Direct Electrolytic Reduction of Solid Ta 2 O 5 to Ta with SOM Process
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TANTALUM is an attractive kind of metal due to its excellent chemical and physical properties including good corrosion resistance, great ductility, high melting point, and so on. As a result, more and more industrial applications have been developed using tantalum-related materials, such as oil and gas pipelines, jet engine turbine blades,[1,2] and electrolytic capacitors that are made of fine tantalum powders.[3] Recently, tantalum has been commercially produced by the Hunter process, in which K2TaF7 is reduced with sodium metal in a molten salt solvent.[4,5] The chemical reaction in the process is expressed as: K2 TaF7 þ 5Na ! Ta þ 5NaF þ 2KF:
½1
This commercial process is carried out in a metallic container at a temperature of around 1073 K (800 C). Another alternative commercial production process for tantalum powder is metallothermic reduction of Ta2O5. However, both processes are expensive, labor-intensive, and high energy-consuming, as well as its large amount
CHAOYI CHEN and XIAQIONG YANG, Doctors, and JUNQI LI, and XIONGGANG LU, Professors, are with the School of Materials and Metallurgy, Guizhou University, Guiyang 550025, Guizhou, P.R. China, and also with the Key Laboratory of Metallurgical Engineering and Process Energy Saving, Guizhou University. SHUFENG YANG, Associate Professor, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted April 24, 2015. Article published online March 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
of environmentally harmful by-products. Furthermore, to obtain pure tantalum with low oxygen content, additional refining processes must be applied.[6,7] Therefore, the application of Ta has been limited due to its high cost of extraction and secondary refining process. On the other hand, many researchers are making great efforts to develop a novel compact process in order to produce pure tantalum powder more efficiently. The recent progress in electrochemical reduction of solid metal oxides in molten chloride salts has demonstrated great promise toward the advanced technology of extracting metals and alloys. This novel electrolytic method, called the Fray–Farthing–Chen (FFC) Cambridge process,[8] has attracted a great deal of attention in the metallurgical industry. This process has been used to prepare titanium, yttrium, niobium, chromium, and silica with the constant voltage electrolysis (2.8 to 3.1 V) in molten CaCl2.[9–14] However, this methodology also possesses some inherent disadvantages. For instance, the applied voltage must be lower than the decomposition voltage of molten salt. The lower voltage may cause the slow rate of reaction because an inevitable side reaction can decrease electric current efficiency and then reduce the efficiency of the whole process. The solid-oxide–oxygen-ion conducting membrane (SOM) process is a promising, environment-friendly, and energy-efficient alternative process for the extraction of high-pu
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