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All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Changes in mineralogical and leaching properties of converter steel slag resulting from accelerated carbonation at low CO{sub 2} pressure

Highlights: > Accelerated carbonation studied to improve environmental properties of steel slag. > Carbonation found to occur predominantly at surface of the steel slag grains. > Combined geochemical modelling and mineral analysis revealed controlling processes. > Enhanced V-leaching with di-Ca silicate (C2S) dissolution identified as major source. > Identified mineral transformations provide guidance for further quality improvement. - Abstract: Steel slag can be applied as substitute for natural aggregates in construction applications. The material imposes a high pH (typically 12.5) and low redox potential (Eh), which may lead to environmental problems in specific application scenarios. The aim of this study is to investigate the potential of accelerated steel slag carbonation, at relatively low pCO{sub 2} pressure (0.2 bar), to improve the environmental pH and the leaching properties of steel slag, with specific focus on the leaching of vanadium. Carbonation experiments are performed in laboratory columns with steel slag under water-saturated and -unsaturated conditions and temperatures between 5 and 90 {sup o}C. Two types of steel slag are tested; free lime containing (K3) slag and K1 slag with a very low free lime content. The fresh and carbonated slag samples are investigated using a combination of leaching experiments, geochemical modelling of leaching mechanisms and microscopic/mineralogical analysis, in order to identify the major processes that control the slag pH and resulting V leaching. The major changes in the amount of sequestered CO{sub 2} and the resulting pH reduction occurred within 24 h, the free lime containing slag (K3-slag) being more prone to carbonation than the slag with lower free lime content (K1-slag). While carbonation at these conditions was found to occur predominantly at the surface of the slag grains, the formation of cracks was observed in carbonated K3 slag, suggesting that free lime in the interior of slag grains had also reacted. The pH of the K3 slag (originally pH {+-} 12.5) was reduced by about 1.5 units, while the K1 slag showed a smaller decrease in pH from about 11.7 to 11.1. However, the pH reduction after carbonation of the K3 slag was observed to lead to an increased V-leaching. Vanadium leaching from the K1 slag resulted in levels above the limit values of the Dutch Soil Quality Decree, for both the untreated and carbonated slag. V-leaching from the carbonated K3 slag remained below these limit values at the relatively high pH that remained after carbonation. The V-bearing di-Ca silicate (C2S) phase has been identified as the major source of the V-leaching. It is shown that the dissolution of this mineral is limited in fresh steel slag, but strongly enhanced by carbonation, which causes the observed enhanced release of V from the K3 slag. The obtained insights in the mineral transformation reactions and their effect on pH and V-leaching provide guidance for further improvement of an accelerated carbonation technology.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2019

Crystallization Behavior of Liquid CaO-SiO{sub 2}-FeO-MnO Slag in Relation to Its Reaction with Moisture

To help maintain the sustainability of the steel industry, we are developing a novel process to recover thermal energy (in the form of hydrogen) and valuable metal elements contained in steelmaking slags by reacting molten slags with moisture. The process is dependent on the structure and properties of the slag, of which the crystallization tendency is key, since surface phases affect the slag reactivity with the gas and enable selective formation of solid phases containing transition metals. In this paper, the precipitated phases of the molten synthetic CaO-SiO{sub 2}-FeO-MnO slags after reacting with moisture were calculated by using thermodynamic package FactSage 7.0. Laboratory experiments were conducted to reveal the crystallization behavior of the targeted metal oxides in the slags with the aim of crystallization control. A hot stage-equipped confocal laser scanning microscope (CLSM) was used to in-situ observe the crystal precipitation on the surface of the liquid slag after reacting with moisture. Time temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams were created from the precipitation behavior of crystals during cooling in the temperature range of 1873 K to 1173 K (1600 °C to 900 °C). The microstructures of the reacted slags were analyzed with a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS) and the phases present in the slag were characterized by X-ray diffraction (XRD). TTT curves of the reacted slags (with moisture) indicated that the nose temperature and critical time for nucleation located at 1473 K (1200 °C) and 89 seconds for the slag with basicity of 1.00. Further increasing the slag basicity to 1.25 and 1.50 increased the nose temperature to 1523 K and 1698 K (1250 °C and 1425 °C), respectively. CCT curves of the reacted slags (with moisture) indicated that the crystallization temperatures of precipitated phases increased with decreasing the cooling rate from 800 to 10 K/min, and the crystallization temperatures of primary phases increased with increasing slag basicity. Both magnetite (Fe{sub 3}O{sub 4}) and monoxide ((FeO){sub x}(MnO){sub 1−x}) phases were detected by SEM-EDS and XRD in the reacted slags (with moisture). The amount of magnetite in the reacted slags increased from 25 to 32 pct to 36 pct and that of monoxide decreased from 5 to 2 pct to 1 pct with the slag basicity increasing from 1.00 to 1.25 to 1.50.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effect of Coal Properties and Operation Conditions on Flow Behavior of Coal Slag in Entrained Flow Gasifiers: A Brief Review

Integrated gasification combined cycle (IGCC) is a potentially promising clean technology with an inherent advantage of low emissions, since the process removes contaminants before combustion instead of from flue gas after combustion, as in a conventional coal steam plant. In addition, IGCC has potential for cost-effective carbon dioxide capture. Availability and high capital costs are the main challenges to making IGCC technology more competitive and fully commercial. Experiences from demonstrated IGCC plants show that, in the gasification system, low availability is largely due to slag buildup in the gasifier and fouling in the syngas cooler downstream of the gasification system. In the entrained flow gasifiers used in IGCC plants, the majority of mineral matter transforms to liquid slag on the wall of the gasifier and flows out the bottom. However, a small fraction of the mineral matter (as fly ash) is entrained with the raw syngas out of the gasifier to downstream processing. This molten/sticky fly ash could cause fouling of the syngas cooler. Therefore, it is preferable to minimize the quantity of fly ash and maximize slag. In addition, the hot raw syngas is cooled to convert any entrained molten fly slag to hardened solid fly ash prior to entering the syngas cooler. To improve gasification availability through better design and operation of the gasification process, better understanding of slag behavior and characteristics of the slagging process are needed. Slagging behavior is affected by char/ash properties, gas compositions in the gasifier, the gasifier wall structure, fluid dynamics, and plant operating conditions (mainly temperature and oxygen/carbon ratio). The viscosity of the slag is used to characterize the behavior of the slag flow and is the dominating factor to determine the probability that ash particles will stick. Slag viscosity strongly depends on the temperature and chemical composition of the slag. Because coal has varying ash content and composition, different operating conditions are required to maintain the slag flow and limit problems downstream. This report briefly introduces the IGCC process, the gasification process, and the main types and operating conditions of entrained flow gasifiers used in IGCC plants. This report also discusses the effects of coal ash and slag properties on slag flow and its qualities required for the entrained flow gasifier. Finally this report will identify the key operating conditions affecting slag flow behaviors, including temperature, oxygen/coal ratio, and flux agents.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2012

Effects of Measurement Materials and Oxygen Partial Pressure on the Viscosity of synthetic Eastern and Western United States Coal Slags

The viscosity of the molten ash (slag) resulting from the mineral constituents in carbon feedstock used in slagging gasifiers is critical for controlling the gasification process. The viscosity of two synthetic slags with compositions resembling the mineral impurities in average eastern and western coal feedstock was examined at temperatures from 1300–1500 °C using a rotating bob viscometer. A few combinations of atmospheres and experimental materials were investigated with respect to one another to determine slag viscosity. A CO/CO{sub 2} atmosphere (CO/CO{sub 2} = 1.8, corresponding to a P{sub O{sub 2}} = 10–8 atm) is required to sustain ferrous ions in FeO-containing slags, an environment that is oxidizing to most metals. Iron oxide in the slag prevents usage of Fe parts. In unpurified Ar, the Fe metal surface oxidizes. Using purified argon prevents iron measurement components from oxidation; however, the metallic surfaces act as nucleation sites for the reduction of the Fe oxide in the slag into metallic Fe. Dissolution of ceramic materials into the slag, including Al{sub 2}O{sub 3} and ZrO{sub 2}, occurs in both atmospheres. Therefore, evaluating slag properties in the laboratory is challenging. The measured viscosities of two synthetic slags in this study diverged depending upon material selection. This difference is likely attributable to container/spindle-slag interactions. Viscosity measurements of the eastern coal slag using all ceramic parts agreed best with FactSage prediction above 1350 °C, with an average activation energy of 271.2 kJ. For western coal slag, the dissolution of container/spindle materials was substantial during the measurement, with precipitation of crystalline phase noted. The experimental viscosity data of the western coal slag agreed best with Kalmanovitch prediction above 1350 °C. The activation energy changed dramatically for both data sets of western coal slag, likely indicating the Newtonian-to-non-Newtonian transition.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2019

Crystallization Characteristics and In-Mold Performance of Electroslag Remelting-Type TiO{sub 2}-Bearing Slag

The non-isothermal crystallization characteristics of the electroslag remelting (ESR)-type TiO{sub 2}-bearing slag were studied. The effect of crystallization characteristics of the slag on the in-mold performance was verified by ESR pilot trials operating in a static mode. The results showed that increasing TiO{sub 2} content decreased the crystallization temperature and liquidus temperature of the slag and suppressed the crystallization tendency of the slag. There is no change in the types of the crystalline phases in the slag with different TiO{sub 2} contents during cooling, i.e., 11CaO·7Al{sub 2}O{sub 3}·CaF{sub 2}, CaTiO{sub 3}, and CaF{sub 2}. The sequence of crystal precipitation during cooling was 11CaO·7Al{sub 2}O{sub 3}·CaF{sub 2} to CaTiO{sub 3}, and then CaF{sub 2} with increasing TiO{sub 2} content from 4.2 to 12.6 mass pct. With adding 16.8 mass pct TiO{sub 2} in the slag, the precipitation of 11CaO·7Al{sub 2}O{sub 3}·CaF{sub 2} and CaTiO{sub 3} took place simultaneously, followed by CaF{sub 2}. With the increase in the TiO{sub 2} content from 4.2 to 16.8 mass pct in the slag, the dominant crystalline phase changed from 11CaO·7Al{sub 2}O{sub 3}·CaF{sub 2} to CaTiO{sub 3}. The morphology of 11CaO·7Al{sub 2}O{sub 3}·CaF{sub 2} and CaTiO{sub 3} changed from faceted and bonelike to elliptically faceted and blocky, respectively. The morphology of CaF{sub 2} were spherical in all cases, irrespective of TiO{sub 2} contents in the slag. The decrease in the crystallization tendency of the slag with increasing TiO{sub 2} contents was attributed to the decrease in the activity of CaO in the slag. The increasing of TiO{sub 2} content in the slag are favorable for providing thin slag skin and stable heat flux across the slag skin, which improved the surface quality of the as-cast ESR ingot as demonstrated by ESR pilot trials operating in a static mode.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermodynamic Properties and Viscosities of CaO-SiO{sub 2}-MgO-Al{sub 2}O{sub 3} Slags

In this work, the effects of Al{sub 2}O{sub 3}, basicity, and MgO/Al{sub 2}O{sub 3} ratio on the heat capacity, enthalpy change, and viscosity of CaO-SiO{sub 2}-MgO-Al{sub 2}O{sub 3} slags at 1773 K, 1823 K, and 1873 K (1500 °C, 1550 °C, and 1600 °C) were investigated. The heat capacity of the slag increased with increasing Al{sub 2}O{sub 3}, basicity, and MgO/Al{sub 2}O{sub 3} ratio in the experimental temperature range. The enthalpy change increased with increasing Al{sub 2}O{sub 3} content and MgO/Al{sub 2}O{sub 3} ratio, and decreasing basicity. Under the fixed heat quantity of the slag, the addition of Al{sub 2}O{sub 3} would result in a decrease of slag temperature and increase of slag viscosity. The effect of decreasing temperature on the viscosity was more significant than increasing Al{sub 2}O{sub 3} content. Increases of slag basicity led to the increased slag temperature and the decreased viscosity which was due to the increased slag temperature. The viscosity change rate with different temperatures was more pronounced than that of different slag compositions. The change of the slag temperature at fixed heat quantity would be opposite due to different expressions of MgO/Al{sub 2}O{sub 3} ratio. In addition, when the heat decrement of the slag was fixed, the slag temperature rose first with the increase of MgO/Al{sub 2}O{sub 3} ratio and then decreased, while the viscosity decreased gradually. The viscosity fluctuation caused by different heat decrements reduced as well with increasing MgO/Al{sub 2}O{sub 3} ratio.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Development of Kinetic Model for Reactions Between Cu-Containing Multicomponent Slag and Liquid Sulfide Using Coupled Reaction Model

Copper smelting slag contains less than 2 mass pct of Cu oxide and 30–50 mass pct of Fe{sub x}O. Each year, the grade of copper ore decreases, while the amount of slag generated in the copper smelting process increases. In this study, a coupled reaction model to simulate the reaction between multicomponent slag and FeS-based matte was developed using reported thermodynamic data and double-film theory for the recycling of copper smelting slag. The activity coefficients of oxides in the multicomponent slag were calculated using a regular solution. The activity coefficients of Cu{sub 2}O in the slag and those of FeS, Cu{sub 2}S, and CaS in the FeS-based matte used previously reported data. The behaviors of Cu in slag and matte were confirmed by comparing the simulated results and the reported solubility of Cu in the slag in the temperatures ranging from 1473 K to 1573 K. In addition, the effect of the input amount of FeS on the copper content of the slag was verified by comparing the results of reaction model to experimental results. Using the reaction model, the influences of the initial ratio of FeO/SiO{sub 2} of the slag, temperature, and matte composition on the behaviors of Cu in the slag and matte were investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Iron Phase Evolution on Copper Separation from Slag Via Coal-Based Reduction

Copper slag, a by-product of copper pyrometallurgy, inevitably contains a certain amount of copper. Oxygen-enriched smelting technologies increase the copper content in slag indirectly because of the production of higher-grade matte. The effect of iron phase evolution on the copper content in slag during the slag cleaning process in an electric furnace was investigated using the method of combining theory with experiments. Based on the analysis, the main factors that impede the separation of slag and copper/matte were determined. Subsequently, the properties of slag were analyzed after decreasing the magnetite content within the slag. The experimental results showed that decreases in magnetite content were beneficial for the separation of copper and slag because of the decrease of slag viscosity. Nevertheless, Cu-Fe alloys formed when magnetite was completely reduced to metallic iron, and the foaming slag was formed at 1250 °C. Furthermore, the distribution of copper in the reduced slags was studied in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Efficient Recovery of Copper and Cobalt from the Matte–Slag Mixture of ISA Furnace by Injection of Coke and Pyrite

During ISA copper smelting process, ISASMELT furnace discharges a large amount of matte and slag mixture and the separation of them has an important influence on the recovery of valuable metals. This paper presented a reduction-sulfurization sedimentation process for recovering copper and cobalt from the matte–slag mixture of ISA furnace. Firstly, matte–slag mixture and traditional static sedimentation slag are characterized to determine their mineral composition and occurrence state. It indicates that Cu is primarily lost in slag in the form of sulfide, while Co is mostly lost in the form of oxide. With coke and pyrite as the reducing agent and vulcanizing agent, an orthogonal laboratory experiment was conducted to determine the effects of the smelting temperature and additive dosage on the recovery process. The optimum slag cleaning conditions were found to be: a coke dosage of 2 pct, a pyrite dosage of 2 pct, and a smelting temperature of 1260 °C lasting for 2 hours. In order to improve on the low utilization ratio of additives associated with the industrial sedimentation process, an innovative additive introducing method was put forward which enables the additives to mix and react with slag more adequately. A powder injection device was purpose-made to inject additives into molten slag in dispersion state, and a laboratory experiment was carried out to simulate this process. By injecting 2 pct coke and 2 pct pyrite, the contents of Cu and Co in cleaned slag decreased to 0.46 and 0.01 pct, respectively. It proves that the injection of additives into molten slag is an effective method to recover Cu and Co from the matte–slag mixture of ISA furnace.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Wetting and Erosion of ZrO{sub 2}-Graphite Refractory by CaO-SiO{sub 2} and CaO-Al{sub 2}O{sub 3}-Based Mold Slags for Submerged Entry Nozzle

To moderate the erosion in a slag-line zone and extend the service life of a submerged entry nozzle during continuous casting of high-Mn and high-Al steel, the current study developed a low-reaction CaO-Al{sub 2}O{sub 3}-based slag with lower SiO{sub 2} and higher Al{sub 2}O{sub 3} contents than that of conventional CaO-SiO{sub 2}-based slag. The static sessile drop technique and dynamic rotating cylinder method were utilized to investigate the erosion behavior of ZrO{sub 2}-graphite refractory by the above slags. The main results indicated that silicate-based slag could wet the ZrO{sub 2}-graphite substrate completely, while the aluminate-based slag spread rapidly in the first 20 minutes, and then a slag drop with an unchanged profile formed on the substrate. The equilibrium contact angles with the substrate at 1623 K were 0 and 79 deg for the CaO-SiO{sub 2} and CaO-Al{sub 2}O{sub 3} based slags, respectively. The comparison of the influences of the rotating speed and the temperature revealed that the decrease of erosion rate could be realized by the developed slag. An examination of ZrO{sub 2}-saturation limit also indicated that the low-reaction slag could dissolve much more ZrO{sub 2} oxidation, accompanied by a less-negative effect on the viscosity of the molten slag.

36 MATERIALS SCIENCE↗
DOE OSTI2011

A Brief Review of Viscosity Models for Slag in Coal Gasification

Many researchers have defined the phenomenon of 'slagging' as the deposition of ash in the radiative section of a boiler, while 'fouling' refers to the deposition of ash in the convective-pass region. Among the important parameters affecting ash deposition that need to be studied are ash chemistry, its transport, deposit growth, and strength development; removability of the ash deposit; heat transfer mechanisms; and the mode of operation for boilers. The heat transfer at the walls of a combustor depends on many parameters including ash deposition. This depends on the processes or parameters controlling the impact efficiency and the sticking efficiency. For a slagging combustor or furnace, however, the temperatures are so high that much of the coal particles are melted and the molten layer, in turn, captures more particles as it flows. The main problems with ash deposition are reduced heat transfer in the boiler and corrosion of the tubes. Common ways of dealing with these issues are soot blowing and wall blowing on a routine basis; however, unexpected or uncontrolled depositions can also complicate the situation, and there are always locations inaccessible to the use of such techniques. Studies have indicated that slag viscosity must be within a certain range of temperatures for tapping and the membrane wall to be accessible, for example, between 1300 C and 1500 C, the viscosity is approximately 25 Pa {center_dot} s. As the operating temperature decreases, the slag cools and solid crystals begin to form. In such cases the slag should be regarded as a non-Newtonian suspension, consisting of liquid silicate and crystals. A better understanding of the rheological properties of the slag, such as yield stress and shear-thinning, are critical in determining the optimum operating conditions. To develop an accurate heat transfer model in any type of coal combustion or gasification process, the heat transfer and to some extent the rheological properties of ash and slag, especially in high-temperature environments need to be understood and properly modeled. The viscosity of slag and the thermal conductivity of ash deposits are among two of the most important constitutive parameters that need to be studied. The accurate formulation or representations of the (transport) properties of coal (and biomass for co-firing cases) present a special challenge of modeling efforts in computational fluid dynamics applications. In this report, we first provide a brief review of the various approaches taken by different researchers in formulating or obtaining a slag viscosity model. In general, these models are based on experiments. Since slag behaves as a non-linear fluid, we discuss the constitutive modeling of slag and the important parameters that must be studied.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2025

In situ high-temperature Raman spectroscopy for online EAF slag analysis

Real-time monitoring of slag chemistry is critical for optimizing Electric Arc Furnace (EAF) steelmaking operations, where dynamic variations in slag composition directly influence slag foaming, refractory degradation, and thermal efficiency. Conventional techniques such as X-ray fluorescence (XRF), Fourier-transform infrared (FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) are commonly used to analyze slag composition, but their offline nature and equipment constraints limit their applicability for online monitoring in harsh industrial environments. To address this challenge, we present an in situ, high-temperature analytical approach that integrates Raman spectroscopy with a custom designed fiber-optic probe for real-time slag characterization at 1550 °C. The system enables non destructive spectral acquisition from molten slags, providing molecular-level insights into silicate polymerization and iron oxidation states. Eight synthetic slag samples were evaluated, and key Raman features—such as Q n silicate units and FeO₄/FeO₆ coordination environments—were identified and quantitatively correlated with slag basicity and Fe₂O₃ content. The results demonstrate agreement between Raman spectral ratios and bulk slag chemistry, validating the method’s capability to track compositional and structural changes under molten temperature. This work establishes the feasibility of deploying fiber-optic Raman probe for online EAF slag monitoring and highlights their potential to support closed-loop control strategies, thereby enhancing process stability, refractory protection, and steel quality in industrial steelmaking applications.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

New Slag for Nickel Matte Smelting Process and Subsequent Fe Extraction

Large quantities of residual slag with high Fe content are left behind in pyrometallurgical nickel processing. How to separate and recover iron from the high fayalite residue is still a problem because iron mainly exists as iron-magnesium silicate in slag. This article suggests that the slag for nickel smelting is adjusted by increasing the amount of lime, partly instead of quartz, to form liable reduction compounds for subsequent Fe extraction. Based on the thermodynamic analysis and phase diagram of the FeO-SiO{sub 2}-CaO-MgO slag system, the new slag composition was chosen, the suitable melting point of relevant slags was measured, and the phase change was observed. The new slag with a composition of 7 to 9 pct MgO, 10 to 15 pct CaO and 1.5 to 1.8 Fe/SiO{sub 2} was found to work well for nickel matte smelting. With the increase of CaO and decrease of SiO{sub 2} content in slag, Ca(Fe, Mg)Si{sub 2}O{sub 6} and Mg(Ca)Fe{sub 2}O{sub 4} are the main existing phases instead of (Fe, Mg){sub 2}SiO{sub 4} in the residual smelting slag. More than 50 pct of Fe-containing phases are in the form of MgFe{sub 2}O{sub 4} when 15 pct CaO is added, which is what we want because the MgFe{sub 2}O{sub 4} is much better for magnetic separation and reduction than the original (Fe, Mg){sub 2}SiO{sub 4}. In this way, Fe can be much more easily extracted from residual slag and have multipurpose benefits.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Study of Non-Newtonian Behavior of CaO-SiO{sub 2}-Based Mold Slag and Its Effect on Lubrication in Continuous Casting of Steel

To understand the effect of the non-Newtonian behavior of mold slag on lubrication during continuous casting of steel, the current study investigated the rheology of CaO-SiO{sub 2}-based mold slag using a rotational viscometer with variable rotating speed. The constitutive equations at different temperatures were determined. Subsequently, the obtained viscosity as a function of temperature and shear rate was incorporated into a validated mathematical model. The main results demonstrated that slag viscosity decreased dramatically with increased shear rate as the experimental temperature approached the break temperature of mold slag, meaning that the mold slag had a property of shear thinning. The predicted evolution of slag viscosity during mold oscillation was consistent with the shear thinning as the shear rate exerted on the slag channel varied. The calculated slag consumption was higher than that for slag treated as a Newtonian fluid, which agreed well with measurements using a miniature continuous caster. This study finds that the consideration of the non-Newtonian behavior of mold slag is significant to understand the lubrication mechanism and optimize related parameters via simulation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Slag-Steel Reaction on the Initial Solidification of Molten Steel during Continuous Casting

With the mold simulator technique, the effect of slag-steel reaction on the initial shell solidification as well as the heat transfer and lubrication behavior of the infiltrated mold/shell slag film was studied in this article. The results showed that the Al{sub 2}O{sub 3} content, the CaO/SiO{sub 2} ratio, and the viscosity of mold flux were increased with the progress of the slag-steel reaction during casting. The slag-steel reaction has two major effects on the initial shell solidification: one is increasing the mold heat flux and shell thickness by the decrease of slag film thickness. The other is the reduction of mold heat flux by the increase of crystal fraction in slag film. Mold flux with a lower basicity, viscosity, and crystallization temperature would result in a larger liquid slag consumption and the uneven infiltration of slag into the mold and shell gap that eventually leads to the irregular solidification of initial shell with a poor surface quality, such as slag entrapment and depressions as well as glaciation marks. Conversely, mold flux with a higher viscosity, basicity, and crystallization temperature would result in a smaller liquid slag consumption, which would cause the poor mold lubrication, the longitudinal shell surface defects, and drag marks.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study of the Mechanism of Liquid Slag Infiltration for Lubrication in Slab Continuous Casting

Consistent and uniform lubrication of the solidifying shell, especially in the meniscus, is crucial for the smooth continuous casting operation and production of strands free of surface defects. Thus, the current study established a coupled model to study the inflow behavior of liquid slag to the mold-strand channel, taking the solidification of steel and slag and the periodic oscillation of mold into account. The difficulties and solutions for the simulation were described in detail. The predicted profiles of the slag rim and initial shell were in good agreement with the reports. The main results indicated that liquid slag could be squeezed out and back into the slag pool in a negative strip period while a large amount of liquid slag could infiltrate into the mold-strand channel. Thus, the amount of slag consumed in the negative strip period was relatively small compared with that in the positive strip period. The predicted variation of slag consumption during mold oscillation was periodic, and the average value was 0.274 kg/m{sup 2}, which agreed well with the slag consumption in industrial practice. The current model can predict and optimize the oscillation parameters aiming at stable lubrication conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of FeO and CaO/Al{sub 2}O{sub 3} Ratio in Slag on the Cleanliness of Al-Killed Steel

The slag composition plays a critical role in the formation of inclusions and the cleanliness of steel. In this study, the effects of FeO content and the C/A (CaO/Al{sub 2}O{sub 3}) ratio in the slag on the formation of inclusions were investigated based on a 10-minute slag–steel reaction in a MgO crucible. The FeO content in the top slag was shown to have a significant effect on the formation of MgO·Al{sub 2}O{sub 3} spinel inclusions, and critical content exists; when the initial FeO content in the slag was less than 2 pct, MgO·Al{sub 2}O{sub 3} spinel inclusions formed, and the T.O (total oxygen) was 20 ppm; when the initial FeO content in the slag was more than 4 pct, only Al{sub 2}O{sub 3} inclusions were observed and the T.O was 50 ppm. It was clarified that the main source of Mg for the MgO·Al{sub 2}O{sub 3} spinel inclusion formation was the top slag rather than the MgO crucible. In addition, the cleanliness of the steel increased as the initial FeO content in the top slag decreased. As regards the effects of the C/A ratio, the MgO amount in the observed inclusions gradually increased, whereas the T.O content decreased gradually with the increasing C/A ratio. Slag with a composition close to the CaO-saturated region had the best effect on the inclusion absorption.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Lime Dissolution in Foaming BOF Slag

The paper describes the dissolution mechanisms of lime into liquid and foaming slags relevant to the BOF process. Two different master slags are employed, representing two different periods of the converter process: an early stage where the FeO content is fixed to 45 wt pct, and a later stage where the FeO content is fixed to 25 wt pct. For these master slags, the ratio between CaO/SiO{sub 2} is varied to examine the effect of basicity on lime dissolution. Calcium silicates are formed and peeled off, or partially peeled off, from the interface between the lime cube and the slag in all cases. The main difference for the dissolutions in pure liquid slag and foaming slag is the controlling step for dissolution. In liquid slag, the controlling mechanism is the removal of the calcium silicate layers, while in foaming slag, the controlling mechanism is the contact area between the lime and the liquid slag phase of the foam. The strong convection in the foam enhance the dissolution process, in some cases, the lime even dissociates into small pieces.

36 MATERIALS SCIENCE↗