Method for preparing polymorphic phosphorus compound fertilizer

The invention discloses a method for preparing a polymorphic phosphorus compound fertilizer, and relates to the technical field of phosphorus compound fertilizer production. Low-grade phosphoric acid and wet-process phosphoric acid are used as raw materials, a segmented manufacturing process is adopted, firstly, the low-grade phosphoric acid or wet-process phosphoric acid is combined with a catalyst, ammonia gas and potassium salt in a mixing reactor to prepare an intermediate material with a relatively high polymerization degree, and the intermediate material is used as a return material of a granulator; in a tubular reactor, mixed phosphoric acid is used as a raw material, sulfuric acid, ammonia gas, a catalyst and washing water are combined to prepare ammonium phosphate slurry with a certain polymerization degree, and the ammonium phosphate slurry is used as a granulation binder and is sprayed on a returned material to prepare a compound fertilizer product containing ammonium polyphosphate, ammonium metaph