Study on the Anode-to-Cathode Distance in an Aluminum Reduction Cell
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UCTION
THE metal capacity from the aluminum reduction cell increases proportionally with a current increase as long as the current efficiency (CE) is constant. The electrical power loss from the bath resistance as a result of the current increase can be reduced by decreasing the anode-to-cathode distance (ACD). This is preferred to make sure that the pots stay inside an operational resistance load window where sufficient frozen side ledge is retained. When the average of the ACD is reduced by current increase actions, it is important to reduce the variation/noise in the ACD resistance. If not, the process will be directed toward an unstable state with reduced current efficiency and higher probability for dynamic short circuiting, which is caused by magneto hydrodynamic waves and the risk of growing spikes at the anodes wear surface. Over the last 10 years, there has been focus on current increase actions and also on reducing the noise in the ACD resistance in the aluminum industry. One of the major improvements of reducing the noise was the implementation of slots in the wear surface as illustrated in Figure 1.[1] The produced carbon dioxide creates DAG HERMAN ANDERSEN, Principal Engineer and Ph.D. Candidate, is with Primary Metal Technology, NO-6882 Øvre A˚rdal, Norway, and with the Department of Structural Engineering, 7491 Trondheim, Norway. ZHILIANG L. ZHANG, Professor of Mechanics and Materials, is with the Department of Structural Engineering, Norweigian University of Science and Technology (NTNU), N-7491 Trondheim, Norway. Contact e-mail: zhiliang. [email protected] Manuscript submitted November 30, 2009. Article published online January 7, 2011. 424—VOLUME 42B, APRIL 2011
bubbles in the bath and the slots function as an escape route for the bubbles. The low-frequency noise components in the liquid metal have also been studied in a magnetohydrodynamic (MHD) aspect that has influenced design optimizations of the cell and also the bus bar system.[2] There is also a focus on how the inhomogeneous density of the bath creates inhomogeneous ACDs.[3] This gives motivation for alumina distribution to the cell through individual feeder control to reduce the density differences in the bath. Still, variations exist in the ACD resistance. In normal situations, it is reported that the standard deviation in the current load on individual anodes in the same cell often are more than 10 pct of the average current.[4] The actions for reducing the noise in the ACD resistance by slots in the anode have led to negative effects on other parameters, like current efficiency and increased inhomogeneous anode consumptions.[5] It is emphasized that each slot implementation must be treated individually for each plant[6] to avoid pitfalls. This article will focus on one special effect on the ACD variation, that is, the initial electric current distribution (the anode in an initial state in the bath as explained previously). The first section of the article defines the k0 parameter to describe the degree of initial inhomogeneous current densities in the a
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