GB784391A - Production of silica-containing aerogels - Google Patents

Abstract

A silica or silica-alumina aerogel is prepared by heating a hydrogel, which is substantially free from metal impurities removable by washing with distilled water (considered to be less than 0.001 per cent), in a closed system to a temperature of at least 340 DEG C. and then releasing water vapour from the closed system, at a rate such that the gel structure is not injured, until substantially all of the water is released from the closed system. The temperature of heating may be as high as, e.g. 500 DEG C., but the range of 370-450 DEG C. is preferred (370 DEG C. being the critical temperature of water). The pressure is preferably greater than 10,000 p.s.i. The gel preferably occupies 50-75 per cent of the volume of the system. The silica hydrogel is prepared either (a) by gelling in situ a hydrosol substantially free from such metal impurities, or (b) by washing a hydrogel containing such impurities with distilled water, demineralized water, acidified distilled water, acidified demineralized water, or acidified natural water containing 10-300 parts per million of inorganic matter. In method (a), the hydrosol may be prepared by passing a water-soluble silicate through a bed of cation-exchange material in hydrogen form so as to produce a product having a pH of 1.5-3.0, preferably 2.0-2.5, and a silica content of 5-15 per cent; or by reacting sodium silicate with sulphuric, phosphoric or hydrochloric acid so as to produce a product having a pH of 2-4 and a silica content of 5-15 per cent, preferably 6-10 per cent cooling to below 10 DEG C., removing precipitated electrolyte by filtration or centrifugation, passing through a bed of cation-exchange resin in hydrogen form and then optionally through a bed of anion-exchange resin. Various cation-exchange resins are specified. In method (b) the pH of acid wash water may be 1.5-5 and is preferably 2-3. Acids specified are sulphuric, hydrochloric, phosphoric formic and acetic acid. Specification 649,896 and U.S.A. Specifications 2,093,454 and 2,249,767 are referred to.