Effect of mixing acidic and alkaline pressure oxidation discharges with different ratios on gold thiosulfate leaching efficiency

Abstract

Thiosulfate gold leaching has been successfully commercialized in Goldstrike mine by Barrick Gold Corp. In this plant, alkaline and acidic pressure oxidation (POX) discharges have been mixed and fed to the calcium thiosulfate (CaTS) gold leaching resin in leach process. In this study, the effect of various mixing ratios of the alkaline and acidic POX treated ores have been investigated via employing electrochemical methods and leaching tests. The open circuit potential (OCP) measurements showed that increasing the percentage of the acidic POX discharge mostly decreases the OCP value. The cyclic voltammetry (CV) study with varying ratios of the two POX discharge slurries showed the oxidation peaks in the anodic cycle which are related to gold and thiosulfate oxidation processes. Chronoamperometry (CA) tests were carried out at the peak potentials identified in the CV tests, specifically at 0 and +0.1 V. The CA test results suggest that the higher alkaline POX discharge in the CaTS slurry results in a higher gold oxidation rate. The results were validated with gold analysis of the CA test solutions.Unlike the electrochemical results, leaching results showed that in the presence of acidic POX discharge ore, gold recovery was 75.5% while increasing the alkaline POX discharge ratio declined the gold recovery to 65.3%. Higher gold oxidation rate associated with the slurries containing higher alkaline:acidic POX discharge ratios, observed in the electrochemical tests, confirmed that the alkaline POX discharge could provide more oxidative media for gold oxidation owning to its higher copper. However, the lower gold leaching recovery which is obtained by increasing the alkaline:acidic POX discharge ratio, is linked to the mineralogy of the alkaline and acidic POX discharges and the degree of liberation of the gold particles. Acidic POX process produced more liberated gold particles while Alkaline POX discharge ore consists of more locked gold in the pyrite crystal structure and iron hydroxide precipitates which are known to lock the gold particles and reduce gold liberation.