Density and Molar Volume of KF-NaF-AlF3 Melts with Al2O3 and CaF2 Additions

The ability to dissolve alumina makes the sodium cryolite melt applicable for the primary aluminum production despite the high liquidus temperature of the melt. Numerous cryolite based compositions are used by aluminum smelters nowadays. The majority of industrial baths contain various salt additions intended to improve physical and chemical properties of the melt, as well as the overall process parameters. Nevertheless, the question of an appropriate medium for the inert anode application remains relevant in the aluminum industry because of high corrosion activity of fluoride melts at temperatures around 950 ºC. Lowering the operating temperature by means of reducing the liquidus temperature seems an advantageous approach to extend the implementation of the technology with the oxygen evolving inert anode.

Recently KF-NaF-AlF₃ melts with low cryolite ratio have been recognized a prospective medium for the new environmentally benign technology for the primary aluminum production [1,2]. The change in the chemical composition of the melt affects noticeably its physical and chemical properties. The electrolyte density is one of the properties dramatically affecting the technological parameters of the electrolytic cell.

The density of conventional electrolytes for the aluminum production has been well studied [3,4] however, the need to scrutinize the physical and chemical properties of low melting electrolytes is clearly warranted. Yan with coauthors [5] addressed the KF-NaF-AlF₃ melt with a different NaF content at (([KF] + [NaF])/[AlF₃]) 1.3 and  1.41 but data presented concerning the influence of alumina and calcium fluoride are scarce.

The aim of the present work was to study the density and molar volume of the KF-NaF-AlF₃ system at CR=1.3, 1.5, with Al₂O₃ and CaF₂ additions by the Archimedean method in the temperature range
920–1150 K.

In the present work the cryolite ratio was defined as a ratio between molar concentrations of the alkali metal fluoride and aluminum fluoride (CR=N_MF/N_AlF3) – for the mixed cryolite melts (KF-NaF-AlF₃) was calculated as CR=(N_KF+N_NaF)/N_AlF3.

Analysis of the data obtained for the density of low-melting cryolite mixtures illustrates the existence of some general regularities and specific features of the melts under study. Over a broad interval of the SCR, the molar volume of the melt rises upon the replacement of sodium fluoride with the potassium one (Fig. 1). The molar volume was shown to decrease with the CR rise. The calcium fluoride additives cause the density increase and molar volume depression (Fig. 2). The alumina additions up to 6 mol.% did not show to significantly change either the density or the molar volume of the melts.

References

 

1. Redkin, A.; Apisarov, A.; Dedyukhin, A.; Kovrov, V.; Zaikov, Yu.; Tkacheva O.; and Hryn J. Recent Developments in Low-Temperature Electrolysis of Aluminum. ECS Trans. 2013, 50 (11), 205-213.
2. Tkacheva, O. J. Hryn, J. Spangenberger, B. Davis, T. Alcorn  Operating parameters of aluminum electrolysis in KF-AlF₃ based electrolytes. Light Metals. – 2012. -  P. 675-680
3. Fellner, P.; Haarberg, G.M.; Hives, J.; Kvande, H.; Sterten A.;, Thonstad, J. Aluminium Electrolysis.
   3 rd Edition – Aluminium; Verlag: Dusseldorf, 2001.
4. Fernandez, R.; Ostvold, T. Surface Tension and Density of molten Fluorides and Fluoride Mixtures Containing Cryolite. Acta Chemica Scandinavica  1989, 43, 151-159.
5. Yan, H.; Yang, J.; Li, W. Surface Tension and Density in the KF-NaF-AlF₃-Based Electrolyte.
   J. Chem. Eng. Data  2011, 56, 4147-4151.