Influence of Al ₂ O ₃ on Element Migration and Crystallization Fractionation in Mold Slag Film by Soret Effect Driven

ABSTRACT The Soret effect, which drives element migration and leads to crystallization fractionation in minerals. During the continuous casting of steel, there is a temperature difference of approximately 700°C between the mold wall and the solidifying shell accompanying Soret effect, but its influence on slag film crystallization remains unclear. In this study, the Double Hot Thermocouple Technique (DHTT) was used to simulate the thermal environment in the range of 700–1400°C to observe the crystallization behavior of slag films containing 0 and 3 wt% Al 2 O 3 under the Soret effect. Further, crystalline phases and elemental distribution were characterized by Energy Dispersive x‐ray Spectroscopy (SEM‐EDS) and Electron Probe x‐ray Micro‐analyzer (EPMA), while slag structure was analyzed using Raman spectroscopy and Magic‐Angle Spinning Nuclear Magnetic Resonance (MAS‐NMR). The results show that adding 3 wt% Al 2 O 3 changed the crystallization direction and fractionation of the slag film. Without Al 2 O 3 , crystals precipitated at the solid–liquid interface and grew toward both low‐ and high‐temperature regions, forming the cuspidine in the low‐temperature region and wollastonite in the high‐temperature region. In contrast, with 3 wt% Al 2 O 3 , nepheline precipitated first in the low‐temperature region, followed by the formation of melilite, cuspidine, and wollastonite as the temperature increased. This difference occurs because Al 2 O 3 modified elemental migration and slag structure under the Soret effect, causing CaO to migrate toward the high‐temperature region, while SiO 2 , Na 2 O, and MgO moved toward the low‐temperature region. Mechanistically, Al 2 O 3 existed as a [AlO 4 ] 5− network former, generating (1Al) structures that coordinate with Na + to form Na 2 O·Al 2 O 3 ·2SiO 2 and Na 2 O·CaO·Al 2 O 3 ·4SiO 2 , while residual and bond with Ca 2+ to yield 3CaO·2SiO 2 ·CaF 2 and CaO·SiO 2 . These findings clarify the Soret‐driven mechanism of element migration and crystallization fractionation regulated by Al 2 O 3 , providing new insights for the optimization of mold flux.