THE FATE OF PHOSPHATES IN COAL MINING WASTE: A STUDY OF RARE EARTH ELEMENT MOBILITY IN WASTE PILES

Coal mining waste is present throughout the Illinois Basin in the form of waste piles scattered around old coal mining sites. These piles are known to contain critical minerals and other materials that have potential use as a recovered resource, such as lanthanides, carbon, clays, silica, and other resources that can have economic use. Specifically, rare earth elements (REE), which will include yttrium, are known to be either at or above upper continental crust concentrations with respect to the materials in the piles. In this study, we collected coal waste samples from abandoned mining sites throughout the state of Illinois. The samples were analyzed via X-ray Absorption Near Edge Structure (XANES) and ICP-MS. We hypothesize that older coal waste sites will have more dissolved XANES features compared to younger coal waste piles.

XANES spectroscopy can be used to detect crystal defects. As phosphate crystals weather, there will be more defects present, such as vacancies, interstitials, dislocations, and impurities. These defects introduce changes in the absorption spectra, which are detected by XANES. By comparing the XANES spectra of a defective crystal with that of a defect-free crystal, differences in absorption features or shifts in energy can be observed, providing insight into the presence and nature of crystal defects.

While various mineral groups can contribute to REE counts, this study will focus on the phosphates. Acidic conditions, which are formed by pyrite oxidation in the coal mining waste, which results in acid mine drainage (AMD), enhance mineral dissolution and release of REEs via AMD. Phosphate minerals, which compared to silicates and oxides, exhibit higher dissolution and leaching rates of REEs.

Our results found that the REE content coal piles ranged from 75 to 300 ppm. Younger coal piles were found to contain more intact monazite crystals compared to older coal waste piles, which can be detected via XANES. This implies that over time, phosphate minerals weather and contribute less to REE counts, with REEs either being incorporated into other minerals via adsorption or leaving the waste pile via AMD.