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Aug 11, 2020

Why it pays off to invest in multi-stage crushing process

Janne Lahtela
Janne Lahtela
Product Specialist, GP Cones and C Jaws
Saku Pursio
Saku Pursio
Development Engineer
Getting the maximum value for your money when investing in crushers is a must for anyone operating a quarry. Those producing fine aggregate might ask: How many crushers do I need? When should I invest in multiple crushers? What is the optimal crushing process?
A stationary crushing plant.

Two of the most common crushing applications are two- and three-stage crushing processes. When deciding on whether to have two or three crushing stages, certain factors need to be considered in order to optimize the crushing process and achieve the best capacity and cost-efficiency. These are the

  • type and size of the crushed rock;
  • desired end-product size; and
  • desired quality of the end product.

An optimal crushing process with the right number of crushers helps to avoid breakages, optimize the wear of parts and operate the crushers in their optimal operating range.

One of the key things is to fully understand and know your operation’s characteristics, as every quarry is unique. There are multiple different crushing processes and the way that they should be operated to achieve the optimal crushing process depends on many factors. This is one example of many applications and illustrates one potential case where this kind of crushing process could be optimal to produce the desired end product.

When and why to choose three crushers instead of two

Feed material characteristics impact on what kind of crusher should be used and what kind of settings are suitable for processing the feed material. For example, the size and type of feed material determines the achievable reduction ratio of the crusher.

For example, when producing 0/32 mm end product, two crushers may be enough to produce the desired end product cost-efficiently and in accordance with the quality requirements. However, when the desired size of the end product gets smaller, for example, 0/16 mm, two crushers may not be enough, and it might be worth considering adding a third crusher to the crushing process for optimal aggregates production.

But how to determine the number of crushers in practice? Here's a practical tip: Check the maximum feed size of the primary crusher and compare that to the desired final product size. As a guiding principle, a reduction ratio of 4 is generally considered as the maximum of any jaw or cone crusher. If the ratio of feed material and end product size is 16:1, you need 2 crushers, and this is calculated by multiplying 4 with 4. If the ratio would be, let’s say 20:1, you would need 3 crushers.

Some easily crushable rock types can be crushed in two stages and some harder rock types in turn require to be processed in three stages or more. If you attempt to process these types of rocks with crushers that are not optimal for processing them, the crushers will not be operated in their best operating range. This leads to increases in operative costs due to such things as higher wear of wear parts.

Improved end-product quality compensates equipment costs

One common counterargument for not investing in additional crushers is naturally to save money. However, that is a devious misconception as producing fine aggregate with only two crushers oftentimes has a negative effect on both final product quality and quantity/capacity. It is possible to meet the size range even without an optimized process, but in order to meet the customers’ quality requirements, the process needs to be designed and built to meet those as well.

The argument on costs can also be examined from the equipment use point of view. All crushers are built to perform optimally in certain conditions, and if they are run constantly in non-optimal conditions, there will be an inevitable increase in cost per ton of production. This includes excess wear, but it can also be seen as not getting the most out of the investment. Luckily, there is always the option of optimizing the process.

Less stress and wear on the primary crusher

So, if your target is to produce fine end products (for example, 0/16 mm) and aim for optimal crusher use and avoid breakages and rapid wear of wear parts, adding a third crusher as part of the crushing process would make production more optimal. Crushing in three stages enables the primary jaw crusher to be operated with a larger setting by accepting more and larger size feed material which in turn increases the life of wear parts and lowers operating costs as wear cost per crushed ton is higher. With three crushers, the settings can be also adjusted and fine-tuned more easily compared to two-stage crushing, which makes it easier to make finer end products.

Designing a crushing process for fine aggregate production

Here we are going to explain the benefits of three crushers in an example scenario producing 0/8, 8/16 and 16/32 end products (for example, 0).

Let’s start with the primary jaw crusher, the Nordberg C120. For a two-stage crushing plant with the feed material properties in this example, it is not possible to squeeze the CSS any tighter than 100 mm as the power limit is the limiting factor. In the three-stage plant, we can open the C120’s CSS to 150 mm, as the GP300S secondary cone crusher with its large feed opening can handle the coarser product of the jaw with ease, without any downtime caused by bridging or blockages.

Bruno: 2-stage crushing process with C120 jaw crusher and GP330 cone crusher
Two stage crushing process with C120 jaw crusher and GP330 cone crusher.
Bruno: 3 stage crushing process with C120 jaw crusher, GP300S secondary cone crusher and GP330 cone crusher.
Three stage crushing process with C120 jaw crusher, GP300S secondary cone crusher and GP330 cone crusher.

A more open CSS of the jaw increases the capacity of the plant, and the feed flows visibly easier through the jaw. In this example, increasing the CSS of the C120 from 100 mm to 150 mm increases the crushed ton per one set of fixed jaw die by over 60%. This significant increase is enabled by the large feed opening of the GP Secondary cone crusher. Speaking of the GP300S, in this example, the fines are scalped away, but due to the GP Secondaries’ steep cavity profile, the crusher would be tolerant to fines being fed straight into the crusher as well.

One wisdom relating to all cone crushers is that, generally speaking, the end product’s quality is the best around the product size which is closest to the CSS used in the crusher. Knowing this, with three crushers, it is possible to tune the GP300S’s CSS close to 16/32 mm, and the tertiary GP330 cone crusher to 8/16 mm, maximizing the possibility of excellent product size for all of the end product gradations. With just two crushers, this wouldn’t be possible, and some compromises have to be made.

In addition to improved quality, a third crusher will also increase the capacity of the process. As can be seen from the example, the overall end product tonnage in a two-stage plant is around 300 mtph, and in a three-stage one it is almost 500 mtph. Even though the quality and capacity of the process increases, the energy consumption will still stay the same as both plants would still have the same 1.06 kWh/t consumption.

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