GB709465A - Improvements in or relating to the production of silica hydrosols - Google Patents

Abstract

Substantially pure silica hydrosols having a high silica content are produced by a batch process which comprises making an intimate mixture of an alkali metal silicate and a quantity of cation exchange material in excess of that required to convert all the alkali metal silicate to silica hydrosol. The amount of exchange material used is preferably such that only 60-80 per cent. of its capacity is utilized. Before use the exchange material is treated with a dilute solution of a mineral acid and washed with water to a pH of 4.5-5.5. All the silicate solution may be added to the granular exchange material and the mixture stirred or agitated by blowing in a gas; alternatively the silicate solution may be added dropwise to a water slurry of the exchange material; mixing may also be effected by circulation in a vessel containing a draft tube. The mixture of silicate solution and exchange material is preferably maintained at a pH (e.g. less than 4) below that at which rapid gelation of the hydrosol occurs; for this purpose hydrochoric, sulphuric, or nitric acid may be added to the mixture as described in Specification 709,466, or the water in which the exchange material is slurried may be acidified. The hydrosol obtained may be passed through a stationary column of granular cation exchange material to remove a further amount of alkali metals ions; before this further treatment an acid (e.g. acetic, hydrochloric, nitric, sulphuric, or sulphurous) may be added to the sol to reduce the pH to 2-3. After use the exchange material is washed with water, the wash liquor being used in a subsequent process, and is then regenerated, either in the same vessel or in a separate column, by treatment with acid. Examples describe the treatment of sodium silicate solutions with "Amberlite" (Registered Trade Mark) exchange resins. Hydrosols containing up to about 10 per cent silica and having alkali metal contents below about 0.1 per cent. on the dry basis may be obtained without concentration and without deposition of silica on the exchange material. The sols may be converted to silica gel or used in the preparation of silica-alumina gel catalysts. Specification 645,703 also is referred to.