Production of Niobium Powder by Direct Electrochemical Reduction of Solid Nb_2O_5 in a Eutectic CaCl_2-NaCl Melt

摘要

The method of electro-deoxidation was used to reduce solid Nb_2O_5 to niobium metal in a CaCl_2-NaCl eutectic melt. The direct electrochemical reduction of Nb_2O_5 was achieved by electrolysis in the eutectic melt at 1123 and 1173 K, respectively, at a controlled potential of 3.1 V, below the decomposition potential of the salts. Analysis of the anodic reaction gases carried by a flow of dry, high-purity argon confirmed that the preferred cathodic reaction is the oxygen ionization of the cathode to form oxygen ions that are successively dissolved in the chloride melt and then discharged at the graphite-rod anode. Chlorine ions are unlikely to be discharged at the anode. The niobium metal powders prepared contained as low as 2311 mass ppm oxygen, clearly demonstrating that the electro-deoxidation method is applicable to the reduction of solid Nb_2O_5 in chloride melts. The kinetics and mechanisms of reduction of the porous pellets of Nb_2O_5 are discussed in detail based on the measured current-time behavior, together with the microstructural analysis of the sintered Nb_2O_5 pellets and the phases present in the partially reduced samples obtained at various times of reduction.