Bubble bursting phenomenon in Gas/Metal/Slag systems
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8/28/03
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Bubble Bursting Phenomenon in Gas/Metal/Slag Systems ZHIJUN HAN and LAURI HOLAPPA Iron droplets can be ejected into the surrounding atmosphere or entrained into the slag phase when gas bubbles pass through the metal surface or the metal/slag interface. The phenomena occurring during passage of single bubbles through the free surface and the interface were investigated by using the in-situ X-ray transmission technique. The mass of droplets ejected into the atmosphere attained a maximum value at a certain bubble size, which depended on the surface tension of the iron melt. Bubble bursting on the free surface of iron melt ejected numerous fine iron droplets called “film droplets” and a few much larger jet droplets. Two different groups of iron droplets were also observed as entrained in the slag due to bubble passage through the iron/slag interface, although the physical phenomena are to some extent different from bubble bursting to the gas phase.
I.
INTRODUCTION
G AS injection is commonly used in steelmaking processes. Iron droplets can be ejected into the surrounding atmosphere or entrained into the slag phase when gas bubbles pass through the gas/metal or metal/slag interfaces. These two cases are called “bubble bursting on the free surface of liquid iron” and “bubble bursting at the interface of slag/iron systems” in this article and were investigated herein. Metal droplets formed by bursting of gas bubbles on the free surface of liquid iron have proven to be one of the main mechanisms of dust formation in pneumatic steelmaking.[1,2] Metal droplets dispersed in slag due to gas bubbles bursting at the interface of slag/iron systems can greatly influence the heat and mass transfer in many metallurgical processes.[3,4,5] Therefore, these are of considerable practical importance and of great interest to metallurgists. However, most of the experiments were conducted in low-temperature media and the high-temperature experimental data are very rare. For bubble bursting on the free surface of water, Newitt et al.[6] and Resch et al.[7] have shown that for small air bubbles, there are two sources of ejection. The first is the rupture of the liquid film that protrudes above a bubble as it comes to rest at the bath surface. It produces very fine droplets with diameter less than 500 mm. The second is the collapse of the cavity that remains after rupture of a bubble cap; depending on its shape and size, it can produce droplets with diameter up to several millimeters. In pure water or seawater, a critical bubble size has been observed that produces a maximum amount of ejections.[8,9,10] For the critical bubble size, the submerged portion of the floating bubble is almost a hemisphere. At smaller size, the protruding portion is smaller than the subZHIJUN HAN, Postdoctoral Researcher, and LAURI HOLAPPA, Professor, are with the Laboratory of Metallurgy, Helsinki University of Technology, FIN-02015 HUT, Finland. Contact e-mail: [email protected] This article is based on a presentation given in the Mills
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