Improvements in or relating to coated phosphoric acid anhydride

Particles of phosphoric acid anhydride having individual, substantially uniform, coatings of a slowly water-soluble complex ammonia-phosphoric acid anhydride reaction product are produced by reacting anhydrous gaseous ammonia with a mildly agitated bed of phosphoric acid anhydride particles while the bed is isolated from atmospheric moisture and maintained at a temperature above - 33 DEG C. but below the sublimation temperature of phosphoric acid anhydride. The phosphoric acid anhydride to be coated may be in the form of finely divided particles or it may be in the form of substantially opaque agglomerated spheroids as produced by the method of Specification 951,383. In the preferred embodiment either a batch or continuous method may be used, and the apparatus is the same as that used in Specification 957,844. In a continuous process solid uncoated phosphoric acid anhydride (P2O5) particles are fed continuously through line 6 (see Fig. 1) to a pre-established bed 7 of coated P2O5, the bed being maintained in the expanded state by a recirculating fluidization gas containing NH3. A make-up stream of NH3 is injected into the recirculating gases either through line 4 or 24. During the reaction the bed is cooled by vertical cooling plates 8 which are located above the horizontal gas distributing grid 12 and the fluidizing gas supplied to the windbox 13 ascends through grid 12 and the bed 7. The gases leaving the bed 7 pass through pipe 15 and any large particles in the gases are separated in a cyclone and returned to the bed 7 via pipe 16. The gases pass on via 17 to the wet scrubber where particles too small to be separated by the cyclone are removed by a liquid e.g. polyphosphoric acid and the gases then flow via pipe 23 and the blower back to wind-box 13. The coated P2O5 particles are continuously withdrawn through pipe 33 at a rate substantially equal to the feed rate. Static superatmospheric pressure is preferably used throughout the entire apparatus. In a batch method the P2O5 feed is prepared by oxidation of a stream of elemental phosphorus entering the combustion chamber through line 1 with air entering through line 2 and the resulting gaseous P2O5 is passed through pipe 3 into bed 7 where it condenses to an agglomerated form. The combustion chamber is then turned off, the bed is fluidized and ammonia is passed in as in the continuous method.