Modelling of bath/ledge heat transfer in Hall-Heroult cells
Abstract (Summary)Heat transfer through the sidewall accounts for a significant proportion of the energy loss from an aluminium reduction cell. At the same time, the ledge formed on the sidewall has important consequences with regards to the cell service life and the dynamic heat balance during various disturbances associated with the smelting operation such as alumina feeding, anode changing, metal tapping and anode effect. Several studies have been carried out to investigate the ledge heat transfer. However, the reported heat transfer coefficients in the literature not only vary over a wide range of values but provide insufficient information on the ledge heat transfer. A heat transfer probe and measurement techniques were developed for studying the ledge heat transfer in a full-scale 3-D air-water model. Quantitative measurements were conducted to determine the bath/ledge heat transfer characteristics at various positions, and under different operating conditions such as anode-ledge distance, current density and bath depth. Variation of the heat transfer were also examined as a function of the anode bottom inclination, the anode slot width and the position on the side ledge relative to the anode slot. The results illustrate that the ledge profile in an operating cell will take on a different shape in compliance with the heat transfer variation. A similitude analysis was carried out to interpret the measured results in a meaningful manner for use in a reduction cell. As a result, the heat transfer coefficient in industrial cells under various operating parameters, and at different positions on the side ledge, can be estimated using the empirical correlations presented. Gas bubble behaviour and bubble impingement on the side ledge were observed in the water model. Observations made on the 2-D and 3-D water models indicate that anode gas evolution in the 3-D model cell will reflect more closely the flow pattern in actual cells and hence provide more reliable quantitative results. A simple 2-D thermal model for the prediction of ledge thickness and profile as a moving boundary was developed and solved simply and efficiently with a commercial spreadsheet software using the finite difference method. The ledge profile was predicted using the ledge heat transfer coefficients measured extensively from the full-scale 3-D physical model. The results show that the ledge shape is highly sensitive to the positional variation of the heat transfer coefficient. It is also shown that the ledge heat transfer coefficients obtained from industrial measurements assuming 1-D heat flow are much lower than the actual values in a Hall-Heroult cell. A transient thermal model derived by considering the Stefan problem$/sp[*]$ for the sidewall/ledge region was developed. A fixed-grid and deforming-grid spacing were respectively superimposed on the sidewall and the ledge region in order to track the moving front of the phase change zone. Various aspects of the process dynamics with respect to the variation of ledge thickness and sidewall shell temperature were presented. The model considered dynamic heat loss through the sidewall which results in a closer approximation to the real situation. ftn$/sp[*]$N.B.: In the strict sense the problem of the ledge is not a classical Stefan problem. The classical Stefan problem involves conduction on both sides of the interface. The ledge problem involves conduction on the side and convection on the other.
School Location:New Zealand
Source Type:Master's Thesis
Keywords:engineering chemical 0542 metallurgy 0743
Date of Publication:01/01/1996