Ammonium fluoride process for defluorinating phosphoric acids and production of ammonium fluosilicate

#The field of this invention is the upgrading of phosphoric acid made by the `wet` process. Such phosphoric acid contains fluorine in amounts sufficient to render the acid unacceptable for many uses such as animal feed additives, for use in foods for human consumption or as a raw material for the preparation of phosphate compounds used in detergent formulations. Also, the presence of fluorine in `wet` process acid causes post-precipitation of complex fluorides which hamper production of `merchant` acid grades of phosphoric acid. For some end uses, the presence of fluorides in the acid is not detrimental but it is likewise not beneficial and may become a source of fluoride pollution. It has therefore been recognized that valuable fluorides may be obtained from the acid and their value exceeds the cost of extraction by many fold.

#Many different methods have been used to remove fluorine from phosphoric acid. The use of salts to precipitate insoluble fluorine containing compounds from the acid has not been overly successful since most precipitating agents also precipitate substantial quantities of phosphoric anhydride and such losses cannot be born economically.

#Fluorine containing phosphoric acid has been subjected to vacuum concentration which removes some of the contained fluorine as a mixture of hydrogen fluoride and silicon tetrafluoride. Such methods create difficult recovery problems, give rise to waste disposal and pollution problems and do not remove sufficient fluorine to greatly enhance the quality of the concentrated phosphoric acid. The commercial practice of this vacuum process on acid containing 160 pounds of fluorine per ton of phosphoric anhydride will remove less than 100 pounds of fluorine leaving about 70 pounds in the acid.

#FIG. 1, comprising pages 1 and 2 of the drawings, shows a flow sheet of a first embodiment of the present invention.

#FIG. 2, comprising pages 3 and 4 of the drawings, shows a flow sheet of a second embodiment of the present invention.

#Referring to FIG. 1 on page 1, and that part of page 1 labeled "Phosphoric Acid Concentration", wet process phosphoric acid 10 containing 28% P.sub.2 O.sub.5 and 2.66% fluorine is mixed with washed silica 11 and fed to the first stage vacuum concentrator 12. Heat is added to the concentrator by the addition of steam 13 to heat exchanger 14. The vapor stream 15 contains silicon tetrafluoride (SiF.sub.4) and water vapor. The first stage of the vacuum concentrator is operated at a temperature of about 190.degree. F. and at a vacuum of about 6 inches of mercury absolute.

#The vapors 15 next enter scrubbing tower 16 which is a spray column. The scrubbing liquor 17 is cooled to a proper temperature in heat exchanger 18 and contains either fluosilicic acid, ammonium fluosilicate and ammonium bifluoride or ammonium fluosilicate and ammonium fluoride depending on the scrubbing solution used. The temperature to which the scrubbing liquor 17 is cooled should be sufficient to permit the absorption and condensation of essentially all of the fluorine containing compounds and yet the liquor temperature should be high enough to allow a substantial amount of the water vapor to remain in the vapor state. In this way the scrubbing liquor does not become unduly dilute. The total fluorine concentration in the scrubbing liquor should be between about 3.5 and 15% by weight. Its pH is controlled in the range 2.5 to 4.2 by the addition of ammonium bifluoride or ammonium fluoride through line 91. The discharge from scrubbing tower 16 passes down a barometric leg 19 to a seal pot 20.

#A preferred method of maintaining vacuum in the first stage vacuum concentrator comprises the use of a total surface condenser 21 followed by a knock-out drum, not shown but connected to line 23, followed in turn by a two-stage stream jet ejector with intermediate condenser also not shown. The water discharged from condenser 21 through line 24 may be discarded or used a make-up water anywhere in this or adjacent processes. This water will contain but traces of fluorine which is a major advantage of the practice of the present invention since fluorine pollution is materially reduced. Line 24 is preferably connected to a barometric leg to maintain the vacuum in condenser 21.