Patent GB-789747-A (GB)
Title: Diammonium phosphate fertilizer production
IPC: C05B7/00
Filed: 1955-04-19
Abstract: <PICT:0789747/III/1> A chemically stable slurry containing a major proportion of diammonium phosphate suitable for direct conversion into granules is continuously prepared by reacting with agitation at atmospheric pressure, wet process phosphoric acid with ammonia in a mol. ratio of NH3 to (PO4) up to 11 : 6 thereby forming a slurry of diammonium phosphate and monoammonium phosphate in a mol. ratio up to 5 : 1, releasing reaction heat by controllably evaporating water at a temperature up to 110 DEG  C. to establish and maintain said slurry at a weight ratio of water to solids up to 1 : 4 and thereafter cooling the slurry to a temperature above that at which it becomes too viscous to flow but below that at which thermal decomposition occurs.  The reaction is preferably performed in two steps in successive reactors by reacting anhydrous ammonia in a first reactor to provide 0.85 to 1 mol. of ammonia per mol. of phosphoric acid, thereby converting the latter into mono-ammonium phosphate, whilst removing water and reaction heat by evaporation of excess moisture. The slurry is maintained at a 100-110 DEG  C.  The slurry is then passed to a second stage reactor where more ammonia is introduced in a mol. ratio of NH3 to PO4 not exceeding 5 : 6, whereby diammonium phosphate and mono-ammonium phosphate in a mol. ratio not exceeding 5 : 1 are formed.  Heat of reaction is released by evaporation, thereby maintaining the reaction products at a temperature at or below 110 DEG  C. and yielding a slurry having a water to solids weight ratio not in excess of 1.4.  A third reaction tank is preferably provided for final cooling and for allowing the slurry to become more uniform in composition, the slurry being air-cooled to a temperature not exceeding 85 DEG  C. in this latter reaction vessel.  If the wet process phosphoric acid employed has a concentration of above 40 per cent P2O5 it may be necessary to dilute the acid fed to the first stage in order to dissipate the excess heat of reaction by evaporation. The final nitrogen concentration of the slurry may be increased by adding sulphuric acid to the first stage reaction vessel together with additional ammonia.  The invention is illustrated in the Figure where 16 indicates a first reaction vessel to which wet process phosphoric acid is metered through line 14 and ammonia gas delivered through conduit 19.  The reaction vessel is stirred and the reaction products are maintained at 100-110 DEG  C., whilst the water content of the slurry is reduced.  The slurry is then passed to a second reaction vessel 24 where further ammonia is added and the excess moisture is removed to give the water to solids ratio not in excess of 1 : 4, the temperature being maintained at or below 110 DEG C.  Reaction slurry from the tank 24 is removed through conduit 30 to a third stage reactor tank 32 which is fed with cooling air from line 36.  Slurry leaving the third reaction vessel 32 through line 38 is passed to a paddle mixer 40 together with recycled fine solids from a conveyer 44, the weight ratio of fines to slurry lying in the range of 5 : 1 to 12 : 1.  The moisture content of the slurry is 15 to 20 per cent.  Moist granules are discharged through line 46 to a rotary kiln 48 which is heated by a concurrent flow of hot gases from a burner 50. Granules leave by line 56 and are passed to a sieve 66 which delivers the product size granules through a line 72 whilst passing fines through line 74; oversize granules pass through a crusher 82, both fines and crushed oversize passing through line 44 back to the paddle mixer 40.  Dust from the drier 48 passes through line 58 to a recovery system 60 and the dust recovered is fed to line 44.  Complete fertilizers are obtained by feeding potassium chloride from a bin 86 into the recycled fines line, and in a similar manner urea and ammonium nitrate and sulphate are fed from bins 90 and 94 if required.