Comminution (CRUSHING)


Crushing of ‘run of mine’ is the second step in cement manufacturing. The extent of size reduction in crushing operation depends on one hand the size of 'run off mine’ as received from quarries and the size of crushed stone as required by the mills selected for grinding on the other. Size of 'run off mine' stone depends on whether mining is 'manual' or 'mechanised'. Manual mining the stone size is generally of the order of - 300 mms. While as in mechanised mining uses shovels, therefore, size of stone is directly related to size of shovel used, that is the size of its bucket. Ball mills require for grinding size of feed generally less than 20 mm.; smaller the better. Vertical Roller Mills, Roller Presses and Horo mills can receive stone between 75 to 100 mm size.

Reduction Ratio The degree of reduction in the ratio of the largest linear dimension of material before crushing to the largest linear dimension after crushing, and is expressed as:

Reduction Ratio  (n) =  Dmax/dmax

where Dmax is largest linear dimension of material before crushing (Feed Size).
And    dmax is largest linear dimension after crushing (Product size).
Reduction ratio decides the number of stages of crushing required and the type of crushers to be used.

Power Consumption. Crushing operation consumes about 1.5 to 2 units of power per ton of limestone crushed. In terms of cement it would be about 2 to 3 units per ton of crushed material.

Venting in crusher section. Crushers and feed transition points are vented by collecting vent gases through a bag filter. Rotors of hammer crushers and impactors behave like impellers of fans and displace considerable quantities of air that needs to be vented.

Key performance Indicators of crusher Crusher operation is monitored through following KPIs

Production rate                tonnes/hour
Run factor                       %run time
Involuntary downtime     hours
Feed moisture                %
Product screen analysis. PSD
Power consumption         units/ton

Types of Crushers
Commonly used crushers in Cement Industry to crush are:

Crusher Type Sub Type
Jaw Crushers Single toggle Jaw Crusher
Double toggle Jaw Crusher
Hammer crushers Unidirectional or Reversible
Full or Partial grate bars
Single or Double rotor
Impactors Single or Double rotor
Gyratory crushers

Jaw crushers and gyratory crushers are used as Primary crushers in two stage crushing. Size reduction ratio is limited to 5 to 6: I. Hammer crushers and Impactors can be used both as Primary crusher in single stage crushing and as Secondary crusher in two stage crushing operation.

Jaw Crushers. Crushing is performed in jaw crushers by compression of stones between fixed and movable jaws moving back and forth like a human jaw. Reciprocating motion is imparted by an eccentric and a toggle. Jaws can be straight or curved design, plain or corrugated. Angle between the fixed and moving jaws grips the stone and crushes it and pushes it down for further reduction in the next stroke. Because of the 'throw' of the moving jaw there is always about 10-15 % oversize in the product. Crushing faces (liners) are generally made of manganese steel and are replaced when worn out.
Single Toggle Jaw Crushers also have a measure of vertical jaw motion which adds attrition to compression. Sticky materials can be handled as well, but wear rate is higher.
Double Toggle Jaw Crushers have a simple compression motion with jaw angle of 15-20°. They are effective for hard and abrasive materials with low (<5%) moisture and give a reduction ratio of about 6:1.

Hammer Crushers. In hammer crushers, hammers are mounted on discs strikes the stone fed into the crusher against breaking impact plates. Stone is broken into smaller pieces and gets further reduced because of reducing clearance between tips of hammers and breaking impact plates. The feeder elevation and, therefore, the velocity with which the material enters the circle is critical; if it is too low, the material may bounce on top of the hammers and if it is too high, it may penetrate through the circle and can damage the rotor discs. Moreover, the feed must enter the crusher so that rotating hammers catch it at a point where impact is maximum. Grate bars (fully or partially) are placed in the bottom through which crushed material smaller than the spacing in the grate, passes. Hammer crushers are available in unidirectional and bi-directional configuration. In unidirectional hammer crushers hammers are to be reversed to bring second face in use after first face is worn out. In reversible hammer crushers, feed falls right on top and crusher can run in either direction, it has the advantage that both faces of hammers are used for breaking stone without need to take a stoppage and reverse the hammers.
Hammers and grate bars are made of manganese steel. It is generally recommended to keep one set of hammers ready in stock to replace the worn out. During installation, care is taken to 'match' the hammers so that rotor is not unbalanced. An overhead traveling crane with a hoist needs to be installed to facilitate maintenance of hammers. For high capacity crushers a hydraulic puller will be installed to take out shaft from the side. In some designs there are two rotors in the same casing rotating in opposite directions.  Size of the product is controlled in two ways - by adjusting breaker plates and - by adjusting spacing between installed grate bars. Therefore, hammer crusher is better suited for running in closed circuit crushing to maintain a product size, particularly when subsequent grinding mill is a ball mill.

Impactors. Impactors have blow bars fitted on a rotor along its width. They push material on impact breaker plates and in a series of impacts between plates and blow bars stone is crushed to desired size. Impactors are preferred when the grinding mills are vertical mills or when a roller press is used as pre grinder with ball mill. Blow bars will be made of manganese steel and rotor will be cast steel mounted on a shaft of forged steel. In some designs stone is broken to required size in two stages in the same casing. Product of stage one is further broken in stage two to desired size.

Gyratory Crushers. operate by pressure between a gyrating cone and a stationary or spring-loaded crushing ring. Hard, abrasive materials like silcastone can be handled with reduction ratios of about 5:1. Compression type crushers normally produce a cubic product with a low proportion of fines; being choke fed, plugging will result with feed moisture in excess of about 5%. Wear is low but reduction ratios are usually less than 8:1.