The 17 metals known as rare earth elements are a critical but hidden part of our lives. They are used in batteries, computers, and cell phones. As the demand for electronics booms around the world, so does the need for these elements. But mining or recycling rare earths is complex, expensive, and harmful to the environment.

Swedish researchers have now developed a simple, environmentally friendly method that efficiently extracts rare earth elements (REEs) from solution using magnetic nanoparticles. The method removes 20 times more metals from solution than previously reported nanoparticle techniques, and does it faster. Plus, the straightforward method should be easy to scale up for industrial use, they say in an article published in Scientific Reports.

“This new approach appears to offer huge capacity using quite simple materials and condition controls,” says Vilas Pol, a professor of chemical engineering at Purdue University who was not involved in the work. “It can potentially provide a principal breakthrough in both REE extraction from ore leachates and from solutions in REE recycling.”

The elements in the rare earth group are not really rare, but are often found together in geological deposits. But these deposits have low REE concentrations, so mining generally results in large amounts of unwanted materials. Plus, extracting REEs from ores or recycling them from waste involves dissolving their compounds in ionic liquids or other solvents. “It generally requires big volumes of solvents in high concentrations, generating lots of hazardous waste,” says Elizabeth Polido Legaria, a graduate student in chemistry and biotechnology at the Swedish University of Agricultural Sciences and lead author of the article.

Researchers have been working on a more sustainable method that uses silicon dioxide or polymer nanoparticles to pull REEs from solution. The particles’ large surface area is decorated with organic molecules that bind exclusively with rare earths, making them efficient at adsorbing large amounts of the metals. However, the efficiency depends on the amount of organic molecules on them.

Legaria and her colleagues decided to investigate whether the nanoparticles would take up more REEs at higher pH levels. To their surprise, this led to a new technique that turned out to be far more efficient and easy to use than using regular silica nanoparticles.

The team made magnetic iron oxide nanoparticles covered with silica, and added them to aqueous solutions of three REEs—lanthanum, dysprosium, and neodymium. When they added ammonium hydroxide drop-by-drop to slowly increase the pH of the solution up to 9, rare earth hydroxides precipitated out of the solution and started forming crystals on the silica nanoparticles surface. The REE-loaded nanoparticles are easily separated from the solution using a magnet. “From an industrial point of view this is very beneficial,” Legaria says.

The researchers also devised an environmentally friendly way to separate the REEs. This is typically done by mixing and shaking the loaded nanoparticles with acids and then filtering out the particles, giving large amounts of waste acid solutions. The Swedish team instead mixed REE-bearing nanoparticles with mild ammonium sulfate and shook the mixture in an orbital shaker machine in three steps, twice for three hours and then one last time for 20 hours. They were able to reuse the nanoparticles three times without any loss in efficiency.

“The process itself is very easy and since no specific organic ligands are needed, the material preparation is much simpler, and therefore the production can be scaled up,” Legaria says. The researchers are now testing the process on real-life REE ore samples. They are also developing industrial contacts and planning to create automated equipment to build a demonstration plant.

“The real advantage is that this method can be applied in relatively dilute solutions where REE concentration is quite low and where all the other methods cannot be applied or are uneconomical,” says Dimitris Panias, a professor of mining and metallurgical engineering at the National Technical University of Athens in Greece, not associated with the study. Before the method can become practical, scientists will need to devise inexpensive, large-scale methods to make the silica-iron nanoparticles, and develop an effective technique to separate the various REEs.

Read the article in Scientific Reports.