The reduction of zinc from slags by an iron-carbon melt

An experimental investigation was undertaken to study the mechanism of reduction of zinc from slags in the presence of a carbon-saturated iron melt. Batch tests were performed at 1400 °C, and the variation of the zinc and iron concentration in the slag during reduction was determined by sampling the slag at intervals during the test. In graphite crucibles, zinc in slags containing iron was reduced faster than zinc in iron-free slags, both when an iron bath was present and when it was absent. Zinc was reduced faster from slags containing iron when an iron bath was present than when an iron bath was absent. The dominant mechanism of reduction of zinc from slags containing iron appears to be the reaction of Zn²⁺ ions with Fe²⁺ ions to form zinc vapor and Fe³⁺ ions. When an iron bath is present, the Fe³⁺ ions are reduced back to Fe²⁺ predominantly by reaction with iron from the bath. Mass transfer of Fe³⁺ ions in the slag appears to be the rate-controlling step. Reduction of iron from slag by carbon occurred in parallel with the reduction of zinc, and whether there was a net increase or decrease of iron in the slag depended on the relative rates of production and consumption of iron. Lead and copper in the slag were reduced to low levels. The lead volatilized and the copper dissolved in the alloy.     

Kinetics of iron oxide reduction from CaO-MgO-FeOn-SiO2 slags by silicon dissolved in liquid iron

At steelmaking temperatures, the kinetics of slag-metal reactions is usually determined by mass transfer. This occurs in two ways: normal mass transfer which is induced by stirring, and mass transfer by interfacial convection induced by interfacially active elements like oxygen and sulphur. In the present work, mass transfer during the reduction of iron oxide from a basic slag by silicon dissolved in liquid iron was studied under defined conditions of gas stirring by argon in MgO crucibles with 1500 g iron and 250 g slag. The variations of the FeO content in the slag and the silicon content in the iron during the reaction were measured by sampling. Trials were carried out with stirring gas flow rates between 1 and 20.4 l/h(STP). The experimental data were evaluated with the multi-component transport model in order to determine the mass transfer coefficients of the reaction components. Simultaneously, the coefficients of normal mass transfer were calculated with the boundary layer theory of liquid-liquid mass transfer for non-turbulent flow conditions. The measured mass transfer coefficients were by a factor 2.5 larger than the theoretically calculated. The difference indicates the presence of mass transfer by interfacial convection. Mass transfer by interfacial convection is superimposed to normal mass transfer.     

Foaming during reduction of iron oxide in molten slag

Abstract

Coal based direct liquid ironmaking processes, also known as smelting reduction (SR) processes, are now receiving serious consideration in the industry. However, although several processes have reached pilot plant trials, only one, namely Corex, has been commercialised. In all SR processes, most of the iron oxide is reduced in the molten slag phase and foaming is an important phenomenon. Better understanding of the foaming behaviour will result in design of a more efficient and economic reactor. Accordingly, some basic studies on the foaming behaviour of some typical SR process slags have been undertaken in this work. Experimental data on foaming in a 50 kW capacity plasma reactor are discussed in this paper. The data generated are analysed in terms of foaming index, which is equal to the residence time or travelling time of the gas in the foam layer. The influence of FeO percentage in the slag, slag basicity, crucible surface area, initial slag height, and CaF₂addition in the slag on foaming has been studied.     

Kinetics of manganese oxide reduction from basic slags by carbon dissolved in liquid iron

The reduction of manganese oxide from a basic slag by carbon dissolved in liquid iron is a slow reaction, failing to approach equilibrium closely in 20 hr. Furthermore, the rate of stirring has no apparent effect on the reaction rate. This identifies the rate-controlling step as a chemical reaction at the interface. Only the model for the reactionO ²⁻ =O + 2e ⁻ gave a consistent interpretation as the melt geometry, and concentration of manganese oxide and carbon were varied. The rate constant for this reaction was found to be 1.28 × 10⁻⁵ mole per sq cm per min at 155O°C. The effect of temperature is substantial with a calculated energy of activation for the system of 25 kcal per mole. Formerly Graduate Student, The University of Michigan This paper is based on a portion of a thesis submitted by W. L. DAINES in partial fulfillment of the requirements for the degree Doctor of Philosophy at The University of Michigan.     

The kinetics of reduction of iron oxide from molten slag

This investigation primarily consists of measurement of the rate of reduction of FeO from sulfur-free slag. Preliminary measurements of the effect of sulfur on the rate of reduction and the sulfide capacity of the iron-rich slags have also been made. The results show that with increasing SiO₂ contents the rate of reduction is decreased. The influence of sulfur could not definitely be clarified.     

Photoelectric measurement of the foaming process in the reduction of slags containing iron oxide

A measuring method for determining foaming volume is being developed. The volume increase and the progress of the foaming process can be continuously determined and quantified by optical means through evaluation of brightness differences. The influences of various reducing agents on the foaming process are considered. CO evolution resulting from the reaction between liquid iron oxide and carbon constitutes the decisive factor for foaming of this type. In contrast, the temperature-dependent formation of O₂ which occurs during conversion of haematite to magnetite only produces a small increase in volume during the melting process. No foaming is observed during reduction by the gas phase at the surface of the specimen. The foaming behaviour of slags in the basic FeO_n-CaO-SiO₂ system is investigated. Maximum foaming occurs in the moderate basicity range. Similarly, foaming may also be maximized with moderate gangue contents in the oxide melt. Relatively high volume increases are likewise achieved within certain temperature ranges. The rate of gas formation constitutes the main criterion determining the foaming intensity. The highest volume increase values observed lie in the region of 1100% related to the initial volume. The foaming process always exhibits a reliable degree of reproducibility.     

Kinetic experiment of dissolving coke in molten iron

The liquid iron carburization in the blast furnace process can almost come to a saturation state, while, that in flash ironmaking process is unexpected, since there is no solid charge that will press the hot metal and no carburization reaction that occurs between the bottom deadman. In order to study the kinetics of coke dissolution into the molten iron in flash iron- making process, a series of experiments were carried out. Reduced iron powder and chemically pure graphite were used as raw materials, and the tubular resistance furnace was employed to heat the samples up to 1855K and melt them, and the high- purity N2 was injected as the protective gas throughout the experiments. The experimental results show that without regard to the factor kt of dissolution rate, the values of kt under natural convection condition are 4. 20??m/s in group 1, 5. 28??m/s in group 2 and 6. 50??m/s in group 3. On the contrary, when taking this factor into account, kt decreases with time increase and only can be controlled by the mass transfer. Besides, when there is sulfur in the iron- carbon melts, it shows that kt also decreases with the increase of sulfur content.     

Evaluation of the reduction of iron oxide from liquid slags using a graphite rotating disk

The rotating disk methodology has been used for examination of the reduction of FeO from CaO-FeO-SiO₂ liquid slags (20 and 60 pct FeO) with a CaO/SiO₂ ratio equal to 0.66 and 1.27, in the temperature range 1350 °C to 1420 °C. It has been found that the reduction proceeds under diffusion control. The calculated diffusion coefficients fall in the range 0.76·10⁻⁷ to 1.6·10⁻⁶ cm²/s. Comparison of these values with those given in the literature suggests that the calculated coefficients are related to the diffusion of oxygen ions in the slag. The calculated thickness of the limiting diffusion layer, δ, ranges from 0.65·10⁻³ to 5.25·10⁻³ cm, depending on the reduction conditions. The largest decrease in the limiting diffusion layer thickness takes place at low rotational speeds, i.e., 100 and 400 rev/min. The maximum value of the mass transfer coefficient is 1.71·10⁻³ cm/s for reduction from slag with a CaO/SiO₂ ratio of 1.27, 60 pct FeO, at 1420 °C and 2000 rev/min, and the minimum value is 0.27·10⁻⁴ cm/s for reduction from slag with a CaO/SiO₂ ratio of 0.66, 20 pct FeO, at 1350 °C and 100 rev/min. Good agreement has been found between experimental and calculated reduction rates at low disk rotations (100 and 400 rev/min).     

Multicomponent diffusion in molten slags

A method for investigating the diffusion of metal cations in multicomponent slag systems was developed. This method used microprobe analysis and allowed the analysis of the diffusion of multiple species within a single system. This project focused on the diffusion behavior of manganese, iron, calcium, and silicon in silicate slags, in order to simulate industrial steel and ferromanganese production. The molecular structure of CaO-Al₂O₃-SiO₂ slags was investigated with Raman spectroscopy, and oxidation states of manganese and iron in slags of varying composition were determined. This study identified the variation in diffusivity of slag components with changes in composition and temperature of multicomponent slag systems. An empirical model based on the correlation between optical basicity and diffusivity was developed to predict the multicomponent diffusivity of ionic species in molten silicate slags. The model takes into account properties of the bulk slag and the network forming ability of the diffusing species. The relative rate of diffusion of metal cations is proportional to the optical basicity coefficient of that species, while the rate of diffusion of all species increases exponentially with the calculated optical basicity of the bulk slag.     

Investigation of iron oxide reduction by TEM

An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction; during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite; at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction in the hydrogen-argon mixture was slower than in pure hydrogen.     

Kinetic study of the reduction of (Nb2O5) in slag by [Si] in molten iron

The kinetic study of the reduction of (Nb₂O₅) by [Si] has been made. The effects of agitation conditions, temperature and reactants’ concentration on the reduction rate were investigated; the control step of the reducing process was analyzed. The results showed that the reaction is obviously affected by agitation and that the reaction rate is controlled by diffusion step. On the basis of analyzing the mathematical model of the reaction, it was further confirmed that the diffusion of [Nb] in molten iron acted as the control step for the overall reduction process. The activation energy for the diffusion of [Nb] is 51.1 kJ/mol.     

The activity coefficient of cobalt oxide in silica-saturated iron silicate slags

The solubility of cobalt oxide in silica-saturated iron silicate slags (1.16 to 10.00 wt pct) in equilibrium with cobalt-gold-iron alloys (1.10 to 6.52 wt pct cobalt) and oxygen pressures of 10₋₉ to 10₋₁₀ atm (1 atm = 1.013 x 105 Pa) has been investigated at 1573 K. The activity coefficient of cobalt oxide, γ_CoO, has been calculated relative to pure solid cobalt oxide as standard, namely, γ_CoO = 0.91 ± 0.09 and a relationship derived between weight percent cobalt in slag, Co (wt pct), oxygen pressure, pO₂, and activity of cobalt relative to liquid cobalt,aCo, namely, Co (wt pct) = 1.32 x 10⁶ pO ₂ ^1/2 a_Co ± 10 pct Both errors are calculated at the 95 pct confidence level. Formerly Senior Lecturer in Pyrometallurgy, Murdoch University, Murdoch, Western Australia Formerly Postgraduate Student, Murdoch University, Murdoch, Western Australia     

Results demonstrating techniques for enhancing electrochemical reactions involving iron oxide in slags and C in liquid iron

Two techniques are described for the enhancement of the kinetics of reduction of iron oxide from slags by carbon in molten iron. Laboratory experiments have shown that the rate of iron oxide reduction by carbon-saturated iron can be increased by 5 to 10 times when the reaction is carried out under a reduced-pressure atmosphere. This effect is thought to be the result of the increased volumetric gas evolution through the slag layer and the associated increase in slag stirring. A model is presented, which relates the mass-transfer coefficient for ferrous ions in the slag to its stirring that is controlled by varying the ambient pressure. Additional laboratory experiments examined the electrochemical nature of iron oxide reduction from slag by carbon in liquid iron. Results indicate that the reduction of iron oxide from slag is increased in the presence of an applied electric field. The external circuit allows for the separation of the half-cell reactions associated with iron oxide reduction and decarburization and increases the reaction area available for the individual reactions. These results have significant implications for several important slag metal reactions, which occur during ironmaking and steelmaking operations.     

A coupled diffusion model for the reduction of agglomerates of iron oxide granules

A coupled diffusion model is presented to describe the reduction of agglomerates of ferrous oxide grains. The reducing gas (CO) is imagined as diffusing first through the boundary layer created by gas flow at the pellet surface, then through, the agglomerate and finally diffusing into individual grains toward a moving boundary where the reduction occurs. The model is a general one applicable to packed beds of ore particles, briqueties or with some simplication to spherical pellets. Analytic solutions are obtained for platelet grains in slabs and the reduction of spherical pellets of spherical grains is computed for a range of values of the ratio of diffusivities of CO in the grains and pellet.     

MIVM对含V2O3多元熔渣体系组元活度的预测

 为了研究钒在攀钢转炉中的热力学行为及分配,根据钒渣的成分对CaO-MgO-Al2O3-V2O3-SiO2熔渣中各组元的活度进行研究。应用分子相互作用体积模型（MIVM）及其伪多元法预测MgO-Al2O3-SiO2、CaO-V2O3-SiO2、CaO-MgO-Al2O3-SiO2渣系中某组元的活度值,并与试验值对比。结果表明,MIVM伪多元法预测值与试验值吻合良好,MgO-Al2O3-SiO2渣系中Al2O3的平均相对误差为15%;CaO-V2O3-SiO2渣系中V2O3的平均相对误差为33%;CaO-MgO-Al2O3-SiO2渣系中SiO2的平均相对误差为13%。在此基础上,利用MIVM预测CaO-MgO-Al2O3-V2O3-SiO2熔渣中各组元的活度,并将V2O3的预测值与试验值对比分析,结果表明,预测值与试验值吻合良好,都呈正偏差,平均相对误差为44%,V2O3的活度随碱度的提高而下降。     

Settling of copper drops in molten slags

The settling of suspended metal and sulfide droplets in liquid metallurgical, slags can be affected by electric fields. The migration of droplets due to electrocapillary motion phenomena may be used to enhance the recovery of suspended matte/metal droplets and thereby to increase the recovery of pay metals. An experimental technique was developed for the purpose of measuring the effect of electric fields on the settling rate of metallic drops in liquid slags. Copper drops suspended in CaO−SiO₂−Al₂O₃−Cu₂O slags were found to migrate toward the cathode. Electric fields can increase the settling rate of 5-mm-diameter copper drops 3 times or decrease the settling until levitation by reversal of the electric field. The enhanced settling due to electric fields decreases with increasing Cu₂O contents in the slag.     

On the mechanism of iron oxide reduction by carbon

A theoretical analysis has been made to determine the conditions under which the reduction of iron oxide by carbon takes place according to the 2 step mechanism involving the Boudouard reaction. This is based on the concept of minimum temperature of reduction (T _min) below which the Boudouard reaction does not affect the reduction process. The effect of variables such as carbon reactivity, total pressure and so forth onT _min has been studied. TheT _min can be used to determine if metallization is possible under a given set of conditions.