Preparation of monodispersed SiO2-Al2O3 microspheres based on fly ash by thermally induced phase separation

In this study, we successfully prepared fly ash-based SiO ₂-Al ₂O ₃ microspheres using phase separation followed by acid leaching. Thermal analysis and X-ray diffraction were used to identify the optimal temperature range of heat treatment. The chemical composition and surface property of the microspheres were analyzed using X-ray fluorescence and contact angle measurements, and scanning electron microscopy was used to characterize the appearance of the microspheres. The results showed that the SiO ₂-Al ₂O ₃ microspheres were composed of 60–80 wt% SiO ₂ and 10–12 wt% Al ₂O ₃ with a uniform particle size, a hydrophilic surface, and high alkali resistance. Heat treatment studies revealed that the phase separation temperature should not exceed 900 °C to avoid crystallization. The mean particle size of the SiO ₂-Al ₂O ₃ microspheres decreased with an increase in the heat treatment temperature. Additionally, observing the evolution of the phase structure at 800 °C showed that there was fusion between the SiO ₂-Al ₂O ₃-enriched phases via interfacial tension in the later stage, resulting in larger spherical phase structures. Further, an increase in the Al ₂O ₃, Fe ₂O ₃ , and CaO composition decreased the SiO ₂-Al ₂O ₃ microspheres particle size, and therefore inhibited phase separation.