There are different type crushers that achieve various objectives in material processing. Every application calls for a certain type crusher or a combination of multiple crushing stages to achieve a certain aggregate production goal.
A primary crusher provides an initial reduction in size. A mobile jaw crusher is considered a primary crusher in quarry operation. This stage aims at taking large chunks and bringing the material down efficiently to a consistent and manageable size for further processing through a secondary crusher. The material produced is not sought to be a final usable material.
Secondary crushers are designed to take a reasonable feed size and producing a final product size and adding a product shape. Common mobile secondary crusher types are horizontal shaft impact crushers (also referred to as an HSI) and cone crushers. When the feed material is not hard or big a secondary crusher can be used without a a primary jaw crusher. This is often the case in concrete recycling, limestone, shale, or sand & gravel applications. However, cone crushers - in most cases - will need a primary jaw crusher due to its infeed limits.
Taking small material and producing fines is very hard to do. For impact crushers it is easier to produce a 3/4"- out of a 25" rock than taking a crushed 1" feed and producing a 1/2" or even smaller product. A vertical shaft impact curhser (VSI) or a specialized horizontal shaft impact crusher with a grinding path (such as the RM V550OG! Mobile Impact Crusher) can be used to produce high amounts of fine material.
The reduction ratio is the ratio of the max infeed size compared to the crusher max output size. It varies depending on the crusher type, settings, and material. The higher the reduction ratio the smaller the output product.
A reduction ratio of 1:10 means that a 20" chunk entering a crusher leaves as 2" crushed product.
| Crusher Type | Reduction Ratio | Example |
|---|---|---|
| Jaw crusher | 3:1 - 5:1 | 20" Diameter Rock -> 4" Output |
| Impact crusher | 5:1 - 10:1 | 20" Diameter Rock -> 2½" Output |
| Cone Crusher | 3:1 - 6:1 | 4" Pre-Crushed Rock -> 1" Output |
While the reduction ratio indicates the efficiency it doesn't tell anything about gradation. Understanding the reduction ratios helps to identify the right combination of crushing stages to achieve a certain production goal based on input and output size.
The gradation is a material quality characteristic that determines gradability, compaction, and structural load bearing characteristics. Different mobile crusher types generate different gradations. While an impact crusher produces a higher amount of fines and a good mix between different particle sizes. Jaw crushers produce mostly coarse material. A sieve analysis shows you how many different particle sizes are in the finsihed product.
For example if your impact crusher puts out a 2"- (50 mm-) top size product 50% of the particles could be smaller than 1" (25 mm).
Impact Crushers are on the rise because they are the most versatile type crusher. They can process reasonable sized soft- medium-hard rock and can be used in concrete & asphalt recycle applications. A spinning rotor with hammers impacts the material and throws it against an impact wall (also called apron) which causes the material to shatter. As a result you get many different size cuboid particles that make high-quality aggregates.
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The on-board screen and return conveyor enable you to reduce the material, screen a finished product, and recirculate the oversize material to be recrushed to the desired output size in a second pass.
The most common mobile jaw crusher type is a single toogle jaw crusher which works like a giant nut cracker. It features a stationary plate and a moving piece that form a V-shaped chamber. The material that enters the crusher from the top is compressed and crushed in between both plates until it is small enough to fit through the gap (closed side setting) on the bottom of the crusher. The tighter the closed-side setting, the smaller the product, the lower the throughput of the machine.
Aggregates produced by a mobile jaw cruhser are elongated, lack fines for compaction, and don't grade very well. Thus, mobile jaw crushers are not ideal for secondary crushing applications.
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Common applications of a mobile jaw crusher are crushing hard rock at the face of a quarry or heavy-duty concrete recycling applications.
Similar to jaw crushers, cone crushers work by compression, which means they reduce materials by squeezing them until they break apart. Cone crushers are built with a rotating mantle in a concave bowl lined with manganese. Cone crushers can accept medium-hard to very hard and abrasive feeds that might be dry or wet, though not sticky. Their output will be a relatively cubical product, with a reduction ratio of about 6-to-1 through 3-to-1.
Typically, mobile cone crushers require a primary jaw crusher. In some applications if the feed is small enough they can operate as a primary crusher.
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