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UBC Theses and Dissertations

Fundamental aspects of thiosulfate leaching of silver sulfide in the presence of additives Deutsch, Jared Luke


Thiosulfate is a promising alternative for leaching silver sulfide ores such as those at the epithermal Yanacocha deposit. These ores suffer from low silver recovery, high mercury extraction and environmental challenges when treated with conventional cyanide. In this study, cupric-ammonia, ferric-ethylenediaminetetraacetic acid (EDTA), ferric-oxalate and ferric-citrate are tested with thiosulfate in a series of rotating disk experiments with silver sulfide and with ore from Yanacocha. This thesis publishes experimental evidence that supports the use of thiosulfate with different additives as a potential alternative to conventional cyanidation for silver sulfide leaching. The leaching of silver sulfide by cupric-ammonia thiosulfate can occur either by the substitution of cupric or cuprous for silver. The cupric catalyzed reaction is favored due to a thermodynamic barrier to the cuprous reaction. Rotating disk experiments demonstrate that cupric-ammonia leaching is under mixed chemical/diffusion control. The leaching rate is maximized by stabilizing cupric in solution with ammonia and increasing the availability of thiosulfate for silver dissolution. The addition of EDTA to this system decreased the leaching rate of the silver sulfide disk by lowering the cupric reactivity, but accelerated silver leaching of the ore, likely due to the prevention of passive oxide film formation on sulfides. Ferric complexes used were found to be very unreactive towards thiosulfate, but are reduced by sulfides present in the ore. Ferric-EDTA was the most effective oxidant of the three for leaching silver sulfide with thiosulfate. Silver recovery of the ground ore in batch leaching tests is low due to quartz locking of silver, with cupric-ammonia and ferric-EDTA leaches exposed to air recovering 31% and 26% after 24 hours, respectively. Cyanidation recovered 34% silver with a 95% confidence interval of 28-37%. The slightly lower recovery by thiosulfate may be due to silver minerals which are not amenable to thiosulfate leaching.

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