Magnetic Separation Techniques

Magnetic separation is a method of separating a mixture by using a magnet to pull out the components that are attracted to it, such as iron, steel, nickel and cobalt. In brief: a magnet lifts iron filings out of a mixture with sand, and large electromagnets separate steel cans from household waste at recycling plants.

The technique works only if at least one component is magnetic and the others are not. It needs no water, heat or chemicals, so it is clean, fast and cheap. This article explains which materials respond to magnets, how to carry out a separation, how industrial separators work, a worked example with masses and the usual errors.

How does magnetic separation work?

A magnet produces a magnetic field that pulls on certain materials. Iron, nickel and cobalt, and alloys containing them, are strongly attracted. Most other materials, such as sand, plastic, glass, aluminum and copper, are not attracted. When a mixture is brought near a magnet, the attracted material leaves the mixture and sticks to the magnet, while the rest stays behind.

The separation is purely physical. The magnetic material is not changed and can be removed from the magnet afterward, often by pulling the magnet away or switching off the electromagnet.

Which materials can be separated with a magnet?

How do you separate a mixture with a magnet?

1. Spread the dry mixture in a thin layer on a tray or sheet of paper.
2. Wrap the magnet in a thin plastic bag or a piece of paper, which makes it easier to remove the attracted particles.
3. Move the magnet slowly over the mixture, close to but not touching the particles.
4. Lift the magnet and hold it over a clean container. Remove the bag or paper so that the magnetic material falls off.
5. Repeat until no more material is attracted.

How is magnetic separation used in industry?

Recycling plants use overhead electromagnets or magnetic drums over conveyor belts. As waste moves along, steel and iron objects are lifted off the belt and dropped into a separate bin, while non-magnetic waste continues. Aluminum cans are then separated by another method known as an eddy current separator.

Mining companies use magnetic separation to concentrate iron ores such as magnetite. Food manufacturers use magnets to catch tiny metal fragments in flour, sugar and spices, which protects both consumers and machinery.

Worked example: a three-component mixture

A mixture of 12.0 g of iron filings, 30.0 g of sand and 8.0 g of salt must be separated.

1. Magnet. Pass a magnet over the dry mixture and collect the iron filings. Mass of iron recovered, for example, 11.8 g, which is 11.8 ÷ 12.0 × 100 = 98.3% of the iron.
2. Water. Add water to the sand and salt. The salt dissolves, and the sand does not.
3. Filtration. Filter to collect the sand. After drying, the sand mass is, for instance, 29.5 g.
4. Evaporation. Evaporate the filtrate to recover the salt, for example 7.7 g.
5. Check the total. 11.8 + 29.5 + 7.7 = 49.0 g, compared with 50.0 g at the start, so 1.0 g (2%) was lost.

The order matters. The magnet is used first, while the mixture is dry, since water would make the separation messy and could start the rusting of iron.

How was magnetic separation developed?

The magnetic mineral lodestone has been known since antiquity. Magnetic separation of ores began on an industrial scale in the 19th century: in the 1800s, several inventors built magnetic separators for iron ore, and the technology expanded quickly as electromagnets improved. Modern high-gradient magnetic separators can even remove weakly magnetic particles from water and clay, using strong magnetic fields.

What are the limits of the method?

Only magnetic materials can be removed, and the method cannot separate two non-magnetic components. Some magnetic components of a mixture may be trapped between non-magnetic ones and left behind. A weak magnet may fail to lift pieces that are large or covered by other material.

Which mistakes are common?

Mistake 1: Assuming that all metals are attracted

Aluminum, copper, zinc, gold and silver are not attracted. Only iron, nickel, cobalt and some of their alloys are.

Mistake 2: Using the magnet on a wet mixture

In a wet mixture, particles stick together and iron may rust. Dry the sample first.

Mistake 3: Testing stainless steel as proof of iron content

Some stainless steels contain a lot of iron but are non-magnetic because of their crystal structure.

Mistake 4: Placing strong magnets near electronic devices

Strong magnets can damage some devices and interfere with medical implants. Keep them away from phones, bank cards and pacemakers.