Open-pit mining

Open-pit mining refers to the removal of mineral resources from the Earth without the use of either tunnels or wells. A gravel pit represents the simplest example of an open-pit mine. Although some mining engineers distinguish between strip mining and open-pit mining, the methods employed in both are similar. The major difference is that strip mines are generally shallow, while a pit may eventually descend to hundreds of meters below the original surface of the Earth.

Background

Open-pit mining is the method mine owners prefer to use when the body lies close enough to the surface of the Earth to allow the removal of the in continuous layers. It is both the safest and most economical method of extracting mineral resources from a site. It has been estimated that worldwide 70 percent of all minerals mined are obtained through processes. There is a strong economic incentive for mine operators to use the open-pit method. Mining by the open-pit method allows the mining company to extract 100 percent of the ore-bearing rock. In underground mines using tunnels and shafting, the recovery rate of ore-bearing is generally 60 percent or less. Open-pit mining is also considerably safer than underground mining, as the ore is removed with power shovels and large trucks. Although underground mining also uses mechanized equipment, the workers are still exposed to risks such as cave-ins and explosions not present in open pits.

Methods

A few minerals are soft enough to be mined without the use of explosives. More commonly, mining proceeds through a series of drilling holes, placing explosive charges in the holes and blasting, and then removing the shattered rock with extremely large power shovels and trucks. Most equipment used in open-pit mining is gargantuan in size. Power shovels, excavators, and draglines are custom-assembled at the mine since they are too large to transport other than in pieces. In the past, when a mine closed, this equipment was abandoned at the site. Today it is more likely to be salvaged as scrap metal. In a few former mining districts, specialized equipment has been left in place and preserved as part of historic landmarks.

In some cases the ore body lies close enough to the surface that mining begins directly. More often, a layer of waste material known as the must be removed before the ore itself is exposed. The overburden consists of the and underlying dirt and rock that contains no extractable ore. When mining begins, the layer of topsoil will be removed carefully and piled separately from other waste material, as the topsoil will be needed for use in the restoration process when the mine site is exhausted.

Depending on the mine site, the type of ore mined, and other factors, the mining may proceed in parallel strips or may be done in a circular pattern that gradually expands in diameter. Coal is often mined in strips, as the mineral frequently occurs in layers that can cover a wide area but are only a meter or so in thickness. The mine operator removes the overburden from a strip of coal, excavates the coal, and then repeats the process in a strip running next to the first strip. When the ore body is exhausted, the overburden will be backfilled into the stripped area as part of the restoration process. In the past, before legislation required mine operators to practice restoration, a strip-mined area often consisted of a devastated landscape dominated by alternating trenches and ridges, with the land left unusable for either agriculture or wildlife habitat. Beginning with the passage of environmental regulations in the 1960s, in the United States, strip-mined areas have been backfilled, leveled, and seeded with grass and trees.

Metals such as iron are generally mined in pits that become both deeper and wider over time. The mining operation will commence as closely as possible to the known center of the site and expand both out and down as ore is removed. As time passes, the pit may become surrounded by large piles of tailings, or waste rock. The final depth of the pit will be determined by factors such as the thickness of the ore body and the stability of the surrounding walls of the pit. A pit developed to excavate material such as gravel often has walls composed of soft materials, such as a mixture of sand and gravel, and at risk of collapsing into the pit. Gravel pits therefore are generally quite shallow. Even a pit mine for hard minerals, such as copper or iron, where the mineral is found in rock, may eventually reach a depth where the height of the walls makes it unsafe to dig the pit any deeper. If the ore is sufficiently valuable, mining of the remaining ore body may continue using shaft mining.

In the past, large open-pit mines often operated next to or within communities, such as Butte, Montana, and Bisbee, Arizona, both of which had enormous copper mines. At one time the Anaconda operation at Butte was reputed to be the largest open-pit mine in the world. As the mines expanded, homes and businesses were forced to relocate to accommodate the mine’s operation. By the end of the twentieth century, however, the public had grown less tolerant of mining. Property owners have sued mining companies over quality-of-life issues such as noise pollution and dust. Developers now generally try to avoid opening new open-pit mines close to towns or suburbs.

Environmental Issues

Open-pit mining raises a number of obvious environmental protection questions. Even at its most innocuous, the mining process tends to be both noisy and dusty. Truck or rail traffic to and from the mine can create a nuisance as well as a safety hazard for area residents. As the pit is deepened, it may affect the local water table. Water can seep into the pit, lowering the for the surrounding area, and can then present a hazard to local streams as it is pumped out loaded with sediment. Strip mining on hillsides can lead to and contamination by mine spillage into the local streams.

Depending on the ore being mined, the open-pit mine may present potentially life-threatening problems in addition to dust and noise. The mine itself may be relatively harmless, but processing plants built next to the mine to remove the ore from waste rock may involve the use of dangerous chemicals and produce toxic by-products. Precious metals mining can be particularly hazardous. In gold mining, for example, the ore often occurs in such small amounts within the ore body that remarkably large amounts of ore must be processed to obtain the precious metal using a method that employs cyanide and mercury. If these substances leak into the environment, they can poison streams and kill wildlife kilometers away from the mine itself. Iron ore mining can release sulfides into the environment, as can mining coal. Although not as toxic as cyanide and mercury, sulfides raise the acidity of water and can make lakes and streams uninhabitable by aquatic life.

In the United States, Canada, and many other nations, mine owners are now required by law to restore an open-pit mining site as closely as possible to its original condition. Toxic wastes must be removed or neutralized and the pit filled in. Restoration efforts at strip-mining sites in eastern states that enjoy a relatively wet climate, such as Tennessee and Ohio, have been successful. Phosphate mining areas in Tennessee, for example, have been restored for use in agriculture. Gravel pits and quarries may be used as small wetlands. Water has always been prone to build up in abandoned pit mines. Engineering consulting firms exist that specialize in preparing abandoned pits to become wetlands and ponds. These firms clean up the site, slope the walls to make the pit safer, remove any potentially dangerous mining debris, and plant the species of vegetation most beneficial to wildlife native to the region. Restoration efforts in arid climates have been less successful. Lack of rain makes restoring native vegetation difficult and even if dumps and mine pits are bulldozed to less artificial contours, the scars from mining will be visible for centuries.

Bibliography

Bell, Fred J., and Laurance J. Donnelly. “Quarrying and Surface Mining.” In Mining and Its Impact on the Environment. New York: Taylor & Francis, 2006.

Cameron, Eugene N. At the Crossroads: The Mineral Problems of the United States. New York: Wiley, 1986.

Chinese Organizing Committee of the Fourteenth World Mining Congress, ed. Mining for the Future: Trends and Expectations. New York: Pergamon Press, 1990.

Hartman, Howard L., and Jan M. Mutmansky. Introductory Mining Engineering. 2d ed. Hoboken, N.J.: J. Wiley, 2002.

Hustrulid, William, and Mark Kuchta. Open Pit Mine Planning and Design. 2d ed. New York: Taylor and Francis, 2006.

Institution of Mining and Metallurgy. Surface Mining and Quarrying: Papers Presented at the Second International Surface Mining and Quarrying Symposium. London: Author, 1983.

"Open-Pit Mining." LibreTexts, 6 May 2022, geo.libretexts.org/Bookshelves/Geology/Fundamentals‗of‗Geology‗(Schulte)/12%3A‗Geological‗Implications/12.10%3A‗Open-Pit‗Mining. Accessed 27 Dec. 2024.

Smith, Duane A. Mining America: The Industry and the Environment, 1800-1980. Lawrence: University Press of Kansas, 1987. Reprint. Niwot: University Press of Colorado, 1993.

Tatiya, Ratan Raj. Surface and Underground Excavations: Methods, Techniques, and Equipment. London: A. A. Balkema, 2005.

Twitty, Eric. “The Technology of Open Pit Mining and Blasting.” In Blown to Bits in the Mine: A History of Mining and Explosives in the United States. Ouray, Colo.: Western Reflections, 2001.