The mechanism of boron removal from silicon alloy by electric field using slag treatment

Stubborn nonmetallic impurity boron is the most difficult to be removed by the metallurgical route for solar grade silicon (SoG-Si) production. To break through the restriction of separation of trace impurity, a novel approach by applying an electrical potential between the slag and the solvent phases to enhance silicon alloy refining using slag was developed. The effect of the external potential and refining time on boron removal has been investigated. For silicon, Al-Si, or Sn-Si system, it can be found that the removal efficiency of B substantially increased with the augment of the external potential. When the external potential was increased to 6 V, the partition ratio of boron increased to 5.7, which was about ten times than that without the external potential applied. Under the constant external potentials, the boron content also decreased with increasing refining time in the Al-Si system, and three stages of average removal velocity of boron were presented within 2 h. In addition, the synergistic removal mechanism of boron with electric field was investigated for the first time. For the electrode reaction, B is oxidated to the oxidizing products, then the oxide crosses the boundary layer rapdily and finally is absorbed by slag owing to the transfer of O2− at the anode, at the same time Si is produced at the cathode, which provides theoretical support for this new purification process.