When a magnetizable solid is subjected to a magnetic induction field, the local magnetic moments are oriented in the direction of the induction fields, and the whole solid becomes similar to a magnet, each pole is attracted by the opposite pole of any magnet in the vicinity.
When we want to attract a product which is not highly magnetizable, we must use a powerful magnet, and vice versa. In the way of magnets, we could use:
– permanent magnets;
– electro-magnets, whose intensity can be varied from one machine to another;
– cylinders of mild steel, situated in the air gap of powerful magnets, and
magnetized by induction (“induced rotors”).
The general principle is to attract the magnetizable particles with a metal surface which is, itself, magnetized. Then, the metal surface moves out of the induction field and the particles that it had attracted detach, generally aided by centrifugal force (if the attracting surface is a cylinder) or a fluid current.
Whether the product is dry or dispersed in water, the principles remain the same. However, humid separation is limited by the viscosity of the liquid – particularly in the case of fine particles, whose velocity of displacement is reduced. Dry separation circumvents this problem but, in order to treat particles which are smaller than 20?m, the product’s humidity must be less than 1%.