Research On Removal Of Phosphorus From Metallurgical Grade Silicon By Secondary Refining

Nowadays, the global conventional energy source is threatening to be exhausted as well as accused of pollution to environment. As an alternative, new energy resource received increasing concerns, of which solar energy is considered as the most important one in the 21st century owing to its abundant resources, pollution-free and safety. As a result, there is an increasing demand of solar grade silicon (SG-Si) owing to the rapid development of solar energy industry. In addition, the high cost restricte the development of Photovoltaic (PV) industry. Therefore, it is very important to find new methods with low cost and high efficiency as the alternative. The metallurgy methods of preparing solar grade silicon have characteristics of low energy consumption, low cost and environmental friendly. The research of preparation of solar grade silicon with metallurgy method will have the important significance for China to breake the international monopoly, possess ourself intellectual property rights and develop PV industry of our county.In this dissertation metallurgical grade silicon (MG-Si) is adopted as raw material, to research phosphorus removal in silicon from many aspects.(1) The result shows SiP is the form of phosphorus in P-Si alloy. Impurities in MG-Si are distributed for dark and white inclusion. It is known from the chemical composition of inclusions that dark impurities are slag inclusion, and white are intermetallic compound. Prior combining of phosphorus and other elements in silicon is Ca3P2>Mg3P2>MnP3>Co2P> AlP>SiP.(2) Experiment of phosphorus removal by reduction refining with adding calcium is carried out. It is resulted from the thermodynamics analysis that form of phosphorus removal is CajP2 generated by phosphorus and calcium added in silicon, which makes the solid-liquid distribution coefficient of phosphorus decreased. Phosphorus can be removed with pickling impurity. Then with the best refining condition-refining temperature is 1813K, weight ratio of calcium and silicon is 9.5%, MG-Si is refined. After refining phosphorus content in silicon decreased from 94ppmw to 15ppmw.(3) Research of gas blowing refining:During the refining process, nozzle types can effect dephosphorization ratio, and the best nozzle types is C types which has 15 little holes at the bottom and the side of high purity graphite tube. Tempurature is the mainly effect factor during the process. High temperature goes against phosphuorus removal, and when the refining temperature is 1793K, dephosphorization ratio achieves 88.30%. High refining gas temperature is good for phosphorus removal, and the best refining gas temperature is 373K. The best gas flow rate is 2L/min. Then in the best gas blowing refining condition, phosphorus content is decreased from 94ppmw to 11ppmw.