Separation of Nonmagnetic Fine Narrow Fractions of PM10 from Coal Fly Ash and Their Characteristics and Mineral Precursors

Nonmagnetic fine narrow fractions of particles with mean diameters of 2, 3, 6, and 10 μm were for the first time separated from fly ash produced by pulverized combustion of Ekibastuz coal using aerodynamic classification with subsequent magnetic separation. These fractions were characterized by the size distribution, bulk density, and chemical and phase compositions. The particle size distributions correspond to d50 values of 1.9, 2.3, 5.1, and 9.2 μm. As the fraction particle size increases, the bulk density was found to rise gradually from 0.90 to 1.07 g/cm3. The main components of the chemical composition were SiO2 (65–70 wt %) and Al2O3 (23–28 wt %). The phase composition was represented by the glass phase (64–69 wt %), mullite (17–21 wt %), and quartz (10–18 wt %). The main morphological particle types were microspheres with a nonporous smooth surface and microspheres with a porous shell. With an increase in the fraction particle size, the percentage of microspheres with a porous shell increases. The largest fraction contains particles with a network structure. Single-particle scanning electron microscopy–energy dispersive X-ray spectroscopy analysis of nonporous microspheres with a diameter of 1–2 μm, approximate in composition to the internal coal minerals, indicated that, depending on the content of SiO2, Al2O3, and FeO, they form several groups differing in mineral precursors. Thus, for microspheres of group 1 (SiO2 + Al2O3 > 95 wt %), the mineral precursors are NH4-illite and montmorillonite; group 2 (SiO2 + Al2O3 = 90–95, FeO ≤ 4 wt %)—minerals of the isomorphic montmorillonite-illite series, including phases with a low level of iron cation substitution; group 3 (SiO2 + Al2O3 = 90–95, 4 < FeO ≤ 6 wt %) and group 4 (SiO2 + Al2O3 < 90, 3 < FeO ≤ 9 wt %)—minerals of the illite-montmorillonite series, with a high level of iron cation substitution and with Fe3+ in interlayer sites.