Partitioning of hazardous elements during preparation of high‑uranium coal from Rongyang, Guizhou, China

The Late Permian coal from Rongyang Mine in southwestern China is characterized by high content of sulfur (5.44%) and elevated concentrations of trace elements U (70.5 μg/g), V (283 μg/g), Cr (63.2 μg/g), Co (12.8 μg/g), Cu (59.5 μg/g), Se (5.28 μg/g), and Mo (87.5 μg/g). Because of their potential adverse effects on human health and environments, it is important to understand the pathways of these elements during preparation. This study focuses on the distribution of hazardous elements in the size- and density-fractionated samples of this coal. The results indicated that: (1) The minerals in the Rongyang coals are mainly composed of quartz, pyrite, marcasite, calcite, anatase, illite, and kaolinite, along with traces of coquimbite, bassanite, roemerite, gypsum, dolomite, rutile, and siderite. Most of the minerals can be effectively liberated from cleaned coal by gravity separation. (2) Elements U, V, Cr, Co, Cu, Se, and Mo with a high content in the Rongyang coal are derived from exfiltrational hydrothermal fluids during syngenetic or early diagenetic stages, and are associated with inorganic components such as clay minerals, pyrite, anatase, and guadarramite as well as organic components. Selenium mainly occurs in pyrite (particularly epigenetic pyrite) in the coal. (3) Uranium is the easiest to remove through gravity separation (the removability is 68.33%) and has lowest concentration (10.5 μg/g) in the 3-6 mm size fractions compared to the others size fractions. It was also found that uranium cannot be removed through gravity separation completely, due to its association with the organic components and fine-grained minerals. The U concentration in the cleaned coal is still much higher than the average value for world hard coals. The ash from the cleaned coal with particle size of 6-13 mm and < 0.5 mm could be potentially used for industrial extraction of uranium. (4) The minerals that are epigenetic in origin are easily liberated during coal cleaning. Therefore, hazardous elements Co, Ni, Cu, As, Se, Cd, Sb, Hg, Tl, and Pb that are associated with epigenetic pyrite are thus relatively easy to remove, whereas elements V, Cr, and Mo that are evenly distributed in inorganic and organic components, are more difficult to be removed.