GB1348705A - Process for making concentrated silica sols - Google Patents

Abstract

1348705 Silca sols E I DU PONT DE NEMOURS & CO 6 July 1971 [14 July 1970] 31585/71 Heading C1A Concentrated silica sols are prepared by a process in which water and a soluble sodium salt are removed from a starting aqueous silica sol. having a pH from 9 to 11, the starting sol containing from 1 to 40% by weight of silica particles having an average diameter of from 5 to 50 millimicrons and the sodium salt in concentration at most 0À25 N, the process comprises adjusting the sodium salt concentration to from N=0À005 to N=0À25-0À0065 C, where N is the normality of the sodium salt in the sol and C is the concentration of silica in grams per 100 millilitres of the solid then/before and/or while concentrating the sol by filtering an aqueous solution of the sodium salt from the sol using a microporous membrane having pores of smaller diameter than the silica particles until the desired concentration of silica in the sol is reached. Preferably the sodium salt concentration is adjusted by filtering an aqueous solution of the sodium salt from the starting sol using the microporous membrane and adding water to the sol to maintain the volume substantially constant. The sodium salt may be replaced at least in part by a potassium, lithium or ammonium salt, a solution containing potassium, lithium or ammonium ions being added during the filtration and N representing the combined normality of the sodium and replacement salts.