Gyratory, jaw and roll sizers have all been installed underground to act as primary crushers prior to the transportation of the ore to the surface. There are a few gyratory crusher installations in hard rock mines (the latest, 1998, being Phelps Dodge Henderson mine 1,371 mm x 1,880 mm [54 ft x 74 in] in Colorado) but the greatest percentage of crushers working underground are jaw crushers . Until a few years ago, double toggle crushers were the underground crushing choice, however, these are now being replaced with more efficient and less expensive single toggle crushers.
FINE CRUSHING (Secondary, Tertiary and Quaternary)
Fine crushing circuits can be more challenging to design than primary crusher installations. There are more equipment options, and each has different installation and maintenance requirements. The process flowsheet dictates the expected performance of the items in the flow stream. The designer must configure the equipment and structures into a balanced, economical plant design. Screens, feeders, stockpiles, bins, conveyors and crushers must all be interfaced with the most economical supporting structures and buildings.
An open-circuit crusher is easier to design and lay out than a closed-circuit design, since it has fewer equipment items and structures. Figures 13, 14, and 15 show some typical secondary crusher open circuit layouts. Figure 16 shows some typical open circuit secondary and tertiary crushers. Provisions should be made for possible future conversion of an open-circuit plant to a closed-circuit version.
Cone crushers remain the choice for most secondary and tertiary operations, with some gyradisc and vertical-impact crushers also utilized on certain ore types. Water-flush cone crushers have been introduced in secondary and tertiary installation, which requires careful design of the water systems to and from the crusher.
Most open-circuit secondary and tertiary crushers include scalping screens to remove fine material prior to the secondary crushers. Closed-circuit crushers use tertiary screens to control the final product size. (See Figure 17 and 18 for typical closed circuit secondary and tertiary crushers.)
In some larger installations, the secondary and tertiary crushers are located in one plant area and the screens in another. (See Figure 18f.) Crushers and screens in these plants have common bins feeding to the multiple crushers or screens. Most plants now have the screen feeding the fine crusher, providing for easier access to service the screens.
As with primary crushers, fine crushers must meet the design parameters listed above. Design details that are helpful to fine crusher plant layout area listed in the sections that follow.
Typical closed circuit secondary and tertiary crushers
Coarse Ore Stockpile
• The primary crushing circuit is normally separated from the secondary crushing circuit by provision of a coarse ore stockpile, which retains the primary crushed ore. For mines with short lives and small tonnage rates, operators may eliminate the coarse ore stockpile to reduce costs.
• Stockpile live capacity is a source of controversy. Coarse ore stockpiles were originally designed for three days’ capacity (a long weekend), but this is impractical for some of today’s mines with high daily throughputs. Now, it is possible to size a stockpile by simulation, using pile specific criteria.
• There are many types of coarse ore stockpiles (e.g., conical, elongated, radial, covered or open, heated) with just as many types of withdrawal arrangements and feeders. (See chapter on bins, stockpiles, and feeders.)
• The stockpile provides improved overall system utilization by de-linking the primary and subsequent crushing operations.
• Coarse ore stockpiles also provide the capability to continue operation of the secondary crushing facilities should the primary crusher become inoperative, by bulldozing the stockpile into the reclaim openings.
Feed to Screens or Direct to Secondary Crusher
• The conveyor or feeder transporting ore directly to a secondary crusher should be retractable (or tilt at the head end) to permit crane access to the crusher.
• The conveyor feed chute to a secondary screen should have sufficient height to allow the material to feed to the total width of the screen as rapidly as possible. This is especially critical when feeding from a conveyor to the new wider banana screens.
• The installation of a bin feeding to a single secondary crusher or screen can allow for the future installation of an additional secondary crusher and screen.
• Screens are being manufactured wider so the feed must have more height to allow the material to spread out across the width of the screen.
• The chute feeding the screen should have easy man-access for replacement of liners and easy removal of material build-up.
• Most screen installations should be totally enclosed. Screen manufacturers have yet to develop a cover that allows for ease of maintenance and access.
• Adequate platforms should be provided for access around the entire screen, to facilitate rapid inspection and changing of screen decks.
• The discharge chute from the screen should seal against the screen, and be designed as one piece for ease of removal. When the chute is removed, there should be sufficient opening in the floor to lift out the crusher bowl and head.
• The discharge chute should have a full man-door.
• Permanent floodlighting should be installed inside the chute for ease of inspection.
• Some cone crushers require servicing and removal of components from the bottom. The design and layout should allow for such service requirements.
• There have been many changes recently in the manufacture of cone crushers. A careful assessment of each suppliers’ requirements for lubrication, water and air services to the crusher is mandatory.
• Automatic bowl adjustment is now common on most cone crushers and is definitely going to be used for on-stream electronic control adjustment to maximize input and product size control.
Tertiary Feed Bins
• Most manufacturers ask for controlled ore feed rate to their tertiary crushers. The secondary crushed material (and in the case of a closed circuit, the recirculated material) is usually stored in a bin, and fed to the tertiary screen or directly to the crusher with a variable speed feeder.
• When the feed comes from a bin via a feeder directly to a crusher, the feeder design should be retractable so that the tertiary crusher head and bowl may be removed by a service crane.
• Feed to the tertiary feed bins is dependent on the number of crushers being fed. A single point discharge will be adequate with three crushers, although feed distribution is not good. Five crushers can be fed with a two point feed discharge system, with the main feed conveyor feeding directly to the bin or using a flop gate to a fixed horizontal conveyor to feed to the other discharge point (or a tripper conveyor can be utilized) See Figures 18d, 18e and 18f.
Feeders from a Tertiary Bin
• A belt feeder will maximize live volume in the bin
• Vibratory feeder/magnetic/mechanical is less expensive but provides less live storage in the bin.
• Provide easy stair access to each level.
• Provide access to both sides of the take-away conveyor.
• Braces and structures must be kept away from equipment service and maintenance areas.
• As with primary crushers, structural costs for fine crushing circuits are very high. It is therefore very important to select the most economical structure for the support of the crusher and ancillary equipment. The designer should spend most of his effort on selecting a structure that best suits the design parameters.
• The selection of the types of flooring in a crushing plant is always controversial. Some selections are grating, checker plate and concrete. Checker plate flooring allows for easier clean-up and does not allow for smaller rocks or spillage to any floors below. (The most suitable flooring for conveyor galleries in northern climates is wood.)
• There are many choices for dust collection/suppression systems, including bag filters, scrubbers, cartridge collectors, surfactants, water sprays and sonic fog. Whether one system or a combination is selected, care must be taken to provide service access.
• Control, lube, compressor and electrical rooms should all be well-ventilated.
• Bin air/dust evacuators are required.
• In cold climates, determine whether dust-collector air must be preheated.
• A collector fan silencer should be considered, as fan noise can be deleterious in closed areas.
• If an air system is selected for crusher-oil cooling, it will require venting and hot-air evacuation.
Primary crushing will see the introduction of bigger, 1,800 mm (72 in) gyratory crushers, and lowspeed, high-tonnage roll sizers will become more generally accepted. These larger crushers are required to handle the higher throughputs, and 400- to 500-tonne capacity trucks, expected on some future projects. Fine crushing has already seen the introduction of 10 ft cone crushers.
Much wider banana screens, with sizes up to 4 x 8 m, are also being introduced. Vertical impact crushers are still finding their place for certain crushing applications. Safety and the working environment remain the two areas of plant design, which require more attention.
Noise abatement and the reduction of dust emissions remain the goals of most operators and crushing plant designers. As designers become better trained and familiar with 3D software, then 3D plant design may become the method of choice to optimize the most economical plant designs. The biggest change, which is now being introduced into the design of crushing plants is the design of the mining process as one complete system, from mine quarry and pit to the final product.
Digital image analysis is now allowing electronic monitoring of the size distribution of the material being handled at any point in the product stream. This monitoring and analysis of the size distribution from the pit face to the mill or heap, now allows for the adjustment of the crusher discharge openings as production continues. We can now plan for a more uniform product, increased production, less wear, and longer mine life.