CHOICE OF MESH OPENING RELATED TO THE DESIRED SEPARATION SIZE
The vibrating screen function consists in separating materials into size fractions, avoiding the excessive contamination of one fraction with particles belonging to another. The product sizes thus obtained are measured in laboratory screens where the screening media are oriented horizontally and very long screening time warrants the passage of all particles with dimensions below those of the mesh opening used. The actual separation process in vibrating screens differs from the laboratory equipment performance. The screen slope and the particle trajectory travel reduce the projection (X) of the free passing area (A) according to this figure:
Consequently, the passing particles are slightly smaller than the mesh opening itself.
Also, the thickness and material of the separa- ting media influence the passing material size..
In order to obtain well-defined separation, the mesh opening must always be slightly larger than the specified separation size.
In summary, we have the following definitions:
Product size – the lab screen mesh opening through which the tested material passes.
Equivalent opening – vibrating screen-opening mesh that achieves a specified product size.
Equivalent opening > product size For practical reasons, it is considered acceptable that a product contain 3 to 5% material with dimensions slightly greater than those for the specified size, and this is taken into account in vibrating screen capacity determination factors.
For example, if we wish to obtain a 20-mm product, the screen opening mesh has to be larger and the product will contain 3% particles of slightly larger than 20 mm.
On the other hand, if we decide to use an opening mesh equal to the desired size, 20 mm, the passing material should be contamination-free; however, what is retained will be highly contaminated with fines and never reach an acceptable efficiency.
The reason for this phenomenon is that actually we reduce the product size, and the efficiency should be measured in relation to the smaller separation size.
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