CONCLUSION
The same conclusion than for the thermal bake-out simulation can be drawn [2]: although it had
been previously demonstrated that a slice model might be used for the thermo-electric design of
a cell [15], the significant difference of behaviour of the cathode block with the change in
boundary conditions on plane P3 makes a slice model ill-suited to assess the mechanical
behaviour during the preheating of a Hall-Héroult cell, unless the shell provided no confinement
along the length of the pot, or otherwise if it were infinitely rigid. At least a quarter-cell must
therefore be modeled to obtain a representative stress state in the lining.
However, these results strongly hint that some form of expansion relief must be provided along
the length of the cell to prevent overstressing the cathode blocks.
Another interesting result from the model is that for our simple cathode block slot design, the
relative displacement of the cathode block and the rodded bar during preheating causes a
localised contact zone where current can flow into the bar. This is certainly far from the ideal,
and a detailed study of cathode block-to-bar connection geometry would undoubtedly yield
interesting findings
It can be argued that our arbitrary scenario involving the use of shunts is suboptimal, since the
resulting heat-up rate as shown in Figure 8 is slightly increasing with time. In reality, the cell
resistance should have been left to decrease further before removing the next shunt.
Nevertheless, this illustrates how a numerical model can be used to help design better
preheating procedures taking into account the coupled effects of thermal, electrical and
mechanical phenomena.
Once validated on in situ measurements, the industrial application of such a model could also
involve the development of control metrics. For instance, by relating appropriate results from the
model considered as the expected "theoretical results" and the corresponding measurement
on an actual cell, it may be possible to develop early warning signals and react accordingly.