Selective separation could help alleviate the critical shortages of rare earth metals and other key metals

Shown are rare earth oxides of neodymium, praseodymium and dysprosium – all critical components for magnets – which have been processed with sulfidation technology. The purple areas are neodymium-rich sulfide, the green areas are praseodymium oxysulfide, and the orange areas are dysprosium-rich sulfides and oxysulfides. Credit: Courtesy of the researchers

A new way of processing rare earth and other key metals to separate them from other materials could reduce environmental impact and costs.

New processing methods developed by

“> WITH Researchers could help alleviate the looming shortage of the vital metals that power everything from phones to car batteries by making it easier to separate these rare metals from mining ores and recycled materials.

Selective adjustments within a chemical process known as sulfidation enabled the metallurgy professor Antoine Allanore and his doctoral student Caspar Stinn to successfully separate and separate rare metals such as the cobalt in a lithium-ion battery from mischmetal materials.

As they report in the journal Nature, their processing techniques allow the metals to remain in solid form and be separated without dissolving the material. This avoids traditional but costly liquid separation processes that require a lot of energy. The researchers developed processing conditions for 56 elements and tested these conditions on 15 elements.

Their sulfidation approach, they write in the paper, could reduce the capital cost of metal separation by between 65 and 95 percent from mixed metal oxides. Their selective processing could also reduce greenhouse gas emissions by 60 to 90 percent compared to traditional liquid-based separation.

“We were thrilled to find replacements for processes that have really high water consumption and very high greenhouse gas emissions, such as recycling lithium-ion batteries, recycling rare earth magnets, and separating rare earths,” says Stinn. “These are processes that make materials for sustainability applications, but the processes themselves are very unsustainable.”

The results offer a way to alleviate the growing demand for minor metals such as cobalt, lithium, and rare earth elements, which are used in “clean” energy products such as electric cars, solar cells, and power-generating windmills. According to a report by the International Energy Agency for 2021, the average amount of minerals needed for a new unit of power generating capacity has increased by 50 percent since 2010 as renewable energy technologies using these metals expand their reach.

Possibility of selectivity

For more than a decade, the Allanore Group has studied the use of sulfide materials in the development of new electrochemical routes for metal fabrication. Sulfides are common materials, but MIT scientists are experimenting with them under extreme conditions such as very high temperatures – from 800 to 3,000 degrees

“> Fahrenheit – that are used in production facilities, but not in a typical university laboratory.

“We look at very well-established materials under conditions that are unusual compared to before,” explains Allanore, “and that’s why we find new applications or new realities.”

When synthesizing high temperature sulfide materials to aid electrochemical production, Stinn says, “We learned that we can be very selective and very controlled about which products we make. And with this understanding we realized: ‘Okay, maybe there is an opportunity here for selectivity in the separation.’ “

The chemical reaction exploited by the researchers converts a material containing a mixture of metal oxides into new metal-sulfur compounds or sulfides. By changing factors such as temperature, gas pressure, and the addition of carbon in the reaction process, Stinn and Allanore found that they could selectively produce a variety of sulfide solids that can be physically separated by a variety of methods, including crushing the material and sorting out different sizes Sulfides or the use of magnets to separate different sulfides from each other.

Current methods for separating rare metals rely on large amounts of energy, water, acids, and organic solvents, which have costly environmental impacts, says Stinn. “We are trying to use abundant, economical and readily available materials for sustainable material separation, and we have expanded this area to include sulfur and sulfides.”

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

Use of existing technology

Metals like cobalt and rare earths are only found in small amounts in mined materials, so industries must process large amounts of material to extract or recycle enough of these metals to be economically viable, explains Allanore. “It is very clear that these processes are not efficient. Most of the emissions stem from the lack of selectivity and the low concentration at which they are operated. “

By eliminating liquid separation and the additional steps and materials required to dissolve and then re-precipitate individual elements, the MIT researchers’ process significantly reduces separation costs and emissions.

“The nice thing about the separation of substances through sulfidation is that many existing technologies and process infrastructures can be used,” says Stinn. “There are new conditions and new chemistries in established reactor types and equipment.”

The next step is to show that the process can work for large amounts of raw materials – for example, the separation of 16 elements from rare earth mining streams. “Now we’ve shown that we can handle three, four or five of these together, but we haven’t yet processed any actual power from an existing mine on a scale that meets the requirements for the mission,” says Allanore.

Stinn and colleagues from the laboratory have built a reactor that can process around 10 kilograms of raw material per day, and the researchers are starting discussions with several corporations about the possibilities.

“We are discussing what would be needed to demonstrate the effectiveness of this approach with existing mineral and recycling streams,” says Allanore.

Reference: “Selective sulfidation of metal compounds” by Caspar Stinn and Antoine Allanore, December 16, 2021, Nature.
DOI: 10.1038 / s41586-021-04321-5

This research was supported by the US Department of Energy and the US National Science Foundation.