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What is magnetic separation?

The magnetic separation is a physical separation technique which is based mixtures or using the magnetic properties of the material , such as its sensitivity to the attraction or repulsion magnets.

It is also known as magnetization, because the objects that can be separated by this technique are magnetized by the magnetic field of the magnets. As a result of this natural process, these objects, such as iron, are strongly attracted to the magnet, causing them to leave their free mixture.

The most classic example of a magnetic separation is that of the mixture of iron and sulfur filings (upper image). Iron filings respond to the magnetic field of the magnet, and depending on the magnetic orientations of the filings, they are directed to either of the two poles of the magnet (north or south).

Therefore, the iron filings leave the mass of sulfur, an element that, unlike iron, does not feel any attraction to the magnet. And this is how magnetic separation works, being widely used to obtain all those objects or contaminants that can be extracted with powerful or small magnets.

Magnetic separation process

Theoretical principles

Ferromagnetism

What objects can be magnetically separated? The intuitive answer would be metals. But are all metals attracted to magnets? Or better yet: can there be bodies other than metals that respond to the magnetic field of magnets? For example, mineral fragments from some rocks.

It all depends on the magnetic nature of the solids subjected to the separation. These must be ferromagnetic. Iron is very ferromagnetic, and therefore it is quite sensitive to being magnetized by magnets. This is the reason why we can extract iron objects, as in the case of a mixture of iron and sulfur.

But iron is not the only ferromagnetic metal: we also have nickel, cobalt, and other less common metals such as gadolinium and dysprosium. These metals, depending on their ferromagnetism, respond more easily to the magnetic field of a specific magnet; the stronger the response, the faster and more effective your separation will be.

Paramagnetism and diamagnetism

Those objects that are paramagnetic instead of being ferromagnetic, such as platinum, aluminum and magnesium, need magnets that provide very strong magnetic fields in order to be separated. This is because they are not susceptible to being magnetized, and therefore, they hardly feel attracted to magnets.

On the other hand, diamagnetic objects, unlike the previous ones, feel a weak repulsion by magnets. Therefore, a magnet would not be able to extract them from their mixtures. Among some diamagnetic objects we have: silver, copper, wood, gold, mercury, diamond, etc.

Practice

For magnetic separation to be possible, there must be ferromagnetic objects or components in the mix. In its default , paramagnetic components, which however will require stronger magnets.

Depending on the dimensions of these components and their magnetic sensitivity, the characteristics of the magnet with which the separation will be carried out will vary.

Similarly, the mixture with all its components can be suspended in water containers, deposited in rotating drums, or passed through a movable ramp under a roof of magnets.

Once the magnets have acted on the mixture, the ferromagnetic components will be stuck on or very close to them. The rest of the mixture is then removed, with small impurities that can be separated by applying other more refined magnetic separations.

Examples of magnetic separation

The following examples will emphasize the action of magnets in different applications of magnetic separation.

Iron blends

Iron and gold separation

As we saw at the beginning, a magnet can attract iron filings from a portion of solid sulfur. Similar experiments are reproduced using flour, sugar, sand, salt, or even liquids like water or oil.

With magnetic separation it is possible, for example, to collect iron waste under the sand on the beach or to separate iron from gold, a common process in jewelry.

In any mixture where there is iron, it can be separated by using a magnet; even when the iron particles are microscopic. The latter is very important in the food and pharmaceutical industries, which must decontaminate their fumes.

Iron minerals

Not only iron is ferromagnetic, but also some of its main minerals or ores. For example, magnetite, Fe 3 O 4 , and pyrrhotite, Fe 7 S 8 , are also ferromagnetic, and therefore sensitive to the magnetic field of magnets. Consequently, we can obtain these minerals from their pulverized and processed rocks.

Similarly, paramagnetic minerals can be obtained by magnetic separation; such as ilmenite, FeTiO 3 , siderite, FeCO 3 , and chromite, FeCr 2 O 4 . The difference is that they need more powerful magnets to magnetize and extract them.

Therefore, magnetic separation is important in iron mining.

Scrap metal recycling

A crane with a magnet to attract iron

Although the magnetic separation of the iron in the mixtures has already been specified, this same procedure cannot be left without mentioning but applied in the fields of demolitions or the scrap yard.

For example, as seen in the image above, a crane can carry a powerful magnet to lift and attract very heavy steels from the rubble.

Also, anyone who has seen the movie Toy Story 3 will have noticed an example of magnetic separation when Buddy and his friends tried to save themselves from the garbage disposal. Each of the toys clung to the ferromagnetic objects they found before they ascended by the effect of the upper magnet; a feat that Rex, the toy dinosaur, had difficulty performing.

This is so because iron and iron alloys (which are many in addition to steel), can be recycled and used for the manufacture of other instruments. Therefore, magnetic separation is an essential technique for recycling scrap metal and its most valuable components.

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