The nucleation of co bubbles in molten ironcarbon drops reacting with oxidizing gases
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I.
INTRODUCTION
THEoxidation of carbon is undoubtedly the most important steelmaking reaction. There has been considerable interest in the kinetics of the decarburization reaction, as indicated by the large number of investigations that have been conducted on this subject. The current state of knowledge may be summarized by stating that the mechanism of the reaction in the early stages of decarburization is fairly well understood, and the appropriate rate limiting factors have been recognized. However, there are some features of the reaction as it occurs during the late stages of steel refining and during solidification, which are imperfectly understood. These are: i. carbon monoxide gas bubble evolution from molten iron, during carbon oxidation, which is the cause of the foam formation in oxygen steelmaking operations, t1'2'3] and ii. the formation of gas pores in solidification processes. [4'5'6] While the phenomenon of carbon boil was observed in many of the decarburization studies, particularly those studying levitated and free fall drops, the conditions at which bubble formation will occur are not clearly understood. Distin et al. [71 and Baker et al. ts] found that bubble formation occurred after oxide formation on the surface, while Baker et al.,[9] Robertson,[lO] See and Warner, [H] and Roddis It2] found that carbon boil starts before oxide formation. On the other hand, Kaplan and Philbrook113]reported that they did not observe any gas bubble formation in their experiments even after oxide formation. In general, decarburization of molten iron with oxidizing gases involves diffusion of carbon to a gas-liquid interface and absorption of oxygen into the melt. The reaction between dissolved oxygen and carbon in molten iron, resulting in carbon monoxide, belongs to a general class of gas-liquid reactions where diffusion and growth take place in supersaturated solutions. [~4'~5'16]The reaction involved in decarburization is of the type: __A+ B ~ AB (gas)
[ 1]
N. EL-KADDAH is Associate Professor, Department of Metallurgical Engineering, The University of Alabama, Tuscaloosa, AL 35487-9644. D. G. C. ROBERTSON is Professor, Department of Metallurgical Engineering, The University of Missouri-Rolla, Rolla, MO 65401-0249. Manuscript submitted February 4, 1987. METALLURGICAL TRANSACTIONS B
which has not previously been dealt with in the chemical engineering literature. The purpose of this work is to examine the supersaturation of molten iron with respect to carbon monoxide caused by the counter diffusion of carbon and oxygen during decarburization. The present paper is concerned with decarburization of molten iron droplets. It also presents a new approach to determine the criteria for supersaturation in gas-liquid reactions involving a gaseous product. II.
FORMULATION
Consider a stagnant molten iron-carbon alloy drop in an oxidizing gaseous environment. At the gas-liquid interface carbon will react to form CO by reactions such as 1
C + 502 ( g a s ) = C O (gas)
[2]
C + C02 ( g a s ) = 2 C O (gas)
[3]
Since o
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