Part 1: Model Development and Validation

Neslihan DOGAN, Alan BROOKS and Akbar RAMDHANI

(Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Australia.)

(April 15, 2011)

A comprehensive model of oxygen steelmaking that includes the kinetics of scrap melting, flux dissolution, slag chemistry, temperature profile of the system, formation and residence of metal droplets in the emulsion, and kinetics of decarburization reaction in different reaction zones was developed. This paper discussed the development and the application of the model into an industrial practice. The results from the model were consistent with the plant data from the study of Cicutti. The model suggested that 45% of the total carbon was removed via emulsified metal droplets and the remaining was removed from the impact zone during the entire blow. It was found that the residence time of droplets as well as decarburization reaction rate via emulsified droplets was a strong function of bloating behavior of the droplets.

This model is the first attempt in the open literature that allows for the decarburization kinetics of the impact zone to be predicted separately from decarburization kinetics of the emulsion.

Oxygen steelmaking is currently the dominant technology for producing steel from pig iron. The process is complex since it involves simultaneous multi-phase interactions, chemical reactions, heat transfer and complex flow patterns at high temperatures. The transient nature of the process also adds more complexities. The severe operating conditions make it difficult to take measurements and directly observe the process. Furthermore, experimental results are not always adequate in providing an evaluation of important parameters of the system. Mathematical modelling has been widely used to describe the complicated nature of the process; improve understanding of the system and optimise the process control.

There have been a number of process models1–18) developed to describe the kinetics of oxygen steelmaking process with an emphasis on the evolution of bath temperature, metal and slag chemistry. The details of these models are not available in the open literature and they are generally used for internal research requirements at steel plants. Additionally, these models and other previous models represent the system by using practical equations in order to achieve the process control. These simplified models might be suitable for industrial applications and provide reasonable approximations.

However, to the authors’ knowledge these models ignore the important process variables and changes in process conditions. For example, recent findings such as bloated droplet theory are not included in the previous models.

When the oxygen absorption rate is higher than the rate of consumption of this oxygen at the surface, oxygen builds up in the metal droplet and increases the internal pressure of CO gas. As the internal pressure of CO exceeds the surface energy of the metal droplet, CO gas formation occurs inside metal droplet. Once the internal nucleation started, metal droplet becomes “bloated” and surface area increases, therefore, reaction kinetics increases since the turbulence caused by the generation of CO bubbles inside the metal droplet promotes the mass transfer. Bloating behaviour recognised by Molloseau and Fruehan resulted in dramatic affects on the residence time of the droplets which increases up to 120 s. This behaviour has a significant impact on the decarburization rates in the emulsion, thereby the overall kinetics of the oxygen steelmaking process. Although these models played an important role in the development of holistic model for oxygen steelmaking process, there is still work required for the development of robust and accurate models.

The principle aim of the present study is to develop a comprehensive model of decarburization in oxygen steelmaking.

The model focuses on the decarburization reaction in different reaction zones to predict the carbon content of liquid steel throughout the blow and is validated with a set of industrial data. In this Part 1 paper, the development of the global model incorporating the bloated droplet theory will be discussed.

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