Mechanical Reduction Methods

 There are four basic ways to reduce a material - by impact, attrition, shear or compression- and most crushers employ a combination of all these crushing methods.

Compression. As the name implies, crushing by compression is done between two surfaces. Gyratory and double toggle jaw crushers using this method of compression are suitable for reducing extremely hard and abrasive rock However, single toggle jaw crushers employ attrition as well as compression and are less suitable for abrasive rock since the rubbing action accentuates the wear on crushing surfaces.

As a mechanical reduction method, compression should be used as follow:

- when the material is hard and tough

- when the material is abrasive

- when the material is not sticky

- when a uniform product with a minimum of fines is desired

- when the finished product is to be relatively coarse, i.e. -38 mm or larger top size

- when the material will break cubically

Impact. Refers to the sharp, instantaneous impingement of one moving object against another. Both objects may be moving, such as a baseball bat connecting with a fast ball, or one object may be motionless, such as a rock being struck by hammer blows.

There are two variations of impact: gravity impact and dynamic impact Material dropped onto a hard surface such as a steel plate is an example of gravity impact Material dropping in front of a moving hammer (both objects in motion) illustrates dynamic impact When rock is 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 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 hammermills, but where jaw crushers and gyratory crushers cannot be used because of particle shape requirements, high moisture content or capacity.

Attrition. A term applied to the reduction of materials by scrubbing it between two hard surfaces. Hammermills operate with close clearances between the hammers and the screen bars and they reduce by attrition combined with shear and impact reduction. Though attrition crushing consumes more power and exacts heavier wear on hammers and screen bars, it is practical for crushing the less abrasive materials such as low silica limestone or coal.

Attrition crushing is most useful in the following circumstances:

- when material is friable and/or non-abrasive
- when a closed-circuit system is not desirable to control top size
- when a maximum of fines is required.

Shear. This consists of a trimming or cleaving action rather than the rubbing action associated with attrition. Shear exploits the fact that the ratio of compression strength too tensile and shear strength in the majority of rocks is approximately 10:1. Low speed sizers break the rocks in tension and shear by its chopping action.

Shear crushing is normally called for under these conditions:

    when material is somewhat friable and has relatively low silica content.
    when the material is soft to medium hardness.
    for primary crushing with a reduction ratio of 6 to 1.
    when a minimum of fines is desired.
    when a relatively coarse product is desired