Mössbauer Research and Magnetic Properties of Dispersed Microspheres from High-Calcium Fly Ash

 https://doi.org/10.3390/magnetochemistry11090072

High-calcium fly ash (HCFA), produced from the lignite combustion, has emerged as a global concern due to its fine particle size and adverse environmental impacts. This study presents the characteristics of dispersed microspheres from HCFA obtained using modern techniques, such as XRD, SEM-EDS, ⁵⁷Fe Mössbauer spectroscopy, DSC-TG, particle size analysis, and magnetic measurements. It is found that an increase in microsphere size is likely due to the growth of the silicate glass-like phase, while the magnetic crystalline phase content remains stable. According to the ⁵⁷Fe Mössbauer spectroscopy, there are two substituted Ca-based ferrites—CaFe₂O₄ and Ca₂Fe₂O₅ with a quite different magnetic behavior. Besides, the magnetic ordering temperature of the brownmillerite (Ca₂Fe₂O₅) phase increases with the average diameter of the microspheres. FORC analysis reveals enhanced magnetic interactions as microsphere size increases, indicating an elevation in the concentration of magnetic microparticles, primarily on the microsphere surface, as supported by electron microscopy data. The discovered the magnetic crystallographic phases distribution on the microsphere’s surface claims the accessibility for further enrichment of the magnetically active particles and the possible application of fly ashes as a cheap source for magnetic materials synthesis.