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. ii 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. iii 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. iv Publications included in this 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. 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%) 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. Contributor Statement of contribution Nestor Cruz (Candidate) Conception and design (80%) Analysis and interpretation of data (70%) Wrote the paper (70%) v 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.