Slag System Research Articles

Thermodynamic evaluation of viscosity behavior for CaO–SiO2–Al2O3–MgO slag systems examined at the temperatures range from 1500 to 1700 °C

This work showed an application of computational tools to understand systematically the behavior of viscosity on CSAM systems relevant to industrial uses. Consequently in this study, the viscosity experimental data obtained from the literature were compared with the thermodynamic calculated results via the software FactSage v.7.3 for melts in CaO–SiO2–Al2O3–MgO slag system with the range of compositions slags cover 0–100 wt% CaO, 0–100 wt% SiO2, 0–100 wt% Al2O3 and 0–15 wt% MgO at temperature ranges of 1500–1700 °C. Using open-source software in Python, the results of viscosity, liquid, and solid fraction of the slag, as a function of composition and temperature, are represented by multiple color maps and by iso-viscosity contours. The results of the viscosity values indicated that the effect of all the oxides in the CSAM slag system follows the well-known behavior trend observed in the literature. Viscosities of the slag were found to increase with increasing SiO2 contents and decrease with increasing basicities (high CaO). The increase in Al2O3 content increases the viscosity values. An increase of 0–15% MgO depolymerized the slag melt and decreases the viscosity. However, above 5% MgO content occur a decrease in the liquid zone (single phase) and a liquid fraction (two-phase region) of the slag. For a constant MgO concentration, the increase in temperature generates an expansion of low-viscosity zones associated with an increase in the liquid phase of the slag. From the comparison between the calculated and experimental viscosities data keeps up within 30% average relative deviation (Δ), the predictions are considered acceptable for viscosity in the CSAM slag system at high temperatures.

Effect of alkaline oxides on aluminate slag structure by first principles calculation

To guide the research and development of CaO-Al2O3-based mold fluxes, the influence mechanism of 8 mol% alkaline oxides (Na2O, Li2O, BaO, MgO) on the multi-scale structure characteristics of aluminate slag (CaO/Al2O3 = 1.3) at 1873 K was analyzed by first-principles calculation. With the addition of alkaline oxides, the stability of Al-O and Ca-O bonds in slag system was Na2O-Al2O3-CaO (NAC) ≈ BaO-Al2O3-CaO (BAC) > Li2O-Al2O3-CaO (LAC) ≈ Al2O3-CaO (AC) > MgO-Al2O3-CaO (MAC), and the proportion of average coordination number for Al were AC > MAC > LAC > BAC > NAC. The alkaline cations played two roles of structure depolymerization and charge compensation, as the ability was ranked as NAC > BAC > MAC > LAC, and Na+ > Li+ > Ba2+ > Mg2+, respectively. At the electronic level, a small number of electrons were localized between O atoms, which surrounded Al to form a closed structure. Adding appropriate amount of Na2O/BaO was necessary to effectively regulate the low/non-reactive mold flux.

Synergetic effect of superabsorbent polymer and shrinkage reducing admixture on the properties of alkali-activated slag binders

This research is to investigate the synergistic effects of superabsorbent polymer (SAP) and shrinkage-reducing admixture (SRA) on autogenous shrinkage, hydration and microstructure of alkali-activated slag system (AAS). Results revealed that the addition of SRA reduced the absorption capacity of SAP, resulting in a smaller size distribution of macrovoids throughout the AAS matrix. Nevertheless, the presence of SAP affected the ability of SRA to effectively reduce the surface tension of the pore solution of AAS. Instead, the addition of SAP resulted in an increase in surface tension by up to 14%. This phenomenon can be explained by the hydrophobic interactions between SAP networks and SRA molecules. The combination of SAP and SRA in AAS exhibited greater effectiveness in controlling autogenous shrinkage compared to using SRA alone, resulting in a reduction of around 53% compared to 46%. On the other hand, incorporating only SAP proved to be more effective, leading to a higher reduction of 90%. However, this combined effect is reported to be counteractive and dependent on the content of each component. Moreover, the hybrid system of SAP and SRA demonstrates a pronounced retardation effect on AAS hydration. The XRD patterns showed no new reaction products were formed. As revealed by SEM analysis, the microstructure of AAS containing SAP and SRA exhibited a distribution of microvoids ranging from 5 to 50 μm. This can explain the reduction of the compressive strength by incorporation a high content of SAP combined with SRA. However, incorporating a lower dosage of SAP is advantageous when combined with SRA in AAS systems. Finally, the integration of SAP and SRA had a positive influence on the properties of AAS mortars, thus compensating for the limitations of each constituent.

Effect of Al2O3 on Inclusion Removal in H13 Steels Using High-Basicity LF (Ladle Furnace) Refining Slags

In this experiment, a quaternary fluorine-free refining slag system of CaO-SiO2-Al2O3-MgO was selected, with basicity ratios of 2, 4, and 6 and calcium-aluminum ratios of 1.5, 2.1, and 3. High-temperature “slag-steel equilibrium” experiments were conducted to investigate the influence of different basicity ratios and calcium–aluminum ratios on the morphologies, compositions, sizes, and quantities of the inclusions in H13 steel, aiming to improve the cleanliness of H13 steel to meet practical industrial requirements. The experimental results showed that with the increase in the basicity ratio and the calcium–aluminum ratio, the morphologies of the inclusions changed from elliptical to regular circular, with more regular edges. As the basicity ratio increased from 2 to 6, the densities of the inclusions showed a decreasing trend, with values of 40, 35, 30, 25, 32, and 30 inclusions/mm2. When the basicity ratio remained the same, the average size of the inclusions in the steel decreased first and then increased with the increases in the calcium–aluminum ratios, with sizes of 1.59 μm, 1.23 μm, and 1.38 μm, respectively. Among these, when the basicity ratio was 6 and the calcium–aluminum ratio was 2.1, the control effect on the densities and sizes of the inclusions was the best, yielding an inclusion density of 25 inclusions/mm2 and a size of 1.15 μm. Additionally, reducing the Al2O3 content in the slag could reduce the Al2O3 contents in the inclusions, which also promoted improvements in the elastic deformation capacities of the inclusions. With increases in the calcium–aluminum ratios in the slag system, the masses of the inclusions decreased due to the reduced Al contents in the steel. The Al contents in the steel also had an impact on the compositions of the inclusions.

Simultaneous recovery of valuable metals W, Co, Ta, and Nb from tungsten leaching residue by sodium-based slag refining

As a hazardous solid waste in the tungsten (W) materials industry, the utilization of W leaching residue (W-residue) has become a challenge. In this study, we propose a sustainable strategy for the simultaneous extraction of valuable elements W, Ta, Nb, and Co from W-residue by a sodium-based slag refining. The slag system was designed to provide a slag-liquid phase to facilitate the mass transfer and reduction reaction of the valuable metallic compounds during the refining process. The characteristics of Na2O-SiO2-NaF slag system, including the liquidus temperature and viscosity, were predicted using FactSage thermodynamics software and evaluated experimentally. Using 0.5Na2O-0.3SiO2-0.1NaF slag, the recovery efficiency of the valuable elements from W-residue can reach 94%(W), 66%(Co), 53%(Ta), and 77%(Nb), and a total recovery rate was reached 75%. The mechanism illustrated that the valuable elements (W, Co, Ta, and Nb) in W-residue can be transformed to (W, Ta, Nb, Ti)C and (Fe, Co)Si, which co-precipitated as alloy phases during the slag refining process. The present work offers a strategy, not merely limited to waste utilization of W-residue, to supplement the recycling route of W materials industry.

Effect of TiO<sub>2</sub> Addition and Cooling Rate on Crystallization Behavior of Separated Slag Containing Low-grade RE

With the continued exploitation and utilization of high-grade rare earth ores, it is increasingly important to extract rare earths from separated slag containing low-grade rare earth. The X-ray powder diffraction, scanning electron microscopy, electron probe micro-analyzer and confocal laser scanning microscopy were used to explore the influence of TiO2 and cooling rate on the crystallization of CaO-SiO2-TiO2-10 wt% P2O5-8 wt% Nb2O5-5 wt% CeO2-5 wt% CaF2 slag system. In this study, the britholite was precipitated selectively as the Ce-enriched phase. When TiO2 was added at less than 12 wt%, the britholite was promoted to crystallize meanwhile the Ca2Nb2O7 was suppressed. However, CaTiSiO5 inhibited the growth of britholite when the TiO2 content exceeded 15 wt%. The non-isothermal crystallization kinetics had also been investigated for the TiO2 content and cooling rate varied from 0-18 wt% and 10-40°C/min, respectively. The continuous cooling transformation diagram and the relative crystallinity of the primary crystals were also constructed. Based on the observation and measurement of crystallization process, the modified Avrami model was applied to determine the crystallization mode of britholite with 9 wt% TiO2 addition. It was constant nucleation rate and one-dimensional growth with diffusion controlled. Considering the nucleation and growth of crystals, 20–30°C/min was preferred to be the reasonable parameter during cooling stage.

Elucidating the impact of chemical variability on the nano-structure of Fe-rich slags

Fe-rich silicate slags are promising resources in the production of alternative cementitious materials. In this investigation, molecular dynamics simulations have been employed to provide a comprehensive structural description of a range of binary FeO-SiO2 and ternary CaO-FeO-SiO2 glasses, which are the basis of the slag systems that are observed in the non-ferrous metallurgy. The impact of the chemical variations on the structure of the glasses is assessed. Fe atoms have a slightly more profound role as network modifiers, and Ca as charge compensators for lower SiO2 content. There is a preference of Fe atoms to be associated with free oxygens, as compared to Ca atoms, forming small scale clusters. The obtained results enrich our understanding of the relationship between the composition, the structure and the reactivity of such complex glasses.

Mechanisms of chloride transport in low carbon marine concrete: An alkali-activated slag system with high limestone powder

Mechanisms of chloride transport in low carbon marine concrete: An alkali-activated slag system with high limestone powder

Development of Molecular Dynamics and Research Progress in the Study of Slag.

Molecular dynamics is a method of studying microstructure and properties by calculating and simulating the movement and interaction of molecules. The molecular dynamics simulation method has become an important method for studying the structural and dynamic characteristics of slag systems and can make up for the shortcomings of existing detection methods and experiments. Firstly, this paper analyzes the development process and application fields of molecular dynamics, summarizes the general simulation steps and software algorithms of molecular dynamics simulation methods, and discusses the advantages and disadvantages of the algorithms and the common functions of the software. Secondly, the research status and application progress of molecular dynamics simulation methods in the study of phosphate, silicate, aluminate and aluminosilicate are introduced. On this basis, a method of combining molecular dynamics simulation with laboratory experiments is proposed, which will help obtain more accurate simulation results. This review provides theoretical guidance and a technical framework for the effective analysis of the microstructure of different slag systems via molecular dynamics, so as to finally meet the needs of iron and steel enterprises in producing high-quality steel grades.

A novel method for recovering valuable metals from spent lithium-ion batteries inspired by the mineral characteristics of natural spodumene

Based on the unique crystal structure characteristics of natural spodumene (LiAlSi2O6) with six coordination of Li at the M2 position and monoclinic crystal system, an innovative approach is developed for the simultaneous recovery of Co, Ni, and Cu, while selectively enriching Li into the slag. The slag system was designed and controlled according to thermodynamic simulations and calculations to provide the theoretical basis for subsequent experiments. The effects of the ratio of Al2O3 to SiO2, temperature, and ratio of raw material to oxidizer addition on metal recovery were systematically investigated. The results showed that Li, Co, Ni, and Cu recovery efficiencies could exceed 99%, while Li was concentrated in the slag with a high grade of Li2O (6.0–9.0 wt%) under the experimental conditions, which is about 1.0–1.6 times that of the commercially available spodumene concentrate (Li2O, 5.5 wt%). By sulfation roasting-leaching of this lithium-rich slag, a leaching efficiency of Li can be achieved to 93%. This study provides valuable insights into shortening the recycling process of spent lithium-ion batteries and increasing the comprehensive utilization rate of high-value metals.

Experiment and thermodynamic modelling of phase equilibria in PbO−“CuO0.5” and PbO−“CuO0.5”−“FeO1.5” slag systems with metal

The phase equilibria of the PbO−“CuO0.5” and PbO−“CuO0.5”−“FeO1.5” slag systems in equilibrium with solid metallic copper and/or liquid metallic lead−copper alloy were studied as part of the investigation of the 19-element PbO−ZnO−Cu2O−FeO−Fe2O3−CaO−SiO2−Al2O3−MgO−S−(As, Sn, Sb, Bi, Ag, Au, Ni, Cr, Co as minor elements) multicomponent slag/matte/metal/speiss system, supporting the operation and development of the existing and emerging pyrometallurgical processes. In the experimental portion of this study, samples underwent high temperature equilibration followed by quenching, after which the compositions in the liquid slag, solid oxide and metallic phases were directly measured by electron probe microanalysis (EPMA). Phase equilibria data on the massicot (PbO), spinel ((Fe, Cu)Fe2O4), cuprite (Cu2O), lead ferrite (Pb2+xFe2O5+x), magnetoplumbite (Pb1+xFe12−xO19−x), copper plumbite (Cu2PbO2), and delafossite (CuFeO2) primary phase fields were obtained between 688 and 1000 °C. A key finding of the experimental study of the PbO−“CuO0.5” subsystem was that the copper plumbite (Cu2PbO2) phase melted incongruently, contradicting previous studies that suggested it apparently decomposed below the binary eutectic temperature to form massicot (PbO) and cuprite (Cu2O). Based on the results of past and present experimental studies, a self-consistent set of thermodynamic model parameters describing all phases in the system was derived using the FactSage software package.

The Viscosity of Na2O-B2O3-CaO-SiO2-Al2O3 Slags

The molding powder is an important supplement material in the continuous casting process, which plays a decisive role in the condition of continuous casting and the surface quality of the slab. The viscosity of the Na2O-B2O3-CaO-SiO2-Al2O3 slag system was studied by using the internal cylinder rotation method, and the viscosity data within the range of experimental slag composition were analyzed. The result shows that the viscosity of slag decreases gradually with the increase of borax (Na2O·2B2O3) content, where the content of O2- in the slag increases and the polymerization degree of the network structure decreases in the slag. When the Al2O3 contents increase, the amount of free oxygen decreases in the slag, which increases the degree of polymerization of the slag structure and results in the viscosity increasing. The calculation result of the prediction equation for viscosity has a small deviation.

In-situ observation and crystallization kinetics during the solidification of Cr-containing stainless steel slags

For a sustainable recycling process of Cr-containing stainless steel slags, high chromium recovery through selective crystallization could be achieved when thermophysical properties are controlled based on the crystallization kinetics. The in-situ observation and crystallization kinetics of synthesized Cr-containing slags has been studied. The crystallization characteristics in the CaO–SiO2–Al2O3–MgO–Cr2O3 slag system were investigated by using CLSM (confocal laser scanning microscopy) and DSC (differential scanning calorimetry). Analysis of crystallization kinetics was divided into spinel phase and calcium silicate phase. The growth mechanism of primary spinel phase has been demonstrated through CSD (crystal size distribution) theory with varying temperatures and cooling rates. The crystallization kinetic analysis of calcium silicate phase was performed using the modified Avrami equation and the combined Avrami and Ozawa equation. The Friedman differential isoconversional method was applied for determining the apparent activation energy during non-isothermal crystallization. Observations of crystalline phase morphologies and the precipitated phase was identified by using SEM-EDS (scanning electron microscopy-energy dispersive spectroscopy) and XRD (X-ray diffraction) analyses. The change in the crystallization characteristics were compared with the slag melt structures analyzing by Raman spectroscopy.

The effect of temperature rise inhibitor on the hydration and strength development of slag/fly ash blended cement paste

This study compares the effects of Temperature Rise Inhibitor (TRI) on the exothermic process and development of compressive strength of slag/fly ash blended cement systems. Isothermal calorimetry results reveal that, even if there is an enhancement effect on cement hydration due to the filler effect, TRI can still effectively lower the hydration heat flow of composite cementitious materials during the early age. By adjusting the content of TRI and SCMs, these two opposite effects will reach a balance at a critical dosage. However, a distinct effect of TRI on compressive strength development can be observed in these two systems. In the slag system, the compressive strength of the TRI-blended sample can catch up with the reference within 28 days. While in the fly ash system, it takes longer to recover to the normal level. In addition to the difference in SCMs’ intrinsic hydration reactivity in alkaline cement paste, competitive adsorption of TRI between cement and SCMs particle and the change of pore structure further enhance the variation in later strength development.

Unveiling the effect of MnO/SiO2 ratios on the viscosity and structure of mold fluxes for high-Mn cryogenic steels

The serious reaction between Mn in molten steel and SiO2 in calcium silicate-based mold flux during continuous casting of Al-free high-Mn cryogenic steels could change the mold flux properties and in-mold performance. In this study, the combination of rotating viscometer, FTIR spectroscopy, Raman spectroscopy and O1s X-ray photoelectron spectroscopy (XPS) was employed to get a deep insight into the effect of the reaction levels characterized by MnO/SiO2 ratios on the viscosity and structure of mold fluxes. The results exhibit that the transformation of mold flux from Newtonian fluid to non-Newtonian fluid can be reflected by viscosity change with varying rotation speeds. The viscosity decreases continuously with increasing MnO/SiO2 ratio but the temperature dependence of viscosity decreases at high MnO/SiO2 ratio. Structural information from FTIR revealed the slag system is mainly a silicate structure. The deconvoluted results of Raman spectra show an increase in NBO/Si, amounts of Q0 ([SiO4]4–) and Q1 ([Si2O7]6–) units with increasing MnO/SiO2 ratio, whereas a decrease in amounts of Q2 ([SiO3]2–) and Q3 ([Si2O5]2–) units in the meantime, this indicates the depolymerization of the silicate structure with proceeding of the slag/steel reaction. According to semiquantitative analysis of XPS spectra, the relative proportion of bridge oxygen (O0) decreases at the cost of non-bridged oxygen (O–), which suggests that the silicate structure gradually depolymerizes. The structural analysis supports the change of viscous behavior of mold flux with slag/steel reaction.

Phase Diagram of CaO–Al2O3–V2O5 Slag System at 1500 °C and 1600 °C

Phase Diagram of CaO–Al2O3–V2O5 Slag System at 1500 °C and 1600 °C

Development of a Process to Recycle NdFeB Permanent Magnets Based on the CaO-Al2O3-Nd2O3 Slag System

Nd, Pr and Dy are critical raw materials as major components for rare earth permanent magnets (REPM). These are integral for several components placed for example within electric vehicles and wind turbine generators. REE primary production is mainly realized in China (~80%) and no REPM recycling industry has been established. Hydrometallurgical recycling routes lead to iron dissolution (66 wt. % Fe in REPM), while pyrometallurgical approaches that utilize SiO2 risk contaminating the produced iron phase. A two-step process is presented that (i) creates an FeOx-CaO-Al2O3-REE2O3 molten slag at 1500 °C through oxidative smelting and (ii) separates an iron-depleted slag phase (CaO-Al2O3-REE2O3) and a molten iron phase via carbothermic or metallothermic reduction at 1700–2000 °C. The slag has been designed as a selective collector phase and the REE2O3 loading within the bulk slag can reach up 25 wt. % REE2O3 at 1700 °C. The contained minerals within the slag exhibit >40 wt. % REE (a higher REE concentration than in the initial REPM). The resulting phases are characterized via ICP-OES, CS and SEM-EDX. In addition, the first results with regard to the downstream hydrometallurgical processing of the CaO-Al2O3-REE2O3 slag are presented aiming at the recovery of REE2O3, as well as of CaO and Al2O3. The latter compounds are to be reused during the first process step, i.e., the oxidative smelting of REPM. Slag leaching with methane sulfonic acid (MSA) and separation with alternative methods, such as solvent extraction, seems promising. Future work will include slag filtration with the aim to separate REE-rich solid phases (minerals) from the slag and also molten salt electrolysis of the produced REE2O3 oxides.

Modeling viscosity of SiO2-Al2O3-CaO based slags using Arrhenius and VFT models as well as the CALPHAD method

The first step of modeling the viscosity of slags is to choose a temperature-related viscosity model which plays a significant role in determining the framework of the entire model. In the present work, two temperature-related viscosity models, the Arrhenius and the VFT (Vogel-Fulcher-Tammann) models are compared for calculating the viscosity of slag systems taking the experimental data for the SiO2-Al2O3-CaO system as basis. The results show that the VFT model shows a better performance in describing experimental data for the temperature dependence of dynamic viscosity among all the investigated systems compared to the Arrhenius model. Such a good performance by the VFT model is explained by the actual network structure of liquid oxide systems described well by this model. Moreover, a linear relationship between the two semi-empirical parameters of the VFT model (ln A and T0) is found for the first time, which can decrease the number of independent parameters when modeling the viscosity of slags by this model. In addition, the viscosities of the CaO-Al2O3 and SiO2-CaO systems are modelled by the combined VFT-CALPHAD (CALculation of PHAse Diagrams) model over wide temperature and composition ranges.

Influence of B2O3 on the thermophysical properties of molten blast furnace slag

The thermophysical characteristics of blast furnace molten slags have crucial implications for their smelting process and the subsequent utilization of resources. To efficiently recover molten slag waste heat resources, the thermodynamic properties and viscous flow characteristics of molten blast furnace slag systems with various B2O3 contents were investigated. The stability of the slag at high temperatures was evaluated by calculating its heat capacity and critical heat release. The structural evolution of the melt under varying B2O3 contents was comprehensively studied using Raman spectroscopy and Nuclear Magnetic Resonance. From the perspective of viscous flow characteristics, the impact of B2O3 resulted in a reduction in viscosity of the slag, while the electrical conductivity of the slag increased. Concerning the thermodynamic properties, the break temperature decreased from 1377 °C to 1280 °C with an increase in B2O3 content in the range of 0 to 6 wt.%, while heat capacity and critical heat release of the slag both increased. The introduction of B2O3 facilitated smoother the blast furnace smelting conditions while enhancing the superiority of the granulated product and the efficiency of heat recovery. Adding B2O3 to the molten slag disrupted part of chain structure (Si-O-Si) to create a more flowable cyclic borosilicate structure (Si-O-B).

Deportment of Metals from E-Waste PCBs towards Alloy and Slag Phases during Smelting Using CaO-Al2O3-SiO2-B2O3 Slags

Printed circuit boards (PCBs) from antiquated electronic goods were processed by a pyrometallurgical route to produce and separate alloy and slag phases. The process involved initial size reduction of PCBs, followed by pyrolysis at 500 °C for 6 h and finally smelting of the solid materials in an electric furnace. A preliminary smelting test was performed at 1600 °C to estimate the composition of the slag generated. In later kilogram-scale smelting experiments, B2O3 flux was added along with CaO and SiO2 to decrease the liquidus temperature required to smelt the PCBs. The level of fluxing was adapted from earlier phase equilibria studies of the CaO-Al2O3-SiO2-B2O3 slag system. Results showed that the flux decreased the melting temperature and assisted the recovery of most valuable metals within the alloy phase at 1350 °C smelting temperature. The alloy phase recovered 99.8% of Cu, 99% of Sn, and 100% of precious metals (Au, Ag, Pt). A fluxing strategy for smelting high Al2O3 containing e-waste PCBs was proposed based on the experimental findings of this research.