Kinetics of Boron Removal from Metallurgical Grade Silicon using a Slag Refining Technique Based on CaO-SiO2 Binary System

The removal of non-metallic impurity boron in metallurgical grade silicon (MG-Si) can be carried out by using a slag refining technique based on CaO-SiO₂ system. However, the boron removal in depth in the slag refining process is limited by the kinetic conditions. The refining agents based on the binary and ternary slag systems CaO-SiO₂, CaO-SiO₂-LiF, CaO-SiO₂-Li₂O, and CaO-SiO₂-K₂CO₃ were used to remove boron in this paper. The corresponding kinetic equations of boron removal using these slags were established by fitting the relationship between refining time and boron concentration in the refined silicon. It was determined that the rate coefficients of boron removal (k_B) are 0.25, 0.24, 0.26, and 0.57, respectively, using CaO-SiO₂, CaO-SiO₂-LiF, CaO-SiO₂-Li₂O, and CaO-SiO₂-K₂CO₃ slag refining. It was found that the mass transfer of boron in silicon is the rate controlling step for boron removal using 40%CaO-40%SiO₂-20%LiF, 40%CaO-40%SiO₂-20%Li₂O, and 40%CaO-40%SiO₂-20%K₂CO₃ slag refining. It is opposite to 50%CaO-50%SiO₂ slag refining that the mass transfer of boron in slag becomes the controlling step for boron removal.