Alkaline Element Leaching from Fly Ash for Direct CO2 Fixation

Fly ash (FA), a major by-product of coal combustion, has long been regarded as a challenging industrial solid waste. Its inherent abundance of alkaline-earth oxides positioned it as a promising candidate for CO2 sequestration through mineral carbonation. This study systematically investigated the effects of key operational parameters, including time, stirring rate, ultrasonic treatment, and solid-to-liquid ratio, on the leaching efficiency of calcium ions and subsequent CO2 fixation. Ultrasonic treatment, a solid-to-liquid ratio of 1:7, a stirring speed of 600 rpm, and 7% monoethanolamine (MEA) collectively enhanced the calcium leaching efficiency (χe) to 16.7%, thereby supplying a substantial reservoir of calcium ions for CO2 fixation. Additionally, the CO2 injection into fly ash slurry and the slurry spraying into CO2 gas were investigated to optimize reactor configurations. The latter method demonstrated superior performance, attaining a CO2 fixation efficiency of 7.23%. This corresponds to a carbonation conversion efficiency (ηc) of approximately 44.5%, indicating that nearly half of the leached calcium ions were successfully converted into stable carbonates. Advanced characterization techniques (SEM-EDS, XRD, FTIR) confirmed the formation of stable carbonates and highlighted the role of additives in enhancing reactivity. The environmental benefit of this approach is addressing fly ash wastes and transforming fly ash into a CO2 fixation material. These findings provided critical insights for calcium leaching and CO2 fixation of fly ash.