The interaction of clay minerals with gypsum and its effects on copper–gold flotation

Highlights

• Kaolinite affects flotation through entrainment.

• Bentonite network structures decrease flotation kinetics.

• Gypsum in ores with a high clay mineral content can change the flotation outcome.

• Gypsum inhibits the formation of bentonite network structures.

• Flotation in the presence of clay minerals depends on clay particle associations.

Abstract

The interaction of two clay minerals, kaolinite and bentonite with gypsum and its effects on the flotation of a copper–gold ore was investigated in this study. It was found that bentonite increased the viscosity more than kaolinite when mixed with the copper–gold ore at low shear rates. The detrimental effect of these clay minerals on flotation was attributed to the entrainment of clay particles when kaolinite was added to the ore and to a decrease in true flotation by bentonite. Bentonite formed a sponge-like structure with predominant edge–edge (E–E) interactions which might affect hydrodynamics in the flotation cell and have a detrimental effect on flotation recovery. Kaolinite did not form a particular network structure and its aggregates mostly consisted of face–face (F–F) type associations which did not affect flotation hydrodynamics. The addition of gypsum to the ore–bentonite mixture inhibited the formation of interconnected network structures. This led to lower viscosity values with flotation behaviour similar to that of mixtures with kaolinite. In this case, there was an improvement in recovery, but the grade decreased due to entrainment. The addition of gypsum to the ore–kaolinite mixture created aggregates with long strings further enhancing particle entrainment with more mass transported to the froth.

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.