Instabilities of the metal surface in electrolytic alumina reduction cells
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/ ~ N electrolytic aluminum c e l l , working under completely stationary conditions, would have a constant flow pattern in the m e t a l and in the bath, and t h e r e would be a permanent curvature of the m e t a l surface. The anodes would burn off according to this curvature, and the interpolar distance and the current distribution would be constant and even under the whole anode a r e a . This favorable s t a t e can n e v e r be fully obtained by c e l l s in n o r m a l operation, because various kinds of mechanical or electrical disturbances such as gas escape, current variations, crustbreaking and so forth, will start waves on the m e t a l surface. Once started, these waves will continue in a r a t h e r complicated way dynamically, because they influence the current distribution and thereby c r e a t e electromagnetic forces. These disturbances and surface waves may have a s m a l l e r or g r e a t e r influence on cell operation and current efficiency, depending on t h e i r frequency of occurence, t h e i r magnitude, and on the a b i l ity of the cell to damp the waves down quickly. Measurements and observations in n o r m a l s e r v i c e have shown that such waves often o c c u r as transient oscillations of f i x e d frequencies, one type at a low frequency in the o r d e r of 1 per min, and another at a higher frequency in the o r d e r of 1 per s. It is the intention of this work to obtain a better understanding of the low frequency oscillations in the m e t a l surface, which are shown to be of an electromagnetic nature, and to explain the mechanism by which they are created and maintained. Simulation studies have been conducted on a s e m i dynamic computer m o d e l for calculating flow and surface. The key to the wave mechanism is the generation of uneven current distribution in the bath under the anode as the wave moves along, and the corresponding horizontal equalizing currents in the m e t a l pad. T H E SIMULATION MODEL The main part is a m o d e l for calculating stationary flow and m e t a l surface in aluminum electrolytic cells.1 The m o d e l has been made semidynamic by the introTHORLEIF SELE is Chief Engineer, Ardal og Sunndal Verk, Head Office, S6rkedalsveien 6 , Oslo, Norway. Manuscript submitted April 2 8 , 1977. METALLURGICAL
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duction of an automatic adjustment mechanism for current distribution which is activated each time a new stationary condition has been calculated. It is not a real dynamic model, because time is not a parameter, but w e use a point to point calculation between stationary values. It works in the following way: 1) Calculate an initial stationary m e t a l surface. All the l a t e r adjustments of current distribution are made in relation to deviations from this initial surface. 2) Introduce a disturbance. 3) Calculate new stationary state. 4) Remove the disturbance. 5) Adjust current distribution. ~ 6 ) Calculate new stationary state. Steps 5 and 6 are now repeated until the transient dies down, and surface deviations and flow velocities m
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