| The continuous development of the world economy and technology has increased our dependency on and requirement for energy. To ensure the stability of economic development, developed countries around the world have been promoting energy security to a national strategic height. However, issues such as the reserves of conventional energy and carbon emission continue to arise. The result is a gradual depletion of energy, which is becoming obvious year by year. To solve energy security problems, many countries are now paying more attention to the development and application of new energy. As one of the main forms of new energy, solar energy has been receiving unprecedented attention. The development of the photovoltaic industry in China began in2009, and by the end of2011, the photovoltaic installed capacity had increased from300MW to3GW, which is20times that of the former. Hence, China has become one of the emerging leaders in the photovoltaic industry. According to the energy (solar energy) development planning and prediction by China, the United States, and the European Union, solar energy will continue to develop in the future, and by2015, the electricity cost of solar energy is predicted to be the same as that of conventional energy.However, high purity polycrystalline silicon, which is a main raw material of solar cells, is seriously lacking, failing to meet the requirements of the rapidly developing photovoltaic industry. So far, China's dependence on foreign polycrystalline silicon is about50%. In China, the Siemens method is mainly used to prepare high purity polycrystalline silicon. However, this method cannot keep up with the continuously increasing demand for solar cells on its own because of limitations in technique and the energy it requires. Therefore, a new, large-scale method that entails low cost and green manufacturing processes must be developed. The metallurgical method is proposed and developed in this context. In this integration method, phosphorous, boron, and metal impurities are removed using several different techniques based on the characteristics of these impurities.Electron beam melting is an effective method for removing volatile impurities in silicon, and has been successfully applied to industrial production. However, this method lacks impurity removal mechanisms, as removal efficiency is only improved when larger power is consumed or when a longer melting time is employed. Electron beam melting does not effectively target the characteristics of the impurities, resulting in a serious waste of energy and time, which in turn restricts the application of electron beam melting technology. Based on the characteristics of the electron beam and the impurities in silicon, this paper classifies impurities from the point of evaporation. Impurities that can be removed by electron beam melting are verified. The migration regularity of the impurities in molten silicon, free surface layer, and gas phase is investigated by analyzing the thermodynamics and kinetics of volatile impurities in molten silicon to confirm the decisive step. According to these results, the removal effects during electron beam melting and solidification as well as during electron beam candle melting are investigated concretely, and the removal efficiency and energy utilization are determined. The conclusions are as follows:1) Electron beam melting is the main process used to remove volatile impurities like P, AI, and Ca at0.1×10-4,0.6×10-4, and0.5×10-4wt%, respectively, to meet the solar cell requirements. The impurity content is reduced when power and melting time are increased. Under this experimental condition, the evaporation reaction of P, Al, Ca, etc. is first order reaction. The relationships between mass transfer coefficient and temperature Inkp=4.46-18263/T,InkAl=1.51-15006/T,InkCa=2.85-15587/T, respectively. The migration of impurities in the free surface layer is the decisive step for removal.2) The solidification process after melting also has an important influence on impurity removal. The solidification rate can be controlled by adjusting the reduction rate of the beam current. If the beam current is linear with time, the solidification rate increases exponentially; if the beam current is exponential with time, the solidification rate is uniform. Under this experimental condition, when the beam current is reduced with the speed of-dI/dt=8/3exp(-t/50)mA/s, the solidification curve is approximately linear, and the solidification rate is0.09mm/s. If the beam current is reduced too quickly, the top and bottom surface of the molten pool solidifies before the inner part does, leading to the formation of a dividing line inside. With the reduction of the beam current and the extension of the solidification time, the dividing line moves gradually toward the top surface until it disappears. Volatile impurities P, Al, and Ca have obvious segregation effects during the solidification process with the reduction of the beam current, which leads to the enrichment of impurities in the liquid phase. Then, the impurity content increases, thereby improving the removal effect. Under the same experiment time, the removal effect by electron beam melting and uniform solidification process is better than that by evaporation alone. Moreover, the energy consumption of the former is lower than that of the latter.3) Based on the characteristics of electron beam melting and solidification, electron beam candle melting is proposed to effective connect with directional solidification for impurity removal with high efficient and low energy consumption. During electron beam candle melting. electron beam irradiates the silicon surface with an annular pattern to form a molten pool with a largest surface area and a deepest depth, and then the silicon ingot by directional solidification is refined with a periodical melting mode. Compared with electron melting, the molten pool during electron beam candle melting is far from the water-cooled copper crucible, leading to the fully utilization of the energy. So massive ingot can be melted with a lower power and the loss of silicon also maintains at a low level. The results show that volatile impurities P, Al, Ca et al. can be removed effectively by electron beam candle melting, which is positively correlated with melting time. Compared with electron beam melting, it has higher removal efficiency with the same melting time or power.
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