Modeling Energy Dissipation in Slag-Covered Steel Baths in Steelmaking Ladles

Physical and mathematical modeling of energy dissipation phenomena in a gas-stirred ladle with, and without, an overlying second-phase liquid have been carried out at relatively low gas flow rate and specific energy input rate. Data from the literature are applied to infer the extent of energy dissipation caused by various mechanisms. An analysis reveals that bubble slippage and friction at the vessel walls dominate energy dissipation in such systems, each contributing roughly one third of the input energy. The remainder is dissipated because of turbulence in the bulk of the liquid, the formation of a spout, and interactions between the upper phase and the bulk liquid when an overlying liquid is present. Remarkably, the overlying liquid despite its small volume (similar to 3 pct to 13 pct of the bulk), is found to dissipate about 10 pct of input energy. To understand the way the total input energy is dissipated via the overlying liquid, flow and mixing studies were carried out with different types of upper phase liquids. Tracer dispersion studies conducted with Petroleum ether as the overlying liquid show reasonably intense flow within the upper phase with no noticeable entrainment around the spout. In contrast, a thick layer of highly viscous upper phase liquid such as mustard oil shows extensive deformation of the upper phase around the spout, but no discernable motion within. However, remarkably, the thickness of the upper phase rather than its physical properties was found to influence bath hydrodynamics and mixing most significantly. A mechanism based on the rerouting of the surfacing plume and the attendant reversal of flow in the vicinity of the spout is advocated to explain energy dissipation caused by the overlying liquid. This finding is rationalized with our experimental results on composition adjustment with sealed argon bubbling (CAS) alloy addition procedures reported more than two decades ago, wherein flow reversal caused by the baffle in the immediate vicinity of the surfacing plume was shown to cause significant energy dissipation, leading to much sluggish flow and slower mixing in the bulk of the liquid, in comparison with an equivalent unbaffled situation.