It was formerly the custom to consider one set of product gradation, or screen analysis, curves as being suitable to represent the products of both primary (unscreened) and secondary (screened) feeds, making no allowance ore the undersize material which is always present, to some extent, in quarry-run and mine-run materials. The average quarry does not produce as much on this undersize rock as the average mine, but the usual practice in mining operations is to scalp off most of the undersize ahead of the primary crusher. For example, jaw crusher usually is used as primary crusher.
