Author: K. Adham
Abstract
Particle size classification yields products with a specific size distribution or with a minimum amount of the fine or course material. Classification can be done in a variety of process equipment. Equipment selection depends upon the physical properties of material, the required cut size and the process economics.
Dry screens can be used for many applications, but are usually less effective for separating the fine fractions (smaller than 500µm). Air classifiers are commonly used for separating out fine and coarse material. These pneumatically transport the solids in air to separate the fines, which are more easily elutriated than the coarse particles.
The fluidized bed (FB) classifier can be used to remove fine particles from the bulk of the material while the main charge is not pneumatically transported, but is merely suspended in a fluid-like state. FB classifiers are especially useful for recovering fines from coarse products, with a minimum loss of the coarse fraction, while using a small amount of low-velocity classifying air. Fines elutriation from FBs has been studied extensively, but most of the available correlations only apply to units with straight freeboards. Little information is available for designing FB classifiers with expanded freeboards. Here, a simple method is provided for the preliminary sizing of such units.
