Thiosulphate leaching of silver from an arsenical refractory ore

Thiosulphate leaching has received particular attention in recent years as an environmentally benign alternative to cyanide leaching. In this study, thiosulphate leaching combined with ultra-fine grinding (UFG) was studied for silver extraction from an arsenical refractory ore. Direct cyanidation of the ore (d80: 50 µm) led to a low silver extraction of ≤ 70% over 24 h. leaching period. Extraction of silver in cyanide leaching was improved to 85% after ultra-fine grinding. In comparison, up to 98% of silver extraction was achieved in thiosulphate leaching system after ultra-fine grinding of the ore down to 5 μm (d80) in a stirred mill. Furthermore, thiosulphate leaching is more selective than cyanide leaching with only negligible leaching of arsenic, antimony and other metals observed in this study. This is of particular importance considering the environmental concern for the presence of arsenic in cyanide leaching solutions and effluents. Effects of thiosulphate (0.2-0.6 M Na2S2O3), ammonia (0.04-0.52 M NH3) and copper (0.02-0.11 M CuSO4) on the extraction of silver were investigated using a Box-Behnken design. Statistical analysis of the experimental data (%Ag extraction at 8 h.) confirmed that all the parameters have quadratic effects in that the extraction of silver tends to improve with increasing their concentration up to a critical level above which a reverse trend occurs. The addition of a small amount (0.1 M EDTA, 50 mg/L CMC) of ethylenediaminetetraacetic acid (EDTA) and carboxymethyl cellulose (CMC) improved (by ∼20-30%) silver extraction and reduced thiosulphate consumption by ∼20%. Using nickel sulphate in place of CuSO4 also improved silver extraction with concurrent decrease in thiosulphate consumption. Recovery of silver from real pregnant leach solutions using sodium borohydride (NaBH4) as the precipitating agent was also investigated in detail. A high silver recovery of ≥98% was achieved under suitable conditions. These findings highlight that ammoniacal thiosulphate leaching in combination with ultra-fine grinding can be used as a suitable and environmentally sound alternative to cyanide leaching and the additives such as EDTA and CMC, and nickel in place of copper can improve the extraction of silver from refractory silver ores.