Al2O3 Slag Research Articles

Influence of Silicon on Desulfurization of Al-Killed Steel by CaO-Al2O3Slag Contained FeO and MnO

Industry trials of slag-washing desulfurization in LD tapping process are performed to study the influence of silicon on desulfurization of Al-killed steel using CaO–Al2O3 slag contained FeO and MnO. Influence of silicon on desulfurization is found to be great, while the influences of aluminum and manganese on desulfurization are negligible. Oxygen activity at slag–steel interface together with the (FeO + MnO) content in slag, decreased with the rise of silicon content, which is considered as the main reason of different desulfurization efficiencies between Si-free steel and Si-containing steel. Oxygen activity at slag–steel interface is reduced more sharply by increasing the silicon content than that by increasing the aluminum content. As a result, desulfurization efficiency can be enhanced significantly with the rise of silicon content, rather than the aluminum content. Furthermore, the influence of silicon on desulfurization can be magnified if the slag contained FeO and MnO, while negligible for the slag with few FeO and MnO.

Reduction Smelting Low Ferronickel from Pre-concentrated Nickel-Iron Ore of Nickel Laterite

AbstractThe research of smelting low ferronickel from pre-concentrate nickel-iron ore with 2.76 % Ni and 38.00 % Fetotal was carried out to find an effective way for stainless steel enterprises to use the low-nickel laterite reasonable. The results show that Ni and Fe both have a certain degree of enrichment, and impurities and harm elements have different degrees of reduction after pre-concentration of nickel-iron ore. Most valuable metal did not compound with impurities which greatly accelerated the speed and extent of melt separation reduction. Good alloy of 6.58 % Ni with the overall recoveries of 93.38 % and 89.95 % Fetotal with the overall recoveries of 89.57 % was manufactured under the following conditions: 10 % coke, 1.0 binary basicity, 18 % MgO and 3 % Al2O3 in slag, melting at 1,550 °C for 10 min. The product can be used for the feed of producing stainless steel.

Effect of Basicity, Al2O3and MgO content on the softening and melting properties of the CaO-MgO-SiO2-Al2O3high alumina quaternary slag system

The flow characteristic temperatures of the selected quaternary CaO-MgO-SiO2 -Al2 O3 slags have been experimentally measured by performing softening and melting tests using a high temperature microscope in accordance with German standard 51730. An effective statistical model is developed through constructing empirical equations by regression analysis method to predict Softening temperature (ST), Hemispherical temperature (HT), and Flow temperature (FT) based on Al2 O3 (25%–30%) content, CaO/SiO2 (C/S ratio: 0.9–1.4), and MgO (4%–12%) content. The effect of basicity, MgO and Al2 O3 content on characteristic temperature are also studied. FactSage 6.4 data base is used to calculate liquidus temperature of the above quaternary slags and compared with the experimental and regression results. The experimental results show that, increase in Al2 O3 content increases ST and HT while decreases FT of the slags. The ST, HT and FT increase with increase in C/S ratio. Increase in MgO content decreases both ST and HT while increases FT of the slags. The liquidus temperature obtained from FactSage 6.4 are found to correspond well with the experimental and regression data. Under the range of composition examined, a high Al2 O3 (30%) and high C/S ratio (1.4) coupled with low MgO (4%) content is beneficial for the blast furnaces running with high Alumina slags, as it ensures the formation of a short slag.

Effect of Slag Composition on Inclusions in Si-Deoxidized 18Cr-8Ni Stainless Steels

Laboratory experiments and thermodynamic calculation for the Si-deoxidized 18Cr-8Ni stainless steel with CaO-Al2O3-SiO2-CaF2-MgO-MnO slag were performed to investigate the effect of slag composition on inclusions in Si-deoxidized 18Cr-8Ni stainless steels. A high-basicity slag improved the cleanness of stainless steel, while a low basicity slag lowered the Al2O3 content in inclusions, lowering the melting temperature of inclusions and improving the deformability of inclusions. Approximately, 15 pct content of MgO in the initial slag was beneficial to reduce the corrosion of refractory and would not influence the composition of inclusions. The Al2O3 in slag should be lowered to a minimum to reduce the Al2O3 content in inclusions in Si-deoxidized 18Cr-8Ni stainless steel. The $$ \log X_{{{\text{Al}}_{ 2} {\text{O}}_{ 3} }}^{i} + \log X_{\text{MgO}}^{i} - \log X_{MnO}^{i} - \log X_{{{\text{SiO}}_{2} }}^{i} $$ of inclusions was expressed as a linear function of $$ \log a_{{{\text{Al}}_{ 2} {\text{O}}_{ 3} }}^{\text{s}} + \log a_{\text{CaO}}^{\text{s}} - \log a_{MnO}^{\text{s}} - \log a_{{{\text{SiO}}_{2} }}^{\text{s}} $$ of the slag, which was used to predict the composition of inclusions based on the activity of Al2O3, CaO, SiO2, and MnO in the slag. Moreover, the equilibrium curve of Al-O and the stability diagram of inclusions in Si-deoxidized 18Cr-8Ni stainless steel at 1873 K (1600 °C) were calculated.

Vapor pressure measurement of lead and lead chlorides in FeOT–CaO–SiO2–Al2O3 system

Vapor pressure of lead and lead chlorides from FeOT–CaO–SiO2–Al2O3 slag system was measured by using Knudsen effusion method. The results suggest that the vapor pressures of lead and lead chlorides increase with increasing temperature. For the slag systems without chlorine, the logarithm of vapor pressure (lnp) shows highly linear dependency on the reciprocal of temperature (1/T), and higher vapor pressure is observed in the condition where more metallic lead vapor is formed. In this case, the vapor pressure of lead increases with increasing slag basicity (w(CaO)/w(SiO2)), increasing FeO content and w(Fe2+)/w(Fe3+) ratio. For the case of slag system with chlorine addition, the total pressures of PbCl2 and PbCl increase with decreasing basicity and FeO content of slag.

Reduction and melting behavior of carbon composite lateritic bauxite pellets

Direct reduction of low-grade lateritic bauxite was studied at high temperature to recover Fe and beneficiate Al2O3 slag. The results show that a metallization rate of 97.9% and a nugget recovery rate of 85.1% can be achieved when the reducing and melting temperatures are 1350 and 1480°C, respectively. Moreover, a higher-grade calcium aluminate slag (Al2O3 = 50.52wt%) can also be obtained, which is mainly composed of α-Al2O3, hercynite (FeAl2O4), and gehlenite (Ca2Al2SiO7). In addition, high-quality iron nuggets have been produced from low-grade lateritic bauxite. The nugget is mainly composed of iron (93.82wt%) and carbon (3.86wt%), with almost no gangue (slag).

Study on electrical conductivities of CaO–SiO2–Al2O3 slags

The composition dependence of electrical conductivities for CaO–SiO2–Al2O3 slags has been studied experimentally in the present work. From the experiments results, it can be seen that the electrical conductivity decreases firstly and then increases with increasing CaO/Al2O3 ratio while keeping the content of SiO2 constant, and there is a minimum value of electrical conductivity near CaO/Al2O3 = 1, which is resulted from the charge compensation effect of Al3+ ion. It was also found that electrical conductivity increases with gradually replacing SiO2 by CaO at fixed Al2O3 content, because the concentration of Ca2+ ions acting as the charge carriers increases. However, different variation behaviours of electrical conductivity occur by gradually substituting Al2O3 for SiO2, according to the content of Al2O3 relative to that of CaO.

Effect of Slag on Inclusions During Electroslag Remelting Process of Die Steel

Many factors influence the non-metallic inclusions in electroslag steel including furnace atmosphere and inclusions’ content in the consumable electrode, slag amount and its composition, power input, melting rate, filling ratio, and so on. Fluoride containing slag, which influences the non-metallic inclusions to a great extent, has been widely used for the electroslag remelting process. The current paper focuses on the effect of fluoride containing slag on the inclusions in electroslag ingots based on the interaction of the slag-metal interface and electroslag remelting process. In this work, die steel of CR-5A and several slags have been employed for investigating the effect of slag on inclusions in an electrical resistance furnace under argon atmosphere in order to eliminate the effect of ambient oxygen. Specimens were taken at different times for analyzing the content, dimensions, and type of non-metallic inclusions. Results of quantitative metallographic analysis indicate that a multi-component slag has better capacity for controlling the amount of inclusions; especially protective gas atmosphere has also been adopted. The findings of inclusions in electroslag steel by SEM–EDS analysis reveal that most non-metallic inclusions in electroslag steel are MgO-Al2O3 inclusions for multi-component slags, but it is Al2O3 inclusions when remelting using conventional 70 wt pct CaF2-30 wt pct Al2O3 slag. The maximal inclusions’ size using multi-component slags is less than that using conventional binary slag. Small filling ratio as well as protective gas atmosphere is favorable for controlling the non-metallic inclusions in electroslag steel. All the results obtained will be compared to the original state inclusions in steel, which contribute to choice of slag for electroslag remelting.

Effects of basicity and MgO content on the viscosity of the SiO2-CaO-MgO-9wt%Al2O3 slag system

The effects of basicity and MgO content on the viscosity of SiO2-CaO-MgO-9wt%Al2O3 slags with basicity from 0.4 to 1.0 and MgO content from 13wt% to 19wt% were investigated using the rotating cylinder method. A correlation between the viscosity and the slag structure was determined by Fourier transform infrared (FTIR) spectroscopy. It is indicated that the complex network structure of the slag melt is depolymerized into simpler network units with increasing basicity or MgO content, resulting in a continuous decrease in viscosity of the slag. The viscosity is strongly dependent on the combined action of basic oxide components in the slag. Under the present experimental conditions, increasing the basicity is found to be more effective than increasing the MgO content in decreasing the viscosity of the slag. At higher temperatures, the increase of basicity or MgO content does not appreciably decrease the viscosity of the slag, as it does at lower temperatures. The calculated activation energy of viscous flow is between 154 and 200 kJ·mol−1, which decreases with an increase in basicity from 0.4 to 1.0 at a fixed MgO content in the range of 13wt% to 19wt%.

Inclusion Evolution and Estimation during Secondary Refining in Calcium Treated Aluminum Killed Steels

The objective of this study was to investigate the evolution of inclusions during secondary refining in aluminum killed steels. As calcium treatment is also used to modify solid alumina inclusions to liquid or partially liquid calcium aluminates for keeping free from nozzle clogging, a thermodynamic model has been calculated using Factsage to predict the type of calcium aluminates and sulfides present in the steel. The results showed that alumina generated from deoxidation had transformed to MgOAl2O3 spinel with a small amount of CaO by using high basicity and high Al2O3 slag during ladle furnace (LF) refining. Though soft blowing after LF refining is benefit for the removal of inclusions by floatation, vigorous stirring which may make slag layer exposing an “eye” to air reoxidation should be avoided. After calcium treatment, the inclusions were restricted mainly to globular calcium aluminates and the sulfur in the steel was associated with these inclusions as calcium sulfide. The ND (number density) of inclusions seems to appear a similar tendency to the size distribution of smaller inclusions, and the similar change between the AF (area fraction) of inclusions and the total oxygen content of the steel can be also found by careful comparison. Predicted inclusion type in the steel were validated with measurement based on samples collected from industrial heats and a reasonable range of Ca content was also given for the experimental steel.

The mechanism of boron removal in the CaO–SiO2–Al2O3 slag system for SoG-Si

The dissolution behavior of boron in a molten slag was evaluated at 1773 K to establish the boron removal mechanism from silicon refining for preparing higher purity solar-grade silicon. From the experimental results, a mechanism of boron removal from silicon into the CaO–SiO2–Al2O3 slag system was confirmed to proceed via BO33− and B62− following a linear relationship with oxygen partial pressure in the slag system. In addition, the distribution ratio of boron was seen to increase with increasing slag basicity.6B+1/3O2−=B62−+1/6O2(g)The boride capacity of the CaO–SiO2–Al2O3 slag can be expressed as a linear function of the activity of CaO and the optical basicity. It is suggested that the proposed dissolution mechanism of boron could be appropriate to slag systems, depending on oxygen partial pressure and basicity.

Effects of B2O3and TiO2on Crystallization Behavior of Slags in Al2O3-CaO-MgO-Na2O-SiO2System

Nowadays, there are increasing demands for developing mould fluxes without fluoride due to environmental concerns. Slag bearing titanium oxide and/or boron oxide proved to be promising substitute for traditional mould fluxes with fluoride. In the present work, Crystallization behaviors of some slags in Al2O3–CaO–MgO–Na2O–SiO2–B2O3–TiO2 system were investigated using single hot thermocouple technique (SHTT), differential thermal analysis (DTA), and X-ray diffraction (XRD) technique. It was found that liquidus temperature of slag decreases with increases of B2O3 content. The XRD analysis on the crystallized samples shows that crystallization products are perovskite, gehlenite, and wollastonite. Time–temperature–transformation diagrams (TTT) for various slags have been constructed using SHTT technique. It was found that B2O3 can significantly increase the incubation time for crystallization of slags. Incubation time for crystallization of slags at nose temperature decreases with increase of titanium oxide content from 5% to 10%. The morphology of crystals was also investigated using SHTT technique. At higher temperature, dendrites and columnar crystal tend to form and grow with time progress, while at lower temperature, small crystals appear and the amount of crystal increase with time increasing.

Experimental Study to Improve the Castability of Aluminum Killed Cold Heading Steel

AbstractExperimental study on the LF refining of aluminum killed cold heading steel shows that calcium content in the molten steel increased to about 0.0010% at the end of refining, and the aluminum deoxized products were transformed from Al2O3 to the complex inclusions CaO–MgO–Al2O3 with lower melting point by the high basicity, high Al2O3, and strong deoxidizing slag. The inclusions are in liquid state and can be easily floated up during LF refining and continuous casting. The total oxygen content of the steel falls to about 0.0020%. The experimental technology uses only 50 m calcium wire to the 80‐t heat or even without calcium treatment. As compared to the traditional technology with higher amount of calcium for treatment, which forms CaS and CaO–MgO–Al2O3 inclusions with high melting point, the experimental technology improves the castability and reduces the manufacturing cost.

Calculation Model of Mass Action Concentrations for CaO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Slag System

According to the coexistence theory of melted slag structure, a calculation model of mass action concentrations for CaO-Al2O3-SiO2 slag system has been established, by which the mass action concentrations of all structure units in CaO-SiO2-Al2O3 systems are calculated. The effects of Al2O3 content and slag basicity on the mass action concentrations are investigated. The results indicate that both basicity and the content of Al2O3 have remarkable effects on mass action concentrations of CaO, 2CaO•SiO2, CaO•SiO2, CaO•Al2O3 and 2CaO•SiO2•Al2O3, etc. Furthermore, the effects of Al2O3 content on mass action concentrations are relevant with slag basicity. The basicity has more influences on mass action concentrations of CaO, CaO•SiO2, 2CaO•SiO2 and 3CaO•SiO2, whereas the basicity has little effect on mass action concentrations of other structure units in CaO-SiO2-Al2O3 system.

Crystallization Control in Metallurgical Slags Using the Single Hot Thermocouple Technique

AbstractWith the single hot thermocouple technique (SHTT) the solidification behavior of metallurgical slags has been studied by in situ observation, constructing time–temperature–transformation (TTT) or continuous‐cooling‐transformation (CCT) diagrams. The SHTT is a unique apparatus that enables measurement of the slag sample temperature using a thermocouple while the sample is heated or cooled simultaneously. Due to the low heat capacity of the system sample/thermocouple high heating or cooling rates can be easily obtained (>3000°C/min). The following findings are reported in the present paper: (i) For the CaO–Al2O3 slag – 44% CaO, 56% Al2O3 (wt%) – the CCT diagram shows large differences between liquidus and the temperature for first crystals precipitation, even at low cooling rates, for example, 168°C below the liquidus when cooling at a rate of 6°C min−1. (ii) For the CaO–SiO2 slag – % CaO/% SiO2 (wt%) = 0.7 – no crystal is observed for continuous cooling, even at low cooling rates, such as 10°C min−1. During isothermal experiments crystallization was observed only at 1000°C with an incubation time of 76 s (average of six experiments, standard deviation 27 s). However, crystallization becomes much more intense for the CaO–SiO2 slag when increasing the temperature after reaching lower temperatures (<1000°C), where probably the conditions for nucleation are better.

Influence of ladle slag additions on BOF process performance

A plant trial has been performed, with recycling of ladle slag to the LD converter at SSAB EMEA, Luleå, Sweden. The effect of ladle slag addition on the slag and steel composition, together with the slag weight, was investigated with sampling both during the blow and at blow end. The addition of ladle slag resulted in an increase in slag weight between 1 and 2 t throughout the blow and an increase of 3 wt-% in the slag Al2O3 content. This indicated that the ladle slag melted during the initial stages of the blow and enhanced the early slag formation. Lime additions were reduced with 3·5 kg t−1 liquid steel without drawbacks on the phosphorus or sulphur refining. Heats with added ladle slag had an increase in blowing time with ∼4% and an increased tendency for slopping. However, this can be handled by different lance and addition programmes.

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

Abstract In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al2O3 slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al2O3 slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.

Dissolution kinetics of chromium ore in slag system for stainless steelmaking

To reveal the smelting reduction mechanism of chromium ore for producing stainless steel in converter, the dissolution behaviour of chromium ore in CaO–SiO2–MgO–Al2O3 slag system was studied by laboratory experiments, and the effects of different temperatures and slag compositions on the dissolution rate of chromium ore in slag were investigated. A kinetic model for the dissolution process of chromium ore was developed for the first time. The dissolution process of chromium ore is controlled by the surface dissolution reaction under the conditions of the present experiments. The effects of temperature, slag composition and particle size of chromium ore on the dissolution rate of chromium ore are relatively great. The chromium ore sand is suggested to be used as raw material in the smelting reduction converter.Afin de révéler le mécanisme de réduction de la fusion du minerai de chrome pour la production d'acier inoxydable dans un convertisseur, on a étudié le comportement de dissolution du minerai de chrome dans le système de laitier CaO-SiO2-MgO-Al2O3, par des expériences en laboratoire. On a examiné l'effet des différentes températures et compositions de laitier sur la vitesse de dissolution du minerai de chrome dans le laitier. Pour la première fois, on a développé un modèle cinétique du procédé de dissolution du minerai de chrome. Le procédé de dissolution du minerai de chrome est contrôlé par la réaction de dissolution à la surface sous les conditions des présentes expériences. Les effets de la température, de la composition du laitier et de la taille de particule du minerai de chrome sur la vitesse de dissolution du minerai de chrome sont relativement importants. On suggère que le sable de minerai de chrome soit utilisé comme matériau brut dans le convertisseur de réduction de fusion.

A Thermodynamic Model of Sulfur Distribution Ratio between CaO–SiO2–MgO–FeO–MnO–Al2O3 Slags and Molten Steel during LF Refining Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the sulfur distribution ratio between ladle furnace (LF) refining slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating the mass action concentrations of structural units in LF refining slags, i.e., CaO-SiO2-MgO-FeO-MnO-Al2O3 hexabasic slags, based on the ion and molecule coexistence theory (IMCT). The calculated mass action concentrations of structural units in CaO-SiO2-MgO-FeO-Al2O3-MnO slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than mass percentage of components, in the slags can represent their reaction abilities. The calculated total sulfur distribution ratio shows a reliable agreement with the measured or the calculated sulfur distribution ratio between the slags and molten steel by other models under the condition of choosing oxygen activity based on (FeO)-[O] equilibrium. Meanwhile, the developed thermodynamic model for calculating sulfur distribution ratio can quantitatively determine the respective contribution of free CaO, MgO, FeO, and MnO in the LF refining slags. A significant difference of desulfurization ability among free component as CaO, MgO, FeO, and MnO has been found with approximately 87-93 pct, 11.43-5.85 pct, 0.81-0.60 pct and 0.30-0.27 pct at both middle and final stages during LF refining process, respectively. A large difference of oxygen activity is found in molten steel at the slag-metal interface and in bulk molten steel. The oxygen activity in molten steel at the slag-metal interface is controlled by (FeO)-[O] equilibrium, whereas the oxygen activity in bulk molten steel is controlled by [Al]-[O] equilibrium. Decreasing the high-oxygen-activity boundary layer beneath the slag-metal interface can promote the desulfurization reaction rate effectively or shorten the refining period during the LF refining process.

A New Approach for the Diffusion Coefficient Evaluation of Sulfur in CaO-SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub> Slag

The Diffusion coefficient of sulfur in a ternary slag with composition of 51.5% CaO- 9.6% SiO2- 38.9% Al2O3 was measured at 1723 K by chemical diffusion from the variation of concentration of sulfur in silver metal. A MATLAB program was developed to find the concentration variation of sulfur in silver metal using various critical parameters like the diffusion coefficient of sulfur in slag available in literature, sulfur partition ratio, sulfide capacity of the slag and the its density. The PS2 and PO2 pressures were calculated from the Gibbs energy of the equilibrium reaction between CaO in the slag and solid CaS and confirming the same by using ThermoCalc. The density of the slag at 1723 K was obtained from earlier experiments. Initially the order of magnitude for the diffusion coefficient was taken from the works of Saito and Kawai but later was modified so that the concentration changes of Sulfur obtained from the program agreed with the experimental results. The diffusion coefficient of sulfur in 51.5% CaO- 9.6% SiO2- 38.9% Al2O3 slag at 1723 K was estimated as 4.14x10-6 cm2/sec.