1. Boron Removal from Metallurgical-Grade Silicon Using CaO-SiO2 Slag
Ding Zhao1,2, Wei Kuixian1,2, Ma Wenhui1,2,*, Wu Jijun1,2, Xie Keqiang1,2, Zhou Yang1,2
1 The National Engineering Laboratory for Vacuum Metallurgy, Faculty of Metallurgical and Energy Engineering,
Kunming University of Science and Technology, Kunming , P. R. China;
2 Engineering Research Center for Silicon Metallurgy and Silicon Materials of Yunnan Provincial Universities, Kunming , P. R. China
Introduction
Boron is one of the most difficult impurities to be removed from metallurgical grade silicon (MG-Si) because of its stable physical properties. CaO-SiO2 binary slag becomes the most common refining slag for boron removal. In this paper, the optical basicity proposed by Duffy and Ingram, which relied on the ability to contribute electron of oxygen ion was introduced to represent the basicity of slag.
Effect of slag basicity on LB
The optical basicity can be calculated as the equation shown as follows :
where Λ represents optical basicity of the oxides, and X represents the equivalent fraction of cation. The optical basicity of initial CaO-SiO2 binary slag in this work are summarized in Table 1.
The effect of slag basicity on LB is obtained by varying the CaO/SiO2 ratio. Fig.3 depicts the relationship between LB and slag basicity.
With increasing optical basicity of the slag, the actual value of LB decreases at first until it reaches the local minimum 0.72 while the optical basicity is At slag basicity of 0.56, while the CaO/SiO2 ratio is 1:3, the local minimum value of LB=0.72 is obtained, therefore, this acid-base reaction can be deduced as follows:
After that, slag basicity heighten with increasing CaO, which results in that more CaB2O4 exists the slag. So LB expresses a clear upward trend in the range 0.56<Λ<0.74, and it reaches the local maximum When Λ>0.74, the decrease of SiO2 has caused reduction in the oxygen potential, which leads LB to decrease sharply.
Fig. 2 XRD patterns of slag phases after refining by CaO-SiO2 binary slag
Experimental
The experiments are conducted in an electromagnetic induction furnace. During melting, the quartz tube is flushed with argon gas. The heat-preservation temperature is fixed at 1823 K.
Conclusions
(1) The results suggest a strong dependence on both oxygen potential and the basicity. The increasing the basicity of slag is not always effective in achieving high removal of boron from silicon, since an excess amount of basic oxides lowers the oxygen potential of the slag.
(2) The distribution coefficient of boron LB is calculated at 1823 K for CaO-SiO2 slag with a optical basicity between 0.48 and The highest value of LB is obtained for a optical basicity of 0.74, when the slag composition is 65%CaO-35%SiO2.
(3) The product of acid-base reaction is proved to be CaO·3SiO2 when slag refining MG-Si using CaO-SiO2 slag.
Table 1 Optical basicity of initial CaO-SiO2 binary slag used in the experiments
No.
Starting Slag (Wt Pct) Λ 1
100%SiO2
0.48 2
13% CaO-87%SiO2
0.52 3
25%CaO-75 %SiO2
0.56 4
37%CaO-63%SiO2
0.60 5
45%CaO-55%SiO2
0.64 6
50%CaO-50%SiO2
0.66 7
55%CaO-45%SiO2
0.69 8
60%CaO-40%SiO2
0.71 9
65%CaO-35%SiO2
0.74 10
70%CaO-30%SiO2
0.77 11
75%CaO-25%SiO2
0.80
Results and discussion
Mechanism
The schematic diagram of slag refining using CaO-SiO2 binary slag system is shown in Fig. 1.
A new slag-silicon experiment which uses the same quality of silicon master alloy and 45%CaO-55%SiO2 binary slag to serve as raw material is carried out. The XRD profile of its slag phase is shown in Fig.2. The peaks of CaB2O4 are observed.
Fig.1 Schematic diagram of slag refining using CaO-SiO2 binary slag system
Acknowledgements
The authors would like to thank National Natural Science Foundation of China [u , ].
Corresponding author: Wenhui Ma,
Fig.3 Distribution coefficients for boron between slag and silicon phase in varying optical basicity (CaO-SiO2) slags