Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Kinetics of Ca and B Transfer
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SLAG treatment, or slag refining as it is sometimes referred to, is a metallurgical refining process step in the production of solar-grade silicon.[1] The established industrial process for slag refining is a batch process conducted in graphite vessels. Boron concentrations in silicon of less than 0.3 ppm are achievable. Currently, gas stirring is used to improve the mass transfer rate of boron to the slag phase. There is considerable economic incentive, however, to employ more efficacious methods of agitation, such as mechanical stirring, to improve reaction rates. In general, the use of impeller stirring in industrial metallurgical practice is very limited due to many practical challenges dealing with high-temperature melts such as finding a suitable refractory material that has high mechanical strength at elevated temperature combined with satisfactory chemical inertness and resistance to erosion. In the case of silicon refining, fortunately, it is possible to employ graphite as a refractory material, which has exceptional physicochemical properties at high temperature.[2] Due to its ease of machinability, the use of graphite as a refractory affords the possibility of developing increasingly complex reactor designs. Probably the first published study that investigated the kinetics of deboronization of silicon was by Nishimoto et al.[3] They concluded that boron oxidation is a first-order reaction. By conducting experiments with JESSE F. WHITE, Senior Research Engineer, is with Elkem AS, Technology, Kristiansand, Norway. DU SICHEN, Professor, is with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted November 21, 2013. Article published online September 11, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
varying slag-metal ratios, they deduced that mass transport in the slag phase is the rate-controlling step. It was demonstrated that it takes nearly 2 hours to attain equilibrium under static conditions. They calculated a mass transfer coefficient for boron mass transfer. Other researchers have since corroborated these results.[4] In a similar study, Safarian et al. studied the kinetics of boron extraction using Na2O-SiO2 slags.[5] A novel approach was taken by Islam et al., who studied the kinetics of boron removal by application of an electrical potential across the slag and silicon phase.[6] It must be pointed out though that industrial metallurgical reactors are by no means static systems, as reaction rates are usually maximized through forced convection such as via gas stirring. Therefore, mass transfer coefficients derived from static laboratory measurements are generally of limited value for reactor design and scale-up. There are preceding publications that employed mechanical stirring to study mass transfer rates between liquid slag and metal, though mainly in the area of ferrous metallurgy. Fulton, Grant, and Chipman used a graphite crucible and impeller to transfer silicon between carbon-saturated liquid iron an
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