H. Hsu-Kim, A. Mishrra, A. Altmann, B.C. Hedin
Duke University,
United States
Keywords: rare earth, mine drainage, waste valorization, metal resources
Summary:
Legacy coal mining has left behind extensive acid mine drainage (AMD) across the Appalachian Basin, offering an untapped opportunity for recovering critical minerals such as rare earth elements (REEs), cobalt (Co), nickel (Ni), and zinc (Zn). This study seeks to establish the variety of AMD-based feedstocks, with a special focus on chemical composition of AMD and how this knowledge can inform the design of extraction processes. AMD fluids and associated residuals were collected from eight coal mine sites in southwestern Pennsylvania. These sites represent a variety of treatment systems including drainable limestone beds, settling ponds, vertical flow ponds, and active chemical neutralization. The samples were characterized for elemental composition, mineralogy, and particle morphology with the goal of identifying trends in elemental enrichment and partitioning behavior of critical minerals. The results showed the total REE concentrations in the treatment residuals ranged from 270 to 1450 mg/kg in the solids and between 200 ug/L to 1600 ug/L in the AMD fluids. Whereas in the same samples, Co can co-occur at concentrations ranging from vary anywhere from 80 to 730 mg/kg and 143 – 980 ug/L. Furthermore, AMD fluids and solids were commonly enriched in Fe, Mn, and Al and that AMD site could be categorized by their relative balance of these metals. While REEs were most concentrated in Fe-rich fluids, high REE accumulation was observed in Al-rich treatment residuals. In Fe-rich treatment residuals, REE accumulation linearly increased with the Al content. In contrast, Co was preferentially enriched in Mn-rich and directly correlated with the Mn content in the solids, but no such trends were observed in the AMD fluid. Likewise in the vertical flow ponds (VFP), we observed depth-dependent stratification of enrichment of target elements. Specifically, the REE concentrated near the surface of the VFP media and coincided with Al accumulation, while transition metals Co, Ni and Zn were co-enriched at deeper layers along with Mn. These observations highlight the importance of major metals such as Fe, Mn, and Al and their deposition as hydroxides that are the likely host phases for critical metals retention. By linking compositional trends in AMD fluids and solids to critical mineral enrichment behavior, this study provides insight into where and how valuable elements accumulate during treatment. These findings can guide targeted recovery strategies that leverage treatment residuals as secondary sources of critical minerals from legacy mine waste.