Silicon and aluminum in Bulgarian coals

The presence and distribution of Si and Al in 19 Bulgarian coal basins and deposits, belonging to 8 coal provinces, were studied. Coals are of different rank (lignite to anthracite) and age (Late Carboniferous to Pliocene). The presence of both studied major elements depends strongly on: i) their concentration within the basin’s provenance; ii) the extent of terrigenous supply; iii) environmental acidity during peat formation; and iv) the presence and composition of epigenetic mineralization within the coal seams. Because of the presumed abundant terrigenous supply, the concentrations of both Si and Al are higher than the world average for most of the studied coals. Exceptions are the Maritsa-West, Burgas, Staniantsi, Oranovo, Chukurovo and Pernik Basins, for which reduced terrigenous supply can be suggested. For most of the basins, both Si and Al demonstrate good positive correlation with the ash yields, thus arguing for predominant inorganic affinity. Relatively low Si/Al ratios (typically 0.6) imply the predominant role of the clay mineral assemblages as the main source of the elements in coal. Quartz is presumed to have dominant role only in Maritsa-West Basin. Poorer or negative correlations with the ash yields in part of the studied basins (i.e. Svoge, Maritsa-West, Sofia, Burgas and Dobrudzha Basins) suggest either mixed or predominantly organic affinity of the elements. Release of Si and Al from the clay minerals during the peat formation/diagenesis and subsequent formation of organo-metallic complexes is considered to responsible for such affinity.