(57) Disclosed are differential rate screening processes and apparatuses for continuously
screening undersize particles in different size classes to different degrees of incompletion
to provide a product having a preselected distribution of particle sizes substantially
different from the distribution of particle sizes in a feed of particulate material.
An input stream of feed material (62') is introduced onto a screening member (60')
having apertures of sufficient size to pass a plurality of size classes, and is separated
into at least a throughs stream (250') and one other stream (252') by causing undersize
classes to pass through screen apertures (64') and into a throughs stream (250') in
proportions relative to one another substantially different from the proportions of
the same undersize classes relative to one another in the input stream (62'). The
overs, throughs or both from one rate screening member (60') may be introduced as
an input stream onto another rate screening member (68'). A sufficient population
of undersize particles are provided in each undersize class and differentials between
relative proportions of undersize classes in an input stream (62') and relative proportions
of undersize classes in a throughs stream (250') are controlled so as to provide a
product having substantially the desired particle size distribution. Various means
such as a shroud (130') are provided for causing differentials between relative proportions
of undersize classes in an input stream (62') and relative proportions of undersize
classes passing through a screening member (60', 68') and into a throughs stream (250',
258'), and for controlling these differentials. An output stream from one rate screening
member (60') may be blended with an output stream from another rate screening member
(68') and at least a portion of an output stream from one rate screening member may
be recycled to a crusher and then sent to the same or a different rate screening member.
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