Preliminary data on critical element contents in lignite from the Maritsa East basin, Bulgaria

The studied samples include three composite coal, three lithotypes (lyptain, xylain, and fusain), and a black clay parting from the Troyanovo 1 and Troyanovo North mines in the largest Maritsa East lignite basin in Bulgaria. Mineral matter in composite coal samples, lithotypes, and clay is represented by clay minerals (montmorillonite, kaolinite, illite), quartz, pyrite, and gypsum in various quantities. A total of 66 elements were measured in all samples. The measured concentrations were compared to worldwide values of brown coal and upper continental crust and coefficients of enrichment (K1 and K2, respectively) were established. Most of the studied critical elements (REY, platinum-group metals, Li, Si, Mg, Ge, Ga, Nb, Sb, In, Co, Be, W) have low concentrations (K1 and K2 <2). The content of rare earth elements, yttrium, and scandium in the studied composite coal and lithotype samples is lower than concentration in world low-rank coal (65 ppm) and lower than in the studied black clay (145 ppm). The K1 and K2 coefficients of Pd and Pt, Te, Re, and Au are anomalously high. The mode of occurrence of most trace elements is mineral matter (sorbed in clay minerals, trace elements in pyrite, and as discrete phases). Some elements in low-ash samples (lithotypes) demonstrate affinity to organic matter: Te, Re, As, Mo, Ca, P, Au, Ba, Sr, Cd, etc. The lithotypes show enrichment in HREE (Gd–Lu) and Y in the following decreasing order: xylain > fusain ≫ liptain. The anomalously high contents of Te, Re, Pd, Pt, Au, Se, As, Mo, and others require further investigation.