| Fossil fuels such as coal, natural gas, oil in global are drying up and the energy crisis is becoming more and more severe. Solar energy, wind energy, water energy, geothermal energy and biologic energy are displacing the traditional energy. In these renewable energy sources, photovoltaic for solar energy is rapidly developed. In 2015, the production of solar cells in world was up to 54.5GW. In the production of solar cells from solar grade silicon (SoG-Si), the cost of crystalline silicon material is 70% of the battery production. A metallurgical route with low cost, low energy consumption and low pollution is being paid attention in the world. In the current technology, the removal of non-metallic impurity boron in depth from metallurgical grade silicon (MG-Si) to SoG-Si is the key to the metallurgical route.In this paper, the raw materials of MG-Si with boron and phosphorus contents of 14.7 ppmw and 64 ppmw were obtained from a certain industrial silicon factory. The innovative research on secondary refining of MG-Si by a united blowing-slagging refining was brought forward. The process and technology of boron and phosphorus removal are studied. At the same time, the active process and mechanism of boron and phosphorus removal between slagging reagent and gaseous oxidant medium by slag refining, gas blowing and united blowing-slagging refining technique are analyzed.Based on the calcium silicate slag, a ternary slag CaO-SiO2-CaCl2 was obtained by adding CaCl2 to the binary CaO-SiO2 slag. The effects of slag compositions, refining time, refining temperature and ratio of slag to silicon on boron and phosphorus removal efficiencies by binary and ternary slag refining are studied, as well as, the effects of gas compositions, gas amount and blowing rate on boron and phosphorus removal efficiencies by the united refining. The boron concentration in refined silicon is reduced from 14.7 ppmw to 4.59 ppmw with slag/silicon mass ratio of 1:1, refining temperature of 1550℃ and refining time of 3h by binary CaO-SiO2 slag refining. It can be reduced to 3.12 ppmw with a distribution coefficient (LB) of 2.47 using the ternary CaO-SiO2-CaCl2 slag refining with a CaCl2 addition of of 15% in slag. The results display that a phosphorus removal efficiency of lower than 10% is reached using a single slag refining. As well, the removal efficiencies of boron and phosphorus using the single O2 or H2O blowing refining is not very good. The boron and phosphorus are respectively reduced to 9.8ppmw and 52.63ppmw after 3h of refining time at 1550℃ with a H2O/O2 ratio (vol.) of 2 and a total gas rate of 100ml/min. As for the united gas blowing-slag refining, the the removal efficiency of boron and phosphorus improves with the increase of water vapour ratio in mixed gases, the prolonging of blowing time and increase of gas blowing rate when the slag composition is 42.5%CaO-42.5%SiO2-15%CaCl2. At the same time, when a total 40%Ar-40%H2O-20%O2 gas blowing rate of 100ml/min and a slag composition of 42.5%CaO-42.5%SiO2-15%CaCl2 are used in the united refinig at 1550℃, the boron and phosphorus are reduced to 0.75ppmw and 34.15ppmw, respectively, with 3h of refining time. The removal efficnencies of boron and phosphorus reaches 95% and 47%. The main impurities in silicon are silicon dioxide and calcium. |
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