The Process of Stratification
Before a product can be separated by weight the product must be fluidized and then stratified vertically. On
Gravity Separators lighter particles are moved upward through the fluidized bed. Heavier particles sink downward.
This produces stratified layers with lighter particles in the upper layers and the heavier particles in the
lower layers. Air is used as the fluidizing medium for the process of stratification. Stratification occurs by
forcing air upward through the particle mixture so that the particles rise or fall by their weight relative to the air
flow. Figure 1 represents a cross section of the Gravity Separator directly over a fan. A particle mixture
consisting of heavy and light material has been introduced on top of the screen deck with the fans off. In
Figure 2 the fan has been turned on and adjusted, so that the heaviest particles sink to the surface of the deck
and the lightest particles are lifted to the top of the material completely free of the deck surface. Proper
regulation of the air-flow at this time is critical; or the result is a situation seen in Figure 3, where all particles
are lifted free of the deck surface by excess air. This results in a boiling turbulent action and can actually remix
the stratified material.
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