subject: Crushing And Screening Process [print this page] Crushing And Screening Process Crushing And Screening Process
Crushing and screening processes
The technical term for the crushing, or size reduction of mineral, is comminution. There are four main types of crushing plant in common use in quarries:
Jaw crushers employ the pressure exerted by moving jaws to break the rock and are generally used as primary crushers to reduce run of quarry stone to a manageable size of 300 400mm.
Impact crushers either subject rock to impacts by swiftly rotating components, or throw rock at fixed members, to break the rock into smaller aggregates and are commonly used as secondary or tertiary crushers.
Gyratory or cone crushers are similar in design and operation and reduce the size of the material through pressure exerted by a crushing head rotating eccentrically in a hollow cone shaped anvil. Gyratory crushers tend to be used for primary size reduction and cone crushers in the secondary or tertiary circuit.
Attrition crushers subject aggregates to abrasive forces generated by grinding to produce aggregate products of small size, such as fines.
The selection of a particular type of crusher will depend on the:
size of material fed into the crusher (feed size)
throughput of material through the crusher
composition of rock to be crushed
required product size and shape
economic and power requirements
The quality of the crushed aggregate is important in determining choice of crusher. The quality of aggregates for specific uses, such as in road construction, are specified by British Standard. Aggregates with high cubicity are premium products required for asphalt and concrete applications because of properties such as their superior strength compared to other particle shapes
The size and shape of crushed aggregate may be determined by choice of plant, the incorporation of different crushing stages, and the re-crushing of poor quality aggregate to produce smaller sized aggregate of the required shape. 3.13 Primary crushers reduce run of the quarry stone (boulders) to rock in the size range 300 400mm.
Secondary crushers reduce primary crusher material to product i.e. 50 60 mm (larger plant) or 40 mm (smaller plant).
Tertiary crushing reduces secondary crusher material to products in the size range below 20mm.
Quaternary crushing produces fines for asphalt and concrete needs, and manufactured sands.
Hard rocks may pass through all four stages whereas some material may only pass through one or two stages.
The raw mineral undergoing size reduction may be washed to remove impurities such as silt or mud. This forms a slurry which is removed for treatment. Many sites make use of settlement lagoons where silt is allowed to settle from the waste water which may then be recycled.
Associated with the size reduction of minerals are screening processes to sieve mineral into desired size fractions at different points in the processing circuit. Separation is achieved by passing the crushed stone over plate or mesh decks with each deck containing apertures of a different size. It is common for multi-screen vibrating decks to be used with conveyor or chute discharge to bins or external stockpiles.
