Refining of metal droplet in slag using the concept of Gibbs’ Free Energy Minimization at the slag-metal interface

Abstract

Slag-metal emulsion plays an important role in the oxidation kinetics of different metalloids in oxygen steelmaking. The importance of droplet generation rate, droplet size and its residence time in the slag-metal emulsion on the overall reaction kinetics has become evident in recent times. Residence times of the droplets are strongly dependent on the decarburization rate, the CO bubbles giving a buoyant force to the droplets. The present work aims at developing a mathematical model for predicting the temperature and composition evolutions of the slag and the metal phases as the blow proceeds in an LD convertor. The process dynamics are modelled by dividing the LD convertor into three separate continuously stirred tank reactors (CSTRs). Oxidation reactions are assumed to be primarily taking place at the interface between the slag and the metal phases in the emulsion. Among the different mass-transfer and reaction steps controlling the kinetics, the mass transfer of FeO in the slag phase and that of the metalloids within the metal droplet are only assumed to be rate-controlling. For a Fe-C-X (X=Mn, Si etc.) droplet, simultaneous removal of elements have been modelled by the Fee Energy Minimization at the slag-metal interface. Using this concept, the FeO reaching the interface is partitioned among the metalloids such that the Gibbs Free Energy Change for the entire process is the minimum. Effects of droplet size, mass-transfer co-efficient and initial carbon content on the mean residence time of metal droplets in the slag-metal emulsion have also been identified.