Simplifying the crushing process

How many types of crushers are there? This question can be answered in a variety of ways depending on what someone’s really asking.

The answer might be three if you’re referring to stations in a complete crushing plant – primary, secondary and tertiary crushers. Of course, there are also different styles of crushers: compression-style jaw and cone crushers, for example, which fit into the various stations within a crushing circuit.

The number of crusher types, in terms of style and configuration, can be more difficult to quantify, as there are many ways to customize crushers. Still, there are four basic designs – jaw, cone, gyratory and impact crushers – that operate within many crushing plants.

To provide more depth to the question presented at the outset of this article, here’s a breakdown of the fundamental details you should know about crushing to ensure your operation has the proper machinery.

Stages of rock crushing

It’s common to use multiple crusher types within a project and set them up as stations in a circuit format to perform the necessary material reduction work. In many cases, primary, secondary and tertiary stations are established to reduce the rock to the desired size, shape and consistency.

Of course, not all projects require all of the stages. Sometimes, primary crushing alone may offer enough reduction to meet your goals.

For instance, if the final size of your product only needs to be between 4 in. and 6 in., a primary jaw crusher can accomplish this goal. However, you will likely require a much finer product, and that means incorporating up to three stations, including a variety of crusher types.

1. Primary crushing. As the first stage in a crushing circuit, primary crushing reduces material to a size and shape that can be handled by a secondary crusher.

Typically, the minimum setting on most primary crushers is about 4 in. to 6 in. Jaw, gyratory and impact crushers are most often appropriate as primary crushers, although there can be overlap between primary and secondary machines.

2. Secondary crushing. Reduction ratios become an important consideration in secondary crushing. Knowing just how fine you need a final output to be, along with the feed requirements of your tertiary crushing station, will help to determine how much reduction needs to take place within this stage.

Cone crushers are often placed within a secondary crushing station because they are versatile in terms of feed and discharge openings. With cone crushers, though, it is essential to operate them at consistent settings to keep productivity up.

3. Tertiary crushing. The goal of the tertiary, or final, reduction stage is to size and shape rock – or other material – into marketable products. Again, there may be overlap between stages regarding which crusher styles work best between secondary and final crushing. Cone crushers, vertical shaft impactors (VSIs) or even high-pressure grinding roll crushers can be used in the tertiary position.

How rock impacts crusher choice

Sandstone, limestone and granite are arguably the three most common aggregate used in the construction industry. But these rocks each have very different hardness and abrasiveness characteristics.

The type of rock you need to process will dictate the types of crushers needed in the crushing circuit. The more you know about the aggregate you wish to crush – along with its end use – the easier it will be to select the best equipment to achieve project goals.

1. Jaw crushers. In compression crushing, jaw crushers tend to be more of a blunt instrument compared to cone crushers. This is why jaw crushers are often used in the primary circuit stage.

Jaw crushers are also known as “rock breakers” and are used to break up larger, harder materials into more manageable pieces. They tend to do well with different types of materials, and they don’t display as much wear and tear as impact crushers.

Jaw crushers also produce minimal fine materials and dust, although the finished product with this type of crusher almost always requires secondary crushing.

2. Gyratory crushers. These are actually quite similar in concept and design to jaw crushers. Both feature a conical head and concave surface (often lined with manganese steel) and break material apart by compression through what is known as “eccentric movement.”

Like jaw crushers, gyratory crushers are often utilized in primary crushing stations, although they may sometimes be used as secondary crushers.

3. Cone crushers. Along with jaw crushers, cone crushers are classified as compression-style crushers. These are typically used with more abrasive and harder materials like granite. These types of machines are very powerful and reduce materials by squeezing them until they break apart.

Cone crushers are also extremely versatile, and newer-generation cone crushers can produce a more cubical product similar to impact crushers.

4. Impact crushers. Impact-style crushers, including VSIs and horizontal shaft impactors, are best used with less abrasive rock types like limestone.

These types of machines break material apart by throwing it into an anvil or by striking it with a blow bar.

Some operations also use impact-style crushers after they’ve already used a different type of crusher that produces a more elongated stone. This helps to further shape the crushed material into a finer consistency with a more cubical nature.

Impact crushers tend to be less expensive than compression crushers and have a higher reduction ratio. They can also break sedimentary deposit-type rocks, like limestone, along natural lines, rounding off sharp angles and weak edges. This produces an end result that is more sand-like in nature.

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Greg Donecker is president of Kemper Equipment.