Image Number 8 for United States Patent #6241796.
A process for the preparation of granular fertilizer grade DAP (a product composed of ammonium phosphates, principally diammonium phosphate, resulting from the ammoniation of phosphoric acid, as defined in Official Publication No. 52 of the Association of American Plant Food Officials, dated 1999) comprising partially preneutralizing orthophosphoric acid with ammonia, completing the ammoniation of the orthophosphoric acid with ammonia in a rotary ammoniator-granulator to provide granular DAP, sizing the granular DAP to provide the granular DAP product, reducing the particle size of the oversized granular DAP, and recycling the undersized granular DAP and the sized-reduced oversized granular DAP to the ammoniator-granulator. The ammoniacal nitrogen concentration of the granular DAP recycle, hence the granular DAP product, is enhanced by subjecting the finely-divided recycled granular DAP to anhydrous gaseous ammonia which is at a super atmospheric pressure and which is at a temperature sufficient to maintain said anhydrous ammonia in the gaseous state. The increase in the ammoniacal nitrogen concentration is a function of the absolute ammonia pressure, the initial moisture content of the granular DAP recycle, and the contact time of the ammonium with the granular DAP recycle.