REMOVING BORON FROM METALLURGICAL GRADE SILICON BY A HIGH BASIC SLAG REFINING TECHNIQUE

a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization/The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, China b Key Laboratory of Non-Ferrous Metals Vacuum Metallurgy of Yunnan Province, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, China c Engineering Research Center for Silicon Metallurgy and Silicon Materials of Yunnan Provincial Universities, Kunming University of Science and Technology, China


Introduction
Silicon is widely used as a photoelectric conversion material due to its relatively low cost and high performance [1] and silicon-base solar cells are a conventional and low-cost semiconductor material applied in photovoltaic (PV) technology with at least 90% of photovoltaic market [2,3].Presently, the market demand for solar grade silicon (SoG-Si) has been growing rapidly.In order to meet the high demands on the purity of solar grade silicon (SoG-Si) for a target purity of 99.9999% (6 nines), some inexpensive metallurgical purification techniques have been developed to purify metallurgical grade silicon (MG-Si) to SoG-Si [4][5][6].
It is well known that boron is an especially obstinate impurity element that is difficult to be removed from MG-Si as a result of its large sgregation coefficient (0.83) in silicon and its low volatility compared to silicon [7].The molten slag refining is a traditional method using in the purification of steel making [8] and it was also proved that the molten slag refining based on calcium sillicate system is an efficient agent of removing boron from molten metallurgical grade silicon (MG-Si).Impurity boron in MG-Si can be oxidized into boric oxide, which will then enter a basic slag based on calcium sillicate system in the form of lime borate [9][10][11].
In previous studies, Diet [12] described that impurity boron in silicon can be reduced from 18×10 - 6 to 1×10 -6 using a calcium silicate slag in an arc furnace.Johnston et al. [13] studied the effects of basicity and oxygen potential of Al 2 O 3 -CaO-MgO-SiO 2 and Al 2 O 3 -BaO-SiO 2 slags on removing boron and found that the removal efficiency of boron reached to 80%.Li et al. [14] reported that boron in MG-Si was successfully reduced from 15×10 -6 to 2×10 -6 using a ternary slag CaO-SiO 2 -Al 2 O 3 by an electromagnetic induction slag melting (EISM) method.In this paper, the mechanism of removing boron using molten slag based on CaO-SiO 2 system was studied and then the effects of slag composition and slag basicity on removing boron were primarily investigated by using an high basic slag refining technique in an induction heating method.

Experimental
The metallurgical grade silicon block with a boron concentration of 18×10 -6 was pulverized into powder

J. Min. Metall. Sect. B-Metall. 50 (1) B (2014) 83 -86
Letter to Editor with a particle size of 50-200μm.At the same time, the reagent grade chemicals of CaO, SiO 2 , Li 2 O and K 2 O were prepared and the different composition of binary and ternary slag systems CaO-SiO 2 , CaO-SiO 2 -Li 2 O and CaO-SiO 2 -K 2 O were obtained in advance for slag refining.The metallurgical grade silicon powder and the slags were respectively mixed with different ratio of slag to silicon.Then, the mixture was loaded to a graphite crucible, which was put into a quartz tube of the high frequency induction furnace.The quartz tube was blowed argon (Ar) for protection.The crucible was heated up for removing boron at 1600 o C for 2h.The experimental installation was shown in Figure 1.Lastly, the experiment was completed and the refined slag and silicon from cooled sample was drawn out for a chemical analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Elan-5000A USA).

Distribution ratio of boron between calcium sillicate slag and silicon
The boron in silicon (expressed as [B]) can be oxidized into boric oxide by free oxygen ion and oxygen.The generated boric oxide will enter a basic slag phase based on calcium oxide in the form of negative ion and a calcium borate phase is finally generated via the ionic Eq. ( 1). (1) The deboronization ability of calcium sillicate slag from MG-Si is usually expressed with the distribution coefficient of boron (L B ) defined as Eq. ( 2) [15]. (2) Where w [B] and w (B) represent the equilibrium concentrations of boron in the refined silicon and the refined slag, respectively.In order to study the distribution ratio of boron between CaO-SiO 2 binary slag and refined silicon, the experiments of removing boron from MG-Si with different composition of CaO/SiO 2 were carried out at 1600 o C. The mass ratio of slag to MG-Si and the refining time were 1:1 and 3h, respectively.The boron concentrations in the refined silicon and the refined slag were then obtained.The distribution coefficient of boron are calculated and shown in Figure 2.
The concentration of boron in silicon reduces with the increase of the ratio of CaO in calcium sillicate slag and on the contrary it increases in slag.However, the variation trend of boron concentrations in silicon and slag are reversed when the mass ratio of CaO/SiO 2 is more than 1.5.It is found that the concentration of boron in refined silicon reduces to about 4×10 -6 with 1.5 of CaO/SiO 2 mass ratio and it was also found from the distribution coefficient curve that the maximal value of L B reaches to 1.57 with 1.5 of CaO/SiO 2 mass ratio .This shows that a satisfied result of removing boron from MG-Si is difficult using a calcium sillicate binary slag and the optimal composition is 60%CaO-40%SiO 2 .
It is reported that the efficiency of removing boron can be improved by increasing the basicity of slag [13].The basicity values of some oxides are listed in Table 1 and it is calculated that the optical basicity of CaO-SiO 2 binary slag is 0.71 according to Eq. ( 3). ( Where, Λ represents the basicity of molten slag.are the optical basicities of oxides CaO and SiO 2 .and are mole fractions of CaO and SiO 2 in slag, respectively.

Effects of slag basicity on removing boron
In order to study the effect of slag basicity on removing boron, the basic oxides Li 2 O and K 2 O were added to a calcium sillicate with the compositon of 45%CaO-55%SiO 2 .The compositions of ternary refining slag systems are shown in Table 2.
The refining experiments were carried out in the induction furnace at 1600 o C with the amounts of Li 2 O and K 2 O in ternary slag varying from 2.5-20% and 3-40%, respectively.The mass ratio of ternary slag to MG-Si and the refining time were 1:1 and 2h, respectively.The basicities of ternary slags CaO-SiO 2 -Li 2 O and CaO-SiO 2 -K 2 O can be calculated as Eqs.( 4) and ( 5). Figure 3 shows the variation trend of boron concentration in the refined silicon with 2.5-20% of Li 2 O in the ternary refining slag CaO-SiO 2 -Li 2 O.It is calculated according to Eqs. ( 4) and ( 5) that the basicity of this ternary slag varies from 0.65 to 0.73.It is found that the boron concentration in the refined silicon reduces with the increase of Li 2 O mass ratio in slag and it reaches to about 4.5×10 -6 with 10% of Li 2 O mass ratio.It shows that the addition of Li 2 O increases the basicity of slag, which improves the deboronization ability of calcium sillicate slag.However, it is not helpful any more when it is higher than 10%.The boron concentration in the refined silicon increases again.It is concluded that the oxidizability of slag weakens with more basic oxides CaO and Li 2 O and less acid oxide SiO 2 in slag.So the optimal composition of lithia slag for removing boron is 40.5%CaO-49.5%SiO2 -10%Li 2 O, where the basicity of slag is 0.68.
The oxidizability of molten slag may be expressed as Eq. ( 6).(6) The molten silicon and slag are considered as a dilute solution.The equilibrium constant of Eq. ( 6) might be written as Eq. ( 7) on condition that the standard state for the Henry's law is chosen.(7) Where and represent the concentration of calcium borate in slag and the concentration of boron in silicon, respectively.Parameters a and f are

J.J. Wu et al. / JMM 50 (1) B (2014) 83 -86
CaO SiO   the activity and the activity coefficient.For the Henry's law, it is considered as and .The distribution coefficient of boron might be written as Eq. ( 8).(8) Obviously, it is helpful for removing boron using slag refining with a higher basicity.The oxide K 2 O with a basicity value of 1.4 as listed in Table 1 was added to the calcium sillicate slag for removing boron.Figure 4 shows the variation of boron concentration in the refined silicon using different compositon of ternary slag CaO-SiO 2 -K 2 O.The mass ratio of K 2 O in slag varies from 3% to 40% and the basicity of slag increases from 0.64 to 0.72.It is found that the efficiency of removing boron improves greatly.The boron concentration in silicon reduces from 3.87×10 -6 to 1.4×10 -6 .The efficiency of removing boron reaches to 92.2%.On the contrary however, it reduces again when the ratio of K 2 O in slag exceeds 30% although the basicity of slag increases continuously.It is concluded that the oxidizability of slag is weakened with SiO 2 mass ratio of 37% in slag.So it is thought that the optimal composition for a potash slag is 32%CaO-38%SiO 2 -30%K 2 O for removing boron from MG-Si, where the basicity of slag is 0.7.

Conclusions
1) The boron concentration in refined silicon is reduced with the increase of ratio for CaO in slag.The maximal distribution coefficient of boron between calcium sillicate slag and silicon is 1.57 when the mass ratio of CaO/SiO 2 is 1.5 and the composition is 60%CaO-40%SiO 2 with a basicity of 0.71.
2) It shows a positive act for the oxidizability of calcium sillicate slag with a higher slag basicity and it will weaken with a low mass ratio of SiO 2 in slag.
3) The additions of Li

Figure 1 .Figure 2 .
Figure 2. Distribution coefficient of boron with differentCaO/SiO 2 composition basicity of oxide B. are the mole fraction of oxide B and the mole fraction of oxygen ion in oxide B, respectively.represents the amount of oxygen atom in the molecule of oxide B.

1 Figure 3 .
Figure 3. Effects of Li 2 O concentration in the ternary system CaO-SiO 2 -Li 2 O on removing boron
2 Figure 4. Effects of K 2 O concentration in the ternary system CaO-SiO 2 -Li 2 O on removing boron