Recovery of Erbium Metal Via Electrorefining in Room Temperature Ionic Liquids

Erbium is a rare earth element with a variety of technologically important applications.  Here we describe our efforts to develop room temperature ionic liquids specifically designed to facilitate electroplating of erbium metal from erbium chloride or erbium triflate, in an effort to recycle erbium waste converted into these salts.  A variety of pyrrolidinium, ammonium, phosphonium, and chloroerbate ionic liquids are investigated.  While all tested ionic liquids possess the required voltage window for theoretical erbium electroreduction, choice of chemistry is shown to drastically affect both incorporation of organics into the electrodeposited film, and the solubility of erbium salt in the ionic liquid, limiting deposition rates.  A plan-view SEM micrograph of an amorphous erbium film electroplated onto platinum in butyltrimethylammonium bis(trifluoromethanesulfonyl)imide (BTMA NTf₂) is shown in Figure 1.  Analysis by energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy reveals incorporation of fluorocarbons into the film.  These undesirable processes are not readily evident from the simple cyclic voltammograms (Figure 2).  Detailed analysis of these CVs suggests reversible reduction of an intermediate followed by irreversible reduction to an erbium film contaminated with fluorocarbons.  

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.