Research on the mechanism of heavy metal solidification by supercritical CO ₂ mineralization

Fly ash mineralization is a feasible means to achieve carbon reduction and solidify heavy metals. However, the slow diffusion reaction restricts its industrial application. This study employs supercritical CO2 – which features high diffusivity and low viscosity – to enhance fly ash mineralization. Experiments on three typical fly ashes show that supercritical CO2 significantly improves mineralization efficiency. However, the associated heavy metal solidification mechanism remains unclear. Subsequently, heavy metal leaching experiments are carried out. With the increase of pressure, the leaching concentration of heavy metals shows two changing trends. The leaching concentrations of Pb, Cr, Ni, and Cu continue to decline and reach the lowest point at 8 MPa. The leaching concentrations of Cd and Zn are the lowest at 7 MPa, but increase at supercritical 8 MPa. Supercritical CO2 promotes mineralization and also enhances the solidification effect on heavy metals. However, the solubility of supercritical CO2 increases several times, which will cause a significant drop in the pH value of the solution and lead to local acidification. Heavy metals sensitive to pH value will re-leach, increasing the leaching concentration.