Geochemistry, mineralogy and technological properties of coals from Rio Maior (Portugal) and Peñarroya (Spain) basins

Abstract

Abstract This study focuses on the geochemistry and mineralogy of Pliocene lignites from the Rio Maior Basin (Portugal) and of Late Westphalian bituminous and anthracite coals from the Penarroya Basin (Spain). Major mineral phases present in the Rio Maior lignites are quartz and clay minerals with minor amounts of other detrital minerals depending on the sedimentary environment and the source of the detrital input. The concentration of Rb, Cs, Sc, V, U, and Gd in lignites is higher than the maximum value established for the usual worldwide concentration range for coal. Geochemical affinity study for major and trace elements in Rio Maior established three major associations: (i) aluminium–silicate affinity (Fe, Mg, Na, K, Li, Rb, Sc, Zr, V, Cr, Co, Ni, Se, Pb, Sb, Gd, and Tb); (ii) the sulphur affinity (Mo, and As); and (iii) organic affinity Sr, Ca, Mn, Ba, and U. Ce and Cs affinities could not be explained with the available data. In the Penarroya bituminous coals the main mineral phases identified were quartz and clay minerals (kaolinite and illite), minor traces of anatase and epigenetic carbonates. Elements with inorganic affinity (Fe, Mg, Na, K, Ti, Li, Rb, Cs, Ba, Sc, Zr, V, Cr, Co, Ni, Pb, U, Se, Ce, Gd, Tb, Cu) have a high to medium aluminium–silicate affinity. Anomalous concentrations of Li, Rb, Cs, Sc, V and Cr could be related to the input of detrital minerals from the Precambrian and Lower Palaeozoic rocks in the northeastern part of Penarroya Basin. Anthracites from the Penarroya Basin have the following mineral phases: calcium, iron, and magnesium carbonates, clay minerals, and quartz as well as traces of apatite and of pyrite. Carbonates, and some kaolinite, are clearly epigenetic as a consequence of fluid circulation related to magmatic activity. Two main geochemical associations were found: (i) aluminium–silicate affinity (Na, K, Ti, Rb, Cs, Ba, Sc, Cr, Mo, Ni, Cd, As, Sb, Se, Ce, Gd, Tb, Cu); and, (ii) carbonate affinity (Ca, Mg, Fe, V, Mn, Sr). An anomalous amount of Mn and V is considered to be related with the circulation of magmatic fluids. The geochemical and mineralogical variations observed were primarily related to coal rank in the Penarroya Basin, whilst paleoenvironmental depositional conditions were more important in the Rio Maior Basin. The chemical composition of the high temperature ashes is related to the trend followed by the ash fusion temperatures and to the predictive indices associated with slagging and fouling propensities. These indices indicate low to medium slagging and fouling propensities for all the coals studied.