Process for making calcined alkali phosphates of high citrate solubility

Abstract

Calcined alkali phosphates of high citrate solubility are made by subjecting natural calcium phosphates including tricalcium phosphate to a thermal decomposition with an aqueous alkali hydroxide solution in a revolving furnace at 900.degree. to 1300.degree.C in the presence of silicic acid. The amounts in the starting mixture are selected to provide for one mol of P.sub.2 O.sub.5 per 1.1 to 1.5 mol of Me.sub. 2 O (Me being an alkali metal) and an amount of silicic acid sufficient to form calcium orthosilicate by combining with 1 mol of CaO derived from the initial tricalcium phosphate and with the lime which is not part of a phosphoric acid compound. The alkali hydroxide solution is introduced at a 30 to 80% concentration directly into the revolving furnace so as to cause the solution to impinge upon the mass while the same is at a temperature of at least 400.degree.C. Thus a rapid evaporation of the water, a speedy course of the decomposition reaction and a good homogenization due to the revolving movement of the furnace are accomplished. The decomposition is then completed in the calcination zone of the furnace.