C. Laliwala, O.O. Amusat, A.I. Torres
Carnegie Mellon University,
United States
Keywords: recycling, rare earth elements, process design and optimization
Summary:
The rare earth elements (REEs) are a group of elements encompassing the lanthanide series, scandium, and yttrium. These elements are valued for their magnetic properties and are used to produce rare earth permanent magnets (REPMs), the strongest magnets commercially available [1,2]. REPMs find uses everywhere, including energy applications, everyday consumer technologies, and defense-related systems [1,2]. Currently, the United States is dependent on offshore sources of these materials, producing only approximately 15% of globally mined REEs. Furthermore, domestic processing capabilities remain limited, forcing the export of nearly all mined material for separation and refining [1]. With demand for these materials projected to increase, it is necessary to strengthen domestic supply chains [2]. To help achieve this goal, we explore the economics of REE recovery from end-of-life (EOL) products. EOL products can serve as a complementary source of REEs. One advantage of targeting EOL products is that they often contain high concentrations of the REEs of interest for permanent magnet production, such as neodymium and dysprosium. Targeting EOL products also offers some distinct advantages over conventional mining, including the lengthy times required for new mines [3]. In this work, we aim to explore potential processing pathways for the recovery of REEs from EOL products containing REPMs. To achieve this, a master flowsheet encompassing processing alternatives for recovering REEs as either mixed or individual rare earth oxides is first constructed. For the potential feedstocks, we consider hard disk drives and motors from electric and hybrid electric vehicles. We then utilize superstructure optimization to identify the optimal processing pathway that maximizes an economic objective function and minimizes process impacts. A set of equivalent trade-off solutions are generated based on these criteria. We find the process that maximizes the economic objective function to consist of automatic disassembly, hydrogen decrepitation, copper nitrate dissolution, oxalic acid precipitation, and calcination to produce mixed REOs. Notably, the optimal processing pathway incorporates the commercially proven Acid-Free Dissolution Recycling process (copper nitrate dissolution followed by oxalic acid precipitation and calcination) owned by Critical Materials Recycling [4]. After identifying the optimal processing pathway that maximizes the economic objective, a detailed process simulation is developed to further improve process performance, utilizing a novel precipitation/dissolution unit model [5]. To increase profits, the production of iron as a byproduct is considered. Process optimization is then utilized to identify operating conditions that minimize operational expenses while maintaining >98% and >99% product yield and purity.