Element geochemical characteristics for the roof and floor rocks of coal seams in the Shendong mining area: Emphasis on hazardous element fluorine

The Shendong mining area produces over 200 million tonnes of coal annually, resulting in notable gangue output, including materials from coal roof, floor and gangue itself. The hazardous elements, particularly fluorine (F), present in the gangues pose challenges for energy utilization and safe use of mine water. Despite this, studies on the elemental geochemical characteristics of gangues in the Shendong mining area are limited. This study collected 19 samples of coal roof and floor and employed methods such as SEM-EDS, XRD, XRF and ICP-MS to analyse the mineral compositions and quantify major-element oxides, rare earth elements and yttrium (REY), and most trace elements. Additionally, fluorine concentration was determined using the alkali fusion–ion selective electrode method. Results indicate that fluorine has the highest average concentration (655 μg/g) among the trace elements, with an enrichment coefficient of 1.21. The major minerals identified in the samples include clay minerals, quartz, and feldspar. The REY distribution patterns in the investigated samples predominantly show L-type enrichment. Combined analysis of B/Ga, Sr/Ba, V/Cr, U/Th, Ni/Co and δU suggest that the coal roof and floor formations occurred in a continental freshwater and oxic sedimentary environment. The material source analysis indicates that coal roof and floor rocks are primarily derived from felsic rocks in the upper continental crust, especially a mixed provenance of granite and calcareous mudstone. Principal component analysis (PCA) reveals that the fluorine primarily exists on the surface of illites in an adsorption state, with Cu, Cs, Se, Th and Rb exhibiting similar states of occurrence. Correlation analyses between fluorine concentrations in the coal roof and floor rocks and parameters of ∑LREY, La_N/Lu_N, and δCe suggest that fluorine tends to be enriched in weathering residues; additionally, a weakly oxidising environment appears to facilitate fluorine enrichment in these coal roof and floor rocks. The findings in this study can guide the development of effective strategies to reduce fluorine contamination in the mine water; additionally, they aid in developing approaches to address fluorine-related issues during combustion, thereby promoting the safe and efficient utilization of coal gangue as a supplemental fuel resource.