Electrochemical aspects of leaching copper from chalcopyrite in ferric and cupric salt solutions

Abstract

Electrochemical aspects of semiconductors are used to interpret well established observations on the kinetics of leaching of chalcopyrite. The oxidation of this n-type semiconductor is dominated by a surface film which is thermally unstable and breaks down in CS₂, acetone or acidified water, or under dry nitrogen, over comparable time periods. The film is thought to be a semiconductor metal-deficient polysulfide which slows transport of Cu⁺ and Fe²⁺ products, slows electron transfer to oxidants such as Fe³⁺ and Cu²⁺, and dramatically slows supply of holes and thus electron transfer from reduced species such as Fe²⁺ on corroding chalcopyrite. Thus the Fe³⁺/Fe²⁺ couple (especially as sulfate) is much less reversible on corroding chalcopyrite than on pyrite or platinum. The couples Cu²⁺/Cu⁺,I₃^-/I^- and Fe(CN)₆^3-/Fe(CN)₆^4- are more reversible than Fe³⁺ /Fe²⁺ but all couples are much less reversible on chalcopyrite than on pyrite.

A layer of sulfur forms on corroding chalcopyrite, but this is not the species which slows transport of ions and transfer of electrons. A mixture of Fe³⁺/Cu²⁺ chlorides is one of the more effective oxidants for CuFeS₂ because of relatively fast electron transfer from corroding chalcopyrite to Cu²⁺ and oxidation of Cu⁺ by Fe³⁺. Catalysis by iodine and by inclusion of Ag₂S or FeS₂ in natural chalcopyrite is explained by the electrochemical model.