The easy way is to just show up, offload stuff, get paid the bulk rate (which isn't much), and let them deal with it.
If you can sort the metals ahead of time into pure types (which, probably, isn't going to be that easy, but maybe pull out the scrap iron from the auto parts if it's flywheels & such), you can get a little bit more.
It depends on how optimal you want to be. If you just want to be rid of it and be paid for your time, drop it off as-is. Spending too much time sorting it probably isn't worth it for that sort of material.
It also depends on the metals involved. Aluminum is worth a lot more than iron, so it might be worth pulling the aluminum bits out.