An electrochemical study of gold leaching in thiosulfate solutions containing copper and ammonia

The electrochemistry of gold in thiosulfate solutions containing copper and ammonia was studied using a combination of standard electrochemical techniques and the rotating electrochemical quartz crystal microbalance. It was found that the cathodic half reaction, the reduction of copper(II), occurs readily in the potential region where gold should be oxidised to gold thiosulfate. However, the gold oxidation half reaction in thiosulfate solutions alone is hindered; the presence of ammonia is required for the reaction to occur at an appreciable rate. It was also shown that copper(II) affects the gold oxidation half reaction; this is why copper(II) is more effective than other oxidants at leaching gold in thiosulfate solutions. One complication with using copper(II) as an oxidant is that it also reacts with thiosulfate, which results in a decrease in copper(II) concentration. Such a decrease in [Cu(II)] was shown to cause a decrease in the rate at which the gold oxidation half reaction proceeds. The effect of temperature and thiosulfate concentration on the oxidation of gold was also investigated.