Determination of rare earth elements in combustion ashes from selected Polish coal mines by wavelength dispersive X-ray fluorescence spectrometry

• Novel WDXRF method for determination of REEs in the lowest possible range of contents
• Characteristics of hard coal ashes from various Polish coal mines in terms of REEs content
• Application of the hierarchical clustering analysis in the WDXRF data set analysis

The aim of the experimental works presented in this paper was to develop a method using wavelength dispersive X-ray fluorescence spectrometry (WDXRF) in order to determine the content of 16 rare earth elements (REEs) and the concentration of the said elements in 169 samples of combustion ash of coals coming from ten Polish coal mines, as well as to validate the method. It was found out that there is a clear diversity in the levels and ranges of the variability of REEs occurrence in coal ashes. The average content of cerium, lanthanum, and scandium amounts to 198.8 μg ∙ g− 1, 76.5 μg ∙ g− 1, and 52.4 μg ∙ g− 1 respectively, whereas for such metals as europium, holmium, lutetium, terbium, and thulium, the average content does not exceed the level of 5 μg ∙ g− 1 (the average content for these metals amounts to 1.2 μg ∙ g− 1, 1.4 μg ∙ g− 1, 0.3 μg ∙ g− 1, 1.3 μg ∙ g− 1, and 0.6 μg ∙ g− 1, respectively). In addition, this paper presents an analysis of data obtained by means of hierarchical clustering analysis. Simultaneous interpretation of the dendrogram of objects (coal ash samples) and the color map of the experimental data allowed a more in-depth analysis of the relationships between the clustered coal ash samples from different coal mines and the content of the rare earth elements.