Mining processes

Mining techniques are methods used to extract ore from the earth. Surface mining techniques are employed when ore is found at close depths. Underground methods are utilized when the valued minerals are concentrated too far beneath the land surface to recover profitably using other techniques.

Surface Mining Methods

There are eleven varieties of surface-mining methods. These are open-pit, mountaintop removal, conventional contour, box-cut contour, longwall strip, multiseam, block-cut, area, block-area, multiseam scraper, and terrace mining. Open-pit mining involves removing overburden and ore in a series of benches from near surface to pit floor. Expansion can occur outward and downward from the initial dig-in point. Many metallic deposits and industrial minerals are mined by this method; one example is the Bingham Canyon mine in Utah. The chief disadvantage of this method is the near impossibility of reclaiming large pits, such as at Bingham, where nearly all the extracted material has been processed or transported away.

Mountaintop removal mining was popular among mining companies, if not among environmentalists, in eastern Kentucky. It involves removing an entire mountaintop, dumping the overburden in valleys on the first cut, and leaving a fairly flat floor upon which to reclaim overburden from succeeding cuts. A postmining surface of gently rolling terrain will replace the rugged premine surface. The disadvantages of this method include limited valley fill material, increased capital costs for additional equipment, and a need for extensive mine planning to ensure maximum productivity at minimum cost. It is, however, more effective than is contour mining.

In conventional contour mining, waste material is dumped downslope from the active pit, often resulting in toxic material resting on native ground and causing erosion, poor vegetative reestablishment, and potential acid-mine drainage. Mining continues into the hillside until the volume of waste rock makes further mining uneconomical. This type of mining results in a long strip-like bench running around the hillside and has been discouraged by stricter environmental laws in Kentucky and West Virginia. It is generally favored by smaller operators because expenses are lower and premine and reclamation planning efforts are reduced, but the method disturbs much more acreage than other steep-slope methods. A method similar to conventional contour mining, but with somewhat improved environmental impacts, is box-cut contour mining, in which a boxlike initial pit is dug to receive waste from later mining. Spoil segregation and terrace regrading help to inhibit toxic down slope material and aid revegetation. The low wall of the box-cut helps to support most of the spoil.

Longwall strip mining has been employed in West Virginia. This innovative method involves removing a small pit by conventional stripping methods, followed by the setup of a continuous miner (or excavator) and a conveyor. Although production costs are higher, land may be more easily restored. This mining method can also help to prevent acid-mine drainage when geology is favorable.

Multiseam mining is done in many locations throughout the world. Frequently, a bottom and top coal seam are separated by about 2 to 6 meters of rock or more. The distance means that this “interburden” material also must be removed, and often drilled and blasted as well. Yet, even though the overall ratio of waste rock to coal may be higher, it may be beneficial and economical to extract both seams at once. Permitting and assessment costs are lower as well. Block-cut mining is also suitable in areas of moderate to steep topography. A single box- or block-cut is excavated and mining progresses outward from the initial cut in two directions. Environmental disturbances are minimized as the first cut is regraded and revegetated during mining of subsequent cuts.

Large areas of flat or gently rolling terrain are ideal for area mining, in which draglines or large stripping shovels remove overburden. Succeeding pits are dug normally or parallel to the strata-bound ore body. This method is the most common type of mining attempted in the major lignite and subbituminous coal fields of western North Dakota, Montana, and northeastern Wyoming.

Block-area mining is similar in many respects to both area and block-cut methods. It is designed to recover seams as thin as 0.3 meter. Capital requirements are less than in some methods, and overburden removal can be sequenced with reclamation regrading.

Multiseam scraper mining involves uncovering large blocks of coal (up to 122 meters wide by 305 meters long) by scraper. After the coal is removed, scrapers again remove the parting between succeeding seams until all the coal is mined. This method is ideal for the coal and thin parting sequences found in the upper Midwest.

Terrace mining involves overburden and ore removal in a series of benches, or terraces. These benches may follow natural features, as in stream or lake terraces. This type of mining has been effective in the recovery of diamonds, sand, and gravel.

Costs of Surface Mining

Surface mining can be a costly and complex operation. Surface mines are frequently located far from populated areas. In arctic regions, production may be inhibited for much of the year by unfavorable weather. Some surface mines experience difficult mining conditions year-round.

Surface mining has left ugly scars on the landscape in many countries, to include the eastern United States. This prompted environmental legislation. Since the 1950s, the production and disposal of toxic materials has been regulated in the United States, and surface and groundwater problems are being addressed.

Underground Mining Methods

When the valuable mineral to be extracted occurs at a depth too great to allow profitable surface mining, underground methods are used. The depth at which profitability decreases depends on the value of the mineral deposit to be mined. Thus, some metallic minerals have been surface (open-pit) mined to depths that equal or exceed the deepest underground coal mines. Several physical factors influence the selection of underground mining methods, including size, shape, continuity, and depth of the mineral deposit; range and pattern of ore quality; strength, hardness, and structural characteristics of the deposit and the surrounding and overlying rock; groundwater conditions; subsurface temperatures (some mines are so hot they must be air-conditioned); local topography and climate; and environmental protection considerations. If solution mining (flooding the pit) is being considered, the chemical composition of the ore mineral will be an important consideration as well. Additionally, technologic and economic factors must be considered, such as availability of workers and worker safety; availability of water, power, and transportation; and, most important, a market for the mineral.

Once a decision has been made to develop an underground mining operation, one of four methods are chosen. The first involves predominantly self-supporting openings. The second is dependent on artificial supports. The third is a caving method, and the fourth involves solution mining. In solution mining, workers do not work underground to mine the valuable minerals. Instead, wells are drilled and liquid flows or is pumped down the wells and through the deposit to dissolve the mineral. Later, the mineral is recovered by processing the pregnant solution (liquid containing the dissolved mineral) after it has been pumped to the surface. Copper and uranium have both been recovered this way.

Self-Supporting Methods

Methods involving predominantly self-supporting openings include open stoping, sublevel stoping, shrinkage stoping, and room-and-pillar mining. Prior to production of the mineral in marketable quantities, the mine must be developed. Development work begins with gaining access to the mineral deposit by sinking a shaft, driving a horizontal tunnel-like excavation (adit) into a hillside, or driving a sloping tunnel-like excavation (decline) from the surface. Once accessed, portions of the sought-after deposit are blocked out by bounding them with a three-dimensional network of horizontal tunnel-like excavations (levels) and vertical shaft-like excavations (raises or winzes). Each of these blocked-out areas defines a stope to be mined by the stoping method selected for the particular mine.

Open stoping is any mining method in which a stope is created by the removal of a valuable mineral without the use of timber, or other artificial supports, as the predominant means of supporting the overburden. Open stopes include both isolated single openings, from which pockets of ore have been extracted, and pillared open stopes, in which a deposit of considerable lateral extent has been mined with ore pillars left for support. In sublevel stoping, large blocks of ore between the levels and raises bounding the stope are partitioned into a series of smaller slices or blocks by driving sublevels. The ore is drilled and blasted by miners in the sublevels in a sequence that causes the sublevels to retreat en echelon (in step formation), with each successively lower sublevel being mined slightly ahead of the one immediately above it. This process allows all the broken ore to fall directly to the bottom of the stope, where it passes through funnel-like openings (called draw points) to the haulage level below for transport out of the mine.

In shrinkage stoping, the ore in the stope is mined from the bottom upward, with the broken ore allowed to accumulate above the draw points to provide a floor from which blasting operations can be conducted. Periodically, the broken ore is drawn down from below (shrunk) to maintain the proper headroom for blasting. Room-and-pillar mining is a form of pillared open stoping employed where the mineral deposit is relatively thin, flat-lying, and of great lateral extent. It is employed most commonly in the mining of coal but is also used to mine other nonmetals, such as rock salt, potash, trona, and limestone.

Artificially Supported Methods

Stull stoping, cut-and-fill stoping, square-set stoping, longwall mining, and top slicing are among the methods predominantly dependent on artificial support. Stull stoping is a method that can be considered transitionary between those methods involving predominantly self-supporting openings and those predominantly involving artificially supported openings. It is used chiefly in narrow, steeply sloping (dipping) vein deposits. Timbers (called stulls) are placed for support between the lower side wall (called the footwall) and the upper side wall (called the hanging wall) of the vein.

The cut-and-fill stoping method, like shrinkage stoping, has a working-level floor of broken rock; however, in this case, the broken rock is waste rather than ore. Instead of draw points, heavily timbered ore chutes are constructed through the waste rock to pass the broken ore to the haulage level for transport from the mine. The solid ore is fragmented by blasting so that it falls on top of the waste rock floor, where it is collected and dumped down the ore chutes. Waste rock, often from development work elsewhere in the mine, is placed in the stope to build up the floor and maintain the desired headroom for blasting.

Square-set stoping is a method employing extensive artificial support. “Square sets” (interlocking cube-shaped wooden frames of approximately 2.5 to 3 meters on a side) are constructed to form a network that resembles monkey bars on a playground to provide support for the stope. As mining progresses, the square sets are filled immediately with waste rock—except for those sets kept open for ventilation, ore passes, or passageways for miners (called manways). In the longwall method, a massive system of props or hydraulic supports is used to support the roof over a relatively long, continuous exposure of solid mineral. This method allows for virtually complete extraction, and the overlying roof behind the line of support collapses into the mined void as the mining process advances. A method similar to longwall is top slicing, in which ore is extracted in horizontal timbered slices, starting at the top of the ore deposit and working downward. A timbered mat is placed in the first cut, and the overburden is caved. As subsequent cuts advance, caving is induced by blasting out props (timbers) behind the face, while working room is continuously maintained under the mat. Top slicing differs from longwall mining in that several levels are developed en echelon rather than progressing on a single level.

Caving Methods

Two caving methods are commonly used in underground mining: sublevel caving and block caving. In sublevel caving, work begins in the uppermost sublevel of the stope and progresses downward. As the ore is blasted and collapses onto the sublevel floor for removal, the wall rock of the stope immediately caves behind the ore. The broken ore is removed through the sublevels. In block caving operations, the ore body is induced to cave downward because of gravity for the entire height of the stope (usually more than 30 meters). The process is initiated by undercutting the block of ore and allowing it to collapse. The collapsed ore is removed through draw points at the bottom of the stope.

Adaptation to Other Uses

The relevance of mining methods to society is not limited simply to the production of useful and valuable minerals. The ideas and concepts developed for underground mining have been applied and adapted to other uses as well. Techniques used in underground mining are used in the construction industry, especially on projects involving underground openings for transportation, electric power generation, fuel storage, and military purposes. The construction of highway, railroad, and subway tunnels can be cited as examples of transportation applications. Large hydroelectric dams and similar projects utilize rock tunnels and chambers to channel the falling water used to turn the turbines that generate the electricity. Underground caverns created through underground mining technology are being utilized for petroleum and liquefied natural gas storage, and mine-like underground excavations in rock are also being investigated for the deep underground storage or disposal of nuclear wastes. Military applications include underground chambers for housing intercontinental ballistic missiles, and nuclear weapon-proof command facilities. One of the earliest military applications of mining was tunneling under enemy fortifications, either to collapse walls (“sapping”) or to place explosives. On June 7, 1917, the British set off 455 tons of explosives under German lines at Messines, Belgium, killing nearly 10,000 German troops, one of the largest non-nuclear explosions ever.

Principal Terms

dragline: a large excavating machine that casts a rope-hung bucket, collects the excavated material by dragging the bucket toward itself, elevates the bucket, and dumps the material on a spoil bank, or pile

grade: the classification of an ore according to either material content or value

level: all connected horizontal mine openings at a given elevation; generally, levels are 30 to 60 meters apart and designated by their vertical distance below the top of the shaft

ore: any rock or material that can be mined at a profit

overburden: the material overlying the ore in a surface mine

panel: an area of underground coal excavation for production rather than development; the coal mine equivalent of a stope

pillar: ore, coal, rock, or waste left in place underground to support the wall or roof of a mined opening

raise: a vertical or steeply inclined excavation of narrow dimensions that connects subsurface levels; unlike a winze, it is bored upward rather than sunk

scraper: a digging, hauling, and grading machine that has a cutting edge, a carrying bowl, a movable front wall, and a dumping or ejecting mechanism

shaft or winze: a vertical or steeply inclined excavation of narrow dimensions; shafts are sunk from the surface, and winzes are sunk from one subsurface level to another

stope: an underground excavation to remove ore, as opposed to development; the outlines of a stope are determined either by the limits of the ore body or by raises and levels

vein: a well-defined, tabular mineralized zone that may or may not contain ore bodies

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