Rock crusher
From Wikipedia, the free encyclopedia
A rock crusher is a machine designed to take large rocks and reduce them to smaller rocks, gravel, or rock dust. Rock crushers produce aggregates and ready-to-process mining ores, as well as rock fill material for landscaping and erosion control. They can be used with virgin rock or other materials such as reclaimed concrete. Rock crushers can be mobile (although usually very heavy) machines or they can be fixed installations.
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[edit] Process
Crushing is the first step in converting shot rock or demolition rubble into usable products, by taking large rocks and breaking them into smaller pieces. Crushing is sometimes continued until only the sand-like 'fines' remain, and in mining applications it is usually followed by milling. At some operations, all the crushing is accomplished in one step, by a single crusher. At other operations, crushing is done in two or more steps, with a primary crusher that is followed by a secondary crusher, and sometimes a tertiary or even quaternary crusher. Each crusher is designed to work with a certain maximum size of raw material, and often delivers its output to a screening machine which sorts and directs the product for further processing.
In operation, the raw material (of various sizes) is usually delivered to the primary crusher's hopper by dump trucks, excavators or wheeled front-end loaders. A feeder device such as a conveyor or vibrating grid controls the rate at which this material enters the crusher, and often contains a preliminary screening device which allows smaller material to bypass the crusher itself, thus improving efficiency. Primary crushing reduces the large pieces to a size which can be handled by the downstream machinery.
[edit] Types
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[edit] Jaw
The jaw crusher squeezes rock between two ridged surfaces (jaws) which taper to form a funnel. In most designs one jaw is fixed while the other oscillates at a rate of somewhere around 3 times a second. Raw material enters the jaw crusher from the top. Pieces of rock that are larger than the opening at the bottom of the jaw lodge between the two metal plates of the jaw, and the motion of the oscillating jaw against the fixed jaw continues to pound the lodged pieces until they are broken into pieces small enough to drop through the opening at the bottom.
[edit] Gyratory
A gyratory crusher breaks rock by squeezing it between an eccentrically gyrating spindle (which is covered by a wear resistant mantle) and the enclosing concave hopper. As run-of-mine rock enters the top of the gyratory crusher, it becomes wedged and squeezed between the mantle and concaves. Large pieces of ore are broken once and then fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough to fall through the narrow opening at the bottom of the crusher.
[edit] Impact
There are two types of impact crushers. The Horizontal Shaft Impactor and the Vertical Shaft Impactor.
[edit] Horizontal Shaft Impactor (HSI)
The HSI crushers break rock by impacting the rock with hammers that swing on a rotating shaft. The practical use of HSI crushers is limited to soft materials and non abrasive materials, such as limestone, phosphate, gypsum, weathered shales.
[edit] Vertical Shaft Impactor (VSI)
VSI Crushers use a different approach involving a high speed rotor with wear resistant tips and a crushing chamber designed to 'throw' the rock against. The VSI crushers utilize velocity rather than surface force as the predominant force to break rock. In its natural state, rock has a jagged and uneven surface. Applying surface force (pressure) results in unpredictable and typically non-cubicle resulting particles. Utilizing velocity rather than surface force allows the breaking force to be applied evenly both across the surface of the rock as well as through the mass of the rock. Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is 'thrown' by a VSI Rotor against a solid anvil, it fractures and breaks along these fissures. Final particle size can be controlled by 1) the velocity at which the rock is thrown against the anvil and 2) the distance between the end of the rotor and the impact point on the anvil. The product resulting from VSI Crushing is generally of a consistent cubicle shape such as that required by modern SUPERPAVE highway asphalt applications. Using this method also allows materials with much higher abrasiveness to be crushed than is capable with an HSI and most other crushing methods.
VSI Crushers generally utilize a high speed spinning rotor at the center of the crushing chamber and an outer impact surface of either abrasive resistant metal anvils or crushed rock. Utilizing cast metal surfaces 'anvils' is traditionally referred to as a "Shoe and Anvil VSI". Utilizing crushed rock on the outer walls of the crusher for new rock to be crushed against is traditionally referred to as "rock on rock VSI".
[edit] Cone Crusher
A cone crusher is similar in operation to a gyratory crusher, with less steepness in the crushing chamber and more of a parallel zone between crushing zones. A cone crusher breaks rock by squeezing the rock between an eccentrically gyrating spindle, which is covered by a wear resistant mantle, and the enclosing concave hopper, covered by a manganese concave or a bowl liner. As rock enters the top of the cone crusher, it becomes wedged and squeezed between the mantle and the bowl liner or concave. Large pieces of ore are broken once, and then fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough to fall through the narrow opening at the bottom of the crusher.
[edit] Technology
For the most part advances in crusher design have moved slowly. Jaw crushers have remained virtually unchanged for sixty years. More reliability and higher production have been added to basic cone crusher designs that have also remained largely unchanged. Increases in rotating speed, have provided the largest variation. For instance, a 48 inch (120 cm) cone crusher manufactured in 1960 may be able to produce 170 tons/hr of crushed rock, whereas the same size cone manufactured today may produce 300 tons/hr. These production improvements come from speed increases and better crushing chamber designs.
The largest advance in cone crusher reliability has been seen in the use of hydraulics to protect crushers from damage from uncrushable objects entering the crushing chamber. Foreign objects, such as steel, can cause extensive damage to a cone crusher, and more costs in lost production. The advance of hydraulic relief systems has greatly reduced downtime and improved the life of these machines.
