A Primer on Hydrometallurgical Rare Earth Separations BEN KRONHOLM, 1,3 CORBY G. ANDERSON, 2,4 and PATRICK R. TAYLOR 2 1.—Molycorp Inc., Greenwood Village, CO, USA. 2.—Kroll Institute for Extractive Metallurgy, Colorado School of Mines, Golden, CO, USA. 3.—e-mail: Benjamin.Kronholm@molycorp.com. 4.—e-mail:cganders@mines.edu This article delineates the history and details of hydrometallurgical rare earth separations and technologies. It covers the history, development, application, and recently published research into this key aspect of rare earths separation and recovery. INTRODUCTION The lanthanides or ‘‘rare earths’’ have found increasing applications since the 1960s when developments in process technology allowed their purification on a large scale. 1 The rare earths (RE or TREO) have useful materials properties primarily originating from their f-orbital chemistry and oxide stability. 1 These elements find uses in catalysts, phosphors, magnets, ceramics, specialty alloys, and electronics. Increased demand, coupled with supply chain uncertainties, has led to increased interest in RE production. This article will review current and past advancements in rare earth processing, spe- cifically, purification of individual elements from mixed rare earth solutions. LANTHANIDE SEPARATIONS The term ‘‘rare earth’’ is a misnomer as it is relatively abundant; the ‘‘rare’’ originates from difficulty in isolating individual lanthanide ele- ments. Chemically, rare earth elements are very similar, and most exist solely in a 3+ oxidation state with the exceptions of Ce(IV), Pr(IV), Tb(IV), Sm(II), Eu(II), and Yb(II). 1 Of the elements with multiple oxidations states, Ce(IV) and Eu/Sm/ Yb(II) have low enough redox potential to exist in aqueous solution without decomposing water. Separation based on selective oxidation/reduction followed by precipitation has been implemented in the production of cerium and europium. RE exhibit little difference from one another in covalent bonding as they are f-block elements; f-orbitals fall within the 4d,5s, and 5p orbitals and are therefore largely unreactive. 2 Aside from selective oxidation/ reduction, the primary difference between the lanthanides that can be utilized for their separation is the lanthanide contraction. As one moves from left to right on the Periodic Table, the ionic radius of lanthanides decreases. This is a result of poor shielding of the nucleus by f-orbital electrons that translates into an increasingly effective nuclear charge as this orbital is filled. The lanthanide contraction allows for separation by fractional crystallization ion-exchange and solvent extraction. These four modes of separation, selective oxidation/ reduction, fractional crystallization, ion exchange, and solvent extraction, will be discussed in the following sections. OXIDATION/REDUCTION Although RE elements are chemically similar and therefore difficult to separate, several elements can be isolated based on their redox properties. Cerium and europium have both been isolated on an industrial scale using oxidation to +4 and reduction to +2, respectively. Cerium(III) chloride, nitrate, and sulfate can be oxidized to the sparingly soluble cerium(IV) oxide or hydrate. 3–5 This precipitate can then be isolated yielding a high-purity cerium solid and cerium-free liquor. This oxidation has been reported using permanganate, chlorate, persulfate, and hypochlorite. The oxidant used is dependent on the solubility and reactivity of the RE counter-ion. Cerium purities of greater than 99% have been produced using chemical oxidation. An electro- chemical oxidation process has also been proposed wherein a rotating cathode would be utilized to oxidize trivalent cerium to ceric hydroxide at a pH of 5.5–6.5. 6 Cerium purity of >95% was reported using this method. Europium can be reduced to the 2+ state by chemical reduction using zinc amalgam, 7 zinc powder, 8 or electrochemical reduction 9 and precipitation as a sulfate. Much like cerium, the JOM, Vol. 65, No. 10, 2013 DOI: 10.1007/s11837-013-0718-9 Ó 2013 TMS (Published online August 21, 2013) 1321