Optimization of microwave-assisted synthesis process for water-soluble ammonium polyphosphate from urea phosphate and urea

Abstract 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.