Analysis of bloated droplet theory using steelmaking process models

There is significant evidence that droplets generated in steelmaking "bloat" due to the inability of gas generated from the decarburisation reaction to escape from the surface of liquid metal droplets. A model to describe this behavior was developed by Brooks, Subagyo, Coley and Pan based on their own experimental work and calculations and previous studies by Fruehan and co-workers. This approach has been successfully incorporated into an overall process model of oxygen steelmaking. A unique feature of this model is an evaluation of the decarburization kinetics of individual metal droplets in the emulsion and comparing this to the overall kinetics of oxygen steelmaking. The model suggests that the droplets become bloated and remain in the emulsion for long periods (30+ seconds). This paper evaluates the effects of droplet size and volume fraction on the bloating behavior of droplets and critically examines the repercussions of the new theory on plant design and operation. Introduction One of the main goals of the oxygen steelmaking process is to effectively reduce the carbon concentration of the liquid iron. It is understood that most carbon removal reactions occur in the emulsion phase via a reaction between the metal droplets and slag phase.(Meyer et al., 1968, Price, 1974) An improved understanding of decarburization reaction and the factors controlling the overall rate should provide better control of the process and increase the productivity. In the literature, there is a limited knowledge on how to relate the carbon removal rate within the droplets to the overall kinetics of the process under full scale operating conditions. A computer based model which incorporates the bloated droplet theory under dynamic conditions was developed to evaluate its influence on the overall kinetics of the process. The model focused on the decarburization reaction in different reaction zones to predict the carbon content of liquid steel throughout the blow. The system includes 17 submodels and 2 reaction zones. Two reaction zones, namely, the emulsion and impact zone are considered to investigate the kinetics and mechanism of carbon removal reactions since it is well known that these reactions take place via direct oxygen absorption at the impact area and FeO reduction in the emulsion phase. The reaction zones are linked to each other by material streams which are slag constitutes and liquid metal droplets. The input mass flows, process conditions and calculation sub-models to be considered for each reaction zone are hot metal, scrap and flux charges, hot metal, N. Dogan, G. A. Brooks, M. A. Rhamdhani 2 scrap and slag compositions, oxygen blowing conditions, lance height, gas flow rates, temperature of the bath, the slag and the impact zones, flux dissolution, scrap melting, ejected metal droplets behaviour such as droplet generation rate, droplet size, residence time in the emulsion, decarburization rates in the emulsion and impact zones and are illustrated in Figure 1. Fig. 1: The schematic description of the system Once all the variables deemed to be important to the each sub-model have been identified, inter-relationships among them are developed. These sub-models were modelled individually in which they include the calculation procedure, assumptions and boundary conditions to represent each process variable considered. In the following step, the sub models were linked to each other dynamically to be used as input data or boundary condition and sub-models formed the global model of the oxygen steelmaking process. Most of the sub-models such as flux dissolution, droplet generation, scrap melting, decarburization reactions in the emulsion and at the impact zone that form the global model and the global model itself have been validated against industrial data in the open literature to investigate the feasibility of the application of the developed models. The results have been published elsewhere.(Dogan et al., 2009a, 2009b, Dogan et al., 2011a, Dogan et al., 2011b, Dogan et al., 2011c) This model is the first attempt to determine the role of emulsion quantitatively based on the bloated droplet theory, using a theoretical model under full scale operating conditions. In this paper the effects of droplet size and gas fraction in the emulsion on bloating behaviour of droplets will be discussed. Metal Droplets Metal Return Slag Flux