Evaporation of Phosphorus in Molten Silicon with Electron Beam Irradiation Method

The evaporation behavior of phosphorus in molten silicon has been investigated during electron beam irradiation to produce solar grade silicon (SOG-Si) from metallurgical grade silicon (MG-Si) by a sequential metallurgical process. The batch experiments showed that the evaporation rate of phosphorus increased in proportion to an increase of the power of irradiated electron beam and was found to be the first order with respect to phosphorus concentration. The removal rate of phosphorus was controlled by the free evaporation from the surface of molten silicon. The electron beam irradiation enables us to secure a higher temperature of free liquid surface, which results in efficient dephosphorization. On the other hand, the continuous flow experiment indicated that the phosphorus concentration at the outlet increased because as the silicon feed rate was raised, the residual time of the molten silicon in the hearth was proportionately shortened. The flow of the molten silicon in the hearth did not behave with a complete mixed flow type reaction but close to a plug flow type reaction. With a 150 kg scale pilot manufacturing plant, MG-Si containing phosphorous of about 25 mass ppm was successfully purified to less than 0.1 mass ppm.