Researchers have discovered a possible new source of rare earth elements—phosphate rock waste—and an environmentally friendly way to get them out.
Rare earth elements like neodymium and dysprosium are essential for technologies such as solar and wind energy, advanced vehicles, and modern electronics like smartphones. But a shortage of rare earth element production in the United States puts our energy security at risk. China produces roughly 90 percent of all such elements.
Recovering them from phosphogypsum—waste from phosphoric acid production—is a potential solution. Each year, the US mines an estimated 250 million tons of phosphate rock to produce phosphoric acid for fertilizers. The US mined approximately 28 million metric tons in 2017.
Rare earth elements generally amount to less than 0.1 percent in phosphate rock. But worldwide, about 100,000 tons of these elements per year end up in phosphogypsum waste. That’s almost as much as the approximately 126,000 tons of rare earth oxides produced worldwide each year.
Conventional methods to extract rare earth elements from ores generate millions of tons of toxic and acidic pollutants.
But instead of using harsh chemicals to extract the elements, another method might use organic acids that bacteria produce, say co-lead authors Paul J. Antonick and Zhichao Hu, members of the thermodynamics team of senior author Richard E. Riman, professor in the materials science and engineering department at the Rutgers University School of Engineering.
The researchers explored using mineral and organic acids, including a bio-acid mixture, to extract six rare earth elements (yttrium, cerium, neodymium, samarium, europium, and ytterbium) from synthetic phosphogypsum.
Scientists working with David Reed at Idaho National Laboratory produced the bio-acid mixture—consisting primarily of gluconic acid, found naturally in fruits and honey—by growing the bacteria Gluconobacter oxydans on glucose.
The results suggest that the bio-acid did a better job extracting rare earth elements than pure gluconic acid at the same pH (2.1), or degree of acidity.
The mineral acids (sulfuric and phosphoric) failed to extract any rare earth elements in that scenario. When the researchers tested the four acids at the same concentration, only sulfuric acid was more effective than the bio-acid.
As a next step, researchers will test bio-acid on industrial phosphogypsum and other wastes generated during phosphoric acid production that also contain rare earth elements. For their initial study, the researchers evaluated phosphogypsum made in the lab, so they could easily control its composition. Industrial samples are more complex.
The research appears in the Journal of Chemical Thermodynamics. Additional researchers are from Rutgers; the Idaho National Laboratory; the University of California, Davis; Lawrence Livermore National Laboratory; and OLI Systems Inc. in Cedar Knolls, New Jersey.
Source: Rutgers University