Abstract: The freezing acidolysis solution of the nitric acid-phosphate fertilizer process has a high calcium content, which makes it difficult to produce fine phosphate and high water-soluble phosphate fertilizer products. Here, based on the potential crystallization principle of calcium sulfate in NH₄NO₃-H₃PO₄-H₂O, the deep decalcification (i.e. calcium removal) technology to achieve α-high-strength gypsum originated from freezing acidolysis-solutions has been firstly proposed and investigated. Typically, calcium can be removed from the factory-provided freezing acidolysis-solution by neutralizing it with ammonia, followed by the addition of ammonium sulfate solution. As a result, the formation of calcium sulfate in the reaction system undergoes the nucleation and growth of CaSO₄·2H₂O (DH), as well as its dissolution and crystallization into short columnar α-CaSO₄·0.5H₂O (α-HH). Remarkably, with the molar ratio of SO₄^2-/Ca²⁺ at 1.8, the degree of neutralization (NH₃/HNO₃ molar ratio) at 1.7, the reaction temperature of 94 ℃, and the reaction time of 300 min, the decalcification rate can reach 86.89%, of which the high-strength α-CaSO₄·0.5H₂O (α-HH) will be obtained. Noteworthy, the deep decalcification product meets the standards for the production of fine phosphates and highly water-soluble phosphate fertilizers. Consequently, the 2 h flexural strength of α-HH is 5.3 MPa and the dry compressive strength is 36.8 MPa, which is up to the standard of commercial α-HH.

Key words: Nitrophosphate, Ammonia neutralization, Deep decalcification, High-strength α-hemihydrate gypsum, Controlled crystallization at atmospheric pressure

摘要： The freezing acidolysis solution of the nitric acid-phosphate fertilizer process has a high calcium content, which makes it difficult to produce fine phosphate and high water-soluble phosphate fertilizer products. Here, based on the potential crystallization principle of calcium sulfate in NH₄NO₃-H₃PO₄-H₂O, the deep decalcification (i.e. calcium removal) technology to achieve α-high-strength gypsum originated from freezing acidolysis-solutions has been firstly proposed and investigated. Typically, calcium can be removed from the factory-provided freezing acidolysis-solution by neutralizing it with ammonia, followed by the addition of ammonium sulfate solution. As a result, the formation of calcium sulfate in the reaction system undergoes the nucleation and growth of CaSO₄·2H₂O (DH), as well as its dissolution and crystallization into short columnar α-CaSO₄·0.5H₂O (α-HH). Remarkably, with the molar ratio of SO₄^2-/Ca²⁺ at 1.8, the degree of neutralization (NH₃/HNO₃ molar ratio) at 1.7, the reaction temperature of 94 ℃, and the reaction time of 300 min, the decalcification rate can reach 86.89%, of which the high-strength α-CaSO₄·0.5H₂O (α-HH) will be obtained. Noteworthy, the deep decalcification product meets the standards for the production of fine phosphates and highly water-soluble phosphate fertilizers. Consequently, the 2 h flexural strength of α-HH is 5.3 MPa and the dry compressive strength is 36.8 MPa, which is up to the standard of commercial α-HH.

关键词: Nitrophosphate, Ammonia neutralization, Deep decalcification, High-strength α-hemihydrate gypsum, Controlled crystallization at atmospheric pressure