Eddy current separators...

[Steelforbrains]

Can anyone explain to me how an eddy current separator works to separate aluminum scrap from ferrous and plastic scrap? The separator that I have seen flings the aluminum off of a belt conveyor into a chute. From what I understand the head pulley on the conveyor has a permanent magnet inside of it, that rotates at a much higher speed than the head pulley. This rotating magnet induces an eddy current into the scrap and I guess that sets up an opposing magnetic field in the part which is repelled from the head pulley at great speed. My question is, why is it that the aluminum is thrown so much further than the other materials on the belt? It is simply due to the fact that the ferrous material is heavier and requires more force to throw it as far? I assume that non metallic objects would fall the shortest distance, followed by heavier metal ojects and lighter metals would go the farthest. Please feel free to critique my reasoning and keep in mind that I am not an electrical engineer.

I have also seen another type of separator that is used to separate different alloys/grades of aluminum. This system supplements the preceding system. In this setup there is a long row of what appear to be aluminum panels that hang side by side. The panels pivot at the top and when they detect certain alloys they are actuated to swing forward (in the direction of material flow) to toss the aluminum even further. There is no contact involved between that panels and the aluminum. How does a system like this measure the alloy content, and what causes the attractive force between the alloy and the paddle? This all happens at a very fast pace, I would say several hundred fpm.

Feel free to speculate, discuss, or even guess as to how this works...
Thanks,
Rob Brooker
Conveyor Services Inc.

 

[itsmoked]

Reporting back...

The magnet does seem to be repulsed! As it drops next to a vertical sheet of aluminum the bottom of the magnet always separates from the panel. Often with just the top remaining in contact, the rest at an angle of about 20-30 degrees away from the panel. Often the magnet will entirely leave the panel to free fall, 25% of the time.

resqcapt19; twould seem it is a repulsive force. Thanks for the heads up.

 

[fsmyth]

I just did a *highly* scientific experiment.

Using a A.C.-powered tape eraser (which I have not used for
YEARS), it strongly attracts ferrous plates, and less
strongly repulses stainless steel (unknown alloy), and
even less strongly repulses aluminum plate (97% Al).
So there.
<als>
As a P.S., I have used the same eraser to wipe hard drives,
which usually had an alloy shell, so the attraction must be
considerably greater. Now there's you a project: calculate
the attenuation in the housing.
<als>