A crusher is a machine designed to reduce large rocks into
smaller rocks, gravel, or rock dust.
Crushers may be used to reduce the size, or change the form,
of waste materials so they can be more easily disposed of or recycled, or to reduce
the size of a solid mix of raw materials (as in rock ore), so that pieces of
different composition can be differentiated. Crushing is the process of
transferring a force amplified by mechanical advantage through a material made
of molecules that bond together more strongly, and resist deformation more,
than those in the material being crushed do.
Crushing devices hold material
between two parallel or tangent solid surfaces, and apply sufficient force to
bring the surfaces together to generate enough energy within the material being
crushed so that its molecules separate from (fracturing), or change alignment
in relation to (deformation), each other. The earliest crushers were hand-held
stones, where the weight of the stone provided a boost to muscle power, used
against a stone anvil. Querns and mortars are types of these crushing devices.
Mining operations use crushers, commonly classified by the
degree to which they fragment the starting material, with primary and secondary
crushers handling coarse materials, and tertiary and quaternary crushers
reducing ore particles to finer gradations. 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.
Typically, crushing stages are followed by milling stages if the materials need
to be further reduced. Additionally rockbreakers are typically located next to
a crusher to reduce oversize material too large for a crusher. Crushers are
used to reduce particle size enough so that the material can be processed into
finer particles in a grinder. A typical processing line at a mine might consist
of a crusher followed by a SAG mill followed by a ball mill. In this context,
the SAG mill and ball mill are considered grinders rather than crushers.
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 an apron feeder, 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.
Some crushers are mobile and can crush rocks as large as 60
inches. Primarily used in-pit at the mine face these units are able to move
with the large infeed machines (mainly shovels) to increase the tonnage
produced. In a mobile road operation, these crushed rocks are directly combined
with concrete and asphalt which are then deposited on to a road surface. This
removes the need for hauling over-sized material to a stationary crusher and then
back to the road surface.