Remediation of reduced sulfur species effects on gold and silver recovery during cyanide leaching

Cyanidation circuits that treat sulfidic ores may contain one or more of the following reduced sulfur species: sulfide (HS-), polysulfides (S-n(2-)), 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 hexathionate) present in leach solutions react with cyanide ions rapidly and consume the available cyanide causing lower metal recoveries at cyanide leaching plants. When higher polythionate ions are degraded to trithionate, cyanide consumption by these ions could be lowered, and metal recoveries would increase. This study presents the sulfur species formed and a practical polythionate degradation method that can be applied within the cyanidation circuits. The possibility of increased gold and silver recoveries and decreased cyanide consumptions by controlling the higher polythionates (SnO62-, n = 3,4,5,6) with the addition of sulfite salts to the cyanide leach slurries was studied. Experimental results showed that average gold and silver recoveries increased by 13.3% and 28.8%, respectively at 500 ppm (mg/L) NaCN and 300 ppm (mg/L) sulfite (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 higher thiosulfate, trithionate and available cyanide concentrations.

Automated summary

Cyanidation circuits processing sulfidic ores contain various reduced sulfur species, with higher polythionates reducing metal recoveries. By degrading higher polythionates and adding sulfite salts to the cyanide leach slurries, gold and silver recoveries can be increased while reducing cyanide consumption.