Microporous membrane process for making concentrated silica sols

Aqueous silica sols having a pH from 8 to 11 containing 1 to 40% by weight of colloidal silica and soluble salts can be purified and concentrated by filtering the soluble salt from the sol using a microporous membrane filter while maintaining the salt concentration within a range determined by the concentration of silica in the sol. In one embodiment, colloidal silica particles are grown to a larger size by starting with a sol of nuclei, and adding sodium silicate solution and acid.