Process for preparing sodium ammonium hydrogen-phosphate and ammonium chloride from wet process phosphoric acid

Claim 1

A process for producing sodium ammonium hydrogen-phosphate and ammonium chloride from wet process phosphoric acid, the process comprising the steps of: (a) introducing ammonia into a wet process phosphoric acid solution until the pH of said solution ranges from 3.5 to 6.0 to cause precipitation of impurities contained in said solution, said solution being maintained at a temperature above 60° C. upon completion of the introduction of ammonia; (b) separating at least a portion of the precipitated impurities from said solution, the resulting solution having a molar ratio of NH 3 /H 3 PO 4 of from 0.7 to 1.5; (c) adding the solution from which said impurities have been separated, to a circulating mother liquor maintained at temperatures within which sodium ammonium hydrogenphosphate is hard to precipitate; (d) introducing sodium chloride into the solution in step (a) or into the mother liquor in step (c); (e) additionally introducing ammonia into the liquor until the pH of said liquor ranges from 5.5 to 9.0, the total amounts of ammonia introduced in steps (a) and (e) and of sodium chloride introduced in step (d) being such that the molar ratios of H 3 PO 4 :NH 3 :NaCl are in the range of 1:1.6-2.5:0.2-2.8; (f) cooling of the liquor to a temperature of 30° C. to 60° C. to permit sodium ammonium hydrogenphosphate to separate as crystals; (g) removing said crystals from the liquor; (h) adding to the liquor from which said crystals have been removed, an acidic compound selected from the group consisting of phosphoric acid, acidic ammonium phosphate, hydrochloric acid, sulfuric acid, nitric acid and a mixture thereof, thereby adjusting the pH of the liquor to 5.6-6.4; (i) cooling the resulting liquor to a temperature which is lower than the temperature of step (f) and which ranges from 5° C. to 50° C. to precipitate ammonium chloride as crystals; (j) removing the ammonium chloride crystals from the liquor; and (k) recycling the resulting solution for use as a circulating mother liquor.