Monday, September 20, 2010

Crushers and Rockbreakers

1. Introduction
Hard rock mines use rockbreakers and crushers for two fundamental reasons.
• To facilitate the transport of ore from the mine to the mill.
• To initiate the size reduction process required to concentrate the ore.

Crushers
Only three types of crushers are normally used in the hard rock mining industry.
• Jaw crushers
• Gyratory crushers
• Cone crushers
In the crushing circuit, gyratory and jaw crushers are normally employed as primary crushers, while cone crushers serve as secondary or tertiary crushers. Primary crushers normally operate in open circuit while cone crushers operate in either open or closed circuit. (The coarser portion of the product is separated and re-circulated through the crusher in a closed circuit.) In some primary crusher installations, fine-sized material is scalped from the feed before it enters the crusher and reunites with the crushed product after bypassing the crusher (short circuit).

Product Size
In the past, primary crushers provided a product of 4-6 inches (100-150 mm) to feed secondary (cone) crushers. Small modern mines continue with the traditional primary crusher product size that is then directed to secondary and tertiary cone crushers to reduce the particle size enough to permit a single grinding stage.

Medium and large sized mines are typically designed to provide feed directly to an autogenous mill. For this reason, the desired product from the primary crusher has increased to 8-12 inches (200- 300mm) to provide a grinding medium (cone crushers are eliminated). However, the cone crusher has found a new role at some larger mines crushing the coarse fraction of the output from an autogenous mill for subsequent re-circulation. This procedure permits the reduction ratio (ratio between size of feed and product) required of a subsequent ball mill to be kept within practical limits.

Before the advent of the modern rockbreaker, over-size material feeding the primary crusher was a greater problem than it is today and primary crushers were designed largely on the basis of gape (minimum dimension of feed opening) rather than capacity. As a result, typical practice underground was to crush the daily requirement in 1 or 1½ shifts and provide sufficient storage in passes and bins to feed the crusher continuously throughout the operating shift(s).

Rockbreakers enable smaller crushers to be employed for the same service, but the old principals of underground primary crusher sizing persist, perhaps to reduce the cost of operating labor and the frequency of maintenance and repairs.

Truck Haulage
Underground mines served by truck haulage from ramp or adit do not require an underground crusher, but there is a limit to the amount of traffic that such an entry can bear. Several current trends are allowing the vertical depth to which ore bodies may be practically exploited by truck haulage to increase.
• Larger trucks (up to 70-tonne capacity)
• Road trains (truck tractor and trailers)
• Twin entries (permitting one-way traffic)
• Block signals (and even transponders) to control traffic
Nevertheless, when high production is to be transported over a long distance, a crusher-fed belt conveyor remains a more economical alternative.

Belt Conveyor
A primary crusher is invariably required before a belt conveyor transports ore, except over a relatively short distance without a turning point (transfer to a second leg of belt conveyor). Operators report that sizing is not the only important function of the crusher that feeds a belt conveyor. The crusher helps to ensure the removal of treacherous tramp metal (such as rock bolts and rebar).

Practice in large open pits has been to provide huge in-the-pit portable crushing plants with belt conveyor haulage to surface and the concentrator. Where the topography is in steep relief, there is still occasional application for glory holes (ore passes of large diameter) within (or near) the pit that typically feeds an underground crusher (Cananea, El Teniente, Carol Lake).

Rockbreakers
Rockbreakers are employed to reduce the size of oversize ore and rock (shot muck). Two categories of oversize muck exist, based on size.
• The larger size consists of shot muck that is too big for handling with an excavator (shovel or LHD unit) at the pit bench or the underground face. Traditionally, the large size muck was broken up by secondary blasting (a few open pits employed a drop hammer). Today, some mines employ a mobile rockbreaker for this purpose. A recent innovation is a portable drill that splits the oversize by hydro-fracture (ultra-high water pressure) after drilling a single hole.
• The smaller size consists of shot muck that is too large to fit in the mouth of a primary crusher or pass through a grizzly (Texas gate). Once “tapped” by hand with a sledgehammer, the muck is now broken with a rockbreaker that normally consists of a percussive breaker unit attached to a pedestal-mounted boom (stationary). The breaker unit is similar to (but much larger than) a common pavement breaker, except that is normally hydraulic rather than pneumatic. This type of rockbreaker operates over a horizontal grizzly, sized to fit the application.

Installation Cost
The installation cost of an underground crusher is considerably higher than one on surface. This is one reason that mine planners investigate options that use rockbreakers alone to eliminate the requirement for an underground crusher.

Small Mines
In the past, the smaller hard rock mines served by shaft entries (that did not employ bulk mining methods) rarely had a primary crusher underground. Today, these small mines typically use only rockbreakers for underground sizing. Some medium sized mines employing blasthole mining methods are also successful with the use of only rockbreakers.

Large Mines
Some of the largest underground mines in the world do not need underground crushers. In these cases, the required capacity of the ore handling conveyances is so large that there is little problem handling run-of-mine (uncrushed) ore that has been “sized” with rockbreakers. For example, it makes little sense to haul ore in 15-ton capacity rail cars to an underground crusher to facilitate hoisting in 30-ton capacity skips.

Run-of-Mine Ore Hoisting
In the past, a number of open pits hoisted run-of-mine ore with an inclined skipway up the wall of the pit. This technology is apparently obsolete, but the principle of hoisting run-of-mine ore in large skips designed for this purpose has been re-visited. The result is a proposal for medium-sized underground mines to apply this procedure to vertical shaft hoisting with two parts of line (“crusher-saver hoisting”).

2. Rules of Thumb
Crusher Selection
• For a hard rock mine application below 600 tonnes/hour, select a jaw as the primary crusher. Over 1,000 tph, select a gyratory crusher. Between these capacities, you have a choice. Source: Chris Ottergren
• For a hard rock mine application below 540 tonnes/hour, a jaw crusher is more economical. Above 725 tonnes/hour, jaw crushers cannot compete with gyratory crushers at normal settings (6 -10 inches). Source: Lewis, Cobourn and Bhappu
• For an underground hard rock mine, a gyratory crusher may be more economical in the case where its required daily production exceeds 8,000 tonnes of ore. Source: Jack de la Vergne
• If the hourly tonnage to be crushed divided by the square of the required gape in inches is less than 0.115, use a jaw crusher; otherwise use a gyratory. (If the required capacity in metric tph is less than 162 times the square of the gape in metres, use a jaw crusher.) Source: Arthur Taggart
• Nearly all crushers produce a product that is 40% finer than one-half the crusher setting. Source: Babu and Cook
• The product of a jaw crusher will have a size distribution such that the -80% fraction size (d80) is slightly less than the open-side setting of the crusher. For example, if the open-side setting is 6 inches, the d80 product size will be 5¾ inches. Source: Unknown
• In a hard rock mine, the product from a jaw crusher will tend to be slabby, while the product from a gyratory crusher may tend to be blocky, the latter being easier to convey through transfer points on a conveyor system. Source: Heinz Schober
• Impact crushers (rotary or hammer mills) have the capacity for high reduction ratios (up to 40:1), but are rarely applied to hard rock mines. Since they depend on high velocities for crushing, wear is greater than for jaw or gyratory crushers. Hence, they should not be used in hard rock mines that normally have ores containing more than 15% silica (or any ores that are abrasive). Source: Barry Wills

Crusher Design
• The approximate capacity of a jaw crusher for hard rock application at a typical setting may be obtained by multiplying the width by 10 to get tonnes per hour. For example, a 48 by 60 crusher will have a capacity in the order of 600 tph when crushing ore in a hard rock mine. Source: Jack de la Vergne
• The capacity of a jaw crusher selected for underground service should be sufficient to crush the daily requirement in 12 hours. Source: Dejan Polak
• For most applications, 7:1 is the maximum practical reduction factor (ratio) for a jaw crusher, but 6:1 represents better design practice. Source: Jack de la Vergne
• For most applications, 6:1 is the maximum practical reduction factor (ratio) for a cone crusher, but 5:1 represents better design practice. Source: Jack de la Vergne
• Corrugated liner plates designed for jaw crushers (to avoid a slabby product) result in shortening liner life by up to two-thirds and they are more prone to plugging than smooth jaws. Source: Ron Doyle

Crusher Installation
• The crushed ore surge pocket beneath a gyratory crusher should have a live load capacity equal to 20 minutes of crusher capacity or the capacity of two pit trucks. Various Sources.
• It will take six months to excavate, install, and commission an underground crusher station for a typical jaw crusher. For a very large jaw crusher or a gyratory crusher, it can take nine months. Source: Jim Redpath
• The desired grizzly opening for an underground jaw crusher is equal to 80% of the gape of the crusher. Source: Jack de la Vergne
• The combination of a jaw crusher and a scalping grizzly will have 15% more capacity than a stand-alone jaw crusher. Source: Ron Casson
• As a rule, scalping grizzlies are rarely used anymore for (large) primary crushers. The exception is when ore contains wet fines that can cause acute packing in a gyratory crusher. Source: McQuiston and Shoemaker
• The product from a jaw crusher will tend to be less slabby and more even-dimensioned without a scalping grizzly, since slabs do not pass through so readily under this circumstance. Source: A. L. Engels
• Removal of the scalping grizzly for a primary jaw crusher can cut the liner life by 50%. It also makes it more difficult to clear a jam when the jaws are filled with fines. Source: Ron Doyle

Rockbreakers
• The capacity of a hydraulic rockbreaker is higher (and the operating cost lower) than a pneumatic rockbreaker. For these reasons, most new installations are hydraulic, despite the higher capital cost. Source: John Kelly
• For underground production rates less than 2,000 tpd, it may be economical to size the ore underground with rockbreakers only, otherwise, an underground crusher is usually necessary when skip hoisting is employed. Source: John Gilbert
• The operating cost for a stand-alone rockbreaker will be approximately 30% higher than it is for a crusher handling the same daily tonnage. Source: John Gilbert
• The capacity of one rockbreaker on a grizzly with the standard opening (± 16 by 18 inches) is in the order of 1,500-2,000 tpd. Source: John Gilbert.
• For skips that fit into a standard 6 by 6 shaft compartment, the maximum particle size that is normally desired for skip hoisting is obtained when run-of-mine muck has been passed through a grizzly with a 16-18 inch opening. Skips hoisted in narrow shaft compartments may require a 12-14 inch spacing, while oversize skips may handle muck that has passed a 24-30 inch spacing. Source: Jack de la Vergne
• A pedestal mounted rockbreaker installed should be equipped with a boom that enables a reach of 20 feet (6m). Source: Peter van Schaayk

3. Tricks of the Trade
• A crushing circuit over-designed for the application will not increase the operating cost and provides for future expansion. A crushing circuit under-designed for the application (particularly with respect to reduction ratios) can increase operating costs dramatically. Source: Jack de la Vergne
• A new jaw crusher manufactured in China may cost less and be more rugged and reliable than a rebuilt crusher purchased from the North American after market. Source: Luis Browne
• The foundation design for primary jaw crushers should provide walk-in capability for maintenance on the toggles and lower jaw liners. The incorporation of a cylinder-actuated sliding floor beneath the foundations provides a safe working platform in an underground mine. Source: Peter White
• Never dump a large piece of rock into a full jaw crusher; allow jaws to empty first. Source: John Smith
• Normally, a jaw crusher will operate best if it is kept just full but not flood fed. If full, it will experience less plate wear because a greater portion of the breakage is autogenous (i.e. attrition between particles of ore). Attrition is increased (and jamming less likely) if the fines are first scalped from the ore. Dribble feeding increases wear on the jaw plates due to bouncing and lack of attrition. Various Sources
• A gyratory crusher usually operates best at 80% of rated maximum speed. Source: Allis Chalmers
• An easier way than using lead to determine the closed side setting of a cone crusher is to throw in an empty soft drink or similar can and measure its thickness after it has passed through the crusher. Source: Dave Assinck
• If the automatic lubricating oil temperature of a crusher exceeds 120 degrees F., there is likely to be a fault or restriction in the oil flow. Source: Unknown
• A difference exceeding 2 degrees C. between the inlet and outlet lubricating oil of a crusher is an indication of a problem developing. If it reaches 3 degrees C., the crusher should be shut down and all bearings examined. Source: Unknown

4. Jaw Crushers
Jaw crushers are used as primary crushers in some open pit mining operations and in almost all medium-sized underground hard rock mines. Outside of North America, jaw crushers are occasionally used as a secondary crusher. One of the largest capacity jaw crushers is a custom built 63 by 83 (1,600 by 2,100) unit that operates in an open pit with a stated capacity of 1,200 tph when set at 16 inches (400 mm). The largest jaw crushers normally found underground in hard rock mines are 48 by 66 units that can be opened up to a maximum setting of about 10 inches. (Jaw crushers manufactured in China and Eastern Europe may be opened wider than those normally built in North America, Western Europe, or Brazil.)

Capacity
Jaw crusher capacity for a particular application may be obtained by consulting manufacturers’ catalogues that contain capacity tables (accurate to ±20%) for the different standard-sized models. For most applications, jaw crushers come in standard sizes that are similar from one manufacturer to another. Traditionally, these sizes denote the gape and width expressed in inches.

A concern often exists that the ore to be crushed from a hard rock mine is much stronger (higher compressive and impact strength) than the reference material (usually limestone) on which the tables in manufacturers’ catalogues are based. The fact is that it makes little difference to the capacity of a crusher. Hard rock ores may tend to reduce volumetric capacity by a small amount but compensation for this is obtained because a broken ore fed to the crusher usually has a higher bulk density than broken limestone. In other words, the capacities indicated in the tables normally require no modification for a hard rock mine application.

Power Consumption and Drive Capacity
The power consumption and drive capacity (HP) required for a proposed jaw (or any other) crusher may be determined with the application of Bond’s law. The procedure is fully explained in Chapter 26 – Mineral Processing.

Jaw Crusher Types
The two distinct types of jaw crusher employed in hard rock mines are single toggle and double toggle. For primary jaw crushers it has been general practice for underground hard rock mines in North America to employ the double toggle, Blake type. In northern Europe, especially Sweden, the single toggle is typically employed, overhead eccentric type (LKAB, Kiruna). Either single or double toggle crushers are used for most mobile “in-the-pit” crushers, but single-toggle crushers are used exclusively for semi-portable crushers when employed underground.

Single Toggle
The single toggle crushers weigh half to two thirds as much as double toggle and are usually much less expensive; however, they have higher maintenance and repair costs. Higher costs are logical, based on the operating mechanism. At least a portion of these costs may be alleviated if the foundation or supporting steel is designed more robust than required for a comparable double toggle crusher.

The single toggle crusher drives the swing (moving) jaw in two ways:
• The moving jaw is supported directly by means of a single eccentric drive shaft that causes it to move in an elliptical path at the top end.
• The single toggle diminishes and restricts this motion to near linear at the bottom (discharge) end of the moving jaw. The toggle is typically designed to shear or break if tramp iron enters the jaws, and other safeties fail to stop the crusher. Some manufacturers refer to the swing jaw of a single toggle crusher as the “pitman,” which can be confusing.

Double Toggle
The double toggle crusher drives the moving jaw through a separate eccentric drive shaft that moves the front and rear toggles up and down at each revolution of the shaft, by means of a pitman (connecting rod). The moving jaw is supported by and pivots about a separate shaft. The front toggle is connected to the moving jaw at a right angle. A special front toggle (shear toggle) can be supplied as an option designed to break if tramp iron enters the jaws, and other safeties fail to stop the crusher.


Figure 25-1 Jaw Crusher Types

5. Gyratory Crushers
Gyratory crushers operate on the principal of a mortar and pestle. They depend on a reciprocating eccentric mechanism similar to the single-toggle jaw crusher; however, the main shaft is vertical and driven from beneath through a right angle pinion shaft and gears that permits the drive motor to be mounted horizontally. A tapered “head” on the main shaft rotates with an eccentric motion around a conical bowl crushing the muck. The setting of the gap in the annular space between the head and the liners on the bowl controls the size of the product that falls through. The throw of the eccentricity is usually between 1 and 1½ inches. This throw makes up the difference between the “open-side” and “closed side” setting.

Before the advent of in-the-pit crushing and conveying, stationary gyratory crushers were the default selection for open pits due to high capacity, flood feed ability, and uniform product dimensions. More recently, custom-built gyratory crushers (sectionalized) have been applied to larger underground mines to enable component transport through a mine entry. The heaviest component of even a custom built gyratory is heavier than that of a jaw crusher of the same minimum dimension (gape) of feed opening. For example, the bottom shell section of one name brand 42-65 gyratory weighs 56,400 Lbs. while the swing jaw of a 42 by 48 jaw crusher may weigh 24,000 Lbs. The feed openings of the two crushers are comparable, but in this case the gyratory has triple the capacity of the jaw crusher.

6. Cone Crushers
Cone crushers are a type of gyratory crusher that serve as secondary or tertiary units in a crusher circuit. In simple terms, a cone crusher may be thought of as a small gyratory crusher; however, the standard cone crusher (invented by Edgar Symonds) is different from a gyratory, as shown below.
• No spider exists to interfere with the feed.
• The cone is constructed at a flatter angle.
• No upper bearing exists for the gyratory shaft.
• The head gyrates much faster.
• Springs attached to the frame act as a safety device allowing the crushing bowl to rise if tramp iron is encountered. Following are different types of cone crushers.
• Short-head Crusher – a stubby version of the standard cone crusher normally used for fine crushing in closed circuit.
• Hydrocone® Crusher – a proprietary cone crusher that may be used as a standard or a short head, depending on the crushing chamber design.
• A Gyradisc® Crusher – a proprietary cone crusher specifically designed for very fine crushing.
• A Water Flush® Crusher – a proprietary cone crusher that employs water as the medium, as opposed to crushing dry.

7. Jaw Crusher versus Gyratory Crusher
When designing a large underground mine (or a relatively small open-pit mine), a common question is whether to select a jaw or a gyratory crusher as the primary. The decision can often be made by using rules of thumb, but further investigation is usually desirable. The following observations are useful in a more comprehensive determination:
• The installed cost of a jaw crusher is less than a gyratory in part due to less foundation work. Thus, portable and self-propelled units most often use a jaw crusher.
• For a stationary installation in an underground mine, jaw crushers are the conventional choice for the following reasons.
− Approximately 50 feet (15m) less vertical distance is required in the crusher room
− Liners are more easily and more quickly replaced when worn
− Large feed opening (gape) in relation to capacity
− Usually better able to handle fines and sticky feed
− Good competition exists between suppliers of new units
− An ample supply of second-hand units are usually available
− Pirate parts are available for major brand names at reduced cost
− A jaw crusher is more readily sold in the after market
− The components are easier to transport underground
• Gyratory crushers are employed at most large open pit operations. Additionally, they have been installed underground in some of the larger mines (Kidd Creek, Brunswick, Henderson, El Teniente, Palabora) and for a few open pit and quarry/glory hole operations (Cadomin, Glensada).
• Gyratory crushers offer the following advantages compared to jaw crushers.
− High capacity (At 3,600 tph, the largest gyratory crusher on the market has three times the capacity of the largest jaw crusher available).
− Product of uniform dimensions (cubical product better than a slabby one).
− Ability to flood feed (careful feed regulation not normally required).
− May usually be started up when fully loaded (choke start).

8. Locating a Primary Crusher Underground
Typically, two potential locations exist for an underground crusher in a mine employing a production shaft.
• Near the shaft
• Under the ore body
Classic mine design employs a crusher near the shaft, which is considered advantageous for steeply dipping ore bodies of simple geometry. In cases where rail haulage is employed in conjunction with a multiple ore pass system, the rail haulage is typically extended to dump above a crusher near the shaft. Placing the crusher in a location near the shaft has also been employed where the configuration of the ore body makes it impossible to place ore passes close enough to the draw points to allow LHD equipment to economically tram the ore. In such a case, haulage trucks are required. If haulage trucks are used, they may as well tram the extra distance required to an ore pass leading to a crusher near the shaft.

An indirect determining factor is the pre-production schedule. A crusher room near the shaft is accessed for excavation more rapidly than one that is remote. In very deep mines, the crusher should be located near the shaft, well away from the influence of ground stress due to mining activity.

9. Crushing Waste Rock
In addition to crushing the ore, many mines crush a substantial amount of waste rock that has been stripped from a pit or results from underground development. The product is used at the mine to provide road dressing, concrete aggregate, and underground backfill. Some mines sell surplus crushed rock to local clients as a sideline operation.

Waste rock crushing plants on surface typically incorporate a screening facility to separate the fines and sort the remainder. It is rarely practical to install a separate crushing and screening plant in an underground mine. Underground mines usually direct as much waste rock as possible to supplement the backfill operations rather move it to the surface. The remainder is either truck hauled to surface, batched (campaigned) through an ore crusher, or sized with a separate rockbreaker circuit.

Underground mines require significant quantities of crushed rock for road dressing (ballast) on lateral haulage ways and ramps. Usually, it is difficult to recover enough desirable waste rock from the materials handling system underground. Moreover, minus 6 or 8 inch material is not particularly suitable for ballast. One practical means of obtaining road dressing underground at the size desired (-2½ inches) is to install a tuning fork grizzly (or similar device) at a suitable location to divert a portion of the undersize material for use as roadbed dressing.

10. Rockbreakers
Oversize muck (ore and waste rock) is the cause of hang-ups, feeder problems, crusher jams, and damage to chutes and gates. In hard rock mines, mechanical rockbreakers are used to size lumps of muck in preference to secondary blasting. The rockbreakers may be mobile or stationary for surface operations but are most always pedestal mounted at one location in an underground hard rock mine.

The capacity of a rockbreaker is dependent on the following items.
• Characteristics of the breaker (energy per blow and blows per minute).
• Size of the grizzly (or “Texas gate”) openings.
• Amount of oversize in the ore stream.
• Work index and cleavage of the oversize muck to be broken.
• Geometry of the installation.
• Operator ability.
• Visibility (remote operation via camera versus direct).

A capacity of up to 180 tph over a standard 400 by 600 (16 by 24 inch) grizzly has been reported at one underground mine in Northern Quebec. The capacity for bulk mining methods using large LHD equipment is reported as low as 1,000 tonnes per day (tpd) through a similar opening for a three-shift operation at some mines.

The standard grizzly opening (16 inch by 18 inch) is normally employed to size ore for hoisting and is often employed to size waste rock for a conveyor load out to a shaft pocket. A larger opening is employed to feed a crusher (typically and the grizzly opening dimension is made equal to 80% of the gape of the crusher).

Hydraulic rockbreakers (as opposed to pneumatic) are invariably selected for new installations. Despite the higher capital cost, hydraulic breakers have greater capacity and significantly less operating cost. A rockbreaker should normally have a boom reach of at least 6m (20 feet).

If production is increased, it is conceivable that a second rockbreaker may be found desirable at the same location. To prepare for such an event, allowance should be made in the design layout for installing a second rockbreaker at the same grizzly in the future.