Europium (Eu) and yttrium (Y) are two rare earth metals that are commonly used in sustainable technology and high-tech applications. As these metals are difficult to mine, there is a great interest in recycling them. They can be recovered from red lamp phosphor, a powder that is used in fluorescent lamps such as neon tubes.

“Separating the two rare earth elements is a complicated process. The traditional method dissolves europium and yttrium in aqueous acid,” explained Prof Tom Van Gerven of the University of Leuven’s Department of Chemical Engineering, senior author on the study published in the journal Green Chemistry.

“An extractant and a solvent are then added to the aqueous liquid, leading to two separate layers known as phases: an aqueous layer containing the rare earth metals and a solvent layer with the extractant. When the two layers come into contact, one of the two rare earth metals is extracted to the solvent, while the other rare earth metal remains in the aqueous layer.”

“But this process leaves much to be desired in terms of efficiency and purity: it needs to be repeated dozens of times to recover a high percentage of a particular rare earth metal, and there will still be traces of yttrium in the europium-containing liquid and vice versa.”

Prof Van Gerven and co-authors have now managed to recover europium from the liquid mixture with UV light instead of a solvent.

“The UV light influences the electrically charged particles known as ions. Both europium and yttrium have three positive charges per ion. When we shine UV light upon the solution of europium and yttrium, we add energy to the system,” said Bart Van den Bogaert, a PhD student at the University of Leuven.

“As a result, one positive charge per europium ion is neutralized. When we add sulfate, only the europium reacts with it. The result is a precipitate that can easily be filtered, while the yttrium remains in the solution.”

The advantages of UV light are that it does not leave behind any harmful chemicals in the liquid and that the separation efficiency and purity in synthetic mixtures are very high: more than 95 percent of the europium is recovered from the solution.

The precipitate itself is 98.5 percent pure, so it contains hardly any traces ofyttrium. A similar purity was obtained with industrial mixtures, but the efficiency of the separation still needs to be improved.