GB710015A - An improved process for the production of finely divided silica - Google Patents

Abstract

<PICT:0710015/III/1> Finely divided non-gelatinous silica is produced by reacting an alkali metal silicate with a hydrogen ion donor in the presence of an indifferent electrolyte, e.g. an alkali metal salt of a strong acid, wherein a solution or slurry of the silicate and a solution of the hydrogen ion donor, which latter may contain some of the indifferent electrolyte, are introduced simultaneously but separately into a solution containing at least 1 per cent. by weight of the indifferent electrolyte which is stirred in a reaction vessel, the concentration and quantity of the silicate and acid solutions introduced being such that in the reaction vessel the sum of the concentrations of the alkali metal salt formed and of the indifferent electrolyte, including any of the latter introduced with the acid, is at all times throughout the reaction greater than 1 per cent. of the solution in the reaction vessel. The preferred concentration in the reaction vessel is 5-10 per cent. Preferred electrolytes are sodium chloride or sulphate, although alkali metal carbonates, which are not attacked in the presence of the silicate may also be used. Hydrogen donors are acids, e.g. hydrochloric or sulphuric, acid salts, e.g. sodium bisulphate, or ammonium salts, particularly of strong acids, e.g. ammonium chloride, in which latter case the ammonia formed may be collected. Preferably the electrolyte is the same salt as is formed during the reaction, so that the resulting solution, after silica separation, may be used as the electrolyte in subsequent reactions, after adjustment of concentration, if necessary. 6-25 per cent. sodium silicate solution from tank Z1 passes through pipe L1 to the bottom of vessel A, which contains the electrolyte solution, while 1/2 -5N acid from tank Z2 is fed on to centrifugal disc S, so that the acid is added at a point as far removed as possible from the silicate and in fine subdivision, while the electrolyte solution is stirred. In an alternative apparatus (not shown) for continuous operation an outlet is provided near the mid-point of vessel A, and the silica containing solution is withdrawn therethrough, at a rate equivalent to the rate of addition of the reactants to a washing vessel wherein the silica is washed with acid, before it is separated on a filter or centrifuge. The pH of the precipitating zone may be adjusted to say 8.5 by controlling the rates of addition of the reactants.