The basics of crushing

No matter where an operation is located or what geologic material is available, the goal is always the same: produce required aggregate sizes at the lowest cost per ton.

To do that, an operation must be organized and efficient, especially when it comes to the crushing process.

Crushing is at the heart of every aggregate operation. Put simply, crushing aggregate is the process of “making little ones out of big ones.” Mark Krause, managing director of North America for McLanahan Corp., recently explained the basics of crushing in an introductory-level webinar. Presented here are some key points from the presentation.

Types of operations

There are two types of aggregate operations: open circuit and closed circuit. An open circuit operation is where feed material is only run through the crusher once. This usually occurs in a primary and sometimes secondary operation where uniformity of product size is not as critical.

A closed circuit operation is where material is continuously returned to the crusher until it’s of a size that will pass through the product screen.

Types of crushers

The two most common types of crushers are compression crushers and impact crushers. The four factors affecting the crusher type, be it compression or impact, for an aggregate operation are:

◾ Type of material (abrasiveness)
◾ Hardness of material (friability)
◾ Density (toughness)
◾ Fragmentation (particle shape)
◾ Condition

Compression crushing is when material is squeezed between two surfaces. The crusher reduces the material as it advances downward through the chamber by means of squeezing the material between moving and stationary pieces of steel. These include gyratory crushers, jaw crushers, roll crushers and cone crushers.

Compression crushing quick facts

1. The output is changed by adjusting the gap setting at the discharge point of the crusher.

2. Rule of thumb: Twice the setting is the top size discharging from the crusher.

3. The tighter the setting, the lower the throughput.

4. Material breaks to fill the air pockets or voids in the crushing chamber.

Impact crushing

Impact crushing is when the material is hit or thrown. This crushing process uses the theory of mass and velocity to reduce the feed material. Therefore, the faster the speed of the crusher means the finer the output. However, the faster the speed also means exponential wear on the machine.

The first break of the feed will occur along the natural fracture line, while the second break will be perpendicular to the first. These include horizontal shaft impactors, vertical shaft impactors, cage mill and hammermills.

Impact crushing quick facts

1. The faster the speed, the finer the gradation.

2. Setting has some effect on gradation and throughput, but speed is by far the most important factor.

3. Rotor penetration is critical for best performance.

4. Double the speed equals four times the wear.

Crushing efficiency

There are four factors that most affect the crushing process:

◾ Feed size of the material (available)
◾ Product size (desired)
◾ Expected capacity (expected)
◾ Reduction ratio (dictates)

The size of feed material is crucial when it comes to crushing. If the feed size is too large, the material will percolate around the feed opening, causing the top of liners to wear faster than the bottom. Production will also decrease as the feed opening is bridged over and blocked by oversized material.

In contrast, undersized feed material can also be detrimental to production. If the material is too small, the majority of crushing will take place at the lowest portion of the cavity, causing the bottom of the liners to wear away faster than the top of the liners. In turn, production will decrease to a poor ratio of reduction.