Numerical investigation of desulfurization behavior in electroslag remelting process

When a chemical reaction occurs between two immiscible liquids, mass transfer is continuously taking place at the liquid–liquid interface. Several studies have shown that if the species being exchanged between the two liquids are surface-active, a very pronounced decrease in interfacial tension can occur which can lead to a phenomenon called spontaneous emulsification. In steelmaking, this behavior has been observed for several reactions that involve the transfer of impurities from molten steel to a molten-oxide slag but little quantification has been made. This work focuses on spontaneous emulsification due to the dephosphorization of a Fe-P drop immersed in a basic oxygen furnace type slag. An Au-image furnace attached to a confocal scanning laser microscope was used to rapidly heat and cool the samples at different times, and X-ray computerized tomography was used to perform the surface area calculations of the samples where the slag/steel reaction was allowed to occur for distinct times. The results show that the surface area of the metal drop rapidly increases by over one order of magnitude during the first 60 seconds of the reaction while the chemical reaction is occurring at a fast rate. Once the reaction slows down, approximately after 60 seconds, the droplets start to coalesce back together minimizing the surface area and returning to a geometry close to its equilibrium shape.

Spontaneous Emulsification of a Metal Drop Immersed in Slag Due to Dephosphorization: Surface Area Quantification

A multi-zone kinetic model coupled with a dynamic slag generation model was developed for the simulation of hot metal and slag composition during the basic oxygen furnace (BOF) operation. The three reaction zones (i) jet impact zone, (ii) slag–bulk metal zone, (iii) slag–metal–gas emulsion zone were considered for the calculation of overall refining kinetics. In the rate equations, the transient rate parameters were mathematically described as a function of process variables. A micro and macroscopic rate calculation methodology (micro-kinetics and macro-kinetics) were developed to estimate the total refining contributed by the recirculating metal droplets through the slag–metal emulsion zone. The micro-kinetics involves developing the rate equation for individual droplets in the emulsion. The mathematical models for the size distribution of initial droplets, kinetics of simultaneous refining of elements, the residence time in the emulsion, and dynamic interfacial area change were established in the micro-kinetic model. In the macro-kinetics calculation, a droplet generation model was employed and the total amount of refining by emulsion was calculated by summing the refining from the entire population of returning droplets. A dynamic FetO generation model based on oxygen mass balance was developed and coupled with the multi-zone kinetic model. The effect of post-combustion on the evolution of slag and metal composition was investigated. The model was applied to a 200-ton top blowing converter and the simulated value of metal and slag was found to be in good agreement with the measured data. The post-combustion ratio was found to be an important factor in controlling FetO content in the slag and the kinetics of Mn and P in a BOF process.

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Dynamic Model of Basic Oxygen Steelmaking Process Based on Multi-zone Reaction Kinetics: Model Derivation and Validation

Slag foaming in an electric arc furnace (EAF) is examined using a mathematical model of the EAF to compute the CO generation rate and slag chemistry and laboratory measurements of the foam index. The foam height limited by either the foam stability or from the amount of slag present was computed for a high productivity furnace using relatively large amounts of pig iron, carbon and oxygen. It was found that for this case the foam is usually simply limited by the amount of slag in the furnace. The effects of the amounts of pig iron, carbon, oxygen and slag were determined. As the amount of carbon as pig iron, coke and coal and oxygen are reduced, the foam height is limited by the CO generation rate and is reduced. The foam decreases towards the end of the process due to lower CO generation rates.

Slag Foaming in an Electric Arc Furnace

Slag foaming practice in the steelmaking process

In the current study, the rates of dephosphorization and rephosphorization of liquid iron with simulated steelmaking slags were investigated at 1873 K (1600° C). The experiments were conducted in an induction furnace with supplemental heating to maintain a consistent temperature within both the metal and slag phases. An integrated form of the rate equation was used to evaluate the results, assuming mass transfer in both the slag and metal was rate controlling. The results of the current and previous studies indicate that the mass transfer parameter, the slag-metal surface area, and the overall mass transfer coefficient (A*k0), decreased as the reaction proceeded. It is proposed that initially when the rate and oxygen flux are high, the interfacial energy decreases, and the interfacial fluid velocity increases causing disruption of the slag metal interface. The consequent increases in interfacial area and interfacial fluid flow cause A*k0 to be high initially and then decrease as the oxygen flux decreases.

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag

A decarburization and slag formation model for the electric arc furnace was developed, which includes the rate phenomena for decarburization and the reaction between carbonaceous materials and iron oxide in slag, mass balance for each species in the metal, slag and gas phases, and the melting behavior of pig iron, scrap and fluxes. The model was applied to a two bucket charge operation electric arc furnace and the dynamic metal and slag compositions were calculated as a function of time. The effects of melting patterns for raw materials, carbon–FeO reaction rate, and the post combustion ratio were examined. The critical parameters, which most strongly influence chemistry development, were identified. These parameters were fitted to an industrial case, such that the model could accurately predict slag and metal chemistry development. This model could be utilized to optimize the operation in order to improve yield, energy efficiency, and increase consistency of metal and slag chemistries from heat to heat.

Christopher P. Manning

Papers

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag

Development of a Decarburization and Slag Formation Model for the Electric Arc Furnace