Slags mass transfer

The comparison of the magnitude of the two resistances clearly indicates whether tire metal or the slag mass transfer is rate-determining. A value for the ratio of the boundary layer thicknesses can be obtained from the Sherwood number, which is related to the Reynolds number and the Schmidt number, defined by... 

Between the bath and the metal-slag zone, mass transfer of Fe droplets takes place. During a major part of the process, the decarburisation reaction is limited by the oxygen supply. After reaching the critical point, it is limited by the carbon transport to the reaction zone. Therefore, at the critical point, we have... 

As discussed above, Equation (8) is valid for process conditions up to the critical point. The decrease in decarburisation rate from the critical point onwards results in a decrease in CO production and, therefore, less stirring is caused by the gas hubbies. Hence, mass transfer in tbe form of metal droplets in the slag decreases. 

As discussed above, the ideal reaction zones in the cell model are not only coupled by defined mass transfer but also by heat transfer. Energy losses from the converter are calculated at every time step and enthalpy is withdrawn from various reaction zones, mainly from the metal-slag reaction zone. In Figure 9, the evolution of the temperature in the hot spot and of the mean temperature of the hot metal bath... 

Steps 2 and 4 depend on the mass transfer coefficient in the metal, k, and in the slag, ks, respectively. With the assumptions that steps 1, 5 and 3 are must faster than step 2, the final concentration can be calculated. 

Here, we have assumed that resistance is in the melt boundary layer. However, resistance in the slag boundary layer can also be taken into account. Then kt is replaced by a total mass transfer coefficient that is obtained by summing the two resistances. 

Figure 11 The effects of time (a) on interfacial tension and (b) on the mass transfer of A1 from steel (to the slag) [12].

It has been proposed [12,13] that the mass transfer of oxygen from the slag could be responsible when the oxygen stream flux exceeds a value of 0.1 g atom m s. However, other workers have recorded [14] that there was no dramatic decrease in Yms with a flux of 0.2 g atom m s. It has been proposed that the kinetics could be controlled by one of the following steps ... 

Recent work has indicated that the mass transfer of oxides (SiOa, FeO, MnO) in the slag is the rate controlling step [4]. 

It has been reported that there was some lowering of the dynamic interfacial tension Yms when a molten, 321-stainless steel was in contact with a mould flux [4] due to mass transfer between the metal and slag phase and this would promote entrapment. 

Mass and heat transfer during post-combustion in thoroughly mixed metal-slag-gas systems... 

After the reactor was cooled to room temperature, it was opened and the mass of metal was mechanically freed of frozen slag. Ninety percent of the zinc in the alloy was removed by distillation in a retort heated to 1150°C at a vacuum lower than 0.2 Torr. The retort was then filled with argon or helium to prevent oxidation of the spongy thorium and cooled to room temperature. The thorium was transferred to a beryllia crucible in an induction-heated vacuum furnace for melting, evaporation of the residual zinc, and casting into a graphite mold. Thorium metal yield was 94 to 96 percent. 

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