Crushing is accomplished by compression of the ore against a rigid surface or by impact against a surface in a rigidly constrained motion path. Crushing is usually a dry process and carried out on ore in succession of two or three stages, namely, by primary, secondary, and tertiary crushers.
Primary crushers are heavy-duty rugged machines used to crush ROM ore of (−) 1.5 m size. These large-sized ores are reduced at the primary crushing stage for an output product dimension of 10–20 cm. The common primary crushers are of jaw and gyratory types.
The jaw crusher reduces the size of large rocks by dropping them into a “V”-shaped mouth at the top of the crusher chamber. This is created between one fixed rigid jaw and a pivoting swing jaw set at acute angles to each other. Compression is created by forcing the rock against the stationary plate in the crushing chamber. The opening at the bottom of the jaw plates is adjustable to the desired aperture for product size. The rocks remain in between the jaws until they are small enough to be set free through this opening for further size reduction by feeding to the secondary crusher.
The type of jaw crusher depends on input feed and output product size, rock/ore strength, volume of operation, cost, and other related parameters. Heavy-duty primary jaw crushers are installed underground for uniform size reduction before transferring the ore to the main centralized hoisting system. Medium-duty jaw crushers are useful in underground mines with low production and in process plants. Small-sized jaw crushers are installed in laboratories for the preparation of representative samples for chemical analysis.