Study on the Kinetics of Aluminum Removal from Liquid Silicon to Slag with Mechanical Stirring
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INTRODUCTION
SILICON is generally produced in large electric arc furnaces and tapped into ladles, where oxidative refining takes place to remove impurities such as Ca and Al. Current refining ladles are semiclosed on the top, lined with refractory, and equipped with bottom-blowing. Almost all companies worldwide use this technique. The last major improvement to oxidative refining took place a couple of decades ago with the introduction of the bottom plug. While bottom-blowing does help to improve the kinetic condition for refining, the process has still very poor mass transfer and lack of control of the heat balance. The poor kinetic conditions lead to an unnecessarily long process time, which in turn results in the loss of SiO2 to the surroundings and short life-time of refining ladle. In order to develop a new type of refining vessel, a long-term project is currently being carried out in the present laboratory. To improve reaction rates, more efficient methods of agitation, such as mechanical stirring are considered. The facility of using mechanical stirring in the ladle to enhance the slag-metal reaction is one of the important topics of this project. In many cases of slag-metal reactions, the rate limiting step is the mass transfer to and/or from the interface where the chemical reaction takes place rapidly. Consequently, most of the kinetic studies have addressed the mass transport of reactants to the interface and the product away from the interface without stirring. A number of studies have been carried out focusing on the
JAEWOO LEE, Researcher, and DU SICHEN, Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden. Contact e-mail: [email protected] JESSE F. WHITE, and KJETIL HILDAL, Senior Research Engineers, are with Elkem AS, Technology, Kristiansand, Norway. Manuscript submitted March 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
boron and phosphorus transfer. Johnston and Barati[1] investigated the distribution of impurity elements between slag and MG-silicon doped with B and P at 1773 K (1500 C) under argon atmosphere. The reaction time needed for the system to reach equilibrium was determined to be 2 hours. Nishimoto et al.[2] studied the removal of boron from molten silicon to CaO-SiO2 slag at 1823 K (1550 C). It was found that the rate of boron removal was first-order and was controlled by mass transfer in the slag phase. Experimental data showed that equilibrium can be obtained within 2 hours. Kinetic studies on aluminum removal from silicon are scarce. Weiss and Schwerdtfeger[3] investigated the equilibrium between silicon and CaO-SiO2-Al2O3 slag using silica crucibles. High-purity silicon and slag samples were held at 1773 K (1500 C) under argon atmosphere for different periods of time and subsequently quenched. The aluminum content in silicon was found to increase with increasing alumina content in slag. It needed at least 3 hours for the system to reach equilibrium. These studies[1–3] indicate that the slag-metal reaction i
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