Stable preparation of highly water-soluble ammonium polyphosphate by ion regulation

[Display omitted] •Highly water-soluble ammonium polyphosphate was prepared stably via a new method.•The addition of Mg2+ increased the activation energy of the polymerization reaction.•The addition of SO42− induced transformation of crystalline type-I oligomeric APP.•This method enabled resource utilization of impurity ions in wet-process phosphoric acid.•This method is easier to control, cheaper, and convenient. Water-soluble ammonium polyphosphate (APP) is an ideal phosphorus source to replace conventional phosphate fertilizers and improve phosphorus utilization. The industrial preparation of highly water-soluble APP is currently difficult owing to the associated harsh operating conditions and high raw material costs. Thus, a new strategy to stabilize the preparation of highly water-soluble APP was developed based on the addition of Mg2+ and SO42− ions to the raw materials. Product characterization demonstrated that Mg2+ inhibited the formation of water-insoluble long-chain polymers and stabilized the formation of crystalline type-I oligomeric APP, while the addition of SO42− induced the transformation of this oligomeric APP to amorphous acidic APP or polyphosphoric acid, further improving the product solubility. These effects were further investigated using reaction kinetics and density functional theory calculations. Compared with the pure phosphoric acid–urea system, the addition of Mg2+ and SO42− stabilized the average degree of polymerization of the product and simplified control of the production process. A pilot test showed that the use of this approach with wet-process phosphoric acid (WPA) as a raw material yields a product with superior solubility than the prevalent 18-58-0 (N-P2O5-K2O). In addition, the solubility of Ca2+ was increased from 2.3 g to 7.3 g/100 g APP, and the process cost was reduced by 17.7%. This study achieved the low-cost and stable preparation of highly water-soluble APP products from WPA, which could promote the industrial production of agricultural polyphosphates.