Efficient separation and comprehensive extraction of aluminum, silicon, and iron from coal fly ash by a cascade extraction method

A cascade extraction method was constructed by combining alkali desilication with carbonation and acid leaching to achieve efficient separation and comprehensive recovery of aluminum, silicon, and iron products in coal fly ash (CFA). Alkaline desilication obtained the maximum SiO₂ extraction efficiency of 65.7% after response surface methodology optimization, transformed mullite phase to hydroxysodalite [Na₈Al₆Si₆O₂₄(OH)₂(H₂O)₂], and generated fiber-formed nanostructures, which were beneficial for resource extraction. After being treated by dosing Ca(OH)₂ to recover CaSiO₃ crystal, the supernatant of desilication was recycled to reduce alkali consumption and waste liquid discharge. The desilicated CFA was activated by carbonate calcination to convert mullite into the nepheline phase, thereby elevating the dissolution efficiency of Al₂O₃ and Fe₂O₃ to 90.4% and 92.3% by acid leaching. Acid leaching also converted silicious compounds in activated CFA to silicic acid with a purity of 93.9%. Stepwise precipitation obtained Fe(OH)₃ and Al(OH)₃ products with purities of 70.6% and 90.8%. The generated CaSiO₃, silica gel, Fe(OH)₃, and Al(OH)₃ can be recycled as flame retardants, and the recovered Al(OH)₃ could be converted to NaAlO₂ for chloride removal from saline wastewater. The cascade extraction is economically beneficial and environmentally friendly without solid waste production for utilization of CFA.