GB391495A - Improvements in or relating to the production of mono-calcuim phosphate and di-calcium phosphate - Google Patents

Abstract

Dicalcium phosphate, and optionally monocalcium phosphate, are obtained by extracting tricalcium phosphate with sufficient nitric acid to produce a phosphoric acid concentration in the mixture such that monocalcium phosphate is capable of existing therein in the solid phase, partially neutralizing the resulting solution, which may contain solid monocalcium phosphate, by adding di- or tricalcium phosphate or both, and treating the magma thus produced with ammonia so as to precipitate successively mono- and dicalcium phosphate, the separation of the former being optional, but being effected, if at all, when the concentration of the mother liquor is just above the equilibrium point between the two solid phases, mono- and dicalcium phosphate. A preferred temperature range of 30-75 DEG C. is specified. The use of a 40 per cent or stronger nitric acid is desirable, and in such case, the magma or mother liquor is diluted before precipitation of dicalcium phosphate with water, or preferably with the calcium nitrate-ammonium nitrate or ammonium nitrate solution obtained in the process. The ammonia may be added dissolved in this diluting liquor, or be diluted with an inert gas, the mixture being forced into the air-filled space above the level of the liquid contained in an agitated closed vessel, or be passed in small bubbles through the liquor. Preferably when the ammoniation is half completed, sodium silicate may be added. The raw phosphate should be added to the acid, and when the solution is nearly saturated with monocalcium phosphate, gangue may be filtered off, and the remainder of the phosphate then added. Dicalcium phosphate separated in the last stage of the process may be returned for use in the initial partial neutralization of the acid solution; the ammonium nitrate-calcium nitrate mother liquor obtained on separation of the dicalcium phosphate may be treated with ammonia and carbon dioxide, or ammonium carbonate, bicarbonate or carbamate. The resulting ammonium nitrate solution is concentrated, and may then be admixed with the calcium carbonate produced, and granulated, for example by stirring or spraying. In the Specification as open to inspection under Sect. 91 the initial partial neutralization of the acid solution with di- or tricalcium phosphate or both is not essential.