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Hard rock quarries
Diagram of jaw crusher. 
Diagram of horizontal shaft impactor crusher. 
Diagram of vertical shaft impactor crusher. 
Diagram of cone crusher. |
Simplified diagram of hard rock processing system (click to enlarge). The final aggregate product should have the particle size distribution suitable for the desired market. Crushing starts with primary crushing; usually using jaw, gyratory, roller or impactor-type crushers depending on the characteristics of the rock. Jaw and gyratory crushers can both handle hard, abrasive rocks, whilst impactors are restricted to non-siliceous rocks e.g. limestones, although they can handle high throughputs (up to 1000 tonnes per hour) and lump sizes in excess of 1 m. Roller crushers are rarely installed in new quarries but may still be found in some of the older limestone quarries. Material is loaded into the primary crusher by dump trucks, and the crushed material is then transported to the processing plant by conveyor.Scalping screen Scalping screens are used to allow rock fragments and fines that do not require crushing to bypass further crushing stages. Most quarries will install a scalping screen after the primary crusher. Material is passed over a vibrating mesh that allows particles less than 20 mm in diameter to pass through. This material bypasses the remaining crusher stages and is fed directly to the sizing screens. In 'dirty' quarries, where clay is present within the deposit, a scalping screen is often used to remove excessive amounts of clay before the mineral is fed into the primary crusher. However the material removed at this stage will usually still contain some useable rock and is therefore washed to recover the maximum amount of mineral before being fed into the sizing screens. Secondary crushers The secondary crusher breaks rock fragments down into sizes that are suitable for the market. A variety of crushers can be used depending on the rock type. Horizontal shaft impactors break the rock on impact with a blade on a rotating barrel. Material bounces round within the crushing chamber until it is small enough to escape. The size of the final product can be adjusted by adjusting the gap between the rotor blades and breaker bars attached to the chamber edge. Rock on metal impact is suitable for use with soft rocks, such as limestones, however high wear rate on the rotor tips makes this method unsuitable for abrasive igneous rocks. Vertical shaft impactors depend on rock on rock impact to break fragments and can be used on all except the hardest of rock types. While impact crushers have the advantage of giving the cubical shape that is required for the concrete industry, they can produce large quantities of fines, up to 40% of output in some instances, resulting in excessive waste. The cone crusher is favoured in igneous rock quarries. Material is fed into the top of the cone and moves along the gap between the cone and the fixed outer wall. As the cone is rotated on an off-centre axis the size of the gap gets bigger and smaller, resulting in the crushing action. Sizes from 50 mm down can be created and the size range can be adjusted by adjusting the height of the cone within the concaves. However, the requirement for smaller final products will result in the production of greater quantities of fines. It is normal in some hard rock quarries to crush the rock a third time before sizing (tertiary crushing). Each stage of crushing (primary and secondary) can be expected to reduce the size of the material feeding through it by between 60% and 80%. This reduction ratio can be increased by demanding more of each crusher, however this increases the quantity of fines produced, decreases the throughput and increases the power requirements and costs of crushing. Tertiary crushing is therefore used to produce the final aggregate size and shape. | |
