N. N. Anshits

Composition and morphology of narrowly sized ferrospheres isolated from various types of fly ash

We report a systematic study of the interrelationship between the composition and microstructure of ferrospheres isolated from all known types of power plant fly ash. Our results demonstrate that the macro-component composition of narrow ferrosphere size fractions corresponds to the compositions of the phase boundaries between the primary crystallization fields of wüstite, fayalite, Fe-cordierite, and hercynite in the phase diagram of the FeO-SiO2-Al2O3 system. An increase in iron content leads to a monotonic variation in the main morphological species of the ferrospheres in the following sequence: porous, glassy, dendritic, skeletal-dendritic, and blocky. The relationships identified in this study allow one to find a fly ash source suitable for the preparation of functional materials of controlled composition and morphology.

Mössbauer Study of Magnetic Microspheres Isolated from Power Plant Fly Ash

The phase composition of magnetic microspheres isolated from Ekibastuz coal fly ash in six narrow size ranges is determined by Mössbauer spectroscopy. The major phase in the microspheres is nonstoichiometric, cation-substituted (Al, Mg, and Ti) magnetite. Their magnetic properties are shown to depend on the Fe content and cation distribution over the spinel sites.

Influence of the composition and structure of the glass-crystalline shell of cenospheres on helium permeability

We studied the interrelation between the composition, morphology, and helium permeability of the shell of narrow fractions of nonmagnetic nonperforated cenospheres extracted from cenosphere concentrates of fly ash of the sialic type with the use of technological stages of hydrodynamic, magnetic separation, sizing, and aerodynamic classification. For the interval of variation of Al2O3 content from 20 to 38 wt %, the regression equation [SiO2]/[Al2O3] = 5.06 − 0.1[Al2O3] is established, with the correlation coefficient equal to −0.98. It is found that, together with the growth in the concentration of Al2O3 in the indicated interval, the content of the originally mullite phase increases from 1.3 to 42.4 wt %, and this is accompanied by the growth of the helium permeability of the glass-crystalline shell of the cenospheres.

Polyfunctional microspherical materials for long-term disposal of liquid radioactive wastes

The possibility of immobilizing liquid radioactive wastes into polyfunctional microspherical materials of the block and powdered types is demonstrated. These materials are intended for reprocessing radioactive wastes of different compositions and make it possible to perform a multistage process of conditioning radioactive wastes under relatively mild conditions (at temperatures below 1000°C) with the conversion of water-soluble cesium and strontium compounds into water-insoluble mineral forms in the course of solid-phase transformations. Owing to the aluminosilicate composition of microspherical components of energy ashes (cenospheres), the cenospheres can serve as precursors of aluminosilicate granitoid minerals. Different techniques are proposed and conditions are experimentally determined for transforming precursors into final mineral-like materials of the predicted structure types chosen in the framework of the geoecological approach. The framework structures of aluminosilicates and phosphates thus formed can fix cesium and strontium in the crystal lattice and ensure the geochemical equilibrium between the matrix and incorporating granitoid rocks under conditions of long-term disposal.

Composition and Structure of Block-Type Ferrospheres Isolated from Calcium-Rich Power Plant Ash

Polished sections of individual ferrospheres 30 to 40 μm in size, with single-block and blocky structures and a variable glass phase content, have been studied using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer system. The results demonstrate that the single-block globules consist of sintered magnetite crystallites containing Al2O3, MgO, and CaO as impurities and are formed from the pyrite of the initial coal. Characteristically, the ferrospheres with a variable glass phase content differ in the composition of local areas on polished sections of the globules, which attests to inhomogeneity of the melt droplets they formed from. We have identified groups of globules whose overall composition, as well as the composition of their local areas, meet general equations for the interrelation between the concentrations of their components: SiO2 = f(FeO) and SiO2 = f(Al2O3). Comparison of the coefficients of the SiO2 = f(Al2O3) dependence for the globules with the silicate modulus (SiO2/Al2O3) of the aluminosilicate mineral components of the coal indicates that the formation of this type of globules involves pyrite–anorthite or pyrite–albite associates containing quartz impurities. The composition of the spinel ferrite in the globules produced with the participation of anorthite comprises FeO, Al2O3, MgO, and CaO in concentrations of 85–96, 1.7–10, 0.1–1.8, and 0.3–2.8 wt %, respectively. In the albite-based globules, the respective concentrations are 81–92, 0.7–5.9, 1.0–5.7, and 2.2–5.6 wt %. The crystallite size and shape are determined by the size of the local melt areas where the total concentration of spinel-forming oxides exceeds 85 wt %.

Composition and structure of the shells of aluminosilicate microspheres in fly ash formed on the combustion of Ekibastuz coal

A systematic study of the chemical and phase composition and structure of the shells of narrow fractions of nonmagnetic cenospheres separated from a concentrate of fly ash cenospheres from the combustion of pulverized SS (weakly caking coal) coal from the Ekibastuz Basin was carried out. It was found that the separated narrow fractions were characterized by a high Al2O3 content of 33–38 wt %. The phase composition included 57–73 wt % vitreous phase, 25–40 wt % mullite, and 1.2–2.5 wt % quartz. An increase in the average diameter, thickness, and porosity of the glass-crystalline shell of globules with the concentration of aluminum was observed. Globules of the following two types were identified in the obtained fractions: spherical globules with an annular structure and foamy globules with a network structure formed from different illite and kaolinite mineral precursors, respectively.

Nature of the active sites of ferrospheres in the oxidative condensation of methane

The catalytic properties of ferrospheres containing 76–97 wt % Fe2O3 in the oxidative condensation of methane were compared with their phase composition and the distribution of iron cations over the crystallographic positions of iron-containing phases in a steady state. It was established that the reaction route of methane oxidation changed at a Fe2O3 content of 89 wt %. Deep oxidation was the main reaction route on ferrospheres with a Fe2O3 content of <88.8 wt %. At a Fe2O3 content of ≥89 wt %, the yield of C2 hydrocarbons sharply increased and the contribution of deep oxidation decreased. The yield of C2 hydrocarbons correlated with the amount of defects in the structure of iron spinel, which are iron ions with the tetrahedral cation of Ca2+ and the octahedral cation vacancy among the nearest neighbors.

Mössbauer and magnetic study of microspheres extracted from fly ashes of power stations

Phase composition of six narrow size fractions of microspheres extracted from fly ashes of Ekibastuz coals has been determined by Mössbauer spectroscopy. The basic phase is defect magnetite substituted by Al, Mg, and Ti ions. The magnetic properties of microspheres depend on the content of iron and its distribution over spinel positions.

Structure–Composition Relationship of Skeletal and Dendritic Ferrospheres Isolated from Calcium-Rich Power Plant Ash

This paper presents a systematic SEM–EDS study of polished sections of individual skeletal and dendritic ferrospheres in the–0.04 + 0.032 mm size fraction, isolated from fly ash from the combustion of brown coal from the Berezovskoe field. The ferrospheres are characterized by a wide range of variations in the macrocomponent composition of local areas. We have identified groups of globules whose overall composition as well as the composition of local areas on their polished sections can be represented by general equations for component concentrations: SiO2 = f(FeO), SiO2 = f(Al2O3), and CaO = f(SiO2). Such equations make it possible to identify the nature of the mineral precursors involved in the formation of the globules. FeO-rich skeletal ferrospheres with low CaO concentration originate from the thermochemical transformation of pyrite and illite associates. Skeletal and dendritic ferrospheres with monotonically increasing CaO and SiO2 concentrations are formed from pyrite and montmorillonite associates, with the participation of a melt containing quartz and decomposition products of Ca-humates of the initial coal. Skeletal and dendritic spinel ferrite crystallization is due to a magnesium aluminate spinel “seed,” resulting from the thermal transformation of illite and montmorillonite from the parent coal. The observed increase in glass phase concentration and the change from the skeletal type of crystallization to a dendritic in the ferrospheres containing ≤64 wt % FeO and ≥6.5 wt % CaO are due to the low concentration of the spinel-forming cations Fe2+ and Fe3+ in the melt and the increase in the percentage of [Fe3+O2]− and [Fe23+ O5]4− ferrite complexes with an increase in the degree of oxidation of the melt.

Structure–Composition Relationship of Platelike Ferrospheres in Calcium-Rich Power Plant Ash

The structure–composition relationship for polished sections of individual platelike ferrospheres in the–0.04 + 0.032 mm size fraction isolated from fly ash from the combustion of brown coal has been studied systematically by scanning electron microscopy and energy dispersive X-ray spectroscopy. We have identified groups of globules whose overall composition, as well as the composition of local areas on their polished sections, can be represented by general equations for component concentrations: CaO = f(FeO) and SiO2 = f(FeO). Analysis of the structure–composition relationship for the globules leads us to conclude that their structure is determined by transformations that occur in the CaO–FexOy system in response to an increase in its oxidation potential. It has been shown that the platelike globules containing 68–73 wt % FeO are made up of Ca2Fe2O5 and CaFe2O4 crystallites resulting from the oxidative transformation of Fe, Ca, and Mg complex humates in the parent brown coal. The ferrospheres containing 79–90 wt % FeO have a fragmentary core–shell structure, where the platelike shell consists of Ca2Fe2O5 and CaFe2O4 crystallites, and the core consists of Fe2O3 and a Ca-, Mg-, and Al-promoted magnetite. Precursors for the formation of this type of globule are pyrite associates with complex humates. It has also been demonstrated that the low concentration of aluminum and silicon oxides in the composition of the globules and the viscosity of their melt have no effect on the structure of the platelike ferrospheres.