Properties of Heat-Insulating Materials (A Review)

Abstract

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...

Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition

“…They are highly porous and lightweight refractories having much lower thermal conductivity and heat capacity than other refractories [1]. Different types of insulating firebricks are mainly manufactured by using the raw materials such as diatomite, perlite, expanded vermiculite, calcium silicate, fireclay, kaolin, quartz, alumina and lightweight refractory aggregates by conventional methods [1,2]. Porosity is usually created by adding a combustible material to the raw material mixture.…”

“…During firing, the combustible material burns out, and leaves a large fraction of pores within the fired body. Different types of pore-formers such as sawdust, foam polystyrene, fine coke, binders and organic foams, or granular materials such as hollow microspheres and bubble alumina are commonly used to obtain decreased density or to produce porous bodies in the insulating material [2][3][4][5][6][7][8]. Insulating firebricks that have a highly porous structure (between 45% and 90% porosity) exhibit low thermal conductivity values.…”

“…Insulating firebricks that have a highly porous structure (between 45% and 90% porosity) exhibit low thermal conductivity values. The thermal conductivity not only depends on their total porosity, but also their pore size and shape, chemical and mineralogical composition [2,5].…”

Impact fracture toughness and morphology of diatomite‐filled polypropylene composites

“…It is a new kind of filler or reinforced material extensively used in polymer composites, coat and paint, catalyst carrier, adsorbent carrier, surface activator, etc. Recently, diatomite is used extensively more and more in sound materials, construct materials and heat insulation materials [2,3].…”

Ceramoconcrete refractory and heat-insulating components in ferrous and nonferrous metallurgy

“…This area of demand is far from being satisfied. Everything now proposed is either composed of shaped bricks of standard dimensions or else of formed components made by low-cement or ultralow-cement concrete technologies [1]. The organization has its own production of expanded vermiculite, so VKVS is used in making blocks as the binding agent, while vermiculite is the filler.…”