Chemistry of inorganic compounds – Silicon or compound thereof – Halogen containing
Patent
1975-06-04
1977-09-06
Stern, Edward
Chemistry of inorganic compounds
Silicon or compound thereof
Halogen containing
423321R, 423326, 423462, C01B 2518, C01B 3320, C01C 100
Patent
active
040468607
ABSTRACT:
A process for removing fluorine from phosphoric acid solutions comprising combining relatively dilute (e.g. 28% P.sub.2 O.sub.5) fluorine-containing phosphoric acid with up to about 2% colloidal silica, heating said slurry to a temperature of about 190.degree. F under reduced pressure in phosphoric acid concentrators and removing silicon tetrafluoride in a vapor stream. The liquid stream is concentrated (50% to 54% P.sub.2 O.sub.5) phosphoric acid having a fluorine content of less than about 0.5%. The gaseous silicon tetrafluoride which also contains water vapor is alternatively scrubbed with a solution of ammonium bifluoride to convert said silicon tetrafluoride to fluosilicic acid and ammonium fluosilicate or with a solution of ammonium fluoride to convert said silicon tetrafluoride to ammonium fluosilicate. The solution of fluosilicic acid and ammonium fluosilicate, or ammonium fluosilicate alone if ammonium fluoride is used for scrubbing, is reacted with ammonia thereby converting the fluorine-containing salts to ammonium fluoride and regenerating the colloidal silica for recycle to the phosphoric acid concentrators. The ammonium fluoride is heated to recover all free ammonia which is also recycled and to convert the ammonium fluoride to ammonium bifluoride which may be sold thus resulting in a ground pollution abatement program which can operate at a profit.
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