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The chemical and phase composition, morphology, and shell structure of narrow fractions of nonmagnetic and magnetic low-density nonperforated cenospheres separated from fly ash concentrates produced via the pulverized combustion of coal from the Kuznetsk Basin (Russia) have been studied. Narrow fractions of nonmagnetic cenospheres contain 2.6–3.5 wt % Fe2O3 and include globules with a uniform smooth or relief surface and shells with different degrees of porosity. For nonmagnetic cenospheres, the aluminum concentration increase leads to an increase in the mullite phase content and a decrease in the average sphere diameter, glass-crystalline shell thickness and porosity, and the crystalline quartz content. The quartz phase comprises two modifications with different lattice parameters. The narrow fractions of magnetic cenospheres contain 3–21 wt % Fe2O3 and include globules with thick porous shells covered by heterogeneous regions of ferrospinel on their outer surface. In magnetic cenospheres, an increase in the iron concentration leads to an increase in the ferrospinel phase content and crystallite size, accompanied by a decrease in the degree of substitution of magnesium and aluminum for iron.

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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.

The relationship between the chemical composition and shell structure of cenospheres with a low bulk density of 0.40−0.45 g/cm 3 and a high Al 2 O 3 content of 33−38 wt % has been systematically studied. It was established that the composition of the narrow fractions of cenospheres can be described by the general regression equation SiO 2 /Al 2 O 3 = 4.34 − 0.08[Al 2 O 3 ] with the correlation coefficient of r = −0.99. The phase composition includes the glass phase (57−73 wt %), mullite (25−40 wt %), and quartz (1.2−2.5 wt %). An increase in the Al 2 O 3 content leads to an increase in the size of particles and the porosity of their shells. In the obtained fractions of cenospheres, there are two types of globules: spherical globules with a singlering structure and foamy globules with a network structure. It is established that the composition of individual particles with a network structure localized in the range of the Al 2 O 3 content from 43 wt % to 51 wt % and can be described by the regression equation SiO 2 /Al 2 O 3 = 2.71 − 0.04[Al 2 O 3 ] with the correlation coefficient of r = −0.97. The framework of these particles permeated by mullite microcrystallites and coated with a nanoscale surface film. The structure-forming mineral precursor of these particles is kaolinite. The gross composition of the shell of individual globules with a single-ring structure localized in the range of the Al 2 O 3 content from 26 wt % to 42 wt % and can be described by the general regression equation SiO 2 /Al 2 O 3 = 4.71− 0.09[Al 2 O 3 ] with the correlation coefficient of r = −0.98. The outer and inner surfaces of the shell are covered by large and small in-plane localized mullite crystals hidden by the nanoscale film. The spherical shape and their crystalline framework are formed from the illite melt with inclusions of products of the thermal conversion of other mineral forms. ■ INTRODUCTIONIn the process of generating power from coal, large quantities of coal combustion products (CCPs) are produced. According to the various estimates, combustion of coal in the Russia alone generates ∼25 million tons of CCPs per year but utilization rate does not exceed 15%. 1 The complex composition of CCPs, including the fly ash, has proven to be a barrier to its bulk utilization in many fields. Extraction of concentrates of microspherical components with specific characteristics from fly ashes of variable composition provides wider opportunities for the multicomponent use of fly ashes produced from coal combustion. 2,3 The unique properties of cenospheres, namely, their low density, sphericity of particles, and high strength and nontoxicity, make them useful for a variety of fields of applications. Thus, concentrates of cenospheres are used as fillers of lightweight composite materials, such as concretes, 4,5 polymers and resins, 6−8 and metal alloys. 9,10 Also, they are been studied for the production of ceramic composite foams with different properties. 11,12 In recent years, new functional materials have been developed based on a d...

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