Crushers

The main purpose of a primary crusher is to reduce the material to a size that allows its transportation on a conveyor belt. In most crushing installations a jaw crusher takes care of primary crushing. Plants with very high capacities that are common in mining and less popular in aggregates production, normally use a primary gyratory crusher. When the processed material is easy to crush and not very abrasive, an impact crusher may be the best choice for primary crushing.

One of the most important characteristics of a primary crusher is its capacity for accepting feed material without bridging. A large primary crusher is, naturally, more expensive than a smaller one. Therefore, the investment cost calculations for primary crushers are compared together against the total costs of primary stages, including quarry face clearing, blasting, and drilling costs. In many cases, dump trucks transport the rock to a stationary primary crusher. This may be an expensive solution. Amortization, fuel, tires, and maintenance costs can be included when the vehicles are in high demand. In modern aggregates operations, the use of mobile primary crushers that can move alongside the rock face is, in many cases, the most economical solution.

Primary crushing with jaw crushers

In terms of the size of the feed opening, the client gets a better return on investment when the primary crusher is a jaw crusher. That means less drilling and blasting because the crusher accepts larger boulders. The disadvantage of this type of crusher, when high capacity is required, is the relatively small discharge width, limiting the capacity as compared with the discharge circuit of a gyratory crusher. Jaw crushers are mainly used in plants producing up to approximately 1600 t/h.

Primary crushing with gyratory crushers

The primary gyratory crusher offers high capacity thanks to its generously dimensioned circular discharge opening (which provides a much larger area than that of the jaw crusher) and the continuous operation principle (while the reciprocating motion of the jaw crusher produces a batch crushing action). The gyratory crusher has no rival in large plants with capacities starting from 1200 t/h and above. To have a feed opening corresponding to that of a jaw crusher, the primary gyratory crusher must be much taller and heavier. Also, primary gyratories require quite a massive foundation.

Primary crushing with impact crushers

The primary impact crusher offers high capacity and is designed to accept large feed sizes. The primary impact crushers are used to process from 200 t/h up to 1900 t/h and feed sizes of up to 1830 mm (71") in the largest model. Primary impact crushers are generally used in nonabrasive applications and where the production of fines is not a problem. Of all primary crushers, the impactor is the crusher that gives the best cubical product.

The purpose of intermediate crushing is to produce several coarse-grade products – for example, road base aggregates – or to prepare material for final recrushing.

Generally the goal of secondary, tertiary, and quaternary crushing is to obtain the best possible size reduction at the lowest cost. 

If the intermediate crushing is done with the purpose of producing railway ballast, the quality of the product is important. In other cases, there normally are no quality requirements, except that the product be suitable for fine crushing.

Cone crushers are often used for intermediate crushing, due to their high capacity and low operating costs.

Fine crushing with cone crushers

Due to their design, cone crushers are generally a more expensive investment than impactors are. However, when correctly used, a cone crusher offers lower operating costs than a conventional impact crusher. Therefore, clients crushing hard or abrasive materials are advised to install cone crushers for the final crushing and cubicising stage.

Cone crushers can in most cases also give a good cubic shape to fine grades. They can be adapted to different applications. This is an important factor, as client-specific needs often change during a crusher’s lifetime.

For cone crushers there are few rules to be followed of optimum cubical shape. These ‘Ten Golden Rules’ are:

1. Full crushing chamber. This means that cone head must be covered by rock.
2. Stable and continuous feed.
3. Material below setting in the feed 10-30% (but no filler and fines 0-4 mm normally).
4. Maximum feed size. Reduction ratio must be limited to 3 (-4). Recommended max feed size is 50 mm.
5. Correct feed distribution. Feed distribution should be non segregated and evenly distributed around crushing cavity.
6. Setting closer to required product
7. Correct choke point. This means the right selection of cavities for feed in question.
8. Crusher itself. New generation cones will produce considerably better shape than so called old generation. This is due to improved crusher kinematics and shape of cavity. 
9. Closed circuit. This improves shape by attrition, gives constant feed curve and recrushing of flaky product. In secondary stages closed circuit calibrates feed to tertiaries.
10. Flow sheet in general. Important, especially in production of very high quality (shape) aggregate is that selective circuits are used, meaning that secondary and tertiary products are not mixed.

Fine crushing with impact crushers

The impactor family consists of two main types of impact crushers.

The conventional type has horizontal shaft configuration, known as HSI. The other type consists of a centrifugal crusher with vertical shaft, generally known as VSI. Impactor operation is based on the principle of rapid transfer of impact energy to the rock material. Impactors produce cubic products, and they can offer high reduction ratios as long as the feed material is not too fine. This means that in certain cases it is possible to use a single impact crusher to carry out a task normally done in several crushing stages using compressing crushers (i.e., jaw, gyratory, and/or cone crushers). Impactors are mostly used for nonabrasive materials.

The two main types of impactors can be further subdivided, into various groups.

Conventional horizontal-shaft impact crushers are available in various sizes and models, from high-capacity primary crushers for large limestone quarries to specially designed machines for the crushing of materials such as slag.

There are two main categories of VSI crushers – machines with impact wear parts around the body and machines that use a layer of accumulated material. The first type is in many respects similar to the conventional impactor with horizontal shaft and rotor. The second type became quite popular in the past decade and is known as the Barmac crusher. The difference between a conventional impactor and a VSI of the Barmac type is that the latter offers lower operating costs, but its reduction ratio is lower also. In a Barmac VSI, the material undergoes an intense rock-on-rock crushing process. In the other crushers, most of the reduction is done by the impact of stone against metal.

Customers operating old, rebuilt, or expanded plants often have problems with the shape of the product. In these cases, the addition of a Barmac VSI in the final crushing stage offers a solution to product shape problems.

The same applies to many mobile crushing units. As the number of crushing stages is normally small with this type of plant, it is almost impossible to obtain a good product shape unless the rock is relatively soft and thus more suited for the production of cubic product. A centrifugal crusher in the final stage can help to solve the problem.