Numerical Investigation on the Impact of Anode Change on Heat Transfer and Fluid Flow in Aluminum Smelting Cells
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ALUMINUM is used in various products and industries such as transportation, construction, and medicine. More than 40 million tons of aluminum are produced per year, mostly in China, Russia, and Canada.[1] The Hall–He´roult process is used to transform alumina (Al2O3) into aluminum as shown in Figure 1. A direct electric current passes through a bath layer between anodes and cathodes. The overall electrochemical reaction taking place in the aluminum smelting cell can be expressed as 2Al2 O3 þ 3C þ electricity ! 4Al þ 3CO2 : The carbon entering into this reaction comes from the anodes. Therefore, anodes are constantly consumed by the process and need to be periodically replaced. Typically, an anode can last approximately 20 days in a cell. Considering a cell with 40 anodes, this means that QIANG WANG, Ph.D. Candidate, JIANPING PENG, Assistant Professor, and BAOKUAN LI, Professor, are with the School of Materials and Metallurgy, Northeastern University, Shenyang, 110819, China. Contact e-mail: [email protected], LOUIS GOSSELIN, Professor, is with the Department of Mechanical Engineering, Universite´ Laval, Quebec City, QC G1V 0A6, Canada. MARIO FAFARD, Professor, is with the NSERC/Alcoa Industrial Research Chair MACE3 and Aluminum Research Centre REGAL, Universite´ Laval, Quebec City, QC G1V 0A6, Canada. Manuscript submitted May 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
almost every day one pair of anodes needs to be replaced in the cell.[2,3] Anode change can significantly affect the thermal balance within the cell. New anodes are usually in room temperature much colder than old anodes. The bath in the cell is kept only 5 K to 15 K (5 C to 15 C) above its primary temperature of crystallization, which would be frozen if cold anodes are added. Moreover, the influence can prolong more than 24 hours.[4,5] However, a well-maintained thermal balance of the cell provides the foundation for better operation and higher energy efficiency. Therefore, it is necessary to understand the effect of new anode on heat transfer and magnetohydrodynamic (MHD) flow in the cell. Considering the complex transient and coupled phenomena taking place in the replacement of anodes, the understanding of heat transfer and fluid flow is challengeable. It is difficult to observe the varying of the temperature distribution in the entire cell by measurement. With the continuous improvement of computational resources, numerical simulation has become an attractive method to achieve this objective. Cheung et al.[6] have developed a thermal model to estimate temperature field and bath ledge thickness in a cell under different anode current distributions. The flow field however was ignored. Dupuis et al.[7] and Safa et al.[8] investigated thermal behavior coupled with MHD flow. They used finite element method to calculate electromagnetic and temperature fields, and then the fluid flow and phase change were studied. Their results indicated
Fig. 1—Schematic of aluminum reduction cell.
that the velocity field had a strong effect on the ledge shape, and the fluid
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