Calcium Silicate Heat Insulation Materials

Calcium silicate heat insulation material is a synthetic material produced from silica (rebned diatomaceous ore or milled silica sand) and of calcium oxide (high-quality calcium carbonate chalk or lime) with the addition of organic bbers (in order to keep shape after pressing). Pulp sluny from the mixer is squeezed out, pressed, and after exposure at air placed in the autoclave with steam pressure about 10 atm and temperature 120-130 °C. The synthesis of xonolite [207] mineral takes place at the above-menti(Mied crHiditimis, and actually it is the end of the technological process. The dimensiOTis of calcium sibcate boards may be up to 2,500 mm x 1,500 mm. 

At preheating and during startup of the reduction cell, calcium silicate boards lose physical water and chemically bonded water (in total, up to 10 %), and the burning out and degradation of organic bbers take place. 

Usually in calcium silicate slabs, there are no geometrical defects (the torque sides of blocks may be grinded) however, the technology of the material is complex and needs accurate processing There are no problems in the service stability of calcium sdicate at high temperatures if there are no problems with 

2 Refractories and Carbon Cathode Materials for Aluminium Reduction Cells 

Thermal Conductivity of Heat Insulation Refractory Materials for Reduction Cells 

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This classification is rather tentative, as the final heat treatment of calcium silicate heat insulation refractory materials is about 180 °C, while the service temperature is above 800 °C. The exfoliation of vermiculite grains takes place at 650-800 °C, but the heat treatment of vermiculite heat insulation slabs takes place at 250-280 °C, with a service temperature above 800 C. 

Although in refractory practice there are hundreds of heat insulation materials, the list of heat insulation materials for the lining of reduction cells is rather limited. For one thing, economic considerations add some limitations, but for another, the heat insulation materials in reduction cells should withstand mechanical compression loads without deformation at temperatures up to 900 °C for a long time, and numerous inexpensive fiber heat insulation materials don t correspond to this requirement. In the Hall-Heroult reduction cell, the heat insulation materials should withstand the pressure of the layer of the electrolyte, the layer of molten aluminium, cathode carbon blocks (taking into account collector bars), and the refractory layer. Currently, only four or five heat insulation materials are used in the lining of reduction cells diatomaceous (moler) and perlite bricks, vermiculite and calcium silicate blocks (slabs), and sometimes lightweight fireclay bricks (but their thermal conductivity is relatively big, while the cost is not small) and fiber fireclay bricks. 

Fig. 2.85 Thermal conductivity of heat insulation materials 7, 2—diatomaceous bricks 3— lightweight fireclay brick 4—diatomaceous brick 5— vermiculite slab 6—calcium silicate slab...

In Chap. 1, we showed that the temperature dependence of refractory and heat insulation materials usually is a complex function. Yet for diatomaceous, vermic-ulite, lightweight fireclay, and calcium silicate materials in temperature intervals, the temperature dependence is linear (Fig. 2.91). The linear character of the temperature dependence of thermal conductivity reveals the peculiarities of heat transfer in lightweight refractory materials [206] (Fig. 2.85). 

The range of heat insulation materials is rather broad. Heat insulation materials that can withstand relatively high mechanical loads - this list includes lightweight fireclay bricks, vermiculite slabs, perlite bricks, calcium silicate boards, and diatomaceous bricks - are used for the heat insulation of the bottom. The pressure on the walls is lower, so it is possible to use fiber-based boards (in addition to the above-mentioned materials). Sometimes lightweight castables with fillers, such as lightweight fireclay, vermiculite, and fiber, are also used for heat insulation. 

Container Insulation Tanks containing materials above atmospheric temperature may require insulation to reduce loss of heat. Almost any of the commonly used insulating materials can be employed. Calcium silicate, glass fiber, mineral wool, cellular glass, and plastic foams are among those used. Tanks exposed to weather must have jackets or protective coatings, usually asphalt, to keep water out of the insulation. 

These can be inorganic materials such as calcium silicate, mineral wool, diatomaceous earth or perlite and mineral wool. If provided as an assembly they are fitted with steel panels or jackets. These are woven noncombustible or flame retardant materials the provide insulation properties to fire barrier for the blockage of heat transfer. 

Wollastonite and diopside are utilized as abrasion-resistant materials for road surfaces and fillers for rubber and plastics (Jacob, 1976), and also as thermally insulating materials for a variety of applications (Demidenko et al., 2001). Currently, these calcium and caldum magnesium silicates are synthesized starting with mixtures of moist lime (CaO) or burned dolomite (CaO + MgO) and finely ground quartz flour. The mixtures are heated hydrothermally between 200 and 250 °C for 7-16 h to yield xonotlite or mixtures of xonotlite and serpentine according to >... 

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