GB1436115A - Crystallization process - Google Patents

Abstract

1436115 Purifying wet process by acid ALBRIGHT & WILSON Ltd 26 April 1973 [2 Oct 1972] 44924/75 Divided out of 1436113 Heading C1A Wet process phosphoric acid is purified by subjecting it to extraction with an organic solvent, back extracting the acid with water to give an aqueous purified phosphoric acid, concentrating the aqueous acid to give an acid of H 3 PO 4 content of 84À9-92%, preferably 85- 88%, cooling the concentrated acid, e.g. to 8-12‹ C. to form crystals of H 3 PO 4 . ¢H 2 O and a mother liquor and separating the crystals from the liquor. The oganic solvent may be tributyl phosphate, butanol, isoamyl alcohol, diisopropyl ether or preferably an unsubstituted acyclic dialkyl ketone of 5 or 6 C atoms, e.g. diethyl ketone, methyl N-propyl ketone or especially methyl isobutyl ketone. The ratio of organic extractant to wet process acid is preferably in a ratio of above 0À3 : 1. The extract is preferably scrubbed prior to the back extraction, suitably with diluted mother liquor separated and recycled from the final stage. The aqueous purified acid may be concentrated by the use of vacuum evaporators or by the addition of P 2 O 5 or concentrated thermal H 3 PO 4 . The extraction and/or back extraction are preferably done in more than one stage. The purified phosphoric acid is suitable for use in foodstuffs and contains less than 100 p.p.m. metals. The stream of acid from the extraction which contains phosphate is suitable for use in fertilizer production.