Synergistic H2S production and CO2 capture from copper smelting neutralized gypsum slag: Process optimization, reaction mechanism and CaCO3 crystal regulation.

The non-ferrous metallurgy industry faces three interconnected environmental challenges: massive stockpiling of neutralized gypsum slag (NG), high CO2 emissions, and reliance on externally supplied hazardous H2S for wastewater treatment. Here, we develop a closed-loop management strategy that addresses all three simultaneously by converting NG into a CO2 capture carrier and on-site H2S source. NG (mainly CaSO4·2H2O) is first converted to CaS via carbothermal reduction (optimized at 30 % carbon, 900 °C). The resulting CaS then reacts with CO2 and water to co-produce H2S and CaCO3. Systematic optimization yields a peak H2S concentration of 4.16 % under mild conditions (25 °C, liquid-solid ratio 50/1). A two-step reaction mechanism (CaS → Ca(OH)2 → CaCO3) is elucidated. The CaCO3 byproduct undergoes a phase transition from metastable vaterite to stable calcite, enabling value-added utilization. This integrated strategy offers three managerial benefits: (i) eliminating NG landfilling, (ii) mineralizing CO2 into stable carbonate, and (iii) producing H2S exactly where it is needed for heavy metal precipitation. The process is compatible with existing copper smelting lines, providing a practical pathway toward circular economy and carbon neutrality in non-ferrous metallurgy.