GB773549A - Improvements in the production of very finely divided solid substances with a low bulk density - Google Patents

Abstract

Aerogels are continuously prepared by pumping a colloidal solution through a heated tube (2) at at least the critical pressure of the dispersion medium and released at the end of said tube &c. by pressure reducing means (3) to a substantially lower pressure e.g. atmospheric, separating the mixture of solid substance and vapour before condensation of the vapour. The temperature at least at the end of the tube &c. is not less than the critical temperature. The numerals relate to the drawing in which the separation of aerogel and dis <PICT:0773549/III/1> persion medium vapour takes place in a cyclone 4 which may be heated to prevent condensation and the separated materials are collected in vessels 7 and 6. A stream of inert gas such as nitrogen is led through the container 7 for aerogel. The heating may be electrical or with hot gases or by the use of baths e.g. of diphenyl or lead. Preferably two or more cyclones are used in series. In examples, (1) a 6 per cent silica sol in methyl alcohol (produced by peptizing a gel) is expanded at 270 DEG C. and 90 ats. to give particles of size 13-20 millimicrons (2) a 10 per cent aqueous methyl alcohol silica sol (produced by peptizing a hydro-gel with ammonia under pressure at 170 DEG , concentrating to 30 per cent and adding methyl alcohol) is expanded 320 DEG C. and 150 ats. to give particles of size 10-30 millimicrons (3) an aqueous 5 per cent titanium dioxide sol (obtained by pressing a gel through a nozzle at 80 ats.) is expanded at 420 DEG C. and 230 ats. (4) a 6 per cent aqueous alumina sol (obtained by peptizing a gel with formic acid) is expanded at 320 DEG C. and 150 ats. to give needle-shaped boemite rods 1-2 by 0.025-0.1 microns in size. Other suitable starting materials include sols of metal oxides and mixtures or compounds thereof with silica in water and/or organic liquids. Silica sols may be obtained by dialysis or electrodialysis of soluble silicates or treatment thereof with ion exchangers, by removal by cooling or precipitation of salts from the reaction product of waterglass and an acid, by hydrolysis of silica acid esters or by dissolving elementary silicon in ammoniacal or amine-containing liquids. Gels may be pressed under high pressure through nozzles or slits and the resulting colloid mixed if desired with substances altering the pH, or with organic acids which alone are insufficient to cause peptization. Metal oxide, e.g. alumina, sols produced by hydrolysing an alcoholate by electrolysis of salt solutions by dissolving metals in water or aqueous organic liquids, and by reaction of metal halide solutions with alkylene oxides ar ealso mentioned. Specification 649,896 and U.S.A. Specification 2,589,705 are referred to.