Crushing & Screening

Processing involves a series of crushing and screening stages designed to produce material with a specified range of sizes. In a hard rock quarry the primary crusher first roughly crushes material; secondary crushers then reduce the aggregate size and tertiary crushers produce the final size and shape of aggregate products. Crushed materials are sized by passing over various screens and /or classifiers. Within a sand and gravel quarry the process is simpler involving washing and sizing, and crushing oversized gravel material.

Dry aggregates are sometimes washed to remove fine particles adhering to the surfaces of the aggregate. The water used is commonly recycled via settling ponds or lagoons where the heavier particles will sink if the water is allowed to stand. The sized materials are then stockpiled before being sold for their end use.

Stockpiling of materials, which involves moving the processed aggregate to storage within the site, requires double handling of material and therefore represents a significant cost to the mineral operator. Storage in covered bays, hoppers or in the open requires significant areas of land which must be included in the planning consent. Aggregates are sold by weight, therefore processing plants commonly incorporate weigh-hoppers for proportioning and blending materials to meet specifications, and weighbridges to weigh loaded vehicles.