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Effects of Reduced Sulfur Species on the Recovery of Gold and Silver by Cyanide Leaching
AuthorDuru, Naci U.
AdvisorNesbitt, Carl C.
Mining and Metallurgical Engineering
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Cyanidation circuits that treat sulfidic ores may contain one or more of the following reduced sulfur species: sulfide (HS-), polysulfides (Sn2-), thiocyanate (SCN-), thiosulfate (S2O32-), trithionate (S3O62-), tetrathionate (S4O62-), pentathionate (S5O62-), hexathionate (S6O62-). The actual speciation of the reduced sulfur species in the circuits will primarily depend on the redox potential, pH, cyanide levels and existing sulfur species in recycled process water, as well as the presence of other reactive metal species. Higher polythionates (i.e. tetra-, penta- and hexa-thionate) present in leach solutions react with cyanide ions rapidly and consume the available cyanide causing lower metal recoveries at cyanide leaching plants. However, if higher polythionate ions could be degraded to trithionate, cyanide consumption would be lowered and metal recoveries would be increased.The aim of this study is to investigate the possibility of increased gold and silver recoveries and decreased cyanide consumptions by controlling the higher polythionate levels with the addition of sulfite salts to the leach slurries. Experimental results showed that average gold and silver recoveries increased by 13.3% and 28.8%, respectively at 500 ppm NaCN and 300 ppm SO32- concentration levels compared to the control experiments where no sulfite was added to the leach slurries. Additionally, higher sulfite concentrations in the leach slurries resulted in lower tetrathionate and thiocyanate and, higher thiosulfate, trithionate and available cyanide concentrations.