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[3K62101-08-02] High-Purity Tantalum Powder Production by Electrochemical Reduction of TaS2
Keywords:Tantalum powder, sulfurization, 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 NaCl, KCl or NaF. In this study, a new processing method for synthesis of Ta sulfide and electrochemical reduction of TaS2 is disclosed. TaS2 has been prepared by the high-efficiency carbo-sulfurization of Ta2O5 or sulfurization of TaC and TaN by S2 gas in a dual-zone furnace. Then, the sulfides were electrochemically reduced by in-situ generated Ca from the decomposition of CaS in the electrolyte.
For preparation of the sulfide, Ta2O5 was mixed with graphite and carbo-sulfurized at 1573 K for 18 ks using S2 gas. Alternatively, TaC and TaN powder were sulfurized by S2 gas at 1473 K for 10.8 ks. The samples sulfurized from TiN and TiC consisted of about 0.0043 mass% N and less than 0.01 mass% C, respectively. TaS2 synthesized from Ta2O5, TaC and TaN were not contaminated with undesired impurities such as carbon. The high-quality sulfide was loaded into a cathode made of Ti basket and reduced by Ca in molten CaCl2 - 0.5 mol% CaS at 1173 K by varying the supplied charge (Q/Q0) ranging from 100% to 600%. The results indicated that the quality of Ta powder can be controlled by applying the supplied charge and CaS molar ratio in the electrolysis. The sulfur removal was achieved because high-purity plate-like Ta particles consisted of coral structures with 0.01 mass% S could be successfully produced at Q ≧ 2Q0. The oxygen and nitrogen contents of the product also reduced to 0.17 mass% O and 0.01 mass% N. This product is useful for manufacturing high-voltage Ta capacitors, and direct electrometallurgical reduction of TaS2 can be applied for production of high-purity Ta powder and its alloys.
For preparation of the sulfide, Ta2O5 was mixed with graphite and carbo-sulfurized at 1573 K for 18 ks using S2 gas. Alternatively, TaC and TaN powder were sulfurized by S2 gas at 1473 K for 10.8 ks. The samples sulfurized from TiN and TiC consisted of about 0.0043 mass% N and less than 0.01 mass% C, respectively. TaS2 synthesized from Ta2O5, TaC and TaN were not contaminated with undesired impurities such as carbon. The high-quality sulfide was loaded into a cathode made of Ti basket and reduced by Ca in molten CaCl2 - 0.5 mol% CaS at 1173 K by varying the supplied charge (Q/Q0) ranging from 100% to 600%. The results indicated that the quality of Ta powder can be controlled by applying the supplied charge and CaS molar ratio in the electrolysis. The sulfur removal was achieved because high-purity plate-like Ta particles consisted of coral structures with 0.01 mass% S could be successfully produced at Q ≧ 2Q0. The oxygen and nitrogen contents of the product also reduced to 0.17 mass% O and 0.01 mass% N. This product is useful for manufacturing high-voltage Ta capacitors, and direct electrometallurgical reduction of TaS2 can be applied for production of high-purity Ta powder and its alloys.
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