|How are aggregates processed?
Few deposits can be used 'as dug' as aggregate. Processing provides scope for adjusting the grain-size distribution of the 'as-dug' material to match market requirements for the final saleable product.
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.
New plant machinery (foreground) replacing old, worn out plant machinery (background).
Processing leads to the production of fine-grained aggregate/dust, which in some cases also has commercial value – e.g. agricultural limestone, or fillers for paint, paper etc. However, many fines can be difficult to sell and become a waste product.
Rock that is contaminated with clay (known as 'scalpings' ) may be initially screened out and sold as a low-grade, low value fill material. This has become more difficult because of competition from recycled materials and secondary products, which have the cost advantage of being exempt from the Aggregates Levy. Washing plants are sometimes built to treat this material and upgrade its quality. Fines and scalpings are often used in site restoration.
Aggregate processing plant comprises equipment for crushing, (also known as comminution) and sizing ( screening and classification) and materials handling and transportation (feeders and conveyors). Aggregates are frequently processed wet, both to minimise dust and to facilitate the sizing of fine particles and removal of undesirable contaminants such as clays. Plants therefore also contain equipment for washing, dewatering of the aggregate products, silt handling and recycling of water. Mineral separation processes are occasionally employed to remove material with undesirable physical or chemical properties such as lignite.
Processing plants may be fixed or mobile. Fixed installations require permanent foundation and are appropriate to large hard rock quarries with larger reserves and an extended life supplying a variety of markets. The relatively short life of sand and gravel quarries mean that processing plant may be re-used. Modular construction facilitates rapid installation and removal of equipment. In-quarry use of mobile processing plant is also increasingly common.
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