Combined silicon refining for boron removal and Si-Kerf recycling using K2O-CaO-SiO2 slags

It is imperative in the photovoltaic (PV) industry to establish a circular materials flow for more sustainable solar silicon production. In the present work, a series of K2O-SiO2 binary and K2O-CaO-SiO2 ternary slags were examined for the combination of silicon purification and Si-kerf waste recycling. The study revealed that the mass transfer mechanism of the slag composition variation is predominantly due to the silicothermic reduction of K2O. The boron removal degree achieved high level at approximately 80% in most of tests, with gasification as potassium metaborate confirmed as an important B removal mechanism, especially in slags with high K2O content. In the case of ternary slags, the results indicated that an increase in CaO enhances boron partitioning in slag phase and a high K2O content also led to boron accumulation in the slag phase due to rapid reaction kinetics. Molecular dynamics simulations reveal that calcium and potassium ions exhibit distinct behaviours in slag phase. Potassium ions preferentially modify bridging oxygen, while calcium ions contribute more to depolymerizing the tectosilicate network and generating more non-bridging oxygens, which may further aid in the boron stabilization in slag phase. The oxidation layer of Si-kerf was successfully and enabled the coalescence of the silicon nanoparticles into the Si melt. The presence of SiC clusters in slag phase was also observed, indicating effective SiC removal and a feasible Si-kerf waste recycling.