Removal of Phosphorus in Silicon by the Formation of CaAl 2 Si 2 Phase at the Solidification Interface
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WITH the rapid increasing demand for the solar energy installation, the development of production of polysilicon with high purity becomes an urgent mission. Compared to the traditional chemical manufacturing technology, metallurgical process is considered as a potentially innovative way to get high purity silicon for its advantages such as low investment, low cost, short process, and environment-friendly. Corresponding to types of impurities, several technologies are combined together, making this process long and complicated. Profound removal of the non-metallic impurity B and P is the key. Firstly, B/P has relative high segregation coefficient that make them hard to be segregated.[1] In our previous work,[2] it was indicated that acid leaching was not very effective for P removal, because only the impurities segregated on the surface of Si had the chance to be removed effectively by acid leaching. Secondly, their low content and relative high solubility in silicon cause them homogeneously distributed, which is bad for
LIYUAN SUN and GUOYU QIAN, Postdoctoral Fellow, ZHI WANG, Professor, and DONG WANG, Research Assistant, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), No. 1 Bei Er Jie, Zhong Guan Cun, Beijing, 100190, China. Contact e-mail: [email protected] HANG CHEN, formerly Master Student with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), is now Researcher with Zijin Mining Group Co. Ltd State Key laboratory of Comprehensive Utilization of Low-grade Refractory Gold Ores, Shanghang, 364200, Fujian, China. Manuscript submitted March 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
their effective concentration. Thus, B/P is hard to be deeply removed for their removal limitation. As a new effective method for removing impurities B/ P, solvent refining has been developed recently[3–6] for its impact of lowering segregation coefficients[7,8] of impurities. By alloying with some solvent, Metallurgical-Grade silicon (MG-Si) can be purified by recrystallization in alloy melt at a temperature significantly lower than the melting point of Si 1687 K (1414 °C). To improve the segregation properties of impurity elements, much research has been conducted to optimize the solvent selection such as Al,[4,5,9–12] Cu,[13–15] Sn.[2,16] However, it is revealed that the computed value of impurity removal is lower than the experimental one. The original existence state of impurities in silicon may have been changed during the refining process, and impurity reconstitution becomes the main reason for the impurity removal besides segregation. To directly ‘‘get’’ the impurity B/P, ‘‘impurity getter’’ is applied to create heterogeneous impurity phase which will be removed easier by acid leaching. Fe, T
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