Factors of Crusher Selection

The type of material will have a large bearing on the selection of equipment to produce the required products in a cost effective manner. We look at some of these factors. This is an extract from the CMPA’s Conduct Crushing Operations Reference Manual.

MATERIAL HARDNESS

Material hardness is determined by having an impact test carried out by a laboratory. The impact value is an indication of the energy required to crush the material.

A general rule of thumb is:

For example, if the impact value was greater than 27, consideration should be given to a double toggle jaw crusher in the primary position.

When very hard materials are crushed they can become flaky and may need to be put through an impactor to shape up the final product.

The hardness of the materials listed above is only an indication based on a number of test results and as such can vary from site to site. It is advisable to have the impact value checked prior to investing in a new crusher or when moving to a new site.

ABRASIVENESS

Just because a material is hard, does not generally mean it is abrasive. Th ere are many sites processing a very hard stone that do not have a problem with high wear due to abrasion.

Abrasiveness is generally caused by the amount of free silica in the material. For instance, new basalt and limestone have very little if any free silica and as such have very low abrasion; however, quartzite has a very high ratio of free silica and is one of the most abrasive materials to be crushed.

The most common method for checking the abrasiveness of material is by carrying out an abrasive index test. This test establishes the abrasiveness of the material against wear characteristics of known materials in the field. This test can normally be carried out by your crusher supplier.

The higher the abrasive index the greater the abrasion.

For instance, in most cases you would not select a horizontal impactor or steel on steel VSI impactor when the material is above an abrasive index of 2500, unless it was the only way you could produce the product in specification. The cost would be far greater due to wear of the crushing components than for example with a cone crusher, per ton of production.

The selection of a single or double toggle jaw crusher needs to be considered when handling materials above an abrasive index of Excessive wear of the crushing surfaces could be caused by the crushing action of a single toggle jaw crusher.

FEED SIZE

The maximum feed size will determine the size of the crusher that can be selected.

For instance, river or hill gravel or material with a maximum size of 300mm could be crushed in a cone, gyratory or impactor, whilst material above this size could be crushed in an impactor, jaw crusher or primary gyratory.

WET OR STICKY MATERIAL

Wet sticky materials will cause problems in most crushers as they slow throughput and can even choke the crusher. Such material could also block chutes and hoppers and reduce screening efficiency.

Material with moisture content over 3% would need to considered very carefully with your supplier prior to selecting the appropriate crusher.

Efficient scalping prior to the primary crusher to remove finer fractions will greatly assist in improving the performance of the crusher and reduce contamination of the final products.

BULK DENSITY

All crusher capacities are based on a bulk density of 1600kg per cubic metre. Bulk density of material will have a large influence on the size of the crusher selected to handle the desired tonnage. Your crusher supplier will assist you with this.

If the material bulk density is greater than 1600kg per cubic metre you will have less volume through the crusher for a particular tonnage.

Likewise, if the bulk density is less than 1600kg per cubic metre you will need a greater volume through the crusher to achieve the same tonnage.

ROQ GRADING

The grading of the ROQ feed will have a large influence on the size and number of crushers required.

Crusher capacities are based on a graded feed from the maximum size down to a minimum size approximately equivalent to the CSS.

For instance, if the ROQ feed is from a gravel pit and you are producing a product less than 20mm is size and the raw feed contains 70% of natural occurring material of less than 20mm, this leaves only 30% to be crushed.

Another example is when the feed material is all large in size, one size spalls without a graded feed into the crusher which will slow the crushing process by as much as 50% and may require a larger machine to handle the capacity.

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