Factors influencing the time of gelation of acid ‘modified’ sodium silicate

Abstract

In one method of preparing ‘modified’ sodium silicate, a coagulant of increasing importance in water treatment processes, a solution of sodium silicate is treated with an acid, allowed to age and finally diluted according to a recommended procedure. The changes which occur during the ageing time are retarded by the final dilution. If the ageing time is much too long, the product gels; if too short, the diluted sol is a less efficient coagulant than one aged for the correct time. The time of gelation is therefore of considerable importance in the technology of making the reagent.The literature contains scattered references to the effect of such variables as concentration on the gel time of ‘modified’ sodium silicate, but few systematic measurements appear yet to have been reported on the influence of other factors. The results of such a study are given in this report. The data are set out in graphical form from which most of the facts required in practice can be obtained. The conclusions of especial interest are: (a) silica sols exhibit the greatest instability when the pH value is about 7.5 whatever acid is used for neutralization; (b) the presence of neutral salts hastens gelation; (c) the effect of using sodium silicate containing more Na₂O (i.e. of a lower grade) and neutralizing to the same residual alkalinity can be explained quantitatively in terms of the extra sodium salt introduced by such a method of preparation.