Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching

Thiosulfate leaching is considered a promising alternative to cyanidation for gold extraction because it can be achieved at a low cost. However, existing leaching systems struggle to balance leaching efficiency with thiosulfate consumption. Herein, a novel synergistic Cu-CH3NH2-NH3 leaching system was proposed, balancing thiosulfate consumption and gold leaching efficiency through a mixed-ligand strategy. Thermodynamic analysis revealed that the steric hindrance and electron-donating effects of methylamine effectively block the oxidative decomposition pathway of thiosulfate by Cu(II), significantly reducing thiosulfate consumption. However, this also reduced the dissolution rate of gold. By introducing ammonia to adjust the Cu(II) coordination environment, the system achieved a gold leaching rate of 88.6% with a thiosulfate consumption of 14.2 kg/t-ore, significantly outperforming the traditional Cu-NH3 system. In this system, the gold leaching process mainly is catalyzed by the mixed-ligand complex Cu(NH3)x(CH3NH2)4−x2+. Within the coordination sphere, the methyl group of CH3NH2 inhibits the axial attack of S2O32− on Cu(II) via electron-donating and steric hindrance effects, thereby blocking the redox pathway of S2O32−; simultaneously, NH3 provides active sites to promote the gold oxidation. This study provides a vital theoretical basis and technical support for developing green, low-cost, and high-efficiency gold leaching processes.