Removal of Pb and Bi in silicon by Ca solvent refining through the solidification process

This study addresses the challenge of efficiently removing lead and bismuth impurities from metallurgical-grade silicon by combining secondary fractionation (two-stage solidification-driven impurity segregation) and acid leaching processes. Specifically, within the Si-Ca alloy, lead and bismuth ifirst undergo primary fractionation (rejection into residual liquid during primary Si precipitation) and then secondary fractionation (selective segregation into the newly formed CaSi₂ phase during eutectic reaction). Thermodynamic calculations reveal that calcium addition significantly reduces the activity coefficients and segregation fractionation coefficients of lead and bismuth by forming CaSi 2 which supports strong affinity to these impurities. Subsequently, an optimal acid leaching process demonstrates that for the Si-5 %Ca alloy, the highest removal efficiency for lead and bismuth (94.11 % and 96.56 %, respectively) could be achieved with 6 mol/L HCl for 6 h at 80℃. Furthermore, it shows that slow cooling rate at 2 ℃/min during the primary crystalline silicon precipitation phase is helpful to impurity removal by promoting impurity fractionation into the CaSi 2 phase. However, the rapid cooling (quenching) during the complete solid-state cooling phase after eutectic reaction is essential to suppress impurity back-diffusion into the silicon matrix. Compared to the existing direct acid leaching process, the Si-Ca solvent refining method demonstrates significant advantages in removal efficiency, energy consumption, and operation easiness, providing an alternative pathway for the efficient purification of industrial silicon. • The Pb and Bi containing phase in the silicon solidification process was studied. • Pb and Bi impurities were captured in the CaSi₂ phase with excellent acid solubility through the Ca refining process. • By using the Ca refining process, 94.11 % of Pb and 96.56 % of Bi was removed in the subsequent acid washing process.