Abstract
The highly elevated concentrations of the rare earth elements and yttrium (REY), U, Mo, Se, and Pb in late Permian coals in some coalfields in Guangxi Province, South China, have been widely investigated; however, the mode of occurrence and enrichment mechanism of these critical elements are still under debate. This study investigates the mineralogical and geochemical compositions of coals and non-coal rocks from the Xian’an Coalfield in Guangxi Province to discuss the geological factors influencing the distribution of critical elements. The mineral in the studied coals consists mainly of quartz, and to a lesser extent, muscovite and kaolinite, with a trace amount of anatase. The coals are significantly enriched in REY, Pb, Se, Mo, and U and display the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and U–Mo-enrichment horizon (Horizon II) adjacent to the host rocks or partings. The REY, U, Se, and Pb show organic association while Mo is primarily hosted by Fe-sulfides within Horizon I. The U and Mo have a phosphate affinity within Horizon II. Both the input of pyroclastic and epiclastic materials and the leaching of acidic solutions jointly govern the distribution of the REY–U–Se–Mo–Pb-enrichment horizon (Horizon I) and the U–Mo-enrichment horizon (Horizon II). The concentrations of REY in Horizon I exceed the cutoff grade of REY, and, therefore, the coals in the Horizon I can be regarded as promising raw materials of REY.
Highlights
Rare earth elements and yttrium (REY) in coals have attracted increasing attention in recent years [1,2,3,4,5], mainly due to the concentrations of Rare Earth Elements and Y (REY) in some coals close to or exceeding the cutoff grade of REY in coal ashes [6,7,8] and the supply shortage of conventional types of REY ore since 2009 [7]
The high volatile matter yields in the coal samples are ascribed to relatively high ash yields, which contribute to some proportion of volatile matter as indicated by the linear correlation of volatile matter and ash yield (r = 0.81) in coal samples except for sample SL-13
A threefold geochemical classification (LREY, MREY, and HREY) and the three enA threefold geochemical classification (LREY, MREY, and HREY) and the three enrichrichment types (L-type, M-type, and H-type) of REY proposed by Seredin and Dai [7] as ment types (L-type, M-type, and H-type) of REY proposed by Seredin and Dai [7] as well well as the upper continental crust [52]–normalized REY pattern were used in the present as the upper continental crust [52]–normalized REY pattern were used in the present study study to monitor the fractionation among REY
Summary
Rare earth elements and yttrium (REY) in coals have attracted increasing attention in recent years [1,2,3,4,5], mainly due to the concentrations of REY in some coals close to or exceeding the cutoff grade of REY in coal ashes [6,7,8] and the supply shortage of conventional types of REY ore since 2009 [7]. The REY-rich coals are mostly enriched in critical REY (Nd, Eu, Tb, Dy, Y, and Er), which could alleviate the raw materials crisis of critical REY in the near future [7]. The REY-rich coals can be regarded as alternative REY raw materials [1,7]. Multiple geological factors jointly govern the abnormal enrichment of REY in the REY-rich coals [5,7,18,32]
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