Electrical conductivity of molten cryolite-based mixtures obtained with a tube-type cell made of pyrolytic boron nitride
- PDF / 596,438 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 184 Views
I.
INTRODUCTION
INVESTIGATIONS of the electrical conductivity of molten cryolite-based systems have been an area of active research in recent years. Melts based on sodium cryolite (Na3AIF6) serve as solvent for alumina in the electrolyte used in the production of aluminium: ~) Cryolite is a high melting compound (1012 ~ and it is advantageous to use a lower melting electrolyte.t2.3.4] This can be done by the use of an acid (AlFj-rich) bath and/or by the introduction of certain additives, such as LiF, CaF2, and MgF:. With decreasing temperature, one might expect an increase in the current efficiency, lower energy consumption, and possibly, prolonged cell life and easier adaptation of inert electrode materials. However, the electrical conductivity of the electrolyte decreases both with increasing contents of most additives (AIF3, CaF2, MgF2, and KF) and with decreasing temperature.t n This unfavorable effect can partly be compensated by the addition of LiF. In industrial production of aluminum, it is desirable to have an electrolyte with a high elec-
J. HI~'ES, Assistant Professor, and P. FELLNER, Professor, are with the Department of Inorganic Technology, S}ovak Technical University. 8 t 2 37 Bratislava, Slavakia. J. THONSTAD, Professor. and A. STERTEN, Associate Professor, are with the Department of Electrochemistry, Norwegian Institute of Technology, University of Trondheirn, N-7034 Trondheim, Norway. Manuscript submitted November 8, 1994. METALLURGICAL AND MATERIAI.S TRANSACTIONS B
trical conductance. Thus, it is not surprising that the literature dealing with this topic is rather extensive.lJ.5-s] The major difficulty in determining conductivities of fused fluorides is to select a suitable material for the conductance cell. It should not be attacked by the melt, it should be an electrical insulator at temperatures around 1000 ~ and it should possess dimensional stability at these temperatures. It seems that pyrolytic boron nitride is the most suitable material for this ptu'pose. [x6'9] Most of the available conductivity data for molten cryolite-based mixtures were obtained with all-metal cells (platinum and its alloys). The measured electrolyte resistance of such cells is usually less than 1 ohm, and electrode polarization becomes a major problem t~~ in determining the true resistivity of the electrolyte. Recently, a novel experimental technique with moving electrodes (continuously varying cell constant) for tube-type cells made of pyrolytic boron nitride was developed by Wang et aLtS1 and by Kim and SadowayfOJ to increase the reproducibility of the measured data. However, this technique was not adopted in the present work. In 1973, ChoudharyCl~l published an equation based on a regression analysis of the conductivity data for cryolite melts available at that time: In r = 2.0156 - 0.0207 (pct A1203) - 0.0049 [1] (pct CaF2) - 0.0166 (pct MgF2) + 0.0178 (pct LiF) + 0.0063 (pct NaCI)+ + 0.0077 (pct Li3AlF6) + 0.2175 CR - 2068.4/T where CR denotes the NaF/AIF3 molar ratio, the concenVOLUME 27B, APRIL 1096--.255
Data Loading...
