Study On Boron Removal Efficiency Of Metallurgical Grade Silicon Slag Refining

Solar grade silicon (SoG-Si) is the basic material of photovoltaic power generation and the metallurgical method will gradually replace the improved Siemens process and fluidized bed preocess and become the main technology of polycrystalline silicon material production. However, there is still some breakthrough in key technology such as the removal of non-metallic impurities B, P, C, O for the metallurgical method, which is the major bottleneck of the techique.In this paper, it has been mainly studied that the boron removal from metallurgical grade silicon (MG-Si) using the binary slag SiO2-CaO and the ternary slags CaO-K2CO3-SiO2and then the kinetic principle and process of boron removal during the slag refining process from metallurgical grade silicon are discussed. The refining temperature range and slag composition have been selected reasonably basde on the SiO2-CaO binary phase diagram. The slag refining experiments for boron removal from MG-Si using slag refining have been conducted at different conditions of SiO2-CaO slag composition and refining time. Finaly, the relative kinetic equations are obtained by the deducing kinetic model.The ternary slag agents of CaO-K2CO3-SiO2were proposed and selected for boron removal from MG-Si by slag refining in the experiments. The effect of slag compositon, refining time and ratio of slag to silicon on boron removal have been obtained. It is found that that the concentration of boron in silicon is decreased with the prolong of refining time with the same ratio of slag to silicon and slag composition, but the trend was gently after3hours’refining time. Similarily, the boron content in silicon decreases with the increase of K2CO3in slags and ratio of slag to silicon. The boron concentration can be reduced from22ppmw to1.91ppmw after2hours refining when the mass concentration of K2CO3was20%. It is found that B2O3generated by oxidition exists in the form of borate K2O2B2O3in the refined slag CaO-K2CO3-SiO2according to the thermodynamic analysis.The diffusion coefficient of impurity boron in silicon melt is determined and estimated in the resistance furnace. The boron content of different distances from B-Si interface in sample is determined with the diffusion of3h at1823K. It is calculated that the diffusion coefficient boron in melt silicon is D=2.3×10-10m2·s-1by the Fick’s second law. In addition, it is gotten that the mass transfer coefficient of boron in silicon melt is1.7×10"4m·s-1using the experimental data of binary50%SiO2-50%CaO slag refining in the resistance furnace. Finally, an effective boundary layer thickness (8=1.35μm) between the slag and the silicon melt is derived by the relationship between the mass transfer coefficient and the diffusion coefficient of boron in silicon.The binary slag of SiO2-CaO refining experiments were conducted in the induction furnace and the atmosphere furnace, respectively. By comparison, it is shown that the rate of boron removal obtained in the induction furnace is significantly higher than the rate obtained in the furnace atmosphere under the same refining time and temprature. It could be found from the binary SiO2-CaO slag refining experiments in the atmosphere furnace that the mass transfer coefficients of boron in slag is4.0×10"4m-s’1with the slag refining process. It follows that the mass transfer process of boron in slag is the control step of boron removal during the slag refining. The mass transfer coefficients of boron in CaO-SiO2-Li2O and CaO-SiO2-LiF, respectively, are2.5×10-6m·s-1and1.7×10-6m·s-1. While the mass concent of K2CO3increases from5%to20%in CaO-K2CO3-SiO2slag, the mass transfer coefficient of boron increases from9.6×10-7m·s-1to8.06×10"6m·s-1, which indicates that the addition of another basic component to slag can improve the mass transfer of impurity boron in slag and the boron removal efficiency as well.