Electrolytic reduction of Eu(III) to Eu(II) in acidic chloride solutions with titanium cathode
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INTRODUCTION
IN recent years, the importance of rare earth metals has increased in the field of functioning materials, such as magnetic materials, superconducting materials, and so on, which are recognized as the supporting elements of modem technology. However, the physical and chemical properties of these rare earths are similar, and thus, their mutual separation is generally very difficult. Although solvent extraction has been employed in the separation and concentration of rare earth elements, the separation of these elements is still difficult. Rare earths are present inherently as trivalent ions in an aqueous solution, some of which can be reduced to divalent ions. Table I shows the standard redox potentials M3+/M2ยง of several rare earth metal ions. tl,2j The redox potentials shown in brackets have not been confirmed. As is obvious in this table, the reduction potential of Eu(III) to Eu(II) is larger than that of the other redox systems. Thus, Eu(III) can be selectively reduced more easily than the other trivalent rare earth ions. In a typical industrial separation process for europium, the rare earth chloride solution is treated by solvent extraction, and then the separated solution containing middle rare earth group ions (Sm(III), Eu(III), and Gd(III)) is subjected to Eu(III) reduction by zinc powder or zinc amalgam. The Eu(II) thus formed is precipitated as sulfate by means of a TETSUJI HIRATO, Associate Professor, and YASUHIRO AWAKURA, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto, 606-01 Japan. HIROSHI KAJIYAMA, formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University, is Research Engineer, Fukuyama Research Department, Steel Research Center, NKK Corp., Hiroshima, 721 Japan. HIROSHI MAJIMA, formerly Professor, Department of Metallurgy, Kyoto University, is Professor Emeritus, Kyoto University, and Standing Advisor, Titan Kogyo K.K., Ube-city, 775 Japan. Manuscript submitted April 26, 1994. METALLURGICALAND MATERIALSTRANSACTIONSB
fractional crystallization method, t3~However, the zinc introduced during reduction of Eu(III) must be removed from the resultant solution. Recently, Lu et al. reported the electrolytic reduction of Eu(III) using a graphite electrode.t4J Furthermore, the electrolytic reduction of U(VI) to U(IV) has been studied in our laboratory using a titanium cathode with a high hydrogen overvoltage55-8~ Since reduction of U(VI) with a high efficiency was achieved using a titanium cathode, similar efficiencies might also be expected for the reduction of Eu(IlI). Previously, we studied the cathodic polarization characteristics of Eu(III) on titanium cathode and found that the electrolytic reduction of Eu(III) could proceed in aqueous 0.1 kmol m -3 EuC13-0.1 kmol m 3 HC1 solution, tg~ In the present study, the feasibility of rare earth separation by electrolytic reduction of Eu(III) with a titanium cathode is examined by cathodic polarization characteristics of Eu(III) on a titanium electrode and batch-t
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