Sizing processes, when combined with comminution or attrition scrubbing, are useful to reject undesirable clays and weaker portions of the aggregate into the finer size fractions.
A grizzly screen is commonly used for this purpose ahead of the primary crusher to remove friable materials such as clays, shales, gypsum and porous weathered stone from a quarried rock. Igneous rocks often contain weak, weathered or altered minerals in joints, veins and fissures and sedimentary rocks such as limestones are commonly interbedded with bands of shale and clays. In desert environments scalping and sizing may be able to reject deleterious, friable components of the rocks such as concretions containing gypsum and salt. It should be noted that the thin bands of shale, not uncommonly interbedded with limestones and gritstones, pose a particular problem of contamination of concrete and roadstone aggregates. The shale tends to break into particles of slabby shape that are not easily removed by the scalping screening and are, therefore, carried through to the product. Furthermore, the rock will not readily disintegrate when subjected to the washing process. The only solution may be selective extraction in the quarry.
Vibrating screens are employed for scalping at later stages in processing and, ultimately, classifiers are used to remove fine clay particles from sand products.
The 'scalpings' may not always be waste but form a discrete quarry product sold as fill material for example. It may also be useful for the restoration of the quarry or it may be separately processed. The decision as to whether scalpings should be processed by washing depends upon several factors:
- the quantity of scalpings generated in order to clean the coarse stone;
- market for scalpings;
- space and permission for disposal without sterilizing reserves;
- supply of water and space for silt lagoons;
- market for washed scalpings, especially fine aggregates and sand since the revenue must justify the costs.
However, moist plastic clays tend to adhere strongly to larger pieces of rock and to smear over the surfaces. As a consequence, scalping may be inadequate to clean the stone to meet the specifications for, say, concrete aggregates.