Institute of High-Temperature Electrochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia:

A. V. Rudenko, Post-Graduate Student, e-mail: a.rudenko@ihte.uran.ru
A. A. Kataev, Junior Researcher
I. D. Zakiryanova, Leading Researcher
O. Yu. Tkacheva, Leading Researcher

During the electrolytic or metal-thermal obtaining of Al – Sc alloy, the molten sodium cryolite (Na₃AlF₆) is used as a dissolvent medium, which gives an opportunity to use the scandium oxide as an initial raw material for the second component. The value of Sc₂O₃ solubility in salt solution is an important characteristics of the process. The co-solubility of Sc₂O₃ and Al₂O₃ in the molten sodium cryolite NaF – AlF₃ with a cryolite ratio of 2.3 was studied by thermal analysis and Raman spectroscopy. The liquidus temperature of the quazi-binary phase diagrams [(NaF – AlF₃) – Sc₂O₃] – Al₂O₃ and [(NaF – AlF₃) – Al₂O₃] – Sc₂O₃ was measured. All phase diagrams have an eutectic minimum, which shifts toward a decrease in the concentration of the added oxide as the concentration of the “fixed” oxide increases. The Sc₂O₃ solubility in the NaF – AlF₃ melt is 4.5 mol.% at 980 ^оC and depends on the Al₂O₃ solubility at a certain electrolyte composition and temperature, but at the same time the co-solubility of scandium and aluminum oxides exceeds the solubility of each oxide individually. This phenomenon can be explained by the different mechanism of the Al₂O₃ and Sc₂O₃ interaction with the molten sodium cryolite. The Raman spectra of the quenched samples (NaF – AlF₃) – Sc₂O₃ and (NaF – AlF₃) – Sc₂O₃ – Al₂O₃ revealed that, unlike Al₂O₃, the chemical dissolution of Sc₂O₃ proceeds in two stages with the formation of the complex ions ScF₆^3–, Al₂OF₆^2– and Sc₂OF₆^2–. However, at high concentrations of dissolved alumina, only oxyfluoride complexes of aluminum and scandium were detected in the spectra.

1. Makhov S. V., Moskvitin V. I. Modern technology of obtaining of aluminiumscandium ligature. Tsvetnye Metally. 2010. No. 5. pp. 95–96.
2. Yang Sh., Gao B., Wang Zh., Shi Zh., Ban Y., Kan H., Cao X., Qiu Zh. Preparation of Al – Sc alloys by molten salts electrolysis. Innovations in Electrometallurgy (TMS Annual Meeting). 2007. pp. 54–57.
3. Shtefanyuk Yu., Mann V., Pingin V., Vinogradov D., Zaikov Yu., Tkacheva O., Nikolaev A., Suzdaltsev A. Production of Al – Sc alloy by electrolysis of cryolitescandium oxide melts. Light Metals. 2015. January. pp. 589–593.
4. Moskvitin V. I., Makhov S. V. About the possibility of obtaining of aluminiumscandium ligature in aluminium electrolyzer. Tsvetnye Metally. 1998. No. 7. pp. 43–46.
5. Yatsenko S. P., Skachkov V. M., Yatsenko A. C. Receipt of ligature on basis of aluminium by method of high-temperature exchange reaction in molten salts. V. Injection of technological powders in liguid aluminium. Rasplavy. 2011. No. 4. pp. 41–46.
6. Zaikov Yu., Tkacheva O., Suzdaltsev A., Kataev A., Shtefanyuk Yu., Pingin V., Vinogradov D. Lab scale synthesis of Al – Sc alloys in NaF – AlF₃ – Al₂O₃ – Sc₂O₃ melt. Advance materials research. 2015. Vol. 1088. pp. 213–216.
7. Napalkov V. I., Makhov S. V. Alloying and modifying of magnesium and aluminium. Moscow : MISIS. 2002. 376 p.
8. Schwellinger Р. Method for the production of an aluminum-scandium master alloy. Patent WO 2006/079353 A1. Alcan Technology & Managements Ltd, Germany ; Publ. 25 Jan. 2005.
9. Tian C., Hu X., Lai Y., Yang S., Te S., Li J. Solubility of Sc₂O₃ in Na₃AlF₆ – К₃AlF₆ – AlF₃ melts. Proceedings 6-th International Symposium on High Temperature Metallurgical Processing. TMS, 2015. pp. 105–112.
10. Tkacheva O. Yu., Kataev A. A., Redkin A. A., Rudenko A. V., Dedyukhin A. A., Zaykov Yu. P. Fluxes for producing the aluminum–boron alloys. Rasplavy. 2016. No. 5. pp. 387–396.
11. Skybakmoen Е., Solheim A., Sterten A. Alumina solubility in molten salt systems of interest for aluminum electrolysis and related phase diagram data. Metallurgical and materials Transactions B. 1997. Vol. 28B. pp. 81–86.
12. Fenerty A., Hollingshead E. J. Liquidus curves for aluminum cell electrolyte. 3. Systems cryolite-alumina with aluminum fluoride and calcium fluoride. Journal of The Electrochemical Society. 1960. Vol. 107. pp. 993–997.
13. Pshenichnyy R. N., Omelchuk A. A. Interaction of rare-earth oxides with binary molten mixtures of zirconium and alkali metal fluorides. Zhurnal neorganicheskoy khimii. 2012. Vol. 57, No. 1. pp. 123–127.
14. K. Nakamoto. Infrared and Raman spectra of Inorganic and Coordination Compounds. Moscow : Mir, 1991. 220 p.
15. Auguste F., Tkatcheva O., Mediaas H., Østvold T., Gilbert B. The Dissociation of Fluoroaluminates in FLiNaK and CsF – KF Molten Mixtures: A Raman Spectroscopic and Solubility Study. Inorganic Chemistry. 2003. Vol. 42, No. 20. pp. 6338–6344.
16. Todorov N. D., Abrashev M. V., Marinova V., Kadiyski M., Dimowa L., Faulques E. Raman spectroscopy and lattice dynamical calculations of Sc₂O₃ single crystals. Physical Review B. Vol. 87. DOI: 10.1103/PhysRevB.87.104301.
17. Brooker M. H., Berg R. W., Barner J. H., Bjerrum N. J. Matrix-isolated Al₂OF₆^2– ion in molten and solid LiF/NaF/KF. Inorganic Chemistry. 2000. Vol. 39. pp. 4725-4730.
18. Aleksandrov K. S., Voronov V. N., Vtyurin A. N., Krylov A. S., Molokeev M. S., Pavlovskiy M. S., Goryaynov S. V., Likhacheva A. N., Ancharov A. I. Pressure-induced phase transition in the cubic ScF₃ crystal. Fizika tverdogo tela. 2009. Vol. 51, No. 4. pp. 764–770.
19. Hu X, Qu J., Gao B., Shi Z., Liu F., Wang Z. Raman spectroscopy and ionic structure of Na₃AlF₆ – Al₂O₃ melts. Transactions of Nonferrous Metals Society of China. 2011. Vol. 21. pp. 402–406.