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UBC Theses and Dissertations

Comparative study of auxiliary oxidants in the cyanidation of silver sulfide Jiang, Hao


Noticeably lower extraction of silver than gold in conventional cyanidation process has been commonly observed. Acanthite (Ag₂S) is one of the typical silver minerals with low solubility in aqueous medium, which result in low overall silver extraction. This thesis discusses the performance of hydrogen peroxide (H₂O₂), calcium peroxide (CaO₂), lead nitrate (Pb(NO₃)₂) and LeachWELL™ as potential auxiliary oxidants to improve the conventional cyanidation process. The dissolution of synthetic silver sulfide in cyanide solution with different oxidants was studied with stirred reactor leaching tests. The addition of H₂O₂ increased final silver extraction, and also raised cyanide and hydroxide consumption. The addition of CaO₂ had few effects on silver extraction. LeachWELL™ had a better performance than H₂O₂ and CaO₂ in terms of increasing silver extraction and reducing reagent cost. Furthermore, Pb(NO₃)₂ was also tested on synthetic silver sulfide leaching separately and was shown to have a similar effects as LeachWELL™ on this system. The effect of leaching parameters in the LeachWELL™-cyanide system, such as LeachWELL™ concentration, cyanide concentration, temperature and pH, were also studied. It was indicated that the increase of LeachWELL™ and cyanide concentration could further accelerate leaching and the performance of LeachWELL™ in this system was not affected by pulp pH. The extraction of silver from synthetic silver sulfide in the LeachWELL™–cyanide system was influenced to different extents by the presence of non-silver sulfide minerals. In general, most of the non-silver sulfides interfered with silver extraction via consumption of LeachWELL™ and cyanide. However, galena acted as a promoter and enhanced silver extraction. The performance of H₂O₂, CaO₂ and LeachWELL™ on natural acanthite sample cyanidation was also tested. Leaching of the natural lead-bearing acanthite sample with no auxiliary oxidant presented higher silver recovery than that achieved when leaching synthetic Ag₂S. The addition of H₂O₂ or CaO₂ further accelerated leaching while the addition of LeachWELL™ had no significant effect. It could be concluded that with enough lead in the leaching system the presence of sufficient oxidant could further enhance cyanidation of acanthite.

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