| Περίληψη: | Coal waste dumps are prone to self-heating due to weathering and oxidation of the organic matter. External and internal factors such as the coal rank and moisture, dump shape and size, the contained minerals, etc., influence self-heating and self-ignition processes. The current study aims to characterize the petrographic and geochemical alterations in coal wastes underwent self-heating in the Bytom dump, Upper Silesia. Coal wastes derived from the pre-mining and mining activities at the Radzionków Mine, had been dumped there from 1980 till 1995.
About 18 coal waste samples obtained at a depth of 0.3 m from the surface of the Bytom dump were macroscopically described, whereas the temperature at several locations on the surface and beneath the surface of the dump was measured. Proximate and ultimate analyses were conducted to determine moisture, ash yield, loss on ignition (LOI), and inorganic/organic carbon contents. Furthermore, petrographic, X-ray diffraction and fluorescence (XRD, XRF), and Gas Chromatography-Mass Spectrometry (GC-MS) analyses were conducted to determine alterations in the organic and inorganic matter.
Temperatures beneath the dump surface range from 30-922oC. The self-heated coal samples are solid, hard and have a dark brown-grey colour, while the inherent moisture is 1-2 wt.%, and the ash yield 65.9-87.8 wt.%. The petrographic analysis under the microscope proves that mineral matter dominates most of the samples (50.0-86.8 vol.%), while the organic matter, particularly vitrinite, reveals signs of alteration such as irregular cracks, devolatilisation pores, oxidation rims, etc. The most common mineral phases contained in the samples are quartz, dolomite, kaolinite, and illite. Additionally, the contents of 10 major and minor elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe), and 20 trace elements (S, Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Hf, W, Pb, La, Ce, Th) are determined. The vitrinite reflectance values (Rr) range from 0.57-2.12%. Extract yield in the coal waste samples varies from 0.010-10.113 wt.% depending on the thermal influence. The main groups of chemical compounds are n-alkanes, hopanes, and light aromatic hydrocarbons.
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The results prove that self-heating is the dominating process altering the coal wastes in the Bytom dump. Alterations of the organic matter are visible in both the petrographic and the geochemical properties of the samples. The samples experienced a high heating rate up to temperatures >500oC. Lastly, based on the aromatic hydrocarbon indexes the samples are distinguished into two groups, those with Rr<1.1% not showing any correlation between biomarker indexes and the vitrinite reflectance, and those with Rr>1.1% correlating to each other. Specifically, DNR, TNR-1, TNR-2 show a decrease with increasing reflectance >1.1%. The remaining indexes show low concentration of compounds, mainly due to the leaching of the coal wastes, thus, these indexes are less reliable to make any conclusion.
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