Process for production of ammonium polyphosphates

In a process for production of ammonium polyphosphate solution which comprises the following steps:

• a. providing a concentrated wet-process phosphoric acid containing 60-70% P.sub.2 O.sub.5 ;
• b. continuously reacting said concentrated acid with substantially anhydrous ammonia in an elongated tubular reactor at a temperature of 550.degree.-650.degree.F. for 5-60 seconds, sufficient ammonia being reacted with the acid to produce a reaction melt having a nitrogen to P.sub.2 O.sub.5 weight ratio between about 0.12 and 0.18, whereby the polyphosphate content of the melt is increased to 60-90% of the total P.sub.2 O.sub.5 without formation of insoluble reaction products;
• and
• c. continuously quenching the resulting reaction melt in aqueous ammonia of sufficient concentration to form a clear solution containing 10-13% nitrogen and 33-37% P.sub.2 O.sub.5 having 60-90% of the total P.sub.2 O.sub.5 in polyphosphate form, the improvement comprising:
• 1. maintaining 2-10% sulfate ion in the concentrated wet-process phosphoric acid of step (a) as a corrosion inhibitor;
• 2. preheating the concentrated wet-process phosphoric acid utilized in step (b) in a conventional steam-heated heat exchanger to a temperature of 200.degree.-290.degree.F.;
• 3. further preheating said concentrated wet-process phosphoric acid to a temperature of 320.degree.-500.degree.F. by passing said melt from said reactor to an indirect heat exchanger;
• 4. preheating the substantially anhydrous ammonia utilized in step (b) in a conventional steam-heated heat exchanger to a temperature of 200.degree.-440.degree.F.;
• 5. further preheating said ammonia to a temperature of 320.degree.-540.degree.F. by indirectly heat-exchanging the ammonia with the hot reaction melt in step (b).