O. M. Sharonova

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.

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 of individual ferrospheres of different morphological types

We have studied the composition of three types of individual spheres differing in microstructure (single-block, platelike, and skeletal–dendritic spheres) in the–40 + 32 μm size fraction of ferrospheres containing 92.72 wt % FeO. The results demonstrate that the single-block spheres have the highest Fe content (95–97% FeO), a basicity factor Mb > 48, and an Fe/O atomic ratio of 0.68–0.71. They consist of block individuals of a partially martitized (oxidized to hematite) spinel ferrite. The platelike spheres feature high calcium content (11–12% CaO); lower basicity factor, Mb ≃ 20–21; and a higher degree of oxidation of Fe, with an Fe/O atomic ratio of 0.63–0.66. They consist of spinel ferrite blocks with large percentages of FeO (up to 90%) and MgO (up to 6%), surrounded by regions with a platelike structure, reduced FeO content (down to 57%), and high CaO content (up to 34%). The skeletal–dendritic spheres feature increased percentages of SiO2 (≃ 4.7 %) and Al2O3 (3.6–4.6%) and low basicity factor: Mb ≃ 10–11. They consist of unidirectional or branched crystalline spinel ferrite individuals with partial Mg2+ substitution for Fe2+, which prevents oxidation of the spinel to hematite, as evidenced by the large Fe/O atomic ratio: 0.72–0.73.

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.

Composition and structure of calcium aluminosilicate microspheres

The composition was studied of calcium aluminosilicate microspheres of three morphological types in high-calcium fly ash from combustion of brown coal from the Kansk-Achinsk basin in slag-tap boilers at temperatures from 1400 to 1500°С and sampled in the first field of electrostatic precipitators at the Krasnoyarsk Cogeneration Power Station no. 2 (TETs-2). Gross compositions and the composition of local areas were determined using a scanning electron microscopy technique and an energy-dispersive analysis with full mapping of globules. With a high content of basic oxides Oох (68 to 79 wt %) and a low content of acid oxides Kох (21 to 31 wt %), type 1 microspheres are formed. They consist of heterogeneous areas having a porous structure and crystalline components in which the content of CaO, SiO2, or Al2O3 differs by two to three times and the content of MgO differs by seven times. With a lower content of Oох (55 to 63 wt %) and an elevated content of Kох (37 to 45 wt %), type 2 microspheres are formed. They are more homogeneous in the composition and structure and consist of similar crystalline components. Having a close content of Oох (46 to 53 wt %) and Kох (47 to 54 wt %), type 3 microspheres, which are a dense matter consisting of amorphous substance with submicron- and nanostructure of crystalline components, are formed. The basic precursor in formation of high-calcium aluminosilicate microspheres is calcium from the organomineral matter of coals with various contribution of Mg, Fe, S, or Na from the coal organic matter and Al, Fe, S, or Si in the form of single mineral inclusions in a coal particle. On the basis of the available data, the effect was analyzed of the composition of a CaO–MgO–Al2O3–SiO2–FeO system on the melting and viscous properties of the matter in microspheres and formation of globules of different morphology. The results of this analysis will help to find a correlation with properties of microspheres in their use as functional microaggregates in cement or polymeric composite materials, or in the production of ceramic membranes or zeolite sorbents.