An experimental investigation into the mass transfer of phosphorus in molten silicon under vacuum induction refining has been carried out. In a pilot-scale experiment, in the temperature range 1773 K (1500 A degrees C) to 1873 K (1600 A degrees C) and a vacuum of 0.1 to 0.035 Pa smelting for 7200 seconds (2 hours), phosphorus is decreased from 15 ppmw to 0.08 ppmw, which achieved the target for solar-grade silicon of less than 0.1 ppmw. Lab-scale experiments are used to determine the effects of vacuum, refining time, and temperature on the rate of mass transfer of phosphorus during vacuum refining. Hardly any phosphorus was removed when the vacuum pressure is greater than 100 Pa. Mass-transfer coefficients are nearly independent of pressure at 1783 K (1510 A degrees C) when pressures are below 0.1 Pa and are highly correlated with vacuum pressures above 0.1 Pa. A model of vacuum refining of inductively stirred silicon melts is discussed to explain the transfer path of phosphorus out of the melt.
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