The initial materials for obtaining silica by a wet process are solutions of alkali silicate, preferably sodium silicate, from which amorphous silica is precipitated by adding acid. After is has been filtered, washed and dried, the precipitated product consists of 86 - 88 percent SiO2 and 10 - 12 percent water, the latter being present both in the molecular structure and physically bound on the surface.
Metal silicates such as calcium silicate and aluminum silicate are obtained by replacing a part or the total amount of the acid with metal salts, which from relatively insoluble precipitates with waterglass.
In order to remove as much water as possible and wash out the salt resulting form the precipitation, the silica suspension has to be filtrated. The filter cake still contains considerable quantities of water, and so at the drying stage it is necessary to evaporate up to six times the amount by weight of water. For this reason, drying accounts for a considerable proportion of the production costs. Drying is carried out differently depending on the target properties of the end product. The drying step is often followed by the grinding classifying and/or granulating steps.
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