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

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)63-/Fe(CN)64- 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.