Manufacture of water-insoluble ammonium polyphosphate

Claim 1

A process for preparing a crystalline water-insoluble ammonium polyphosphate of the general formula: H.sub.(n-m+2) (NH.sub.4).sub.m P.sub.n O.sub.(3n+2) wherein n is an integer having an average value greater than 10, m /n has an average value in the range of about 0.7 to 1.1, and the maximum value of m is equal to n+2, which process comprises feeding a melt of urea and a condensed phosphoric acid or a solid urea phosphate into the inlet of a jacketed oval-shaped tubular shell reaction zone maintained at a temperature in the range of about 285° C. to about 400° C. to provide a fuming viscous melt which adheres to surfaces of said reaction zone; venting gaseous products from said fuming viscous melt from a vent in the entry end of said reaction zone; constantly removing adherents from said surfaces to provide a powdery solid and advancing said solid to an exit end of said reaction zone at a rate consistent with formation of ammonium polyphosphate by driven paddles; and controlling release of said solid from said exit while recycling said gaseous products vented from said entry end of said reaction zone to said exit end thereof to provide a solid product having a pH in the range of about 4.5 to 6.5 as it exits said reaction zone.