Purification of metallurgical-grade silicon via acid leaching, calcination and quenching before boron complexation

Abstract

The effects of hydrofluoric acid concentration ([HF]), silicon particle size, calcination and quenching (C&Q), in addition to complexation with various ligands on the purification of metallurgical grade silicon (MG-Si), which was achieved via leaching with mixed nitric acid (HNO3) and HF acid, were investigated by observing the removal efficiency of boron and the rest of the impurities, and the purified silicon recovery. The investigation suggested that higher [HF] and smaller silicon particle sizes were beneficial for the removal of boron and all other impurities within a certain range but were not advantageous for recovering the purified silicon. The selected MG-Si particles were within 44–74micron (μm) and the leaching acid was composed of 2.0M HNO3+2.0M HF. This investigation also revealed that more impurities and cracks were exposed on the surface of MG-Si after C&Q, which aided the removal of boron and other impurities during acid leaching. Additionally, acid leaching with six boron complex ligands increased the boron removal efficiency and the purified silicon recovery. The maximum boron removal efficiency was 91.5% (from 128.0ppmw (parts per million (weight)) to 10.9ppmw), the total impurity removal efficiency was 94.8% (from 9881.0ppmw to 499.5ppmw) and the purified silicon recovery was 87.1%; these results were obtained by acid leaching using 10% (w/v) glycerin at 50°C for 80min. The results agreed with the HyperChem software calculation data to indicate that the boron complexation with glycerin had the highest bond energy and the least steric hindrance of the six ligands, which implied that the glycerine-boron complex was the most stable. The microstructure of the silicon before and after C&Q and acid leaching indicated that many tunnels formed in the silicon, which aided the acid leaching process to a large degree.