Electrochemical Behavior of Titanium(II) Ion in a Purified Calcium Chloride Melt
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DUCTION
TITANIUM is ranked as the ninth most abundant element and fourth among common metals in the earth’s crust. Primary titanium metal, sponge titanium, is produced through a process invented by Dr. Kroll in the 1940s.[1] The current industrial practice includes multiple steps. The procedure is complex and is a batch process, factors that drive up the production cost substantially. Much research has been focused on developing a technique to produce low-cost and highpurity titanium.[2–8] Electrolysis from TiO2[9–13] and related methods[14–17] have been investigated extensively. In the oxide electrolysis process, TiO2 serves as the cathode and is reduced in a CaCl2 electrolyte. Unfortunately, very long electrolyzing times are required for significant conversion, and current efficiencies are low. Furthermore, production is still restricted to a batch process. Japanese research groups[14–17] proposed a similar method involving a combination of electrolysis and calciothermic reduction. Calcium, electrodeposited on the cathode, is used to reduce the TiO2. However, problems remain associated with the high oxygen content of titania and with difficulties in the extraction of the product titanium from its mixture with TiO2, CaO, Ca, and solvent CaCl2. Zhu etc. proposed USTB process.[18] High-purity titanium was prepared from titanium dioxide through a combination of low-temperature carbothermic reduction and electrolysis. The problem for the process is unstable dissolution of anode. Therefore, we reported a new process in which a dual
MIN HO KANG, formerly Visiting Scientist with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China, is now an Associate Professor with the School of Metallurgical Engineering, Kim Chaek University of Technology, Pyongyang, DPR Korea. JIANXUN SONG, Ph.D. Student, and HONGMIN ZHU and SHUQIANG JIAO, Professors, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing. Contact e-mail: [email protected] Manuscript submitted June 3, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
electrolytic cell had been used to prepare titanium from ores or titanium oxides.[19] The flowchart for the process is shown in Figure 1. The reaction of the first step can be expressed as follows: The cathode reaction: Ca2þ þ 2e ¼ CaðliqÞ
½1
The anode reaction: 2Cl 2e ¼ Cl2 ðgÞ
½2
The reaction between the chlorine gas and the mixture of TiO2 and carbon powder at the bottom of the electrolytic cell is as follows: TiO2 þ 2C þ 2Cl2 ðgÞ ¼ TiCl4 þ 2COðgÞ
½3
TiCl4 is reduced in situ by the cathodic dissolved calcium: 2Ca þ TiCl4 ¼ 2CaCl2 þ Ti
½4
Ca þ TiCl4 ¼ CaCl2 þ TiCl2
½5
The results show that DG0 of the Reaction [4] and [5] are 712.8 and 419. J/mol at 1123 K (850 °C), respectively. Therefore, the Reaction [4] can be taken place first than Reaction [5]. However, the formed metallic Ti by Reaction [4] can react with TiCl4 according to the Reaction [6]. Ti þ x=ð4 xÞTiCl4 ¼ 4=ð4 xÞTiClx ðx ¼ 2; 3
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