Patent GB-1092055-A (GB)
Title: Process for the production of phosphoric acid
IPC: C01B25/22
Filed: 1965-02-26
Abstract: <PICT:1092055/C1/1> Phosphoric acid and gypsum are prepared by acidulation of calcium phosphate rock by a process which comprises reacting the said rock with a first mixture of phosphoric acid at a concentration of 36 to 45% P2O5 and sulphuric acid at a concentration in a range producing - 1% to + 1% free sulphuric acid, forming a slurry of calcium sulphate hemihydrate crystals and removing said hemi-hydrate crystals from said phosphoric acid mixture and producing a phosphoric acid product at a concentration within a range of 40% to 50% P2O5, withdrawing the phosphoric acid from said process and mixing the hemihydrate with a second mixture of phosphoric acid at a concentration within the range of 20% to 30% P2O5 and sulphuric acid within a range of 5% to 20% to produce gypsum crystals which are separated from the acid mixture. In an exemplification of the process phosphate rock, suitably ground, is fed into the primary digester 10 where it is dispersed in a slurry of calcium sulphate hemihydrate, mono-calcium phosphate and phosphoric acid.  Sulphuric acid from line 85 is added to vacuum cooler 20 where it is mixed with phosphoric acid (20-30% P2O5) entering through line 76 from the first filter 50, and with slurry from 10.  Cooler 20 is of size to permit a residence time of 3-15 minutes.  In 20 flash evaporation occurs with substantially no loss of P2O5.  Thus cooled mixture flows via pipe 22 to a mixing tank 15. Conduit 16 to digester 10 is at a lower level than conduit 17 to digester 11 so that a smaller portion of the mixture flows to 10.  Digester 10 may operate at 90-110 DEG C. while the P2O5 concentratio is maintained at 36-45% with a slight deficiency of sulphuric acid.  Digester 11 is at 65-80 DEG C. and the sulphuric acid deficiency. is made up to obtain a concentration of from -1% to +1%.  Temperatures are controlled by varying the quantity of filtrate from filter 50, the quantity of sulphuric acid added to 20 and the proportions of slurry recirculating.  From digester 11 the mixture passes to filter 30 and product acid is withdrawn at 31. Unwashed filter cake is passed to recrystallizing tank 35 which is large enough to permit several hours retention.  The phosphoric acid concentration in 35 is 20-30% with sulphuric acid concentration 5-15%, the temperature being 72-85 DEG C. The hemi-hydrate dissolves and is recrystallized as dihydrate.  The mixture is recirculated via line 71 to vacuum cooler 45 and mixed with filtrate from filter 51. The mixture passes from 45 via line 47 to tank 39 whence the bulk is recirculated to tank 36 maintained at 60 DEG C. where any remaining insoluble phosphate is reacted and crystallization to gypsum occurs. The gypsum slurry passes to tilting pass filter 50-53 and leaves 53 as product.  Filter 30 may be replaced by a classification separator the smaller passed out with the product acid.  The first digestion stage may be carried out independently of temperature control at up to the boiling point, in closed tanks, the effluent vapours being condensed and scrubbed, with fluorine recovery if nesessary.