Solubilities and raman spectra of NdOCl in some chloride melts of interest for the electrowinning of magnesium from its
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I. INTRODUCTION
A new process for electrochemical production of magnesium from MgO dissolved in NdCl3- and MgCl2-containing melts has been proposed.[1] Some preliminary electrolysis experiments on this process have been reported.[2] A better knowledge of the chemistry of this solvent is, therefore, important in elucidating and maybe improving the previously mentioned production processes. The NdCl3-Alk.Cl (Alk. 5 Na, K, Rb, and Cs) phase diagrams are well established.[3,4] The mixtures have been studied by high-temperature calorimetry[5] and Raman spectroscopy[6] showing mainly octahedral NdCl32 6 species in Alk.Cl-rich melts. It is well known that oxide species of neodymium will dissolve readily in liquid NdCl3. A phase diagram is reported showing a eutectic point of 727 8C at 93 mol pct NdCl3 to 7 mol pct Nd2O3 corresponding to 80.4 mol pct NdCl3 to 19.6 mol pct NdOCl, which forms when Nd2O3 dissolves in liquid H. MEDIAAS, Researcher, is with Sintef Applied Chemistry, N-7465 Trondheim, Norway. O. TKATCHEVA, Dr., is with the Institute of High Temperature Electrochemistry, Ekaterinburg, Russia. V. DRACOPOULOS, Dr., and G.N. PAPATHEODOROU, Professor, are with the Department of Chemical Engineering, University of Patras and Institute of Chemical Engineering and High Temperature Chemical Processes, FORTH, 26500 Patras, Greece. G.J. KIPOUROS, Professor, is with the Department of Mining and Metallurgical Engineering, Dalhousie University (DalTech), Halifax, NS, Canada B3J 2Z4. T. ØSTVOLD, Professor, is with the Institute of Chemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS B
NdCl3.[7] In the original article, as well as in the American Ceramic Society compilation of phase diagrams, the NdCl3NdOCl phase diagram is reported to have a eutectic composition of 16.5 mol pct NdOCl. This is obviously due to a calculation error, as it is clear that Nd203 and NdCl3 were used in the experiments, and that the eutectic melt contained 7 mol pct Nd2O3. An investigation of the NdCl3-NdOClMgCl2 phase diagram[8a] agreed with the previously published data for the NdCl3-NdOCl system. On the other hand, a recent investigation of the NdCl3-NdOCl phase diagram has shown a lower NdOCl solubility in NdCl3.[9] A phase diagram is also published for the quasi binary MgCl2-NdOCl system[8b] showing an extensive solubility of NdOCl in MgCl2. The latter phase diagram may not be correct because it does not take into account the equilibrium MgCl2 (l) 1 NdOCl(l, s) 5 MgO (l, s) 1 NdCl3 (l)
[1]
and the possible precipitation of MgO and/or NdOCl. Thermodynamic data[10] show that DG81 ' 4.9 kJ at 1100 K if MgO (s) and NdOCl (s) are present. The solubility of MgO i
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