Progress in Slag Foaming in Metallurgical Processes
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I.
INTRODUCTION
SLAG foaming is an integral feature of many metallurgical processes; it may be the bane of the metallurgist, leading to slag overflowing from the vessel or it may be a desirable feature providing major benefits to the process. The beneficial effects are obtained in electric arc furnaces (EAFs); the slag conditions are designed to cause the slag to foam up over the arcs, leading to the following: Higher electrical efficiency because the arc heat is captured by the slag
Shielding of the water-cooled panels and roof from the radiation, decreasing damage to them
Lower levels of electrical harmonics transmitted to the electrical grid from the EAF
Lower levels of acoustic noise In slag-based smelting-reduction processes, such as the direct iron ore smelting (DIOS) and american iron and steel institute (AISI) steelmaking processes, intense foaming occurs in the vessel that is central to the operation of the process. Thus, it is clear that a good understanding of the slag foaming is necessary to use it properly in metallurgical processes. Professor Fruehan and his colleagues contributed some of the early and most insightful work on slag foaming. Slag foaming has been studied extensively over the past two decades[1–9] with the goal of quantifying the phenomenon and developing useful models that can be applied to control slag foaming in metallurgical processes. Ito and Fruehan[1] were the first to apply the foaming index to slag systems. The foaming index is defined as the ratio of the foam height to the superficial gas velocity. The superficial gas velocity is given by TAI XI ZHU, Ph.D. Candidate, is with the Materials Science and Engineering Department, Steel Research Centre, McMaster University, Hamilton, ON L8S 4L8, Canada. Contact e-mail: zhutx@ mcmaster.ca K.S. COLEY, Professor and Associate Dean Academic, is with the Faculty of Engineering, Steel Research Centre, McMaster University. G.A. IRONS, Professor and Director, is with the Steel Research Centre, McMaster University. Manuscript submitted December 5, 2011. Article published online March 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
Vsg ¼
Qg Ac
½1
Dh Vsg
½2
and the foaming index is R¼
where Qg is the flow rate of gas through the slag and Ac is the cross-sectional area of the vessel. The foam height Dh is defined as the change in slag height from flat bath to the steady-state foamed condition. Usually, there is an unfoamed reservoir of slag below the foam. To make use of the foaming index, it must be independent of the superficial gas velocity. Ito and Fruehan[1] observed the foaming index to be constant above a superficial gas velocity of 1.0 cm/s for a range of crucible diameters from 25 to 50 mm, and they concluded that the foaming index is a function of slag properties and independent of the superficial gas velocity. This conclusion was confirmed by the work of Ozturk and Fruehan[6] and Zhang and Fruehan.[8] Ozturk and Fruehan[6] found foam height to be dependent linearly on superficial gas velocity for 48CaO32SiO2-10-Al2O3-10FeO (wt
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