Patent GB-516470-A (GB)
Title: Process for producing salts
IPC: C01D5/14
Filed: 1938-06-20
Abstract: 516,470. Alkali salts ; precipitating. RAND ALL, M: June 20, 1938, No. 18269. Convention date; June 19, 1937. [Classes 1 (i), 1 (ii) and 1 (iii)] Water soluble compounds of high purity and of a definite stateof hydration are made by adding an alkali hydroxide in liquid form and an acid in relatively concentrated fluid form, which hydroxide and acid will react to produce the desired compound and water to a relatively large volume of a substantially non aqueous water miscible medium in which the reactants are soluble but in which the resultant water soluble compound is only moderately soluble. Fluid form is defined as gaseous, liquid, or dissolved. The conditions that govern the reaction are that the water contained in the solvent for the reactants together with the water contained in the large body of medium and the water produced by the reaction must be such that the final concentration of water will not be sufficient to dissolve an uneconomical quantity of the product and is also that which gives a vapour pressure between those of a mixture of the desired hydrated salt and the next higher hydrate and the mixture of the desired hydrated salt and the next lower hydrate, or anhydrous compound if a lower hydrate does not exist. If the anhydrous salt is desired the vapour pressure must be less than that from a mixture of the anhydrous salt and the lowest hydrate while if the highest hydrate is required the vapour pressure must be below that of a mixture of the highest hydrate and the saturated solution. Suitable mediums are methyl or ethyl alcohol or a mixture thereof, acetone or ether or their mixtures with ethyl alcohol or with ether carrying less than its saturation quantity of water. The reactants or their solutions in the medium may be led into a body of liquid ; the medium or the two solutions may be mixed without any separate body of medium. The reaction may be carried out at elevated temperature which is assisted by the heat of reaction and may be at increased or reduced pressure. The process may be a batch or continuous process and in the latter case the product may settle in the reaction chamber or its suspension may be led off for separate settlement. In order to keep the water concentration within the required limits a part of the medium may be led off and replaced by fresh medium of lower water content. The part of the medium led off, if the reaction proceeds at high temperature, may be passed through a heat exchanger or led direct together with any vapours produced into a rectifying column where it is regenerated for re-use. The. precipitated product may be removed through a salt trap. Specific compounds produced are potassium sulphite and sodium metabisulphite from sulphur dioxide and alcoholic solutions of potassium hydroxide and sodium hydroxide respectively. The sulphur dioxide is in slight excess in the former and insufficiency in the latter reactions. The concentration of alcohol is 66.3 over proof. Other compounds produced are potassium cyanide from caustic potash and hydrogen cyanide gas, potassium selenite from caustic potash and alcoholic selenium dioxide, and sodium mono-hydrogen phosphite from caustic soda and alcoholic phosphorous acid. The concentration of the alkali hydroxide is preferably kept high to inhibit the solution of other alkali salts which may be present as impurities. The Specification as open to inspection under Sect. 91 comprises also a process which is not restricted to alkali hydroxides in liquid form but includes the reaction of a base and an acid in relatively concentrated form. This subjectmatter does not appear in the Specification as accepted.