Turning Coal Waste into Treasure Extracting Critical Rare Earth Elements for Clean Energy
Chemical engineering student Lawrence Ajayi, PhD’28, received the Certificate of Merit (CoM) from the American Chemical Society’s (ACS) Division of Environmental Chemistry (ENVR) for his research on “Effect of alkali-treatment on microwave-assisted rare earth elements extraction from coal mine tailings,” which he presented at the ACS Fall 2025 National Meeting. Ajayi’s co-authors were ChE Assistant Teaching Professor Brian Lejeune, COS Research Professor Jason Guo, and ChE/COS Assistant Professor Damilola Daramola.
Abstract
Rare earth elements (REEs) are critical materials for decarbonized energy generation and advanced technologies, including wind turbines, electric vehicles, medical imaging, and video displays. As demand for these technologies grows, the need for REEs continues to rise due to their unique physicochemical properties. While REEs have been identified in non-traditional sources such as coal-related waste, their extraction via acid leaching is often hindered by the complex and refractory nature of aluminosilicate minerals present in these wastes. To overcome this challenge, researchers have explored hydrothermal alkali treatment as a means of dissolving aluminosilicates and liberating encapsulated REEs. While this pretreatment step has been effective in improving REE recovery during acid leaching, it also facilitates the formation of geopolymeric or zeolitic phases, depending on reaction conditions. The structural characteristics of these newly formed compounds can significantly influence their dissolution behavior in subsequent acid digestion, thereby affecting REE extraction efficiency.
In this study, we investigate the structural transformations induced by alkali treatment in coal tailings and their implications for REE recovery. Tailings from a coal preparation plant in the Midwest, United States, were used as the raw material. The tailings were treated with 5M NaOH in a microwave digester at 180°C, under varying experimental conditions to assess the effects of raw material concentration, reaction time, and multi-cycle treatments. Structural modifications in the alkali-treated tailings were analyzed using 27Al, 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and X-ray diffraction (XRD) and then compared to untreated samples. Additionally, the impact of these structural transformations on REE recovery was assessed by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Furthermore, the dissolution behavior of structurally modified aluminosilicate minerals will be examined by measuring leached silicon and aluminum concentrations using inductively coupled plasma optical emission spectrometry (ICP-OES). The findings from this study will provide insights into strategies for enhanced REE recovery from coal mine tailings.