Direct mineral carbonation of fly ash under high pressure using acid mine drainage: Effects of solid-to-liquid ratio, stirring speed and CO2 pressure

CO₂ emissions contributing to global warming and waste related to energy generation using coal pose an issue in developing countries. Mineral carbonation (MC) of fly ash (FA) using acid mine drainage (AMD) can reduce CO₂ and the negative environmental impact of FA including AMD from coal mining activity. The present study examined the direct carbonation (DC) of FA using pure water and AMD in a 600 mL autoclave pressure reactor. A preliminary study was conducted to observe the leaching behavior of FA in water and AMD. DC was conducted to determine the effect of solid-to-liquid ratio (0.2 and 0.5 g mL⁻¹), stirring speed (100 and 400 rpm) and CO₂ pressure (1–4 MPa) on the carbonation performance. The maximum calcium carbonate content in the carbonated FA was 4.33 wt% with a 60% conversion of calcium to CaCO₃ for DC with pure water. The electricity required by the process was 18.9 kWh t⁻¹-CO₂, corresponding to 0.019 t-CO₂ emitted t⁻¹-CO₂ fixed in FA. DC using AMD was effective and the maximum CaCO₃ content in FA was 6.68 wt%. This was due to the additional calcium content and enhanced calcium extraction provided by AMD. The actual CO₂ uptake capacity was 29.4 g-CO₂ kg⁻¹ fly ash. Studies on AMD as a reaction solvent for MC are few but can improve the carbonation performance of FA. Hence, the method can be viable for mining industries to mitigate negative environmental impacts and generate additional revenue through carbon credits.