Magnesium Ammonium Phosphates, Related Salts and Their Behavior in Compound Fertilizers

Abstract Fluorine forms a previously unknown compound MgNH4HFPO4 (MAFP) when monoammonium phosphate made from wet-process phosphoric acid is treated with magnesium hydroxide above about 75°C in the presence of moisture. When treatment is conducted with a large quantity of water, fluorine forms a gelatinous material which resembles MAFP in chemical composition. Wet-process phosphoric acid ammoniated and reacted with magnesium hydroxide above about 75°C gives a very thick slurry, which almost solidifies due to the formation of gelatinous material, while the acid treated in the same way after being defluorinated gives a less viscous slurry in which most of the magnesium is present in the form of MgNH4PO4·H2O (MH). Fluorine shows less effect at lower temperature. When treated below about 60°C, most of the magnesium combines to form MgNH4PO4·6H2O (M6H) and Mg(NH4)2(HPO4)2·4H2O (M4H). M6H and M4H decompose during drying at about 100°C, losing ammonia and water of crystallization to form amorphous materials. The decomposition is promoted by the presence of potassium salts resulting in the formation of water-insoluble potassium. Another previously unknown compound, MgKHFPO4 (MKFP), which is isomorphic with MAFP has also been identified. MKFP is less soluble in water while MAFP decomposes in water to precipitate amorphous material and M6H. Solid solutions of MAFP and MKFP are often present in commercial fertilizers, and provide a small amount of water-insoluble potassium. M4H decomposes rapidly in water to precipitate M6H. M6H and MH are less soluble in pure water but are considerably soluble in water which contains acidic salts, such as monoammonium phosphate.