Computational Modeling of Thermochemical Evolution of Aluminum Smelter Crust
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INTRODUCTION
MODERN Hall-Heroult cells operate at higher amperages and in a very energy intensive manner. During cell operation, nearly 50 pct of the input energy is dissipated as heat loss from the cell surfaces to the surroundings, and the top heat loss accounts for almost 50 pct of this total heat loss.[1,2] Because the anode cover must transmit a large proportion of the cell heat loss, it has a great impact on the cell heat balance as well as on the environmental performance in terms of fugitive emissions from the process. The crust, which is the bottom consolidated part of the anode cover, undergoes high temperature service conditions due to the large heat flow from the hot bulk electrolyte. The lack of thermochemical stability of the crust at the service temperatures generated is a crucial problem for modern high amperage reduction cells. Frequently, crust and cover are observed to ‘fall in’ to the cell, leaving large holes for heat and fluoride emissions to escape. A significant proportion of the manual labor employed in smelters goes toward fixing (termed ‘dressing’) the anode cover on hundreds of cells each day. An important cause of crust deterioration is the melting of bath components within the crust, particularly the loss of the previously solidified bath from within the crust. In the experiments performed in an industrial reduction cell by Liu et al.,[3] loss of a large amount of
QINSONG ZHANG, Engineer, is with the Shenyang Aluminium & Magnesium Engineering & Research Institute Co. Ltd, Shenyang, 110001 Liaoning, P.R. China. Contact e-mail: [email protected] MARK P. TAYLOR and JOHN J.J. CHEN, Professors, are with the Department of Chemical and Materials Engineering, Light Metals Research Centre, The University of Auckland, Private Bag 92019, Auckland, New Zealand. Manuscript submitted July 28, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
the lower crust material was observed immediately after an anode effect. In related testing of the crust reported also in Reference 3, this rapid loss of crust material was found to be due to the melting of bath components and weakening of the crust structure through its high liquid fraction, before the anode effect occurred. Crust thermochemical stability is thought to be closely related to changes in thermal and chemical behaviors of the material as it ages after anode setting. In a previous publication by these authors,[4] chemical compositions in industrial crust samples were measured. There are decreasing trends for the cryolite content and cryolite ratio from the bottom to the top of the industrial crusts tested. However, understanding of the crust thermochemical behaviors is still somewhat limited due to its complexity. The liquid electrolyte content in the crust clearly will play an important role in the crust thermochemical and mechanical behavior, and melting of bath components in the original cover material during its preheating in the cell could be a source of liquid electrolyte in the crust. However, another important liquid electrolyte source may be li
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