Factors controlling the occurrence and distribution of iron in Bulgarian coals

Abstract

Coal of varying age (Upper Carboniferous to Pliocene) and coalification rank (lignite to anthracite) from nineteen coal-bearing basins and deposits, belonging to 8 coal-bearing provinces from Bulgaria, were examined for the occurrence and distribution of Fe. For the majority of the coals, the average Fe concentrations vary from 1.1 to 10.2 wt% and are higher than the World’s average. An exception exists only for Karlovo and Dobrudzha Basins, where slight depletion of Fe (<1 wt%) was established. On contrary, carbonaceous shales from the majority of the basins show Fe depletion in comparison to the Clarke values for clays. Based on the negative correlations with the ash yields, organic affinity of the element can be suggested for most of the studied coals. An exception exists for the Burgas Basin, for which predominantly inorganic affinity of the element can be suggested, whereas for Stanyantsi, Samokov, Bobov Dol, Dobrudzha and Svoge Basins mixed organic/inorganic affinity of Fe can be presumed. The element most probably predominantly occurs in the form of organometallic compounds, especially in the coals that formed in preferentially acidic peat-forming environment. Nevertheless, small contribution from biogenic iron cannot be excluded. The mineral form of Fe is probably mainly associated with sulfides, carbonates, and sulfates, although the presence of Fe oxides, silicates and hydroxides is also established. The analysis of Fe occurrence and its organic/inorganic affinity in Bulgarian coals reveal the following controlling factors: i) Fe concentration within the basin’s provenance; ii) the type (i.e. terrigenous or groundwater) of element supply; and iii) environmental conditions during peat-formation.