Stone crushing can be classified into four stages depending on the degree to which the starting material is fragmented. These four stages are primary, secondary, tertiary and quaternary stages. Primary and secondary stages involve crushing of coarse materials while the tertiary and quaternary stages involve the reduction of ore particles to finer degrees. Activities at the primary stage will depend on gyratory, jaw or impact crushers. Cone crushers, roll crushers and impact crushers are mostly used at the secondary stages. The tertiary and quaternary stages mostly require the utilization of cone crushers, although some functions may require vertical-shift impact crusher. In order to control the size and quality of the product and at the same time reduce wastage, you must ensure that the reduction of aggregates is evenly spread over the four stages.
A gyratory crusher consists of a concave surface and a conical head constructed from heavy steel casting. It works by using a mantle that gyrates within a concave bowl. This rock crushing equipment uses compressive force to fracture the rock and this happens when the mantle makes contact with the bowl during gyration. Gyratory crushers are often built into a cavity in the ground and are mostly used to crush rocks that have high compressive strength.
A jaw crusher also uses compressive force and the materials are allowed into a gap at the top of the equipment between two jaws. One of the jaws is fixed while the other reciprocates by moving back and forth relative to the stationary one. The gap between the two jaws is known as the crushing chamber. The moving jaw exerts a compressive force against the stone in the chamber causing it to fracture and reduce. The rock remains in the jaws until is small enough to move down the chamber to the opening at the bottom. Jaw crushers can work on a range of stone from the softer ones like limestone to harder basalt or granite.
A cone crusher is similar to a gyratory crusher because it operates using a mantle that rotates within a bowl, but it has less steepness in the crushing chamber. It has a short spindle which is supported by a curved universal bearing located beneath the cone. They use compression force to break the rock between the gyrating spindle and the enclosing bowl liner. The rock becomes wedged and squeezed as it enters the top of this rock crushing equipment. The cone crusher breaks large pieces of ore once into smaller particles that fall to a lower position where they are broken again. The pieces are continually crushed until they are small enough to move through the narrow opening at the bottom of the crusher.
Roll crushers are a compression-type reduction crusher with two drums rotating about a shaft. The gap between the drums is adjustable. The particles are drawn into the crushing chamber by the rotating motions of the rolls and a friction angle is formed between the particles and the rolls. The stone fractures from the compression forces presented by the rolls as they rotate. The crushed particles are then forced between the rotating surfaces into the smaller gap area. Roll crushers are mostly used in smaller scale production to crush ores that are not too abrasive. This type of rock crushing equipment gives a very fine product size distribution with very little dust production.
Impact crushers do not use force to crush materials, instead, they use impact. The material is contained within a cage that has openings on the bottom or side to allow for the crushed materials to escape. Impact crushers can be classified into two categories: vertical shaft impact crushers (VSI) and horizontal shaft impact crushers (HSI).
VSI crushers use high-speed rotors with wear resistant tips that catch and throw the feed stone against anvils lining the crushing chamber. Rock is fractured along its natural fissures when it’s thrown against the anvils to produce materials with consistent cubical shapes.
The HSI crusher has a shaft that runs on a level plane through the crushing chamber. It works by impacting the rock with hammers that are fixed on a spinning rotor. It also works on the principle of throwing the stone to break the rock. Horizontal shaft impact crushers can be primary or secondary. They are better suited for softer, less abrasive stone in the primary stage and more abrasive and harder stone in the secondary stage.
Selection of Rock Crushing Equipment
Choosing the right rock crusher is based on different factors and these include:
- Ore characteristics
- Operational considerations
- Production requirements
- Maintenance requirements
- Project location
- Expansion plans
Rock crushing equipment with good performance can improve productivity and the quality of the final products. Choosing the right crusher is therefore very important.