Crushers play an important role in the material handling process for underground mine operations. There are two possible locations for an underground crusher: near the shaft and under the ore body. The location is mainly determined by the steepness and shape of the ore body, the depth of the ore body and the production schedule of the mine. For underground crushing, three stages of material handling are defined: pre-crushing, crushing, and post-crushing. The pre-crushing stage prepares the material for the crusher, the crushing stage reduces the size of the material, the post-crushing stage transports the material to surface for further processing. The main types of crushers are the gyratory crusher, the jaw crusher and the impact crusher. Each of these types uses a different technique to reduce the size of the material. The costs for crushers are divided in the capital costs and the operating costs. The capital costs include the crusher cost, the installation cost and a couple of other costs like the cost of the motor. Crushers produce fine material that will become airborne when air movement is present. Dust can be a health and safety hazard and therefore needs to be controlled. Two ways of dust control are wet dust suppression and airborne dust capture.
Location of the Crusher In an Underground Mine
The location of the crusher in an underground mine operation has a great impact on the design and development of the mine. The reason for this great impact is the role of the crusher on the design of material handling and skipping. Two possible locations for an underground crusher exist: near the shaft and under the orebody. Factors that determine the location of the shaft are the steepness of the orebody, the production schedule of the mine and the ground stress. It will take approximately six to twelve months to excavate, install and commission an underground crusher station (de la Vergne, 2003).
Near the Shaft
In classic mine design, the crusher is located near the shaft. This has the advantage that the material can go directly from the crusher to the skip. For mine operations that use truck haulage, the extra travel distance for the trucks is compensated by the advantages of more rapid access for excavation of the crusher and the expenses involved in moving the crusher (de la Vergne, 2003).
Under the Orebody
By mining rule of thumb, production shaft should be positioned in the footwall side of the orebody in the host rock from its stability point of view. For the same reason ore handling and hoisting equipment including the underground primary crusher will also be located in the footwall side of the ore body.
The material handling portion of crushing is the means by which the ore travels from the grizzly all the way to the skip in an underground mining operation. This can easily be broken down in to three sections, pre-crushing, crushing, and post crushing. Pre-crushing is the movement of material through the grizzly, down the ore pass, and finally into the crusher itself. The crushing section is the ores movement through the crusher. This will vary, of course, depending on which crusher is utilized. Lastly, the post-crushing phase is the movement of the material when it exits the crusher and gets released in to the ore bin and then continues its path towards the loading zone at the skip.
At the start of the material handling process the ore needs to pass from the haulage equipment in to the primary feeding mechanism. A simple hopper at the end of the ore pass can perform this task. The hopper will increase the potential catchment area of the ore when dumped from the trucks ensuring minimal spillage of material. A hopper is a very simple apparatus, but its presence has significant performance implications. Without the use of a hopper at the beginning of the material handling process productivity would be lost due large amounts of material missing the entrance to the feed, resulting in expensive labor for cleaning the ore pass of debris.
At the bottom of the hopper there is a grizzly. The grizzly is responsible for preventing over size material from entering the ore passes and the crusher where the material is liable to do damage to the equipment and underground construction. A standard opening for the grizzly, as defined by McIntosh Engineering, is ±16 by 18 inches. The rock that is stopped by the grizzly is often broken in to more manageable pieces by using a hydraulic or pneumatic rock breaker.
In most underground mining applications a Ross Chain Feeder is installed. The Ross Chain Feeder serves the purpose of slowing the violent rush of dense heavy rock through the feeder system all the way in to the crusher. This will provide significant wear protection on the feeder system due to drastically reducing the impact velocity of the ore with the feeder apparatus. The Feeder Chain also serves to slow the ore to ensure that a back up or overload does not occur in the crushing system and the crusher’s output surge pile does not become largely unmanageable.
There are several types of crusher that are used in underground mining operations. The most common types of crushers are the gyratory crusher, the jaw crusher and the cone crusher. The technology and selection criteria will be discussed in section three.