Crushing is the first operation in any mineral processing plant generally with probable exceptions such as plants processing beach sands or where minerals are just hand picked [for example the old practice of hand picking of high grade fluorspar lumps]. According to Gaudin, “Crushing may be defined as that operation or that group of operations in a mineral dressing plant whose object is to reduce large lumps to fragments, the coarsest particle in the crushed product being 1/20 inch or more in size”. Crushers handle largest tonnage in mineral processing business. The ratio of the largest size of the particle in the feed to crusher to the size of largest particle in the crushed product is known as reduction ratio.

Sizers are crushers whose objective is to crush the feed such that the major portion of the product is in a particular size range. Toothed rolls are generally used for this purpose. For example high grade lime stone is crushed to -80 mm to +40 mm size range or -60 mm to +30 mm for making active lime suitable for Steel Melting Shop, SMS. Jaisalmer [India] lime stone eminently suits the purpose of SMS.

Crushing operations are generally carried out in stages. [1] Jaw or gyratory crushers are used in the primary crushing stage [2] cone crushers are used in the secondary or tertiary crushing and [3] impactors are used for extra fine crushing. It is customary to introduce screens in the crushing flow sheets to improve the efficiency of the sizing operations. Gape is the opening of the mouth of a crusher through which ore is fed to the crusher. Set is the opening of the crusher through which the product is discharged.

Q & A: [1] What is the objective of Crushers? Answer: The objective of crushers is to reduce the size of the ore from large boulders to fragments.

[2] What is the objective of sizers? Answer: The objective of sizers is to obtain maximum portion of the crushed product in a desired size range.

[3] What is the reduction ratio? Answer: Reduction ratio is the ratio of the top size in the feed to a size reduction equipment to the top size in the product of that size reduction equipment.

[4] What is gape? Answer: Gape is the feed opening of a crusher in inches or centimeters.

[5] What is set? Answer: Set is the size of the discharge opening of a crusher in inches or centimeters.

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