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Electrochemical production of Al–Sc alloy in CaCl2–Sc2O3 molten salt
Journal of Alloys and Compounds 474 (2009) 124–130
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical production of Al–Sc alloy in CaCl2 –Sc2 O3 molten salt Masanori Harata a,1 , Kouji Yasuda b,2 , Hiromasa Yakushiji c,3 , Toru H. Okabe b,∗ a
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Technical Research & Development Department, Production Division, Pacific Metals Co., Ltd., 5-2 Toyama Shinden, Kawaragi, Hachinohe, Aomori 031-8617, Japan b
a r t i c l e
i n f o
Article history: Received 27 April 2008 Received in revised form 19 June 2008 Accepted 19 June 2008 Available online 13 August 2008 Keywords: Scandium Al–Sc alloy Molten salt Electrolysis
a b s t r a c t In order to develop a new production process for Al–Sc alloys, a fundamental study on the electrolysis in CaCl2 –Sc2 O3 melts was conducted using a small-scale laboratory cell. Al–Sc alloys were electrochemically produced by cathodically polarizing an Al liquid electrode in CaCl2 –Sc2 O3 melts at 1173 K. Metallic-colored spherical samples were produced by the electrolysis and were analyzed by XRD, EPMA, XRF, and ICP–AES. The electrolyzed samples consisted of Al and Al3 Sc phases. The purity of the obtained Al–Sc alloys was greater than 99 mass%, and the calcium content was less than 0.65 mass%. This study demonstrates the feasibility of Al–Sc alloy production directly from Sc2 O3 by electrochemical methods. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, scandium (Sc) has been focused on as an alloying element for aluminum (Al) alloys. The addition of a small amount of Sc to Al alloys improves the strength, weldability, resistance to recrystallization, and corrosion resistance [1,2]. Owing to these attractive properties, the demand for Sc as an alloying element in Al alloys is expected to increase in the future. However, the application fields and the number of uses of Al–Sc alloys are presently limited because of the extremely high price of Sc metal, which is due to the difficulties in the reduction process and the small production volume of the starting material. Therefore, the development of a new and inexpensive production process for Sc metal or Al–Sc alloys is required. Currently, metallic Sc is commercially manufactured by the calciothermic reduction of its fluoride salt (ScF3 ) produced from its oxide (Sc2 O3 ); the major chemical reactions are expressed as fol-
∗ Corresponding author at: Institute of Industrial Science, The University of Tokyo, Fw-302, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6312, +81 3 5452 6314 (Okabe Lab.); fax: +81 3 5452 6313. E-mail addresses: [email protected] (M. Harata), [email protected] (K. Yasuda), h-yakushiji@pacific-metals.co.jp (H. Yakushiji), [email protected] (T.H. Okabe). 1 Presently at Toyota Industries Corporation, Japan. Tel.: +81 3 5452 6314; fax: +81 3 5452 6313. 2 Tel.: +81 3 5452 6820; fax: +81 3 5452 6313. 3 Tel.: +81 178 47 7231; fax: +81 178 22 7350. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.110
lows [3]: Sc2 O3 (s) + 6HF(g) → 2ScF3 (s) + 3H2 O(g) ScF3 (l) + 3/2Ca(l) → Sc(l) + 3/2CaF2 (l)
(at 973 K)
(1)
(at 1873 K)
(2)
In this process, the starting material, Sc2 O3 , is first converted to scandium fluoride (ScF3 ), because it is difficult to directly reduce the thermodynamically stable Sc2 O3 to metallic Sc even by using metallic calcium (Ca) as a reductant. The fluorination is carried out by treating Sc2 O3 with hydrogen fluoride (HF) gas at 973 K. The obtained ScF3 is then reduced to Sc metal by using the Ca reductant at 1873 K. While the utilization of fluoride metallurgy enables the production of Sc metal with low oxygen content, the high-temperature operation under highly a corrosive environment significantly increases the production cost of Sc metal. Furthermore, contamination from the reactor material and reductant is inevitable in the reduction at high temperature, particularly in the fluoride reactor system. With regard to the starting material of Sc metal, its price is expected to decrease. At present, the starting material is produced in the form of Sc2 O3 from the byproducts of uranium (U), tungsten (W), or tantalum (Ta) smelting [3]. The production volume of the byproduct Sc2 O3 is small, and this results in a very small production volume of Sc metal and alloys. This small scale of Sc2 O3 production gives rise to the extremely high price of Sc and its compounds. In recent years, it has been reported that Sc2 O3 can be also efficiently recovered in the nickel (Ni) smelting process, since this smelting process has been changing from a pyrometallurgical process to a hydrometallurgical process [4]. Once the effective recovery of Sc
M. Harata et al. / Journal of Alloys and Compounds 474 (2009) 124–130
125
from the Ni smelting process is established, it is expected that a significantly large amount of Sc can be supplied to the market and this increase in production volume will lead to a reduction in production cost. Currently, the majority of Sc metal is consumed as the additives in Al alloys, and this application will expand if the cost of Sc metal is lowered. With this background, the direct production of Al–Sc alloys from Sc2 O3 has been investigated. In our previous studies [5,6], Al–Sc alloys were produced by the calciothermic reduction of Sc2 O3 , in which Al was used as the collector metal for Sc. It was demonstrated that Al–Sc alloys could be produced even at low temperatures (1273 K) directly from Sc2 O3 . However, excess Ca reductant remained in the alloy at above 0.7 mass%, and an additional process for removing Ca is required if low-Ca-content Al–Sc alloys are required. In this study, we aimed to produce an Al–Sc alloy with a low Ca content directly from the oxide feed by electrolysis in calcium chloride (CaCl2 )–Sc2 O3 molten salt. 2. Molten salt electrolysis Fig. 1 shows the concept employed in this study. Liquid Al and solid Sc2 O3 particles sink under molten CaCl2 due to the difference of the densities. Al–Sc alloys are produced by cathodically polarizing an Al liquid electrode at the bottom of a carbon crucible (Fig. 1(a)). The possible reaction scheme for Al–Sc formation depends on the solubility of Sc2 O3 ; however, there has been no report on its solubility in molten CaCl2 . (A) When Sc2 O3 dissolves into molten CaCl2 , an Al–Sc alloy is formed by the reduction of the Sc3+ ions in the melt. Cathode : Sc3+ + 3e− → Sc(l, in Al liq.) 2−
Anode : C(s) + xO
(1)
(in salt) → COx (g) + 2xe
−
(2)
(B) When Sc2 O3 is insoluble in molten CaCl2 , the Sc2 O3 powder added to the electrolyte sinks to the bottom of the crucible and comes into contact with the Al cathode. Then, the solid Sc2 O3 is electrochemically reduced on the surface of the Al liquid electrode, forming an Al–Sc alloy. This reaction is similar to the electrochemical reduction process investigated by Fray et al. [7–9]. Cathode : Sc2 O3 (s) + 6e− → 2Sc(l, in Al liq.) + 3O2−
(3)
Anode : C(s) + xO2− → COx (g) + 2xe−
(4)
Fig. 1. (a) Concept of production of Al–Sc alloys by molten salt electrolysis. (b) Possible mechanism of reduction of Sc2 O3 by electrolysis.
(C) Calciothermic reduction can also occur in the case where Sc2 O3 is insoluble. Ca metal is produced in the cathodic reaction when the potential of the cathode is sufficiently negative. The electrochemically produced Ca is soluble in molten CaCl2 [10,11], Table 1 Materials used in this study Materials
Form
Purity (%)
Supplier
Note
Sc2 O3 CaCl2
Powder Powder
99.9 up 95.0 up
Pacific Metals Co., Ltd. Kanto Chemicals, Inc.
13.7 m (average particle size) Vacuum dried at 423 K
Ni Mo Al Al C C
Rod Wire Shot Powder Rod Crucible
99.9 99.9 99.99 up 99.99 up 99.9 99.9
Shinkinzoku Industry Co., Ltd. Shinkinzoku Industry Co., Ltd. Koujundo Chemical Lab. Co., Ltd. Koujundo Chemical Lab Co Ltd Sogo Carbon Co., Ltd. Tokai Techno Service Co., Ltd.
3 mm 1 mm 2–5 mm 3 m diameter Graphite, 6 mm Graphite, 37 mm × 50 mm
Mullite Mullite Ethanol
Tube Tube Liquid
HB grade, 6 mm HB grade, 12 mm
99.5 up
Nikkato Corp. Nikkato Corp. Kanto Chemicals, Inc.
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M. Harata et al. / Journal of Alloys and Compounds 474 (2009) 124–130
and the Ca dissolved in the CaCl2 also acts as a reductant. Cathode : Ca2+ + 2e− → Ca(l) −
(5)
−
(6)
C(s) + xO2− → COx (g) + 2xe−
(7)
Anode : 2Cl → Cl2 (g) + 2e
The produced Ca metal chemically reduces the Sc2 O3 powder to produce Sc metal. This type of reduction process has been investigated by Oki et al. [12] and Ono et al. [13,14] to study the reduction of titanium oxide. When Al exists in the system, the Sc metal produced by the calciothermic reduction reacts to form an Al–Sc alloy according to the following reaction: Sc2 O3 (s) + Al(l) + 3Ca(l) → Al–Sc(l) + 3CaO(l)
(8)
In any of the abovementioned reaction schemes, when the cathode potential is adjusted to the optimum value which facilitates high Ca activity, an Al–Sc alloy is expected to be produced by the reduction of Sc2 O3 in the molten CaCl2 . 3. Experimental The materials used in this study are listed in Table 1, and a schematic illustration of the experimental apparatus used for the cyclic voltammetry measurements is shown in Fig. 2(a). CaCl2 powder (40 g, dried in a vacuum for several days and charged into a carbon crucible) was used as the electrolyte. The crucible was placed inside a stainless steel chamber, which was installed in an electric furnace and attached to a gas supply/vacuum system. The pressure in the chamber was monitored by using a digital pressure gauge (Cosmo Co., DP-310) and maintained at 1 atm. The salt was melted at 1173 K under an argon (Ar) atmosphere. CaCl2 –Sc2 O3 molten salt was synthesized by charging Sc2 O3 powder into the CaCl2 salt. As the working electrodes, a carbon (C) electrode was used in the anodic region, and molybdenum (Mo) and Al liquid electrodes were used in the cathodic region. The appearance and configuration of the Al liquid electrode is shown in Fig. 2(b). The Al liquid electrode was prepared by holding Al metal in a mullite tube, which was fixed to a smaller mullite tube. Electrical contact was provided through a Mo wire. A Ni rod was used as a quasi-reference electrode. While Mo does not reacts with either Ca or Sc, Al reacts with both Ca and Sc and forms several intermetallic compounds, as described in the phase diagrams [15]. The reduction behaviors of Sc and Ca were investigated by a comparison between the voltammograms for Mo and Al. After performing cyclic voltammetry, potentiostatic electrolysis and galvanostatic electrolysis were carried out. Fig. 3 shows a schematic illustration of the experimental apparatus for the molten salt electrolysis. CaCl2 –Sc2 O3 molten salt containing 1.37–8 mol%Sc2 O3 was used as the electrolyte. The Al metal placed at the bottom of the carbon crucible was used as the working electrode and was polarized at the cathodic potential, and a carbon rod was used as the counter electrode. Representative experimental conditions for the galvanostatic electrolysis are shown in Table 2. After the electrolysis, the samples in the crucible were gradually cooled down to room temperature in the electric furnace. The samples were recovered by leaching in water. The Al–Sc alloy samples obtained by the electrolysis were analyzed by X-ray diffraction (XRD; Rigaku Co., RINT 2500, Cu K␣), electron-probe microanalysis (EPMA; JEOL Ltd., JXA-8800RL), X-ray fluorescence spectrometry (XRF; JEOL Ltd., JSX–3210), and inductively coupled plasma-atomic emission spectrometry (ICP–AES).
Fig. 2. (a) Schematic illustration of the experimental apparatus for cyclic voltammetry and (b) appearance and configuration of the Al working electrode.
Table 2 Experimental conditions for molten salt electrolysis Exp #.
A B C D E F G H I J
Molten salt system
CaCl2 –1.37 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3
Mass of feed samples, wi (g)
Electrolysis
Sc2 O3
CaCl2
Al
Current, i (A)
Temperature, T (K)
Time, t (s)
0.69 1.02 1.02 1.02 2.09 2.09 2.09 2.09 4.36 4.36
40 40 40 40 40 40 40 40 40 40
2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66
−1 −0.25 −1 −1 −0.25 −0.5 −1 −1 −0.25 −0.25
1173 1173 1173 1173 1173 1173 1173 1173 1173 1173
1800 7200 1800 3600 3600 3600 1800 3600 3600 7200
Working electrode: aluminum. Counter electrode: carbon. Mass of molten salt: 40 g CaCl2 .
M. Harata et al. / Journal of Alloys and Compounds 474 (2009) 124–130
127
Fig. 5. Cyclic voltammograms in pure CaCl2 molten salt and CaCl2 –2 mol%Sc2 O3 molten salt at 1173 K. (i) CaCl2 molten salt, WE: Al, CE: C, scan rate, ve = 20 mV/s, (ii) CaCl2 –2 mol%Sc2 O3 , WE: Al, CE: C, scan rate, ve = 20 mV/s.
4. Result and discussion Fig. 3. Schematic illustration of the experimental apparatus for molten salt electrolysis.
4.1. Cyclic voltammetry The electrochemical behavior in the molten salt was investigated using cyclic voltammetry. Here, the anodic and cathodic regions were investigated by carbon and Mo electrodes, respectively. In pure molten CaCl2 (Fig. 4(a)), Ca deposition and Cl2 evolution occur at −1.8 V and +1.4 V with respect to the Ni quasireference electrode, respectively. The decomposition voltage of
Fig. 4. Cyclic voltammograms in (a) pure CaCl2 molten salt and (b) CaCl2 –2mol%Sc2 O3 molten salt at 1173 K. (a) (i) WE: C, CE: Mo, scan rate, ve = 100 mV/s, (ii) WE: Mo, CE: C, scan rate, ve = 100 mV/s, (b) (i) WE: C, CE: Mo, scan rate, ve = 20 mV/s, (ii) WE: Mo, CE: C, scan rate, ve = 20 mV/s.
Fig. 6. (a) Appearance of the sample obtained by the potentiostatic electrolysis. (b) XRD pattern of the sample obtained by the potentiostatic electrolysis (temperature, T = 1173 K, Electrolysis time, t = 3600 s, potential vs. Ni, E = −0.9 V).
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M. Harata et al. / Journal of Alloys and Compounds 474 (2009) 124–130
Fig. 7. Photograph and EPMA elemental mapping of sectioned alloy sample obtained by the electrolysis. (a) Sectioned sample, (b) aluminum, (c) scandium and (d) calcium. (Exp. A, temperature, T = 1173 K, Electrolysis time, t = 1800 s, Current, i = −1.00 A).
molten CaCl2 calculated from the standard Gibbs energy of formation is 3.2 V at 1173 K [16], which agrees well with the voltammetric results. In the CaCl2 –2 mol%Sc2 O3 molten salt (Fig. 4(b)), Ca deposition and Cl2 evolution also occur at −1.8 V and +1.4 V, respectively. Further, the oxidation current for COx evolution is observed at +0.4 V [17,18], and the redox current related to the Sc species is observed at around −1.2 V. The electrode behavior for the Al liquid electrode was investigated in CaCl2 molten salt and CaCl2 –2 mol%Sc2 O3 molten salt. As shown in Fig. 5, the redox current is observed at around −1.4 V in CaCl2 molten salt, which is more positive compared with that for Mo by 0.4 V. This current corresponds to the formation and deformation of an Al–Ca alloy. In the CaCl2 –2 mol%Sc2 O3 molten salt, the redox current is observed around −0.7 V. A comparison of these voltammograms indicates that an Al–Sc alloy is produced at around −0.7 V. The anodic current observed at more positive than −0.3 V in the both voltammograms is possibly due to the anodic dissolution of Al metal. 4.2. Molten salt electrolysis In order to confirm that the reduction current at around −0.7 V corresponds to the formation of an Al–Sc alloy, potentiostatic electrolysis was carried out at −0.9 V for 1 h. Fig. 6 shows the
appearance and XRD pattern of a representative sample obtained by the potentiostatic electrolysis. The obtained metallic-colored spherical sample consisted of Al and Al3 Sc, confirming that an Al–Sc alloy is produced at this potential. The use of Al metal as a collector metal assisted the reduction of Sc2 O3 by lowering the activity of Sc through a formation of Al–Sc alloy; similar effect was previously reported in the direct and electrochemical reduction of solid oxides [19,20]. Based on the results from the cyclic voltammetry and potentiostatic electrolysis, galvanostatic electrolysis was also performed at the currents from −0.25 A to −1 A. In all the galvanostatic electrolysis experiments, the potential value was more negative than −0.9 V, which was sufficiently negative to produce Al–Sc alloys. In fact, the existence of an Al3 Sc phase in Al was confirmed by XRD. Fig. 7 shows the EPMA elemental mapping of a sectioned Al–Sc alloy sample. The segregation of Sc at the surface of the alloy sample is observed. This segregation is probably due to the slow cooling of the samples in the furnace. The maximum solubility of Sc in the Al–Sc liquid alloy is approximately 6 mass% at 1173 K and it equilibrates with Al3 Sc [15]. At room temperature, however, the solubility of Sc in the Al solid solution decreases down to less than 0.01 mass% [21,22]. The composition of the surface of the Al–Sc alloy sample was determined by XRF and is listed in Table 3. The concentration of Sc is 0.81–32.3 mass%, which basically increases with the addition of
M. Harata et al. / Journal of Alloys and Compounds 474 (2009) 124–130
129
Table 3 Analytical results by XRF for the samples obtained by the electrolysis Exp. #
Molten salt system
Current, i (A)
Time, t (s)
Aa Ab Ba Ca
CaCl2 –1.37 mol%Sc2 O3 CaCl2 –1.37 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3
−1 −1 −0.25 −1
1800 1800 7200 1800
Da
CaCl2 –2 mol%Sc2 O3
−1
3600
a
CaCl2 –4 mol%Sc2 O3
−0.25
3600
Fa Ga
CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3
−0.5 −1
3600 1800
Ha
CaCl2 –4 mol%Sc2 O3
−1
3600
a
I Ib
CaCl2 –8 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3
−0.25 −0.25
3600 3600
Ja
CaCl2 –8 mol%Sc2 O3
−0.25
7200
E
Concentration of element i, Ci (mass%)* Al
Sc
Ca
Fe
88.3 96.9 97.4 83.3 (Bal.) 95.6 (Bal.) 98.9 (Bal.) 92.4 93.2 (Bal.) 85.7 (Bal.) 67.0 83.1 (Bal.) 89.4 (Bal.)
11.5 3.12 2.16 16.3 (1.9) 3.90 (0.1) 0.81 (0.2) 6.73 6.24 (0.4) 13.8 (1.6) 32.3 16.5 (0.6) 9.57 (1.8)
0.14 <0.01 0.21 0.28 (<0.01) 0.46 (<0.01) 0.08 (<0.01) 0.45 0.35 (<0.01) 0.39 (<0.01) 0.65 0.10 (<0.01) 0.39 (<0.01)
<0.01 <0.01 0.26 0.19 (0.01) <0.01 (0.03) 0.21 (0.03) 0.47 0.25 (0.03) 0.09 (0.02) <0.01 0.27 (0.02) 0.60 (0.05)
Values in parenthesis are determined by ICP–AES, and listed for reference. * The values exclude carbon and gaseous elements. a Surface of the sample was analyzed. b Sectioned sample was analyzed.
Sc2 O3 to the melt. It should be noted that the concentration of Ca is less than 0.65 mass%. Further, the results of the bulk analysis for the obtained alloy were determined by ICP–AES and are listed for reference in Table 3. These results show that the Ca content in the bulk Al–Sc alloy is less than 0.01 mass%. These values are substantially lower than those obtained by calciothermic reduction in the previous studies (11.1–22.5 mass%) [5,6]. It was difficult to evaluate the current efficiency because some of the samples could not be recovered after the electrolysis and also because segregation of Sc in the alloy occurred during cooling. The details of the electrolysis, such as the current efficiency, solubility of Sc2 O3 , and reduction mechanism of Sc2 O3 , are currently under investigation. From the experimental results demonstrated in this study, the feasibility of direct and (semi-)continuous production of Al–Sc alloy
from Sc2 O3 by the electrolysis in CaCl2 molten salt can be proposed. Fig. 8 shows an example of a continuous electrochemical cell based on the results of this study. In this process, CaCl2 –Sc2 O3 molten salt is used as the electrolyte, and carbon and aluminum are used as the anode and cathode, respectively. The electrolyzed Sc-rich alloy is transferred to another container; the liquid state of the product is promising for the development of a (semi-)continuous production process. The alloy is stirred to obtain homogeneity and then rapidly quenched to form a solid alloy in order to avoid segregation. The obtained alloy can be used as the mother alloy to structural Al–Sc alloys after diluting it with pure Al metal. This process enables a (semi-)continuous electrolysis only by supplementing the consumed Sc2 O3 and Al electrode. 5. Conclusions This study investigated a new production process for Al–Sc alloys by the electrolysis of CaCl2 –Sc2 O3 molten salt. The electrochemical reduction behavior of Sc2 O3 was investigated by cyclic voltammetry. The formation of an Al3 Sc phase in the Al matrix was confirmed by XRD, and the segregation of Sc at the surface of the alloy sample was confirmed by EPMA. The concentration of Sc in the alloy sample was 0.81–32.3 mass%. Furthermore, the concentration of Ca was less than 0.65 mass%, which is significantly smaller than the Ca concentration in the Al–Sc alloy produced by the calciothermic reduction of Sc2 O3 in the presence of Al. This study demonstrated that an Al–Sc alloy with a low Ca content can be produced directly from the oxide by the electrolysis in CaCl2 –Sc2 O3 molten salt. Acknowledgements
Fig. 8. Schematic illustration of the apparatus for (semi-)continuous production of Al–Sc alloy directly from Sc2 O3 .
The authors are grateful to Profs. M. Maeda, Y. Mitsuda, and K. Morita of The University of Tokyo, and Prof. T. Uda of Kyoto University for their generous support and valuable discussions during the course of this project. We are also thankful to Mr. S. Ito and Mr. K. Miura of Pacific Metals Co., Ltd., for their support and useful inputs during the study. Thanks are also due to Mr. I. Maebashi and Dr. O. Takeda for their technical assistance. The authors gratefully
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acknowledge Pacific Metals Co., Ltd., for their financial support and supply of samples.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical production of Al–Sc alloy in CaCl2 –Sc2 O3 molten salt Masanori Harata a,1 , Kouji Yasuda b,2 , Hiromasa Yakushiji c,3 , Toru H. Okabe b,∗ a
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan c Technical Research & Development Department, Production Division, Pacific Metals Co., Ltd., 5-2 Toyama Shinden, Kawaragi, Hachinohe, Aomori 031-8617, Japan b
a r t i c l e
i n f o
Article history: Received 27 April 2008 Received in revised form 19 June 2008 Accepted 19 June 2008 Available online 13 August 2008 Keywords: Scandium Al–Sc alloy Molten salt Electrolysis
a b s t r a c t In order to develop a new production process for Al–Sc alloys, a fundamental study on the electrolysis in CaCl2 –Sc2 O3 melts was conducted using a small-scale laboratory cell. Al–Sc alloys were electrochemically produced by cathodically polarizing an Al liquid electrode in CaCl2 –Sc2 O3 melts at 1173 K. Metallic-colored spherical samples were produced by the electrolysis and were analyzed by XRD, EPMA, XRF, and ICP–AES. The electrolyzed samples consisted of Al and Al3 Sc phases. The purity of the obtained Al–Sc alloys was greater than 99 mass%, and the calcium content was less than 0.65 mass%. This study demonstrates the feasibility of Al–Sc alloy production directly from Sc2 O3 by electrochemical methods. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, scandium (Sc) has been focused on as an alloying element for aluminum (Al) alloys. The addition of a small amount of Sc to Al alloys improves the strength, weldability, resistance to recrystallization, and corrosion resistance [1,2]. Owing to these attractive properties, the demand for Sc as an alloying element in Al alloys is expected to increase in the future. However, the application fields and the number of uses of Al–Sc alloys are presently limited because of the extremely high price of Sc metal, which is due to the difficulties in the reduction process and the small production volume of the starting material. Therefore, the development of a new and inexpensive production process for Sc metal or Al–Sc alloys is required. Currently, metallic Sc is commercially manufactured by the calciothermic reduction of its fluoride salt (ScF3 ) produced from its oxide (Sc2 O3 ); the major chemical reactions are expressed as fol-
∗ Corresponding author at: Institute of Industrial Science, The University of Tokyo, Fw-302, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6312, +81 3 5452 6314 (Okabe Lab.); fax: +81 3 5452 6313. E-mail addresses: [email protected] (M. Harata), [email protected] (K. Yasuda), h-yakushiji@pacific-metals.co.jp (H. Yakushiji), [email protected] (T.H. Okabe). 1 Presently at Toyota Industries Corporation, Japan. Tel.: +81 3 5452 6314; fax: +81 3 5452 6313. 2 Tel.: +81 3 5452 6820; fax: +81 3 5452 6313. 3 Tel.: +81 178 47 7231; fax: +81 178 22 7350. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.110
lows [3]: Sc2 O3 (s) + 6HF(g) → 2ScF3 (s) + 3H2 O(g) ScF3 (l) + 3/2Ca(l) → Sc(l) + 3/2CaF2 (l)
(at 973 K)
(1)
(at 1873 K)
(2)
In this process, the starting material, Sc2 O3 , is first converted to scandium fluoride (ScF3 ), because it is difficult to directly reduce the thermodynamically stable Sc2 O3 to metallic Sc even by using metallic calcium (Ca) as a reductant. The fluorination is carried out by treating Sc2 O3 with hydrogen fluoride (HF) gas at 973 K. The obtained ScF3 is then reduced to Sc metal by using the Ca reductant at 1873 K. While the utilization of fluoride metallurgy enables the production of Sc metal with low oxygen content, the high-temperature operation under highly a corrosive environment significantly increases the production cost of Sc metal. Furthermore, contamination from the reactor material and reductant is inevitable in the reduction at high temperature, particularly in the fluoride reactor system. With regard to the starting material of Sc metal, its price is expected to decrease. At present, the starting material is produced in the form of Sc2 O3 from the byproducts of uranium (U), tungsten (W), or tantalum (Ta) smelting [3]. The production volume of the byproduct Sc2 O3 is small, and this results in a very small production volume of Sc metal and alloys. This small scale of Sc2 O3 production gives rise to the extremely high price of Sc and its compounds. In recent years, it has been reported that Sc2 O3 can be also efficiently recovered in the nickel (Ni) smelting process, since this smelting process has been changing from a pyrometallurgical process to a hydrometallurgical process [4]. Once the effective recovery of Sc
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125
from the Ni smelting process is established, it is expected that a significantly large amount of Sc can be supplied to the market and this increase in production volume will lead to a reduction in production cost. Currently, the majority of Sc metal is consumed as the additives in Al alloys, and this application will expand if the cost of Sc metal is lowered. With this background, the direct production of Al–Sc alloys from Sc2 O3 has been investigated. In our previous studies [5,6], Al–Sc alloys were produced by the calciothermic reduction of Sc2 O3 , in which Al was used as the collector metal for Sc. It was demonstrated that Al–Sc alloys could be produced even at low temperatures (1273 K) directly from Sc2 O3 . However, excess Ca reductant remained in the alloy at above 0.7 mass%, and an additional process for removing Ca is required if low-Ca-content Al–Sc alloys are required. In this study, we aimed to produce an Al–Sc alloy with a low Ca content directly from the oxide feed by electrolysis in calcium chloride (CaCl2 )–Sc2 O3 molten salt. 2. Molten salt electrolysis Fig. 1 shows the concept employed in this study. Liquid Al and solid Sc2 O3 particles sink under molten CaCl2 due to the difference of the densities. Al–Sc alloys are produced by cathodically polarizing an Al liquid electrode at the bottom of a carbon crucible (Fig. 1(a)). The possible reaction scheme for Al–Sc formation depends on the solubility of Sc2 O3 ; however, there has been no report on its solubility in molten CaCl2 . (A) When Sc2 O3 dissolves into molten CaCl2 , an Al–Sc alloy is formed by the reduction of the Sc3+ ions in the melt. Cathode : Sc3+ + 3e− → Sc(l, in Al liq.) 2−
Anode : C(s) + xO
(1)
(in salt) → COx (g) + 2xe
−
(2)
(B) When Sc2 O3 is insoluble in molten CaCl2 , the Sc2 O3 powder added to the electrolyte sinks to the bottom of the crucible and comes into contact with the Al cathode. Then, the solid Sc2 O3 is electrochemically reduced on the surface of the Al liquid electrode, forming an Al–Sc alloy. This reaction is similar to the electrochemical reduction process investigated by Fray et al. [7–9]. Cathode : Sc2 O3 (s) + 6e− → 2Sc(l, in Al liq.) + 3O2−
(3)
Anode : C(s) + xO2− → COx (g) + 2xe−
(4)
Fig. 1. (a) Concept of production of Al–Sc alloys by molten salt electrolysis. (b) Possible mechanism of reduction of Sc2 O3 by electrolysis.
(C) Calciothermic reduction can also occur in the case where Sc2 O3 is insoluble. Ca metal is produced in the cathodic reaction when the potential of the cathode is sufficiently negative. The electrochemically produced Ca is soluble in molten CaCl2 [10,11], Table 1 Materials used in this study Materials
Form
Purity (%)
Supplier
Note
Sc2 O3 CaCl2
Powder Powder
99.9 up 95.0 up
Pacific Metals Co., Ltd. Kanto Chemicals, Inc.
13.7 m (average particle size) Vacuum dried at 423 K
Ni Mo Al Al C C
Rod Wire Shot Powder Rod Crucible
99.9 99.9 99.99 up 99.99 up 99.9 99.9
Shinkinzoku Industry Co., Ltd. Shinkinzoku Industry Co., Ltd. Koujundo Chemical Lab. Co., Ltd. Koujundo Chemical Lab Co Ltd Sogo Carbon Co., Ltd. Tokai Techno Service Co., Ltd.
3 mm 1 mm 2–5 mm 3 m diameter Graphite, 6 mm Graphite, 37 mm × 50 mm
Mullite Mullite Ethanol
Tube Tube Liquid
HB grade, 6 mm HB grade, 12 mm
99.5 up
Nikkato Corp. Nikkato Corp. Kanto Chemicals, Inc.
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and the Ca dissolved in the CaCl2 also acts as a reductant. Cathode : Ca2+ + 2e− → Ca(l) −
(5)
−
(6)
C(s) + xO2− → COx (g) + 2xe−
(7)
Anode : 2Cl → Cl2 (g) + 2e
The produced Ca metal chemically reduces the Sc2 O3 powder to produce Sc metal. This type of reduction process has been investigated by Oki et al. [12] and Ono et al. [13,14] to study the reduction of titanium oxide. When Al exists in the system, the Sc metal produced by the calciothermic reduction reacts to form an Al–Sc alloy according to the following reaction: Sc2 O3 (s) + Al(l) + 3Ca(l) → Al–Sc(l) + 3CaO(l)
(8)
In any of the abovementioned reaction schemes, when the cathode potential is adjusted to the optimum value which facilitates high Ca activity, an Al–Sc alloy is expected to be produced by the reduction of Sc2 O3 in the molten CaCl2 . 3. Experimental The materials used in this study are listed in Table 1, and a schematic illustration of the experimental apparatus used for the cyclic voltammetry measurements is shown in Fig. 2(a). CaCl2 powder (40 g, dried in a vacuum for several days and charged into a carbon crucible) was used as the electrolyte. The crucible was placed inside a stainless steel chamber, which was installed in an electric furnace and attached to a gas supply/vacuum system. The pressure in the chamber was monitored by using a digital pressure gauge (Cosmo Co., DP-310) and maintained at 1 atm. The salt was melted at 1173 K under an argon (Ar) atmosphere. CaCl2 –Sc2 O3 molten salt was synthesized by charging Sc2 O3 powder into the CaCl2 salt. As the working electrodes, a carbon (C) electrode was used in the anodic region, and molybdenum (Mo) and Al liquid electrodes were used in the cathodic region. The appearance and configuration of the Al liquid electrode is shown in Fig. 2(b). The Al liquid electrode was prepared by holding Al metal in a mullite tube, which was fixed to a smaller mullite tube. Electrical contact was provided through a Mo wire. A Ni rod was used as a quasi-reference electrode. While Mo does not reacts with either Ca or Sc, Al reacts with both Ca and Sc and forms several intermetallic compounds, as described in the phase diagrams [15]. The reduction behaviors of Sc and Ca were investigated by a comparison between the voltammograms for Mo and Al. After performing cyclic voltammetry, potentiostatic electrolysis and galvanostatic electrolysis were carried out. Fig. 3 shows a schematic illustration of the experimental apparatus for the molten salt electrolysis. CaCl2 –Sc2 O3 molten salt containing 1.37–8 mol%Sc2 O3 was used as the electrolyte. The Al metal placed at the bottom of the carbon crucible was used as the working electrode and was polarized at the cathodic potential, and a carbon rod was used as the counter electrode. Representative experimental conditions for the galvanostatic electrolysis are shown in Table 2. After the electrolysis, the samples in the crucible were gradually cooled down to room temperature in the electric furnace. The samples were recovered by leaching in water. The Al–Sc alloy samples obtained by the electrolysis were analyzed by X-ray diffraction (XRD; Rigaku Co., RINT 2500, Cu K␣), electron-probe microanalysis (EPMA; JEOL Ltd., JXA-8800RL), X-ray fluorescence spectrometry (XRF; JEOL Ltd., JSX–3210), and inductively coupled plasma-atomic emission spectrometry (ICP–AES).
Fig. 2. (a) Schematic illustration of the experimental apparatus for cyclic voltammetry and (b) appearance and configuration of the Al working electrode.
Table 2 Experimental conditions for molten salt electrolysis Exp #.
A B C D E F G H I J
Molten salt system
CaCl2 –1.37 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3
Mass of feed samples, wi (g)
Electrolysis
Sc2 O3
CaCl2
Al
Current, i (A)
Temperature, T (K)
Time, t (s)
0.69 1.02 1.02 1.02 2.09 2.09 2.09 2.09 4.36 4.36
40 40 40 40 40 40 40 40 40 40
2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66
−1 −0.25 −1 −1 −0.25 −0.5 −1 −1 −0.25 −0.25
1173 1173 1173 1173 1173 1173 1173 1173 1173 1173
1800 7200 1800 3600 3600 3600 1800 3600 3600 7200
Working electrode: aluminum. Counter electrode: carbon. Mass of molten salt: 40 g CaCl2 .
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Fig. 5. Cyclic voltammograms in pure CaCl2 molten salt and CaCl2 –2 mol%Sc2 O3 molten salt at 1173 K. (i) CaCl2 molten salt, WE: Al, CE: C, scan rate, ve = 20 mV/s, (ii) CaCl2 –2 mol%Sc2 O3 , WE: Al, CE: C, scan rate, ve = 20 mV/s.
4. Result and discussion Fig. 3. Schematic illustration of the experimental apparatus for molten salt electrolysis.
4.1. Cyclic voltammetry The electrochemical behavior in the molten salt was investigated using cyclic voltammetry. Here, the anodic and cathodic regions were investigated by carbon and Mo electrodes, respectively. In pure molten CaCl2 (Fig. 4(a)), Ca deposition and Cl2 evolution occur at −1.8 V and +1.4 V with respect to the Ni quasireference electrode, respectively. The decomposition voltage of
Fig. 4. Cyclic voltammograms in (a) pure CaCl2 molten salt and (b) CaCl2 –2mol%Sc2 O3 molten salt at 1173 K. (a) (i) WE: C, CE: Mo, scan rate, ve = 100 mV/s, (ii) WE: Mo, CE: C, scan rate, ve = 100 mV/s, (b) (i) WE: C, CE: Mo, scan rate, ve = 20 mV/s, (ii) WE: Mo, CE: C, scan rate, ve = 20 mV/s.
Fig. 6. (a) Appearance of the sample obtained by the potentiostatic electrolysis. (b) XRD pattern of the sample obtained by the potentiostatic electrolysis (temperature, T = 1173 K, Electrolysis time, t = 3600 s, potential vs. Ni, E = −0.9 V).
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Fig. 7. Photograph and EPMA elemental mapping of sectioned alloy sample obtained by the electrolysis. (a) Sectioned sample, (b) aluminum, (c) scandium and (d) calcium. (Exp. A, temperature, T = 1173 K, Electrolysis time, t = 1800 s, Current, i = −1.00 A).
molten CaCl2 calculated from the standard Gibbs energy of formation is 3.2 V at 1173 K [16], which agrees well with the voltammetric results. In the CaCl2 –2 mol%Sc2 O3 molten salt (Fig. 4(b)), Ca deposition and Cl2 evolution also occur at −1.8 V and +1.4 V, respectively. Further, the oxidation current for COx evolution is observed at +0.4 V [17,18], and the redox current related to the Sc species is observed at around −1.2 V. The electrode behavior for the Al liquid electrode was investigated in CaCl2 molten salt and CaCl2 –2 mol%Sc2 O3 molten salt. As shown in Fig. 5, the redox current is observed at around −1.4 V in CaCl2 molten salt, which is more positive compared with that for Mo by 0.4 V. This current corresponds to the formation and deformation of an Al–Ca alloy. In the CaCl2 –2 mol%Sc2 O3 molten salt, the redox current is observed around −0.7 V. A comparison of these voltammograms indicates that an Al–Sc alloy is produced at around −0.7 V. The anodic current observed at more positive than −0.3 V in the both voltammograms is possibly due to the anodic dissolution of Al metal. 4.2. Molten salt electrolysis In order to confirm that the reduction current at around −0.7 V corresponds to the formation of an Al–Sc alloy, potentiostatic electrolysis was carried out at −0.9 V for 1 h. Fig. 6 shows the
appearance and XRD pattern of a representative sample obtained by the potentiostatic electrolysis. The obtained metallic-colored spherical sample consisted of Al and Al3 Sc, confirming that an Al–Sc alloy is produced at this potential. The use of Al metal as a collector metal assisted the reduction of Sc2 O3 by lowering the activity of Sc through a formation of Al–Sc alloy; similar effect was previously reported in the direct and electrochemical reduction of solid oxides [19,20]. Based on the results from the cyclic voltammetry and potentiostatic electrolysis, galvanostatic electrolysis was also performed at the currents from −0.25 A to −1 A. In all the galvanostatic electrolysis experiments, the potential value was more negative than −0.9 V, which was sufficiently negative to produce Al–Sc alloys. In fact, the existence of an Al3 Sc phase in Al was confirmed by XRD. Fig. 7 shows the EPMA elemental mapping of a sectioned Al–Sc alloy sample. The segregation of Sc at the surface of the alloy sample is observed. This segregation is probably due to the slow cooling of the samples in the furnace. The maximum solubility of Sc in the Al–Sc liquid alloy is approximately 6 mass% at 1173 K and it equilibrates with Al3 Sc [15]. At room temperature, however, the solubility of Sc in the Al solid solution decreases down to less than 0.01 mass% [21,22]. The composition of the surface of the Al–Sc alloy sample was determined by XRF and is listed in Table 3. The concentration of Sc is 0.81–32.3 mass%, which basically increases with the addition of
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Table 3 Analytical results by XRF for the samples obtained by the electrolysis Exp. #
Molten salt system
Current, i (A)
Time, t (s)
Aa Ab Ba Ca
CaCl2 –1.37 mol%Sc2 O3 CaCl2 –1.37 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3 CaCl2 –2 mol%Sc2 O3
−1 −1 −0.25 −1
1800 1800 7200 1800
Da
CaCl2 –2 mol%Sc2 O3
−1
3600
a
CaCl2 –4 mol%Sc2 O3
−0.25
3600
Fa Ga
CaCl2 –4 mol%Sc2 O3 CaCl2 –4 mol%Sc2 O3
−0.5 −1
3600 1800
Ha
CaCl2 –4 mol%Sc2 O3
−1
3600
a
I Ib
CaCl2 –8 mol%Sc2 O3 CaCl2 –8 mol%Sc2 O3
−0.25 −0.25
3600 3600
Ja
CaCl2 –8 mol%Sc2 O3
−0.25
7200
E
Concentration of element i, Ci (mass%)* Al
Sc
Ca
Fe
88.3 96.9 97.4 83.3 (Bal.) 95.6 (Bal.) 98.9 (Bal.) 92.4 93.2 (Bal.) 85.7 (Bal.) 67.0 83.1 (Bal.) 89.4 (Bal.)
11.5 3.12 2.16 16.3 (1.9) 3.90 (0.1) 0.81 (0.2) 6.73 6.24 (0.4) 13.8 (1.6) 32.3 16.5 (0.6) 9.57 (1.8)
0.14 <0.01 0.21 0.28 (<0.01) 0.46 (<0.01) 0.08 (<0.01) 0.45 0.35 (<0.01) 0.39 (<0.01) 0.65 0.10 (<0.01) 0.39 (<0.01)
<0.01 <0.01 0.26 0.19 (0.01) <0.01 (0.03) 0.21 (0.03) 0.47 0.25 (0.03) 0.09 (0.02) <0.01 0.27 (0.02) 0.60 (0.05)
Values in parenthesis are determined by ICP–AES, and listed for reference. * The values exclude carbon and gaseous elements. a Surface of the sample was analyzed. b Sectioned sample was analyzed.
Sc2 O3 to the melt. It should be noted that the concentration of Ca is less than 0.65 mass%. Further, the results of the bulk analysis for the obtained alloy were determined by ICP–AES and are listed for reference in Table 3. These results show that the Ca content in the bulk Al–Sc alloy is less than 0.01 mass%. These values are substantially lower than those obtained by calciothermic reduction in the previous studies (11.1–22.5 mass%) [5,6]. It was difficult to evaluate the current efficiency because some of the samples could not be recovered after the electrolysis and also because segregation of Sc in the alloy occurred during cooling. The details of the electrolysis, such as the current efficiency, solubility of Sc2 O3 , and reduction mechanism of Sc2 O3 , are currently under investigation. From the experimental results demonstrated in this study, the feasibility of direct and (semi-)continuous production of Al–Sc alloy
from Sc2 O3 by the electrolysis in CaCl2 molten salt can be proposed. Fig. 8 shows an example of a continuous electrochemical cell based on the results of this study. In this process, CaCl2 –Sc2 O3 molten salt is used as the electrolyte, and carbon and aluminum are used as the anode and cathode, respectively. The electrolyzed Sc-rich alloy is transferred to another container; the liquid state of the product is promising for the development of a (semi-)continuous production process. The alloy is stirred to obtain homogeneity and then rapidly quenched to form a solid alloy in order to avoid segregation. The obtained alloy can be used as the mother alloy to structural Al–Sc alloys after diluting it with pure Al metal. This process enables a (semi-)continuous electrolysis only by supplementing the consumed Sc2 O3 and Al electrode. 5. Conclusions This study investigated a new production process for Al–Sc alloys by the electrolysis of CaCl2 –Sc2 O3 molten salt. The electrochemical reduction behavior of Sc2 O3 was investigated by cyclic voltammetry. The formation of an Al3 Sc phase in the Al matrix was confirmed by XRD, and the segregation of Sc at the surface of the alloy sample was confirmed by EPMA. The concentration of Sc in the alloy sample was 0.81–32.3 mass%. Furthermore, the concentration of Ca was less than 0.65 mass%, which is significantly smaller than the Ca concentration in the Al–Sc alloy produced by the calciothermic reduction of Sc2 O3 in the presence of Al. This study demonstrated that an Al–Sc alloy with a low Ca content can be produced directly from the oxide by the electrolysis in CaCl2 –Sc2 O3 molten salt. Acknowledgements
Fig. 8. Schematic illustration of the apparatus for (semi-)continuous production of Al–Sc alloy directly from Sc2 O3 .
The authors are grateful to Profs. M. Maeda, Y. Mitsuda, and K. Morita of The University of Tokyo, and Prof. T. Uda of Kyoto University for their generous support and valuable discussions during the course of this project. We are also thankful to Mr. S. Ito and Mr. K. Miura of Pacific Metals Co., Ltd., for their support and useful inputs during the study. Thanks are also due to Mr. I. Maebashi and Dr. O. Takeda for their technical assistance. The authors gratefully
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acknowledge Pacific Metals Co., Ltd., for their financial support and supply of samples.
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