Mineralogy of the coal waste dumps from the Czech part of the Upper Silesian Basin: Emphasized role of halides for element mobility

Abstract

Both unburning and burning coal mine dumps present serious hazards to the natural environment and human health. Bituminous coal has been produced in the Czech part of the Upper Silesian Basin since 18th century, with total production over 100 million metric tons of waste material (barren rock) deposited in dumps. Today, each dump has undergone remediation and reclamation works, and many of them were totaly removed. Only three dumps with signs of thermal activity (burning) remain, with the Heřmanice dump as the most important. The powder X-ray diffraction (XRD) in combination with electron dispersive spectroscopy (EDS) and backscattered electron (BSE) imaging were applied to identify and describe the products of both unburning and burning dumps. The ones without thermal activity released mostly sulfates such as jarosite group minerals, hexahydrite, ferrohexahydrite, gypsum, thenardite, konyanite, and baryte due to pyrite weathering processes, accompanied by ferrihydrite, schwertmannite, goethite, and aragonite. As expected, the mineral associations of thermally active dumps were more variable. Apart from hot gas vent products such as sulfur, salammoniac, and cryptohalite, we identified a zoned sulfate cap (crust) with an accumulation of newly formed phases (e.g., aluminopyracmonite, pyracmonite, godovikovite, mascagnite, koktaite, boussingaultite, and clairite). Both the gas vents and sulfate crust contained a number of rare and exotic phases, such as Bi, bismuthinite, demicheleite-(Br), demicheleite-(I), fluorite, selenium, NH4Br, NH4I, BiOCl, BiBr3, BiI3, native iodine, CdS, CdIn2S4, (NH4,K)AlF4, (NH4,K)3AlF6, Ce-dominant REE-sulfate, and others. Since coal ash and barren rock are not geochemicaly anomalous, the presence of such various phases could be atributed to the very effective leaching and transport of chemical elements by halide elements, which originated in brines in the roof of the Carboniferous rocks circulating in the strata prior to extraction and emplacement in the dump. This mineralization is to a great extent similar to that of natural volcanic vents, and must be studied by specific methods (low-vacuum electron microscopy with energy-dispersive detectors, use of natural unpolished samples). The composition of altered rock clasts within the dumps was monitored only occasionally, but newly formed feldspars (probably sanidine, albite, andesine), hematite, diopside to hedenbergite, cordierite, magnetite-magnesioferrite minerals, anhydrite, TiO2 microcrystals, Mg-rich fayalite, and in case of more intensively burnt rocks also wollastonite, barium feldspar (celsian-hyalophane series), and a phase of Ca-SiO4-PO4 system were also observed. It is clear that the burning of coal dumps presents a specific problem, and future steps towards understanding their potential environmental risks would require complex studies including the determination of their mass and chemical element balance, the estimation of the dump volume affected by the thermal mobilization of chemical elements, and the migration pathways of the potentially-hazardous elements. However, it is clear that the most problematic part incudes just the few uppermost meters with vents of hot gases and the sulfate cap, which are readily soluble in water after the end or lateral shift of burning.