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The dissolution mechanism of Sc2O3 in CaO–CaCl2 flux for the calciothermic production of Al–Sc alloy
Journal of Alloys and Compounds 658 (2016) 595e597
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Letter
The dissolution mechanism of Sc2O3 in CaOeCaCl2 flux for the calciothermic production of AleSc alloy a b s t r a c t Keywords: Scandium Solubility Calciothermic reduction Ionic state
The effect of oxygen partial pressure and basicity on the solubility of Sc2O3 in the CaOeCaCl2 flux is studied. As a result, the solubility of Sc2O3 only depends on basicity of the flux and the stable ionic state of which is ScO2:5 2 . The thermodynamic simulation on the alloy composition of calciothermic reduction is also studied and shows consistency with experimental results. © 2015 Published by Elsevier B.V.
1. Introduction
3. Results and discussion
Recently, recovery of scandium from lateritic nickel oxide is developed by several processes [1e3]. Amongst which, calciothermic reduction is particularly paid attention for recovery of various metals including titanium, hafnium, and rare earth metals [4e10]. To the best of our knowledge, however, studies on calciothermic reduction of scandium is restricted in qualitative confirmation of the possibility of the process. The purpose of this study is to quantitatively illuminate the mechanism of dissolution for the scandium oxide into the CaOeCaCl2 molten flux and to suggest the alternative reaction mechanism of calciothermic reduction.
General mechanism of dissolution of an element into a flux is categorized into oxidation and reductive refining as Eqs (1) and (2).
2. Experimental procedure Reagent grade powders of CaCO3, CaCl2, and Sc2O3 were used. CaCO3 was calcined at 1273 K for 6 h and the powder mixture was weighed and placed in Pt and carbon crucible (14F 12F 60 mm) depending on the oxygen partial pressure control. The crucible was placed in vertical super kanthal furnace at 1273 K under Ar, COeCO2, and CeCO atmosphere (all gases used in research purity, 99.9999%) to maintain pO2 of 1014e1021 atm. After 20 h, crucible was pulled out of the furnace and quenched by water (Pt crucible) and Ar gas (C crucible). The calciothermic reduction of Sc2O3 from molten CaOeCaCl2 flux to an AleSc alloy is induced by Ca vapor at 1273 K. Details of experimental procedure is described in previous study of Harata et al. [2]. Al2O3 crucible is applied to prevent the metal-crucible reaction. Reaction took place in stainless chamber for 24 h. Concentration of Ca and Sc in Al is analyzed by ICP-OES (Agilent OES-750, Santa Clara, California, USA). The sample was leached in 15 ml of aqua regia in glass container for 20 min and then diluted by 85 ml of distilled water. Ca and Sc standard solution (Sigma Aldrich, Saint Louis, Missouri, USA) was used for calibration. http://dx.doi.org/10.1016/j.jallcom.2015.10.211 0925-8388/© 2015 Published by Elsevier B.V.
M þ mO2 þ nO2 ¼ MO2n 2mþn M þ mO2 ¼ M 2m þ
m O 2 2
(1) (2)
Various works are dedicated to the transition between the two mechanisms occurs at particular values of oxygen partial pressure for sulfur, phosphorus, and boron [11e14]. To determine the dissolution mechanism of scandium into the CaOeCaCl2 flux, the effects of oxygen partial pressure and basicity on the solubility of Sc2O3 were investigated. The effect of oxygen partial pressure on the solubility is expressed in logarithmic scale in Fig. 1. The solubility of Sc2O3 lied between 2.89 105 and 4.0 104 in mole fraction. It is notable that the solubility of Sc2O3 was independent of oxygen partial pressure between 1014 and 1021. The result corresponds with the high affinity of scandium with oxygen leaving the oxidation of scandium occurs simultaneously, as in Eq. (3).
3 Sc þ O2 ¼ ScO1:5 ; 4
ln K1273K ¼ 218:035
(3)
Hence the dissolution mechanism of Sc2O3 to the stable ionic state of Sc2O3 was of our interest and further discussion lies on the determination of coefficient of oxidation refining reaction formula, as in Eq. (4) by investigating the effect of basicity on the solubility of Sc2O3.
ScO1:5 þ nO2 ¼ ScO2n ð1:5þnÞ
(4)
The effect of basicity on the solubility of scandium oxide is expressed in Fig. 2. The activity of CaO is taken as the measurement of oxygen ion activity i.e. basicity, and calculated from the result of
596
Letter / Journal of Alloys and Compounds 658 (2016) 595e597
The linear relationship from Eq. (6) suggests that the logarithm of the activity coefficient of ScO2:5 2 also has a in a linear relationship with logarithmic basicity. The calculated activity coefficient of ScO2:5 2 relative to solid CaO in flux is expressed by the dashed line in Fig. 2. Harata et al. [3] once suggested calciothermic reduction by a Ca reductant as shown in Eq. (7).
3CaðgÞ þ Sc2 O3 ðfluxÞ ¼ 2ScðalloyÞ þ 3CaOðfluxÞ
(7) 2
Noting that the stable ionic state of Sc2O3 is ScO2:5 in CaOeCaCl2 flux, Eq. (7) may rewritten as an ionic formula, as Eq. (8).
3 2 ScO2 þ O2 ; 2:5 ¼ Sc þ O 4
K¼
aSc aO2 pO2 3=4 aScO2
(8)
1:5
Wagner [16] once discussed in his pioneering work on basicity of the flux and slag that the activity of individual ions may not be evaluated. Hence, Wagner suggested to formulate the molecular equation, leading the activity of basic oxide be a measurement of basicity. Since oxygen partial pressure between AleSceCa melt and CaOeCaCl2 is determined by CaeCaO equilibrium, pO2 , aO2 , and aScO2 in Eq. (8) is replaced as Eq. (10), noting that CaeCaO equi1:5 librium is determined as Eq. (9).
1 Ca þ O2 ¼ CaO 2 Fig. 1. The effect of oxygen partial pressure on the solubility of Sc2O3 in CaOeCaCl2 flux.
activity measurements by Suzuki et al. [15] with the aids of FactSage™ 6.4. software, assuming the regular solution model [16]. The logarithm of activity of CaO appears to have a positive linear relationship with that of ScOð1:5þnÞ 2n solubility. The method of least squares is applied to give the relationship as following:
log XScO2n
ð1:5þnÞ
¼ 0:95 log aCaO 3:57
(5)
Given some extent of errors, it is plausible to take the slope of the plot as unity which gives the coefficient n in Eq. (4). Therefore,
ScO1:5 þ O2 ¼ ScO2 2:5
Ca þ CaScO2:5 ¼ Sc þ
(9) 5 CaO; 2
5
K0 ¼
aSc aCaO 2 aCa aCaScO2:5
(10)
The Henrian activity of Ca and Sc was once published by Cacciamani et al. [17] and Sommer [18], and the activity of CaO is calculated from Eq. (9). Since the flux was in equilibrium with solid Sc2O3, the activity of CaScO2.5 was assumed to be unity relative to solid Sc2O3. From the calculation of equilibrium constant in Eq. (10), a thermodynamic simulation on the Ca and Sc contents in the alloy was performed and is shown in Fig. 3. The calciothermic experimental results are also described by solid circles. The results follow the relationship suggested in Eq. (10), albeit deviates from
(6)
Fig. 2. The effect of basicity on the solubility and the activity coefficient of Sc2O3 in CaOeCaCl2 flux.
Fig. 3. The effect of basicity and oxygen partial pressure on the calcium/scandium ratio in AleSceCa alloy.
Letter / Journal of Alloys and Compounds 658 (2016) 595e597
the simulation in low-calcium compositions: The deviation originates from calcium loss from alloy to flux, due to difference in CaO activity in the flux. Although the thermodynamic calculation seems plausible, further study for confirmation of reaction mechanism of the calciothermic reduction of Sc2O3, noting that the Ca exists in various valencies in CaOeCaCl2 flux [19e21]. 4. Conclusion The dissolution mechanism of Sc2O3 in CaOeCaCl2 for calciothermic production of AleSc alloy was suggested. Sc2O3 exists in CaOeCaCl2 flux in the form of a Sc2 O5 4 complex ion and the solubility of which depends only on the basicity of the flux. References [1] M. Harata, K. Yasuda, H. Yakushiji, T.H. Okabe, J. Alloys Compd. 474 (2009) 124e130. [2] M. Harata, T. Nakamura, H. Yakushiji, T.H. Okabe, Miner. Process. Extr. Metall. Trans. Inst. Min. Metall. C 117 (2008) 95e99. [3] N. Iyatomi, M. Nanjo, Bull. Res. Inst. Miner. Dress. Metall. 45 (1989) 66e76. [4] T.H. Okabe, T. Oda, Y. Mitsuda, J. Alloys Compd. 364 (2004) 156e163. [5] A.M. Abdelkader, A. Daher, J. Alloys Compd. 469 (2009) 571e575. [6] R.O. Suzuki, M. Aizawa, K. Ono, J. Alloys Compd. 288 (1999) 173e182. [7] R.A. Sharma, R.N. Seefurth, J. Electrochem. Soc. 135 (1988) 66e71.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
597
M. Ohtsuka, D.-Y. Kim, K. Itagaki, J. Alloys Compd. 230 (1995) 46e52. R.O. Suzuki, K. Teranuma, K. Ono, Metall. Trans. B 34 (2003) 288e295. Z. Li, K. Yasuda, K. Itagaki, J. Alloy Compd. 193 (1993) 26e28. C.J.B. Fincham, F.D. Richadson, Proc. R. Soc. A 223 (1954) 40e62. H. Momokawa, N. Sano, Metall. Trans. B 13 (1982) 643e644. S. Tabuchi, N. Sano, Metall. Trans. B 15 (1984) 351e356. E.J. Jung, B.M. Moon, S.H. Seok, D.J. Min, Energy 66 (2014) 35e40. R.O. Suzuki, M. Aizawa, K. Ono, J. Alloys Compd. 288 (1999) 173e182. C. Wagner, Met. Trans. B 6B (1975) 405e409. G. Cacciamani, P. Riani, G. Borzone, N. Parodi, A. Saccone, R. Ferro, A. Pisch, R. Schmid-Fetzer, Intermetallics 7 (1999) 101e108. F. Sommer, Z. Metall. 74 (1983) 100e105. H. Fischbach, Steel Res. 7 (1985) 365e368. V. Dosaj, C. Aksaranan, D.R. Morris, J. Chem. Soc. Faraday Trans. 1 71 (1975) 1083e1098. S. Shaw, R. Watson, ECS Trans. 16 (2009) 3010e3308.
Yong-Uk Han, Jae Woong Koh, Byong Pil Lee, Dong Joon Min* Department of Materials Science and Engineering, Yonsei University, South Korea *
Corresponding author. E-mail address: [email protected] (D.J. Min). 15 September 2015 Available online 28 October 2015
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Letter
The dissolution mechanism of Sc2O3 in CaOeCaCl2 flux for the calciothermic production of AleSc alloy a b s t r a c t Keywords: Scandium Solubility Calciothermic reduction Ionic state
The effect of oxygen partial pressure and basicity on the solubility of Sc2O3 in the CaOeCaCl2 flux is studied. As a result, the solubility of Sc2O3 only depends on basicity of the flux and the stable ionic state of which is ScO2:5 2 . The thermodynamic simulation on the alloy composition of calciothermic reduction is also studied and shows consistency with experimental results. © 2015 Published by Elsevier B.V.
1. Introduction
3. Results and discussion
Recently, recovery of scandium from lateritic nickel oxide is developed by several processes [1e3]. Amongst which, calciothermic reduction is particularly paid attention for recovery of various metals including titanium, hafnium, and rare earth metals [4e10]. To the best of our knowledge, however, studies on calciothermic reduction of scandium is restricted in qualitative confirmation of the possibility of the process. The purpose of this study is to quantitatively illuminate the mechanism of dissolution for the scandium oxide into the CaOeCaCl2 molten flux and to suggest the alternative reaction mechanism of calciothermic reduction.
General mechanism of dissolution of an element into a flux is categorized into oxidation and reductive refining as Eqs (1) and (2).
2. Experimental procedure Reagent grade powders of CaCO3, CaCl2, and Sc2O3 were used. CaCO3 was calcined at 1273 K for 6 h and the powder mixture was weighed and placed in Pt and carbon crucible (14F 12F 60 mm) depending on the oxygen partial pressure control. The crucible was placed in vertical super kanthal furnace at 1273 K under Ar, COeCO2, and CeCO atmosphere (all gases used in research purity, 99.9999%) to maintain pO2 of 1014e1021 atm. After 20 h, crucible was pulled out of the furnace and quenched by water (Pt crucible) and Ar gas (C crucible). The calciothermic reduction of Sc2O3 from molten CaOeCaCl2 flux to an AleSc alloy is induced by Ca vapor at 1273 K. Details of experimental procedure is described in previous study of Harata et al. [2]. Al2O3 crucible is applied to prevent the metal-crucible reaction. Reaction took place in stainless chamber for 24 h. Concentration of Ca and Sc in Al is analyzed by ICP-OES (Agilent OES-750, Santa Clara, California, USA). The sample was leached in 15 ml of aqua regia in glass container for 20 min and then diluted by 85 ml of distilled water. Ca and Sc standard solution (Sigma Aldrich, Saint Louis, Missouri, USA) was used for calibration. http://dx.doi.org/10.1016/j.jallcom.2015.10.211 0925-8388/© 2015 Published by Elsevier B.V.
M þ mO2 þ nO2 ¼ MO2n 2mþn M þ mO2 ¼ M 2m þ
m O 2 2
(1) (2)
Various works are dedicated to the transition between the two mechanisms occurs at particular values of oxygen partial pressure for sulfur, phosphorus, and boron [11e14]. To determine the dissolution mechanism of scandium into the CaOeCaCl2 flux, the effects of oxygen partial pressure and basicity on the solubility of Sc2O3 were investigated. The effect of oxygen partial pressure on the solubility is expressed in logarithmic scale in Fig. 1. The solubility of Sc2O3 lied between 2.89 105 and 4.0 104 in mole fraction. It is notable that the solubility of Sc2O3 was independent of oxygen partial pressure between 1014 and 1021. The result corresponds with the high affinity of scandium with oxygen leaving the oxidation of scandium occurs simultaneously, as in Eq. (3).
3 Sc þ O2 ¼ ScO1:5 ; 4
ln K1273K ¼ 218:035
(3)
Hence the dissolution mechanism of Sc2O3 to the stable ionic state of Sc2O3 was of our interest and further discussion lies on the determination of coefficient of oxidation refining reaction formula, as in Eq. (4) by investigating the effect of basicity on the solubility of Sc2O3.
ScO1:5 þ nO2 ¼ ScO2n ð1:5þnÞ
(4)
The effect of basicity on the solubility of scandium oxide is expressed in Fig. 2. The activity of CaO is taken as the measurement of oxygen ion activity i.e. basicity, and calculated from the result of
596
Letter / Journal of Alloys and Compounds 658 (2016) 595e597
The linear relationship from Eq. (6) suggests that the logarithm of the activity coefficient of ScO2:5 2 also has a in a linear relationship with logarithmic basicity. The calculated activity coefficient of ScO2:5 2 relative to solid CaO in flux is expressed by the dashed line in Fig. 2. Harata et al. [3] once suggested calciothermic reduction by a Ca reductant as shown in Eq. (7).
3CaðgÞ þ Sc2 O3 ðfluxÞ ¼ 2ScðalloyÞ þ 3CaOðfluxÞ
(7) 2
Noting that the stable ionic state of Sc2O3 is ScO2:5 in CaOeCaCl2 flux, Eq. (7) may rewritten as an ionic formula, as Eq. (8).
3 2 ScO2 þ O2 ; 2:5 ¼ Sc þ O 4
K¼
aSc aO2 pO2 3=4 aScO2
(8)
1:5
Wagner [16] once discussed in his pioneering work on basicity of the flux and slag that the activity of individual ions may not be evaluated. Hence, Wagner suggested to formulate the molecular equation, leading the activity of basic oxide be a measurement of basicity. Since oxygen partial pressure between AleSceCa melt and CaOeCaCl2 is determined by CaeCaO equilibrium, pO2 , aO2 , and aScO2 in Eq. (8) is replaced as Eq. (10), noting that CaeCaO equi1:5 librium is determined as Eq. (9).
1 Ca þ O2 ¼ CaO 2 Fig. 1. The effect of oxygen partial pressure on the solubility of Sc2O3 in CaOeCaCl2 flux.
activity measurements by Suzuki et al. [15] with the aids of FactSage™ 6.4. software, assuming the regular solution model [16]. The logarithm of activity of CaO appears to have a positive linear relationship with that of ScOð1:5þnÞ 2n solubility. The method of least squares is applied to give the relationship as following:
log XScO2n
ð1:5þnÞ
¼ 0:95 log aCaO 3:57
(5)
Given some extent of errors, it is plausible to take the slope of the plot as unity which gives the coefficient n in Eq. (4). Therefore,
ScO1:5 þ O2 ¼ ScO2 2:5
Ca þ CaScO2:5 ¼ Sc þ
(9) 5 CaO; 2
5
K0 ¼
aSc aCaO 2 aCa aCaScO2:5
(10)
The Henrian activity of Ca and Sc was once published by Cacciamani et al. [17] and Sommer [18], and the activity of CaO is calculated from Eq. (9). Since the flux was in equilibrium with solid Sc2O3, the activity of CaScO2.5 was assumed to be unity relative to solid Sc2O3. From the calculation of equilibrium constant in Eq. (10), a thermodynamic simulation on the Ca and Sc contents in the alloy was performed and is shown in Fig. 3. The calciothermic experimental results are also described by solid circles. The results follow the relationship suggested in Eq. (10), albeit deviates from
(6)
Fig. 2. The effect of basicity on the solubility and the activity coefficient of Sc2O3 in CaOeCaCl2 flux.
Fig. 3. The effect of basicity and oxygen partial pressure on the calcium/scandium ratio in AleSceCa alloy.
Letter / Journal of Alloys and Compounds 658 (2016) 595e597
the simulation in low-calcium compositions: The deviation originates from calcium loss from alloy to flux, due to difference in CaO activity in the flux. Although the thermodynamic calculation seems plausible, further study for confirmation of reaction mechanism of the calciothermic reduction of Sc2O3, noting that the Ca exists in various valencies in CaOeCaCl2 flux [19e21]. 4. Conclusion The dissolution mechanism of Sc2O3 in CaOeCaCl2 for calciothermic production of AleSc alloy was suggested. Sc2O3 exists in CaOeCaCl2 flux in the form of a Sc2 O5 4 complex ion and the solubility of which depends only on the basicity of the flux. References [1] M. Harata, K. Yasuda, H. Yakushiji, T.H. Okabe, J. Alloys Compd. 474 (2009) 124e130. [2] M. Harata, T. Nakamura, H. Yakushiji, T.H. Okabe, Miner. Process. Extr. Metall. Trans. Inst. Min. Metall. C 117 (2008) 95e99. [3] N. Iyatomi, M. Nanjo, Bull. Res. Inst. Miner. Dress. Metall. 45 (1989) 66e76. [4] T.H. Okabe, T. Oda, Y. Mitsuda, J. Alloys Compd. 364 (2004) 156e163. [5] A.M. Abdelkader, A. Daher, J. Alloys Compd. 469 (2009) 571e575. [6] R.O. Suzuki, M. Aizawa, K. Ono, J. Alloys Compd. 288 (1999) 173e182. [7] R.A. Sharma, R.N. Seefurth, J. Electrochem. Soc. 135 (1988) 66e71.
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
597
M. Ohtsuka, D.-Y. Kim, K. Itagaki, J. Alloys Compd. 230 (1995) 46e52. R.O. Suzuki, K. Teranuma, K. Ono, Metall. Trans. B 34 (2003) 288e295. Z. Li, K. Yasuda, K. Itagaki, J. Alloy Compd. 193 (1993) 26e28. C.J.B. Fincham, F.D. Richadson, Proc. R. Soc. A 223 (1954) 40e62. H. Momokawa, N. Sano, Metall. Trans. B 13 (1982) 643e644. S. Tabuchi, N. Sano, Metall. Trans. B 15 (1984) 351e356. E.J. Jung, B.M. Moon, S.H. Seok, D.J. Min, Energy 66 (2014) 35e40. R.O. Suzuki, M. Aizawa, K. Ono, J. Alloys Compd. 288 (1999) 173e182. C. Wagner, Met. Trans. B 6B (1975) 405e409. G. Cacciamani, P. Riani, G. Borzone, N. Parodi, A. Saccone, R. Ferro, A. Pisch, R. Schmid-Fetzer, Intermetallics 7 (1999) 101e108. F. Sommer, Z. Metall. 74 (1983) 100e105. H. Fischbach, Steel Res. 7 (1985) 365e368. V. Dosaj, C. Aksaranan, D.R. Morris, J. Chem. Soc. Faraday Trans. 1 71 (1975) 1083e1098. S. Shaw, R. Watson, ECS Trans. 16 (2009) 3010e3308.
Yong-Uk Han, Jae Woong Koh, Byong Pil Lee, Dong Joon Min* Department of Materials Science and Engineering, Yonsei University, South Korea *
Corresponding author. E-mail address: [email protected] (D.J. Min). 15 September 2015 Available online 28 October 2015