Clean enhancing elimination of boron from silicon kerf using Na2O-SiO2 slag treatment

Silicon kerf waste can be used as a secondary resource for the recovery of high purity silicon for solar cells production. Boron is amongst the most deleterious impurities in silicon and its concentration should be strictly controlled. Slag treatment is considered an effective method to extract impurities, especially boron, from silicon kerf. The study investigated the feasibility and optimization of a process for producing high-purity silicon from sawing waste by slag treatment. The slag system studied was Na2O-SiO2 and the effects of holding time, slag composition, and slag:kerf ratio on boron distribution were investigated. The optimum conditions for refining of silicon kerf at 1923 K were found to be a slag of 65 wt.% Na2O−35 wt.% SiO2 and treatment for 30 min. This resulted in reduction of boron concentration in silicon from 8.6 to 1.0 ppmw, corresponding to a removal efficiency of 88%. Investigation of the boron transfer mechanism indicated that mass transfer of boron in slag is likely the rate controlling step of the overall process. The mass transfer coefficients for boron in silicon and slag were 3.6 × 10−6 cm⋅s−1 and 5.8 × 10−6 cm s−1, respectively.