Factors Leading to the Formation of a Resistive Thin Film at the Bottom of Aluminum Electrolysis Cells
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HALL–HEROULT cells from the aluminum electrolysis industry often encounter incomplete dissolution of alumina through many types of events enabling the agglomerations to sink and form sludge.[1] This phenomenon will depend heavily on the alumina physical properties.[2] Secondary feeding from the anode cover material, a mixture of crushed bath and alumina protecting the anode, and backfeeding of the sludge itself add uncertainty to the alumina concentration in the bath.[3] Moreover, changes in the heat losses at the cell wall may influence the amount of sludge generated from the side towards the center of the cell.[4] The presence of this sludge can lead to the formation of resistive deposits, an increase in the length of the current path, the deterioration of the cathode block, and even the premature stoppage of the cell.[5] Figure 1 presents the different zones found in the Hall–Heroult cell. The study of deposits formation at the center of the cell is necessary to extend the general understanding of the path that the smelter grade alumina (SGA) follows
MARC-ANDRE´ COULOMBE, Graduate Student, and GERVAIS SOUCY, Professor, are with the Department of Chemical and Biotechnological Engineering, Universite´ de Sherbrooke, 2500 Boulevard de l Universite´, Sherbrooke, QC, J1K 2R1, Canada. Contact e-mail: [email protected] LOIG RIVOALAND and LYNNE DAVIES, Researchers, are with Rio Tinto (Arvida Research and Development Centre), 1955 Boulevard Mellon, Jonquie`re QC, G7S 4K8 Canada. Manuscript submitted February 16, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
from the top of the bath to the bottom of the cell. It is known that SGA, while in bath, will undergo a c-Al2O3 to a-Al2O3 phase transformation and agglomeration that can lead to incomplete dissolution.[6] Electrolytic bath will infiltrate the agglomerate and may cause the SGA to transport some bath material under the metal pad if its weight overcomes the surface tension between the bath and the metal.[7] Previous work on CFD modeling showed that alumina could be mixed in the bath through gas release beneath the anode, the interaction of the magnetic and electric fields in the bath, the drag in the metal from MHD-induced movement and thermal convection.[8] The previous study also showed that mixing via center and end channels was a faster process than alumina consumption. It could then be concluded that while SGA concentrated in the bath under the SGA feeders, this same SGA will also tend to spread along the central channel. Extended works performed by Geay et al. on metal/bath liquid height differential along cathode blocks demonstrated that most of the material that sinks under the metal pad will preferentially deposit in the central channel.[1] The study suggested three reasons why sludge accumulations were higher in the central channel: – SGA feeding takes place along the central channel and when alumina does not dissolve properly, it produces sludge in that area. – The crust is fragile in that area and can form sludge after its collapse. – During cov
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