Aluminium smelter, along with ferrochrome and other electric arc furnace types, are not the load you can turn off fast or easy. Turn off the power for over 5 minutes and you will be spending a lot of money to strip the furnace out, move the 20 tons of solidified metal and slag to a corner of the plant ( as in move the whole furnace and the solid chunk inside it) and build a new pot in place of it. Then you have to break up the block of metal and glass, and dispose of it.
Power companies that supply smelters have a multi page contract for power, and there are massive penalties for unscheduled interruptions, and the power company will load shed pretty much every other customer before they will drop that smelter. Turning off a pot line is going to take a week, ramping down the feed and the pool of molten metal to the point where you can handle the blob when you switch off and it cools down. You only do that when you are relining the unit or upgrading it.