Unlocking Refractory Gold: Synergistic Pretreatment Strategies for High-Efficiency Thiosulfate Leaching

This study evaluates four physicochemical pretreatments—ultra-fine grinding, roasting, alkaline pressure oxidation (POX), and oxidative ammoniacal pre-leaching—for improving gold extraction from a refractory sulfide concentrate produced trough flotation. The gold extraction by direct cyanidation is only ~48.6%, mainly due to the encapsulation of gold by associated minerals. Ultra-fine grinding increased the BET surface area eight-fold but depressed gold dissolution from 74% to 18% due to accelerated thiosulfate decomposition and copper (I) passivation in the presence of a bigger surface area. Oxidative roasting at 750 °C converted pyrite–pyrrhotite to hematite without liberating additional gold, indicating limited benefit from thermal treatment. POX was conducted at 190 °C and 10 bar O2 dissolved 33% of the solids and yielded only 26% of gold in a thiosulfate leaching step with 50% of the thiosulfate consumption. In contrast, a two-step oxidative ammoniacal conditioning (0.4 M NH3 + 10 mM Cu2+ for 42 h) followed by thiosulfate leaching boosted gold extraction from 71% to 85% while cutting thiosulfate consumption from 48.4 to 29.0 kg t−1. The results demonstrate that among the pretreatments investigated, oxidative ammoniacal pre-leaching provides the most effective and environmentally benign route to unlock encapsulated gold and enhance reagent efficiency for thiosulfate processing of refractory gold ore.