A slag heap (also called bing, Boney piles, culm, waste coal, Terekons (Russian), gob piles, or slate dumps) is a pile built of accumulated tailings, which are by-products of mining. These waste materials are mostly composed of shale, as well as smaller quantities of carboniferous sandstone and various other residues.
Slag heaps may be conical in shape, hence appearing as conspicuous features of the landscape, or they may be much flatter and eroded, especially if vegetation is established thereon. The highest in Europe is in Loos-en-Gohelle in the former mining area of Pas-de-Calais. It comprises a range of five cones, of which two reach 180m (590 ft), surpassing the highest peak in Flanders, Mont Cassel.
The oldest coal-based slag heaps may still contain enough coal to begin spontaneous slow combustion. This results in a form of vitrification 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 and 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 slag heaps became colonised with now more or less forgotten varieties of fruit trees. One can even observe the proliferation of buckler-leaved sorrel (French sorrel - Rumex 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 slag heap is significantly warmer than its surroundings, which contributes to the diverse ecology of the area. In this way, the slag heap of Pinchonvalles, at Avion, hosts 200 different varieties of higher plants. Some thirty species of birds nest there.
Some cultivate vines, as in the case of slag heap No7 of the coal-mining region of Mariemont-Bascoup near Chapelle-lez-Herlaimont (province of Hainaut) which produces some 3,000 litres of wine each year.
Finally, some slag heaps provide for various sporting activities. The slopes of the slag heaps of 11/19 at Loos-en-Gohelle, or again, at Noeux-les-Mines, are used for winter sports, for example ski and luge, since the provision of a piste on the flank of the heap.
In Belgium, a long distance footpath along the slag heaps (terrils) (GR-412, Sentier des terrils) has been opened in 2005. It leads from Bernissart in western Hainaut to Blegny in the province of Liège.
Heaps 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 vegetation to take root; this encourages erosion and creates dangerous, unstable slopes. Existing techniques for regreening slag heaps include the use of geotextiles to control erosion as the site is resoiled and simple vegetation such as grass is seeded on the slope.
The piles also create acid rock drainage, which pollutes streams and rivers.
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.