The machine you atlre talking about is probably simier than you think....o ce they are cut up, the only thing that is magnetic most likely in the pile is the bit with the nail on it....so vyt really small and then as it runs along the conveyor you just pass a 2nd conveyor close to the 1st and that one has a magnetic strip...it will pick up any puece with metal on it...carries it up to the top wherw the magnetic field stops and it just pours out into a separate container...easy cheesy. That it how they separate ferrous metals from non ferrous (well the 1st pass at least).