A Mathematical model for prediction of currents, magnetic fields, melt velocities, melt topography and current efficienc
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ALMOSTall the aluminum manufactured in the U.S. is produced in a Hall-Hrroult cell in which aluminum oxide, dissolved in a molten cryolite electrolyte, is electrolytically reduced to metal. Approximately 11.4 GW of electrical energy are consumed in this operation and there is, therefore, considerable incentive for improving the performance of the cell. Part of a cell is shown in cross section in Fig. l. The molten cryolite is seen to be floating on a pool of molten aluminum. Dipping into the cryolite is a carbon anode and the surface of the molten metal forms the cathode at which the metal is generated. The anode reaction is the generation of carbon dioxide. Cell performance is adversely affected by electromagnetic stirring forces within the molten cryolite and aluminum. A typical modern cell would have a current of 100 to 200 kA flowing in a predominantly downward direction from anode to cathode. This current, and currents flowing in adjacent cell components, generate strong magnetic fields, of the order of 10 millitesla (mT) acting predominantly in the horizontal direction. The interaction of these currents and fields causes a circulation of both cryolite and metal with velocities of the order of 10 cm/s. Deterioration of the carbon cell lining is promoted by this circulation which also has the effect of reducing the current efficiency of the cell. This reduction in current efficiency arises from the fact that the metallic aluminum generated in the cell has a small solubility in molten cryolite. This dissolved aluminum is transported to the anode region where it is reoxidized by carbon dioxide bubbles) This transport and reoxidation of product, illustrated in Fig. 1, results in a loss of current efficiency, which in a typical cell is 85 to 95 pct. The transport of the dissolved aluminum is proJ. W. EVANS is Principal Investigator, Molecular and Materials Research Division, Lawrence Berkeley Laboratory and Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California, Berkeley 94720. Y. ZUNDELEVICH is Senior Principal Staff Engineer, Air Products & Chemicals,Incorporated, Corporate Engineering Department, P.O. Box 538, Allentown, PA 18105. D. SHARMA is Manager of Advanced Technology, Dames and Moore, 1626 Cole Blvd., Golden, CO 80401. Manuscript submitted October 14, 1980. METALLU RGICAL TRANSACTIONS B
moted by the turbulence within the cryolite caused by the electromagnetic stirring. This investigation has, as one objective, the prediction of the electromagnetic stirring and current efficiency from first principles, thereby providing a means for developing cell designs or operating procedures with higher current efficiencies. A second objective is the prediction of the topography of the interface between the electrolyte and the aluminum. The electromagnetic forces cause a bowing of this interface such that the interface is typically higher in the center of the cell than at the walls. Reduction of the distance between anodes and the interface results in a reduction of
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