Selecting the right crusher

By |  July 25, 2018
Photo courtesy of Bluegrass Materials

The primary crusher should make it possible for material to be transported on a conveyor belt. Photo courtesy of Bluegrass Materials.

Selecting the appropriate crusher is important for every operation.

Understanding the stages of crushing and the types of crushers that best fit each stage simplifies equipment selection. Each type of crusher is different and used to achieve a certain end result.

Similarly, a certain output is expected at the end of each crushing stage for the next phase of the process. Aggregate producers who pair the correct crusher to the correct stage will be the most efficient and, in turn, the most profitable.

Crushing stages

Most aggregate producers are well acquainted with the selection of crushing equipment and know it is possible to select a piece of equipment based solely on spec sheets and gradation calculations. Still, theoretical conclusions must always be weighed against practical experience regarding the material at hand and of the operational, maintenance and economical aspects of different solutions.

Generally, material reduction is handled in stages. There are some single-crusher systems, but the most common systems involve at least two or three crushing stages.

Primary crushing. The duty of the primary crusher is, above all, to make it possible to transport material on a conveyor belt. In most aggregate crushing plants, primary crushing is carried out in a jaw crusher, although a gyratory primary crusher may be used. If material is easily crushed and not excessively abrasive, an impact breaker could also be the best choice.

The most important characteristics of a primary crusher are the capacity and the ability to accept raw material without blockages. A large primary crusher is more expensive to purchase than a smaller machine. For this reason, investment cost calculations for primary crushers are weighed against the costs of blasting raw material to a smaller size.

In most cases, trucks transport raw materials to a fixed primary. The costs of fuel, tires, maintenance and return on investment should also be considered.

A pit-portable primary crusher can be an economically sound solution in cases where the producer is crushing at the quarry face. In modern plants, it is often advantageous to use a moveable primary crusher so it can follow the movement of the face where raw material is extracted.

Intermediate (secondary) crushing. The purpose of intermediate crushing is to produce various coarser fractions or to prepare material for final crushing. If the intermediate crusher is used to make railway ballast, product quality is important.

In other cases, there are normally no quality requirements, although the product must be suitable for fine crushing. In most cases, the objective is to obtain the greatest possible reduction at the lowest possible cost.

Fine (tertiary) crushing. In this crushing stage, the quality and quantity of fine products are determined. Quality requirements can be stringent for final products, especially within the aggregate industry.

In most cases, the fine crushing and cubicization functions are combined in a single crushing stage. The selection of a crusher for tertiary crushing calls for both practical experience and theoretical know-how. This is where producers should be sure to call in an experienced applications specialist to make sure a system is properly engineered.

Crusher types

Jaw crushers. A jaw crusher is a compression type of crusher. Material is reduced by squeezing the feed material between a moving piece of steel and a stationary piece. The discharge size is controlled by the setting or the space between those two pieces of steel. The tighter the setting, the smaller the output size and the lower the throughput capacity.

As a compression crusher, jaw crushers generally produce the coarsest material because they break the rock by the natural inherent lines of weakness. Jaw crushers are an excellent primary crusher when used to prepare rock for subsequent processing stages.

Cone and gyratory crushers. These are also compression-type machines that crush material between a moving and a stationary piece of steel. The setting between the two pieces controls the output.

Although the chamber is round in shape, the moving piece of steel is not meant to rotate. Instead, a wedge is driven around to create compression on one side of the chamber and discharge opening on the opposite side. Cone crushers are used in secondary and tertiary roles as an alternative to impact crushers when shape is an important requirement, but the proportion of fines produced needs to be minimized.

Impact crushers. An impact crusher uses mass and velocity to break down feed material. First, the feed material is reduced as it enters the crusher with the rotating blow bars or hammers in the rotor. The secondary breakage occurs as the material is accelerated into the stationary aprons or breaker plates.

Impact crushers tend to be used where shape is a critical requirement and the feed material is not very abrasive. The crushing action of an impact crusher breaks a rock along natural cleavage planes, giving rise to better product quality in terms of shape.

Final thoughts

When selecting the crusher to best suit your operation, it’s important to understand your needs for each stage.

During the primary stage, look to get material to a size your conveyors and other processing machines can handle.

Some operations need further crushing in the secondary and tertiary crushing stages. These stages are used to better control and size the output.
Understanding how to best match the type of crusher to the right crushing stage will ensure crushing efficiency throughout your operation and overall profitability.


Gerry Mangrich is a process engineering specialist at McLanahan Corp.

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