Ammonium phosphate fertilizer monoammonium

Ammonium phosphate fertilizers are relatively impure (purity ca. 85%), due to their being prepared with nonpurified wet-process acid. Commercial monoammonium phosphate contains 11 to 13% N and 48 to 53% P2O5 (theoretically 12.2% N, 61.7% P2O5). Commercial diammonium phosphate contains 16 to 18% N and 46 to 48% P2O5 (theoretically 21.2% N, 53.7% P2O,). 

The routes by which mineral phosphates are processed into finished fertilizers are outlined in Eigure 7. World and U.S. trends in the types of products produced are shown in Eigures 8 and 9, respectively. Most notable in both instances is the large, steady increase in the importance of monoammonium and diammonium phosphates as finished phosphate fertilizers at the expense of ordinary superphosphate, and to some extent at the expense of triple superphosphate. In the United States, about 65% of the total phosphate appHed is now in the form of granular ammonium phosphates, and additional amounts of ammonium phosphates are appHed as integral parts of granulated mixtures and fluid fertilizers. 

The ammonium phosphates took over the lead in 1967. Diammonium phosphate (DAP) is made from wet process phosphoric acid of about 40% P2O5 content and ammonia. The usual finishing NH3 H3P04 mole ratio is 1.85-1.94 1. Monoammonium phosphate (MAP) is made with a final NH3iH3P04 ratio of 1 1. Current production percentages for phosphate fertilizers are DAP, 67% MAP, 26% and TSP, 7%. 

Ammonium Phosphate. Ammonium phosphates have a long history of use as a fertilizer. They are not as easily leached from the soil as ammonium nitrate, but cannot be made in high nitrogen concentrations. There are three forms of ammonium phosphate monoammonium phosphate (MAP, NH4H2P04), diammonium phosphate (DAP, [NH4]2HP04), and triammonium phosphate (TAP, [NH4]3P04). Unfortunately, triammonium phosphate is unstable and loses ammonia easily. Ammonium phosphates are produced by reacting ammonia with phosphoric acid (Reaction 3.8). 

Three granular fertilizers were made triple superphosphate, monoammonium phosphate (11-47-0), and ammonium phosphate-sulfate (16-20-0). This technology, with various modifications, was later used in numerous fertilizer plants around the world and was one of the more important granulation processes for many years. 

Although ammonium phosphate had long been known to be an effective fertilizer and small quantities had been produced in several countries from time to time, it did not become a popular fertilizer until the 1960s. Ammonium phosphates (diammonium phosphate and monoammonium phosphate) are now the leading form of phosphate fertilizer in the world. 

Ammonium phosphates, particularly DAP, are the most popular phosphate fertilizers worldwide because of their high analysis and good physical properties. The compositions of the pure salts - monoammonium phosphate (MAP) and DAP - are given in Table 12.4. 

Several systems have been patented for removal of the unreacted ammonia from the crude product gas. The early Andrussow processes used dilute sulfuric acid ( 6%) as absorbent and produced ammonium sulfate as a by-product. However, the high cost of processing this material has made it increasingly difficult to compete in the already oversupplied fertilizer market. Unless there are special circumstances, present economics and environmental considerations favor the use of a recycle system to recover the unconverted amonia. The typical ammonia recovery systems use monoammonium phosphate at 80°C as the absorbent to react with the unconverted ammonia in the crude product gas to form diammonium phosphate. The diammonium phosphate solution then undergoes a thermal reversal process to liberate ammonia for recycle. The regenerated monoammonium phosphate solution is reused in the absorption system. Polyhydroxyboric acid complex is an alternative absorbent for ammonia recovery that works on the same principles. 

---