Thiosulphate leaching of silver from an arsenical refractory ore

• Published in: Minerals engineering 2018-06, Vol.122, p.285-295
• Main Authors: Celep, Oktay, Altinkaya, Pelin, Yazici, Ersin Y., Deveci, Haci
• Format: Article
• Language: English
• Subjects: Refractory ore; Silver; Silver recovery; Thiosulphate; Ultrafine grinding
• ISSN: 0892-6875; 1872-9444
• DOI: 10.1016/j.mineng.2018.04.011

•Ultra-fine grinding improves the extraction of Ag from an arsenical refractory ore.•Significance of parameters is in the order of [CuSO4] > [Na2S2O3] > [NH3].•Thiosulphate is more selective than cyanide with negligible leaching of As, Sb and Zn.•Addition of EDTA, CMC or NiSO4 (in place of CuSO4) improves Ag extraction.•Sodium borohydride (NaBH4) can be used for recovery of Ag from pregnant leach solutions. 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 su