GB649897A - Improvements in or relating to the preparation of silica sols - Google Patents

Abstract

Stable silica aquasols of high concentration are obtained by first preparing a silica aquagel, having a pH of 1.5 to 7, from a water-soluble silicate and an acid, and then allowing the gel to synerize, crushing it into lumps, washing it with water to remove the electrolytes formed, covering it with an aqueous solution containing 0.03 to 0.5 per cent of a substance which ionizes or hydrolyses in water to produce hydroxyl ions, and heating the treated gel at at least 80 DEG C. without evaporation of water until substantially all the gel is converted to a sol. Specified aqueous solutions of a substance giving rise to hydroxyl ions are those of caustic soda, caustic potash, alkaline earth metal hydroxides and sodium and potassium carbonates and phosphates. Heating of the treated gel is effected in an autoclave or other closed vessel. The aquagel is prepared, for example, from sodium or potassium silicate and sulphuric or other mineral acid or acetic or tartaric acids. The aquasols prepared as above may be converted into organo-aquasols by the addition of a water-miscible organic liquid, e.g. acetone, methyl, ethyl and propyl alcohols, ethylene glycol mono-ethyl ether, ethylene glycol mono-methyl ether mono-acetate, and methyl and ethyl carbitol, and part or all of the water may then be removed by distillation, if the organic liquid has a boiling point higher than that of water. In the case when an organic liquid, e.g. ethyl alcohol, having a boiling point lower than that of water, is employed, another organic liquid, e.g. benzene, is added in an amount sufficient to permit removal of the water in the form of a ternary constant boiling mixture. A silica organosol in a water-immiscible organic liquid may be obtained by the addition of a water-immiscible organic liquid to the organosols prepared as above and the water-miscible organic liquid then removed by distillation.