Abstract
Rare earth elements (REE) are necessary for advanced technological and energy applications. To support the emerging need, it is necessary to identify new domestic sources of REE and technologies to separate and recover saleable REE product in a safe and economical manner. Underclay rock associated with Central Appalachian coal seams and prevalent in coal utilization waste products is an alternative source of REE to hard rock ores that are mainly composed of highly refractory REE-bearing minerals. This study utilizes a suite of analytical techniques and benchtop leaching tests to characterize the properties and leachability of the coal seam underclays sampled. Laboratory bench-top and flow-through reactor leaching experiments were conducted on underclay rock powders to produce a pregnant leach solution (PLS) that has relatively low concentrations of gangue elements Al, Si, Fe, and Th and is amenable to further processing steps to recover and produce purified REE product. The leaching method described here uses a chelating agent, the citrate anion, to solubilize elements that are adsorbed, or weakly bonded to the surface of clay minerals or other mineral solid phases in the rock. The citrate PLS produced from leaching specific underclay powders contains relatively higher concentrations of REE and lower concentrations of gangue elements compared to PLS produced from sequential digestion using ammonium sulfate and mineral acids. Citrate solution leaching of underclay produces a PLS with lower concentrations of gangue elements and higher concentrations of REE than achieved with hydrochloric acid or sulfuric acid. The results provide a preliminary assessment of the types of REE-bearing minerals and potential leachability of coal seam underclays from the Central Appalachian basin.
Highlights
Rare earth elements (REE) are essential for the development of low-carbon, renewable energy technologies
The citrate pregnant leach solution (PLS) produced from leaching specific underclay powders contains relatively higher concentrations of REE and lower concentrations of gangue elements compared to PLS produced from sequential digestion using ammonium sulfate and mineral acids
The results provide a preliminary assessment of the types of REE-bearing minerals and potential leachability of coal seam underclays from the Central Appalachian basin
Summary
Rare earth elements (REE) are essential for the development of low-carbon, renewable energy technologies. The results confirm that organic anions, including anions of citric acid, can act as assistant leaching agents both through the complexation of REE in solution and the interaction with the clay surface to promote changes in the zeta potential of the clay. Citric acid anion recovery of REE from coal seam underclay is a promising method that may liberate higher concentrations of ion exchangeable REE from the clay compared to traditional lixiviants such as ammonium sulfate or sodium chloride. A mildly acidic organic acid-based ionic recovery solution with the presence of a monovalent salt likely liberates clay surface adsorbed REE and some inorganic REE mineral phases embedded in the clay matrix. Characterizes and tests the leaching behavior of underclay rock from geologic formations associated with coal production in West Virginia, Pennsylvania, and Ohio
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