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Eight fractions of ferrospheres in a range of sizes from 0.4 to 0.02 mm recovered from high-calcium fly ash have been studied. The major component composition of obtained fractions can be described by two linear regression equations, [CaO] = 54.50 – 0.54­[FeO] and [SiO2] = 27.71 – 0.29­[FeO] with the correlation coefficients of −0.96 and −0.88, respectively. On the basis of SEM-EDS study of the structure of 540 ferrospheres, it was found that the fraction −0.04 + 0.032 mm contains the individual globules with block-like, plate-like, and dendritic structures in concentrations of 60, 10–13, and 13–15%, respectively. The block-like globules containing 94–95 wt % FeO mainly consist of intergrown blocks (“single-block type”) of the ferrospinel which is subjected to partial martitization in regions with CaO content more than 0.9 wt %. The composition of the local sites of the plate-like globules containing 79–90 wt % FeO and 3.5–14.0 wt % CaO are characterized by the general dependence [CaO] = 87.4 – 0.93­[FeO] with the correlation coefficient −0.96. These globules consist of the fragments of the “core–shell” type with the size ranging from 3 to 6 μm. The composition of the core with the block-like structure corresponds to a region of ferrospinel crystallization on the phase diagram Fe x O y –CaO system. The composition of the shell with a plate-like structure corresponds to a region of the crystallization of the Fe2O3, CaFe2O4, and CaFe4O7 phases. The composition of the local sites of the dendritic individual globules containing ∼90 wt % FeO and 4.8–5.5 wt % SiO2 are characterized by the general dependence [SiO2] = 61.3 – 0.63­[FeO] with the correlation coefficient −0.94. It was shown that the structure of ferrospinel aggregates depends on the concentrations of Al2O3 and MgO.

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