§ 4.2.1 Magnetic Separation of Free Metals
As discussed in the section on asteroid resources, asteroids are rich in free nickel-iron metal granules. The Moon has trace but extractable quantities of these granules as well, left over from asteroid impacts and preserved on the waterless, rustless Moon.
Though there are big differences in concentration and size of granules between the two sources, the basic process is the same.
After grinding, the streams of material are put through magnetic fields to separate the nickel-iron metal granules from the silicate grains. Repeated cycling through the magnetic field gives highly pure bags of free nickel iron metal. One of several alternative ways is to drop a stream of material onto magnetic drums, as shown in the figure below. This method also shows an impact grinder discussed in the next paragraph. The silicates and weakly magnetic material deflect off the drum whereas the magnetic granules and material holding magnetic grains stick to the magnetic drum until the scrape off point.
An optional additional piece of equipment is an "impact grinder" or "centrifugal grinder" whereby a very rapidly spinning wheel accelerates the material down its spokes and flings it against an impact block. Any silicate impurities still attached to the free metal are shattered off. It's feasible to have drum speeds sufficient to flatten the metal granules by impact. A centrifugal grinder may be used after mechanical grinding and sieving, and before further magnetic separation. In fact, most of the shattered silicate will be small particles which could be sieved out.
Magnetic beneficiation can be used not only for separating pure nickel-iron metal granules, but also for minerals which have weak magnetic properties. This is done at Earth mines. In space, where gravity is lower and more sensitive processes are possible, magnetic beneficiation can play a significantly greater role.
The quick & dirty theory
There are minerals that are attracted, repulsed, and unaffected by magnetic fields, based on their "permeability" to magnetic fields. This is often illustrated by showing a picture of magnetic field lines and grains which attract lines by bending them into the grain (concentrating), grains which repel the lines, and grains which aren't affected. The degrees of magnetic permeability differ from mineral to mineral. Particles which concentrate the lines of force and become polarized and consequently attracted are called "paramagnetic". Those which disperse the lines are called "diamagnetic".
Based on magnetic behaviour, paramagnetic materials are sub-classified as ferro-magnetic and feebly magnetic.
Magnetic separators are classified as drum, pulley, disc, ring and belt separators. They are all based on the same principle, and all use a provision for feed to run into and through the magnetic field and various means for discharging separately the magnetic and nonmagnetic portions.
See also the PERMANENT section on electrostatic beneficiation, a similar process for separation of minerals based upon electrostatic, instead of magnetic, properties.
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