Pneumatic classification

Pneumatic classification, or selection, is very often carried out at room temperature with a suspension of particles in a gas, in order to obtain two separate fractions, one containing particles larger than a given size and the other those smaller. It is based on the difference that can exist between the falling velocities of particles in a gas. These particles are generally sufficiently fine for their movements to obey Stokes' law.

Pneumatic classification differs from dedusting, the aim of which is the almost total elimination of solid particles contained in a carrier gas (see "Dedusting and dedusting" file [J 3 580] ). For example, in the case of dedusting, there is no optimum specification for the material to be treated, whereas in the case of pneumatic classification, there is a relationship between the fineness of the classifier feed and that of the fine product separated.

All pneumatic separators operate with a gas stream. Although every effort is made to make the best possible use of the forces involved, pneumatic classification, which may be neat in theory, is imperfect in practice. In fact, forces external to the gas-particle mixture, due to acceleration fields created by gravity, magnetism or electrostatics, or forces internal to the gas-particle mixture, due to centrifugation, diffusion, electrostatic coagulation, etc., may come into play. These forces may add to or oppose each other, may or may not be modulated, and may or may not create disruptive effects. These effects are reflected in the precision of the granular cut, the assembly of the equipment and the economy of the process.

Readers are referred to the "Fragmentation. Theoretical aspects" [J 3 050] .