Steel composition can depend on mass transfer with slag. It is typically the case during the secondary metallurgy step for the desulfurization of steel. In a continuous casting mold, the lubrication efficiency is strongly affected by the slag viscosity, which is dynamically modified before reaching the chemical equilibrium with steel. To correctly predict the time-evolution of the steel or slag composition, the mass transfer coefficient should be correctly assessed.

The aim of this work is to detect and explain the occurrence of different mass transfer regimes in an industrial ladle when the argon flow rate used for stirring is increased. Those regimes are already observed in different water models, see Kim and Fruehan (1987) or Joubert et al. (2022). OpenFOAM software is used for the flow modelling and the compressible multiphase VoF solver is selected, with static refinement in the interface region (steel / slag and steel / bubbles). Turbulence is described by a hybrid RANS-LES approach, as developed by Shur et al. (2008).

A set of 12 argon flow rates from 106.5 to 1200 STL is chosen. We check that the prediction of the open eye geometry is in correct agreement with published visualizations in an industrial configuration. For each of the gas flow rates, it is possible to get the local mass transfer coefficient k (m/s) and the local surface area A (m2). In this work, k is obtained by a correlation based on the local shear stress, Banerjee (2004). Specific algorithms are developed to get the gradient of tangential velocity at the steel slag interface and the local surface area.

When they are averaged over the steel/slag interface, opposite behaviors are observed for k and A. As expected, A continuously increases with argon flow rate. But a significant increase of A is detected above a critical flow rate corresponding to the creation of a large amount of slag droplets. Same phenomenon is observed in water model simulations, Joubert (2022). For k, non-intuitive results are predicted since k, globally, decreases with the gas flow rate. Even if, locally, important shear stresses are produced with high gas glow rate, the progressive intumescence development when the gas bubbles cross the top interface diverts the flow into the core region of the ladle. Then, dead regions are created near the slag / steel interface outside the open eye, accompanied by low values of the shear stresses. When averaged over all the interface, the contribution of these dead zones is predominant and drives the k value. Regarding the surface mass transfer kA, which is ultimately the main coefficient affecting the chemical kinetics, it is predicted to have the same behavior as A.

Prediction with OpenFOAM can reproduce complex phenomena in a ladle. Similar mass transfer regimes are observed in water models and industrial configurations. Significant differences are detected for k and A when gas flow rate increases.