PREPARATION OF SILICA GELS

摘要

1284085 Silica gel NATIONAL DISTILLERS & CHEMICAL CORP 5 Aug 1969 [6 Aug 1968] 39239/69 Heading C1A A silica gel is stable at temperatures up to 1100‹ C. in a fluidized bed and has a surface area of from 200-500 m.2/gm. the majority which surface is provided by pores of a diameter from 300-600 Š. The above gel may be prepared by a process which comprises precipitating a silica hydrogel by reducing the pH of an aqueous silicate solution to provide a silica hydrogel slurry having an SiO 2 concentration of from 5 to 12% by weight and a pH from 3 to 8, the temperature during precipitation being maintained between 0‹ and 10‹ C., (b) heating the product of step (a) at a pH of from 3-8 for 1-6 hours, (c) washing the product of step (b) to remove sodium until the filtrate contains less than 20 p.p.m. by weight of the sodium salt, (d) cooling said hydrogel particles to a temperature sufficient to maintain the water in the pores in a frozen state and (e) vacuum subliming the frozen water from said gel. The pH adjustment in step (a) may be accomplished by reducing the pH of an aqueous solution of sodium silicate with an acid or an ion exchange resin. If desired, 40% of the necessary amount of pH reducing agent may be added over a period of 30-120 minutes, e.g. 1 hour, and the remainder added during a subsequent period of from 20 to 90 minutes, e.g. 45 minutes. The preferred concentration of silica in the slurry is 8-9% by weight and the preferred temperatures in steps (a) and (b) are 5‹ and 90‹ C. respectively. The preferred pH range is 5-6, and step (b) maybe conducted for amount 4 hours. The hydrogel may be washed with a solution of aluminium chloride, aluminium sulphate, ammonium chloride, ammonium nitrate, HCl or H 2 SO 4 . The gel may be comminuted to a particle size of less than 200 microns before or after step (d). The dried gel may be calcined at a temperature of 260-650‹ C. In step (e) the hydrogel may be cooled to a temperature of from -100‹ to 10‹ C.