An electrochemical study of gold cyanidation in the presence of carbon coatings

The anodic and cathodic behaviour of gold with clay and different carbon coatings was investigated using the potentiodynamic method. The overall dissolution reactions were also examined by combining the anodic and cathodic polarisation curves. The presence of clay coatings affected neither the anodic curve shapes nor the peak positions for gold oxidation nor changed the cathodic curve appearance for oxygen reduction. The peak current densities slightly reduced in the presence of clay coating and this trend became more noticeable with an increase in the coating thickness. Non-conductive clay only acted as a barrier to the diffusion of reagents to the gold surface, but did not galvanically interact with gold. Different carbon coatings, natural carbonaceous matter from Stawell gold ore, activated carbon and graphite, performed similarly in affecting the anodic gold oxidation, i.e., the shift of rest potential to a more anodic position, the narrowing of the first peak region (a more cathodic overpotential region), the shift of current peaks to more anodic positions and the high current densities at high overpotentials over 0.7 V. The carbon coatings changed the wave appearance for oxygen reduction. In the presence of the carbon coatings, the current densities for oxygen reduction became lower at less cathodic potentials and became higher at more cathodic potentials, and the current densities for oxygen reduction increased with an increase in the thickness of the carbon coatings. The presence of the carbon coatings did not change the reaction control mechanisms for either gold oxidation or oxygen reduction. The combination of anodic and cathodic reactions indicated that the carbon coatings significantly reduced the gold oxidation rates with increasing coating thickness, in agreement with the leach results. The oxidation potentials shifted to a more anodic direction with increased thickness of the carbon coatings. The magnitude of the decrease in the gold oxidation rates was comparable for the different carbon coatings at a similar thickness, despite their microstructures, surface areas and activities being significantly different. The results showed that the carbon coatings galvanically interacted with gold.