In recent years phenol-formaldehyde resins have been increasingly used as binders in the production of refractories which are being successfully exploited in oxygen converters, electric furnaces, and steelcasting ladles, and also as steelcasting products.Detailed studies have been made of the properties of such resins which are of interest for refractories technology [I], the processes of polycondensation oligomers [2][3][4], the coking of the resulting polymers [5][6][7], and oxidative destruction [2][3][4][5][6][7][8].

Bauxite concentrate is a promising raw material for producing high-alumina refractories that are used in the ladles designed for out-of-furnace treatment of steels [i].The purpose of this work is to study the structure and the properties of the fused refractories made from a bauxite concentrate obtained by hydrochloric acid treatment [2].Mullite-based solid solutions (having a basicity >1.5) constitute the main crystalline phase of the refractories obtained from bauxite and its concentrate [2][3][4]. The phase diagram of the AI203-SiO2 system gives an approximate phase constitution of the aluminosilicate refractories (including the fused refractories) as a functionof the chemical composition. According to this diagram, mullite is the only aluminosilicate that is stable at high temperatures. It was established [5][6][7][8][9][10][11][12][13] that the solid solutions of mullite whose composition changes from 3A1203"2Si02 up to 2A!203"Si02 are obtained as a result of a heterovalent substitution 2Si~++C~--~-~2AI~++ 0.In sillimanite, the type (i) substitution leads to a mullite structure having a statistical (random) distribution of oxygen vacancies [] [8, 9, 13]. The following structural formula of mullite was previously suggested [Ii]:where AIVI and AIIV indicate the coordination~ (sites) of aluminum atoms in the octahedral and tetrahedral positions, respectively; and x is the number of oxygen atoms lost per unit structural cell of sillimanite. Cameron [10][11][12] noted that sucha scheme of isomorphous substitution does not restrict the extent of mullitic solid solutions right up to sillimanite [14] on one hand and up to aluminum oxide [15][16][17][18] on the other.The possibility of obtaining mullites of variable composition by electromelting alumina and silica was shown earlier [19].Electromelting of bauxite concentrate and the compositions of the Al2Os-bauxite concentrate-SiO 2 system was carried out in a 'Kristall' unit using a cold crucible having an internal diameter of 65 mm. In order to melt (cast) the specimens of this system, we used high purity (99.99%) aluminum oxide and silica. Electromelting was carried out in air by successive fusion of AI203 and the mixtures of A1203, bauxite concentrate, and silica and an ingot measuring approximately 600 mm in length was obtained. Samples were cut out from the external region of the ingot using a diamond tool. The ingot was subjected to the maximum cooling rate (10-20~ Phase analysis of aluminosilicate refractories is difficult because not only chemical, but also crystallographic transformations occur during the process of their production. In view of this, the methods of rational chemical and x-ray phase analysis are of limited use because chemical analysis requires corrections for the solubility of the phase being analyzed as a function of the particle size, the temperature, and the duration of acid treatment; and x-ray phase analysis needs corrections for the degree of crystallinity of the phases. These facts prompted the application of infrared spectroscopy.Qu...

Plastic forming of bodies is one of the main industrial methods of obtaining silicon carbide products. This method accounts for approximately 60% of their total production. Selection of the composition of the bodies used for obtaining highquality preforms is based on ensuring the maximum packing density of the polyfraction silicon carbide by optimizing its granulometrie composition and determining the appropriate filler-to-binder ratio experimentally.This paper deals with a study of.the filler-plasticizer systems as applied to the bodies based on polycrystalline silicon carbide and aims at determining the conditions required for obtaining dense packing in the cured preforms and establishing the relation between these conditions and the technological features of production of the silicon carbide products.Resol phenol-formaldehyde resin (PFR) is used as a binder in the plastic bodies. It not only bonds the filler grains and plasticizes the body but also gives a dense coke residue due to thermal destruction during the subsequent stages of the process [ 1]. In order to increase the content of the coke residue and to improve the plasticizing ability of the binder, commercial-grade carbon is introduced into the body. At the present time, the P-803 commercial-grade carbon (GOST 7885-86) is being used independently of the quantity and the granulometric composition of the original silicon carbode. Besides this, silicon can be introduced as one of the carbide-formers into the binder.We carried out thermogravimetric studies on the systems consisting of commercial-grade carbon and bakelite,* commercial-grade carbon and silicon carbide, and bakelite and silicon carbide. With respect to the thermal effects, the jointly cured systems differ from the original components and the mechanical mixtures (Fig. 1). This can be attributed to the chemical interaction of PFR with the filler and commercial-grade carbon during the curing process. The conclusions drawn from the data of thermogravimetric analysis regarding the superficial interaction of the binder and the filler agree with the results of EPRspectroscopy of the PFR-oxide systems [2]. In this investigation, the PFR-based binder mixed with commercial-grade carbon or with its substituting product is treated as a plasticizer having specific properties. Figure 2 shows the micrographs of the cured preforms of the silicon carbide products obtained according to the method of plastic forming maintaining different contents of primary silicon carbide (SIC'). It can be seen that virtually each and every filler grain is isolated from the other grains and that the intergranular spaces are filled with the plasticizer; the presence of pores is also observed. The significant increase of the pore size with increasing quantity of silicon carbide in the original charge indicates deficiency of the binder.In order to determine the analytical form of the relationship between the parameters characterizing the structural features of the filler-plasticizer system, we shall examine the relative differenc...

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