MIT creates groundbreaking REE separation

Massachusetts Institute of Technology researchers have devised a unique method to help curb the looming shortages of critical minerals and metals by making it easier to separate them from ore and recycled materials with a chemical process called sulfidation.

This processing technique, as written about in a paper they published in the journal "Nature," allows the metals to remain in solid form and be separated without dissolving. This would avoid the typical but costly liquid separation methods that require significantly higher energy.

So far, the researchers have developed processing conditions for 56 elements and tested these conditions on 15.

"We are excited to find replacements for processes that had really high levels of water usage and greenhouse gas emissions, such as lithium-ion battery recycling, rare-earth magnet recycling, rare-earth separation," said Caspar Stinn, a graduate student of MIT department of materials science and co-author of the research paper. "Those processes that make materials for sustainability applications, but the processes themselves are very unsuitable."

According to the team, their sulfidation approach could reduce capital costs of metal separation between 65% and 95% from mixed-metal oxides. In addition, compared to traditional liquid-based separation, their selective processing could also reduce greenhouse gas emissions by 60% to 90%.

The findings offer a new way to alleviate the growing demand for minor metals such as cobalt, lithium, and rare earth elements that are used in clean energy products like electric cars, solar cells, and electricity-generating windmills.

In a 2021 report by the International Energy Agency, the average amount of minerals needed for a new unit of power generation capacity has risen by 50% since 2010, as renewable energy technologies using these metals expand their reach.

"We are looking at very well-established materials in conditions that are uncommon compared to what has been done before," said MIT Professor of Metallurgy Antoine Allanore, co-writer of the research paper. "And that is why we are finding new applications or new realities."

For more than a decade, the Allanore group has been studying the use of sulfide materials in developing new electrochemical routes for metal production. Sulfides are common materials, but the MIT scientists have been experimenting with them under extreme conditions like very high temperatures – from 800 to 3,000 degrees Fahrenheit – used in manufacturing plants but not in a typical university lab.

The chemical reaction exploited by the researchers reacts to a material containing a mix of metal oxides to form new metal-sulfur compounds and sulfides. By altering factors like temperature, gas pressure and the addition of carbon in the reaction process, Allanore and Stinn found that they could selectively create a variety of sulfide solids that can be physically separated by a variety of methods, including crushing the material and sorting different-sized sulfides or using magnets to separate different sulfides from one another.

Present methods of rare metal separation rely on large quantities of energy, water, acids, and organic solvents, which have costly environmental impacts, said Stinn.

"We are trying to use materials that are abundant, economical, and readily available for sustainable materials separation, and we have expanded that domain to now include sulfur and sulfides."

Allanore and Stinn used selective sulfidation to separate out economically important metals like cobalt in recycled lithium-ion batteries. They also used their techniques to separate dysprosium – a rare earth element used in applications ranging from data storage devices to optoelectronics – from rare earth-boron magnets or from the typical mixture of oxides available from mining minerals such as bastnaesite.

Metals like cobalt and rare earths are only found in small amounts in mined minerals, so industries must process large volumes of material to retrieve or recycle enough of these metals to be economically viable.

"It's quite clear that these processes are not efficient. Most of the emissions come from the lack of selectivity and the low concentration at which they operate," explained Allanore.

By eliminating the need for liquid separation and the extra steps and materials it requires to dissolve and then reciprocate individual elements, the MIT researchers' process significantly reduces the costs incurred and emissions produced during separation.

"One of the nice things about separating materials using sulfidation is that a lot of existing technology and process infrastructure can be leveraged," said Stinn. "It's new conditions and new chemistries in established reactor styles and equipment."

The next step is to show that the process can work for large amounts of raw material, scaling up – separating out 16 elements from rare earth mining streams, for example.

"Now we have shown that we can handle three or four or five of them together, but we have not yet processed an actual stream from an existing mine at a scale to match what's required for deployment," added Allanore.

The team in the lab have built a reactor that can process about 10 kilograms (22 pounds) of raw material per day, and the researchers are starting conversations with several corporations about the extent of the technology and its possibilities.

"We are discussing what it would take to demonstrate the performance of this approach with existing mineral and recycling streams," finished Allanore.

As for the extent, the world will have to wait and see, yet using a "dry-clean" filtering process may be just what the green energy transition needs to begin surmounting the demand for these critical minerals.