TABLE OF CONTENTS Industrial sizing is extensively used for size separations from 300 mm down to around 40 m, although the efficiency decreases rapidly with fineness. Dry screening is generally limited to material above about 5 mm in size, while wet screening down to around 250 m is common. Although there are screen types that are capable of efficient size separations down to 40 m, sizing below 250 m is also undertaken by classification (Chapter 9). Selection between screening and classification is influenced by the fact that finer separations demand large areas of screening surface and therefore can be expensive compared with classification for high-throughput applications.
The types of screening equipment are many and varied. Likewise, there are a wide range of screening objectives. The main purposes in the minerals industry are:
Sizing or Classifying, to separate particles by size, usually to provide a downstream unit process with the particle size range suited to that unit operation;
Scalping, to remove the coarsest size fractions in the feed material, usually so that they can be crushed or removed from the process;
Grading, to prepare a number of products within specified size ranges. This is important in quarrying and iron ore, where the final product size is an important part of the specification;
Media recovery, for washing magnetic media from ore in dense medium circuits;
Dewatering, to drain free moisture from a wet sand slurry;
Desliming or de-dusting, to remove fine material, generally below 0.5 mm...
