Preparation of High Purity Vanadium Metal for Hydrogen Storage Using the Direct Reduction and Simultaneous Molten Salt Electrolysis

提出了用钒氧化物熔体（V_2O_5）直接还原法制备金属钒的新工艺。将(A)和(B)两个步骤结合起来：(A)固体氧化物被溶解在熔盐中的钙直接还原，(B)副产物CaO被电化学分解为金属钙。这种结合下文称为OS过程，除固体低氧化物外，还扩展到熔融氧化物V_2O_5。将OS工艺应用于熔体CaCl_2中，研究了电极排列的优化，以便与存在于阴极附近的熔体V_2O_5和Ca有效反应。取Mo或V作为阴极材料。采用坩埚式电极时，电流效率降低。阳极碳与阴极的距离更近，例如在几毫米内，我们获得了仅含2000 ppm氧的优异质量的金属钒。研究了还原过程中的相变，并分析了不同电极布置下的电化学行为和电流效率。还原速率与所提供的电流很好地对应。在反应初期可以获得较高的电流效率。而在还原后期，得到的钒金属以脱氧为主，电流和电流效率均下降。在整个实验时间内，氧含量的下降速率可以用一个独特的公式来表示。这表明，OS过程的速率决定步骤是Ca或CaO向熔融的CaCl_2体中的扩散。

A New process was developed that the metallic vanadium (V) was prepared by the direct reduction from the molten vanadium oxide melt (V_2O_5). Two steps (A) and (B) were combined: (A) The solid oxides was directly reduced by calcium dissolved in the molten salt, and (B) the by-product CaO was electrochemically decomposed into the metallic calcium. This combination is hereafter called as OS process, which is extended to the molten oxide V_2O_5, in addition to the solid lower oxides.By applying OS process in the molten CaCl_2, the optimization of electrodes arrangement was studied in order to react efficiently with the molten oxide V_2O_5 and Ca existing near the cathode. Mo or V was taken for the cathodic material. The current efficiency decreased when the crucible-type electrode was used. The anodic carbon was set more closely to the cathode, such as in a few mm, and we obtained the excellent quality of vanadium metal containing only 2000 ppm oxygen. The phase transformation during the reduction was studied in addition to the analysis of electrochemical behavior and current efficiency at the various electrode arrangements. The reduction rate corresponded well to the supplied current. A high current efficiency could be achieved at the initial stage of reaction. At the later stage of reduction, however, the deoxidation from the obtained vanadium metal was dominant, and both the current and the current efficiency decreased. The decreasing rate of oxygen content could be expressed in an unique formula against the supplied electric charge over the whole experimental time. This implies that the rate-determining step in OS process is the diffusion of Ca or CaO into the molten CaCl_2 bulk.