Optimization and Characterization of Polyphosphate Fertilizers by Two Different Manufacturing Processes

To explore how different reaction parameters affect the major features of short-chain ammonium polyphosphate (APP) fertilizers, a batch of manufacturing experiments were conducted under two different manufacturing processes [phosphoric acid (PA)–urea and monoammonium phosphate (MAP)–urea]. The APP features including polymerization degree, polymerization rate, solubility, and N and P recovery rates were significantly varied and influenced by the molar ratio of raw materials (P:N), reaction temperature, time, and pressure under different manufacturing conditions. In the MAP–urea process, the optimized APP products were gained under the combination condition of molar ratio = 1.6:1, T = 130 °C, and t = 45 min, while this happened in molar ratio = 1:1.7, T = 180 °C, and t = 60 min in the PA–urea process. Comprehensively, the features of APP fertilizers produced by the MAP–urea process were better than those produced by the PA–urea process. Our results provide valuable references for manufacturing high-quality short-chain APP fertilizers.