Experimental pyrolysis of metalliferous coal: A contribution to the understanding of pyrometamorphism of organic matter and sulfides during coal waste heaps fires

Abstract Thermal metamorphism of original coal adjacent to burning centers in coal waste heaps is a common phenomenon, and semi-coke and coke are formed during this process. In this study, changes taking place during the pyrolysis of both organic and inorganic constituents of coal in burning heaps are interpreted, and the volatility of major and trace elements is assessed. Experimental pyrolysis of bituminous coal from the coal and uranium Novator Mine (Czech Republic) was carried out at temperatures of 200 °C, 500 °C, 700 °C and 900 °C. Changes occurring at various temperatures of pyrolysis in the coal matrix were studied by methods of coal petrography, organic and inorganic geochemistry. Due to the high contents of sulfides of Fe, Cu Zn and Pb in coal, changes in their chemical composition which occur during their melting were also studied. The results showed that with rising temperature the amount of gas produced during the pyrolysis increased significantly while the amount of released nitrogen from coal organic substance and CO2 decreased and, conversely, the H2 content was increasing significantly. The largest amount of tar originated at a temperature of 500 °C. Coke and semi-coke appeared in the solid residue already at a temperature of 500 °C and, as the temperature increased, its porosity and optical reflectance were gradually increasing. The results of gas chromatography/mass spectroscopy (GC/MS) showed that the pyrolysis was associated with a rapid decline in the extractability of solid products of pyrolysis. The maximum concentration of polycyclic aromatic hydrocarbons (PAHs) in coke was recorded at 500 °C (28.5 rel.% of identified compounds), and then dropped to 11 rel.% at 900 °C. The concentration of Hg in the solid residue after the pyrolysis declined significantly even at temperatures of Galena was partially melted during the pyrolysis already at a temperature of 700 °C to form metallic Pb. However, a significant increase in the volume of melts occurred when the temperature reached 900 °C with the formation of mineral assemblage consisting of galena – metallic Pb – pyrrhotite – sphalerite – intermediate Cu-Fe solid solution phase (iss) – bornite. Consequently, the melting of sulfides during the pyrolysis can be one of the reasons for high volatility of some chalcophile elements in the processes of coal pyrolysis in burning heaps.