GB812083A - Improvements in or relating to crystalline tripolyphosphate and method of preparing the same - Google Patents

Abstract

Crystalline mixed alkali metal tripolyphosphates having alkali metals M1, M2 and phosphorus present in the molar ratio 3-3.2M1 : 2-1.8M2 : 3P are obtained by reacting a solution of an alkali metal orthophosphate with a compound (other than a phosphate) of another alkali metal, the proportions being such as to provide the above molar ratios, evaporating if desired and heating the mixture at 500-1200 DEG F. If desired phosphoric acid may also be present in the reactants to obtain the desired ratios. Specified alkali metals are sodium, potassium and lithium, orthophosphates referred to are the mono-alkali metal salts, and other compounds mentioned are sodium carbonate, hydroxide, formate, acetate and oxalate. In the preferred procedure, an aqueous solution of impure phosphoric acid (obtained, e.g. from phosphate rock with sulphuric acid and containing iron and aluminium in solution) is reacted with potassium chloride (e.g. sylvite) or sulphate, the mixture fired at 1400-2000 DEG F., quenched in water and autoclaved at 165-225 DEG C. to produce mono-potassium orthophosphate from which impurities may be separated by filtration or centrifuging. Three mols. sodium carbonate are then added per mol. of the orthophosphate and the mixture evaporated to 75 per cent solids. The resulting paste which has the components present in amounts corresponding to the formula K3Na2P3O10 ma be heated in a rotary kiln at 1000 DEG F. Alternatively the aqueous solution may be spraydried and then heated at 1000 DEG F. or sprayed directly into a rotary furnace.