Abstract
The burning of slag heaps is a specific feature of regions where coal is (or was) exploited. This spectacular phenomenon is relatively common and closely followed, on the one hand because of the risk of explosion linked to it and on the other hand because of the environmental consequences. From the viewpoint of petrographers, unusual ultrahigh temperature at ambient pressure paragenesis develop, as well as complex mineral blooms, thus providing opportunities for their study.
Slag heaps
In order to understand the unusual mineralogy, complex and variable that results from burning sslag heaps, it seems important to give the characteristics of them. It is a hill on which is spilled all rubble related to mining: sterile (sandstone, arkose, ...) still containing remains of carbonaceous organic materials, and also various waste: used rails, demolition wastes... Historically, the first slag heaps (18th-19th century) were flat and did not reach great heights (10-30 m) because they were built by dumping trucks, their volume did not exceed the million m3. Coal separation processes were then not very effective. In the past, coal sorting was manual and rocky fragments of large size(up to 15-20 cm); consequently the old slag heaps have a lot of empty volumes and contain coal-rich residues (up to 20-25%). Thus, they are more likely to catch fire. Technical advances have resulted in the elevation of the slag heaps as cones, reaching 100 m height for a radius of 100 to 200 m and a volume of 1 to 3 million m3. They are built through conveyor belts, cable or mobile ramps. In addition, improving coal cleaning processes has led to the reduction of the coal dust concentration in the heaps.
The causes of combustion
Except for the deliberate burning to turn their content into reusable materials, burning of slag heaps is generally an accidental phenomenon, caused most frequently by lightning, a nearby forest fire, or by a short circuit. The auto-ignition and spontaneous combustion of the coal-rich residues, as well as the heat input due to the exothermic decomposition of pyrite, are the internal and typical causes of the combustion of the majority of slag heaps in the world. Moreover, the weakly compacted nature of the deposits facilitates air calls as well as its flow to the internal parts. Mildew on the wood pieces can also contribute to the internal temperature rise (Figure 1). It was estimated in the Nord-Pas-de-Calais, that one slag heap out of three come into combustion, which in the early 1980s accounted for 74 slag heap. Combustion of slag heaps is thus a complex phenomenon.
Figure 1 : Schematic of a slag heaps and representation of the different factors causing their combustion.
Durée
It is hard to precisely know when the combustion of a slag heap starts, but once started, it can last for decades. It is the case for the slag heap of Ricamarie (42), who burns since the '50s, or the Avion slag heap (62) who's in combustion since dacades. In Tchelyabinsk bassin (Russia), some slag heaps have been burning for 30 to 50 years.
Exemples
Many examples are known around the world, in exploited or former coal bassin: Silesia (Poland), Tchelyabinsk or south of Ural (Russia), Wallonia (Belgium), Douro (Portugal)…
In France, many slag heaps are in active combustion, or have recenly been, one can cite the slag heaps from Haveluy (59) (Figure 2), Avion, Ricamarie, or la Taupe (Brassac-les-Mines bassin, 43), Champclauzon (La Grand-Combe bassin near Alès, 30), Rochebelle à Alès (30), Gardanne (13)…
Figure 2 : Slag heap n°157 from Nord-Pas-de-Calais coal bassin, in combustion, stands as a volcano above Haveluy (59) near Valenciennes (a). One can observe fumaroles as well as calcinated vegetation (b), flames are sometimes visible.
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To validate your visit:
1-. With informations found in the description, Sainte-Marie slag heap is old or new?
2-. Walking around it, is the slag hill currently in combustion, has it already been in combustion, or it has never been ?
3-. Dig a small hole with you hands and touch the bottom, what about the temperature, is it varying?
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