Multiscale in-situ elemental characterization of critical elements in low rank coal, implications for modes of occurrence

Abstract

This study investigated the distribution characteristics and modes of occurrence of rare elements (Li, V, Co, Mo, Ni, Sb, W, Nb), dispersed elements (Ga, Ge), and rare earth elements (REEs) in two selected coal samples: the Al-Li-Ga-rich coal from the Jungar Coalfield in the Ordos Basin, China, and the Ge-rich coal from the Shengli Coalfield in the Erlian Basin, China. Multiscale in-situ elemental characterization techniques, including BSE-EDS, EMPA mapping, and LA-ICP-MS mapping, were used for this purpose. The results showed that in the Al-Li-Ga-rich Jungar coal, Li was mainly associated with kaolinite with various origins, dominated by epigenetic kaolinite, followed by syngenetic kaolinite. Gallium was highly concentrated in organic matter, led by collodetrinite, followed by collotelinite. The majority of Nb was associated with organic matter, while a minority was distributed in Al-bearing minerals, including boehmite and epigenetic and syngenetic kaolinite. In the Ge-rich Shengli coal, Ge, W, and Sb were consistently distributed in coal and concentrated in organic matter, mainly semifusinite. V was widespread in organic matter, including textinite, fusinite, and semifusinite. Cobalt, Mo, and Ni occurred in a similar distribution and were associated with organic matter, led by collodetrinite in the Al-Ga-rich coal and cutinite in the Ge-rich coal. REEs, except for La and Ce, were associated with organic matter, led by vitrinite in Al-Ga-rich coal and huminite in Ge-rich coal. Lanthanum and Ce were highly concentrated in La, Ce-rich minerals, even if they were widespread in organic matter as well. The visualized results of this study confirm most of the previous findings obtained from indirect analyses but also reveal some differences. Gallium was rarely detected in Al-rich minerals such as boehmite and kaolinite, but highly concentrated in organic matter. Lithium was detected in both epigenetic and syngenetic kaolinite. Ge, W, and Sb were organically associated and concentrated in semifusinite in Shengli Ge-rich coal. Overall, this study provides a way to determine the modes of occurrence of trace elements in organic matter in coal, and these findings could be applied to coal from other geological backgrounds.