Separation of Phosphorus from silicon by induction vacuum refining

Phosphorus can be expected to separate preferentially from silicon melt by induction vacuum refining (IVR). In the present study, thermodynamic and kinetics of phosphorus evaporating from silicon melt has been investigated. Due to the phosphorus concentration in metallurgical grade silicon (MG-Si) being around 100 ppmw level, we believe that phosphorus vaporizes from silicon as gas species P and P2 in the vacuum refining process. A numerical model has been developed for phosphorus separation by IVR, and the mass transfer coefficients for three possible rate limiting steps are developed. Experiments have been carried out to verify the IVR model. Such key factors as vacuum pressure, temperature, and melt geometry have been investigated by comparing the experimental data with simulation curve. The effect of vacuum pressure on phosphorus removal can be divided in three regions: independent region, sensitive region, and inert region. Temperature shows nonlinear relationship with phosphorus evaporation. The melt geometry is defined as ratio of diameter to height of melt (d/h) and it is favorable to set d/h at around 4. The numerical results agree well with experimental data.

► P in Si melt is believed to evaporate as gas species P and P2 under vacuum condition. ► Three mass transfer rate limiting steps on phosphorus evaporation have been considered. ► The effect of vacuum on P removal contains independent, sensitive, and inert region. ► Temperature shows nonlinear relationship with phosphorus evaporation. ► The ratio of diameter to height of melt (d/h) is favorable to set at around 4.