Mining

I

Introduction

Mining, in its broadest sense, the process of obtaining useful minerals from the earth’s crust. The process includes excavations in underground mines and surface excavations in open-pit, or opencut (strip) mines. In addition, recent technological developments may soon make economically feasible the mining of metallic ores from the seafloor. Mining normally means an operation that involves the physical removal of rock and earth. A number of substances, notably natural gas, petroleum, and some sulfur, are produced by methods (primarily drilling) that are not classified as mining. See Gases, Fuel; Petroleum; Sulfur. See also separate articles on the minerals mentioned in this article.

A mineral is generally defined as any naturally occurring substance of definite chemical composition and consistent physical properties. An ore is a mineral or combination of minerals from which a useful substance, such as a metal, can be extracted and marketed at a price that will recover the costs of mining and processing and yield a profit. The naturally occurring substances are usually divided into metalliferous ores, such as the ores of gold, iron, copper, lead, zinc, tin, and manganese, and nonmetalliferous minerals, such as coal, quartz, bauxite, trona, borax, asbestos, talc, feldspar, and phosphate rock. Building and ornamental stones, which form a separate group, include slate, marble, limestone, traprock, travertine, and granite.

Most minerals are found in veins, or tabular-shaped deposits of nonsedimentary origin, often dipping at high angles; in beds, or seams, which are tabular deposits conforming to the stratification of enclosing rocks; and as masses, or large ore bodies of irregular shape standing at any angle. Gold, diamonds, tin, and platinum are often found in placers, or deposits of sand and gravel containing particles of the mineral.

II

Mining Operations

Mining operations generally progress through four stages: (1) prospecting, or the search for mineral deposits; (2) exploration, or the work involved in assessing the size, shape, location, and economic value of the deposit; (3) development, or the work of preparing access to the deposit so that the minerals can be extracted from it; and (4) exploitation, the work of extracting the minerals.

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In the past, ore bodies were discovered by prospectors in areas where veins were exposed on the surface, or by accident, as when gold was discovered in California in 1848. Today, however, prospecting and exploration are skilled occupations involving expert scientific personnel. Teams of geologists, mining engineers, geophysicists,and geochemists work together to discover new deposits. Modern prospecting methods include regional geological studies to define areas where mineralization is likely to have occurred; broad surveys by sophisticated instruments mounted in airplanes and artificial earth satellites (see Remote Sensing) to discover anomalies in the earth’s magnetic field, electrical fields, or radiation patterns in order to define the most promising locations; visual examinations of the surface area for coloring, rock formations, and plant life; chemical analyses of soil and water in the area; and surface work with geophysical instruments (see Geophysics).

These modern techniques can reveal deep-seated as well as near-surface prospects, and they serve as a basis for preliminary estimates of the economic potential of the prospect. The subsequent exploration work includes digging pits, sinking exploration shafts, and core-drilling operations, all of which tend to define the physical limits of the ore body and permit a more reliable estimate of its economic value. The findings may dictate the method used to reach the ore body, the extent of the development work, and the best method of exploitation.

The decision to develop an ore body is reached as soon as sufficient information is available to indicate a profitable return on the financial investment. Complete certainty about the full potential of the mineral is not crucial at this point; exploration work can continue over many years while the deposit is being mined.

After the decision is made to mine an ore deposit, the mode of entry and the extent of lateral or subsidiary development must be determined. If the ore body lies at or near the surface and extends to a depth of no more than a few hundred feet, it may be developed by an open-pit excavation, using power shovels and large trucks. If, however, it is deep or steeply inclined, access may be made through a vertical or inclined shaft, an adit, or crosscut tunnels. The topography of the region, the geometry and physical nature of the ore body, and the proposed method of exploitation have a bearing on this decision. When the terrain is nearly flat, entry must be made through a shaft. In mountainous regions, access to the ore body may be gained through an adit, a nearly horizontal tunnel from which crosscuts may be driven at right angles to reach the ore. Shaft sinking involves a larger outlay of capital and higher operating costs than an adit or crosscut opening. A shaft requires hoisting equipment to raise the ore and rock to the surface, pumping equipment to dispose of any water present, and structural support for the rock and the mechanical equipment operating in the shaft. In an adit, drainage occurs naturally in all workings above the adit as a result of gravity, and structural support is usually not as costly or extensive.

The problems encountered in the sinking of a shaft may be great, especially if water-bearing strata need to be pierced. The water-bearing strata must be cemented or frozen before excavation begins, and lining the shaft with concrete becomes necessary. Even in dry strata, deep shafts are often lined in order to withstand the lateral pressures in the rocks through which they are sunk. After the shaft or adit is completed, lateral development takes place, and crosscuts are driven to reach the ore deposit at different levels. An extensive mine may have a main hoisting shaft and one or more auxiliary shafts or adits for supplies and ventilation. Many state mining laws require mines to be equipped with at least two points of entry and egress to improve the degree of safety for miners.

The method chosen for mining will depend on how maximum yield may be obtained under existing conditions at a minimum cost, with the least danger to the mining personnel. The conditions include the shape, size, continuity, and attitude of the ore body; the mineralogical and physical character of the ore, and the character of the wall rock or overlying material; the relation of the deposit to the surface, to other ore bodies, and to existing shafts on the same property; the skill of available labor; and regional economic conditions. These variables are interdependent and of varying importance, but maximum profit and maximum extraction are closely related, because a method that sacrifices part of the ore body often yields maximum profit. In view of these considerations, open-pit mining tends to be more economical than underground mining, except in regions where climatic conditions are so severe that surface mining is often impossible.

III

Coal Mining

Coal has been mined for more than 1,000 years, and large-scale mining was practiced as early as the 18th century. The first coal mine in America was opened in Virginia, in the Appalachian bituminous field, during the 1750s; the mining of anthracite began in the late 1700s. Extensive mining in the United States commenced about 1820; until 1854 more than half of all the coal that was produced in the U.S. was Pennsylvania anthracite. In 2000, anthracite production was about 4.15 million metric tons, compared to about 970 million metric tons of bituminous coal and lignite.

Two principal systems of coal mining are used: surface, or strip, mining and underground, or deep, mining. Strip mining, which is a form of quarrying, is possible only when the coal seam is near the surface of the ground. In large surface mines, huge power shovels and draglines are used to remove the earth and rock (overburden) from above the seam; modern shovels have bucket capacities of as much as 290 metric tons. Smaller shovels then load the coal directly into trucks. The chief advantage of strip mining over underground mining is the enormous saving of time and labor. The daily output per person in strip mines is many times that in underground mines.

As a supplement to strip mining, or when other mining techniques are not adequate, augers are used to bore horizontally into exposed coal seams. The loosened coal then flows into a conveyor for loading into trucks. A newer development is a boring machine, called a push-button miner, that can tunnel as deep as 300 m (1,000 ft) into the coal seam, dumping the coal into mobile conveyors pulled by the machine.

In underground, or deep, mining, the coal seam is reached through vertical or inclined shafts, or, if the deposit is located in a mountain, through level or nearly level tunnels. The coal deposit is usually marked out in “rooms,” which vary in size according to local conditions. The coal is cut and blasted away, with pillars of coal left to support the roof. In the longwall system of working, a machine with steel teeth is raked along the face, and the broken coal drops onto a conveyor belt. As the machine moves forward, steel supports are advanced to support the roof directly over the working face. The roof behind the coal face is allowed to collapse.

In the conventional method of mining, power cutters have supplanted the traditional tool of the miner, the pick. The miner makes an undercut with these cutters about 15 cm (about 6 in) wide and as much as 2.7 m (9 ft) deep across the face of the coal seam, often close to the floor of the room. Deep holes are then drilled at the top of the face, and charges of safety-approved explosives or cartridges of compressed air are tamped into them. The explosive blast brings down and partially shatters a large chunk of the coal face, which is then loaded by machines into low, electrically propelled shuttle cars that bring the coal to a central loading point. From there it is hauled to the surface by either rail cars or giant conveyor belts. Most of the U.S. underground production of bituminous coal is mined by so-called continuous-mining machines, which eliminate the separate steps of cutting, drilling, blasting, and loading. These huge machines, capable of mining up to 10.8 metric tons of coal per minute, tear coal from the face and load it onto built-in conveyor belts. The belts transport the coal to waiting shuttle cars or mine conveyor belts that carry it to the surface. The coal is then transferred to a preparation plant, where it is screened, washed, sorted into various sizes, and sometimes crushed before shipment.

A recent development, called the shortwall system, combines the continuous-mining machine with the use of longwall steel supports at the face. The continuous miner operates under the protective canopy of the supports and the roof is allowed to cave in as in the longwall system.

Among the chief problems in underground mines are ventilation and roof support. Ventilation is important because of the presence in coal mines of dangerous gases such as methane and carbon dioxide. Large fans and blowers must be used to maintain the circulation of pure air. In order to prevent the spread of coal dust, which can be highly explosive, mine interiors are frequently sprayed with limestone dust, a process known as rock-dusting. To provide support for the roofs of tunnels and work spaces, steel roof bolts that bind together the overlying rock layers are inserted into the mine ceiling.

In the U.S., mining employment generally declined through the late 1960s. In the early 1970s, however, with the Arab oil embargo (see Energy Supply, World: The Energy Crisis), coal reemerged as an important fuel. As production increased, so did employment—to more than 200,000 men and women in the early 1980s. By 1990, the coal mining employment dropped to 147,000 workers and by 2000 to 80,000.

IV

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.

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