Fundamental Studies on the Recovery of Rare Earth Elements from Acid Mine Drainage

The Rare Earth Elements (REEs) are a group of 17 chemically similar elements that are essential raw materials for modern society. Given their increasing importance in civil and defense applications, many national governments, including the US, have denoted the REEs as critical materials and emphasized the need to identify and develop both conventional and unconventional resources to meet the growing demand. To address this challenge, the US Department of Energy has recently spearheaded efforts to evaluate the technical and economic feasibility of recovering REEs from coal and coal byproducts, which have been shown to contain REEs at an elevated concentration relative to surrounding strata. One promising resource identified in this program is acid mine drainage (AMD) and its subsequent treatment precipitates (AMDp). Prior studies have shown that the AMD produced from the coal measures acts as a heap leach reactor that mobilizes common transition metals as well as REEs. Per Section 404 of the Clean Water Act, mine operators are required to collect and treat AMD prior to discharge by raising the pH and in turn removing the transition metals. During this process, the REEs tend to deport with bulk transition metal, and are ultimately recovered to the AMDp. While a select number of studies have evaluated the content and quantity of REEs in AMD, the scientific literature is generally lacking fundamental information on the mechanisms that control the deportment of REEs from the initial coal strata to the AMD and ultimately to the AMDp. The research in this study seeks to overcome this knowledge gap through a combination of both fundamental and empirical studies. Initially, thermodynamic simulations were conducted to evaluate the solubility of REEs in aqueous systems. HSC Chemistry 9.0 was used to generate Eh-pH diagrams for both REEs and other transition metals. Data from these simulations were then used to inform experimental investigations on the leaching of REEs from AMDp as well as the selective separation of REEs from major metals by staged precipitation. Collectively, this data showed that the REEs in AMDp are readily soluble in nitric acid and subsequent pH adjustment with NaOH prompted significant separation of REEs from iron and other major contaminants. Lastly, to better understand the initial leaching of REEs into AMD, humidity cell tests were conducted over a period of 80 weeks using feed material from a coarse refuse site in Central Appalachia. Data showed that the leaching of REEs was hindered until nearly full depletion of the calcium was achieved, and the REE leaching was typically accompanied by release of iron and aluminum. Altogether, these results provide insights on the mechanisms controlling content and characteristics of REEs in AMD. This information thus supports the development and design of integrated processes to recover REEs from AMD, while prompting superior economic and environmental outcomes for localities impacted by AMD