Comparative Evaluation of Cyanide-Free Lixiviant Systems for Gold Recovery from Pyrite Concentrate

The development of less hazardous alternatives to cyanide is an important objective in the hydrometallurgical processing of refractory gold-bearing sulfide materials. This study comparatively evaluates sodium thiosulfate, ammonium thiosulfate, and thiourea systems for gold extraction from a gold-bearing pyrite concentrate. The effects of lixiviant concentration, catalyst or oxidant dosage, temperature, and leaching time on gold dissolution were investigated. Copper sulfate was used as a catalyst in the sodium and ammonium thiosulfate systems, whereas ferric chloride was employed as an oxidizing agent in the thiourea system. Increasing the temperature from 25 to 45 °C, raising the lixiviant concentration within the range of 0.2–0.3 mol L⁻¹, and extending the leaching duration enhanced gold transfer into solution in all investigated systems. Among the tested lixiviants, ammonium thiosulfate exhibited the highest performance, achieving a maximum gold extraction of 84% under the selected optimal conditions. Gold extraction reached 79% in the thiourea system and 69% in the sodium thiosulfate system. The superior performance of ammonium thiosulfate was attributed to favourable gold–thiosulfate complex formation and the catalytic action of copper ions. The leaching kinetics were interpreted using the shrinking-core model, while the temperature dependence of the process was assessed using the Arrhenius equation. The comparative results indicate that ammonium thiosulfate is the most effective of the investigated cyanide-free lixiviants, whereas thiourea also provides competitive extraction performance. These systems offer a potential basis for developing more environmentally compatible hydrometallurgical processes for recovering gold from refractory pyritic concentrates.