Foaming and Structure of Plasma‐Heated Tundish Fluxes: Role of Alkali Metal Carbonate Additions

ABSTRACT Optimizing the foaming characteristics of tundish fluxes is crucial for effectively enhancing plasma‐heating efficiency, necessitating a systematic investigation into the evolution of these fluxes under plasma‐heating conditions. In this study, the foaming process of slags was observed, and the influence of plasma heating on slag structure was analyzed using DSC, high‐speed camera imaging, XRD, Raman spectroscopy, and 27 Al MAS‐NMR. The results indicate that, during plasma heating, the gas source for slag foaming primarily originates from carrier gas entrainment and CO 2 released from carbonate pyrolysis. Under a fixed addition level of 5 wt%, the foaming duration of the K 2 CO 3 , Na 2 CO 3 , Rb 2 CO 3 , Cs 2 CO 3 , and Li 2 CO 3 systems decreased sequentially from 136 to 111 s, among which the N‐K system exhibited the most stable foaming behavior. The N‐K system was characterized by the highest BO/Si ratio (3.804) and an appropriate AlO 4 fraction (66%), forming a moderately polymerized bridging‐oxygen network. This structure reduced surface tension while maintaining suitable viscosity, thereby lowering the energy required for foam formation and extending foam lifetime. In contrast, Li + led to an excessively dense network, whereas Cs + resulted in an overly loose structure, both of which were unfavorable for foam stability. Further analysis revealed that the ultra‐high‐temperature and electric field environment under plasma heating promoted structural evolution of all slag systems toward more defined types, with silicate and aluminate units transforming into species and AlO 4 units, respectively. These findings suggest that high temperature provides the energetic basis for structural reconstruction, while the electric field and the size effect of alkali metal ions synergistically govern the pathway and extent of structural evolution. Collectively, these results provide a theoretical foundation for compositional optimization of tundish fluxes under high‐efficiency plasma‐heating conditions and offer data support for subsequent industrial applications.