- Email: [email protected]
Purification of metallurgical-grade silicon by Sn–Si refining system with calcium addition
Separation and Purification Technology 118 (2013) 699–703
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Purification of metallurgical-grade silicon by Sn–Si refining system with calcium addition Lei Hu a,b, Zhi Wang a,⇑, Xuzhong Gong a, Zhancheng Guo a,c, Hu Zhang a a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Beiertiao, Zhongguancun, Haidian District, Beijing 100190, China b University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China c State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 9 June 2013 Received in revised form 7 August 2013 Accepted 8 August 2013 Available online 15 August 2013 Keywords: Purification Metallurgical-grade silicon Solvent refining Si–Sn system Calcium addition
a b s t r a c t Purification of metallurgical-grade silicon (MG–Si) by a combination of solvent refining and super gravity separation and acid leaching has been studied. MG–Si was alloyed with tin, and based on this system, the removal of main impurities in MG–Si by solvent refining was investigated. Furthermore, phosphorus removal by calcium addition in molten Si and Sn–Si melt was also studied. Inductively Coupled Plasma (ICP) chemical analysis revealed main impurities including B and P could be efficiently removed by the Sn–Si process and acid leaching. The content of P further reduced when Ca was added to the Sn–Si refining system. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM–EDS) analysis showed that the formation of compounds between P and several elements in the grain boundaries during the solvent refining process was an important routine of P removal. The maximum weight percent of P in P-containing impurity phases reached to 17.8% in the refined Si after the Sn–Si refining process with Ca addition. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Increased attention on environmental protection and the exhaustion of traditional energy resources has urged each country to exploit green and renewable energies. Beside other renewable energy sources, solar energy is the most abundant of renewable and clean energy sources. With the rapid development of the photovoltaic (PV) industry, there is an immense need for silicon with required chemical purity (solar-grade silicon (SoG–Si)) for PV application [1]. Currently, the production cost of SoG–Si, via the traditional Siemens process or its modified alternatives, is fairly expensive for PV application, so other alternative methods based on metallurgical purification techniques such as acid leaching [2], directional solidification [3], plasma treatment [4], vaporization refining [5], slagging [6], solvent refining [7] have been carried out to lower the production cost of SoG–Si. Solvent refining draws ever more attention, and it is a purification process in which Si recrystallization takes place from the supersaturated melt depending on the segregation behavior of different elements [8]. In effect, making alloys from MG–Si with an element in the solvent refining process decreases the segregation coefficients of the target impurities, as the Si solidifies, the impurities will favour to stay in the molten alloy phase, thus enhancing ⇑ Corresponding author. Tel.: +86 010 82544818. E-mail address: [email protected] (Z. Wang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.08.013
the removal efficiency of impurities in Si. So far, some attempts have been made with aluminum, tin, copper, iron or nickel as the alloying element to take advantage of the properties of such an alloy to improve the solvent refining process. For Al–Si system [9,10], the main problem is that it is difficult to separate primary Si and Al–Si melt because of their similar densities, so the use of gravity force is not effective as a separation method. For Cu–Si, Fe–Si and Ni–Si systems [11–14], complex compounds formed between the solvent and Si in the refining process not only result in the difficulty in separating refined silicon from the alloy, but also cause serious loss of silicon. In contrast, Sn–Si system is a relatively effective system in terms of separation owing to larger density difference between Sn (7.01 g/cm3) and Si (2.13 g/cm3) [15–17]. In addition, the melting point of Sn–Si alloy is very low and no compounds form between Sn and Si in the refining process. In our previous work [17], impurities removal from metallurgical-grade silicon by combined Sn–Si and Al–Si refining processes was studied. The final content of P reduced to 0.46 ppmw which met the requirement of SoG–Si by the combined refining method, and the removal mechanism of P was explored. It was found that P formed impurity phases with other elements in the solvent refining process. The removal effect of P was satisfactory, however, the combined refining method was relatively cost for P removal, so other methods had to be considered based on solvent refining. In Min and Sano’s study [18], it was reported that Ca3P2 was a stable compound and Ca had great affinity for P. The stability of
700
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703 Table 1 The contents of main impurities in the MG–Si (ppmw). Impurity
MG–Si (ppmw)
Impurity
MG–Si (ppmw)
Al B Ca Cu Fe
925.8 10.3 34.6 19.6 924.6
Mn Ni P Ti V
130.0 97.8 108.5 124.4 226.1
After that, they were cooled under the cooling rate of 3 K/min. After reaching room temperature, the crucibles were taken out from the furnace. 2.2. Super gravity separation
Fig. 1. Ellingham diagram for stability of phosphides.
this compound is shown as the phosphides Ellingham diagram that is plotted with Factsage software [12,19] in Fig. 1. Shimpo et al. [20] had conducted some experiments, it was found that the removal efficiency of P reported to be 80.4% as 5.9 at.% of Ca was added to molten silicon. In the work of Meteleva-Fischer et al. [21], it was concluded that it was very effective for the removal of P in MG–Si by the refining process in which Ca was added to MG–Si, and the removal efficiency of P achieved near complete with 4 wt.% addition of Ca. Based on the discuss above, in this paper, Sn was chosen as solvent and alloyed with MG–Si to trap the impurities and later refined silicon was recovered by super gravity separation and then treated by acid leaching. The purpose of adding Ca to the Sn–Si refining process was to better achieve the reconstruction process of P so that P mainly formed impurity phases with Ca. The effects of Ca addition on P removal both in MG–Si remelting process and in solvent refining process were also studied, respectively. In addition, the reconstruction process of impurity phases in refined Si was investigated.
The solidified alloy in A1 was placed in an alumina filter, and the filter was put into the rotor furnace, and then heated to 673 K at the rate of 10 K/min, and held for 30 min. After that, the centrifugal machine was started, and the rotate speed was regulated to 1000 r/min (the centrifugal force was 250 g) and held for 5 min. After reaching room temperature, the silicon was removed from the filter. The solidified alloy in A2 was conducted with the same operation. The sketch of centrifugal apparatus can be seen in our previous work [17]. 2.3. Acid leaching The recovered silicon from 2.2 and the remelted Si in A3 were crushed to the particle size less than 500 lm, then 2 g silicon powder of each sample was leached by aqua regia (20 wt.%, 120 mL) and HF (5 wt.%, 120 mL) in sequence. Acid leaching condition was as follows: leaching time was 6 h, leaching temperature was 343 K, and the silicon powder and the acid in closed bottle were continuously agitated. After acid leaching, the remaining silicon powder was washed with deionized water until the solution was almost neutral, and then it was dried. Meanwhile, crude MG–Si powder (150–500 lm) was conducted with the same acid leaching experiment. 2.4. Characterization
2. Experimental Metallurgical techniques used in the purification process involve alloying MG–Si with Sn (including adding calcium to the alloy), prolonged solidification of the mixture, separation of refined silicon from the alloy by using the super gravity, and acid leaching of refined silicon. The sketch of the purification process can be seen in our previous work [17].
The surface morphologies and the impurity phase of refined Si samples were examined by Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM–EDS) analysis. In addition, the contents of impurities in silicon samples after acid leaching were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP–OES). Furthermore, the compositions of Si samples were investigated by XRD (X-ray Diffraction) analysis.
2.1. Alloying and solidification procedure
3. Results and discussion
MG–Si lumps were crushed and pulverized, and then blended silicon powder (150–500 lm) with Sn of analytical purity to form a mixture of Sn–12 wt.% Si. The composition of the MG–Si is summarized in Table 1. A total of about 6 g MG–Si and 44 g Sn were imposed in two same alumina crucibles (A1 and A2) respectively and then additional 0.25 g Ca (99.5%), about 0.25 g, was added to the A2 crucible. In addition, about 6 g MG–Si and 0.25 g Ca (the weight percent of Ca in the mixture was 4%) were imposed in one alumina crucible (A3). Ca granules were supplied by Alfa Aesar Co., Ltd. The crucibles were placed in an electrical furnace (GSL-1700X, Kejing Co., Ltd., Hefei, China), heated to 1773 K at the rate of 5 K/min in argon atmosphere, and then held for 2 h for homogenization of the melt.
3.1. Refined silicon composition and microconstituents analysis The chemical composition of the refined silicon after the Sn-Si refining process and acid leaching was analyzed by ICP, and the contents of B, P and typical metallic impurities together with their removal efficiencies are given in Table 2. As can be seen, the total removal efficiency of the metallic impurities was above 99.4%, furthermore, the content of B reduced from 10.3 ppmw to 3.12 ppmw, and that of P reduced from 108.5 ppmw to 28.9 ppmw. Since the segregation coefficients of most of metallic impurities are much less than 1 in MG–Si [22], this kind of impurities will be segregated in the grain boundaries of Si in the form of an alloy or intermediate compounds. Therefore, acid leaching is an effective
701
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703 Table 2 The contents and removal efficiencies of impurities in the refined Si after the Sn-Si refining process and acid leaching. Content (ppmw)
Removal efficiency (%)
Element
Content (ppmw)
Removal efficiency (%)
B P Fe Al Ca
3.12 28.9 1.97 9.20 0.98
69.71 73.36 99.79 99.01 97.18
Ti Cu Mn Ni V
0.05 0.33 0.21 0.46 0.65
99.96 98.57 99.84 99.53 99.71
The content of P, ppmw
Element
150 120 90 60 30 0
1600 Si 1400 1200 1000 800 600 400 V Al Fe P 200 Ti 0 0 2
6
keV
The contents of P in different Si samples are shown in Fig. 3. S1 and S2 represent the MG–Si after acid leaching, and the refined Si after the MG–Si remelting process with 4 wt.% Ca addition and acid leaching, respectively. As can be seen, the content of P in S2 reduced significantly compared with that of P in S1, and the removal efficiency of P in S2 was up to 94.8%. Shimpo et al. [20] reported that compared with S1, the increase of the removal efficiency of P in S2 had two reasons. One was the activity coefficient of P in molten silicon decreased, and the other was the amount of secondary phases increased. In Shimpo’s work, it was observed that P-containing phase was alongside CaSi2 phase by SEM–EDS analysis and identified as Ca3P2 by XRD analysis, and
Si
2#
1500 1000 500
10
S4
3.2. Effect of Ca addition on P removal by remelting MG–Si
2000
8
S3
process was an important routine of P removal which was confirmed in our previous work [17].
1#
V Fe TiTi Mn Fe
S2
Fig. 3. The contents of P in different Si samples.
2500
4
S1
Different Si samples
Counts
Counts
method to remove the segregated impurities. However, for nonmetallic impurities, especially B and P, their segregation coefficients are 0.8 and 0.35 [22] respectively, are almost completely soliddissovled inside Si matrix, so acid leaching is not effective for the removal of them, but the solvent refining process is an effective method to removal B and P because it decreases their segregation coefficients as Si solidifies. For the Sn–Si system, the segregation coefficient of B decreased to 0.038 at 1500 K [15], which is much less than 0.8 at the melting point of silicon. For the Al–Si system, the segregation coefficients of B and P at infinite dilution were determined to be 0.22 and 0.085 at 1273 K [9], respectively. With the decrease of the segregation coefficient, B and P could be segregated in the grain boundaries like metallic impurities. The microstructure of impurity phases in the refined Si after the Sn–Si refining process and EDS analysis of different areas are demonstrated in Fig. 2. It can be seen that P was found to be contained in some impurity phases in the grain boundary, and it was expected that P may form compounds with other elements and segregated in the grain boundaries of refined Si when Si recrystallized from the melt. Thus, the formation of P-containing impurity phases segregated in the grain boundaries during the solvent refining
0
Al P Fe 0
2
Fe Fe 4
6
8
10
keV
Fig. 2. The microstructure of impurity phases in the refined Si after the Sn–Si refining process and EDS analysis.
702
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703
(a)
6000
Al
5000
Counts
4000
1#
(b)
Si
3000 2000
Ca
1000 0
P 0
2
4
6
8
10
keV Fig. 4. The microstructure of impurity phases in the refined Si after the MG–Si remelting process with 4 wt.% Ca addition (a) and EDS analysis (b).
Al–Ca-Si phase was CaAl2Si2 phase in Fig. 4. Based on the results above, P maybe also have high removal efficiency in the refined Si after the Sn–Si refining process with Ca addition and acid leaching.
Intensity (counts)
5000 4500
Si
4000
CaSi 2
3500
Al 0.7Fe 3 Si 0.3
3000
3.3. Effect of Ca addition on P removal in solvent refining process
2500 2000 1500 1000 500 0 25
30
35
40
45
Angle-2Theta. (degree) Fig. 5. XRD pattern of the refined Si after the MG–Si remelting process with 4 wt.% Ca addition.
Table 3 EDS quantitative analysis. EDS analysis
#1
Element (Atom%) Ca
Al
Si
P
20.14 ± 0.21
39.50 ± 0.18
39.76 ± 0.14
0.60 ± 0.46
Ca3P2 could be removed by acid leaching. However, some results of our work were different from those. SEM micrograph of impurity phases in the silicon particles from solidified Si–4 wt.% Ca alloy is shown in Fig. 4, the lighter areas were impurity phases, and the darker area was Si phase. Qualitative analysis was performed to determine the composition of lighter areas, it was found that the impurity phase was CaSi2, however, no P was detected in this area of CaSi2 phase by EDS analysis, and Ca3P2 was not detected by XRD analysis (Fig. 5). The result was the same with Johnston and Barati’s work [23]. Ca3P2 was not detected perhaps because P was not added to MG–Si in our work, while 3 wt.% P was added to MG–Si in Shimpo’ study. Furthermore, it was also found that the impurity phase was not just CaSi2 phase by SEM-EDS analysis, and besides, homogeneous Al–Ca–Si phase was linked to CaSi2 phase, and P was detected in the area of Al–Ca–Si phase (Fig. 4(b)). Note that the mole ratio of Al, Ca and Si was about 2:1:2 (Table 3), in other words, the Al–Ca–Si phase was chemically similar to the ternary compound CaAl2Si2. Anglézio et al. [24] reported that it was verified that the formulae found in synthetic alloys were also applicable for the intergranular compounds in the industrial silicon, and some amount of P was contained in CaAl2Si2 phase, so it was inferred that the
The comparison of the removal efficiencies of P by the Sn–Si refining process with and without Ca addition is demonstrated in Fig. 3. S3 and S4 represent the refined Si after the Sn–Si refining process and acid leaching, and the refined Si after the Sn–Si refining process with Ca addition (the weight ratio of Ca and Si was 4:96, which was the same with that in Section 3.2) and acid leaching, respectively. It was found that in the Sn–Si system, the content of P further decreased with Ca addition when comparing S3 with S4, and the removal efficiency of P increased from 73.4% to 86.5%. It was inferred that in the Sn–Si refining process with Ca addition, the amount of secondary phases containing P also increased. Fig. 6 shows the X-ray mapping of an impurity phase area in the refined Si after the Sn–Si refining process with Ca addition. It can be seen that the darker area was Si phase, and the lighter area was the impurity phase in Fig. 6(a), which attached to the surface of Si phase, and the impurity phase was Ca–P–Al–Sn phase as shown in Fig. 6(b)–(f). Most notably, P was enriched apparently in the triangle area, which indicated that the content of P in this area was very high. The maximum weight percent of P in the area reached to 17.8% by EDS quantitative analysis. Moreover, Ca was also contained and abundant in the impurity phase. In addition, a certain amount of Al and Sn existed in the triangle area. It is known that Ca has great affinity for P and Ca3P2 is a stable compound [19], however, in the complex solvent refining system which is very different from Si system, it was expected that Ca3P2 was not likely to form alone due to the effect of other metallic elements in MG–Si, and P could form multicomponent compounds with Ca as the melt solidified. Therefore, P and Ca probably formed compounds with several elements when Ca was added to the solvent refining system. The formation of P-containing impurity phases segregated on the surface of the refined Si during the solvent refining process, which changed the segregation characteristics of P, was very beneficial for soliddissovled P removal by acid leaching. As can be seen in Fig. 3, the removal efficiency of P in the refined Si after the MG–Si remelting process with Ca addition and acid leaching (S2) was higher than that of P in the refined Si after the Sn–Si refining process with Ca addition and acid leaching (S4). The standard free energy of the formation of Ca2Sn was close to that of Ca3P2 in high temperature [18], and in the Sn–Si refining system with Ca addition, solvent Sn was the major element, so the existence of Sn could be considered to dilute the concentration
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703
703
Fig. 6. The X-ray mapping of an impurity phase area in the refined Si after the Sn–Si refining process with Ca addition and EDS analysis (a, b, c, d, e, f).
of Ca in molten Si and decrease the reaction chance of Ca and P, which lead to the result. However, seen another way for the result, although the removal efficiency of P in S2 was higher than that of P in S4, the fundamental goal of the MG–Si remelting process with Ca addition was to better remove P only, however, for the solvent refining process, it can remove P and other impurities effectively at the same time. 4. Conclusions In this study, MG–Si was purified by a combination of solvent refining and gravity separation and acid leaching. The results showed that the total removal efficiency of the metallic impurities was above 99.4%, and the content of B reduced from 10.3 ppmw to 3.12 ppmw, and that of P reduced from 108.5 ppmw to 28.9 ppmw by the Sn–Si refining process and acid leaching. The formation of compounds between P and several elements in the grain boundaries during the solvent refining process was further confirmed to be an important routine of P removal. Due to the increase of CaAl2Si2 phase, the content of P in the refined Si after the MG–Si remelting process with Ca addition and acid leaching reduced significantly compared with that of P in the MG–Si after acid leaching. The removal efficiency of P increased from 73.4% to 86.5% when Ca was added in the Sn–Si refining system. The maximum weight percent of P in P-containing impurity phases reached to 17.8% in the refined Si after the Sn–Si refining process with Ca addition. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51174187) and National Key Technologies R&D Program (2011BAE03B01). References [1] S. Pizzini, Towards solar grade silicon: challenges and benefits for low cost photovoltaics, Sol. Energy Mater. Sol. Cells 94 (2010) 1528–1533. [2] K. Visnovec, C. Varuawa, T. Utigrad, A. Mitrašinovic´, Elimination of impurities from the surface of silicon using hydrochloric and nitric acid, Mater. Sci. Semicond. Process. 16 (2013) 106–110. [3] Y. Tan, S.Q. Ren, S. Shi, S.T. Wen, D.C. Jiang, W. Dong, M. Ji, S.H. Sun, Removal of aluminum and calcium in multicrystalline silicon by vacuum induction melting and directional solidification, Vacuum 99 (2014) 272–276.
[4] J.T. Wang, X.D. Li, Y.M. He, N. Feng, X.Y. An, F. Teng, C.T. Gao, C.H. Zhao, Z.X. Zhang, E.Q. Xie, Purification of metallurgical grade silicon by a microwaveassisted plasma process, Sep. Purif. Technol. 102 (2013) 82–85. [5] J.L. Sun, J. Zhang, H.W. Wang, T.M. Wang, Z.Q. Cao, Y.P. Lu, T.J. Li, Purification of metallurgical grade silicon in an electron beam melting furnace, Surf. Coat. Technol. 228 (2013) S67–S71. [6] L. Zhang, Y. Tan, J.Y. Li, Y. Liu, D.K. Wang, Study of boron removal from molten silicon by slag refining under atmosphere, Mater. Sci. Semicond. Process. 16 (2013) 1645–1649. [7] T. Yoshikawa, K. Morita, An evolving method for solar-grade silicon production solvent refining, JOM 64 (2012) 946–951. [8] J. Dietl, in: Proc. of the Symp. on Matericals and New processing Technologies for Photovoltaics, vol. 11, 1983, p. 52. [9] T. Yoshikawa, K. Morita, Refining of silicon during its solidification from a Si-Al melt, J. Cryst. Growth 311 (2009) 776–779. [10] J.W. Li, Z.C. Guo, H.Q. Tang, Z. Wang, S.T. Sun, Si purification by solidification of Al–Si melt with super gravity, Trans. Nonferr. Met. Soc. China 22 (2012) 958– 963. [11] A.M. Mitrašinovic´, T.A. Utigard, Refining silicon for solar cell application by copper alloying, Silicon 1 (2009) 239–248. [12] S. Esfahani, M. Barati, Purification of metallurgical silicon using iron as an impurity getter part I, Met. Mater. Int. 17 (2011) 825–829. [13] S. Esfahani, M. Barati, Purification of metallurgical silicon using iron as an impurity getter part II, Met. Mater. Int. 17 (2011) 1009–1015. [14] Z. Yin, A. Oliazadeh, S. Esfahani, Solvent refining of silicon using nickel as impurity getter, Can. Metall. Q 5 (7) (2011) 166–172. [15] 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. [16] 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. [17] L. Hu, Z. Wang, X.Z. Gong, Z.C. Guo, H. Zhang, Impurities removal from metallurgical-grade silicon by combined Sn–Si and Al–Si refining processes, Metall. Mater. Trans. B 44 (2013) 828–836. [18] D. Min, N. Sano, Determination of standard free energies of formation of Ca3P2 and Ca2Sn at high temperatures, Metall. Mater. Trans. B 19 (1988) 433–439. [19] C.W. Bale, P. Chartrand, S.A. Degterov, G. Eriksson, K. Hack, R.B. Mahfoud, J. Melancon, A.D. Pelton, S. Petersen, FactSage thermochemical software and databases, Calphad 26 (2002) 189–228. [20] T. Shimpo, T. Yoshikawa, K. Morita, Thermodynamic study of the effect of calcium on removal of phosphorus from silicon by acid leaching treatment, Metall. Mater. Trans. B 35 (2004) 277–284. [21] Y.V. Meteleva-fischer, Y. Room, B. Kraaijveld, H. Kuntzel, Microstructure of metallurgical grade silicon during alloying refining with calcium, Intermetallics 25 (2012) 9–17. [22] T. Yoshikawa, K. Morita, S. Kawanish, T. Tanaka, Thermodynamics of impurity elements in solid silicon, J. Alloys Compd. 490 (2010) 31–41. [23] M.D. Johnston, M. Barati, Calcium and titanium as impurity getter metals in purification of silicon, Sep. Purif. Technol. 107 (2013) 129–134. [24] J.C. Anglézio, C. Servant, F. Dubrous, Characterization of metallurgical grade silicon, J. Mater. Res. 5 (1990) 1894–1899.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Purification of metallurgical-grade silicon by Sn–Si refining system with calcium addition Lei Hu a,b, Zhi Wang a,⇑, Xuzhong Gong a, Zhancheng Guo a,c, Hu Zhang a a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Beiertiao, Zhongguancun, Haidian District, Beijing 100190, China b University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China c State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 9 June 2013 Received in revised form 7 August 2013 Accepted 8 August 2013 Available online 15 August 2013 Keywords: Purification Metallurgical-grade silicon Solvent refining Si–Sn system Calcium addition
a b s t r a c t Purification of metallurgical-grade silicon (MG–Si) by a combination of solvent refining and super gravity separation and acid leaching has been studied. MG–Si was alloyed with tin, and based on this system, the removal of main impurities in MG–Si by solvent refining was investigated. Furthermore, phosphorus removal by calcium addition in molten Si and Sn–Si melt was also studied. Inductively Coupled Plasma (ICP) chemical analysis revealed main impurities including B and P could be efficiently removed by the Sn–Si process and acid leaching. The content of P further reduced when Ca was added to the Sn–Si refining system. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM–EDS) analysis showed that the formation of compounds between P and several elements in the grain boundaries during the solvent refining process was an important routine of P removal. The maximum weight percent of P in P-containing impurity phases reached to 17.8% in the refined Si after the Sn–Si refining process with Ca addition. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Increased attention on environmental protection and the exhaustion of traditional energy resources has urged each country to exploit green and renewable energies. Beside other renewable energy sources, solar energy is the most abundant of renewable and clean energy sources. With the rapid development of the photovoltaic (PV) industry, there is an immense need for silicon with required chemical purity (solar-grade silicon (SoG–Si)) for PV application [1]. Currently, the production cost of SoG–Si, via the traditional Siemens process or its modified alternatives, is fairly expensive for PV application, so other alternative methods based on metallurgical purification techniques such as acid leaching [2], directional solidification [3], plasma treatment [4], vaporization refining [5], slagging [6], solvent refining [7] have been carried out to lower the production cost of SoG–Si. Solvent refining draws ever more attention, and it is a purification process in which Si recrystallization takes place from the supersaturated melt depending on the segregation behavior of different elements [8]. In effect, making alloys from MG–Si with an element in the solvent refining process decreases the segregation coefficients of the target impurities, as the Si solidifies, the impurities will favour to stay in the molten alloy phase, thus enhancing ⇑ Corresponding author. Tel.: +86 010 82544818. E-mail address: [email protected] (Z. Wang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.08.013
the removal efficiency of impurities in Si. So far, some attempts have been made with aluminum, tin, copper, iron or nickel as the alloying element to take advantage of the properties of such an alloy to improve the solvent refining process. For Al–Si system [9,10], the main problem is that it is difficult to separate primary Si and Al–Si melt because of their similar densities, so the use of gravity force is not effective as a separation method. For Cu–Si, Fe–Si and Ni–Si systems [11–14], complex compounds formed between the solvent and Si in the refining process not only result in the difficulty in separating refined silicon from the alloy, but also cause serious loss of silicon. In contrast, Sn–Si system is a relatively effective system in terms of separation owing to larger density difference between Sn (7.01 g/cm3) and Si (2.13 g/cm3) [15–17]. In addition, the melting point of Sn–Si alloy is very low and no compounds form between Sn and Si in the refining process. In our previous work [17], impurities removal from metallurgical-grade silicon by combined Sn–Si and Al–Si refining processes was studied. The final content of P reduced to 0.46 ppmw which met the requirement of SoG–Si by the combined refining method, and the removal mechanism of P was explored. It was found that P formed impurity phases with other elements in the solvent refining process. The removal effect of P was satisfactory, however, the combined refining method was relatively cost for P removal, so other methods had to be considered based on solvent refining. In Min and Sano’s study [18], it was reported that Ca3P2 was a stable compound and Ca had great affinity for P. The stability of
700
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703 Table 1 The contents of main impurities in the MG–Si (ppmw). Impurity
MG–Si (ppmw)
Impurity
MG–Si (ppmw)
Al B Ca Cu Fe
925.8 10.3 34.6 19.6 924.6
Mn Ni P Ti V
130.0 97.8 108.5 124.4 226.1
After that, they were cooled under the cooling rate of 3 K/min. After reaching room temperature, the crucibles were taken out from the furnace. 2.2. Super gravity separation
Fig. 1. Ellingham diagram for stability of phosphides.
this compound is shown as the phosphides Ellingham diagram that is plotted with Factsage software [12,19] in Fig. 1. Shimpo et al. [20] had conducted some experiments, it was found that the removal efficiency of P reported to be 80.4% as 5.9 at.% of Ca was added to molten silicon. In the work of Meteleva-Fischer et al. [21], it was concluded that it was very effective for the removal of P in MG–Si by the refining process in which Ca was added to MG–Si, and the removal efficiency of P achieved near complete with 4 wt.% addition of Ca. Based on the discuss above, in this paper, Sn was chosen as solvent and alloyed with MG–Si to trap the impurities and later refined silicon was recovered by super gravity separation and then treated by acid leaching. The purpose of adding Ca to the Sn–Si refining process was to better achieve the reconstruction process of P so that P mainly formed impurity phases with Ca. The effects of Ca addition on P removal both in MG–Si remelting process and in solvent refining process were also studied, respectively. In addition, the reconstruction process of impurity phases in refined Si was investigated.
The solidified alloy in A1 was placed in an alumina filter, and the filter was put into the rotor furnace, and then heated to 673 K at the rate of 10 K/min, and held for 30 min. After that, the centrifugal machine was started, and the rotate speed was regulated to 1000 r/min (the centrifugal force was 250 g) and held for 5 min. After reaching room temperature, the silicon was removed from the filter. The solidified alloy in A2 was conducted with the same operation. The sketch of centrifugal apparatus can be seen in our previous work [17]. 2.3. Acid leaching The recovered silicon from 2.2 and the remelted Si in A3 were crushed to the particle size less than 500 lm, then 2 g silicon powder of each sample was leached by aqua regia (20 wt.%, 120 mL) and HF (5 wt.%, 120 mL) in sequence. Acid leaching condition was as follows: leaching time was 6 h, leaching temperature was 343 K, and the silicon powder and the acid in closed bottle were continuously agitated. After acid leaching, the remaining silicon powder was washed with deionized water until the solution was almost neutral, and then it was dried. Meanwhile, crude MG–Si powder (150–500 lm) was conducted with the same acid leaching experiment. 2.4. Characterization
2. Experimental Metallurgical techniques used in the purification process involve alloying MG–Si with Sn (including adding calcium to the alloy), prolonged solidification of the mixture, separation of refined silicon from the alloy by using the super gravity, and acid leaching of refined silicon. The sketch of the purification process can be seen in our previous work [17].
The surface morphologies and the impurity phase of refined Si samples were examined by Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM–EDS) analysis. In addition, the contents of impurities in silicon samples after acid leaching were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP–OES). Furthermore, the compositions of Si samples were investigated by XRD (X-ray Diffraction) analysis.
2.1. Alloying and solidification procedure
3. Results and discussion
MG–Si lumps were crushed and pulverized, and then blended silicon powder (150–500 lm) with Sn of analytical purity to form a mixture of Sn–12 wt.% Si. The composition of the MG–Si is summarized in Table 1. A total of about 6 g MG–Si and 44 g Sn were imposed in two same alumina crucibles (A1 and A2) respectively and then additional 0.25 g Ca (99.5%), about 0.25 g, was added to the A2 crucible. In addition, about 6 g MG–Si and 0.25 g Ca (the weight percent of Ca in the mixture was 4%) were imposed in one alumina crucible (A3). Ca granules were supplied by Alfa Aesar Co., Ltd. The crucibles were placed in an electrical furnace (GSL-1700X, Kejing Co., Ltd., Hefei, China), heated to 1773 K at the rate of 5 K/min in argon atmosphere, and then held for 2 h for homogenization of the melt.
3.1. Refined silicon composition and microconstituents analysis The chemical composition of the refined silicon after the Sn-Si refining process and acid leaching was analyzed by ICP, and the contents of B, P and typical metallic impurities together with their removal efficiencies are given in Table 2. As can be seen, the total removal efficiency of the metallic impurities was above 99.4%, furthermore, the content of B reduced from 10.3 ppmw to 3.12 ppmw, and that of P reduced from 108.5 ppmw to 28.9 ppmw. Since the segregation coefficients of most of metallic impurities are much less than 1 in MG–Si [22], this kind of impurities will be segregated in the grain boundaries of Si in the form of an alloy or intermediate compounds. Therefore, acid leaching is an effective
701
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703 Table 2 The contents and removal efficiencies of impurities in the refined Si after the Sn-Si refining process and acid leaching. Content (ppmw)
Removal efficiency (%)
Element
Content (ppmw)
Removal efficiency (%)
B P Fe Al Ca
3.12 28.9 1.97 9.20 0.98
69.71 73.36 99.79 99.01 97.18
Ti Cu Mn Ni V
0.05 0.33 0.21 0.46 0.65
99.96 98.57 99.84 99.53 99.71
The content of P, ppmw
Element
150 120 90 60 30 0
1600 Si 1400 1200 1000 800 600 400 V Al Fe P 200 Ti 0 0 2
6
keV
The contents of P in different Si samples are shown in Fig. 3. S1 and S2 represent the MG–Si after acid leaching, and the refined Si after the MG–Si remelting process with 4 wt.% Ca addition and acid leaching, respectively. As can be seen, the content of P in S2 reduced significantly compared with that of P in S1, and the removal efficiency of P in S2 was up to 94.8%. Shimpo et al. [20] reported that compared with S1, the increase of the removal efficiency of P in S2 had two reasons. One was the activity coefficient of P in molten silicon decreased, and the other was the amount of secondary phases increased. In Shimpo’s work, it was observed that P-containing phase was alongside CaSi2 phase by SEM–EDS analysis and identified as Ca3P2 by XRD analysis, and
Si
2#
1500 1000 500
10
S4
3.2. Effect of Ca addition on P removal by remelting MG–Si
2000
8
S3
process was an important routine of P removal which was confirmed in our previous work [17].
1#
V Fe TiTi Mn Fe
S2
Fig. 3. The contents of P in different Si samples.
2500
4
S1
Different Si samples
Counts
Counts
method to remove the segregated impurities. However, for nonmetallic impurities, especially B and P, their segregation coefficients are 0.8 and 0.35 [22] respectively, are almost completely soliddissovled inside Si matrix, so acid leaching is not effective for the removal of them, but the solvent refining process is an effective method to removal B and P because it decreases their segregation coefficients as Si solidifies. For the Sn–Si system, the segregation coefficient of B decreased to 0.038 at 1500 K [15], which is much less than 0.8 at the melting point of silicon. For the Al–Si system, the segregation coefficients of B and P at infinite dilution were determined to be 0.22 and 0.085 at 1273 K [9], respectively. With the decrease of the segregation coefficient, B and P could be segregated in the grain boundaries like metallic impurities. The microstructure of impurity phases in the refined Si after the Sn–Si refining process and EDS analysis of different areas are demonstrated in Fig. 2. It can be seen that P was found to be contained in some impurity phases in the grain boundary, and it was expected that P may form compounds with other elements and segregated in the grain boundaries of refined Si when Si recrystallized from the melt. Thus, the formation of P-containing impurity phases segregated in the grain boundaries during the solvent refining
0
Al P Fe 0
2
Fe Fe 4
6
8
10
keV
Fig. 2. The microstructure of impurity phases in the refined Si after the Sn–Si refining process and EDS analysis.
702
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703
(a)
6000
Al
5000
Counts
4000
1#
(b)
Si
3000 2000
Ca
1000 0
P 0
2
4
6
8
10
keV Fig. 4. The microstructure of impurity phases in the refined Si after the MG–Si remelting process with 4 wt.% Ca addition (a) and EDS analysis (b).
Al–Ca-Si phase was CaAl2Si2 phase in Fig. 4. Based on the results above, P maybe also have high removal efficiency in the refined Si after the Sn–Si refining process with Ca addition and acid leaching.
Intensity (counts)
5000 4500
Si
4000
CaSi 2
3500
Al 0.7Fe 3 Si 0.3
3000
3.3. Effect of Ca addition on P removal in solvent refining process
2500 2000 1500 1000 500 0 25
30
35
40
45
Angle-2Theta. (degree) Fig. 5. XRD pattern of the refined Si after the MG–Si remelting process with 4 wt.% Ca addition.
Table 3 EDS quantitative analysis. EDS analysis
#1
Element (Atom%) Ca
Al
Si
P
20.14 ± 0.21
39.50 ± 0.18
39.76 ± 0.14
0.60 ± 0.46
Ca3P2 could be removed by acid leaching. However, some results of our work were different from those. SEM micrograph of impurity phases in the silicon particles from solidified Si–4 wt.% Ca alloy is shown in Fig. 4, the lighter areas were impurity phases, and the darker area was Si phase. Qualitative analysis was performed to determine the composition of lighter areas, it was found that the impurity phase was CaSi2, however, no P was detected in this area of CaSi2 phase by EDS analysis, and Ca3P2 was not detected by XRD analysis (Fig. 5). The result was the same with Johnston and Barati’s work [23]. Ca3P2 was not detected perhaps because P was not added to MG–Si in our work, while 3 wt.% P was added to MG–Si in Shimpo’ study. Furthermore, it was also found that the impurity phase was not just CaSi2 phase by SEM-EDS analysis, and besides, homogeneous Al–Ca–Si phase was linked to CaSi2 phase, and P was detected in the area of Al–Ca–Si phase (Fig. 4(b)). Note that the mole ratio of Al, Ca and Si was about 2:1:2 (Table 3), in other words, the Al–Ca–Si phase was chemically similar to the ternary compound CaAl2Si2. Anglézio et al. [24] reported that it was verified that the formulae found in synthetic alloys were also applicable for the intergranular compounds in the industrial silicon, and some amount of P was contained in CaAl2Si2 phase, so it was inferred that the
The comparison of the removal efficiencies of P by the Sn–Si refining process with and without Ca addition is demonstrated in Fig. 3. S3 and S4 represent the refined Si after the Sn–Si refining process and acid leaching, and the refined Si after the Sn–Si refining process with Ca addition (the weight ratio of Ca and Si was 4:96, which was the same with that in Section 3.2) and acid leaching, respectively. It was found that in the Sn–Si system, the content of P further decreased with Ca addition when comparing S3 with S4, and the removal efficiency of P increased from 73.4% to 86.5%. It was inferred that in the Sn–Si refining process with Ca addition, the amount of secondary phases containing P also increased. Fig. 6 shows the X-ray mapping of an impurity phase area in the refined Si after the Sn–Si refining process with Ca addition. It can be seen that the darker area was Si phase, and the lighter area was the impurity phase in Fig. 6(a), which attached to the surface of Si phase, and the impurity phase was Ca–P–Al–Sn phase as shown in Fig. 6(b)–(f). Most notably, P was enriched apparently in the triangle area, which indicated that the content of P in this area was very high. The maximum weight percent of P in the area reached to 17.8% by EDS quantitative analysis. Moreover, Ca was also contained and abundant in the impurity phase. In addition, a certain amount of Al and Sn existed in the triangle area. It is known that Ca has great affinity for P and Ca3P2 is a stable compound [19], however, in the complex solvent refining system which is very different from Si system, it was expected that Ca3P2 was not likely to form alone due to the effect of other metallic elements in MG–Si, and P could form multicomponent compounds with Ca as the melt solidified. Therefore, P and Ca probably formed compounds with several elements when Ca was added to the solvent refining system. The formation of P-containing impurity phases segregated on the surface of the refined Si during the solvent refining process, which changed the segregation characteristics of P, was very beneficial for soliddissovled P removal by acid leaching. As can be seen in Fig. 3, the removal efficiency of P in the refined Si after the MG–Si remelting process with Ca addition and acid leaching (S2) was higher than that of P in the refined Si after the Sn–Si refining process with Ca addition and acid leaching (S4). The standard free energy of the formation of Ca2Sn was close to that of Ca3P2 in high temperature [18], and in the Sn–Si refining system with Ca addition, solvent Sn was the major element, so the existence of Sn could be considered to dilute the concentration
L. Hu et al. / Separation and Purification Technology 118 (2013) 699–703
703
Fig. 6. The X-ray mapping of an impurity phase area in the refined Si after the Sn–Si refining process with Ca addition and EDS analysis (a, b, c, d, e, f).
of Ca in molten Si and decrease the reaction chance of Ca and P, which lead to the result. However, seen another way for the result, although the removal efficiency of P in S2 was higher than that of P in S4, the fundamental goal of the MG–Si remelting process with Ca addition was to better remove P only, however, for the solvent refining process, it can remove P and other impurities effectively at the same time. 4. Conclusions In this study, MG–Si was purified by a combination of solvent refining and gravity separation and acid leaching. The results showed that the total removal efficiency of the metallic impurities was above 99.4%, and the content of B reduced from 10.3 ppmw to 3.12 ppmw, and that of P reduced from 108.5 ppmw to 28.9 ppmw by the Sn–Si refining process and acid leaching. The formation of compounds between P and several elements in the grain boundaries during the solvent refining process was further confirmed to be an important routine of P removal. Due to the increase of CaAl2Si2 phase, the content of P in the refined Si after the MG–Si remelting process with Ca addition and acid leaching reduced significantly compared with that of P in the MG–Si after acid leaching. The removal efficiency of P increased from 73.4% to 86.5% when Ca was added in the Sn–Si refining system. The maximum weight percent of P in P-containing impurity phases reached to 17.8% in the refined Si after the Sn–Si refining process with Ca addition. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51174187) and National Key Technologies R&D Program (2011BAE03B01). References [1] S. Pizzini, Towards solar grade silicon: challenges and benefits for low cost photovoltaics, Sol. Energy Mater. Sol. Cells 94 (2010) 1528–1533. [2] K. Visnovec, C. Varuawa, T. Utigrad, A. Mitrašinovic´, Elimination of impurities from the surface of silicon using hydrochloric and nitric acid, Mater. Sci. Semicond. Process. 16 (2013) 106–110. [3] Y. Tan, S.Q. Ren, S. Shi, S.T. Wen, D.C. Jiang, W. Dong, M. Ji, S.H. Sun, Removal of aluminum and calcium in multicrystalline silicon by vacuum induction melting and directional solidification, Vacuum 99 (2014) 272–276.
[4] J.T. Wang, X.D. Li, Y.M. He, N. Feng, X.Y. An, F. Teng, C.T. Gao, C.H. Zhao, Z.X. Zhang, E.Q. Xie, Purification of metallurgical grade silicon by a microwaveassisted plasma process, Sep. Purif. Technol. 102 (2013) 82–85. [5] J.L. Sun, J. Zhang, H.W. Wang, T.M. Wang, Z.Q. Cao, Y.P. Lu, T.J. Li, Purification of metallurgical grade silicon in an electron beam melting furnace, Surf. Coat. Technol. 228 (2013) S67–S71. [6] L. Zhang, Y. Tan, J.Y. Li, Y. Liu, D.K. Wang, Study of boron removal from molten silicon by slag refining under atmosphere, Mater. Sci. Semicond. Process. 16 (2013) 1645–1649. [7] T. Yoshikawa, K. Morita, An evolving method for solar-grade silicon production solvent refining, JOM 64 (2012) 946–951. [8] J. Dietl, in: Proc. of the Symp. on Matericals and New processing Technologies for Photovoltaics, vol. 11, 1983, p. 52. [9] T. Yoshikawa, K. Morita, Refining of silicon during its solidification from a Si-Al melt, J. Cryst. Growth 311 (2009) 776–779. [10] J.W. Li, Z.C. Guo, H.Q. Tang, Z. Wang, S.T. Sun, Si purification by solidification of Al–Si melt with super gravity, Trans. Nonferr. Met. Soc. China 22 (2012) 958– 963. [11] A.M. Mitrašinovic´, T.A. Utigard, Refining silicon for solar cell application by copper alloying, Silicon 1 (2009) 239–248. [12] S. Esfahani, M. Barati, Purification of metallurgical silicon using iron as an impurity getter part I, Met. Mater. Int. 17 (2011) 825–829. [13] S. Esfahani, M. Barati, Purification of metallurgical silicon using iron as an impurity getter part II, Met. Mater. Int. 17 (2011) 1009–1015. [14] Z. Yin, A. Oliazadeh, S. Esfahani, Solvent refining of silicon using nickel as impurity getter, Can. Metall. Q 5 (7) (2011) 166–172. [15] 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. [16] 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. [17] L. Hu, Z. Wang, X.Z. Gong, Z.C. Guo, H. Zhang, Impurities removal from metallurgical-grade silicon by combined Sn–Si and Al–Si refining processes, Metall. Mater. Trans. B 44 (2013) 828–836. [18] D. Min, N. Sano, Determination of standard free energies of formation of Ca3P2 and Ca2Sn at high temperatures, Metall. Mater. Trans. B 19 (1988) 433–439. [19] C.W. Bale, P. Chartrand, S.A. Degterov, G. Eriksson, K. Hack, R.B. Mahfoud, J. Melancon, A.D. Pelton, S. Petersen, FactSage thermochemical software and databases, Calphad 26 (2002) 189–228. [20] T. Shimpo, T. Yoshikawa, K. Morita, Thermodynamic study of the effect of calcium on removal of phosphorus from silicon by acid leaching treatment, Metall. Mater. Trans. B 35 (2004) 277–284. [21] Y.V. Meteleva-fischer, Y. Room, B. Kraaijveld, H. Kuntzel, Microstructure of metallurgical grade silicon during alloying refining with calcium, Intermetallics 25 (2012) 9–17. [22] T. Yoshikawa, K. Morita, S. Kawanish, T. Tanaka, Thermodynamics of impurity elements in solid silicon, J. Alloys Compd. 490 (2010) 31–41. [23] M.D. Johnston, M. Barati, Calcium and titanium as impurity getter metals in purification of silicon, Sep. Purif. Technol. 107 (2013) 129–134. [24] J.C. Anglézio, C. Servant, F. Dubrous, Characterization of metallurgical grade silicon, J. Mater. Res. 5 (1990) 1894–1899.