Physical modeling studies of electrolyte flow due to gas evolution and some aspects of bubble behavior in advanced hall
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I.
INTRODUCTION
P A R T I m showed the importance of anode submergence, electrode inclination, and the surface tension of the electrolyte in determining the flow pattern and, hence, "bubble effect" in advanced Hall cells equipped with planar (flat) anodes. It was also pointed out that anode design might have some influence on the gas fraction of bubbles in the anode-to-cathode gap (ACG). Because inert anodes are not consumed during the electrolytic reaction, unlike conventional carbon anodes, they provide the opportunity to experiment with their design. Therefore, in Part II, a new anode design intended to facilitate the removal of bubbles from the ACG will be presented. The performance of an advanced Hall cell equipped with this anode will be evaluated in terms of (simulated) electrolyte flow pattern and interpolar resistance. Further, the results obtained here will be used for comparison with advanced Hall ceils with flat anodes, results of which were presented in Part I. The distinguishing feature of this new anode was that it had several grooves (channels) running along the length (near-horizontal configuration) or height (near-vertical configuration) of the anode. The rationale behind the use R. SHEKHAR, Assistant Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016, India. J.W. EVANS, Professor, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted June 25, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS B
of grooves was that they would provide a channel for the escape of gas bubbles, thereby reducing the bubble void fraction in the ACG. Furthermore, the grooves would result in increased surface area for the anodic reaction, compared to a flat anode, provided that the bubble void fraction inside the grooves was not very high. First, resuits pertaining to advanced Hall cells with anodes in the horizontal/near-horizontal configuration will be presented. This will be followed by a discussion of cells equipped with near-vertical electrodes.
II. EXPERIMENTAL APPARATUS AND ARRANGEMENT
A. Horizontal~Near-Horizontal Configuration 1. Anode design A porous graphite slab, 30.8 x 90.2 x 5.7-cm thick, was machined into the shape depicted in Figure 1. Grooves 0.95-cm wide and ranging in depth from 2.5 to 3.8 cm, along the length of the anode, were milled in the front face of the graphite. This variation in groove depth (Figure l(b)) was designed to promote preferential gas flow in one direction when the electrodes were in a horizontal position. The groove dimensions were selected on the basis of input from the U.S. Department of Energy, the aluminum industry, the constraints imposed by the fabrication process, and the ability of the fins to withstand normal handling without any breakage. These dimensions were also rationalized by secondary current VOLUME 25B, JUNE 1994--341
distribution calculations, p] Holes 0.63 cm in diameter, indicated by dashed lines, were drilled in the oth
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