Patent GB-757835-A (GB)
Title: Improvements in or relating to the production of monoammonium phosphate and other products from raw phosphate
IPC: C05B11/06
Filed: 1954-06-18
Abstract: Calcium nitrate monoammonium phosphate, and a nitrogen - and phosphoric acid - containing fertilizer are produced by dissolving raw phosphate, which may be calcined, in 53-63, preferably 58 per cent, nitric acid, and dividing the resulting solution into two parts (A) and (B).  85-95 per cent of the calcium is crystallized as calcium nitrate tetrahydrate from part (A) e.g. 80 per cent of the nitric acid-raw phophate solution, by cooling, e.g. to -10 DEG C. in two stages, and is separated, so that the calcium content of the remaining solution is at least stoichiometrically equivalent to its fluoride ion content; the remaining solution is neutralized with ammonia, preferably gaseous, when fluorine, remaining calcium, iron and aluminium are precipitated as fluorides and phosphates, and separated; monoammonium phosphate is then crystallized by cooling, e.g. to 40 DEG C., the residual solution.  The monoammonium phosphate crystals are recrystallized by dissolving in the wash water from the subsequent washing, the solution is cooled and filtered, and the crystals are washed and dried.  The second part (B), e.g. 20 per cent of the nitric acid-raw phosphate solution, is mixed with some or all the mother liquor from monoammonium phosphate crystallization with (A), and, if necessary, the precipitated fluorides and phosphates of calcium, iron, and aluminium from (A), and is processed to a nitrogen - and phosphoric acid - containing fertilizer by neutralization with ammonia, ammonium carbonate, alkali metal hydroxide, alkaline earth metal carbonate, calcium oxide, limestone, or dolomite.  The resulting mixture, preferably after evaporation, may be treated with a potassium salt, e.g. sulphate or chloride, to make a complete fertilizer.  The quantitative ratio (A) : (B) is selected so that the fertilizer obtained has the desired N : P2O5 ratio.  The nitric acid should be in a quantity which exceeds that stoichiometrically equivalent to the calcium in the raw phosphate by at least 0.15 mol N2O5 per mol P2O5.  The calcium nitrate crystals may be washed with nitric acid, which is then used for raw phosphate treatment, and then with water.  Some of the mother liquor from the monoammonium phosphate crystallization from (A) may be recycled to the cooling step for the monoammonium phosphate-containing solution.  The precipitated fluorides and phosphates of calcium, iron and aluminium may be treated with nitric or sulphuric acid, calcium fluoride and other insolubles separated, and the resulting solution mixed with (B) to reduce the fluoride content of the fertilizer.  Heat from the raw phosphate-nitric acid treatment is used to heat the solution from calcium nitrate crystallization, and heat from the ammonia neutralization is used for water evaporation.ALSO:Calcium nitrate, monoammonium phosphate, and a nitrogen- and phosphoric acid-fertilizer are produced by dissolving raw phosphate, which may be calcined, in 53-63, preferably 58 per cent, nitric acid, and dividing the resulting solution into two parts (A) and (B).  85-95 per cent of the calcium is crystallized as calcium nitrate tetrahydrate from part (A), e.g. 80 per cent of the nitric acid-raw phosphate solution, by cooling, e.g. to -10 DEG  C. in two stages, and is separated, so that the calcium content of the remaining solution is at least stoichiometrically equivalent to its fluoride ion content; the remaining solution is neutralized with ammonia, preferably gaseous, when fluorine, remaining calcium, iron, and aluminium are precipitated as fluorides and phosphates, and separated; monoammonium phosphate is then crystallized by cooling, e.g. to 40 DEG  C., the residual solution. The monoammonium phosphate crystals are recrystallized by dissolving in the wash water from the subsequent washing, the solution is cooled and filtered, and the crystals are washed and dried.  The second part (B), e.g. 20 per cent of the nitric acid-raw phosphate solution, is mixed with some or all the mother liquor from monoammonium phosphate crystallization with (A), and, if necessary, the precipitated fluorides and phosphates of calcium, iron, and aluminium from (A), and is processed to nitrogen- and phosphoric acid-containing fertilizer by neutralization with ammonia, ammonium carbonate, alkali metal hydroxide, alkaline earth metal carbonate, calcium oxide, limestone, or dolomite.  The resulting mixture, preferably after evaporation, may be treated with a potassium salt, e.g. sulphate or chloride, to make a complete fertilizer.  The quantitative ratio (A) : (B) is elected so that the fertilizer obtained has the desired N : P2O5 ratio.  The nitric acid should be in a quantity which exceeds that stoichiometrically equivalent to the calcium in the raw phosphate by at least 0.15 mol. N2O5 per mol. P2O5.  The calcium nitrate crystals may be washed with nitric acid, which is then used for raw phosphate treatment, and then with water.  Some of the mother liquor from the monoammonium phosphate crystallization from (A) may be recycled to the cooling step for the monoammonium phosphate-containing solution.  The precipitated fluorides and phosphates of calcium, iron and aluminium may be treated with nitric or sulphuric acid, calcium fluoride and other insolubles separated, and the resulting solution mixed with (B) to reduce the fluoride content of the fertilizer.  Heat from the raw phosphate-nitric acid treatment is used to heat the solution from calcium nitrate crystallization, and heat from the ammonia neutralization is used for water evaporation.