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.
Types of Primary Crushers
There are several types of primary crushers, including:
- Gyratory Crushers
- Jaw Crushers
- Hammermills
- Horizontal Shaft Impact Crushers
- Sizers
- Roll Crushers
- Cone Crushers
- Feeder-Breakers
Gyratory Crushers
Gyratory Crushers are an excellent choice for a primary crusher, 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 crushing chamber and compressed between the wobbling mantle in the center 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.
One 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.
Gyratory Crushers typically have a higher capital cost and require a large housing structure that can become a significant engineering investment.
Jaw Crushers
Jaw Crushers are another excellent choice for primary crushing applications because they can accept the toughest, hardest, 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 (called 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 piece of steel and the stationary piece of steel. Most of the crushing action occurs in the bottom third of the chamber.
In a cycle of the rotating pitman, first the material in the chamber is squeezed to crush the material, then the chamber opens, which allows the material to fall farther down the chamber before the next squeezing cycle comes around.
Sized material discharges out the bottom of the crusher on the open stroke. The product size can be adjusted by moving the pitman closer to or farther 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.
Hammermills
Hammermills can be used as a primary crusher in many types of aggregate and industrial applications, including limestone, marble, shale, gypsum, phosphate, bauxite, aluminum dross, uranium, cryolite, clinkers, quicklime, petroleum coke, coal and more.
Using a combination of impact, shear and attrition reduction methods, Hammermills tend to generate finer material in addition to size reduction, which may not be desirable for every application. The material enters the crusher and shatters upon impact with swinging hammers that are secured to a spinning rotor. The particles rebound off the breaker plates into the hammer circle and are carried into the grate assembly and across the grate surface, grinding down larger particles. Once the material reaches the appropriate size, it exits through the openings in the grate assembly.
Output gradation changes are made through speed changes as well as different combinations of grates with various sizes of openings. They are a good tool to use in lower abrasive applications.
Common configurations of Industrial Hammermills are:
- Standard
- Non-Clog
- Centerfeed
Of these three types, the Standard and Non-Clog Hammermill types are considered primary crushers due to the fact that they can accept large feed sizes up to 72” and are capable of milling down to a nominal 3”.
The differences between Standard Hammermills and Non-Clog Hammermills are the breaker plate and the types of materials they process. Standard Hammermills feature a stationary breaker plate and are perfect for free-flowing friable materials with large feed size. Non-Clog Hammermills feature a moveable breaker plate and are perfect for wet, sticky, friable materials with large feed size.
Aggregate Hammermills can be used to generate high-quality, cubical aggregate material with superior soundness or to generate pulverized lime.
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 of the rotor. Additional gradation control is provided by adjusting the grates or aprons within the machine. A tighter setting yields a smaller output. A tighter setting also 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 thus 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.
Sizers
Direct-Drive Crushing Sizers are used for the primary crushing of soft to medium hard material that can be slightly abrasive, dry, or wet and sticky. They are best suited for heavy mining applications and provide a typical reduction ratio of 4:1.
Primary sizers feature two inward rotating rolls that crush the material between the teeth and roll bodies. Already sized material passes between or flows around the rolls, minimizing the generation of fines.
These types of crushers offer a low profile and high throughput capacities with low roll speeds. Also, like gyratories, they can accept fines in the feed where other compression machines, like jaws, require some pre-screening.
Roll Crushers
Similar to Sizers, Roll Crushers can be used for the primary crushing of soft to medium hard material that can be slightly abrasive, dry, or wet and sticky. They are typically used in heavy mining applications and have a typical ratio of reduction of 4:1.
Primary Roll Crushers are available in several designs:
Roll Crushers are compression-type machines, so output gradation is controlled mainly through the gap setting between the rolls. A wider setting results in a coarser product. A wider setting also provides greater throughput.
Single Roll Crushers feature a low-speed crushing roll that crushes material against a curved crushing plate. They require lower headroom than other primary crushers and produce minimal fines. However, Single Roll Crushers provide limited product sizing and are limited in throughput capacity.
Double Roll Crushers feature two rotating roll shafts that crush material between the teeth and roll bodies. These types of Roll Crushers produce a cubical product with minimal fines generation, and can handle higher capacities.
In the correct application, Triple and Quad Roll Crushers are capable of performing both primary and secondary stages of material reduction in one machine. The top stage of a Triple Roll Crusher has a 6:1 ratio of reduction, whereas the bottom stage has a 4:1.
In a triple-roll configuration, the primary stage of crushing occurs in the top stage of the crusher and features a single roll configuration that performs the initial crushing. A double-roll arrangement on the bottom stage of the machine accepts the sized material from the top stage and sizes it further. It is ideal for handling larger feed sizes due to the design and larger feed opening.
In a quad-roll configuration, both the top and bottom stages have a 4:1 reduction ratio. The primary crushing stage at the top of the machine and the secondary stage at the bottom of the machine both consist of a double-roll arrangement. Due to the feed opening and design of the top stage, these types of Roll Crushers are best for handling medium sized feed material.
Like the other Roll Crushers listed, Triple and Quad Roll Crushers produce a cubical product with minimal fines generation.
Cone Crushers
Cone Crushers are compression-type crushers that are typically found in secondary crushing stages but can be used as the primary crusher in certain applications. They handle medium hard to very hard, abrasive materials that can be dry or wet but not sticky. Cone Crushers are typically found in quarried materials and sand and gravel applications. They 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.
Feeder-Breakers
Feeder-Breakers, which combine both feeding and crushing into one machine, are used for the primary crushing of friable materials that can be slightly abrasive, dry or wet, but not sticky. They are typically used in heavy mining applications to provide the primary crushing and to make a beltable product to be conveyed for further downstream processing.
These high capacity machines feature a chain and flight drag conveyor that meters material toward a horizontal pick. Run-of-mine material is conveyed toward the rotating pick roll, which crushes the material against the flights and conveyor deck.
Since Feeder-Breakers size in one dimension only, secondary and even tertiary crushing stages may be necessary to achieve the desired product size. They can be fed by a shovel, truck, loader or dozer and can be made portable for underground operations.
How To Choose the Right Primary Crusher
While all these machines can be used as primary crushers, they can’t be used interchangeably in every industry. Some types of primary crushers are best suited for hard material, while others are best at handling more friable or wet/sticky material. Some crushers require pre-screening, and some accept all-in feed. Some crushers produce more fines than others.
Primary Crushers Used in Aggregate Applications
The types of primary crushers typically found in aggregates applications include:
- Jaws
- Gyratories
- Impactors
- Cones
Primary Crushers Used in Mining Applications
The types of primary crushers typically found in mining applications include:
- Roll Crushers
- Sizers
- Feeder-Breakers
- Jaws
- Cones
- Impactors
The right primary crusher for an application depends on several factors:
- Material to be crushed
- Feed size
- Desired product size
- Capacity required
- Compressive strength of the feed
- Moisture content
The material and its characteristics, e.g., its hardness, density, shape and condition, will affect the type of crusher that needs to be used. Knowing the material characteristics as well as the advantages and limitations of the different crusher types will help to determine the best primary crusher for a given application.
