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Chromium oxide refractory products are irreplaceable for lining glass-melting tank furnaces for producing alkali-free borosilicate glass at 1550-1600~In mastering the production of chromium oxide blocks (measuring 710 x 360 • 90 mm in the raw state) from finely milled masses, a significant quantity of reject crock was obtained because of the prevalence of overpressing cracks. It therefore seemed of interest to study the structure--mechanical characteristics of finely dispersed chromium oxide masses.The present paper deals with a study of the effect of some surface-active materials on the structuremechanical properties of finely milled chromium oxide masses. It is well known [1][2][3][4][5] that the surface-active material forms a shell on the surface of the mineral particles and helps to increase the plasticity of the powdered masses thus ensuring that a dense and strong system is achieved after pressing.As the starting materials we used industrial chromium oxide, grade OkhM (GOST 2912-73) with a concentration of CrO 2 of > 99%; the material consisted of tablet-shaped crystals predominantly 2-20 ~m in size. The maximum size of the crystals was from 40 to 150 #m. The chromium oxide was milled in a ball mill for 18 h using the dry method (material : ball ratio = 1 : 1) to a grain size of ~-10 ~m. Two hours before the end of the milling, a sintering additive of TiO z [6,7] was added to the mill. The mixture thus obtained was moistened with water, an aqueous solution of sulfite alcohol distillery waste (SAD), or methyl cellulose in various concentrations up to a moisture content of 6%. The specimens were pressed under a pressure of 300 kgf/cm2.The structure--mechanical constants of fine-grain, semidry CrO 2 masses were determined in the apparatus designed by D. M. Tolstoi using the method described in [3,9]. The values of the rapid era p and slow esl elastic deformations; the plastic deformation epl; and the rate of deformation d~/d~ after the flow had been established were determined from the deformation curve of the dependence ~ = fff), where e is the deformation of pure shear and ~-is the time. We then calculated the rapid E 1 and slow E 2 elastic deformations, the greatest plastic viscosity ~ 1, the nominal static ultimate limit of flow Pk,, the elasticityk, the Volarovich plasticity Pkl/~i, and the relaxation time el.The characteristics of the structure--mechanical properties of chromium oxide masses as a function of the SAD concentration are given in Table 1.The masses moistened with water alone have very good structure--mechanical properties. It is clear that with this particular ratio of ~i, El, and E2, the deformation is associated mainly with the rotation at the contacts of the particles without any slip. When the mass is moistened with a 12.5% ~ iution of SAD, the

It was previously shown [1][2][3][4][5][6] that satisfactory results can be obtained with the sintering of chromium oxide in vacuum furnaces, in a tube containing a current of inert gas, or in other vessels when the firing zone contains materials which reduce the partial pressure of the oxygen P02 by bonding it. One such material is solid carbon, which starts to react with oxygen from the air even at 500-600~Below 800-900~ C02 predominates in the reaction products, and CO at higher temperatures [7]. Over heated carbon, carbon dioxide and monoxide are located in equilibrium between themselves and with the solid phase, and also with oxygen formed by the partial dissociation of carbon dioxide according to the equation 2C02 ~_ 2CO + 02.The low P02 in the gas medium provides rapid sintering of powdered pressings made of chromium oxide placed in direct proximity to solid carbon (but without contacting it).We studied the relationship between the sintering capacity of Cr202 grade OKhM-I,* milled in a ballmill for 18 h, with an addition of 4% by weight of Ti02 in a carbon-containing vessel (corundum crucible) at 1650~ with a soak of 5 h, and the amount and variety of solid carbon. Two series of experiments were made. In the first the ratio of the volume of specimen to the size of the fired volume was 1:2 and in the second 1:7; the variable was the ratio K of the weight of Kryptol with a grain size of 1-5 mm to the weight of the specimen. Firing was done in a chamber furnace at the Experimental Factory of the Ukrainian Institute. The results are shown in Fig. 1.In the first series of experiments the sintering characteristics of the chromium oxide specimens (linear shrinkage and open porosity) reach constant values even for minimum K values, equal to 3 9 10 -3 (see Fig. la). In the second series the degree of sintering is increased with increase in K, but more slowly, reaching the same level when K ~ 5 9 10 -2 (see Fig. lb). This is apparently due to the fact that in the second case the low partial pressure of oxygen needed to sinter Cr202 has to be created in a vessel which at the commencement of firing contains a larger quantity of free oxygen than in the first case. The location of the specimens of Cr202 in crucibles in direct proximity to the Kryptol does not exclude possible flow of oxygen into the firing space; the absence of oxygen flow ensures the better sintering of the specimens (see points with arrows in Fig. 1). A study was also made of the possibility of using crushed metallurgical coke in the firing space to reduce PC),-Specimens were placed in a closed container in direct proximity to the coke and fired under the same Z o * conditions (1650 C, soaking for 5 h). The open porosity of the specimens after firing was 10-15% higher than that of specimens fired in the presence of Kryptol.The cause of this, probably, lies in the different rates of interaction of the Kryptol and the coke with the oxygen. The first is preserved in amounts of 80% up to the end of firing, which ensures a low partial pressure of oxygen at...

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