Process for preparing aqueous colloidal silica sols of high purity from alkali metal silicate solutions

A process for preparing an aqueous colloidal silica sol, the process comprising the following steps:

• a. Preparing an acidic aqueous alkali metal silicate solution with a pH of less than 2 by mixing a water-soluble alkali metal silicate or an aqueous alkali metal silicate solution with an acidifier, wherein the concentration of the alkali metal silicate in the acidic aqueous alkali metal silicate solution is 1 to 8% by weight (based on the SiO2 content), and wherein the weight ratio between the alkali metal silicate (based on the SiO2 content) and the acidifier in the acidic aqueous metal silicate solution is within a range from 0.4 to 40, and contacting the acidic aqueous alkali metal silicate solution with a resin containing one or more chelate-forming functional groups selected from the group consisting of phosphonic acid groups, substituted or unsubstituted amino phosphonic acid groups, sulphonic acid groups, and carboxyl groups, wherein the chelate-forming functional groups have binding affinity to divalent or higher-valency metal ions;
• b. Contacting the acidic alkali metal silicate solution from process step a) with a basic anion exchange resin of the hydroxyl type and an acidic cation exchange resin of the hydrogen type in any sequence, the anion exchange resin and the cation exchange resin being used in spatial separation or in a mixture;
• and
• c. Forming a stable colloidal silica sol by establishing a temperature, concentration and pH suitable for nucleation and particle growth in the aqueous silica solution obtained from step b).