Vermiculite, insulation material

Zonollte. [W.R. Grace/Construction Prods.] Vermiculite, insulation materials. 

Vermiculite. A micaceous mineral is roasted to cause exfoliation for use as an insulating material. 

Individuals residing or working in buildings with insulation or other building materials that may potentially contain asbestiform minerals (for example, vermiculite from the Libby, Montana, mine) are encouraged to ensure that the insulation material is solidly contained and not able to be disturbed and become airborne. If the material is to be removed, special procedures must be followed that minimize the generation of dust and specify appropriate locations for disposal. Individuals can obtain information about asbestos removal and disposal procedures from the 10 regional offices of the EPA. 

Smith et al. used polystyrene beads in sulfur to form an insulating material (7). Unfortunately, the beads are melted by the molten sulfur. It was believed that exfoliated vermiculite might serve the same purpose... 

Amosite and crocidolite make up nearly all the difference. Tremolite and anthophyllite make up a very small percentage. Tremolite is found in extremely small amounts in certain chrysotile deposits. Actinolite exposure is probably greatest from environmental sources, but has been identified in vermiculite containing, sprayed-on insulating materials which may have been certified as asbestos-free. 

Mg,Fe,AI)3(AI,Si),0, (0H),. nH20. Heating to about 300 C causes the mineral to exfoliate and expand due to the steam formation from water between the layers. Vermiculite is used in light concrete and as insulating material. 

Fig. 1.2 The records of the cold crushing strength of (a) vermiculite heat insulation materials (in the customer s specification, the CCS should be above 1 MPa) (b) carbon (30 % graphite) cathode materials (in the customer s specification, the CCS should be in the range 25-50 MPa)...

Some producers indicate in the specifications that the time of exposure at max temperature is 6 or 12 h (because the reheat change may not take place within 2 h or may take place but not through the end). Unfired refractory or heat insulation materials may sinter at service, and their dimensions may diminish. An example is unfired vermiculite materials on a sodium silicate binder. For such materials, exposure at the service temperature should be 50 h or even 100 h during testing. Usually, the value for the reheat change according to the procedure just described is like the value obtained in a dilatometer, but sometimes the values might differ a httle. 

Fig. 1.15 Temperature deformation curve for vermiculite heat insulation material. Note long temperature interval between Tj (maximum expansion) and T2 (softening point - 0.5 % deformation) and short temperature interval between T2 (0.5 % deformation) and T3 (2 % deformation)...

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. 

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. 

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. 

AlSi)40io] (0H)2 4H2O, a type of hydrated biotite mica. Vermiculite expands at heating to 650-1,000 °C up to 18-25 times. Before expansion, the density is 2.4— 2.7 g/sm after expansion, the density is 0.15-0.3 g/sm. The blowup (thermal expansion) of vermiculite is usually performed in fluidized bed furnaces. The expanded vermiculite has a low thermal conductivity and is a good heat insulation material. 

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). 

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...

The thermal conductivity of diatomaceous and vermiculite heat insulation materials is similar. Diatomaceous materials have a very fine pore structure in such materials, the radiation effect on the thermal conductivity is low. An interesting dependence of thermal conductivity vs. temperature appears in Fig. 2.86—diatomaceous bricks with density 400, 500, and 600 kg/m have different values of thermal conductivity at 200 °C, but rather similar values at the temperature of service—400-600 °C. At such temperatures, the thermal conductivity of materials... 

Fig. 2.90 Linear change at heating for different unfired vermiculite heat insulation materials...

It is possible to use various heat insulation materials for the melting and holding furnaces. Fiber materials are usually not used in the bottom, but they may be used in the walls and in the roof. Materials that may withstand relatively high pressures at relatively high temperatures (diatomaceous bricks, vermiculite slabs, calcium... 

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. 

The asbestos minerals are crystalline fibrous silicates, some of which have been exploited commercially, especially as an insulating material. These minerals comprise sheets, or chains of silicate tetrahedra in which oxygen is either bound to two silicon atoms or to one silicon atom, and possesses a negative charge. There are two major groups of asbestos mineral serpentines and amphiboles. White asbestos or chrysotile is a serpentine mineral, whereas the other forms of asbestos, crocidolite (blue asbestos), amosite (brown asbestos), anthophyllite, tremolite, and aetinolite, are amphiboles. Tremolite can also be found as a contaminant in other minerals, such as chrysotile, talc, and vermiculite. 

Normally, castables are 25 percent cements and 75 percent aggregates. The aggregate is the more chemically resistant of the two components. The highest-strength materials have 30 percent cement, but too much cement results in too much shrinkage. The standard insulating refractory, 1 2 4 LHV castable, consists of 1 volume of cement, 2 volumes of expanded clay (Haydite), and 4 volumes of vermiculite. 

Vermiculite Silicate minerals used for heat insulation, plaster, packing material, and as planting medium. 

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