Naked Science Forum
General Science => General Science => Topic started by: chris on 29/04/2017 09:49:31
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I received this email from Harrison:
"What materials can eddy currents sort?"
I must admit that I'm not really sure what is being asked here: is this an electromagnetism question, or a geology question about hydrology? Can anyone help?
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You can use it to sort out things like copper and aluminium from a waste stream after you have pulled out the ferrous objects, as the eddy current separation will take these common metal types out as the rest of the stream follows a gravity fall, you can use this to change them to side chutes for separation. Then you can further sort them using a further eddy current sorter as they do have different conductivity, so you can increase the concentration of the metals in the streams.
After that you either smelt them again and accept the impurity level increase, or do more refining and get a pure metal back. For copper that is electrolytic refining, and for aluminium it is just resmelting in an inert atmosphere and skim off the impurities off the surface.
Of course you do need a waste stream high in metal, and all preferably shredded to a uniform size so particles are mostly purer metal. The scrap stream out is going to be mostly plastic and glass, so you can burn it in an incinerator to get power, though you will need a high temperature incinerator and precipitatorsand filters to get the more toxic gases, like dioxins, to either decompose or get filtered out of the hot gas.
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I'm intrigued by the point you make about further differential sorting based on conductivity. How does that work?
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From the electromagnetic viewpoint:
Magnetic fields are often used in waste recycling to separate out metals from the waste stream.
- Iron-containing items like food cans are attracted to a steady magnetic field, and can be separated
- Aluminium/Aluminum cans are not attracted to a steady magnetic field. However, a moving magnetic field will induce eddy currents in the can. Because these cans are fairly light, the magnetic field produced by these eddy currents produces a force that diverts the aluminium cans
- The familiar metal detectors often seen used on a beach (or used when searching for gold nuggets deposited by an eddy in a river) generate an oscillating magnetic field, which induces eddy currents in any buried metal; this does not occur for non-conductive dirt or sand. Unfortunately, you are more likely to find a buried can than a gold nugget!
- The Wikipedia article (below) has one type of separation that I did not know about - a permanent magnet inside a vending machine is used to distinguish the conductivity of real coins from fake coins.
See: https://en.wikipedia.org/wiki/Eddy_current
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Who is eddy currents?