GB967934A - Silica sols and refractory materials prepared therewith - Google Patents

Abstract

Silica sols in a dispersion medium comprising an oxygen-containing solvent miscible or partially miscible with water, and substantially no inorganic salts, are prepared by conventionally forming a silica organo-hydrosol sol containing inorganic salts, adding a quantity of said solvent such as to precipitate or flocculate said salts. The original sol may be prepared by acidifying an inorganic silicate, e.g. with hydrochloric or sulphuric acid and adding a salt of the acid used together with an organic dispersing medium for the silica to salt out the water; the organo-hydrosol may be separated by decantation and subjected to osmosis to remove water. The purified sol produced by the method of the invention may be dehydrated and/or concentrated (a) by addition of ethyl silicate, (b) by addition of SiCl4 and subsequently removing the hydrochloric acid formed by blowing in dry air or by passing through an anion exchanger in hydroxyl form, or (c) by adding an organic solvent miscible with the oxygen-containing organic solvent but which does not serve as a dispersion medium for the silica, and separating the layers which form; (c) may if desired by followed by (a) or (b). Specified oxygen-containing solvents are methyl, ethyl, n- and i-propyl, t-butyl alcohols, acetone and dioxane. Specified solvents miscible therewith are turpentine, trichlorethylene, diisobutylene and benzene. The sols may be stabilized by addition of aluminium salts, e.g. the chloride, acetate or isopropylate. The organic liquid in the original sol may be acetone, t-butyl alcohol and i-propyl alcohol. The sols may be employed as bonding agents for metals or refractory materials, e.g. to form furnace elements or catalysts. For furnace elements the powdered refractory, e.g. a mixture of zirconiferous sand and zirconia, may be incorporated with the sol and a small amount of magnesia added as a setting accelerator, to form a paste which can be poured into moulds subjected to vibration or compacting by a ram. After hardening, the product is dried to remove solvent, and produce a dense element. For elements, such as catalyst spheres, of greater porosity, powdered alumina may be mixed with the sol and magnesia added as a setting accelerator; the mixture is poured into moulds, removed therefrom after hardening, which may be after 1 minute, and calcined to give spheres having a porosity of 42%.ALSO:Silica sols composed of colloidal silica, organic solvents and water (see Division C1) may be applied alone or mixed with powdered materials, e.g. metals or metallic oxides, to form sealing or protective coatings on parts to be heated to a high temperature, by spraying, brushing or impregnating in vacuo.