Synergistic effect of cupric and ferrous ions on active-passive behavior in anodic dissolution of chalcopyrite in sulfuric acid solutions

Copper extraction during the oxidative leaching of chalcopyrite (CuFeS ₂) by ferric ions or by dissolved oxygen in sulfuric acid solutions is known to be faster at low redox potentials but slower at potentials above a critical value. The present study shows that this phenomenon occurs only when cupric and ferrous ions coexist, based on electrochemical measurements. Using a chalcopyrite electrode prepared from an ore lump sample, effects of the addition of 0.1 kmol m^-3 cupric and ferrous ions on anodic polarization curves and AC impedance spectra were investigated in 0.1 kmol m^-3 sulfuric acid with stirring at 298 K in nitrogen. Without cupric and/or ferrous ions, the anodic current increased monotonically with increasing applied potential. When cupric and ferrous ions coexisted, active-passive behavior was observed, i.e., the current increased with increasing potential to reach a maximum and it suddenly decreased at a certain potential, whereby the current became less dependent on the potential. In the active region, i.e., at low potentials, the anodic current with coexisting cupric and ferrous ions was larger than that without these ions. These results agree well with the results of leaching experiments reported previously, and indicate that coexisting cupric and ferrous ions promote the anodic dissolution of chalcopyrite at low potentials. The analysis of the AC impedance spectra indicates that a high-resistance passive layer grows on the chalcopyrite surface without cupric and/or ferrous ions, and that coexistence of these ions causes a formation of another product layer and inhibits the passive layer growth in the active region. To interpret the active-passive behavior of chalcopyrite, a reaction model, assuming the formation of intermediate Cu₂S in the active region, is discussed based on the experimental results.