I'm absolutely no expert here, but tin comes to mind. It was a critical ore with limited sources during the bronze age, and required a substantial trading econ

I'm absolutely no expert here, but tin comes to mind. It was a critical ore with limited sources during the bronze age, and required a substantial trading economy to exist to allow bronze-based technology to flourish. Extracting resources from the detritus of the 20th and 21st centuries would surely be a major activity. But to efficiently do so requires a bunch of power and chemistry and reagents/catalysts that are themselves probably a heap of work to acquire. Getting iron and mild steel from rebar, fallen bridges, rail lines, cars, etc, would likely be plausible though. It wouldn't be high-quality steel, but for blacksmithing-type stuff, it'd work ok. Probably sufficient for sand-casting too? Aluminium and copper are probably recyclable too, although I'm not sure how aluminium would work with different source alloys?