The raw material for the wet production of silica is alkali silicate, preferably sodium silicate, from which amorphous silica is precipitated by the addition of an acid. The silica suspension is conveyed to filter presses in which the salt, formed as a result of the precipitation, is washed out and as much water as possible removed. The filter cake still contains a considerable amount of water which has to be evaporated at the drying stage. The method employed in the drying depends on the targeted properties of the silica.
Afterwards the silica may be ground, classified and / or granulated. After having been filtered, washed and dried, the silica contains 86 – 88 percent of SiO2 and 10 – 12 percent of water which is either bound in the molecular structure or physically adsorbed 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 a solution of metal salts which creates a very weakly soluble precipitate.
The variation of production parameters such as precipitation temperature, pH-value, electrolyte concentration and the precipitation time in the different precipitation techniques, allows the production of silica with various surface characteristics.