Aluminum smelters use the Hall-Heroult process, where alumina is dissolved in molten cryolite and reduced in massive “pots” which are large electrolytic cells. Each pot contains a carbon cathode lining that must be kept at around 950C during operation. If the pot cools down, the frozen electrolyte and solidified aluminum contract at different rates than the carbon and steel shell, cracking the lining.

Once it’s cracked, the pot has to be completely cleaned out and relined which takes weeks. A smelter usually has hundreds of pots so this alone takes a while as the liner and anything in it are basically frozen solid and need to be broken apart and torn out. Once relined the pots must be brought back up slowly and the chemistry balanced. The pots also draw a ton of power and are wired in series so they have to all be brought up slowly together (or in batches).

That assumes it was a clean shutdown with nothing else clogged up in the system. “Cleaning” in smelting means that the hardware involved needs to be replaced because it fused to molten metal while cooling down.