Purification of metallurgical-grade silicon using zirconium as an impurity getter

Zr was employed as an impurity getter to enhance removal of impurities from MG-Si because of its strong affinity for a wide range of impurities. Different lixiviants, HCl + HF, HF and aqua regia, were employed to investigate the leaching behavior of MG-Si at 348 K for 2.5 h with and without Zr addition. Typical precipitates at Si grain boundaries before and after acid leaching were observed and analyzed via scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The leaching results show that the order of efficiency of the lixiviants, from highest to lowest, is HCl + HF > HF > aqua regia. HCl + HF is the optimal lixiviant for purification of MG-Si with and without Zr addition. The presence of Zr in MG-Si can significantly enhance the efficient extraction of impurities, especially P and Al, which cannot be removed efficiently using conventional hydrometallurgical technologies. The total fraction of impurities removed (Fe, Al, Ca, B, P, Zr, Ti, V, Mn, Cu and Ni) increased from 94.2 to 97.5% after HCl + HF leaching. In addition, 99.9% of the impurity getter Zr was removed, with its concentration decreasing from 50,000 ppmw to 49 ppmw after HCl + HF leaching. The residual amount of Zr in the refined Si can be further removed by directional-solidification purification because of its extremely small segregation coefficient.