Manufacturing of Dysprosium-Iron Alloys by Electrolysis in Fluoride-Based Electrolytes: Oxide Solubility Determinations
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AS in the case of aluminum metal production, the light rare earth (RE) metals (i.e., Nd, Pr, La, Ce), as well as some alloys with Fe, are currently obtained by electrolysis from molten fluoride-based electrolytes using oxide raw materials. For the time being, China is the sole manufacturer of RE metals and RE alloys by electrolysis. The technology relies on an electrolytic process using a vertical electrode set-up cell, using graphite anodes and inert (W) or consumable (Fe) cathode materials, where the RE metal or RE alloy is deposited in a liquid form.[1] The electrolyte typically consists on a REF3-LiF mixture and RE oxide (REO) is used as raw material, while the operating temperature can vary from 1223 K to 1373 K (950 °C to 1100 °C).[2] The hurdles to an effective and environmental electrolytic RE production have been identified to be, mainly, anode effects, evolution of fluorine-containing compounds and side cathode reactions. These challenges are mostly controlled by the amount of REO species
ANA MARIA MARTINEZ, ANNE STØRE, and KAREN SENDE OSEN are with the SINTEF Materials and Chemistry, Sem Saelands vei 12, 7465 Trondheim, Norway. Contact e-mail: anamaria. [email protected] Manuscript submitted August 18, 2017.
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dissolved in the fluoride-based electrolyte and their dissolution rate. Accordingly, it is necessary that the amount of dissolved oxides matches the supply of electrolytic current. Therefore, it is very important to have a good and exact knowledge of the oxide saturation levels in the electrolyte at the working temperature. Some investigations of REO solubility in molten fluorides have been previously reported. However, the obtained results are quite scattered, and studies devoted to dysprosium are, to the authors’ knowledge, scarce, if inexistent. Stefanidaki et al.[3] reported that the solubility of Nd2O3 decreased when increasing the LiF content in the molten electrolyte, at the investigated temperatures ranging from 1023 K to 1173 K (750 °C to 900 °C). Solubility values in the order of 0.91 wt pct were found for the eutectic NdF3 (20 mol pct)-LiF (80 mol pct) mixture at 1133 K (860 °C). Based on Raman spectroscopic studies, the authors claimed that complex ions of and Nd2OF(x1) may be formed the form NdOF(x1) x x+3 in the molten fluoride electrolyte, most likely the mononuclear compounds NdOF43 and NdOF54 and the binuclear complexes Nd2OF106 and Nd2OF84. The suggested dissolution reaction is given by the following equations for the monomer and for the dimer, respectively: Nd2 O3 þ NdF3 þ 3ðx 1ÞLiF 3NdOFðx1Þ x þ 3ðx 1Þ Liþ
½1
ðx1Þ
Nd2 O3 þ 4NdF3 þ 3ðx 1ÞLiF 3Nd2 OFxþ3 þ 3ðx 1Þ Liþ
½2
Keller and Larimer[4] reported a much higher solubility than Stefanidaki et al., i.e., 2 wt pct at 1073 K (800 °C). The same authors determined the saturation concentrations in the CaF2 (70 wt pct)-LiF (30 wt pct) mixtures at ca. 1317 K (1044 °C). The values obtained ranged from 0.33 to 0.60 wt pct. Morrice et al.[5,6] reported a Nd2O3 solubility valu
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