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

The properties and transformation of ionic solutes in molten NaF–AlF3–Al2O3 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–AlF3–Al2O3 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–AlF3 salts with saturated alumina at 1300 K. Various models were built that contained Na3AlF6, Na2AlF5, NaAlF4, NaF, and various combinations of oxyfluoro-aluminates, such as Na2Al2OF6, NaAlOF2, Na2Al2O2F4, Na3Al3O3F6, Na6Al2OF10, Na4Al2O2F6, Na2Al3O4F3, Na2AlOF3, and Na3Al3O4F4. 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–AlF3–Al2O3 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–AlF3–Al2O3 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–AlF3–Al2O3 salts.