Vermiculite-Based Heat Insulation Materials

Vermiculate ore is a result of the erosion and disintegration of black mica and the subsequent geological hydrothermal interaction. Probably the best-known deposit of vermiculite ore is in South Africa. Vermiculite is magnesium potassium hydro alumino silicate, with approximate formula (Mg , Fe , Fe )3 

Vermiculate-based heat insulation materials may be made with firing and without firing. The first step in processing is refining and expansion (blowup). In fired processing, the expanded vermiculite is mixed with clay and shaped in blocks by pressing and then fired in a tunnel kiln up to 900-1,000 °C. Fired materials do not contain water and do not absorb water from the atmosphere the safe service temperature of fired vermiculite blocks is superior to the safe service temperature of unfired ones. 

The safe service temperature of unfired vermiculite heat insulation materials is determined by the binder and may vary considerably, depending on processing and the sodium/potassium ratio (Fig. 2.90). The materials with sodium liquid glass binder have a lower safe service temperature than materials with potassium liquid glass binder, because of different high-temperature deformations (Fig. 2.84). 

Other than safe service temperature, usually in vermiculite-based materials there are no defects, and the dimensional tolerances at pressed materials are perfect (Fig. 2.84). If something is not going steadily in the fluidized furnaces, the density of grained vermiculite might increase, which leads directly to an increase in the thermal conductivity of materials. On the contrary, if some problems appear with temperature in a fluidized bed furnace and vermiculite doesn t exfoliate properly, it might start exfoliating during the service, which is not desirable. 

Beginning from 1,000 °C, the slow transformation of vermiculite to clinoenstatite occurs, which may lead to cavities and decay of the thermal conductivity properties. That is why although the melting point of vermiculite is 1,350 °C, even for fired materials, the safe service temperature should be considered as 1,000-1,100 °C. For unbred vermiculite-based materials, the question about safe service temperature is sufficiently trickier. 

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Pores do not affect the values of linear coefficients of thermal expansion if the continuous media are solid particles. If the material consists of particles that are not bonded together and the continuous media are pores (as in vermiculite-based heat insulation materials), linear coefficients of thermal expansion depend on the structure of pores, the dimension of the particles, and so forth. 

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. 

For unheated heat insulation and refractory materials, the temperature dependence of thermal expansion is essential. Figure 2.90 in Sect. 2.7 shows the temperature dependencies of linear coefficients of thermal expansion for several vermiculite-based materials. The materials cannot be recommended for use in the high-temperature devices due to high-volume increases upon heating. The excep-ti(Mis are materials 2 and 3, which have uniform temperature dependencies of linear coefficients of thermal expansion. 

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