1088
Effect of Composition and Temperature on Current Efficiency for Aluminium Electrolysis from Cryolite-Based Molten Alumina Electrolytes

Tuesday, 30 May 2017: 17:20
Marlborough A (Hilton New Orleans Riverside)
T. Morishige (Kansai University, Norwegian University of Science and Technology (NTNU)), G. M. Haarberg (Norwegian University of Science and Technology), H. Gudbrandsen (SINTEF Materials and Chemistry, 7456 Trondheim, Norway), E. Skybakmoen, and A. Solheim (SINTEF Materials and Chemistry)
Aluminium smelting from molten cryolite alumina electrolytes is the industrial production process known as the Hall-Héroult process. The current efficiency for aluminium electrolysis in modern cells can be as high as 96 %. The main reason for loss in current efficiency is the socalled back reaction between dissolved aluminium and the anode product of CO2as the following reaction:

Al(diss) + 3/2 CO2 = 1/2 Al2O3+ 3/2 CO

The current efficiency can be improved by suppression of metal dissolution from deposited metal. Decreasing temperature of the cell may also give higher current efficiency. Addition of LiF leads to decreasing the liquidus temperature and also to decrease the metal solubility into the electrolyte. However, low superheat may cause solidification of melt. A laboratory cell to determine the current efficiency was recently modified to obtain more reliable results. The base composition of the electrolyte was Na3AlF6-11.55wt.%AlF3-5wt.%CaF2-4wt.%Al2O3 and electrolysis was conducted at 965 ºC. Electrolysis was performed at constant current density (~0.9 A cm-2) and the current efficiency was determined from the weight of deposited aluminium. The current efficiency at the base condition was 95% in this study. Effects of electrolyte composition by some percent of LiF addition on the current efficiency was studied. The current efficiency and the metal shape on the cathode was changed by the addition of LiF. The loss in current efficiency is strongly related to the content of dissolved aluminium in the electrolyte. Realistic values for the current efficiency were obtained.

References

[1] J. Thonstad, P. Fellner, G.M. Haarberg, J. Híveš, H. Kvande and Å. Sterten. Aluminium electrolysis: Fundamentals of the Hall-Héroult process 3rd edition, Aluminium-Verlag, Düsseldorf (2001). ISBN 3-87017-155-3

[2] P.A. Solli, T. Haarberg, T. Eggen, E. Skybakmoen and Å. Solheim. Light Metals 1994, 195-203 (1994).

[3] X. Wang, R.D. Peterson and N.E. Richards. Light Metals 1991, 323-330 (1991)

[4] E.W. Dewing. Metall. Mater. Trans. B., 22B, 177-182 (1991).

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See more of: F01: Electrochemical Engineering General Session
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