Production of High Purity Rare Earth Metals by the Continuous Processing Composed of Calciothermic Reduction and Vacuum Melting.

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1. Preliminary ExperimentsDTA revealed calciothermic reduction of NdF_3 starts at 1093 K.The phase diagram of the NdF_3-CaF_2 system was estimated by thermal analysis. The analytical procedures of oxygen content in neodymium using the device in our laboratory were also studied as a function of flux and temperature.2. Optimization of Calciothermic Reduction of NdF_3Calciothermic reduction of NdF_3 were studied for various temperatures, reaction periods, and quantities of calcium addition. Optimum conditions for obtaining a lump of neodymium were considered not only based on purity and reduction ratio but also separation between slag and metal, and that between slag and crucible.3. Optimization of Vacuum MeltingUsing commercially produced neodymium and calcium, model experiments of vacuum melting were conducted for various temperatures, reaction periods, initial impurity contents, and stirring condition. Optimum conditions were discussed not only for the removal of volatile impurities but also precipitation of crucible material dissolved during the calciothermic reduction and the vacuum melting.4. Optimization of the Continuous ProcessingOn the basis of the previous experimental results, optimum conditions for the continuous processing were determined as follows : calciothermic reduction is carried out for 0.9 ks at 1750 K by the addition of calcium 1.2 times as much as the stoichiometric moles : and vacuum melting is carried out at first for 1.2 ks at 1500 K with stirring, then, by decreasing temperature down to just above the melting point of neodymium, for 3.6ks.