase slurry viscosity and/or entrainment. A common solution to this problem relies on trial and error to blend the high clay content ores with easy to process ores to reduce the concentration of clay minerals to a point where the negative impacts are less significant. This solution does not deal with the root cause of the problem and the continuous depletion of the easy to process ores means that the problematic ores have to be processed at greater amounts. The findings in this thesis are crucial to deal with the high slurry viscosities and entrainment caused by the presence of the clay minerals in the ores. Knowledge on how the clay particles associate to form network structures, increasing viscosity and affecting true flotation, can lead to find the proper dispersants to break those structures. Similarly, understanding of the entrainment of clay minerals is useful to find the most suitable reagents to deal with the clay agglomerates that are prone to high entrainment.

8.2 Conclusions

The first hypothesis tested stated that kaolinite and bentonite affect the rheological behaviour of copper-gold flotation slurries, and the influence of bentonite, a swelling clay, is stronger. This is due to the swelling behaviour of bentonite and the formation of the house of cards network structure which can be modified by pH modifiers or other minerals. To asses this hypothesis the rheology of kaolinite and bentonite suspensions, and the rheology of copper-gold ore-clay mixtures was tested at different concentrations, and those results were complement with settling tests, gel point measurements and Cryo-SEM imaging. The rheology results for the kaolinite and bentonite suspensions in Chapter 4 and Chapter 5 were a baseline to identify the difference in behaviour between these two clay minerals in the presence of flotation reagents and other gangue minerals. 144

Results showed that bentonite had a much greater effect on viscosity than kaolinite, and that pH modifiers and calcium bearing gangue minerals can change the particle associations of these clays as it was confirmed with Cryo-SEM imaging in chapters 6 and 7. Lime increased the viscosity of kaolinite slurries while soda ash decreased it, and for the bentonite slurries soda ash increased the viscosity more than lime. This effect was caused by the different structure of clay minerals and their interaction with Na+ and Ca2+ cations. However, the mixtures of bentonite-ore did not follow the interaction between the pH modifiers, lime and soda ash, and the pure bentonite suspensions. It was found that the effect of soda ash and lime depended on the bentonite concentration in the mixture with ore. For 5 and 15 wt.% bentonite concentrations, soda ash did not increase viscosity as much as lime, but at a 25 wt.% concentration soda ash increased viscosity more than lime as it happened on the pure bentonite suspensions. This may be related to the gel point of the slurries as shown in Chapter 7. The type of clay particles associations is responsible for this difference in rheology between kaolinite and lime with pH modifiers. At the pH values tested, kaolinite particles did not form any network structures, and aggregates were not interconnected as in the case of bentonite. This was supported by Cryo-SEM images in Chapter 6 and 7 that showed that bentonite particles arranged predominantly in E-E associations to form a house of cards network structure, and kaolinite formed loose aggregates in the form of face-face (F-F) and edge-edge (E-E) associations. In Chapter 5 it was demonstrated that the Ca2+ cations from calcium bearing minerals such as gypsum changed rheological behaviour of clay suspensions, but different to pH modifiers, the quantity of Ca2+ from gypsum was so high that, in the case of bentonite, interfered with the swelling of the clay mineral decreasing the viscosity of the slurry. These Ca2+ cations had the same effect in both pure bentonite suspensions and on bentonite-ore mixtures. Cryo-SEM images and gel points in Chapter 7 supported this. These findings prove that kaolinite and bentonite in slurries have rheological interactions with some flotation reagents and gangue minerals that are a source of ions, and that bentonite, a swelling clay, has a stronger effect on viscosity. These findings add some knowledge that is lacking about the way kaolinite and bentonite can change rheology in a flotation context. This information is important to design a systematic way to deal with high viscosities in flotation.

The second hypothesis tested was that kaolinite and bentonite affect grade/recovery in the flotation of gold and copper, due to the formation of network structures in the flotation pulp and particle entrainment. This is dependent on clay type, and pH modifiers. The assessment of this hypothesis was done by conducting flotation experiments of a copper-gold ore and its mixtures with either kaolinite or bentonite at pH 10 adjusted with lime or soda ash as presented in chapters 6 and 7. One 145

important observation on the rheology measurements was that the rheograms for the slurry with 15 wt.% bentonite and the slurry with 30 wt.% kaolinite were very similar, but the flotation response was totally opposite. The differences between those rheograms were found at low shear rate values where the structures in the mixture with bentonite had higher viscosities across a wider range. This is important because the shear rate distribution in a flotation cell is not uniform and shear rate values can be very low in the quiescent zone. A slurry with very high viscosities at low shear rates may cause the shear rate values in quiescent zone to approach zero and this could have a huge impact on the hydrodynamics in the flotation cell. This may be the case of the complex bentonite particle associations where a porous or sponge-like structure causes high viscosities at low shear rate values. Flotation results showed that this type of structures greatly affected flotation kinetics leading to low recoveries. On the other hand, the less complex, loose kaolinite aggregates did not affect recovery, but increased entrainment. To demonstrate the effect of pH modifiers on flotation, lime or soda ash were used to adjust pH to 10 for the mixtures of the copper-gold ore with either kaolinite or bentonite. Rheology tests in Chapter 7 showed that the bentonite and kaolinite particle associations were stronger when lime was used to adjust pH to 10, but Cryo-SEM images in the same chapter indicated that there were no noticeable differences in the size or shape of kaolinite aggregates between the two pH modifiers. However, the bentonite sponge-like structure appeared less compact when soda ash was used to adjust pH. These variations in particle associations between pH modifiers were not enough to cause differences in the overall copper or gold recovery in the flotation of the copper-gold ore mixed with kaolinite or bentonite, but grade was affected with lime addition in the mixture containing kaolinite. This was more evident for the first concentrate. In general lime caused higher entrainment than soda ash, but the difference was more accentuated when kaolinite was present in the slurry. The Ca2+ cations from lime made stronger kaolinite aggregates that may not break easily during the transport to the froth and can be recovered in the concentrate. These results add some fundamental information to deal with the problems in the flotation of ores with high clay content. For instance, it has been reported in industrial practice that when floating high clay content ores sometimes no froth is formed or in other cases froth is abundant and too stable. This behaviour is related to the type of clay particle associations in the slurry as shown in this study. Absence of froth indicates that a house of cards network structure may be present in the flotation pulp. This knowledge should help find the proper reagents to deal with clay network structures or aggregates.

The third hypothesis tested stated that gypsum can interact with clay minerals by releasing cations (i.e. Ca2+) that modify network structures or aggregates, changing viscosity and impacting flotation 146

outcomes. This was tested in Chapter 6 by conducting flotation experiments using mixtures of a copper-gold ore with clay mineral and gypsum. Two mixtures were used as baselines, one with 30 wt.% kaolinite and another one with 15 wt.% bentonite, to compare the gypsum effect on the flotation of clay mineral-ore mixtures. A 5 wt.% gypsum in the mixtures produced significant changes in flotation outcome for both mixtures with kaolinite and bentonite, but the bentonite- gypsum interaction was much stronger on flotation than the kaolinite-gypsum interaction. This was the result of the interaction of the clay minerals with the Ca2+ cations from gypsum. The quantity of these cations released by gypsum in a suspension was measured in Chapter 5 using dissolutions tests, and it was proven that gypsum can contribute high amounts of them. From the flotation results, rheology measurements, and Cryo-SEM images in Chapter 6 it was inferred that the quantity of these cations in the flotation slurry was enough to prevent the association of the bentonite particles in a sponge-like structure. It seems that the concentration of the Ca2+ cations inhibited swelling of the bentonite, and compressed the double layers of the bentonite particles to form aggregates comparable to the kaolinite aggregates. In contrast, the impact of gypsum on the kaolinite aggregates was less noticeable since there was not a significant difference between the aggregates with and without gypsum in the slurry. Rheology measurements confirmed the change in particle associations in the mixture with bentonite by showing that viscosity values decreased when gypsum was added. The viscosity changes were minimal in the mixture with kaolinite when gypsum was added, and this agreed with Cryo-SEM images. The changes in the bentonite particle associations in the mixture with gypsum caused a better stable froth during flotation as shown by the mass and water recovery data, and the overall copper recovery and gold recovery were improved from 83% and 64% to 93% and 85%, respectively. The negative effect of gypsum was on the copper and gold grade that decreased from 8.4% to 1.8% for copper, and from 5.2 ppm to 1.4 ppm for gold. Presence of gypsum in the mixture with kaolinite did not change flotation outcome as much. The most significant change was on the copper recovery by entrainment, from 3.2% to 3.8%, since gypsum increased mass and water recovery. In general, this study demonstrates that gangue minerals can be a source for interactions in flotation slurries with high clay content, and confirms that network structures such as the sponge-like structure have great impact on flotation, and can be manipulated with ions. This is an important clue to find dispersants that can target specific clay particle associations.

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8.3 Recommendations for processing copper-gold ores with high clay contents in flotation

The goal of this thesis was to understand some of the most important interactions involving kaolinite and bentonite in the flotation of a copper-gold ore, and to determine the effect of these interactions on flotation performance. The findings in this work can be used to establish guidelines in the processing of ores with high clay contents, and the following are the recommendations when dealing with this type of ores.

1) Detailed study of the clay minerals present in the ore

It was demonstrated that swelling and non-swelling clay minerals interact differently with water, flotation reagents and other gangue minerals, and these interactions can affect grade and recovery in distinct ways depending on the type of clay mineral. Although the focus in this thesis was on kaolinite and bentonite, it is expected that other clay minerals with similar structures (non-swelling and swelling) will behave in a close manner. Therefore, it is suggested that the first step to address the flotation of high clay content ores is to use a reliable method to characterize the clay minerals present in the ore and according to this make a systematic approach to optimize flotation by either using dispersants to control particle associations or to make changes in plant operation.

2) Investigation on the mineral composition of the problematic ores

It was also demonstrated that other gangue minerals such as gypsum can have a strong interaction with clay minerals affecting flotation outcome. This finding suggests that analysis of other gangue minerals will be useful to determine their possible contribution to interactions in flotation. In this study, the effect of gypsum in the slurries with clay minerals was mostly due to its solubility in water. This means that a simple preliminary assessment of physical properties of the gangue minerals in the ore can give key information about possible interactions with clay minerals. In fact, this criterion was followed in this study and it suggested that gypsum could have an effect on clay particle association due to its moderate solubility in water.

3) Use of in situ tests to predict rheology and effect on flotation

Rheology measurements were essential in this thesis, and proved that recovery is significantly reduced when clay mineral network structures are present in the slurry, but using this tool to identify these structures may not be practical for in situ measurements. Good and reliable

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rheometers can be expensive to purchase and maintain. A possible solution for this is to use simple measurements such as settling tests. It was observed that slurries with low viscosities and no network structures settle fast. A high settling rate should indicate that viscosity is not an issue for flotation.

8.4 Recommendations for future work

This study provides the foundation to address the detrimental effect of clay minerals in flotation, and to make a good use of this knowledge the following work is recommended:

1) Rheology measurements to identify the type of clay particle associations in the slurry and development of an equation to predict rheological behaviour of clay minerals in flotation

One of the important findings in this research was that rheograms or yield stress and viscosity alone are not enough to identify the type of clay minerals associations, and this was only possible by using Cryo-SEM imaging. It was observed that two slurries with similar rheograms can have opposite flotation responses and this was due to the type of clay aggregates. Rheology is a complex field, and oscillatory rheology could provide the required information for the identification of network structures. This type of rheology measurements could be used to determine the solid-like or liquid-like behaviour of slurries. It is expected that a slurry with a house of cards or sponge-like structure will have a solid-like behaviour. If this proves to be correct, it will be an important tool to predict the effect of high clay content in flotation slurries. One disadvantage with these measurements is that it requires a good rheometer. Usually they are expensive and need careful maintenance and this can be impractical at a mine site.

It would be ideal to have a model to predict rheological behaviour of mineral slurries with high clay content, but this might be very challenging given the different types of clay minerals and their properties. It is known that the rheology of suspensions depends on the balance of Brownian diffusion, hydrodynamic interaction and inter-particle forces (Tadros, 2010). These forces are determined by three main parameters: volume fraction, particle size and shape distribution and the net energy of interaction, which is the balance between repulsive and attractive forces (Tadros, 2010). There are some simple equations to predict viscosity such as the Einstein equation used for dilute suspensions with non-interacting particles, or the Bachelor equation that takes into account hydrodynamic interaction between particles. More complicated equations are necessary for concentrated suspensions where viscosity becomes a complex function of volume fraction and the 149

suspensions show non-Newtonian behaviour. In this thesis it was found that for non-problematic ores, flotation slurries had low viscosities and close to Newtonian behaviour. In this case the Einstein or Bachelor equations are probably enough to predict viscosity. However, the presence of clay minerals in a flotation cell can add strong hydrodynamic interactions, and electrostatic and Van der Waals forces due to the shape, surface charges and small size of clay particles. Hydration forces could be also significant if swelling clays are present. Most likely an equation to predict rheological behaviour of flotation slurries with high clay content should include all these forces contributed by clay minerals as well as the particle size and shape of other minerals in the slurries.

2) Evaluation of other tests to identify particle associations

It is recommended to explore simple methods to identify particle associations. An example of this is the settling test. It was mentioned that problematic slurries for flotation settled very slowly and had high viscosities, while slurries that settled fast showed low viscosity values and no issues on flotation. This hints that settling tests could correlate with viscosity of high clay content ores and predict the presence of a network structure such as the house of cards. When a network structure is formed, settling of the slurry is very low and or it does not happen at all. In these structures particles are interconnected forming a self-supporting network, and slurries have low gel points. Gel point could be another test to consider, but it has a disadvantage in the sense that important information is lost since time is not measured, contrary to the settling tests. Other interesting observation during experimentation in this study is that when drying the concentrates and tails from flotation a crack pattern was observed and it depended on the clay mineral present in the slurry. The dried samples from slurries with bentonite formed cracks with a close to hexagonal pattern, but samples with kaolinite did not form cracks with this pattern. It dried more in a continuous, uniform way, and this was similar for the bentonite sample with gypsum which did not show the crack pattern observed in the slurry with bentonite. This might be correlated to the type of particle associations. Some research in other fields about dry patterns for slurries containing clays is found in literature, however, it is understood that this proposal about the connection with clay particle associations is new.

3) Assessment of reagents to manipulate clay particle associations

This study provides an understanding to address the difficulties faced in the flotation of ores with high clay contents. It was demonstrated that clay minerals affect flotation by either diminishing flotation kinetics through network structures or increasing entrainment of loose clay aggregates, and

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the concentration of ions in flotation slurries was one of the main factors for the clay mineral interactions. Dispersants, depressants or even electrolytes can be used to inhibit the formation of complex clay network structures such as the house of cards in the case of bentonite or to prevent the formation of clay aggregates that are easily entrained in the case of kaolinite. These reagents can modify the charge density at the solid-liquid interface or the electrical double layers of the clay particles, and the knowledge contributed in this study will help selecting the proper dispersants to achieve this. The effect coagulation effect of Ca2+ cations from gypsum on flotation outcome can be comparable to the flocculation effect of dispersants on clay associations with the difference that these cations are not selective on the mineral surfaces as the dispersants can be. For instance, the Ca2+ from gypsum improved the recovery in the mixtures with bentonite, but added the high entrainment problem. An ideal dispersant should be able to cause an improvement in recovery and at the same time prevent the entrainment of clay aggregates. It is important to mention that during experimentation in this study it was observed that Ca2+ cations from gypsum inhibited the formation of bentonite network structures in an effective way only if gypsum was present in the dry mixture or was added to the copper-gold ore slurry before adding the clay mineral. This indicates that in some cases the use of dispersants might be more effective if added before the water the clay-water interactions occur.

It was found in literature that some studies have been done to assess the effect of some reagents to manipulate clay particle interactions in flotation. For instance, an urea-based polymer was used as depressant for the flotation of coal, potash (Tao et al., 2007), and phosphate (Tao et al., 2010). This is a low molecular weight polymer that acts as a clay binder and is the condensation product of urea and formaldehyde reacted under acidic conditions (Tao et al., 2007; Tao et al., 2010). According to the authors this depressant agglomerated the clay particles reducing their surface area and this minimized the adsorption of flotation reagents and slime coatings. This type of depressant could also have an effect on the clay mineral network structures that are problematic for flotation.

Similarly Wei et al. (2013) used a lignosulfonate-based polymer to study its effect on the recovery of copper and gold in the flotation of an ore with high clay content. They found that the effectiveness of this dispersant depended on the grinding media, and suggested that the negative impact of clay minerals on flotation was a combination of increase in viscosity and slime coatings. On the other hand, Oats et al. (2010) used sodium silicate and sodium hexametaphosphate as dispersant in coal flotation in the presence of clay minerals, and reported that these two inorganic compounds did not enhance the recovery significantly. These researchers mentioned that while

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bentonite significantly depressed the coal flotation, kaolinite or illite did not show any effect on the coal flotation.

Although most of the research found in literature has focused on the use of clay dispersants in coal flotation, those findings provide insights into the possible effect of dispersants on copper and gold flotation. There is also abundant information about the use of polymers to modify the surface properties of clay minerals, but the focus is on agriculture and other industrial areas (Theng, 2012c). Some of the main points to consider when using polymers are to know the type of clay minerals involved and their surface properties, and to be aware that water will compete strongly with the polymer for adsorption sites on the clay solid surfaces when hydrogen bonding occurs. Water can add hydrogen as bond donor and acceptor and may promote or inhibit polymer adsorption (Theng, 2012c), although in some cases where there is no isomorphous substitution (e.g., talc and pyrophyllite), clay minerals are essentially hydrophobic. In contrast, the basal aluminol surface of 1:1 clay minerals, such as kaolinite, can form hydrogen bonds with water molecules (Theng, 2012c). With respect to the 2:1 phyllosilicates that have a negative surface charge, the presence of charge-balancing inorganic cations which attract water molecules, makes them hydrophilic, and to adsorb a single polymer, many of those water molecules must be desorbed (Theng, 2012c). Polymers can act as flocculants or have a dispersive effect on suspended solids depending on the quantities added among other factors (Theng, 2012c), and their proper selection in the flotation of ore with high clay content requires a substantial knowledge of their structure and properties. This alone could be a subject of research for a doctoral thesis.

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