Trace and major element content and distribution in Neogene lignite from the Sofia Basin, Bulgaria

Abstract

Lignite samples from the Balsha seam in the Sofia Neogene Basin were collected and ashed. The coal ash was analysed for major and trace element contents. The concentration of most of the elements in the coal ash was higher than the Clarke value: Ag, Be, Cd, Co, Cr, Cu, In, Mn, Mo, Ni, P, Pb, Sc, Sn, Ti, V, and Zn. Forty-nine trace elements were determined. Aluminium, Si, K, REE, Bi, Zn, Ta, Tl, Sn, Sr, Cs, U, Th, Rb, Au, and Hf have positive correlation coefficients with the ash content, i.e. respectively high inorganic affinity, but Ti, Mn, Ga, Tb, Cu, and Cd have intermediate (organic and inorganic) affinity. Calcium, Mg, Fe, S, Ge, As, Be, Ag, Mo, W, In, Sb, Na, P, Sc, V, Cr, Ba, Pb, Ni, Co, Y, and Zr have high organic affinity. Some elements with organic affinity was also related with carbonate (Ca, Mg, Fe, Na, Ba, P) or sulphide (Fe, S, Be, As, Sb, Ge, Na, Mo, In, W, Ni, Ag, Tb) minerals. Five groups of elements may be separated according to the form of occurrence in coal: The first two (La–Eu–Tb–Ce–Ta–Rb–Cs–Th–Sm–U–Yb–Sr–Mn–Hf–Zn and Al–Bi–K–Au–Sn–Tl–Si associations) contain elements with positive correlation with the ash content, i.e. they show mainly inorganic affinity. The elements, which show negative correlation, were separated in three other groups (P–Sb–Ba–Na–S–Mg–Ca–As–Ge–In–W–Mo–Fe–Be; Zr–Y–Cd–Sc; and Pb–Ag–Cu–Cr–Co–Ni–V–Ti associations). Four types of distribution of the elements along the Balsha seam profile were recognised: (1) elements concentrated in the low part of the seam; (2) elements which have maximum concentration in the middle part of the seam; (3) elements which tend to concentrate in the upper part of the coal seam; (4) irregular distribution of the element contents. It is established that most of the elements show high concentrations near the bottom, the top, or at the clay layers of the Balsha seam.