Analysis of the occurrence of critical elements and raw materials in Polish lignite deposits with particular emphasis on coal ashes

Abstract

Fly ash collected from landfills at power plants using lignite from the following mines: Turów in Bogatynia, Bełchatów in Rogowiec, and Pątnów in Konin was subjected to analysis. Petrographic studies were performed using a reflected light optical microscope. The chemical composition (the presence of minor, trace, and rare elements) was determined using ICP-MS and X-ray spectroscopy. Petrographic examination using a JEOL JXA-8230 electron microprobe analyzer allowed determining critical elements in the selected minerals. Silicate and aluminosilicate enamel, mullite aggregates, magnetite-hematite iron microspheres and fine unburned carbon particles were found in the analyzed fly ash samples; in addition, small amounts of gypsum and anhydrite were also recorded. The results of oxide analyses of fly ash samples, e.g. CaO content higher than 10% wt, suggest their calcium character. In order to present interesting concentrations of the analyzed critical elements, their maximum contents were compared to their Clarke values (for sedimentary rocks and crustal rocks). The comparison to Clarke values of sedimentary rocks, based on calculation of the enrichment factor (EF), has shown that the content of majority of critical elements in lignite and its ashes is similar to the mean values for sedimentary rocks; the contents higher than Clarke value did not exceed the mentioned values and were observed in the case of tungsten (6.2 - the Turów deposit) and silver (5 to 7 in the Turów deposits). In addition, the comparison of analytical content to Clarke values of crustal rocks did not fundamentally alter these relations; the maximum contents higher than the Clarke value were recorded for tungsten (9.4) and silver (9.3) in the Turów deposit. The microanalysis of iron microspheres has revealed the presence of Ga. In the case of the platinum group, the presence of Pd, Pt, In, Nb, V, Cu, and Ti was confirmed.