A review of chloride assisted copper sulfide leaching by oxygenated sulfuric acid and mechanistic considerations

The beneficial effect of chloride on sulfide leaching in sulfuric acid has been widely reported over the last 3–4 decades but the reasons have not been resolved. A review of recent literature shows that sulfide leaching is complex due to alternative reaction paths, competitive reactions and interim compounds formed on solid surfaces or in solutions. This study focuses on analysis of rate data for covellite leaching by oxygenated sulfuric acid in the presence of sodium chloride and comparison with the leaching data of chalcocite. The published results for initial copper leaching from covellite are analysed on the basis of a shrinking particle kinetic model to determine the apparent rate constants and reaction orders with respect to the concentrations of chloride, dissolved oxygen, and hydrogen ions. The first stage leaching of chalcocite to an intermediate CuS appears to be controlled by the mass transport of oxygen to the sulfide surface. A comparison has been made between the second stage leaching of chalcocite and the initial leaching of pure covellite to produce elemental sulfur by considering the effect of temperature on dissolved oxygen concentration and apparent rate constants. The Arrhenius plots gave comparable values for activation energy for covellite (101 kJ mol− 1) and second stage leaching of chalcocite (96 kJ mol− 1). A linear correlation of stability constants was used to determine equilibrium constants for the formation of CuS2 and Cu(OH)(Cl). The peak potentials of voltammograms of covellite are in reasonable agreement with the predicted potentials based on thermodynamics of a range of solid phases including CuS, CuS2, Cu(OH)Cl, and Cu2Cl(OH)3. These observations are used to propose a reaction mechanism via mixed-ligand complex species.