Fabrication de phosphates d'ammonium cristallins

摘要

Crystalline ammonium phosphates e.g. diammonium phosphate are produced by reacting wet process phosphoric acid with impure ammonia obtained by destructive distillation of coal e.g. in a coke oven or gas works, the reaction taking place at 30 DEG to 75 DEG C. in the presence of a polyphosphate prepared by the neutralization of a phosphoric acid having at least 75% P2O5 with e.g. potassium hydroxide or ammonia, and the N : P ratio of the liquor before crystallization being 1.5 to 2.0 : 1. Any commercial grade acid may be used, concentrated say to 50% P2O5. A mixture of acid and recycled mother liquor may be sprayed into the gas stream in an absorber or the gas stream passed into the mixture in a saturator. Gas stream A from a coke oven is fed into tar stripper 1a, passes into absorber 2 fed with acid and polyphosphate to give N : P of 1.9 : 1, and maintained supersaturated with respect to diammonium phosphate by varying the acid concentration or by adding ammoniacal liquor from 1a. Ammoniacal liquor from 1a is also distilled with lime in evaporator 3 to give ammonia gas for feedback to 2. Crystals are separated in 4 by filtration or centrifuging, and mother liquor recycled to 2. Precipitated sulphides may be removed from mother liquor prior to recycle by e.g. centrifuging in separator 5. Polyphosphate make-up may be added to 2 or recycling mother liquor. Gas stream B passes through tar stripper 1b and reacts in absorber 6 operated at N : P of 1.6: 1 and unsaturated with respect to diammonium phosphate. The reaction liquor is freed from sulphides in filter 7 and supersaturated with respect to diammonium phosphate at ambient temperature in evaporator 8 operated at 110 DEG to 120 DEG C. The supersaturated liquor is cooled to ambient temperature in chiller 9, crystals recovered in 4, mother liquor recycled to 6. Ammoniacal liquor from 1b may be added to 6 or treated as before to provide ammonia gas for 6. Gas stream C is passed to water scrubber 10, the ammoniacal liquor decanted from tarry matter, concentrated to at least 10% ammonia in dephlegmater 11 and reacted in absorber 12 operated at N : P 1.4: 1 by controlling the acid concentration and adjusting with ammoniacal liquor from 1a and 1b, concentrated in 11. The liquor is concentrated in evaporator 13 operated at 110 DEG to 120 DEG C. to provide a liquor which after adjustment of the N : P ratio by the addition of further ammonia would be supersaturated with respect to diammonium phosphate, freed from sulphides in separator 14 and cooled in chiller 15. The N : P ratio is adjusted before, during, or after cooling to 1.9: 1 by addition of ammoniacal liquor, and the adjusted liquor is fed to 4 to recover crystals, mother liquor being recycled to 12 with polyphosphate make-up.