Rare earth elements (REEs) are widely used in the defense, energy, electronics, and automotive industries and are particularly critical to clean energy technologies. Instability in the REE supply chain in recent years has generated considerable interest in alternative sources. The reclamation of REEs from coal combustion fly ash is one promising resource that could provide environmental and economical advantages over conventional sources such as mining. While several previous studies have documented instances of REE enrichment in fly ash, a systematic evaluation is needed in order to identify suitable fly ash feedstocks for REE recovery. The objective of this study is to characterize a broad selection of U.S. fly ashes of varied geological origin in order to rank their potential for REE recovery. More than 125 ash samples obtained from 20 power plants and representing 5 major coal basins were analyzed for total REEs, nitric acid-extractable REEs, major oxides, and other relevant fly ash parameters. The total concentration of REEs as well as the concentration of high value REEs (Nd, Eu, Tb, Dy, Y and Er) varied considerably and depended on the origin basin of the feed coal. In Appalachian Basin coal ashes, the average total REE content was 530 mg kg<sup>-1</sup> and significantly richer in REEs than both Illinois and Powder River basin ashes (360 and 310 mg kg<sup>-1</sup>, respectively). This suggests that ashes from power plants that burn Appalachian coal could be prioritized for REE recovery. The fraction of high value REEs in the fly ashes studied was considerably higher than conventional REE ores (30-40% in fly ashes compared to &lt;15% at REE mines). Powder River Basin ashes had the highest nitric acid-extractable REE content relative to their total REE content, with 71.9% recovered. This is likely due to the higher fraction of calcium oxide in Powder River Basin ashes, which is more easily solubilized by nitric acid. Overall, this study shows that Appalachian basin fly ashes are the best potential source for REE, though high-calcium ashes such as those from Powder River basin sources may be promising for their ease of extraction.

Recovery of rare earth elements from coal fly ash: Identification of candidate feedstock materials

251st National Meeting (2016)

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