Transformation of waste into wealth: Thermochemical conversion of coal fly ash to extract Al2O3 via alkali desilication and soda–lime assisted sintering

Waste utilization is at the forefront of protecting the environment and developing value-added materials for society. The extraction of alumina from industrial byproducts like coal fly ash (CFA) is an environmentally sustainable method of resource exploitation. This study demonstrates the efficient extraction of alumina from CFA through combined alkali desilication, soda-lime assisted sintering and leaching. Alkali disilication effectively improved the Si:Al ratio, while soda-lime assisted sintering converts mullite into soluble aluminate phases followed by effective alkali leaching expedites to obtain the predominantly of 71.4% γ-phase and 28.6% α-phase of alumina with crystallite size of 63.85nm. X-Ray Diffraction (XRD) and Simultaneous Thermal Analyzer (STA) confirms the sequential phase transformation of desilicated coal fly ash (DSFA) to soluble aluminates (NaAlO 2 and CaAl 2 O 4 ) at 1150°C. Eventually, the alumina was extracted from the soluble NaAlO 2 and CaAl 2 O 4 phases using different leachants and analyzed to validate the process. The morphological characteristics were analyzed with Scanning Electron Microscopy instrument. The effects of alkali-to-ash ratio and temperature on sintering, the effect of leaching temperature and time were studied to optimize the process parameters. Due to improved aluminate solubility and secondary precipitation suppression, the mixed NaOH + Na 2 CO 3 leaching system produced the best extraction efficiency, according to the results. As the leaching temperature and duration to equilibrium increased, so did the extraction efficiency. The innovation ensures 90% extraction efficiency, and such effort may be employed in reducing the thermal power plant waste. Thermochemical transformation of CFA into alumina is demonstrated graphically. The process involves the alkali pre-disilication of CFA to remove glassy silica phase, resulting enhancement of alumina phases. To bring the alumina into soluble NaAlO 2 and CaAl 2 O 4 phases, DSFA was sintered at 1150 o C with mixed Na 2 CO 3 –CaO, followed by leaching to extract alumina. Current process explains a coherent and viable way of extracting alumina from CFA, which supports the waste to wealth approach. • Alumina was extracted from CFA via consolidated desilication, sintering and, leaching processes. • Upgradation of Si:Al ratio was achieved by removing amorphous silica through NaOH desilication • Under optimum sintering conditions, aluminosilicates were converted into soluble NaAlO 2 and CaAl 2 O 4 phases. • Alumina was extracted from soluble NaAlO 2 and CaAl 2 O 4 via DI water and alkali leaching. • All the variables were optimized to achieve maximum yield of alumina and minimizing energy consumptions.