The interaction of clay minerals with gypsum and its effects on copper–gold flotation
Introduction
Clay minerals can be detrimental to mineral flotation and high viscosity and high particle entrainment promoted by clay minerals are among the main contributing factors (Forbes et al., 2014, Jorjani et al., 2011, Wang and Peng, 2014, Wei et al., 2013, Zhang and Peng, 2015). Incremental reagent consumption has also been suggested as a factor affecting flotation of ores with a high clay content (Connelly, 2011). Bentonite has a 2:1 layer structure and is a swelling clay mineral while kaolinite does not swell and has a 1:1 structure. Both have a small particle size. Zhang and Peng (2015) found 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.
The factors influencing hydrodynamics are flotation cell characteristics, impeller properties, and slurry properties such as density and rheology (Shabalala et al., 2011). In this paper 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, which decreases the bubble-particle collision efficiency (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). It is reported that a high yield stress can form a “cavern” around the impeller (Bakker et al., 2010) and this decreases 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). Patra et al., 2012a, Patra et al., 2012b found that network structures formed by fibrous materials in flotation also interfered with the hydrodynamic conditions and then decreased mineral flotation recovery. Their studies suggest that network structures formed by bentonite may have a similar effect on mineral flotation.
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).
Calcium bearing minerals can be present in ores and associate with clay minerals. In the previous work, it was found that among the calcium bearing minerals gypsum had the strongest rheological interaction with bentonite and kaolinite (Cruz et al., 2013). Gypsum acted as a potential source for Ca2+ and SO42− ions released into the process water. It is known that multivalent cations are more strongly attracted to clay particles than monovalent cations (i.e. Na+ ions) (Lagaly and Dékány, 2013a). Ca2+ cations can increase the viscosity of kaolinite slurries by attaching to the clay surfaces modifying the double layers and contributing to the formation of network structures (Abdi and Wild, 1993, Wild et al., 1993). In bentonite slurries a high concentration of Ca2+ cations can replace Na+ cations in the interlayer space deactivating sodium bentonite such that it is no longer possible for hydration and swelling to occur (Alther, 1986, Bradshaw et al., 2013, Meer and Benson, 2007). It is also reported that the presence of gypsum and lime can increase water absorption and swelling pressure of bentonite (Abdi and Wild, 1993, Wild et al., 1993). The Ca2+ from lime and gypsum and SO42− anions play a role on this effect.
In this study, the interaction of clay minerals with gypsum in the formation of network structures and its effects on copper–gold flotation were investigated.
Section snippets
Materials
For the flotation experiments sodium isopropyl xanthate (SIPX), and the promoter Aero 3894 were used as the 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. The pH in flotation was adjusted using AR grade hydrated lime (Ca(OH)2). These reagents are used in the industrial flotation of the copper–gold ore examined in this study.
Flotation
Six flotation tests were conducted using the copper–gold ore and its mixtures with quartz, clay minerals and gypsum. Fig. 1 shows the mass recovery as a function of water recovery from these tests. As can be seen, flotation of the ore or baseline resulted in a 5% mass recovery and 31% water recovery at the completion of flotation. The flotation of the ore and its mixture with 30 wt.% quartz produced very similar mass and water recoveries. The addition of 15% bentonite reduced the water recovery
Discussion
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 was a significant difference in the distribution of apparent viscosities at low shear rates. It seems that this difference in apparent viscosity was 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
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
Acknowledgments
The authors gratefully acknowledge the financial support of this study from the Australian Research Council, Newmont Mining Corporation and Newcrest Mining Limited as well as the discussion and suggestion from Dr. Ronel Kappes at Newmont Mining Corporation and Dr. David Seaman at Newcrest Mining Limited. The first author also thanks the scholarship provided by the University of Queensland.
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