Roll crushers will only crush material down to a minimum particle size of about 10 Mesh (2 mm). A roll crusher crushes using compression, with two rolls rotating about a shaft, towards the gap between the rolls. The gap between the rolls is set to the size of product desired, with the realization that the largest feed particle can only be 4 times the gap dimension. The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. The two rolls force the particle between their rotating surface into the ever-smaller gap area, and it fractures from the compressive forces presented by the rotating rolls. Some major advantages of roll crushers are they give a very fine product size distribution and they produce very little dust or fines.
Roll crushers are effectively used in minerals crushing where the ores are not too abrasive and they are also used in smaller scale production mining of more abrasive metal ores, such as gold. Coal is probably the largest user of roll crushers, currently, though. Coal plants will use roll crushers, either single roll or double roll, as primary crushers, reducing the ROM coal. Usually, these crushers will have teeth or raised forms on the face of the roll. (Roll crushers used for minerals and metal ores have smooth faced rolls.)
Hammer Mills
A hammer mill crushes material that is friable, by impacting it against a rotating hammer (typically travelling between 750 RPM and 1800 RPM). Then the material is forced against a rugged solid plate called a "breaker plate" which further degrades the particle size. Finally, the material is forced over a discharge grate by the hammers, where crushed finer particles drop through the discharge grate and larger particles travel around for another crushing cycle, until they fall through the discharge grid.
During the entire time the material is travelling around in the hammer mill, the hammers, and the side of the mill casing, causing breakage of the articles, are constantly impacting it. This sequence repeats itself between 750-1800 times each minute; until the particle is ground fine enough to fall through the discharge grid. One big drawback to hammer mills is they wear due to the abrasiveness of the material being crushed and the high speed of the mill. It should not be used to grind any material harder than a medium hard limestone. They are used most in crushing coal, but are also common in limestone operations. Large particles cannot usually be crushed in hammer mills, so they are almost never used for run of mine material or primary crushers, but secondary or tertiary crushers. Six to eight inches is a typical top size of feed for hammer mills.
Hammer mills are relatively inexpensive, as crushers go, can produce relatively fine material (1/4" to -10 mesh) from 6" or 8" feed in one step, and they take up relatively little floor space. They are easily repaired, and simple to operate. Production size hammer mills can produce products ranging from 1" to as fine as 10 mesh, or finer, depending upon the material being crushed.
Jaw Crushers
A jaw crusher is one of the main types of primary crushers in a mine or ore processing plant. The size of a jaw crusher is designated by the rectangular or square opening at the top of the jaws (feed opening). For instance, a 24 x 36 jaw crusher has an opening of 24" by 36", a 56 x 56 jaw crusher has a opening of 56" square. Primary jaw crushers are typically of the square opening design, and secondary jaw crushers are of the rectangular opening design. However, there are many exceptions to this general rule.
A jaw crusher reduces large size rocks or ore by placing the rock into compression. A fixed jaw, mounted in a "V" alignment is the stationary breaking surface, while the movable jaw exerts force on the rock by forcing it against the stationary plate. The space at the bottom of the "V" aligned jaw plates is the crusher product size gap, or the size of the crushed product from the jaw crusher. The rock remains in the jaws until it is small enough to pass through the gap at the bottom of the jaws.
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| Posted : 8/12/2005 |
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