[Filligree]

While I can’t speak to aluminium specifically, smelters in general have the problem that they’re pumping molten metal around. And the thing that keeps it molten inside the pipes is, usually, just inertia.

Pausing the plant doesn’t just mean pushing a button. Safely shutting it down is a long process of draining every single part of molten metal first, and might not even be doable; almost any other factory would have an easier time.

[pfdietz]

Molten cryolite rather; I think the metal pooled in the bottom of a pot could be drained off or even allowed to solidify (it would still be conductive.)

A related problem is that the walls of the pot cannot be allowed to get too hot. If this were not the case, they could simply insulate the pots to reduce heat losses to some arbitrarily low level.