A Case Study of Variation in Aluminum Smelting Cell Thermal State with Control Implications
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NTRODUCTION
ALUMINUM is produced by the electrochemical reduction of alumina (aluminum oxide) dissolved in an electrolyte, which consists mainly of cryolite (sodium aluminum hexafluoride). In order to increase the efficiency of the process, solubility of the metal in the electrolyte needs to be reduced. This is achieved by adding aluminum fluoride (AlF3) to the electrolyte, termed and measured as excess (xs) AlF3 in the smelters. This addition reduces the electrolyte’s liquidus point and hence the operating/electrolyte temperature. In most smelters today, concentration of xs-AlF3 is maintained in the range of 7 to 14 pct. As shown in the sodium fluoride–aluminum fluoride phase diagram (showing pure cryolite at 25 mol pct AlF3) in Figure 1,[1] a small change in the concentration of xs-AlF3 will give rise to a large change (decrease/increase) in the electrolyte’s liquidus point. The cell thermal state is defined principally by the electrolyte temperature and xs-AlF3 concentration. Variation in these parameters has the following impact on the smelting process. (1) Good operational practice requires a continuous layer of the frozen electrolyte on the inner surface of the cell walls. This layer protects the walls from erosion that occurs when the walls are exposed to the molten electrolyte. If the electrolyte temperature goes above specification, it will eventually melt the frozen layer. This will result in the cell wall refractory being eroded, thereby decreasing the life span of the cell.[2,3] G. TANDON, Doctoral Researcher, and J.J.J. CHEN, Professor, Department of Chemical and Materials Engineering, and M.P. TAYLOR, Honorary Professor, Department of Chemical and Materials Engineering, and Director of Light Metals Research Centre, are with University of Auckland, Private Bag 92019, New Zealand. Contact e-mail: [email protected] Manuscript submitted November 10, 2006. Article published online July 20, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
(2) High xs-AlF3 concentration (>14 pct) will significantly reduce the melting temperature of cryolite as well as the electrolyte temperature. As the temperature is lowered, there will not be sufficient energy to dissolve the added alumina. This will result in the formation of sludge—agglomeration of undissolved alumina and cryolite.[4,5] Sludge is a hindrance to the flow of current. The severity of the preceding consequences has provided the impetus for identifying the causes of the variation in the cell thermal state and recommending the necessary actions that will minimize it. This is the objective of the present study. If the recommended actions are implemented, then they will result in maintaining the thermal state within specifications with a greater probability that they will remain there in the future.
II.
EXPERIMENTAL
In order to provide a rigorous base for the process observability, an industrial smelting cell (187 kA amperage, end riser, and prebake technology) was monitored for a period of 2 months for variation in the cell thermal state.
III.
RESULTS AND ANALYSI
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