Trans. Nonferrous Met. Soc. China 24(2014) 1231-1236

Boron removal in purifying metallurgical grade silicon by CaO-SiO₂ slag refining

Ji-jun WU^1,2,3, Yan-long LI^1,3, Wen-hui MA^1,2,3, Kui-xian WEI^1,2,3, Bin YANG^1,2, Yong-nian DAI^1,2,3

1. State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province/National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China;

2. Yunnan Provincial Key Laboratory of Nonferrous Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China;

3. Engineering Research Center for Silicon Metallurgy and Silicon Materials of Yunnan Provincial Universities, Kunming University of Science and Technology, Kunming 650093, China

Received 8 April 2013; accepted 26 August 2013

Abstract: Boron removal from metallurgical grade silicon (MG-Si) using a calcium silicate slag was studied. The results show that it is impossible basically to remove boron using a pure SiO₂ refining. The oxidizing ability of CaO-SiO₂ slag for boron removal was characterized by establishing the thermodynamic relationship between the distribution coefficient of boron (L_B) and the activities of SiO₂ and CaO. The experimental results show that the distribution coefficient and the removal efficiency of boron are greatly improved with the increase of CaO proportion in the slag. The maximal value of L_B reaches 1.57 with a slag composition of 60%CaO-40%SiO₂ (mass fraction). The boron content in the refined silicon is reduced from 18×10^-6 to 1.8×10^-6 using slag refining at 1600 °C for 3 h with a CaO-SiO₂/MG-Si ratio of 2.5, and the removal efficiency of boron reaches 90%.

Key words: metallurgical grade silicon; boron removal; thermodynamics; distribution coefficient; slag refining

1 Introduction

Renewable energy is the only way to substitute fossil fuel [1,2]. In recent years, solar cell has been paid close attention for its potential advantages. Traditionally, the off-spec silicon feeds from semiconductor industry produced by the Simens process have been supplied to the photovoltaic industry [3,4]. The metallurgical route is an increasingly popular method in the range from purifying metallurgical grade silicon (MG-Si) to solar grade silicon (SoG-Si). However, boron removal from MG-Si is still a puzzle as a result of the low vapour pressure for boron [5] and the large segregation coefficient for boron to silicon [6]. Boron, which will accelerate the auger recombination and reduce the mobility ratio of minority carriers in silicon wafer [7,8] on account of the existence of the recombination centers such as Fe-B pairs, B-O metastable complexes and Cu-related extended defects [9], is a trouble maker to the photoelectric conversion.

As a rule, the boron content in SoG-Si is required to be lower than 0.5×10^-6. To meet this requirement, a purification process for boron removal must be operated. These methods include gas blowing, thermal plasma and slag refining. Through blowing the reactive gases such as O₂, H₂ and H₂O into MG-Si melt, the gaseous boric species B_xO_y can be generated and volatilized. It is similar to the thermal plasma refining, which usually has preferable boron removal efficiency [10,11]. Slag refining employed in the steelmaking industry is also an efficient way to remove boron from MG-Si and these slags include CaO-SiO₂, CaO-SiO₂-CaF₂, CaO-SiO₂- Al₂O₃, CaO-SiO₂-MgO and CaO-SiO₂-Na₂O [12,13]. KHATTAK and SCHMID [14] studied the boron removal using a CaO-SiO₂ slag refining and the boron content in MG-Si was reduced from 18×10^-6 to 1.0×10^-6. JOHNSTON and BARATI [15] and LUO et al [16] found that the slag baisicity interpreted as the concentration of free oxygen ions in slag had an important effect on boron removal.

In the present work, the boron removal from MG-Si using CaO-SiO₂ slag was studied, and the role of CaO in slag was illustrated. The distribution of boron between slag and silicon (L_B) was determined using different slag composition. Simultaneously, the oxidizing ability of CaO-SiO₂ slag to impurity boron was calculated and characterized. The boron removal was finally carried out with different ratios of slag to silicon and refining time.

2 Experimental

The MG-Si with a boron content of 18×10^-6 was used in this work. It was smashed and ground to powders with sizes of 50-200 μm. The MG-Si powder and the reagent grade chemicals of CaO and SiO₂ were mixed and loaded in a graphite crucible. The crucible was put into a quartz tube of intermediate frequency inductive furnace. Argon with 99.9% purity was blown into quartz tube for protection, and the temperature was determined by an infrared thermometer. The schematic diagram of experimental apparatus is shown in Fig. 1.

Fig. 1 Schematic diagram of apparatus for molten slag refining

Boron removal using the pure SiO₂ and CaO-SiO₂ binary slag refining was carried out under different conditions of slag composition, mass ratios of slag to silicon and refining time, respectively. The boron contents in the refined slag and silicon samples were determined by inductively coupled plasma mass spectrometry (ICP-MS, Elan-5000A, USA).

3 Results and discussion

3.1 Boron removal using pure oxide SiO₂

Boron removal from MG-Si using pure SiO₂ refining was carried out with different refining temperatures and mass ratios of SiO₂ versus MG-Si. TEIXEIRA and MORITA [17], JOHNSTON and BARATI [18] described that boron in silicon (expressed as [B]) could be oxidized into boric oxide by free oxygen ion (O^2-) and oxygen (O₂). Then, the generated boric oxide would enter a basic slag phase based on calcium oxide in the form of negative ion , and a calcium borate phase was finally generated via the ionic equation as follows:

                   (1)

The studies focused on the free oxygen ion released by the basic oxides CaO and oxygen decomposed by SiO₂. In thermodynamics, the activity of free oxygen ion and the partial pressure of oxygen were crucial for boron removal. However, the oxygen partial pressure would be restricted by the decomposition of SiO₂.

In addition, TEIXEIRA et al [19] detailed that boron in silicon was directly oxidized into BO_1.5 by SiO₂ and it was expressed as:

                   (2)

It is found that this reaction would only take place at a higher temperature than 1961.8 °C according to the standard Gibbs free energy of formation, and it could be thought that the oxidation of boron by the pure SiO₂ was especially difficult. Experimentally, boron removal was done in the intermediate frequency furnace at the refining temperatures of 1500 and 1750 °C, respectively. The mass ratios of SiO₂ to MG-Si were set as 1:10, 2:10, 3:10 and 4:10. Figure 2 shows the refined sample using pure SiO₂ at 1500 °C.

It is seen from the longitudinal section of refined sample in Fig. 2(a) that SiO₂ distributes on the top of refined silicon and takes on the state of granule, which implies that SiO₂ is unmelted during the refining process. The cross section image of sample in Fig. 2(b) shows a same distribution for SiO₂ as the longitudinal section. The boron removal using SiO₂ with a refining time of 2 h and different mass ratios of SiO₂ to MG-Si at 1500 and 1750 °C, respectively, are shown in Fig. 3.

Fig. 2 Silicon sample refined by pure SiO₂

Fig. 3 Effects of mass ratio of SiO₂ to MG-Si and refining temperature on boron removal

It is found that the boron content is reduced from 18×10^-6 to 16×10^-6 at 1500 °C and the removal efficiency of boron is only 11%. However, it is reduced to 11.3×10^-6 at a refining temperature of 1750 °C and the removal efficiency reaches 37%. Though it is hardly effective for boron removal using pure SiO₂ refining, it is more helpful for a higher refining temperature. The result is also in accordance with the thermodynamic calculation. It is thought that the liquid SiO₂ may make the oxidation of boron in molt silicon more easily compared with the solid state one. As shown in the SiO₂-B₂O₃ binary phase diagram [20], the acidic B₂O₃ generated in the interface of SiO₂ and MG-Si can not be easily dissolved into a same acidic slag. Therefore, the diffusion of B₂O₃ towards slag phase is obstructed on account of the accumulation of B₂O₃ in the interface of slag and silicon.

3.2 Boron removal using CaO-SiO₂ binary slag

The basic oxide CaO was added to SiO₂, however, the oxidation of boron became possible. At the moment, Eq. (2) might be written as:

   (3)

It is found that the theoretical initial temperature of Eq. (3) is greatly reduced compared with that of Eq. (2) according to the standard Gibbs free energy of formation. The molten silicon and slag are considered as a dilute solution. On condition that the standard state for the Henry’s law is chosen, the equilibrium constant (K_p) of Eq. (3) might be written as:

            (4)

where w(Ca_3/2BO₃) and w[B] are the mass fraction of calcium borate in slag and the mass fraction of boron in silicon, respectively; a and f are the activity and the activity coefficient, respectively. For the Henry’s law, it is considered that  and a_Si(l)=1.  can be substituted with the concentration of boron in slag (w(B)), and it can be expressed as:

                      (5)

The distribution coefficient of boron between slag and silicon (L_B) is expressed as:

                             (6)

Lastly, the distribution coefficient of boron might be written as:

      (7)

The activities of SiO₂ and CaO in CaO-SiO₂ binary system (0.4<<0.7) were determined by ZOU et al [21], as shown in Fig. 4.

Fig. 4 Activities of CaO and SiO₂ in CaO-SiO₂ binary system at 1600 °C [21]

It is found that the activity of SiO₂ increases with the rise of its proportion in CaO-SiO₂ slag. It is completely opposite to CaO for this case. As shown in Fig. 5, the thermodynamic relationship displays that the value of item  in Eq. (7) decreases gradually when the mole fraction of SiO₂ in slag is larger than 0.4.

Fig. 5 Relationship between item and mole fraction of SiO₂ in slag

Figure 6 shows the refined silicon sample using CaO-SiO₂ binary slag at 1600 °C. It is seen that the CaO-SiO₂ slag is melted completely and separated out from silicon melt. After refining, the refining slags separated are distributed on the top of silicon (Fig. 6(a)) and between silicon and graphite crucible (Fig. 6(b)).

Fig. 6 Refined silicon sample in crucible using CaO-SiO₂ slag

The SEM images of silicon sample before and after refining are shown in Fig. 7. It is found that the amounts of impurities in MG-Si are greatly reduced after refining using CaO-SiO₂ slag. Unexpectedly, the content of calcium in silicon increased from 0.015% to 0.042% in the study by DIETL [22]. Obviously, the calcium derived from refining slag should not be ignored using CaO-SiO₂ system. However, it can be removed using hydrometallurgical treatment or vacuum volatilization [23,24].

Fig. 7 SEM images of refined silicon using CaO-SiO₂slag before (a) and after (b) refining

3.3 Effect of technological conditions on boron removal

In order to display the oxidizing ability of CaO-SiO₂ slag for boron removal, the distribution of boron between CaO-SiO₂ slag and silicon was studied with different slag compositions. The mass ratios of slag to silicon and the refining time are set as 1:1 and 3 h, respectively, at 1600 °C. The distribution coefficient of boron was calculated according to Eq. (7), as shown in Fig. 8.

It is found that the distribution coefficient of boron increases with the increase of CaO proportion in slag. It reaches 1.57 when the mass ratio of CaO/SiO₂ is 1.5, the slag composition is 60%CaO-40%SiO₂. Unexpectedly, the distribution coefficient of boron reduces when CaO/SiO₂ composition is exceeded to 1.5, which is in accordance with the studies by SUZUKI et al [25]. It is presumed that the increase of CaO/SiO₂ composition results in the decrease of SiO₂ activity, which is crucial to the oxidizing ability of CaO-SiO₂ slag. Therefore, the distribution coefficient of boron declines as well with the continuous increase of CaO in slag.

Fig. 8 Distribution coefficient of boron (L_B) with different CaO/SiO₂ compositions

Figure 9 shows the effects of slag/silicon mass ratio on boron removal. The slag composed of 60%CaO- 40%SiO₂ is used to refine MG-Si. The refining time and temperature are set as 3 h and 1600 °C, respectively. It is found that the removal efficiency of boron increases with the increase of ratios for slag to silicon. The boron content in silicon is reduced from 18×10^-6 to 1.8×10^-6 and the removal efficiency reaches 90% when the mass ratio is 2.5. However, the trend is unobvious when the ratio is larger than 2.5. It is thought that the superfluous CaO-SiO₂ slag is non-effective for boron removal in refining process. The optimal ratio of slag to silicon for boron removal is about 2.5 using a 60%CaO-40%SiO₂ slag refining at 1600 °C for 3 h.

Fig. 9 Effects of mass ratio of CaO-SiO₂ slag to MG-Si on boron removal

Figure 10 shows the effects of refining time on boron removal from MG-Si. The 60%CaO-40%SiO₂ slag was used and the mass ratio of slag to silicon and the refining temperature were set as 2.5 and 1600 °C, respectively. It is found that the boron content in refined silicon decreases rapidly with the increase of refining time. It is reduced to 2.2×10^-6 for 2.5 h and 1.8×10^-6 for 3 h, respectively. However, the decreasing trend for boron removal becomes unobvious after longer refining time. It is thought that the driving force of boron diffusion towards slag phase decreases and the equilibrium for boron in slag and silicon is reached. Therefore, it is optimal for boron removal at 1600 °C for 3 h in the intermediate frequency inductive furnace using 60%CaO-40%SiO₂ slag refining.

Fig. 10 Effects of refining time on boron removal

4 Conclusions

1) Boron removal from metallurgical grade silicon using a CaO-SiO₂ slag refining was studied. It is non-effective basically to remove boron using a pure SiO₂ refining. However, the removal efficiency of boron is greatly improved with the addition of basic CaO in slag, which is consistent with the thermodynamic calculation.

2) The oxidizing ability of CaO-SiO₂ slag for boron removal was calculated and characterized. The distribution coefficient of boron between slag and silicon increases when the mole fraction of CaO in slag rises.

3) The mass fraction of boron in MG-Si can be reduced from 18×10^-6 to 1.8×10^-6 using 60%CaO- 40%SiO₂ slag refining for 3 h when the mass ratio of slag to MG-Si is set as 2.5.

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CaO-SiO₂熔渣精炼去除冶金级硅中杂质硼

伍继君^1,2,3，李彦龙^1,3，马文会^1,2,3，魏奎先^1,2,3，杨 斌^1,2，戴永年^1,2,3

1. 昆明理工大学 复杂有色金属资源清洁利用省部共建国家重点实验室/真空冶金国家工程实验室，昆明 650093；

2. 昆明理工大学 云南省有色金属真空冶金重点实验室，昆明 650093；

3. 昆明理工大学 云南省高校硅冶金与硅材料工程研究中心，昆明 650093

摘  要：利用CaO-SiO₂熔渣去除冶金级硅(MG-Si)中的杂质硼。热力学分析和实验结果表明：纯SiO₂基本上不能去除冶金级硅中的杂质硼。通过建立硼的分配系数与熔渣中SiO₂和CaO活度之间的关系，从热力学上对CaO-SiO₂熔渣的除硼能力进行表征。结果表明：随着渣中CaO配比的升高，硼的分配系数和去除效率大大提高。当熔渣组成为60%CaO-40%SiO₂(质量分数)时，硼的分配系数达到最大值1.57。当渣硅比为2.5，精炼温度为1600 °C以及精炼时间为3 h时，利用60%CaO-40%SiO₂熔渣可以将冶金级硅中的硼含量从18×10^-6降低至1.8×10^-6，去除效率达到90%。

关键词：冶金级硅；除硼；热力学；分配系数；熔渣精炼

(Edited by Chao WANG)

Foundation item: Projects (51104080, u1137601) supported by the National Natural Science Foundation of China; Project (14118557) supported by the Personnel Training Foundation of Kunming University of Science and Technology in China

Corresponding author: Ji-jun WU; E-mail: dragon_wu213@126.com

DOI: 10.1016/S1003-6326(14)63183-6