Enhancement of gold extraction by citric acid-assisted microwave roasting of Carlin-type gold ores and sulfide/thiosulfate leaching

The occurrence of most gold in an encapsulated state often results in poor extraction efficiency, and efficient pretreatment is the key to effective gold recovery from Carlin-type gold ores. This study investigated the enhancement of citric acid (CA) assisted microwave (MW) roasting on gold extraction from Carlin-type gold ores by sulfide/thiosulfate leaching, primarily analyzing in-situ phase transformation, microstructure characterization, and the thermal decomposition behavior of solids. The results show that gold extraction increases significantly with increasing CA dosage, furnace temperature, and roasting time when heated by MW, reaching a maximum of 87.7% at a CA dosage of 2% and a temperature of 500°C for 60 min in the presence of air. Moreover, extraction is significantly higher with CA than without CA at roasting temperatures below 300°C. The phase transition of gold-bearing pyrite is controlled mainly by temperature when heated by MW without CA, resulting in surface oxidation at 300°C, with significant oxidation at 400°C, and complete transformation into Fe₂O₃ and SO₂ at temperatures over 500°C. Interestingly, CA and its decomposition products of CO₂ and H₂O induce the generation of unstable intermediates at 100°C, such as FeCO₃, FeC₂₁H₂₁O₇, and FeOOH. The decomposition of these intermediates at higher temperatures results in the rapid formation of a hematite layer with developed pores at 300°C, which provides more diffusion channels for SO₂ and O₂, producing coral-like porous hematite particles. Both number of pores and the pore size are larger at 500°C. The combined effect of MW and CA enhances exposed gold content over 70% which in turn increases gold extraction. This work provides significant insights into the enhancement process of gold extraction of CA-assisted MW roasting.