![SL500 01](https://upload.wikimedia.org/wikipedia/commons/1/19/SL500_01.jpg)
Longwall mining is a form of underground
coal mining where a long wall of coal is mined in a single slice (typically thick). The longwall panel (the block of coal that is being mined) is typically long (but can be upto long) and wide.
History
![LongwallOld](https://upload.wikimedia.org/wikipedia/commons/9/91/LongwallOld.png)
The basic idea of longwall mining was developed in
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
in the late 17th century. Miners undercut the coal along the width of the coal face, removing coal as it fell, and used wooden props to control the fall of the roof behind the face. This was known as the
Shropshire
Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to ...
method of mining. While the technology has changed considerably, the basic idea remains the same, to remove essentially all of the
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when ...
from a broad coal face and allow the roof and overlying
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
to collapse into the void behind, while maintaining a safe working space along the face for the miners.
Starting around 1900, mechanization was applied to this method. By 1940, some referred to longwall
mining
Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...
as "the conveyor method" of mining, after the most prominent piece of machinery involved. Unlike earlier longwall mining, the use of a
conveyor belt parallel to the coal face forced the face to be developed along a straight line. The only other machinery used was an
electric
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
cutter to undercut the coal face and electric
drills for blasting to drop the face. Once dropped, manual labor was used to load coal on to the conveyor parallel to the face and to place wooden roof props to control the fall of the roof.
Such low-technology longwall mines continued in operation into the 1970s. The best known example was the New Gladstone Mine near
Centerville, Iowa, "one of the last advancing longwall mines in the United States". This longwall mine did not use a conveyor belt, instead relying on ponies to haul coal tubs from the face to the
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
where a hoist hauled the tubs to the surface.
Longwall mining has been extensively used as the final stage in mining old
room and pillar
Room and pillar or pillar and stall is a variant of breast stoping. It is a mining system in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars. To do this, "rooms" of ore are dug out ...
mines. In this context, longwall mining can be classified as a form of
retreat mining
Retreat mining is the removal of pillars in the underground mining technique known as room and pillar mining.
In the first phase of room and pillar mining, tunnels are advanced into the coal or ore body in a rectangular pattern resembling city ...
.
Layout
Gate roads are driven to the back of each panel before longwall mining begins. The gate road along one side of the block is called the maingate or headgate; the road on the other side is called the tailgate. Where the thickness of the coal allows, these gate roads have been previously developed by continuous miner units, as the longwall itself is not capable of the initial development. The layout of Longwall could be either "advancing" type or of "retreat" type. In the advancing type, the gate roads are formed as the coal face advances. In thinner seams the advancing longwall mining method may be used. In the retreat type, the panel is a face connecting them both. Only the maingate road is formed in advance of the face. The tailgate road is formed behind the coal face by removing the
stone
In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...
above
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when ...
height to form a roadway that is high enough to travel in. The end of the block that includes the longwall equipment is called the face. The other end of the block is usually one of the main travel roads of the mine. The cavity behind the longwall is called the goaf, goff or gob.
Ventilation
Typically, intake (fresh) air travels up the main gate, across the face, and then down the tail gate, known as 'U' type ventilation. Once past the face the air is no longer fresh air, but return air carrying away
coal dust Coal dust is a fine powdered form of which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal. It is a form ...
and mine gases such as
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
,
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, depending on the
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
of the coal. Return air is extracted by ventilation
fans
Fan commonly refers to:
* Fan (machine), a machine for producing airflow, often used for cooling
** Hand fan, an implement held and waved by hand to move air for cooling
* Fan (person), short for fanatic; an enthusiast or supporter, especially wit ...
mounted on the surface. Other ventilation methods can be used where intake air also passes the main gate and into a bleeder or back return road reducing gas emissions from the goaf on to the face, or intake air travels up the tail gate and across the face in the same direction as the face chain in a homotropal system.
To avoid spontaneous
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
of coal in the goaf area, gases may be allowed to build up behind seals so as to exclude oxygen from the sealed goaf area. Where a goaf may contain an explosive mixture of methane and oxygen, nitrogen injection/inertisation may be used to exclude oxygen or push the explosive mixture deep into the goaf where there are no probable ignition sources. Seals are required to be monitored each shift by a certified mine supervisor for damage and leaks of harmful gases.
Equipment
![Schildausbau](https://upload.wikimedia.org/wikipedia/commons/5/5d/Schildausbau.jpg)
A number of
hydraulic jacks, called powered roof supports, chocks or shields, which are typically wide and placed in a long line, side by side for up to in length in order to support the roof of the coalface. An individual chock can weigh 30–40 tonnes, extend to a maximum cutting height of up to and have yield rating of 1000–1250 tonnes each, and hydraulically advance itself at a time.
![Longwall with hydraulic chocks, conveyor and shearer](https://upload.wikimedia.org/wikipedia/commons/9/91/Longwall_with_hydraulic_chocks%2C_conveyor_and_shearer.jpg)
The coal is cut from the coalface by a machine called the shearer (power loader). This machine can weigh 75–120 tonnes typically and comprises a main body, housing the electrical functions, the tractive motive units to move the shearer along the coalface and pumping units (to power both hydraulic and water functions). At either end of the main body are fitted the ranging arms which can be ranged vertically up down by means of hydraulic rams, and on to which are mounted the shearer cutting drums which are fitted with 40–60 cutting picks. Within the ranging arms are housed very powerful electric motors (typically up to 850 kW) which transfer their power through a series of lay gears within the body and through the arms to the drum mounting locations at the extreme ends of the ranging arms where the cutting drums are. The cutting drums are rotated at a speed of 20–50 revs/min to cut the mineral from coal seam.
![Longwall2](https://upload.wikimedia.org/wikipedia/commons/8/8b/Longwall2.png)
The shearer is carried along the length of the face on the armoured face conveyor (AFC); using a chain-less haulage system, which resembles a ruggedised
rack and pinion system specially developed for mining. Prior to the chainless haulage systems, haulage systems with chain were popular, where a heavy duty chain was run along the length of the coal face for the shearer to pull itself along. The shearer moves at a speed of depending on cutting conditions.
The AFC is placed in front of the powered roof supports, and the shearing action of the rotating drums cutting into the coal seam disintegrates the coal, this being loaded on to the AFC. The coal is removed from the coal face by a
scraper chain conveyor to the main gate. Here it is loaded on to a network of conveyor belts for transport to the surface. At the main gate the coal is usually reduced in size in a crusher, and loaded on to the first conveyor belt by the beam stage loader (BSL).
As the shearer removes the coal, the AFC is snaked over behind the shearer and the powered roof supports move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the goaf increases. This goaf collapses under the weight of the overlying strata. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle on to the collapsed goaf. This collapsing can lower surface height, causing problems such as changing the course of rivers and severely damaging building foundations.
["LONGWALL MINING"](_blank)
BHP Billiton Mitsubishi Alliance (2005) accessed 19 December 2011
Comparison with room and pillar method
Longwall and room and pillar methods of mining can both be used for mining suitable underground coal seams. Longwall has better
resource recovery
Resource recovery is using wastes as an input material to create valuable products as new outputs. The aim is to reduce the amount of waste generated, thereby reducing the need for landfill space, and optimising the values created from waste. Reso ...
(about 80% compared with about 60% for
room and pillar
Room and pillar or pillar and stall is a variant of breast stoping. It is a mining system in which the mined material is extracted across a horizontal plane, creating horizontal arrays of rooms and pillars. To do this, "rooms" of ore are dug out ...
method),
fewer roof support consumables are needed, higher volume coal clearance systems, minimal manual handling and safety of the miners is enhanced by the fact that they are always under the hydraulic roof supports when they are extracting coal.
Automation
Longwall mining has traditionally been a manual process in which alignment of the face equipment was done with string lines. Technologies have been developed which automates several aspects of the longwall mining operation, including a system that aligns the face of the retreating longwall panel perpendicularly to the gate-roads.
Briefly,
Inertial navigation system
An inertial navigation system (INS) is a navigation device that uses motion sensors (accelerometers), rotation sensors ( gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (dir ...
outputs are used in a
dead reckoning
In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
calculation to estimate the shearer positions. Optimal
Kalman filter
For statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estima ...
s and smoothers can be applied to improve the dead reckoning estimates prior to repositioning the longwall equipment at the completion of each shear.
Expectation-maximization algorithms can be used to estimate the unknown filter and smoother parameters for tracking the longwall shearer positions.
Compared to manual control of the mine equipment, the automated system yields improved production rates. In addition to productivity gains, automating longwall equipment leads to safety benefits. The coalface is a hazardous area because methane and carbon monoxide are present, while the area is hot and humid since water is sprayed over the face to minimize the likelihood of sparks occurring when the shearer picks strike rock. By automating manual processes, face workers can be removed from these hazardous areas.
Environmental impacts
As with many mining techniques, it is important to consider the local and regional environmental impacts from longwall mining.
Subsidence
Longwall mine
subsidence (LWMS) is an anthropogenic process that has many ecological and environmental impacts, particularly on
soil health
Soil health is a state of a soil meeting its range of ecosystem functions as appropriate to its environment. In more colloquial terms, the health of soil arises from favorable interactions of all soil components (living and non-living) that belong ...
and water movement in a region where LWMS is heavily done. This is important to consider as some longwall mine sites can span lengths of several kilometers. That being said, hydrological flow systems, root systems from trees, and vegetative species can suffer from the amounts of soil being removed beneath them, and these stresses lead to surface erosion.
Abandoned mines are also an issue with regards to areas where residential development has moved in. Houses erected near abandoned longwall mines face the threat of future damage from sinkholes and poor soil quality, even up to thirty years after mine abandonment.
Since longwall mining is namely, very long, it can effect areas of over . Over these largest spans, it has been observed that longwall mines underlying mountain sides demonstrate more visible subsidence in mountain landscapes than it does for valley landscapes.
There have been cases of surface subsidence altering the landscape above the mines. At Newstan Colliery in
New South Wales
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, image_map = New South Wales in Australia.svg
, map_caption = Location of New South Wales in AustraliaCoordinates:
, subdivision_type = Country
, subdivision_name = Australia
, established_title = Before federation
, es ...
, Australia "the surface has dropped by as much as five metres in places" above a multi-level mine.
In some cases the subsidence causes damage to natural features such as drainage to water courses
or man-made structures such as roads and buildings. "
Douglas Park Drive was closed for four weeks because longwall panels ... destabilised the road. In 2000, the State Government stopped mining when it came within 600 metres from the twin bridges. A year later there were reports of 40-centimetre gaps appearing in the road, and the bridge had to be jacked sideways to realign it."
A 2005 geotechnical report commissioned by the
Roads & Traffic Authority
The Roads & Traffic Authority (RTA) was an agency of the New South Wales Government responsible for major road infrastructure, licensing of drivers, and registration of motor vehicles. The RTA directly managed state roads and provided fundin ...
warned that "subsidence could happen suddenly and occur over many years".
However, there are several mines, which were successfully mined with little to no measurable surface subsidence including mines under lakes, oceans, important water catchments and environmentally sensitive areas. Subsidence is minimised by increasing the block's adjacent chain pillar widths, decreasing extracted block widths and heights, and by giving consideration to the depth of cover as well as competency and thickness of overlying strata.
Fracturing and water quality
Longwall mining can result in geological disruptions in the rockbed, and can in turn affect water movement and result in water moving away from the surface, through the mined area, and into the
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...
. A resulting loss in surface water can negatively impact riparian ecosystems.
On top of this, if there are present dams near to the longwall mining site, this could doubly impact the
riparian ecosystems as it would have a reduced inflow rate as well as the loss to the underlying rock fracturing.
As of 2014, measures were taken by the Australian government to mitigate water impacts from longwall mining. Legislative assemblies have called for action to improve mine infrastructure to minimize disturbances.
As a result of bedrock cracking from mining beneath waters such as lakes or rivers, there have been groundwater losses of varying extents. Mines within a few hundred meters of the surface are susceptible to receiving great inputs of water from these bodies. Moreover, after mining interference disturbing the natural landscape near the mines, the natural water flow paths can be redirected which results in additional erosion across a stream or river bank. Additional mining in concentrated areas continuously move these water flow paths, which take years to return to their original states.
Ecosystem impacts
Many ecosystems rely on the annual consistency of water inputs and outputs, and disturbing these patterns can result in unsustainable conditions for species reliant on water for species reproduction. Longwall mining can also result in localized water temperature change, stimulating algal bloom which can use up available oxygen required for other species health.
Longwall mining has limited available research on the impacts of nearby forests, however emerging satellite imagery studies have shown possible relations to drier surface soil near regions where longwall mining has recently occurred. In addition to drier soils, forest canopy moisture has been observed to be reduced.
Gas emissions
Longwall mines have been observed to release
methane gas
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...
, a common
greenhouse gas into the environment, however the increase of a typical longwall mine face from was not found to release significantly more methane.
Methane emissions
Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...
from closed longwall mines can continue for up to fifteen years, however it is possible to measure the volume of potential methane emissions based on water flow in the closed mines.
In Canada
Canada has some of the largest coal reserves in the world, and until 2013 there had been no longwall mines in Canada for over 25 years. A mine was opened by
HD Mining in 2015 in British Columbia, causing disputes because of the hiring of foreign workers instead of Canadians and because of its potential impact on the environment.
This mine expected to have
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
emissions of 17 megatons per year; however, a carbon cap was placed on it by the
Canadian federal government
The government of Canada (french: gouvernement du Canada) is the body responsible for the federal administration of Canada. A constitutional monarchy, the Crown is the corporation sole, assuming distinct roles: the executive, as the ''Crown ...
to keep the emissions at 500,000 tonnes per year.
See also
*
Drift mining
Drift mining is either the mining of an ore deposit by underground methods, or the working of coal seams accessed by adits driven into the surface outcrop of the coal bed. A drift mine is an underground mine in which the entry or access is abov ...
*
Shaft mining
Shaft mining or shaft sinking is the action of excavating a mine shaft from the top down, where there is initially no access to the bottom. Shallow shafts, typically sunk for civil engineering projects, differ greatly in execution method from ...
*
Slope mining
Slope mining is a method of accessing valuable geological material, such as coal or ore. A sloping access shaft travels downwards towards desired material. Slope mines differ from shaft and drift mines, which access resources by tunneling stra ...
*
Strip mining
Surface mining, including strip mining, open-pit mining and mountaintop removal mining, is a broad category of mining in which soil and rock overlying the mineral deposit (the overburden) are removed, in contrast to underground mining, in which ...
*
Glossary of coal mining terminology
Notes
External links
University of Wollongong (Australia) Educational resource on longwall mining.
* Bauer, Robert A
Planned Coal Mine Subsidence in Illinois: A Public Information BookletIllinois Department of Natural Resources, Illinois State Geological Survey, Circular 573, 2008
Hidden Costs of Clean Coal ProjectThe Center for Public Integrity
International Longwall NewsNews and information resource dedicated solely to longwall mining
Mine Subsidence BoardNSW Government
The Mine Subsidence Technological SocietyThe Institute of Engineers, Australia
{{Mining techniques
Coal mining
Underground mining