Process for the production and concentration of silica sols

A continuous process for the production of aqueous, alkaline silica sols having a water content of 50 to 95% by weight, based on the weight of the sol, and containing nonaggregated, spherical SiO.sub.2 particles, comprising:

• A) producing an acidic fresh sol as a starting material, which contains 4 to 8% by weight of SiO.sub.2 in the form of particles having an average diameter of 2 to 4 nm at a pH value of 2 to 4, by contacting an alkali metal silicate solution with a cation exchange resin in the H-form, and
• B1) adding said acidic fresh sol to a starting medium consisting of an aqueous, alkaline, colloidal silica sol solution having a pH value of .gtoreq.8, a SiO.sub.2 content of 2 to 14% by weight and an average particle size of 4 to 16 nm, and
• B2) simultaneously adding alkaline agents to said starting medium at temperatures of .gtoreq.60.degree. C., whereby the quantity of alkaline agents added are such that the pH value does not fall below 8 during the whole process and whereby
• the addition of acidic fresh sol and alkaline agents to the starting medium being continued until the particles have an average diameter of .gtoreq.4 nm, and
• c) then concentrating the aqueous, alkaline silica sol produced in B1) and B2),
• wherein
• the process of adding acidic fresh sol and alkaline agents to the starting medium under B1) and B2) is carried out continuously in a multistage reaction cascade so that stationary-operating states are established in the reactors of the multistage reaction cascade, and the first reactor of the cascade contains the starting medium, which is continuously reproduced in the course of the continuous process, and each subsequent reactor in the cascade is supplied with overflowing material from the preceding reactor so that the starting medium is formed in the second and the subsequent reactors of the cascade, which starting medium is continuously reproduced in the course of the continuous process, and fresh sol and alkaline agents can optionally be added to one or more of the reactors in said reactor cascade, and
• the product obtained from the reactor cascade is concentrated by ultrafiltration, for which an alkaline, stable membrane having a cutoff value of lower than 30,000 daltons is used.