Study on the foaming of CaO-SiO2-FeO slags: Part II. Dimensional analysis and foaming in iron and steelmaking processes

An empirical equation for the foaming index Σ of a CaO-SiO₂-FeO slag was obtained by dimensional analysis. The effect of second-phase particles on slag foaming was well described by calculating the viscosity of the mixture using the modified Einstein equation. The anticipated foaming in basic oxygen furnace (BOF), electric arc furnace (EAF), and bath-smelting processes was estimated using the parameter Σ for various operating conditions and slag compositions. For BOF operations, it is predicted that foaming is most extreme in the middle of the blow, and a stable foam in EAF is achieved with less basic slags with low FeO contents. For bath smelting, foam heights of 5 m are possible, and a higher degree of prereduction prior to smelting will reduce foaming (because of smaller gas evolution) and possibly increase production rates. Running the process at a higher pressure will also reduce foam heights, because the volume of gas generated is less. Kimihisa Ito, Research Associate, formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

Formation and effects of slag foaming in smelting reduction

Research was carried out on the foaming phenomenon in the bath of the smelting reduction process with a hydrodynamic model and with high-temperature experiments. There are two mechanisms of slag foaming: when foam is generated by injected gas, the height of the slag foaming can be regulated. Contrarily, the production of gas during the reduction of iron oxides causes slag foaming which can hardly be controlled. High temperature as well as low slag basicity reduce the foam stability. In smelting reduction a large slag volume is expected. A rinse-spot in the slag layer, which can prevent slag foaming, disappears under these conditions. Wall-effects play an important role for the foam stability.     

Role of prereduced pellets in the slag foaming in modern EAFs

The problems of electric arc furnace slags are considered, and the role of prereduced pellets in the slag foaming in electric arc furnaces is studied. The optimum rate of loading of prereduced pellets into a furnace that ensures effective steelmaking slag foaming is determined as a function of the degree of pellet prereduction.     

Slag foaming in smelting reduction processes

In order to understand the effect of slag composition on foaming in the smelting reduction process, slag foaming was quantitatively studied for CaO–SiO₂–FeO slags in the temperature ranging 1250–1400 °C. It was found that slag foaming could be characterized by a foaming index Σ which is equal to the retention or travelling time of the gas in the slag and by the foam life. The effects of P₂O₅, S, MgO and CaF₂ on foaming were studied. As expected slag foaming increased with increasing viscosity and decreasing surface tension. The results were extrapolated to bath smelting process to predict the foam height. Slag foaming heights as high as 3–5 meters are predicted for a typical operation.     

Process dynamics of electric arc furnace during direct reduced iron melting

A phenomenological mathematical model to simulate steelmaking operations in an electric arc furnace (EAF) has been developed. This model has been validated with industrial data from a Mexican company that employs direct reduced iron (DRI) as the main iron unit. The slag and steel composition can be predicted in real time with the aid of several components integrated into the simulator. The model starts with the optimization of mass and energy balances that yields the requirement of materials to produce a given quality of steel with the minimum cost. The rate of reduction of iron oxide is computed based on two reactions occurring between the FeO and carbon dissolved in the metal and FeO with the carbon particles injected into the slag. A mechanism of consumption of carbon particles has been completed, which includes the effect of surfactant species in the slag and operational variables of the industrial practice of carbon injection. A new concept, called dynamic foaming index, (DFI) is proposed to quantify the actual foaming behavior of industrial slags throughout a commercial heat. The results prove the versatility and robustness of the present mathematical model and provide a knowledge base to assist in the design, operation, and optimization of metallurgical practices for EAF steelmaking.     

Picture analysing method of slag foaming behaviour

Hot tests of foaming behaviour of steelmaking slags were conducted on a laboratory scale up to 1760 °C using a Tammann furnace. The foaming behaviour of the slags was quantified on the basis of a new measuring method. The volume increase and the progress of the foaming process can be continuously observed and calculated by means of picture analysis. The gas content of foaming slags was compared with the results of the measurements performed in steel plants. The influence of the magnesia content on the foaming behaviour is investigated. The chemical composition of the slag is beside the CO evolution the decisive factor influencing slag foaming behaviour. The highest volume increase values observed lie in the region of 2500 % in relation to the initial volume.     

Study on the foaming of CaO-SiO2-FeO slags: Part I. Foaming parameters and experimental results

In order to understand the effect of slag composition on foaming in iron and steelmaking processes, slag foaming was quantitatively studied for CaO-SiO₂-FeO slags in the temperature range of 1250 °C to 1400 °C. It was found that slag foaming could be characterized by a foaming index (Σ), which is equal to the retention or traveling time of the gas in the slag, and the average foam life ( τ). The effects of P₂O₅, S, MgO, and CaF₂ on foaming were studied. As expected, slag foaming increased with increasing viscosity and decreasing surface tension. It was found that suspended second-phase solid particles such as CaO, 2CaO SiO₂, and MgO stabilized the foam and had a larger effect on foaming than changes in viscosity and surface tension for the slags studied. Kimihisa Ito, Research Associate, formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

The structure of foaming BOF-converter slag

The structure of foaming synthetic BOF-converter slags was studied by freezing the foam and using ocular examination. The foams were generated by CO gas formed due to the reaction between FeO in the slag and carbon in the hot metal. The character of the foams varied a lot with slag composition. Slag with lower viscosity resulted in foams with small gas bubbles, while slag having high viscosity resulted in very big bubbles.     

Foaming and the Rate of the Carbon-Iron Oxide Reaction in Slag

Foaming in the electric arc furnace is achieved by injecting carbon into slag, where the resulting reaction of the carbon with FeO dissolved in the slag generates gas (CO) that causes the slag to foam. In this research, the rate of the reaction of FeO in slag with carbon and the resulting foam height were measured. In these experiments, the FeO content of the slag ranged from 15 to 45 mass pct, and several different types of carbon were used including graphite, coals, and chars. The rate of the slag-carbon reaction and the consequent CO generation increased with FeO content of the slag from 15 to 45 mass pct. However, the slag foam height reached a maximum at about 25 mass pct FeO and decreased at higher FeO contents. The decrease in foaming is apparently due to a decrease in the foam index or foamability caused by a decrease in viscosity and an increase in density of the slag with FeO content. The results of this work indicate that the foam height is influenced more significantly by the decrease in the foam index compared to the increase in the CO gas generation rate at higher FeO contents. The decrease in the foam index with FeO agrees with that predicted from the slag properties.     

Theoretical analysis of the interfacial phenomena during the injection of carbon particles into EAF slags

A theoretical analysis of FeO reduction through the injection of carbon fines in electric arc furnace slags, involving the interfacial phenomena at the liquid‐gas‐solid interface, has been performed using basic principles of transport phenomena and physical chemistry of steelmaking. It was found that small angle contacts between slag and carbon favour FeO reduction. Moreover, FeO in basic slags are more prone to be reduced because the interfacial liquid‐gas interface has more free reaction places. In acid slags FeO reduction is difficult because the gas‐liquid interface is partially filled by polymeric silicates. When the particle size is smaller than 100 μm the influence of slag basicity is considerably decreased. Practical applications of these results can be found in electric arc furnace shops aiming at the mastering of slag foaming practices and energy saving.     

酸性含钛炉渣泡沫化问题的研究

为解决攀钢熔分深还原电炉冶炼钒钛磁铁矿金属化球团过程中出现的泡沫化严重的问题,采取提高炉渣二元碱度、控制金属化球团w(FeO)和残碳量、减少低温电炉加料量等措施,使炉内泡沫化严重的现象得到控制,保证了冶炼过程的连续进行。同时钒还原率提高了13%,冶炼时间缩短了45min。     

Slag foaming in bath smelting

Slag foaming measurements in terms of the foaming index (∑) were conducted on bath smelting-type slags (CaO-SiO₂-FeO, CaO-SiO₂-MgO-Al₂O₃-FeO) at 1773 K. It was found that the slag foam stability decreases with increasing FeO (FeO > 2 pct) content and basicity. For the slag system (CaO-SiO₂-FeO), no stable foam was observed at very low FeO content (<2 pct). As pct FeO increases, the slag foaming index goes through a maximum and then decreases; a similar phenomenon was observed for CaO-SiO₂-NiO slags with respect to the NiO content. The foaming index determined from the normal small-scale experiments (3.8-cm ID diameter) were confirmed on a larger scale (9.2-cm ID diameter), indicating that the foaming index is independent of container size. Measurements were also made for the actual compositions for bath smelting slags. For these slags, the foaming index is higher than those of simple CaO-SiO₂-FeO slags, because MgO and Al₂O₃ may increase their viscosities. The foam index is believed to be a function of the physical properties of the slag. Consequently, a dimensional analysis was performed, and a correlation was developed relating the foaming index to the viscosity, surface tension, and density of the slag. An estimation of slag foaming in actual pilot plant trials was also made from the results of the present study. Good agreement was observed between the predicted and observed foam heights and indicated coke in the slag can reduce the foam height by more than 50 pct. R. Jiang, Formerly Graduate Student, Carnegie Mellon University, is deceased.     

The fluctuations in slag foam under dynamic conditions

In the present article, slag foaming phenomenon under dynamic conditions is critically analyzed on the basis of the results of high-temperature X-ray image analysis experiments. The results indicate that the mismatch between the gas generation rate and gas escape rate has a serious impact on the foam height. This mismatch is attributed to the chemical reaction rate, which has to be considered in modeling slag foaming under dynamic conditions. The results further imply that a critical ratio of bubble size/crucible size exists, where wall effects are likely to become prominent.     

Effect of carbonaceous particles on slag foaming

Use of carbonaceous particles such as coke or coal char in controlling slag foaming is of great practical significance for bath-smelting and other steelmaking processes. The foamability of the liquid slag in terms of the foam index has been determined with the presence of different amounts of coke and coal char particles. Different sized and shaped particles were used in the experiments. It was found that the foam index decreased significantly as the ratio of the total cross-sectional area of the particles to the liquid slag surface area increased. When the foam was generated by argon gas injection through an alumina nozzle (i.d. = 1.5 mm), a liquid slag, CaO-SiO₂-CaF₂-(Al₂O₃), depending on the alumina content, could have an initial foam index of about 2 to 4 seconds at 1500 °C without any carbonaceous particles. When the slag surface was covered only 15 ~20 pct with either coke or coal char particles, the foam was totally suppressed regardless of the initial foam index. In order to understand the mechanism of the antifoam effect of the carbonaceous particles, interactions of a coke sphere, an iron ore pellet, an alumina tube, and a coal char particle with the liquid slag foam were examined by X-ray observation. It was concluded that the antifoam effect of coke or coal char particles is primarily contributed by the nonwetting nature of the carbonaceous materials with the liquid slag. Possible mechanisms of carbonaceous particles rupturing a slag film could be (1) the rapid thinning of the liquid slag film driven by a difference between the instantaneous contact angle and the equilibrium contact angle or (2) the “dewetting” of the liquid slag from the interface when the film is “bridged” by the particle. Formerly Graduate Student and Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University.     

Smelting reduction reactions by solid carbon using induction furnace: foaming behaviour and kinetics of FeO reduction in CaO–SiO2–FeO slag

Abstract

Smelting reduction process technology is progressing rapidly, and research to understand the reduction of FeO in molten slag and the associated foaming behaviour has gained importance. The present paper reports experimental data on the reduction of FeO in molten slag generated in a 30 kW capacity induction furnace. The influence of FeO content in the slag and temperature on the foaming and kinetics is discussed. The foaming index, a parameter describing the travel time of gas in the reactor, is shown to decrease with an increase in the superficial gas velocity. The quantitative dependence of the foaming index on slag properties viscosity, surface tension and density has been studied. The data have also been analysed to give an estimation of the activation energy for the reduction reaction. The reduction reaction, initiated by direct slag–graphite contact, produces CO gas, which spreads into the molten slag bath causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction is found to depend directly on the initial FeO content. An increase in temperature increases the rate of reduction, which has an activation energy of 118 kJ mol^?1 of FeO. The results indicate that transport of FeO in the liquid phase is the rate controlling step. The major findings are in agreement with those reported by earlier investigators.     

Bubble Formation through Reaction at Liquid‐Liquid Interfaces

Slag foaming is an important phenomenon in steelmaking processes with both beneficial as well as negative effects. The present work is part of the wider project on the modelling of slag foaming, with special reference to dynamic conditions. Since bubble formation is the first step to foam formation, the present work was carried out in an attempt to simulate the bubble formation in slag/metal reactions in steelmaking processes by water‐modelling experiments. The bubble formation due to the gas produced through chemical reaction at the interface between oleic acid and sodium bicarbonate solution was systematically monitored. The chemical reaction rate was varied by varying the concentration of sodium bicarbonate. The bubbles were observed to be generated in the heavier aqueous phase just below the water‐oil interface. The bubbles penetrated the interface and escaped through the oil phase. The rate of the reaction was estimated from the volume of the gas that passed the water/oil interface. It was observed that the bubble formation and bubble growth mechanism were influenced by the reaction rate while the bubble size seemed to be unaffected by the reaction rate.     

Modelling of slag foaming

Abstract

In this work, silicone oils of different viscosities were used to simulate slag foaming. The experimental results showed that the variations of foaming height with superficial gas velocity and liquid viscosity do not show simple increasing trends. At a constant viscosity, foaming height increased first and then started decreasing with increasing superficial velocity. Similarly, a maximum foaming height was observed at an optimum viscosity when a constant gas flowrate was applied. The foaming height started decreasing with further increasing viscosity. Based on the experimental data, a semiempirical equation of foaming height was developed. The predictions of the model agreed well with experimental data. The model could also reasonably well explain the industrial pilot trial experiments. The experimental results with paraffin particle additions indicated that a small amount of particle addition moved the onset of foaming to lower superficial velocity. However, the increase in viscosity due to the presence of solid particles was found not the main reason for the increase in foaming height.     

Applications of Computational Fluid Dynamics (CFD) in iron- and steelmaking: Part 2

Abstract

After presenting a review of some applications of computational fluid mechanics (CFD) to ironmaking processes in Part 1, the authors now explore the use and extent of CFD in steelmaking and steel casting processes. Steelmaking processes generally include the basic oxygen furnace, electric arc furnace or equivalent, the ladle and continuous casting and incorporating a tundish and moulds. All these steelmaking processing steps involve highly coupled complex transport phenomena. The use of CFD to model such processes has been an active area of research for the last three decades. Many models have been developed to predict mixing behaviour, slag foaming, gas–liquid interactions, multiphase flows, as well as heat and mass transfer aspects. In the present review, the role of CFD in modelling steelmaking operations is reviewed, discussed and critiqued.     

电炉高效埋弧泡沫渣技术开发

电弧炉在各个不同的熔炼阶段，分别加入或喷入不同比全铁复合发泡剂后，使炼钢的全程实现了泡沫渣埋弧操作，不仅提高了热效率，而且还取得了较好的冶金效果。     

Investigation into rheological characteristic and foaming behaviour of molten slags

The rheological characteristic and foaming behaviour of molten slags have been investigated at different temperatures and with different additives, such as coke, coal and CaO. In the experiment of rheological characteristic, a modified rotational viscometer with variable revolution's speed was applied. Therefore, constitutive equations and rheological parameters of molten slags have been established. The tolerance-analysis method is applied to judge whether a molten slag belongs to Newtonian fluid or not. The results of foaming behaviour in the blowing gas show that addition of coarse grain coke and coal decreases the foam height. However, the carbonaceous materials of fine grain and CaO powder can cause the increase in foam height. It has also been found from the experimental results that the foam index ∑ is only available for slag of Newtonian fluid in blowing gas. The reason may be well explained with the rheological characteristic of molten slags.