GB1071060A - Continuous method for making silica sols - Google Patents

Abstract

A continuous method for producing colloidal silica sols comprises repeating in a continuous manner the steps of: (a) contacting an aqueous solution of alkali metal silicate containing at least 5% by weight solids expressed as SiO2 with a resinous body of an acid-regenerable cation-exchange material in the hydrogen form by flowing the solution through the resin; (b) collecting the resultant acid silica sol effluent; (c) terminating the flow of alkali metal silcate solution when the capacity of the resin is exhausted as indicated by a first drop in conductivity of the effluent; (d) backwashing the resin with water to remove entrapped unreacted alkali metal silicate; (e) regenerating the resin with a mineral acid to an original cation-exchange capacity efficiency of at least 60%; and (f) rinsing the resin with water which is free of ions causing hardness in order to free the resin of residual acid. It is preferred that the silicate solution contains from 5 to 8% by weight solids expressed as SiO2; that the flow of silicate solution is terminated when the conductivity of the effluent first measures between 150 and 400 mmhos; that backwashing is for a time sufficient to reduce the alkalinity of the backwash effluent to below 25 grains per gallon when measured as CaCO3; that the acid regenerant is dilute sulphuric acid; that rinsing is for a time sufficient to reduce the acidity of the rinse effluent to below 20 grains per gallon when measured as CaCO3; and that, in the rinsing, which may be in two stages, the rate of flow of water is sufficient firstly to displace the acid from the resin and latterly to scour the resin. The silica sol may be concentrated by known methods, for example those disclosed in U.S.A. Specifications 2,574,902 and 2,929,790.