Investigation on the Ionic Composition and Spectroscopic Properties of Molten NaF–AlF3–Al2O3 Salts at 1300 K

• Published in: Metallurgical and materials transactions. B, Process metallurgy and materials processing science Process metallurgy and materials processing science, 2022-02, Vol.53 (1), p.474-484
• Main Authors: Tan, Min, Li, Tao, Cui, Henan, Zhu, Yulin, Shang, Bo, Dang, Jie
• Format: Article
• Language: English
• Subjects: Aluminates; Aluminum fluorides; Aluminum oxide; Characterization and Evaluation of Materials; Chemistry and Materials Science; Composition; Cryolite; Electrolysis; High temperature; Ions; Materials Science; Metallic Materials; Molten salts; Nanotechnology; Original Research Article; Quantum chemistry; Raman spectra; Structural Materials; Surfaces and Interfaces; Thin Films
• ISSN: 1073-5615; 1543-1916
• DOI: 10.1007/s11663-021-02384-3

The properties and transformation of ionic solutes in molten NaF–AlF 3 –Al 2 O 3 salts play important roles in the aluminum electrolysis process. Various oxyfluoro-aluminate complexes have been reported, while the structural features and ionic composition of molten NaF–AlF 3 –Al 2 O 3 salts are still debatable. In this work, thermodynamic calculations were performed to study the effect of the cryolite ratio (CR) on the composition of molten NaF–AlF 3 salts with saturated alumina at 1300 K. Various models were built that contained Na 3 AlF 6 , Na 2 AlF 5 , NaAlF 4 , NaF, and various combinations of oxyfluoro-aluminates, such as Na 2 Al 2 OF 6 , NaAlOF 2 , Na 2 Al 2 O 2 F 4 , Na 3 Al 3 O 3 F 6 , Na 6 Al 2 OF 10 , Na 4 Al 2 O 2 F 6 , Na 2 Al 3 O 4 F 3 , Na 2 AlOF 3 , and Na 3 Al 3 O 4 F 4 . Quantum chemical calculations were applied to obtain the Raman spectra of oxyfluoro-aluminates, which were utilized to simulate and parse the high-temperature Raman spectra of molten NaF–AlF 3 –Al 2 O 3 salts. Moreover, a method was proposed to predict the characteristic bands of Raman spectra of molten salts using the calculated Raman spectra of oxyfluoro-aluminates involved in various models, which agrees well with the fitting results of experimental Raman spectra of molten NaF–AlF 3 –Al 2 O 3 salts. The composition of fluoro- and oxyfluoro-aluminates in molten salts with a wide CR range was illustrated based on the thermodynamics, quantum chemical calculations, and Raman spectroscopic analysis of molten NaF–AlF 3 –Al 2 O 3 salts.