MINING

process of extracting useful minerals from the surface of the Earth, including the seas. A mineral, with a few exceptions, is an inorganic substance occurring in nature that has a definite chemical composition and distinctive physical properties or molecular structure. (One organic substance, coal, is often discussed as a mineral as well.) Ore is a metalliferous mineral, or an aggregate of metalliferous minerals and gangue (associated rock of no economic value), that can be mined at a profit. Mineral deposit designates a natural occurrence of a useful mineral, while ore deposit denotes a mineral deposit of sufficient extent and concentration to invite exploitation. When evaluating mineral deposits, it is extremely important to keep profit in mind. The total quantity of mineral in a given deposit is referred to as the mineral inventory, but only that quantity which can be mined at a profit is termed the ore reserve. As the selling price of the mineral rises or the extraction costs fall, the proportion of the mineral inventory classified as ore increases. Obviously, the opposite is also true: a mine may cease production because (1) the mineral is exhausted or (2) the prices have dropped or costs risen so much that what was once ore is now only mineral. William Andrew Hustrulid excavation of materials from the Earth's crust, including such minerals of organic origin as coal and petroleum. Mankind has found uses for minerals found in the Earth since prehistoric times. Materials like flint, for making fire, and pigment-bearing minerals like ochre and manganese ore were probably the first substances that people mined. The common names of archaeological periods like the Bronze Age and the Iron Age indicate significant use of those metals. Bronze (copper alloyed with tin) dates to roughly 3700 BC, and iron dates to as early as 2800 BC; both were first used in Mediterranean civilizations. Stone obtained from surface excavation, a process called quarrying, was responsible for the great Egyptian pyramids, some of which used as many as 2,300,000 blocks of limestone weighing 16 tons (14,500 kg) each. An early method of mining, first recorded in Roman times, involved the use of log fires built at the base of rock faces. The intense heat caused expansion and cracking of the rock. The thorough treatise on mining entitled De re metallica (1556) by the German scholar Georgius Agricola has provided the best source of information on early mining techniques, many of which are still used or were used until relatively recently. These techniques included picks and hammers, ventilation and pumping systems, and cart-like trucks for hauling minerals. Later advances of great significance included the use of explosive gunpowder, which was replaced by dynamite in the mid-19th century and supplemented by ammonium nitrate fuel-blasting agents in the mid-20th century. Drilling techniques have also enhanced mining capability. The first rotary drill may have been used in England in 1813. Thirty years later mechanical piston drills came into use, and the air drill, powered by compressed air, appeared in Germany in 1853. Modern mining is a costly and complicated business. It begins with the locating of probable mineral veins that can produce enough of the desired substance to justify the high cost of extraction. Prospecting and exploration require a vast body of knowledge in the earth sciences to find likely mining locations. Geological evidence relates the age of sample rocks to the long-term mineralization processes of the Earth's crust. Geochemical evaluations are madeas well as seismic tests, magnetic analyses, and electrical studiesto determine the geophysical character of a section of earth. Also commonly used is a hollow cylinder drill that brings up core sample materials from deep in the ground. Once the approximate location and size of a vein or deposit are determined, mining engineers decide the best way to mine it. The familiar type of mine opening directly into the base of a hill or mountain on a horizontal plane is called an adit or crosscut tunnel. A straight vertical shaft may be drilled to one side of a large deposit, with horizontal tunnels running at periodic levels into the deposit. The cavernous openings where minerals are excavated are called stopes. Many times a vein of ore will run from the surface, where it appears as an outcrop, deep into the ground on a diagonal. An inclined shaft runs parallel to the vein with horizontal levels connected to the vein at intervals. Ore that is found at the surface is mined from an open pit or from long strip excavations in a process called strip mining. Excavating a mine and extracting mineral substances involves different combinations of drilling, blasting, hoisting, and hauling. Drilling a shaft itself may involve using a large circular bit that has a series of grinding wheels for cutting into rock. Drills are also commonly used for placing explosives. Drag-bit rotary drills use an abrasive, shaving action from diamond blades or steel shot, and the percussion drill pounds at the rock with a tungsten cutting edge that rotates at the same time. By carefully setting explosives in drilled holes, mining engineers can now use blasting agents of ammonium nitrate, which is effective at only 40 percent the strength of dynamite. In some hard rocks, heat applied through a flame-jet or jet-piercing drill causes the rock surface to chip or spall. Careful consideration is given to the nature of the excavated sections to determine what support is needed in the mine. In some, the ceiling on an inclined shaft may not need support because of the lower ground weight from above in such a shaft. The ceilings of open stopes, the large cavities dug in the middle of deposits, may only need support from pillars of ore or waste rock. Rock ceilings with faults and fissures may be covered with shotcrete, a sprayed concrete that sets in and reinforces the cracks. Elsewhere, steel rods may be imbedded in the rock with large nuts threaded on the exposed end to hold up weakened segments of the ceiling. Where the span of the ceiling and the ground weight above dictate, structural support is provided. This may range from a variety of timber supports to collapsible steel arches. Timber arrangements include simple beams, or stulls, with diagonal supports into the side walls and a wood ceiling; an inclined beam against the hanging wall and wedged against the opposing foot wall with another beam; a four-piece set that consists of a complete beam frame (top, bottom, and sides); the timber square set linking frames together; and a post that supports the ceiling. In loose ground, ceiling sections are short and built at angles, and at the digging face a top section of planking called the breast boards keeps the earth from sliding backward. Supported stopes like these move slowly and produce a lower volume of ore. In unsupported stopes a number of methods help speed up production. These include slicing broad sections from the stope ceiling and also allowing large sections to cave in. Removal of minerals from mines usually depends on trains of steel boxes drawn along tracks by an electric locomotive; vehicles running on treads or rubber wheels are also used. In some mines, large hoe-like scrapers pull rubble out by cable. Hand carrying, or mucking, is still practiced where mechanical means cannot be used. A vital consideration in mining is how to ventilate the underground tunnels and caverns, not only to provide fresh air to miners but also to disperse harmful gases. Some mines become unbearably hot and need cooling. Sometimes ventilation ducts arranged in certain combinations can create natural drafting of air; usually, powerful fans are necessary. In addition, electric lighting must be provided. These two support systems often constitute a large part of mining costs. Additional reading A comprehensive and fascinating tour of mining history from the Stone Age to the present is offered by Cedric E. Gregory, A Concise History of Mining (1980). Russel Madigan, Of Minerals and Man (1981), describes the history and changing uses through history of many of the major minerals. Agricola, Georgius Agricola de re metallica, trans. by Herbert Clark Hoover and Lou Henry Hoover (1912, reissued 1950; originally published in Latin, 1556), is a classic work on early mining in Europe. Mining practices of the 1990s are thoroughly described in Howard L. Hartman (ed.), SME Mining Engineering Handbook, 2nd ed., 2 vol. (1992).United States Forest Service, Anatomy of a Mine from Prospect to Production (1977), is a brief but excellent introduction to metal mining. Works on surface mining include Eugene P. Pfleider (ed.), Surface Mining (1968, reprinted 1972); and Bruce A. Kennedy (ed.), Surface Mining, 2nd ed. (1990). W.A. Hustrulid (ed.), Underground Mining Methods Handbook (1982), is a comprehensive reference describing all aspects of underground mining. Louis W. Cope and Lee R. Rice (eds.), Practical Placer Mining (1992), provides a very useful reference. John L. Mero, The Mineral Resources of the Sea (1964), is still a highly informative source.A broad view of the mining industry, focusing on financial and administrative aspects, is provided in Raymond F. Mikesell and John W. Whitney, The World Mining Industry: Investment Strategy and Public Policy (1987). The United States Bureau of Mines regularly publishes technical reports on a wide variety of mining topics; its Mineral Facts and Problems (quinquennial) is a massive review, while the Minerals Yearbook is a good source of current information. Stanley J. Lefond (ed.), Industrial Minerals and Rocks (Nonmetallics Other Than Fuels), 5th ed., 2 vol. (1983), provides a good overview of this important segment of the mining business.Current thinking on mining theory, policy, research, and practice is found in such periodicals as Mining Magazine (monthly); Mining Journal (weekly); Mining Engineering (monthly); Canadian Mining Journal (monthly); Institution of Mining and Metallurgy Transactions, Section A: Mining Industry (quarterly); AMC Journal: A Publication of the American Mining Congress (monthly); South African Mining, Coal, Gold & Base Minerals (monthly); Engineering and Mining Journal (monthly); Stone World (monthly); and Dimensional Stone Magazine (10/yr.). William Andrew Hustrulid