High-Purity Rare Earth Oxides Produced via Precipitation Stripping
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INTRODUCTION
THE rare earth elements (scandium, yttrium, and the lanthanide series) have many important technological uses: as phosphors,[1] electroluminescent materials,[2] biological sensors,[3] scintillation crystals,[4] and many more. The vast majority of these applications use the lanthanide ions as light-emitting or luminescent centers. The luminescence of the lanthanide ions is more efficient when the materials have very high purity. Because the rare earth elements all are found in the same minerals and all have very similar chemical and physical properties, the separation and purification of the elements is not trivial. The most common method for separating and purifying the rare earths is solvent extraction. This process involves mixing an aqueous solution of the rare earths with an immiscible organic solution containing an extractant and an inert diluent. All of the rare earth ions are extracted into the organic phase; however, the extent of extraction tends to increase with decreasing ionic radii.[5] Through a series of extractions, the rare earths can be separated and purified based on their ionic radii. The most common extractants for the rare earths are organophosphorus compounds such as di-2-ethylhexyl phosphoric acid (DEHPA) and mono-2-ethylhexyl-(2ethylhexyl) phosphonic acid (MEHEHP, Figure 1). Kerosene is the most common organic solvent used as the inert diluent. The extracted rare earth ions exist in the organic phase as phosphate or phosphonate complexes. The ions are usually stripped from the organic phase by mixing the kerosene solution with high concentration aqueous mineral acids (typically, >6 M hydrochloric or nitric PETER M. SMITH, Assistant Professor of Chemistry, is with the Department of Chemistry, Westminster College, New Wilmington, PA 16105. Contact e-mail: [email protected] Manuscript submitted January 17, 2007. Article published online September 6, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
acid). This removes the rare earth ion from the organic phase, draws it into the aqueous phase, and, in turn, regenerates the acidic extractant. The highly acidic aqueous solution of the purified rare earth is then neutralized, generally with aqueous ammonia, and the rare earths are precipitated as the oxalate by addition of oxalic acid. Finally, the oxalates are calcined at high temperature converting them into the rare earth oxides. Precipitation stripping is a process that combines the acid stripping and precipitation steps by mixing the rareearth-loaded organic phase with an aqueous solution of an acidic precipitating agent, usually oxalic acid. The rare earth is stripped from the organic phase directly as the solid oxalate salt: 2 M(HR2 Þ3 þ 3 H2 C2 O4 ! M2 ðC2 O4 Þ3 # þ6 H2 R2 ½1 where the overbar indicates that the species is in the organic phase. Precipitation stripping has been used to strip rare earth ions from organic solutions.[6] This technique has even been demonstrated to be feasible in industrial counter-current solvent extraction processes.[7] Precipitation stripping
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