A multiphase fluid mechanics approach to gas holdup in bath smelting processes
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INTRODUCTION
[N several new smelting-reduction processes for ironmaking, coal and partially prereduced iron ore are added to a slag phase.l~:-;~ The iron oxide quickly dissolves in the slag to be reduced by the coal, (FeO) + C~.,.~~ Fe + CO
[1]
to produce iron that collects at the bottom of the vessel. Reduction can also be accomplished by carbon dissolved in iron by a similar reaction: (FeO) + C ~ Fe + CO
[2]
Oxygen is delivered by a top oxygen lance which combusts the carbon (and other volatile hydrocarbons) in the coal: 2C~.,.) + O, =* 2CO
[3]
In a similar manner to oxygen steelmaking, oxygen will also be absorbed into the slag where it will react by equations similar to Eqs. [l] and [2] to tbrm carbon monoxide. Unreacted oxygen, or oxygen introduced later, reacts with the evolved carbon monoxide to produce carbon dioxide (postcombustion), 2C0 + O2 ~ 2C0_~
[4]
that generates additional heat for the process. The objective of these new ironmaking processes is to supplant conventional blast furnaces by much smaller, more intensive reactors. Consequently, the intensity of gas evolution in the slag is unprecedented in a metallurgical reactor. The slag layer is lifted due to this extremely high gas evolution rate and may even flow over the top of the vessel; this phenomenon is commonly called slag foaming. It is gen-
H. GOU. formerly Research Associate with the Department o f Materials Science and Engineering, McMaster University, is Process Engineer with Hatch Associates Ltd.. Mississauga, ON, Canada L5K 2R7. G.A. IRONS. Dofaseo/NSERC Professor of Process Metallurgy, and W.-K LU. Professor. are with the Department of Materials Science and Engineering, McMaster University.. Hamilton, ON, Canada L85 417. Manuscript submitted July 20, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS 13
erally accepted that slag tbaming is one of the key limiting factors in the development of these new processes, t4-7] Recently, there have been several studies on the slag foaming phenomenon related to the smelting-reduction processes, both in laboratory-scale fumacest,*~41 and in operational smeltersY -'.~1 In some laboratory-scale studies, the foaming was generated by passing an inert gas through a molten slag layer at low flow rate.t"-tu In others, the gas was generated from reduction reactions taking place between iron oxide-containing stag and carbonaceous materials, u'-,~,~l In these studies, the superficial gas velocities (volumetric flow rate per unit cross-sectional area of the vessel) were of the order of a few centimeters per second. X-ray photographs clearly show that the foams resemble soap or beer bubble foams. ]t~J It was found that the extent of foaming depended on the superficial gas flow rate and slag properties, particularly slag viscosity and surface tension t~.9,~0.t3.ul Ogawa e t al. ~14) investigated gas bubble formation and size in a foaming slag to understand foaming phenomena and control. Based on experimental evidence and a static force balance between buoyancy and surface tension forces, they devel
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