Metals can be sorted fairly efficiently. Most types of steel can be pulled out with magnets. Nonmagnetic metals can be sorted from plastic by having a conveyor

Metals can be sorted fairly efficiently. Most types of steel can be pulled out with magnets. Nonmagnetic metals can be sorted from plastic by having a conveyor belt chuck the incoming stream over an gap with a magnetic field applied to it. The magnetic field induces a current in the flying metal, causing the metal itself to become magnetic. This causes the metal to slow down (eddy current braking), and now you have physically separate streams of plastic etc. (fast) and metal (slow).