Study on the removal of volatile impurities from diamond wire saw silicon powder via vacuum refining

Abstract

With the increasing annual output of diamond wire saw silicon powder (DWSSP) from photovoltaic (PV) wafer production, the recovery and purification of silicon from DWSSP are crucial for producing high-purity silicon. This study investigated the migration mechanism of impurity elements during the vacuum refining process of DWSSP through kinetic analysis. The dynamic relationship between impurity removal efficiency and the mass transfer coefficient was examined using a volatile mass transfer model at the gas–liquid interface. At a melting temperature of 1700 K, the removal of impurity elements (including Na, K, and Ca) was controlled by diffusion in the melt boundary layer. In contrast, P and Mn removal were dominated by surface volatilization. Comparative experiments confirmed that the combination of acid leaching pretreatment and vacuum refining eliminated the oxide encapsulation effect, thereby reducing impurity levels. After acid leaching, DWSSP was processed under optimized parameters (a vacuum of 10⁻³–10⁻² Pa, a furnace chamber melting temperature of 1823 K, and a holding time of 120 min). The residual concentrations of P, Na, K, Ca, and Mn were 0.84, 4.33, <2.80, and 0.22 ppmw, respectively, with the Log Reduction Value (removal rates) of 1.058(91.30 %), 2.166(99.32 %), 1.809(97.80 %), 1.087(91.71 %), and 2.482(99.65 %). These results indicate that acid leaching and fire vacuum melting can effectively eliminate impurity elements from DWSSP, and the recovered silicon is likely to achieve a 6 N purity level. which holds significant practical implications for promoting the sustainable development and low-carbon transformation of the PV industry.