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
THE electrolytic production of aluminum in HallH6roult cells is an energy-intensive process accounting for nearly 4 pct of the electrical energy generated in the United States. High electrical energy costs provide great incentive for the minimization of this energy consumption. Reduction in energy consumption can be achieved in two ways: improvements in current efficiency and reduction in cell voltage. The latter has the greater potential, since efflciencies in modern cells exceed 94 pct, while actual cell voltages exceed the thermodynamic minimum of 1.75 V by a factor of over 2. Figure 1 is a schematic (side) view of a typical Hall cell. Here current of the order of 150 kA is passed through the anode bus structure into the cryolite bath. Cryolite is a solvent for alumina fed to the cell. Carbon anodes, either prebaked or Soderberg (monolithic blocks of carbon baked from carbon paste within the cell), dip into the cryolite. Beneath the cryolite is a layer of molten aluminum. The cathodic reaction, that is, the reduction of aluminum ions, occurs at the interface between the two liquids. The largest single part of the cell voltage is the drop 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 about 1.8 V caused by passage of current through the electrolyte-filled gap between the carbon anode and the metal pool which is the cathode. This anode-to-cathode distance (ACD) is 4 to 5 cm in present-day cells. Reducing this ACD, for the purpose of decreasing energy consumption, is not performed in practice because of the danger of shorting the cell through contact between the anodes and the metal pool. Shorting could occur in industrial cells because of (a) uncertainty concerning the position of the bottom of the consumable carbon anode, and (b) variation in the position of the metal/electrolyte interface under the influence of electromagnetic forces. The need to reduce ACD has led to the concept of advanced Hall cells that use inert anodes (to replace the consumable carbon anodes presently being used) and wettable refractory cathodes (on which a thin film of aluminum will form rather than the pool of aluminum in existing cells). In recent years, most of the work on advanced Hall cells has been devoted to research and development of materials for the inert anode 171and refractory cathode. 12,31There is a reasonable expectation that these materials will become available during this decade, and the question of how a cell is to be designed using them must be addressed. Unfortunately, very little attention has been paid to the operational aspects of advanced Hall cells. These aspects will be the subject of this article.
II.
PREVIOUS INVESTIGATIONS
The most significant work pertaining to the operation of
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