Process for the production of water soluble acids having a dissociation constant of less than 10 from their sparingly soluble salts

摘要

Water-soluble inorganic acids having a dissociation constant of less than 1 X 10-2.5 are prepared from their sparingly soluble salts by treating an aqueous dispersion of such a salt with an acid-cation exchange resin, the acid groups of which have a dissociation constant greater than that of the acid to be prepared, the individual granules of the ion-exchanger being moved relatively to one another and to the dispersion and running off the acid solution so obtained. The treatment may be effected in a slowly rotating drum filter or in a column of large diameter having an air delivery device working on the air-lift pump principle and fitted into the centre of the column. The salt concentration in the dispersion may amount up to 400 gms/litre. After the conversion of the salt to the free acid and removal of the resulting acid solution, the ion-exchanger may be washed, providing a wash liquor utilizable in forming more salt suspension. A suitable ion-exchanger is a cross-linked polystyrene having nuclear sulphonic acid groups. The invention also includes the preparation in the same manner, of organic water-soluble acids having a dissociation constant below 1 X 10-2.5 and examples of this preparation are given (see Group IV (b)).ALSO:Water-soluble organic acids having a dissociation constant of less than 1 X 10-2.5 are prepared from their sparingly soluble salts by treating an aqueous dispersion of such a salt with an acid cation-exchange resin, the acid groups of which have a dissociation-constant greater than that of the acid to be produced, the individual granules of the ion-exchanger being moved relatively to one another and to the dispersion and running off the acid solution so obtained. Suitably the exchanger granules are submitted to a whirling action which may be effected by using, as a reaction chamber, either a column of large diameter having an air delivery device working on the air-lift pump principle fitted into the centre of the column or a slowly rotating drum filter. The salt concentration in the dispersion to be treated may amount up to 400 gms/litre. After the conversion the acid solution is run off and the exchanger is then washed and the washing liquid may be used in preparing aqueous salt dispersions for subsequent batches. Finally the exchanger is regenerated preferably by means of hydrochloric acid. Examples describe the isolation of tartaric and citric acids from aqueous dispersions of crude tartar and calcium citrate respectively using a cross-linked polystyrene ion-exchanger having nuclear sulphonic acid groups. The recovery of lactic acid is also mentioned.