Start-up of a Hall-Héroult cell is a delicate task. Modern practices
for high amperage cells involve preheating the lining before the
molten electrolyte is poured in. The optimum preheating method
for a rapid production of metal and a long pot life is elusive.
complex phenomena taking place during start-up. The adequate
modeling of the mechanical response of the lining is critical to
detect risks of cathode block cracking or the development of gaps
where liquids could leak. Taking into account the ramming paste
baking, the quasi-brittle nature of carbon and the contact
interfaces are examples of key elements to consider.
simulations of different thermal bake-out scenarios were
performed using the in-house code FESh++. Potential industrial
application of the model is discussed.
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.
The requirements for preheating, summarized in [2], are the
following:
in. Freezing leads to an uneven current
distribution and a potentially harmful unstable
early operation.
blocks when bath is poured in. Large
gradients may induce cracks.
large thermal gradients within the cathode blocks will
occur and may also induce cracks.
flash pyrolysis when bath is poured in.
times to ensure no gap is present in the lining as this
would allow bath or metal penetration.
out using gas-fired or oil-fired burners has been shown to provide
the most uniform temperature distribution in the lining [1,3]. The
desired heat-up curve also varies, using one or more surface
temperature ramps, and sometimes ending with a soaking time at
constant temperature [1,2,4].
determine the optimal thermal preheating cycle [2]. Surface and
sub-cathodic temperature measurements were performed for
different ramps and final temperature. It was concluded that the
best results were obtained with a low heat-up rate and a high final
block temperature.
possible calls for the shortest possible preheating time. This
results in less than optimal heat-up curves for the cathode blocks
and lining, and the mechanical response of the cell then becomes a
critical limiting factor.
early operation of a cell. Predicting its mechanical response is not
a simple task.
interaction of the lining with the pot shell, the intrinsic mechanical
behaviour of the lining materials, the transformations within the
materials, and the cell construction are important and are changing
during the bake-out period.
provide insights into these complex phenomena and help in
designing the optimal procedure for a given cell.
lining, but is also an integral part of the cell design. In addition to
its thermal purpose, it must also maintain the lining in firm
compression without inducing cracks or excessive deformation.
It is observed that in operation the pot shell deforms. The problem
is in reality strain-driven. The shell and the lining deformation
and stresses result from the complex interaction of their expansion
(thermal and chemical) and their stress-strain response. Although
during preheating the shell is not likely to deform very much, this
does not change the fundamental nature of the problem. The
dilatometric response of the materials and their stress-strain
behaviour must therefore be known accurately.