Been thinking along the lines of Planet of the Apes -but without any man made catastrophe, meteorite impact and climate stabilization- what sort of structures or evidence of our civilization would remain in five or six million years if we all just vanish? Would an intelligent species of evolved insects -or something else- find buried artifacts in the ground like we did the dinosaurs? Would ancient structures like the pyramids or Great Wall remain for that long, and would any modern things like the Eiffel tower, Burj Khalifa, or even some present day Doomsday Vaults survive that amount of time? In our digitized age, I assume that not much would remain, other than satellites maybe or I suppose any modern species that just happens to get preserved like a dinosaur did.
But even rusted iron can be re-smelted, no?
Rust needs to be reduced back to ore, using a reactive, usually coke. Coke is purified coal. Coal is a fossil fuel. You can do it with charcoal, which can be made by burning wood, so it’s possible without coal, just not as efficient. This assumes you can gather the rust - it tends to break down and disperse into the environment, but if you broke up concrete to get at rusted rebar and could collect the rust, you could reduce it with charcoal.
Again, it’s a matter of scale. We mine iron and deposits because we can get large amounts in seams. If you’re trying to harvest rust and reduced it with charcoal, you’re producing iron on the scale of making knives and swords, not cars, or combine harvesters, or more rebar.
It’s a chicken-egg problem. We have been able to come as far as a have because oil, coal, and iron were just laying around on the surface, in huge quantities. Those are gone, and now you need the big tools first to get at the reserves that are left.