Date of Award

Spring 2012

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

Kenneth Coley

Language

English

Committee Member

Gordon Irons and Nikolas Provatas

Abstract

Considerable attention has been paid to the reaction between molten iron oxide containing slag and iron droplets or solid carbon due to the critical roles it plays in various metallurgical processes. However, during the last two decades, most of the studies have been carried out on iron droplets, for which the size remains constant. Another important phenomenon, that the droplet will swell has not been paid the same attention. Knowledge of the extent of droplet swelling is essential in predicting residence times in BOF steelmaking. The objective of this study is to develop the understanding for droplet swelling and produce a predictive model that predicts droplet swelling over the range of oxygen steelmaking conditions.

Several workers have observed swelling of high carbon droplets when exposed to oxidizing slags. In the present work, the measurements on swelling rates were made using X-ray fluoroscopy. Comparing the swelling rate with the total volume of gas evolved during the reaction, it is shown that only a small percentage of the gas generated is retained in the droplet to contribute to swelling. The gas generation rate is shown to be controlled by the rate of nucleation of CO bubbles inside the droplet. The critical supersaturation pressure for nucleation is found to be two orders of magnitude less than predicted from theory, which is in keeping with many other studies on nucleation of gases in liquids. However, the effect of surface tension, temperature and saturation pressure shows quantitative agreement with theory.

In order to predict the droplet swelling rate, CO bubble escape rate has to be known. In this research, the escape mechanism has been proposed; it is the film rupture around the iron droplet surface. The rupture rate is mainly influenced by viscosity, surface tension and bubble size. For a given experimental condition, the calculated film thickness is 1.5 μm at the maximum drop diameter, assuming the bubble radius is 0.3 mm. The CO escape rate is 2.51 cm3/s, it agrees well with 1 to 12 cm3/s when gas escapes from steelmaking slags considering the differences of surface tensions and viscosities between metal and slag.

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