GB1020071A - Process for obtaining concentrated phosphoric acid - Google Patents

Abstract

Phosphonic acid containing at least 70% P2O5, part of which is in the form of polyphosphoric acids, is prepared from aqueous solutions of phosphoric acid containing 25-56% P2O5 e.g. those obtained by the action of H2SO4 on phosphate rock by a four-step process: (1). Removal of calcium sulphate by known extraction processes at 20-25 DEG C. (2) Extraction of phosphoric acid with a polar organic solvent e.g. n-butanol, tertiary amyl alcohol, cyclopentanol, cyclohexanol, dihydroxyethyl-n-hexylether (3) Addition of sufficient water to form an azeotropic mixture. (4) Distillation of the azeotrope at atmospheric or subatmospheric pressure. In an example an aqueous solution of phosphoric acid ofter removal of CaSO4 contains 32% P2O5 and less than 5% each of fluorine, sulphate ion, and calcium, aluminium and iron oxides. The solution is extracted with tertiary amyl alcohol yielding a precipitate containing iron, alu-minium phosphate and an organic extract containing 93.5-94% of the initial P2O5; water is added to the latter and the azeotropic mixture is distilled at 250-300 mm. Hg to give phosphoric acid residue containing 69.6% P2O5, of which 1.6% is polyphosphoric acid. Cerium and lanthanum are obtained in the precipitate from step (2) and may be extracted, when phosphate rocks containing these rare earths are treated.