Effect of supercritical CO₂ carbonation parameters on heavy metal solidification and environmental stability of coal fly ash

Fly ash carbonation is an effective method to achieve carbon reduction and stabilize the heavy metals. Supercritical CO₂ can facilitate the carbonation process due to its strong diffusivity, but the effect of parameters on the stabilisation characteristics under supercritical conditions is unclear. In this paper, the heavy metal leaching characteristics of carbonized fly ash under supercritical CO₂ conditions were comprehensively investigated. The experimental results showed that, increasing the pressure, applying mechanical force, increasing the rotational speed, raising the temperature, increasing the liquid-to-solid ratio, and adding calcium oxide all reduced the heavy metals leaching concentration. Compared with the original ash, Pb decreased by 38.65%, Cd by 31.66%, Hg by 47.06%, and As by 4.91% under supercritical conditions. After the fly ash carbonation, the influence of environmental factors such as the acid–base value, temperature, moisture, reaction time, and additives on the release characteristics were investigated. It was found that carbonized fly ash had good stability. At 30 °C, that leaching rate of Ni, Cr, Cd, Pb, Hg, and As after carbonation were 28.80%, 8.62%, 26.60%, 22.19%, 100.00% and 17.47%, respectively, lower than those of the original sample. The leaching rate of heavy metals increased with increasing temperature, and moisture aggravated the precipitation of heavy metals, which also increased with the reaction time. The addition of gypsum decreased the heavy metals leaching. The extraction experiments showed that after carbonation the heavy metals changed from active forms to stable residue states, except for Hg and As. Supercritical CO₂ carbonation has a good effect on the stabilization of heavy metals.