Simulator of Non-homogenous Alumina and Current Distribution in an Aluminum Electrolysis Cell to Predict Low-Voltage Ano
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MARY aluminum production contributes significantly to greenhouses gases (GHG) emissions due to the production of carbon dioxide (CO2), inherent to the chemical reactions occurring in the electrolysis cells using carbon anodes. However, the aluminum industry is also known as one of the two most important anthropogenic emitters of perfluorocarbons (PFC) along with the semiconductor industry. These types of gas, namely the tetrafluoromethane (CF4) and the hexafluoroethane (C2F6), are known to be produced when the cell’s current distribution diverges from the normal operating range due to the lack of alumina in the electrolyte, thus leading to a harmful event called anode effect. In the 1990s, many researchers[1–4] demonstrated that a relationship exists between the polarized anode
LUKAS DION, LA´SZLO´ I. KISS, SA´NDOR PONCSA´K are with the GRIPS, Universite´ du Que´bec a` Chicoutimi, 555 boul. de l’Universite´, Chicoutimi, QC G7H 2B1, Canada. Contact e-mail: [email protected] CHARLES-LUC LAGACE´ is with the Aluminerie Alouette Inc., 400 Chemin de la Pointe-Noire, C.P. 1650, Sept-Iˆles, QC G4R 5M9, Canada. Manuscript submitted September 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
effect duration and the amount of PFC gas generated during the occurrence of the anode effect. However, such relation is consistent with a specific type of anode effects which perturbs strongly the cell behavior by significantly increasing the global cell voltage above the normal operation level, thus generating important amounts of heat while increasing the level of cell instability. Since the cell voltage is a well-followed indicator, such events can be easily detected by the cell control system and are classified as high-voltage anode effect (HVAE). However, recent studies (early 2010s)[5–10] demonstrated that PFC emissions can also be generated locally by similar mechanisms under only a few number of anodes. Redistribution of the current might prevent propagation of this phenomenon and only a local increase in resistance is observed with no significant increase in the global cell voltage.[11] Depending on the cell technologies, the annual emissions resulting from low-voltage anode effects (LVAE) can be even more important that the emission level resulting from HVAE.[12] Even though LVAE detection is very difficult currently in a fully operating smelter, some key indicators presented in the literature can be used in order to create a simulation tool which could help to improve the electrolysis process while reducing the risk of LVAE. To achieve this goal, a simulator could provide information about the evolution of non-homogeneity of the alumina
distribution inside the electrolytic bath as well as information regarding non-uniform current distribution among the anodes. In this article, the different mathematical sub-models used in the development of this simulator are presented and discussed, focusing on alumina feeding, alumina dissolution, the diffusive and convective transport within the electrolytic bath, alumina consumption by electrolysis,
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