Crushers and Their Types

Crushers and Their Types

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.

• Impact – In crushing terminology, impact refers to the sharp, instantaneous collision of one moving object against another. Both objects may be moving, or one object may be motionless. There are two variations of impact, namely (i) gravity impact, and (ii) dynamic impact. Material dropped onto a hard surface such as a steel plate is an example of gravity impact. Gravity impact is most often used when it is necessary to separate two materials which have relatively different friability. The more friable material is broken, while the less friable material remains unbroken. Separation can then be done by screening. Material dropping in front of a moving hammer (both objects in motion), illustrates dynamic impact. When crushed by gravity impact, the free-falling material is momentarily stopped by the stationary object. But when crushed by dynamic impact, the material is unsupported and the force of impact accelerates movement of the reduced particles toward breaker blocks and/or other hammers. Dynamic impact has definite advantages for the reduction of many materials.
• Attrition – It is a term applied to the reduction of materials by scrubbing it between two hard surfaces. Hammer mills operate with close clearances between the hammers and the screen bars and materials reduce by attrition combined with shear and impact reduction. Though attrition consumes more power and exacts heavier wear on hammers and screen bars, it is practical method for crushing the less abrasive materials such as limestone and coal.
• Shear – It consists of a trimming or cleaving action rather than the rubbing action associated with attrition. Shear is usually combined with other methods. For example, single roll crushers employ shear together with impact and compression. Shear crushing is normally called for under the conditions when material is somewhat friable or when a relatively coarse product is desired. It is usually employed for primary crushing with a reduction ratio of 6 to 1.
• Compression – As the name implies, crushing by compression is done between two surfaces, with the work being done by one or both surfaces. Jaw crushers using this method of compression are suitable for reducing extremely hard and abrasive materials. However, some jaw crushers employ attrition as well as compression and are not as suitable for abrasive materials since the rubbing action accentuates the wear on crushing surfaces. As a mechanical reduction method, compression is to be used if the material is hard and tough, if the material is abrasive, if the material is not sticky, and where the finished product is to be relatively coarse.

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 crusherstypes 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.

• The three-stage breaking action – Initially, the material is gripped by the leading faces of opposed rotor teeth. These subject the material to multiple point loading, inducing stress into the material to exploit any natural weaknesses. At the second stage, material is broken in tension by being subjected to a three point loading, applied between the front tooth faces on one rotor, and rear tooth faces on the other rotor. Any lumps of material that still remain oversize, are broken as the rotors chop through the fixed teeth of the breaker bar, thereby achieving a three dimensional controlled product size.
• The rotating screen effect – The interlaced toothed rotor design allows free flowing undersize material to pass through the continuously changing gaps generated by the relatively slow moving shafts.
• The deep scroll tooth pattern – The deep scroll conveys the larger material to one end of the machine and helps to spread the feed across the full length of the rotors. This feature can also be used to reject oversize material from the machine.

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.

• Production requirement – It includes output size and shape, and the required capacity.
• Ore characteristics – Include material specification, feed (input) size, material friability, and material abrasiveness.
• Operational considerations – It includes power demand, equipment availability (hours/annum), availability and cost of replaceable parts, reduction ratio, maintenance requirements, needed manpower, approachability of parts for maintenance, availability of spares, safety and environment.
• Equipment ruggedness – Can the crusher pass uncrushable debris without damage to the crusher.
• Capital cost of the crusher and the total cost of the crusher station

Different types of crushers are optimal for distinct crushing needs

All rock crushers can be classified as falling into two main groups. Compressive crushers that press the material until it breaks, and impact crushers using the principle of quick impacts to crush the material. Jaw crushers, gyratory crushers, and cone operate according to the compression principle. Impact crushers, in turn, utilize the impact principle.

Jaw crushers

Jaw crushers are mainly used as primary crushers. Their main purpose is to reduce the material to a small enough size that it can be transported by conveyors to the next crushing stages.

As the name suggest, jaw crushers reduce rock and other materials between a fixed and a moving jaw. The moving jaw is mounted on a pitman that has a reciprocating motion, and the fixed jaw stays put. When the material runs between the two jaws, the jaws compress larger boulders into smaller pieces.  

There are two basic types of jaw crushers: single toggle and double toggle. In the single toggle jaw crusher, an eccentric shaft is on the top of the crusher. Shaft rotation causes, along with the toggle plate, a compressive action.

A double toggle crusher has two shafts and two toggle plates. The first shaft is a pivoting shaft on the top of the crusher, while the other is an eccentric shaft that drives both toggle plates.

The chewing movement, which causes compression at both material intake and discharge, gives the single toggle jaw better capacity, compared to a double toggle jaw of similar size. Metso’s jaw crushers are all single toggle.

Gyratory crushers

Gyratory crushers are frequently used in the primary crushing stage and a little less often in in the secondary stage.

Gyratory crushers have an oscillating shaft. The material is reduced in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly.

The fragmentation of the material results from the continuous compression that takes place between the liners around the chamber. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners.

The gyratory crushers are equipped with a hydraulic setting adjustment system, which makes it possible to regulate the gradation of the crushed material.

Cone crushers

Cone crushers resemble gyratory crushers from technological standpoint, but unlike gyratory crushers, cone crushers are popular in secondary, tertiary, and quaternary crushing stages. Sometimes, however, the grain size of the processed material is small enough by nature and the traditional primary crushing stage is not needed. In these cases, also cone crushers can carry out the first stage of the crushing process.     

Cone crushers have an oscillating shaft, and the material is crushed in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly.

An eccentric shaft rotated by a gear and pinion produces the oscillating movement of the main shaft. The eccentricity causes the cone head to oscillate between open side setting and closed side setting discharge opening.

The fragmentation of the material results from the continuous compression that takes place between the liners around the chamber. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners. This is called interparticular crushing also.

The cone crushers are equipped with a hydraulic setting adjustment system, which adjusts closed side setting and thus affects product gradation.

Depending on cone crusher, setting can be adjusted in two ways. The first way is for setting adjustment to be done by rotating the bowl against the threads so that the vertical position of the outer wear part (concave) is changed. One advantage of this adjustment type is that liners wear more evenly.

Another principle is that of setting adjustment by lifting or lowering the main shaft. An advantage of this is that adjustment can be done continuously under load.

To optimize operating costs and improve the product shape it is recommended that cone crushers are always be choke fed, meaning that the cavity should be as full of rock material as possible. This can be easily achieved by using a stockpile or a silo to regulate the inevitable fluctuation of feed material flow. Level monitoring devices detect the maximum and minimum levels of the material, starting and stopping the feed of material to the crusher, as needed.

Impact crushers

Impact crushers are versatile crushing machines that can be used in any stage of the crushing process. However, the features and capabilities of different impact crusher types vary considerably.

Impact crushers are traditionally classified to two main types: horizontal shaft impact (HSI) crushers and vertical shaft impact (VSI) crushers. These different types of impact crushers share the crushing principle, impact, to reduce the material to smaller sizes, but features, capacities and optimal applications are far from each other.

Horizontal shaft impact (HSI) crushers are used in primary, secondary or tertiary crushing stage. HSI crushers reduce the feed material by highly intensive impacts originating in the quick rotational movement of hammers or bars fixed to the rotor. The particles produced are then further fragmentated inside the crusher as they collide against crusher chamber and each other, producing a finer, better-shaped product.

Vertical shaft impact (VSI) crushers, on the other hand, are used in the last stage of the crushing process, especially when its required that the end product has a precise cubical shape. 

VSI crusher can be considered a ‘stone pump’ that operates like a centrifugal pump. The material is fed through the center of the rotor, where it is accelerated to high speed before being discharged through openings in the rotor periphery. The material is crushed as it hits of the outer body at high speed and due to rocks colliding against each other.

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.

• Impact – In crushing terminology, impact refers to the sharp, instantaneous collision of one moving object against another. Both objects may be moving, or one object may be motionless. There are two variations of impact, namely (i) gravity impact, and (ii) dynamic impact. Material dropped onto a hard surface such as a steel plate is an example of gravity impact. Gravity impact is most often used when it is necessary to separate two materials which have relatively different friability. The more friable material is broken, while the less friable material remains unbroken. Separation can then be done by screening. Material dropping in front of a moving hammer (both objects in motion), illustrates dynamic impact. When crushed by gravity impact, the free-falling material is momentarily stopped by the stationary object. But when crushed by dynamic impact, the material is unsupported and the force of impact accelerates movement of the reduced particles toward breaker blocks and/or other hammers. Dynamic impact has definite advantages for the reduction of many materials.
• Attrition – It is a term applied to the reduction of materials by scrubbing it between two hard surfaces. Hammer mills operate with close clearances between the hammers and the screen bars and materials reduce by attrition combined with shear and impact reduction. Though attrition consumes more power and exacts heavier wear on hammers and screen bars, it is practical method for crushing the less abrasive materials such as limestone and coal.
• Shear – It consists of a trimming or cleaving action rather than the rubbing action associated with attrition. Shear is usually combined with other methods. For example, single roll crushers employ shear together with impact and compression. Shear crushing is normally called for under the conditions when material is somewhat friable or when a relatively coarse product is desired. It is usually employed for primary crushing with a reduction ratio of 6 to 1.
• Compression – As the name implies, crushing by compression is done between two surfaces, with the work being done by one or both surfaces. Jaw crushers using this method of compression are suitable for reducing extremely hard and abrasive materials. However, some jaw crushers employ attrition as well as compression and are not as suitable for abrasive materials since the rubbing action accentuates the wear on crushing surfaces. As a mechanical reduction method, compression is to be used if the material is hard and tough, if the material is abrasive, if the material is not sticky, and where the finished product is to be relatively coarse.

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.

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 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.

• The three-stage breaking action – Initially, the material is gripped by the leading faces of opposed rotor teeth. These subject the material to multiple point loading, inducing stress into the material to exploit any natural weaknesses. At the second stage, material is broken in tension by being subjected to a three point loading, applied between the front tooth faces on one rotor, and rear tooth faces on the other rotor. Any lumps of material that still remain oversize, are broken as the rotors chop through the fixed teeth of the breaker bar, thereby achieving a three dimensional controlled product size.
• The rotating screen effect – The interlaced toothed rotor design allows free flowing undersize material to pass through the continuously changing gaps generated by the relatively slow moving shafts.
• The deep scroll tooth pattern – The deep scroll conveys the larger material to one end of the machine and helps to spread the feed across the full length of the rotors. This feature can also be used to reject oversize material from the machine.

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.

• Production requirement – It includes output size and shape, and the required capacity.
• Ore characteristics – Include material specification, feed (input) size, material friability, and material abrasiveness.
• Operational considerations – It includes power demand, equipment availability (hours/annum), availability and cost of replaceable parts, reduction ratio, maintenance requirements, needed manpower, approachability of parts for maintenance, availability of spares, safety and environment.
• Equipment ruggedness – Can the crusher pass uncrushable debris without damage to the crusher.
• Capital cost of the crusher and the total cost of the crusher station

Different types of crushers are optimal for distinct crushing needs

All rock crushers can be classified as falling into two main groups. Compressive crushers that press the material until it breaks, and impact crushers using the principle of quick impacts to crush the material. Jaw crushers, gyratory crushers, and cone operate according to the compression principle. Impact crushers, in turn, utilize the impact principle.

Jaw crushers

Jaw crushers are mainly used as primary crushers. Their main purpose is to reduce the material to a small enough size that it can be transported by conveyors to the next crushing stages.

As the name suggest, jaw crushers reduce rock and other materials between a fixed and a moving jaw. The moving jaw is mounted on a pitman that has a reciprocating motion, and the fixed jaw stays put. When the material runs between the two jaws, the jaws compress larger boulders into smaller pieces.  

There are two basic types of jaw crushers: single toggle and double toggle. In the single toggle jaw crusher, an eccentric shaft is on the top of the crusher. Shaft rotation causes, along with the toggle plate, a compressive action.

A double toggle crusher has two shafts and two toggle plates. The first shaft is a pivoting shaft on the top of the crusher, while the other is an eccentric shaft that drives both toggle plates.

The chewing movement, which causes compression at both material intake and discharge, gives the single toggle jaw better capacity, compared to a double toggle jaw of similar size. Metso’s jaw crushers are all single toggle.

Gyratory crushers

Gyratory crushers are frequently used in the primary crushing stage and a little less often in in the secondary stage.

Gyratory crushers have an oscillating shaft. The material is reduced in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly.

The fragmentation of the material results from the continuous compression that takes place between the liners around the chamber. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners.

The gyratory crushers are equipped with a hydraulic setting adjustment system, which makes it possible to regulate the gradation of the crushed material.

Cone crushers

Cone crushers resemble gyratory crushers from technological standpoint, but unlike gyratory crushers, cone crushers are popular in secondary, tertiary, and quaternary crushing stages. Sometimes, however, the grain size of the processed material is small enough by nature and the traditional primary crushing stage is not needed. In these cases, also cone crushers can carry out the first stage of the crushing process.     

Cone crushers have an oscillating shaft, and the material is crushed in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly.

An eccentric shaft rotated by a gear and pinion produces the oscillating movement of the main shaft. The eccentricity causes the cone head to oscillate between open side setting and closed side setting discharge opening.

The fragmentation of the material results from the continuous compression that takes place between the liners around the chamber. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners. This is called interparticular crushing also.

The cone crushers are equipped with a hydraulic setting adjustment system, which adjusts closed side setting and thus affects product gradation.

Depending on cone crusher, setting can be adjusted in two ways. The first way is for setting adjustment to be done by rotating the bowl against the threads so that the vertical position of the outer wear part (concave) is changed. One advantage of this adjustment type is that liners wear more evenly.

Another principle is that of setting adjustment by lifting or lowering the main shaft. An advantage of this is that adjustment can be done continuously under load.

To optimize operating costs and improve the product shape it is recommended that cone crushers are always be choke fed, meaning that the cavity should be as full of rock material as possible. This can be easily achieved by using a stockpile or a silo to regulate the inevitable fluctuation of feed material flow. Level monitoring devices detect the maximum and minimum levels of the material, starting and stopping the feed of material to the crusher, as needed.

Impact crushers

Impact crushers are versatile crushing machines that can be used in any stage of the crushing process. However, the features and capabilities of different impact crusher types vary considerably.

Impact crushers are traditionally classified to two main types: horizontal shaft impact (HSI) crushers and vertical shaft impact (VSI) crushers. These different types of impact crushers share the crushing principle, impact, to reduce the material to smaller sizes, but features, capacities and optimal applications are far from each other.

Horizontal shaft impact (HSI) crushers are used in primary, secondary or tertiary crushing stage. HSI crushers reduce the feed material by highly intensive impacts originating in the quick rotational movement of hammers or bars fixed to the rotor. The particles produced are then further fragmentated inside the crusher as they collide against crusher chamber and each other, producing a finer, better-shaped product.

Vertical shaft impact (VSI) crushers, on the other hand, are used in the last stage of the crushing process, especially when its required that the end product has a precise cubical shape. 

VSI crusher can be considered a ‘stone pump’ that operates like a centrifugal pump. The material is fed through the center of the rotor, where it is accelerated to high speed before being discharged through openings in the rotor periphery. The material is crushed as it hits of the outer body at high speed and due to rocks colliding against each other.

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