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Effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt
Journal of Crystal Growth 405 (2014) 23–28
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt Wenzhou Yu a,b, Wenhui Ma a,b,n, Guoqiang Lv a, Haiyang Xue a, Shaoyuan Li a,b, Yongnian Dai a,b a
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2014 Received in revised form 19 July 2014 Accepted 21 July 2014 Communicated by P. Rudolph Available online 1 August 2014
The effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt during the process of electromagnetic separation was investigated. It is shown that the enrichment of primary silicon in Al–Si melt strongly depends on the melt flowing and viscosity gradient. The efficient enrichment of primary silicon was achieved by implementing a high current intensity, which induced a high intense melt flowing. Also, the remaining primary Si in Al–Si alloy could be precipitated by gradually decreasing the current intensity. Additionally, the inductively coupled plasma mass spectrometry (ICP-MS) results show that Si purification is attributed with the enrichment of primary silicon. In this work, the impurity content in primary silicon is 43.3 ppmw, which is much smaller than 777.6 ppmw in metallurgical silicon. Therefore, a potential low-cost technology would be provided for the Si purification. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Magnetic fields A1. Purification A1. Stirring B1. Alloys B2. Semiconducting silicon
1. Introduction The massive and low-cost solar grade silicon (SOG-Si) feedstock is essential for the widespread use of solar cells. Normally, the photovoltaic (PV) industry is dependent on the high-cost semiconductor-grade silicon (SEG-Si) that is produced by the Siemens technology. To reduce the cost of raw materials, some low-cost technologies for SOG-Si production should be proposed. Metallurgical technologies [1–7] are considered as a candidate to produce low-cost SOG-Si by removing the impurities from the metallurgical Si (MG-Si). However, Si has to repeatedly be in the molten stage in each step to ensure the impurities can be efficiently removed, which results in high energy consumption. Recently, a new method called “solvent refining” has been proposed for further cost reduction of the PV industry as the temperature of melting can be significantly reduced. Yoshikawa and Morita [8–11] and Gu et al. [12] proposed an Si–Al solvent to refine Si by using the enhanced segregation tendency of impurities between solid silicon and the Si–Al melt. Zhao et al. [13] and Ma et al. [14,15] reported that B could be efficiently removed from silicon using Sn–Si system. Additionally, Cu–Si [16] and
Zn–Si [13] solvent refining methods were also investigated for Si purification. Basically, for the low-cost SOG-Si production, the purification process of MG-Si should obey the following principles: (1) the Table 1 The impurity content in MG-Si (ppmw). Impurity
Fe 379
Ar in
Ti 76.8
Ca 189
B 27.9
P 104.9
Ar out Al-Si melt Induction coil
Pulling system n
Corresponding author. E-mail address: [email protected] (W. Ma).
http://dx.doi.org/10.1016/j.jcrysgro.2014.07.035 0022-0248/& 2014 Elsevier B.V. All rights reserved.
Fig. 1. The schematic diagram of the experiment.
Total 777.6
24
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 2. The cross-section of solidified samples: (a) without electromagnetic stirring; (b) electromagnetic separation (12 A); (c) electromagnetic separation (15 A); and (d) electromagnetic separation (18 A).
melting point of the melt should be lower than that of Si; (2) the process should efficiently remove most impurities simultaneously; and (3) it should be environment friendly which ensures that all the final products are utilized rationally as much as possible. For these reasons, Si purification in Al–Si melt is an attractive and promising technology for industrial production. During the solidification of hypereutectic Al–Si melt, Si crystals initially precipitate from the supersaturated melt with decreasing temperature, and the impurities are enriched in the melt because of the segregation behavior. Generally, acid leaching is always used to collect the Si crystals by removing the attached Al–Si alloy. However, this can result in considerable loss of Al and Si which is
unfavorable for the cost reduction and being environmentfriendly. Therefore, an efficient separation of Si crystals from Al– Si melt before acid leaching is very important. To improve the enrichment of silicon crystals during the solidification of the Al–Si melt, various magnetic fields such as the fixed alternating magnetic field at induction heating [8,17,18], the high gradient magnetic field [19] and the rotating magnetic field [20] have tried to control the migration of primary silicon crystals. In this paper, the migration characteristics of silicon crystals under induction heating have been investigated. Furthermore, the effect of electromagnetic stirring on the enrichment of silicon was also discussed thoroughly.
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 3. The simulation diagram about the movement of primary silicon in the Al– Si melt.
25
of a coil zone of a 60 kW high frequency induction furnace, then were heated at 1323 K (the current intensity is 32 A) for 1 h until completely melted. After melting, the alloys were pulled upward by a pulling system. The pulling rate was controlled at 7 μm/s, the temperature gradient was 35–40 K/cm and the pulling distance was 10 cm from the initial place. Meanwhile, the current intensity was controlled in the range of 12–18 A during the process of pulling up, which can influence the electromagnetic stirring. During the pulling upward process, the alloys solidified continuously from top to bottom due to natural cooling. The Ar gas was poured through the quartz tube to prevent the melt from being oxidized. After solidification, the samples were cut in the longitudinal direction to discuss the enrichment of primary silicon crystals. The surface of the samples was ground by SiC paper for metallographic observation. Macro- and micro-structures of solidified samples were respectively observed by SONY digital camera and Olympus PME3 light optical microscope (LOM) along with a KAPPA image analyzer. The distribution of impurities in the Al–Si alloy was characterized by a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS). The Si enrichment region was cut from the sample and then measured by atomic absorption spectroscopy (AAS) for the Si content. To discuss the effect of purification, the enriched primary silicon was leached at first by aqua regia (343 K, V(HCl):V(HNO3):V(H2O) ¼3:1:40, 200 mL) for 2 h with a magnetic stirrer, then was thoroughly rinsed with deionized water until the solution was neutral. The contents of the main impurities in primary Si were determined by ICP-MS.
3. Results and discussion
Fig. 4. Calculated migration velocity of primary phase VP vs viscosity μ.
Fig. 5. Macro- and micro-structures of solidified sample with an improved electromagnetic separation process (18–12 A).
2. Experimental The Al–45 wt% Si alloys were prepared by a mixture of 36 g MG-Si powders and 44 g pure Al powders (99.99 wt%) in a graphite crucible, and then were melted in the electrical heatresistant furnace. The contents of the main impurities in MG-Si are listed in Table 1, which were determined by inductively coupled plasma mass spectrometry (ICP-MS). Fig. 1 shows the schematic diagram of the electromagnetic separation experiment. The alloys were placed first in the middle
3.1. Effect of electromagnetic stirring on the enrichment of primary silicon Fig. 2 shows the cross-section of solidified samples. In the case of the sample without electromagnetic stirring, silicon crystals distribute evenly in the alloy, as shown in Fig. 2(a). The reason is that the density of solid primary silicon and the Al–Si melt has no significant difference [21]; hence the primary silicon cannot be enriched under the gravity force. As shown in Fig. 2(b–d), the separation interface between the Si enrichment region and Al–Si alloy can be clearly seen in the samples. The process of electromagnetic separation should be associated with the fact shown in Fig. 3(a) where melt flowing is induced in the Al–Si melt due to the electromagnetic field [8]; thus the solidified primary silicon crystals are carried downward or upward in the melt. Meanwhile, an axial temperature gradient appears by pulling the sample up from the hot zone, which results in an axial viscosity gradient of the melt. When the primary silicon crystals are carried to the cold zone of which the viscosity is high, it will agglomerate at this zone. The similar description of the separation process has been exhibited in Ref. [8], however, in which the effect of the viscosity gradient was absent. This may be the extensible explanation of the Si separation theory based on Ref. [8]. Therefore, the electromagnetic stirring together with the viscosity gradient plays a critical role in primary Si enrichment. Furthermore, with the increase of current intensity, the position of the separation interface transfers gradually upward in the solidified samples, as shown in Fig. 2(b–d). Since the migration of the primary silicon crystal is attributed to electromagnetic stirring, it is noted that the position of the separation interface is also related to the melt flowing. According to the theoretical expression of Leenov et al. [22] about the electromagnetic (EM) force acting on a single sphere
26
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 6. (a) SEM photographs of the hypereutectic Al–Si alloy, and (b, c, d and e) EDS analysis for the elements C, O, Ca and Fe, respectively.
particle in a conductive liquid, the EM force is expressed as F ¼J B
ð1Þ
where F is the EM force (N), J is the induced current intensity (A) and B is the magnetic flux density vector (T). In the experiment, the effect of the electromagnetic force on the migration velocity is so great that the gravity of the primary phase can be neglected. The primary phase will migrate with the migration velocity [23]: 2
VP ¼
dp F
24μ
ð2Þ
where μ is the movement viscosity of the melt and dp is the particle diameter.
The electromagnetic force F is in the range of 0–9.0 106 N/m3 (J ¼0–3.75 106 A/m2 at B ¼0–2.4 T). For the primary silicon, the average particle diameter dp is approximately 20 μm. Under such conditions, the calculated value of μ as a function of VP is shown in Fig. 4. It is seen that the migration velocity of the primary phase is inversely proportion to the movement viscosity of the melt. Additionally, the migration velocity of the primary phase increases with increasing the electromagnetic force. Therefore, it is reasonable that the higher current intensity would result in stronger intense melt flowing, and the solidified silicon crystals can be carried more easily to the Si enrichment region. It is concluded that a high current intensity is beneficial to the agglomeration of the Si crystals. However, a great number of remaining primary
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
27
Fig. 7. Microstructures of Si enrichment region under the different current intensities: (a) 12 A; (b) 15 A; (c) 18 A; and (d) 18–12 A.
silicon crystals failed to be pushed to the enrichment region, such as region B and region C in Fig. 2(c and d). From region A in Fig. 2(b), the structure is an Al–Si eutectic structure without any remaining primary silicon crystals in this region. Region B exhibits a hypereutectic structure with some polygonal primary silicon crystals. For region C, there seems to be more remaining primary silicon crystals than that in region B, which indicates that the remaining silicon crystals are related to the current intensity. Obviously, higher current intensity will increase the temperature of the melt. According to the Al–Si binary diagram [24], the primary silicon solidified continuously along with the decreasing temperature. This implies that the high temperature suppresses the solidification of Si crystals from the Al–Si melt. After the current is missing, the subsequent solidified primary silicon cannot be pushed to the enrichment region because of the absence of the electromagnetic stirring, which results in the remainder of primary silicon crystals. Therefore, for an efficient enrichment of primary silicon crystals, the relation between the current intensity and the temperature should be known. Fig. 5 shows the macro- and micro-structures of solidified sample with an improved electromagnetic process (18–12 A). It is revealed that the Al–Si alloy shows a perfect eutectic structure without any remaining primary silicon crystals in this region by gradually decreasing the current intensity from 18 A to 12 A. Meanwhile, the separation interface is very clear. This can be concluded that the high current intensity is beneficial to the enrichment of primary silicon crystals because of the high intense melt flowing, and the low current intensity in favor of the elimination of remaining primary Si in Al–Si alloy owing to the low temperature. It is proved that an enhanced agglomeration of primary silicon crystals in Al–Si melt can be achieved by controlling the current intensity. This provided a highly effective approach for Si separation in Al–Si alloy.
3.2. Si purification by the enrichment of Si crystals Some research works[8,25] have studied the removal of impurities thermodynamically during the solidification of silicon from the Al–Si melt. It is estimated that the impurities can be concentrated in the Al–Si melt by the solid/liquid segregation. However, few references have confirmed it by characterization. Fig. 6 shows the distribution of impurities in hypereutectic Al–Si alloy. From Fig. 6, it can be seen that the impurities such as C, O, Fe and Ca tend to enrich in the Al–Si alloy and the grain boundaries between the primary silicon and Al–Si alloy. On the other hand, the distribution of impurities such as B, P and Ti is difficult to be found in this system due to the limited low content. In addition, few impurities have been observed in primary silicon. It is clearly proved that the impurities can be removed from the primary silicon during the crystal growth. For the purpose of Si purification in Al–Si system, the primary silicon crystals should be enriched to reduce the attachment of Al– Si alloy as much as possible. Fig. 7 shows the microstructures of the Si enrichment region under different current intensities. It can be found that the Al attachment decreases with the increasing current intensity, which indicates that the more the intense melt flowing, the beneficial the agglomeration of Si crystals is. In this case, the Si crystals may obtain more chance to collide with each other and finally agglomerate to become Si bulk, which automatically decreases the Al attachment in the Si enrichment region. Fig. 8 shows the Si content in the Si enrichment region under different current intensities. It is clearly seen that the Si content increases by the increasing current intensity, and for the improved electromagnetic process (18–12 A), the Si content in the enrichment region is obviously higher than that of the others. The contents of main impurities in the Si enrichment region are shown in Table 2. The results reveal that the contents of main impurities
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W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
43.3 ppmw, which is obviously improved compared with the 777.6 ppmw in metallurgical silicon.
Acknowledgment The authors are grateful for financial support from the National Natural Science Foundation of China (u1137601). References
Fig. 8. Silicon content in enrichment region under the different current intensities.
Table 2 Impurity contents of Si enrichment region after acid leaching (ppmw). Current intensity (A)
Fe
Ca
Ti
B
P
Total
12 15 18 18–12
34.1 20.2 13.3 7.8
10.8 8.6 7.4 5.1
19.8 11.3 14.5 11.3
13.2 12.4 10.8 8.7
10.2 10.3 10.9 10.4
88.1 62.8 56.9 43.3
decrease with increasing current intensity. For the electromagnetic process (18-12 A), the impurity content in primary silicon is 43.3 ppmw, which is obviously improved compared with the 777.6 ppmw in metallurgical silicon. Therefore, it is concluded that the Si purification is attributed with the enrichment of primary silicon. This provides a low-cost technology which will be a potential route for the Si purification. Finally, for the demand of SOG-Si, the purity of Si must be deeply improved. The primary silicon should be enriched more by controlling the current intensity better, and the acid leaching should be carried out more effective to reduce the attachment of Al–Si alloy in future. Of course, if needed, the metallurgical method such as slag refining, vacuum distillation etc. could be combined to remove the impurities. 4. Conclusions 1. The electromagnetic stirring plays an important role in the enrichment of primary silicon crystals from the Al–Si melt during the process of electromagnetic separation. The high current intensity is beneficial to the agglomeration of primary silicon crystals and the low current intensity in favor of the elimination of remaining primary Si in Al–Si melt. 2. An improved electromagnetic separation process (18–12 A) consisting of high current intensity step and low current intensity step was proved to obtain an enhanced enrichment of primary silicon crystals. 3. Si purification is attributed with the enrichment of primary silicon. In this work, the impurity content in primary silicon is
[1] J. Dietl, Hydrometallurgical purification of metallurgical-grades silicon, Sol. Cells 10 (1983) 145–154. [2] K.X. Wei, W.H. Ma, B. Yang, D.C. Liu, Y.N. Dai, K. Morita, Study on volatilization rate of silicon in multicrystalline silicon preparation from metallurgical grade silicon, Vacuum 85 (2011) 749–754. [3] N. Nakamura, H. Baba, Y. Sakaguchi, Y. Kato, Boron removal in molten silicon by a steam-added plasma melting method, Mater. Trans. 45 (2004) 858–864. [4] J.J. Wu, W.H. Ma, B.J. Jia, B. Yang, D.C. Liu, Y.N. Dai, Boron removal from metallurgical grade silicon using a CaO–Li2O–SiO2 molten slag refining technique, J. Non-Cryst. Solids 358 (2012) 3079–3083. [5] L.J. Liu, S. Nakano, K. Kakimoto, Carbon concentration and particle precipitation during directional solidification of multicrystalline silicon for solar cells, J. Cryst. Growth 310 (2008) 2192–2197. [6] M.A. Martorano, J.B. Ferreira Neto, T.S. Oliveira, T.O. Tsubaki, Refining of metallurgical silicon by directional solidification, Mater. Sci. Eng. B 176 (2011) 217–226. [7] K. Hanazawa, N. Yuge, Y. Kato, Evaporation of phosphorus in molten silicon by an electron beam irradiation method, Mater. Trans. 45 (2004) 844–849. [8] T. Yoshikawa, K. Morita, Refining of silicon during its solidification from a Si–Al melt, J. Cryst. Growth 311 (2009) 776–779. [9] T. Yoshikawa, K. Morita, Refining of Si by the solidification of Si–Al melt with electromagnetic force, ISIJ Int. 45 (2005) 967–971. [10] T. Yoshikawa, K. Morita, Continuous solidification of Si from Si–Al melt under the induction heating, ISIJ Int. 47 (2007) 582–584. [11] T. Yoshikawa, K. Morita, Removal of B from Si by solidification refining with Si– Al melts, Metall. Mater. Trans. B 36 (2005) 731–736. [12] X. Gu, X.G. Yu, D.R. Yang, Low-cost solar grade silicon purification process with Al–Si system using a powder metallurgy technique, Sep. Purif. Technol. 77 (2011) 33–39. [13] L.X. Zhao, Z. Wang, Z.C. Guo, C.Y. Li, Low-temperature purification process of metallurgical silicon, Trans. Nonferr. Met. Soc. China 21 (2011) 1185–1192. [14] X.D. Ma, T. Yoshikawa, K. Morita, Phase relations and thermodynamic property of boron in the silicon-tin melt at 1673 K, J. Alloys Compd. 529 (2012) 12–16. [15] X.D. Ma, T. Yoshikawa, K. Morita, Si growth by directional solidification of Si– Sn alloys to produce solar-grade Si, J. Cryst. Growth 377 (2013) 192–196. [16] A.M. Mitrašinović, T.A. Utigard, Refining silicon for solar cell application by copper alloying, Silicon 4 (2009) 239–248. [17] W.Z. Yu, W.H. Ma, G.Q. Lv, Y.S. Ren, H.Y. Xue, Y.N. Dai, Si purification by enrichment of primary Si in Al–Si melt, Trans. Nonferr. Met. Soc. China 23 (2013) 3476–3481. [18] J.Y. Li, Y. Liu, Y. Tan, Y.Q. Li, L. Zhang, S.R. Wu, P.J. Jia, Effect of tin addition on primary silicon recovery in Si–Al melt during solidification refining of silicon, J. Cryst. Growth 371 (2013) 1–6. [19] X. Li, Z.M. Ren, Y. Fautrelle, Effect of a high gradient magnetic field on the distribution of the solute Si and the morphology of the primary Si phase, Mater. Lett. 63 (2009) 1235–1238. [20] Y.J. He, Q.L. Li, W. Liu, Effect of combined magnetic field on the eliminating inclusions from liquid aluminum alloy, Mater. Lett. 65 (2011) 1226–1228. [21] T. Magnusson, L. Arnberg, Density and solidification shrinkage of hypoeutectic aluminum–silicon alloys, Metall. Mater. Trans. A 32 (2001) 2605–2613. [22] D. Leenov, A. Kolin, Theory of electromagnetophoresis (I)-magnetohydrodynamic forces experienced by spherical and symmetrically oriented cylindrical particles, J. Chem. Phys. 22 (1954) 683–688. [23] Z.M. Xu, T.X. Li, X.P. Zhang, Y.H. Zhou, Movement velocity of primary iron-rich phase in Al–Si melt under electromagnetic force field, J. Shanghai Jiaotong Univ. 35 (2001) 668–671. [24] J.L. Murray, A.J. McAlister, Alloy Phase Diagr. 5 (1984) 74. [25] K. Morita, T. Yoshikawa, Thermodynamic evaluation of new metallurgical refining processes for SOG-silicon production, Trans. Nonferr. Met. Soc. China 21 (2011) 685–690.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt Wenzhou Yu a,b, Wenhui Ma a,b,n, Guoqiang Lv a, Haiyang Xue a, Shaoyuan Li a,b, Yongnian Dai a,b a
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2014 Received in revised form 19 July 2014 Accepted 21 July 2014 Communicated by P. Rudolph Available online 1 August 2014
The effect of electromagnetic stirring on the enrichment of primary silicon from Al–Si melt during the process of electromagnetic separation was investigated. It is shown that the enrichment of primary silicon in Al–Si melt strongly depends on the melt flowing and viscosity gradient. The efficient enrichment of primary silicon was achieved by implementing a high current intensity, which induced a high intense melt flowing. Also, the remaining primary Si in Al–Si alloy could be precipitated by gradually decreasing the current intensity. Additionally, the inductively coupled plasma mass spectrometry (ICP-MS) results show that Si purification is attributed with the enrichment of primary silicon. In this work, the impurity content in primary silicon is 43.3 ppmw, which is much smaller than 777.6 ppmw in metallurgical silicon. Therefore, a potential low-cost technology would be provided for the Si purification. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Magnetic fields A1. Purification A1. Stirring B1. Alloys B2. Semiconducting silicon
1. Introduction The massive and low-cost solar grade silicon (SOG-Si) feedstock is essential for the widespread use of solar cells. Normally, the photovoltaic (PV) industry is dependent on the high-cost semiconductor-grade silicon (SEG-Si) that is produced by the Siemens technology. To reduce the cost of raw materials, some low-cost technologies for SOG-Si production should be proposed. Metallurgical technologies [1–7] are considered as a candidate to produce low-cost SOG-Si by removing the impurities from the metallurgical Si (MG-Si). However, Si has to repeatedly be in the molten stage in each step to ensure the impurities can be efficiently removed, which results in high energy consumption. Recently, a new method called “solvent refining” has been proposed for further cost reduction of the PV industry as the temperature of melting can be significantly reduced. Yoshikawa and Morita [8–11] and Gu et al. [12] proposed an Si–Al solvent to refine Si by using the enhanced segregation tendency of impurities between solid silicon and the Si–Al melt. Zhao et al. [13] and Ma et al. [14,15] reported that B could be efficiently removed from silicon using Sn–Si system. Additionally, Cu–Si [16] and
Zn–Si [13] solvent refining methods were also investigated for Si purification. Basically, for the low-cost SOG-Si production, the purification process of MG-Si should obey the following principles: (1) the Table 1 The impurity content in MG-Si (ppmw). Impurity
Fe 379
Ar in
Ti 76.8
Ca 189
B 27.9
P 104.9
Ar out Al-Si melt Induction coil
Pulling system n
Corresponding author. E-mail address: [email protected] (W. Ma).
http://dx.doi.org/10.1016/j.jcrysgro.2014.07.035 0022-0248/& 2014 Elsevier B.V. All rights reserved.
Fig. 1. The schematic diagram of the experiment.
Total 777.6
24
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 2. The cross-section of solidified samples: (a) without electromagnetic stirring; (b) electromagnetic separation (12 A); (c) electromagnetic separation (15 A); and (d) electromagnetic separation (18 A).
melting point of the melt should be lower than that of Si; (2) the process should efficiently remove most impurities simultaneously; and (3) it should be environment friendly which ensures that all the final products are utilized rationally as much as possible. For these reasons, Si purification in Al–Si melt is an attractive and promising technology for industrial production. During the solidification of hypereutectic Al–Si melt, Si crystals initially precipitate from the supersaturated melt with decreasing temperature, and the impurities are enriched in the melt because of the segregation behavior. Generally, acid leaching is always used to collect the Si crystals by removing the attached Al–Si alloy. However, this can result in considerable loss of Al and Si which is
unfavorable for the cost reduction and being environmentfriendly. Therefore, an efficient separation of Si crystals from Al– Si melt before acid leaching is very important. To improve the enrichment of silicon crystals during the solidification of the Al–Si melt, various magnetic fields such as the fixed alternating magnetic field at induction heating [8,17,18], the high gradient magnetic field [19] and the rotating magnetic field [20] have tried to control the migration of primary silicon crystals. In this paper, the migration characteristics of silicon crystals under induction heating have been investigated. Furthermore, the effect of electromagnetic stirring on the enrichment of silicon was also discussed thoroughly.
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 3. The simulation diagram about the movement of primary silicon in the Al– Si melt.
25
of a coil zone of a 60 kW high frequency induction furnace, then were heated at 1323 K (the current intensity is 32 A) for 1 h until completely melted. After melting, the alloys were pulled upward by a pulling system. The pulling rate was controlled at 7 μm/s, the temperature gradient was 35–40 K/cm and the pulling distance was 10 cm from the initial place. Meanwhile, the current intensity was controlled in the range of 12–18 A during the process of pulling up, which can influence the electromagnetic stirring. During the pulling upward process, the alloys solidified continuously from top to bottom due to natural cooling. The Ar gas was poured through the quartz tube to prevent the melt from being oxidized. After solidification, the samples were cut in the longitudinal direction to discuss the enrichment of primary silicon crystals. The surface of the samples was ground by SiC paper for metallographic observation. Macro- and micro-structures of solidified samples were respectively observed by SONY digital camera and Olympus PME3 light optical microscope (LOM) along with a KAPPA image analyzer. The distribution of impurities in the Al–Si alloy was characterized by a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS). The Si enrichment region was cut from the sample and then measured by atomic absorption spectroscopy (AAS) for the Si content. To discuss the effect of purification, the enriched primary silicon was leached at first by aqua regia (343 K, V(HCl):V(HNO3):V(H2O) ¼3:1:40, 200 mL) for 2 h with a magnetic stirrer, then was thoroughly rinsed with deionized water until the solution was neutral. The contents of the main impurities in primary Si were determined by ICP-MS.
3. Results and discussion
Fig. 4. Calculated migration velocity of primary phase VP vs viscosity μ.
Fig. 5. Macro- and micro-structures of solidified sample with an improved electromagnetic separation process (18–12 A).
2. Experimental The Al–45 wt% Si alloys were prepared by a mixture of 36 g MG-Si powders and 44 g pure Al powders (99.99 wt%) in a graphite crucible, and then were melted in the electrical heatresistant furnace. The contents of the main impurities in MG-Si are listed in Table 1, which were determined by inductively coupled plasma mass spectrometry (ICP-MS). Fig. 1 shows the schematic diagram of the electromagnetic separation experiment. The alloys were placed first in the middle
3.1. Effect of electromagnetic stirring on the enrichment of primary silicon Fig. 2 shows the cross-section of solidified samples. In the case of the sample without electromagnetic stirring, silicon crystals distribute evenly in the alloy, as shown in Fig. 2(a). The reason is that the density of solid primary silicon and the Al–Si melt has no significant difference [21]; hence the primary silicon cannot be enriched under the gravity force. As shown in Fig. 2(b–d), the separation interface between the Si enrichment region and Al–Si alloy can be clearly seen in the samples. The process of electromagnetic separation should be associated with the fact shown in Fig. 3(a) where melt flowing is induced in the Al–Si melt due to the electromagnetic field [8]; thus the solidified primary silicon crystals are carried downward or upward in the melt. Meanwhile, an axial temperature gradient appears by pulling the sample up from the hot zone, which results in an axial viscosity gradient of the melt. When the primary silicon crystals are carried to the cold zone of which the viscosity is high, it will agglomerate at this zone. The similar description of the separation process has been exhibited in Ref. [8], however, in which the effect of the viscosity gradient was absent. This may be the extensible explanation of the Si separation theory based on Ref. [8]. Therefore, the electromagnetic stirring together with the viscosity gradient plays a critical role in primary Si enrichment. Furthermore, with the increase of current intensity, the position of the separation interface transfers gradually upward in the solidified samples, as shown in Fig. 2(b–d). Since the migration of the primary silicon crystal is attributed to electromagnetic stirring, it is noted that the position of the separation interface is also related to the melt flowing. According to the theoretical expression of Leenov et al. [22] about the electromagnetic (EM) force acting on a single sphere
26
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
Fig. 6. (a) SEM photographs of the hypereutectic Al–Si alloy, and (b, c, d and e) EDS analysis for the elements C, O, Ca and Fe, respectively.
particle in a conductive liquid, the EM force is expressed as F ¼J B
ð1Þ
where F is the EM force (N), J is the induced current intensity (A) and B is the magnetic flux density vector (T). In the experiment, the effect of the electromagnetic force on the migration velocity is so great that the gravity of the primary phase can be neglected. The primary phase will migrate with the migration velocity [23]: 2
VP ¼
dp F
24μ
ð2Þ
where μ is the movement viscosity of the melt and dp is the particle diameter.
The electromagnetic force F is in the range of 0–9.0 106 N/m3 (J ¼0–3.75 106 A/m2 at B ¼0–2.4 T). For the primary silicon, the average particle diameter dp is approximately 20 μm. Under such conditions, the calculated value of μ as a function of VP is shown in Fig. 4. It is seen that the migration velocity of the primary phase is inversely proportion to the movement viscosity of the melt. Additionally, the migration velocity of the primary phase increases with increasing the electromagnetic force. Therefore, it is reasonable that the higher current intensity would result in stronger intense melt flowing, and the solidified silicon crystals can be carried more easily to the Si enrichment region. It is concluded that a high current intensity is beneficial to the agglomeration of the Si crystals. However, a great number of remaining primary
W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
27
Fig. 7. Microstructures of Si enrichment region under the different current intensities: (a) 12 A; (b) 15 A; (c) 18 A; and (d) 18–12 A.
silicon crystals failed to be pushed to the enrichment region, such as region B and region C in Fig. 2(c and d). From region A in Fig. 2(b), the structure is an Al–Si eutectic structure without any remaining primary silicon crystals in this region. Region B exhibits a hypereutectic structure with some polygonal primary silicon crystals. For region C, there seems to be more remaining primary silicon crystals than that in region B, which indicates that the remaining silicon crystals are related to the current intensity. Obviously, higher current intensity will increase the temperature of the melt. According to the Al–Si binary diagram [24], the primary silicon solidified continuously along with the decreasing temperature. This implies that the high temperature suppresses the solidification of Si crystals from the Al–Si melt. After the current is missing, the subsequent solidified primary silicon cannot be pushed to the enrichment region because of the absence of the electromagnetic stirring, which results in the remainder of primary silicon crystals. Therefore, for an efficient enrichment of primary silicon crystals, the relation between the current intensity and the temperature should be known. Fig. 5 shows the macro- and micro-structures of solidified sample with an improved electromagnetic process (18–12 A). It is revealed that the Al–Si alloy shows a perfect eutectic structure without any remaining primary silicon crystals in this region by gradually decreasing the current intensity from 18 A to 12 A. Meanwhile, the separation interface is very clear. This can be concluded that the high current intensity is beneficial to the enrichment of primary silicon crystals because of the high intense melt flowing, and the low current intensity in favor of the elimination of remaining primary Si in Al–Si alloy owing to the low temperature. It is proved that an enhanced agglomeration of primary silicon crystals in Al–Si melt can be achieved by controlling the current intensity. This provided a highly effective approach for Si separation in Al–Si alloy.
3.2. Si purification by the enrichment of Si crystals Some research works[8,25] have studied the removal of impurities thermodynamically during the solidification of silicon from the Al–Si melt. It is estimated that the impurities can be concentrated in the Al–Si melt by the solid/liquid segregation. However, few references have confirmed it by characterization. Fig. 6 shows the distribution of impurities in hypereutectic Al–Si alloy. From Fig. 6, it can be seen that the impurities such as C, O, Fe and Ca tend to enrich in the Al–Si alloy and the grain boundaries between the primary silicon and Al–Si alloy. On the other hand, the distribution of impurities such as B, P and Ti is difficult to be found in this system due to the limited low content. In addition, few impurities have been observed in primary silicon. It is clearly proved that the impurities can be removed from the primary silicon during the crystal growth. For the purpose of Si purification in Al–Si system, the primary silicon crystals should be enriched to reduce the attachment of Al– Si alloy as much as possible. Fig. 7 shows the microstructures of the Si enrichment region under different current intensities. It can be found that the Al attachment decreases with the increasing current intensity, which indicates that the more the intense melt flowing, the beneficial the agglomeration of Si crystals is. In this case, the Si crystals may obtain more chance to collide with each other and finally agglomerate to become Si bulk, which automatically decreases the Al attachment in the Si enrichment region. Fig. 8 shows the Si content in the Si enrichment region under different current intensities. It is clearly seen that the Si content increases by the increasing current intensity, and for the improved electromagnetic process (18–12 A), the Si content in the enrichment region is obviously higher than that of the others. The contents of main impurities in the Si enrichment region are shown in Table 2. The results reveal that the contents of main impurities
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W. Yu et al. / Journal of Crystal Growth 405 (2014) 23–28
43.3 ppmw, which is obviously improved compared with the 777.6 ppmw in metallurgical silicon.
Acknowledgment The authors are grateful for financial support from the National Natural Science Foundation of China (u1137601). References
Fig. 8. Silicon content in enrichment region under the different current intensities.
Table 2 Impurity contents of Si enrichment region after acid leaching (ppmw). Current intensity (A)
Fe
Ca
Ti
B
P
Total
12 15 18 18–12
34.1 20.2 13.3 7.8
10.8 8.6 7.4 5.1
19.8 11.3 14.5 11.3
13.2 12.4 10.8 8.7
10.2 10.3 10.9 10.4
88.1 62.8 56.9 43.3
decrease with increasing current intensity. For the electromagnetic process (18-12 A), the impurity content in primary silicon is 43.3 ppmw, which is obviously improved compared with the 777.6 ppmw in metallurgical silicon. Therefore, it is concluded that the Si purification is attributed with the enrichment of primary silicon. This provides a low-cost technology which will be a potential route for the Si purification. Finally, for the demand of SOG-Si, the purity of Si must be deeply improved. The primary silicon should be enriched more by controlling the current intensity better, and the acid leaching should be carried out more effective to reduce the attachment of Al–Si alloy in future. Of course, if needed, the metallurgical method such as slag refining, vacuum distillation etc. could be combined to remove the impurities. 4. Conclusions 1. The electromagnetic stirring plays an important role in the enrichment of primary silicon crystals from the Al–Si melt during the process of electromagnetic separation. The high current intensity is beneficial to the agglomeration of primary silicon crystals and the low current intensity in favor of the elimination of remaining primary Si in Al–Si melt. 2. An improved electromagnetic separation process (18–12 A) consisting of high current intensity step and low current intensity step was proved to obtain an enhanced enrichment of primary silicon crystals. 3. Si purification is attributed with the enrichment of primary silicon. In this work, the impurity content in primary silicon is
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