MIT researchers develop optimized sulfidation separation process for rare earths and other key metals

New processing methods developed by MIT researchers could help alleviate the looming scarcity of the essential 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 sulfidation process enabled the metallurgy professor Antoine Allanore and his doctoral student Caspar Stinn to selectively separate rare metals such as cobalt in a lithium-ion battery from mixed-metal materials. An article about their work is published in the journal Nature.

The processing techniques allow the metals to remain in solid form and separate 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.

Shown are rare earth oxides of neodymium, praseodymium and dysprosium, 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. Image: Courtesy of the researchers

Your sulfidation approach 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.

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

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. Sulphides are common materials, but MIT scientists are experimenting with them in extreme conditions like temperatures of 800 to 3,000 degrees Fahrenheit that are used in manufacturing facilities but not in a typical university laboratory.

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 could 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 try to use abundant, economical and readily available materials for sustainable material separation, and we have expanded this area to include sulfur and sulfides.

—Caspar Stinn

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 from rare earth boron magnets or from the typical mixture of oxides obtained from mining minerals such as bastnaesite.

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.

—Antoine Allanore

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 sulphidation is that many existing technologies and process infrastructures can be used. There are new conditions and new chemistries in established reactor types and equipment.

—Caspar Stinn

The next step is to show that the process can work for large amounts of raw materials – for example by separating 16 elements from rare earth mining streams.

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.

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

resources

• Cite this article Stinn, C., Allanore, A. (2021) “Selective Sulphidation of Metal Compounds”. Nature doi: 10.1038 / s41586-021-04321-5