mining

mining Mining is the process of extracting solid natural resources from the shallow parts of the Earth's crust. Mining has been carried out by man since it was first realized that materials within the ground could be valuable, whether as fuel, for tools, or as decorative materials in jewellery or for religious use. Mining is known to have taken place for thousands of years; there are early indications of the perceived importance of geology at a number of sites. In the remains of the gold–silver workings at Cassandra in Greece there is evidence that miners dug in search of faulted vein segments at some time prior to 300 bc. Their Athenian contemporaries, faced with the depletion of silver and lead ore at Laurium, recognized the significance of marble near a schist contact and sank more than a thousand shafts through barren rock, some to depths of 100 m, in search of hidden orebodies.

The materials that are recovered by mining operations can be divided into six general categories: coal and low-grade coals; metalliferous materials (non-ferrous); iron, including all types of iron ore; rock, including construction materials such as building stone; precious stones; and evaporite deposits.

The two main methods of extraction are opencast mining and underground mining. Opencast mining entails the excavation of open pits from the ground surface so that the raw materials can be transported from the mine to a processing plant or export point. Underground mining entails sinking shafts to reach the target resource and driving tunnels and adits, either inclined or horizontally.

Whether to extract a resource by opencast or underground mining depends on the depth of the ore or rock below the ground surface and the concentration of the material per unit volume. In historical times, underground mining was carried out at extremely shallow depths because of the lack of mechanical equipment for removing the overburden. As a result of advances in mechanization, underground mining is no longer carried out on an industrial scale at shallow depths. There is no comprehensive formula for the depth of a mine: each mine and its ore are unique. For example, as a result of highly efficient production methods, mines in the Witwatersrand gold fields of South Africa operate at depths in excess of 3000 m where the specific geological conditions are suitable.

The ‘bigger, faster, and deeper’ requirements of modern mining are often met by scaling up the mining equipment and giving that equipment enough room to work. This ‘up-scaling’ is generally accommodated easily in open-pit mines, but in underground workings the creation of large caverns brings with it problems of rock mechanics and increased risk of instability.

In subsurface ore mining, headings are driven into new ground, level drifts follow the ore, level crosscuts connect drifts, and vertical or inclined raises connect the workings from level to level. Underground mining systems consist of workings that may be naturally supported, may require artificial support, or may be allowed to collapse as part of a caving method, according to the orebody geometry and the engineering properties of the strata. Because very few mineral deposits are uniform, most mines make use of more than one method.

The three main underground mining methods are stoping, room-and-pillar mining, and longwall mining.

In stoping, ore is blasted or excavated in a vertical or inclined stope and extracted from the base of the stope for removal from the mine. Various methods are used, including sub-level open stoping, longhole stoping, and cut-and-fill stoping, according to the geometry of the orebody and the relative strengths of the orebody and wall rock.

Room-and-pillar mining is best suited to gently dipping and relatively uniform bedded deposits, provided that the deposits are not too deep and the rock is strong enough for the pillars to support the overlying strata. At depths beyond 1000 to 2000 m, hard-rock room and pillar mining methods are considered dangerous because of rock bursts and other results of high stress concentration. The limiting depth in coal mines is much less, in the region of 200 to 300 m, since the rocks of the Coal Measures tend to be weaker. Room-and-pillar mining was used extensively in the UK and Europe for the extraction of building stone, such as the Jurassic oolitic limestones of the Bath area.

Longwall mining began in the coalfields as a method of extracting seams at depths in excess of 200 m. It is by far the most common method of working in European coal mines where the shallower seams have been depleted. Longwall mining is best suited to deposits ranging in thickness from 1 to 2.5 m, dipping at less than 12°, with relatively incompetent rock and a fairly competent floor. The method lends itself to mechanization, especially in coal mines where fast-moving shearing machines move across a 100- to 200-m face and load the coal into conveyors without the need for blasting.

Other forms of mining take place, most notably solution mining (leaching) or brining of evaporite deposits, which is a long-established method. The process is extensively used in the mining of halite in Cheshire.

Brian J. Mcconnell