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.     

Effect of FeO and CaO on the Sulfide Capacity of the Ferronickel Smelting Slag

The effect of FeO and CaO on the sulfide capacity in MgO-SiO₂-FeO based slags equilibrating with Fe-Ni alloys at 1773 K and 1873 K (1500 °C and 1600 °C) was investigated. The sulfide capacity in the MgO-SiO₂-FeO and MgO-SiO₂-CaO-FeO slags increased with higher FeO content and higher temperatures due to an increase in the activity of O^2? and a decrease in the activity coefficient of sulfide ion in slag. The sulfide capacity of the MgO-SiO₂-CaO-FeO slag also increased with an increase in the CaO content due largely to the increase in the activity of O^2?. Furthermore, CaO and FeO seem to be more effective than MgO in increasing the sulfide capacity in the MgO-SiO₂-CaO-FeO slag system. In addition, the comparison of the experimental results with the theoretical estimate using the modified empirical optical basicity showed relatively good linear agreement.     

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.     

Effect of FeO and Al2O3 on the MgO Solubility in CaO–SiO2–FeO–Al2O3–MgO Slag System at 1823 K

The MgO solubility in CaO–SiO₂–FeO–Al₂O₃–MgO quinary slag system at 1823 K was measured to evaluate the effect of FeO and Al₂O₃ on the MgO solubility. It was found that the MgO solubility was decreased with higher optical basicity, FeO concentration, and increased with higher Al₂O₃ concentration. The MgO solubility was affected by activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ). Increase of the activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ) with higher FeO or lower Al₂O₃ decreased the MgO solubility. The increment in MgO solubility is remarkably reduced beyond a critical $X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } /(X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } + X_{{\rm FeO}} )$ ratio. The significant decrease of the increment of MgO solubility is caused by the change of the molten slag structure. The excess stability function of Al₂O₃ and the Fourier transform infrared (FT‐IR) analysis were applied to indirectly verify the existence of the spinel structure in the CaO–SiO₂–FeO–Al₂O₃–MgO slag system.     

The sulphide capacity of CaO-SiO2-Al2O3-MgO(-FeO) smelting reduction slags

The effects of MgO and FeO contents on the sulphide capacity of Corex slags were investigated at 1773 K using gas/slag equilibrium technique as a fundamental study for stabilising Corex operational conditions. The gradual substitution of MgO for CaO at a fixed basicity [B = {(%CaO)+(%MgO)}/(%SiO₂)] decreased the sulphide capacities of CaO-SiO₂-Al₂C₃-MgO slags. The addition of FeO into the CaO-SiO₂-Al₂O₃-MgO slags at the fixed (%CaO)/(%SiO₂) ratio, MgO and Al₂O₃ contents significantly increased the sulphide capacities. The sulphide capacity decreased according to the increasing Al₂O₃ content at the fixed (%CaO)/(%SiO₂) ratio, MgO and FeO content. Based on the previously reported and present values of sulphide capacities, the sulphide capacity as a function of slag composition on the mole fraction base at 1773 K was expressed by the formula of log C_S = X_CaO – 3.89 X_SiO2 – 3.18 X_Al2O₃ – 0.55 X_MgO + 2.43 X_FeO – 2.61. In addition, the relationship between the sulphide capacity and optical basicity could be represented as the formula of log C_S = 12.51 A – 12.24 and the theoretical optical basicity of FeO was found be 0.94 from the correlation. The effect of FeO on the sulphur distribution ratio was estimated using the present sulphide capacity values and the oxygen activity in liquid iron, which could be determined with the help of Fe/FeO equilibrium. FeO activity in slag was well described by the quadratic formalism based on the regular solution model. The sulphur distribution ratio according to FeO content varies in an opposite way, compared with that of the sulphide capacity.     

MgO solubility in BOF slag equilibrated with ambient air

The MgO solubility in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂‐(MnO)‐(Al₂O₃) slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron content was in the range of 10 to 35 %. Moreover, 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the MgO solubility. The effect of slag composition on the MgO solubility was discussed and quantified by means of a newly developed formula. As the basicity in slag increases, the MgO solubility decreases. The effect of iron oxide content is observed to be dependent on the basicity of slag. An increase in iron oxide content makes the MgO solubility higher for basic slag but lower for acidic slag. It is revealed that the MgO solubility in steelmaking slag is controlled by the complex anion formation reaction of iron oxide. Both Al₂O₃ and P₂O₅ increase the MgO solubility by diluting the basic oxides as SiO₂ does, while manganese oxide affects the MgO solubility in a similar manner as iron oxide. The MgO solubility can be described as a function of slag composition, X = (%CaO) + 0.45(%Fe₂O₃+ %FeO) + 0.55(%MnO), in the equation of (%MgO) = 0.00816X^2‐1.404X + 62.31. Based on the results, the guidance for addition of MgO‐containing material could be suggested for best slag coating practice.     

Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

An analysis of slag stratification in nickel laterite smelting furnaces due to composition and temperature gradients

The density of FeO-MgO-SiO₂ and FeO-Fe₂O₃-SiO₂ based slags has been analyzed in terms of smelting of lateritic ores for the production of ferronickel. The density of these slags decreases with increasing MgO, SiO₂, and Fe₂O₃ contents as well as with increasing temperature. During the electric furnace smelting of calcined and prereduced garnieritic ores, the slag temperature decreases from the upper layer down toward the slag/metal interface. Together with precipitation of either olivine or silica, this leads to the formation of a dense and stagnant slag layer at the slag/metal interface. For limonitic ores, the use of deep electrode immersion and high currents leads to slag reduction and increased slag temperatures toward the bottom part of the slag layer. The reduction of Fe₂O₃ to FeO increases the slag density. In this manner, it may be possible to maintain a hot slag layer in the region of the slag/metal interface, without buoyancy-induced flow.     

Rationale on composition of slags in oxygen steelmaking processes

Abstract

A study of plant analytical data on slags for the BOS and OBM processes revealed consistent interrelations between the concentrations of FeO, CaO, and SiO₂ in the slag at the end of the oxygen blow. Over a wide composition range, the amounts of CaO and SiO₂ decrease with increasing FeO, slag basicity increases with increasing FeO, and the amount of MgO decreases with increasing slag basicity. At all levels of lime and silica contents, the amount of FeO in the slag varies by 5–6% between the low and high contents. Consequently, the slag/ metal distribution ratios (%P)/[%P] and (%S)/[%S] as functions of basic and/or acidic oxide contents are within a composition range bordered by the equilibrium curves for the low and high FeO contents.     

Measurements of oxygen pressure in a copper flash smelting furnace by an EMF method

Oxygen pressures in a copper flash smelting furnace were measured by means of the following galvanic cell: Fe, FeO/ZrO₂ + MgO/barO in slag or matte. Measured oxygen pressures were normalized to 1523 K with respect to the reaction: 4 FeO(l) + O₂(g) = 4 FeO_1.5(l). Vertical and horizontal variations of normalized oxygen pressures in the reaction shaft and in the settler were studied. The equilibrium relation between normalized oxygen pressure and the ratio of ferric to ferrous oxide content in the furnace slag was confirmed, and the activity coefficient ratio of these oxides was determined.     

电渣重熔六元渣系FeO活度的研究

利用熔渣分子—离子共存理论,研究了电渣重熔20%CaO-20%Al_2O_3-60%CaF_2渣系在冶炼过程中,由于吸收MgO、FeO、SiO_2等夹杂物后,在渣中形成了一定浓度的FeO,而使渣系具有向钢液传递[O]的能力,考察了1 550℃下FeO、MgO质量分数以及二元碱度w(CaO)/w(SiO_2)对FeO活度的影响;分析了该渣系在1 550、1 600、1 650、1 700、1 750和1 800℃下FeO活度随温度的变化情况,构建了20%CaO-20%Al_2O_3-60%CaF_2为基础渣系的六元渣系的FeO活度的模型.研究表明:FeO活度随二元碱度w(CaO)/w(SiO_2)的增加而先增大至趋于平缓后略微减小,在碱度为3.8达到最大;FeO活度随FeO质量分数增加而线性增加,高碱度时,随FeO质量分数增加FeO活度相近;碱度为1时,FeO的活度随MgO的质量分数增加而增大,随温度升高而增加,且MgO含量越高,FeO活度越大;当碱度增加到4、7、10时,FeO的活度随MgO的质量分数增加而减小,相同质量分数的MgO时,碱度越大,FeO活度值越小;碱度为4,MgO的质量分数为1%时,FeO活度达到最大值,高碱度时,温度升高,FeO活度基本保持不变,且同一温度下,碱度越大,FeO活度反而降低.工业试验表明,该模型可以直接利用渣系对金属熔体中氧含量变化进行预测,并对减小钢液中氧含量具有指导意义.     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

Reoxidation of Al‐Killed Steel by Ca(OH)2 in the High Basicity Slag

In the present work, reoxidation of Al‐killed steel by Ca(OH)₂ in high basicity slag was investigated by using laboratory experiments at 1873 K in MgO crucibles with various amount of Ca(OH)₂ addition into slag. The CaO–SiO₂–Al₂O₃–MgO–Ca(OH)₂ slags were used to study the effect of Ca(OH)₂ on total oxygen content, aluminum loss, and FeO content in the slag. It was shown that total oxygen content decreased with the time when no Ca(OH)₂ was added into the slag, but it first increased and then decreased with the time when the addition of Ca(OH)₂ was made. Moreover, aluminum loss and FeO content in the slag increased with increasing Ca(OH)₂ content.     

Effect of slag composition on the cleanliness of drill rod steel

In order to obtain the 55SiMnMo drill rod steel with a high cleanliness, the slag refining has been simulated by laboratory experiments. More desired spherical-shaped complex inclusions with an average diameter of about 2.7?μm, total oxygen of 4?ppm and Mg of 10?ppm after refining were obtained with initial slag basicity of 2.1 and Al₂O₃ 15?wt-%. The relationship between the slag composition and the melting temperature and viscosities of slag was achieved based on a calculation by Factsage Software and Einstein–Roscoe Equation. The refractory–slag–metal–inclusion multiphase reactions were investigated from the viewpoint of thermodynamics and kinetics by the estimation of viscosities, MgO solubility, Al₂O₃ activity in slag and sulphur capacity of slags. It is experimentally confirmed that the corrosion of MgO crucible by slag was affected by the MgO solubility and viscosity of slag. The factors facilitating to obtain low oxygen and control sulphur content were also analysed. Finally, the composition transformation of inclusions during slag refining and cooling process was discussed based on thermodynamic calculation.     

Thermodynamic behaviour of carbonate in CaO based slag

Carbonate solubility was measured for CaO bearing slag systems at 1600 °C under different thermodynamic conditions by using equilibration techniques. Carbonate solubility increased with activity of (CaO). The reaction mechanism of the carbonate dissolution in slag can be expressed as a reaction between CO₂ gaseous phase and oxygen ion to form carbonate (CO₃^2-). Carbonate capacities of various slags depended not only on oxygen ion but also activity coefficient of carbonate ion. The activity coefficient of carbonate ion in CaO-SiO₂ slag changed with CaO content, but that of CaO-Al₂O₃ slag did not change remarkably. Substitution of MgO by CaO for solubility of carbonate had a similar effect as in the case of carbide in CaO-SiO₂-MgO slag. The critical oxygen potential for carbide and carbonate stability was found to be 10-¹⁰ bar.     

Thermodynamic Behaviour of Chlorine in CaO‐SiO2‐MgO‐Al2O3(‐CaF2) Slags

The solubility of chlorine in CaO‐SiO₂‐Al₂O₃‐MgO(‐CaF₂) slag was measured at 1673 ‐1823 K. By estimating the chloride capacity of slags, thermodynamic behaviour of chlorine in the molten slag was investigated. Chloride capacity increased with increasing CaO / SiO₂ ratio (C/S). An increase in MgO content decreased chloride capacity at C/S≥1.0 because it lowered the activity of Ca²⁺ which seemed to have strong affinity with Cl^? in molten slag. Also, the chloride capacity decreased with increasing Al₂O₃ content. The affinity between the Ca²⁺ and Cl^? ions was confirmed by measuring the infrared spectra of slags. The dissolution reaction of chlorine into slag was exothermic and its molar enthalpy was evaluated from the experimental results at 1673 ‐ 1823 K. Based on the result obtained in the present study, the quantitative prediction of chlorine distribution during the blast furnace process was performed. It was estimated that almost all chlorine in the blast furnace would be absorbed into molten slag even if the PCI ratio was increased or low quality coal with chlorine content less than 1.0 mass% was injected.     

A Sulfide Capacity Prediction Model of CaO-SiO2-MgO-FeO-MnO-Al2O3 Slags during the LF Refining Process Based on the Ion and Molecule Coexistence Theory

A sulfide capacity prediction model of CaO-SiO₂-MgO-FeO-MnO-Al₂O₃ ladle furnace (LF) refining slags has been developed based on the ion and molecule coexistence theory (IMCT). The predicted sulfide capacity of the LF refining slags has better accuracy than the measured sulfide capacity of the slags at the middle and final stages during the LF refining process. Increasing slag binary basicity, optical basicity, and the Mannesmann index can lead to an increase of the predicted sulfide capacity for the LF refining slags as well as to an increase of the sulfur distribution ratio between the slags and molten steel at the middle and final stages during the LF refining process. The calculated equilibrium mole numbers, mass action concentrations of structural units or ion couples, rather than mass percentages of components, are recommended to represent the slag composition for correlating with the sulfide capacity of the slags. The developed sulfide capacity IMCT model can calculate not only the total sulfide capacity of the slags but also the respective sulfide capacity of free CaO, MgO, FeO, and MnO in the slags. The comprehensive contribution of the combined ion couples (Ca²⁺ + O²⁻) and (Mn²⁺ + O²⁻) on the desulfurization reactions accounts for 96.23 pct; meanwhile, the average contribution of the ion couple (Fe²⁺ + O²⁻) and (Mg²⁺ + O²⁻) only has a negligible contribution as 3.13 pct and 0.25 pct during the LF refining process, respectively. The oxygen activity of bulk molten steel in LF is controlled by the [Al]–[O] equilibrium, and the oxygen activity of molten steel at the slag–metal interface is controlled by the (FeO)–[O] equilibrium. The ratio of the oxygen activity of molten steel at the slag–metal interface to the oxygen activity of bulk molten steel will decrease from 37 to 5 at the initial stage, and further decrease from 28 to 4 at the middle stage, but will maintain at a reliable constant as 5 to 14 at the final stage during the LF refining process. The proposed high-oxygen potential layer of molten steel beneath the slag–metal interface can be quantitatively verified.     

Reduction Behaviour of BOF type Slags by Solid Carbon

Experiments were carried out on a system with artificially prepared slags in a graphite crucible, in order to examine the possibility of recycling BOF slags produced in the steelmaking process. More than 80% of FeO and P₂O₅ was reduced within 20 minutes and the FeO reduction rate was greater than that of P₂O₅. P₂O₅ reduction began after more than 60% of FeO was reduced. Increasing slag basicity enhanced the reduction of FeO and P₂O₅. Temperature also improved slag reduction. The overall reduction rate was controlled by the chemical reaction at the slag/carbon interface. The reduction rates of FeO and P₂O₅ were second and first order with respect to their respective contents. Most of the reduced phosphorus is believed to vaporize in the form of P₂ gas.     

Phase Equilibria in the System “FeO”-CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO at Different CaO/SiO<Subscript>2</Subscript> Ratios

The “FeO”-containing slags play an important role in the operation of an ironmaking blast furnace (BF), in particular the primary slags such as the system “FeO”-CaO-SiO₂-Al₂O₃-2 mass pct MgO with CaO/SiO₂ weight ratios of 1.3, 1.5, and 1.8 saturated with metallic iron. To investigate the characteristics of such a slag system and its behavior in BF, the phase equilibria and liquidus temperatures in the slag system have been experimentally determined using the high-temperature equilibration and quenching technique followed by an electron probe X-ray microanalysis (EPMA). Isotherms between 1553 K and 1603 K (1280 °C and 1330 °C) were determined in the primary phase fields of dicalcium silicate, melilite, spinel, and monoxide [(Mg,Fe²⁺)O]. Pseudo-ternary phase diagrams of (CaO + SiO₂)-Al₂O₃-“FeO” with a fixed MgO concentration at 2 mass pct and at CaO/SiO₂ ratios of 1.3, 1.5, and 1.8 have been discussed, respectively, simplifying the complexity of the slag system for easy understanding and applying in BF operation. It was found that the liquidus temperatures increase in melilite and spinel primary phase fields, but decrease in dicalcium silicate and monoxide primary phase fields with increasing Al₂O₃/(CaO + SiO₂) ratio. In addition, the liquidus temperatures decrease with increasing “FeO” concentration in dicalcium silicate and melilite primary phase fields, while showing an increasing trend in the spinel and monoxide primary phase fields. The data resulted from this study can be used to improve and optimize currently available database of thermodynamic models used in FactSage.     

Activity of MnO in MnO‐CaO‐MgO‐SiO2‐Al2O3 Slags at 1500 °C

A thermodynamic study was made on the MnO‐CaO‐MgO‐SiO₂‐Al₂O₃ slags that are typical of the production of ferromanganese in submerged arc furnaces. The Al₂O₃ content of the slags was kept constant at 5 per cent by mass. The activity‐composition relationship in Pt‐Mn binary alloys were re‐determined for calibration purposes at 1300, 1400 and 1500°C and po₂ values between 5.40×10^?6 and 4.54×10^?13 atm. A linear regression equation was derived to predict the activity coefficients of manganese, in Pt‐Mn alloys at 1500°C. The effect of concentration, basicity ratio and CaO‐to‐MgO ratio on MnO activities in above mentioned complex slags was investigated at 1500 °C and at two different po₂ values of 4.76×10^?7 and 5.80×10^?8 atm. It was found that a_Mno values increase with increasing MnO, and tend to increase with an increasing CaO‐to‐MgO ratio. The a_MnO values also increase with increasing basicity ratio. The activity coefficient of MnO increases with an increase in its mole fraction in the slag. Quadratic multivariable regression model equations which represent the activity data successfully and which can be used to predict the MnO activities in the compositional range of this study were developed. The MnO activity data was interpreted in terms of a slag model which describes the thermodynamic properties of the slag successfully.