Differential rate screening

Abstract

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 is introduced onto a screening member having apertures of sufficient size to pass a plurality of size classes, and is separated into at least a throughs stream and one other stream by causing undersize classes to pass through screen apertures and into a throughs stream in proportions relative to one another substantially different from the proportions of the same undersize classes relative to one another in the input stream. The overs, throughs or both from one rate screening member may be introduced as an input stream onto another rate screening member. A sufficient population of undersize particles are provided in each undersize class and differentials between relative proportions of undersize classes in an input stream and relative proportions of undersize classes in a throughs stream are controlled so as to provide a product having substantially the desired particle size distribution. Various means are provided for causing differentials between relative proportions of undersize classes in an input stream and relative proportions of undersize classes passing through a screening member and into a throughs stream, and for controlling these differentials. An output stream from one rate screening member may be blended with an output stream from another rate screening member 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.