The Leaching Performance and Mechanism of Calcium Ions from Coal Fly Ash Under Sequential Alkaline-Acid Processing

Liquid-phase mineralization of CO2 using coal fly ash (CFA) is an efficient approach to permanent CO2 sequestration. To address the low leaching efficiency of calcium ions (Ca2+) in carbon mineralization, this study systematically investigates the leaching performance and leaching mechanism of calcium ions from CFA by using a sequential alkaline-acid processing (i.e., alkaline activation followed by acid leaching). The effects of NaOH concentration, acid concentration, acid type (HCl/CH3COOH), reaction time, and grinding duration on leaching efficiency are studied. The reaction products are characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). A kinetic model is proposed to analyze the reaction dynamics and leaching mechanisms. The results show that the maximum Ca2+ leaching efficiency for untreated CFA is 43.7% after 40-min acid leaching with 7 mol/L HCl and 1:1.5 S/L ratio. The leaching efficiency can be enhanced to 72.1% after 50-min alkaline activation with 11 mol/L NaOH. Grinding the CFA can further increase the leaching performance of Ca2+. It is shown that the leaching efficiency can be enhanced to 58.75% and 82.3% after 90-min grinding, respectively, for cases without and with 50-min alkaline activation using 9 mol/L NaOH. It is also shown that a peak leaching efficiency of 86.51% can be obtained when 8 mol/L CH3COOH is used for the acid system. The mechanism for the enhancement of leaching efficiency is that both NaOH activation and mechanical grinding can break down the calcium and aluminum silicate vitreous matrix of CFA, facilitating calcium release. Ca2+ leaching performance exhibits two regimes. The leaching efficiency is significantly time-dependent in the first regime, and it remains almost constant in the second regime after the efficiency reaches a pseudo-maximum value. The contribution of this study is that a theoretical foundation is provided for enhancing the Ca2+ recovery from CFA, which makes it practical for large-scale CFA utilization and permanent CO2 sequestration in industry applications.