drainage (mining)

drainage in mining, removal of water seeping into shafts and other underground mine workings from the surrounding ground. Unless seeping water is removed continually, it may endanger haulage and mining equipment, weaken supporting structures, and, in some instances, flood the mine completely. Water in a mine is drained into sumps, or reservoirs, usually excavated below the lowest working level of the mine, and is then removed by pumps. Methods to minimize seepage include the sealing of visible fissures through which water enters and the injection of concrete into the ground surrounding the shafts.

mine drainage Mining activities often result in the production of large amounts of waste rock. These dumps of rock, or tailings, contain all the minerals that are not required, together with small quantities of the ore minerals that are the object of mining activities. Where metals are mined, the ore minerals in the waste are typically sulphides (e.g. pyrite, FeS₂, and sphalerite, ZnS). Breakdown of sulphide minerals occurs when an increased surface area of mineral is available to interact with atmospheric oxygen and bacteria, and can result in the formation of acid and transport of the metals away from the tailings dump.

When sulphide-rich minerals are exposed to oxygen, they start to break down as a result of hydrolysis of the metal ion. For example,Zn²⁺ + H₂O = ZnOH⁺ + H⁺.

This reaction generates acidity (H⁺). Breakdown of pyrite, FeS₂, a common sulphide mineral, results in a very acidic solution:2FeS₂ + 7.50₂ + 4H₂O = Fe₂O₃ + 4SO₄²⁻ + 8H⁺.

Most metals are soluble in acidic solutions, and large amounts of metals can therefore be dissolved in such drainage. When the acidity of these fluids is decreased, for example by mixing with river water, the metals become less soluble and start to precipitate out of solution. This results in the red coloration that is typically seen where acidic mine drainage comes into contact with river water and iron oxyhydroxides, FeOOH, are precipitated.

Elizabeth H. Bailey

coal industry. This has the longest continuously recorded history of any British industry, beginning with monastic and manorial documents of the 13th cent. Pits were small, shallow, or outcrops, capital investment was minuscule, drainage was an insignificant problem, and markets were generally local. However, by the 14th cent. a water-borne export trade from the Forth and Tyne had developed. Expansion occurred in the 16th and 17th cents. as a consequence of the growth of markets. By 1690 coal output was approaching 3 million tons, with Northumberland and Durham accounting for 45 per cent of production.

Growth in output resulted from greater investment by landowners with coal on their estates. Supply was partly affected by the Reformation and secularization of ecclesiastical land. New owners exploited coal reserves more actively and sought markets within and beyond their localities. Where land was leased for mining, leases were often longer, encouraging investment and entrepreneurship. Mining methods improved considerably, and coal-mining was a major stimulus of invention, especially in the use of water-wheels to drive drainage pumps. Ultimately, between 1698 and 1705 the problem of drainage led to steam pumps devised by Thomas Savery and Thomas Newcomen. Demand from London accounted for the dominance of the seasale mines of the Great Northern Coalfield; domestic consumers were very important, but the industries of London—smiths, glass-makers, owners of sugarhouses, brick and tile kilns, breweries and distilleries—should not be ignored. A range of industries from lime-burning to dyeing encouraged landsale and seasale, the former operating close to the pit-head, thereby offsetting transport costs.

Demand for coal escalated in the 18th cent. as the iron and metal industries altered their methods of production. Smelting by coke, achieved by Abraham Darby I in 1709, spread rapidly after 1760. In the late 18th cent. the Cranage brothers, Peter Onions, and Henry Cort applied coke to the puddling process in producing bar iron. New industries, notably gas, chemicals, the railways, and steam-power installations, also widened demand. Domestic consumers increased in number as urbanization proceeded. By 1800 output exceeded 11 million tons and in 1854 approached 65 million tons. It has been estimated that annual gross capital formation increased nearly sixfold between these years from over £480,000 to over £2.5 million. Technical and commercial constraints on growth were removed in this period. The steam-engine solved the problem of mine drainage; ventilation was improved in deeper pits by the sinking of air shafts; George Stephenson's and Humphry Davy's safety lamps began the improvement in underground lighting and mining safety, although gases remained a hazard; a labour-intensive industry which used ponies underground, as well as women and children in coal haulage, mining gained from the wire rope, the cage, and the rotary steam-engine, all features of substantial collieries by 1855. Cheaper transport was achieved by river improvement, canals, and railways. One per cent of national income in 1800, the coal industry accounted for about 6 per cent by 1900. Coal represented 5 per cent of the total value of exports up to the 1870s and by the early 20th cent. about 10 per cent. The industry was Britain's major male employer by 1914, when output reached 287.4 million tons, the highest figure ever attained.

After the war (1914–18) coal went into structural decline, encountering massive labour problems and declining productivity, interrupted only by the Second World War (1939–45) and its immediate aftermath. Nationalized in 1947, the industry was extensively modernized at great public cost. Demand fell as alternative fuels gained ground. High-costing pits closed after 1959, and declining productivity was reversed. The number of collieries fell from 901 in 1951 to 438 in 1967, a process hastened in the 1970s and 1980s; one consequence was a substantial fall in the mining labour force. Despite a growing emphasis on open-cast working and new mechanized mines, such as the Selby complex, in an abortive attempt to compete with foreign imports, the industry continued its decline in the 1990s, returning to private ownership in 1994–5. By 1997 there were only 20 large deep mines still in operation.

John Butt