Microwave-assisted acid extraction of rare-earth elements from non-ore sources

Over the last 15 years, the development of catalysts, solar panels, and other advanced technologies has doubled the demand for rare-earth elements (REEs). Bastnasite, monazite, and loparite and the lateritic ion-adsorption clays are the primary REE containing minerals. However, the global distribution of ores and the associated geopolitical environment have raised concerns over the security of these vital resources. As a result, REE containing waste, such as fly ash from coal generation, is increasingly being considered as alternative sources of these metals. In 2017 alone, over 38 million tons of fly ash were produced. The concentration of REEs found in coal fly ash can be greater than many ore materials (>1.5%). There are two primary technological challenges that inhibit acquiring REE from fly ash and other non-ore sources: (1) most REEs are bound in recalcitrant lattice structures that limit extraction and (2) once released from the source material, REEs are difficult to selectively concentrate or isolate. As part of the Center for Rare Earths from U.S. Extractions (REUSE) at Wayne State University, we take aim at the first of these challenges by systematically assessing the extraction of REE from fly ash using standard microwave-assisted digestion with boric, hydrochloric, nitric, and sulfuric acids. Yields from these extraction processes for 16 REEs (Sc, Y, La, Ce, Pr, Nd, Sn, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) will be presented. The results of these extractions and the associated chemistry of extraction solutions will be discussed with a focus on implications for further concentration and isolation of REEs.