Pyrohydrolysis of β-AlF₃ and FeF₃ with and without Ammonia

In this work, the pyrohydrolysis of β-AlF₃ and FeF₃ in Ar + 10 vol % H₂O with and without 20 vol % NH₃ were investigated using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray powder diffraction. The results indicate that β-AlF₃/FeF₃ absorbs NH₃ and H₂O to form NH₄AlF₄/NH₄FeF₄ at 150–300/150–250°C, respectively. No pyrohydrolysis of β-AlF₃ occurs at 350°C and β-AlF₃ transforms to α-AlF₃ at T ≥ 400°C under NH₃ + H₂O condition. Under the same condition, Fe₂O₃ with minor Fe₃O₄ form during the pyrohydrolysis of FeF₃ between 300–400°C, and the content of Fe₃O₄ increases with the increment of temperature. While pure finer Fe₂O₃ forms quickly during the pyrohydrolysis of FeF₃ in H₂O gas between 300–400°C. Based on the above results, the optimal pyrohydrolysis of FeF₃ to produce finer Fe₂O₃ was further analyzed.