Study On Thermodynamics And Mass Transfer Of Boron Removal From Metallurgical Silicon Refined By Na₂O-SiO₂ And CaO-SiO₂ Slag Systems

Boron removal is the key process which effects the solar cell efficiency in the metallurgical route of polysilicon production.Slag refining is one of the most effective and feasible way to remove boron from metallurgical-grade silicon.In this paper,slag refining is applied to removal boron from polysilicon by choosing slag agent and slag refining factors.Thermodynamic and kinetic analysis was based on Na2CO3-SiO2,CaO-SiO2-CaF2 and CaO-SiO2-CaCl2 slag systems.Furthermore,advanced technique was adopted to enhance boron removal efficiency by multiple slag refining,recycle slag refining,and interval slag refining.(1)The boron removal mechanism was researched from 1773-1973 K in Na2Co3-SiO2 slag systems.The final boron content is determined by the weight ratio of Na2Co3/Sio2 which achieves an optical effect at 2.The migration of impurities and oxidation reaction can be carried out more quickly at high temperature,thus improving the efficiency of slag refining.The equilibrium state is reached after 40-minute slag refining.The major metal impurities are Si,Fe,Ti,and V.During the slag refining process,impurity B was oxidized into B2O3 by Na2O on the slag silicon interface,and there were two ways to remove them.One was to enter the slag gas interface to evaporate.The second is to react with Na to produce borate,which evaporates from the slag phase in the form of Na2B2O4.The results show that the value of the equilibrium constant of B at 1773 K is between 1.3 × 10-6 and 3.1 × 10-6 m/s,and the kB value is between 1.9 × 10-6 and 3.6 × 10-6 m/s.The volatilization of boron mainly depends on the evaporation of Na2B2O4 in the slag/gas interface.In the industrial application,the back diffusion of impurity B can be prevented by triple slag refining with Na2CO3-SiO2 slag,which can reduce the content of the impurity B to less than 0.1 ppmw.(2)The boron removal mechanism was investigated between 1773-1973 K with CaO-SiO2-CaF2(CaCl2)slag system.The boron removal was consistent with that of Na2CO3-SiO2 slag.The addition of CaF2 can reduce the viscosity of slag system because of the depolymerization of F-,improving the B removal efficiency.The addition of CaCl2 can promote the chlorination of B2O3 into BOCl;In the slag refining process,impurity B was oxidized into B2O3 at the slag/silicon interface and entered the slag phase in the form of calcium borate.Adding CaCl2 can promote B2O3 to be chlorinated into BOCI and volatilize into the gas phase.The mass transfer coefficient of B from silicon to slag phase is 3.64 × 10-4-10-6 cm/s(1873 K)and 4.41 ×10-4-10-6cm/s(1973K)respectively.Calcium slag can reduce the content of impurity B to 0.3 ppmw.(3)Boron content can reduce below 0.1 ppmw after 3-times slag refining by Na2CO3-SiO2 slag refining;Boron content can reduce below 0.2 ppmw after 3-times slag refining by CaO-SiO2-CaCl2 slag systems with active CaCl2 addition which promote the chlorination of B2O3;the majority route of boron removal is evaporation;Interval slag refining with CaO-SiO2-CaCl2 slag systems can reduce the totally slag usage and refining times as well as increase the silicon yield.