GB778252A - Improvements in or relating to the production of di-calcium phosphate - Google Patents

Abstract

<PICT:0778252/III/1> <PICT:0778252/III/2> In the manufacture of dicalcium phosphate by attacking natural phosphate with hydrochloric acid sufficient to dissolve the phosphate values, and neutralizing the solution with calcium carbonate to precipitate dicalcium phosphate, the calcium chloride solution obtained is decomposed with ammonia and carbon dioxide and the ammonium chloride solution thus produced is decomposed into hydrochloric acid and ammonia which are reintroduced into the treatment cycle. The dicalcium phosphate is preferably precipitated in two stages with calcium carbonate the first precipitation being suitable for agricultural use and the second being substantially pure dicalcium phosphate. The resulting calcium chloride solution may then be treated with excess calcium carbonate to precipitate any remaining phosphate ions and the calcium chloride solution is then treated with ammonia and carbon dioxide for form ammonium chloride and precipitate calcium carbonate, the latter being re-cycled, and the ammonium chloride solution is then reacted with calcined magnesia to form basic magnesium chloride and evolve ammonia, which is re-cycled. The basic magnesium chloride is then calcined at a higher temperature whereby hydrochoric acid gas and water vapour are driven off leaving calcined magnesia which is re-cycled to the ammonium chloride decomposition step. Carbon dioxide is evolved in a pure form from the reactors where the calcium carbonate neutralization occurs and such carbon dioxide may be employed to carbonate the calcium chloride solution. As shown in the drawings, Figs. 1 and 2 phosphate rock is fed by a conveyer 1 to the top of a reactor 2, at the bottom of which acid enters through line 5. The reaction solution passes through a decanter 3 and then through a duct 6 to a decanter 7 and filter 94 whereby the solution overflowing from decanter 7 and that resulting from the washing of the solids on the filter 94 are together passed through line 15 to a precipitation vessel 13, and the residual solids from the filter 94 are discharged at 12. Vessel 13 is fed with the clarified phosphate solution from line 15, and through line 16 with a calcium carbonete suspension in an amount to leave excess P2O5 in the solution. The suspension thus obtained is passed through line 17 to a decanter 14 with its associated filter 20 whereby the dicalcium precipitate is recovered from the filter 20 at 25 and the remaining liquid from the decanter 14 and from the washing of the filter 20 is passed by pump 24 through line 27 to a further precipitation vessel 23 which is fed also with a calcium carbonate suspension through a line 28. In this and in a subsequent precipitation vessel 26 practically pure dicalcium phosphate is precipitated which is removed in a decanter 30 with its associated filter 32, at 35. The liquid leaving the decanter 30 and the filter 32 passes through line 42 to a further precipitation vessel 38 where the residual P2O5 is extracted by a large excess of calcium carbonate fed through line 43. The suspension then passes to a decanter 40 from the bottom of which a calcium carbonate sludge is removed which is returned to the first reaction vessel 13 by line 16. The residual solution from the decanter 40 is filtered at 41 and thereafter pumped by pump 47 through line 46 to a storage vessel 48. The calcium chloride solution from the vessel 48 is fed to two columns 49 and 50 (Fig. 2) through lines 51 and 57. The column 49 is also fed with carbon dioxide gas from line 52 and ammonia from duct 53 whereby the calcium chloride solution is converted to a calcium carbonate precipitate which is withdrawn from the bottom of the column through line 54, and an ammonium chloride solution. The carbonation column 50 (Fig. 2) operates similarly, being fed with calcium chloride solution through line 57, carbon dioxide through line 58 and ammonia through a line 56. The calcium carbonate suspension in ammonium chloride solution is withdrawn through line 59 and passed to a filter 60 where the calcium carbonate is removed at 62 and the ammonium chloride solution passed through a line 64 to a kiln 66. Kiln 66 is fed with magnesia at a temperature of about 1200 DEG F. from a hopper 68 and the ammonium chloride from line 64 is sprayed thereon whereby basic magnesium chloride is formed and withdrawn through a hopper 69 and ammonia is evolved which is withdrawn through line 73 and cyclone 72 for passage through line 74 to a rectifying column 80. The basic magnesium chloride withdrawn from kiln 66 is passed via conveyers 70 and hopper 71 to a kiln 67 operating at 1200 DEG F. to split it into hydrochloric acid gas and magnesia, the latter being recycled through hoppers 77 and 68 to the first kiln 66. The kiln 67 is fed with combustion gases from a burner 76 and the hydrochloric acid gas evolved is withdrawn through line 78 and passed to an adiabatic absorption column 79 which is fed through a line 86 with calcium chloride liquor from the storage tank 48. The hydrochloric acid solution formed is passed through a cooler 89 and recycled through a line 4 to the base of the initial reaction vessel for the phosphate rock and hydrochloric acid. Carbon dioxide gas in a pure state is evolved in vessels 13, 23, 26 and 38 being withdrawn from the ducts 90, 91, 92 and 93, such withdrawn carbon dioxide being in part supplied to the carbonating columns 49 and 50, supplemented if desired by the carbon dioxide in the effluent from the adiabatic absorption column 79 derived from the combustion gases in kiln 67.