Screening - Screening surfaces and equipment

Screening and sifting are unitary operations that separate a population of material fragments according to their size into two (or more) parts. This is achieved by using a surface equipped with openings that let through fragments of smaller dimensions than those of the openings, while retaining fragments of larger dimensions.

Although, theoretically, the term sieving is reserved for fine separations below the millimeter and the term screening for coarse separations, the term screening usually covers both fine and coarse separations.

Any set of particles can be subject to such a process. Particles can be of mineral (aggregates or fragmented rocks), vegetable (grains, flours), natural or artificial origin (various manufactured objects).

The purpose of screening can vary:

• Separate larger fragments before reprocessing to improve fragmentation;

• conversely, eliminate the finest fractions that may be a nuisance or that can be treated separately (e.g. agglomeration);

• prepare particle size classes to meet commercial requirements (aggregates, coal) ;

• improve the efficiency of a physical (e.g. gravimetric) or physico-chemical (e.g. flotation) treatment process by judicious selection of the particle size ranges to be treated.

Unless otherwise indicated, the percentages given in this text are by mass.

Descriptions of the main screening machines and techniques are the subject of this first dossier [J 3 101]; the criteria for selecting and designing screens, and their use in the specific field of mechanical ore preparation, are covered in the following dossier [J 3 102] .