A Modeling Approach for Time-Dependent Geometry Applied to Transient Heat Transfer of Aluminum Electrolysis Cells
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UCTION
THE aluminum electrolysis process is continuously improving in order to reach a better energetic efficiency and environmental balance, while increasing the production at the same time. To achieve these goals, the design of the aluminum electrolysis cells (AEC) is optimized by performing modeling and experimental investigations. The industrial aluminum production is carried out by reducing alumina (Al2O3) with carbon anodes in a liquid electrolytic bath. The liquid bath is primarily composed of a mixture of Na3AlF6 (cryolite), AlF3 (aluminum fluoride), CaF2 (calcium fluoride), and dissolved Al2O3, but it usually also contains additives in the form of lithium, potassium, and magnesium fluoride salts. Eq. [1] is the overall electrochemical reaction occurring in AEC[1]: current
2Al2 O3ðdissolvedÞ þ 3CðsÞ ! 4AlðlÞ þ 3CO2ðgÞ :
½1
FRANC¸OIS ALLARD and MARTIN DE´SILETS are with the Department of Chemical Engineering and Biotechnological Engineering, Universite´ de Sherbrooke, C1 - Pavillon J. Armand Bombardier, 2500 Boulevard de l’Universite´, Sherbrooke, QC J1K 2R1, Canada. Contact e-mail: [email protected] ALEXANDRE BLAIS is with Rio Tinto Aluminium (Arvida Research and Development Centre); 1955 Boulevard Mellon, Jonquie´re, QC G7S 4K8, Canada. Manuscript submitted September 9, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The liquid aluminum is continuously produced by the electrochemical process and is extracted periodically by siphoning during industrial operations. Carbon dioxide (CO2) is produced by the reaction of oxide ions at the surface of the carbon anodes immersed in the liquid bath. In order to feed the electrochemical reaction, an electric current is supplied to the AEC and is divided between the several consumable carbon anodes. The total current intensity varies from 200 to 600 kA depending on the technology. The main components of the AEC are described in Figure 1. The heat balance of the AEC is rigorously controlled in order to maintain a protective layer of frozen bath, usually called side ledge, on the side walls. This layer protects the side components from the highly corrosive and erosive behavior of the liquid bath. The thermal conductivity of the side ledge has been recently determined by laser flash analysis at temperature up to 550 C.[2–4] The side ledge grows by the primary crystallization of Na3AlF6 at a temperature close to 950 C, while the AEC operates with a bulk liquid bath at around 965 C. The side ledge formed at equilibrium conditions demonstrates a composition close to pure Na3AlF6, while with higher cooling rate the AlF3, CaF2, and Al2O3 species from the bath can be trapped inside its microstructure.[5–7] A ledge toe is also formed on the cathode blocks by both the solidification of Na3AlF6 and the precipitation of Al2O3 at temperature also close to 950 C, depending on the local chemical composition.[6] When the AEC is quickly cooled down, the side ledge grows by forming an open crystalline layer, in which mass transfer can occur with the bulk liquid
Fig. 1—
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