Surface oxidation of chalcopyrite (CuFeS2) in alkaline solutions

The surface oxidation of chalcopyrite in solutions of pH 9.2 and 12.7 has b een investigated using electrochemical techniques, X-ray photoelectron spec troscopy, and Anger electron spectroscopy. The results show that the oxidat ion process consists essentially of three potential- and pH-dependent stage s. In 0.1 M Na2B4O7 (pH 9.2), on increasing the electrode potential From -0 .6 to +0.02 V l's. SCE, the iron in the top layer of the chalcopyrite surfa ce is oxidized, forming a monolayer of Fe(OH)(3) and Fe2O3. The copper and sulfur remain unoxidized as a phase we designate CuS2*, which together with Fe(OH)(3) and Fe2O3 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, CuS2* is no longer stable and is oxidized to CuO , S, and SO42- ions. The passivating film then decomposes, greatly accelera ting the oxidation rate of the underlying CuFeS2. In 0.05 M NaOH (pH 12.7), the oxidation mechanism is similar to that in 0.1 hi borax solution. Howev er, because the equilibrium potentials are lower, the corresponding current peaks appear at less positive potentials. In addition, higher concentratio ns of OH- ions enhance the dissolution rates of iron and copper oxides and hydroxides, so increasing reaction rates. (C) 2000 The Electrochemical Soci ety. S0013-4651(99)11-028-0. All rights reserved.