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...
Corrosion mechanisms are proposed for aluminosilicate refractories by molten aluminum taking account of new ideas about physicochemical reaction of molten aluminum with refractory. With a negative volumetric effect for aluminum reactions with components of aluminosilicate refractories reaction products will not form a continuous film, and with a positive volumetric effect the reaction products will split pores and cause material cracking. A porous aluminum oxide film will not be a barrier for the reaction of aluminum with refractory. The possibility for aluminosilicate refractories of wetting by molten aluminum and the size of pores permeable for penetration of molten aluminum in relation to presence or absence of antiwetting additions are determined.Action of aluminum on refractory materials involves chemical reaction (increasing with introduction of alloying additions), erosive action, thermal shock (with metal pouring), and mechanical action (with loading of ingots). Account should also be taken of the action of fluxing additions (although considerably less than in ferrous metallurgy).Integration into the world market has required in our country both examination of ideas about the reaction of aluminosilicate refractories laid down in 1960 -80s, and introduction of new refractory materials for the aluminum industry, more resistant to the action of molten aluminum. Ideas formulated in the 1960 -80s about the physicochemical reaction of aluminum with refractories required considerable reworking. Not all possible reactions with refractory components were considered for the alloys based on aluminum and fluxing addition components, and almost no attention was devoted to wetting of refractories by molten aluminum, and the structure of refractories for producing aluminum alloys.In previous work for introduction of corundum refractories into the melting and casting units of the aluminum industry no consideration was given to the fact that magnesium is one of the main components of aluminum alloys of the 5 -6th group, and it reacts with Al 2 O 3 with formation of aluminum-magnesium spinel. A first approximation is given in Table 1 [ 1 -3] for information about the possible reaction of molten aluminum with oxides comprising refractory materials. On the basis of data about enthalpy values for oxide formation it is may be noted that all oxides placed to the right of Al 2 O 3 cannot be reduced by molten aluminum. Conversely, TiO 2 , SiO 2 , and FeO have a capacity to react with molten aluminum: 4Al + 3SiO 2 ® 2Al 2 O 3 + 3Si.(1)In the presence of magnesium spinel formation is possible:Data in Table 1 do not take account of the following situations:in molten aluminum there are always both impurities (alkali metals, gases) and additions (magnesium, strontium, zirconium, etc.) that on one hand are corrosive agents, and on the other hand in view of their activity, strengthen the corrosive action of aluminum; oxides in a bonded state (for example SiO 2 in calcium silicate, magnesium silicate and even in aluminum silicate) are l...
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