INTRODUCTION
It is well accepted that start-up and early operation have a strong influence on the performance
and life of a Hall-Héroult cell [1]. Generally, a preheating phase is necessary during start-up to
ensure a smooth transition to normal operation.
As summarized in [2], some of the requirements for preheating are the following:
The cathode block temperature must be high enough to:
o
Minimize bath freezing when bath is poured in. Freezing leads to an uneven
current distribution and a potentially harmful unstable early operation.
o
Avoid large thermal gradients in the cathode blocks before bath is poured in.
Large gradients may induce cracks.
If the preheating rate is too fast or not uniform enough, large thermal gradients within the
cathode blocks will occur and may also induce cracks.
The paste temperature must be sufficiently high to avoid flash pyrolysis when bath is
poured in.
The lining must be maintained in compression at all times to ensure no gap is present in
the lining as this would allow bath or metal penetration.
Complex phenomena are taking place during the start-up and early operation of a cell, for
instance the transformations within the materials, the interaction of the lining with the pot shell
and the electrical contact between the carbon electrodes and their metal conductors.
Direct electrical coke-bed preheating is one of the most common techniques. This practice
typically results in a non-uniform surface temperature distribution and can generate large
thermal gradients in the cathode blocks. To assess the risk of cell failure, the mechanical
response of the cell must be studied.
Due to the intrinsic mechanical behaviour of the lining materials and the cell construction
peculiarities, this is not a trivial task. Numerical modeling is therefore an interesting tool to help
provide insights into these complex phenomena and help in designing the optimal procedure for
a given cell.