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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.

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.

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 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 focusses on three main numerical methods, i.e., the Reynolds-Averaged Navier-Stokes (RANS), Large Eddy Simulation (LES), and Direct Numerical Simulation (DNS) methods. The formulation and variation of different RANS methods are evaluated. The advantage and disadvantage of RANS models to characterize turbulent flows are discussed. The progress of LES with different subgrid scale models is presented. Special attention is paid to the inflow boundary condition for LES modelling. Application and limitation of the DNS model are described. Different experimental techniques for model validation are given. The consistency between physical experimentation/modelling and industrial cases is discussed. An emphasis is placed on the model validation through physical experimentation. Subsequently, the application of a turbulence model for three specific flow problems commonly encountered in metallurgical process, i.e., bubble-induced turbulence, supersonic jet transport, and electromagnetic suppression of turbulence, is discussed. Some future perspectives for the simulation of turbulent flow are formulated.

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

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.

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

To assess the viability of an alumina-chromia refractory alternative for secondary copper smelters, the degradation behavior of alumina-chromia refractories in a secondary copper smelting slag and a Cu-CuxO mixture was studied via lab scale refractory finger testing. Microstructural characterization of the worn refractory samples resulted in a comprehensive understanding of the corrosion and penetration resistance and a comparison with the degradation of magnesia-chromite refractories. Alumina grains in the refractory are preferentially corroded by the slag. The alumina-chromia refractory exhibits superior infiltration resistance to the slag and the Cu-CuxO mixture compared to magnesia-chromite refractories.

Due to the scarcity of Sb reserves and the challenging recovery of Sb from waste plastics, it is of great significance to recover Sb from metallurgical residues. A comprehensive characterization of Sb-containing metallurgical residues is essential to develop efficient and clean technologies to recover Sb and other associated metals. This paper reports a complete characterization of three typical Sb-containing metallurgical residues, namely crude antimony trioxide, lead softening slag, and antimony slag. The particle size distribution, chemical composition, morphology, mineralogical composition, and thermal transitions of these residues were determined. The phase equilibria in the antimony slag at 1000 °C were experimentally studied to figure out the Sb distribution within different phases. Feasible Sb recovery technologies were proposed and critically discussed based on the characterization results.

Recycling of Nd-Fe-B magnets is one of the few solutions to alleviate the supply risks of certain rare earth elements (REE) such as Nd and Dy. One of the most promising solutions with regards to extraction of end-of-life (EOL) magnets is to apply hydrogen decrepitation and to physically separate the Nd-Fe-B as a demagnetized hydrogenated powder. Such a powder can be directly recycled into new sintered magnets, or indirectly recycled by a conventional oxidation roasting and acid leaching flowsheet. The oxidation stability of powders, the microstructure after oxidation and the role of compositional variations can be of importance for the success of such subsequent direct or indirect recycling routes. In this study, magnets with varying compositions were subjected to different hydrogen treatments (e.g. hydrogenation, partial or full degassing and vacuum or conventional disproportionation) and subsequently studied for their comparative oxidation behavior and microstructural changes. While initial magnet composition was found to have little or no effect on oxidation behavior, the treatment type was found to have a direct impact on both the particle size distribution and the oxidation mechanism. The general oxidation rate was found to be the equally highest for hydrogenated and partially degassed samples followed by fully degassed and then disproportionated samples. After complete oxidation, complex REE-Fe-O compounds were formed more extensively in hydrogenated and fully degassed NdFeB samples than in the disproportionated samples.

This study investigates the origin of mechanically entrained metal droplets in liquid slag due to their interaction with solid spinel particles. Two possible mechanisms were proposed previously: separately formed droplets and spinel particles get attached to each other due to agitation of the slag and metal phases; or the spinel particles form by a chemical reaction together with a new droplet or alongside a droplet that was already present in the system. In this study, an inert tracer element was added to the metallic phase in adapted sessile drop experiments. For this purpose, Cu-Ag alloys, with various Ag-contents, were produced. The results showed that the small entrained metal droplets within the slag droplet contained Ag, but in very low amounts with respect to the amount of Ag in the Cu-Ag alloy. This indicates that the entrained metal droplets are formed due to a sequential combination of the two origins: first, very small metal droplets are dispersed in the slag drop, due to the emulsification process. Then, these metal droplets are nucleation sites for the Cu-spinel reactive formation.

Both primary and secondary copper productions encounter a limitation in the process efficiency due to droplet losses in slags. One of the causes for the mechanical entrainment of these droplets is their interaction with solid spinel particles, hindering the sedimentation of the copper droplets. Previous experiments with synthetic slags provided insights into this interaction and yielded two possible mechanisms: separately formed droplets and particles become attached to one another due to fierce agitation of the slag and metal phases; or the spinel particles and metal droplets form as the result of a chemical reaction together with a new droplet or alongside a droplet that was already present in the system. This experimental study aims to investigate the hypothesis of the formation of copper droplets sticking to spinel particles due to a chemical reaction further. A slag that is initially free from Cu droplets was prepared. After creating controlled conditions to induce the chemical reaction, the formation of sticking droplets was observed. The results from this experiment therefore confirm the previously proposed reactive mechanism. ispartof: Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science vol:48 issue:5 pages:2459-2468 status: published

The slag samples taken from landfill, which originated from different metallurgical processes, have been characterized in this study. The slags were categorized as electric arc furnace (EAF) slag, argon oxygen decarburization/metal refining process slag and vacuum oxygen decarburization slag based on chromium content and basicity. EAF slags have higher potential in metal recovery than the other two slags due to its higher iron and chromium contents. The size of the iron-chromium-nickel alloy particles varies from a few μm up to several cm. The recoveries of large metal particles and metal-spinel aggregates have potential to make the metal recovery from landfilled slags economically viable.

BOF steel slag is a main by-product in steelmaking. The valorization of BOF slag is good for the industry and the environment. In the present study, the carbo-thermic reduction of BOF slag has been investigated, the reduction of Fe and P containing phases (i. e. oxide and compounds) is discussed, and the effects of Al2O3 additions on the reduction process are described. XRD and EPMA have been applied for characterizing the slag. We conclude that, by controlling the test conditions, it is possible to purify the recovered Fe.

Basic oxygen furnace (BOF) steel slag is a main byproduct in steelmaking, and its valorization is therefore of considerable interest, from a metal-recovery perspective and from a residue-utilization perspective. In the present study, the carbothermic reduction of BOF slag was investigated systematically. The reductions of Fe- and P-containing phases (i.e., oxide and compounds) are discussed. Effects of Al2O3 and SiO2 additions on the solidification microstructure and mineralogy associated with the reduction processes were also investigated. The formation and growth of the extracted metallic phase are discussed, and the mineralogy of the residue slag is determined. We conclude that by controlling the additions under a rapid cooling condition, it is possible to extract metallic iron as high-grade metal and simultaneously to utilize the remaining slag for construction applications.

NdFeB magnets currently dominate the magnet market. Supply risks of certain rare earth elements (REE), e.g. Nd and Dy, demand efficient recycling options that are applicable to different types and compositions with minimum use of chemicals and energy and with minimum waste generation. In this study, a combined pyro- and hydrometallurgical method is presented that is adjustable to all NdFeB magnets regardless of their composition. After completely transforming powdered samples into a metal nitrate mixture at room temperature for 1 h, a low-temperature (e.g. 200 °C) calcination and water leaching treatment resulted in 95–100% extraction efficiencies for Nd, Dy, Pr and Gd. The major impurity Fe (63.9 wt.%) almost completely remained in the resultant residue (e.g. b1% extraction) forming a mixture of hematite and goethite as a useful by-product for related iron industries. Al and Co, representing minor impurities with individually b2 wt.%, also mostly remained in the residue (i.e. b40% extraction), thereby enabling the production of a liquid with very high REE purity. Such a solution then can be directly treated with subsequent shortened downstream processes without pre-treatments required for impurity removal. Due to decomposition reactions of impurities (including Fe) during the low-temperature calcination stage (e.g. 200 °C), the majority of consumed acid (i.e. the only chemical used) is recyclable resulting in a potentially environmentally-friendly and cost-effective flowsheet. ispartof: Hydrometallurgy vol:167 pages:115-123 status: published

© 2019, The Minerals, Metals & Materials Society and ASM International. The influence of the Al 2 O 3 level in CaO-SiO 2 -MgO-Al 2 O 3 (CSMA) stainless steel refining slags on the degradation of magnesia-doloma refractories was investigated through static refractory finger corrosion tests. The tests were performed at 1620 °C under Ar atmosphere, using slags with Al 2 O 3 contents of 5, 10, and 20 wt pct, respectively. The results indicate that the formation of 2CaO·SiO 2 at the slag/refractory interface was suppressed by increasing the Al 2 O 3 content to 20 wt pct, thereby changing the corrosion mechanism from an indirect dissolution to a direct dissolution of CaO and MgO from the refractories. The increased solubility limit of MgO by the Al 2 O 3 -rich CSMA slags results in an overall higher corrosion rate of the MgO-doloma refractory. ispartof: METALLURGICAL AND MATERIALS TRANSACTIONS B-PROCESS METALLURGY AND MATERIALS PROCESSING SCIENCE vol:50 issue:4 pages:1822-1829 status: published

© 2019 Iron and Steel Institute of Japan. All Rights Reserved. The influence of temperature, slag basicity and Cr 2 O 3 content on the viscosity of stainless steelmaking slags has been experimentally determined. The viscosity is mostly affected by the combination of basicity and Cr 2 O 3 content. The results are discussed by considering the Iida model for the fully liquid slag and the Herschel-Bulkley equation for the solid-bearing slag. For the liquid slag, the predicted viscosity values by the original Iida model do not agree very well with the measured ones, because the model does not include the amphoteric behavior of Cr 2 O 3 and underestimates the basic character of Al 2 O 3 . With the presently optimized parameters in the Iida model, the calculation is consistent with the measured viscosity values of the fully liquid slag with an error of 30%, which is fairly acceptable. At high basicity and high Cr 2 O 3 contents, the presence of solid particles in slag has a paramount influence on the viscosity. It was also found that the solid-bearing slags exhibit evident Bingham pseudoplastic behavior with a residual torque of 0.12 mNm and 0.99 mNm in the investigated samples. ispartof: ISIJ INTERNATIONAL vol:59 issue:3 pages:404-411 status: published

In both primary and secondary copper production, copper losses in slags are a decisive factor confining the process efficiency. An important cause for mechanical entrainment of metal droplets in slags is their attachment to solid spinel particles present in the slag phase, hindering sedimentation. To further optimize the production process, it is important to gain insights in the fundamental mechanisms governing this attachment. In the present study, the influence of the spinel composition on the attachment of copper droplets is investigated. First, the attachment is studied in an industrially relevant synthetic PbO–CaO–SiO2–Cu2O–Al2O3–FeO–ZnO slag system. Second, the wetting of copper on two spinel substrates (ZnFe2O4 and MgAl2O4) has been studied in the absence of a slag system, by sessile-drop experiments. Based on the results of both types of experiments, a clear influence of the spinel composition on the sticking behavior of copper droplets is noted. These observations might be transferred to industrial processes to adapt processing parameters to diminish copper losses in industrial slags.

Bauxite residue (red mud) with a high alumina content requires large amounts of fluxes during smelting, which increases the overall process cost, the energy consumption during smelting, and the acid consumption during slag leaching and decreases the throughput of bauxite residue in the smelting phase. Therefore, alumina was removed (or recovered) from bauxite residue by alkali roasting prior to smelting. The sample after alumina removal could be smelted at 1500 °C without the need of adding any flux to obtain a clear slag–metal separation. After smelting, the slag was leached with acids (HCl, H2SO4, or HNO3). The recovery yields of the rare-earth elements (REEs), except scandium, drastically decreased in the low-alumina slags, compared to the original bauxite residue. These poorer recovery yields are due to the formation of a perovskite (CaTiO3) phase, which binds REEs except scandium. CaTiO3 is a stable phase and does not dissolve in acids under normal conditions. Therefore, slag was quenched to suppress the CaTiO3 formation. The REEs and titanium could also be dissolved in acidic solutions at 25 °C by quenching the molten slag. Removal of alumina from bauxite residue by alkali roasting with sodium carbonate requires high temperatures (>900 °C). Therefore, sodium carbonate was replaced with sodium hydroxide to decrease the energy consumption by decreasing the roasting temperature (

The effect of surfactant Te on the behavior of alumina inclusions at advancing solid-liquid interfaces of liquid steel was studied by adding Te to liquid steel before Al deoxidation at 1873 K. After water-quenching, the spatial distribution homogeneity of alumina inclusions in the steel matrix was characterized using the Dirichlet tessellation method. The deterioration of this homogeneity with increasing the addition of Te indicates that Te facilitates pushing of alumina inclusions. This phenomenon was discussed based on the thermodynamics of an asymmetric thin liquid film confined by an advancing solid-liquid interface and a particle. The surface excesses of Te at the solid-liquid and particle-liquid interfaces were theoretically demonstrated to decrease when an alumina inclusion moves towards the solid-liquid interface, thereby weakening the effect of Te on the solid-liquid and particle-liquid interfacial energies. Based on this, effect of surfactants was incorporated in the models predicting the critical velocity VC.

An important cause for the mechanical entrainment of copper droplets in slags during primary and secondary copper production is their interaction with solid spinel particles, hindering the sedimentation of the copper droplets. In the present study, the interactions between the three phases involved (slag–Cu droplets–spinel solids) were investigated using an adapted sessile drop experiment, combined with detailed microstructural investigation of the interaction zone. An industrially relevant synthetic PbO-CaO-SiO2-Cu2O-Al2O3-FeO-ZnO slag system, a MgAl2O4 spinel particle, and pure copper were examined with electron microscopy after their brief interaction at 1523 K (1250 °C). Based on the experimental results, a mechanism depending on the interlinked dissolved Cu and oxygen contents within the slag is proposed to describe the origin of the phenomenon of sticking Cu alloy droplets. In addition, the oxygen potential gradient across the phases (i.e., liquid Cu, slag, and spinel) appears to affect the Cu entrainment, as deduced from a microstructural analysis.

The present study reports on the mineralogy of BOF slag after melting and solidification under Ar and air atmosphere. Results indicate that free lime can be effectively stabilized in air without additional stabilizers like SiO2. The stabilization mechanism was analyzed, and the role of the oxygen partial pressure was explored. Wustite is believed to be oxidized to hematite under high oxygen partial pressure and then stabilizes the free lime by forming brownmillerite.

The long-term storage of bauxite residue (red mud) is harmful to the environment and the tailing ponds also cover large areas. At the moment there is no large-scale utilization of bauxite residue. However, some bauxite residues contain considerable concentrations of rare-earth elements (REEs) and the recovery of these REEs together with(out) other metals and utilization of the generated residue in other applications (e.g., building materials) can solve the storage problem of bauxite residue. This paper reviews the recovery of REEs, possibly alongside other valuable metals, from bauxite residue. REEs can be recovered from bauxite residue by direct leaching or by smelting followed by leaching. The main disadvantages of direct acid leaching are the consumption of large amounts of acid for neutralization, the handling of large volumes of effluents, and the difficulty in using the bauxite residue after leaching. Recovery of iron prior to leaching can improve the economics of the process. However, high alumina in the bauxite residue increases the flux and acid consumption. Therefore, alumina needs to be removed by alkali roasting prior to smelting in order to decrease the flux and acid consumption. The alkali roasting–smelting–leaching process allows recovery of aluminum, iron, titanium, and REEs from bauxite residue. The residue generated in this process can be used in building materials and cementitious binders. Other processes with commercial potential are the Orbite, the pressure leaching, and the acid baking processes.

A new process is suggested in the present work turning bauxite residue (BR) into a ready-made mortar for the synthesis of inorganic polymers, effectively transforming the Bayer process into a zero-waste process. This was achieved by firing BR at 1100 C which supports the formation of liquid phase and results after subsequent fast cooling in a semivitreous material. Based on thermodynamic calculations, the process was subsequently improved by adding minor quantities of C and silica to BR before firing which leads to the carbothermic reduction of ferric iron into ferrous iron; the new blends demonstrated an increase in the melt formation and eventually in the content of amorphous phase after solidification. The resulting material was activated using a K-silicate solution and was cured at 60 C for 72 h. The synthesized materials were water insoluble and dense, demonstrating compressive strength exceeding 40 MPa for the silica-rich blend. These inorganic polymers can find applications in construction, such as pavement tiles or floor/roofing tiles. A possible implementation of the process to transform BR within the alumina refinery is also represented. ispartof: Journal of Sustainable Metallurgy vol:2 issue:4 pages:394-404 status: published

Used direct-bonded magnesia–chromite refractory bricks from a secondary Cu smelter were collected to characterize the degradation occurring during application. Based on this post-mortem analysis, lab scale experiments were designed using the direct-bonded refractory type and a fused grain magnesia–chromite refractory. Firstly, the infiltration behavior of a Cu–CuxO and a Cu–CuxO–PbO mixture was investigated. Secondly, the influence of temperature and the prior infiltration of a Cu–CuxO mixture on the infiltration of the ZnO containing fayalite slag was determined. The combination of the post-mortem study and the lab scale tests allowed a more comprehensive understanding of the evolution of the refractory microstructure during degradation. Likewise, it allowed to evaluate how certain parameters, such as temperature and refractory type, affect the infiltration and degradation behavior. As a consequence, conclusions can be drawn about measures to minimize refractory wear in copper smelter linings.

The corrosion behavior of MgO in iron-saturated ZnO-rich fayalite (ZFS) slags having various FeO/SiO2 ratio and CaO/SiO2 ratio was investigated using MgO crucible tests for 12 h at 1200°C. The FeO/SiO2 and CaO/SiO2 ratios in the ZFS slags were varied from 1.0 to 2.2, and from 0.04 to 0.32, respectively. In all of the tests, it was observed that MgO dissolves into ZFS slags and that (Zn,Fe,Mg)2SiO4 olivine and (Zn,Fe,Mg)O solid solution are formed at the crucible/slag interface. The MgO dissolution decreased with the FeO/SiO2 ratio up to a value of 1.7 and then slightly increased, whereas it continuously increased with the CaO/SiO2 ratio. There is no obvious relationship between the amount of olivine and the FeO/SiO2 ratio or CaO/SiO2 ratio. In comparison, the formation of (Zn,Fe,Mg)O solid solution is enhanced by increasing the FeO/SiO2 ratio or CaO/SiO2 ratio in ZFS slags. The results suggest that MgO corrosion is the lowest for FeO/SiO2 and CaO/SiO2 ratios around 1.7 and 0, respectively.

A ZnO-containing fayalite-based slag can be formed in copper smelting from secondary raw materials and its high viscosity is a common issue that hinders slag tapping. In this work, the crystallization behavior of the industrial slag was observed in situ by confocal laser scanning microscopy. Solid precipitation was found to be the major cause of the poor slag fluidity. The phase relations in the industrial slag system ZnO-“FeO”-SiO2-Al2O3-CaO (CaO/SiO2 = 0.05, Al2O3/SiO2 = 0.15) were investigated by quenching the samples after reaching equilibrium at 1423 K (1150 °C) under iron saturation. The equilibrium composition of the phases was determined with electron probe micro-analysis. The effect of individual components, such as FeO, ZnO, and CaO on the phase equilibrium of the slag system has been quantitatively studied. The relation between the solid-phase fraction and the chemical composition of the slag has been revealed. Suggestions to modify the slag composition toward low viscosity are provided.

To determine the effect of ZnO in ZnO-containing fayalite (ZFS) slags on the degradation of magnesia-chromite refractories, the corrosion behaviour of a direct-bonded magnesia-chromite refractory was studied by rotating refractory finger tests at 1200 °C under reducing atmosphere. In addition, MgO crucible tests at these conditions were performed to investigate in particular the effect of ZnO on the slag/MgO interaction. The dissolution of MgO from periclase into the ZFS slags is not enhanced by increasing the ZnO content in the slags. (Fe,Mg,Zn)2SiO4 olivine and (Fe,Mg,Zn)O solid solution are the only two products in the slag/MgO interfacial reactions, even when the ZnO content in the ZFS slag reaches 16.7 wt%. The diffusion of ZnO into the chromite grains to form the (Fe,Mg,Zn)(Fe,Al,Cr)2O4 spinel was favoured with increasing the ZnO content in ZFS slags.

The corrosion behavior of alumina-silica (Al 2 O 3 –SiO 2 ) refractories by liquid carbon steel was studied. This was performed by a refractory crucible test in a vacuum induction furnace at 1520–1545 °C under reducing atmosphere. The results indicate that Mn, Si and a minor amount of Fe in the steel were oxidized at steel/refractory interface, combing with refractory components to form a MnO–SiO 2 –Al 2 O 3 –based slag. This slag was liquid at the testing temperature, and infiltrated along the open-pore network and the grain boundaries into the Al 2 O 3 –SiO 2 refractories. Both Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the refractories dissolved into the infiltrating slag. As a result, the refractories were structurally weakened and residual Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the hot face of the refractories were washed away into the liquid steel, affecting steel quality.

Bauxite residue (red mud) that is generated from karst bauxite ores is rich in rare-earth elements (REEs). The REEs can be recovered from bauxite residue by direct acid leaching but the extraction yields are generally low. The extraction yields can be increased by increasing the acid concentration but this will increase the dissolution of iron as well. Large amounts of iron in the leach solution create problems in the further recovery processes. Therefore, a combined sulfation–roasting–leaching process was developed to selectively leach the REEs while leaving iron undissolved in the residue. In this process bauxite residue was mixed with water and concentrated H2SO4 followed by drying, roasting and then leaching of the roasted product with water. Most of the oxides were converted to their respective sulfates during the sulfation process. During subsequent roasting, unstable sulfates (mainly iron(III) sulfate) decompose to their respective oxides. Rare-earth sulfates, on the other hand, are stable during roasting and dissolve during water leaching, leaving the iron oxides in the residue. The effect of the roasting temperature, roasting time and amount of acid on leaching of the different elements was studied. Decreasing the roasting temperature increased the dissolution of the REEs, but also that of iron and aluminum. Increasing the amount of acid led to higher REEs extraction. Acid to bauxite residue mass ratio beyond 0.75 at 650 °C increases the iron and aluminum dissolution due to increase in the iron(III) and aluminum sulfate amounts. The extraction of REEs slightly increased (

CaO-SiO2-Nd2O3 slags were equilibrated at 1773 K and 1873 K (1500 °C and 1600 °C) for 24 hours in Ar, and quenched in water to determine the operative phase relations. The composition and crystallinity of the phases in equilibrium were determined by EPMA-WDS and EBSD, respectively. Based on these analyses, the liquid stability region was accurately determined, and a large part of the isothermal section of the phase diagram was constructed. Data resulting from this work can be used to generate a thermodynamic database for rare-earth oxide-containing systems and to support further investigation on separation of rare earths from metallurgical slags or other residues through high-temperature processing.

Ladle metallurgy (LM) slag is generated during the refining process step in stainless steel making. In view of its very fine particle size, due to the transformation of β- to γ-C 2 S, and the Cr content, the slag is challenging in terms of handling and disposal and is used in only few applications. The current work explores the utilisation of the LM's fine fraction in Ordinary Portland Cement (OPC) clinker production. Three sets of samples containing LM slag, in 0 wt% (OPC – reference), 6 wt% (OPC6) and 14 wt% (OPC14), were investigated. The sintering temperature was 1450 °C with 40 min soaking time. The clinkers containing LM slag, showed a slight increase in C 3 S and a decrease in C 3 A and C 4 AF. The formation of the C 3 S initiated at a lower temperature probably due to the presence of F and C 2 S. Longer setting times were found for OPC6 and OPC14. The compressive strength results were found to be comparable at 2 d (22–25 MPa), 7 d (36–38 MPa) and 28 d (44 MPa) of curing, falling into 42.5R CEM I category. The emission of kgCO 2 /t of clinker during production, is estimated to decrease at least 12% for OPC14. A processing step for the pre-treatment of the slag is also presented, aiming to reduce the initial Cr content.

Surface active elements in liquid steel may affect the morphology of non-metallic particles during deoxidation. The effect of Te on the morphology of alumina particles was therefore studied by adding Te to molten iron before Al deoxidation at 1873 K. Dendritic, spherical, faceted, plate-like and clustered particles were identified in all samples. Te, however, considerably changes the rela-tive frequencies of the morphologies, decreasing the amount of dendritic and spherical particles whereas increasing the amount of faceted and plate-like particles. This effect is closely related with the supersaturation degree, stirring, and Te content. The way Te influences the morphology of the alumina particles and the factors influencing Te effectiveness are discussed. ispartof: ISIJ INTERNATIONAL vol:59 issue:9 pages:1529-1536 status: published

This study evaluates commercially available magnesia–chromite, magnesia–carbon and magnesia–doloma bricks for their use in a Vacuum Oxygen Decarburisation ladle. The corrosion behaviour of these bricks by stainless steelmaking slags is, therefore, investigated through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (5.3 and 3.0×10 −11 atm). The results reveal that magnesia–carbon bricks are severely corroded due to the high dissolution of MgO, while magnesia–chromite and magnesia–doloma refractories exhibit an excellent corrosion resistance. The MgO enrichment in the slag is believed to be the reason of the low wear rate of the MgO–doloma refractories. Rebonded and direct-bonded MgO–chromite refractories show similar corrosion resistance against the slags because of the ‘secondary chromite inactivation’. Decreasing the slag basicity enhances the dissolution of MgO into the slag, thereby increasing the corrosion of the magnesia-based refractories.

Bauxite residue, the waste product from the Bayer process, typically consists of approximately 40 wt% Fe2O3, which makes it a potential resource for Fe. This study presents an integrated process in which Fe is extracted in a first stage, followed by valorisation of the remaining residue towards alkali-activated binders. About 75 wt% of residue was melted with the addition of C, B2O3 and SiO2 at 1250 °C. During this treatment, and after water- quenching, a significant amount of Fe was formed as well as an almost fully vitrified slag. The slag was used to produce inorganic polymers with satisfactory compressive strengths. However, tincalconite, a boron salt eventually precipitated, demonstrating limited incorporation of B in the binder.

To investigate the densification of porous alumina plugs, hydrogen measurements, using the Hydris probe system, were performed in molten steel with different oxygen levels at around 1600 °C. The oxygen contents in molten steel were controlled by Al and Fe-Si alloy additions to 664, 296, 92, 51 and 2.5 ppm, respectively. High oxygen levels (> = 92 ppm) in molten steel were found to favor the infiltration of steel into porous alumina plug. The infiltrating steel interacts with SiO2-containing phases and Al2O3 inside the plug, forming liquid FeOnAl2O3-SiO2 slag and FeAl2O4 (hercynite). These newly formed phases, along with the infiltrating steel fill the porous structure of the alumina plugs. As a result, severe densification was observed inside the alumina plugs in contact with molten steel containing high oxygen levels. In comparison, no densification occurred in the plugs contacting with the steel having low oxygen contents of 51 and 2.5 ppm. ispartof: Journal of the European Ceramic Society vol:38 issue:6 pages:2662-2670 status: published

Valorization of slag from metallurgical industrial processes becomes ever more important as this may help optimizing and reducing the use of natural resources such as rock and sand. In view of this objective, metallurgical processes have to be analyzed in order to modify the slag composition so that it will meet the expected requirements of potential users of these resources. This paper gives the results of a thermodynamic analysis of the flash smelting process to understand the impact of operational parameters on slag chemistry and elemental partitioning in the process. It is shown that an increase of the matte grade and temperature leads to higher deportment of Pb, Zn and As to the slag. With increasing matte grade the volatilization of these elements decreases. Furthermore, the results indicate an increasing solubility of Cu and S in slag at higher temperatures.

In Europe, 50% of the copper originates from recycling. High-Cu containing scraps are regularly rich in impurities like Fe, Ni, Sn and Pb. Many high-Cu containing scraps are fed directly into anode-furnace casting installations or furnaces with limited refining capability. The impurities levels of the anodes produced in these processes should however remain acceptable for further electro-refining.The distribution of Sn between copper and slag has been investigated for conditions relevant to copper converting. Unlike previous studies, this work focuses on the behavior of Sn relevant to fire refining conditions, which are essential for anode refineries.In this study, liquid Cu–1Sn and CuOx–FeOy–SiO2 slag, with CaO additions, were equilibrated at 1300 °C in vacuum sealed SiO2 ampoules. After metal–slag equilibration, the samples were quenched in water. The composition of the slag and Cu metal was investigated with Electron Probe X-ray Microanalysis. The distribution coefficients of Sn between slag and Cu metal were determined as a function of slag composition.

In order to reduce the burden of electric arc furnace dust on steel recycling plants, a new technology called in-process separation (IPS) was proposed. By separating Zn vapor from the solid particles in the electric arc furnace off-gas, production of Zn-containing electric arc furnace dust may be prevented. However, the spinel formation reaction at high temperatures and its impact on the technology’s feasibility still require better understanding. This paper demonstrates a model that was developed to provide this insight, based on the results of our previous work on the kinetics of the spinel formation.This model evaluates the performance of IPS based on four key process parameters: the position of the separation unit, the time–temperature profile inside the off-gas duct, the solid particle size distribution, and the initial Zn and Fe content of the off-gas. The impact on the efficiency of each parameter was demonstrated using the results of the model for a specific off-gas treatment setup.

During acid leaching of bauxite residue (red mud), the increase in dissolution of rare-earth elements (REEs) is associated with an increase in iron dissolution, which poses problems in the downstream processing. Therefore, it would be beneficial to remove iron from bauxite residue by smelting reduction. The slag generated in the smelting reduction process could then be further processed for recovery of REEs. Smelting experiments were carried out at temperatures between 1500 and 1600 °C. Wollastonite (CaSiO3) was used as a flux and graphite as a reducing agent. The addition of wollastonite decreases the slag melting temperature and the viscosity, facilitating slag-metal separation, whereas a graphite content higher than the optimum level alters the slag chemistry and hinders the slag-metal separation. The optimum conditions were found to be for heating at 1500 °C: 20 wt% of wollastonite and 5 wt% of graphite. More than 85 wt% of the iron was separated from the slag in the form of a nugget. A further 10 wt% of the iron could be extracted from the slag by subsequent grinding and magnetic separation. The slag obtained after iron removal was treated with HCl, HNO3, and H2SO4 acids to extract REEs. Room-temperature leaching was found to be not beneficial for REEs extraction. High-temperature leaching enhanced the recovery of REEs. More than 95 % of scandium, >70 % of REEs, and about 70 % of titanium could be leached at 90 °C. The selectivity of REEs over iron during slag leaching was clearly improved.

In order to study the role of ZnO in the degradation of magnesia-chromite refractories, the chemical corrosion behaviour in a ZnO-containing fayalite slag (FeO–SiO 2 –ZnO–Al 2 O 3 ) is investigated using a rotating magnesia-chromite refractory finger corrosion test under a reducing atmosphere at 1200 °C. The results show that, likewise to ZnO-free fayalite slag, periclase is predominantly corroded. Both ZnO and FeO diffused into the original periclase and chromite grains thereby forming, respectively, (Zn,Fe,Mg)O solid solution and (Zn,Fe,Mg)[Cr,Al,Fe] 2 O 4 spinel. Concurrently, ZnO and FeO were also incorporated into the (Zn,Fe,Mg) 2 SiO 4 olivine during its formation. None of these phase formations resulted in new crack generation in the refractory. Considering the infiltrated slag is almost completely depleted in ZnO at 4 mm from the hot face, the severe slag penetration up to the centre of the refractory sample is not caused by the ZnO presence in the initial slag.

Furnace relinings represent a major operating cost in pyrometallurgy. External cooling is, therefore, often used to reduce the chemical wear by limiting the slag infiltration depth, reducing the reaction kinetics and lowering the solubility of refractory components into the liquid slag. In this paper a new experimental setup is used to study the reaction between a synthetic PbO–SiO2 based slag and a magnesia–chromite refractory under a temperature gradient. Forsterite (Mg2SiO4) is formed throughout the sample, removing SiO2 from the infiltrated liquid slag. The resulting change in slag composition causes the liquidus temperature and the viscosity of the liquid to decrease partially countering the effect of the applied temperature gradient and resulting in the complete infiltration of the sample. The extent to which external cooling prolongs the lifetime of an industrial furnace thus depends on the slag properties and how they are modified after reaction with the refractory.

Metal slags generated from metallurgical processes are a group of promising recyclable sources of metal. Each year, a large amount of copper slag is produced worldwide. Copper slag usually contains a certain quantity of valuable metals, such as copper, nickel, cobalt and iron (Gorai et al., 2003). Therefore, it can be treated as a secondary resource for metal extraction rather than as a waste. Numerous studies have been carried out on metal recovery from copper slag using hydro- and/or pyrometallurgical methods (Altundogan et al., 2004, Banza et al., 2002, González et al., 2005, Das et al., 2010, Kucharski et al., 2014, Yang et al., 2014, Heo et al., 2014). Characterisation of feed material to determine the expected concentrate grade and metal recovery is essential for the design of separation flow sheets and achieving sustainable development in the utilisation of resources (Jan D. Miller et al., 2009). Extensive research has been performed to characterise copper slags in view of metal recovery through various techniques (Li et al., 2009, Das et al., 2010, Cardona et al., 2011). For the sake of metal recovery, the mineralogy and particle size of metal phases are the key metallurgical parameters. Quantification of minerals can be performed by quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). It is an automated image analysis system using backscattered electron (BSE) and energy dispersive X-ray (EDX) signals from a scanning electron microscope (SEM).QEMSCANhasbeen widely applied inthe studyof mineral properties (Pascoe et al., 2007, Liu et al., 2005, Matjie et al., 2011).X-ray computed tomography (CT) is a non-destructive characterisation technology, which can provide direct volumetric data acquisition and quantitative analysis in three dimensions (3D) with a reasonably short analysis time and limited sample preparation (Jan D. Miller and Lin, 2004). CT scanning has been applied to characterise different properties of rocks and ores (J. D. Miller et al., 2003, Remeysen, 2006, Ghorbani et al., 2011). CT scanning and QEMSCAN are powerful characterisation tools that have not yet been applied in the study of copper slag. In this work, the species and distribution of copper bearing phases were characterized through QEMSCAN. In addition, the distribution of copper bearing grains was studied through CT scanning. Once the mineralogy and distribution of copper bearing particles are understood, the suitable slag processing techniques and conditions can be designed in view of copper recovery. ispartof: Transactions of the Institutions of Mining and Metallurgy, Section C: Mineral Processing and Extractive Metallurgy vol:124 issue:2 pages:83-87 status: published

To better understand the phenomena of ZnFe2O4 spinel formation in electric arc furnace dust, the dust was characterized with particle size analysis, X-ray fluorescence (XRF), electron backscatter diffraction (EBSD), and electron probe micro-analysis (EPMA). Different ZnFe2O4 formation reaction extents were observed for iron oxide particles with different particle sizes. ZnO particles were present as both individual particles and aggregated on the surface of larger particles. Also, the slag particles found in the off-gas were shown not to react with the zinc vapor. After confirming the presence of a ZnFe2O4 formation reaction, the thermodynamic feasibility of in-process separation – a new electric arc furnace dust treatment technology – was reevaluated. The large air intake and the presence of iron oxide particles in the off-gas were included into the thermodynamic calculations. The formation of the stable ZnFe2O4 spinel phase was shown to be thermodynamically favorable in current electric arc furnace off-gas ducts conditions even before reaching the post combustion chamber.