Electrocapillarity in the aluminum reduction cell
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I.
INTRODUCTION
IN a Hall-HGroult cell for the electrolysis of alumina,
the interfacial tension between aluminum and the electrolyte influences the stability and the shape of the interface which in turn may control the rate of dissolution of aluminum into the electrolyte. 1,2The interfacial tension also determines, in part, the ability of the interface to support undissolved alumina particles. 3 This phenomenon prevents or decreases the formation of an alumina sludge between the carbon cathode and liquid aluminum. In a previous study by the present authors, 4 the interfacial tension between aluminum and electrolytes with different additives was measured. As indicated by Grjotheim et al.,5 information concerning the effect of electrolysis on interfacial tension is very limited. Thus the present investigation was undertaken to study the effect of current flow on the interfacial tension. When a potential difference is applied to the interface between two phases in contact with each other, various situations are encountered which depend upon the system. The mercury/water interface is nearly an ideal polarizable system, while the aluminum/cryolite interface is k n o w n 6'7 to be nearly an ideal nonpolarizable electrode with a high exchange current density of 12 to 36 A/cm2. 8'9 During electrolysis, mass transfer occurs across the interface and the composition of the electrolyte near the interface changes. The cathodic overvoltage in aluminum/cryolite melts increases with increasing cathodic current density. The overvoltage was found by Thonstad and Rolseth 8'9 and Piontelli and co-workers6'7 to be mainly a diffusion overvoltage. For a cathodic current density of 1 A / c m z, the charge-transfer overvoltage was only - 3 . 5 mV compared to the total overvoltage of - 1 9 0 mV.
II.
EXPERIMENTAL
A. Method and Apparatus The experimental assembly has been described in detail earlier;4 thus only a brief outline is given here. The sessile drop technique was used to measure the interfacial tension between aluminum and cryolite. This method has been successfully used in several related studies. 1~ Figure 1 shows the experimental cell. A boron nitride (BN) cylinder with a wall thickness of 1.5 to 2.0 mm was fitted inside a graphite crucible with an inner diameter of about 2.3 cm and a height of 8 cm. The required amounts of aluminum and premixed electrolyte were placed
Stainless Steel Anode Connector Stainless Steel Crucible Holder AI203 Insulator BN Insulator
Graphite Anode Anode Gas (c0 2 9 co)
Electrolyte Graphite Crucible Aluminum Cathode
T. UTIGARD, formerly Graduate Student, is now Research Engineer, Alusuisse, CH-3965 Chippis, Switzerland. J.M. TOGURI is Professor, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON M5S 1A4, Canada. Manuscript submitted October 25, 1985.
METALLURGICALTRANSACTIONS B
Fig. 1--Crucible assembly.
VOLUME 17B, SEPTEMBER 1986--547
inside the graphite crucible. An Inconel cylinder was connected to the upper section of the graphite crucible and a graphite
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