Factors for increasing strength of composite materials based on fine high-calcium fly ash

Industrial high-calcium fly ashes obtained by burning Kansk-Achinsk coal at a thermal power plant and selected from different fields of electrostatic precipitators of an ash collecting plant were studied as the basis for composite binders (CB). The main factors influencing the properties of such CBs are the particle size, the concentration of superplasticizer at a water:binder (w/b) ratio of 0.25, and the proportion of HCFA in the mixture with cement. In particular, for cementless CBs at w/b 0.4, it was found that a change in the particle size d90 from 30 μm to 10 μm leads to an increase in compressive strength by more than 2 times – from 5.5–14 MPa to 11–36 MPa, accordingly, with a curing age of 3–300 days. The 0.12% additive of Melflux 5581F superplasticizer at w/b 0.25 increases the compressive strength – up to 14–32 MPa and up to 24–78 MPa, accordingly. The HCFA-cement blends were investigated in the range of 60–90% HCFA and the maximum compressive strength 77 MPa at 28 days of hardening was found at 80% HCFA. On the basis of 80% HCFA blend with the 0.3% addition of Melflux 5581F and 5% silica fume, the specimens of ultra-high strength (108 MPa at 28 days of hardening) were obtained

Hydrothermal synthesis and sorption performance to Cs(I) and Sr(II) of zirconia-analcime composites derived from coal fly ash cenospheres

The paper is concerned with (i) the hydrothermal synthesis of hydrous zirconium dioxide (HZD) bearing analcime (HZD-ANA, zirconia-analcime) and (ii) its sorption properties with respect to Cs+ and Sr2+. The HZD-ANA particles were synthesized from coal fly ash cenospheres composed of aluminosilicate glass with (SiO2/Al2O3)wt.=3.1 and characterized by PXRD, SEM-EDS, STA, and low-temperature N2 adsorption. The non-radioactive simulant solutions of different acidity (pH=2–10) and Cs+/Sr2+ content (0.5–50.0 mg/L) were used in the work. The effect of synthesis conditions on the HZD-ANA particle size, zirconia content and localization as well as the sorption behavior with respect to Cs+ and Sr2+ (capacity, KD) were clarified. It was found that the small-sized HZD-ANA composites surpasses the Zr free analcime and large-sized HZD-ANA material in the Cs+ and Sr2+ sorption parameters (KD ~104–106 mL/g). The conditions to synthesize the zirconia-analcime composite of the highly enhanced sorption ability with respect to Sr2+ (KD ~106 mL/g) were determined. The high-temperature solid-phase re-crystallization of Cs+/Sr2+-exchanged HZD-ANA composites was shown to occur at 1000 °C resulting in a polyphase system based on nepheline, tetragonal ZrO2, and glass phase

Chemistry for Sustainable Development 16 (2008) 521-528 Content of Various Size and Density Particles in Cenosphere Concentrates of Volatile Coal Combustion Ashes from the Kuznetsk Coalfield

Abstract Using an aerodynamic method with the subsequent particle size analysis of the fractions obtained a separation process was carried out for cenosphere concentrates from volatile ashes of the Kuznetsk field coal combustion at the Moscow Thermal and Electric Plant TETs-22, the Belovo State Regional Power Station as well as for several cenosphere concentrate fractions obtained at the Novosibirsk Thermal and Electric Plant TETs-5. As much as 90 fractions with the various particle size (70-250 mm) and bulk density (0.16-0.52 g/cm 3 ) have been isolated from the concentrate of the Moscow TETs-22. Irrespective of particle size the bulk density of fractions with the maximal yield is equal to 0.33-0.35g/cm 3 , whereas the ratio of the apparent wall thickness to the particle diameter amounts to 0.042-0.043. The process of the aerodynamic separation of cenospheres from different concentrate sources can be satisfactorily described by theoretical dependences for the carryover of spherical particles; the deviations are caused, first of all, by a nonspherical shape of cenospheres

Influence of Temperature and Duration of α-Fe2O3 Calcination on Reactivity in Hydrogen Oxidation

Исследовано влияние температуры и продолжительности прокаливания однофазных образцов гематита со структурой α-Fe2O3 на окислительную способность в отношении водорода в режиме температурно-программируемой реакции в интервале 40–900 °C. Показано, что температура прокаливания является существенным фактором, влияющим на реакционную способность решеточного кислорода в окислении водорода. Образцы α-Fe2O3, прокаленные при 800–900 °C, проявляют наибольшую активность, процесс восстановления α-Fe2O3 в этих образцах протекает через стадию восстановления до магнетита, с последующим полным восстановлением до металла через совмещение стадий восстановления оксидов. Прокаливание образцов α-Fe2O3 при 1000–1100 °C приводит к существенному снижению окислительной способности, восстановление α-Fe2O3 начинается при температурах на 50–100 °C выше, идет без выделения отдельных стадий восстановления, полное восстановление α-Fe2O3 до металла в исследованных условиях не происходит. Установлено, что с увеличением температуры прокаливания образцов возрастает рентгенографическая плотность α-Fe2O3, что свидетельствует о снижении степени разупорядочения кристаллической решетки, которое приводит к росту энергии связи решеточного кислорода и существенному снижению реакционной способности α-Fe2O3 в окислении водородаThe effect of temperature and duration of calcination of single-phase samples of hematite with the α-Fe2O3 structure on the oxidizing ability with respect to hydrogen in the temperature-programmed reaction mode in the temperature range of 40–900 °C was studied. It is shown that the calcination temperature is a significant factor affecting the reactivity of lattice oxygen in the oxidation of hydrogen. Samples of α-Fe2O3, calcined at 800–900 °C, show the highest activity, the process of α-Fe2O3 reduction in these samples proceeds through the stage of reduction to magnetite, followed by complete reduction to metal through the combination of reduction stages of oxides. The calcination of α-Fe2O3 samples at 1000–1100 °C leads to a significant decrease in the oxidizing ability, the α-Fe2O3 reduction initiates at temperatures 50–100 °C higher, proceeds without separating individual reduction stages of oxide, there is no complete reduction of α-Fe2O3 under the studied conditions. It has been established that with an increase in the calcination temperature of the hematite samples, the X‑ray density of α-Fe2O3 increases, which indicates a decrease in the degree of crystal lattice disorder and an increase in the binding energy of lattice oxygen and manifests itself in a significant decrease in the reactivity of α-Fe2O3 in the oxidation of hydroge

The Composition, Structure, and Helium Permeability of Glass-Crystalline Shells of Cenospheres

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.The results of studying the relationship of the composition, structure, and the diffusive properties of glass-crystalline shells of cenospheres relative to helium are presented in a wide range of variations of macrocomponent and phase compositions, wt %: 58–68 of SiO2, 21–37 of Al2O3, 30–93 of glass phase, 1–50 of mullite, 0–15 of cristobalite, 1–7 of quartz, and 0–2 of anorthite. It is found that the helium permeability coefficients of the cenosphere glass phase exceed the similar values of silicate glass. The high level of permeability of cenosphere glass-crystalline shells ensures significant potential for their application as membrane elements of the diffusion–sorption technology of the selective recovery of helium from gas mixtures

Composition−Structure Relationship of Skeletal−Dendritic Ferrospheres Formed during Industrial Combustion of Lignite and Coal

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.The structure−composition relationship of skeletal−dendritic ferrospheres (FSs) isolated from fly ash from the coal and lignite combustion has been studied systematically by scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy. It is shown that illite is the aluminosilicate precursor determining the structure of globules in both cases. The formation of skeletal−dendritic globules occurs due to the “seed” of Al, Mg-ferrospinel that is formed in the thermochemical conversion of illite from initial coals. The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs, the composition of which is represented by linear regression equations. An analysis of SEM images of polished sections from six globule groups reveals that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from the coarse-grained crystalline skeletal type to the finecrystalline dendritic type with a high content of the glass phase. The observed change in the structure is explained by expansion of the liquation region in the FeO−Fe2O3−SiO2 system, a rise in the oxidation potential, an increase in the proportion of ferrite complexes [Fe3+O2]− and [Fe23+O5]4− in high-calcium melts, and a decrease in the concentration of ferrospinel-forming Fe2+ and Fe3+ ions

Effect of HF Modification on the Catalytic Properties of Ferrospheres in the Oxidative Coupling of Methane

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.The effect of the HF modification of ferrospheres separated from fly ash after the combustion of brown coal on their chemical, phase compositions and catalytic properties in the oxidative coupling of methane was studied. The modification led to a change in the phase composition in comparison with that of the initial ferrospheres: a CaF2 phase appeared, the hematite phase content increased, and the ferrospinel content decreased. The yield of C2 hydrocarbons at 750°C increased by a factor of 1.5–2.0, and the fraction of ethylene in them increased to 30 or 65% at 750 or 850°C, respectively. It was assumed that an increase in the efficiency of HF-modified ferrospheres in the formation of ethane and its dehydrogenation into ethylene was due to the formation of oxyfluoride-type active sites. The pyrohydrolysis of fluorine-containing catalyst components at 850°C due to interaction with water vapor in a reaction atmosphere led to the formation of systems active in deep oxidation; this manifested itself in a sharp decrease in selectivity for the formation of C2 hydrocarbons and an increase in selectivity for CO2

Composition–Structure Relationship of Skeletal–Dendritic Ferrospheres Formed during Industrial Combustion of Lignite and Coal

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.The structure−composition relationship of skeletal−dendritic ferrospheres (FSs) isolated from fly ash from the coal and lignite combustion has been studied systematically by scanning electron microscopy (SEM) and energy-dispersive Xray spectroscopy. It is shown that illite is the aluminosilicate precursor determining the structure of globules in both cases. The formation of skeletal−dendritic globules occurs due to the “seed” of Al, Mg-ferrospinel that is formed in the thermochemical conversion of illite from initial coals. The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs, the composition of which is represented by linear regression equations. An analysis of SEM images of polished sections from six globule groups reveals that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from the coarse-grained crystalline skeletal type to the finecrystalline dendritic type with a high content of the glass phase. The observed change in the structure is explained by expansion of the liquation region in the FeO−Fe2O3−SiO2 system, a rise in the oxidation potential, an increase in the proportion of ferrite complexes [Fe3+O2]− and [Fe23+O5]4− in high-calcium melts, and a decrease in the concentration of ferrospinel-forming Fe2+ and Fe3+ ions

Narrow Dispersed Fractions of High-Calcium Fly Ash Produced from the Pulverized Combustion of Irsha-Borodinsky Coal

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.By means of airflow aerodynamic classification, high-calcium fly ash (with a bulk density of 1.14 g/cm3 and size distribution parameters of dav = 5 µm and d90 = 14 µm) produced from the combustion of Irsha-Borodinsky coal and sampled from the fourth field of the electrostatic precipitator at the Krasnoyarsk TPP-2 has been separated. The obtained morphologically homogeneous fractions of spherical particles with a narrow distribution are characterized by dav = 1, 2, 3, 4, and 10 μm and d90 = 3, 4, 5, 9, and 16 μm. It has been established that the main chemical component of the obtained narrow fractions is represented by CaO with a content of 34–43 wt %; the content of the other components is as follows: 15–34 wt % of SiO2, 13–16 wt % of Fe2O3, 9–10 wt % of MgO, 8–10 wt % of Al2O3, 2–10 wt % of SO3. The phase composition has 35–49 wt % of crystalline calcium-containing compounds, including 11–15 wt % of aluminum substituted calcium ferrite and 8–11 wt % of tricalcium aluminate, which are the main phases of Portland cement. The content of crystalline quartz amounts to 2–7 wt %, and that of the amorphous glass phase is 41–51 wt %. It has been established that, as the average fraction size dav obtained after the aerodynamic separation of ash increases from 1 to 10 μm, the bulk ash density exhibits an increase from 0.89 to 1.50 g/cm3 and the content of the magnetic component also increases, amounting up to 4 wt %. At the same time, the chemical composition of the fractions exhibits an increase in the content of SiO2 as well as a decrease in the content of Al2O3 and SO3. The content of СаО and Fe2O3 in the fractions having dav = 1–3 μm increases, then it exhibits an abrupt decrease with increasing particle size. As far as the phase composition is concerned, an increase in the content of crystalline quartz is observed, and the total fraction of calcium-containing phases gradually decreases. At the same time, the percentage of calcium sulfate decreases and the percentage of free calcium oxide exhibits a considerable increase. The content of calcium hydroxide increases in the fractions having dav = 1–2 μm and then decreases with increasing particle size

Composition, Structure, and Formation Routes of Blocklike Ferrospheres Separated from Coal and Lignite Fly Ashes

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.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