opencut mining[′ō·p·ən¦kət ′mīn·iŋ]
(also open-pit mining or strip-mining), the recovery of minerals from the earth’s surface.
The oldest opencut stone quarrying dates to the sixth millennium B.C. Complex ores for smelting bronze were extracted by the opencut method in the fourth millennium B.C. in India, on the Sinai Peninsula, in the Caucasus, and in northern Ethiopia. Opencut mining of iron ores has been known since the second millennium B.C. in the Middle East and India and, since somewhat more recent times, in southern Europe. In the Middle Ages, the opencut method was used on a considerably larger scale for the extraction of nonferrous metals in Spain (Ríotinto), marble in Italy, and copper and iron ores in Russia (the Urals). Opencut mining was widely used for placer deposits in 18th-century Russia, at first in the Urals and later in Siberia. Vigorous development of opencut mining began in the early 20th century in the USA and Germany in connection with advances in technology. In prerevolutionary Russia, semimechanized opencut mining was prevalent in the Urals, Krivoi Rog, and Siberia.
In the USSR, the first large pits for the recovery of coal, ferrous and nonferrous metal ores, and nonmetallic ores were developed between 1928 and 1941. Opencut mining played an important role during the Great Patriotic War (1941–45) by making it possible for enterprises to begin operations rapidly and to have a significant increase in production capacity.
A special feature of the postwar period has been the mechanization of all production processes, a transition to higher-capacity machines and equipment, and standardization of excavator and transportation equipment. As of 1972, opencut mining accounted for 60–65 percent of world consumption of ore and nonmetalliferous raw materials and 30–35 percent of the consumption of solid fuel. This is explained by the economic efficiency of opencut mining. For example, the cost of opencut coal recovery is lower by a factor of 2.5–3.0 (for ore the factor is 1.5–2.0) than the cost of underground mining of deposits, and labor productivity is two to three times higher. With the use of high-capacity mining and transportation equipment, automation, and computerization, the opencut method can be used for the development of large deposits having ore with a low metal content, thereby increasing the reserves of scarce raw-material resources. Mineral losses are reduced by a factor of 4–5 in comparison with underground mining. Therefore, the general trend in the mining industry is toward increased use of the opencut recovery method (see Table 1).
|Table 1. Proportion of opencut mining in total extraction of minerals in USSR (percent)|
|Iron ore ...................||48.9||57.1||79.2|
In the USSR, opencut mining makes possible the development of large complexes for the recovery, processing, and consumption of raw material. The complexes are characterized by high production concentration, a well-developed transportation network, minimum distance for transporting raw materials, and low production costs. Thus, enterprises with an annual production capacity of 45–60 million tons of coal are being developed based on deposits of the Kansk-Achinsk Coal Basin and on the Ekibas-tuz coal deposit. In the iron-ore industry, enterprises extracting up to 30 million tons of raw ore per year are in operation. In the building-materials industry, enterprises with an annual asbestos ore output of 30–35 million tons are in operation, and enterprises with an annual capacity of 10–12 million tons of granite for the production of crushed stone are under construction (as of 1974).
The volume of barren rock removed to spoil heaps, or dumps, during opencut mining usually considerably exceeds the volume of the mineral recovered. The relation of these volumes is characterized by the stripping ratio, which sometimes reaches 25—that is, 25 tons of overburden excavated for each ton of mineral recovered. Efficient planning of mining operations according to periods is accomplished in accordance with mining operation charts and calendar schedules. The stripping methods and the system of opencut development depend on the shape and depth of bedding of the deposit and on the quantity and physicome-chanical properties of the overburden.
Opencut mining results in a change in landforms, the agricultural properties of the land, and the hydrogeological conditions of the region. The land is completely or partially reclaimed, depending on the value of the disrupted area.
Opencut mining consists of the following stages: preparation of the surface, drainage of the mineral deposits, fundamental mining operations, stripping operations (removal of barren rock overlying or enclosing the mineral), and recovery operations. The stripping and recovery operations include the processes of breaking out, excavating, hauling, and unloading the mineral. These main production processes are combined into a single process through full mechanization and automation. The secondary processes in opencut mining include clearing of benches, construction and repair of rail lines and motor-vehicle roads, and water drainage. Breaking out is the separation of a mass of rock and earth from a larger mass, and its simultaneous crushing, by means of blasting and drilling operations. Excavation and loading are usually performed by excavators and loaders. The rock and earth mass is removed from the face by quarry transport. A mass composed of rock that is not hard does not require preliminary loosening; in this case, stoping and loading are combined in a single process performed by excavators, scrapers, loaders, bulldozers, or other mechanical equipment or by hydromechani-zation. Dredges are used effectively for mining placers. Minerals are transported to storage areas or to processing plants, and the waste rock is transported to spoil banks.
Opencut mining production processes may be of the cyclical (batch), flow, or combination cyclical-flow type. In the cyclical operation, the excavating and transporting processes are interrupted by production pauses. In cyclical-flow operations, excavation is performed by cyclical-operation machines (mechanical shovels or loaders), and removal is performed by belt conveyors, a combination of conveyor and truck transport (sometimes using self-propelled crushing units or stationary and semistationary crushing, grading, or combination crushing-grading plants), or rail transport. In flow production, the processes of stoping, excavating, transporting, and unloading are performed by continuous-operation machinery, such as multibucket excavators, belt conveyors, or hydromechanization. Automatic control systems for the separate processes, with computer processing of data on the progress of the processes, have been designed and developed for cyclical and cyclical-flow production. Automatic control systems exist for equipment used in the flow process, particularly the continuous-operation equipment. In the USSR, equipment for continuous-operation processes is made up from complexes of machinery using rotary bucket excavators and having a theoretical capacity of 630, 1,250, 1,500, 2,500, 5,000, 10,000, or 12,500 m3/hr. Rotary bucket excavators with a normal cutting force are the most widely used type of continuous-operation equipment. The development and improvement of flow mining operations are associated with the use of semistationary and self-propelled crushers and crushing-screening units with capacities of up to 2,000 tons per hour, and with the use of reliable conveyors with belts capable of moving large pieces of abrasive material.
Among the factors taken into account in selecting the proper parameters for opencut mining operations and equipment are climatic conditions, the region of mining development, the properties of the rock, the mineral reserves, the shape of the deposit, and the requirements for quality of the finished product.
The sequence of opencut mining operations that provides economic and safe exploitation of a deposit is called the mining method. Several opencut mining methods exist. In the USSR, the most common classification of methods is that of N. V. Mel’-nikov (1950), which is based on the method of removing the overburden to the spoil heaps and on the type of equipment used; the classification consists of five groups.
Nontransport methods are those in which the overburden is transported from the face to a worked-out area by a stripping excavator. In spoil-transport methods, the overburden is moved to the spoil heaps by conveyor bridges or spoil spreaders. The rock is usually loaded onto the belt conveyors of the conveyor bridges and cantilever spoil spreaders by multibucket excavators (sometimes by mechanical shovels). In transport methods, rock is moved to inside spoil heaps, which are located in a worked-out area, or outside spoil heaps, which are located outside the quarry, by rail, truck, conveyor, skip, and combined haulage. In special methods, the stripped overburden is removed by dragline cableway excavators, bulldozers, wheeled scraper-excavators, or hydromechanization. In combined methods, the stripped overburden of the upper zone of the deposit is removed by transport facilities to inside or outside spoil heaps and the rock of the lower zone is removed to inside spoil heaps by excavators, conveyor bridges, or spoil spreaders.
The large volume of the stripping operations and the complex conditions of mining in quarries have predetermined the prevailing use of the transport mining method, which in the USSR will occupy a dominant position in the opencut operations of all sectors of the mining industry. High-efficiency nontransport and spoil-transport methods are being used successfully in the mining of stratified deposits of coal, manganese ores, and mining and chemical raw materials.
Classifications of opencut mining methods have also been developed by E. F. Sheshko (1950), V. V. Rzhevskii (1963), and others.
Opencut mining technology is being improved through full mechanization and optimization of mining operations and equipment, the development and introduction of new, efficient production-process flow systems, primary use of simple explosives and the continuous-operation method, expansion of the area of use of nontransport mining methods and cyclical-flow technology (with ordinary and specially designed equipment), and the use of optimum combined transport systems.
The prospects for strip-mining of coal in the USSR are based on deposits situated in the eastern part of the country (mainly the Kansk-Achinsk, Kuznetsk, and Ekibastuz basins), where about 98 percent of the geological reserves of coal suitable for strip-mining are concentrated. Opencut recovery of iron ore is centered on deposits in the Ukraine (the Krivoi Rog basin), the Central Zone (the Kursk Magnetic Anomaly), Kazakhstan (the Sokolov, Sarbai, Kachar, Lisakovsk, and Aiat deposits), and the Urals. Nonferrous metal ores are mainly recovered by the open-cut method in Siberia and Kazakhstan.
In other countries, opencut (or strip) mining is used for about 30 percent of the coal, about 75 percent of the iron ore, up to 80 percent of the nonferrous metal ores, more than 90 percent of nonmetalliferous minerals (asbestos, graphite, kaolin, mica, and talc), and almost 100 percent of the nonmetalliferous materials. Most opencut mining is done in the USA; the opencut method is also used for recovery of minerals in Australia, South America (Brazil and Venezuela), Canada, China, and Europe (the German Democratic Republic, the Federal Republic of Germany, Poland, and Czechoslovakia).
The most common method in ore recovery is the transport method, which uses high-capacity equipment, such as dump trucks with capacities of more than 100 cu m and heavy-duty excavators with power-shovel bucket capacities of up to 20 cu m. The nontransport mining method, with heavy-duty excavators (stripping excavators with bucket capacities of up to 150 cu m and draglines with capacities of up to 160 cu m), is commonly used for mining coal in the USA; high-capacity spoil-transport complexes are used in the German Democratic Republic and the Federal Republic of Germany. The cyclical-flow system, in which stationary or self-propelled crushing-grading plants are located in the quarry, is used in the extraction of nonmetalliferous building materials.
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