Study on some phenomena of slag in steelmaking process

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: The present work is to study slag phenomena in steelmaking process. In order to attain thegoal, a number of high temperature experiments and simulation experiments were carried out.Four master slags were used to study the effect of CaF2 on slag viscosity. Experimentalresults indicated the effect of CaF2 on slag viscosity depended strongly on the composition ofmaster slag. For high basicity slags, CaF2 mainly suppressed the precipitation of solid phasesat lower temperatures, leading to a lower viscosity compared to CaF2-free slags. For slagswith higher SiO2 contents, CaF2 both lowered the viscosity of the liquid phase and suppressedthe precipitation of solid phases.The viscosities of solid-liquid mixtures were experimentally determined for silicon oilparaffinsystem at room temperature and solid-liquid oxide mixture at steelmakingtemperature. The results of both measurements indicated that the increasing trend of mixturesviscosity with particle fraction can be described by Einstein-Roscoe equation.Silicone oils of different viscosities were used to simulate slag foaming. The experimentalresults showed, at a constant viscosity, the foaming height increased first with superficialvelocity before reaching a maximum value. Thereafter, the foaming height decreased with thefurther increase of the superficial velocity. Similar, a maximum foaming height was observedat an optimum viscosity when a constant superficial gas velocity was applied. Based on theexperimental data, a semi-empirical equation of foaming height was developed. Thepredictions of the model agreed well with experiment data. The model could also reasonablywell explain the industrial pilot trial experiments.Water-silicon oils model and liquid alloy (Ga-In-Sn)- 12% HCl acid model were employed tosimulate the formation of open-eyes in a gas stirred ladle. The experimental results indicatedthat the viscosity of the top liquid and the interfacial tension between the two liquids had onlylittle effect on the open-eye size. A semi-empirical model was developed to describe the sizeof open-eye as functions of the gas flow rate, bath height and slag height. The two sets ofparameters obtained for the water and Ga-In-Sn models were very different. Industrial trialswere also conducted to examine the applicability of the models. Liquid alloy model couldwell predict the formation of an open-eye during ladle treatment.Two cold models, sodium tungstate-oil model and water-oil model, were carried out tosimulate the formation of droplets in a gas stirred ladle. The experimental results showed thatthe gas flow rate and interfacial tension had strong impact on the size of droplets. A semiempiricalmodel was developed to describe the size of droplets for water model. Meanwhile,the parameter obtained for water model can be used for sodium tungstate-silicone oil system.The results indicate the model has strong compatibility.