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Thiosulfate degradation during gold leaching in ammoniacal thiosulfate solutions : a focus trithionate Ahern, Noelene

Abstract

Thiosulfate has shown considerable promise as an alternative to cyanide for gold leaching. However, one of the main limitations of the thiosulfate system is the high consumption of thiosulfate. Besides increasing the cost of the process, the degradation products of thiosulfate have been claimed to passivate gold surfaces and the polythionates often produced are loaded onto resins proposed for gold recovery. The thiosulfate degradation process is not completely understood. Of the degradation products, trithionate is a concern in the resin recovery of gold and is persistent in gold leach solutions. Very little is known about the expected behaviour of trithionate, both with respect to its formation and its interaction with other solution species. The focus of this work was therefore to further the understanding of the behaviour of trithionate in gold leach solutions. Experimental work was carried out to determine the kinetics of trithionate degradation in systems resembling gold leaching solutions, and a kinetic model was derived for trithionate degradation. The rate of degradation of trithionate in aqueous ammoniacal solutions was expressed by Equation 1. [the equation is not included] In some cases, the presence of lower concentrations of thiosulfate catalysed the reaction while excess thiosulfate inhibited it. However, under typical gold leaching conditions, thiosulfate was not expected to have a significant effect so was excluded from Equation 1. Cupric copper was not found to have any significant effect on the rate of trithionate degradation under the conditions tested. This observed trithionate degradation rate equation was integrated with known kinetic behaviour of thiosulfate and tetrathionate based on literature findings to develop an overall model for the thiosulfate degradation and the resulting solution speciation of the sulfur oxyanions in the absence of ores. The model was evaluated against experimental data and its shortcomings were identified. The model parameters were adjusted to obtain a best fit to the experimental data. It was found that the best-fit parameters varied with the experimental conditions, indicating inadequacies in the model. The main concern was that the understanding of the thiosulfate degradation reactions is limited, and the way in which thiosulfate degradation was described had a major impact on the model output. In particular, the effects of copper species and pH on thiosulfate degradation have not been adequately addressed in the literature. Even taking into consideration the limitations of the model, based on the model output, decreasing the cupric concentration and increasing the ammonia concentration should help to minimise thiosulfate degradation. Solution recycle can also be used to minimise thiosulfate degradation but can result in a build up of trithionate. Limiting the reaction time would also be useful. This investigation has led to an improved understanding of the behaviour of trithionate in gold leach solutions and the model of the thiosulfate degradation system is a first step in developing a useful assessment method for thiosulfate degradation and solution speciation under gold leaching conditions. Further research is required to refine the model, particularly with respect to thiosulfate degradation to trithionate and tetrathionate.

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