Scientists produce rare earth elements separation technology

New rare earth element separation technology provides insight into how to cost-effective separate in-demand rare-earth elements can be, dramatically shifting the industry to benefit producers in the United States.

A new technology for rare-earth elements chemical separation has been licensed to Marshallton Research Laboratories, a North Carolina-based manufacturer of organic chemicals for various industries. It was established by scientists from Oak Ridge National Laboratory and Idaho National Laboratory in the Department of Energy’s Critical Materials Institute (CMI).

CMI is supported by the Office of Energy Efficiency and Renewable Energy’s Advanced Manufacturing Office. They work to decarbonise industries and increase the competitiveness of US manufacturing and clean energy sectors through processes of innovations, collaborations, research and technical assistance and workforce training. CMI pursues ways to eliminate and reduce reliance on rare earth metals and other materials critical to the success of clean energy technologies to encourage better use of materials. Their technology provides critical insight into how to cost-effectively separate in-demand rare-earth elements, which could dramatically shift industry to benefit producers in the United States.

Rare earth element properties

The unique electronic properties of rare-earth elements (REE’s) that are a grouped in 17 metallic elements, including 15 lanthanides as well as yttrium and scandium, make them critical for producing electronics, optical technologies, alloys, and high-performance magnets. These powerful, permanent magnets are vital to clean energy technology and defence applications.

However, individual rare earth elements do not occur in minable concentrations in the Earth’s crust but are instead naturally mineralised together require chemical separation in order for use in technological applications. Unfortunately, their physical and chemical similarities make them extremely difficult and costly to separate while generating a lot of waste. Extraction and separation of REEs for technological applications occurs largely overseas.

Extraction agents

In order to achieve the growing requirements for these materials and to limit the nation’s reliance on foreign sources, ORNL and INL scientists working under the banner of CMI have applied their extensive expertise in chemical synthesis, separations, and engineering to design and produce new extraction agents. They are based on diglycolamide (DGA) ligands and a corresponding process for separating lanthanides that outperforms current technology.

“At Marshallton, our purpose is to become a domestic, strategically reliable supplier of DGA extractants for rare-earth elements,” said Mac Foster, co-owner of Marshallton and a collaborator on the technology. “We expect to service pilot-plant and commercial operations in ore processing, recovery from mining tailings and recycling. We’re excited to further explore what these new extractants can achieve.”

Rare earth elements are typically separated using liquid extraction, which uses ligands, organic molecules composed of carbon, hydrogen, oxygen, and nitrogen atoms, as extractants to selectively bind the REE ions. An oily solvent containing the extractant is then mixed vigorously with a REE-rich aqueous solution and allowed to separate in the same manner as oil and vinegar for a salad dressing. During this process, the REEs are transferred into the organic solvent, thus forming complexes with the extractant molecules. DGAs display a higher affinity for lanthanides with smaller ionic radius, allowing individual REEs to be separated from one another in multiple stages.

“Our goal was to identify an extractant that surpasses the performance of the state-of-the-art ligands that are currently used in industry,” explained ORNL’s Santa Jansone-Popova. “The compound widely used is a phosphorous based extractant, called PC88A, and since its selectivity is relatively low, a lot of separation stages are required along with generation of additional waste throughout the process.”

Selectivity and separation factors

Selectivity is the degree to which a solvent favours one metal over another and is described by a unit called separation factor. For instance, when seeking to separate adjacent lanthanides neodymium and praseodymium, that are both used in high-powered magnets, the phosphorus-based extractant’s separation factor is around 1.2, which is especially low. “You have to run the extraction many, many times to separate adjacent lanthanides completely,” Jansone-Popova said. “We need to improve the economics of the process, reduce the waste, reduce the complexity – limit the steps it takes to achieve separation.”

ORNL’s Chemical Science Division had been investigating an alternative DGA known as TOGDA, which has a separation factor of 2.5, which is a big improvement over the phosphorus-based extractant. Nevertheless, a key variable in the economics of the process is loading capacity, meaning how many grams per litre of extractants can be held in the organic solvent without adverse reactions. TODGA could only handle about one-fifth of what the phosphorous-based extract could.

Kevin Lyon, an INL chemical engineer with expertise in applied solvent extraction who tested and developed the process design for the licensed technology added that “the extractant concentrations we were limited to were not adequate compared to the industry standard. At higher concentrations, we run into things like gelling or precipitation, which are detrimental to the process. If you think of the process as a conveyor belt, we want to be able to load that conveyor belt up as high as we can, or at least competitive with what industry does, to make it cost effective.”

Chemically modifying the structure of DGAs

Jansone-Popova recognised that by chemically modifying the structure of DGAs, she might improve their properties and their efficiency in extracting REEs. Her team at ORNL began a systematic approach to applying structural changes to the DGA ligands. They did this by adding a range of substituents known as alkyls, that are fatty organic groups that exclusively contain hydrogen and carbon atoms. These groups can be arranged into different structural configurations. For example, their length and shape can be altered, branches created, or linear chains transformed into cyclic arrangements.

The ORNL team sent the trial ligands off to Lyon to test under their industrial operating conditions using a counter-current solvent extraction system which involves a series of vessels that mix and settle the materials to separate out rare earth element compounds through a sequence of liquid-liquid extraction stages. During this mixing process, the ligands attract the metal ions using electron-rich donor groups, binding the metal ions in a synchronised manner. Extracting certain lanthanides depends on ligands having the right number and arrangement of functional groups, that are atoms within a molecule that can maintain functionality independently of other atoms in the molecule. It also relies on the size of the ligands and their ability to mix with the oily organic solvent.

The ORNL team have also designed, synthesised, and established a library of chemically modified ligands, in collaboration with Lyon. Aiming to narrow the field of novel agents for industrial application that could potentially outperform state-of-the-art technology in REE selectivity. Each agent performs differently based on its physical arrangement and the electronic activity it prompts. “The TOGDA extractant, when saturated with REE ions, would rapidly transform from the liquid phase into a gel or precipitate,” Jansone-Popova explained. “The new DGA ligands allow the system to remain homogenous even at higher extractant concentrations and maintain good selectivity.”

In separating REEs, the new ligands achieved a selectivity range of 2.5–3.1, a staggering improvement for these critical materials. The team then took on the challenge of scaling up the process to be viable for industry use. “The process was very iterative; minute changes in the structures of these molecules have impact,” Lyon said. “The bottom line is that a new technology has to be economically viable. We’re very driven by input from industry and the methods they use.”

ORNL’s Bruce Moyer, who leads the CMI focus area for diversifying supply and is a collaborator on the licensed technology said that “most rare earth elements separation extractants have a separation factor of about 1.5 for adjacent lanthanides across the series, if we get to 2, that’s good. If we get to 2.5, that’s really starting to save some money. If we can get to 3, we’re really happy. We’ve gotten to 6.7 with one of Santa’s ligands.”

In his role at CMI, Moyer oversees a portfolio of research projects investigating how to expand the supply of rare earth elements through innovative processes. “CMI’s goal is to provide the best separation technology to industry,” said Moyer. “We’ve selected these DGAs because they have the potential to reduce the consumption of chemicals and production of waste, thereby lowering costs. They’re more selective, which reduces the number of stages needed, reducing the overall capital cost of building a plant.”

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