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Some aspects of the use of thiourea in gold processing

Zhang, Hong-Guang (1997) Some aspects of the use of thiourea in gold processing. PhD thesis, Murdoch University.

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Thiourea is a potential alternative for cyanide in gold processing. In an acid solution of thiourea containing an oxidant such as iron(III) ions, gold can be readily dissolved as the gold thiourea complex. Although the thiourea leaching process has been studied quite extensively, the fundamental electrochemistry associated with this process remained unexamined. In addition, the method for the recovery of gold from thiourea solutions has received relatively little attention. This thesis presents a study of these aspects.

The results from a detailed electrochemical study of the dissolution of gold and the oxidation of thiourea in acidic thiourea solutions are presented in the first part of the thesis. The cyclic voltammetry of the gold/thiourea system for various conditions was conducted and the relevant reactions were identified. The electrochemical dissolution of gold was found to be controlled by a slow surface reaction but could be catalysed by formamidine disulphide, an oxidation product of thiourea. Passivation was observed at higher potentials. The oxidation of thiourea on a gold electrode occurs at a much more positive potential than on a platinum electrode. The strong chemical adsorption of thiourea on gold is considered to have inhibited both oxidation processes on the gold electrode.

In the next part of the thesis, the results from an investigation of the recovery of gold from acidic thiourea solutions by mild steel cementation and activated carbon adsorption are presented. The kinetic and electrochemical measurements showed that the cementation process was controlled by the diffusion of the gold thiourea complex to the mild steel surface. Thiourea, sulphuric acid, ferrous and silver ions do not interfere with the rate of the reaction in the concentration range studied, while cupric ion substantially reduces the reaction rate. The results were interpreted using the Evans’ diagram. The adsorption of the gold thiourea complex onto activated carbon was found to be an exothermic process. The equilibrium and the kinetics of the adsorption were investigated with various factors such as pH, thiourea concentration and temperature. At relatively low gold loadings, the gold thiourea complex was adsorbed intact, while at high gold loadings, decomposition into metallic gold was observed.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ritchie, Ian and La Brooy, Steve
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