Electrically Enhanced Boron Removal from Silicon Using Slag
- PDF / 381,106 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 1 Downloads / 218 Views
ment for removal of impurities from metallurgical grade silicon (MG-Si) is based on a principle of liquid-liquid refining applied in steel industry. Slag is used to absorb selected impurities (such as boron and phosphorus) from the silicon phase. In the case of removal of boron from silicon, the slag must be oxidizing to boron, that is the slag must have good capacity to dissolve boron and immiscible and non‘‘reactive’’ with molten silicon. In terms of practical application the slag must also differ markedly in density from the molten silicon. It has been suggested that suitable slags for purifying MG-Si would be CaO-SiO2 or MgO-SiO2 based with relatively high SiO2.[1,2] The selection of the slags depends on a number of factors. For example, the slag’s boron capacity, i.e., amount of boron retained by the slag system is a
M.S. ISLAM, Doctoral Student, M.A. RHAMDHANI, Director Pathways Engineering, and G.A. BROOKS, Professor, are with the Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Melbourne, VIC 3122, Australia. Contact e-mail: [email protected] Manuscript submitted August 27, 2013. Article published online November 9, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
function of the CaO/SiO2 ratio (basicity), the oxygen partial pressure, and the state of the boron in the slag phase, which is influenced by the structure of the slag. The slag’s boron capacity is often represented by the boron partition ratio as defined in Eq. [1]. LB ¼ ðwt pctBÞ=½wt pctB
½1
where (wt pctB) and [wt pctB] represent the concentration of boron in slag and silicon phase, respectively. There have been a number of studies on the thermodynamics of silicon melt in equilibrium with different slag systems. Texeira and Morita[1] investigated the use of CaO-SiO2 slag to remove B from Si and reported the boron partition ratios for slags with CaO/SiO2 ratios of 0.55 to 1.21 at 1823 K (1550 °C). The highest value of boron partition ratio of 5.5 was attained for a basicity of 1.21, and the results suggested a strong dependence on both the oxygen partial pressure and the basicity. Teixeira et al.[3] investigated the boron partition ratio for CaO-SiO2-(25 and 40 pct) CaF2 slag systems at 1823 K (1550 °C) for the basicity of 0.3 to 7.0. They reported the values of partitioning ratio in the range of 1 to 5.5. Johnston and Barati[2] investigated the possibility of refining using Al2O3-CaO-MgO-SiO2 and Al2O3-BaO-SiO2 slags at 1773 K (1500 °C). They confirmed that boron partition ratio increases through a local maximum with increasing basicity and with increasing oxygen partial pressure. Suzuki and Sano[4] investigated the slag system of CaO-SiO2 with additions of CaF2, BaO, and MgO, while Fujiwara et al.[5] focused on the CaO-SiO2-Al2O3. The general observation made was that the boron partition ratio reached maximum when the CaO/SiO2 ratio close to 2. There are only limited studies on the kinetics of the boron removal from silicon to the slag in the literature. Nishimoto and Morita[6] carried out boron removal kinetic
Data Loading...
