Research On The Removal Of Boron From Metallurgical Grade Silicon Slag Refining Depth

With the persistent increase in global economy demand,solar photovoltaic(PV)power generation has rapidly developed as part of the new energy industry.Solar grade silicon(SoG-Si)is the main raw material used for the fabrication of solar cells.In recent years,a metallurgical route with low cost,low energy consumption and more environmentally friendly has become the primary process for the production of solar grade polysilicon.The challenge of this metallurgical route is the effective removal of boron,phosphorus,and other impurities in metallurgical grade silicon(MG-Si).Solar grade silicon requires a final boron concentration of less than 0.3 ppmw in silicon chip because boron will reduce the minority carrier lifetime,and thus affect the conversion efficiency of solar cell.In this paper,the optimum conditions for boron removal from MG-Si by two new ternary slag systems(CaO-SiO2-ZnCl2 and CaO-SiO2-ZnO)were studied.The diffusion coefficient and mass transfer coefficient of B2O3 in CaO-SiO2 binary slag were also studied experimentally,and the limiting step of boron removal process was clarified.MG-Si was refined using the CaO-SiO2-ZnCl2 and CaO-SiO2-ZnO ternary systems by the electromagnetic induction heating and resistance heating respectively.The results show that:(1)When ZnCl2 addition is 20 wt.%,the CaO-SiO2-ZnCl2 ternary slag system has the best boron removal effect and it could reduce boron to 11.95ppmw by resistance heating.When ZnO addition is 10 wt.%,the CaO-SiO2-ZnO ternary slag system has the best boron removal effect and it could reduce boron to 7.51ppmw by resistance heating.The boron removal efficiency enhances with the increase of mass ratio between slag and silicon.When the mass ratio is 2;1,the boron is reduced to 7.92ppmw and 12.09ppmw,respectively.When the refining time is 3 h,boron is reduced to 8.91 ppmw and 8.02ppmw,respectively.When the refining temperature is 1823K,the boron content decreases to 11.95ppmw and13.41ppmw,respectively.(2)The 46%CaO-46%SiO2-8%ZnO ternary slag can reduce boron from 12.94ppmw to 2.18ppmw by the electromagnetic induction heating.After acid leaching,boron can be further reduced to 1.52ppmw with a removal efficiency of 88.25%.(3)Treated by acid leaching,the minimum zinc content in refined silicon is only 4ppmw.The residual zinc in silicon is reduced to lower than 0.05ppmw after a vacuum treatment.The diffusion coefficient of B2O3 in 37%CaO-63%SiO2 binary slag was experimentally determined by the capillary-bath diffusion device.The mass transfer coefficient of B2O3 in binary slag is calculated according to the results of 37%CaO-63%SiO2 binary slag refining experiment,and the boundary layer thickness between reaction interface and slag is obtained.The results show that the diffusion coefficient of B2O3 in 37%CaO-63%SiO2 slag is 5.265×10-9 m2/s at 1723K in a 4mm pore graphite capillary.The mass transfer coefficient of B2O3 in 37%CaO-63%SiO2 binary slag is 6.2×10-6 m/s according to the slag refining experiment in the corundum crucible with diameter of 35mm.The effective boundary layer thickness between the reaction interface and the slag is 0.849mm.It is found that the mass transfer process of B2O3 in slag is the limiting step of slag refining.