Crushing is often one of the first steps in the production of rock, coal and other minerals, as mined material can consist of boulders that are too big to fit through the processing plant. The type of crusher required depends on the material being reduced. McLanahan specializes in a variety of crushing solutions for reducing minerals – from very friable minerals, such as coal and salt, to hard rock, such as granite, and ore-bearing minerals, such as copper and iron – at the primary, secondary and tertiary stages using compression, impact, attrition or shear forces.
Crushers that employ the forces of compression squeeze material between two surfaces, one stationary and one mobile, in order to achieve reduction. As feed material advances downward through the chamber, it is crushed between the moving piece of steel and the stationary plate. Only material that has reached the desired size passes on to the next stage in the process; whereas the larger material remains subjected to repeated pressure in the chamber until it, too, reaches the desired size. Gradation is controlled by adjusting the spacing between the stationary plate and the moving plate at their closest point.
Compression crushers can be used at the primary, secondary or tertiary stage, depending on the application. They are ideal for hard, tough and abrasive materials that are not sticky.
Impact crushing reduces material by utilizing the theory of mass versus velocity in two ways. In one method, material can be broken by its collision with hammers that are fixed to a spinning rotor. The material is broken mainly by its initial impact with the hammer and then further reduced by its impact against the breaker plates. Inter-particle collisions and particle-on-particle attrition also break down the material.
The other method involves the material being thrown at high speed against a solid anvil, breaking the material along its natural fissure lines. The particle size is controlled by how fast and how far the material is thrown.
With both types of impact crushing, material that has reached the desired size falls through the chamber, while the larger pieces remain subject to further impact. Both hard rock and soft material can be reduced using impact crushing.
Crushers that utilize shear forces to achieve the desired size and shape reduce material by trimming or cleaving. Material trapped between a solid plate and a rotating roll is shorn by its contact with the teeth on the roll. Oftentimes, shear crushing is combined with other crushing methods, such as compression, attrition and impact, for mineral size reduction.
Shear crushing is ideal for crushing friable material or creating a coarse product at the primary crushing stage.
Attrition crushers employ the theory of mass and velocity with a grinding action to reduce feed material. These types of crushers scrub material between two hard surfaces to achieve reduction in size and shape. Particles are reduced by their contact with other particles or by their contact with a rigid face.
Crushers that use the forces of attrition provide a high reduction ratio. They are best suited for friable, less abrasive materials.