Abstract: Impure wet-process phosphoric acid is stabilized against sludge formation by adding to a solution of said acid about 0.1 to 1% by weight of an inorganic oxidizing agent and about 0.1 to 1% by weight of silica, aging the thus treated solution concentrated to a P.sub.2 O.sub.5 content of 35% to 42% by weight for a time sufficient to form a solid phase and a substantially solid-free liquid phase of phosphoric acid improved in stability and separating the liquid and solid phases.

Abstract: Stable sludge-free ammonium polyphosphate fertilizer solution is prepared by introducing ammonia and a blend of impure wet process phosphoric acid having a P.sub.2 O.sub.5 content of 58 to 68% and solvent extracted wet process phosphoric acid of reduced Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO and free H.sub.2 O content into a reactor wherein the mixed reactants are retained for at least about 0.10 second after which the reaction product is immediately quenched and the resulting solution aged to effect precipitation and nucleation of suspended insoluble metallic phosphates and insoluble organic matter, and filtered.