Innovative separation model for boron removal from silicon during slag refining based on ion and molecule coexistence theory

Abstract

How to reduce boron concentration to an extremely low level has become a major obstacle during silicon waste recycling. As a simple and efficient method of removing impurities, slag refining plays an increasingly important role for industrial silicon waste recovery. The deboration ability of slag is usually evaluated by boron distribution ratio LB. However, the development of a separation prediction model is challenged by the limitations on the recognition of impurity structure in slag and complexity of slag-silicon reactions. Herein, a novel thermodynamic model for predicting boron distribution ratio between CaO-SiO2 based slags and molten silicon has been developed based on ion and molecule coexistence theory (IMCT), i.e., IMCT-LB model. The reaction abilities of structural units are represented by the calculated mass action concentration. The predictive boron distribution ratio between CaO-SiO2, CaO-Al2O3-SiO2, CaF2-CaO-SiO2 slags and silicon correspond well with experimental data, reflecting the reasonability of IMCT-LB model. According to IMCT, deboration reactions between slags and silicon can be represented by ion couples (Ca2+ + O2–), simple molecule Al2O3 with SiO2 to form eight deboration products as B2O3, 3CaO · B2O3, 2CaO · B2O3, CaO · B2O3, CaO · 2B2O3, 2Al2O3 · B2O3, 9Al2O3 · B2O3, CaO · 2SiO2 · B2O3. With the developed IMCT-LB model, the respective boron distribution ratio LB,i of the eight deboration products can be determined quantitatively. Furthermore, the effect of temperature, mass ratio of CaO to SiO2 (%CaO/%SiO2), Al2O3 and CaF2 content of CaO-SiO2 based slags on LB are fully discussed.