2.2.2.4 Primary and secondary crushing, grinding and screening

Coarse primary crushing of relatively dry/brittle clays may be effected by large toothed kibbler rollers, usually located in the quarry, or by large jaw-crushers. Very hard raw materials (often used to manufacture refractory products) are also reduced in size using jaw or cone-crushers, both of which compress the lumps of mineral between a stationary and a moving hard surface. Hammer mills achieve size reduction by impact forces – pieces of raw material fed into the mill are shattered when struck by the rapidly rotating hammers.

Edge runner mills (e.g. dry or wet pan grinders) are suitable for reducing the particle size of more plastic raw materials. Pans with perforated base plates ensure a defined maximum particle size, and wet pans have the added function of allowing the tempering of clays with added water. Crushing rollers are widely employed in the heavy clay industry to disintegrate, flatten and homogenise clay particles. Pairs of parallel, smooth hard-steel rollers are driven in opposing directions so that the raw material, fed between the rollers, undergoes nipping, shearing and flattening. Size control is achieved by setting the gap between the rolls. Clay shredders (and also roller mills) are useful in the case of plastic raw materials, and consist of a rotating slotted drum fitted with cutter knives. The clay lumps are forced through the slots

in the form of shavings.

Impact rotor crushers have two rotors carrying bolted-on impactors or ‘shoes’. They rotate towards each other and achieve continuous turning, mixing and disintegration of the feed material.