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Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon alloy by alternating electromagnetic directional solidification
Accepted Manuscript Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon alloy by alternating electromagnetic directional solidification Yunfei He, Xi Yang, Yu Bao, Shaoyuan Li, Zhengjie Chen, Wenhui Ma, Guoqiang Lv PII: DOI: Reference:
S1383-5866(18)32239-1 https://doi.org/10.1016/j.seppur.2018.10.033 SEPPUR 15020
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
28 June 2018 8 October 2018 16 October 2018
Please cite this article as: Y. He, X. Yang, Y. Bao, S. Li, Z. Chen, W. Ma, G. Lv, Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon alloy by alternating electromagnetic directional solidification, Separation and Purification Technology (2018), doi: https://doi.org/ 10.1016/j.seppur.2018.10.033
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Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon
alloy
by
alternating
electromagnetic directional solidification Yunfei He a,b,c Xi Yang d , Yu Bao a,b,c, Shaoyuan Li a,b,Zhengjie Chen a,b , Wenhui Ma a,b,c*, Guoqiang Lv a,b,* a.
The National Engineering Laboratory for Vacuum Metallurgy, Kunming
University of Science and Technology, Kunming 650093, China b.
Faculty of Metallurgical and Energy Engineering, Kunming University of
Science and Technology, Kunming 650093, China; c.
State Key Laboratory of Complex Nonferrous Metal Resources Cleaning
Utilization in Yunnan Province, Kunming 650093, China d.
Yunnan Provincial Energy Research Institute Co., Ltd., Kunming 650000, China
*Corresponding author: Tel.: +86-871-65161583; Fax: +86-871-65107208. E-mail address: [email protected] (W. Ma); [email protected]
Abstract Hypereutectic aluminum (Al)–silicon (Si) alloys with different Si contents were used to evaluate the effects of Si content on separation and purification by alternating electromagnetic directional solidification (AEM–DS) at 3 kHz. A relatively high pulling speed of 40 µm/s was used for improved energy efficiency. After cooling, samples underwent AEM–DS, and efficient separation was obtained. The Si content in Si-rich areas exceeded 85 wt.% (considerably higher than the content in others) and exhibited not a slight relevance with the original Si contents in the hypereutectic Al–Si alloy. With regard to purification, impurities— 1
particularly metallic impurities (Fe, Ti, and Ca)—can be removed to a low level. Electron probe microanalysis (EPMA) indicated that B content was lower in the Si-rich area than in the Al–Si alloy, suggesting that the segregation of B in solid primary Si was higher than that in Al–Si melt during AEM–DS. Moreover, the contents of metallic impurities (Fe, Ti, and Ca) and B decreased when the initial Si content in the Al–Si melt decreased. This finding was attributed to a lower silicon content, which indicated a lower melt temperature, thus allowing segregation at a lower temperature.
Keywords: Hypereutectic Al–Si alloys; Silicon content; Separation; Purification; Alternating electromagnetic field.
1. Introduction The large demand for clean energy has led to considerable growth in the photovoltaic (PV) industry in recent decades. Solar cells, which are mainly fabricated from Si, comprise more than 90% of PV materials. A traditional and chemical method to produce solar cells, referred to as the Siemens method, is currently used by a large number of manufacturers. However, this chemical approach leads to environmental pollution and involves large costs, necessitating the development of a clean PV industry. Low-cost and environment-friendly methods have to be established to replace the Siemens method. A metallic technique is a desirable choice because it produces SOG–Si releases with much less 2
pollution and is low-cost. Numerous metallic techniques have been developed, such as vacuum refining [1], slag refining [2], directional solidification [3–4], plasma refining [5], and solvent refining [6]. Among
these
methods,
solvent
refining
by
alternating
electromagnetic directional solidification (AEM–DS) exhibits the greatest potential because its temperature requirement is lower than that required to melt Si; in addition, aluminum can absorb typical metallic impurities, allowing the application of directional solidification to purify Si. An alternating electromagnetic field is used to stir the Al–Si melt, which enhances material transfer. Moreover, after solidification, the Si-rich area is at one end and the Al–Si alloy is at another end, facilitating the peeling process. Its continuous production can thus be achieved. Our research team has conducted several studies on the aggregation of Si from a hypereutectic Al–Si melt by AEM–DS [7–10] and proposed a corresponding mechanism [11–12]. AEM–DS was used to separate the Si phase from the hypereutectic Al–Si melt. This approach provides a highly effective method to separate and purify Si in the engineering field. Nevertheless, systematic studies have to be conducted on the influence of Si content on separation and purification, which shows potential for future application.
2. Experimental 2.1. Alloying and electromagnetic stirring separation 3
Fig. 1. Schematic diagram of the experimental devices:(1) argon, (2) gas switch, (3) gas flow meters, (4) control system, (5) water cooled induction coils, (6) Al-Si melt, (7) pulling device equipped with step-machine, (8) glass, (9) vacuum gauge, (10) vacuum pump, (11) bubble checking, (12) infrared pyrometer.
Fig. 1 illustrates the experimental device. Al particles and M–G Si ingots (the main impurities in metallic Si are shown in Table I) were placed in a high-purity, high-density graphite crucible (I.D., 28 mm; O.D., 36 mm; H.D., 130 mm) to prepare 35 wt.%, 45 wt.%, 55 wt.%, and 65 wt.% Si. Each Al–Si component weighed 90 g and was placed in the experimental apparatus, which was an alternating-frequency induction furnace with a 50 kw power and at 3 kHz frequency. The coils had an inner diameter of 100 mm and an outer diameter of 130 mm and contained hollow copper coils for cooling water flow. The bottom of the crucible was initially set to the level of the bottom coils. A vacuum device was then used to remove air from the furnace chamber, and the chamber was filled with high-purity Ar gas (99.99%). After melting and holding at 4
temperatures higher than the melting temperature for 30 min, the samples were cooled, using a pulling speed of 40 µm/s, which was much higher than the speeds applied in other studies; However, our samples show improved separation. The reason is that the frequency (3 kHz) used in our study exerted a deeper skin effect and largely contributed to the melt flow [13]. After cooling, the samples were cut axially and polished. The samples are presented in Figs. 2 and 3.
5
Fig. 2. Samples of different Si contents dealt with different furnaces without pulling: (i) resistance furnace: (a) Al–35w.%Si, (b) Al–45wt.%Si, (c) Al–55wt.%Si, (d) Al–65wt.%Si. (ii) 3 kHz induction furnace: (e) Al–35wt.%Si, (f) Al–45wt.%Si, (g) Al–55wt.%Si, (h) Al–65wt.%Si.
6
Fig. 3. Samples of different Si contents dealt with 3 kHz induction furnace: (a) Al–35wt.%Si, (b) Al–45wt.%Si, (c) Al–55wt.%Si, (d) Al–65wt.%Si.
2.2. Acid leaching After the pulled samples were cooled, Si-enriched areas were found at the bottom of the samples. Cracks between Al–Si alloys and Si-enriched areas provided ease in peeling the former Si-enriched areas from the samples. These Si-rich layers were then crushed into powders with sizes of less than 200 lm. These powder particles were then dissolved using a combination of HCl and HF for 3 h in a water bath heated at 80 ºC, filtered, and dried. Aqua regia was used to conduct another cycle of acid leaching under similar conditions. After acid leaching, the powder particles were collected and then washed with deionized water until a neutral solution was obtained. These powder particles were then dried in a drying box. The acid leaching process is illustrated in Fig. 4. For comparison with metallic Si, raw M–G Si 7
materials were treated under similar conditions.
Fig. 4. Process of acid leaching
2.3. Characterization To characterize the microstructures of the solidified samples and other features, the solidified ingots were cut vertically and polished. The microstructures of the samples, particularly the Si-rich areas, were observed by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy. The Si content in the Si-rich area was determined by X-ray fluorescence spectrometry (XRFS). Moreover, impurities in the Si-rich area were characterized by EPMA, and their distribution was confirmed. The impurity contents in Si after Al–Si solvent refining by AEM–DS with acid leaching and raw Si materials were detected by inductively coupled plasma mass spectrometry.
3. Results and discussion 3.1. Separation of the primary Si phase from Al–Si melts The longitudinal sections of the samples after cooling and polishing 8
are shown in Figs. 2 and 3. Fig. 2 shows the samples without pulling. Fig. 3 shows the samples with pulling under alternating electromagnetic fields. Al–Si alloy with different Si contents at 3 KHz: a–d show samples pulled at 40 μm/s, which is higher than the pulling speed used in the studies by He et al. (10 μm/s) [13], Shaodong Hu et al. (5, 10, and 15 μm/s) [14], T. Yoshikawa and K. Morita (0.25–1.0 mm/min or 4.17–16.7 μm/s) [15], Yu et al. (7, 16, 19, and 25 μm/s) [16], and Lei et al. (1.36–0.22 mm/min or 22.7–3.7 μm/s) [17]. In the current study, 40 μm/s (with a corresponding cooling rate of 18 °C–30.4 °C/min) was used to achieve efficient separation of components. Applying a high pulling speed results in energy efficiency and thus benefits the manufacturing industry. Fig. 2a–h present the samples without directional solidification, similar to a previous study conducted by our research group [12]. Primary Si plates were evenly distributed in all samples in which no separation occurred. Moreover, the samples under an alternating electromagnetic field showed considerably smaller Si plates [Fig. 2(a)– (d); Fig. 2(e)–(h)] because the melt flow can influence Si growth [18–19]. Samples a–d and samples e–h are shown in Fig. 2; the primary Si plates in the samples are increased because of the increase in Si content. Fig. 3 shows samples e–h (the pulled samples) which exhibit Si separation at the bottom where temperature initially decreases. Cracks distinctly appear between the Si-rich areas and Al–Si alloys in all samples. The Si-rich areas can be 9
easily peeled out from the samples. No studies have examined the effect of Si content on Si separation and purification during AEM–DS, which can be used in industrial manufacturing in the future (considering kinds of silicon content can be existed in future production). To evaluate the effects of Si content on purification and separation, the primary Si content of Si-rich areas were determined by XRFS. The results are shown in Fig. 5. AEM–DS at 3 kHz markedly affects Si separation. With the microstructures and Si content of Si-rich areas considered, Al–Si alloys with different Si contents markedly fluctuate in the Si-rich areas. To compare the microstructures of the ingots with different Si content, the cross-sections of the Si-rich areas 3 mm below the cracks were compared with one another (Fig. 3).
10
Fig. 5. Si content in the Si-rich layer of Al–Si alloy with different Si contents.
Fig. 6. Microstructures in different positions: (a) Al–45wt.%Si under 3 kHz AEM. Microstructures in different positions in the Si-rich layer of the Al–Si alloy with different Si contents under AEM–DS, (b) Al–35wt.%Si, (c) Al–45wt.%Si, (d) Al–55wt.%Si and (e) Al–65wt.%Si.
The microstructures of the ingots are shown in Fig. 6b–e; their corresponding positions are 1, 2, 3, and 4 in Fig. 3. The microstructures of the four samples show the same dense and eutectic Al–Si alloy mixed 11
in the Si plates. Fig. 6a is position 1 in Fig. 2f. In samples under an alternating electromagnetic field without directional solidification, primary Si plates are evenly distributed with the eutectic Al–Si alloy surrounding them, and a considerably larger amount of metallic impurities (white areas denote metallic atoms) is in the eutectic Al–Si alloy; meanwhile, the primary Si shows no metallic impurities. In the samples under AEM–DS litter metallic impurities are found in the eutectic Al–Si alloy structures in the bottom areas (corresponding to Fig. 6b–e). In addition, the Si-enriched areas at the bottom are dense. Cracks appear between the Si-rich areas and the eutectic Al–Si alloy, indicating that Si achieves satisfactory separation results in the samples. More primary Si plates can be detected in the structures of the Si-rich areas.
12
Fig. 7. Silicon plates statistics of microstructures of different positions :(a) Al-45.wt%Si under 3 kHz AEM(shown in Fig.6a). Microstructures of positions in Si-rich layer of Al–Si alloy with different Si contents dealt with AEM-DS: (b) Al–35wt.%Si(shown in Fig.6b), (c) Al–45wt.%Si(shown in Fig.6c), (d) Al–55wt.%Si(shown in Fig.6d), and (e) Al–65wt.%Si(shown in Fig.6e).
The quantitative descriptions of the Si plate distribution are based on the imaging results. In the SEM images, different shades denote different 13
elements. The image function and image processing are based on different shades. The process can be described as follows: First, the SEM images are imported into COMSOL Multiphysics 5.3 by using the image function of the software. The image function can sort images based on the shades of the imported images. Second, the range is adjusted. Third, the filtering tools of the software are used to subtract the shades of the eutectic Al–Si. Last, the images are exported. The processed images are shown in Fig. 7. Fig. 7a-e correspond to those in Fig. 6a-e, respectively. The values of image function for Fig. 7a-e are 0.26701, 0.50076, 0.50224, 0.51178, and 0.49905, respectively. The results are consistent with the XRFS result in Fig. 5. In the refining stage, the hypereutectic Al–Si melt is stirred under the alternating electromagnetic field, and the temperature gradient from top to bottom are the two main requirements for achieving good Si separation results at the bottom. The generated melt flow can transport primary Si plates to the low-temperature area where viscosity is high and Si plates can be obtained. When the melt is markedly stirred, most of the primary Si plates can be transported to the bottom and lead to excellent Si separation during AEM–DS. When the melt flow is not sufficient, efficient Si separation cannot be achieved, as demonstrated by He et al. [13] and Hu et al. [14]. The Si-enriched areas at the bottom can be easily peeled out and 14
crushed lightly because of low Al content. On the one hand, persistent alternating electromagnetic field (lower frequency leads to deeper penetration) induces the melt to flow continually and transports Si plates to the low-temperature area continuously. On the other hand, owing to variations in Si content, the heights of the samples differ, and the cracks appear as arcs (as shown in Fig. 3). The arc formation is attributed to the wide heat distribution in the graphic crucibles. During AEM–DS, the primary Si is first formed at the bottom where temperature is low. However, high heat conduction occurs in graphite crucibles, which leads to a large horizontal temperature gradient and agglomeration of the primary Si in the low-temperature area. The sizes of the primary Si are similar with an increase in Si content because a strong melt flow leads to sufficient Si supply. Moreover, the sizes of the primary Si can increase when the temperature gradually decreases. During AEM–DS, a large amount of primary Si tends to precipitate, nucleate, and grow. The Si is then transported to a low-temperature, high-viscosity area at the bottom. When the primary Si particles increase to a sufficient amount and size, they come in contact with one another, exhibiting primary Si separation. This process consists of loose and mesh-like Si plates and can also grow larger by absorbing Si atoms from the hypereutectic Al–Si alloy in the environment until these areas freeze, as shown in Fig. 6b–e. 15
3.2. Impurity removal 3.2.1. Distribution of impurities under different conditions Direct acid leaching [20], Al–Si solvent refining [21-22], thermodynamics of impurities in Al and Si [23], and their segregation ratios [24] were studied. In the current study, a frequency of 3 kHz and a pulling speed of 40 μm/s were used to improve separation and purification. After cooling of the Al–Si alloy, its different contents slightly fluctuated but were higher than 85 wt.%, which is significantly higher than those achieved in other studies [7–8,25–27]. Alternating electromagnetic fields can induce melt flow. When the temperature of the melts decreases, Si is segregated from Al–Si melts, which is a purifying process. Impurities between solid Si and Al–Si melts have low segregation ratios. Wang et al. [28] used Al–Si solvent refining to obtain purified Si without an electromagnetic field, and temperature gradient could be formed by a vertical Bridgman-type furnace. However, the separation result is not satisfactory, which leads acid, Al, and Si waste during acid leaching. The effect of electromagnetic field is to effectively separate these purified Si plates to one end during directional solidification, leaving the impurities in the melts. The considerable Si separation benefited from the large melt flow combined with an appropriate temperature gradient. Moreover, after cooling of samples, the primary Si was crushed below 200 orders and 16
then treated with two kinds of efficient acid solutions. The acid leaching process is illustrated in Fig. 4. The remaining impurities in the Si-enriched area can be effectively removed. Table 1 shows the impurities in metallic Si and the metallic Si directly leached by acid. Table.1 mpurity contents of metallic silicon and leached directly by acid Fe
Ti
Ca
Al
P
B
Content in metallic silicon(ppmw)
4700
1800
260
1600
82
68
After acid leaching
563
230
78
240
66
51
Removal fraction(%)
88.02
87.22
70.00
85.00
19.51
25.00
Fig. 8 shows the impurities of different Si contents in the enriched area after acid leaching. Typical metallic impurities (Ti, Ca, and Fe) can be effectively removed by acid leaching after AEM–DS. Comparatively, the removal of typical nonmetallic impurities (B and P) was not apparent.
17
Fig. 8. The impurities contents of the purified Si separated from the different silicon contents Al–Si alloys after acid leaching
Fig. 9 shows the main properties of the impurities in the Si. Fig. 9a [24] presents the abilities of the elements dissolved in the solid Si, which shows Fe and Ti having small mole fractions in solid Si, whereas Al can dissolve more efficiently in solid Si. This observation explains why a large amount of Al can be detected after acid leaching. Fig. 9b [29] shows the variations in metallic impurities related to temperature change; a lower temperature results in a smaller segregation ratio. However, the results of the refined M–G Si after acid leaching (Fig. 8) show that the expected effect of purification is not consistent with their extremely low segregation ratios. Fig. 9c [30–31], for boron and phosphorus, shows that the segregation ratios decrease when the temperature decreases.
18
Fig. 9. Properties of impurities:(a) abilities of impurity element dissolved in solid Si, (b) Temperature dependence of metallic impurities’ segregation ratios between solid Al-Si solvent and Si, (c) Temperature dependence of B and P between solid Al-Si solvent and Si.
3.2.2. Distribution of impurities in MG–Si and Si-enriched area of Al–Si samples after AEM–DS. To elucidate the removal of impurities, the M–G directly treated with acid leaching was compared with the M–G Si composite with Al subjected to AEM–DS and then treated with acid leaching. As shown in Table 1 and Fig. 8, the Al–Si solvent subjected to AEM–DS can effectively remove impurities from Si to Al. After acid leaching, impurities are removed with eutectic Al–Si alloys. EPMA mapping was used to detect impurities in the M–G Si and Si-enriched area after AEM–DS with solvent refining (M–G Si and Al) of Al–45 wt.% Si and 19
thus clarify the distribution of impurities. The results (Fig. 10) indicate that the impurities (Al, Ti, Fe, Ca, P, and B) have two types of distribution, as shown in Fig. 10. On the one hand, metallic impurities (Al, Ti, Fe, and Ca) are consistently distributed in the boundaries of primary Si grains and can form several intermetallic compounds [32]. On the other hand, B and P are distributed in the entire sample because the segregation coefficients of metallic impurities are smaller than 1 in the primary Si, whereas those of nonmetal impurities are larger (P and B have segregation coefficients of 0.35 and 0.8, respectively).
Fig. 10. Impurity distribution in MG-Si measured by EPMA-mapping.
20
Fig. 11. Impurity distribution in silicon enriched area of Al-45wt.%Si sample measured by EPMA-mapping.
These distributions of impurities also influence the subsequent removal of impurities—that is, acid leaching. The primary Si grain with smaller sizes exert enhanced removal effects because more grain boundaries can be explored in the acid solution. However, acid leaching exerts no apparent effect on the removal of non-metallic impurities because the large segregation coefficients subject that B and P can distribute in primary Si homogeneously, which is the reason acid leaching exerts no removal effect, as shown in Table 1. Fig. 11 presents notable occurrences. The non-metallic impurity B also shows its nonhomogeneous distribution. This occurrence is attributed to the decrease in temperature, which leads to a linear reduction in the 21
segregation coefficients. During directional solidification, a temperature gradient is formed, which facilitates the reduction of segregation coefficients in the removal of the nonmetallic impurity B. Meanwhile, metallic impurities can also show smaller segregation coefficients, which can prompt a decrease in the contents of metallic impurities during directional solidification. Consequently, the effects of removing impurities become apparent during Al–Si refining with AEM–DS, including nonmetallic and metallic impurities. Alternating electromagnetic fields are known to effectively separate Si from hypereutectic Al–Si alloy during directional solidification. The influence of alternating electromagnetic field on the removal of impurities was evaluated by Zou et al. [21] Comparison of the samples with and without an alternating electromagnetic field indicates that without an alternating electromagnetic field, variations in impurity contents are not apparent. An alternating electromagnetic field with directional solidification exhibits considerable purification and separation effects. On the one hand, Al exhibits its ability to capture impurities because of its effective attractive interaction with impurities. An alternating electromagnetic field induces melt flow, which can enhance the transport of impurities and allow for increased interaction between the impurities and Al. On the other hand, removal is also determined in the solid stage; once the temperature gradient is formed, impurities remain in 22
the Al area, and the alternating electromagnetic field is expected to facilitate the transport of solid primary Si to a lower-temperature area from the hypereutectic Al–Si alloy during AEM–DS. Moreover, the formation of complex compounds should also be discussed. Yu et al. [16] suggested that impurities are attached to eutectic Al–Si alloy. Moreover, some eutectic alloys can be enveloped in primary Si inter part, as shown in Fig. 12a-b. The EDS results (Point 1 in Fig .12b) show that impurities are also enveloped in the Si plates. The enveloped impurities during acid leaching cannot be removed effectively because the impurities are not explored. Intermetallic compounds, such as AlFeSi [33], can also be formed by the quasiperitectic reaction L Al
at 612 °C. At 756 °C, Al8SiFe2 is formed. These
intermetallic compounds can limit the removal of impurities. To summarize, impurities enveloped in Si and intermetallic compounds exhibited limited removal of impurities.
23
Fig. 12. (a) Microstructure of silicon enriched area in Al-45wt.%Si sample, (b) an enlargement of the rectangle region marked in (a), (c) EDS analysis of the formed impurities--intermetallic compound, enveloped in silicon plates.
3.2.3 Impurity removal efficiency of different Si contents The liquidus temperatures of different Si contents in hypereutectic Al–Si alloy are as follows: Al–35 wt.% Si, 887 °C; Al–45 wt.% Si, 986 °C; Al–55 wt.% Si, 1087 °C, and Al–65 wt.% Si, 1180 °C. [34]. The segregation coefficients of Ti, Fe, and Ca, including those of B and P, can decrease with a reduction in temperature. The temperature and these impurities (Fe, Ca, Ti, P, and B) exhibit a linear relationship, as shown in Fig. 9b-c. The Al–Si binary diagram shows that different Si contents in the hypereutectic Al–Si alloy indicate different temperatures for primary Si solidification. The higher the Si content in the alloy, the higher the solidification temperature. During directional solidification, the Al–65 24
wt.% Si hypereutectic melt was chosen as the typical melt to explain the directional solidification process. When the temperature of the melt was 1180 °C, the Si started to precipitate and flow with the melts. During directional solidification, the temperature decreases, which means that the Si content of hypereutectic Al–Si melts decreases (to 1087 °C) and reduces the Si content in the hypereutectic Al–Si alloy (Al–55 wt.% Si). The first solidified Si contains more impurities than the subsequent one because a higher temperature facilitates the formation of intermetallic compounds. A higher Si content also leads to the formation of more Si plates, thereby limiting the melt flow. More impurities can be enveloped in these Si plates, influencing acid leaching.
4. Conclusion The effects of hypereutectic Al–Si alloy with different Si contents on impurity removal during AEM–DS. A high pulling speed of 40 μm/s was used in this study. Significant separation occurred when a 3 kHz frequency induction furnace was used. The results show that the Si content of Al–Si melts slightly influence the microstructures of primary Si and Si content in the Si-enriched area. For impurity removal, typical impurities can be reduced to a markedly low content. The removal efficiency of B and metallic impurities (Ca, Ti, and Fe) can decrease when the initial Si content is increased. When the Si content is higher, segregation occurs at a higher temperature (complex intermetallic 25
compounds can be formed), and some impurities can be encapsulated in the primary Si because the large number of Si plates limits the transport of impurities, which negatively influences Si purification.
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Highlights ►A larger pulling out speed was used to separate silicon from Al-Si melts. ►Different silicon contents show litter influence in the silicon enriched area. ► Silicon contents can reach 85wt.% after AEM-DS. ► Impurities removal was influenced by silicon contents.
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S1383-5866(18)32239-1 https://doi.org/10.1016/j.seppur.2018.10.033 SEPPUR 15020
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
28 June 2018 8 October 2018 16 October 2018
Please cite this article as: Y. He, X. Yang, Y. Bao, S. Li, Z. Chen, W. Ma, G. Lv, Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon alloy by alternating electromagnetic directional solidification, Separation and Purification Technology (2018), doi: https://doi.org/ 10.1016/j.seppur.2018.10.033
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Effects of silicon content on the separation and purification of primary silicon from hypereutectic aluminum–silicon
alloy
by
alternating
electromagnetic directional solidification Yunfei He a,b,c Xi Yang d , Yu Bao a,b,c, Shaoyuan Li a,b,Zhengjie Chen a,b , Wenhui Ma a,b,c*, Guoqiang Lv a,b,* a.
The National Engineering Laboratory for Vacuum Metallurgy, Kunming
University of Science and Technology, Kunming 650093, China b.
Faculty of Metallurgical and Energy Engineering, Kunming University of
Science and Technology, Kunming 650093, China; c.
State Key Laboratory of Complex Nonferrous Metal Resources Cleaning
Utilization in Yunnan Province, Kunming 650093, China d.
Yunnan Provincial Energy Research Institute Co., Ltd., Kunming 650000, China
*Corresponding author: Tel.: +86-871-65161583; Fax: +86-871-65107208. E-mail address: [email protected] (W. Ma); [email protected]
Abstract Hypereutectic aluminum (Al)–silicon (Si) alloys with different Si contents were used to evaluate the effects of Si content on separation and purification by alternating electromagnetic directional solidification (AEM–DS) at 3 kHz. A relatively high pulling speed of 40 µm/s was used for improved energy efficiency. After cooling, samples underwent AEM–DS, and efficient separation was obtained. The Si content in Si-rich areas exceeded 85 wt.% (considerably higher than the content in others) and exhibited not a slight relevance with the original Si contents in the hypereutectic Al–Si alloy. With regard to purification, impurities— 1
particularly metallic impurities (Fe, Ti, and Ca)—can be removed to a low level. Electron probe microanalysis (EPMA) indicated that B content was lower in the Si-rich area than in the Al–Si alloy, suggesting that the segregation of B in solid primary Si was higher than that in Al–Si melt during AEM–DS. Moreover, the contents of metallic impurities (Fe, Ti, and Ca) and B decreased when the initial Si content in the Al–Si melt decreased. This finding was attributed to a lower silicon content, which indicated a lower melt temperature, thus allowing segregation at a lower temperature.
Keywords: Hypereutectic Al–Si alloys; Silicon content; Separation; Purification; Alternating electromagnetic field.
1. Introduction The large demand for clean energy has led to considerable growth in the photovoltaic (PV) industry in recent decades. Solar cells, which are mainly fabricated from Si, comprise more than 90% of PV materials. A traditional and chemical method to produce solar cells, referred to as the Siemens method, is currently used by a large number of manufacturers. However, this chemical approach leads to environmental pollution and involves large costs, necessitating the development of a clean PV industry. Low-cost and environment-friendly methods have to be established to replace the Siemens method. A metallic technique is a desirable choice because it produces SOG–Si releases with much less 2
pollution and is low-cost. Numerous metallic techniques have been developed, such as vacuum refining [1], slag refining [2], directional solidification [3–4], plasma refining [5], and solvent refining [6]. Among
these
methods,
solvent
refining
by
alternating
electromagnetic directional solidification (AEM–DS) exhibits the greatest potential because its temperature requirement is lower than that required to melt Si; in addition, aluminum can absorb typical metallic impurities, allowing the application of directional solidification to purify Si. An alternating electromagnetic field is used to stir the Al–Si melt, which enhances material transfer. Moreover, after solidification, the Si-rich area is at one end and the Al–Si alloy is at another end, facilitating the peeling process. Its continuous production can thus be achieved. Our research team has conducted several studies on the aggregation of Si from a hypereutectic Al–Si melt by AEM–DS [7–10] and proposed a corresponding mechanism [11–12]. AEM–DS was used to separate the Si phase from the hypereutectic Al–Si melt. This approach provides a highly effective method to separate and purify Si in the engineering field. Nevertheless, systematic studies have to be conducted on the influence of Si content on separation and purification, which shows potential for future application.
2. Experimental 2.1. Alloying and electromagnetic stirring separation 3
Fig. 1. Schematic diagram of the experimental devices:(1) argon, (2) gas switch, (3) gas flow meters, (4) control system, (5) water cooled induction coils, (6) Al-Si melt, (7) pulling device equipped with step-machine, (8) glass, (9) vacuum gauge, (10) vacuum pump, (11) bubble checking, (12) infrared pyrometer.
Fig. 1 illustrates the experimental device. Al particles and M–G Si ingots (the main impurities in metallic Si are shown in Table I) were placed in a high-purity, high-density graphite crucible (I.D., 28 mm; O.D., 36 mm; H.D., 130 mm) to prepare 35 wt.%, 45 wt.%, 55 wt.%, and 65 wt.% Si. Each Al–Si component weighed 90 g and was placed in the experimental apparatus, which was an alternating-frequency induction furnace with a 50 kw power and at 3 kHz frequency. The coils had an inner diameter of 100 mm and an outer diameter of 130 mm and contained hollow copper coils for cooling water flow. The bottom of the crucible was initially set to the level of the bottom coils. A vacuum device was then used to remove air from the furnace chamber, and the chamber was filled with high-purity Ar gas (99.99%). After melting and holding at 4
temperatures higher than the melting temperature for 30 min, the samples were cooled, using a pulling speed of 40 µm/s, which was much higher than the speeds applied in other studies; However, our samples show improved separation. The reason is that the frequency (3 kHz) used in our study exerted a deeper skin effect and largely contributed to the melt flow [13]. After cooling, the samples were cut axially and polished. The samples are presented in Figs. 2 and 3.
5
Fig. 2. Samples of different Si contents dealt with different furnaces without pulling: (i) resistance furnace: (a) Al–35w.%Si, (b) Al–45wt.%Si, (c) Al–55wt.%Si, (d) Al–65wt.%Si. (ii) 3 kHz induction furnace: (e) Al–35wt.%Si, (f) Al–45wt.%Si, (g) Al–55wt.%Si, (h) Al–65wt.%Si.
6
Fig. 3. Samples of different Si contents dealt with 3 kHz induction furnace: (a) Al–35wt.%Si, (b) Al–45wt.%Si, (c) Al–55wt.%Si, (d) Al–65wt.%Si.
2.2. Acid leaching After the pulled samples were cooled, Si-enriched areas were found at the bottom of the samples. Cracks between Al–Si alloys and Si-enriched areas provided ease in peeling the former Si-enriched areas from the samples. These Si-rich layers were then crushed into powders with sizes of less than 200 lm. These powder particles were then dissolved using a combination of HCl and HF for 3 h in a water bath heated at 80 ºC, filtered, and dried. Aqua regia was used to conduct another cycle of acid leaching under similar conditions. After acid leaching, the powder particles were collected and then washed with deionized water until a neutral solution was obtained. These powder particles were then dried in a drying box. The acid leaching process is illustrated in Fig. 4. For comparison with metallic Si, raw M–G Si 7
materials were treated under similar conditions.
Fig. 4. Process of acid leaching
2.3. Characterization To characterize the microstructures of the solidified samples and other features, the solidified ingots were cut vertically and polished. The microstructures of the samples, particularly the Si-rich areas, were observed by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy. The Si content in the Si-rich area was determined by X-ray fluorescence spectrometry (XRFS). Moreover, impurities in the Si-rich area were characterized by EPMA, and their distribution was confirmed. The impurity contents in Si after Al–Si solvent refining by AEM–DS with acid leaching and raw Si materials were detected by inductively coupled plasma mass spectrometry.
3. Results and discussion 3.1. Separation of the primary Si phase from Al–Si melts The longitudinal sections of the samples after cooling and polishing 8
are shown in Figs. 2 and 3. Fig. 2 shows the samples without pulling. Fig. 3 shows the samples with pulling under alternating electromagnetic fields. Al–Si alloy with different Si contents at 3 KHz: a–d show samples pulled at 40 μm/s, which is higher than the pulling speed used in the studies by He et al. (10 μm/s) [13], Shaodong Hu et al. (5, 10, and 15 μm/s) [14], T. Yoshikawa and K. Morita (0.25–1.0 mm/min or 4.17–16.7 μm/s) [15], Yu et al. (7, 16, 19, and 25 μm/s) [16], and Lei et al. (1.36–0.22 mm/min or 22.7–3.7 μm/s) [17]. In the current study, 40 μm/s (with a corresponding cooling rate of 18 °C–30.4 °C/min) was used to achieve efficient separation of components. Applying a high pulling speed results in energy efficiency and thus benefits the manufacturing industry. Fig. 2a–h present the samples without directional solidification, similar to a previous study conducted by our research group [12]. Primary Si plates were evenly distributed in all samples in which no separation occurred. Moreover, the samples under an alternating electromagnetic field showed considerably smaller Si plates [Fig. 2(a)– (d); Fig. 2(e)–(h)] because the melt flow can influence Si growth [18–19]. Samples a–d and samples e–h are shown in Fig. 2; the primary Si plates in the samples are increased because of the increase in Si content. Fig. 3 shows samples e–h (the pulled samples) which exhibit Si separation at the bottom where temperature initially decreases. Cracks distinctly appear between the Si-rich areas and Al–Si alloys in all samples. The Si-rich areas can be 9
easily peeled out from the samples. No studies have examined the effect of Si content on Si separation and purification during AEM–DS, which can be used in industrial manufacturing in the future (considering kinds of silicon content can be existed in future production). To evaluate the effects of Si content on purification and separation, the primary Si content of Si-rich areas were determined by XRFS. The results are shown in Fig. 5. AEM–DS at 3 kHz markedly affects Si separation. With the microstructures and Si content of Si-rich areas considered, Al–Si alloys with different Si contents markedly fluctuate in the Si-rich areas. To compare the microstructures of the ingots with different Si content, the cross-sections of the Si-rich areas 3 mm below the cracks were compared with one another (Fig. 3).
10
Fig. 5. Si content in the Si-rich layer of Al–Si alloy with different Si contents.
Fig. 6. Microstructures in different positions: (a) Al–45wt.%Si under 3 kHz AEM. Microstructures in different positions in the Si-rich layer of the Al–Si alloy with different Si contents under AEM–DS, (b) Al–35wt.%Si, (c) Al–45wt.%Si, (d) Al–55wt.%Si and (e) Al–65wt.%Si.
The microstructures of the ingots are shown in Fig. 6b–e; their corresponding positions are 1, 2, 3, and 4 in Fig. 3. The microstructures of the four samples show the same dense and eutectic Al–Si alloy mixed 11
in the Si plates. Fig. 6a is position 1 in Fig. 2f. In samples under an alternating electromagnetic field without directional solidification, primary Si plates are evenly distributed with the eutectic Al–Si alloy surrounding them, and a considerably larger amount of metallic impurities (white areas denote metallic atoms) is in the eutectic Al–Si alloy; meanwhile, the primary Si shows no metallic impurities. In the samples under AEM–DS litter metallic impurities are found in the eutectic Al–Si alloy structures in the bottom areas (corresponding to Fig. 6b–e). In addition, the Si-enriched areas at the bottom are dense. Cracks appear between the Si-rich areas and the eutectic Al–Si alloy, indicating that Si achieves satisfactory separation results in the samples. More primary Si plates can be detected in the structures of the Si-rich areas.
12
Fig. 7. Silicon plates statistics of microstructures of different positions :(a) Al-45.wt%Si under 3 kHz AEM(shown in Fig.6a). Microstructures of positions in Si-rich layer of Al–Si alloy with different Si contents dealt with AEM-DS: (b) Al–35wt.%Si(shown in Fig.6b), (c) Al–45wt.%Si(shown in Fig.6c), (d) Al–55wt.%Si(shown in Fig.6d), and (e) Al–65wt.%Si(shown in Fig.6e).
The quantitative descriptions of the Si plate distribution are based on the imaging results. In the SEM images, different shades denote different 13
elements. The image function and image processing are based on different shades. The process can be described as follows: First, the SEM images are imported into COMSOL Multiphysics 5.3 by using the image function of the software. The image function can sort images based on the shades of the imported images. Second, the range is adjusted. Third, the filtering tools of the software are used to subtract the shades of the eutectic Al–Si. Last, the images are exported. The processed images are shown in Fig. 7. Fig. 7a-e correspond to those in Fig. 6a-e, respectively. The values of image function for Fig. 7a-e are 0.26701, 0.50076, 0.50224, 0.51178, and 0.49905, respectively. The results are consistent with the XRFS result in Fig. 5. In the refining stage, the hypereutectic Al–Si melt is stirred under the alternating electromagnetic field, and the temperature gradient from top to bottom are the two main requirements for achieving good Si separation results at the bottom. The generated melt flow can transport primary Si plates to the low-temperature area where viscosity is high and Si plates can be obtained. When the melt is markedly stirred, most of the primary Si plates can be transported to the bottom and lead to excellent Si separation during AEM–DS. When the melt flow is not sufficient, efficient Si separation cannot be achieved, as demonstrated by He et al. [13] and Hu et al. [14]. The Si-enriched areas at the bottom can be easily peeled out and 14
crushed lightly because of low Al content. On the one hand, persistent alternating electromagnetic field (lower frequency leads to deeper penetration) induces the melt to flow continually and transports Si plates to the low-temperature area continuously. On the other hand, owing to variations in Si content, the heights of the samples differ, and the cracks appear as arcs (as shown in Fig. 3). The arc formation is attributed to the wide heat distribution in the graphic crucibles. During AEM–DS, the primary Si is first formed at the bottom where temperature is low. However, high heat conduction occurs in graphite crucibles, which leads to a large horizontal temperature gradient and agglomeration of the primary Si in the low-temperature area. The sizes of the primary Si are similar with an increase in Si content because a strong melt flow leads to sufficient Si supply. Moreover, the sizes of the primary Si can increase when the temperature gradually decreases. During AEM–DS, a large amount of primary Si tends to precipitate, nucleate, and grow. The Si is then transported to a low-temperature, high-viscosity area at the bottom. When the primary Si particles increase to a sufficient amount and size, they come in contact with one another, exhibiting primary Si separation. This process consists of loose and mesh-like Si plates and can also grow larger by absorbing Si atoms from the hypereutectic Al–Si alloy in the environment until these areas freeze, as shown in Fig. 6b–e. 15
3.2. Impurity removal 3.2.1. Distribution of impurities under different conditions Direct acid leaching [20], Al–Si solvent refining [21-22], thermodynamics of impurities in Al and Si [23], and their segregation ratios [24] were studied. In the current study, a frequency of 3 kHz and a pulling speed of 40 μm/s were used to improve separation and purification. After cooling of the Al–Si alloy, its different contents slightly fluctuated but were higher than 85 wt.%, which is significantly higher than those achieved in other studies [7–8,25–27]. Alternating electromagnetic fields can induce melt flow. When the temperature of the melts decreases, Si is segregated from Al–Si melts, which is a purifying process. Impurities between solid Si and Al–Si melts have low segregation ratios. Wang et al. [28] used Al–Si solvent refining to obtain purified Si without an electromagnetic field, and temperature gradient could be formed by a vertical Bridgman-type furnace. However, the separation result is not satisfactory, which leads acid, Al, and Si waste during acid leaching. The effect of electromagnetic field is to effectively separate these purified Si plates to one end during directional solidification, leaving the impurities in the melts. The considerable Si separation benefited from the large melt flow combined with an appropriate temperature gradient. Moreover, after cooling of samples, the primary Si was crushed below 200 orders and 16
then treated with two kinds of efficient acid solutions. The acid leaching process is illustrated in Fig. 4. The remaining impurities in the Si-enriched area can be effectively removed. Table 1 shows the impurities in metallic Si and the metallic Si directly leached by acid. Table.1 mpurity contents of metallic silicon and leached directly by acid Fe
Ti
Ca
Al
P
B
Content in metallic silicon(ppmw)
4700
1800
260
1600
82
68
After acid leaching
563
230
78
240
66
51
Removal fraction(%)
88.02
87.22
70.00
85.00
19.51
25.00
Fig. 8 shows the impurities of different Si contents in the enriched area after acid leaching. Typical metallic impurities (Ti, Ca, and Fe) can be effectively removed by acid leaching after AEM–DS. Comparatively, the removal of typical nonmetallic impurities (B and P) was not apparent.
17
Fig. 8. The impurities contents of the purified Si separated from the different silicon contents Al–Si alloys after acid leaching
Fig. 9 shows the main properties of the impurities in the Si. Fig. 9a [24] presents the abilities of the elements dissolved in the solid Si, which shows Fe and Ti having small mole fractions in solid Si, whereas Al can dissolve more efficiently in solid Si. This observation explains why a large amount of Al can be detected after acid leaching. Fig. 9b [29] shows the variations in metallic impurities related to temperature change; a lower temperature results in a smaller segregation ratio. However, the results of the refined M–G Si after acid leaching (Fig. 8) show that the expected effect of purification is not consistent with their extremely low segregation ratios. Fig. 9c [30–31], for boron and phosphorus, shows that the segregation ratios decrease when the temperature decreases.
18
Fig. 9. Properties of impurities:(a) abilities of impurity element dissolved in solid Si, (b) Temperature dependence of metallic impurities’ segregation ratios between solid Al-Si solvent and Si, (c) Temperature dependence of B and P between solid Al-Si solvent and Si.
3.2.2. Distribution of impurities in MG–Si and Si-enriched area of Al–Si samples after AEM–DS. To elucidate the removal of impurities, the M–G directly treated with acid leaching was compared with the M–G Si composite with Al subjected to AEM–DS and then treated with acid leaching. As shown in Table 1 and Fig. 8, the Al–Si solvent subjected to AEM–DS can effectively remove impurities from Si to Al. After acid leaching, impurities are removed with eutectic Al–Si alloys. EPMA mapping was used to detect impurities in the M–G Si and Si-enriched area after AEM–DS with solvent refining (M–G Si and Al) of Al–45 wt.% Si and 19
thus clarify the distribution of impurities. The results (Fig. 10) indicate that the impurities (Al, Ti, Fe, Ca, P, and B) have two types of distribution, as shown in Fig. 10. On the one hand, metallic impurities (Al, Ti, Fe, and Ca) are consistently distributed in the boundaries of primary Si grains and can form several intermetallic compounds [32]. On the other hand, B and P are distributed in the entire sample because the segregation coefficients of metallic impurities are smaller than 1 in the primary Si, whereas those of nonmetal impurities are larger (P and B have segregation coefficients of 0.35 and 0.8, respectively).
Fig. 10. Impurity distribution in MG-Si measured by EPMA-mapping.
20
Fig. 11. Impurity distribution in silicon enriched area of Al-45wt.%Si sample measured by EPMA-mapping.
These distributions of impurities also influence the subsequent removal of impurities—that is, acid leaching. The primary Si grain with smaller sizes exert enhanced removal effects because more grain boundaries can be explored in the acid solution. However, acid leaching exerts no apparent effect on the removal of non-metallic impurities because the large segregation coefficients subject that B and P can distribute in primary Si homogeneously, which is the reason acid leaching exerts no removal effect, as shown in Table 1. Fig. 11 presents notable occurrences. The non-metallic impurity B also shows its nonhomogeneous distribution. This occurrence is attributed to the decrease in temperature, which leads to a linear reduction in the 21
segregation coefficients. During directional solidification, a temperature gradient is formed, which facilitates the reduction of segregation coefficients in the removal of the nonmetallic impurity B. Meanwhile, metallic impurities can also show smaller segregation coefficients, which can prompt a decrease in the contents of metallic impurities during directional solidification. Consequently, the effects of removing impurities become apparent during Al–Si refining with AEM–DS, including nonmetallic and metallic impurities. Alternating electromagnetic fields are known to effectively separate Si from hypereutectic Al–Si alloy during directional solidification. The influence of alternating electromagnetic field on the removal of impurities was evaluated by Zou et al. [21] Comparison of the samples with and without an alternating electromagnetic field indicates that without an alternating electromagnetic field, variations in impurity contents are not apparent. An alternating electromagnetic field with directional solidification exhibits considerable purification and separation effects. On the one hand, Al exhibits its ability to capture impurities because of its effective attractive interaction with impurities. An alternating electromagnetic field induces melt flow, which can enhance the transport of impurities and allow for increased interaction between the impurities and Al. On the other hand, removal is also determined in the solid stage; once the temperature gradient is formed, impurities remain in 22
the Al area, and the alternating electromagnetic field is expected to facilitate the transport of solid primary Si to a lower-temperature area from the hypereutectic Al–Si alloy during AEM–DS. Moreover, the formation of complex compounds should also be discussed. Yu et al. [16] suggested that impurities are attached to eutectic Al–Si alloy. Moreover, some eutectic alloys can be enveloped in primary Si inter part, as shown in Fig. 12a-b. The EDS results (Point 1 in Fig .12b) show that impurities are also enveloped in the Si plates. The enveloped impurities during acid leaching cannot be removed effectively because the impurities are not explored. Intermetallic compounds, such as AlFeSi [33], can also be formed by the quasiperitectic reaction L Al
at 612 °C. At 756 °C, Al8SiFe2 is formed. These
intermetallic compounds can limit the removal of impurities. To summarize, impurities enveloped in Si and intermetallic compounds exhibited limited removal of impurities.
23
Fig. 12. (a) Microstructure of silicon enriched area in Al-45wt.%Si sample, (b) an enlargement of the rectangle region marked in (a), (c) EDS analysis of the formed impurities--intermetallic compound, enveloped in silicon plates.
3.2.3 Impurity removal efficiency of different Si contents The liquidus temperatures of different Si contents in hypereutectic Al–Si alloy are as follows: Al–35 wt.% Si, 887 °C; Al–45 wt.% Si, 986 °C; Al–55 wt.% Si, 1087 °C, and Al–65 wt.% Si, 1180 °C. [34]. The segregation coefficients of Ti, Fe, and Ca, including those of B and P, can decrease with a reduction in temperature. The temperature and these impurities (Fe, Ca, Ti, P, and B) exhibit a linear relationship, as shown in Fig. 9b-c. The Al–Si binary diagram shows that different Si contents in the hypereutectic Al–Si alloy indicate different temperatures for primary Si solidification. The higher the Si content in the alloy, the higher the solidification temperature. During directional solidification, the Al–65 24
wt.% Si hypereutectic melt was chosen as the typical melt to explain the directional solidification process. When the temperature of the melt was 1180 °C, the Si started to precipitate and flow with the melts. During directional solidification, the temperature decreases, which means that the Si content of hypereutectic Al–Si melts decreases (to 1087 °C) and reduces the Si content in the hypereutectic Al–Si alloy (Al–55 wt.% Si). The first solidified Si contains more impurities than the subsequent one because a higher temperature facilitates the formation of intermetallic compounds. A higher Si content also leads to the formation of more Si plates, thereby limiting the melt flow. More impurities can be enveloped in these Si plates, influencing acid leaching.
4. Conclusion The effects of hypereutectic Al–Si alloy with different Si contents on impurity removal during AEM–DS. A high pulling speed of 40 μm/s was used in this study. Significant separation occurred when a 3 kHz frequency induction furnace was used. The results show that the Si content of Al–Si melts slightly influence the microstructures of primary Si and Si content in the Si-enriched area. For impurity removal, typical impurities can be reduced to a markedly low content. The removal efficiency of B and metallic impurities (Ca, Ti, and Fe) can decrease when the initial Si content is increased. When the Si content is higher, segregation occurs at a higher temperature (complex intermetallic 25
compounds can be formed), and some impurities can be encapsulated in the primary Si because the large number of Si plates limits the transport of impurities, which negatively influences Si purification.
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Highlights ►A larger pulling out speed was used to separate silicon from Al-Si melts. ►Different silicon contents show litter influence in the silicon enriched area. ► Silicon contents can reach 85wt.% after AEM-DS. ► Impurities removal was influenced by silicon contents.
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