Extraction of gold/silver from copper and pyrite bearing gold ores/concentrates using thiosulphate leaching

Cyanide leaching is the most common method for the extraction of gold from ores. However, in recent years, environmental, technical, economic, social factors and ever increasing legislative regulations have placed high pressure on gold production by traditional cyanide leaching process. Because of these reasons, alternative reagents to cyanide are extensively investigated. As an environmentally friendly alternative to cyanide, thiosulphate has emerged potentially as the most promising reagent with its technical merits including high leaching kinetics and high selectivity. In this thesis, thiosulphate leaching of gold/silver from a copper-rich gold ore and a pyritic gold concentrate was investigated. Recovery of Au/Ag from pregnant leach solutions by sodium sulphide (Na2S) was also tested. In leaching tests, effects of concentrations of ammonia (NH3), thiosulphate (S2O3) and copper (CuSO4) were investigated for extraction of gold/silver using statistical-experimental design methods. The results obtained for both materials showed that increasing ammonia concentration positively influenced the leaching performance whilst the increase in thiosulphate concentration exerted a negative effect. A maximum gold recovery of, 81.5% was obtained from copper-bearing gold ore. (24 h). In the leaching of pyritic gold concentrate, complete extraction of gold was achieved over 8 h. Precipitation/recovery of metals (Au, Ag, Cu ve Zn) from pregnant leach solutions was also studied using the addition of Na2S (2.0-24.2 mM). Recovery of Au and Ag was observed to increase with increasing the concentration of sodium sulphide in the range of 2.0-24.2 mM. Complete recovery of silver was obtained at 24.2 mM Na2S at which 34.8% of Au and 83.5% of Cu precipitated. Increasing the Na2S addition was observed to have a limited effect on the precipitation of zinc (30-36.6%). These findings indicated that silver can be selectively recovered over gold from leach solutions.