PRODUCTION OF HIGH-PURITY SILICON BY CARBOTHERMIC REDUCTION OF SILICA USING AC-ARC FURNACE WITH HEATED SHAFT

To produce inexpensive solar-grade silicon of high purity, a AC-arc furnace has been developed based on a thermodynamic consideration of carbothermic reduction of silica to metallic silicon. The furnace is featured by a closed type shaft and a hearth with arc electrodes and feeding nozzles. The carbon pellets as a reducing agent are fed from the top of the shaft, whereas silica powder is transferred to the hottest arc spot in the hearth. The resulting species of SiO and SiC, generated through the reactions of SiO2 + C --> SiO + CO and SiO + 2C --> SiC + CO, react in the lower part of the shaft to yield silicon, SiO + SiC --> 2Si + CO. Melting silicon is accumulated in the hearth and pulled out from the tapping hole. The productivity has been about 2 kg/h and the silicon yield has been found to be typically 83%. Analyses have shown that the impurities in the silicon were below 0.1 ppmw for B, 12 ppmw for Fe and below 5 ppmw for the other elements. Single-crystalline solar cells fabricated starting from this silicon after purifications of decarburization and unidirectional solidification have recorded a conversion efficiency of 16.5%. This value was equivalent to that of solar cells made from semiconductor-grade silicon.