Dual lixiviant leaching process for extraction and recovery of gold from ores at room temperature

A comprehensive investigation of Au leaching from gold ores with a solution containing thiourea and ammonium thiocyanate was performed. A gold ore A with 18 g/t of Au and 1.15 wt.% of sulfur and another gold ore B with 55.7 g/t of Au and 11.67 wt.% of sulfur were used. The percentages of extracted Au were 57%, 66% and 95% for the leaching solutions containing thiourea only, ammonium thiocyanate only and ammonium thiocyanate plus thiourea, respectively, under optimal conditions. There were some interactions among thiourea, ammonium thiocyanate and ferric ion, which affected gold extraction greatly. The addition of thiocyanate reduced greatly the consumption of thiourea and the process cost accordingly. The optimal concentrations of thiourea, ammonium thiocyanate and ferric sulfate for ore A leaching were 0.13 M, 0.78 M and 0.028 M, respectively. The optimal pH, temperature, time, ratio of liquid to solid, agitation speed and particle size for ore A leaching were 1.5, 21 °C, 6 h, 10, 250 rpm and 54–74 μm, respectively. Au extraction for ore A was 95% under above optimal conditions. Au extraction was 97% when sulfur content of ore B was 1.48 wt.%. The preferential consumption of ferric ions by pyrite over by gold was the main reason for resulting in the decrease of gold extraction. Desulfurization is necessary to achieve a high gold extraction for this leaching system of thiourea-thiocynate-Fe3 +. After Au extraction, about 90% of Au was recovered as insoluble metallic Au by cementation using 5 g/L of Al powder. The cementation filtrate was reused to leach Au ore and about 91% of Au was extracted after pH adjustment. Thus, thiocyanate and thiourea could be reused, which reduced the process cost. The leaching speed of Au of this method was more than 4 times faster than that of cyanidation process. This process concerning extraction and recovery of Au from Au ore was effective, fast, environmentally friendly and also likely economical.