A Proposal for a Novel Method to Measure the Diffusivity of Species in Slag
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THE application of thermodynamics to the understanding of the metallurgical process of ironmaking and steelmaking has received great attention in the view of the obvious advantages. However, studies on microkinetics involving slag have received comparatively less attention. In recent years, significant advancements in the understanding of the kinetic phenomena in pyrometallurgical processes could be reached in view of improved experimental techniques as well as the availability of computational tools. The current industrial scenario demands extremely controlled amounts of impurities in steel for attaining the desired properties of the end product. During major processes like desulphurization, dephosphorization, and deoxidation of steel, the impurity atoms are transported from the metal phase to the slag bulk and even to the gas phase through the intermediate slag. Hence, for a stringent control of such impurities, a deep understanding of the transport of species through the slag phase is required. LUCKMAN MUHMOOD, Doctoral Student, and SESHADRI SEETHARAMAN, Professor, are with the Division of Materials Process Science, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden. Contact e-mail: [email protected] NURNI NEELAKANTAN VISWANATHAN, Associate Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 40076, India. Manuscript submitted August 4, 2010. Article published online January 15, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
In metal–slag–gas reactions taking place in any steelmaking process, the rate of such reactions would depend on the rate of the slowest process in the system. The process of transport of an impurity species as, for example, sulfur from the molten metal to the gas phase through a slag intermediate phase can be divided as follows: (a) Transport through the bulk molten metal (b) Interaction at the slag–metal interface. For sulfur, the reaction would be expressed as follows: 2 Sðliquid metalÞ þ O2 ðslagÞ ¼ Oðliquid metalÞ þ SðslagÞ
½a
(c) Transport of these species through the slag media, which is a diffusion process (d) Interaction at the gas–slag interface through which the species enters the gas phase. The interfacial reaction in the case of sulfur would be expressed as follows: 2 1 1 S2 ðslagÞ þ =2O2ðgasÞ ¼ OðslagÞ þ =2S2ðgasÞ
½b
(e) Gas phase mass transfer of the species involved It is well known that the transport of species through gas media is fast and that the interactions at the interface are electrochemical in nature and are likely to take place rapidly[1] if the charge transfer is enabled suitably. The only remaining process would be the transport through the metal and slag media. It has been well established that the diffusion coefficients of a VOLUME 42B, APRIL 2011—393
species example (e.g., S in the metal phase) is about two orders of magnitude higher than the corresponding species through slag.[2] Thus, the rate-determining step i
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