Underground mining

Underground mining is the process of making selective excavations beneath the Earth’s surface to remove valuable natural materials. A number of methods are used, and underground mining is done throughout the world, depending on mineral concentrations.

Background

The natural materials produced from mining operations are necessary for modern life. Practically all raw materials used in metallurgy (metals, coal, fluxes) are produced from underground mines. Many or energy resources can be mined by either surface or underground means, and the decision to mine using underground techniques is made when economics or environmental conditions will not permit surface mining. Examples include when the amount of material overlying the mineral deposit is too costly to remove and dispose of or when the direction of mining makes continued surface mining impossible, such as mining under populated areas or under bodies of water.

History

Mining is thought to have begun with Paleolithic humans, perhaps as early as 450,000 years ago with the excavation of flint implements. The oldest mine of record, a hematite (iron ore) mine in Swaziland, is believed to be 40,000 years old. Underground mines had reached depths of 240 meters by ancient Egyptian times. The first recorded mining by Europeans in the New World was in 1524. From the beginnings of the Industrial Revolution to the present, mining techniques have evolved to increase output at greater depths while providing a safer working environment underground.

Slope, Shaft, and Drift Methods

Three primary methods are used to gain access to underground mineral deposits: the shaft, slope, and drift methods. Shaft access is by a vertical shaft driven from the surface to the deposit. The shaft is usually large enough to transport personnel, equipment, and raw mined product. A raise, the opposite of a shaft, is driven upward from the mineral deposit. Raises are driven only in special situations, because they require specialized machinery that can be operated in the confines of an underground mine.

A slope is an opening driven into the mineral deposit at an angle. Slopes can be used for transport and often contain a conveyor belt which can be reversed to allow haulage into and out of the mine. A drift mine is the least expensive type, because the drift is a near-horizontal opening driven into the mineral deposit. If a mineral deposit reaches the surface (outcrop), the driven drift removes the mineral from the outset of mining. Such a configuration is desirable but not often found in mineral exploration.

Room and Pillar

The types of underground mining methods are as numerous as the special situations that may be encountered. Most methods, however, are highly mechanized and assume the general classification of room and pillar, stope and pillar, shrinkage stoping, and sublevel caving.

Room and pillar mining provides the familiar “checkerboard” pattern often associated with underground mining, in which pillars provide roof support. This method is used in obtaining relatively flat-lying, tabular, and thin mineral deposits such as coal and certain evaporates such as potash and trona. The pillars are designed to optimize the percentage of coal extracted without endangering the mine to premature roof failures. The extraction rate can vary from 25 percent to 65 percent of the coal in place.

Two basic forms of room and pillar mining are called conventional and continuous mining. Conventional mining refers to the use of mobile equipment arranged to complete a cycle of cutting the coal, drilling and blasting with explosives, loading, hauling, and roof support. At least five working places are required for smooth operations. Although conventional room and pillar mining is still practiced in small mining operations, it has mostly been replaced by continuous mining techniques. Continuous mining refers to the use of a continuous miner, a large machine which essentially removes the mineral using tungsten carbide bits mounted on a ripper or boring configuration. The continuous miner replaces the coal cutter, coal drill, explosives, and loading machine in the conventional mining configuration, needing only a roof support machine to provide a total mining operation. In room and pillar mining, pillars can be extracted later in the life of the mine, increasing the recovery of minerals to more than 70 percent.

Stope and Pillar

“Stope” is a mining term for an artificial excavation in an underground mineral deposit. Stope and pillar mining is similar to room and pillar mining but is used in hard-rock deposits, sometimes in a sloping deposit. The pillars are usually random-sized and irregularly shaped, with their location more dependent on being in low-grade than on a systematic, invariant plan. Pillars are relatively small because they are made of and are extremely strong. Pillar recovery, common in room and pillar mining, is almost unknown in stope and pillar mining.

Shrinkage Stoping

Shrinkage stoping has faded from popularity but was a very popular hard-rock mining technique during the late nineteenth and early twentieth centuries. In its simplest form, a shrinkage stope is an underground cavity between two raises where the mineral deposit is removed overhand, and the broken material falls to form the floor of the cavity and supports continued mining activity as the mining progresses upward. Because the broken material “swells” over its volume as a solid material, a certain amount must be drawn off through chutes at the bottom of the stope, or “shrinkage.” When the stope has reached its vertical limit, all the broken mineral can be drawn off and processed.

Shrinkage stoping, like all mining methods, has its advantages and disadvantages. It requires that 60 percent of the broken mineral be retained as a floor for the mine and support of the opening being excavated but affords storage capacity and an opportunity to control the withdrawn mined material.

Sublevel Caving

Sublevel caving is used in vertical or near-vertical mineral deposits and, like shrinkage stoping, uses gravity as a conveyor of the broken mineral. It is also a safe mining method because no personnel work in the mined area. All mining is done from the safety of raises adjacent to the mined area, and long boreholes are drilled into the mineral deposit for blasting the material into chutes located below the mined area. Drilling accuracy is critical to this mining method, because errant drill holes can stray outside the mineral deposit, and incomplete fragmentation from blasting can cause the mineral to stick in the chutes and require expensive and dangerous removal using explosives.

A specialized form of caving used in coal mining is called longwall mining. A longwall is created between two series of excavated rooms about 240 meters apart. An excavator continually shears the coal across the longwall until the block of coal is removed. These blocks of coal are frequently close to 2 kilometers in length. Longwall mining originated in Europe in the seventeenth century and accounts for nearly 100 percent of the coal mined outside the United States. The method gained popularity in the United States beginning in the 1960s. Its recent popularity has increased through the use of self-advancing hydraulic supports, armored conveyors, continuous mining machines, and roof control and caving technology developed from sound rock mechanics principles. Longwall mining will almost certainly continue to be a popular worldwide coal-mining technique.

Rock Mechanics

Underground mining employs a highly specialized discipline called rock mechanics. Rock mechanics studies the failure criteria for rock masses, even heterogeneous and already fractured masses. Its value lies in the design of underground openings that will allow the maximum amount of material to be excavated while providing support against collapse of the opening. Its applications range from small-hole drilling to large excavations. Modern rock mechanics uses finite analysis for predicting the type of rock failure expected, and when processed interactively by digital computers, it can accurately predict how and when a network of underground openings will fail.

The Underground Mining Environment

The underground miner is faced with a potentially hazardous environment. As mining depth increases, so does rock stress and temperature, so at great depth the mine environment may be extremely hot and rock failure problematic. In addition, potentially toxic gases and dusts may be prevalent. For example, uranium mining poses a hazard to miners, which varies with the purity of the deposit. Dusts and gases from blasting activity are introduced into the mining environment. In some coal mines, the emission of methane, a potentially explosive gas, is inevitable and the gas must be continuously removed before and during mining operations.

Federal and state regulations concerning mine health and safety have been continuous over the twentieth century and have resulted in a dramatic reduction in underground mining mishaps. Foremost among these are regulations dealing with the mine atmosphere, where dust and ventilation air specifications guarantee a healthy work environment. Where mine temperatures are above comfortable working conditions, the ventilation air is either dehumidified and cooled or the mining personnel work for shorter intervals with frequent breaks in air-conditioned areas underground.

Dust has been a continual problem in underground mining. Its generation is particularly prevalent in modern mining practices such as longwall mining, and it is also endemic to the production of the fine-sized product in demand by the power industry. Certain dusts have been found to cause long-term human disabilities such as black lung disease (from coal dust) and silicosis (from finely ground sandstones). The coal mining industry has paid hundreds of millions of dollars into funds to support victims of these disabilities. The dust menace has also been countered through the use of respirators by personnel exposed to dusty environments and with atomizing water sprays for allaying dusts at the point of production.

The number of underground mines worldwide decreased in the twentieth century, but output from the remaining and newly created mines increased because of improved efficiency.

The future of underground mining will depend on finding and developing remaining commercial mineral resources in remote parts of the world and extracting material from greater depths in the Earth’s crust. Extraction techniques involving robotics and other modern technologies will be necessary to support such ventures. As environmental regulations place greater burdens on surface mining worldwide, underground mining is likely to see a revival.

Bibliography

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