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ispartof: ISIJ INTERNATIONAL vol:62 issue:8 pages:1573-1585 status: published

ispartof: JOURNAL OF SUSTAINABLE METALLURGY vol:8 issue:1 pages:297-309 status: published

Freeze lining is a solidified layer of slag formed on the inner side of a water-cooled pyrometallurgical reactor, which protects the reactor walls from thermal, physical, and chemical attacks. Because of the freeze lining's high thermal resistance, the reactor heat losses strongly depend on the freeze lining thickness. In a batch process such as slag fuming, the conditions change with time, affecting the freeze lining thickness. Determining the freeze lining thickness is challenging as it cannot be measured directly. In this study, a conceptual framework based on the morphology and microstructure of freeze lining and the rheology of the slag is discussed and experimentally evaluated to determine the freeze lining thickness. It was found that the bath/freeze lining interface lies just below critical viscosity temperature. The growth of the freeze lining is primarily controlled by the mechanical and thermal degradation of the crystals forming at the interface. The bath/freeze lining interface temperature for the measured slag lies in the range of 1035–1070°C.

ispartof: ISIJ INTERNATIONAL vol:61 issue:9 pages:2400-2409 status: published

We demonstrate the opportunities of first-principles density functional theory (DFT) calculations for the development of new metallurgical refining processes. As such, a methodology based on DFT calculations is developed to discover new pyrometallurgical refining processes that use the addition of a third element to remove an impurity from a molten host material. As a case study, this methodology is applied to the refining of lead. The proposed method predicts the existing refining routes as well as alternative processes. The most interesting candidate for the removal of arsenic from lead is experimentally verified, which confirms the suitability of the remover element. The method is therefore considered as a useful approach to speed up the discovery of new pyrometallurgical refining processes, as it provides an ordered set of interesting candidate remover elements.

The mineralogy of a particular slag can be modified by steering the slag cooling process to valorise a particular slag in high added value applications. However, without online monitoring of the crystallization degree and temperature, it is challenging to control slag crystallization precisely. Since the electrical conductivity is sensitive to a minor change in the slag microstructure, electrical conductivity may be used to monitor the change in the composition of the liquid phase and the precipitation during slag solidification. In this work, an innovative experimental setup was developed using a confocal scanning laser microscope (CSLM) to measure the electrical conductivity of the slag while simultaneously observing slag solidification. Two types of high-temperature electrical conductivity cells are designed to determine the electrical conductivity of the slag. Each method’s advantages and disadvantages are identified, and their potential applications are recommended. The techniques have been confirmed to be accurate and reliable for the electrical conductivity measurement of slag during the in situ observation of slag solidification, providing a powerful tool for slag valorization.

ispartof: ISIJ INTERNATIONAL vol:61 issue:5 pages:1357-1362 status: published

Valorization of slag from metallurgical processes becomes ever more important as a measure to reduce the environmental impact of metal production operations. To this end, the slag composition has to be changed so that the expected technical and environmental requirements of potential end-users can be met. This paper provides a thermodynamic analysis of the copper flash smelting process which was done based on calculation with the FactSage thermodynamic software package. The purpose was to understand the effect of operational parameters on slag chemistry and elemental partitioning in the process. The results of the thermodynamic analysis are used to understand the content changes of impurities considered as harmful to the environment in the slag. Laws and standards regulating the influence of pollutants were analyzed to understand the impact of content changes on application of slag in different markets. A further important aspect is to increase Cu recovery to the matte as recovering more copper is the major economic driver. The study confirms that an increase in the matte grade and temperature lead to higher partitioning of Pb, Zn, and As to the slag. With increasing matte grade, the volatilization of these elements decreases, and with higher operating temperature solubility of Cu and S in slag will increase. The increase of the Fe/SiO2 ratio maximizes the distribution coefficient (wt.% in matte/wt.% in slag ratio) of Pb and reduces the distribution coefficient of As to the slag. Using the current evaluation, the sensitivity of slag quality and metal recovery on operating parameters and slag marketability is estimated. By example of two fictitious smelters it is shown how conditions in existing smelters can be modified to improve the slag marketability and Cu recovery. It is demonstrated that a suitable compromise between slag quality for improved saleability, metal recovery, and optimal operation of the metallurgical process needs to be identified and implemented.

The oxidation balance is arguably one of the most important parameters during pyrometallurgical processes. In many cases, iron is the most abundant multivalent element present in silicate slags. Within these molten oxide mixtures, the oxygen balance is dominated by the relative fractions of Fe3+ and Fe2+ present. Measuring the concentration of ferrous and ferric iron in quenched slags can provide useful information about the oxidation state of the liquid slag at the moment of freezing. These measurements are not always straightforward, especially in lead-rich slags, as present in modern secondary smelting processes. Building on previous work, this article aims towards a practical and reliable methodology for determining the Fe3+ concentration in lead-rich silicate slags using continuous wave electron paramagnetic resonance (CW-EPR). The method is demonstrated for different Fe3+/ $$\Sigma $$ Fe ratios and a basic model is proposed that relates the slag composition to the EPR spectrum, optimized for a high-lead ternary PbO–SiO2–Fe2O3 slag.

The valorization of Basic Oxygen Furnace (BOF) slag is important as the global annual production of this slag amounts to about 200 million tonnes. Al2O3 can be added to modify the composition and mineralogy of the slag. Al2O3 dissolution can provide long term volumetric stability to the solidified slag and tailor its microstructure/mineralogy for high-added value non-binder applications e.g., aggregates. To ensure the effectiveness of this modification, a fast dissolution of the additives is required. In this study, the dissolution of alumina particles in molten BOF slag between 1500 °C and 1600 °C was experimentally investigated. The alumina particles dissolve indirectly in the BOF slag. A continuous layer of CaAl4O7 (CA2) and CaAl2O4 (CA) solids are formed at the alumina/slag interface. Its thickness remains between 3 and 7 µm. The dissolution process starts rapidly wherein the initial particle diameter is reduced from 1092 µm to almost half within 60 seconds after particle addition. The dissolution rate however then drops rapidly due to the formation of an alumina enriched zone around the dissolving particle. The diffusion coefficient of Al2O3 in liquid slag varies from 1.5 × 10−3 mm2/s at 1600 °C to 9.8 × 10−5 mm2/s at 1500 °C. The activation energy of the Al2O3 dissolution process is determined to be 751 ± 54 kJ mol−1 under the present experimental conditions. Clustering of dissolving particles is observed between 1550 °C and 1500 °C, which has an adverse effect on the dissolution rate.

This paper reports on the experimental investigation of the phase relations in the CaO-SiO2-Dy2O3 system. CaO-SiO2-Dy2O3 slags were equilibrated at 1773 and 1873 K for 86400 s in Ar and then quenched in water to determine the phase relations of the system. The composition of the equilibrated phases was measured by EPMA-WDS and XRD. The presence of ternary compounds and the solid solution and liquid regions were determined to construct the isothermal sections at 1773 and 1873 K of the ternary phase diagram. The data from this work will support further investigations on the feasibility to recover REEs through pyrometallurgical processing.

The mixing behavior of compositional modifiers into viscous liquid BOF slag was studied using fluid flow modeling under industrially relevant conditions. The Volume of Fluid-Large Eddy Simulation-Discrete Phase Model was used to simulate the gas-solid particle-liquid slag turbulent flow. The area fraction of the particle migration zone was calculated to quantify the mixing process. The results show that increasing gas flow rate and/or depth of lance submergence can shrink the quiescent region at the bottom of the slag pot, shortening the mixing process. Influence of the lance submergence depth is more significant on the particle mixing process. The input energy flux can be used as a measure to quantitatively evaluate the mixing behavior. Additionally, it was confirmed that increasing the slag viscosity prolongs the mixing process. Particle size in the range that was studied has little influence on the particle migration.

The pursuit of low-CO2 technologies has led to a surge in research on alternative cementitious materials, of which alkali-activated materials are a large family. In recent years alkali-activated materials have expanded to encompass Fe-rich precursors in addition to the more commonly employed aluminosilicate precursors. The formation mechanism of alkali-activated materials from two Fe-rich synthetic slags has been assessed by employing in-situ X-ray total scattering and subsequent pair distribution function analysis. The evolution of the local atom-atom correlations reveals three reaction stages. After the dissolution of Fe-silicate clusters from the slag, a binder phase is formed with Fe in both Fe2+ and Fe3+ oxidation states. The Fe2+ state is present in the form of trioctahedral layers, similar to those in Fe(OH)2, while the Fe3+ is likely located in the polymerized silicate network. Exposure to air causes the Fe2+ species to transition to the Fe3+ state.

In order to better understand the metallic droplet behavior during a slag treatment process, a physical modeling based on the similarity principle was performed in a transparent scaled-down vessel at room temperature. Paraffin oil, 20 wt pct copper sulfate solution, and compressed air were used to simulate the molten slag, metallic droplet, and carrier gas, respectively. The droplets injected into paraffin oil during the experiment were captured by a high speed camera and were analyzed by Image Pro Plus software to obtain the droplet size distribution. The critical droplet size in the physical modeling and slag treatment process is quantitatively correlated. The results show that droplet breakage phenomenon is dominant over its coalescence in the current industrial practice, and droplet breakage is enhanced with increasing gas flow rate and/or lance depth. No significant effect of the nozzle configuration was found on the droplet breakage and coalescence. The droplet size distribution varies with the lance position. Gas flow rate and lance depth are the most important factors for droplet breakage, the extent of which can be reduced through a proper selection of the operational conditions. A linear relationship between the droplet size and the input energy flux is obtained.

Basic oxygen furnace (BOF) steel slag is a major waste of the steelmaking industry. Utilization of BOF slag contributes to the sustainability of the steel industry by alleviating its environmental impact. Vitrification during cooling is an effective method to promote the cementitious activity of slags with the aim to apply slags in high value-added applications. In the present study, an Al2O3- and SiO2-modified BOF slag was water granulated at a pilot scale. The amorphous and mineral fractions were measured quantitatively. The critical cooling rate to vitrify the modified slag was calculated through the Time-Temperature-Transformation diagrams constructed using in situ confocal scanning laser microscopy. To provide an insight into the crystallization behavior during the granulation process, a mathematical model was developed. The model was validated by comparing the amorphous fraction obtained from experiments with that from simulation. Temperature profiles of the slag particles with varied sizes were calculated with the aid of COMSOL Multiphysics software. The effect of particle size on the vitrified fraction was discussed in detail and the temperature gradient from surface to center of the particle was identified. The results provide novel fundamental understanding of the vitrification process regarding to the slag valorization, which will help the industrial implementation of granulation for pollution remediation and other engineered environmental systems such as chemical industry and pharmaceutical industry. ispartof: Metallurgical and Materials Transactions B - Process Metallurgy and Materials Processing Science vol:50 issue:3 pages:1260-1268 status: published

ispartof: ISIJ INTERNATIONAL vol:61 issue:5 pages:1337-1347 status: published

In entrained flow gasifiers, the production of oxidic slag accompanies the gasification process. This slag forms a layer on the refractory walls, flows downwards gravitationally, and is collected in a water quench. Hence, the slag flow must be constant, since a slag blockage represents a worst-case-scenario. Crystallization of the slag increases slag viscosity, subsequently leading to a possible slag blockage. Therefore, crystallization processes in oxidic slags need to be understood and hence investigated. In this study, three artificial, coal ash related oxidic slag systems were analyzed on their crystallization behavior. Therefore, their melt behavior was investigated via hot-stage microscopy and differential thermal analysis (DTA). Additional thermochemical calculations were performed to predict crystallized phases. Subsequently, quenching experiments were conducted to generate supercooled crystallization in the slag samples. These samples were analyzed afterward via X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the morphologies of crystals were characterized/described. In-situ observations on crystallization growth were performed by using a confocal laser scanning microscope (CLSM). Finally, crystallized phases were compared with results obtained from thermochemical calculations, and the impact of kinetics on the distributed phases was discussed. The knowledge on the crystallization behavior of various phases can be transferred to other slag systems and can improve general crystallization predictions made by thermochemical calculations.

Crystallization tendency and the state of oxidic slags are important influencing factors for slag viscosity and therefore slag tapping. During the gasification process slag is constantly produced and flows down the gasifier walls until it is intercepted and stored for further treatment or a possible valorisation. The kinds of fuel used for gasification lead to different kinds of slag composition likewise. Slag viscosity is strongly influenced by the composition of the slag and the temperature. However, the process of crystallization represents another major influencing factor. Slag crystallization is also affected by several parameters, whose effects need to be considered in experimental studies. This study investigated the impact of the sample state, the chosen crucible material, and the oxygen partial pressure on the crystallization of a Fe-rich oxidic slag in the confocal laser scanning microscope (CLSM) experiment. Using sample powder may lead to heterogeneous nucleation and crystallization during heating due to heterogeneous melting of the compounds. The selection of crucible material is crucial to prevent alloy formation of Fe-species with Pt-crucibles under reducing atmosphere. Finally, the partial pressure of oxygen influences the crystallization tendency as well as the morphology of the crystallized phases. These parameters impede the reproducibility of results as well as the comparison with results deriving from other experiments like sample quenching. This contribution provides an enhanced understanding of the stated parameter effects on the experimental investigation of oxidic slags using the CLSM setup and yields advice to prevent experimental influence on the results.

Submerged plasma slag fuming technology is a process intensification of the traditional (coal-fired) slag fuming technology. A general dynamic model of the plasma-driven slag fuming process has been developed in FactSage 7.0 based on Metallo Belgium’s industrial scale submerged plasma slag fuming furnace. This model describes the chemical reactions, phase equilibria and heat transfer in each sub-unit operation, and the heat and mass transfer between the subsequent sub-unit operations of the slag fuming process. The model was validated on several process batches. Upon assessing the simulation results, a deviation was found from the industrial process, which can be explained using kinetic factors.

In the present work, the formation and growth of Al-Ti-O complex inclusions in the steel after SiMn and Ti deoxidation is systematically investigated. The evolution with time of the size, composition and morphology of the Al-Ti-O complex inclusions were studied. After adding 70TiFe alloy, the inclusion composition changed from Si-Mn-Al to Ti-Al-O-N. The growth of the inclusions after a deoxidation time of 30 s is mainly controlled by turbulent collisions. The three-dimensional morphology and shape factor of the inclusions is determined to research the growth pattern of Al-Ti inclusions.

The presence of arsenic (As) in crude antimony trioxide (Sb2O3) impedes the Sb recovery and results in an environmental issue. Selective As removal from crude antimony trioxide would therefore enable using this residue in commercial Sb2O3 production. This work uses a nitric acid leaching method to remove As from crude antimony trioxide. Arsenic oxide and antimony oxide are respectively converted to soluble H3AsO4 and insoluble antimony(III) oxide hydroxide nitrate Sb(III)OHN during HNO3 leaching. The As content in the crude antimony trioxide decreases from 3.0 wt% to 0.4 wt%. The As removal ratio reaches 86%, while the Sb leaching ratio is limited to 0.3%, confirming the high selectivity of HNO3 leaching. The leaching residue is composed of cervantite (Sb2O4) and Sb(III)OHN. By calcining the leaching residue in the air at 200 °C, it can be converted to a mixture of cervantite and valentinite (Sb2O3). Finally, cervantite is decomposed above 1070 °C in N2 into Sb4O6 that can be condensed as Sb2O3. A novel flowsheet is proposed for both As removal and Sb recovery from the crude antimony trioxide.

Basic oxygen furnace (BOF) steel slag is a main byproduct that is produced during the converter steelmaking process. The volume instability and fast crystallization of BOF slag limits its added-value application. This article aims to understand the effect of Al2O3 on the mineralogical modification and crystallization kinetics of a high basicity BOF steel slag. Continuous cooling transformation and time–temperature–transformation curves were constructed to determine the crystallization characteristics of BOF slag. The critical cooling rate to vitrify the slag was experimentally obtained. The crystallization sequence was clarified by integrating in situ and post-mortem observations with thermodynamic calculations. The results suggest that the addition of Al2O3 can effectively remove free lime, decrease the melting point, and improve the glass formation ability of the high basicity BOF slag. Undercooling the slag is enhanced by increasing the cooling rate and/or adding Al2O3. By steering the addition of Al2O3 and the cooling rate, BOF slag can be modified to obtain a more amorphous phase, presenting an enhanced potential to be a binder for added-value applications.

Valorization of basic oxygen furnace (BOF) slag is of significant importance for mitigation of the steel production’s environmental impact. The present work aims to investigate the influence of SiO₂ addition and oxygen partial pressure on the mineralogical modification of a typical industrial BOF slag. The slag basicity (mass ratio of CaO/SiO₂) was varied from 1.8 to 4.4 by mixing specific amounts of SiO₂ with the master BOF slag. The original and modified slags were remelted and solidified under argon or air atmosphere followed by slow cooling. The experimental observations were then compared with the results of thermodynamic modeling to achieve a thorough understanding. With decreasing the basicity, free lime was eliminated, as it forms dicalcium silicate (Ca₂SiO₄). With increasing oxygen partial pressure, wustite was oxidized to hematite, which combined with free lime to form calcium aluminoferrite (C₂AF). The effects of SiO₂ addition and oxygen partial pressure were finally evaluated with respect to the energy consumption for the BOF slag valorization. The modified slag is suitable as a precursor for construction applications as binders. ispartof: Metallurgical and Materials Transactions B - Process Metallurgy and Materials Processing Science vol:50 issue:1 pages:210-218 status: published