Relationship between viscosity, foaming, and structure of tundish fluxes for plasma heating with addition of alkali metal carbonates

Optimizing the foaming characteristics of tundish coating agent is a challenging task, which affects the heating efficiency of plasma and increases the smelting cost. In this work, the foaming behavior of CaO-Al₂O₃-SiO₂₋MgO-Fe₂O₃ slag systems modified with Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, and Cs₂CO₃ was systematically investigated using the electrical current method. Additionally, the continuous cooling viscosity of the slags was monitored to evaluate their temperature-dependent rheological properties. The structural characteristics of the slags were analyzed using Raman spectroscopy and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR). The results demonstrate that the addition of Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, and Cs₂CO₃ significantly enhances slag foaming performance. The stable bubble volume fraction increases from 17.93 % to 85.92 %, while viscosity rises from 0.1174 Pa·s to 0.2054 Pa·s. The BO/Si ratio increases from 0.739 to 2.215, indicating enhanced polymerization of the [SiO₄]^4- tetrahedral network and a higher degree of polymerization (DOP). Concurrently, the area fraction of AlO₄ decreases from 71 % to 34 %, while that of AlO₅ increases from 27 % to 64 %. This shift reflects greater polymerization of the aluminosilicate network, improving thermal stability and effectively enhancing slag foaming performance. Among the studied systems, the K₂CO₃-modified slag exhibits optimal industrial applicability, achieving a maximum foaming efficiency of 30.33 %/min at 1600 °C, an average bubble volume fraction of 80.87 % during defoaming, and a viscosity of 0.1765 Pa·s. Combined with its favorable raw material costs, this system is well-suited for industrial applications. The findings of this study may provide a viable pathway to improve the efficiency of plasma heating technologies.