GB418788A - Improved process of producing a soluble phosphate fertilizer - Google Patents

Abstract

Phosphates are rendered soluble by heating them in the presence of silica and steam or water vapour to temperatures above that at which b -tricalcium phosphate is converted in a -tricalcium phosphate, say to 1300--1600 DEG C. It is stated that when the apatite structure of the phosphate is destroyed the tricalcium phosphate may be present in either the a -modification which is readily soluble, or in the b -modification which is more difficultly soluble, and that the b -modification passes into the a -modification at about 1180 DEG C. The process may also be carried out under reducing conditions, and hydrogen-containing gases may be present in such amount as to provide by their combustion the necessary steam. The heat treatment may take place in a shaft furnace or a rotary kiln, and be effected initially in the absence of steam. Instead of adding silica to the phosphate to be treated phosphorites rich in silica may be used. To prevent reconversion of the a - to the b -modification, the heated product is cooled rapidly, as by quenching. Examples are given showing (1) the beneficial influence during the heat treatment of the presence of silica, hydrogen, and water vapour, (2) the requisite amount of silica needed, and (3) the benefit of the quenching step.