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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Improving the Modeling of Slag and Steel Bath Chemistry in an Electric Arc Furnace Process Model

An improvement to the thermochemical module of the electric arc furnace (EAF) process model developed by Meier based on the work of Logar, Dovžan, and Škrjanc is presented. Different models for the calculation of activities in melt and slag are implemented, and separate reaction zones are defined for the interaction of slag and melt, the injection of oxygen, and the injection of carbon. For each zone, equilibrium compositions and reaction rates are calculated. Furthermore, diffusion of species is considered as a rate-limiting factor in reactions between slag and melt, and diffusion rates are calculated from the bulk melt to the reaction zone where reactions with the slag take place. Oxygen and sulfur dissolved in the melt and CaS in the slag are added as new species not previously considered in the EAF model. The treatment of carbon is revised to reduce model complexity and improve accuracy. The improved model is validated using extensive data from an industrial EAF, and results are compared to measured data as well as results obtained with the unmodified model. The different models for the determination of thermodynamic activities and their impacts on the duration of the simulation as well as its results are evaluated.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Insight into the Relationship Between Viscosity and Structure of CaO-SiO{sub 2}-MgO-Al{sub 2}O{sub 3} Molten Slags

This article elucidates the quantitative relationship between viscosity and structure in a basic slag system of CaO-SiO{sub 2}-MgO-Al{sub 2}O{sub 3} and focuses on the role of Al{sub 2}O{sub 3}. Slag viscosity was measured by the rotating cylinder method, and structural information was obtained using Fourier transformation infrared, Raman and magic angular spinning nuclear magnetic resonance (MAS-NMR) techniques. The results show that, as the Al{sub 2}O{sub 3} content increased, slag viscosity increased initially and decreased afterwards, directly indicating that Al{sub 2}O{sub 3} had an amphoteric effect on slag viscosity. The Raman spectra verified that with increasing Al{sub 2}O{sub 3} content, the concentrations of Q{sup 0}(Si) and Q{sup 2}(Si) decreased first and then increased, while that of Q{sup 1}(Si) kept increasing and that of Q{sup 3}(Si) increased first and then decreased. The {sup 27}Al MAS-NMR spectra proved that the mole ratios of AlO{sub 5} and AlO{sub 6} to AlO{sub 4} kept increasing with the increase of Al{sub 2}O{sub 3} content, and, overall, Al{sub 2}O{sub 3} changed from a network former to a network modifier. The relationship between the viscosity and structure of the molten slags was further analyzed quantitatively based on the modified (NBO/T), denoted as (NBO/T)′, and we found a fine linear correlation between the logarithm of viscosity and (NBO/T)′. Moreover, the variations of thermodynamic properties of this system also indirectly supported the present experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Distinctive microstructural features of aged sodium silicate-activated slag concretes

Electron microscopic characterisation of 7-year old alkali-activated blast-furnace slag concretes enabled the identification of distinct microstructural features, providing insight into the mechanisms by which these materials evolve over time. Backscattered electron images show the formation of Liesegang-type ring formations, suggesting that the reaction at advanced age is likely to follow an Oswald supersaturation–nucleation–depletion cycle. Segregation of Ca-rich veins, related to the formation of Ca(OH){sub 2}, is observed in microcracked regions due to the ongoing reaction between the pore solution and available calcium from remnant slag grains. A highly dense and uniform interfacial transition zone is identified between siliceous aggregate particles and the alkali activated slag binders, across the concretes assessed. Alkali-activated slag concretes retain a highly dense and stable microstructure at advanced ages, where any microcracks induced at early ages seem to be partially closing, and the remnant slag grains continue reacting.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Carbon sequestration of steel slag and carbonation for activating RO phase

Carbonation of Ca/Mg minerals in industrial alkaline residues is a technology to sequester CO{sub 2} and reduce its emissions to the atmosphere. In this work, BSE-EDS were used to determine the mineral phase in steel slag; compositions of RO phase were identified and simulated. The carbon sequestration of steel slag was studied, and RO phase was activated by carbonation. Result shows that the amount of CO{sub 2} sequestered in steel slag and RO phase increases as carbonation time increases. Under autoclaving condition, the hydration ratio of MgO in RO phase was 20.10%. Through carbonation, up to 58.83% of MgO in RO phase can be converted into MgCO{sub 3}, and the activation of RO phase by carbonation was manifested. The mechanical properties and volume stability of carbonated steel slag were improved, proving that the positive effect of carbonation on steel slag when applied in cement and cementitious materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Factors affecting the reactivity of slag at early and late ages

This study investigates the difference between the reactivity of slag in NaOH solution (w/s = 100) and in cement paste (w/b = 0.4). The reactivity of slag is much higher in NaOH solution than in cement paste. When the NaOH solution is saturated with portlandite, the rate of reaction of slag decreases to approach that in cement paste. This suggests that the slower reaction of slag in cement paste might be due to the presence of calcium ions released by the clinker phases. At 28 days, relative humidity measurements indicate that water is only present in pores less than 6 nm. Above 6 nm, hydration cannot take place due to the lack of water. From a thermodynamic point of view, the small size of water filled pores may slow down the slag reaction due to the high concentration of species in solution required for hydrate growth to balance the increase of the surface energy.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Analysis of slags using laser-induced breakdown spectroscopy

The feasibility of laser-induced breakdown spectroscopy (LIBS) for the analysis of gasification slags was investigated by comparing LIBS results to the results of an ICP-OES analyzer. A small amount of slag sample was placed on a piece of double sided adhesive tape attached to a glass microscope slide and analyzed for Al, Ca, Fe, Si, and V which are major elements found in slags. The partial least squares regression (PLS-R) and univariate simple linear regression (SLR) calibration methods indicated that apart from V (accuracy up to + 20%) the accuracy of analysis varies within 0.35–6.5% for SLR and 0.06–10% for PLS-R. A paired-sample t-test within the 95% confidence level yielded p-values greater than 0.05, meaning no appreciable statistical difference was observed between the univariate SLR with internal standardization and the multivariate PLS-R for most of the analytes. From the results obtained in this work, LIBS response varies depending on the element and the technique used for quantitative analysis. Simultaneous use of the univariate calibration curves with internal standard (intensity ratio) and PLS regression in multi-elemental analysis can help reduce the matrix effect of slags associated to their high variation in concentration. Overall, these results demonstrate the capability of LIBS as an alternative technique for analyzing gasification slags. Estimated limits of detection for Al, Ca, Fe, Si and V were 0.167, 0.78, 0.171, 0.243 and 0.01 wt.%, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Fiber-Optic Raman Probe for On-line EAF Slag Analysis

Here, in Electric Arc Furnace (EAF) steelmaking, the push for improved efficiency requires accurate analysis of the chemical composition of its slag system to control slag foaming, provide refractory protection, and maintain high furnace iron yield. Therefore, the ability to obtain real-time slag chemistry data would provide a useful tool to improve the control and efficiency of the process. The work reported here aims to assess the structure and chemistry of EAF slags at high temperatures using a portable fiber-optic Raman probe. As a result, the ability to relate Raman spectra peaks to chemistry and structure is demonstrated in experimental result with EAF slags at 1550°C.

47 OTHER INSTRUMENTATION↗
DOE OSTI2026

Evidence for carbon dioxide removal via enhanced rock weathering with steel slag, though not basalt, in a midwestern U.S. field trial

Enhanced weathering is an emergent pathway for permanent atmospheric carbon dioxide removal (CDR). However, despite a dramatic increase in academic and commercial research, there remain relatively few published examples of field evidence demonstrating the effectiveness of enhanced weathering. Here, we present results from a three-year field trial that evaluated steel slag and crushed basalt applied as amendments in a conventional agricultural system in the Midwestern United States. Steel slag applied to initially acidic soil increased porewater pH and alkalinity and increased soil pH and Ca-saturation. Together, changes in porewater chemistry and soil properties provide strong evidence for steel slag weathering and CDR. However, steel slag applied to soils with a neutral initial pH did not generate significant changes in soil or porewater chemistry. In addition, coarse-grained crushed basalt did not generate significant change in any of the soils. Strong acid effects were apparent in all 3 years of monitoring soil porewater chemistry. Overall, our results demonstrate clear evidence of CDR from applying steel slag amendments to acidic cropland soils while also highlighting the difficulty of greenhouse gas reduction accounting from enhanced weathering and the variable outcomes that can occur depending on feedstock and soil type.

Geosciences↗
DOE OSTI2019

Slags as Evidence for Copper Mining above Casaccia, Val Bregaglia (Central Alps)

Slags from the remote Mota Farun locality above Casaccia (Val Bregaglia, Swiss Alps) have been analyzed with scanning electron microscopy, X-ray powder diffraction and microfocus synchrotron X-ray diffraction to determine mineralogical composition and microstructures. Non-magnetic slag samples are largely composed of euhedral and dendritic iron-rich olivine in a glassy matrix. Locally there are zones with globular inclusions rich in bornite ((Cu5Fe)S4) and locally metallic copper. Some regions display dendritic pentlandite ((Fe,Ni)9S8). Magnetic samples are mainly composed of fayalite (Fe2SiO4) and wüstite (FeO), with minor magnetite (Fe3O4). The mineralogical composition indicates that slags were the product of copper smelting. The slag compositions and morphologies are analogous to slags described from the Oberhalbstein (Graubünden, Switzerland) and the Trentino Alps (Italy) which are attributed to metallurgical exploitations of the Late Bronze Age. While the origin of the ore could not be determined, it may be related to ore deposits of chalcopyrite in greenschists and serpentinites in the vicinity, such as Alp Tgavretga (Septimer Pass) and Val Perossa (Val Bregaglia).

58 GEOSCIENCES↗
DOE OSTI2018

Sintering and collapse of synthetic coal ash and slag cones as observed through constant heating rate optical dilatometry

Ash agglomeration and slagging are responsible for costly maintenance problems in coal-fired boilers. Boiler design and operation rely on critical ash fusion temperatures defined by geometric criteria that explicitly ignore shrinkage or warping. Reproducibility of ash fusion test (AFT) results is known to be a major problem, given the subjective nature of some criteria used in critical temperature reporting. Decades of faulty predictions based on AFT data underscore the need for an objective measure of the likelihood for agglomeration and slagging as a function of coal ash composition and temperature. Heating microscopes and ash fusibility determinators are AFT instruments that are widely available and readily adapted for optical dilatometry of ash cones through digital image analysis. Optical dilatometry enables the continuous measurement of cone height and face area, and the calculation of changes in cone volume and density as a function of temperature during constant heating. A strong correlation was found between the sintering onset temperatures of two synthetic ash compositions measured through optical dilatometry and their corresponding solidus temperatures predicted through thermophysical modeling. The sintering onset temperatures of synthetic slag cones appears consistent with glass transition temperatures estimated using the Kauzmann-Beaman rule. Cone collapse is highly correlated with softening, hemispherical, and fluid temperatures for ash cones. Rapid changes in solid, liquid, and gas volume fractions near the collapse temperature suggest that a transition from capillary to slurry state may play an important role in triggering collapse. Slag cone collapse is better explained by a decrease in viscosity as a function of temperature in the glass working range. Furthermore, the sintering onset temperature and cone collapse temperature are readily understood through thermophysical modeling based on ash composition and provide two additional critical temperatures in the modeling of coal ash agglomeration and slagging.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2020

Controlled evolution of rare earth phosphate in coal ash slag

Coal ash slag as a waste by-product, is generated from commercial processes that include gasification facilities, power plants, and steelmaking industries. Some is recycled into construction materials, insulation fibers, and used as a soil neutralizer, but the majority is still discarded. Coal ash slag exhibit trace amounts of critical materials including rare earth elements (REEs). An economically viable extraction of REEs from the coal ash would lower the production costs, addressing the availability and environmental concerns. In nature, most REEs exist as phosphate such as monazite but its synthetic evolution in coal ash slag has never been reported. In industrial processes, P may originate from coal, additives, and/or refractory liners. REEs may be extracted by first concentrating REEs into phosphates, sulphide, or fluorides [1], or carbonates [2]. In this work, P-enriched coal ash slag bearing REEs was first melted above 1600 °C, then crystallization behaviors in the melt during controlled cooling was studied. REE phosphate coprecipitated with mullite and hematite with mullite being the first crystal to form on cooling. The REEs were rejected from the molten slag, mullite, and hematite, and tended to segregate only to the phosphate phase with REE concentration as high as 58 wt.% in the structure.

Nakano, Jinichiro↗
DOE OSTI2021

Controlled evolution of rare earth phosphate in coal ash slag

Coal ash slag as a waste by-product, is generated from commercial processes that include gasification facilities, power plants, and steelmaking industries. Some is recycled into construction materials, insulation fibers, and used as a soil neutralizer, but the majority is still discarded. Coal ash slag exhibit trace amounts of critical materials including rare earth elements (REEs). An economically viable extraction of REEs from the coal ash would lower the production costs, addressing the availability and environmental concerns. In nature, most REEs exist as phosphate such as monazite but its synthetic evolution in coal ash slag has never been reported. In industrial processes, P may originate from coal, additives, and/or refractory liners. REEs may be extracted by first concentrating REEs into phosphates, sulphide, or fluorides [1], or carbonates [2]. In this work, P-enriched coal ash slag bearing REEs was first melted above 1600 °C, then crystallization behaviors in the melt during controlled cooling was studied. REE phosphate coprecipitated with mullite and hematite with mullite being the first crystal to form on cooling. The REEs were rejected from the molten slag, mullite, and hematite, and tended to segregate only to the phosphate phase with REE concentration as high as 58 wt.% in the structure.

Nakano, Jinichiro↗
DOE OSTI2024

Effect of Si Content in Electrode and SiO2 Additions to the Slag during Electroslag Remelting

Evolution of Si concentration in 316 stainless steel electrodes was observed during a melting campaign consisting of recycling electroslag remelted (ESR) ingots to make new electrodes using vacuum induction melting (VIM). This campaign consisted of iterations of VIM + ESR operations to optimize melting parameters. The effect of Si content on the melt parameters and ingot quality was further evaluated and additions of SiO2 to the slag chemistry were studied using research-scale experiments, x-ray diffraction (XRF), combustion analysis, visual inspections, and computational tools. The Si concentration was found to decrease by approximately 600 ppm following ESR of 150 lb. research-scale electrodes. Eventually, this led to failure of the slag skin and direct ingot/crucible contact. Additions of SiO2 to the slag at levels matching the original calculated Si concentration in the electrode did not eliminate the slag skin failure and the current during steady state increased to maintain a constant melt rate. In this investigation, we propose a mechanism of slag skin failure consisting of local concentration of current density due to absence, or breakage, of the SiO2 layer around the molten metal drop during ESR. This theory was reinforced by additional experiments in which Nb was added to the electrode to change the structure of the oxide layer around the drops.

Jablonski, Paul↗
DOE OSTI2025

CFD modeling and simulation for corrosive wear of refractory in molten slag

The development of high-wear resistant refractories having minimal production costs is facilitated by characterizing the wear mechanisms associated with their corrosive wear. Static cup testing is a commonly used method for comparing the corrosion resistance performance of two or more refractory materials. Although the static cup test conditions are not as severe as dynamic tests, this study shows that the thermal gradient present within the system during heating and cooling stages serves to generate movement of the slag leading to mechanical wear. The thermal gradient within the refractory, and between the slag and the refractory, occurs during the ramp stage of the test and lasts until the soaking stage is reached bringing the system to a thermal equilibrium. Using computational fluid dynamics (CFD) capabilities embedded within ANSYS software, this study modelled and quantified the convection currents within the slag and associated shear stresses generated on the refractory walls due to the thermal gradient. A traditional ladle furnace was employed as a case study to verify the results of the studied CFD model. The corrosion rate of the refractory lining was found to depend on the mass transfer coefficient of the refractory dissolution into the slag, and a velocity term which governs the extent of corrosion at any given location. This velocity term is a function of slag viscosity, as well as the concentration gradient and/or temperature gradient at the triple points. In this study, wall shear stress was used as a reliable proxy for identifying high-velocity regions prone to excessive corrosive wear. Elevated wall shear stress near the slag/air and slag/molten steel interfaces align with observed corrosion grooves, which reflects the intensified corrosive wear at these locations.

Ramteke, Rajat Rajat Durgesh Ramteke [University o↗
DOE OSTI2017

A Structural Study on the Foaming Behavior of CaO-SiO{sub 2}-MO (MO = MgO, FeO, or Al{sub 2}O{sub 3}) Ternary Slag System

The foaming index of the CaO-SiO{sub 2}-MO (MO = MgO, FeO, or Al{sub 2}O{sub 3}) ternary slag system with a fixed CaO/SiO{sub 2} ratio is measured to understand the effect of the ionic structure. At 1773 K (1500 °C), the foaming index of the slag increases with Al{sub 2}O{sub 3} addition and decreases with MgO or FeO addition at a fixed CaO/SiO{sub 2} ratio. It is verified that the previous correlation between the foaming index and the physical properties could also be valid for the CaO-SiO{sub 2}-(MgO or Al{sub 2}O{sub 3}) system. Raman spectroscopy for the CaO/SiO{sub 2} = 1.0 slag is applied to explain the foaming behavior from an ionic structural perspective. From the ionic structural viewpoint, the fractional change in each silicate anion unit is identified by de-convoluted Raman spectra. The Raman spectra indicate that the silicate network structure is polymerized with Al{sub 2}O{sub 3} as an aluminosilicate structure; in contrast, de-polymerization occurs by MgO or FeO addition. Furthermore, the relationship between the silicate structure and the thermodynamic stability function is discussed. Since the ionic structure of the molten slag affects various physical/thermodynamic properties, the foaming behavior could be successfully interpreted from the ionic structural viewpoint.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Rheological Evolution of Ti-Bearing Slag with Different Volume Fractions of TiN

The effect of the volume fraction of TiN (0 to 4.27 vol pct) on the rheological behaviors of Ti-bearing slag, including the viscosity and the constitutive equation, was investigated using a Brookfield rheometer at 1773 K. With the increasing TiN, the viscosity and the shear stress increased substantially, and the slag is converted from Newtonian to non-Newtonian. When the slag samples were dispersed with TiN (1.00 to 4.27 vol pct), the measured viscosities were greater than those estimated by the Einstein–Roscoe equation, and the complex internal structure was suggested to be responsible for the extra enhancement of the viscosity beyond that caused by TiN itself. The viscosity curves of the slag samples containing TiN (1.00 to 4.27 vol pct) were divided into three regions depending on the critical shear rate and the initial shear rate. In the Newtonian region, the apparent volume fraction of TiN was defined and used to develop a semiempirical model based on the Einstein–Roscoe equation to describe the viscosity. In the two non-Newtonian regions, the shear rate, the yield stress, and the viscosity differences between the non-Newtonians regions and the Newtonian regions were applied in developing the viscosity models.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In Situ Observation of Calcium Aluminate Inclusions Dissolution into Steelmaking Slag

The dissolution rate of calcium aluminate inclusions in CaO-SiO{sub 2}-Al{sub 2}O{sub 3} slags has been studied using confocal scanning laser microscopy (CSLM) at elevated temperatures: 1773 K, 1823 K, and 1873 K (1500 °C, 1550 °C, and 1600 °C). The inclusion particles used in this experimental work were produced in our laboratory and their production technique is explained in detail. Even though the particles had irregular shapes, there was no rotation observed. Further, the total dissolution time decreased with increasing temperature and decreasing SiO{sub 2} content in the slag. The rate limiting steps are discussed in terms of shrinking core models and diffusion into a stagnant fluid model. It is shown that the rate limiting step for dissolution is mass transfer in the slag at 1823 K and 1873 K (1550 °C and 1600 °C). Further investigations are required to determine the dissolution mechanism at 1773 K (1500 °C). The calculated diffusion coefficients were inversely proportional to the slag viscosity and the obtained values for the systems studied ranged between 5.64 × 10{sup −12} and 5.8 × 10{sup −10} m{sup 2}/s.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Prediction and Measurement of the Kinetics of Isothermal Crystallization and Devitrification of Mold Powder Slags

The rate and extent of formation of crystalline phases in the slag film that develops between the mold and the strand in steel continuous casting determine heat transfer and lubrication between them and therefore play crucial roles in shell integrity. In this work, a commercial mold powder for casting medium carbon (MC) thin steel slabs is selected for carrying out detailed comparisons between predicted and experimental results of the rates of isothermal crystallization and devitrification of liquid and glassy slag, respectively. Predictions are obtained with a kinetic model that combines the induction period (of Šimon and Kolman) and the Kissinger methods to evaluate time–temperature–transformation (TTT) diagrams for the devitrification and crystallization reactions associated with the peaks appearing in non-isothermal thermo-analysis traces. To obtain experimental validation of the diagram, a high-temperature confocal laser scanning microscope (HT-CLSM) is used to rapidly heat or cool a sample, ~ 140 mg, of glassy powder or liquid slag, respectively, to selected isothermal conditions, where it is held for a prescribed length of time before quenching. Identification and quantification of the phases present in the quenched samples are done by a quantitative X-ray powder diffraction analysis (QXRPDA) method that uses a PONKCS (partial or not known crystal structure) procedure based on Le Bail and Rietveld strategies for refinement of diffraction patterns. The many experimental results indicate that the predicted TTT diagrams for devitrification and crystallization of the mold slag give a good estimation of the rate of the transformations that take place at different temperatures and reveal the differences in the reactions that occur along both paths.

36 MATERIALS SCIENCE↗