Boron Removal From Si And Si-Sn Alloy Using Slag Refining Technique

Solar grade silicon is an important photovoltaic material.The low-cost,high-efficiency,high-quality solar-grade polysilicon preparation method plays an important role in the development of China's photovoltaic industry.This study investigated solvent refining,slagging refining,and gas blowing refining methods.The purpose is to efficiently remove boron impurities in industrial silicon to meet the requirements of solar grade silicon.A combination of solvent refining,slag treatment,and gas blowing was employed to investigate the possibility of removing boron(B)from silicon(Si)to solar application levels.Metallurgical grade silicon(MG-Si)was alloyed with pure tin was subjected to refine in liquid CaO-SiO2-CaCl2 slags by using different methods at 1500 ?(1773 K).MG-Si alloyed Sn as impurities getter,was subjected to slag refining,and then treated by slag refining.The distribution of B in Si/slag and Si-Sn alloy/slag systems was measured under different holding time,slag compositions,Sn/Si ratios,and gas flow rates.The results showed that both slag basicity and oxygen potential have a strong influence on the distribution of B in the Si/Slag and Si-Sn alloy/slag system.The addition of CaCl2 to the slag was found to increase the distributions of B.The CaCl2 addition to the CaO-SiO2 slag was an active component which can contribute to the decrease of slag melting temperature,the reduction in the viscosity,and the improvement of mobility,resulting in eliminating B as B oxy-chlorides gas(BOCl)according to the principle of selective chlorination.The B was reduced to 5.5 ppmw with 70%removal efficiency.MG-Si alloyed with Sn lead to enhance the B removal thermodynamically,where the Sn addition led to increase activity coefficient of B,oxygen partial pressure,and decreasing Si activity,consequencely increasing the boron removal efficiency.The lowest boron content reached to 1.1 ppmw with 93.7%removal efficiency.Combined method of slag refining and gas blowing technique was used for B removal from MG-Si and 75wt%Si-Sn alloy by using the 45%Ca0-45%Si02-10%CaCl2 slag and the Ar(-20%O2)gas.Increasing gas flow rate shows enhancement in B removal.At fixed conditions,the Ar-20%O2 mixed gas blowing shows a good removal efficiency of B about 85%and 96%to MG-Si and 75wt%Si-Sn alloy respectively,compared with the Ar gas blowing with 78.6%and 88%to MG-Si and Si-Sn alloy,respectively.The B impurities after the purification found with low intensity in Si matrix but with high intensity in slag phase and Sn phase.Boron removal controlled by mass transfer in slag phase show the mass transfer coefficients of 3.38×l0-4cm·s-1 and 7.2×l0-5 cm·s-1 in Si and Si-Sn alloy,respectively.