Optimization of Gold Leaching from Roasted Pyrite in a Sodium Thiosulfate–Copper Sulfate System

The development of environmentally safer alternatives to cyanidation is particularly important for processing refractory gold-bearing sulfide materials. This study investigates the effects of key operating parameters on gold extraction from roasted pyrite using a sodium thiosulfate–copper sulfate leaching system. The initial pyrite sample contained 3.1 g t⁻¹ Au, 13.7 g t⁻¹ Ag, 30.66 wt.% Fe, and 32.0 wt.% S. X-ray diffraction analysis confirmed that pyrite was the dominant mineral phase. To disrupt the sulfide matrix and improve gold accessibility, the sample was roasted at 600 °C before leaching. Experiments were conducted at pH 9.5 using 0.2–0.4 mol L⁻¹ Na₂S₂O₃, 0–2 mmol L⁻¹ CuSO₄, temperatures ranging from ambient conditions to 45 °C, and leaching times of 6–14 h. Gold dissolution increased substantially when the thiosulfate concentration was raised from 0.2 to 0.3 mol L⁻¹, whereas a further increase to 0.4 mol L⁻¹ produced no significant improvement. The addition of 1 mmol L⁻¹ CuSO₄ enhanced gold oxidation and increased extraction from approximately 68% to 88% at 0.3 mol L⁻¹ Na₂S₂O₃. Increasing the CuSO₄ concentration to 2 mmol L⁻¹ slightly reduced extraction, presumably because excessive Cu²⁺ promoted thiosulfate degradation. Temperature and contact time also significantly affected the process. Gold extraction increased from approximately 74% at ambient temperature to nearly 90% at 45 °C, with the extraction curve approaching a plateau after 12 h. ICP-OES analysis at 267.595 nm detected 1320 ppb Au in the optimized leach solution, with a calibration correlation coefficient of 0.99792. The results identify 0.3 mol L⁻¹ Na₂S₂O₃, 1 mmol L⁻¹ CuSO₄, pH 9.5, 45 °C, and 12 h as the preferred operating conditions for gold recovery from roasted pyrite.