ELECTROWINNING COUPLED TO GOLD LEACHING BY ELECTROGENERATED CHLORINE .1. AU(III)-AU(I)/AU KINETICS IN AQUEOUS CL2/CL- ELECTROLYTES

The effects of Au(I):Au(III) molar ratio, pH and Cl- concentration on the voltammetric behaviour of a Pt electrode were investigated to enable the kinetics and mechanism of gold dissolution and deposition to be determined. At + 0.8 V vs. SCE gold dissolved exclusively as AuCl2- ions, the proportion of AuCl4- ions increasing at higher potentials. A steady state kinetic model of the coupled electrochemical and chemical reactions resulting in gold dissolution as AuCl(n)- ions, is shown to be capable of predicting the experimentally determined reaction order (almost-equal-to 1.7) with respect to chloride ion concentration and the Tafel slope (almost-equal-to 80 mV decade-1 at pH 0). Gold electrodeposition and chlorine reduction were studied using a rotating disc electrode. Gold leaching by Cl2/HClO was found to occur at about a quarter of the mass transport controlled rate at pH 0; dissolution rates decreased with increasing pH. However, higher concentrations of HClO (7.5 < pH < 3.3) than Cl2 (pH < 3.3) are attainable, enabling compensation for the slower intrinsic gold dissolution kinetics at higher pHs and avoiding loss of gaseous chlorine in an open system, since the equilibrium pressure Of Cl2O(g) above such solutions is much lower than that Of Cl2(g).