Study On Boron Removal From Metallurgical Grade Silicon By Slag Refining

The crystalline silicon solar cells become a growing concern worldwide because of the energy crisis and environment pollution caused by traditional energy. As the main material of solar cells, the preparation of high purity silicon has become a main bottleneck to develop a clean, eco-friendly, low-cost preparation technology of solar grade silicon (SoG-Si).In this paper, the quantitative pure boron with purity of95%-97%was doped into metallurgical grade silicon (MG-Si) for Si-B alloy that result from alloying; Combining with binary phase diagram and X-ray diffraction (XRD) analysis combined with binary phase diagram, it was concluded that under the experimental condition, besides simple substance and solid solution, the main combination modes of element boron and element silicon were SiB4and SiB6.The optical basicity was introduced to represent the basicity of CaO-SiO2binary refining slag. In the medium frequency induction furnace taking argon as shielding gas, slag refining MG-Si using different constituents of CaO-SiO2binary slag, the relationship between the slag basicity and distribution coefficient between slag phases and silicon phases were discussed through the analytical results of Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The ecperimental results demonstrated that, with the increasing slag basicity, LB first decreased then increased, the maximum value of1.58was obtained when slag basicity was0.71, and the slag was60%CaO-40%SiO2. After that, the mechanism of boron removal from MG-Si by CaO-SiO2slag refining was discussed and verified in details through the XRD analytical results of slag phases after refining Si-B alloy by CaO-SiO2binary slag.New lithium containing refining slags for the removal of boron from MG-Si melt were proposed, such as CaO-SiO2-Li2O and CaO-SiO2-LiF ternary slag systems. In the medium frequency induction furnace taking argon as shielding gas, the conditional experiment of slag refining MG-Si using CaO-SiO2-Li2O and CaO-SiO2-LiF, the relationship between boron removal from MG-Si and the content of Li2O and LiF, mass ration of slag and silicon, heating preservation time were investigated in details through the analytical results of ICP-AES and ICP-MS. The ecperimental results demonstrated that, the maximum value of LB of CaO-SiO2-Li2O ternary slag1.98was obtained when the Li2O content was2%, the maximum value of LB of CaO-SiO2-LiF ternary slag2.77was obtained when the LiF content was5%; the boron content in the refined silicon phases first decreased then increased as the increasing mass ration of slag and silicon; the boron content in the refined silicon phases decreased as the extension of heating preservation time; and, it should be noted that when the mass ration of CaO-SiO2-Li2O slag to metallurgical-grade silicon was4, the boron contents in silicon was successfully reduced from2.2×10-5to1.3×10-6after refining2h at1873K. After that, the mechanism of boron removal from MG-Si by CaO-SiO2-LiO and CaO-SiO2-LiF ternary slag refining were discussed and verified in details through the XRD analytical results of slag phases after refining Si-B alloy by CaO-SiO2-LiO and CaO-SiO2-LiF ternary slag.