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SIEVE BENDS

Sieve bends are classification devices commonly used ahead of vibrating screens for initial size separation and in dewatering stages to remove the bulk of the water and very fine size particles from the vibrating screen to reduce its load.  They can be used by themselves in one or two-stage units after cyclones or by themselves, and before flotation, again, as a dewatering and classification step.  The sieve bend is a curved, stationary sieve developed by the Dutch State Mines organization.  To im­prove screening efficiency, especially on very fine coals, use of vibrating sieve bends has been done successfully at several operations.

 Equipment Sizing

The following tables are the general sizing characteristics for sieve bends.  As a general rule the size of classification will be 3/4 to 1/2 of the aperture depending on angle.  Sharper and higher angle sieve bends will classify half the sieve bend aperture, and flatter sieve bends will classify 3/4 sieve bend aperture.

 For dense medium sieve bends ahead of drain and rinse screens use the following.

 Equipment Performance

 Sieve bend feed is normally in the range of 25 to 35% solids by weight, an average condition would be 30% solids.  Classifying sieve bends, ahead of desliming screens and handling coarser size material can be fed at higher solids concentrations.  Des­liming sieve bends handling fine feed after cyclones can be fed lower solids con­centration, commonly down to 5 to 10% solids by weight.  Sieve bend handling +28 mesh feed will produce a product oversize of 35 to 50% solids by weight.  Sieve bends handling finer sizes 28 mesh x 0 will produce 35% and lower solids concentra­tions.  Misplaced materials in the sieve bends commonly runs on the order of 10% of the unders in the overs and 5% of the overs in the unders.