• Isotope exchange technique enables direct measurement of the interfacial nitrogen reaction rate constant in molten iron under CO2–CO–Ar atmospheres, eliminating interference from liquid and gas phase mass transfer.
• The interfacial reaction rate constant kc exhibits a strong dependence on the activities of surface-active elements O, C, and S, following a site-blocking type expression: kc = 4.35×10−6/(1+14.25aO +0.25aC +6.50aS)².
• CO2 injection in steelmaking enhances nitrogen removal by generating CO gas that increases stirring intensity and improves kinetic conditions at the melt–gas interface.
• The derived kinetic relationship provides a quantitative foundation for modeling denitrification in CO2 bottom-blowing converter processes, enabling optimized process design.