A crusher is a multi-dimensional machine that normally operates at low speed and is designed for breaking large lumps of ores and stones into smaller size materials. The purpose of crushers is to reduce the size of the materials for making them usable in construction or industrial use, or for the extraction of valuable minerals trapped within an ore matrix.
Crushing is often one of the first steps in the production of rock, coal, and other minerals, as mined material can consist of boulders that are too big to fit through the processing plant. The type of crusher required depends on the material being reduced. Manufacturers offer a variety of crushing solutions for reducing minerals – from very friable minerals, such as coal and salt, to hard rock, such as granite, and ore-bearing minerals, such as copper and iron – at the primary, secondary and tertiary stages using compression, impact, attrition or shear forces.
There are four basic mechanical methods to reduce a material - by compression, impact, attrition, or shear - and most crushers employ a combination of all these crushing methods.
Compression
Crushers that employ the forces of compression squeeze material between two surfaces, one stationary and the other moving, in order to achieve size reduction. As feed material advances downward through the chamber, it is crushed between the moving piece of steel and the stationary plate. Only material that has reached the desired size passes on to the next stage in the process; whereas the larger material remains subjected to repeated pressure in the chamber until it, too, reaches the desired size. Gradation is controlled by adjusting the spacing between the stationary plate and the moving plate at their closest point.
Compression crushers can be used at the primary, secondary, or tertiary stage, depending on the application. 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, where the finished product is to be relatively coarse, and when a uniform product is desired with fewer fines.
Gyratory and double toggle jaw crushers reduce particles using the method of compression and are suitable for reducing extremely hard and abrasive rock. However, single toggle jaw crushers employ compression as well as attrition method and are less suitable for abrasive rock since the rubbing action accentuates the wear on crushing surfaces.
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 that 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 the dynamic impact, the material is unsupported and the force of impact accelerates the movement of the reduced particles toward breaker blocks and/or other hammers.
The dynamic impact has definite advantages for the reduction of many materials and it is specified under the following conditions:
• when a cubical particle is needed.
• when the finished product must be well-graded and must meet intermediate sizing specifications as well as top and bottom specifications.
• when ores must be broken along natural cleavage lines in order to free and separate the mineral from waste.
• when materials are too hard and abrasive for hammer mills, but where jaw crushers and gyratory crushers cannot be used because of particle shape requirements, high moisture content, or capacity.
Attrition
Attrition crushers employ the theory of mass and velocity with a grinding action to reduce feed material. These types of crushers scrub material between two hard surfaces to achieve a reduction in size and shape. Particles are reduced by their contact with other particles or by their contact with a rigid face.
Crushers that use the forces of attrition provide a high reduction ratio. They are best suited for friable and less abrasive materials. They are also useful when a maximum of fines is required and when a closed-circuit system is not desirable to control top size.
Hammer mills operate with close clearances between the hammers and the screen bars and the materials are reduced by attrition combined with shear and impact reduction. Though attrition consumes more power and exacts heavier wear on hammers and screen bars, it is a practical method for crushing the less abrasive materials such as limestone and coal.
Shear
Crushers that utilize shear forces to achieve the desired size and shape reduce the material by trimming or cleaving action rather than the rubbing action associated with attrition. Material trapped between a solid plate and a rotating roll is shorn by its contact with the teeth on the roll. Often the sheer crushing is combined with other crushing methods, such as compression, attrition, and impact, for mineral size reduction. For example, single roll crushers employ shear together with impact and compression.
Shear crushing is normally called for under the conditions when the material is somewhat friable and has relatively low silica content, when the material is soft to medium hardness, or when a relatively coarse product is desired with a minimum of fines. It is usually employed for primary crushing with a reduction ratio of 6 to 1.
Thus, based on the mechanical method of size reduction, crushers can be distinguished as Jaw crushers, cone crushers, gyratory crushers, and roll crushers all operate on the compression principle. Impact crushers utilize the impact principle, while hammermill crushers follow the principle of attrition.
