Understanding Coal Fly Ash Glass Depolymerization and Colloidal Silica Formation toward Controlled Rare Earth Extraction.

In recent years, coal fly ash (CFA) has emerged as a sustainable alternative resource for rare-earth elements (REEs). Acid leaching enables efficient REE recovery, yet large-scale processing is hindered by the formation of silica gels that immobilize REE-bearing leachates. This work investigates the depolymerization of the amorphous aluminosilicate glass phase in CFA and its role in the generation of colloidal silica during acid leaching. Structural and chemical analyses reveal that under highly acidic conditions (pH < 1), proton-mediated rupture of metal-oxygen bonds releases short-chain silica species (e.g., dimers and trimers) from the glass network without complete dissolution of Si-rich particles. The mobilized short-chain silica then undergoes hydrolysis and condensation in the leachate solution to form polymerized siloxane networks (i.e., gels). The structural characterization and chemical speciation analyses presented here help elucidate the roles of ion concentration, pH, acid/ash ratio, and ash crystallinity in initiating gel growth and provide mechanistic insights for improving the feasibility of industrial-scale REE recovery processes to enable CFA as an abundant resource for REEs.