Enhanced Ferrosphere Recovery from High-Calcium Fly Ash: SEM-EDS, XRD, Magnetic Force Microscopy Characterization

Dispersed ferrospheres (FSs) are a valuable component of coal fly ash, whose application potential is determined by their microspherical design, fine particle size, and high concentration of magnetic iron compounds. This study proposes an efficient technological scheme for extracting dispersed FSs from high-calcium fly ash, comprising (i) aerodynamic classification and (ii) dry magnetic separation. The isolated fractions were characterized, including determination of the particle-size distribution, morphology, chemical and phase composition, Mössbauer parameters, magnetic properties, and surface distribution of magnetic phases. It was shown that the average particle diameters of the FS narrow fractions are 3 and 8 µm. The major chemical components are FeO, CaO, and SiO2, whose total content amounts to 81–83 wt %. Regarding the phase composition, Fe-spinel and calcium ferrites are predominant, accounting for 38–46 and 13–16 wt %, respectively. The efficiency of the proposed process for extracting FSs reaches the level achieved by conventional wet magnetic separation. The saturation magnetization of the dispersed FS samples increases by more than an order of magnitude (up to 23–28 emu/g) compared to the initial fly-ash fractions (1.7–1.8 emu/g). For the first time, magnetic topography investigation of single microspheres directly demonstrates that the surface of the aluminosilicate matrix is enriched with magnetic microcrystals formed during coal combustion. The obtained results may prove useful in the design of functional materials with magnetically active surfaces for advanced applications.