Mechanistic Elucidation and Optimization of Alkali Activation for Si-Al Recovery from Coal Fly Ash

Coal fly ash (FA), a major byproduct of coal-fired power plants, remains underutilized despite its high content of silica (SiO₂) and alumina (Al₂O₃). This study investigates alkali activation as a route for selective Si and Al recovery, combining experimental leaching, thermodynamic modeling, and response surface methodology (RSM). Parametric analysis revealed that NaOH concentration, temperature, and reaction time critically influenced dissolution behavior. Optimal conditions5 M NaOH, 140 °C, and 1 hachieved up to 50.4% Si dissolution while minimizing Al solubilization (<3%). Mechanistic evaluation identified a three-stage process: (i) initial extraction, (ii) concurrent dissolution-precipitation governed by zeolite and phillipsite formation, and (iii) diffusion-limited Si release under surface passivation. XRD, SEM-EDS, and equilibrium modeling confirmed sodalite-type precipitation as the principal limitation to Al recovery. RSM-derived regression models (R ² > 0.94) accurately predicted Si and Al dissolution across variable conditions, enabling process optimization with reduced experimental burden. Beyond resource recovery, selective removal of Si and Al facilitates enrichment of rare-earth elements in the residue, enhancing downstream extraction potential. Recovered silica and alumina can be valorized in advanced alloys, ceramics, adsorbents, and photovoltaic applications, positioning alkali activation as a viable pathway for circular utilization of FA within low-carbon materials innovation.