Rare earths are not as rare as their name suggests. Yet they are indispensable to the modern economy. These 17 metals are indeed essential raw materials for digitalization and the energy transition. They are found in smartphones, computers, screens and batteries – without them, no electric motor would work and no wind turbine would turn. Since Europe is almost entirely dependent on imports from China, these raw materials are considered critical.
Rare earths are also crucial because of their extraction. They always occur as compounds in natural ores, but because these elements are chemically very similar, they are difficult to separate. Traditional separation processes are therefore very chemical and energy intensive and require several extraction steps. Extracting and purifying these metals is therefore expensive, resource- and time-consuming, and extremely harmful to the environment.
“In Europe, rare earths are almost never recycled,” says Victor Mougel, professor at the Laboratory of Inorganic Chemistry at ETH Zurich. A team of researchers led by Mougel wants to remedy this situation. “There is an urgent need to find sustainable and simple methods to separate and recover these strategic raw materials from various sources,” says the chemist.
In a study recently published in the journal external pageNature CommunicationsThe team presents a surprisingly simple method to efficiently separate and recover europium, a rare earth metal, from complex mixtures including other rare earth metals.
Inspired by nature
Marie Perrin, a PhD student in Mougel’s group and first author of the study, explains: “Existing separation methods rely on hundreds of liquid-liquid extraction steps and are inefficient. Recycling europium was previously impossible.” In their study, they show how a simple inorganic reagent can significantly improve separation. “This allows us to obtain europium in a few simple steps, and in quantities at least 50 times higher than those of previous separation methods,” explains Marie Perrin.
The key to this technique lies in small inorganic molecules with four sulfur atoms around tungsten or molybdenum: tetrathiometallates. The researchers were inspired by the world of proteins. Tetrathiometallates are found as metal binding sites in natural enzymes and are used as active substances against cancer and copper metabolism disorders.
For the first time, tetrathiometallates are also used as ligands for the separation of rare earths. Their unique redox properties come into play here, reducing europium to its unusual bivalent state and thus simplifying the separation of other trivalent rare earths.