Abstract: An apparatus for reacting phosphoric acid and ammonia to form a molten reaction mixture, to which water can be added to produce an ammonium polyphosphate product, comprises:(A) fluid reactor means having a plurality of inlet means suitable for administration of reactants and positioned to provide for rapid ammoniation of the phosphoric acid with evolution of heat and aperatures to the other end providing exit means for the reaction mixture, said reactor means being adapted to the expulsion of the reaction mixture by steam generated in situ;(B) means for separating the steam from the liquid product solution and removal of said steam from the apparatus; and(C) columnar vessel means having outlet means at one end and enclosing at the other end the exit means of said fluid reactor means and a plurality of barrier means partially blocking said columnar vessel means in spaced-apart relationship and having aperatures therein for passage of reaction product therethrough and inlet means suitable for introduction of a