Coal related fires in Portugal: New occurrences and new insights on the characterization of thermally affected and non-affected coal waste piles

This study presents a comprehensive investigation on the petrographic and geochemical characterization of new occurrences of self-burning coal waste piles in Douro Coalfield (Portugal). It also includes a reappraisal of data about coal waste piles resulting from past mining, aiming to assess their environmental and economic potential. Burned and unburned material from two coal waste piles that ignited in 2017 were studied for petrographic, mineralogical and geochemical characterization, and for identification of thermally induced changes caused by combustion. Geochemical data relative to more than 20 coal waste piles from Douro Coalfield were integrated and enrichment of elements was investigated by comparison with reference values. The results show that the organic fraction in the thermally affected samples presents dark reaction rims, cracks, devolatilization vacuoles, plasticized edges, pyrolytic carbon and lower vitrinite reflectance, despite the presence of unaltered particles. The formation of iron oxides is also attributed to burning process. The petrographic observations allow the identification and characterization of more and less intense thermally affected materials which indicates the heterogeneity of the burning process within the coal waste piles, and gives information about newly formed minerals and attained combustion temperatures. The geochemical composition of the studied samples is generally within the range expected for Douro Coalfield coaly materials. More noticiable differences are reported to the most thermally affected sample. When considering the geochemical data about all coal waste piles from Douro Coalfield it is noticed that the majority of elements concentration increase in the burned material due to C consumption and loss of other elements by volatilization. The decrease in the concentration of As, Cd, Co, Mn, Pb, Sb, Sn, and Zn in burned material is attributed to partial volatilization of these elements at high combustion temperatures. Considering the elements slightly to significantly enriched in Douro Coalfield materials, Ba, Ga, Hf, Li, Nb, Sb, Sc, Ta, V, W, and REE are considered critical raw materials by the European Commission, while As, Co, Cr, Cu, Mo, Ni, Pb, and U represent environmental concerns. Therefore the reutilization of Douro Coalfield materials as a secondary source of critical raw materials could be economically relevant and will allow the mitigation of environmental impacts caused by their deposition.