The dependence on international supplies of rare earth elements (REE) has prompted the United States (U.S.) to explore alternative sources and sustainable technologies for domestic REE production. One potential source is the secondary electronic and industrial wastes across the U.S. These hazardous wastes require high costs for the maintenance and remediation, while their high-REE contents offer unique opportunities for REE recovery. However, current hydrometallurgical technologies for REE extraction are energy intensive and costly with low REE recovery efficiency and the generation of hazardous secondary wastes. Thus, it is beneficial to develop environmentally friendly technologies for REE extraction from the secondary wastes. This project aimed to develop novel functionalized mesoporous carbon fiber (MCF) and silica materials for effective recovery of REE from neodymium magnet (NdFeB) scrap.

Novel MCF and silica materials were synthesized, functionalized with selected organic ligands, and characterized. The functional MCF and silica materials were evaluated using batch experiments for the effectiveness of neodymium (Nd), praseodymium (Pr), and dysprosium (Dy) recovery from acidic citrate extraction simulants and real NdFeB powder extraction solutions. The MCF and mesoporous silica after REE adsorption were evaluated for the regeneration, reuse, and REE harvest. Molecular-level interactions and binding mechanisms between Nd and MCF surface functional ligands were investigated using Nd L3-edge and C, N and O K-edge synchrotron X-ray absorption spectroscopy, which will facilitate the design of more robust mesoporous materials.

Biodegradable organic ligands (e.g., citrate) can assist the extraction of Nd and Pr from NdFeB powder at pH 2-3 and significantly reduce mineral acid use and secondary high acidity waste generation. MCF synthesized using Polymethyl methacrylate-b-polyacrylonitrile block copolymer template was effective at Nd adsorption from deionized water and 50 mM citrate leachate simulant, with Nd binding with MCF as a corner-sharing mode. For real NdFeB extraction solution, diethylenetriaminepentaacetic acid functionalized MCF significantly improved Nd and Pr recovery rate as compared to non-functionalized MCF. Phosphonate (PP)- and polyethyleneimine (PEI) functionalized magnetic mesoporous silica (MMS) exhibited Nd adsorption capacities of 15 mg/g from 50 mM citrate solutions (pH 2.5). The functionalized MMS materials remained effective after four adsorption-desorption cycles. The MMS-PP and MMS-PEI materials show significantly improved Nd, Pr, and Dy recovery efficiency from real NdFeB extraction solutions.

The new functional MCF and silica platform technology (Figure 1) would bring the following technical benefits: 1) improved selectivity and extraction efficiency for REE, 2) improved stability, and reusability of sorbent materials in acidic media, 3) reduced processing steps and costs for REE recovery, 4) reduced production of hazardous wastes and risks to human health and environment, 5) high scalability and robustness of the separation technology. The development and optimization of this technology can facilitate domestic recovery of REE from electronic and industrial wastes in an environmentally friendly manner, while mitigating the environmental hazards of the wastes across the U.S.