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.

水溶性聚磷酸铵（APP）是替代常规磷肥、提高磷利用率的理想磷源。由于相关的苛刻操作条件和高原料成本，目前高水溶性 APP 的工业制备是困难的。因此，基于向原料中添加Mg ²⁺和SO ₄ ^2-离子，开发了一种稳定制备高水溶性APP 的新策略。产品表征表明，Mg ²⁺抑制了水不溶性长链聚合物的形成并稳定了结晶 I 型低聚 APP 的形成，而添加 SO ₄ ²⁻诱导这种低聚 APP 转化为无定形酸性 APP 或多磷酸，进一步提高了产品的溶解度。使用反应动力学和密度泛函理论计算进一步研究了这些影响。与纯磷酸-尿素体系相比，Mg ²⁺和SO ₄ ^2-的加入稳定了产物的平均聚合度，简化了生产过程的控制。一项中试表明，使用这种以湿法磷酸 (WPA) 作为原料的方法可生产出比常用的 18-58-0 (NP 2 O 5 -K 2 O)_具有更高_溶解度的_产品。^{此外，Ca 2+}的溶解度APP从2.3g提高到7.3g/100g，工艺成本降低17.7%。该研究实现了以WPA为原料低成本稳定制备高水溶性APP产品，有望促进农用聚磷酸盐的工业化生产。