Elimination of Boron in Molten Silicon by Reactive Rotating Plasma Arc Melting

Abstract

An argon arc plasma remelting furnace with a rotational torch device was used to remove impurities in metallurgical-grade silicon under 60 kPa argon atmosphere. The radius of the rotation can be varied from 0 to 20 mm and the rate was controlled from 0 to 100 rpm. In the button melting practice, impurities such as Fe, Ti and Al remained unchanged in molten silicon. The Ca content decreased with melting time. The plasma of pure Ar or Ar with 2.6 vol% oxygen did not decrease the boron content remarkably, but the boron content was decreased by the plasma of lower oxygen partial pressure 0.66 vol%O 2 . The rate of silicon loss in the present study was as low as 6%/h. The boron content was decreased from 12 mass ppm to below 1 mass ppm within 15 min by using Ar + 1.24 vol%H 2 O, which was the highest water vapor pressure in the present study. The apparent rate constant for this case was 3.8 x 10 -3 s -1 . By rotating the plasma flame, the loss of silicon was unchanged and the reaction rate of boron removal increased. A continuous casting practice was developed using a water-cooled copper mold for the continuous removal of boron. Silicon ingots 40 mm in diameter were successfully cast.