Electrowinning Coupled to Gold Leaching by Electrogenerated Chlorine

The kinetics of the conventional gold leaching process by dissolved oxygen in cyanide solution suffers from slow kinetics limited by the aqueous solubility of oxygen. The objectives of this Ph.D project were to improve the leaching kinetics of gold by using chlorine/hypochlorite in solutions containing chloride ions as complexants, to recover that gold from very dilute solutions by electrowinning in packed bed electrodes, and to test the feasibility of coupling the two processes, using anodically generated chlorine as the oxidant in an external leach reactor. The thermodynamics of the Au-Cl-H2O and Ag-Cl-HgO systems were summarised in the form of potential-pH and activity-pH diagrams; silver was included in these calculations since it often occurs alloyed with gold and may passivate the leach reaction. Cyclic voltammetry at a (Pt) rotating disc electrode (RDE) was used to study the kinetics of the various reactions in the system. The higher solubility of chlorine/hypochlorite compared with oxygen enabled gold leaching rates more than two orders of magnitude greater than achievable with a conventional alkaline cyanide-dissolved oxygen system. The results demonstrated the feasibility of the coupled leach-electrowinning system.