In aluminum production, the electricity is used for electrolytic reduction, not melting. It's the waste heat from that which keeps the cell hot enough.

> Presumably any process that can use the heat directly rather than converting to electricity would be more cost-effective, as you lose most of the energy in that conversion.

All practical electricity generation technologies do it through heating water just the same, and they produce hot water as a waste product too. So, if your industrial process can use heat directly, it might as well use waste heat from existing power plants.

And some places this is how entire cities are heated...

Well, usually they run out of excess heating in the winter.

Perhaps, Yellowstone and Iceland ought to have some heavy industry.

Iceland has some large aluminum smelters and aluminum is 40% of their export income[1]. They do heat their homes with geothermal. It's basically free and when I was there the lowest setting for the heat in my apartment was 22.4C (72F).

[1]https://en.wikipedia.org/wiki/Economy_of_Iceland

Interesting link. 800,000 mtpy is a lot of aluminium. While the ‘zero environmental imapct’ claim is controversial, the environment impact is definitely lower than many other producers.

> Presumably any process that can use the heat directly

Maybe build a massive seawater pipeline and create desalination plants that can then be redirected to California/Nevada