Preparation Metallurgical Grade Siliconby Molten Salt Electrolysis Method Using Inert Anode

Photovoltaic industry has been developing for decades.At present, photovoltaic power generation is a mature technology in the field of renewable energy. Solar power has many advantages, such as safe, reliable, clean, widely distributed, no mechanical moving parts and so on. Currently, the major obstacles of PV industry are the high cost of polysilicon and the shortage of silicon in the Photovoltaic industry application. To decreasing the cost of polysilicon production of solar grade silicon for reducing the dependence on the electronic silicon has an important significance to the photovoltaic industry.In this thesis, the preparation of Al-Si alloy by molten salt electrolysis with inert anode and the purification of metallurgical grade silicon by acid-disolved are presened. Unlike traditional metallurgical method of preparation of SOG-Si, inert anodes and high purity SiO2 are used instead of carbon anode. Thus. Al-Si alloy with a lower impurity content of P and B can be obtained. This Al-Si alloy can be used as a precuor for solar grade silicon. It is more competitive than the traditional metallurgical grade silicon with higher impurity content of P and B which is obtained from carbon thermal reduction method.Fe-Ni inert anode used in the experiments was prepared by vacuum induction furnace, and anhydrous AIF3 was prepared by vacuum distillation.By using cyclic voltammetry, the electrochemical reduction behavior of Si in cryolite molten salt system was investigated at 1123K. The cathodic reduction peaks of Na, Al is -1.75V,-1.6V(vs.Pt) respectively in (2.2NaF+AlF3)- CaO(4wt.%)-NaCl(6wt.%) molten salt. And the cathodic reduction peaks of Na, Al is-1.45V,-1.6V (vs.Pt), while there are two cathodic reduction peaks of Si. The reduction peaks of generation of Si (?) from Si (?) and Si (?) formation substance Si are -0.55V, 1.15V(vs.Pt) respectively.During the course of preparation of Al-Si alloy, the influence of the mass of liquid cathode Al and current density were investigated. The results showed that the increasing of Al mass improves the quality of the alloy formation, and to generate alloy with more purity. The decrease of Al mass increases the content of Si in the alloys. When the liquid cathode Al mass is 3.75% of the total of electrolyte, the content of Si is 19.45%, and the current efficiency is 45.5%. When the current density is 0.7 A/cm2, the content of Si is 24.5%, and the current efficiency can reach 52.4%. The optimum of electrolysis condition is that liquid cathode Al is 3.75% of the total of electrolyte, and current density is 0.7A/cm2. Threre are two processes of aluminothermical and cathodic reduction to form Si during the course SiO2 electrolysis in molten salt.The Al-Si alloys obtained from SiO2 electrolysis by high purity graphite anode and Fe-Ni anode were dissolved by acid. The obtained silicon powders were analyized by ICP-MS, and the impurity content of P and B is 132ppm and 35ppm respectively by high purity graphite, while the impurity content of P and B is 56ppm and 10pprn respectively by Fe-Ni inert anodes.