Rutgers researchers discover new, environmentally friendly way to extract rare earth elements
A research team at Rutgers offered findings on a new source of rare earth elements, as well as how to extract them.
The term “rare earth element” (REE) refers to a series of chemical elements called lanthanides, which encompass a row in the periodic table. Though they are not actually rare, they are considered so because they do not occur often in concentrated deposits, said Paul Antonick, a graduate student in the Department of Materials Science and Engineering.
“This makes extracting them difficult, because there is a small amount of them dispersed over large areas,” he said.
Furthermore, the current extraction process for REEs is not environmentally friendly. The largest REE extraction site, which is located in Bayan Obo, China, is incredibly polluted, Antonick said. With approximately 90 percent of REEs being produced in China, there is increasing interest in sustainable REE production in the United States. One of the goals of the research team is to develop a more sustainable process so that the United States can compete in its production.
REEs are essential for their uses in magnets, lighting, batteries and electronics. They are also necessary for clean energy technologies, including wind turbines and solar panels. Antonick said in the United States, there is a shortage in the production of REEs, as they are difficult and costly to obtain.
Extracting REEs from phosphate deposits, which was the focus of Antonick’s research team, may lead to a potential solution. In their research, the source of REEs is a waste product called phosphogypsum, which is a by-product of fertilizer production from phosphate rocks. Along with phosphate rock, the compound is composed of gypsum, a type of soft mineral, and water.
“It has been known for a while that phosphogypsum contains higher amounts of rare earth elements than some other potential sources,” Antonick said. “We have produced results which could potentially be used to design a process ... to commercialize rare earth extraction from phosphogypsum.”
Extraction is primarily based on a process called leaching, which is when a solid material is dissolved in a liquid. Since REEs are chemically bonded to other elements to form a compound, which is the case with phosphogypsum, it must be separated.
“Large chunks of the mineral mix are ground up using mechanical grinding and milling and then dumped into some liquid … usually something acidic (that) will dissolve the REE containing compounds, but leave behind the other junk we don't care about,” Antonick said.
His research team ended up creating six samples of phosphogypsum, each containing a different REE. Each sample was then dissolved into certain types of acids known as lixiviants, which are liquids that can be used to extract metals.
The solutions were then mixed for a set period of time, and any undissolved solids were filtered off from the acids. Using light analysis, the research team was able to see how much of each element was in the lixiviant. They observed that every element had a slightly different color released from the analysis.
“By comparing this amount to the original REE concentration in the phosphogypsum sample, we figured out exactly how much REE was leached,” Antonick said.
The methods they used were also less harmful to the environment. One of the lixiviants used was a “bio lixiviant” since it was produced by bacteria growing from sugar, which is a less environmentally toxic way to produce acids, he said.
To see which acid was most efficient at extracting REEs, they tested each lixiviant at the same pH. Some of the acids, such as the bio lixiviant, were capable of extracting REE, while others were not. In an experiment where the acids were tested at the same concentration, the bio lixiviant performed second best.
There are two major applications of these findings. Antonick said the same experiments can also be done on industrial waste from phosphogypsum to see how the results are different for a more complicated and less clean system. The potential REE extraction process could also be more profitable, according to the results.
There is still much to be done before this new method of REE extraction can be commercialized, Antonick said. The research though, can be used to supplement scientific databases to make more accurate predictions about how REEs behave in different acids.
“The best dream scenario would be that cost analysis shows that REE leaching from phosphogypsum is very profitable, at which point a huge new source of REE would be available to the world,” Antonick said.