Acid Leaching of Rare Earth Elements from Coal and Coal Ash: Implications for Using Fluidized Bed Combustion To Assist in the Recovery of Critical Materials

High temperature pretreatment of coal-based mineral matter in an oxidizing environment significantly enhances the leaching characteristics of rare earth elements (REEs). A research study has found that the temperatures used in fluidized-bed combustion (FBC) of coal to produce electricity are near optimum for pre-treating the associated mineral matter prior to leaching to maximize the recovery of critical materials. Tests were performed on representative samples collected from preparation plants treating West Kentucky No. 13, Illinois No. 6, and Fire Clay coal seam sources as well as fly ash and bed ash samples from two FBC power plants. Acid leaching tests using 1.2M HCl at 75℃ were performed on both the coal and the FBC ash samples. Prior to leaching, the coal samples were pretreated at temperatures of 600℃, 750℃, and 900℃ in an oxidizing environment to study the effect on leaching characteristics. The results showed that pretreatment at 600℃ for 2 hours resulted in a significant increase in REE recovery from a range of 20-40% to about 80% for all coal sources. The leaching kinetics are characterized by a quick release of rare earth elements within the first few minutes of the process. For the West Kentucky No. 13 coal source, about 75% of REEs were leached in the first 15 min from the 1.4-1.8 specific gravity (SG) fraction that was calcined at 600℃. Additionally, the leaching kinetics of the major contaminant, i.e., Fe, were much lower than the REEs, which significantly benefits the efficiency of leaching and the downstream upgrading processes. REE leaching characteristics of the FBC ash samples were similar to that of the calcined coals. Mineralogy characterization showed that the degree of crystallinity for both the calcined coal and FBC samples were similar to the original associated mineral matter, which provided evidence for the advantage of using the FBC by- products as REE feedstocks over pulverized coal boilers that utilize temperatures greater than 1200℃. These findings were used to develop a conceptual flowsheet that incorporates FBC technology and its typical combustion environment to enhance the feasibility of recovering critical materials from coal-based sources.