METHOD OF PREPARING FINELY-DIVIDED SOLIDS

1286952 Oxides; finely-divided solids NATIONAL RESEARCH DEVELOPMENT CORP 7 Aug 1969 [13 Aug 1968] 38647/68 Headings C1A and C1N [Also in Division F4] A finely divided solid is made by heating, using a high-temperature and high-enthalpy gas stream, a melt containing a non-volatile inorganic oxide or a precursor thereof centrifugally distributed on the inner surface of a hollow rotating body, so as to produce a non- volatile inorganic oxide in the vapour phase and then condensing the oxide, or a species derived therefrom, by chemical and/or physical modification, in a finely divided form. A precursor is defined as a material from which the oxide is readily derived, e.g. an oxalate or nitrate. The hot gas may be a plasma jet or an electricallyaugmented chemical flame. The melt may contain an additive designed to aid volatilization, e.g. carbon or a free metal. The process may be conducted in a so-called centrifugal liquid-wall furnace. In one embodiment, a water-cooled steel tube 1 contains a rotatable core 2 of the material to be melted and vaporized. The core is heated by gas from plasma jet 3 provided with nozzles 4 for injection of further raw material if continuous operation is desired. Water-cooled quenching section 6 is provided with radially-directed tubes 8 for inlet of quenching gas. The outlet 9 of the quenching section feeds tangentially into electrostatic precipitator 10 comprising a tube 11 and electrostatically charged tungsten wave 12. The size of the particles, e.g. less than 2000 Š, is determined by the rate of gas flow and the positioning of the gas jets. The oxide may be a refractory such as alumina, zirconia, titania, chromium oxides, thoria, magnesia, silica, manganese oxide, zinc oxide or eerie oxide. Modification may be effected by introducing a reactive gas into the oxide vapour. For example, a carbon-containing gas, e.g. methane, may be fed into SiO or SiO 2 vapour to produce SiC, and NH 3 or N 2 /H 2 mixture may be fed in to form nitrides. A lower oxide may be converted to a higher oxide by reaction with Al 2 or O 2 . Further modification may be effected during quenching. By introducing H 2 O vapour (as such, or produced in situ by oxidation of H 2 derived from the plasma jet), a product with a hydroxylated surface may be obtained. Alternatively an organic vapour, e.g. methanol, may be used to impart hydrophobic properties. Hydroxylated SiO 2 may be further modified by reaction with methyl chloride or trimethyl silyl chloride. In ex