Existing waste streams in the U.S., such as those associated with legacy and current coal mining (e.g., underclay layers from below a coal seam), are being evaluated as potential sources of rare earth elements (REEs) and other critical metals. A portion of those efforts are aimed towards the ‘easily accessible’ fractions of the REE which could encompass ion-adsorbed components on clays or oxides, amorphous secondary minerals, or other accessory minerals formed during weathering. Certain sedimentary processes during deposition or subsequent weathering can lead to a substantive enrichment of these REE-bearing phases relative to the parent material. By targeting the easily accessible fractions, more aggressive chemical leaches can be bypassed, and relatively more benign measures undertaken. We investigated a few strategies for leaching easily accessible REE fractions from a representative underclay originating from the Middle Kittanning coal seam in West Virginia. The effect of pH on total REE extractability was assessed for a series of salt solutions (as ion-exchangeable lixiviants), dilute mineral acid solutions, as well as an organic acid ligand solution (citric acid). REE extraction was increased in all solutions at lower pH. Citric acid solutions showed favorable extraction compared to salt solutions of (NH₄)₂SO₄ and NaCl of similar ionic strength. Citric acid at acidic pH values (pH 2-3) also compared favorably to the REE extraction obtained using higher strength (1-2 M) mineral acids (HCl and H₂SO₄). A maximum ~30% REE extraction was obtained from this particular underclay. The results suggest that the presence of a complexing ligand in solution provides a competitive option for the extraction of easily accessible REEs and that pH may control the extractability for certain types of mineralization. For more widespread applications, these results can potentially inform strategies for targeting easily accessible fractions of the REEs with less aggressive methods and lixiviants.

Sophia Bauer

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