The lanthanides, also known as rare-earth elements, are used to make magnets found in computers, cell phones, and many other gadgets. Until recently, China was the main producer of these raw materials, giving that country an advantage in setting market prices. Seeking cheaper sources, the U.S. Department of Energy developed an initiative to recover lanthanides from electronic waste. Now, researchers working as part of the DOE’s Critical Materials Institute have recovered rare earths from scrap magnet waste using a technique called membrane solvent extraction, which requires much less energy and generates less chemical waste than previous recovery techniques (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b01306).
Finding a cost-effective, environmentally sustainable way to isolate rare-earth elements such as neodymium from electronic waste isn’t easy. Pyrometallurgical methods heat the e-waste to high temperatures to separate out the elements, but the large energy requirement can be expensive. In another process, recyclers dissolve the waste in strong acid and then extract the rare-earth elements with a series of solvents, generating large amounts of hazardous chemical waste. Moreover, this method can’t always isolate lanthanides from other common elements in the e-waste, such as iron.
Ramesh Bhave of Oak Ridge National Laboratory and his colleagues decided to apply a method—codeveloped by Bhave and Kamalesh K. Sirkar in the 1980s to remove organic molecules from aqueous solutions—that combines traditional solvent extraction with membrane separation techniques to recover these elements. In just one step, the researchers can isolate rare earths from a feed solution made from acid-digested scrap magnets that contains the lanthanides neodymium, praseodymium, and dysprosium, as well as iron and boron impurities.
The prototype device is an array of eight strawlike membranes made from hollow fiber polypropylene with microscopic pores. A pump pushes a pressurized feed solution through the straws. On the outer surface of the straws, researchers introduce an organic solvent containing an extractant that selects for lanthanides. The extractant solution is immiscible with the feed solution, and the higher pressure inside the straw prevents it from flowing through the pores. However, because the extractant contacts the feed solution at the pore sites, it draws the lanthanides out of the feed solution and through the pores. Meanwhile, an acidic stripping solution that moves through the space surrounding the straws captures these lanthanides and recovers them. The new method could generate 20 to 30% less chemical waste than traditional extraction techniques do, Bhave says.
The researchers then precipitated a mixture of lanthanide oxides from the stripping solution with oxalic acid and filtered, dried, and annealed them. Using X-ray diffraction, they found that the oxides had no detectable impurities, such as iron or boron, and the oxides can be directly reused for some applications without further processing, Bhave says. The system can recover more than 90% of the lanthanides from the scrap magnet waste, comparable to other current methods.
Corby G. Anderson, a researcher at Colorado School of Mines who is also affiliated with the Critical Materials Institute but was not involved with the study, calls the prototype a significant advance in recovering rare-earth elements, with the benefit that it can do this in a single step. Although the technique successfully separates out lanthanides from e-waste, he says the next challenge will be isolating each element individually.