Removal of boron from silicon by slag treatment and by evaporation of boron from slag in hydrogen atmosphere
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Background and objective: Removal of boron is one of the main challenges in the purification of metallurgical grade silicon (MG-Si) for solar cells and a simple low cost method is therefore needed. Boron removal by slag treatment is today regarded as the most promising method, but the efficiency of the refining method is relatively low. Slag refining as a method for boron removal can be improved by optimization of the slag composition by changing the components and/or the composition of the slag. Another method for improving the slag refining efficiency is to remove boron containing species from the slag by evaporation. The aim of this work is to study the refining properties of the Al2O3-MgO-SiO2 slag and the CaO-SiO2-TiO2 slag, respectively. Furthermore, the possibility of evaporating boron from a CaO-SiO2 slag when refined in a hydrogen containing atmosphere will be investigated. Methods: In all, 12 experiments were performed at 1600˚C for determination of the distribution- and mass transfer coefficient of boron in the Al2O3-MgO-SiO2 and CaO-SiO2-TiO2 slag, respectively. In the study of boron evaporation from CaO-SiO2 slag in hydrogen atmosphere 14 experiments were performed. Boron evaporation was investigated in the temperature range 1500-1600˚C, the refining time was 0-6 hours and the slag/silicon mass ratio was varied between 0.25 and 1.5. Results: The distribution- and mass transfer coefficient of boron in the Al2O3-MgO-SiO2 (29%-23%-48%) slag was found to be 1.9 and 2.3x10-6 m/s, respectively. The amount of TiO2 in the CaO-SiO2-TiO2 slag was found to rapidly decrease due to reduction of TiO2 by Si. The distribution- and mass transfer coefficients found for the system were therefore not the same as for the original system. A decrease in the boron content in the slag was observed with increasing refining time in a hydrogen atmosphere at 1600˚C. This indicated that boron evaporated from the system, but the evaporation rate was found to be low. Temperature was not found to significantly influence the evaporation rate of boron in the temperature range 1500-1600˚C. When varying the slag/silicon mass ratio, a considerable increase in the refining efficiency compared to conventional slag refining was observed for the lower slag/silicon mass ratios. The effect diminished for the higher slag/silicon mass ratios. Conclusion: The kinetic and thermodynamic properties of the Al2O3-MgO-SiO2 slag evaluated in this thesis were shown to be in the same range as comparable slags in previous studies. In the CaO-SiO2-TiO2 slag most of the TiO2 was reduced during the refining process which makes it difficult to use this compound as a part of a slag for refining of silicon. When slag refining was performed in a hydrogen atmosphere the refining efficiency compared to conventional slag refining was shown to increase.