Recovering rare earth metals from electronic waste

Contrary to their name, rare earth metals are not particularly scarce, yet they are crucial for the modern economy. These 17 metals are vital raw materials for digitalisation and the energy transition, playing key roles in devices like smartphones, computers, screens, and batteries. They are also essential for the operation of electric motors and wind turbines. Due to Europe’s heavy reliance on imports from China, these materials are deemed critical.

The extraction of rare earth metals poses significant challenges as well. They are always found in compound forms within natural ores, and their similar chemical properties make them difficult to separate. Traditional separation methods are highly chemical- and energy-intensive, involving multiple extraction steps. This makes the process costly, resource-intensive, time-consuming, and extremely detrimental to the environment.

"Rare earth metals are hardly ever recycled in Europe," says Victor Mougel, Professor at the Laboratory of Inorganic Chemistry at ETH Zurich. "There is an urgent need for sustainable and uncomplicated methods for separating and recovering these strategic raw materials from various sources.”

In a recent study published in the journal Nature Communications, the team introduced an unexpectedly straightforward method for efficiently separating and recovering the rare earth metal europium from complex mixtures that include other rare earth metals.

Nature-inspired

Marie Perrin, a doctoral student in Mougel's group and the primary author of the study, explained: "Current separation methods involve hundreds of liquid-liquid extraction steps and lack efficiency, making the recycling of europium impractical." In their research, they demonstrated how a simple inorganic reagent could greatly enhance the separation process. "This enables us to obtain europium in just a few straightforward steps, yielding quantities at least 50 times greater than previous methods," Perrin stated.

The innovation lies in small inorganic molecules with four sulphur atoms surrounding tungsten or molybdenum, known as tetrathiometallates. Inspired by the protein world, the researchers found that tetrathiometallates, which serve as metal binding sites in natural enzymes and are used as active agents against cancer and copper metabolism disorders, could be repurposed.

For the first time, tetrathiometallates are being utilised as ligands for the separation of rare earth metals. Their unique redox properties allow them to reduce europium to its unusual divalent state, simplifying its separation from other trivalent rare earth metals.

"The principle is so efficient and robust that we can apply it directly to used fluorescent lamps without the usual pre-treatment steps," says Mougel.