| Abstract
| - Investigations of the fluoroaluminate complex formation in cryolitic melts have been extended to the eutectic NaF−KF and LiF−NaF−KF solvents. Methods used were Raman and NMR spectroscopy. In order to fully understand the reactions in these solutions, the melting behavior needed to be taken into account. For this purpose, thermal analyses were performed on selected mixtures. The results indicated that AlF63- was the major species in solid and partly molten solutions, whereas AlF52- forms upon total melting.
- The dissolution and complex formation of fluoroaluminates in two eutectic alkalifluoride mixtures, NaF−KF (FNAK)and LiF−NaF−KF (FLINAK), have been investigated by Raman, NMR, and thermal analysis. Melting and dissolutiontook place stepwise. The eutectic alkalifluoride mixtures with minor amounts of dissolved fluoroaluminate saltsstarted melting at around 460 and 740 °C for FLINAK and FNAK mixtures, respectively. Total melting/dissolutionof mixtures with 9−11 mol % aluminum fluoro salts added took place near 780 °C in the FLINAK solvent and atapproximately 900 °C for FNAK solutions. The solidified melts were characterized by Raman bands at 561 (ν1),391 (ν2), and 328 cm-1 (ν5) and a 27Al NMR chemical shift near 0 ppm originating from isolated AlF63- octahedralions. The Raman and NMR signals due to AlF63- were also observed at temperatures where the samples wereonly partly melted. Upon total melting, a pronounced dissociation of AlF63- into AlF52- and fluoride ions took place.At even higher temperatures, the equilibrium was displaced in favor of AlF52- in the FNAK solvent. The AlF52- ionwas characterized by an intensive Raman band at 558 cm-1 and an increasingly positive 27Al chemical shift withraising temperature, e.g., of 16 ppm at 935 °C.
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