Slime coating mitigation on mineral surfaces in froth flotation by saline water

Fine and ultra-fine particle flotation is central to the mining industry as a result of the need to treat low grade and difficult ores which require fine grinding to liberate valuable minerals from gangue minerals. However, the flotation of fine and ultra-fine particles is not efficient and a slime coating with slime gangue particles attaching to valuable minerals is one of the most popular barriers. A slime coating prevents the adsorption of collectors on the surface of valuable minerals resulting in low flotation recovery or low product quality.Saline water is also an important issue in the mining industry. In Western Australia, bore water with a high ionic strength has to be used at mine sites for production, site rehabilitation, and downstream processing because fresh water is not available locally. In Queensland, most mine sites have adopted water re-use as a means for making freshwater savings. However, water re-use results in increased salinity in site water stores, which is driven largely by evaporation and ongoing salt inputs from spoil, minerals and groundwater. Flotation relies on a large amount of water and therefore the impact of saline water on flotation performance has gained more and more attention.In this research, the role of saline water in the mitigation of slime coatings was studied and a generic approach to improve fine and ultra-fine particle flotation by using saline water was developed, building on the previous research. A model slime coating system was established by using copper minerals in the presence of clay particles. It was found that chalcocite flotation was depressed in the presence of bentonite slimes while chalcopyrite flotation was less affected.Electrostatic interactions between copper minerals and clay particles were found to be responsible for the different flotation of chalcocite and chalcopyrite in the presence of clay minerals. It was found that surface oxidation occurred during grinding and changed the surface properties of chalcopyrite and chalcocite. Chalcocite was strongly oxidized while chalcopyrite was slightly oxidized after the grinding with stainless steel media. The different extent of surface oxidation resulted in the different electrical property of chalcocite and chalcopyrite surfaces. The strongly oxidized chalcocite surface became positively charged after grinding at the same pH and electrostatically attractive to bentonite particles resulting in the depressed chalcocite flotation. In contrast, the slightly oxidized chalcopyrite surface remained negatively charged after grinding at pH 9.0 and entropically repulsive from bentonite slime particles. Therefore, bentonite slimes did not influence chalcopyrite flotation as much as chalcocite flotation.Mitigation of slime coatings on chalcocite in flotation by using electrolytes was further investigated and kaolinite was used to introduce the slime. The results indicated that in tap water, the presence of kaolinite slimes depressed chalcocite flotation. With the addition of electrolytes to the flotation system, the flotation of chalcocite in the presence of kaolinite slimes was improved. The effect of cations (Li+, Na+ and K+) and anions (F-, Cl- and I-) on the mitigation of slime coatings on chalcocite surfaces in flotation was also examined in this study. It was interesting to find that larger size ions improved chalcocite flotation in the presence of kaolinite slimes more than smaller size ions.Electrochemical impedance spectroscopy (EIS) was developed to investigate how kaolinite coated chalcocite surfaces and how electrolytes with different size ions mitigated this coating. It was found that the impedance of chalcocite in the presence of kaolinite was higher than that in the absence of kaolinite in the low frequency range due to the formation of kaolinite coatings on chalcocite surfaces decreasing the dielectric constant and increasing the impedance of chalcocite. EIS results indicated that the impedance of chalcocite in electrolyte solutions in the presence of kaolinite was lower than that in deionized water reflecting the mitigation of the slime coating. Electrolytes reduced the electrostatic attraction between kaolinite and chalcocite, resulting in the mitigation of kaolinite fine particles coating on chalcocite surface and consequently the improved chalcocite flotation. In addition, for the cations (Li+, Na+ and K+) and anions (F-, Cl- and I-), the larger ions reduced the impedance of chalcocite more than the smaller ions with less slime coating on chalcocite surfaces presumably due to the greater decrease of electrostatic attraction between chalcocite and kaolinite.