| Popis: |
Sizing, as performed on screens, grades material according to the minimum cross-section presented during the time of passage across the meshes of the screen cloth. The regularity of the industrial product is dimensional, and takes no account of differences between the weights of the particles in a given grade. “Sorting”, or as it is more usually called classification, discriminates between the behaviour of particles in a fluid and grades them according to their surface, volume, and density. Since ores contain particles of varying densities, this is not a sizing operation. The fluid mostly used is water, though high-density salt solutions are employed for special purposes. Material required in a dry state may be sorted by floating it in air currents of controlled strength. Screening is only used for comparatively coarse material, as the rate of treating large quantities of ore becomes slower when fine-meshed screens are used. Wet screening is practised commercially down to 65 mesh, but industrial dry screening is rarely carried below 20 mesh. Classification can be used from coarse-sand sizes down to well below 200 mesh. Fine particles (say −20 mesh) must have fluid mobility if they are to be sorted, and these conditions cannot be contrived with a long-ranged dry feed subjected to screening. |
