Mass transfer coefficients during steel decarburization in a RH degasser

The RH process is a secondary refining process that can simultaneously attain significant levels of removal of interstitial elements, such as carbon, nitrogen and hydrogen, from liquid steel. In the RH process, the decarburization rate plays a very important role in determining the productivity of the equipment. The kinetics of this reaction is controlled by mass transfer in the liquid phase. In the present work, a physical model of a RE degasser has been built and used in the study of the kinetics of decarburization. The effects of the gas flow rate and of the configurations of the nozzles used in the injection of the gas have been analyzed. The decarburization reaction of liquid steel was simulated using a reaction involving CO2 and caustic solutions. The concentration Of CO2 in the solution was evaluated using pH measurements. Based on the experimental results, it was possible to estimate the reaction rate constant. A volumetric mass transfer coefficient was then calculated based on these rate constants and on the circulation rate of the liquid. The logarithm of the mass transfer coefficient showed a linear relationship with the logarithm of the gas flow rate. The slope of the line was found to vary according to the relevance of the reaction at the free surface in the vacuum chamber. A linear relationship between the volumetric mass transfer coefficient and the nozzle Reynolds number was also observed. The slopes of the lines changed according to the relative importance of the two reaction sites, gas-liquid interface in the upleg snorkel and in the vacuum. At higher Reynolds number, the reaction in the vacuum chamber tends to be more significant.