Evaporation Removal of Boron from Metallurgical-Grade Silicon Using CaO-CaCl 2 -SiO 2 Slag
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lability of abundant and low-cost solar-grade silicon (SOG-Si) feedstock is essential for the widespread use of solar cells. In order to avoid high-cost gasification processes for SOG-Si production, such as the Siemens method,[1] highly productive metallurgical purification processes have been developed. In these metallurgical processes, most of the metallic impurities can be removed from Si using a directional solidification process. However, because the segregation coefficients of B and P are close to 1, they are not easily removable with these processes. Since plasma and vacuum melting processes used for the removal of B and P require long operation times and high temperatures, there is a strong demand to further reduce the costs of B- and P-removal processes. Recently, slag treatment as a method for B and P removal has received great attention because of its low cost and mass productivity. B removal by slag treatment has been studied mainly using CaO-SiO2 slag.[2] The partition ratio (LB), which is defined as (mass pct B in slag)/(mass pct B in Si), is not
YE WANG, Second-Year Ph.D. Student, and KAZUKI MORITA, Professor, are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Contact e-mail: [email protected], wangye2003@163. com XIAODONG MA, Postdoctoral Research Associate, is with the School of Chemical Engineering, The University of Queensland, Brisbane, St Lucia, QLD 4072 Australia. Manuscript submitted September 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
high enough for B removal by a single slag treatment. In order to improve B-removal efficiency in slag refining, Nishimoto et al.[3] introduced chlorination in which Cl2 gas was supplied into the CaO-SiO2 slag, and found a decrease in the B content of Si, presumably because of B oxychloride gas formation. Although such removal by gaseous evaporation during slag treatment is considered to be a possible refining technique, the reaction mechanism has not been clarified yet. In addition, considerable Si loss occurs during the reaction with Cl2 gas. In the current study, after confirming the possibility of evaporating B oxychloride species through the reaction of BO1.5 with CaCl2, Si refining by chlorination at the coexistence of the slags was further studied using CaCl2-containing slag. In addition to the general slagrefining effects, the slag is also expected to react with B in Si to form gaseous species. A new process for the removal of B from molten Si has been proposed, and is shown schematically in Figure 1. When oxidizing slag is added onto the surface of molten Si, B in molten Si is partly oxidized at the interface and transferred to the slag as BO1.5: 3 Bðl in SiÞ þ O2 ðgÞ ¼ BO1:5 ðl in slagÞ 4 G ¼ 664:7 þ 0:13T kJ=mol:
½1
Then, BO1.5 in molten slag is chlorinated by CaCl2 according to the principle of selective chlorination, which can be represented as 1 1 BO1:5 ðl in slagÞ þ CaCl2 ðlÞ ¼ BOClðgÞ þ CaOðsÞ 2 2 G ¼ 572:2 0:13T kJ=mol: ½2 Finally, B oxychlorides with high vapor pres
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