Model Implementation of Boron Removal Using CaCl 2 -CaO-SiO 2 Slag System for Solar-Grade Silicon HUI CHEN, YE WANG, WENJIA ZHENG, QINCAN LI, XIZHI YUAN, and KAZUKI MORITA A new CaCl 2 -CaO-SiO 2 slag system was recently proposed to remove boron from metallurgy-grade silicon by oxidized chlorination and evaporation. To further investigate the boron transformation process at a high temperature, a model implementation to present the transfer of boron from molten silicon to the gas phase via slag is introduced. Heat transfer, fluid flow, the chemical reactions at the interface and surface, the mass transfer and diffusion of boron in the molten silicon and slag, and the evaporation of BOCl and CaCl 2 were coupled in this model. After the confirmation of the thermal field, other critical parameters, including the boron partition ratios (L B ) for this slag from 1723 K to 1823 K (1450 °C to 1550 °C), the thicknesses of the velocity boundary layer at the surface and interface, the mass transfer coefficients of the boundary layer at the surface and interface, and partial pressure of BOCl in the gas phase were analyzed to determine the rate-limiting step. To verify this model implementation, boron removal experiments were carried out at various temperatures and with various initial mass ratios of slag to silicon (l). The evaporation rate of CaCl 2 was also measured by thermogravimetry analysis (TGA). DOI: 10.1007/s11663-017-1105-7 Ó The Minerals, Metals & Materials Society and ASM International 2017 I. INTRODUCTION SOLAR energy has received considerable attention over the past few decades, due to its importance as a green and renewable energy. [1] According to research by IHS Technology, the global photovoltaic installations in major regions will grow at an annual rate of more than 10 pct, as shown in Figure 1(a). [2] Silicon accounts for more than 90 pct of the materials used in photovoltaic module production and p-type polycrystalline technol- ogy, with a market share of more than 60 pct for the component production in 2014, as shown in Figure 1(b). [3] Low-cost solar-grade silicon (SOG-Si) is critical for the widespread use of solar cells. [4] To avoid high-cost traditional methods, such as the Siemens method, [5] many metallurgical techniques have been developed to purify silicon, as shown in Figure 2. [6–11] Most of the impurities found in metallurgy-grade silicon can be removed efficiently by employing these techniques. For example, phosphorous and metals can be removed by vacuum treatment [12] and directional solidification, [9] respectively. However, it is difficult to remove boron from silicon because of its low partial pressure in the gas phase and the high segregation coefficient in silicon. Therefore, slag treatment is considered to be an effective method for boron removal from silicon. Many slag systems have been investigated, such as CaO-SiO 2 , [13] CaO-SiO 2 -Al 2 O 3 , [4] CaO-SiO 2 -CaF 2 , [14] and CaO-SiO 2 -Na 2 O, [15] but a common flaw is the low removal efficiency due to the limited partition ratio. Our group has recently proposed a new process for boron removal from molten silicon, using a CaCl 2 -CaO-SiO 2 slag system. [16] The boron impurity could be evaporated as the oxychloride of boron, and the removal efficiency could reach nearly 90 pct. However, the mechanism of boron removal by this slag system is not yet clear, and the partition ratio is lower than the expected value. Here, we report a model implementation of the boron removal using the CaCl 2 -CaO-SiO 2 slag system, and the experimental data that support our model. II. MODEL DESCRIPTION Figure 3 shows the cross-sectional configuration of the shaft furnace system used in our model and HUI CHEN, YE WANG, WENJIA ZHENG, QINCAN LI, and XIZHI YUAN are with the School of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, P.R. China. Contact e-mail: wangye@scu.edu.cn. KAZUKI MORITA is with the Department of Materials Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8654, Japan. Manuscript submitted June 12, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B