Preparation of sodium tripolyphosphate

In a process for the preparation of sodium tripolyphosphate wherein an aqueous solution of phosphoric acid obtained by the wet method which contains magnesium compounds dissolved therein is neutralized by addition of an aqueous solution of sodium hydroxide, the precipitated impurities are separated from the sodium phosphate solution thus obtained, the sodium phosphate solution is evaporated to dryness and the evaporation residue is calcined, the improvement which comprises neutralizing said aqueous solution of phosphoric acid at a temperature not exceeding 85.degree. C., maturing the neutralization product at a temperature of from 70.degree. to 85.degree. C. for a period of at least one hour before separating said precipitated impurities, submitting the sodium phosphate solution, having a sodium phosphate content of at least 40% by weight, prior to the evaporation to dryness, to a heat treatment at a temperature of from 100.degree. to 200.degree. C., at boiling point or under, and at atmospheric or superatmospheric pressure, for a period such as to induce substantially complete precipitation of the magnesium compounds dissolved in said sodium phosphate solution, the precipitate resulting from this heat treatment being separated from the heat treated solution.