Ammonium polyphosphate (APP) exhibits high water solubility, mobility, and slow-release properties, which can greatly improve the utilization of phosphorus by alleviating soil phosphorus fixation. However, obtaining an efficient method of preparing APP that involves low energy consumption and production cost remains a challenge in current industrial production. Herein, by using urea phosphate (UP) as the main feedstock, we established a simple one-pot method for synthesizing highly water-soluble APP and explored the regulatory mechanism of the reaction temperature, the ratio of urea phosphate to monoammonium phosphate (MAP)/potassium dihydrogen phosphate (MKP), and the reaction time on APP performance. After analyzing the products obtained from different reaction conditions, we classified the polymerization process into the following stages: chain generation and chain growth. The chain generation reaction occurs simultaneously with the chain growth reaction, and the chain growth reaction tends to dominate with further reaction time. Furthermore, the optimum processing conditions for industrial production were obtained by comparing product performance, and the resulting APP products exhibit the following characteristics: the N and P contents were 20.6% and 58.8%, respectively, the polymerization rate (Pr) was 93.6%, the degree of polymerization was 4.22, and the solubility was 263.91 g/100 g H2O. Finally, to characterize the stability of the APP liquid fertilizer, we further investigated the hydrolysis behavior of APP and obtained the apparent activation energy (E a = 93.17 kJ/mol) of APP hydrolysis and the evolution of polymerization degree distribution of APP at different pH values. Overall, our results indicate that low temperature and alkaline conditions can greatly prolong the storage time of ammonium polyphosphate-based liquid fertilizers, which provides a basis to compound highly concentrated ammonium polyphosphate-based liquid fertilizers.

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Ammonium polyphosphate (APP) is rich in nitrogen (N) and phosphorus (P), which is a raw material for the high-efficiency water-soluble fertilizer production. In this work, the water-soluble APP was directly synthesized using commercial grade-urea phosphate and urea in a microwave reactor. The effects of the molar ratio of urea to phosphate urea (UP), microwave power and reaction time on the quality of APP were also studied. Single-factor experiments indicate that with the optimal conditions: the molar ratio of 0.4, the microwave power of 720 W, and the reaction time of 9 min, the average polymerization degree of APP was 18.91, and the solubility was 6.31 g/100 g H2O. Orthogonal experiment indicates that the order of significant factors for APP production is molar ratio > reaction time > microwave power. Based on the results of the range analysis and analysis of variance, the optimized conditions were found at the molar ratio of 0.6, the microwave power of 720 W, and the reaction time of 9 min, the average polymerization degree of the APP was 21.7 and the solubility was 6.03 g/100 g H2O at 25 °C. The TGA analysis showed that the synthesized APP had a good thermal stability. Its XRD spectrum was the same as the crystalline form I.