Mathematical modeling of selected ladle operations

Sammanfattning: The traditional approach to modeling ladle processes involvea critical survey of the thermodynamic aspects of the processincluding mass and heat balantes at the macroscopic level. Onthe other hand, the macrokinetics, specific to the reactor haveoften been studied separately, using computational fluiddynamits. Over the years, it has been realized that traditionalmethods have obvious limitations in providing an understandingof heterogeneous processes. In a long range perspective, acomplete description of ladle metallurgy and a fundamentalunderstarrding of the underlying process phenomena require aconsideration of all the individual reactions involved andtheir interrelations. This, in tum, clearly requiressimultaneous solution of thermodynamics, heat and fluid flow.The salient features of the present study are the considerationof fundamental transport equations and establishedthermodynamic relationships so that mathematital models takinginto account both thermodynamics and fluid flow could beachieved.A technique to' incorporate the slag phase in two-phasefluid-flow models for ladle treatment is introduced in thisthesis. This development enabled, for the first time,predictions of fluid flow conditions in the important steelklagregion. Good agreement is shown between predictions andphysical modeling reported in literature, regarding thecriteria for initiation of entrapment of a lighter phase into aheavier phase.Furthermore, a method to combine fluid dynamits andthermodynamics to model slag/metal reactions is introduced andis demonstrated in the case of sulphur refining during gasstirring. By considering the volume of mixing between slag andmetal and the thermodynamic equilibrium in the two-phase zone,the calculations of interfacial area and the mass-transfercoeffcients for different elements are avoided. In this way,the combined effect of fluid-flow and thermodynamic conditionscould be predicted from fundamental transport equations.Comparison between predicted data and plant data demonstratesthat the method could be used to predict the desulphurizationwith an acceptable accuracy.A new method to model ladle processes involving gas-liquidreactions is also introduced in this thesis. In the cases wherethe interfacial reaction and mass transfer in the gas-phase arerapid, the overall mass transfer resistance could be expressedby the mass-transfer in the melt. The hydrogen refinementprocess during vacuum degassing in an ASEA-SKE ladle furnacehas been taken as an example. The predictions have beencompared with data from production scale experiments carriedout at Ovako Steel. Good agreement between the calculatedhydrogen toncentrations and the hydrogen tontent in the sampletaken after vacuum degassing substantiates the feasibility ofthis technique.Key words:Slag-Metal Reactions, Metal-Gas Reactions,Ladle Metallurgy, Sulphur Refining, Hydrogen Refining, VacuumDegassing, Combined Kinetics and Thermodynamics, 3-Phase Model,Gas Stirring, Modeling, CAS-OB, CFD