GB742616A - Process for the production of alkali metal, alkaline earth metal, and metal phosphates from elementary phosphorus - Google Patents

Abstract

Condensed alkali, alkaline earth or other metal phosphates are produced by reacting solid oxides, hydroxides, salts of volatile acids or phosphates (the metal/phosphorus ratio in the latter being greater than that desired in the reaction product), or fused hydrates of said salts, at 200 -1000 DEG C., but under conditions such as substantially to prevent fusion of the product, with hot phosphorus pentoxide vapour produced by combustion of phosphorus, the reactants being present in such proportions that the end product contains more than one equivalent of metal per equivalent of phosphorus. Specified solid reactants are calcium hydroxide, aluminium hydroxide (in an example), sodium and calcium carbonate, tri- and di-alkali metal orthophosphates and tetralkali metal pyrophosphates. Phosphatic rock may also be used. The temperature conditions may be maintained by external cooling with water or air, by introducing additional air or steam into the reaction zone or by introducing the above mentioned solid reactants in additional quantities or as solutions or as melts containing water of crystallization. The temperature preferably is above 350 DEG C. The reactants are preferably introduced in finely divided form, e.g. a major portion having a particle size of less than 200m , preferably 50m . The reaction may be carried out by the use of a rotating tube, the solids being passed in co- or counter-current to the phosphorus pentoxide vapour. A stationary tube having provision for passing the reaction mass through the reaction chamber and for stirring or whirling it as it advances may alternatively be used. In another embodiment a hot solution or melt of a hydroxide or salt containing water of crystallization is sprayed into the upper part of a tower into the bottom of which extends a phosphorus flame, the calcined material first produced reacting with the phosphorus pentoxide vapour. The non-absorbed phosphorus pentoxide may be passed to dust-removing or absorption plant: it may, e.g. be absorbed with dilute phosphoric acid, or sodium carbonate or hydroxide solution. If an orthophosphate solution is thus produced, it may be used as a coolant by spraying into the reaction chamber. If desired the product of any of these methods may be heated below the melting point a second time, if desired with fresh starting material, with a second phosphorus burner or another source of heat. If it is desired to produce a neutral product for use as a fertilizer when the starting material is phosphate rock, the product may be subsequently heated, or allowed to cool only slowly. In the examples, rotary tubes were used and steam introduced as the cooling medium. The reactants were calcined sodium carbonate, disodium orthophosphate, crude phosphate rock and aluminium hydroxide. In the latter case, unabsorbed P2O5 was contacted with further quantities of aluminium hydroxide in a second rotary tube. In some examples the walls of the tube are protected with a layer of tetra-sodium pyrophosphate obtained by heating a disodium orthophosphate melt therein.