Crushers and Their Types
A crusher is a multi dimensional machine which is designed to reduce large size materials into smaller size materials. 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 the case of ore), so that pieces of different composition can be differentiated for separation.
Crushers are normally low speed machines that are designed for breaking large lumps of ores and stones, even aving a size with a diameter of over one and half meter. The purpose of crusher is to reduce the size of the materials for making them usable in construction or industrial use, or for extraction of valuable minerals trapped within a ore matrix.
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.
There are four basic ways to reduce a material, namely (i) impact, (ii) attrition, (iii) shear, and (iv) compression. Most crushers employ a combination of all these crushing methods.
The above four methods for the size reduction of materials are shown in Fig 1.
Fig 1 Methods for size reduction of materials
The reduction of size of the material when it pass through a crusher is expressed as reduction ratio. The reduction ratio is the ratio of the crusher feed size to product size. The sizes are usually defined as the 80 % passing size of the cumulative size distribution.
Types of crushers
Crushers are classified into three types based upon the stage of crushing they accomplish. These are (i) primary crusher, (ii) secondary crusher, and (iii) tertiary crusher. The primary crusher receives material directly from run of mine (ROM) after blasting and produces the first reduction in size. The output of the primary crusher is fed to a secondary crusher, which further reduces the size of the material. Similarly the output of secondary crusher is fed to the tertiary crusher which reduces the material size further. Some of the materials may pass through four or more of the crushing stages before it is reduced to the desired size. The degree of crushing is spread over several stages as a means of closely controlling product size and limiting waste material.
Crushers are also classified by their method of mechanically transmitted fracturing energy to the material. Jaw, gyratory and roll crushers work by applying compressive forces while impact crushers such as hammer crusher apply high speed impact force to accomplish fracturing.
There are several types of crushers which are used in various industries. These are given below.
Jaw crusher
Jaw crusher is used as primary crusher. It uses compressive force for breaking the material. This mechanical pressure is achieved by the two jaws of the crusher. Reduction ratio is usually 6:1. The jaw crusher is consisting of two vertical jaws installed to a V form, where the top of the jaws are further away from each other than the bottom.
One jaw is kept stationary and is called a fixed jaw while the other jaw, called a swing jaw, moves back and forth relative to it, by a cam or pitman mechanism. The volume or cavity between the two jaws is called the crushing chamber. The movement of the swing jaw can be quite small, since complete crushing is not performed in one stroke. The inertia required to crush the material is provided by a weighted flywheel that moves a shaft creating an eccentric motion that causes the closing of the gap.
Feed is entering to crusher from the top and lumps are crushed between jaws. Jaw crushers are heavy duty machines and hence need to be robustly constructed. The outer frame is generally made of cast iron or steel. The jaws themselves are usually constructed from cast steel. They are fitted with replaceable liners which are made of manganese steel, or Ni-hard (a Ni-Cr alloyed cast iron).Usually both jaws are covered with replaceable liners. Also in some types, the liners can be turned upside down after a while, extending the replacement time.
Gyratory crusher
A gyratory crusher is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head with both the surfaces are typically lined with manganese steel liners. The inner cone has a slight circular movement, but it does not rotate. The movement is generated by an eccentric arrangement. The crushing action is caused by the closing of the gap between the mantle line (movable) mounted on the central vertical spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric on the bottom of the spindle that causes the central vertical spindle to gyrate. The vertical spindle is free to rotate around its own axis.
The material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces. A gyratory crusher is used both for primary or secondary crushing.
Cone crusher
Cone crusher is consisting of a crushing chamber, a crushing cone and a operating mechanism. The cone is built in to a vertical shaft, which is supported from the top with a bowl-shaped bearing and from the other end to an eccentric operating mechanism.
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 material by squeezing the material 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 the material enters the top of the cone crusher, it becomes wedged and squeezed between the mantle and the bowl liner or concave. Large pieces of the material 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.
A cone crusher is suitable for crushing a variety of mid-hard and above mid-hard materials. Feed is dropped to the crusher from the top and it is crushed between the crushing chamber and the slowly rotating cone.
Cone crushers are mostly used for the large scale crushing in the mining industry. It has the advantage of reliable construction, high productivity, easy adjustment and lower operational costs. The spring release system of a cone crusher acts as an overload protection that allows tramp to pass through the crushing chamber without damage to the crusher.
Roller crusher
Roller crusher is a crusher that breaks material by squeezing it between two revolving metal cylinders, with axes parallel to each other and separated by a space equal to the desired maximum size of the finished product. It consists essentially of two opposite directions driven cylinders that are mounted on horizontal shafts. The other shaft is mounted permanently in the frame and is leaning on robust springs. The gap between cylinders can be adjusted, so the size of crushed product is easily adjustable. Usually both cylinders are covered with manganese steel liners. Crushing ratio is usually lower than in other crushers. Roll crusher is suitable for fine crushing. The roll crusher uses compression to crush materials. Reduction ratio is 2 to 2.5 to 1. Roller crushers are not recommended for abrasive materials.
Hammer crusher
Hammer crusher consists of a high-speed, usually horizontally shaft rotor turning inside a cylindrical casing. The crusher contains a certain amount of hammers that are pinned to the rotor disk and the hammers are swinging to the edges because of centrifugal force. Feed is dropped to the crusher from the top of the casing and it is crushed between the casing and the hammers. After crushing the material falls through from the opening in the bottom.
Impact crusher
Impact crushers make use of impact rather than compression to crush material. The material is contained within a cage, with openings of the desired size at the bottom, end, or side to allow crushed material to escape. There are two types of impact crushers namely (i) horizontal shaft impact crusher, and (ii) vertical shaft impact crusher.
Impact crushers are often used with materials, which are soft or which are easily cleaving from the surface. The crusher consists of a fast spinning rotor and beaters attached to the rotor. Feed is entering to the crusher from the top and crushing starts immediately when the feed is impacted with beaters towards the crusher’s inner surface. Impact crusher can also be equipped with a bottom screen, which prevents material leaving the crusher until it is fine enough to pass through the screen. This type of crusher is usually used for soft and non abrasive materials.
Mineral sizers
The basic concept of the mineral sizer is the use of two rotors with large teeth, on small diameter shafts, driven at a low speed by a direct high torque drive system. This design produces three major principles which all interact when breaking materials using sizer technology. The unique principles are the three-stage breaking action, the rotating screen effect, and the deep scroll tooth pattern.
Criteria for selection of crusher
The following are the criteria used in the selection of the right type of a crusher for crushing a material.
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