Kinetics of Boron Removal from Metallurgical Grade Silicon using a Slag Refining Technique Based on CaO-SiO2Binary 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 (kB) 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.