Crushing is the essential function in the treatment of all rocks and minerals, whatever is their end use. Today’s crushing methods have come a long way from the beginning of the 1800’s when crushing was carried out by hundreds of men and women equipped with sledgehammers.
In 1960, the feeder-breaker was developed for underground coal mining to follow the continuous miners. In the early 1980’s, low speed sizers were introduced. This represented one of the only fundamental developments in primary crushers in the last century.
Conventional roll crushers are suitable for crushing low to medium strength materials and can provide reduction of higher than 10:1 ratio. Roll crushers are generally designed with high peripheral speeds, and uneven wear can be a major problem. The main feature of the low speed sizer, is that it uses the fact that the ratio of compressive strength to shear strength in the majority or rocks is approximately 10:1. The low speed sizers breaks the rock in tension by its chopping action rather than in compression as conventional crushers do.
2. Crushing Methods
There are four methods to reduce a material;
Most crushers perform a combination of all these crushing methods
Crushing by compression is done between two surfaces. This compression method is perfect for grinding extremely tough and abrasive rock with gyratory and dual toggle jaw crushers. However, jaw crushers use attrition as well as compression and they are less suitable for abrasive rock since the rubbing action causes wear on crushing surfaces.
Compression is ideal;
- When the material is hard and tough
- When the material is abrasive
- When the material is not sticky
- When a uniform product with a minimum of fines is desired
- When the finished product is to be relatively coarse
- When the material will break cubically
Impact crushing is sharp, quick impact of one moving object to another. Both objects may be moving, or one object may be stationary, such as a rock being struck by hammer blows.
There are two types of impact: gravity impact and dynamic impact.
Gravity impact occurs when one object is motionless and the other objects strikes it such as material dropped onto a hard surface such as a steel plate. Material dropping in front of a moving hammer is an example of dynamic impact. When rock is crushed by dynamic impact, with the force of the impact, the rock parts accelerate toward breaker blocks and/or other hammers.
Impact Crushing is ideal;
- When a cubical particle is needed.
- When the finished product must be well graded and must meet sizing
- When ores must be broken along certain line to separate the mineral from waste.
- When materials are too hard and abrasive but jaw crushers cannot be used
Attrition is the reduction of materials by scrubbing it between two hard surfaces. Hammer mills is an example of attrition crushing where close clearances between the hammers and the screen bars provides scrubbing of the rock. Since attrition consumes relatively more energy and causes higher wearing, it is practical to use it for less abrasive material such as coal or limestone.
Attrition crushing is ideal;
- When material is non-abrasive
- When a closed-circuit system is not desirable to control top size.
- When a maximum of fines is required
The difference between shear crushing and attrition is that shearing involves trimming or cleaving rather than the rubbing. Shear uses the fact that the ratio of compression strength too tensile and shear strength in the majority of rocks is approximately 10:1. Low speed sizers break the rocks in tension and shear by its chopping action.
Shear crushing is ideal;
- When material has relatively low silica content.
- When the material is soft to medium hardness.
- When primary crushing with a reduction ratio of 6 to1 is required.
- When a minimum of fines is desired.
- When a relatively coarse product is desired.
3. Crusher Types
3.1 Gyratory Crusher
A Gyratory crusher is used for primary crushing of various materials by squeezing, splitting and bending. A conical shaped element is supported in a flared shell or frame creating a chamber wide at the top and narrow at the bottom. Rock introduced at the top is broken as it passes through the crusher chamber. The centre element is caused to gyrate about its fulcrum point causing it to advance and retreat with relation to the shell.
Typical gyratory crusher capacities are 350 to 10,000 MTPH. The primary gyratory crusher is
preferred for its high capacity and relatively low maintenance.
The major benefits of Gyratory Crushers are;
- Designed for direct dump from trucks up to 300 tons.
- High capacity ratings.
- Lowest maintenance per ton processed of any design crusher.
- Can handle crushing ore hardness up to 600 Mpa (90,000 PSI) compressive strength.
- Easy handling of residue material with hydraulic relief system
Relatively high first investment cost of Gyratory Crushers is a main disadvantage over the other crushers.
3.2 Jaw Crushers
The distinguishing feature of the jaw crusher is that it has two jaws which are placed with an acute angle between them and the two open and close as animal jaws do. The jaws are such that one is fixed while the other is hinged, swinging relative to the immobile jaw.
Photo 1 : Jaw Crusher
Jaw crushers are classified based on the way the swinging jaw is pivoted.
In the Blake Jaw crusher, the swinging jaw is pivoted at the top and thus it has an ore receiving area that is fixed and an exit or discharge opening that is variable.
For the Dodge Toggle Jaw crusher, on the other hand, the pivoting is at the bottom making it to have a receiving area that varies and a delivery opening that is fixed.
Jaw crushers are rated on the basis of the receiving areas, which are the product of the width of the jaws and the gape. The gape is the distance at the feed opening between the jaws. For instance, a 2130 × 1680 mm jaw crusher specification means that the jaw crusher has a width of 2130 mm and a gape of 1680 mm.
Photo 2 : Gyratory Crusher
A rock piece that falls into the crusher’s mouth, is nipped by the jaws moving relative to each other at a rate of 100–350 rev/min determined by the crusher’s size. The piece is released, allowed to fall and arrested again in a repeating pattern.
3.3 Cone Crusher
The design and development of the cone crushers dates back to 1920s. Cone crushers are similar to gyratory crushers except that instead of the spindle being suspended it is supported at the bottom of the gyrating cone.
The head-to-depth ratio is larger than for gyratory crushers and the cone angles are flatter.
Photo 3 : Gyratory Crusher
The Cone crusher has a smaller crushing chamber. Therefore, break the rocks by tightening them between the rotating spindles. In addition, these spindles are completely covered with a sturdy mantle and a manganese bowl liner covers the hopper. When the rocks enter between the lining of the bowl and the mantle, the rocks are tightened.
Photo 3 : Cone Crusher
The broken rock pieces fall to the next position where they break again. Then, the same process repeats until the broken pieces become small enough to pass through the narrow opening in the bottom of the conical crusher.
3.4 Impact Crusher
Impact crushers reduce mineral materials such as concrete, asphalt and natural rock in size to produce a valuable commodity product. A fast spinning rotor throws the material against a solid stationary impact wall. The striking and impacting causes the material to shatter into smaller pieces. The result is a very homogenous and cubical product leaving the crusher box.
Impact crushers are utilized in soft, non-abrasive applications or when crushing availability and maintenance can be economically offset against capital cost.
The main advantages of the impact crushers are;
- An impact crusher can handle a large size reduction – one meter to 75 mm.
- High reduction ratio for amount of investment.
- Impact crushers provides a high degree of fines.
- Can handle up to 2500 MTPH
The cons of the impact crushers are;
- Higher silica ore cause increased wear.
- Power consumption is higher as more fines are produced.
- Cannot handle residue material.
- Requires feeder
4. Crusher Selection
When selecting the type of crusher, the following criteria must be considered:
- Will it produce desired product size at the required capacity?
- Will it accept the largest feed size expected?
- What is its capacity to handle peak loads?
- Will it choke or plug?
- Is the crusher suited to the type of crushing plant design?
- Is the crusher suited for underground or in-pit duty?
- Can it pass uncrushable debris without damage to the crusher?
- How much supervision of the unit is necessary?
- What is the crusher’s power demand per ton per hour of finished product?
- How does the crusher resist abrasive wear?
- Does the crusher operate economically with minimum maintenance?
- Does the crusher offer dependable and prolonged service life?
- Does the crusher have acceptable parts replacement costs?
- Does the crusher have easy access to internal parts?
- How does the initial cost of the machine compare with its long term operating costs?
When determining whether to employ a jaw crusher or a gyratory crusher, the primary consideration revolves around the maximum ore size the crusher must handle and the desired throughput. Typically, gyratory crushers are preferred when there’s a need for high capacity, while jaw crushers are typically chosen when the crusher’s gap size is more critical than capacity. For example, if the task involves crushing material of a specific maximum diameter, a gyratory crusher with the appropriate gap would possess roughly three times the capacity of a jaw crusher with a matching gap. Opt for a gyratory crusher when high capacity is essential. On the other hand, if a large gap is necessary but not high capacity, a jaw crusher might be more cost-effective as it’s a smaller machine, and the gyratory would remain largely idle.
Additional factors come into play, including capital and maintenance costs, where jaw crushers tend to be slightly less expensive than gyratory crushers. However, installation costs may offset this advantage, as they are lower for gyratory crushers since they occupy approximately two-thirds of the volume and have about two-thirds of the weight of a jaw crusher with the same capacity. The circular crushing chamber in gyratory crushers allows for a more compact design, with a larger portion of the total volume dedicated to the crushing chamber. Furthermore, the foundations for jaw crushers need to be more robust due to alternating working stresses.
In some situations, the self-feeding capability of gyratory crushers compared to jaw crushers can lead to cost savings, as it may eliminate the need for expensive feeding devices such as heavy-duty chain feeders. On the other hand, the ease of shipping in sections to remote locations and underground installations can make jaw crushers more favorable in certain cases.
The choice of crusher may also be influenced by the type of material being crushed. Jaw crushers tend to perform better on clay or plastic materials due to their greater throw, while gyratories are well-suited for hard, abrasive materials and often yield a more cubic product when the feed is laminated or “slabby.”
4.1 Crusher Selection Based on the Material Type
a) Gyratory and Double Toggle Jaw Crushers: Tough, abrasive (high silicate), non-sticky, types of material having compressive strengths up to 600 Mpa (90,000 PSI). Examples of
these materials include Taconite, trap rock, granite, hard limestone, porphyry copper and high silica gold ores.
b) Single toggle jaws, and low speed sizers : Medium hard, non-abrasive, and sticky materials having compressive strength up to 200 Mpa (27,500 PSI). Examples include medium hard limestone, bauxite, kimberlite and low silica gold ores.
c) Impact Crusher, Roll crusher, Feeder breakers and Hammermills – Soft friable, non-abrasive (low silicate) materials having compressive strengths below 115 Mpa (16,500 PSI).
Examples include limestone, phosphate rock and gypsum.
4.2 Crusher Selection based on Clay Content
Clay or sticky clay like materials represents a difficulty for almost every type of primary crusher. Since crushing action in gyratory jaw is by compression, the material will pack if it contains significant amount of clay. Also the material flow in gyratory and jaw crushers is by gravity, packed material may limit or even stop the crushing process.
Impact crushers and hammermills cannot be used for materials with clay content as the chamber will be filled with clay and prevent impact crushing from taking place. The crushing chamber will pack quickly.
Feeder Breakers are a good alternative. The flight bars on the feeder will move the material along the pan toward the breaker drum. The rotating drum breaks the material as if is forced along by the flight bars. There is no chamber in which the clay will pack. Clay, however, can cause maintenance problems with the flight bars and there is a possibility of clay packing between the picks of the rotating drum.
The low-speed sizer is the only primary crusher that can handle the clay very easily. The two shafts of the low-speed sizer rotate inward at low speeds. There is no impact that will cause the material to pack in the chamber. The low speed sizer can be provided with scraper bars that are located between the rows of sizing teeth to keep the toothed shafts clean at all times.
4.3 Crusher Selection for Mobile Crushing Plants
All of the family of primary crushers can be used on mobile crushers. Impact crushers and hammermills are compact and generate a high reduction ratio. The high speed of the machines requires special attention to dynamic forces.
The jaw crusher are very good for small tonnage plants and the single toggle jaw crusher has the advantage of being lighter in weight than the double toggle jaw crusher. Large capacity jaw crushers result in large crushing plants.
The selection of the gyratory crusher for this application is dependent on the capacity of the mobile plant. For most of the in-pit, mobile, semi-mobile and movable crushing applications in hard rock mining the capacities exceed 3500 MTPH. This criterion limits the choice of crushing machine to gyratory or low-speed sizer. The gyratory crusher can crush material to 600 Mpa (90,000PSI). The low-speed sizer is limited to 200 Mpa (27,500PSI). The gyratory crusher by its very nature generates a significant out-of-balance force, on the other side, the sizer shafts rotate toward each other and there is very limited out-of-balance generated. While the gyratory crusher is the tallest of all primary crushers, the low-speed sizer is the shortest of all primary crushers.
Novelty Structures supplies various size and types of crushers with different capacities. Please get into contact with our team for more info through email@example.com