Measurement of the Phase Diagram of the SiO 2 -CaCl 2 System and Liquid Area Study of the SiO 2 -CaO-CaCl 2 System
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he photovoltaic industry, the purification of silicon through metallurgical treatments may be a promising way to produce solar grade silicon (6N) from metallurgical grade silicon (3-4N).[1] Among the metallurgical treatments, the slag method[2] was used to remove boron, which is difficult to eliminate because of its high segregation coefficient (LB = 0.8) and low partially pressure.[3] It was proven that the molten slag based on the CaO-SiO2 binary system is an efficient agent for boron removal from molten metallurgical grade silicon. Several slag systems including CaOSiO2,[4–6] CaO-SiO2-CaF2,[7] CaO-SiO2-Al2O3,[8] CaOSiO2-MgO, CaO-Li2O-SiO2,[9] CaO-SiO2-Na2O,[6,10] and CaO-SiO2-Al2O3-CaF2[11] have been investigated; however, they all exhibit limiting boron removal efficiency.
YE WANG, Associate Professor, is with the Department of Metallurgical Engineering, School of Chemical Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: wangye [email protected] KAZUKI MORITA, Professor, is with Department of Materials Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8654, Japan. Manuscript submitted November 29, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
In a previous study, the authors confirmed that the CaO-SiO2-CaCl2 slag achieved effective results in removing boron.[12] Furthermore, Lu and Huang reported 96 pct of boron and 93 pct of phosphorus could be removed by using CaO-SiO2-CaCl2 slag at 1923 K and 1773 K (1650 C and 1500 C), respectively.[13,14] Although it has been proven that boron can be removed as a gas phase (BOCl) by using the CaO-SiO2-CaCl2 slag system at 1723 K (1450 C), the phase diagrams of the SiO2-CaCl2 and CaO-SiO2-CaCl2 systems have not been reported yet. In the present study, to determine the SiO2-CaCl2 phase diagram and liquid area of the CaO-SiO2-CaCl2 slag, high-temperature quenching experiments followed by thermogravimetric and differential thermal analysis (TG-DTA) were performed along with X-ray diffraction (XRD) phase determination and morphological observation. Reagent grade calcium carbonate (CaCO3, ‡99.5 wt pct in purity, Wako, Japan) and silicon dioxide (SiO2, Cica-Reagent, Kanto Chem. Co., Inc. Japan) were used for preparing the starting materials. CaO was prepared by heating CaCO3 for 12 hours (CaCO3 fi CaO + CO2) at 1273 K (1000 C). Appropriate proportions of the starting materials were carefully weighed, mixed, and stored in a desiccator in batches of 3 g. Various compositions of CaO-SiO2 high-purity powders were mixed first and sealed in a graphite crucible (inner diameter: 23.5 mm; outer diameter: 30 mm; length: 60 mm). After that, the sample was premelted by using a vertical SiC resistance furnace equipped with a mullite tube (outer diameter: 60 mm; inner diameter: 53 mm; length: 1000 mm) at 1723 K (1450 C) for 5 minutes followed by argon gas quenching. The temperature was controlled and maintained at 1723 K ± 2 K (1450 C ± 2 C) by using a proportional-integral-derivative controller. Then, the premelted slag was crushed into pieces; 20 mg of slag
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