Magnetic Field-Controlled Chromatography for Fractionation of Rare-Earth Containing Phosphors: An Interview with Laura Kuger

Rare earth-containing materials are important in modern technology because of theirunique luminescent properties. The use of these materials spans diverse fields including lighting, display technologies, and medical imaging. Laura Kuger of the Institute for Functional Interfaces (IFG) together with Matthias Franzrebare examining the feasibility of using magnetic field-controlled chromatography to fractionate various rare earth-containing phosphors from end-of-life fluorescent lamps, aiming to develop more effective recycling processes for these valuable materials (1).

LCGC International sat down with Kuger to discuss her research and the challenges in scaling up magnetic field-controlled chromatography for industrial applications.

Laura Kuger of the Institute for Functional Interfaces (IFG). Photo Credit: © Laura Kuger

Can you explain the significance of rare earth-containing materials in modern technologies and its importance from both technological and economic perspectives?

Rare earth elements are indispensable for a broad range of high-tech and green applications because of their unique properties. For an example, solar cells utilize neodymium, dysprosium, and terbium, and LEDs rely on europium and dysprosium for their luminescence. Wind turbines and electric motors require neodymium and samarium in their powerful magnets. Despite their critical role, demand outpaces supply, with geopolitical tensions exacerbating scarcity. Only a few countries control the majority of reserves and production, leading to volatile prices. Shortages prompt nations worldwide to explore domestic sources, driving competition even more. However, the rare earth market’s dispersed nature and environmental impact pose challenges, so balancing technological advancement with sustainable practices remains imperative. Our approach addresses supply chain diversification through technological advancement, stepping forward in securing a stable and environmentally conscious future for rare earth materials.

What motivated the investigation into magnetic field-controlled chromatography for the fractionation of rare earth-containing phosphors from end-of-life fluorescent lamps?

The investigation of magnetic field-controlled chromatography for the fractionation of rare earth-containing phosphors was driven by their inherent physical nature. They exhibit intrinsic paramagnetic properties, making them suitable candidates for magnetism-based separation techniques. Given their critical role in modern technology and the environmental and geopolitical challenges associated with primary ore extraction and rare earth supply, the exploration of such an approach was a logical step for us. In our recent manuscript, we demonstrated the practical applicability of our technique through the exemplary fractionation of a mixture of particles from a ground fluorescent lamp. This practical application showcased the broader potential of our method to impact the circular economy and the sustainable management of rare earth materials, extending well beyond the scope of recycling fluorescent lamps.

How does the intrinsic magnetization of phosphor particles contribute to the separation process in magnetic field-controlled chromatography?

The fundamental magnetic properties of rare earth elements play a pivotal role in their fractionation using magnetic field-controlled chromatography. Their intrinsic paramagnetism makes them uniquely susceptible to magnetic fields and enables us to overcome competing forces such as gravitation or diffusion. In this way, we can influence the movement of rare earth-containing particles through a magnetizable chromatography column. In this context, the degree of deflection within the column is highly dependent on the magnetic susceptibility of the material. Therefore, our method is directly informed by the nature of rare earth materials, allowing for a more efficient and targeted approach to material separation. It is an example for the synergy between the inherent properties of materials and innovative application of separation technology.

Could you discuss the role of process parameters in controlling the separation outcome in the studied method?

In the development of a magnetic field-controlled chromatography process, a range of process parameters play crucial roles in determining the fractionation outcome. Similar to conventional chromatography methods, choosing an appropriate stationary phase is fundamental. For our technique, the matrix’s purpose is to amplify the magnetic background field, thereby influencing the particle trajectories. Key factors include its geometry and size, its magnetic characteristics—the material’s magnetization curve dictates the degree of background field amplification—and its chemical compatibility, particularly its resistance to oxidation. The affinity is mainly governed by hydrodynamic and magnetic influences, making the residence time and the strength of the background magnetic field critical to the fractionation performance. A unique aspect of our method was the ability to alter the affinity through adjustments in the magnetic background field, very similar to the use of salt or pH gradients in ion exchange chromatography. Successful optimization of this process requires a balance of these parameters to enhance fractionation efficacy. In our recent article, our goal was to offer guidance for navigating these complex considerations, aiming to optimize the process for higher purities and recoveries.

What were the key findings regarding purities and recoveries of rare earth-containing phosphors achieved in this study?

Our process optimization led to notably high purities that have not been previously realized in the research field, reaching up to 95.3%, alongside recoveries of 93.6% for the targeted green cerium- and terbium-containing phosphor. The nuanced optimization of various process parameters and process strategies was central in achieving these outcomes. Given the dualistic influence of both the particle material and the particle size on affinity, further improvements in recoveries seem challenging under the current methodological framework. Our research is therefore now exploring more sophisticated separation and elution mechanisms to push the boundaries of what is currently achievable in the purification of magnetic particles.

The abstract mentions an optimized gradient shape. Can you elaborate on how this optimization was achieved and its impact on the separation process?

The concept of magnetic field-controlled chromatography adheres well to the established principles of chromatography process design. Our initial attempts to purify three distinct components using an isocratic approach revealed only minimal retention time differences, which prompted the need for enhanced resolution. To address this, we borrowed a well-known strategy from traditional chromatography, and implemented a gradient. However, uniquely for our method, this gradient was achieved through gradually reducing the background field, thereby decreasing the magnetic force’s retention effect. Initially, the gradient shape was designed with throughput optimization in mind. Yet, upon further refinement, an optimized gradient shape was developed to enhance both the purities and the recoveries. This was achieved by fine-tuning the gradient in accordance with the specific magnetic background fields required for the elution of the individual components. In this way, we achieved a balance between throughput, purity, and recovery within our fractionation process.

How does the consumption of the aqueous eluent compare to other separation methods, and what implications does this have for the feasibility of the process at an industrial scale?

The consumption of the aqueous eluent in our magnetic field-controlled chromatography process is a significant improvement over conventional recycling approaches for rare earth compounds, which largely rely on chemical extraction methods. These methods typically involve dissolution and precipitation steps, necessitating a high consumption of (toxic) chemicals, leading to increased costs and environmental burdens. In contrast, our elution method leverages a purely physical effect—utilizing a time-variable magnetic field gradient—and thus operates isocratically with respect to the mobile phase composition.Because our process relied on an intrinsic physical mechanism, it inherently reduced the variety and consumption of chemical reagents. Besides lowering the operational costs associated with the purchase and disposal of chemicals, this also mitigated the environmental impact linked to toxic chemical waste.

Furthermore, the ability to recycle the aqueous eluent after fractionation through straight-forward solid-liquid separation technologies, such as microfiltration, enhanced the sustainability and economic viability of our process. The reduced need for fresh solvents and the ability to maintain a closed-loop system for the eluent minimized the environmental footprint and the operational costs of the separation process.

What considerations were made regarding the environmental impact of the separation process, particularly in terms of solvent usage?

In addressing the environmental considerations of our process, particularly regarding solvent usage, we took comprehensive actions to align with sustainable and green chemistry practices. Central to our approach was the development of an aqueous mobile phase system with a minimal amount of a biodegradable, non-toxic organic solvent (ethanol). This choice reflected our broader responsibility as process engineers not just to achieve the highest possible economic indicators, such as productivities and purities, but to also ensure our processes inflict minimal environmental harm. Our method stands in contrast to state-of-the-art practices in rare earth processing, which often require large volumes of toxic solvents with significant environmental and health risks, such as organophosphorus compounds and strong acids. Exemplified by our process design studies, we optimized the processing strategy to reduce mobile phase volumes, thereby curtailing waste and decreasing the energy consumption tied to solvent production and disposal. This marks a significant step forward in sustainable rare earth material recycling.

What would you say is the real potential for developing efficient and effective industrial scale purification processes for rare earth-containing materials? 

The real potential for developing energy-efficient industrial-scale purification processes for rare earth-containing materials is huge, considering the urgent demand for these critical components in green technologies and the challenges associated with their current supply chains. However, both the opportunities and the complexities of scaling up these processes must be considered. With the demand for rare-earth elements projected to rise fivefold between 2005 and 2030, the urge for developing efficient recycling systems have never been clearer. Such developments hold the promise of reducing environmental impacts, alleviating geopolitical dependencies, and enhancing the resilience of supply chains. Yet, realizing this potential is laden with hurdles. Currently, less than 1% of rare earth materials are recycled, attributed to immature recycling technologies, economic unfeasibility, and a lack of supportive policies. The volatility of rare earth prices, which in 2020–2021 saw increases by three to five times due to the Covid-19 pandemic and geopolitical conflicts after years of stability, underscores the economic challenges in establishing a robust circular economy. Addressing these challenges requires harmonized global efforts, including significant investments in research and development (R&D) to develop and optimize recycling technologies. Additionally, a law passed by the European Commission in 2023 requiring that 15% of rare earth consumption be covered by secondary sources by 2030 showcases the kind of policy initiatives needed to drive change. Overall, recycling rare earth materials on a large scale will require concerted efforts in innovation, policy support, and international cooperation.

What challenges do you foresee in scaling up magnetic field-controlled chromatography for industrial applications?

Regarding economic challenges, the complexity and expense of collecting and preparing feedstock for magnetic field-controlled chromatography can significantly influence the cost-effectiveness of the process. High collection and logistics costs could negate the economic benefits gained from the efficiency and selectivity of the separation process. Moreover, variations in the quality and composition of collected materials can affect process consistency and yield, further complicating economic viability.

Scaling up the magnetic field source is not a major concern as the specific hardware and energy costs decrease with increasing process scale, however, the design of the chromatography system to accommodate large-scale operations could become challenging. Ensuring uniform flow rates and maintaining consistent separation quality across larger volumes demand sophisticated system designs and protocols. We are actively exploring continuous chromatography, namely simulated moving bed (SMB) chromatography, as a viable solution to this challenge. We have already conducted successful feasibility studies and, consequently, filed a patent for this technology. Continuous chromatography allows for the use of smaller column sizes while maintaining equivalent throughput, which leads to a reduced overall process volume and enhanced efficiency.

Furthermore, integrating magnetic field-controlled chromatography into established industrial processes could trigger logistical and technical challenges. Retrofitting existing facilities or designing new ones to incorporate this technology necessitates a deep understanding of current industrial workflows and the potential impacts on throughput and product quality. In our opinion, encouraging market adoption not only includes overcoming technical and economic barriers but also fostering confidence in the reliability and long-term benefits of the technology and rare earth recycling in general. Continuous technological development, driven by research and industry collaborations, is essential for expanding process applicability to various materials and territories worldwide.

Reference

(1) Kuger, L.; Franzreb, M. Design of a Magnetic Field-Controlled Chromatography Process for Efficient and Selective Fractionation of Rare Earth Phosphors from End-of-Life Fluorescent Lamps. ACS Sustainable Chem. Eng.2024, 12 (8), 2988–2999. DOI: 10.1021/acssuschemeng.3c05707