Characterization of Ferrospheres Recovered from High-Calcium Fly Ash

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 ram. The composition of the core with the block-like structure corresponds to a region of ferrospinel crystallization on the phase diagram FexOy-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 similar to 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.