The Effect of Secondary Refining on the Removal of Phosphorus from Metallurgical-Grade Silicon by Acid Leaching

The effects of structural composition, particle size, leaching time, temperature, and liquid–solid ratio on the removal of phosphorus from metallurgical-grade silicon (MG-Si) by acid leaching were investigated. Two specimens with different phase constitutions were studied: crude metallurgical-grade silicon (C-MG-Si), which is reduced from electric arc furnace, with a high content of Al and Ca, and a secondary refined metallurgical-grade silicon (S-MG-Si) obtained after secondary refining in ladle. Using scanning electron microscopy-energy dispersive spectroscopy, the phosphorus-containing phase was only detected in the C-MG-Si grain boundaries. The Si₂Al₂Ca phase, which was soluble in HCl, showed an affinity for phosphorus, with up to 0.53 wt pct dissolved in this phase. The optimum conditions for acid leaching were grain size, 75-106 μm; leaching time, 6 hours; leaching temperature, 338 K (65 °C); liquid–solid ratio, 6:1; and HCl concentration, 4.0 mol L⁻¹. Using these conditions, the mass fractions of P in C-MG-Si and S-MG-Si were reduced from 105 × 10⁻⁶ and 76 × 10⁻⁶ to 48 × 10⁻⁶ and 61 × 10⁻⁶, respectively, with removal efficiencies of 54.3 and 19.7 pct, respectively. Besides, the investigation suggested that adding HF was beneficial for the removal of most impurities. After 6 hours of leaching MG-Si with a mixture composed of 4.0 mol L⁻¹ HCl and 3.0 mol L⁻¹ HF, the purity of C-MG-Si and S-MG-Si were increased from 97.55 and 99.31 pct to 99.91 and 99.87 pct, respectively, with extraction efficiencies of 96.14 and 81.59 pct, respectively. The etching results reveal that the HCl-HF mixture was an effective lixiviant for dissolving impurity inclusions in both C-MG-Si and S-MG-Si.