THERMODYNAMIC ANALYSIS OF AN ALTERNATIVE GOLD LEACHING PROCESS: POLYSULFIDES AND ADVANCED OXIDATION PROCESSES

The recovery of precious metals, particularly gold, from electronic waste (e.g., central processing units, CPUs) is crucial due to its economic value and sustainability benefits, as gold content in e-waste can exceed that of primary deposits. Valorizing secondary raw materials also offers significant environmental advantages. While cyanide leaching dominates gold extraction, non-cyanide alternatives such as thiosulfate, thiourea, halides, and thiocyanate, as more environmentally friendly alternatives, are increasingly explored. Recently, advanced oxidation processes (AOPs) have gained attention as sustainable options. This study investigates the use of alkali metal polysulfides (e.g., Na₂Sₓ) combined with oxidizing additives as alternative lixiviants for non-cyanide gold leaching. Thermodynamic modeling was conducted using HSC Chemistry and HydraMedusa software to predict equilibrium compositions and phase stability under varying conditions (additives, concentrations, temperature, pH, and electrochemical potential). Selectivity was also assessed, including potential co-extraction of silver and platinum-group metals (PGMs). Results show that polysulfides alone are insufficient for gold dissolution; however, when combined with oxidizing agents and optimized parameters (Fenton’s reagent, pH 10, Eh between −0.5 and −0.3 V vs. SHE), effective and selective gold leaching is achievable. This approach offers a promising alternative to cyanide-based methods, addressing both toxicity and environmental concerns. The defined reaction parameters will be further validated through experimental investigations.