Most aggregate producers are well acquainted with the selection of crushing equipment, and know it is possible to select a piece of equipment based solely on spec sheets and gradation calculations. However, theoretical conclusions must always be weighed against practical experience of the material at hand and of the operation, maintenance and economical aspects of different solutions.
In general, material reduction is handled in stages. Although there are some single crusher systems, the most common systems involve at least two or three crushing stages.
Primary crushing – The duty of the primary crusher is, above all, to make it possible to transport material on a conveyor belt. In most aggregate crushing plants, primary crushing is carried out in a jaw crusher, although a gyratory primary crusher may be used. If material is easily crushed and not excessively abrasive, an impact breaker could also be the best choice as a primary crusher.
The most important characteristics of a primary crusher include the capacity and ability to accept raw material without blockages. A large primary crusher is more expensive to purchase than a smaller machine; for this reason, investment cost calculations for primary crushers are weighted against the costs of blasting raw material to a smaller size. In most cases, raw material is transported by trucks to a fixed primary crusher. The costs of fuel, tyres, maintenance and return on investment should also be considered. This can be an expensive solution.
A pit-portable primary crusher can be an economically sound solution in cases where the producer is crushing at the quarry face. In modern plants, it is cost-effective to use a mobile primary crusher, so it can follow the movement of the face where raw material is extracted. The most common use of track-mounted primary crushers is in applications intended for short-term contracts or where frequent moves are necessary.
Intermediate (secondary) crushing – The purpose of intermediate crushing is to produce various coarser fractions – base coarse, for example – or to prepare the material for final crushing. If the intermediate crusher is used to make railway ballast, product quality is important. In other cases there are normally no quality requirements, although the product must be suitable for fine crushing. In most cases, the objective is to obtain the greatest possible reduction at the lowest possible cost.
Fine (tertiary) crushing – In this crushing stage, the quality and quantity of fine products are determined. Quality requirements can be stringent for the final products, especially within the quarrying industry. Customer requirements common to both the aggregate and mining industries are capacities and quality (fraction/particle size). The quarrying industry has additional quality demands such as soundness and particle shape (cubicity).
In most cases, the fine crushing and cubicising functions are combined in a single crushing stage. The selection of a crusher for tertiary crushing calls for both practical experience and theoretical know-how. This is where producers should be sure to call in an experienced applications specialist to make sure a system is properly engineered.
Types of crushers
Most crushers used for aggregate production can be categorised as one of three main types:
- Compression crushers – which squeeze the material until it breaks.
- Impact crushers – which use the principle of rapid impact to shatter material.
- A ttrition crushers – which use both impact and grinding to shear material.
Jaw crushers, cone crushers, gyratory crushers and roll crushers all operate on the compression principle. Impact crushers utilise the impact principle, while hammermill crushers follow the principal of attrition.
Jaw crushers – Jaw crushers are often used as primary crushers and are perhaps the most popular crusher worldwide. They are suitable for most any type of material.
Crushing takes place between a stationary jaw plate and a moving jaw plate. The moving jaw plate is mounted on the pitman, which is given a reciprocating motion. Crushing occurs when the pitman moves toward the stationary jaw.
There are two main types of jaw crushers – single toggle and double toggle.
The single toggle jaw crusher features a pitman mounted on an eccentric shaft at the top. At the bottom of the assembly, the pitman is held in position by a toggle plate. The combination of eccentric motion at the top and rocking motion at the bottom provides a positive downward thrust throughout the crushing chamber.
The double toggle crusher has two shafts. One is a pivot shaft from which the pitman hangs, while the other is an eccentric shaft that stimulates the two toggle plates. The pitman is given a purely swinging motion toward the stationary jaw.
Single toggle jaw crushers have better feed acceptance capability than the corresponding double toggle crushers. Jaw crushers are reliable, robust machines, offering a 6:1 reduction ratio in most materials, and will accommodate hard, abrasive materials.
Roll crushers – Roll crushers are a compression-type reduction crusher with a long history of success in a broad range of applications. The crushing chamber is formed by massive drums, revolving towards one another. The gap between the drums is adjustable, and the outer surface of the drum is comprised of heavy manganese steel castings known as roll shells that are available with either a smooth or corrugated crushing surface.
Double roll crushers offer up to a 3:1 reduction ratio in some applications, depending on the characteristics of the material. Triple roll crushers offer up to a 6:1 reduction. As a compression crusher, the roll crusher is suited for extremely hard and abrasive materials. Automatic welders are available to maintain the roll shell surface and minimise labour expense and wear costs.
These are rugged, dependable crushers, but not as productive as cone crushers with respect to volume. However, roll crushers provide very close product distribution and are excellent for chip stone, particularly when avoiding fines.
Cone and gyratory crushers – Cone and gyratory crushers are gyrating shaft machines. They have a main shaft that gyrates and provides the crushing motion. Crushing takes place between a fixed outer crusher member (the concave ring) and a moving inner crushing member (the mantle), mounted on the gyrating shaft assembly.
A roller-bearing-type cone crusher functions the same as a shaft-type cone. The main difference is the rotating wedge found in roller-bearing cones that causes the gyrating motion of the mantle. It commonly features a flatter angle in the crushing chamber compared with shaft type cones.
It is usually recommended that cone crushers operate with the crushing chamber full (ie choke-fed). Cone and gyratory crushers are typically used on abrasive materials of considerable hardness. Due to high investment value, they are used in cases where impact crushers are not appropriate.
Primary impact breakers – Primary breakers are noted for large expansion chambers above one or two revolving rotor assemblies. As the rock falls into the rotor circle it is struck by manganese hammers fixed onto the outer surface of the rotor(s). The rock shatters upon impact with the hammer, sprays against the back wall of the impactor, and then tumbles back into the hammer circle. When crushed to size, material passes the adjustable bar to exit the crusher.
These breakers are ideal for limestone quarries and are known to provide 20:1 reduction or even as high as 40:1 reduction in the case of a double rotor configuration. The expansion chamber allows stone to shatter at its weakest fissures, minimising fines while generating a high percentage of minus 38mm (1” to 1/2”) product with superior soundness. They are also noted for their ability to accept a maximum feed size much larger than other primary crushers.
Primary impact breakers, while productive, may require additional maintenance expense relating to the hammers (blowbars). Some models offer limited access, and in the face of rising labour costs, maintaining an old impact breaker may be more expensive than a primary horizontal shaft impactor.
Primary horizontal shaft impactors – Horizontal shaft impactors (HSI) are noted for their accessibility, with a housing that opens like a clamshell. Primary HSIs are generally equipped with two adjustable curtains (aprons) and a rotor assembly with hammers (blowbars) that are easily accessible and offer multiple wear surfaces. They have a low profile design compared with primary breakers and a limited expansion area above the rotor.
Quarry shot material enters the crusher, and upon impact with the hammers (blowbars), it strikes the curtain(s), and then immediately rebounds back into the hammer circle. In a typical two-curtain impactor, the initial curtain is often set at 304mm (12”), followed by a 127mm (5”) gap setting on the second curtain.
Reduction ratios associated with primary HSI crushers range from 4:1 to 6:1 depending on the material characteristics. The maximum feed size varies with each model but is generally limited to 609mm (24”) to 914mm (36”). The output gradation of the primary HSI crusher is generally coarser than the primary breaker due to the lack of expansion above the rotor, which also affects the capacity. Primary HSI crushers are considerably wider than primary breakers at comparable capacity ratings.
Secondary HSI crushers – These crushers are characterised by a high 10:1 reduction ratio and by their suitability for generating a cubical product. They can also be used for a variety of applications.
Secondary impact crushing is commonly used to improve product soundness and remove deleterious materials. As with primary HSI crushers, these secondary impactors, which open like a clamshell, are equipped with two or more adjustable curtains (aprons) and a rotor assembly with hammers (blowbars) that are easily accessible and offer multiple wear surfaces.
These high production crushers incorporate chromium alloy wear parts that allow for economical use on materials with abrasive characteristics. Some secondary HSI crushers offer curtains that can be added in the field for increased production of chip stone. Secondary HSI crushers have become very versatile, with multiple rotor configurations, special alloy wear parts, and maintenance features designed to reduce downtime and lower the cost of ownership.
Vertical shaft impact crushers – Generally recognised as a tertiary crusher, vertical shaft impact crushers (VSIs) have material fed into the centre of the crusher through a feed tube and onto the centre of a rotating table or rotor. The material is then accelerated to high velocity and is thrown into the anvil ring or outer shell. Crushing takes place upon impact with the anvil ring or against other material that is in the rock shelf (rock on rock). Product gradation in VSIs is controlled primarily by the speed of the table or rotor.
Hammermill crushers – Hammermills are similar to impact crushers in the upper chamber where the hammer impacts the in-feed of material. The difference is that the rotor of a hammermill carries a number of “swing type” or pivoting hammers. Hammermills also incorporate a grate circle in the lower chamber of the crusher. Grates are available in a variety of configurations. The product must pass through the grate circle as it exits the machine, ensuring controlled product sizing.
Hammermills crush or pulverise materials that have low abrasion. The rotor speed, hammer type and grate configuration can be converted for different applications. Hammermills can be used in a variety of applications, including primary and secondary reduction of aggregates, as well as numerous industrial applications.
Gerry Mangrich is the regional sales manager of aggregate processing for McLanahan Corporation. This article originally appeared in the April 2016 issue of Quarry Management (UK). It reappears in Quarry with kind permission.