Manufacturing of Dysprosium-Iron Alloys by Electrolysis in Fluoride-Based Electrolytes. Electrolysis in a Laboratory-Sca
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THE biggest challenge to obtaining an environmentally friendly rare earth (RE) electrolytic production is avoiding the anode effects as well as the evolution of fluorine-containing compounds. These hurdles are mostly controlled by the amount of RE oxide (REO) species dissolved in the fluoride-based electrolyte and their dissolution rate. Accordingly, the amount of dissolved oxides needs to match the electrolytic current supply and at the same time avoid the formation of sludge, i.e., non-dissolved REO, which could accumulate at the bottom of the electrolysis reactor. Apart from good and exact knowledge of the oxide saturation levels in the electrolyte at the working temperature, it is
ANA MARIA MARTINEZ, KAREN SENDE OSEN, ANNE STØRE, HENRIK GUDBRANDSEN, OLE SIGMUND KJOS, ASBJØRN SOLHEIM, and ZHAOHUI WANG are with SINTEF Industry, Sem Saelands vei 12, 7465 Trondheim, Norway. Contact e-mail: [email protected] ALEXANDRE OURY and PATRICK NAMY are with SIMTEC, 8 rue Duploye´, 38100 Grenoble, France. Manuscript submitted September 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
necessary to know the optimal electrolysis parameters, including the limiting current density values for the discharge of the oxide ions at the anode, at a given concentration of dissolved oxygen-containing electroactive species, which gives the threshold for releasing detrimental environmental hazardous perfluorocarbon (PFC) gases, mainly CF4 and C2F6. Dy is a heavy RE element with a high melting point, i.e., 1680 K (1407 °C), which makes it challenging to obtain in the pure form by electrolysis from molten fluoride-based melts. However, it is possible to obtain a liquid Dy-alloy electrolytic product when using a suitable transition metal forming low-melting-point alloys. Using Fe as a consumable cathode in the electrolytic process allows obtaining a liquid Dy-Fe cathode product, free of salt inclusions. Moreover, the Dy-Fe electrolytic product can be used as a master alloy in the manufacturing of Nd-based permanent magnets. It is believed that Dy-Fe alloys are currently being produced industrially by electrolysis in a DyF3-LiF electrolyte in equimolar composition at temperatures of ca. 1323 K (1050 C) using a vertical electrode set-up cell with graphite anodes and a Fe consumable cathode.[1] The electrochemical stability of Dy ions in fluoride-based melts has been studied by Saı¨ la et al.[2] Using
an Mo inert electrode, the authors found that only Dy(III) ions are stable in the CaF2-LiF eutectic mixture at temperatures ranging from 1113 K to 1203 K (840 °C to 930 °C). The Dy(III) ions are reduced to Dy metal in a one-step diffusion-controlled process exchanging three electrons. The authors also studied the electrochemical reduction of Dy(III) ions using reactive electrodes, i.e., Ni and Cu, showing the evidence of formation of intermetallic compounds. Castrillejo et al. studied the electrochemical behavior of Dy in an eutectic LiCl-KCl mixture on both inert (W) and reactive (Al) electrodes.[3] The authors demonstrated that the ele
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