Rare earth elements (REEs) recovery from coal waste of the Western Kentucky No. 13 and Fire Clay Seams. Part I: Mineralogical characterization using SEM-EDS and TEM-EDS

The mineralogy of rare earth elements (REEs, including 15 lanthanides plus Sc and Y) in the Western Kentucky No. 13 and Fire Clay coal waste was studied based on elemental composition analyses using scanning electron microscopy- and transmission electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS and TEM-EDS). A total of 49 and 50 REE-bearing particles were found from the SEM-EDS specimens of the two materials, respectively. For both materials, light REEs were found in apatite, monazite, and crandallite-group minerals, while heavy REEs primarily existed in zircon and xenotime. Based on the number and REE contents of the REE-bearing particles, it was concluded that monazite, xenotime, and crandallite-group minerals were major contributors to the total REE contents in both materials. The number of crandallite-group mineral particles found from the Western Kentucky No. 13 material was more than that of monazite and xenotime particles. However, for the Fire Clay material, the number and size of crandallite-group mineral particles were close to those of monazite and xenotime particles. Since the monazite and xenotime particles contained more REEs relative to the crandallite-group mineral particles, it was inferred that the majority of the REEs, especially the light REEs, existing in the Fire Clay coal waste occurred as monazite and xenotime. In addition, comparing with the Western Kentucky No. 13 material, a larger portion of the heavy REEs in the Fire Clay material were comprised in zircon. SEM images of the REE-bearing particles indicated that both liberated particles and particles locked within dominant minerals (e.g., clays) existed in the two materials. A few of the particles found in the Western Kentucky No. 13 material were completely encapsulated within the dominant minerals. Findings from the current study will promote REE recovery from coal-related materials, particularly coal waste.