Patent GB-784392-A (GB)
Title: Production of inorganic oxide aerogels
IPC: C01B33/158
Filed: 1954-11-25
Abstract: <PICT:0784392/III/1> An aerogel of an inorganic oxide is produced by continuously feeding a sol or a slurried gel of said oxide into one end of a first zone at a temperature not lower than 20 DEG  C. below the critical temperature and at at least the corresponding autogenous pressure of the liquid phase of said sol or slurried gel, continuously releasing the resulting vapour and aerogel particles from the other end of said zone into a second zone maintained at a lower pressure and a temperature at which the condensation of the vapour does not occur, and continuously separating said vapour and aerogel particles at a temperature at which condensation of said vapour on said particles does not occur.  The Specification refers in detail to silica aerogels and states that other inorganic oxides such as those referred to in U.S.A.  Specification 2,093,454, viz. oxides of aluminium, nickel, thorium, titanium, tin, manganese and chromium may also be obtained.  When produced by this method, silica aerogels having a particle size mostly below 15 microns, usually below 5 microns are obtained, which may be used as flatting agents in clear coating compositions or as thickening agents in lubricating oils, particularly for thickening mineral lubricating oils to the consistency of grease.  In the illustrated embodiment, the aerogel precursor is fed continuously from storage vessel 5 by means of pump 7 into tube 8 comprising the first zone which is heated by means of hot fluids such as a mixture of diphenyl and diphenyl ether, molten lead or sodium nitrate, steam or flue gases or (if as is preferred the critical temperature of the liquid phase of the precursor is appreciably lower than that of water) electrically or by direct flame.  The Figure shows a heat exchanger 59 and heating coil 66 for use as the fluid heating means.  In the tube 8 the sol (if used) first gels, and then vaporization of the liquid occurs and the aerogel is formed; this results in turbulent flow in tube 8.  The mixture is continuously released by means of a valve 14 fitting into a seat in the end of tube 8.  In this particular case the valve is loaded by a spring 28 and equipped with means for rotation.  The vapour and aerogel particles pass into the second zone 9, heated by means of hot fluids in jacket 11 to a temperature above the condensation temperature of the vapour, preferably 50 DEG  C. below the critical temperature, and at a lower pressure, e.g. 1-2 atmospheres absolute.  The mixture then passes via insulated pipe 31 tangentially into cyclone separator 32 heated by means of a fluid in jacket 36.  To maintain the vapour and aerogel at a suitable temperature, air or other gas may if necessary be blown through heat exchanger 34 and pipe 35 to mix with said mixture in pipe 31 or zone 9.  From the separator aerogel particles are removed by star valve 39.  Vapours may if desired (e.g. in the case when organosols are initially employed) be condensed by means of heat exchanger 46.  It is preferred to use aerogel procursors in which the liquid phase has a critical temperature of less than 500 DEG  C., preferably 100-400 DEG  C.; and to operate the first zone at not more than 500 DEG  C. and below a pressure of 10,000 lb./sq. in. absolute.  As high a rate of feed precursor as possible should be maintained.  The following silica aerogel precursors are referred to: (1) (a) anhydrous organosols in which the liquid is methanol, ethanol, propanol, butanol, ethyl acetate, lactol spirits, octane, benzene and toluene; the preferred silica content of said sol is 4 to 12 per cent by weight and they generally contain less than 1.5 per cent of electrolyte; (b) organo-aquasols in which the water-miscible organic liquid is methanol, ethanol, propanol, isopropanol, acetone, methyl ethyl ketone, diethyl ketone, beta-methoxy, -ethoxy and -butoxy ethanols; the organo-aquasol preferably contains 50 to 99, preferably 50 to 70 per cent by weight of the organic liquid, the preferred silica content is 4 to 12 per cent by weight and the electrolyte content may be 0.001 to 2 per cent; (c) acidic aquasols free from metallic cation produced (i) by passage of alkali metal silicate through a hydrogen regenerated cation exchange resin and having a pH of 1.5-3, better 2-2.5; the silica content of the sol should be between 5 and 15 per cent, generally 5 to 9 per cent; (ii) by mixing a water-soluble silicate such as sodium silicate with an acid such as sulphuric, hydrochloric or phosphoric to give a sol having a pH of 2-4 and a silica content of 5-15, preferably 6-10 per cent, cooling if necessary to 0-10 DEG  C. to precipitate the salt formed, removing the salt and removing cations in a hydrogen regenerated cation exchanger and if desired, removing anions as in an anion exchanger; the final pH should be at least 1.5; (2) (a) organo gels slurried in a liquid the same as that in the liquid phase of the gel, e.g. methanol, ethanol, propanol, isopropanol, lactol spirits, octanes, benzene, toluene, acetone, and diethyl ketone; the gels may in general be prepared by replacement of water in a hydrogel, if desired, several replacements being effected to give a final gel with the required liquid phase; the silica content of the organo gel should generally be between 8 and 20 per cent by weight, and of the slurry, between 4 and 12 per cent by weight; generally a slurry will contain 30 to 70 per cent of the gel; (b) organo-aquagels slurried in a mixture of water and a water-miscible organic liquid which is the same as that in the gel, e.g. methanol, ethanol, propanol, acetone, diethyl ketone and beta-methoxy ethanol; such slurries may be prepared by washing a hydrogel successively with water and a solution of water and the organic liquid until part of the water in the gel is replaced and then slurrying the gel in said solution; the gel should contain at least 50 per cent of the organic liquid and the electrolyte content may be up to 2 per cent by weight; the silica content of the gel may be 8 to 20 per cent by weight and of the slurry, 4 to 10 per cent by weight; generally a slurry will contain 3 to 70 per cent by weight of the gel; (c) aquagels slurried in water, which should contain less than 0.001 per cent of metallic cations prepared (i) by forming a gel from a silicate and an acid, washing and slurrying in distilled or demineralized water and preferably acidifying to not less than pH 1.5; (ii) by forming a gel as in (i), and washing and slurrying with acidified water at 1.5-3.0 and 1.5-3.5 respectively; the silica content of the gel should generally be 8 to 20 per cent by weight, and of the slurry, 5 to 10 per cent by weight; generally a slurry will contain 30 to 65 per cent by weight of the gel, and it is preferred to use the minimum water to form a pumpable slurry.  The preferred silica precursors are of the type (2) (c). All slurries must of course be pumpable.  Examples using precursors of types (1) (b), (a) and (c), and (2) (c) and (a) above are given in which the organic liquid, if present, is ethanol.  Specifications 649,896, 718,157, [Group IV (b)] and 784,391; and U.S.A. Specifications 2,386,247, 2,285,449 and 2,093,454 are referred to.