background image
One might argue that just as the Moore Law will eventually hit the quantum
mechanics physical barrier in the computer industry, a similar barrier has already stopped
the aluminum industry in its effort to continuously produce higher amperage cell designs.

If it is the case, it has been clearly demonstrated in [1] that if there is a limit, that
limit is not related to the cell heat balance aspect of the cell design.

With its width to length aspect ratio, a 300 kA potshell dissipates less that 20%
extra heat in its end walls as compared to its sidewalls. This in turn represents not much
more that 10% of the global heat dissipated by the cell. A simple reduction of 10% of the
cell current density, for example from 0.73 A/cm
2
to 0.65 A/cm
2
, would be sufficient to
allow the operation of an infinitely long cell with the exact same lining design while
preserving a perfect thermal balance.

On the MHD cell stability aspect, as the cell gets longer and the return line
influence increases, minimizing the longitudinal gradient of the Bz component of the
magnetic field is certainly becoming more challenging. Yet the magnetic compensation
techniques (asymmetric busbar, compensation loop, further extension of the return line
location, etc) are not loosing their efficiency as the cell amperage increases. Some very
well established MHD experts argue that there is a MHD barrier to the continuous
increase of the cell amperage, other are not so sure.

Even if the jury is still out on the possible existence of an MHD related cell
amperage limit, there is no indication that we are currently near that limit as it was not
that difficult to find a stable cell busbar configuration in order to produce a
demonstration 500 kA cell design [3].
MECHANICAL ASPECT OF THE CELL DESIGN

The thermo-mechanical response of the cell results from complex interactions
between the refractory lining and the steel shell. The lining expands thermally, but some
materials ­ prebaked carbon and ramming paste for instance ­ may also experience
irreversible deformations like chemical swelling and contraction, plastic deformation,
creep, etc. [4,5]. The shell also expands thermally, and it may also deform plastically.
The confinement provided by the shell must be able to keep the lining in compression
while avoiding cracking of the cathode blocks.

The mechanical aspect of designing a high amperage cell with a length to width
aspect ratio of 4 or more is not well covered in the literature. When a potshell reaches a
given length, its vertical deformation due to the presence of thermal gradient in the
sidewalls starts to affect the cell operation. The difference of level between the middle of
the shell floor and its corners leads to a metal pad of varying depth. This in turns may