Surface Oxidation of Chalcopyrite (CuFeS[sub 2]) in Alkaline Solutions

The surface oxidation of chalcopyrite in solutions of pH 9.2 and 12.7 has been investigated using electrochemical techniques, X‐ray photoelectron spectroscopy, and Auger electron spectroscopy. The results show that the oxidation process consists essentially of three potential‐ and pH‐dependent stages. In 0 . 1 M Na 2 B 4 O 7 (pH 9.2), on increasing the electrode potential from −0.6 to +0.02 V vs. SCE, the iron in the top layer of the chalcopyrite surface is oxidized, forming a monolayer of Fe ( OH ) 3 and Fe 2 O 3 . The copper and sulfur remain unoxidized as a phase we designate CuS 2 * , which together with Fe ( OH ) 3 and Fe 2 O 3 forms a film retarding the oxidation. As the potential is increased further, deeper layers are involved in the oxidation, but the passivating film is not destroyed. At this stage, the oxidation process is controlled by solid‐state mass transport. When the applied potentials are higher than 0.4 V vs. SCE, CuS 2 * is no longer stable and is oxidized to CuO, S, and SO 4 2 − ions. The passivating film then decomposes, greatly accelerating the oxidation rate of the underlying CuFeS 2 . In 0.05 M NaOH (pH 12.7), the oxidation mechanism is similar to that in 0.1 M borax solution. However, because the equilibrium potentials are lower, the corresponding current peaks appear at less positive potentials. In addition, higher concentrations of OH − ions enhance the dissolution rates of iron and copper oxides and hydroxides, so increasing reaction rates.