GB778810A - Process for manufacturing crystallised pure and anhydrous phosphoric acid - Google Patents

Abstract

<PICT:0778810/III/1> Pure anhydrous crystalline ortho-phosphoric acid, 98 per cent at least, is prepared by heating the surface of a thin layer of an aqueous solution of phosphoric acid to cause superficial evaporation of water until a concentrated solution containing at least 68 per cent P2O5 is obtained, cooling the concentrated solution to form crystals which are allowed to grow until their largest dimension is at least 1 mm., preferably exceeding 2 mm., and centrifuging the crystals from the mother liquor. An oxidizing agent should be added to the aqueous solution to prevent the formation of pyro- or meta-phosphoric acids and to destroy impurities, particularly an oxidizing agent of which some or all the reduction products are volatile, e.g. a chlorate, hydrogen peroxide, a permanganate, a perphosphate, or a compound obtained by anodic oxidation of the starting solution. The amount of oxidizing agent may be in excess of that required to destroy all oxidizable impurities. Aqueous acid solution from 1 passes through pairs of tanks 3, 4 in series in which it is heated by infra red emitters 7 under reduced pressure to 100 DEG C. and above 100 DEG C. respectively. Evaporated water is extracted by an intake fan through 10. The concentrated solution is cooled to 50 DEG C. in heat exchanger 15, and passes to crystallizer 16, where it is cooled to 18-20 DEG C. by circulating water jacket 17 while stirring. The stirrer rotates at 1-2 cm/sec to avoid breaking up crystals. The crystals are removed by air pressure to centrifuge 20, where mother liquor is separated.