Heat-insulating materials for the lining of aluminum electrolysis cells based on diatomite, vermiculite, perlite, and calcium silicate are briefly reviewed. Data on heat conductivity of these materials over the range of 200 -800°C are given, with emphasis placed on the behavior in the electrolysis bath. Properties of heat-insulating materials available from domestic and foreign manufacturers are discussed in terms of the classification temperature and safe operating temperature.Despite the ready availability of a wide range of heat-insulating materials, a mere four or five of them have found application for the heat insulation of electrolysis cells. The insulators in question are lightweight chamotte products and materials based on diatomite, vermiculite, perlite, and calcium silicate. The main reason for the preference given to these materials is that they are capable of sustaining longterm mechanical loading without deformation at temperatures as high as 900°C. HEAT-INSULATING DIATOMITE-BASED MATERIALSThe high-grade diatomites contain more than 70% SiO 2 . A microporous structure (pore size 0.005 -0.01 mm across) is the main property that provides a low heat conductivity of diatomite [1,2]. To further decrease heat conductivity, combustible additives (sawdust) are introduced into the material which, as they burn out during sintering, increase the pore number. Argillaceous species that make up part of the diatomite rock impart plasticity to the molding mix and control the sintering regime and maximum permissible operating temperature of the material. As a rule, diatomite as-recovered from the quarry is used with no other minerals added; for this reason, heat conductivity and operating temperature of diatomite products purchased from various manufacturers may differ appreciably.The raw material recovered from quarries is averaged to obtain the required consistency, and sawdust is added to the mixture. Depending on the mixture consistency, extrusion or slip-casting techniques are used to prepare green performs of the needed shape; next, the green performs are sintered in a tunnel furnace. In recent years, to meet progressively more stringent size tolerances, the usual practice is to surface-finish the sintered components using diamond machining tools. HEAT-INSULATING PERLITE-BASED MATERIALSPerlite is a high-silica mineral of volcanic origin with a glass-like structure. By chemical composition, it is close to granite; it contains more than 1% water. When heated, perlite, owing to the vigorous evolution of chemically bound water, expands in volume to form a lightweight porous material. The swell ratio of perlite, depending on its occurrence, may be as high as 20. Sharp heating to 800 -900°C imparts to perlite a structure with inner isolated pores. For producing perlite-based heat-insulating materials, firing and nonfiring technologies have been developed using, for example, clay or cement binders. The maximum permissible operating temperature for insulating materials based on high-quality perlite may reach 1200°C. H...

The level of the physical and mechanical properties of steels and alloys can be improved by decreasing the degree of their contamination with nonmetallic inclusions. Among the methods and the technological routes available for solving this problem, the filter-refining method appears to be promising. This method involves passing a molten metal through a filtering unit in which the particles of the nonmetallic phase are separated out and are arrested at the developed surface of the filter. This concept is not new; in the Soviet literature, the process is known as the "Firam-process." This method reduces the extent of contamination due to nonmetallic inclusions during the process of pouring molten metals into the ingot molds and the molds used for continuous casting or in foundry.At the present time, a large volume of experimental data is available on this subject. Systematization of these data is required mainly for evaluating the prospects of application of this method for refining steels and alloys under the conditions of mass-scale production.A filter element (filter-assembly) forms the main technological component of any filtering unit. Under industrial conditions, two basically different designs of the filter elements have been tried out. The first design includes a refining chamber having a granular (lump) sorbent. The sorbent granules are obtained by fragmenting (dispersing) large lumps of a refractory ceramic. A ceramic filter element forms the main component of the second design. Different methods are used for producing it (in particular, compaction, slip casting, extrusion of a refractory body, compaction of ceramic granules into a single block containing a developed system of pore channels, and braiding of glass fibers or the fibers of a plastic ceramic material). In recent years, the so-called 'foam' filter elements have gained in importance. They are obtained by impregnating foamed polyurethane with a ceramic suspension, squeezing out the excess suspension, drying, and firing. The filter elements are located in the intermediate spaces during top-pouring of steels [i, 9], in the channels of the bottom-pouring lines [13], in the intermediate ladles [2, 6-8, I0, 14], in the channels of the immersible nozzles (downtubes) used during continuous casting [15], in the short pipes (nozzles) of the RH and DH installations [16], and in the casting equipment [17]. Figure 1 shows the dispositioning of the filter elements during top and bottom pouring and continuous casting.The method proved to be highly effective when refining the plain-carbon steels that were deoxidized (killed) using A1 [5,8,12,14] The maximum weight of the alloy being refined amounted to 250 tons [6] at a pouring rate of 2.5 ton/min [14].The effectiveness of the filter-refining process is indicated by the decrease in the contamination due to nonmetallic inclusions [5,8,9] and the total oxygen content in the alloy [i, 2, 5, 18] and, in a number of cases, by the decrease in the sulfur content [ii]. All et al. [5] and Fukudo [7] observed a decrea...

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Broadening of the use in domestic steel plants of submerged graphite-containing nozzles in casting of various types of steel has revealed a known complication in operation of continuous casting machines related to a decrease in the through cross-section of the channel and the holes of the submerged nozzle.This process is an obstacle to long continuous casting of steel.Clogging of submerged nozzles and prevention (slowing) of this process have more than once been the subject of investigations [1][2][3][4].In a number of works [3, 5] the influence of the submerged nozzle material on formation of alumina-containing deposits has been evaluated.The processes occurring at the temperature of casting of the steel in the refractory itself taking into consideration the presence in it of a deoxidizer, carbon (graphite, coke residue of the carbon binder), have been considered.The reaction has the general form:Refractory Refractory Gaseous Products In the gaseous state AI20 and SiO diffuse into the wall of the nozzle in the direction of growth of the temperature gradient toward the surface of the nozzle channel and then into the molten metal.On the interface of the phases and close to it reactions of oxidation of the gaseous suboxides occur :in which the oxidizer is oxygen of the steel liberated as the result of the decrease in the temperature of the metal in the layer near the wall and also oxygen liberated in occurrence of the reactionOne of the most important sources of alumina inclusinos must be assumed to be products of deoxidation of the steel with aluminum and depending upon the method of deoxidation, use of ladle treatment, and casting conditions products of either primary or secondary oxidation of the components of the steel predominate in the deposits.In continuous casting of steel containing more than 0.02% aluminum clogging of the nozzle channel occurs as the result of the aluminate deposit on its walls.Small amounts of Ca impurity form with the alumina type CaO.6AI203 hard refractory inclusions with a melting point above 1800~[6].The process of precipitation of oxides is promoted by the presence of irregularities on the inner surface of the submerged nozzle channel.Obviously a certain contribution to the process is made by the hydraulics of flow of metal through the channel of the submerged nozzle.Two features must be emphasized. In the first in the near-wall layer of the liquid (in the given case molten metal) the rate of flow is significantly less than along the axis of the channel while in the second in certain situations (condition of the tundish nozzle, conditions of stopping and shutting off the steel tapping hole) an eddy effect may occur in the stream of metal, especially in the lower portion of the nozzle in the zone of the holes.

Refractories containing in their composition carbon and oxygen-free compounds such as carbides, nitrides, borides, etc. are acquiring increasing practical value [i].Present and future areas of their use include the linings of converters and electric arc furnaces, refractories for ladle gate valves and teeming of steel in continuous billet casting machines, etc.In their interaction with molten metals these refractories exhibit specific features, some of which are discussed in this article.As a first approximation as the result of the high surface activity of oxygen and sulfur in it (their surface activity may exceed the content by tens of times) the surface of molten steel may be represented in the form of the highly electronegative anions 0 =-and S =-in coordination with the cations Fe 2+, Mn =+, etc.In interaction with the oxidizable elements of the refractories the anions form intermediate products all the way to oxides, including gaseous ones.The corrosion of carbon-containing refractories under the action of steel under equilibrium conditions may be represented in three stages [2]:at the steel--refractory contact under the action of surface-active oxygen, oxidation of the carbon occurs and a decarburized zone is formed; the decarburized zone with increased porosity rapidly forms a slag and the products of the interaction have decreased refractoriness, mechanical strength, and thermal coefficient of volumetric expansion differing from the original material; for these reasons the slagged layer of the refractory is rapidly washed away by the steel.The increase in metal resistance of a refractory with the addition of oxygen-free additions, particularly carbon, is the result of the following facts: frequently, the carbon addition is made by impregnation or precipitation from the gaseous or liquid phase into the finished refractory and the carbon partially replaces the porous spaces, mechanically preventing penetration of the molten metal; the products of gasification of the carbon pneumatically prevent penetration of the molten metal into the voids; in the microvolumes carbon deoxidizes the molten metal in contact with it and the highly active oxide forms of iron are converted into less active lower oxides all the way to the metallic state, a confirmation of which is the presence of beads of pure iron in the reaction zones of carbon-containing refractories after their wetting even by slag.To the products of gasification of carbon are added the gases entrapped in the void space and the gaseous products of reduction by carbon of the oxides of the refractory occurring according to reactions of the type:(SiO2, AI2Oa, ZrO~, ZrO=.SiO,, MgO era. )ml+Cso~C0g~+SiOg ~ AIOg~ era.These reactions occur both in the contact zone and in the thickness of the refractory with diffusion of CO and gaseous suboxides into the contact zone. In addition, the decomposition of oxygen-free compounds is possible according to reactions of the form SiaN4--+2N2+3Si.All-Union Refractory Institute.

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