Crushing is the first step of mineral processing where the ore and rocks from the mine site is fed to mechanical equipment in order to reduce the size for subsequent stages and thus liberate the valuable mineral from the gangue. Depending upon the requirement, there could be a primary, secondary, tertiary and quaternary stage. In most of the operations primary stage is performed at the mine site.
Primary crushers are commonly designed to operate 75% of the available time, mainly because of interruptions caused by insufficient crusher feed and by mechanical delays in the crusher. Jaw and Gyratory crushers are two most important types of primary crushers but sometimes impact crushers are also used if the ore is soft.
The jaw crusher reduces the size by the compression force when the rock is pressed against the two plates. It consists of a set of vertical jaws, one jaw fixed and the other moved back and forth relative to it. The rock remains in the jaws until it is small enough to pass through the gap at the bottom of the jaws. The speed of jaw crushers varies inversely with the size, and usually lies in the range of 100-350rev/min. The main criterion in determining the optimum speed is that particles must be given sufficient time to move down the crusher throat into a new position before being nipped again.
The gyratory crusher consists essentially of a long spindle, carrying a hard steel conical grinding element, the head, seated in an eccentric sleeve. The spindle is suspended from a 'spider' and, as it rotates, normally between 85 and 150rev/min, it sweeps out a conical path within the fixed crushing chamber, or shell, due to the gyratory action of the eccentric.