Geological controls on the mineralogy and geochemistry of the Beypazari lignite, central Anatolia, Turkey

Abstract The raw coal from the Cayirhan mine, Beypazari basin, Turkey contains an average of 28.5% mineral matter, consisting up to 80% zeolites. The coal seam is split by a 1-m-thick tuffaceous siltstone into the upper, first (Tv) and lower, second (Tb) seams, which contain essentially different zeolites. The first seam contains Ca-rich zeolites (clinoptilolite/heulandite) whilst the second seam contains Na-rich zeolites (analcime). Experimental work has shown that a Na-rich activation solution will produce Na-rich zeolites when the original volcanic glass is Na-rich, but will produce Ca-rich zeolites when the volcanic material is Ca-rich. It is thought that a Na-rich activation solution, derived from contemporaneous volcanics in the Beypazari basin, reacted with volcanic tuffs of different chemical composition to produce the vertical variability in the mineral matter of the two seams. Trace element analyses of the raw coal showed that they are enriched in As, B, Cr, Ni and Zr when compared to world-wide averages of coal. They are also enriched in the major elements Na and K. These elements are probably also derived from a volcanic source. Barium and Sr show a clear affinity for analcime, whilst Li, Cr, Ni, Cu, Zn, Co and Ga show a clear affinity for clinoptilolite/heulandite. Trace elements showing a positive correlation with the organic matter are Mn, B, Be, Ge, Y, Zr, Nb, Hf, W and U. Manganese, U, Th, Sc, Ge and HREE (heavy rare earth elements) show correlations with phosphate. Trace elements showing correlation with both Fe and S contents are Co, Mo, Ta, Pb and IREE (intermediate rare earth elements). Arsenic and V seem to have mixed affinities with sulphides and zeolites, and organic matter and zeolites, respectively. Although a proportion of the As, B and S are retained in the ash during coal combustion, due to the sorption mechanism of CaO derived from the clinoptilolite/heulandite decomposition, significant emissions of these elements may arise from the Cayirhan thermal power station due to the anomalously high concentrations of these elements in the Beypazari coal. The high total alkali metal content of these coals exceeds the recommended maximum for steam coal, but it is believed that sodium occurring in a mineralogical form such as analcime is not as likely to contribute to boiler fouling as organically-associated sodium.

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