Redox reactions ‘mine’ old fluorescent light bulbs for europium

A new redox method can extract and recycle europium from real-life waste such as compact fluorescent lamps. in just three simple steps this protocol yields the lanthanide in quantities at least 50 times higher than state-of-the-art solutions used by the chemical industry. Researchers have already patented the technology and created a start-up to commercialise it.

Rare earths – a set of elements including all of the lanthanides, plus scandium and yttrium – have become tremendously important in our day-to-day lives, with applications that range from small components in electronics to green technologies such as batteries, wind turbines and electric cars. However, these elements are mostly mined outside of Europe and North America and are difficult to extract, refine and recycle. ‘The sourcing and purification of rare earth elements is expensive, labour-intensive and ecologically devastating,’ says first author Marie Perrin from ETH Zurich in Switzerland.

Another factor that hinders the successful separation of rare earths is that they are chemically similar. Sometimes, separating certain lanthanides and actinides takes up to 1000 steps.

Separation shortcut

In contrast, the Zurich team has demonstrated that simple inorganic reagents can improve separation immensely – in just three steps. ‘We reach separations 50 times higher than current industrial processes, reducing the amount of waste as well,’ Perrin says.

The secret lies in sulfur-based ligands – a bioinspired solution that selectively separates europium from complex mixtures. ‘Tetrathiometallates are common in enzymes and also used as drugs to treat cancer and copper metabolism disorders … but their reactivity with rare earth elements had been relatively unexplored,’ explains lead author Victor Mougel.

In this study, the unique redox properties of tetrathiometallates transform europium into its unusual divalent state, simplifying separation from the other trivalent rare earth elements. After leaching all the lanthanides and actinides from a waste sample using triflic acid, reaction with a tungsten tetrathiometallate creates an insoluble precipitate that contains most of the europium. ‘We reach completion after 24 hours, then calcination at 600ºC yields europium oxide in over 90% purity,’ adds Mougel.

‘Rare earths are essential for modern life … but bear a big environmental burden for primary production and purification,’ explains Eric Schelter, an expert in the separation of lanthanides and actinides at Penn State University, US. ‘The selectivity of this reaction evidently enables an efficient separation of europium,’ he adds. ‘The tetrathiotungstate ligands drive a selective redox reaction [towards] the precipitation of a europium coordination polymer.’

A major advantage is that this reaction works with real waste – particularly discarded fluorescent lamps, which use europium salts as phosphors. And, although many factors affect the traditional liquid-liquid purification processes, including the glass of the lamps and the presence of other cations in the phosphor, none of these hindered the recovery reaction, explains Perrin. ‘Our process needs no prior treatment of the phosphor powder, which proves its robustness,’ she adds. Moreover, she argues it makes more sense to ‘mine’ europium from used lamps, instead of natural rare earth ores. ‘The concentration of europium in natural ores is one of the lowest among all rare earth elements, ranging between 0.05 and 0.10% in weight,’ she explains. This contrasts with the concentration in commercial lamps, at an average of 3.4% by weight. ‘From a purely economical perspective, it is much more interesting to recover europium from spent florescent lamps, currently wasted in landfills,’ she adds.

‘Targeted separation of such critical elements, especially from electronic waste, will provide a more diverse and more environmentally benign supply chain,’ says Schelter. ‘It will be interesting to see where this leads. Such methods can inspire new thinking for improved recycling of rare earths.’

The researchers are now working to adapt the technology to separate other lanthanides, such as neodymium, and to further improve the sustainability of the separation process, replacing triflic acid with greener chemicals, such as methanesulfonic acid. ‘We’re currently looking at both the techno-economical assessment [and] the full life cycle assessment, and evaluating the process on a larger scale, thanks to a recycling plant that provided us with a large quantity of lamps,’ says Perrin.

After filing for a European patent, Perrin and Mougel co-founded the startup REEcover with an expert in financing. ‘Together, we hope to turn this groundbreaking technology into a competitive product,’ says Perrin.