Underground mining (soft rock)

Underground mining (soft rock) refers to a group of underground mining techniques used to extract coal, oil shale and other minerals or geological materials from sedimentary ("soft") rocks. Because deposits in sedimentary rocks are commonly layered and relatively less hard, the mining methods used differ from those used to mine deposits in igneous or metamorphic rocks (see Underground mining (hard rock)). Underground mining techniques also differ greatly from those of surface mining.

Methods

* Longwall mining: A set of longwall mining equipment consists of a coal shearer mounted on conveyor operating underneath a series of self-advancing hydraulic roof supports. Almost the entire process can be automated. Longwall mining machines are typically 150-250 metres in width and 1.5 to 3 metres high. Longwall miners extract "panels" - rectangular blocks of coal as wide as the face the equipment is installed in, and as long as several kilometres. Powerful mechanical coal cutters (shearers) cut coal from the face, which falls onto an armoured face conveyor for removal. Longwalls can advance into an area of coal, or more commonly, retreat back between develoipment tunnels (called "gateroads") As a longwall miner retreats back along a panel, the roof behind the supports is allowed to collapse in a planned and controlled manner.

* Room-and-pillar mining or continuous mining: Room and pillar mining is commonly done in flat or gently dipping bedded ores. Pillars are left in place in a regular pattern while the rooms are mined out. In many room and pillar mines, the pillars are taken out, starting at the farthest point from the mine haulage exit, retreating, and letting the roof come down upon the floor. Room and pillar methods are well adapted to mechanization, and are used in deposits such as coal, potash, phosphate, salt, oil shale, and bedded uranium ores.

* Blast mining – An older practice of coal mining that uses explosives such as dynamite to break up the coal seam, after which the coal is gathered and loaded onto shuttle cars or conveyors for removal to a central loading area. This process consists of a series of operations that begins with "cutting" the coalbed so it will break easily when blasted with explosives. This type of mining accounts for less than 5% of total underground production in the U.S. today.

* Shortwall mining– A coal mining method that accounts for less than 1% of deep coal production, shortwall involves the use of a continuous mining machine with moveable roof supports, similar to longwall. The continuous miner shears coal panels 150–200 feet wide and more than a half-mile long, depending on other things like the strata of the Earth and the transverse waves.


From http://en.wikipedia.org/

Underground mining (hard rock)

Underground hard rock mining refers to various underground mining techniques used to excavate hard minerals, mainly those minerals containing metals such as ore containing gold, copper, zinc, nickel and lead, but also involves using the same techniques for excavating ores of gems such as diamonds. In contrast soft rock mining refers to excavation of softer minerals such as salt, coal, or oil sands.



Mine access

Underground access

Accessing underground ore can be achieved via a decline (ramp), inclined vertical shaft or adit.

* Declines can be a spiral tunnel which circles either the flank of the deposit or circles around the deposit. The decline begins with a box cut, which is the portal to the surface. Depending on the amount of overburden and quality of bedrock, a galvanized steel culvert may be required for safety purposes. They may also be started into the wall of an open cut mine.

* Shafts are vertical excavations sunk adjacent to an ore body. Shafts are sunk for ore bodies where haulage to surface via truck is not economical. Shaft haulage is more economical than truck haulage at depth, and a mine may have both a decline and a ramp.

* Adits are horizontal excavations into the side of a hill or mountain. They are used for horizontal or near-horizontal ore bodies where there is no need for a ramp or shaft.

Declines are often started from the side of the high wall of an open cut mine when the ore body is of a payable grade sufficient to support an underground mining operation but the strip ratio has become too great to support open cast extraction methods.

Ore access

Levels are excavated horizontally off the decline or shaft to access the ore body. Stopes are then excavated perpendicular (or near perpendicular) to the level into the ore.

Development mining vs. production mining

There are two principal phases of underground mining: development mining and production mining.

Development mining is composed of excavation almost entirely in (non-valuable) waste rock in order to gain access to the orebody. There are five steps in development mining: remove previously blasted material (muck out round), drill rock face, load explosives, blast explosives, and support excavation.

Production mining is further broken down into two methods, long hole and short hole. Short hole mining is similar to development mining, except that it occurs in ore. There are several different methods of long hole mining. Typically long hole mining requires two excavations within the ore at different elevations below surface, (15 m – 30 m apart). Holes are drilled between the two excavations and loaded with explosives. The holes are blasted and the ore is removed from the bottom excavation.

Ventilation

One of the most important aspects of underground hard rock mining is ventilation. Ventilation is required to clear toxic fumes from blasting and removing exhaust fumes from diesel equipment. In deep hot mines ventilation is also required for cooling the workplace for miners. Ventilation raises are excavated to provide ventilation for the workplaces, and can be modified for use as emergency escape routes. The primary sources of heat in underground hard rock mines are virgin rock temperature, machinery, auto compression, and fissure water. Other small contributing factors are human body heat and blasting.



Ground support

Some means of support is required in order to maintain the stability of the openings that are excavated. This support comes in two forms, local support and area support.

Area ground support

Area ground support is used to prevent major ground failure. Holes are drilled into the back (ceiling) and walls and a long metal bar (or rock bolt) is installed to hold the ground together. There are three categories of rock bolt, determined on the way the behave in the rock. They are:

Mechanical bolts

* Point anchor bolts (or expansion shell bolts) are a common style of area ground support. A point anchor bolt is a metal bar between 20 mm – 25 mm in diameter, and between 1 m – 4 m in length (the size is determined by the mine's engineering department). There is an expansion shell at the end of the bolt which is inserted into the hole. As the bolt is tightened by the installation drill the expansion shell expands and the bolt tightens holding the rock together. Mechanical bolts are considered temporary support as their lifespan is reduced by corrosion as they are not grouted.

Grouted bolts

* Resin grouted rebar is used in areas which require more support than a point anchor bolt can give. The rebar used is of similar size as a point anchor bolt but does not have an expansion shell. Once the hole for the rebar is drilled, cartridges of epoxy resin are installed in the hole. The rebar bolt is installed after the resin and spun by the installation drill. This opens the resin cartridge and mixes it. Once the resin hardens the drill spinning tightens the rebar bolt holding the rock together. Resin grouted rebar is considered a permanent ground support with a lifespan of 20–30 years.
* Cable bolts are used to bind large masses of rock in the hanging wall and around large excavations. Cable bolts are much larger than standard rock bolts and rebar, usually between 10–25 metres long. Cable bolts are grouted with a cement grout.

Friction bolts

* Split-sets are much easier to install than mechanical bolts or grouted bolts. The bolt is hammered into the drill hole, which has a smaller diameter than the bolt. Pressure from the bolt on the wall holds the rock together. Split-set bolts are particularly susceptible to corrosion and rust from water unless they are grouted. Once grouted the friction increases by a factor of 3-4.
* Swellex is similar to a split-set, except the bolt diameter is smaller than the hole diameter. High pressure water is injected into the bolt to expand the bolt diameter to hold the rock together. Like the split-set, swellex is poorly protected from corrosion and rust.

Local ground support

Local ground support is used to prevent smaller rocks from falling from the backs and walls. Not all excavations require local ground support.

* Welded Wire Mesh is a metal screen with 10 cm x 10 cm (4 inch) openings. It is held to the backs using point anchor bolts or resin grouted rebar.
* Shotcrete is a spray on concrete which coats the backs and walls preventing smaller rocks from falling. Shotcrete thickness can be between 50 mm – 100 mm.
* Latex Membranes can be sprayed on the backs and walls similar to shotcrete, but in smaller amounts.

Stope and retreat vs. stope and fill

Stope and retreat

Using this method, mining is planned to extract rock from the stopes without filling the voids; this allows the wall rocks to cave in to the extracted stope after all the ore has been removed. The stope is then sealed to prevent access.

Stope and fill

Where large bulk ore bodies are to be mined at great depth, or where leaving pillars of ore is uneconomical, the open stope is filled with backfill, which can be a cement and rock mixture, a cement and sand mixture or a cement and tailings mixture. This method is popular as the refilled stopes provide support for the adjacent stopes, allowing total extraction of economic resources.



Mining methods

Selective mining methods

* Cut and Fill mining is a method of short hole mining used steeply dipping or irregular ore zones, in particular where the hanging wall limits the use of long hole methods. The ore is mined in horizontal or slightly inclined slices, and filled with waste rock, sand or tailings. Either fill option may be consolidated with concrete, or left unconsolidated. Cut and fill mining is an expensive but selective method, with low ore loss and dilution.

* Drift and Fill is similar to cut and fill, except it is used in ore zones which are wider than the method of drifting will allow to be mined. In this case the first drift is developed in the ore, and is backfilled using consolidated fill. The second drift is driven adjacent to the first drift. This carries on until the ore zone is mined out to its full width, at which time the second cut is started atop of the first cut.

* Shrinkage Stoping is a short hole mining method which is suitable for steeply dipping orebodies. The method is similar to cut and fill mining with the exception that after being blasted, broken ore is left in the stope where it is used to support the surrounding rock and as a platform to work off of. Only enough ore is removed from the stope to allow for drilling and blasting the next slice. The stope is emptied when all of the ore has been blasted. Although it is very selective and allows for low dilution, since the most of the ore stays in the stope until mining is completed there is a delayed return on capital investments.

* Room and Pillar mining : Room and pillar mining is commonly done in flat or gently dipping bedded ore bodies. Pillars are left in place in a regular pattern while the rooms are mined out. In many room and pillar mines, the pillars are taken out starting at the farthest point from the stope access, allowing the roof to collapse and fill in the stope. This allows a greater recovery as less ore is left behind in pillars.

Bulk mining methods

* Block Caving is used to mine massive steeply dipping orebodies (typically low grade) with high friability. An undercut with haulage access is driven under the orebody, with "drawbells" excavated over the undercut. The drawbells serve as a place for caving rock to fall into. The orebody is drilled and blasted above the undercut, and the ore is removed via the haulage access. Due to the friability of the orebody the ore above the first blast caves and falls into the drawbells. As ore is removed from the drawbells the orebody caves in providing a steady stream of ore. If caving stops and removal of ore from the drawbells continues, a large void may form, resulting in the potential for a sudden and massive collapse and potentially catastrophic windblast throughout the mine.

Ore removal

In mines which use rubber tired equipment for coarse ore removal, the ore is removed from the stope (referred to as "mucked out" or "bogged") using center articulated vehicles (referred to as boggers or LHD [short for Load, Haul, Dump]). These pieces of equipment may operate using diesel or electric engines and resemble a low-profile front end loader.

The ore is then dumped into a truck to be hauled to the surface (in shallower mines). In deeper mines the ore is dumped down an ore pass (a vertical or near vertical excavation) where it falls to a collection level. On the collection level it may receive primary crushing via jaw crusher. The ore is then moved by conveyor belts, trucks or occasionally trains to the shaft to be hoisted to the surface in buckets or skips and emptied into bins beneath the surface headframe for transport to the mill.

In some cases the underground primary crusher feeds an inclined conveyor belt which delivers ore via an incline shaft direct to the surface. The ore is fed down ore passes, with mining equipment accessing the ore body via a decline from surface.

Deepest mines

* The deepest mines in the world are the TauTona (Western Deep Levels) and Savuka gold mines in the Witwatersrand region of South Africa, which are currently working at depths exceeding 3,900 meters (about 12,800 feet ). There are plans to extend Mponeng ming, a sister mine to TauTona, down to 4,500 meters in the coming years.
* The deepest hard rock mine in North America is Xstrata's Kidd Mine, which mines copper and zinc ore in Timmins, Ontario. Mining is actively occurring 2,682 m (8,800 feet) below surface. The shaft bottoms out at over 2,926m (9,600 feet) down.
* The deepest hard rock mines in Australia are the copper and zinc lead mines in Mount Isa, Queensland at 1,800 m (5,906 feet).
* The deepest platinum-palladium mines in the world are on the Merensky Reef, in South Africa, with a resource of 203 million Troy ounces, currently worked to approximately 2,200 m (7,218 feet) depth.
* The harshest conditions for hard rock mining are in the Witwatersrand area of South Africa, where workers toil in temperatures of up to 45 °C (113 °F). However, massive refrigeration plants are used to bring the air temperature down to around 28 °C.

From http://en.wikipedia.org/

Surface mining

Surface mining is a type of mining in which soil and rock overlying the mineral deposit are removed. It is the opposite of underground mining, in which the overlying rock is left in place, and the mineral removed through shafts or tunnels.

Surface mining is used when deposits of commercially useful minerals or rock are found near the surface; that is, where the overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would usually be the case for sand, cinder, and gravel). Where minerals occur deep below the surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods are used to extract the valued material. Surface mines are typically enlarged until either the mineral deposit is exhausted, or the cost of removing larger volumes of overburden makes further mining uneconomic.

In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden - the soil and rock above the deposit. Next, huge machines, such as dragline excavators, extract the mineral.



Types of surface mining

There are five main forms of surface mining, detailed below.

Strip mining

"Strip mining" is the practice of mining a seam of mineral by first removing a long strip of overlying soil and rock (the overburden). It is most commonly used to mine coal or tar sand. Strip mining is only practical when the ore body to be excavated is relatively near the surface. This type of mining uses some of the largest machines on earth, including bucket-wheel excavators which can move as much as 12,000 cubic meters of earth per hour.

There are two forms of strip mining. The more common method is "area stripping", which is used on fairly flat terrain, to extract deposits over a large area. As each long strip is excavated, the overburden is placed in the excavation produced by the previous strip.

"Contour stripping" involves removing the overburden above the mineral seam near the outcrop in hilly terrain, where the mineral outcrop usually follows the contour of the land. Contour stripping is often followed by auger mining into the hillside, to remove more of the mineral. This method commonly leaves behind terraces in mountainsides.

Among others, strip mining is used to extract the oil-impregnated sand in the Athabasca Tar Sands in Alberta. It is also common in coal mining. Bucket-wheel excavators are widely used for this purpose, however, they are prone to damage and require many millions of dollars to repair.

Open-pit mining

"Open-pit mining" refers to a method of extracting rock or minerals from the earth from their removal from an open pit or borrow. Although open-pit mining is sometimes mistakenly referred to as "strip mining", the two methods are different (see above).

Mountaintop removal

"Mountaintop removal mining" (MTR) is a destructive form of coal mining that uses three million pounds of explosives per day to blast 600 to 800 feet (240 m) off the top of densely forested Appalachian mountains. The mining waste or "overburden" is dumped by large trucks into mountain streams. MTR involves the mass restructuring of earth in order to reach the coal seam as deep as 1,000 feet (300 m) below the surface. It is used where a coal seam outcrops all the way around a mountain top. All the rock, soil, trees and vegetation above the coal seam are removed and dumped in adjacent lows such as hollows or ravines. Mountaintop removal replaces previously steep forested topography with a monocultural grassland on a relatively level surface. Economic development attempts on reclaimed mine sites include prisons such the Big Sandy Federal Penitentiary in Martin County, Kentucky, small town airports, golf courses such as Twisted Gun in Mingo County, West Virginia and Stonecrest Golf Course in Floyd County, Kentucky, as well as industrial scrubber sludge disposal sites, solid waste landfills, trailer parks, explosive manufacturers, and storage rental lockers.

The technique has been used increasingly in recent years in the Appalachian coal fields of West Virginia, Kentucky, Virginia and Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial.

Advocates of mountaintop removal point out that once the areas are reclaimed as mandated by law, the technique provides premium flat land suitable for many uses in a region where flat land is at a premium. They also maintain that the new growth on reclaimed mountaintop mined areas is better able to support populations of game animals.

Critics contend that mountaintop removal is a disastrous practice that benefits a small number of corporations at the expense of local communities and the environment. A U.S. Environmental Protection Agency (EPA) environmental impact statement finds that streams near valley fills from mountaintop removal contain high levels of minerals in the water and decreased aquatic biodiversity. The statement also estimates that 724 miles (1,165 km) of Appalachian streams were buried by valley fills from 1985 to 2001.

In common with other methods of coal mining, processing the coal mined by mountaintop removal generates waste slurry (also called "coal sludge"), which is usually stored behind a dam on-site. Many coal slurry impoundments in West Virginia exceed 500 million gallons in volume, and some, including the Brushy Fork impoundment in Raleigh County, exceed 7 billion gallons. Such impoundments can be hundreds of feet high and be in close proximity to schools or private residences. The most controversial sludge dam at present sits 400 yards (370 m) above Marsh Fork Elementary School. The sludge pond is permitted to hold 2.8 billion gallons of toxic sludge, and is 21 times larger than the pond which killed 125 people in the Buffalo Creek Flood.

Blasting at a mountaintop removal mine expels coal dust and fly-rock into the air, which can then disturb or settle onto private property nearby. This dust contains sulfur compounds, which corrodes structures and tombstones and is a health hazard.

Although MTR sites are usually reclaimed after mining is complete, reclamation has traditionally focused on stabilizing rock and controlling erosion, but not reforesting the area with trees. Quick-growing, non-native grasses, planted to quickly provide vegetation on a site, compete with tree seedlings, and trees have difficulty establishing root systems in compacted backfill. Consequently, biodiversity suffers in a region of the United States with numerous endemic species. Erosion also increases, which can intensify flooding. In the Eastern United States, the Appalachian Regional Reforestation Initiative works to promote the use of trees in mining reclamation.

Dredging

"Dredging" is a method often used to bring up underwater mineral deposits. Although dredging is usually employed to clear or enlarge waterways for boats, it can also recover significant amounts of underwater minerals relatively efficiently and cheaply.

Highwall mining

Highwall mining is another form of surface mining that evolved from auger mining. In highwall mining, the coal seam is penetrated by a continuous miner propelled by a hydraulic Pushbeam Transfer Mechanism (PTM). A typical cycle includes sumping (pushing forward) and shearing (raising or lowering the cutterhead boom to cut the entire height of the coal seam). As the coal recovery cycle continues, the cutterhead is progressively pushed into the coal seam for 20 feet (6.1 m). Then, the Pushbeam Transfer Mechanism (PTM) automatically inserts a 20-foot (6.1 m) long rectangular pushbeam into the center section of the machine between the powerhead and the cutterhead. The pushbeams system can penetrate nearly 1,000 feet (300 m) into the coal seam. Some highwall mining systems use augers enclosed inside the pushbeams that prevent the mined coal from being contaminated by rock debris during the conveyance process. Using a video imaging and/or a gamma detector, the operator can see and guide the continuous miner's progress. Highwall mining can produce thousands of tons of clean coal in contour-strip operations with narrow benches, previously mined areas, or trench mine applications.

Recovery is much better than augering, but the mapping of areas that have been developed by a highwall miner are not mapped as rigorously as deep mined areas. Very little spoil is displaced in contrast with mountain top removal, however a large amount of capital is required to operate and own a highwall miner.

Mapping of the outcrop as well as core hole data and samples taken during the bench making process are taken into account to best project the panels that the highwall miner will cut. Obstacles that could be potentially damaged by subsidence and the natural contour of the Highwall are taken into account, and a surveyor points the Highwall miner in a line mostly perpendicular to the highwall. Parallel lines represent the panels cut into the mountain (up to 1,000 feet (300 m) deep), because changing the azimuth during mining results in missing a portion of the coal seam. Recently highwall miners have penetrated more than 1050 feet into the coal seam, and today's models are capable of going farther, limited only by the amount of cable on the machine. The maximum depth would be determined by the stress of further penetration and associated power draw.

Environmental and health issues

The large impact of surface mining on the topography, vegetation, and water resources has made it highly controversial.

Surface mining is subject to state and federal reclamation requirements, but adequacy of the requirements is a constant source of contention. Unless reclaimed, surface mining can leave behind large areas of infertile waste rock, as 70% of material excavated is waste.

In the United States, the Surface Mining Control and Reclamation Act of 1977 mandates reclamation of surface coal mines. Reclamation for non-coal mines is regulated by state and local laws, which may vary widely.

Human health

The United Mine Workers of America has spoken against the use of human sewage sludge to reclaim surface mining sites in Appalachia. The UMWA launched its campaign against the use of sludge on mine sites in 1999 after eight UMWA workers became ill from exposure to Class B sludge spread near their workplace.

On August 20, 2004 at 2:30 a.m. a boulder accidentally pushed off an A&G Coal surface mine above the town of Inman, Virginia rolled 649 feet (198 m) down the mountain and into a home. Three-year-old Jeremy Davidson was crushed in his bed while he slept. The Davidson family settled with A&G Coal for $3 million in 2006, and left the region.

Flooding exacerbated by logging operations and coal surface mining, especially mountaintop removal mines has caused damage in Appalachian communities including the communities of Dorothy, Welch, Keystone, Oceana, Davey, Belo and Delbarton, West Virginia. Flash flooding on July 8, 2001 and May 2, 2002 killed eight West Virginians and washed away homes throughout southern West Virginia. Mountaintop removal mining removes all vegetation, and these trees normally absorb up to 75 percent of rainfall in a mature healthy forest, according to the Federal Office of Surface Mining.

The Martin County Sludge Spill was an accident that occurred after midnight on October 11, 2000 when the bottom of a coal sludge impoundment at a surface mine owned by Massey Energy in Martin County, Kentucky, broke into an abandoned underground mine below. The slurry came out of the mine openings, sending an estimated 306 million gallons (1.16 billion liters) of sludge down two tributaries of the Tug Fork River. By morning, Wolf Creek was oozing with the black waste; on Coldwater Fork, a ten-foot (3 m) wide stream became a 100-yard (91 m) expanse of thick sludge. The spill was over five feet deep in places and covered nearby residents' yards. The spill polluted hundreds of miles (300 - 500 km) of the Big Sandy River and its tributaries and the Ohio River. The water supply for over 27,000 residents was contaminated, and all aquatic life in Coldwater Fork and Wolf Creek was killed. The spill was 30 times larger than the Exxon Valdez oil spill (12 million gallons) and one of the worst environmental disasters ever in the southeastern United States, according to the United States Environmental Protection Agency.

Surface mining creates problems for the human health: gases, dust in suspension, noises and vibrations from the machines and explosions, that can create lung or nervous diseases.

From http://en.wikipedia.org/