Preparation of silicon tetrachloride from fly ash by chlorination-volatilization and recovery of phosphorus from water using fly ash and chlorinated residue

Converting silicon (Si) and aluminum (Al) compounds into valuable resources is essential for promoting the sustainable utilization of fly ash from coal- and biomass-fueled power plants. Transforming Si and Al into volatile chlorides enables their application as high-value raw materials for fumed silica and catalysts. While heating pure Si and Al oxides with solid carbon in chlorine gas yields SiCl 4 and AlCl 3 , complex minerals in fly ash, such as quartz and mullite, exhibit distinct reactivities. This study investigated the effects of reaction temperature and time on Si and Al release by treating bituminous coal fly ash mixed with solid carbon under chlorine gas flow. Significant Si and Al volatilization occurred at 1000 °C, yielding high recovery proportions of 83 % for Al and 99 % for Si in the gas cooling and condensation zones. Additionally, calcium-rich fly ashes derived from sub-bituminous coal and woody biomass were evaluated as phosphate recovery materials. The volatilization of Si and Al elevated Ca concentrations in the residue up to 29.8 % and 38.7 % as CaO, respectively, thereby enhancing overall phosphate capture capacities. The resulting chlorinated residues achieved efficient removal performance with high equilibrium capture capacities ( q e = 36.2 and 42.4 mg-P/g for sub-bituminous coal and woody biomass ash residues, respectively). Mechanistically, structural and extraction analyses confirm that calcium compounds in the residues immobilize phosphate primarily as CaHPO 4 via a coupled surface adsorption and localized precipitation process.