Identification, Characterization, and Speciation of Rare Earth Elements in Coal Refuse

Rare earth elements are the 14 lanthanides on the periodic table, plus yttrium and scandium. These elements play a critical role in modern-day technologies such as liquid-crystal displays, GPS systems, and fiber optic cables. A majority of the mining of these elements is from China; however, due to decreasing reserves a need for alternative processes for extracting and processing rare earth elements (REEs) is becoming increasingly important. Special focus has been placed upon the identification of REEs within coal refuse, but the phase designation and speciation is not fully understood. This investigation focuses on the characterization, speciation, and morphology of REEs within fine and coarse coal refuse. During this study, physical and chemical characterization was conducted on coal refuse samples to understand characteristics, which influence REE phase designation. Experimental methods were chosen to specifically evaluate REE content and speciation across four key characteristics: size distribution, density, seam location, and thermal decomposition. Characterization of the refuse material was conducted in two campaigns: (1) an exploratory campaign, which focused on size distribution, and physical imaging of REEs within fine refuse, and (2) a detailed campaign, which utilized sequential chemical extraction methods alongside calcination to understand the phases in which REEs are present in coarse refuse. The results show that REEs within fine coal refuse are smaller than ten microns and found with phosphorus. In general, as size decreased REE content increased, likely due to increased clay content. Further conclusion could not be drawn from simple microscopic analysis. Consequently, detailed chemical characterization was conducted to fully understand REE speciation. The tests showed that a majority of REEs within coarse refuse were within insoluble species. A calcination treatment was found to greatly increase the recovery of REEs from the metal oxide fraction, thus increasing the overall soluble species contained within the coarse refuse material.