Kinetics of enhanced gold dissolution: activation by dissolved silver

Abstract

Coupled oxygen reduction and metal oxidation occur during leaching of gold in cyanide solutions. Both processes are altered by specific surface-active constituents in solution. Dissolved cyanide in solution depresses the rate of oxygen reduction while the presence of silver in solution enhances the rate of reduction. Similarly, the oxidation (dissolution) rate of gold is dramatically increased by dissolved silver in solution. For both cathodic and anodic processes, silver enhancement approaches a maximum near 1 ppm dissolved silver for low cyanide concentrations. High concentrations of cyanide diminish the effect of silver, indicating a competition between silver and cyanide for specific surface sites. These effects are demonstrated using cyclic voltammetry under a variety of conditions. Reaction rates, based upon potentiostatic, current-time data, are well correlated by a paralinear rate model. The model includes steady-state formation of a passivating film at the metal film interface, metal (gold ion) diffusion through the film, and dissolution at the film solution interface. Silver is found to enhance both the film diffusion process and film dissolution.