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The surface chemistry of chalcopyrite during electrochemical dissolution Ghahremaninezhad Gharelar, Ahmad

Abstract

Hydrometallurgy may be an alternative to the currently practiced smelting process for copper extraction from chalcopyrite (CuFeS₂). However, the low temperature hydrometallurgical processes for chalcopyrite continue to face challenges, mostly relating to their slow dissolution rates or high sulfuric acid production. The slow dissolution rate of the mineral is strongly linked to the formation of the passive film on its surface. However, despite 40 years of research on this topic, there is still not a complete agreement between researchers about the composition and stability of chalcopyrite’s passive film in sulfuric acid solutions. In this work, the nature of chalcopyrite’s passive film and its stability were studied by application of a variety of electrochemical techniques. Additionally, the electrochemical results of the chalcopyrite study were compared to those obtained for a pyrrhotite electrode (Fe₁₋xS), as pyrrhotite electrochemistry represents a simplified case of the chalcopyrite system. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the product layers formed on the surface. It is shown that the chalcopyrite electrode is passive for potentials up to 0.90 VSHE. Above this potential, transpassive dissolution occurs. Results of XPS studies have suggested that a metal-deficient sulfide film (Cu₁₋xFe₁₋yS₂₋z) is the most plausible copper and iron containing sulfide phase which passivates the surface of chalcopyrite. In addition, an outer layer of iron oxyhydroxide (FeOOH) forms on the passive film. FeOOH forms via oxidation of the passive film’s ferrous sulfide phases. The thickness of the sulfide passive film was calculated to be approximately 6.7 nm. It is demonstrated that the transpassive dissolution of chalcopyrite is significantly linked to oxidation of sulfur (from sulfide in the passive film to elemental sulfur and maybe sulfur species with higher oxidation states, e.g. thiosulfate). No elemental sulfur or polysulfide species were detected on the surface for potentials below 0.90 VSHE.

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