Wet process for phosphoric acid manufacture

Phosphoric acid, or alkali metal, or ammonium, phosphate is prepared by forming an aqueous solution of calcium phosphate, mixing a precipitant with the solution to form a supersaturated solution of a calcium compound, crystallizing the calcium compound in a suspension thereof, controlling the temperature of the suspension by withdrawing a portion of the mother liquor of the suspension, cooling, and returning to the suspension, and separating the calcium compound from the suspension to leave a solution of phosphoric acid, or phosphate. The aqueous solution of the calcium phosphate may be obtained by digesting rock phosphate with phosphoric acid, or nitric acid. Precipitants may be H2SO4, or alkali metal, e.g. Na or K, or ammonium, sulphates. Ground rock phosphate is digested in 3 at 75-90 DEG C. with excess H3PO4 with stirring. The resulting solution is added, through 12, to a portion of mother liquor, from the suspension in crystallizer 15, circulating through 7, 8 by pump 10. H2SO4 of at least 30% strength is added at 9 so that the solution in 7, 8 is supersaturated with CaSO4, e.g. to the extent of 0,5-5 grms./l. A portion of mother liquor is withdrawn by pump 14 to vacuum evaporator 19 to effect cooling and concentration, the heat of vapourization of the water removed being equivalent to the heat developed in the mother liquor in 7 by addition of H2SO4 and aqueous solution thereto. The liquor cooled 2-5 DEG C. is returned at 20 to the supersaturated solution for return to the suspension in 15, which is at 65-85 DEG C. CaSO4 crystals are withdrawn at 13 and mother liquor separated on filter 17 is passed to 18, whence H3PO4 is recovered as product, and is returned to digester 3.