Petrographic and geochemical characterization of coal waste piles from Douro Coalfield (NW Portugal)

Abstract Mining waste resulting from the Douro Coalfield exploitation was emplaced all over the coalfield area. More than twenty coal waste piles were identified, consisting of lithic fragments, normally carbonaceous shales and lithic arenites, sometimes with organic matter and particles of variable size. The main objectives of this work were the petrographic and geochemical characterization of the waste material, which provide information about the potential environmental problems associated with the waste disposal. The methodologies applied were optical microscopy and scanning electron microscopy with X-ray microanalysis for the petrographic studies, proximate analysis (moisture and ash), carbon content determination by LECO analyzer, and inductively coupled plasma mass spectrometry for the geochemical characterization. Petrographic observations demonstrated that the organic fraction is attributed to vitrinite and inertinite macerals. The mineral matter comprises lithic fragments with organic and inorganic layering, clay minerals, quartz, iron oxides, oxidized framboidal pyrite and, rarely, non-oxidized pyrite. Some evidence of weathering was detected and found to be related to the time of exposure to the weathering processes. The geochemical composition of the samples are relatively similar and, as expected, are between the chemical composition of coal and main lithologies of the Douro Coalfield. Based on Pearson's correlation coefficients, the trace elements with inorganic affinity are Ba, Cr, Ga, Hf, Ni, Rb, Sc, V, and Zr, and the elements with intermediate affinity are S, As, Bi, Pb, U, and W. According to the results of cluster analysis, a sulphide fraction is represented by the association of As, Cd, Cu, Ni, Pb, and Zn. The association of Al, Be, Co, Fe, Ti, and U indicate that these elements are likely to be related with silicate minerals such as clays; the elements Ba, Cr, Li Mn, Rb, Sr, V, Zr, and RRE probably characterize a silicate fraction, perhaps detrital accessory minerals. Generally, the leachable concentrations are not high, and the elements showing highest releases include Ba, Co, Cu, Mn, Ni, and Zn, indicating that these elements are associated with H 2 O-soluble compounds. Comparing the leaching results with the waste acceptance criteria for landfilling the waste material is classified as non-hazardous wastes.