The rationale for selecting a method for mining coal depends on such factors as topography, geometry of the coal seam, geology of the overlying rocks and environmental requirements or restraints. Overriding these, however, are the economic factors. They include: availability, quality and costs of the required work force (including the availability of trained supervisors and managers); adequacy of housing, feeding and recreational facilities for the workers (especially when the mine is located at a distance from a local community); availability of the necessary equipment and machinery and of workers trained to operate it; availability and costs of transportation for workers, necessary supplies, and for getting the coal to the user or purchaser; availability and the cost of the necessary capital to finance the operation (in local currency); and the market for the particular type of coal to be extracted (i.e., the price at which it may be sold). A major factor is the stripping ratio, that is, the amount of overburden material to be removed in proportion to the amount of coal that can be extracted; as this increases, the cost of mining becomes less attractive. An important factor, especially in surface mining, that, unfortunately, is often overlooked in the equation, is the cost of restoring the terrain and the environment when the mining operation is closed down.

Health and Safety

Another critical factor is the cost of protecting the health and safety of the miners. Unfortunately, particularly in small-scale operations, instead of being weighed in deciding whether or how the coal should be extracted, the necessary protective measures are often ignored or short-changed.

Actually, although there are always unsuspected hazards—they may come from the elements rather than the mining operations—any mining operation can be safe providing there is a commitment from all parties to a safe operation.

Surface Coal Mines

Surface mining of coal is performed by a variety of methods depending on the topography, the area in which the mining is being undertaken and environmental factors. All methods involve the removal of overburden material to allow for the extraction of the coal. While generally safer than underground mining, surface operations do have some specific hazards that must be addressed. Prominent among these is the use of heavy equipment which, in addition to accidents, may involve exposure to exhaust fumes, noise and contact with fuel, lubricants and solvents. Climatic conditions, such as heavy rain, snow and ice, poor visibility and excessive heat or cold may compound these hazards. When blasting is required to break up rock formations, special precautions in the storage, handling and use of explosives are required.

Surface operations require the use of huge waste dumps to store overburden products. Appropriate controls must be implemented to prevent dump failure and to protect the employees, the general public and the environment.

Underground Mining

There is also a variety of methods for underground mining. Their common denominator is the creation of tunnels from the surface to the coal seam and the use of machines and/or explosives to extract the coal. In addition to the high frequency of accidents—coal mining ranks high on the list of hazardous workplaces wherever statistics are maintained—the potential for a major incident involving multiple loss of life is always present in underground operations. Two primary causes of such catastrophes are cave-ins due to faulty engineering of the tunnels and explosion and fire due to the accumulation of methane and/or flammable levels of airborne coal dust.

Methane

Methane is highly explosive in concentrations of 5 to 15% and has been the cause of numerous mining disasters. It is best controlled by providing adequate air flow to dilute the gas to a level that is below its explosive range and to exhaust it quickly from the workings. Methane levels must be continuously monitored and rules established to close down operations when its concentration reaches 1 to 1.5% and to evacuate the mine promptly if it reaches levels of 2 to 2.5%.

Coal dust

In addition to causing black lung disease (anthracosis) if inhaled by miners, coal dust is explosive when fine dust is mixed with air and ignited. Airborne coal dust can be controlled by water sprays and exhaust ventilation. It can be collected by filtering recirculating air or it can be neutralized by the addition of stone dust in sufficient quantities to render the coal dust/air mixture inert.

Back

There are underground mines all over the world presenting a kaleidoscope of methods and equipment. There are approximately 650 underground mines, each with an annual output that exceeds 150,000 tonnes, which account for 90% of the ore output of the western world. In addition, it is estimated that there are 6,000 smaller mines each producing less than 150,000 tonnes. Each mine is unique with workplace, installations and underground workings dictated by the kinds of minerals being sought and the location and geological formations, as well as by such economic considerations as the market for the particular mineral and the availability of funds for investment. Some mines have been in continuous operation for more than a century while others are just starting up.

Mines are dangerous places where most of the jobs involve arduous labour. The hazards faced by the workers range from such catastrophes as cave-ins, explosions and fire to accidents, dust exposure, noise, heat and more. Protecting the health and safety of the workers is a major consideration in properly conducted mining operations and, in most countries, is required by laws and regulations.

The Underground Mine

The underground mine is a factory located in the bedrock inside the earth in which miners work to recover minerals hidden in the rock mass. They drill, charge and blast to access and recover the ore, i.e., rock containing a mix of minerals of which at least one can be processed into a product that can be sold at a profit. The ore is taken to the surface to be refined into a high-grade concentrate.

Working inside the rock mass deep below the surface requires special infrastructures: a network of shafts, tunnels and chambers connecting with the surface and allowing movement of workers, machines and rock within the mine. The shaft is the access to underground where lateral drifts connect the shaft station with production stopes. The internal ramp is an inclined drift which links underground levels at different elevations (i.e., depths). All underground openings need services such as exhaust ventilation and fresh air, electric power, water and compressed air, drains and pumps to collect seeping ground water, and a communication system.

Hoisting plant and systems

The headframe is a tall building which identifies the mine on the surface. It stands directly above the shaft, the mine’s main artery through which the miners enter and leave their workplace and through which supplies and equipment are lowered and ore and waste materials are raised to the surface. Shaft and hoist installations vary depending on the need for capacity, depth and so on. Each mine must have at least two shafts to provide an alternate route for escape in case of an emergency.

Hoisting and shaft travelling are regulated by stringent rules. Hoisting equipment (e.g., winder, brakes and rope) is designed with ample margins of safety and is checked at regular intervals. The shaft interior is regularly inspected by people standing on top of the cage, and stop buttons at all stations trigger the emergency brake.

The gates in front of the shaft barricade the openings when the cage is not at the station. When the cage arrives and comes to a full stop, a signal clears the gate for opening. After miners have entered the cage and closed the gate, another signal clears the cage for moving up or down the shaft. Practice varies: the signal commands may be given by a cage tender or, following the instructions posted at each shaft station, the miners may signal shaft destinations for themselves. Miners are generally quite aware of the potential hazards in shaft riding and hoisting and accidents are rare.

Diamond drilling

A mineral deposit inside the rock must be mapped before the start of mining. It is necessary to know where the orebody is located and define its width, length and depth to achieve a three-dimensional vision of the deposit.

Diamond drilling is used to explore a rock mass. Drilling can be done from the surface or from the drift in the underground mine. A drill bit studded with small diamonds cuts a cylindrical core that is captured in the string of tubes that follows the bit. The core is retrieved and analysed to find out what is in the rock. Core samples are inspected and the mineralized portions are split and analysed for metal content. Extensive drilling programmes are required to locate the mineral deposits; holes are drilled at both horizontal and vertical intervals to identify the dimensions of the orebody (see figure 1).

Figure 1. Drill pattern, Garpenberg Mine, a lead-zinc mine, Sweden

Mine development

Mine development involves the excavations needed to establish the infrastructure necessary for stope production and to prepare for the future continuity of operations. Routine elements, all produced by the drill-blast-excavation technique, include horizontal drifts, inclined ramps and vertical or inclined raises.

Shaft sinking

Shaft sinking involves rock excavation advancing downwards and is usually assigned to contractors rather than being done by mine’s personnel. It requires experienced workers and special equipment, such as a shaft-sinking headframe, a special hoist with a large bucket hanging in the rope and a cactus-grab shaft mucking device.

The shaft-sinking crew is exposed to a variety of hazards. They work at the bottom of a deep, vertical excavation. People, material and blasted rock must all share the large bucket. People at the shaft bottom have no place to hide from falling objects. Clearly, shaft sinking is not a job for the inexperienced.

Drifting and ramping

A drift is a horizontal access tunnel used for transport of rock and ore. Drift excavation is a routine activity in the development of the mine. In mechanized mines, two-boom, electro-hydraulic drill jumbos are used for face drilling. Typical drift profiles are 16.0 m² in section and the face is drilled to a depth of 4.0 m. The holes are charged pneumatically with an explosive, usually bulk ammonium nitrate fuel oil (ANFO), from a special charging truck. Short-delay non-electric (Nonel) detonators are used.

Mucking is done with (load-haul-dump) LHD vehicles (see figure 2) with a bucket capacity of about 3.0 m³. Muck is hauled directly to the ore pass system and transferred to truck for longer hauls. Ramps are passageways connecting one or more levels at grades ranging from 1:7 to 1:10 (a very steep grade compared to normal roads) that provide adequate traction for heavy, self-propelled equipment. The ramps are often driven in an upward or downward spiral, similar to a spiral staircase. Ramp excavation is a routine in the mine’s development schedule and uses the same equipment as drifting.

Figure 2. LHD loader

Atlas Copco

Raising

A raise is a vertical or steeply-inclined opening that connects different levels in the mine. It may serve as a ladderway access to stopes, as an ore pass or as an airway in the mine’s ventilation system. Raising is a difficult and dangerous, but necessary job. Raising methods vary from simple manual drill and blast to mechanical rock excavation with raise boring machines (RBMs) (see figure 3).

Figure 3. Raising methods

Manual raising

Manual raising is difficult, dangerous and physically demanding work that challenges the miner’s agility, strength and endurance. It is a job to be assigned only to experienced miners in good physical condition. As a rule the raise section is divided into two compartments by a timbered wall. One is kept open for the ladder used for climbing to the face, air pipes, etc. The other fills with rock from blasting which the miner uses as a platform when drilling the round. The timber parting is extended after each round. The work involves ladder climbing, timbering, rock drilling and blasting, all done in a cramped, poorly ventilated space. It is all performed by a single miner, as there is no room for a helper. Mines search for alternatives to the hazardous and laborious manual raising methods.

The raise climber

The raise climber is a vehicle that obviates ladder climbing and much of the difficulty of the manual method. This vehicle climbs the raise on a guide rail bolted to the rock and provides a robust working platform when the miner is drilling the round above. Very high raises can be excavated with the raise climber with safety much improved over the manual method. Raise excavation, however, remains a very hazardous job.

The raise boring machine

The RBM is a powerful machine that breaks the rock mechanically (see figure 4). It is erected on top of the planned raise and a pilot hole about 300 mm in diameter is drilled to break through at a lower level target. The pilot drill is replaced by a reamer head with the diameter of the intended raise and the RBM is put in reverse, rotating and pulling the reamer head upward to create a full-size circular raise.

Figure 4. Raise boring machine

Atlas Copco

Ground control

Ground control is an important concept for people working inside a rock mass. It is particularly important in mechanized mines using rubber-tyred equipment where the drift openings are 25.0 m² in section, in contrast to the mines with rail drifts where they are usually only 10.0 m². The roof at 5.0 m is too high for a miner to use a scaling bar to check for potential rock falls.

Different measures are used to secure the roof in underground openings. In smooth blasting, contour holes are drilled closely together and charged with a low-strength explosive. The blast produces a smooth contour without fracturing the outside rock.

Nevertheless, since there are often cracks in the rock mass which do not show on the surface, rock falls are an ever-present hazard. The risk is reduced by rock bolting, i.e., insertion of steel rods in bore holes and fastening them. The rock bolt holds the rock mass together, prevents cracks from spreading, helps to stabilize the rock mass and makes the underground environment safer.

Methods for Underground Mining

The choice of mining method is influenced by the shape and size of the ore deposit, the value of the contained minerals, the composition, stability and strength of the rock mass and the demands for production output and safe working conditions (which sometimes are in conflict). While mining methods have been evolving since antiquity, this article focuses on those used in semi- to fully-mechanized mines during the late twentieth century. Each mine is unique, but they all share the goals of a safe workplace and a profitable business operation.

Flat room-and-pillar mining

Room-and-pillar mining is applicable to tabular mineralization with horizontal to moderate dip at an angle not exceeding 20° (see figure 5). The deposits are often of sedimentary origin and the rock is often in both hanging wall and mineralization in competent (a relative concept here as miners have the option to install rock bolts to reinforce the roof where its stability is in doubt). Room-and-pillar is one of the principal underground coal-mining methods.

Figure 5. Room-and-pillar mining of a flat orebody

Room-and-pillar extracts an orebody by horizontal drilling advancing along a multi-faced front, forming empty rooms behind the producing front. Pillars, sections of rock, are left between the rooms to keep the roof from caving. The usual result is a regular pattern of rooms and pillars, their relative size representing a compromise between maintaining the stability of the rock mass and extracting as much of the ore as possible. This involves careful analysis of the strength of the pillars, the roof strata span capacity and other factors. Rock bolts are commonly used to increase the strength of the rock in the pillars. The mined-out stopes serve as roadways for trucks transporting the ore to the mine’s storage bin.

The room-and-pillar stope face is drilled and blasted as in drifting. The stope width and height correspond to the size of the drift, which can be quite large. Large productive drill jumbos are used in normal height mines; compact rigs are used where the ore is less than 3.0 m thick. The thick orebody is mined in steps starting from the top so that the roof can be secured at a height convenient for the miners. The section below is recovered in horizontal slices, by drilling flat holes and blasting against the space above. The ore is loaded onto trucks at the face. Normally, regular front-end loaders and dump trucks are used. For the low-height mine, special mine trucks and LHD vehicles are available.

Room-and-pillar is an efficient mining method. Safety depends on the height of the open rooms and ground control standards. The main risks are accidents caused by falling rock and moving equipment.

Inclined room-and-pillar mining

Inclined room-and-pillar applies to tabular mineralization with an angle or dip from 15° and 30° to the horizontal. This is too steep an angle for rubber-tyred vehicles to climb and too flat for a gravity assist rock flow.

The traditional approach to the inclined orebody relies on manual labour. The miners drill blast holes in the stopes with hand-held rock drills. The stope is cleaned with slusher scrapers.

The inclined stope is a difficult place to work. The miners have to climb the steep piles of blasted rock carrying with them their rock drills and the drag slusher pulley and steel wires. In addition to rock falls and accidents, there are the hazards of noise, dust, inadequate ventilation and heat.

Where the inclined ore deposits are adaptable to mechanization, “step-room mining” is used. This is based on converting the “difficult dip” footwall into a “staircase” with steps at an angle convenient for trackless machines. The steps are produced by a diamond pattern of stopes and haulage-ways at the selected angle across the orebody.

Ore extraction starts with horizontal stope drives, branching out from a combined access-haulage drift. The initial stope is horizontal and follows the hanging wall. The next stope starts a short distance further down and follows the same route. This procedure is repeated moving downward to create a series of steps to extract the orebody.

Sections of the mineralization are left to support the hanging wall. This is done by mining two or three adjacent stope drives to the full length and then starting the next stope drive one step down, leaving an elongated pillar between them. Sections of this pillar can later be recovered as cut-outs that are drilled and blasted from the stope below.

Modern trackless equipment adapts well to step-room mining. The stoping can be fully mechanized, using standard mobile equipment. The blasted ore is gathered in the stopes by the LHD vehicles and transferred to mine truck for transport to the shaft/ore pass. If the stope is not high enough for truck loading, the trucks can be filled in special loading bays excavated in the haulage drive.

Shrinkage stoping

Shrinkage stoping may be termed a “classic” mining method, having been perhaps the most popular mining method for most of the past century. It has largely been replaced by mechanized methods but is still used in many small mines around the world. It is applicable to mineral deposits with regular boundaries and steep dip hosted in a competent rock mass. Also, the blasted ore must not be affected by storage in the slopes (e.g., sulphide ores have a tendency to oxidize and decompose when exposed to air).

Its most prominent feature is the use of gravity flow for ore handling: ore from stopes drops directly into rail cars via chutes obviating manual loading, traditionally the most common and least liked job in mining. Until the appearance of the pneumatic rocker shovel in the 1950s, there was no machine suitable for loading rock in underground mines.

Shrinkage stoping extracts the ore in horizontal slices, starting at the stope bottoms and advancing upwards. Most of the blasted rock remains in the stope providing a working platform for the miner drilling holes in the roof and serving to keep the stope walls stable. As blasting increases the volume of the rock by about 60%, some 40% of the ore is drawn at the bottom during stoping in order to maintain a work space between the top of the muckpile and the roof. The remaining ore is drawn after blasting has reached the upper limit of the stope.

The necessity of working from the top of the muckpile and the raise-ladder access prevents the use of mechanized equipment in the stope. Only equipment light enough for the miner to handle alone may be used. The air-leg and rock drill, with a combined weight of 45 kg, is the usual tool for drilling the shrinkage stope. Standing on top of the muckpile, the miner picks up the drill/feed, anchors the leg, braces the rock drill/drill steel against the roof and starts drilling; it is not easy work.

Cut-and-fill mining

Cut-and-fill mining is suitable for a steeply dipping mineral deposit contained in a rock mass with good to moderate stability. It removes the ore in horizontal slices starting from a bottom cut and advances upwards, allowing the stope boundaries to be adjusted to follow irregular mineralization. This permits high-grade sections to be mined selectively, leaving low-grade ore in place.

After the stope is mucked clean, the mined out space is backfilled to form a working platform when the next slice is mined and to add stability to the stope walls.

Development for cut-and-fill mining in a trackless environment includes a footwall haulage drive along the orebody at the main level, undercut of the stope provided with drains for the hydraulic backfill, a spiral ramp excavated in the footwall with access turn-outs to the stopes and a raise from the stope to the level above for ventilation and fill transport.

Overhand stoping is used with cut-and-fill, with both dry rock and hydraulic sand as backfill material. Overhand means that the ore is drilled from below by blasting a slice 3.0 m to 4.0 m thick. This allows the complete stope area to be drilled and the blasting of the full stope without interruptions. The “uppers” holes are drilled with simple wagon drills.

Up-hole drilling and blasting leaves a rough rock surface for the roof; after mucking out, its height will be about 7.0 m. Before miners are allowed to enter the area, the roof must be secured by trimming the roof contours with smooth-blasting and subsequent scaling of the loose rock. This is done by miners using hand-held rock drills working from the muckpile.

In front stoping, trackless equipment is used for ore production. Sand tailings are used for backfill and distributed in the underground stopes via plastic pipes. The stopes are filled almost completely, creating a surface sufficiently hard to be traversed by rubber-tyred equipment. The stope production is completely mechanized with drifting jumbos and LHD vehicles. The stope face is a 5.0 m vertical wall across the stope with a 0.5 m open slot beneath it. Five-meter-long horizontal holes are drilled in the face and ore is blasted against the open bottom slot.

The tonnage produced by a single blast depends on the face area and does not compare to that yielded by the overhand stope blast. However, the output of trackless equipment is vastly superior to the manual method, while roof control can be accomplished by the drill jumbo which drills smooth-blast holes together with the stope blast. Fitted with an oversize bucket and large tyres, the LHD vehicle, a versatile tool for mucking and transport, travels easily on the fill surface. In a double face stope, the drill jumbo engages it on one side while the LHD handles the muckpile at the other end, providing efficient use of the equipment and enhancing the production output.

Sublevel stoping removes ore in open stopes. Backfilling of stopes with consolidated fill after the mining allows the miners to return at a later time to recover the pillars between the stopes, enabling a very high recovery rate of the mineral deposit.

Development for sublevel stoping is extensive and complex. The orebody is divided into sections with a vertical height of about 100 m in which sublevels are prepared and connected via an inclined ramp. The orebody sections are further divided laterally in alternating stopes and pillars and a mail haulage drive is created in the footwall, at the bottom, with cut-outs for drawpoint loading.

When mined out, the sublevel stope will be a rectangular opening across the orebody. The bottom of the stope is V-shaped to funnel the blasted material into the draw-points. Drilling drifts for the long-hole rig are prepared on the upper sublevels (see figure 6).

Figure 6. Sublevel stoping using ring drilling & cross-cut loading

Blasting requires space for the rock to expand in volume. This requires that a slot a few metres wide be prepared before the start of long-hole blasting. This is accomplished by enlarging a raise from the bottom to the top of the stope to a full slot.

After opening the slot, the long-hole rig (see figure 7) begins production drilling in sublevel drifts following precisely a detailed plan designed by blasting experts which specifies all the blast holes, the collaring position, depth and direction of the holes. The drill rig continues drilling until all the rings on one level are completed. It is then transferred to the next sublevel to continue drilling. Meanwhile the holes are charged and a blast pattern which covers a large area within the stope breaks up a large volume of ore in one blast. The blasted ore drops to the stope bottom to be recovered by the LHD vehicles mucking in the draw-point beneath the stope. Normally, the long-hole drilling stays ahead of the charging and blasting providing a reserve of ready-to-blast ore, thus making for an efficient production schedule.

Figure 7. Long-hole drill rig

Atlas Copco

Sublevel stoping is a productive mining method. Efficiency is enhanced by the ability to use fully mechanized productive rigs for the long-hole drilling plus the fact that the rig can be used continuously. It is also relatively safe because doing the drilling inside sublevel drifts and mucking through draw-points eliminates exposure to potential rock falls.

Vertical crater retreat mining

Like sublevel stoping and shrinkage stoping, vertical crater retreat (VCR) mining is applicable to mineralization in steeply dipping strata. However, it uses a different blasting technique breaking the rock with heavy, concentrated charges placed in holes (“craters”) with very large diameter (about 165 mm) about 3 m away from a free rock surface. Blasting breaks a cone-shaped opening in the rock mass around the hole and allows the blasted material to remain in the stope during the production phase so that the rock fill can assist in supporting the stope walls. The need for rock stability is less than in sublevel stoping.

The development for VCR mining is similar to that for sublevel stoping except for requiring both over-cut and under-cut excavations. The over-cut is needed in the first stage to accommodate the rig drilling the large-diameter blast holes and for access while charging the holes and blasting. The under-cut excavation provided the free surface necessary for VCR blasting. It may also provide access for a LHD vehicle (operated by remote control with the operator remaining outside the stope) to recover the blasted ore from the draw-points beneath the stope.

The usual VCR blast uses holes in a 4.0 × 4.0 m pattern directed vertically or steeply inclined with charges carefully placed at calculated distances to free the surface beneath. The charges cooperate to break off a horizontal ore slice about 3.0 m thick. The blasted rock falls into the stope underneath. By controlling the rate of mucking out, the stope remains partly filled so that the rock fill assists in stabilizing the stope walls during the production phase. The last blast breaks the over-cut into the stope, after which the stope is mucked clean and prepared for back filling.

VCR mines often uses a system of primary and secondary stopes to the orebody. Primary stopes are mined in the first stage, then backfilled with cemented fill. The stope is left for the fill to consolidate. Miners then return and recover the ore in the pillars between the primary stopes, the secondary stopes. This system, in combination with the cemented backfill, results in close to a 100% recovery of the ore reserves.

Sublevel caving

Sublevel caving is applicable to mineral deposits with steep to moderate dip and large extension at depth. The ore must fracture into manageable block with blasting. The hanging wall will cave following the ore extraction and the ground on the surface above the orebody will subside. (It must be barricaded to prevent any individuals from entering the area.)

Sublevel caving is based on gravity flow inside a broken-up rock mass containing both ore and rock. The rock mass is first fractured by drilling and blasting and then mucked out through drift headings underneath the rock mass cave. It qualifies as a safe mining method because the miners always work inside drift-size openings.

Sublevel caving depends on sublevels with regular patterns of drifts prepared inside the orebody at rather close vertical spacing (from 10.0 m to 20 0 m). The drift layout is the same on each sublevel (i.e., parallel drives across the orebody from the footwall transport drive to the hanging wall) but the patterns on each sublevel are slightly off-set so that the drifts on a lower level are located between the drifts on the sublevel above it. A cross section will show a diamond pattern with drifts in regular vertical and horizontal spacing. Thus, development for sublevel caving is extensive. Drift excavation, however, is a straightforward task which can readily be mechanized. Working on multiple drift headings on several sublevels favours high utilization of the equipment.

When the development of the sublevel is completed, the long-hole drill rig moves in to drill a blast holes in a fan-spread pattern in the rock above. When all of the blast holes are ready, the long-hole drill rig is moved to the sublevel below.

The long-hole blast fractures the rock mass above the sublevel drift, initiating a cave that starts at the hanging wall contact and retreats toward the footwall following a straight front across the orebody on the sublevel. A vertical section would show a staircase where each upper sublevel is one step ahead of the sublevel below.

The blast fills the sublevel front with a mix of ore and waste. When the LHD vehicle arrives, the cave contains 100% ore. As loading continues, the proportion of waste rock will gradually increase until the operator decides that the waste dilution is too high and stops loading. As the loader moves to the next drift to continue mucking, the blaster enters to prepare the next ring of holes for blasting.

Mucking out on sublevels is an ideal application for the LHD vehicle. Available in different sizes to meet particular situations, it fills the bucket, travels some 200 m, empties the bucket into the ore pass and returns for another load.

Sublevel caving features a schematic layout with repetitive work procedures (development drifting, long-hole drilling, charging and blasting, loading and transport) that are carried out independently. This allows the procedures to move continuously from one sublevel to another, allowing for the most efficient use of work crews and equipment. In effect the mine is analogous to a departmentalized factory. Sublevel mining, however, being less selective than other methods, does not yield particularly efficient extraction rates. The cave includes some 20 to 40% of waste with a loss of ore that ranges from 15 to 25%.

Block-caving

Block-caving is a large-scale method applicable to mineralization on the order of 100 million tonnes in all directions contained in rock masses amenable to caving (i.e., with internal stresses which, after removal of the supporting elements in the rock mass, assist the fracturing of the mined block). An annual output ranging from 10 to 30 million tonnes is the anticipated yield. These requirements limit block-caving to a few specific mineral deposits. Worldwide, there are block-caving mines exploiting deposits containing copper, iron, molybdenum and diamonds.

Block refers to the mining layout. The orebody is divided into large sections, blocks, each containing a tonnage sufficient for many years of production. The caving is induced by removing the supporting strength of the rock mass directly underneath the block by means of an undercut, a 15 m high section of rock fractured by long-hole drilling and blasting. Stresses created by natural tectonic forces of considerable magnitude, similar to those causing continental movements, create cracks in the rock mass, breaking the blocks, hopefully to pass draw-point openings in the mine. Nature, though, often needs the assistance of miners to handle oversize boulders.

Preparation for block-caving requires long-range planning and extensive initial development involving a complex system of excavations beneath the block. These vary with the site; they generally include undercut, drawbells, grizzlies for control of oversize rock and ore passes that funnel the ore into train loading.

Drawbells are conical openings excavated underneath the undercut which gather ore from a large area and funnel it into the drawpoint at the production level below. Here the ore is recovered in LHD vehicles and transferred to ore passes. Boulders too large for the bucket are blasted in draw-points, while smaller ones are dealt with on the grizzly. Grizzlies, sets of parallel bars for screening coarse material, are commonly used in block-caving mines although, increasingly, hydraulic breakers are being preferred.

Openings in a block-caving mine are subject to high rock pressure. Drifts and other openings, therefore, are excavated with the smallest possible section. Nevertheless, extensive rock bolting and concrete lining is required to keep the openings intact.

Properly applied, block-caving is a low-cost, productive mass mining method. However, the amenability of a rock mass to caving is not always predictable. Also, the comprehensive development that is required results in a long lead-time before the mine starts producing: the delay in earnings can have a negative influence on the financial projections used to justify the investment.

Longwall mining

Longwall mining is applicable to bedded deposits of uniform shape, limited thickness and large horizontal extension (e.g., a coal seam, a potash layer or the reef, the bed of quartz pebbles exploited by gold mines in South Africa). It is one of the main methods for mining coal. It recovers the mineral in slices along a straight line that are repeated to recover materials over a larger area. The space closest to the face in kept open while the hanging wall is allowed to collapse at a safe distance behind the miners and their equipment.

Preparation for longwall mining involves the network of drifts required for access to the mining area and transport of the mined product to the shaft. Since the mineralization is in the form of a sheet that extends over a wide area, the drifts can usually be arranged in a schematic network pattern. The haulage drifts are prepared in the seam itself. The distance between two adjacent haulage drifts determines the length of the longwall face.

Backfilling

Backfilling of mine stopes prevents rock from collapsing. It preserves the inherent stability of the rock mass which promotes safety and allows more complete extraction of the desired ore. Backfilling is traditionally used with cut-and-fill but it is also common with sublevel stoping and VCR mining.

Traditionally, miners have dumped waste rock from development in empty stopes instead of hauling it to the surface. For example, in cut-and-fill, waste rock is distributed over the empty stope by scrapers or bulldozers.

Hydraulic backfilling uses tailings from the mine’s dressing plant which are distributed underground through bore holes and plastic tubing. The tailings are first de-slimed, only the coarse fraction being used for filling. The fill is a mix of sand and water, about 65% of which is solid matter. By mixing cement into the last pour, the fill’s surface will harden into a smooth roadbed for rubber-tyred equipment.

Backfilling is also used with sublevel stoping and VCR mining, with crushed rock introduced as a complement to sand fill. The crushed and screened rock, produced in a nearby quarry, is delivered underground through special backfill raises where it is loaded on trucks and delivered to the stopes where it is dumped into special fill raises. Primary stopes are backfilled with cemented rock fill produced by spraying a cement-fly ash slurry on the rockfill before it is distributed to the stopes. The cemented rockfill hardens into a solid mass forming an artificial pillar for mining the secondary stope. The cement slurry is generally not required when secondary stopes are backfilled, except for the last pours to establish a firm mucking floor.

Equipment for Underground Mining

Underground mining is becoming increasingly mechanized wherever circumstances permit. The rubber-tyred, diesel-powered, four-wheel traction, articulated steer carrier is common to all mobile underground machines (see figure 8).

Figure 8. Small-size face rig

Atlas Copco

Face drill jumbo for development drilling

This is an indispensable workhorse in mines that is used for all rock excavation work. It carries one or two booms with hydraulic rock drills. With one worker at the control panel, it will complete a pattern of 60 blast holes 4.0 m deep in a few hours.

Long-hole production drill rig

This rig (see figure 7 drills blast holes in a radial spread around the drift which cover a large area of rock and break off large volumes of ore. It is used with sublevel stoping, sublevel caving, block-caving and VCR mining. With a powerful hydraulic rock drill and carousel storage for extension rods, the operator uses remote controls to perform rock drilling from a safe position.

Charging truck

The charging truck is a necessary complement to the drifting jumbo. The carrier mounts a hydraulic service platform, a pressurized ANFO explosive container and a charging hose that permit the operator to fill blast holes all over the face in a very short time. At the same time, Nonel detonators may be inserted for the correct timing of the individual blasts.

LHD vehicle

The versatile load-haul-dump vehicle (see figure 10) is used for a variety of services including ore production and materials handling. It is available in a choice of sizes allowing miners to select the model most appropriate for each task and each situation. Unlike the other diesel vehicles used in mines, the LHD vehicle engine is generally run continuously at full power for long periods of time generating large volumes of smoke and exhaust fumes. A ventilation system capable of diluting and exhausting these fumes is essential to compliance with acceptable breathing standards in the loading area.

Underground haulage

The ore recovered in stopes spread along an orebody is transported to an ore dump located close to the hoisting shaft. Special haulage levels are prepared for longer lateral transfer; they commonly feature rail track installations with trains for ore transport. Rail has proved to be an efficient transport system carrying larger volumes for longer distances with electric locomotives that do not contaminate the underground atmosphere like diesel-powered trucks used in trackless mines.

Ore handling

On its route from the stopes to the hoisting shaft, the ore passes several stations with a variety of materials-handling techniques.

The slusher uses a scraper bucket to draw ore from the stope to the ore pass. It is equipped with rotating drums, wires and pulleys, arranged to produce a back and forth scraper route. The slusher does not need preparation of the stope flooring and can draw ore from a rough muckpile.

The LHD vehicle, diesel powered and travelling on rubber tyres, takes the volume held in its bucket (sizes vary) from the muckpile to the ore pass.

The ore pass is a vertical or steeply inclined opening through which rock flows by gravity from upper to lower levels. Ore passes are sometimes arranged in a vertical sequence to collect ore from upper levels to a common delivery point on the haulage level.

The chute is the gate located at the bottom of the ore pass. Ore passes normally end in rock close to the haulage drift so that, when the chute is opened, the ore can flow to fill cars on the track beneath it.

Close to the shaft, the ore trains pass a dump station where the load may be dropped into a storage bin, A grizzly at the dump station stops oversized rocks from falling into the bin. These boulders are split by blasting or hydraulic hammers; a coarse crusher may be installed below the grizzly for further size control. Under the storage bin is a measure pocket which automatically verifies that the load’s volume and weight do not exceed the capacities of the skip and the hoist. When an empty skip, a container for vertical travel, arrives at the filling station, a chute opens in the bottom of the measure pocket filling the skip with a proper load. After the hoist lifts the loaded skip to the headframe on the surface, a chute opens to discharge the load into the surface storage bin. Skip hoisting can be automatically operated using closed-circuit television to monitor the process.

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Underground coal production first began with access tunnels, or adits, being mined into seams from their surface outcrops. However, problems caused by inadequate means of transport to bring coal to the surface and by the increasing risk of igniting pockets of methane from candles and other open flame lights limited the depth to which early underground mines could be worked.

Increasing demand for coal during the Industrial Revolution gave the incentive for shaft sinking to access deeper coal reserves, and by the mid-twentieth century by far the greater proportion of world coal production came from underground operations. During the 1970s and 1980s there was widespread development of new surface coal mine capacity, particularly in countries such as the United States, South Africa, Australia and India. In the 1990s, however, renewed interest in underground mining resulted in new mines being developed (in Queensland, Australia, for instance) from the deepest points of former surface mines. In the mid-1990s, underground mining accounted for perhaps 45% of all the hard coal mined worldwide. The actual proportion varied widely, ranging from under 30% in Australia and India to around 95% in China. For economic reasons, lignite and brown coal are rarely mined underground.

An underground coal mine consists essentially of three components: a production area; coal transport to the foot of a shaft or decline; and either hoisting or conveying the coal to the surface. Production also includes the preparatory work that is needed in order to permit access to future production areas of a mine and, in consequence, represents the highest level of personal risk.

Mine Development

The simplest means of accessing a coal seam is to follow it in from its surface outcrop, a still widely practised technique in areas where the overlying topography is steep and the seams are relatively flat-lying. An example is the Appalachian coalfield of southern West Virginia in the United States. The actual mining method used in the seam is immaterial at this point; the important factor is that access can be gained cheaply and with minimal construction effort. Adits are also commonly used in areas of low-technology coal mining, where the coal produced during mining of the adit can be used to offset its development costs.

Other means of access include declines (or ramps) and vertical shafts. The choice usually depends on the depth of the coal seam being worked: the deeper the seam, the more expensive it is to develop a graded ramp along which vehicles or belt conveyors can operate.

Shaft sinking, in which a shaft is mined vertically downwards from the surface, is both costly and time-consuming and requires a longer lead-time between the commencement of construction and the first coal being mined. In cases where the seams are deep-lying, as in most European countries and in China, shafts often have to be sunk through water-bearing rocks overlying the coal seams. In this instance, specialist techniques, such as ground freezing or grouting, have to be used to prevent water from flowing into the shaft, which is then lined with steel rings or cast concrete to provide a long-term seal.

Declines are typically used to access seams that are too deep for open-cast mining, but which are still relatively near-surface. In the Mpumalanga (eastern Transvaal) coalfield in South Africa, for instance, the mineable seams lie at a depth of no more than 150 m; in some areas, they are mined from opencasts, and in others underground mining is necessary, in which case declines are often used to provide access for mining equipment and to install the belt conveyors used to carry the cut coal out of the mine.

Declines differ from adits in that they are usually excavated in rock, not coal (unless the seam dips at a constant rate), and are mined to a constant gradient to optimize vehicle and conveyor access. An innovation since the 1970s has been the use of belt conveyors running in declines to carry deep-mine production, a system that has advantages over traditional shaft hoisting in terms of capacity and reliability.

Mining Methods

Underground coal mining encompasses two principal methods, of which many variations have evolved to address mining conditions in individual operations. Room-and-pillar extraction involves mining tunnels (or roadways) on a regular grid, often leaving substantial pillars for long-term support of the roof. Longwall mining achieves total extraction of large parts of a coal seam, causing the roof rocks to collapse into the mined-out area.

Room-and-pillar mining

Room-and-pillar mining is the oldest underground coal mining system, and the first to use the concept of regular roof support to protect mine workers. The name room-and-pillar mining derives from the pillars of coal that are left behind on a regular grid to provide in situ support to the roof. It has been developed into a high-production, mechanized method that, in some countries, accounts for a substantial proportion of the total underground output. For instance, 60% of underground coal production in the United States comes from room-and-pillar mines. In terms of scale, some mines in South Africa have installed capacities exceeding 10 million tonnes per year from multi-production section operations in seams up to 6 m thick. By contrast, many room-and-pillar mines in the United States are small, operating in seam thicknesses as low as 1 m, with the ability to stop and restart production quickly as market conditions dictate.

Room-and-pillar mining is typically used in shallower seams, where the pressure applied by overlying rocks on the support pillars is not excessive. The system has two key advantages over longwall mining: its flexibility and inherent safety. Its major disadvantage is that recovery of the coal resource is only partial, the precise amount depending on factors such as the depth of the seam below surface and its thickness. Recoveries of up to 60% are possible. Ninety per cent recovery is possible if pillars are mined out as a second phase of the extraction process.

The system is also capable of various levels of technical sophistication, ranging from labour-intensive techniques (such as “basket mining” in which most stages of mining, including coal transport, are manual), to highly mechanized techniques. Coal can be excavated from the tunnel face by using explosives or continuous mining machines. Vehicles or mobile belt conveyors provide mechanized coal transport. Roofbolts and metal or timber strapping are used to support the roadway roof and the intersections between roadways where the open span is greater.

A continuous miner, which incorporates a cutting head and coal loading system mounted on crawler tracks, typically weighs from 50 to 100 tonnes, depending on the operating height in which it is designed to work, the installed power and the width of cut required. Some are equipped with on-board rockbolt installation machines that provide roof support simultaneously with coal cutting; in other cases, separate continuous miner and roofbolter machines are used sequentially.

Coal carriers can be supplied with electric power from an umbilical cable or can be battery or diesel-engine powered. The latter provides greater flexibility. Coal is loaded from the rear of the continuous miner into the vehicle, which then carries a payload, typically between 5 and 20 tonnes, a short distance to a feed hopper for the main belt conveyor system. A crusher may be included in the hopper feeder to break oversize coal or rock that could block chutes or damage conveyor belts further along the transport system.

An alternative to vehicular transport is the continuous haulage system, a crawler-mounted, flexible sectional conveyor that transports cut coal directly from the continuous miner to the hopper. These offer advantages in terms of personnel safety and productive capacity, and their use is being extended to longwall gateroad development systems for the same reasons.

Roadways are mined to widths of 6.0 m, normally the full height of the seam. Pillar sizes depend on the depth below surface; 15.0 m square pillars on 21.0 m centres would be representative of pillar design for a shallow, low-seam mine.

Longwall mining

Longwall mining is widely perceived to be a twentieth century development; however, the concept is actually believed to have been developed over 200 years earlier. The main advance is that earlier operations were principally manual, while, since the 1950s, the level of mechanization has increased to the stage that a longwall face is now a high-productivity unit which can be operated by a very small crew of workers.

Longwalling has one overriding advantage compared to room-and-pillar mining: it can achieve full extraction of the panel in one pass and recovers a higher overall proportion of the total coal resource. However, the method is relatively inflexible and demands both a large mineable resource and guaranteed sales to be viable, because of the high capital costs involved in developing and equipping a modern longwall face (over US$20 million in some cases).

While in the past individual mines often simultaneously operated several longwall faces (in countries such as Poland, over ten per mine in a number of cases), the current trend is towards consolidation of mining capacity into fewer, heavy-duty units. The advantages of this are reduced labour requirements and the need for less extensive underground infrastructure development and maintenance.

In longwall mining the roof is deliberately collapsed as the seam is mined out; only major access routes underground are protected by support pillars. Roof control is provided on a longwall face by two- or four-leg hydraulic supports which take the immediate load of the overlying roof, permitting its partial distribution to the unmined face and the pillars on either side of the panel, and protect the face equipment and personnel from collapsed roof behind the line of supports. Coal is cut by an electric-powered shearer, usually equipped with two coal-cutting drums, that mines a strip of coal up to 1.1 m thick from the face with each pass. The shearer runs along and loads the cut coal onto an armoured conveyor that snakes forward after each cut by sequential movement of the face supports.

At the face end, the cut coal is transferred to a belt conveyor for transport to the surface. In an advancing face, the belt must be extended regularly as the distance from the face starting point increases, while in retreat-longwalling the opposite applies.

Over the past 40 years, there have been substantial increases in both the length of the longwall face mined and the length of the individual longwall panel (the block of coal through which the face progresses). By way of illustration, in the United States the average longwall face length rose from 150 m in 1980 to 227 m in 1993. In Germany the mid-1990s average was 270 m and face lengths of over 300 m are being planned. In both the United Kingdom and Poland, faces are mined up to 300 m long. Panel lengths are largely determined by geological conditions, such as faults, or by mine boundaries, but are now consistently over 2.5 km in good conditions. The possibility of panels up to 6.7 km long is being discussed in the United States.

Retreat mining is becoming the industry standard, although it involves higher initial capital expenditure in roadway development to the furthest extent of each panel before longwalling can begin. Where possible, roadways are now mined in-seam, using continuous miners, with rockbolt support replacing the steel arches and trusses that were used previously in order to provide positive support to the overlying rocks, rather than passive reaction to rock movements. It is limited in applicability, however, to competent roof rocks.

Safety Precautions

Statistics from the ILO (1994) indicate a wide geographical variation in the rate fatalities occur in coal mining, although these data have to take into account the level of mining sophistication and the number of workers employed on a country-by-country basis. Conditions have improved in many industrialized countries.

Major mining incidents are now relatively infrequent, as engineering standards have improved and fire-resistance has been incorporated into materials such as the conveyor belting and hydraulic fluids used underground. Nonetheless, the potential for incidents capable of causing either personal or structural damage remains. Methane gas and coal dust explosions still occur, despite vastly improved ventilation practices, and roof falls account for the majority of serious accidents on a world-wide basis. Fires, either on equipment or occurring as a result of spontaneous combustion, represent a particular hazard.

Considering the two extremes, labour-intensive and highly mechanized mining, there are also wide differences in both accident rates and the types of incident involved. Workers employed in a small-scale, manual mine are more likely to incur injury through falls of rock or coal from the roadway roof or sidewalls. They also risk greater exposure to dust and flammable gas if ventilation systems are inadequate.

Both room-and-pillar mining and the development of roadways to provide access to longwall panels require support to the roof and sidewall rocks. The type and density of support varies according to the seam thickness, competence of the overlying rocks and the depth of the seam, among other factors. The most hazardous place in any mine is beneath an unsupported roof, and most countries impose strict legislative constraints on the length of roadway that may be developed before support is installed. Pillar recovery in room-and-pillar operations presents specific hazards through the potential for sudden roof collapse and must be scheduled carefully to prevent increased risk to workers.

Modern high-productivity longwall faces require a team of six to eight operators, so the number of people exposed to potential hazards is markedly reduced. Dust generated by the longwall shearer is a major concern. Coal cutting is thus sometimes restricted to one direction along the face to take advantage of the ventilation flow to carry dust away from the shearer operators. The heat generated by increasingly powerful electric machines in the confines of the face also has potentially deleterious effects on face workers, especially as mines become deeper.

The speed at which shearers work along the face is also increasing. Cutting rates of up to 45 m/minute are under active consideration in the late 1990s. The ability of workers physically to keep up with the coal cutter moving repeatedly over a 300 m-long face for a full working shift is doubtful, and increasing shearer speed is thus a major incentive to the wider introduction of automation systems for which miners would act as monitors rather than as hands-on operators.

The recovery of face equipment and its transfer to a new worksite offers unique hazards for workers. Innovative methods have been developed for securing the longwall roof and face coal in order to minimize the risk of rock falls during the transfer operation. However, the individual items of machinery are extremely heavy (over 20 tonnes for a large face support and considerably more for a shearer), and despite the use of custom-designed transporters, there remains the risk of personal crushing or lifting injuries during longwall salvage.

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Mine Development

Pit planning and layout

The overall economic goal in surface mining is to remove the least amount of material while gaining the greatest return on investment by processing the most marketable mineral product. The higher the grade of the mineral deposit, the greater the value. To minimize capital investment while accessing the highest valued material within a mineral deposit, a mine plan is developed that precisely details the manner in which the ore body will be extracted and processed. As many ore deposits are not a uniform shape, the mine plan is preceded by extensive exploratory drilling to profile the geology and position of the ore body. The size of the mineral deposit dictates the size and layout of the mine. The layout of a surface mine is dictated by the mineralogy and geology of the area. The shape of most open-pit mines approximates a cone but always reflects the shape of the mineral deposit being developed. Open-pit mines are constructed of a series of concentric ledges or benches that are bisected by mine access and haulage roads angling down from the rim of the pit to the bottom in a spiral or zigzag orientation. Regardless of size, the mine plan includes provisions for pit development, infrastructure, (e.g., storage, offices and maintenance) transportation, equipment, mining ratios and rates. Mining rates and ratios influence the life of the mine which is defined by depletion of the ore body or realization of an economic limit.

Contemporary open-pit mines vary in scale from small privately-operated enterprises processing a few hundred tonnes of ore per day to expanded industrial complexes operated by governments and multinational corporations that mine more than one million tonnes of material per day. The largest operations can involve many square kilometres in area.

Stripping overburden

Overburden is waste rock consisting of consolidated and unconsolidated material that must be removed to expose the underlying ore body. It is desirable to remove as little overburden as possible in order to access the ore of interest, but a larger volume of waste rock is excavated when the mineral deposit is deep. Most removal techniques are cyclical with interruption in the extraction (drilling, blasting and loading) and removal (haulage) phases. This is particularly true for hard rock overburden which must be drilled and blasted first. An exception to this cyclical effect are dredges used in hydraulic surface mining and some types of loose material mining with bucket wheel excavators. The fraction of waste rock to ore excavated is defined as the stripping ratio. Stripping ratios of 2:1 up to 4:1 are not uncommon in large mining operations. Ratios above 6:1 tend to be less economically viable, depending on the commodity. Once removed, overburden can be used for road and tailings construction or may have non-mining commercial value as fill dirt.

Mining equipment selection

The selection of mining equipment is a function of the mine plan. Some of the factors considered in the selection of mine equipment include the topography of the pit and surrounding area, the amount of ore to be mined, the speed and distance the ore must be transported for processing and the estimated mine life, among others. In general, most contemporary surface mining operations rely on mobile drill rigs, hydraulic shovels, front-end loaders, scrapers and haul trucks to extract ore and initiate ore processing. The larger the mine operation, the larger the capacity of equipment required to maintain the mine plan.

Equipment is generally the largest available to match the economy of scale of surface mines with consideration for matching the capacities of equipment. For example, a small front-end loader can fill a large haul truck but the match is not efficient. Similarly, a large shovel can load smaller trucks but requires the trucks to decrease their cycle times and does not optimize utilization of the shovel since one shovel bucket may contain enough ore for more than one truck. Safety may be compromised by attempting to load only half of a bucket or if a truck is overloaded. Also, the scale of equipment selected must match the available maintenance facilities. Large equipment is often maintained where it malfunctions due to the logistical difficulties associated with transporting it to established maintenance facilities. When possible, the mine’s maintenance facilities are designed to accommodate the scale and quantity of the mine equipment. Therefore, as new larger equipment is introduced into the mine plan, the supporting infrastructure, including the size and quality of haul roads, tools and maintenance facilities, must also be addressed.

Conventional Methods of Surface Mining

Open-pit mining and strip mining are the two major categories of surface mining which account for more than 90% worldwide surface mining production. The primary differences between these mining methods are the location of the ore body and the mode of mechanical extraction. For loose rock mining, the process is essentially continuous with extraction and haulage steps running in series. Solid rock mining requires a discontinuous process of drilling and blasting prior to the loading and hauling stages. Strip mining (or open-cast mining) techniques relate to the extraction of ore bodies that are near the surface and relatively flat or tabular in nature and mineral seams. It uses a variety of different types of equipment including shovels, trucks, drag lines, bucket wheel excavators and scrapers. Most strip mines process non-hard rock deposits. Coal is the most common commodity that is strip mined from surface seams. In contrast, open-pit mining is employed to remove hard rock ore that is disseminated and/or located in deep seams and is typically limited to extraction by shovel and truck equipment. Many metals are mined by the open-pit technique: gold, silver and copper, to name a few.

Quarrying is a term used to describe a specialized open-pit mining technique wherein solid rock with a high degree of consolidation and density is extracted from localized deposits. Quarried materials are either crushed and broken to produce aggregate or building stone, such as dolomite and limestone, or combined with other chemicals to produce cement and lime. Construction materials are produced from quarries located in close proximity to the site of material use to reduce transportation costs. Dimension stone such as flagstone, granite, limestone, marble, sandstone and slate represent a second class of quarried materials. Dimension stone quarries are found in areas having the desired mineral characteristics which may or may not be geographically remote and require transportation to user markets.

Many ore bodies are too diffuse and irregular, or too small or deep to be mined by strip or open-pit methods and must be extracted by the more surgical approach of underground mining. To determine when open-pit mining is applicable, a number of factors must be considered, including the terrain and elevation of the site and region, its remoteness, climate, infrastructure such as roads, power and water supply, regulatory and environmental requirements, slope stability, overburden disposal and product transportation, among others.

Terrain and elevation: Topography and elevation also play an important role in defining the feasibility and scope of a mining project. In general, the higher the elevation and rougher the terrain, the more difficult mine development and production are likely to be. A higher grade of mineral in an inaccessible mountainous location may be mined less efficiently than a lower grade of ore in a flat location. Mines located at lower elevations generally experience less inclement weather-related problems for exploration, development and production of mines. As such, topography and location affect the mining method as well as economic feasibility.

The decision to develop a mine occurs after exploration has characterized the ore deposit and feasibility studies have defined the options for mineral extraction and processing. Information that is necessary to establish a development plan may include the shape, size and grade of minerals in the ore body, the total volume or tonnage of material including overburden and other factors, such as hydrology and access to a source of process water, availability and source of power, waste rock storage sites, transportation requirements and infrastructure features, including the location of population centres to support the labour force or the need to develop a townsite.

Transportation requirements may include roads, highways, pipelines, airports, railroads, waterways and harbours. For surface mines, large land areas are generally required that may have no existing infrastructure. In such instances roads, utilities and living arrangements must be established first. The pit would be developed in connection with other processing elements such as waste rock storage areas, crushers, concentrators, smelters and refineries, depending on the degree of integration required. Due to the large amount of capital necessary to finance these operations, development may be conducted in phases to take advantage of the earliest possible saleable or leasable mineral to help finance the remainder of the development.

Production and Equipment

Drilling and blasting

Mechanical drilling and blasting are the first steps in extracting ore from most developed open-pit mines and are the most common method used to remove hard rock overburden. While there are many mechanical devices capable of loosening hard rock, explosives are the preferred method as no mechanical device can currently match the fracturing capability of energy contained in explosive charges. A commonly used hard rock explosive is ammonium nitrate. Drilling equipment is selected on the basis of the nature of the ore and the speed and depth of the holes necessary to fracture a specified tonnage of ore per day. For example, in mining a 15-m bench of ore, 60 or more holes will generally be drilled 15 m back from the current muck face depending on the length of the bench to be mined. This must occur with enough lead-time to allow for site preparation for subsequent loading and haulage activities.

Loading

Surface mining is now typically conducted utilizing table shovels, front-end loaders or hydraulic shovels. In open-pit mining loading equipment is matched with haul trucks that can be loaded in three to five cycles or passes of the shovel; however, various factors determine the preference of loading equipment. With sharp rock and/or hard digging and/or wet climates, tracked shovels are preferable. Conversely, rubber-tyred loaders have much lower capital cost and are preferred for loading material that is low volume and easy to dig. Additionally, loaders are very mobile and well-suited for mining scenarios requiring rapid movements from one area to another or for ore blending requirements. Loaders are also frequently used to load, haul and dump material into crushers from blending stock piles deposited near crushers by haul trucks.

Hydraulic shovels and cable shovels have similar advantages and limitations. Hydraulic shovels are not preferred for digging hard rock and cable shovels are generally available in larger sizes. Therefore, large cable shovels with payloads of about 50 cubic metres and greater are the preferred equipment at mines were production exceeds 200,000 tonnes per day. Hydraulic shovels are more versatile on the mine face and allow greater operator control to selectively load the from either the bottom or top half of the mine face. This advantage is helpful where separation of waste from ore can be achieved at the loading zone thereby maximizing the ore grade that is hauled and processed.

Hauling

Haulage in open-pit and strip mines is most commonly accomplished by haul trucks. The role of haul trucks in many surface mines is restricted to cycling between the loading zone and the transfer point such as an in-pit crushing station or conveyance system. Haul trucks are favoured based on their flexibility of operation relative to railroads, which were the preferred haulage method until the 1960s. However, the cost of transporting materials in surface metal and non-metal pits is generally greater than 50% of the total operating cost of the mine. In-pit crushing and conveying through belt conveyor systems has been a primary factor in reducing haulage costs. Technical developments in haul trucks such as diesel engines and electrical drives have lead to much larger capacity vehicles. Several manufactures currently produce 240 tonne capacity trucks with expectation for greater than 310 tonne capacity trucks in the near future. In addition, the use of computerized dispatch systems and global satellite positioning technology allow vehicles to be tracked and scheduled with improved efficiency and productivity.

Haul road systems may use single or dual direction traffic. Traffic may be either left or right lane configuration. Left lane traffic is frequently preferred to improve operator visibility of tyre position on very large trucks. Safety is also improved with left hand traffic by reducing the potential for driver-side collision in the centre of a road. Haul road gradients are typically limited to between 8 and 15% for sustained hauls and optimally are about 7 to 8%. Safety and water drainage requires long gradients to include at least 45-m sections with a maximum gradient of 2% for every 460 m of severe gradient. Road berms (elevated dirt borders) located between roads and adjacent excavations are standard safety features in surface mines. They may also be placed in the middle of the road to separate opposing traffic. Where switch-back haul roads exist, increasing elevation escape lanes may be installed at the end of long steep grades. Road edge barriers such as berms are standard and should be located between all roads and adjacent excavations. High-quality roads enhance maximum productivity by maximizing safe truck speeds, reduced down-time for maintenance and reduced driver fatigue. Haul-truck road maintenance contributes to reduced operating costs through reduced fuel consumption, longer tyre life and reduced repair costs.

Rail haulage, under the best of conditions, is superior to other methods of haulage for transport of ore over long distances outside the mine. However, as a practical matter, rail haulage is no longer widely used in open-pit mining since the advent of electrical and diesel-powered trucks. Rail haulage was replaced to capitalize on the greater versatility and flexibility of haul trucks and in-pit conveyor systems. Railroads requires very gentle grades of 0.5 to a maximum of 3% for up-hill hauls. Capital investment for railroad engines and track requirements is very high and requires a long mine life and large production outputs to justify return on investment.

Ore handling (conveyance)

In-pit crushing and conveying is a methodology that has grown in popularity since first being implemented in the mid-1950s. Location of a semi-mobile crusher in the mine pit with the subsequent transport out of the pit by a conveyor system has resulted in significant production advantages and cost savings over traditional vehicle haulage. High cost haulage road construction and maintenance is reduced and labour costs associated with haul truck operation and truck maintenance and fuel are minimized.

The purpose of the in-pit crusher system is primarily to allow transport of ore by conveyor. In-pit crusher systems may range from permanent facilities to fully mobile units. However, more commonly, crushers are constructed in a modular form to allow some portability within the mine. Crushers might be relocated every one to ten years; it may require hours, days or months to complete the move depending on the size and complexity of the unit and the relocation distance.

Conveyors’ advantages over haul trucks include instantaneous start up, automatic and continuous operation, and a high degree of reliability with up 90 to 95% availability. They are generally not impaired by inclement weather. Conveyors also have much lower labour requirements relative to haul trucks; operating and maintaining a truck fleet may require ten times as many crew members as an equivalent-capacity conveyor system. Also, conveyors can operate at grades up to 30% while maximum grades for trucks are generally 10%. Using steeper grades lowers the need to remove low-grade overburden material and may reduce the need to establish high cost haulage roads. Conveyors systems are also integrated into bucket wheel shovels in many surface coal operations, which eliminates the need for haulage trucks.

Solution Mining Methods

Solution mining, the most common of two types of aqueous mining, is employed to extract soluble ore where conventional mining methods are less efficient and/or less economical. Also known as leaching or surface leaching, this technique can be a primary mining method, as with gold and silver leach mining, or it can supplement the conventional pyrometallurgical steps of smelting and refining, as in the case of leaching low-grade copper oxide ores.

Environmental aspects of surface mining

The significant environmental effects of surface mines attract attention wherever the mines are located. Alteration of terrain, destruction of plant life and adverse effects on indigenous animals are inevitable consequences of surface mining. Contamination of surface and underground waters often presents problems, particularly with the use of lixiviants in solution mining and the run-off from hydraulic mining.

Thanks to the increased attention from environmentalists around the world and the use of planes and aerial photography, mining enterprises are no longer free to “dig and run” when the extraction of the desired ore has been complete. Laws and regulations have been promulgated in most of the developed countries and, through the activities of international organizations, are being urged where they do not yet exist. They establish an environmental management programme as an integral element in every mining project and stipulate such requirements as preliminary environmental impact assessments; progressive rehabilitation programmes, including restoration of land contours, reforestration, replanting of indigenous fauna, restocking of indigenous wild life and so on; as well as concurrent and long-term compliance auditing (UNEP 1991,UN 1992, Environmental Protection Agency (Australia) 1996, ICME 1996). It is essential that these be more than statements in the documentation required for the necessary government licenses. The basic principles must be accepted and practised by managers in the field and communicated to workers on all levels.

Regardless of the necessity or economic advantage, all surface solution methods share two common characteristics: (1) ore is mined in the usual way and then stockpiled; and, (2) an aqueous solution is applied to the top of the ore stock which reacts chemically with the metal of interest from which the resulting metal salt solution is channelled through the stock pile for collection and processing. The application of surface solution mining is dependent on the volume, the metallurgy of the mineral(s) of interest and the related host rock, and available area and drainage to develop sufficiently large leach dumps to make the operation economically viable.

The development of leach dumps in a surface mine in which solution mining is the primary production method is the same as all open-pit operations with the exception that the ore is destined solely for the dump and not a mill. In mines with both milling and solution methods, ore is segregated into milled and leached portions. For example, most copper sulphide ore is milled and purified to market grade copper by smelting and refining. Copper oxide ore, which is not generally amenable to pyrometallurgical processing, is routed to leach operations. Once the dump is developed, the solution leaches the soluble metal from the surrounding rock at a predictable rate that is controlled by the design parameters of the dump, the nature and volume of the solution applied, and the concentration and mineralogy of the metal in the ore. The solution used to extract the soluble metal is referred to as a lixiviant. The most common lixiviants used in this mining sector are dilute solutions of alkaline sodium cyanide for gold, acidic sulphuric acid for copper, aqueous sulphur dioxide for manganese and sulphuric acid-ferric sulphate for uranium ores; however, most leached uranium and soluble salts are collected by in-situ mining in which the lixiviant is injected directly into the ore body without prior mechanical extraction. This latter technique enables low-grade ores to be processed without extracting the ore from the mineral deposit.

Health and safety aspects

The occupational health and safety hazards associated with mechanical extraction of the ore in solution mining are essentially similar to those of conventional surface mine operations. An exception to this generalization is the need for non-leaching ore to undergo primary crushing in the surface mine pit before being conveyed to a mill for conventional processing, whereas ore is generally transported by haul truck directly from the extraction site to the leach dump in solution mining. Solution mining workers would therefore have less exposure to primary crushing hazards such as dust, noise and physical hazards.

The leading causes of injuries in surface mine environments include materials handling, slips and falls, machinery, hand-tool use, power haulage and electrical source contact. However, unique to solution mining is the potential exposure to the chemical lixiviants during transportation, leach field activities and chemical and electrolytic processing. Acid mist exposures may occur in metal electrowinning tankhouses. Ionizing radiation hazards, which increase proportionally from extraction to concentration, must be addressed in uranium mining.

Hydraulic Mining Methods

In hydraulic mining, or “hydraulicking”, high pressure water spray is used to excavate loosely consolidated or unconsolidated material into a slurry for processing. Hydraulic methods are applied primarily to metal and aggregate stone deposits, although coal, sandstone and metal mill tailings are also amenable to this method. The most common and best known application is placer mining in which concentrations of metals such as gold, titanium, silver, tin and tungsten are washed from within an alluvial deposit (placer). Water supply and pressure, ground slope gradient for runoff, distance from the mine face to the processing facilities, degree of consolidation of the mineable material and the availability of waste disposal areas are all primary considerations in the development of a hydraulic mining operation. As with other surface mining, the applicability is location specific. Inherent advantages of this method mining include relatively low operating costs and flexibility resulting from the use of simple, rugged and mobile equipment. As a result, many hydraulic operations develop in remote mining areas where infrastructure requirements are not a limitation.

Unlike other types of surface mining, hydraulic techniques rely on water as the medium for both mining and conveyance of the mined material (“sluicing”). High pressure water sprays are delivered by monitors or water cannons to a placer bank or mineral deposit. They disintegrate gravel and unconsolidated material, which washes into collection and processing facilities. Water pressures may vary from a normal gravity flow for very loose fine materials to thousands of kilograms per square centimetre for unconsolidated deposits. Bulldozers and graders or other mobile excavating equipment are sometimes employed to facilitate mining of more compacted materials. Historically, and in modem small-scale operations, the collection of the slurry or runoff is managed with small volume sluice boxes and catches. Commercial-scale operations rely on pumps, containment and settling basins and separation equipment that can process very large volumes of slurry per hour. Depending on the size of the deposit to be mined, the operation of the water monitors may be manual, remotely controlled or computer controlled.

When hydraulic mining occurs underwater it is referred to as dredging. In this method a floating processing station extracts loose deposits such as clay, silt, sand, gravel and any associated minerals using a bucket line, drag line and/or submerged water jets. The mined material is transported hydraulically or mechanically to a washing station which may be part of the dredging rig or physically separate with subsequent processing steps to segregate and complete processing. While dredging is used to extract commercial minerals and aggregate stone, it is best known as a technique used to clear and deepen water channels and floodplains.

Health and safety

Physical hazards in hydraulic mining differ from those in surface mining methods. Due to the minimal application of drilling, explosives, haulage and reduction activities, safety hazards tend to be associated mostly often with high pressure water systems, manual movement of mobile equipment, proximity issues involving power supplies and water, proximity issues associated with collapse of the mine face and maintenance activities. Health hazards primarily involve exposure to noise and dusts and ergonomic hazards related to equipment handling. Dust exposure is generally less of an issue than in traditional surface mining due to the use of water as the mining medium. Maintenance activities such as uncontrolled welding may also contribute to worker exposures.

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