Which method is of separating fine particles from bigger particles?

Answer 1

The process is called "sorting". There are numerous methods to sort particles.

For example the physical size can be used to sort particles by passing particles through some sort of sieve or series of sieves. The larger ones will be trapped and smaller ones will pass through. The bank of sieves is usually vibrated to assist passage of the particles.

Another method uses the difference in volume/area ratio between large and small particles. A simple way is to let particles fall through a rising flow of gas. The smaller particles will experience a low weight force but a high drag force so they will rise in the gas flow, while the larger particles experience a much higher weight force and only slightly larger drag force so will sink in the gas flow. Varying the gas flow rate will select the particle size separation or sort.

A variation on the volume/area ratio method is to use an air flow acting against a "centrifugal" force. (there is no such thing as centrifugal force, but the tangential momentum applied to a spinning wheel is sometimes called "centrifugal force"). In these sorters a disc with radial channels spins. Air is blown towards the centre of the disc and the particles are introduced at the axle where they are tangentially forced towards the outside. Light particles will be blown back and heavy particles will be thrown outwards.

There are other methods to sort particles.