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Sunday, 13 March 2011 15:57

Underground Coal Mining

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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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