Hydrocarbon condensates and argillites in the Eliška Mine burnt coal waste heap of the Žacléř coal district (Czech Republic): Products of high- and low-temperature stages of self-ignition

The Eliška Mine waste coal heap burned intensely in the 1960s and 1980s, and even now, on the surface of the heap, active vents still occur from which warm air (temperature 25–35°C) and water vapor escape. In the neighborhood of active vents, the surficial part of the heap is enriched in hydrocarbon condensates (HCC). Based on differences in optical reflectance and fluorescence color, two types of HCC were identified: The A-type HCC with yellow fluorescence and very low reflectance (~0.30%), which occurs as a binding material or filler of cracks in inorganic and organic particles, and the B-type HCC without fluorescence and with higher reflectance (1.04 to 1.08%). These HCC are probably formed through condensation of organic pyrolysates on the heap surface in various stages of burning, and the two types of HCC most probably reflect a gradual decrease in burning temperature and progressive cooling of the heap surface. This assumption is supported by the chemical composition of HCC extracts, revealing volatile compounds that have formed at various temperatures. Changes in the distribution of n-alkanes, O-, N-, and S-containing compounds also occurred in the chemical fingerprint. The zone enriched in condensates of semivolatile hydrocarbons was also enriched in Hg and S relative to the burnt heap substrate. The underlying argilite zone consisted of a mixture of clay minerals (illite, kaolinite and smectite), jarosite, alunite, and amorphous SiO2, together with mullite, cristobalite, trydimite, and sanidine. This mineral association indicates that argillitization affected the burnt heap substrate (the clinker), which forms a footwall of HCC-enriched and argilite zones. The origin of argilites is thought to be due to the exposure of clinker to water vapor containing high concentrations of H2SO4 and H2S at a late stage of burning.The leaching yields of potentially harmful elements (PHEs) present in the waste samples was determined using batch and percolation tests. The concentrations of Hg, Pb, and Cd in some leachates exceeded the limit values for inert wastes according to EU legislation. However, the concentration of other PHEs was low, probably due to prolonged washout of burnt materials by rainwater at a very low pH (1.5 to 4.6).