Slag heap
Encyclopedia
Overburden
Overburden is the material that lies above an area of economic or scientific interest in mining and archaeology; most commonly the rock, soil, and ecosystem that lies above a coal seam or ore body. It is also known as 'waste' or 'spoil'...
removed during coal and ore mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
. These waste materials are generally composed of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, as well as smaller quantities of carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
and various other residues. Spoil tips are not formed of slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
, but in some areas they are referred to as slag heaps.
Spoil is distinct from tailings
Tailings
Tailings, also called mine dumps, slimes, tails, leach residue, or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore...
, which is the processed material that remains after the valuable components have been extracted from ore.
Spoil tips may be conical in shape, and can appear as conspicuous features of the landscape
Landscape
Landscape comprises the visible features of an area of land, including the physical elements of landforms such as mountains, hills, water bodies such as rivers, lakes, ponds and the sea, living elements of land cover including indigenous vegetation, human elements including different forms of...
, or they may be much flatter and eroded, especially if vegetation has established itself. The highest in Europe is in Loos-en-Gohelle
Loos-en-Gohelle
Loos-en-Gohelle is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:A former coal mining town, three miles northwest of the centre of Lens, at the junction of the D943 and the A21 autoroute. Its nearest neighbours are Lens to the south, Grenay to the...
in the former mining area of Pas-de-Calais. It comprises a range of five cones, of which two reach 180 metres (590.6 ft), surpassing the highest peak in Flanders
Flanders
Flanders is the community of the Flemings but also one of the institutions in Belgium, and a geographical region located in parts of present-day Belgium, France and the Netherlands. "Flanders" can also refer to the northern part of Belgium that contains Brussels, Bruges, Ghent and Antwerp...
, Mont Cassel.
The oldest coal-based spoil tips may still contain enough coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
to begin spontaneous slow combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
. This results in a form of vitrification
Glass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...
of the shale, which then acquires sufficient mechanical strength to be of use in road construction. Some can therefore have a new life in being thus exploited; for example, the flattened pile of residue from the 11/19 site of Loos-en-Gohelle. Conversely, others are painstakingly preserved on account of their ecological wealth. With the passage of time, they become colonised with a variety of flora
Flora
Flora is the plant life occurring in a particular region or time, generally the naturally occurring or indigenous—native plant life. The corresponding term for animals is fauna.-Etymology:...
and fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
, sometimes foreign to the region. This diversity follows the mining exploitation. For example, because the miners threw their apple or pear cores into the wagons, the spoil tips became colonised with fruit trees. One can even observe the proliferation of buckler-leaved sorrel (French sorrel - Rumex
Rumex
The docks and sorrels, genus Rumex L., are a genus of about 200 species of annual, biennial and perennial herbs in the buckwheat family Polygonaceae....
scutatus), the seeds of which have been carried within the cracks in the pine timber used in the mines. Furthermore, on account of its dark colour, the South face of the spoil tip is significantly warmer than its surroundings, which contributes to the diverse ecology of the area. In this way, the spoil tip of Pinchonvalles, at Avion
Avion, Pas-de-Calais
Avion is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:An ex-coalmining industrial town, with a little farming, situated just south of Lens at the junction of the N17, D40 and D55 roads.-Population:...
, hosts 200 different varieties of higher plants. Some thirty species of birds nest there.
Some cultivate vines, as in the case of spoil tip No7 of the coal-mining region of Mariemont-Bascoup near Chapelle-lez-Herlaimont
Chapelle-lez-Herlaimont
Chapelle-lez-Herlaimont is a Walloon municipality located in the Belgian province of Hainaut. On January 1, 2006 Chapelle-lez-Herlaimont had a total population of 14,022. The total area is 18.10 km² which gives a population density of 775 inhabitants per km²....
(province of Hainaut) which produces some 3,000 litres of wine each year.
Loos-en-Gohelle
Loos-en-Gohelle is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:A former coal mining town, three miles northwest of the centre of Lens, at the junction of the D943 and the A21 autoroute. Its nearest neighbours are Lens to the south, Grenay to the...
, or again, at Nœux-les-Mines
Nœux-les-Mines
Nœux-les-Mines is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:Nœux-les-Mines is situated some south of Béthune and southwest of Lille, at the junction of the D937 and D65 roads...
, are used for winter sports, for example ski
Ski
A ski is a long, flat device worn on the foot, usually attached through a boot, designed to help the wearer slide smoothly over snow. Originally intended as an aid to travel in snowy regions, they are now mainly used for recreational and sporting purposes...
and luge
Luge
A Luge is a small one- or two-person sled on which one sleds supine and feet-first. Steering is done by flexing the sled's runners with the calf of each leg or exerting opposite shoulder pressure to the seat. Racing sleds weigh 21-25 kilograms for singles and 25-30 kilograms for doubles. Luge...
, since the provision of a piste
Piste
A piste is a marked ski run or path down a mountain for snow skiing, snowboarding, or other mountain sports. The term is European, from the French for trail or track, synonymous with trail, slope, or groomed run in North America....
on the flank of the heap.
In Belgium, a long distance footpath along the spoil tips (GR-412, Sentier des terrils) was opened in 2005. It leads from Bernissart
Bernissart
Bernissart is a Walloon municipality located in the Belgian province of Hainaut. On January 1, 2006 Bernissart had a total population of 11,458. The total area is 43.42 km², which gives a population density of 264 inhabitants per km²....
in western Hainaut to Blegny
Blegny
Blegny is a Walloon municipality located in the Belgian province of Liège. On January 1, 2006 Blegny had a total population of 12,799. The total area is 26.07 km² which gives a population density of 491 inhabitants per km²....
in the province of Liège
Liège (province)
Liège is the easternmost province of Belgium and belongs to the Walloon Region. It is an area of French and German ethnicity. It borders on the Netherlands, Germany, Luxembourg, and in Belgium the provinces of Luxembourg, Namur, Walloon Brabant , and those of Flemish Brabant and Limburg . Its...
.
In the United States, mining companies have not been allowed to leave behind abandoned piles since the Surface Mining Control and Reclamation Act
Surface Mining Control and Reclamation Act of 1977
The Surface Mining Control and Reclamation Act of 1977 is the primary federal law that regulates the environmental effects of coal mining in the United States....
was passed in 1977.
Future
The question of re-utilisation of spoil tips is sometimes raised.- At Nœux-les-MinesNœux-les-MinesNœux-les-Mines is a commune in the Pas-de-Calais department in the Nord-Pas-de-Calais region of France.-Geography:Nœux-les-Mines is situated some south of Béthune and southwest of Lille, at the junction of the D937 and D65 roads...
, an artificial skiSkiA ski is a long, flat device worn on the foot, usually attached through a boot, designed to help the wearer slide smoothly over snow. Originally intended as an aid to travel in snowy regions, they are now mainly used for recreational and sporting purposes...
slope has been provided on an old spoil tip, the building of which has provided it with a novel use. - Because they are inappropriate for building purposes, old spoil tips can be partially revegetated and provide valuable green spaces.
- Some spoil tips contain residual coal and evolving techniques allow renewed exploitation of this asset.
- There has also been research into various recyclingRecyclingRecycling is processing used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution and water pollution by reducing the need for "conventional" waste disposal, and lower greenhouse...
techniques, which result in spoil being removed from the site and potentially used for other commercial or construction purposes.
Environmental effects
Spoil tips sometimes grew to millions of tons, and, having been abandoned, remain as huge piles today. They trap solar heat, making it difficult (although not impossible) for vegetationVegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
to take root; this encourages erosion and creates dangerous, unstable slopes. Existing techniques for regreening spoil tips include the use of geotextile
Geotextile
Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain...
s to control erosion as the site is resoiled and simple vegetation such as grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
is seeded on the slope.
The piles also create acid rock drainage
Acid mine drainage
Acid mine drainage , or acid rock drainage , refers to the outflow of acidic water from metal mines or coal mines. However, other areas where the earth has been disturbed may also contribute acid rock drainage to the environment...
, which pollutes streams and rivers.
Subterranean combustion
As some spoil tips resulting from industries such as coal or oil shale production can contain a relatively high proportion of hydrocarbons, they can commence spontaneous subterranean combustion, which can be followed by surface fires.Such fires can follow slow combustion of residual hydrocarbons. Their extinction can require complete encasement, which can prove impossible for technical and financial reasons. Sprinkling is generally ineffective and injecting water under pressure counter-productive, because it carries oxygen, bringing the risk of explosion.
The weak environmental and public health impact of these fires leads generally to waiting for their natural extinction, which can take a number of decades.
Landslip
With spoil tips there is a danger of landslip. An example was the Aberfan disaster in WalesWales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
1966, in which a total 144 people were killed, 116 of whom were school children, mostly between the ages of 7 and 10. Five teachers also died in the accident.