Investigating the Influence of the Electrochemical Environment on the Flotation of a Mixed Sulphide Mineral System of Bornite and Chalcocite

Tafirenyika, Tanaka

Investigating the Influence of the Electrochemical Environment on the Flotation of a Mixed Sulphide Mineral System of Bornite and Chalcocite

Master Thesis

2022

Abstract

There is a growing demand for copper driven by its applications in renewable energy and electric vehicles. Sulphide ores are an important source of copper. These ores contain, on average, 2% copper and require extensive processing to extract this as pure copper. Flotation is a critical front-end process used to remove gangue minerals and concentrate the copper minerals. However, flotation is an electrochemically intense process with multiple redox reactions taking place simultaneously. The interdependency of these processes makes it extremely difficult to isolate the effect of one parameter, and hence it is difficult to predict flotation behaviour. The electrochemical activity of sulphide minerals contributes to the overall activity in the flotation pulp. This ability to conduct electrons is called their rest potential, and different sulphide minerals have different natural rest potentials and therefore different extents of activity. The ability to conduct electrons gives rise to galvanic interactions between different minerals, minerals and media or a mineral-media-mineral complexes in solution. This dictates how collectors interact with the mineral surface, and ultimately the flotation response. Other variables in the electrochemical environment such as the dissolved oxygen, (DO), pH, redox potential (Eh) and water composition are also essential in controlling flotation outcomes. The aim of this investigation is to determine the influence of the electrochemical environment on the flotation of two copper sulphide minerals: bornite and chalcocite. The focus areas include; collector-mineral interaction, surface charge and flotation recovery of the bornite and chalcocite under varying pH conditions. For this investigation, the interaction of collector with bornite and chalcocite are considered in 2 water compositions: synthetic plant water (SPW1) and deionized water (DIW) at 5 different pH levels, increasing from 3 to 11. Starvation dosages of sodium isobutyl xanthate (SIBX) are used both in the batch flotation and collector adsorption tests conducted. Zeta potential tests are carried out to determine the surface charge of the minerals under the varying conditions. Further to the pure mineral studies, batch flotation tests are carried out using a synthetic ore under natural pH, Eh and DO conditions, with grinding done in a Magotteaux Mill® to monitor pulp chemical conditions during milling. From pure mineral flotation studies, it was observed that high mineral recovery is possible in both acidic and alkaline conditions via different hydrophobicity inducing mechanisms. In acidic conditions, low pH, two possible processes are occurring, the first being the inhibition of the formation of oxy-hydroxy species that adsorb onto the mineral surface and block sites for collector adsorption. The second is the decomposition of xanthate resulting in the formation of carbon disulphide that is known to be hydrophobic and is speculated to form a film around the mineral surface and render it hydrophobic enhancing flotation. In alkaline conditions, the well-established mechanisms of xanthate ion adsorption and dixanthogen formation take place on the mineral surface and enhances the flotation. Thus, the surface charge of the minerals as the conditions changed from acidic to alkaline resulted in a change in the surfaceactive species from the pure mineral to the oxide. However, for bornite, owing to the mineral structure containing iron, when oxidation occurs, iron hydroxide species form which precipitate at the mineral surface and inhibit collector adsorption, reducing the floatability of the mineral. The surface charge of the minerals' changes with changing pH due to a change in the surface-active species from the pure mineral to the oxide and hydroxide. Flotation of chalcocite and bornite in a mixed mineral system resulted in higher copper recovery compared to the weighted sum recoveries of the individual minerals. This suggests a possible synergistic effect when floating chalcocite and bornite together. Electrochemically active impurities present in the mineral samples made it difficult to decouple the exact nature of the interaction between the two minerals, but nonetheless provides insightful observations as industrial operations have to process equally complex mineral systems.