Chemical and Electrochemical Extraction of Ytterbium from Molten Chlorides in Pyrochemical Processes

Abstract

Abstract The electrochemical behaviour of ytterbium ions was investigated in the eutectic LiCl‐KCl at 723 K at the surface of tungsten, glassy carbon, aluminium and aluminium coated tungsten electrodes. On inert electrodes (W and GC) the electroreduction of Yb(III) to Yb(II) takes place in a quasi‐reversible electrochemical mode within the electroactive window. The electrochemical system Yb(II)/Yb(0) is not observed because it is similar or more negative in potential than the Li(I)/Li(0) couple. As a result Yb metal in contact with the melt could react to give Li(0) according to the reaction: Yb(0)+2Li(I)↔Yb(II)+2Li(0) On the contrary, on reactive Al electrodes the electrochemical system Yb(II)/Yb(0) was observed within the electroactive range. The potential shift of the redox couple is caused by the decrease of Yb activity in the metal phase due to the formation of Yb‐Al alloys at the interface. The formation mechanism of the alloys was determined in a melt containing both Yb(III) and Al(III) ions, using W, Al and Al coated tungsten electrodes. X‐ray diffraction analysis and scanning electron microscopy (SEM) after potentiostatic electrolysis runs allowed the identification of Al 3 Yb and Al 2 Yb. Potentiometric titrations of Yb(III) solutions with oxide donors, using a ytria stabilized circonia membrane electrode “YSZME” as a pO 2− indicator electrode, have shown the formation of ytterbium oxychloride and ytterbium oxide and their solubility products have been determined at 723 K (p k s (Yb 2 O 3 )=7.6±0.3 and p k s (YbOCl)=18.5±0.7.