An electrochemical study of the copper-gold-cyanide system with reference to the processing of copper-gold ores

Gold and copper often occur together in ores, an occurrence which creates processing difficulties since cyanide leach solutions are not selective for gold. Copper contamination can result in serious problems throughout the conventional carbon-inpulp process, often making the operation uneconomic. This thesis describes electrochemical studies of two ways of dealing with copper contamination. One concerns the selective electrodeposition of gold from coppercontaining cyanide solutions; the second concerns the possibility of selectively leaching gold from copper-containing ores using an aerated ammoniacal cyanide solution. In the electrowinning study, a rotating disc electrode technique was combined with a method of chemical analysis to investigate the codeposition behaviour of copper and gold. In these experiments, the variables were gold concentration, agitation, free cyanide concentration, pH and temperature. The experimental results obtained have provided an improved understanding of the electrolytic codeposition of copper with gold in cyanide solutions. The major factors influencing copper codeposition with gold are the free cyanide concentration, the operating temperature and the pH of the electrowinning solution. By controlling these variables in the electrowinning operation, it is possible to minimise the copper deposition. Pulsed potential control was employed as an alternative to constant potential electrodeposition as a means of reducing copper codeposition in the gold electrowinning process. It was demonstrated in this work that by simply varying the parameters of the applied pulsed potential waveform, all other operating conditions being the same as for the constant potential case, copper contamination was greatly reduced. This improvement is due to the high instantaneous gold deposition rate during the cathodic potential pulse period and the copper redissolution during the anodic potential pulse period. A rotating electrochemical quartz crystal microbalance (REQCM) technique was developed in this research work. It was demonstrated that this REQCM technique is capable of measuring the partial current of a metal deposition in the potential region where the hydrogen evolution reaction is involved.. Thus, obtaining information outside the electrochemical window of a solid electrode become possible. The REQCM was used to study the mechanism of copper and gold deposition from cyanide solution. One of the findings from this work is that when using the chronopotentiometric method to study metal dissolution or cementation reactions, the effect of the inert substrate must be taken into account. It was found that the substrate has no influence on the rate if the reaction is diffusion controlled; however, it markedly affects the rate of chemically controlled reactions. The magnitude of the effect is related to the catalytic activity of the substrate. The REQCM technique was also used to study the mechanism of selective leaching of gold from copper contaminated ores using ammoniacal cyanide. Experimental results show that in copper containing cyanide solutions, the copper-cyanide complex species takes part in the gold dissolution reaction. The addition of ammonia to solutions containing copper and cyanide greatly diminishes the rate of gold dissolution. However, when the cyanide to copper ratio is close to 2:1, the rate of gold dissolution increases on addition of ammonia. This may be due to the formation of some goldammonia complex or the involvement of the formation of Cu(II)- cyanide-ammonia complex which can act as an oxidant in the gold leaching process