Alumina Extraction from Coal Fly Ash via Pre-Desilication, Vacuum Reduction, and the Alkali Dissolving Method

The high silica content of coal fly ash (CFA) poses a significant challenge for alumina extraction, resulting in high material and energy consumption. To reduce the silica content and improve alumina extraction efficiency, a novel process combining pre-desilication, vacuum reduction, and alkali dissolving is proposed. In the pre-desilication stage, amorphous silica in CFA is effectively removed by NaOH solution, increasing the Al2O3/SiO2 mass ratio from 0.78 to 1.27. The desilicated coal fly ash (D-CFA) is then subjected to vacuum carbothermal reduction with the addition of Fe2O3 and CaO to produce Fe-Si alloys and CaO·xAl2O3. The resulting CaO·xAl2O3 can be dissolved via alkali dissolving to extract alumina, achieving an alumina dissolving rate of over 90%. The Fe-Si alloys remaining in the dissolved residue are subsequently recovered by magnetic separation. Compared with the process without pre-desilication, the current process reduces material input by 30.25% and energy consumption by 35.18%, demonstrating that this approach offers a low-cost, energy-efficient, and environmentally friendly route for high-value-added utilization of CFA.