The anodic behaviour of covellite in chloride solutions

A detailed study of the anodic behaviour of synthetic covellite in acidic chloride solutions has been conducted as part of an overall program on the fundamental aspects of the heap leaching of copper sulfide minerals. The anodic behaviour in chloride solutions is characterized by active (but slow) dissolution at low potentials below about 0.65 V, passivation at potentials above 0.70 V that continues to potentials greater than 1.4 V above which rapid transpassive dissolution occurs. These characteristics are also typical of chalcopyrite under similar conditions. The anodic characteristics at potentials in the region of the measured mixed potentials in the presence of copper(II) show that there are two peaks in the voltammetric sweeps at about 0.65 V and 0.75 V, the magnitude of which increase with increasing chloride concentration. The two peaks merge into a single peak at very high chloride concentrations. Potentiostatic current-time transients at various potentials in the region of the mixed potential show the slow passivation typical of chalcopyrite and the data can similarly be described quantitatively in terms of rate limiting solid-state diffusion. The rate of oxidative dissolution of covellite under ambient temperature conditions is slow but about an order of magnitude greater than that of chalcopyrite. The voltammetric response of chalcopyrite after pre-treatment with a chloride solution containing a low concentration of copper(II) shows the presence of the same two peaks observed for covellite confirming the surface conversion of chalcopyrite to covellite. A mechanism similar to that previously proposed for the dissolution of chalcopyrite has been described in terms of which the formation of polysulfides (typically CuS2) by dissolution of a fraction of the copper in the covellite lattice is responsible for the passivation that increases slowly as the polysulfide layer increases in thickness. Novel measurements of the mixed potential and the solution potential at a covellite surface at the bottom of a capillary have been used to simulate the situation within ore particles. The results have been simulated using a simple linear diffusion model that show that the potential at the covellite surface can be 50–100 mV lower than that in the bulk of the solution. This has important consequences for the rate of heap leaching.