GB484037A - Method of producing dicalcium phosphate and calcium nitrate solution - Google Patents

Abstract

In the production of dicalcium phosphate and calcium nitrate solution by dissolving phosphate rock in nitric acid and neutralizing the solution by lime or limestone, the addition of the neutralizing agent is regulated so that the dissolved iron and aluminium are to an essential part precipitated from the solution before precipitation of the dicalcium phosphate occurs, this latter being effected by continued addition of the neutralizing agent. Easily filterable crystals of dicalcium phosphate are thereby obtained. Unless a pure product is required, the iron and aluminium precipitate need not be removed prior to the precipitation of the dicalcium phosphate. The solution to be neutralized should have a free P205 content not exceeding 8 grms. per 100 cc., and preferably below 7 grms. per 100 cc. solution, the concentration being adjusted by adding calcium nitrate solution and/or water at or after the dissolving of the rock. The first stage of the neutralization should be effected by supplying the acid solution and lime or limestone simultaneously to a reaction vessel. After the first stage, the free P2O5 content should be at least 1 grm. per 100 cc. Small portions of lime or limestone are then supplied to batches of the partly neutralized solution until the remaining impurities are precipitated, whereupon the addition of the neutralizing agent may be accelerated to precipitate dicalcium phosphate. The ratio of H2SiF6 to Al2O3 should be from 2 : 1 to 3 : 1. If the ratio is too low, a fluor compound is added on or after dissolving the rock; if too high, a complex salt containing aluminium and fluorine is precipitated from the solution according to the method disclosed in Specification 467,843. Reaction is conveniently effected at 30-35 DEG C., a temperature up to 40 DEG C. being however permissible at the beginning. The limestone used is preferably mixed with calcium nitrate solution. The last precipitation may be made with lime or ammonia if the limestone is not sufficiently finely ground. Four examples are given, one demonstrating that small needle-shaped badly-separating crystals of dicalcium phosphate are obtained when the first neutralizing stage is carried so far that the solution then contains less than 1 grm. free P2O5 per 100 cc. solution.