The right crusher depends on many factors. McLanahan helps break down the important requirements.
Primary crushers reduce large run-of-mine material, such as rocks and ore lumps, into smaller sizes for further processing downstream, whether that be transport to additional crushers or to a wet processing plant.
It is the first stage in the crushing process that can sometimes include secondary, tertiary and even quaternary crushing stages, depending on the material and the end product requirements.
There are several different types of primary crushers. The right one for an application depends on the material to be crushed, feed size, desired product size, and the capacity required.
There are four types of primary crushers that are typically found in aggregates applications: jaw crushers, gyratory crushers, horizontal shaft impact crushers, and cone crushers.
Jaw crushers
These are a strong choice for primary crushing applications because they can accept the toughest, hardest and most abrasive materials. They can handle dry-to-slightly-wet material that is not sticky.
These types of compression crushers are best suited for heavy mining, quarried materials, sand and gravel, and recycling applications. They provide a reduction ratio of 3:1 to 6:1.
Jaw crushers feature a moving piece of steel – the pitman – and a stationary piece of steel that form a V-shaped chamber.
Material that enters the chamber is crushed to size between the moving and stationary pieces of steel. Most of the crushing action occurs in the bottom third of the chamber.
In a cycle of the rotating pitman, the material in the chamber is first squeezed to crush the material. The chamber then opens, which allows the material to fall farther down before the next squeezing cycle comes around. Sized material discharges out of the bottom of the crusher on the open stroke. The product size can be adjusted by moving the pitman closer to or further from the stationary piece of steel.
The closer the pitman to the stationary piece of steel, the smaller the product size. The tighter the setting between the two pieces of steel, the lower the throughput of the machine. This adjustment can be made either hydraulically or manually by adding and removing shims.
Gyratory crushers
These are a good choice as they can handle very hard and abrasive material. They are ideal for dry-to-slightly-wet material, but not sticky material.
Gyratories are compression crushers primarily used for heavy mining and quarried materials in extremely high-tonnage applications. They provide a reduction ratio of 4:1 to 7:1.
The main features of a gyratory crusher include a tall crushing chamber lined with replaceable manganese wear plates, a main shaft with head and mantle, and a spider to anchor the top of the main shaft bearing. They have an adjustable main shaft positioning system and drive assembly, and they are available in a single-cylinder or multiple-cylinder design.
Material to be crushed is fed into the top of the chamber and compressed between the wobbling mantle in the centre and the stationary concave liners covering the inner walls of the chamber. When the material is crushed to the desired product size, it is discharged out the bottom of the crusher.
An additional benefit of a gyratory crusher is that there is no need for pre-screening or scalping of fines in the feed. The crusher can handle the all-in feed.
The gradation is controlled by adjusting the spacing between the mantle and the concaves at their closest point. For a smaller product size, the main shaft can be raised to create a smaller space between the mantle and the liners. To create a larger product size, the main shaft can be lowered to create a larger space between the mantle and the liners.
As the machine is set tighter for a finer product, the volumetric throughput of the machine is also lowered. A larger opening equals greater throughput.
Gyratories typically have a higher capital cost and require a large housing structure that can become a significant investment.
Horizontal shaft impact crushers
Horizontal shaft impact crushers, such as the New Holland, Andreas and MaxCap (which is a hybrid of the two) styles, are typically used for the primary size reduction of soft-to-medium hard materials.
These types of impact crushers can handle slightly abrasive, dry or wet material that is not sticky. They provide a reduction ratio of 10:1 to 25:1 and are typically used in quarried materials, sand and gravel, and recycling applications.
Horizontal shaft impactors feature a large expansion chamber, inside of which is a spinning rotor shaft covered with blow bars or hammers. Material that enters the crushing chamber connects with the spinning rotor, which flings the material at the breaker plates lining walls of the chamber.
Much of the breaking of the material occurs upon the initial contact of the material with the rotor and its subsequent impact with the breaker plates. Some additional breakage occurs by the inter-particle collisions as the material is tossed around in the chamber.
Horizontal shaft impactor crushers produce a highly cubical product with enhanced soundness. The gradation is more uniform and can be adjusted by changing the RPM (revolutions per minute) of the rotor.
Additional gradation control is provided by adjusting the grates or aprons within the machine. A tighter setting yields a smaller output and lowers throughput.
One thing to keep in mind, however, is that while horizontal shaft impact crushers produce more fines than any other type of primary crusher in that stage, they may allow for less fines production in the entire circuit.
The more stages of reduction a particle must go through before it is properly sized, the more fines will be produced. Since the primary impactor has a higher reduction ratio, it creates more finished product. This results in less reduction needed down the process and thus may result in fines being produced by the entire circuit.
Cone crushers
These are compression-type crushers that are typically found in secondary crushing stages but can be used as the primary in certain applications. They handle medium-hard-to-very-hard abrasive materials that can be dry or wet but not sticky. Cone crushers are usually found in quarried materials, and in sand and gravel applications, and provide a reduction ratio of 3:1 to 5:1.
Material that enters a cone crusher is compressed between a wobbling mantle and the concaves inside the crushing chamber. The gradation is controlled by adjusting the spacing between the mantle and the concave at their closest point. This is accomplished by raising and lowering the top or bowl of the crusher. A finer setting will create a finer product but at a lower throughput.
Speed can also help to determine the final output gradation. Faster speed, for finer feed, is used to produce a finer output.
Case study
A McLanahan customer was looking to increase their production and had a challenge of dealing with clays on the products they were mining. They wanted something that would deal with the heavy clays while also allowing them to follow the veins throughout the quarry until they could get to a more stationary location.
After a material analysis from McLanahan’s in-house lab and working closely with the local dealer, this customer decided to install a McLanahan Pit Portable MaxCap 600 primary impact crusher for their primary crushing plant.
To feed the MaxCap and round out the portable primary plant, they installed a unique configuration of an apron feeder – referred to as a Texas-style apron – and a wobbler feeder. The apron feeder accepts bucketloads of quarry shot material and conveys it to a wobbler feeder, which is able to scalp out some of the clay before it enters the MaxCap impact crusher.
The MaxCap and wobbler feeder were installed on a five-axle chassis and the apron feeder was installed on a separate quad-axle chassis for ease of movement around the site.
For their secondary crushing plant, the customer installed two McLanahan Universal NGS secondary impact crushers. Both impactors were optioned with a third curtain to provide additional material size reduction and to eliminate the need for a tertiary crushing circuit.
Since start-up, the McLanahan crushers and feeders have been an asset to this customer and their overall production at their quarry. They have seen an increase in production by being able to deal with the clay covered material and process more with these new crushers.
For more information, visit the McLanahan website here.