| With the rapid development of global economy, the energy crisis and environmental pollution have caused wide public concern all over the world. As the representative of renewable energy industry, the photovoltaic power generation is growing very fast in the world. At present, the yield of solar cells in the world shows a rapid growth trend for several years. In2010, the production of solar cell in world was up to52.5GW. The cost of solar grade silicon (SoG-Si) material is about70%in the process of crystalline silicon solar cell production. For its low cost, low energy consumption and low pollution, metallurgical routes for SoG-Si preparation have become the focus of the world. The research shows that the removal of boron from metallurgical grade silicon (MG-Si) is the hard and key part of solar grade silicon making.In this paper, the raw materials with22ppmw boron content were obtained from one of the Yunnan factories. The research on the process of removing boron from metallurgical grade silicon using the method of chloride molten salt refining was done.According to the related knowledge of thermodynamics, we calculated the standard Gibbs free energy of reactions between Cl2and Ca, Fe, Cu, Ti, B, Si. The boron removal from metallurgical grade silicon using the CuCl and FeCl3molten salts was investigated in the medium frequency induction furnace, and the thermodynamically equilibrium relationships between boron and CuCl or FeCl3molten salt were studied, respectively. Under the refining temperature range of1650K to1950K, the results shows that boron in MG-Si was mainly removed in the form of BCl3using chloride molten salt refining method and a small amount of boron can be volatilized in the forms of BCl and BCl2. Further more, the relationships among the equilibrium partial pressures of boron chlorine species, the boron content in refined silicon and refining temperature were thermodynamically studied.Based on the thermodynamic equilibria analysis, a series of experiments were executed in the medium frequency induction furnace, the conclusions were obtained as follows:(1) Under the same refining conditions, the use of CuCl molten salt was greatest way to remove boron from metallurgical grade silicon (MG-Si), FeCl3next, followed by NaCl; The experimental results were were consistent to the thermodynamic calculation.(2) The efficiency of boron removal is obviously improved through prolong the refining time. When the refining time is less than60min, the boron content in the refined silicon phases decreased as the extension of refining time. Nevertheless, when the refining time is more than60min, the decreased trend became unobvious.(3) The efficiency of boron removal by the mass ratio of molten salt to MG-Si is same as the refining time. With the imcrease of mass ratio of molten salt to silicon, the boron content in the refined silicon phases decreased obviously, when the mass ration of molten salt to MG-Si is less than1. When the the mass ration of molten salt to MG-Si is larger than1, this trend is not obvious.Combined with the character of slag oxidation refining for boron removal, a new refining method was adopted for boron removal using FeCl3molten salt adding CaO-SiO2slags. The experimental results showed that the boron content in silicon was successfully reduced from2.2×10-5to0.85×10-6after refining120min at1823K by the mixed refining reagent of35%FeCl3-29%CaO-36%SiO2slags. The mechanism of boron removal from MG-Si by chloride molten salt refining was discussed and verified, the rate of boron removal was studied in kinetic and the corresponding kinetic equation is proposed and established. |
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