Silica-based sols and their production and use

A process for producing an aqueous silica-based sol containing aggregated silica-based particles which comprises:

• (a) providing a reaction vessel equiped with means for stirring;
• (b) adding to said reaction vessel:
• (i) a cationic ion exchange resin having at least part of its ion exchange capacity in hydrogen form, and
• (ii) an aqueous solution of alkali metal silicate to form an aqueous slurry;
• (c) stirring said aqueous slurry in said reaction vessel to allow particle aggregation and until the pH of the aqueous phase is in the range of from 5.0 to 11.5;
• or stirring said aqueous slurry in said reaction vessel to allow particle aggregation or microgel formation corresponding to an S-value up to 45%, and pH of the aqueous phase of at least 5.0;
• (d) adding one or more materials comprising at least one aluminum compound to the aqueous phase obtained after step (c) to form an aqueous phase having a pH of above 9.0;
• (e) separating said ion exchange resin from the aqueous phase after step (c) or after step (d).