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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The structure–composition relationship of blocklike ferrospheres (FSs) isolated from fly ash from the coal and lignite combustion has been studied systematically by scanning electron microscopy and energy dispersive X-ray spectroscopy. Groups of globules for which the gross composition of polished sections corresponds to the general equations for the relationship of the concentrations SiO2 = f(Al2O3) and CaO = f(SiO2) are highlighted from FSs of two series. It is shown that blocklike FSs are formed during the sequential transformation of dispersed products of thermal conversion of mineral precursor associates: pyrite, quartz, and Ca, Al-humates in the case of brown coal; and pyrite, siderite, quartz, and calcite in the case of coal. Anorthite is the aluminosilicate precursor of blocklike FSs of both series. The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs. An analysis of SEM images of polished globule sections demonstrates that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from a large blocklike type to a fine crystalline type with a high glass-phase content. The size and shape of crystallites are controlled by the size of a local melt area where the total concentration of spinel-forming oxides exceeds 85 wt %. An increase in the glass-phase concentration and a decrease in the crystallite size in globules with FeO ≤ 46–50 wt % are explained by expansion of the segregation region in the FeO–Fe2O3–SiO2 system as the oxidation potential rises.

Magnetic Ni(2+)-zeolite/ferrosphere and Ni(2+)-silica/ferrosphere beads (Ni-ferrosphere beads - NFB) of a core-shell structure were synthesized starting from coal fly ash ferrospheres having diameters in the range of 0.063-0.050 mm. The strategy of NFB fabrication is an oriented chemical modification of the outer surface preserving the magnetic core of parent beads with the formation of micro-mesoporous coverings. Two routes of ferrosphere modification were realized, such as (i) hydrothermal treatment in an alkaline medium resulting in a NaP zeolite layer and (ii) synthesis of micro-mesoporous silica on the glass surface using conventional methods. Immobilization of Ni(2+) ions in the siliceous porous shell of the magnetic beads was carried out via (i) the ion exchange of Na(+) for Ni(2+) in the zeolite layer or (ii) deposition of NiO clusters in the zeolite and silica pores. The final NFB were tested for affinity in magnetic separation of the histidine-tagged green fluorescent protein (GFP) directly from a cell lysate. Results pointed to the high affinity of the magnetic beads towards the protein in the presence of 10 mM EDTA. The sorption capacity of the ferrosphere-based Ni-beads with respect to GFP was in the range 1.5-5.7 mg cm(-3).