The kinetics of dissolution of synthetic covellite, chalcocite and digenite in dilute chloride solutions at ambient temperatures

An experimental study of the comparative kinetics of dissolution of various synthetic copper sulphides (covellite, chalcocite, and digenite) in dilute chloride solutions has been carried out at ambient temperatures such as could be encountered in the heap leaching of secondary copper sulphide minerals. The dissolution of sized synthetic covellite particles was carried out in dilute HCl solutions containing known concentrations of copper(II) and iron(III) at controlled potentials. The results show that the rate of dissolution is similar at potentials of 600 and 650 mV, but is predictably less at a potential of 550 mV. The rate of dissolution is remarkably similar to that of chalcopyrite under similar conditions and is largely independent of Cl^- and HCl concentration in the range 0.2 to 2.5 M and 0.1 to 1 M respectively. The effect of temperature is significant and an activation energy of 71.5 kJ mol ^{- 1} was derived which confirms a chemical or electrochemical rate-determining reaction on the mineral surface. A mineralogical study of the residue after leaching shows that most of the sulphur is associated with unreacted covellite and occurs as isolated globules on the surface with over 90% of the unreacted covellite surface free of sulphur. Dissolution of synthetic chalcocite and digenite is rapid compared with that of covellite under the same conditions. At a potential of 500 mV, the relatively rapid initial dissolution of chalcocite and digenite does not proceed beyond about 50% and 45% copper dissolution, respectively, as predicted from the thermodynamics. These results confirm the formation of a covellite-like phase as an intermediate which cannot be leached at a potential of 500 mV. An increase in the potential results in rapid dissolution of this "secondary covellite," relative to primary covellite. These results provide useful information for the conditions that should be used for the heap leaching of ores containing secondary copper sulphide minerals.