Dissolution of Solid FePO₄ in Molten CaCl₂

FePO₄ is a common source of P in secondary resources and can be found in sewage sludge, phosphating slag, and spent LiFePO₄ batteries. Effective circulation of P in FePO₄-bearing solid waste is essential for P sustainability. As a fundamental study to develop a technology for recycling P as P₄ in FePO₄-bearing solid wastes through molten salt electrolysis, this work focuses on investigating the dissolution behavior of solid FePO₄ in molten CaCl₂. Excessive solid FePO₄ was introduced into molten CaCl₂ from 1073 K to 1253 K, and the melt composition was monitored for 156 hours. The result indicates the formation of Ca₂PO₄Cl, FeCl₂, and Cl₂. Ca₂PO₄Cl further dissolves into the melt as soluble ions, while FeCl₂ and Cl₂ escape as volatiles. The dissolution of FePO₄ in molten CaCl₂ is significantly faster compared to other metal phosphates. Concentration analysis reveals an initial rapid increase followed by a subsequent decrease in both P and total Fe concentrations in the melt. At 1123 K, the Fe content reaches a peak value of 4.4 mass pct in 1 hour, indicating a complete reaction between solid FePO₄ and molten CaCl₂. The P content also reaches a peak of 2.1 mass pct in 1 hour, exceeding the solubility of P in molten CaCl₂. This can be attributed to the suspension of fine solid Ca₂PO₄Cl particles in the melt. The dissolution can be described as explosive at the beginning due to the rapid formation of high-pressure gas bubbles (Cl₂ and FeCl₂). Adding Fe(II) accelerates the dissolution of PO₄^3-, although the solubility of P is independent of Fe ions. These findings provide a foundation for developing practical technologies that enable the isolation of P₄ from FePO₄-bearing solid wastes.