GB1159466A - Silica Aquasols and Powders - Google Patents

Abstract

1,159,466. Production of silica aquasols. E. I. DU PONT DE NEMOURS & CO. 29 June, 1967, No. 30142/67. Heading C1A. An aqueous silica sol is prepared by providing a heel of the sol containing aqueous NH 4 OH, gradually introducing Si into the heel, and permitting the Si and water to react in the presence of the NH 3 to form SiO 2 , the concentration and surface area of SiO 2 in the heel and the production rate of SiO 2 in the reaction mixture being such that the SiO 2 forms on the heel particles present so that the SiO 2 particles increase in size. The surface characteristics of the silica sol can be stabilized by treating the sol with a cation exchange resin to provide a sol having a pH of 3 to 4. Further, the surface characteristics of the sol can be altered by subjecting the sol to a period of elevated temperature at a pH of 8 to 11, thereby forming on the porous sol particles a relatively dense outer layer of SiO 2 which is impervious to N 2 molecules. The products of the invention include a silica aquasol consisting of spherical silica particles having a ds of 150-500mÁ and having substantially uniformly distributed porosity or the particles being composed of a dense spherical silica core surrounded by a porous silica coating, and a dry colloidal silica powder consisting of spherical silica particles having a ds of 15-500mÁ and having substantially uniformly distributed porosity or the particles being composed of a dense spherical silica core surrounded by a porous silica coating. Si in finely divided state is used, e.g. 0À5-10Á average diameter and may be rendered active by washing with aqueous HF or by subjecting commercial lump Si to attrition in a ball mill. " ds " is the surface area average diameter or the diameter of a spherical particle which has the same specific surface area as that of the of the total collection of counted particles.