Primary crushing plants represent the first step in size reduction process in mineral processing operations where run-of-mine rocks are processed directly after being blasted. Primary crushing equipment crushes boulders with a maximum size of 1.5 meters to sizes suitable for belt conveyors and SAG mill circuits. Both types of primary crushing equipment, namely jaw and gyratory crushers, accomplish the task through application of compressive force resulting in tensile breakage of the hard rocks. However, there are some differences in the construction, capacities, installation needs, and economics between the two types of heavy equipment.
The jaw crusher consists of a crushing chamber in V-shape having a fixed jaw plate and swing jaw plate. The crushing action is performed with the help of an eccentric shaft that drives a toggle mechanism and provides intermittent motion for crushing action. Energy is stored using heavy flywheels to maintain the momentum of the machine during the returning phase when there is no crushing action. On the other hand, the gyratory crusher contains a central main shaft with a cone shaped mantle inside a fixed concave shell. The eccentric drive rotates the mantle continuously within the shell, and material gets crushed continuously from one side and discharged from another side.
The throughput capacity is the main differentiating feature of these two machines. Single jaw crushers are suitable for low throughput capacity operations handling about 1,000 to 1,600 tonnes per hour because these machines have a rectangular discharge opening and operate in an intermittent cycle. On the other hand, for high throughput capacity mining operations that must handle 1,200 to more than 9,000 tonnes per hour, gyratory crushers should be used since these machines have a circular discharge opening that allows for continuous material flow. Feed type is another consideration that determines the type of machine to use. The jaw crusher is suitable for coarse and blocky feeds as well as ore that has clay due to the open frame nature of the machine.
The costs related to infrastructure and operation have considerable differences between different kinds of machines. The use of jaw crushers requires less initial capital outlay, lightweight structure, and smaller concrete foundations; therefore, jaw crushers are more appropriate for small surface installations or underground workings. On the other hand, the use of gyratory crushers requires greater initial capital outlay, height and larger reinforced concrete foundations. Maintenance also demonstrates the same pattern, because jaw plates can be easily replaced during service at a relatively low price, while gyratory crushers provide lower maintenance costs per ton above about 1,000 tons per hour.
Equipment choice for a primary crushing circuit will be dictated by the targeted hourly tonnes, the sizes of the feed, and capital limitations on site. In the case where production expectations are less than 1,000 tonnes per hour, a single jaw crusher is the most economical solution, as it reduces civil costs at the plant level. For capacities higher than 1,200 tonnes per hour, the use of a primary gyratory crusher is necessary to prevent the need for parallel jaw circuits and circuit availability. Mine processing plant designers make such mechanical and economic considerations in their designs to provide efficient ore preparation services in the life of the mine.
