Process for the manufacture of finely dispersed sparingly soluble salts of oxyacids of phosphorus

In a process for the manufacture of a finely dispersed, sparingly soluble divalent metal salt of an oxyacid of phosphorus, having a maximum particle size of 20 .mu.m, by reacting a compound of a divalent metal with the oxyacid of phosphorus or an alkali metal or ammonium salt thereof, the improvement which comprises reacting an at least 5% by weight aqueous solution or suspension of a compound of a divalent metal selected from the group consisting of Mg, Ca, Sr, Ba, Mn, Zn, Cu, Cd, Pb, Sn, Co and Ni, while maintaining a pH range of between 3 and 9 and while stirring, with an at least 2 mol% solution of at least one trivalent metal selected from the group consisting of Al, Fe

• and Cr in the oxyacid of phosphorus or with an at least 2 mol% solution of at least one trivalent metal selected from the group consisting of Al, Fe and Cr in the oxyacid of phosphorus and an aqueous solution of an alkali metal or ammonium salt of the oxyacid of phosphorus, the compound of the divalent metal and that of the trivalent metal being used in the reaction mixture in a molar ratio of from 9:1 to 50:1, with the resultant formation of the finely dispersed divalent metal salt of the oxyacid of phosphorus, at least 90% of the particles of which salt have a size between 0.05 and 7 .mu.m, and separating and drying the precipitated salt.