Does smelting actually need a lot of labor? I'd imagine this process is very scalable, bauxite flows into the plan, into furnaces, into machines which create ingots or sheets, etc. Labor would mostly be at the input/output stage of unpackaging/packaging, and of running the factory machinery and management.

But this is not like manufacturing phones where labor = O(number of phones produced). Am I wrong? Why is aluminum production not almost purely capital intensive?

If so, it would suggest that the advantage of China isn't cheap labor costs, but either cheap energy, cheap regulations, or cheap currency.

Which really means Chinese aluminum needs to be taxed to price in the full costs of environmental damage. If for example, the aluminum is produced by carbon emissions, or just plain old soot and other pollution, the costs of this should be estimated and other countries which import Chinese aluminum should slap a tariff on it equal to those costs. They should also domestically tax carbon. Actually, I'd argue that you can't really tax carbon domestically unless you tax it internationally.