It is established that the sequence and intensity of polymorphic transformations of silica raw material on heating in the range 1000 -1200°C are explained by features of its chemical and phase composition. Differences in structure and phase changes determine the different degree of activity for this raw material during synthesis of silicate compounds.The main tendencies of contemporary development of science and technology suggest the use of ceramic materials with a high level of functional properties. Of particular importance are calcium silicate ceramics with a wollastonite crystalline phase due to its good mechanical properties, low thermal conductivity, high heat resistance, etc. The ceramics may be used in nonferrous metallurgy, mainly for preparing primary aluminum, in special radio ceramic technology, glazed pottery, porcelain, building and heat insulation ceramics, sanitary objects, etc.Synthesis of wollastonite and its use in ceramic technology in recent years has received considerable attention. The known synthesis methods are very varied: hydrothermal synthesis, solid-phase reaction in the presence of a liquid phase, crystallization from a melt, dehydration of hydrosilicates, etc. Of particular scientific and practical interest is solid-phase synthesis of wollastonite by ceramic technology from a mixture of silica and lime raw materials of natural and technogenic occurrence. It is well known that the rate of intensity of reaction in the solid phase depends on numerous factors, of which the main ones are the condition (degree of perfection) of the crystal lattice of mixture components, surface defects of reagent grains, fineness of the components, etc.In nature and in commercial products silica raw material is encountered both in amorphic and crystalline states, which undoubtedly affects its activity during synthesis of wollastonite and other silicate materials. In addition, the most widespread quartz sands and vein quartz are known and other forms of natural silica-containing material, in particular tripolite, i.e. a soft porous rock composed of very fine (0.005 -0.020 mm) weakly-bonded grains of amorphous silica (opal) of volcanic occurrence; diatomite (kieselgur, rock flour), i.e. light, porous, weakly-cemented sedimentary rock formed from the residues of microscopic radiolaria; opoka, a light porous, but distinguished from diatomite and tripolite, densely-cemented silica rock formed by opal microspherulite material; marshallite (powdery or dusty quartz), i.e. finely-dispersed loose weakly-compacted sedimentary rock consisting mainly of grains of powdery quartz and sometimes with a mixture of chalcedony.In this work as a silica raw material for synthesizing wollastonite natural rocks have been studied in the form of diatomite and opoka of the Inzensk deposit (Ul'yanovsk region), marshallite of the Elbashevsk deposit (Novosibirsk region) and also commercial products, i.e. analytical grade anhydrous silicic acid and microsilica that is a waste product of crystalline silicon and ferrosilicon of the Nove...
The synthesis of mullite in mixtures of kaolinite and alumina in the presence of small amounts of topaz is studied. Topaz is shown to activate the synthesis of both the primary (from kaolinite) and secondary mullite (through binding silica produced by thermal degradation of kaolinite), decrease the temperature of synthesis by 100°C, and increase the total yield of mullite. Products of the thermal degradation of topaz -mullite and gaseous chemically active fluoride compounds -are shown to play the role of mineralizing agents.Most technologies for aluminosilicate ceramics involve clay minerals that play the role of a binder during shaping and of a crystallizing component during calcination. The theoretical yield of mullite, the major phase produced by calcination, amounts to 55 wt.%; in practice, depending on the quality of raw materials, the yield does not exceed 80 -90% of theoretical.It is generally agreed, in the thermal degradation of kaolinite, that the formation of mullite involves the following steps: kaolinite ® metakaolinite ® spinel-type phase ® mullite phase ® mullite. In greater detail, at 925°C, metakaolinite (Al 2 O 3 × 2SiO 2 ) converts to a spinel-type phase with approximate composition 2Al 2 O 3 × 3SiO 2 and releases silica which in part goes into molten silicate. At 1050 -1100°C, the spinel-type converts to a mullite phase; concurrently, silica in the form of cristobalite is released. At 1200 -1400°C, cristobalite continues to form along with mullite of composition 3Al 2 O 3 × 2SiO 2 per crystal unit cell. For preparation of an aluminosilicate ceramic using refractory clays low in glassy material and high in mullite, the precursor mixture must be high in Al 2 O 3 (higher than 90%); for that purpose, native or artificial hydrated aluminum oxide, commercial alumina, or electrofused corundum are most frequently used.In ceramics technologies, beneficiated kaolin is preferably used, free of extraneous inclusions and with properly size-graded particles. In this work, the commercial beneficiated kaolin from Zhuravlinyi Log deposit (Chelyabinsk Region) [1] was further treated for higher quality using an elutriation technique to obtain fractions of size less than 5 mm and of chemical composition (wt.%): 46 SiO 2 , 36.93 Al 2 O 3 , 1.27 Fe 2 O 3 , 0.34 TiO 2 , 0.5 K 2 O, 0.04 Na 2 O, and 14.2 Dm calc .To provide a 100% yield for mullite as an Al 2 O 3 -based additive to kaolin, analytical-grade chemicals (dehydrated aluminum oxide, aluminum oxide "for chromatography", hydrated aluminum oxide, and aluminum sulfate) and commercial-grade amorphous alumina were used.To intensify the synthesis of minerals in ceramic technologies, mineralizing additives are used that produce a modifying effect on the structure and properties of materials, reaction products and molten phases during sintering and, ultimately, on quality characteristics of the finished products. Earlier, a mineralizing action of topaz-based additives (0.5 -1.0%) on the synthesis of mullite in refractory clays has been reported [2]. In this work, we...
Results of a study of the mullitization in stoichiometric pure oxides Al 2 O 3 and SiO 2 doped with topaz are reported. A mineralizing topaz-mediated effect is manifested in the activated synthesis of mullite of short prismatic habit involving products of thermal degradation of topaz (mullite and volatile fluorides). The volatile active fluorides enhance structural imperfections in the reactants and promote their high-temperature interaction. Mullite produced by thermal degradation of topaz plays the role of a seeding agent in the solid-state synthesis of mullite from oxides.Nowadays, progress in science and technology is to a significant extent based on the use of ceramic materials with a high level of functional properties. In this respect, a promising material is high-alumina ceramics with a mullite crystalline phase, which imparts to products high mechanical strength, refractoriness, thermal and chemical corrosion stability, high incipient softening temperature, etc. However, the superior qualities of mullite-containing aluminosilicate ceramics are determined not only by the high concentration of mullite, but also by its structure and morphology (prismatic or acicular habit of mullite particles).The habit of mullite is determined by the composition of the raw mixture and by the temperature of synthesis. The acicular mullite is commonly synthesized from commercialgrade chemicals via gas-phase mass transfer or crystallization from the molten state. In amorphous oxides, mullitization starts at 900°C, proceeds actively at 1000 -1200°C, and is completed at 1400°C within 2 h. If crystalline SiO 2 and Al 2 O 3 are used, the temperatures for the onset and termination of mullitization increase to 1400 and 1600°C, respectively. Lengthy heating at above 1650°C causes complete disappearance of corundum from mixtures of mullitic composition (silica + alumina) [1].Ineffective synthesis and sintering of mullite in a single-step calcination is explained by unidirectional (onesided) diffusion of SiO 2 into the Al 2 O 3 grain (the so-called Kirkendall -Frenkel effect); the diffused SiO 2 particle leaves a pore, which finally results in the buildup of a rigid crystalline mullite framework which in the absence of a sufficient liquid phase (that is, for a total of impurities 0.2 -0.5% or less), prevents the sintering and closure of pores. Under these conditions, the solid-phase mullitization reaction proceeds via diffusion, with the partial diffusion coefficient for silicon much higher than that for aluminum. For this reason, obtaining a dense ceramic from high-purity mullite by a combined process of synthesis and sintering presents a very serious problem [2].To intensify the synthesis of minerals, mineralizers are employed in ceramic technologies; in each particular case, the mechanism of mineralization is determined by the conditions of synthesis [3]. Mineralizers are promoters for mullitization at low temperatures (about 1350°C), while at higher temperatures (about 1700°C) they are inactive [4]; they exert a specific effect on...
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