Several valuable elements, including V, Cr, Fe, Mn, and Ti, exist in vanadium slags. In this paper, an innovative way to extract Fe and Mn selectively and destroy the typical encompassed structure with NH4Cl has been proposed. Fe and Mn are then used to synthesize MnFe2O4, and the residue is an enriched Cr–V–Ti–O mixture. Moreover, the chlorination agent NH4Cl can be recycled in this process. Vanadium slag was mixed with various amounts of NH4Cl and NaCl in a temperature range from 300 °C to 900 °C for 1 to 8 h. The optimal conditions for the chlorination of Mn and Fe are NH4Cl–slag mass ratio of 2:1, NaCl–NH4Cl mass ratio of 0.308:1, 800 °C and 4 h. The manganese and iron chlorination ratio can be reached at 95% and 72%, respectively. The presence of NaCl can enhance the chlorination rate of manganese and iron. Manganese ferrite powders were prepared via hydrothermal treatment of iron and manganese in the leaching solution at a temperature of 140–200 °C. The synthesized MnFe2O4 exhibits high saturation magnetization (Ms, 55.85 emu/g) and low coercivity (Hc, 38.4 Oe). The possible mechanisms involved in these findings are explored.Keywords: Ammonium chloride; Magnetic properties; Mechanisms; Sodium chloride; Vanadium slag;

The carbon deposition behavior on nickel particles was observed within the temperature range from 400 to 800°C in a pure methane atmosphere. The topography, properties, and molecular structure of the deposited carbon were investigated using field-emission scanning electron microscopy (FESEM), temperature-programmed oxidation (TPO) technology, X-ray diffraction (XRD), and Raman spectroscopy. The deposited carbon is present in the form of a film at 400–450°C, as fibers at 500–600°C, and as particles at 650–800°C. In addition, the structure of the deposited carbon becomes more ordered at higher temperatures because both the TPO peak temperature of deposited carbon and the Raman shift of the G band increase with the increase in experimental temperature, whereas the intensity ratio between the D bands and the G band decreases. An interesting observation is that the carbon deposition rate is suppressed in the medium-temperature range (M-T range) and the corresponding kinetic mechanism changes. Correspondingly, the FWHM of the G and D1 bands in the Raman spectrum reaches a maximum and the intensities of the D2, D3, and D4 bands decrease to low limits in the M-T range. These results indicate that carbon structure parameters exhibit two different tendencies with respect to varying temperature. Both of the two group parameters change dramatically as a peak function with increasing reaction temperature within the M-T range.

•The medium frequency arc is associated with the anode charge transfer process.•No carbon is observed for pure CO, but the addition of H2 promotes carbon deposition.•The carbon formation is only on the Ni-YSZ anode surface.H2CO mixtures were fed to anode-supported solid oxide fuel cell (SOFC) Ni-YSZ/YSZ/GDC/BSCF to investigate the cell electrochemical performance and carbon deposition on the anode. Reasonable power density (∼0.8 W cm−2) was achieved with different H2CO ratios as fuels. By analyzing the electrochemical impedance spectroscopy (EIS) data, it was found that the medium frequency resistance increased with the increasing CO content, which indicated it was most likely associated with the anode charge transfer process. When ratio of H2:CO was around 2, CO mainly participates in the water gas shift reaction; and with the increasing CO content in the mixtures, CO also partly was involved in electrochemical oxidation. The cells could operate stably for all H2CO mixtures about 10 h without failure at a constant current of 50 mA cm−2. After the cells were tested, no carbon deposition was observed for the pure CO case, but with the presence of H2, carbon was observed. This phenomenon proves that the presence of H2 in H2CO mixtures promotes carbon deposition of CO. We firstly explored the carbon deposition area and found that carbon formation occurred only on the anode surface, which might be mainly caused by CO reduction reaction due to shifting the reaction equilibrium towards reverse syngas formation.

La0.7Sr0.3Fe0.9Ni0.1O3−δ (LSFNi) and La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3−δ (CLSFNi) are synthesized and applied for use as symmetrical electrodes in direct-methane solid oxide fuel cells (SOFCs). In a symmetric SOFC, the same electrode material is used for both the anode and cathode and must therefore remain active in both oxidizing and reducing atmospheres. LSFNi and CLSFNi retain their stable perovskite phase at high temperature in both oxidizing and moderately reducing environments, with a minor amount of SrLaFeO4 phase (K2NiF4 structure) present under reducing conditions. Symmetric SOFCs incorporating either LSFNi or CLSFNi electrodes give excellent peak power densities of ∼900 mW cm−2 at 850 °C in wet H2/air (3% H2O). In wet CH4/air (3% H2O), the CLSFNi electrode greatly outperforms the LSFNi electrode (522 mW cm−2vs. 221 mW cm−2) due to the enhanced methane reforming activity imparted by the cerium doping. The performance and stability of the CLSFNi symmetric cell under direct CH4 operation are among the best reported to date in the literature, indicating that CLSFNi is a promising electrode material for symmetrical SOFCs.

Experiments were carried out to determine the effect of Al2O3 in the slag of the CaO-SiO2-Al2O3-MgO-CaF2 system on the cleanness of Fe-13Cr stainless steel deoxidized by ferrosilicon. Increasing the Al2O3 content in basicity = 2.28 slag can reduce the usage of CaF2 and benefit the obtainment of a good kinetic condition for inclusion removal, but over 21 pct would lead to a higher total oxygen content in the melt and make the inclusion composition more complex. It is found that increasing basicity in 16 pct Al2O3 slag would have a good deoxidation ability and accelerate the transformation from high Al2O3 inclusions to low melting point CaO-Al2O3-SiO2-MgO system inclusions, but basicity over 2.58 would lead to high content of [Al] in liquid steel, which would promote the formation of MgO-Al2O3 inclusions. Therefore, it is not suitable to add a high content of Al2O3 into high-basicity slag. Adding Al2O3 into slag of 2.28 in basicity until a content of 16 pct could achieve inclusion plastication within 45 minutes without Ca treatment, which has potential application in industrial production.

•Effects of CO2 and H2 in dry reforming CH4 feeding SOFC are investigated.•Concentration and activation polarizations are suppressed by CO2 in H2CO2.•Concentration and activation polarizations are reduced by replacement of CH4 with H2.•Replacement of CH4 with H2 in dry reforming CH4 improves fuel utilization.Recycling and reusing of exhaust gas from solid oxide fuel cell (SOFC) could enhance the fuel utilization of methane. In the present work, the effects of two main components of exhaust gas, viz. hydrogen and carbon dioxide, on the performance of exhaust gas reforming methane fueled cell were investigated. The cell with Ni/8 mol% Y2O3 doped zirconia (Ni/YSZ) as the anode was exposed to H2CO2CH4 atmospheres at 800 °C. DC techniques and impedance spectroscopy was used for characterization. Two kinds of cells with different fuel utilization factors (Uf) were used in experiments, which represented different relationship of power density/voltage with current density. The results indicated that electrooxidation of H* dominated the electrochemical reaction on the anode of the cell at low Uf value, while electrooxidation of C* becomes important at high Uf value. In the case of CO2H2 mixtures, CO2 improved the exchange current density, raised the value of Uf, and depressed anodic activation polarization. As for H2CH4CO2 system, H2, partly replacing CH4, reduced the heat loss in dry reforming, improved the exchange current density, raised Uf value, and depressed the activation and concentration polarization resistances of Ni/YSZ anode. But it had limited effect on the power density of the cell with high Uf value.

The structures of the Cr-containing slags were analyzed by Raman spectroscopy. The results obtained show that when the Cr contents increased from 0 to 5 pct, the depolymerization degree of the silicate matrix is increased. However, increasing Cr content to 7 pct caused the depolymerization degree to decrease. The spectral results are consistent with the results of sulfide capacity measurements. And the relationship between average bridging oxygen and sulfide capacities (logCS) was discussed accordingly.

Unique experiments were designed to study the surface phenomena in steelmaking reactions. The concept of surface sulfide capacities and an understanding of the surface accumulation of surface-active species, based on experimental results, are presented. In order to understand the flow phenomenon at slag/metal interface, experiments were designed to measure the interfacial velocity of S on the surface of an iron drop immersed in an aluminosilicate slag using the X-ray sessile drop method. The oscillation of the iron drop in the slag due to the change in the surface concentration of sulfur at the slag–metal interface was monitored by X-ray imaging. From the observations, the interfacial velocity of sulfur was evaluated. Similar experiments were performed to measure the interfacial velocity of oxygen at the interface as well as the impact of oxygen potential on the interfacial velocity of sulfur. The interfacial shear viscosity and the dilatational modulus were also evaluated. In a study of the wetting of alumina base by iron drop at constant oxygen pressure under isothermal condition, the contact angle was found to be decreased with the progress of the reaction leading to the formation of hercynite as an intermediate layer creating non-wetting conditions. In the case of silica substrate, an intermediate liquid fayalite layer was formed.

Disposal of slags from alloy steelmaking is a serious problem as the toxic metals in the slag such as chromium and vanadium can be leached out. Recovery of the valuable metals needs an effective, economically viable method with a minimum number of unit processes. In the present work, a novel process for simultaneous recovery of iron, vanadium, titanium, chromium, and manganese from vanadium slag is proposed including the chlorination of vanadium slag in molten salt and electrolysis of the salt bath. The optimal conditions for the chlorination are an AlCl3–slag mass ratio of 1.5:1 and a salt bath composition (NaCl–KCl)–AlCl3 mass ratio of 1.66:1, at 900 °C for 8 h. The chlorination ratio of iron, vanadium, chromium, and manganese can reach 90.3%, 76.5%, 81.9%, and 97.3%, respectively, and the titanium volatilization ratio was 79.9%. Metal chlorides in molten salts are electrolyzed at 900 °C with graphite electrodes. Valuable metals (Fe, V, Cr, Mn) were deposited on the cathode in terms of alloy or metal of granular shape. The possible mechanisms involved in these findings were explored. The main compositions of residue are Al2O3 and SiO2, which has the potential usage for landfilling and or building by mixing with other substances for instance.

•The power density of the cell has been enhanced by 140% with the introduction of O3.•The enhancement is related to the ionization reactions and oxygen adsorption.•The rate-determining step of the electrochemical reaction does not changed by O3.•Oxygen vacancy concentration of SSC could be increased in ozone-contained ambient.•The Raman spectra of YSZ/SSC cathode were reported.The electrochemical properties of porous composite cathode consisting of Sm0.5Sr0.5CoO3 (SSC) and 8 mol% Y2O3 doped zirconia (YSZ) have been investigated in Ni/YSZ anode-supported single cells at 600 °C under oxygen with/without ozone conditions. The Microscopic features and X-ray powder diffraction patterns of the cathode don't show obvious change before and after the measurement. However, the power density of the cell increases in ozone-contained ambient, which is approximately 140% higher than that in ozone-free ambient in most. The effect of the ozone on the performance of cathode is also examined by impedance spectra and Raman spectra. The results of the impedance spectra indicate that polarization resistance decreases in the ozone-contained ambient compared with that in ozone-free ambient. And the Raman spectra suggest that the physicochemical properties of SSC/YSZ porous cathode are also modified.

•The oxidation kinetics of ss441 at initial 2 h is not parabolic like.•A non-protective layer is nonselective oxidized at initial oxidization of ss441.•Cr2O3-main-contain layer is forming at the next oxidation stage.•The oxides layer could not protective the alloy effectively at high temperature.The oxidation behavior of the stainless steel 441 in the initial 2 h was investigated gravimetrically at 600 °C–950 °C under various oxygen pressures. The morphology, composition and the growth stress of the scale on the oxidized alloy were analyzed by SEM/EDS and Raman spectra. It was found that the performances of the scale degraded with the temperature and oxygen pressure increasing. The oxidation of stainless steel 441 presented a multi-stages behavior. It was suggested that the first chemical reaction determining stage was the nonselective oxidation of steel surface. The following stage in pure oxygen ambient was the formation of chromium-oxide-mainly-containing layer. Kinetic mechanism of second stage oxidation changed from diffusion determining step (600–800 °C) to interface chemical reaction (900–950 °C) due to the decline of the protection of the oxides layer. Only linear rate law was obeyed in the lower oxygen pressure atmospheres at 800 and 900 °C within the initial 2 h.

The effects of Ce addition on the quantity, size, distribution of inclusions and the content of oxygen, sulfur and other hazardous residual elements in spring steel used as fastener in high speed railway were investigated by metallographic examination, SEM-EDS and composition analysis. The results indicated that the contents of oxygen decreased with the addition of Ce ([Ce]<0.1%) and the content of sulfur continually decreased with increasing content of Ce ([Ce]<1.2%). However, with the further increase of Ce element addition, the content of [O] and T[O] began to increase. The content of Ce Corresponding to the lowest [O] and T[O] lied in the range of 0.10%–0.13% and 0.045%–0.065%, respectively. The addition of Ce in spring steel resulted in the formation of rare earth oxides/oxysulfides and decreased the size of inclusions to less than 3 μm in globular or spheroid shape. Moreover, the residual harmful elements (As, P, Pb and Sn) were found to exist in the Ce-containing inclusions, which had proved that the Ce addition could capture the residual elements and suppress their precipitation behaviors in the grain boundary.SEM-mapping of elements in Ce inclusions

•A special thermodynamic description corresponding to the kinetics was applied.•We reported the relationships of degradation time with temperature and moisture.•”Turning time” in the Fe–16Cr alloy oxidation kinetic model was given.•The oxidation mechanism of Fe–16Cr alloy in the wet air was discussed.Experimental study on the oxidation corrosions of Fe–16Cr alloy was carried out at 800–1100 °C under dry/wet air conditions. Faster oxidation rate was observed at higher temperature and water vapor content. The degradation time td between two stages in oxidation process showed an exponential relationship with elevating corrosion temperature in dry air, and a linear relationship with the water content in the case of water vapor introduced to the system.The mechanism of oxidation corrosions of Fe–16Cr alloy was suggested by the Real Physical Picture (RPP) model. It was found that the break-away oxidation in stage II was controlled by diffusion at initial both in dry and wet air, then became linear with the exposure time, which implied that the oxidation rate was then controlled by chemical reaction of the interface between the metal and the oxidized scale. Moreover, the effect of water in the oxidation process is not only to supply more oxygen into system, but also to modify the structures of oxide scale due to the existence of hydrogen atom, which results in the accelerated corrosions.

In the present study, the applicability of the high-temperature mass spectrometric method combined with Knudsen effusion cell for quantifying the valence states of V in the multicomponent system CaO-MgO-Al2O3-SiO2-VOx up to a maximum temperature of 2050 K (1777 °C) was examined. The valence ratio of V3+/V4+ in slag phase was derived from the partial pressures of VO and VO2 in the effused vapor phase. The results show good agreement with the literature values obtained by other techniques. A correlation between the valence ratio V3+/V4+ and the oxygen partial pressure as well as basicity was achieved based on the present results and accessed data in the literature. The results of the present study demonstrate that the Knudsen cell-mass spectrometric method can be a very effective tool in estimating the valence ratios for of transition metals in metallurgical slags.

The molar sulphide capacities \( C_{\text{S}}^{'} \) = (mol pct S) (\( P_{{{\text{O}}_{2} }} /P_{{{\text{S}}_{2} }} \))1/2 on four binary systems, MgO-SiO2, CaO-SiO2, MnO-SiO2 and FeO-SiO2 are elucidated so as to compare the magnitudes of the basicities of four metallic oxides and to estimate the temperature dependencies of the basicities of metallic oxides. The enthalpy changes of the reaction 2O− = O + O2−, viz. the silicate polymerization reaction (denoted as \( \Updelta H_{(8)}^{^\circ } \)) have been calculated from the slopes of the log \( C_{\text{S}}^{'} \)vs 1/T curves for four binary silicates. The \( \Updelta H_{(8)}^{^\circ } \) value is considered in the present work to be an index of the basicity of silicate melts. The basicities obtained on the basis of the \( \Updelta H_{(8)}^{^\circ } \) values are in the order MgO < CaO < MnO < FeO, which are determined by two effects; (i) ionicity of chemical bonds between metallic and oxygen ions and (ii) clustering of metallic oxides in silicates. It is also found that the basicity of the FeO-SiO2 system is larger at higher temperatures.

With a goal to estimate the sulfide capacities of slags used in the pretreatment of hot metal, the sulfide capacities of CaO-Al2O3-SiO2 slags were measured at 1673 K to 1773 K (1400 °C to 1500 °C). The gas–slag equilibrium technique has been used for this measurement. From the results obtained, it was found that the temperature dependence of the sulfide capacity of this slag is independent of the slag compositions. Therefore, a new empirical model based on optical basicity for sulfide capacity estimation of this slag was developed using the measured values of the current work and literature. With the use of the new model, the isosulfide capacity curves at 1673 K (1400 °C) were mapped.

Precipitation behavior of grain boundary carbides and its influence on mechanical properties and fracture mechanism of the high nitrogen austenitic stainless steel produced by different processing methods were studied. The simulation software Thermo-calc was applied to analyze the effects of element content on precipitation of carbides. The results show that hot-rolled plate has higher strength, but solution-treated one followed by water quenching has excellent combination of strength and ductility (toughness). M23C6 is the main precipitate and deteriorates the toughness of the steel obviously when it precipitates along grain boundaries. In this case, intergranular fracture is the predominant failure mechanism and the fracture surface is characterized by the shape of rock candy. The toughness at −40 °C is decreased by 53% when small amount of carbides precipitates during sand cooling process after solution treatment. The simulation results exhibit that with the decrease of C content, both the precipitation quantity and precipitation temperature of M23C6 decrease. Cr and N have no influence on precipitation quantity of M23C6, but the precipitation temperature will increase with the increase of Cr and the decrease of N.Highlights► The toughness of high nitrogen stainless steel is seriously deteriorated by grain boundary carbides. ► Hot-rolled high nitrogen steel has higher strength. ► Solution-treated one with water quenching has excellent combination of strength and ductility. ► The precipitation quantity and precipitation temperature of M23C6 decrease with decreasing C content.

In view of the importance of the thermodynamic behavior of chromium in the slag phase as well as the serious discrepancies in the earlier reports on the valence state of chromium in slag melts, the oxidation state of chromium oxides in CaO-SiO2-CrOx and CaO-MgO-(FeO-) Al2O3-SiO2-CrOx were investigated experimentally in the present study using two different experimental techniques. The gas–slag equilibrium technique was adopted to study the CaO-SiO2-CrOx system between 1823 K (1550 °C) and 1923 K (1650 °C) and equilibrated with mixtures of CO-CO2-Ar gases corresponding to three different oxygen partial pressures (between 10−4 and 10−5 Pa). After equilibrating, the samples were quenched and subjected to analysis using the X-ray absorption near edge structure method to determine the distribution ratio of Cr2+/Cr3+ in the slags. The second technique examined the applicability of the high-temperature mass spectrometric method combined with the Knuden effusion cell for quantifying the valence states of Cr in the multicomponent system CaO-MgO-(FeO-) Al2O3-SiO2-CrOx up to a maximum temperature of 2000 K (1727 °C). The results showed that the Knudsen cell-mass spectrometric method could be used successfully to estimate the valence ratio for Cr in silicate melts. According to the present study, the Cr2+/Cr3+ ratio increased with increasing temperature and a decreasing slag basicity as well as the oxygen partial pressure prevailing in the system. A mathematical correlation of XCrO/XCrO1.5 as a function of temperature, oxygen partial pressure, and basicity was developed in the present work based on the present results as well as on those assessed from earlier data.

The crystal structure, conductivity, and electrochemical properties of Ce doped La0.7Sr0.3Fe0.9Ni0.1O3−δ (CLSFNi) have been investigated as solid oxide fuel cell (SOFC) cathode. The polarization resistance of symmetric cell was reduced, although the substitution of Ce at La-sites decreased the symmetry of crystal structure and total electrical conductivity. The results indicate that the incorporation of Ce improves the catalyst activity of oxygen reduction reaction. The addition of GDC (Gd0.1Ce0.9O2-δ) can further decrease the polarization resistance (Rp); for example, the Rp values of CLSFNi and CLSFNi7-GDC3 at 750 °C are 0.130 and 0.047 Ω cm2, respectively. The peak power density is 903, 499, 235 mW cm−2 for the La0.7Sr0.3Fe0.9Ni0.1O3−δ (LSFNi) cathode and 1091, 725, 423 mW cm−2 for the CLSFNi cathode at 750 °C, 700 °C and 650 °C, respectively.

It is promising for metal especially ferritic stainless steel (FSS) to be used as interconnector when the solid oxide fuel cell (SOFC) is operated at temperature lower than 800 °C. However, there are many challenges for FSS such as anti-oxidant, poisoning to cathode and high area specific resistance (ASR) when using as SOFC interconnector. The effect of Cr content (12 — 30 mass%) in Fe-Cr alloys on thermal expansion coefficient (TEC), oxidation resistance, microstructure of oxidation scale and ASR was investigated by thermo-gravimetry, scanning electron microscopy, energy dispersive spectroscopy and four-probe DC technique. The TEC of Fe-Cr alloys is (11 — 13) × 10−6 K−1, which excellently matches with other SOFC components. Alloys have excellent oxidation resistance when Cr content exceeds 22 mass% because of the formation of chromium on the surface of alloy. The oxidation rate constants kd and ks decrease rapidly with increasing the Cr content and then increase slowly when the Cr content is higher than 22 mass%. The kinetic results indicate that Cr evaporation must be considered at high temperature for Fe-Cr alloys. After the alloys were oxidized in air at 800 °C for 500 h, log(ASR/T) (T is the absolute temperature) presents linear relationship with 1/T and the conduct activation energy is 0.6 — 0.8 eV (Cr16-30). Optimal Cr content is 22 — 26 mass% considering the oxidation resistance and ASR.

La0.7Sr0.3Fe0.9Ni0.1O3−δ (LSFNi) and La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3−δ (CLSFNi) are synthesized and applied for use as symmetrical electrodes in direct-methane solid oxide fuel cells (SOFCs). In a symmetric SOFC, the same electrode material is used for both the anode and cathode and must therefore remain active in both oxidizing and reducing atmospheres. LSFNi and CLSFNi retain their stable perovskite phase at high temperature in both oxidizing and moderately reducing environments, with a minor amount of SrLaFeO4 phase (K2NiF4 structure) present under reducing conditions. Symmetric SOFCs incorporating either LSFNi or CLSFNi electrodes give excellent peak power densities of ∼900 mW cm−2 at 850 °C in wet H2/air (3% H2O). In wet CH4/air (3% H2O), the CLSFNi electrode greatly outperforms the LSFNi electrode (522 mW cm−2vs. 221 mW cm−2) due to the enhanced methane reforming activity imparted by the cerium doping. The performance and stability of the CLSFNi symmetric cell under direct CH4 operation are among the best reported to date in the literature, indicating that CLSFNi is a promising electrode material for symmetrical SOFCs.