Slag Heap

Waste Rock - Spoil Tip - Overburden

A slag heap is a hill made from waste material, such as rock and mud, left over from mining. The name differs from mining area to mining area, slag heap is the British miner term. Common names are spoil tip, boney pile, culm bank, gob pile, waste tip, bing, mine dump or even mine waste dump.

photography — Botayama at Iizuka City in 1950s, Japan. Public Domain.

There are numerous mining terms for material which is not ore. If something is mined in open cast, it is covered by overburden. If material is transported out of the mine, it is separated, ore from other material, and as a result, the ore is enriched for further processing. The rock which is removed because it does not contain "economic amounts" of mineral is called waste rock. Those waste rocks are transported to piles or dumps, the British miners term is slag heaps.

The idea that such "waste" rock is inert, like granite or quartzite, is too naive. Actually, it is rock with ore, or at least it was in contact with ore. If it contains residuals which are not economically valuable, they may still be poisonous or pose serious environmental hazards. Think about uranium mining or quicksilver mining slag heaps. Some uranium mines have slag heaps which are so radioactive that no plants grow on the rocks. But even "harmless" residuals like iron ore can colour a river red and make the water deadly to most animals and plants. This happened in Spain and led to the river being named Rio Tinto (red river) which is also the name of the mining company. Slag heaps from coal mining can start burning. And when the rocks are soluble, not only is material washed out and pollutes the river, the material leaves holes in unconsolidated piles, causing collapses and rockfalls.

Then there are chemicals which are added by the mining company to separate the ore from the waste rock. Even if the idea is to retrieve most of those reagents, some will react with the waste rock, or the process simply does not allow removing all of it. In some cases, the reagent is too cheap to replace, so it is not retrieved at all to save money. And the list of such chemicals is quite impressive: ammonia, chlorine, hydrochloric acid, sulphuric acid, cyanide, and mercury. In other words: slag heaps are frequently contaminated.

Even coal mine slag heaps, which consist of shale, sandstone, limestone and other sedimentary rocks and some remnants of the coal, can be quite dirty. The mining company deposite any kind of waste with the slag, used rails, demolition waste, sleepers, and sometimes even household waste. The rubbish of the whole mining town was collected and dumped on the heap. And they have another danger, due to the presence of residual hydrocarbons. Carbon-rich deposits with up to 30% carbon are prone to self-ignition and not only the coal but also the pyrite in the rock burns. These slag heaps can burn for up to thirty years, while they release sulphurous vapours and bad odours. Also, it's quite dangerous to cross such burning heaps because there are spots with dangerous heat.

And finally, there are developments in technology, a new process allows extracting a resource from the low grade slag heap economically. Or a material, which was previously not used, is now needed in some way and becomes valuable. This happened, for example, to several metals which were dropped on the heaps in the Middle Ages and later, the new process allowed to extract them from the waste rock. Or fluorite and baryte, which were just rock which does not contain metal, one is helpful as an additive in furnaces, the other is used for shielding X-rays.

However, the slag heaps are extremely popular with mineral and fossil collectors. Especially because it is far too expensive to monitor these huge areas with security guards. And so they can search for minerals on the slag heaps, which is much less dangerous and also less illegal than entering the mine itself. Most slag heaps do not even have a serious fence. There are even slag heaps that are opened up specifically for collectors, the Clara mine in the Black Forest in Germany is particularly famous here. But there are also some so-called ‘fee mining sites’ on slag heaps in the USA.

However, slag heaps that contain few toxic substances can become biodiversity hotspots in just a few decades. Due to the lack of humus, they are colonised by completely different plants and animals than the surrounding area and are an ideal habitat for pioneer species. They are also favoured by xero-thermophilic species that seek out particularly warm or cold habitats. For example, 500 plant species, 90 bird species, 40 butterfly species and 20 beetle species have been recorded on a spoil tip in Belgium.