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UBC Theses and Dissertations

Kinetics of sulphide conversion to thiosulphate and gold dissolution study by electrochemical quartz crystal microbalance Melashvili, Mariam


The kinetics of the oxidation of pyrite sulphide to a thiosulphate reaction were studied using a pyrite concentrate oxidized in an alkaline medium at an oxygen overpressure between 10 and 40 psig at a temperature of 80ºC. Sodium hydroxide was used to neutralize the acid produced as a result of the pyrite oxidation. The focus of this study was the calculation of thiosulphate yield as a function of sulphide sulphur concentration of the pyrite in the feed. A single rate expression combining kinetic constants of all the metastable oxyanions was derived to predict the thiosulphate yield as a function of the known pyrite sulphide sulphur concentration under the experimental conditions that were adopted. [formula omitted] It was found that at 20 psig oxygen overpressure and a temperature of 80ºC, the initial rate of sulphide oxidation and thiosulphate yield was close to 0.08 mol/h and 0.015 mol/h, respectively, at pH values greater than 12. However, a shift from linearity occurred when the pH decreased below 12. It was observed in the experiments that any decrease in pH was accompanied by an increase in solution potential, which enhanced the rate of gold leaching provided sufficient thiosulphate remained in the solution. The oxidation and reduction of gold in thiosulphate as a function of applied potential was studied using cyclic voltammetry. The responses of cyclic voltammetry of gold electrodes in thiosulphate were compared under different conditions involving catalysts such as thallium and thiourea. It was found that the gold anodic current increases in the presence of catalysts which is more likely to be related to the adsorption/desorption phenomenon and not to the depletion of the thiosulphate ion at the reaction interface. Higher anodic current increased the surface gold concentration available for reduction. The large anodic-to-cathodic peak separation that ranged from 0.25V to 0.37V allowed most of the leached gold to diffuse away from the electrode. This indicates the electrochemically-irreversible character of this system even in the presence of catalysts. The relationship between the reduction peak current and concentration of gold thiosulphate in the bulk electrolyte was estimated based on a relevant equation for electrochemically-irreversible reaction.

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