Determination of interfacial reaction rate constant of nitrogen between N2–CO2–CO–Ar and molten iron using isotope exchange technique

Abstract

The application of CO2 in the steelmaking process has yielded promising results, demonstrating a certain capability for nitrogen removal. To accurately determine the kinetic parameters of nitrogen reactions at the iron melt interface under CO2 injection conditions, an isotope exchange technique was employed. This technique was used to monitor the evolution of the nitrogen isotopic composition during the reaction between a 28N2–30N2–CO2–CO–Ar gas mixture and an iron melt of controlled composition. The kinetic parameters of nitrogen were subsequently calculated for various CO2/CO ratios. Furthermore, the dissociation rate determining model was applied to establish the relationship between the interfacial reaction rate constant (kc) and the activity of surfactive elements (O, C, and S) in molten iron (aO, aC, and aS), expressed as .

1. Introduction

To achieve global temperature control targets, carbon dioxide capture and utilization represent a critical pathway for the steel industry to reduce its carbon emissions. Jin et al. [1] investigated the feasibility of CO2 injection in the steelmaking process and found that it could decarburize the molten steel and facilitate the smelting process. Zhu et al. [2–7] proposed a novel converter process using CO2 to replace a portion of the top-blown O2 and all of the bottom-blown N2/Ar, enabling CO2–O2 mixed injection steelmaking. The endothermic reactions that occur upon CO2 injection into molten steel suppress iron evaporation. Compared to pure oxygen injection, CO2 reacts with elements in the steel to form CO. This gas generation enhances stirring intensity, improves kinetic conditions, and promotes nitrogen removal from the molten steel [5]. Nitrogen has both beneficial and detrimental effects in steel. It can serve as an alloying element to enhance steel properties [8–13]; conversely, it can be considered an impurity that degrades performance [14–16]. Therefore, determining the nitrogen–liquid iron interfacial reaction rate constant and identifying its influencing factors are of paramount importance.

However, the interfacial dissolution and reaction behavior between CO2–CO–Ar gas mixtures and nitrogen in molten steel remains unclear. To establish a kinetic model for denitrification using CO2 as a bottom blowing gas, the relationship between the gas mixture composition and the melt composition must be elucidated. In traditional research methods, such as the Sieverts’ technique [17] used to study nitrogen absorption and desorption rates, the effects of diffusion in both the liquid and gas phases are significant and cannot be neglected. In this study, the isotope exchange technique was employed to measure the interfacial reaction rate constant. A key advantage of this method is that the influences of liquid phase and gas phase mass transfer on the interfacial reaction rate can be disregarded once equilibrium is established. The isotopic exchange of diatomic molecules, including oxygen, hydrogen, and nitrogen, has been extensively described in previous studies. Klier et al. [18] proposed a theory describing isotopic exchange reactions in systems containing oxygen molecules and oxides, based on an examination of the behavior of 18O18O, 16O16O, and 18O16O. Bouwmeester’s team [19] developed a stochastic model for surface-limited oxygen isotope exchange in closed gas systems. Boreskov’s team [20–21] investigated the isotope exchange of molecular nitrogen over various metals and nitrides.

Byrne and Belton [22] and Glaws and Fruehan [23–24] introduced isotope gas exchange technique into the kinetics of metallurgical reaction. They investigated the dissociation rate of N2 on high-purity liquid iron and iron–sulfur alloys using the 15N–14N exchange reaction. Notably, Byrne and Belton observed that the rate constant exhibits a functional dependence on the reciprocal of the surfactive element concentration, a finding consistent with that reported by Pelke and Elliott [25]. These findings are considered compelling evidence that the absorption rate of nitrogen is governed by its dissociation rate. Ono et al. [26] reported that in Fe–O systems with an oxygen content exceeding 150 ppm, the nitrogen absorption rate is also controlled by its dissociation rate. This behavior was similarly observed in Fe–S–O systems by Han et al. [27]. This phenomenon originates from the surface active nature of oxygen and sulfur, which can be explained by the site-blocking model [22–24,26–27]. The model assumes that surface chemical reactions can only proceed at accessible active sites that are not occupied by other species. The adsorption of atoms, molecules, or ions on these active sites blocks them, preventing reactant molecules from approaching and reacting, thereby leading to a reduction in the overall reaction rate.

The fundamental kinetic parameters for nitrogen reactions remain undetermined when CO2 is employed as a bottom-blowing gas in converters. Thus, in