High temperature decomposition process for producing an alkali-containing calcined phosphate fertilizer

An alkaline high temperature decomposition process for producing a calcined alkali calcium phosphate fertilizing agent in a rotary kiln which kiln comprises an inlet for solid material and discharge outlet at its end opposite the inlet and which kiln is heated by means of a direct flame-burner situated inside the kiln in the region of its discharge outlet and wherein flue gases from the burner are passed first through a calcining zone and subsequently through a preheating zone of the kiln towards the region of the inlet, said process comprising the steps of

• (a) preparing a precursory mixture comprising a calcium phosphate;
• an alkaline agent selected from the group consisting of alkali-carbonates, alkali hydroxides and mixtures thereof;
• and silicic acid wherein when said alkaline agent is an alkali hydroxide it may be added directly into the rotary kiln prior to said preheating zone;
• (b) introducing the precursory mixture into the rotary kiln at its inlet;
• (c) progressively heating the precursory mixture up to a temperature of between about 900.degree. and 1300.degree. C. by continuously passing it through the kiln countercurrently to the flow of the flue gases from the kiln inlet through the preheating zone and subsequently through the calcining zone towards the discharge outlet;
• (d) introducing an aqueous solution of phosphoric acid directly into the preheating zone;
• (e) mixing the aqueous phosphoric acid solution in sufficiently finely divided form with the heated precursory mixture in a section of the preheating zone wherein the precursory mixture has been heated to a temperature of between about 150.degree. and 900.degree. C. which is sufficiently high to cause rapid vaporization of free and of chemically bound water in the phosphoric acid solution and at such a rate that a complete mixture is obtained wherein the ratio between the calcium phosphate, the alkaline agent and the phosphoric acid is equivalent to a ratio of 1 mole of P.sub.2 O.sub.5 per about 1.1 to 1.8 moles of alkalioxide, and SiO.sub.2 is present in the complete mixture in an amount which is sufficient to combine as calcium orthosilicate with the amount of CaO which exceeds the molar ratio 2 CaO:1 P.sub.2 O.sub.5 ;
• (f) passing the heated complete mixture into the calcining zone of the rotary kiln;
• (g) calcining the preheated mixture in the calcining zone at a temperature of from about 900.degree. to about 1300.degree. C.;
• and
• (h) recovering the calcined mixture at the discharge outlet of the rotary kiln.