Coal fly ash industrial waste-derived products: a review on the extraction of aluminum and silicon for nanoparticle synthesis

C oal fly ash (CFA), a common industrial by-product, poses both environmental challenges and opportunities as a secondary source of silicon and aluminum. This review critically evaluates extraction methods for these elements, focusing on efficiency, selectivity, and scalability. Sintering techniques enhance aluminum recovery by decomposing stable aluminosilicate phases but generally fail to extract silicon, requiring high energy input and large chemical volumes and generating substantial secondary waste. Direct acid leaching selectively dissolves aluminum, often achieving extraction efficiencies above 80%, though crystalline phases such as mullite remain largely unreactive. Alkaline leaching and alkali fusion improve solubilization of both aluminum and silicon, particularly targeting refractory quartz and mullite, but frequently favor silicate recovery and involve significant reagent consumption. Hybrid and combined approaches, including sequential acid-alkali leaching and hydrothermal treatments, achieve more balanced recovery of Al and Si, with moderate to high efficiencies (up to 98%), and reduce energy and chemical demand. Despite progress, most recoveries are still moderate, impurities persist, and scalable, standardized protocols are lacking. While CFA-derived silica nanoparticles are increasingly studied, sustainable alumina nanoparticle synthesis and broader applications remained underexplored. Limited research in regions such as South Africa further restricts local adaptation. Thus, the review highlights the importance of integrated, eco-friendly, and region-specific strategies, reporting both elemental and oxide-based data to enable meaningful comparison and guide future CFA valorization.