The Importance of Slag Structure to Boron Removal from Silicon during the Refining Process: Insights from Raman and Nucl
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INTRODUCTION
THE development of a novel solar cell has become increasingly urgent in recent years to account for the environmental deterioration caused by use of fossil fuel. The removal of impurities in silicon to a trace level is the key to producing solar grade silicon materials. Boron is considered to be one of the most detrimental elements because of its doping effect on silicon,[1] and its removal to a required range (less than 0.3 9 106) has proven a challenging task. Recently, metallurgical refining routes as a simple process have been widely applied,[2–8] including purifying silicon materials, due to their low operational cost and energy consumption compared with the Siemens process. Indeed, oxidizing/slagging treatment is considered to be the most effective way for
GUOYU QIAN, ZHI WANG, XUZHONG GONG, and LIYUAN SUN are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China. Contact email: [email protected] Manuscript submitted March 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
removing boron via oxidation reactions according to Reactions [1] and [2].[9–22]
LB ¼
½B þ ð3=4ÞO2 ¼ ð1=2ÞB2 O3
½1
ð1=2ÞB2 O3 þ ð3=2ÞO2 ¼ BO3 3
½2
3=2 3=4 3=2 cB aO2 pO2 cB aO2 K0 aSiO2 3=4 ðB Þ =K =K : ½B cBO3 cBO3 aSi 3
3
½3 Generally, the thermodynamics partition coefficient between slag and metal, shown in Eq. [3], was used to express the refining capacity of slag to remove boron from silicon. Regarding the behavior of B and Si in silicon, Hu et al.,[14] Li et al.,[15] and Teixeira and Morita[16] have tried to increase LB (partition coefficient of boron between slag and silicon) by increasing cB (activity coefficient of boron in silicon) and reducingaSi (activity of silicon), but only a limited increase was obtained in their works. Therefore, the behavior of slag has caused extensive concern; there have been numerous
studies targeting the removal of boron from silicon using various slags, such as CaO-SiO2,[17] CaO-CaF2CaO-SiO2-Na2O,[19] CaO-SiO2-Al2O3,[20] SiO2,[18] Na2O-SiO2,[21] and CaO-SiO2-MgO[22] at 1723 K to 1873 K (1450 C to 1600 C) under Ar atmosphere. In these studies, basicity (being a function of basic oxide activity) and oxygen potential (PO2 being a function of SiO2 activity) of slags were generally used to evaluate the effect of slag structure on the change of LB. However, the basicity and PO2 could be precisely derived only if the basic oxides and acid oxides existed in the form of molecules in slag. Therefore, understanding the true structural forms generated in slag and the interactions of the various structure species is significantly important, but there is still a lack of theoretical basis. Optical basicity (K), which was developed by Duffy and Ingram,[23,24] has been used to further study the structure of slag. When the complex networks of acidic oxides were broken by basic oxides, more free c
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