[Paper Review] Opportunities for plasma separation techniques in rare earth elements recycling

Renaud Gueroult, Jean-Marcel Rax+1 more|arXiv (Cornell University)|Oct 12, 2017

Extraction and Separation Processes108 references63 citations

TL;DR

The paper evaluates plasma mass separation as a recycling pathway for NdFeB magnets, modeling a mass-filter approach and giving preliminary cost estimates and performance insights.

ABSTRACT

Rare earth elements recycling has been proposed to alleviate supply risks and market volatility. In this context, the potential of a new recycling pathway, namely plasma mass separation, is uncovered through the example of nedodymium - iron - boron magnets recycling. Plasma mass separation is shown to address some of the shortcomings of existing rare earth elements recycling pathways, in particular detrimental environmental effects. A simplified mass separation model suggests that plasma separation performances could compare favourably with existing recycling options. In addition, simple energetic considerations of plasma processing suggest that the cost of these techniques may not be prohibitive, particularly considering that energy costs from solar may become significantly cheaper. Further investigation and experimental demonstration of plasma separation techniques should permit asserting the potential of these techniques against other recycling techniques currently under development.

Motivation & Objective

• Motivate the need for diversified REE supply and reduced environmental impact through recycling of NdFeB magnets.
• Introduce plasma separation as an environmentally friendly alternative to hydro- and pyro-metallurgical recycling.
• Propose a simple mass-filter model to assess REE separation performance from NdFeB waste.
• Provide preliminary cost estimates and discuss potential advantages and challenges for plasma-based recycling.

Proposed method

• Define a plasma mass filter function Gamma_M0(M) to model separation based on atomic mass with a threshold M0 and width parameter alpha.
• Analyze a three-tier separation scenario where heavy (REEs) and light (Fe, B, contaminants) streams are produced (Stage 1).
• Compute REE purity x_RE, separation factor beta_RE, and extraction efficiency r_RE using mass fractions and the filter function (Equations 2–4).
• Explore how filter width and threshold affect purity and efficiency (Cases A, B, C correspond to different alpha values).
• Offer a conceptual flow for advanced separation to isolate individual REEs from the mixed REE stream (Stage 2).
• Provide a preliminary energetic cost framework for plasma processing, including evaporation/ heating and plasma formation costs.

Experimental results

Research questions

• RQ1 Can plasma mass separation effectively separate REEs from light elements in NdFeB magnet waste in a single stage?
• RQ2 How do filter width and threshold (M0, alpha) affect REE purity, extraction efficiency, and separation factor?
• RQ3 What are the potential cost ranges for plasma-based NdFeB magnet recycling, and how do they compare to chemical methods?
• RQ4 Is a multi-stage plasma separation scheme capable of isolating individual REEs with acceptable costs?

Key findings

• A narrow filter (Case A) yields near-perfect REE purity in the heavy stream (x_RE = 1) and very high extraction (r_RE = 1) with extremely large separation factors (beta_RE = 3×10^5).
• A moderate-width filter (Case B) still achieves high purity (x_RE = 0.97) and high extraction (r_RE = 0.99) with a substantial separation factor (beta_RE = 76).
• A broad filter (Case C) markedly reduces purity (x_RE = 0.44) and extraction (r_RE = 0.61) with a small separation factor (beta_RE = 1.57).
• Sensitivity analysis shows that purity is mostly controlled by filter width, while extraction efficiency decreases as the threshold M0 increases in wider filters.
• Preliminary energy-cost estimates place plasma processing at about 800 MJ/kg for NdFeB feed, corresponding to ~$27 per kg at $0.12/kW·h, with solar-energy cost reductions potentially lowering future costs; partial ionization strategies could further reduce costs (to ~$13–$27 per kg depending on composition).
• The study suggests plasma separation could be environmentally advantageous (no chemical waste) and potentially cost-competitive for concentrated feeds like large magnets, warranting further experimental validation.