Selective electro-oxidative separation of Sm, Yb, and Eu in low-temperature molten salt AlCl3-NaCl

Abstract

The variable valence rare earth elements are difficult to separate due to their similar properties and intermediate valence states in molten salts. In order to realize their efficient and selectivity separation, the properties and separation methods of Sm, Yb, and Eu have been investigated in a low-temperature AlCl3-NaCl molten salt. It has been found that the oxidation reactions of different Ln(II) to Ln(III) have different potentials. The Ln(II)/Ln(III) reaction is also irreversible when the AlCl3 content is less than 50 mol%, Ln(III) exists in the form of LnCl3 precipitation. Therefore, selectively modulating the valence of Sm, Yb, and Eu by electrochemical methods can achieve their separation. The LnCl3 produced from the Ln(II) electro-oxidation precipitates and separates from the other unoxidized soluble Ln(II) in AlCl3-NaCl melt. The efficient separation of Ln(II) in AlCl3-NaCl can be achieved by pulse potentiostatic electrolysis. Under these conditions, the separation factors of Sm-Eu, Sm-Yb, and Yb-Eu can reach 103. The maximum removal of Ln(II) exceeded 99 % with a maximum current efficiency of more than 90 %. Most importantly, the separation of variable valence rare earth elements has been realized in a low-temperature AlCl3-NaCl melt.