Fluorine in Lopingian superhigh-organic-sulfur coals from the Lalang Coal Mine, Guangxi, southern China

This work deals with the abundance, mode of occurrence, and origin of fluorine in Lopingian coals from the Lalang Coal Mine, Guangxi, southern China. The Lalang coal has a semi-anthracite rank and is characterized by superhigh-organic-sulfur (SHOS) content (4.46-11.34%). Fluorine concentration was determined by pyrohydrolysis plus a fluorine ion-selective electrode. Fluorine presents a significantly-enriched content in the Lalang coal, up to 2096 μg/g in an individual coal bench. Fluorine is positively correlated with B in the coals present in this study; however, it does not show relevance with the metamorphism degrees of SHOS coals. The highly-elevated F in the Lalang coals occurs primarily in illite, fluorite, fluorapatite, and to a lesser extent, organic matter. The enrichment and occurrences of fluorine in coal are attributed to input from the sediment-source Yunkai Upland region, hydrothermal solutions originated from volcanic activities, marine invasions, original coal-forming flora and fauna, underground water circulating within the deep crust, and meteoric water.