Charge-reversed silica sol

A process for producing a charge-reversed aqueous silica sol comprising the steps of:

• (a) providing a slurry of an acidic cation-exchange solid in an aqueous liquid;
• (b) providing a starting aqueous silica sol with an alkaline pH and comprising one or more monovalent cations;
• (c) contacting the slurry of (a) and the aqueous silica sol of (b) to form a mixture;
• (d) removing the acidic cation-exchange solid from the mixture resulting from (c) to leave a decationised aqueous silica sol with an acidic pH and a reduced content of monovalent cations compared to the starting aqueous silica sol;
• and
• (e) contacting the decationised aqueous silica sol with one or more compounds comprising one or more modifying elements that is capable of adopting a +3 or +4 oxidation state to produce a charge-reversed aqueous silica sol whose silica particles comprise the one or more modifying elements on a surface of the silica particles;
• wherein the starting aqueous silica sol has an S-value of from about 10 to about 50%, and the colloidal silica particles in the starting aqueous silica sol have a surface area of at least about 500 m2 g−1.