DISSOLUTION OF GOLD DURING PYRITE OXIDATION REACTION

摘要

Numerous papers discuss the mechanism of alkaline oxidation of pyrite, but there is limited information available describing the kinetics of the pyrite sulphide to thiosulphate reaction. A previous investigation in this series determined the rate of sulphide oxidation and thiosulphate yield during the reaction of pyrite with sodium hydroxide under various testing conditions. The goal of the current study is to validate these rates using two different gold-containing pyrite concentrates. A further objective of the current work is to investigate the simultaneous dissolution of gold with the thiosulphate that is formed in-situ during pyrite oxidation.It was found that at 20 psi oxygen overpressure and a temperature of 80°C , the initial rates of sulphide oxidation and thiosulphate yield were close to 0.08 mol/h and 0.0155 mol/h, respectively. These rates are in agreement with previously published data. However, a shift from linearity occurred when the pH decreased below 12. A rapid decay of thiosulphate was evidenced at pH 8.3-9.2 while E_h was in the range of 22-141mV. Based on relevant thermodynamic analysis of metastable thiosalts system, such rapid decomposition is not expected at these pH and E_h values. It is believed that the presence of unreacted pyrite acting as a catalyst caused this behaviour. It appears that under mildly alkaline conditions, the rate of oxidation of sulphide to thiosulphate is slower than the rate of thiosulphate degradation, and as a result there is no significant build up of thiosulphate in solution under these conditions. The maximum extraction of gold and silver (96 and 75% respectively) was achieved under conditions of pH 10 - 12.