Use of a thermal plasma process to recycle silicon kerf loss to solar-grade silicon feedstock

The objective of this study was to recycle the dust generated in the slicing of silicon wafers (silicon kerf) by means of a two-stage thermal plasma process. In the first stage, the silicon particles are injected in a plasma jet where silicon oxides and carbon impurities are removed by vaporization. In the second stage, the purified silicon droplets are collected in a silicon bath maintained in a hot-wall crucible. The optimal operating conditions for reduction were determined by injecting into the plasma jet a commercial silicon powder of known degree of oxidation and measuring the attained degree of reduction. Also, silicon powders from wafer slicing were processed in the two-stage plasma set-up. The results of these tests showed that the deoxidation rate of the final silicon ingot was as high as 80% and the initial carbon concentration decreased by 85%. The purification was essentially controlled by the residence time of particles in the hottest zones of the plasma jet and the partial pressure of oxygen in the processing atmosphere.