Process for making stable sludge-free ammonium polyphosphate fertilizer solutions

Claim 1

A process of preparing a stable, sludge-free ammonium polyphosphate fertilizer solution which comprises introducing a blend of impure wet process phosphoric acid having a P 2 O 5 content of from about 58 to 68 percent by weight and a solvent extracted wet process phosphoric acid of reduced Fe 2 O 3 , Al 2 O 3 , MgO and free H 2 O content, the proportions of impure wet process phosphoric acid to solvent extracted wet process phosphoric acid providing a phosphoric acid blend containing the following materials in the defined amounts: ______________________________________ P.sub.2 O.sub.5 ≧ about 59% CaO up to about .06% SO.sub.4 up to about 3.5% F up to about 0.28% Fe.sub.2 O.sub.3 up to about 0.35% Al.sub.2 O.sub.3 up to about 0.45% MgO up to about 0.35% Free Water up to about 14.0% ______________________________________ said phosphoric acid having a S.G. at 60/60 up to about 1.7 into a mixing zone; introducing ammonia vapor into said mixing zone in an amount of from about 0.8 to 1.2 moles per mole of phosphoric acid; conveying said mixed acid blend and ammonia through a reaction zone maintained at a temperature of from 560° to 600° F for from at least 0.10 second to effect reaction between the acid bland and ammonia, the reaction product comprising a hot melt ammonia polyphosphate product; and substantially immediately quenching said hot melt to provide a solution comprising ammonium polyphosphate at least 50% of the P 2 O 5 content of which is in the non-orthophosphate form and suspended insoluble metallic phosphates and insoluble organic materials as solid finely divided impurities, separating steam and unreacted ammonia from said solution of quenched product, condensing and recycling said steam and ammonia for use in said quenching step, aging the resulting solution for about 10 to 20 days to effect precipitation and nucleation of said insolubles into filterable form and filtering the aged solution to provide said stable, sludge-free ammonium polyphosphate solution.