Investigation of Freeze-Linings in Aluminum Production Cells
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RODUCTION
IN the electrochemical production of aluminum using the Hall–He´roult process cryolite-based bath material is solidified on the sidewalls of the cells. This side ledge or freeze-lining plays an important role in protecting the side wall material from direct chemical attack by the molten salt bath. Both the thickness of the freeze-lining and the rate of heat loss through the sidewalls are determined by the rate of heat transfer from the bulk liquid to the deposit/liquid bath interface.[1,2] Aluminum production is an energy-intensive process, and the effect of the ledge profile on the cell performance is significant. As a result, it is of particular importance to be able to predict the ledge behavior. Several heat-transfer models for steady-state conditions have been developed to predict the thickness of the side ledge as a function of the process parameters, such as, bulk bath temperature, outer wall temperature, and thermal conduction of the solids.[3–7] Various experimental techniques have been used to estimate the convection heat-transfer coefficient between the liquid bath and solid surface.[8,9] In those studies, the bath/side ledge interface is assumed to be at the liquidus temperature of the bulk bath. The liquidus temperatures of the multicomponent electrolyte systems have been measured[10–14] and thermodynamically modeled.[10,15] In-situ measurements of liquidus temperatures have also been carried using the Heraeus Cryotherm probe in order to evaluate the dynamic behavior of
the freeze-lining deposits in aluminum production cells.[16–18] In addition, several models have been developed to take into account the fact that the temperature within the cell varies with time, and the sidewall can be dissolved or can grow depending on whether the cell temperature increases or decreases. These dynamic models are used to describe the effects of heat and mass transfer on the behavior of the side ledge.[2,19,20] These models assume that during the growth of the side ledge, the liquid ahead of the bath/deposit interface becomes enriched with solute species, and the liquidus temperature is that of the local liquid, not the bulk liquidus temperature. Freeze-linings formed in the Hall–He´roult process have been experimentally investigated using both industrially[2] and laboratory-prepared synthetic bath samples.[2,20] The microstructures and compositions of cryolite in the side ledge were investigated, and it was concluded that the interface temperature of the bath/ side ledge depended on the rate of the side ledge formation. The aim of the current study is to determine the effects of bath chemistry on the microstructure, stability of the freeze-lining, and bath/deposit interface temperature at steady-state conditions.
II.
EXPERIMENTAL PROCEDURE
A. Cold Finger Experiments ATA FALLAH-MEHRJARDI, PhD in Metallurgical Engineering, Postdoctoral Research Fellow, PETER C. HAYES, Xstrata Professor, and EVGUENI JAK, Professor in Pyrometallurgy, are with the PYROSEARCH, Level 3, School of Chemical Engineering, The University of
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