新材料

[2K0307-09-02] High-Purity Tantalum Powder Production by Electrochemical Reduction of TaS₂

○Eltefat Ahmadi^1,2, Ryosuke O. Suzuki¹, Tatsuya Kikuchi¹ (1. Hokkaido University, 2. Japan Society for the Promotion of Science (JSPS))

司会：安田幸司（京都大学）

キーワード：Tantalum powder, Sulfidation, Capacitor , Electrochemical reduction, Molten salt

The well-matured industrial method for producing Ta powder of capacitor grade is the sodium reduction of potassium heptafluoro tantalate (K₂TaF₇) in molten mixture of NaCl, KCl and NaF. The use of K₂TaF₇ is hazardous to the environment and drives up the process and plant costs making the process less attractive. The present work discloses a new alternative method for synthesis of Ta sulfide and Ca reduction of TaS₂ in molten salt. TaS₂ was prepared via a high-efficiency sulfidation of either Ta₂O₅, TaC or TaN using S₂ gas. Then, the sulfides were here reduced to Ta powders by in-situ generated Ca from the decomposition of CaS in the electrolyte. Ta₂O₅ was mixed with graphite and carbo-sulfurized at 1573 K using S₂ gas. Alternatively, TaC or TaN powder was sulfurized by S₂ gas at 1473 K. The sulfides obtained from TiN and TiC consisted of about 0.0043 mass% N and less than 0.01 mass% C, respectively. Interestingly, TaS₂ samples synthesized from Ta₂O₅, TaC or TaN were not contaminated with carbon. The clean sulfides served as starting materials for production of extra pure Ta powder. The electro-calciothermic reduction of TaS₂ samples was investigated in molten CaCl₂ – x CaS (x = 0.1 and 0.5 mol%) at various supplied charges (Q/Q₀) at 1173 K. The supplied charge and CaS molar ratio have significant effects on efficiency of the process, particularly on the oxygen removal. The sulfur removal was achieved rapidly and high-purity plate-like Ta particles consisted of coral structures with 0.01 mass% S could be successfully produced at Q ≥ 2Q₀. The oxygen, carbon and nitrogen contents of Ta powder, obtained at the optimum conditions, were approximately 0.08 mass% O, 0.07 mass% C and 0.01 mass% N. The electrometallurgical reduction of TaS₂ may be then applied for manufacturing high-voltage Ta capacitor powders.