Metals removal and high-silica powder production from coal fly ash via phase separation process

Under the dual impetus of the sustained improvement of the Chinese economy and the accelerating urbanization process, domestic demand and consumption of coal have maintained a steady growth trend. Consequently, the production and stockpiling of coal combustion byproducts, particularly coal fly ash (CFA), have increased remarkably, while its comprehensive utilization has seen limited improvement in recent years. Containing approximately 30%-60% SiO₂, CFA represents a significant resource for silica recovery and purification to enable high-value applications. This study employed a thermal phase separation-acid leaching method to remove metal oxides and purify SiO₂ from CFA, utilizing B₂O₃ as a phase-separation agent. The research systematically investigated the optimal process parameters for CFA phase separation and metal removal, while elucidating the underlying mechanisms of phase separation and metal migration. Experimental results demonstrated that under optimal conditions (B₂O₃ addition: 25 wt%, temperature: 1100 °C, holding time: 1 h) the removal efficiency of metal oxides exceeded 95%, yielding a high-silica product with a purity of 97.22%. Complementary molecular dynamics (MD) simulations provided atomic-scale insights into the SiO₂-B₂O₃ phase separation process. Analysis of the root mean square deviation (RMSD) of atomic coordinates and radial distribution functions (RDFs) for Si and B atoms confirmed that phase separation occurred rapidly within the high-temperature regime during cooling, following complete melting and homogenization. This work provided a novel approach for the resource utilization of CFA.