- 1 Introduction
- 2 Location of the Crusher In an Underground Mine
- 3 Material Handling
- 4 Technology and Equipment Selection
- 5 Costing
- 6 Dust Control
- 7 Conclusions and Recommendations
- 8 References
Crushers play an important role in the material handling process for underground mine operations. The main purpose of a crusher is the size reduction of the feed. This size reduction is achieved by applying a force to the material, creating cracks in the material which in the end will cause the material to break into smaller pieces. There are two categories of crushers based on the way the force is applied to the material. For crushers of the first category apply force via pressure, the crushers of the second category apply force via impact. In underground mine operations, size reduction is necessary to facilitate the transport of the material to the mill. Resizing of material often is the first step in the concentration process of the ore (de la Vergne, 2003). Since crushing is an important step of the ore concentration process, the selection and sizing of crushers should be given sufficient consideration during the mine design process.
The objective of this article is to outline the factors that determine the selection and sizing of crushers for underground mining operations. This article gives an overview of the different crushers and the parameters that determine the sizing and selection of the crusher. The second section describes the options for crusher location in an underground mine. Section three deals with the handling of the material from the grizzly to the skip and the role of the crusher in this process. The fourth section explains the technology and selection parameters of several types of crushers. In chapter five, details on crusher costs can be found. The last section provides information on various aspects of dust control.
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.
Figure 1 : Crusher Location in an Underground Mine (www.ugdsb.on.ca)
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.
Figure 2: A Ross Chain Schematic (www.flsmidthminerals.com)
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 four.
After the ore is crushed it moves in to a chamber known as the ore bin. The bottom of the ore bin has a controlled hopper that will periodically release ore on to a transportation system. This transportation system can consist of either conveyors or trolleys, which guide the ore to the end of the pass where it will finally be loaded in to a skip so it can move up to the surface. This system will also be controlled using control chains to prevent the violent surge of a heavy rock mass to minimize damage.
Figure 3: An example of a conveyor assisted ore transport system into the skip (www.flsmidthminerals.com)
Technology and Equipment Selection
In any operation that incorporates size reduction, selecting the primary crusher is vital to the overall success. The main parameters to first decide on a general type of crusher are the impact strength, product size and material hardness. The final design for a crusher will be selected primarily based on required capacity, feed and product size. It should be noted that further crusher selection could be defined based upon the location and the degree of mobility.
Technological advancements, from 1830 when the first crusher design was patented, have allowed today’s crushers to take blasted ROM (Run of mine) feed up to 1500mm (60 inches) and reduce them to sizes ranging from -300mm to -38mm (Mular et al, 2002).
Mechanical Reduction Methods
The four main methods for size reduction are summarized in table 1 (Pennsylvania Crusher Corporation, 1995) below for ease of reading.
Table 1: Showing the four types of mechanical reduction methods.
Types of Crushers
The gyratory crusher consists of a wide opening that has a conical shaped mid section leading down to a narrow bottom. Using the center element to rotate and gyrate about its fulcrum, the rock is broken due to the motion of the centerpiece in relation to the outer shell (advance and retreat). These types of crusher have capacities from 350 to 10,000 MTPH (mega tons per hour) (Mular et al, 2002). The advantages and disadvantages of a gyratory crusher are listed in table 2.
Table 2: Advantages and disadvantages of a gyratory crusher.
Figure 4: Typical gyratory crusher schematic
Double Toggle (DT) Design
Commonly called the “DT Jaw Crusher” this design utilizes two plates, one being fixed and the other being free to move back and forth to crush the rock. This machine uses a squeezing motion and as the rock breaks it falls to a lower position so it can be further broken until it passes through the narrow opening at the bottom. The arrangement of all the parts causes the design to produce twice the amount of crushing force. These crushers are generally used for materials around 350 MPa but max at 600MPa. Table 3 shows the advantages and disadvantages of a double toggle jaw crusher.
Table 3: Advantages and disadvantages of a Double Toggle Jaw Crusher
Figure 5: Typical double toggle jaw crusher schematic
Single Toggle (ST) Design
Having very similar features as the DT jaw crusher, the ST jaw crusher has slight differences. The maximum motion occurs at the top of the jaw and is created by an eccentric shaft. Due to the rubbing action of both the vertical and horizontal components of the jaw, the plate wear is accelerated and the power efficiency is lowered. This motion does give an advantage when handling sticky materials where a DT jaw crusher would not do well as its motion is perpendicular to the fixed plate. These crushers are usually used for light or medium hard materials. Advantages and disadvantages of the single toggle jaw crusher are listed in table 4.
Table 4: Advantages and Disadvantages of a single toggle jaw crusher
Figure 6: Typical single toggle jaw crusher schematic
Double Roll and Low Speed Sizers
The double roll crushers have two high-speed, parallel, toothed rollers with a fixed gap between them. As the material passes through them, it is subjected to mostly compression forces and some impacting forces. The low speed sizer is equipped with two toothed rolls. Even though the two rolls revolve slowly in an enclosed chamber, they have a high torque. These teeth are placed in an arrangement such that shear forces are induced on the material. These low speed sizers are used generally for hard non-abrasive sticky materials ranging from 200 to 400 MPa. The advantages and disadvantages of a double roll crusher can be found in table 5.
Table 5: Advantages and disadvantages of double roll crushers and low speed sizers
Figure 7: Typical double roll crusher schematic
Figure 8: Typical low speed sizer schematic
These crushers are made up of one or two heavy rotors with components attached that spin inside a casing. These attached pieces throw the material at the case for secondary impact after the initial impact of the material hitting the rotor. Hammermills are very similar to the impact crushers although they usually use only one rotor and the hammers are pivoted. It should be noted that these crushers cannot be used underground for primary crushers as they cannot handle any steel (i.e. Rockbolts, drill steels). Table 6 lists the advantages and disadvantages of impact crushers.
Table 6: Advatnages and disadvantages of impact crushers
Figure 9: Typical impact crusher schematic
These types of crushers would be implemented in situations where the material has low to medium strength and the material needs to be small enough to be transported on a conveyance system. They are often used for overburden and in underground applications. A list of advantages and disadvantages is shown in table 7.
Table 7: Advantages and disadvantages of feeder breakers
Figure 10: Typical feeder breaker schematic
Important Considerations for Primary Crusher Selection
Questions to consider when selecting a primary crusher are:
• Will it produce product size at the required capacity?
• Will it accept the largest feed size expected?
• What is its capacity to handle peak loads?
• Will it choke of plug?
• Is the crusher suited to the type of crushing plant designed?
• Is the crusher suited for underground or open-pit duty?
• Can it pass uncrushable debris without damaging itself?
• How much supervision will be needed?
• What type of horsepower will need to be provided?
• Is it resistant to abrasive wear?
• Will it require a lot of maintenance?
• If parts break, are they affordable to replace?
• Are the internal parts easy to access if they require repair?
• How does the capital costs compare with the long term operating costs?
This list is based on the SME Mineral Processing Plant Design Proceedings.
After selecting an appropriate type and size of crusher obtaining an estimate of its cost is straightforward. The following empirical formula can be used to provide a preliminary approximation (Mullar, 1998):
Table 8: Costing coefficients
This method of costing is relatively inaccurate as coefficients can vary greatly over time and are only updated sporadically. A more accurate method of approximation involves the use of M&S(Mine/Mill) ratios as seen in the following formula.
These coefficients are generally updated on a quarterly basis and have been shown to produce estimates within ±10 % of the cost if data is less than six years old (Mullar, 2008). This method can also be used to obtain a more accurate result when used in conjunction with equation one.
The installation of a crusher accounts for a significant portion of the capital input required to bring a crusher into operation. This is due to the large amounts of support required to accommodate both geotechnical problems over the course of mine life as well as to resist its own internal forces. The physical installation process is also labour intensive, involving the deconstruction on surface, part by part hoisting and reconstruction in place. Generally, installation can exceed six times the cost of the crusher itself (de la Vergne, 2003).
The above approximation does not account for the many other accessory components that may be required over the course of a mine’s operation, most specifically the motor. In the case of Jaw Crushers, the cost of the motor is generally negligible compared with the cost of the crusher itself due to the significantly smaller power requirements (Infomine, 2008)(Mullar, 1998). Gyratory crushers require larger motors to account for their greater throughput, the cost of which cannot be neglected. Similar methods to those shown above exist for the costing of motors but motor type must also be chosen according to specific operating requirements. Generally large horsepower motors can range up to a million dollars, but considerable variation exists (Mullar, 1998).
The primary operating costs of a crusher are parts and labour for maintenance, which can vary approximately twice as much as the power for the motor. Gyratory crushers generally cost less than $1500 per hour to run, while Jaw crushers cost less than $200, including electricity (Infomine, 2008).
Rules of Thumb
• A 42-inch gyratory crusher produces approximately 2.4 tons per horsepower-hour (2.9 t/kWh).
• When idling jaw crushers consume approximately 50% of the power of full operation and gyratory approximately 30%.
•Installation of an underground jaw crusher may cost up to six times as much as the crusher itself.
• A 48 by 60 jaw crusher produces approximately 1.8 tons per horsepower-hour (2.2 t/kWh) at a 6:1 reduction ratio. (de la Vergne, 2003)
Throughout the process of crushing, large amounts of dust are produced. Dust has the capability of destruction of the equipment. Left uncontrolled the dust is liable to increase wear and cause severe backups in the system thus causing expensive maintenance and heavy losses in production. Even though compressive crushing results in higher dust creation then impact crushing, a large amount of dust is still created and must be managed and controlled in an efficient manner. Regardless of the types of crusher utilized the movement of material through the crusher will create airflow, thus creating airborne particulates. This is not to say that dust always becomes airborne, but it is challenge that must be considered thoroughly. The most common ways of dust management in underground crushing are by water sprays and local exhaust ventilation. Primary considerations while deciding which method of dust controlled shall be used are analysis of material type and arrangement around the crusher unit.
Wet Dust Suppression
One of the commonly utilized methods of dust suppression is achieved through the assistance of water sprays. Water sprayers work by dousing the rock with a small amount of water, which adds significant weight to the dust particulates and as a result prevents them from becoming airborne. Using a larger volume of water leads to less dust creation, but can have adverse affects on downstream operations. An alternative to a larger volume of water for increased suppression is foam. Uniformly wetting the rock prior to crushing is important. Therefore, wetting devices need to be placed periodically throughout the pre-crushing process as opposed to using one large wetting system with higher flow just before the material enters the crushers. This will ensure that the material has been wetted uniformly. A good starting point for water addition is 1% of the weight of the material to be crushed if the material is dry to begin with. Also, the nozzle pressure should be kept below 60 psi in order to avoid stirring the dust (Kissel, 2003).
Airborne Dust Capture
Airborne dust capture uses ventilation principles to capture the dust-rich airstream and transport it from the source through ducts to either exhaust or dust collection areas. A primary concern of dust collection is the rollback effect. Rollback is where there are uneven ventilation areas, resulting in stagnant dust. This occurs largely because of a negative pressure created by the violent flow of rock mass in to the crusher. This high-pressure area can work as an advantage by creating an optimal area location for airflow exhaust. If the collection device is placed in this area then the dust can be intercepted along its natural flight path in to the appropriate gathering facility. The open area in the crusher should be multiplied by 200 ft/min to get the required airflow volume through the crusher unit. (Kissel, 2003)
Conclusions and Recommendations
In most underground mining operations, the material size of the excavated material will be too big for efficient transportation and processing. An underground crusher assures the size reduction to practical sizes and therefore plays an important part in the mining operation as a whole. 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. The location of the crusher has to be determined in an early design stage, since it has a great impact on the other steps in the mine design.
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. A grizzly will prevent oversize material from entering the crusher. A rock breaker reduces the size of this material to manageable sizes. The crushing stage further reduces the size of the material so that the material can be transported to surface using trucks or a conveyance system. The size of the material is critical for mineral processing and the actual extraction of the minerals. The post-crushing stage transports the material to surface for further processing.
There are several types of crushers; 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 selection of the crusher depends on the capacity, maintenance and availability of the crusher and the ability to handle the assigned material.
Another important parameter for the selection of the crusher is the costs. 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. Empirical formulas exist to calculate the capital cost of the crusher. Operation costs can be easily determined using the suggested rules of thumb.
Crushers produce fine material that will become airborne when air movement is present. Fine dust can become a health and safety hazard when present in high amounts. Dust problems can occur in the field of maintenance, less production and health issues like respiratory problems. Dust needs to be controlled to minimize the risks of these hazards and maintain a safe workplace. There are two ways of dust control: wet dust suppression and airborne dust capture. Wet dust suppression simply adds water to the material, causing a significant increase in weight of the particles, therefore preventing the particles from becoming airborne. Airborne dust capture uses ventilation principles to capture the dus-rich air. The dust-rich air is transported to exhaust or dust collection areas.
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Mullar, A.L., Halbe, D.N., Barrat, D.J.; Mineral Processing Plant Design, Practice, and Control; SME, 2002
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