Sometimes, but rather rarely, extensive and fairly level deposits of coal are discovered beneath a few feet of overburden, and may readily be won by opencast workings. It is probable that opencast exploitation of outcrop coal is more successful than the mining of such coal from workings.
Opencast affords greater safety, more complete extraction and maybe less subsidence and less surface damage. Sites are worked where the ratio of overburden to coal is greater than 15 to 1, and depths may exceed 150 ft.
Conditions vary widely, and rarely, indeed, opencast workings are similar.
After the preliminary work of prospecting, drilling, trenching, draining and preparation of any access road, etc., the major task of removing the overburden commences.
Heavy-duty loader excavators, capable of excavating 700-800 tons per hour, are used for removing topsoil from a line of outcrop. They usually employ plough-type blades which excavate and force the soil on to conveyer belts which deliver into bottom-dump trucks. The soil is stored separately in readiness for the final restoration of the site when it is replaced and spread by mobile equipment.
Where a considerable thickness of overburden must be removed, dragline excavators are employed. This method of extraction has been found to be very successful on steep coal measures.
The removal of the coal is usually accomplished by a skimmer scoop if the seam is thin, or by power shovel. The first differs from the second in that the scoop is hauled backwards and forwards along the jib by ropes, and picks up its load in moving towards the end of the jib.
The powerful shovel has a more positive action than a dragline. It is more suitable for digging in hard material, and deals with large lumps more readily. A recent development of opencast coal technique uses the shovel on a semi-quarrying basis, the overburden being dug by shovel and transported from the cut by 4-wheel-driven vehicles.
(1640)
NOTES:
· overburden – покрывающий пласт, наносы;
· trenching – проходка кана или траншей;
· draining – дренирование, осушение;
· plough-type blade – стругообразное лезвие;
· bottom-dump truck – вагонетка с откидным дном;
· skimmer scoop – планировочный экскаватор;
· (power) shovel – механическая лопата, экскаватор;
· jib - 1) стрела, 2) бар.
Text 15. COAL MINING
Coal has been mined for more than 1000 years, and large-scale mining was practiced as early as the 18th century.
Coal is mined by two general methods: surface mining and underground mining. In general, surface mines extract coal deposits located up to 30 m (100 ft) below the surface, while underground mines excavate coal beds further underground.
A. Surface Mining. In surface, or strip mining, workers remove the rocky overburden and then mechanically shovel coal up from the underlying deposit. For increased efficiency, the modern coal industry has developed some of the largest industrial equipment ever made, including shovels (part of a piece of equipment known as a dragline)capable ofholding 290 metric tons of coal.
To reach the coal, bulldozers clear the vegetation and soil. Depending on the hardness and depth of the exposed sedimentary rocks, these rocky layers may be shattered with explosives. To do this, workers drill blast holes into the overlying sedimentary rock, fill these holes with explosives, and then blast the overburden to fracture the rock. Once the broken rock is removed, miners shovel coal from the underlying deposit into giant earth-moving trucks for transport.
Depending on the terrain, a mining company will build either an area mine or a contour mine. Area mining occurs on level ground, where workers use excavation equipment to dig a series of long parallel strips, or cuts, into the earth. The overburden is cleared from each cut, and the material (known as spoil) is stacked alongside the long trench. After the exposed coal is shoveled from the cut, workers dump the spoil back into the trench to help reclaim the mined area.
Contour mining occurs on hilly or mountainous terrain, where workers use excavation equipment to cut into the hillside along its contour to remove the overlying rock and then mine the coal. The depth to which workers must cut into the hillside depends on factors such as hill slope and coal bed thickness. For example, steeper slopes require cutting away more overburden to expose the coal bed.
Once the hill slope prevents miners from cutting further back into the hillside, they often switch to a technique known as auger mining to extract more coal along the contour. In this technique, miners drill a series of horizontal holes into the coal bed with a large auger (drill) powered by a diesel or gasoline engine. These augers are typically about 60 m (200 ft) long and between 0.6 and 2.1 m (between 2 and 7 ft) in diameter. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings. Workers continue drilling until the auger drills through the coal seam or until the maximum auger length or torque (energy required to twist an object) is reached.
B. Underground Mining. To reach coal beds deeper than about 30 m (about 100 ft), miners typically build underground mines. They create two or more shafts tunneled down into the coal seam - shafts built to allow passage for miners and machinery are typically separate from those built for passage of mined coal. Miners construct three types of underground mines - shaft, slope, or drift mine - depending on the depth of the coal deposit, the angle of the coal bed, and the thickness of the coal seam.
Shaft mines are built to reach deep coal beds, usually about 200 m (about 660 ft) or more below the surface. A shaft mine uses two vertical shafts to reach the coal bed.
Slope mines reach coal deposits that have been distorted or tilted by shifts in the earth’s crust. A slope mine uses two angled shafts to reach the coal bed. The passageways of a slope mine typically begin where the inclined coal bed outcrops on the surface and follow the incline into the ground. Some slope mines angle down through the overburden to reach the sloping coal bed, then parallel the bed into the earth. If the grade of the slope mine passageway does not exceed 18°, the coal is usually transported from the mine by conveyor. For steeper grades, coal is typically removed by trolley or mine cars.
Drift mines are built where a coal seam outcrops on a hill or mountainside. The drift mine consists of a single passageway that follows the coal seam back into the mountain. Mining companies favor drift mines because they eliminate the need to tunnel through overlying rock to reach a coal deposit.
A single underground mine may use all three types of passageways (drift, slope, and shaft) to mine a coal deposit. For instance, a company might build a drift mine to excavate coal from a hillside outcrop. As the drift mine follows the coal bed into the earth, workers might dig angled passageways (slope mine) from above down to the coal bed to shorten the transportation distance into the progressing mine. If the overburden becomes too deep above the progressing mine, workers may dig more economical vertical passageways (shaft mine) to provide transportation and ventilation.
Once a coal deposit has been reached by a shaft, slope, or drift mine, workers mine the coal by one of two methods: the room-and-pillar method or the longwall method. Room-and-pillar mines extract coal at greater depths and are usually left standing when the mine is abandoned. Longwall mines are built at shallower depths and are allowed to collapse as the mine progresses.
B 1. Room-and-Pillar Method. As its name implies, the room-and-pillar method uses rows of large pillars of coal to support the roof of a mine. Workers tunnel parallel passageways through the coal seam, and then cut 12- to 24-m (40- to 80-ft) wide pillars at regular intervals out of the separating walls of coal. The percentage of coal recovered from a coal seam mined by the room-and-pillar system depends on the economic incentive to remove as much coal as possible versus the number and size of coal pillars necessary to support the roof. Workers leave the pillars standing in areas where environmental regulations prohibit land subsidence (sinking or settling of land). In areas where land subsidence is acceptable, workers may remove some pillars just before closing the mine.
Companies use two processes, known as conventional mining and continuous mining, to remove coal from room-and-pillar underground mines. Conventional coal mining replaced hand mining (mining with pick and shovel) in the 1930s. In conventional mining, miners use power saws to slice a deep cut, 3 to 4 m (10 to 12 ft) wide, into the bottom of a coal wall. Next, they drill holes into the coal above this cut and fill the holes with explosives. Miners discharge the explosives and chunks of coal collapse from the wall. Rubber-tired electric vehicles (called shuttle cars) or conveyors carry these coal chunks out of the mine.
In continuous mining, a worker uses a machine known as a continuous miner that can be operated by remote control. This mobile machine has a series of metal-studded rotating drums that gouge coal from the face of the coal seam (known as the wall face). One continuous miner can mechanically break apart about 1.8 metric tons of coal per hour. After a wall face has been mined to a certain depth, miners stabilize the adjacent roof by bolting long rods into the mine ceiling, advance the ventilation, and begin a new continuous mining cycle.
B 2. Longwall Mining. Instead of coal pillars, the longwall mining system uses a line of moving hydraulic jacks to temporarily support the roof in the mining area. No coal pillars are present to obstruct work, so a large coal-cutting machine cuts coal continuously along a wall face typically about 180 m (590 ft) wide. This massive coal cutting machine works like a wood power saw, shredding coal from the wall in strips about 50 to 75 cm (about 20 to 30 in) wide. As the coal-cutting machine strips layers of coal from the wall face like a meat cutter, the line of roof-supporting hydraulic jacks moves automatically behind the machine. As the hydraulic jacks move forward, the roof is allowed to collapse behind the equipment.
Longwall mining produces four to five times more coal from a given deposit than the room-and-pillar method because coal pillars are not built. But because longwall mining causes the land to sink, land use regulations prohibit this practice in many areas.
(6860)
NOTES:
· steep – крутопадающий, крутой;
· auger – змеевик, спиральный бур, ложечный бур, шнек;
· torque – момент кручения, момент скручивания;
· slope – наклон, наклонная выработка;
· room-and-pillar method – камерно-столбовая система разработки;
· longwall method – система сплошного забоя;
· hydraulic jack - гидродомкрат.
Text 16. COAL MINING WASTE
Strip mining has resulted in a great deal of damage to the landscape. Many strip mines have removed acres of vegetation and altered topographic features, such as hills and valleys, leaving soil exposed for erosion. Longwall mining, which allows the mine to collapse, results in widespread land subsidence, or sinking. Coal and rock waste, often dumped indiscriminately during surface and underground mining processes, weathers rapidly, producing acid drainage. Acid drainage contains sulfur-bearing compounds that oxidize (join with oxygen) in the presence of water to form sulfuric acid (H2SO4). In addition, weathering of coal mine waste can produce alkaline compounds (basic compounds with pH greater than 7), heavy metals, and sediments. Acid mine drainage, alkaline compounds, heavy metals, and sediment leached from the mine waste into the groundwater or washed away by rainwater are detrimental to streams, rivers, and lakes.
Today, companies in many countries must secure government permits before mining for coal. Now companies must submit plans detailing proposed methods for blasting, road construction, land reclamation, and waste disposal. New land reclamation methods, driven by stringent laws and regulations, require coal businesses to restore strip-mined landscape to nearly pre-mined conditions.
(1140)
Text 17. CLEAN COAL TECHNOLOGY
One method of cleanly harnessing energy is called fluidized bed coal combustion. This method burns coal in a limestone bed that transfers heat to water, generating steam. This steam is pressurized and used to turn a turbine shaft, which subsequently drives an electric generator. The limestone absorbs sulfur dioxide emitted by the coal, subsequently reducing the amount of acid gases released during combustion.
A process called furnace sorbent injection removes acid gas from coal emissions at less cost than expensive scrubbers. This process injects a highly absorbent material (called a sorbent), such as powdered limestone, into the boilers, where the powdered limestone reacts with the acid gases emitted by the burning coal. The used powder is siphoned away through the boiler outtake and is captured (with fly ash) in a baghouse or electrostatic precipitator.
A process called advanced flue-gas desulfurization also removes acid gas from burning coal without expensive scrubbers. Emissions from burning coal are piped into a container called an absorber, where the acid gases react with an absorbing solution (such as a mixture of lime, water, and oxygen). This reaction forms gypsum, a soft white mineral valuable as an ingredient in cement.
(1080)
Text 18. METAL MINING
Metalliferous ores are mined either on the surface of the earth or underground. In opencut mining, the ore is removed from deposits that crop out at the surface, lie on a hillside, or are covered by a shallow overburden that is stripped before or simultaneously with the removal of the ore. In open-pit mining, benches are terraced into the earth or rock along the hillside or in the pit. The ore is usually loosened by blasting and loaded into trucks or rail cars by mechanical loaders, or shovels. As the pit increases in depth, the cost of mining by this method also increases, primarily because of the need to remove ever-increasing volumes of waste rock around the ore body to ensure a safe slope for the pit. The change from open-pit to underground mining occurs at the depth at which the costs of mining by the two methods are equal.
The selection of an underground method of mining depends on a number of conditions, primarily the grade, size, shape, and attitude of the ore body and the strength of the ore and wall rock. Generally, a system is used in which the force of gravity helps in the removal of ore.
The mine development work consists of driving a system of crosscuts that connect the shaft with the ore body at a number of levels, a suitable vertical distance apart. Vertical openings, called raises, are made to connect the various levels. The ore body is thus divided into blocks, which are bounded by the levels and the raises, and is ready for extraction. The ore may be removed from the bottom of the block upwards, the process being known as overhand stoping, or, more rarely, from the top of the block downward (underhand stoping). A stope is the chamber in which the ore is broken and mined. The stopes, on completion of mining, may be allowed to remain empty, if adequately supported, or may be filled with material, usually waste rock, brought down from the surface to support the exhausted stope and ensure safety of mining operations in adjacent stopes.
The specific method or methods used in removing the ore usually depends on local geometry and the physical characteristics of the ore and wall rock. Thick-bedded and massive deposits are usually mined by the so-called caving method. In block caving, the development work is confined below the lowest boundary of the block and consists of a network of operating tunnels, called drifts, and slanting raises, called finger-raises, which emanate from the drifts and terminate at the bottom surface of the block. The finger-raises are widened to funnel-shaped openings under the block. The block is then undercut as the rock supporting it is removed. This causes the collapse of the unsupported ore, which falls by gravity through the finger-raises into the drifts, from which it is scraped into mine cars. Under ideal conditions, no primary blasting is necessary, and in general, this is the cheapest underground mining method for handling large low-grade deposits. If the ore body does not disintegrate readily when the support is removed, blasting is necessary; this method is called forced block caving. The gradual extraction of the ore and the resulting fracturing of the rock around the mine workings cause subsidence at the ground surface, which may be counteracted by filling the resultant surface depressions with waste material from the ore-processing mill.
Placer mining is a special opencut method for deposits of sand, gravel, or other alluvium containing workable amounts of valuable minerals. Native gold is the most important placer mineral, but platinum and tin are also found in gravels. Other minerals found are zircon, diamond, ruby, and other gems, and monazite, ilmenite, and ores of columbium and tantalum. Large placer operations involve excavation by power shovels, bucket-wheel excavators, or dragline conveyors, which deliver the sand and gravel to a system of screens, jigs, and sluices used to recover the ore mineral. Occasionally, the gravel bank is broken down by a high-pressure stream of water delivered through a large nozzle, called a hydraulic giant; this method is known as hydraulicking. More often, the placer is flooded, and the digging and processing equipment is mounted on a dredge. The mechanical excavator is usually of the chain-bucket type, and this method is known as dredging.
(3600)
NOTES:
· raisе – восстающая выработка;
· stope – выемочная камера;
· caving method – система с обрушением;
· blasting – взрывание, взрывные работы;
· placer mining - прииск (золотой);
· jig - отсадочная машина;
· sluice – рудопромывальный желоб;
· dredging - выемка грунта, драгирование.
Text 19. GEM CUTTING
Gemstones are minerals that are treasured for their beauty and durability. A large number of minerals have been used as gems. Their value generally depends on four elements: the beauty of the stone itself; its rarity; its hardness and toughness; and the skill with which it has been cut and polished. Stones such as diamonds, rubies, and emeralds represent one of the greatest concentrations of money value. During times of war or economic disturbance many people convert their wealth into precious stones, which are transportable and more easily sold.
The shaping and polishing of gem materials to enhance their beauty and, in some cases, to remove imperfections is performed by expert workers known as lapidaries. Their trade, although highly skilled, is not as exacting as that of the diamond cutter.
Materials and Equipment. Gems are shaped entirely by being ground on abrasive wheels or revolving abrasive disks. For minerals that are no harder than quartz, natural sandstone wheels are sometimes used, but for the harder stones, such as rubies and sapphires, synthetic grinding wheels of cemented Carborundum (silicon carbide) must be employed.
The first step in the cutting of a gem is to saw it roughly to shape. Thin abrasive disks or metal disks charged with powdered diamond or other abrasives are employed in this process. Wheels (called laps) made of Carborundum or of abrasive-charged cast iron are used to shape the stone. The stone to be shaped is cemented to the end of a wooden stick called a dop and is held against the revolving wheel or lap with the aid of a supporting block placed adjacent to the wheel. This supporting block contains a number of holes in which the end of the dop can be rested. By changing the dop from one hole to another the lapidary is able to control the angle of the facet, or face, being ground. When the stone has been ground to the required shape, it is brought to a high polish on wooden or cloth wheels charged with a fine abrasive such as rouge or tripoli powder.
Gem Cuts. The oldest and simplest of the many standardized shapes or cuts given to gemstones is the cabochon cut, in which the stone is smoothly rounded. The cabochon cut is essential if a star or cat’s-eye is to be visible, and is the most satisfactory cut for opal, moonstone, and colorful opaque gems. Cabochon-cut stones usually are rounded on the back; this is sometimes advantageous in improving appearance, but often is done in order to give the stone extra weight.
Various forms of faceted cuts, in which the gem is given a number of symmetrical plane surfaces, or facets, are universally employed in the cutting of diamonds and are used extensively for other stones as well. The most common cut is the brilliant. In this cut the top of the stone is ground to a flat so-called table from which the sides of the stone slope outward to the broadest portion of the stone, which is known as the girdle. Below the girdle, the sides slope inward at a slightly broader angle to a tiny flat surface, the culet, parallel to the table at the bottom of the stone. The ordinary brilliant-cut stone has 32 facets besides the table in the top portion of the stone (called the crown or bezel) above the girdle, and 24 facets besides the culet on the bottom portion of the stone (called the pavilion or base) below the girdle. In rare cases the number of facets is increased by some multiple of 8. Scientific studies have worked out proportions of the size and inclination of the facets that give the maximum brilliance to a given gem.
In addition to the round brilliant, stones are cut in a variety of square, triangular, diamond-shaped, and trapezoidal faceted cuts. The use of such cuts is largely determined by the original shape of the stone. Large rubies, sapphires, and emeralds are often cut square or rectangular with a large table facet surrounded by a relatively small number of supplementary facets. The emerald cut, which is frequently also used for diamonds, resembles the brilliant, but has a large square or rectangular facet at the top and a total of 58 facets in all, although more or less facets may be used, again added or subtracted in multiples of 8.
(3460)
NOTES:
· lapidary – гранильщик драгоценных камней;
· facet – грань, фаска, фацет;
· grinding – шлифовка, измельчение, истирание;
· abrasive - абразив;
· girdle – тонкий пласт песчаника;
