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Calciothermic reduction of TiO2 and in situ electrolysis of CaO in the molten CaCl2
Journal of Physics and Chemistry of Solids 66 (2005) 461–465 www.elsevier.com/locate/jpcs
Calciothermic reduction of TiO2 and in situ electrolysis of CaO in the molten CaCl2 Ryosuke O. Suzuki* Department of Energy Science and Technology, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Accepted 4 June 2004
Abstract A new cell concept for calciothermic reduction is presented where TiO2 is used as the raw material for reduction. The reduction system consists of a single cell, where both the reduction reaction and the electrolytic reaction for recovery of reducing agent coexist in the same molten CaCl2 bath. TiO2 powder reacts with a few mol%Ca dissolved in the melt. Sufficiently deoxidized titanium metal was obtained at the bottom of the cell. Because the molten CaCl2 has a large solubility for CaO, both mechanisms of the halide flux deoxidation and the electrochemical deoxidation work efficiently once the metallic Ti was precipitated. The reducing agent is in situ recovered by electrolysis of CaO as, At the anode : C C 2 O2K Z CO2 C 4 eK At the cathode : Ca2C C 2 eK Z Ca: Some experimental results are presented for confirmation of this proposal and mechanism. q 2004 Elsevier Ltd. All rights reserved. Keywords: Titanium refining; Titanium oxide; Calcium chloride; Calciothermic reduction; Molten salt electrolysis
1. Introduction Kroll process produces metallic titanium commercially in industry. It consists of a three-step operation; the conversion from TiO2 to TiCl4, the subsequent reduction of TiCl4 to Ti by Mg liquid, and the electrolysis of the byproduct, MgCl2. It takes 2–5 days in this reduction route via TiCl4. A simpler and more compact process in a single step directly from TiO2 is desired to get higher productivity and energy saving. Metallic calcium can reduce TiO2 directly into metallic Ti to the oxygen level of 300–730 mass ppm [1–4], which fits for the industrial standards. The solubility of Ca in Ti is also as low as 50–200 ppm Ca at 1155–1600 K [5]. Alexander proposed the reduction of TiO2 using Ca first
in 1936 [6]. TiO2 C 2 Ca Z Ti C 2 CaO
(1)
However, at least a few thousand ppm O remain in Ti mainly as CaO [2–4]. The by-product, CaO, attached to the surface of Ti particles, and the slow mass transfer through the CaO layer hindered the further deoxidation. The CaO phase was easily captured at the grain boundaries of sintered Ti particles, and hardly removed by acid leaching. Molten CaCl2 can dissolve about 20 mol% CaO [7,8]. When we use this dissolution, the by-product CaO can be removed in situ from the reaction place, and it enhances the reduction [9–15] and subsequent deoxidation [16–18] more effectively. 2. Combination of reduction and electrolysis
* Tel.: C81 75 753 5453; fax: C81 75 753 4745. E-mail address: [email protected]. 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.041
To close the material cycle, the dissolved CaO should return to the reductant Ca, for example, by the molten salt
462
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
At the cathode : Ca2C C 2eK Z Ca
(3)
However, many previous attempts of CaO electrolysis in CaCl2 failed to deposit the pure Ca liquid with a good yield, mainly because the precipitated Ca dissolved immediately to the salt as Ca [20–22] (the solubility is 3.9 mol% Ca in CaCl2 at 1173 K [23–25]). Both the electrolysis and reduction can be combined together in the same bath as illustrated in Fig. 2 [10–15]. Ca in the molten CaCl2 reduces TiO2 powder in the right room of Fig. 2, and the dissolved CaO is electrochemically decomposed in the left room. Oxygen in the reduced Ti particles, [O]Ti, (max. 14 mass%) can be removed by Eq. (4) to !100 ppm oxygen level from bulk Ti samples [16–18,26].
Fig. 1. Proposal of calciothermic reduction using CaCl2.
½OTi C Ca Z Ca2C C O2K
(4)
We can summarize all the working reactions that TiO2 is reduced by carbon, which is the same as the overall reaction in Kroll process where the reductant Mg is circulated.
3. Salt constitution
Fig. 2. Basic concept of cell arrangement.
electrolysis (Fig. 1). When the voltage applied between the consumable carbon anode and the cathode is higher than the decomposition voltage of CaO (1.66 V), but below that of CaCl2 (3.2 V), both CO2 and CO gas evolve from the carbon anode [10–12,14–19]. At the anode : C C 2 O2K Z CO2 C 4 eK and C C O2K Z CO C 2 eK
ð2Þ
The thermochemical activity of CaO, aCaO, becomes lower by dissolution of CaO into CaCl2, and the attainable oxygen level can be lowered [10–19,26]. Because the reduction from TiO2 forms a large amount of CaO during the operation, it is more favorable to adapt a CaCl2-richer composition as the initial point for reduction. However, the amount of TiO2 is restricted there because the applicable amount of Ca is limited. Fig. 3 shows the CaCl2 edge in the isothermal crosssection of CaCl2–CaO–Ca system at 1173 K [7,8,23–25,27]. The reduction of TiO2 powder by Ca was confirmed, using the experimental setup as inserted in Fig. 3 [10–15]. The lighter Ca reacted with the heavier TiO2 particles on the bottom of the vessel, via the molten CaCl2. Fig. 3 shows the final salt compositions of the samples, where the increment of CaO due to reduction was considered.
Fig. 3. Concentrations after reduction experiments at the isothermal section of CaCl2–CaO–Ca system at 1173 K. The open and solid circles show that the oxygen concentrations in Ti after 3.6 ks reduction were !3000 mass ppm and O3000 ppm, respectively.
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
463
When the salt with the higher concentration of CaO was used, the larger current could be supplied under the same experimental geometry and the same applied voltage. This means that Ca could be formed more efficiently at the higher concentration of CaO. However, the dissolution of CaO from the reducing reaction was delayed due to the higher concentration of CaO. The lower Ti oxides were often detected with a-Ti in case of the short time operations. The better oxygen concentration was obtained at 0.5–1.0 mol% CaO. a-Ti single phase was obtained as slightly sintered lump, and its oxygen concentration became lower after 3 h (Run #f-1). In case of pure CaCl2, the current under a constant voltage was limited and a-Ti was not obtained within 3 h. When we define the current efficiency as the ratio, (the necessary charge to attain the analytical oxygen level)/(the applied charge during electrolysis), it was evaluated as 11.8 and 12.9% for Runs #e-1 and #f-1, respectively. These low values show that the parasitic reactions such as reaction with Ca and CO2, dissolution of Ca into the bulk, etc. occurred simultaneously in addition to the reduction. Therefore, it is essential to keep a good balance between the thermochemical reduction of TiO2 and the electrochemical decomposition of CaO.
Fig. 4. Experimental arrangement of Ti net cathode model [14,15,19].
All the samples were identified as a-Ti by X-ray diffraction measurements. The oxygen concentration in Ti was lowered to the level of !1000 mass ppm oxygen at the CaCl2 richer region. A significant increase of oxygen concentration was found at O3 mol% CaO, although CaO was not saturated. Because it took a long time to achieve the equilibrium at the high CaO concentration [23,27], the dissolution speed of CaO may be slowed. Therefore, the desired composition for reduction is hatched in Fig. 3. In the earlier study of Ca reduction at 1273 K without CaCl2, it took at least 6 h to reduce TiO2 into the lower oxides such as TiO [2], and the analytical values !1000 ppm were seldom achieved even in the prolonged treatment [2–4]. In case of this reduction for 86.4 ks, the oxygen content decreased to 420 ppm [13], which is well acceptable for industrial material.
5. Confirmation of Ca reduction Recently Fray et al. proposed the electrolysis of TiO2 in the CaCl2 bath [28–30]. Their process bases on the oxygen extraction from TiO2 cathode, written as, At the anode : 2 O2K Z O2 C 4 eK
4. Reduction and in situ electrolysis
(5) 2K
At the cathode : TiO2 C 4 e Z Ti C 2 O K
Fig. 4 shows an experimental arrangement for the simultaneous reduction and in situ electrolysis [10–15,19]. A carbon crucible was used for anode, and Ti net shaped like basket was used for cathode, in which about 1 g granular TiO2 was filled. By applying a constant voltage below that of theoretical decomposition of CaCl2 at 1173 K, CO2 gas evolved as listed in Table 1.
(6)
Two differences between FFC process and our proposal are the workability of Ca near the cathode, and the evolved gas at the anode. In our process, the evolved oxygen gas may react with the carbon anode to form CO2/CO gas. In order to detect dissolved Ca, therefore, TiO2 powder was placed at the several points in the bath as illustrated in Fig. 5. About 1.0 kg CaCl2 was melted and electrolyzed in
Table 1 Experimental conditions and results using Ti net cathode [19] Run
Molten salt (mol%)
Voltage (V)
Detected gas
Time (ks)
Total charge (C)
Phases identified by XRD
Oxygen concentration (mass ppm)
d-1 d-2 d-3 e-1 e-2 f-1 f-2 g-1 g-2
15% CaO 15% CaO 15% CaO 1% CaO 1% CaO 0.5% CaO 0.5% CaO CaCl2 CaCl2
2.6–2.8 2.6–2.8 2.6–2.8 2.5–2.8 2.6–2.8 2.6–2.9 2.7–2.9 2.6–2.8 2.6–2.9
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2
10.8 3.6 1.8 10.8 3.6 10.8 3.6 10.8 3.6
54,126 23,055 10,800 40,194 14,931 36,978 10,704 25,374 5 439
a-Ti,TiO0.325 Ti6O,TiO TiO0.325,TiO a-Ti a-Ti,TiO0.325 a-Ti a-Ti,TiO0.325 TiO0.325 TiO0.325
– – – 6 200 – 2 000 – – –
Cl2 gas could not be detected. The evolution of O2 and CO was not analyzed.
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R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
Carbon dust was observed in the upper part of the bath and sometimes froze as a crust layer. The carbon contamination in Ti should be minimized for better ductility of Ti. By analyzing the cell behaviors, we believe that the cathode design is the key to solve the matter [15,19].
6. Conclusion
Fig. 5. Positions of TiO2 powder. (a) is in the basket-type cathode, (b) in the Y2O3 crucible isolated electronically from the cathode, (c) in the basket inside the cathode and (d) outside the cathode.
Ca dissolved in the molten CaCl2 could reduce successfully TiO2 powder. Combining the reduction and the electrolysis in the same bath, sufficiently deoxidized titanium metal deposited as the granular sponge. The better-deoxidized Ti was obtained at the lower concentration of CaO, and when it attached to the cathode, although the strongly reducing atmosphere was extended near the cathode.
Acknowledgements The author thanks Prof. Emeritus, K. Ono for discussion, Mr S. Inoue, Mr K. Teranuma and Mr S. Fukui for their experiments. This work was financially supported by Grants-in-Aid for Scientific Research under Contract No.14205109.
References Fig. 6. Electrolysis with stirring. TiO2 powder was filled in the cathodic Ti basket.
the anodic carbon crucible. TiO2 filled in Ti net cathode was reduced to Ti (1600 ppmO), while TiO2 at the positions (b)– (d) was reduced to the mixture of Ti3O, Ti2O, TiO and Ti2O3, although pure Ti was not formed. The electron extraction mechanism (Eq. 6) could work when the electro-conductive material such as the lower oxides attached directly to the cathode, while it cannot explain the reduction at the positions (b)–(d) because they were electronically isolated. TiO2 in the Ti net cathode was rotated in the bath, as shown in Fig. 6. The small propellers were set below the Ti basket to stir the bath. TiO2 was reduced to Ti2O (11 mass%O) with stirring of 1 Hz at 1173 K for 3.6 ks, while a-Ti was obtained without stirring (0.18%O). This is because the Ca concentrated region near the Ti net cathode was scattered into the bulk by stirring. These phenomena suggest strongly that Ca precipitated electrochemically on the cathode surface dissolves into the CaCl2 melt and reduces TiO2. However, the problems encountered were back reaction due to the solubility of metallic Ca in the melt, and parasitic reactions due to CO2 gas bubbles. 2 Ca C CO2 Z C C 2 CaO
(7)
Ca C CO Z C C CaO
(8)
[1] O. Kubaschewski, W.A. Dench, J. Inst. Met. 82 (1953) 87–91. [2] K. Ono, S. Miyazaki, J. Jpn Inst. Met. 49 (1985) 871–875. [3] K. Ono, T.H. Okabe, M. Ogawa, R.O. Suzuki, Tetsu-to-Hagane´ 76 (4) (1990) 568–575. [4] T.H. Okabe, R.O. Suzuki, T. Oishi, K. Ono, Mater. Trans. JIM 32 (5) (1991) 485–488. [5] I. Obinata, Y. Takeuchi, S. Saikawa, Trans. ASM 52 (1960) 1072– 1083. [6] P.P. Alexander, US Patent, 2.038.402, (1936),US Patent, 2.043.363, (1936),US Patent, 2.082.134, (1937). [7] D.A. Wenz, I. Johnson, R.D. Wolson, J. Chem. Eng. Data 14 (2) (1969) 250–252. [8] G.S. Perry, L.G. MacDonald, J. Nucl. Mater. 130 (1985) 234–241. [9] K. Ono, Titanium Jpn 48 (1) (2000) 13–15. [10] K. Ono, R.O. Suzuki, Materia Jpn 41 (1) (2002) 28–31. [11] K. Ono, R.O. Suzuki, J.O.M. Mem, J. Min. Met. Mater. Soc. 54 (2) (2002) 59–61. [12] K. Ono, R.O. Suzuki, Titanium Japan 50 (2) (2002) 105–108. [13] R.O. Suzuki, S. Inoue, Metall. Mater. Trans. B 34 (6) (2003) 277–286. [14] R.O. Suzuki, K. Ono, Proceedings of the 13th International Symposium on Molten Salt, The Electrochemical Society, Penningston, NJ, 2002, pp. 810–821. [15] R.O. Suzuki, K. Ono, Proceedings of Yazawa International Symposium Metallurgical and Materials Processing: Principles and TechnologiesThe Minerals Aqueous and Electrochemical Processing vol. III, Metals and Materials Soc, OH, USA, 2003, pp. 187–199. [16] T.H. Okabe, R.O. Suzuki, T. Oishi, K. Ono, Tetsu-to-Hagane´ 77 (1) (1991) 93–99. [17] T.H. Okabe, T. Oishi, K. Ono, J. Alloys Comp. 184 (1992) 43–56. [18] T.H. Okabe, M. Nakamura, T. Ueki, T. Oishi, K. Ono, Bull. Jpn Inst. Met. 31 (4) (1992) 315–317.
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465 [19] R.O. Suzuki, K. Teranuma, K. Ono, Metall. Mater. Trans. B 34 (6) (2003) 287–295. [20] G.J. Kipouros, R.A. Sharma, J. Electrochem. Soc. 137 (1990) 3333–3338. [21] P.D. Ferro, B. Mishra, D.L. Olson, W.A. Averill, Waste Management 17 (1997) 451–461. [22] P.D. Ferro, B. Mishra, D.L. Olson, W.A. Averill, Trans. Indian Inst. Met. 51 (1998) 69–77. [23] V. Dosaj, C. Aksaranan, D.R. Morris, J. Chem. Soc. Faraday Trans. 71 (1975) 1083–1098. [24] H. Fischbach, Steel Research 56 (7) (1985) 365–368.
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[25] L.-I. Staffansson, D. Sichen, Scand. J. Metall. 21 (1992) 165–171. [26] T.H. Okabe, M. Nakamura, T. Oishi, K. Ono, Metall. Trans. B 24B (1993) 449–456. [27] K.M. Axler, G.L. DePoorter, Mater. Sci. Forum 73-75 (1991) 19–24. [28] G.Z. Chen, D.J. Fray, T.W. Farthing:, Nature 407 (Sep. 2000) 361–364. [29] D.J. Fray:, J. Minerals, Metals Mater. Soc. (JOM) 53 (10) (2001) 26–31. [30] G.Z. Chen, D.J. Fray, T.W. Farthing, Metall. Mater. Trans. B 32B (6) (2001) 1041–1052.
Calciothermic reduction of TiO2 and in situ electrolysis of CaO in the molten CaCl2 Ryosuke O. Suzuki* Department of Energy Science and Technology, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Accepted 4 June 2004
Abstract A new cell concept for calciothermic reduction is presented where TiO2 is used as the raw material for reduction. The reduction system consists of a single cell, where both the reduction reaction and the electrolytic reaction for recovery of reducing agent coexist in the same molten CaCl2 bath. TiO2 powder reacts with a few mol%Ca dissolved in the melt. Sufficiently deoxidized titanium metal was obtained at the bottom of the cell. Because the molten CaCl2 has a large solubility for CaO, both mechanisms of the halide flux deoxidation and the electrochemical deoxidation work efficiently once the metallic Ti was precipitated. The reducing agent is in situ recovered by electrolysis of CaO as, At the anode : C C 2 O2K Z CO2 C 4 eK At the cathode : Ca2C C 2 eK Z Ca: Some experimental results are presented for confirmation of this proposal and mechanism. q 2004 Elsevier Ltd. All rights reserved. Keywords: Titanium refining; Titanium oxide; Calcium chloride; Calciothermic reduction; Molten salt electrolysis
1. Introduction Kroll process produces metallic titanium commercially in industry. It consists of a three-step operation; the conversion from TiO2 to TiCl4, the subsequent reduction of TiCl4 to Ti by Mg liquid, and the electrolysis of the byproduct, MgCl2. It takes 2–5 days in this reduction route via TiCl4. A simpler and more compact process in a single step directly from TiO2 is desired to get higher productivity and energy saving. Metallic calcium can reduce TiO2 directly into metallic Ti to the oxygen level of 300–730 mass ppm [1–4], which fits for the industrial standards. The solubility of Ca in Ti is also as low as 50–200 ppm Ca at 1155–1600 K [5]. Alexander proposed the reduction of TiO2 using Ca first
in 1936 [6]. TiO2 C 2 Ca Z Ti C 2 CaO
(1)
However, at least a few thousand ppm O remain in Ti mainly as CaO [2–4]. The by-product, CaO, attached to the surface of Ti particles, and the slow mass transfer through the CaO layer hindered the further deoxidation. The CaO phase was easily captured at the grain boundaries of sintered Ti particles, and hardly removed by acid leaching. Molten CaCl2 can dissolve about 20 mol% CaO [7,8]. When we use this dissolution, the by-product CaO can be removed in situ from the reaction place, and it enhances the reduction [9–15] and subsequent deoxidation [16–18] more effectively. 2. Combination of reduction and electrolysis
* Tel.: C81 75 753 5453; fax: C81 75 753 4745. E-mail address: [email protected]. 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.041
To close the material cycle, the dissolved CaO should return to the reductant Ca, for example, by the molten salt
462
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
At the cathode : Ca2C C 2eK Z Ca
(3)
However, many previous attempts of CaO electrolysis in CaCl2 failed to deposit the pure Ca liquid with a good yield, mainly because the precipitated Ca dissolved immediately to the salt as Ca [20–22] (the solubility is 3.9 mol% Ca in CaCl2 at 1173 K [23–25]). Both the electrolysis and reduction can be combined together in the same bath as illustrated in Fig. 2 [10–15]. Ca in the molten CaCl2 reduces TiO2 powder in the right room of Fig. 2, and the dissolved CaO is electrochemically decomposed in the left room. Oxygen in the reduced Ti particles, [O]Ti, (max. 14 mass%) can be removed by Eq. (4) to !100 ppm oxygen level from bulk Ti samples [16–18,26].
Fig. 1. Proposal of calciothermic reduction using CaCl2.
½OTi C Ca Z Ca2C C O2K
(4)
We can summarize all the working reactions that TiO2 is reduced by carbon, which is the same as the overall reaction in Kroll process where the reductant Mg is circulated.
3. Salt constitution
Fig. 2. Basic concept of cell arrangement.
electrolysis (Fig. 1). When the voltage applied between the consumable carbon anode and the cathode is higher than the decomposition voltage of CaO (1.66 V), but below that of CaCl2 (3.2 V), both CO2 and CO gas evolve from the carbon anode [10–12,14–19]. At the anode : C C 2 O2K Z CO2 C 4 eK and C C O2K Z CO C 2 eK
ð2Þ
The thermochemical activity of CaO, aCaO, becomes lower by dissolution of CaO into CaCl2, and the attainable oxygen level can be lowered [10–19,26]. Because the reduction from TiO2 forms a large amount of CaO during the operation, it is more favorable to adapt a CaCl2-richer composition as the initial point for reduction. However, the amount of TiO2 is restricted there because the applicable amount of Ca is limited. Fig. 3 shows the CaCl2 edge in the isothermal crosssection of CaCl2–CaO–Ca system at 1173 K [7,8,23–25,27]. The reduction of TiO2 powder by Ca was confirmed, using the experimental setup as inserted in Fig. 3 [10–15]. The lighter Ca reacted with the heavier TiO2 particles on the bottom of the vessel, via the molten CaCl2. Fig. 3 shows the final salt compositions of the samples, where the increment of CaO due to reduction was considered.
Fig. 3. Concentrations after reduction experiments at the isothermal section of CaCl2–CaO–Ca system at 1173 K. The open and solid circles show that the oxygen concentrations in Ti after 3.6 ks reduction were !3000 mass ppm and O3000 ppm, respectively.
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
463
When the salt with the higher concentration of CaO was used, the larger current could be supplied under the same experimental geometry and the same applied voltage. This means that Ca could be formed more efficiently at the higher concentration of CaO. However, the dissolution of CaO from the reducing reaction was delayed due to the higher concentration of CaO. The lower Ti oxides were often detected with a-Ti in case of the short time operations. The better oxygen concentration was obtained at 0.5–1.0 mol% CaO. a-Ti single phase was obtained as slightly sintered lump, and its oxygen concentration became lower after 3 h (Run #f-1). In case of pure CaCl2, the current under a constant voltage was limited and a-Ti was not obtained within 3 h. When we define the current efficiency as the ratio, (the necessary charge to attain the analytical oxygen level)/(the applied charge during electrolysis), it was evaluated as 11.8 and 12.9% for Runs #e-1 and #f-1, respectively. These low values show that the parasitic reactions such as reaction with Ca and CO2, dissolution of Ca into the bulk, etc. occurred simultaneously in addition to the reduction. Therefore, it is essential to keep a good balance between the thermochemical reduction of TiO2 and the electrochemical decomposition of CaO.
Fig. 4. Experimental arrangement of Ti net cathode model [14,15,19].
All the samples were identified as a-Ti by X-ray diffraction measurements. The oxygen concentration in Ti was lowered to the level of !1000 mass ppm oxygen at the CaCl2 richer region. A significant increase of oxygen concentration was found at O3 mol% CaO, although CaO was not saturated. Because it took a long time to achieve the equilibrium at the high CaO concentration [23,27], the dissolution speed of CaO may be slowed. Therefore, the desired composition for reduction is hatched in Fig. 3. In the earlier study of Ca reduction at 1273 K without CaCl2, it took at least 6 h to reduce TiO2 into the lower oxides such as TiO [2], and the analytical values !1000 ppm were seldom achieved even in the prolonged treatment [2–4]. In case of this reduction for 86.4 ks, the oxygen content decreased to 420 ppm [13], which is well acceptable for industrial material.
5. Confirmation of Ca reduction Recently Fray et al. proposed the electrolysis of TiO2 in the CaCl2 bath [28–30]. Their process bases on the oxygen extraction from TiO2 cathode, written as, At the anode : 2 O2K Z O2 C 4 eK
4. Reduction and in situ electrolysis
(5) 2K
At the cathode : TiO2 C 4 e Z Ti C 2 O K
Fig. 4 shows an experimental arrangement for the simultaneous reduction and in situ electrolysis [10–15,19]. A carbon crucible was used for anode, and Ti net shaped like basket was used for cathode, in which about 1 g granular TiO2 was filled. By applying a constant voltage below that of theoretical decomposition of CaCl2 at 1173 K, CO2 gas evolved as listed in Table 1.
(6)
Two differences between FFC process and our proposal are the workability of Ca near the cathode, and the evolved gas at the anode. In our process, the evolved oxygen gas may react with the carbon anode to form CO2/CO gas. In order to detect dissolved Ca, therefore, TiO2 powder was placed at the several points in the bath as illustrated in Fig. 5. About 1.0 kg CaCl2 was melted and electrolyzed in
Table 1 Experimental conditions and results using Ti net cathode [19] Run
Molten salt (mol%)
Voltage (V)
Detected gas
Time (ks)
Total charge (C)
Phases identified by XRD
Oxygen concentration (mass ppm)
d-1 d-2 d-3 e-1 e-2 f-1 f-2 g-1 g-2
15% CaO 15% CaO 15% CaO 1% CaO 1% CaO 0.5% CaO 0.5% CaO CaCl2 CaCl2
2.6–2.8 2.6–2.8 2.6–2.8 2.5–2.8 2.6–2.8 2.6–2.9 2.7–2.9 2.6–2.8 2.6–2.9
CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2
10.8 3.6 1.8 10.8 3.6 10.8 3.6 10.8 3.6
54,126 23,055 10,800 40,194 14,931 36,978 10,704 25,374 5 439
a-Ti,TiO0.325 Ti6O,TiO TiO0.325,TiO a-Ti a-Ti,TiO0.325 a-Ti a-Ti,TiO0.325 TiO0.325 TiO0.325
– – – 6 200 – 2 000 – – –
Cl2 gas could not be detected. The evolution of O2 and CO was not analyzed.
464
R.O. Suzuki / Journal of Physics and Chemistry of Solids 66 (2005) 461–465
Carbon dust was observed in the upper part of the bath and sometimes froze as a crust layer. The carbon contamination in Ti should be minimized for better ductility of Ti. By analyzing the cell behaviors, we believe that the cathode design is the key to solve the matter [15,19].
6. Conclusion
Fig. 5. Positions of TiO2 powder. (a) is in the basket-type cathode, (b) in the Y2O3 crucible isolated electronically from the cathode, (c) in the basket inside the cathode and (d) outside the cathode.
Ca dissolved in the molten CaCl2 could reduce successfully TiO2 powder. Combining the reduction and the electrolysis in the same bath, sufficiently deoxidized titanium metal deposited as the granular sponge. The better-deoxidized Ti was obtained at the lower concentration of CaO, and when it attached to the cathode, although the strongly reducing atmosphere was extended near the cathode.
Acknowledgements The author thanks Prof. Emeritus, K. Ono for discussion, Mr S. Inoue, Mr K. Teranuma and Mr S. Fukui for their experiments. This work was financially supported by Grants-in-Aid for Scientific Research under Contract No.14205109.
References Fig. 6. Electrolysis with stirring. TiO2 powder was filled in the cathodic Ti basket.
the anodic carbon crucible. TiO2 filled in Ti net cathode was reduced to Ti (1600 ppmO), while TiO2 at the positions (b)– (d) was reduced to the mixture of Ti3O, Ti2O, TiO and Ti2O3, although pure Ti was not formed. The electron extraction mechanism (Eq. 6) could work when the electro-conductive material such as the lower oxides attached directly to the cathode, while it cannot explain the reduction at the positions (b)–(d) because they were electronically isolated. TiO2 in the Ti net cathode was rotated in the bath, as shown in Fig. 6. The small propellers were set below the Ti basket to stir the bath. TiO2 was reduced to Ti2O (11 mass%O) with stirring of 1 Hz at 1173 K for 3.6 ks, while a-Ti was obtained without stirring (0.18%O). This is because the Ca concentrated region near the Ti net cathode was scattered into the bulk by stirring. These phenomena suggest strongly that Ca precipitated electrochemically on the cathode surface dissolves into the CaCl2 melt and reduces TiO2. However, the problems encountered were back reaction due to the solubility of metallic Ca in the melt, and parasitic reactions due to CO2 gas bubbles. 2 Ca C CO2 Z C C 2 CaO
(7)
Ca C CO Z C C CaO
(8)
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