The depolymerization and reconstruction mechanism of Si-Al lattice during sodium carbonate roasting of coal fly ash

Coal fly ash (CFA), a typical industrial solid waste rich in Si and Al with associated critical elements (Ga, Ge, REEs), poses environmental risks while having high resource utilization potential. Its valuable elements are hosted in aluminosilicate glass, hindering their recovery. Na 2 CO 3 roasting releases Na 2 O with strong lattice-disrupting capability, leading to the depolymerization of aluminosilicate lattices. In this study, experimental characterization and HSC Chemistry thermodynamic calculations were combined. The experimental and thermodynamic results were used to elucidate the evolution of the aluminosilicate lattice and the reconstruction of mineral phases. The reaction was substantially promoted at a Na 2 CO 3 /CFA mass mixing ratio of 1, and residual mullite, quartz and Na 2 CO 3 were no longer detected when the temperature increased to 850℃. The main reaction occurred at 600–900℃ and proceeded in three stages. (1) Reaction initiation stage (≤ 600℃): Some crystal defects and short-range ordered sodium aluminosilicate structures formed, while the overall lattice was not completely destroyed. (2) Structural reorganization stage (700–800℃): Inert aluminosilicate crystalline phases progressively disappeared, while nepheline-type sodium aluminosilicates and sodium silicate formed, with reaction intermediates coexisting with newly formed phases. (3) Recrystallization and stabilization stage (850–900℃): The roasting reaction was nearly complete, and the crystalline fraction of the roasted products consisted mainly of nepheline-type sodium aluminosilicates and sodium silicate, while a substantial amorphous component remained. The glassy phase and quartz contained in CFA could account for the lower temperature required for Na 2 CO 3 roasting compared with that required for pure minerals. This work provides theoretical support for CFA resource utilization.