Research On Preparation Of High Quality Silicon With Low Boron And Phosphorus With Carbonized Rice Husk By Electrothermal Metallurgy Process

Solar energy has advantages of unlimited, clean, safe and reliable, which is important, new to deal with energy shortage and environmental problems. Silicon material is the basis of the solar photovoltaic industry which is rich in content and resources. However, the development of solar photovoltaic industry is limited by the traditional techniques of Solar Grade Silicon (SoG-Si) such as Siemens Process and Silane Method which have the shortcomings of the pollution and high production costs. Therefore, it is crucial to find a new process which is low production cost, low energy consume and frendly to the environment. Metallurgy process are widely considered the most promising long-term replacement for the traditional process with its advantages of low production cost, low energy consume and relatively safe.It is crucial to control the content of boron and phosphorus in the process of preparation of SoG-Si. P-N junction can not formed when boron and phosphorus exist in silicon crystals at the same time, that is, the process of photoelectric conversion can not be realized. High quality silicon with low boron and phosphorus is up to the standar of Metallurgical-grade Silicon (MG-Si) in purity of silicon, and, at the same time, is lower in the content of boron, phosphorus and iron etc and it can be made into SoG-Si by further processing. Carbonized rice husk consists of carbon and silicon dioxide. In the thesis, the preparation of high quality silicon with low boron and phosphorus with carbonized rice husk and high purity quartz sand by electrothermal metallurgy process is investigated. The impurity content especially the content of boron and phosphorus are controlled from the raw material for silicon. The general research purpose are combining the preparation of MG-Si with the preparation of polycrystalline silicon and MG-Si is replaced by high purity raw materials, then, SoG-Si can be made from high purity silicon with low boron and phosphorus by external refining and directional solidification. The results of the research can lay the foundation for further preparation of SoG-Si. The main contents of the paper are as follows.1. The physicochemical property of carbonized rice husk and purification by pickling are investigated. The technique of XRF, XRD, SEM and ICP-AES are applied in the study. The results show that carbonized rice husk mainly consist of 51.44% carbon and 27.15% SiO2, furthermore, carbonized rice husk presents honeycomb like structure. The TG curve of carbonized rice husk shows that its fire point is about 350℃ and the burning interval is 500℃ to 600℃. Acid pickling and ultraphonic acid pickling are studied in the paper. The optimum conditions of acid pickling are that its particle size are less than 100μm, the concentration of HCl is 5wt%, bath temperature is 60℃, acid pickling time is 8 hours, and solid liquid ratio is 1 to 5. At this condition, the leaching efficiency of Fe is 92.33%, the leaching efficiency of P is 79.10%, the leaching efficiency of B is 27.64%, the leaching efficiency of metallic element is 82.08% and the leaching efficiency of non-metallic element is 79.61%. The optimum conditions of ultraphonic acid pickling are that its particle size are less than 100μm, the concentration of HCl is 5wt%, solid liquid ratio is 1 to 5, ultrasonic power is 300W, ultrasonic frequency is 45k Hz, bath temperature is 60℃ and acid pickling time is 35 minuts. At this condition, the leaching efficiency of Fe is 98.29%, the leaching efficiency of P is 91.25%, the leaching efficiency of B is 99.95%, the leaching efficiency of metallic element is 96.15% and the leaching efficiency of non-metallic element is 91.72%.2. The preparation of pelletizing and the physical properties are also stuied in the paper. The raw materials consist of high purity quartz sand, carbonized rice husk, organic binder, and water, and their molar ratio is 100:85.6:1.87:22.53. The effects of pressing pressure, binder content and water content are studied. When the pressing pressure is 40MPa, binder content is 1% and water content is 12%, the compression strength is 6.41 MPa and pore ration is 35.77%.The roasting condition makes a great difference on the electrical resistivity of pelletizing. Its electrical resistivity decline when the calcination temperature rises. The pelletizing almost can not conduct electricity when the temperature is less than 600℃, and its electrical conductivity is obvious when the temperature is above 600℃. Therefore, its behavior on electrical conductivity can meet the demands of the process of preparation of silicon.3. The possible reactions between C and SiO2 is analyzed, and the Gibbs free energy of each reaction and the temperature was obtained. The results are verified by HSC.4. The results of two experiments show that it is feasible to prepare high quality silicon with low boron and phosphorus with carbonized rice husk by electrothermal metallurgy process. The content of impurity can be controlled, the phosphorus content is 0.024%, the boron content is 0.016% and the purity of silicon is 99.350%, especially, phosphorus content and boron content are lees than the index of industry silicon which the phosphorus content is 120 ppmw to 200ppmw and boron content is 20 ppmw to 60ppmw. The study lays a foundation for further preparation of SoG-Si.