GB1458128A - Process for removing fluorine from phosphoric acid - Google Patents

Abstract

1458128 Removing fluorine from phosphoric acid G E KIDDE 7 Feb 1974 [14 May 1973] 05743/74 Heading C1A Fluorine is removed from unconcentrated wet process phosphoric acid (i.e. containing less than 32% P 2 O 5 ) by heating a slurry of the acid and colloidal silica having a silica content of at least 1% by weight under reduced pressure and scrubbing the vapour thus produced with an aqueous solution of ammonium fluoride and/or ammonium bifluoride to give a solution containing ammonium fluosilicate and/or fluosilicic acid. The aqueous solution discharged from the scrubber is suitably neutralized with ammonia to a pH of 8À8 to 9À2 to precipitate silica and convert fluorine - containing compounds to ammonium fluoride. The resulting ammonium fluoride may then be fed to a stripper held at elevated temperature and reduced pressure to volatilize ammonia and convert the fluoride to ammonium bifluoride which may be recycled or isolated. In a preferred embodiment, wet process H 3 PO 4 containing 28 to 32% P 2 O 5 and fluorine is mixed with washed silica 11 to form a slurry containing at least 2% colloidal silica and is fed to a vacuum concentrator 12 which is operated at a temperature of 190‹ F. and a pressure of 50 to 180 mm of mercury absolute. The vapours 15 are passed to a scrubbing tower 16. A liquor 17 which contains either fluosilicic acid, ammonium fluosilicate and ammonium bifluoride or ammonium fluosilicate and ammonium fluoride and which contains 3À5 to 15% by weight fluorine is used to scrub the vapours. The liquid stream from the first stage vacuum concentrator 12 comprising partially defluorinated phosphoric acid 30 having a P 2 O 5 content in the range 36 to 41% is fed to a second stage 12A which operates in the same manner as the first stage except that the vacuum is preferably higher. The still more concentrated and defluorinated phosphoric acid stream 31 from the second stage is fed to a third identical stage wherein the P 2 O 5 content of the acid is raised to 54% and the fluorine content is reduced to below 0À5%. The scrub liquors from the three scrubbing stages 19, 19A and 19B are fed to a silica regeneration system wherein the solution is first treated with ammonia to adjust the pH to 3À5 to 5 to remove any free acid and is then passed to a second stage where further ammonia is added to give a pH of 8À8 to 9À2. The slurry is filtered 47 and the silica produced is passed to a series of countercurrent washes 51, 55, 60. The silica is dried and recycled to form the initial slurry with the phosphoric acid. A filtrate from the filtration stage 48 is polished and passed to a bifluoride stripper where it is heated at between 170‹ to 230‹ F. and a pressure of 12 to 40 inches Hg. The vapour stream 74 comprising ammonia and water is fed to a still 75 which produces an overhead vapour stream 76 containing 28% ammonia and 72% water. The liquid 84 from the stripper 71 is split into two streams 91, 92. The first stream 91 is recycled to the scrubbing stage. The remainder of the ammonium bifluoride is recovered.