Diammonium Phosphate from Phosphoric Acid and Ammonia

The process under analysis comprises three major sections: (1) Ammoniation-granulation; (2) Drying, cooling and sizing; and (3) Scrubbers.

Ammoniation-granulation. Initially, phosphoric acid, gaseous ammonia, and scrubber liquor are directed to the pre-neutralizer reactor. The main goal of the pre-neutralization is to form an ammonium phosphate slurry to be sent to the pipe reactor downstream, in order to reduce the amount of water vaporized during granulation. From the pre-neutralizer, the ammonium phosphate slurry is fed to the pipe-reactor, along with additional phosphoric acid and gaseous ammonia. The pipe-reactor is a horizontal reaction tube, mounted inside a drum – the granulator’s drum. The reaction mixture from the pipe-reactor is sprayed onto the rolling bed of a rotary ammoniator-granulator - the granulator. The slurry then reacts with additional liquid ammonia and recycled solids from sizing and crushing area.

Drying, cooling and sizing. The diammonium phosphate granules formed in the granulator are dried with moderate heat at about 85°C. At this temperature range, the wet fertilizer entering in the concurrent rotary dryer is not damaged by the hot air. The dried DAP granules are then distributed over screens to separate onsize material from the oversize and undersize material. The undersize material is recycled to the granulator. The oversize material is sent to chain mills, where it is crushed and then recycled along with the undersize material. The sized granules are subsequently cooled in a fluidized bed cooler. Finally, DAP granules are packed in bags and stored in warehouses.

Scrubbers. A scrubbing circuit comprising four scrubbers recovers the unabsorbed ammonia gases from pre-neutralizer, granulator, dryer, and cooler. Phosphoric acid solutions are used in the scrubbers. The scrubber liquor containing the recovered ammonia is recycled to the pre-neutralizer.