Effect of Cr2O3 addition on oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets with simulated coke oven gas

The oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets (CVTP) with Cr₂O₃ addition were studied, and the reduction swelling index (RSI) and compressive strength (CS) of the reduced CVTP with simulated coke oven gas (COG) injection were investigated. The results showed that the CS of the CVTP decreases and the porosity of the CVTP increases with increasing amount of Cr₂O₃ added. The Cr₂O₃ mainly exists in the form of (Cr, Fe)₂O₃ solid solution in the CVTP and as Fe-Cr in the reduced CVTP. The CS of the reduced CVTP increases and the RSI of the reduced CVTP decreases with increasing amount of Cr₂O₃ added. The limited aggregation and diffusion of metallic iron contribute to the formation of dense lamellar crystals, which leads to the slight decrease for reduction swelling behavior of reduced CVTP. This work provides a theoretical and technical basis for the utilization of CVTP and other Cr-bearing ores such as chromite with COG recycling technology.     

Phosphate enrichment mechanism in CaO–SiO2–FeO–Fe2O3–P2O5 steelmaking slags with lower binary basicity

The addition of silica to steelmaking slags to decrease the binary basicity can promote phosphate enrichment in quenched slag samples. In this study, we experimentally investigated phosphate enrichment behavior in CaO–SiO₂–FeO–Fe₂O₃–P₂O₅ slags with a P₂O₅ content of 5.00% and the binary basicity B ranging from 1.0 to 2.0, where the (%Fe_tO)/(%CaO) mass percentage ratio was maintained at 0.955. The experimental results are explained by the defined enrichment degree R_C2S-C3P of solid solution 2CaO·SiO₂–3CaO·P₂O₅ (C₂S–C₃P), where R_C2S-C3P is a component of the developed ion and molecule coexistence theory (IMCT)–N_i model for calculating the mass action concentrations N_i of structural units in the slags on the basis of the IMCT. The asymmetrically inverse V-shaped relation between phosphate enrichment and binary basicity B was observed to be correlated in the slags under applied two-stage cooling conditions. The maximum content of P₂O₅ in the C₂S–C₃P solid solution reached approximately 30.0% when the binary basicity B was controlled at 1.3.     

Fabrication of Fe–TiC–Al2O3 composites on the surface of steel using a TiO2–Al–C–Fe combustion reaction induced by gas tungsten arc cladding

The aim of the present study was to fabricate Fe–TiC–Al₂O₃ composites on the surface of medium carbon steel. For this purpose, TiO₂–3C and 3TiO₂–4Al–3C–xFe (0 ≤ x ≤ 4.6 by mole) mixtures were pre-placed on the surface of a medium carbon steel plate. The mixtures and substrate were then melted using a gas tungsten arc cladding process. The results show that the martensite forms in the layer produced by the TiO₂–3C mixture. However, ferrite–Fe₃C–TiC phases are the main phases in the microstructure of the clad layer produced by the 3TiO₂–4Al–3C mixture. The addition of Fe to the TiO₂–4Al–3C reactants with the content from 0 to 20wt% increases the volume fraction of particles, and a composite containing approximately 9vol% TiC and Al₂O₃ particles forms. This composite substantially improves the substrate hardness. The mechanism by which Fe particles enhance the TiC + Al₂O₃ volume fraction in the composite is determined.     

Complete stabilization of severely As-contaminated soil by a simple H2O2 pre-oxidation method combined with non-toxic TMT-15 and FeCl3·6H2O

The stabilization of severely As-polluted soil has been a challenge, especially for the extremely toxic As(Ⅲ) contaminants. In this study, soil with a high As concentration (26084 mg/kg) was availably stabilized by a H₂O₂ pre-oxidation assisted TMT-15 (Na₃S₃C₃N₃ solution with a mass fraction of 15%) and FeCl₃·6H₂O stabilization method. The results showed that the combination of the two stabilizers (i.e., TMT-15 and FeCl₃·6H₂O) presented a better stabilization behavior than either stabilizer used individually. The use of the H₂O₂ pre-oxidation assisted TMT-15 and FeCl₃·6H₂O stabilization approach not only converted the As(Ⅲ) to As(V) but also reduced the toxic leaching concentration of As to 1.61 mg/L, which is a safe level, when the additions of TMT-15 and FeCl₃·6H₂O were 2 mL and 0.20 g, respectively. Thus, using only a simple H₂O₂ pre-oxidation to combine clean stabilization with non-toxic stabilizers TMT-15 and FeCl₃·6H₂O could render the severely As-contaminated soil safe for disposal in a landfill.     

Activity coefficient of NiO in SiO2-saturated MnO–SiO2slag and Al2O3-saturated MnO–SiO2–Al2O3slag at 1623 K

As a part of the fundamental study related to the reduction smelting of spent lithium-ion batteries and ocean polymetallic nodules based on MnO–SiO₂slags,this work investigated the activity coefficient of NiO in SiO₂-saturated Mn O–Si O₂slag and Al₂O₃-saturated Mn O–SiO₂–Al₂O₃slag at 1623 K with controlled oxygen partial pressure levels of 10^-7,10^-6,and 10^-5Pa.Results showed that the solubility of nickel oxide in the slags increased with increasing oxygen partial pressure.The nickel in the Mn O–Si O₂slag and Mn O–Si O₂–Al₂O₃slag existed as Ni O under experimental conditions.The addition of Al₂O₃in the Mn O–Si O₂slag decreased the dissolution of nickel in the slag and increased the activity coefficient of Ni O.Furthermore,the activity coefficient of Ni O(γN_(i O)),which is solid Ni O,in the Si O₂saturated Mn O–Si O₂slag and Al₂O₃saturated Mn O–Si O₂–Al₂O₃slag at 1623 K can be respectively calculated asγN_(i O)=8.58w(Ni O)+3.18 andγN_(i O)=11.06w(Ni O)+4.07,respectively,where w(Ni O)is the Ni O mass fraction in the slag.     

Hot-corrosion behavior of Cr2O3-CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700℃

The present work investigates the hot-corrosion behavior of carbon nanotube (CNT)-reinforced chromium oxide coatings on boiler steel in a molten salt (Na₂SO₄-60wt%V₂O₅) environment at 700℃ under cyclic conditions. The coatings were deposited via the high-velocity oxygen fuel process. The uncoated and coated steel samples were subjected to hot corrosion in a silicon tube furnace at 700℃ for 50 cycles. The kinetics of the corrosion behavior was analyzed through mass-gain measurements after each cycle. The corrosion products were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis techniques. The results revealed that uncoated steel suffered spallation of scale because of the formation of nonprotective Fe₂O₃ scale. The coated steel samples exhibited lower mass gains with better adhesiveness of oxide scale with the steel alloy until the end of exposure. The CNT-reinforced coatings were concluded to provide better corrosion resistance in the hot-corrosion environment because of the uniform dispersion of CNTs in the coating matrix and the formation of protective chromium oxides in the scale.     

In situ reaction mechanism of MgAlON in Al-Al2O3-MgO composites at 1700℃ under flowing N2

The Al-Al₂O₃-MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M₁, M₂, and M₃, respectively, were prepared at 1700℃ for 5 h under a flowing N₂ atmosphere using the reaction sintering method. After sintering, the Al-Al₂O₃-MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M₁ was composed of MgO and MgAl₂O₄. Compared with specimen M₁, specimens M₂ and M₃ possessed MgAlON, and its production increased with increasing aluminum addition. Under an N₂ atmosphere, MgO, Al₂O₃, and Al in the matrix of specimens M₂ and M₃ reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al-Al₂O₃-MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N₂ atmosphere, the partial pressure of oxygen is quite low; thus, when the Al-Al₂O₃-MgO composites were soaked at 580℃ for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al₂O₃ diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al₂O₃ to form MgAl₂O₄. When the temperature was greater than (1640 ±10)℃, AlN diffused into Al₂O₃ and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and MgAl₂O₄ at high temperatures because of their similar spinel structures.     

高重力燃烧合成WC/CoCrFeNiAl0.2高熵合金复合材料

The WC/CoCrFeNiAl_0.2 high-entropy alloy (HEA) composites were prepared through high-gravity combustion synthesis. The preparation method is presented below. First, using a designed suitable multiphase thermite system, the molten CoCrFeNiAl_0.2 HEA was fabricated using low-cost metal oxides. The molten HEA was subsequently infiltrated into the WC layer to fabricate WC/CoCrFeNiAl_0.2 composites in a high-gravity field. The porosity of the WC/CoCrFeNiAl_0.2 composites was down-regulated, and their compressive yield strength was up-regulated when the high-gravity field was increased from 600gto 1500gbecause this infiltration process of a HEA melt into the WC layer is driven by centrifugal force. The WC particles in the composites exhibited a gradient distribution along the direction of the centrifugal force, which was attributed to the combined action of the high-gravity field and the temperature gradient field. The Vickers hardness of the sample was down-regulated from 9.53 to 7.41 GPa along the direction of the centrifugal force.     

Mineral-phase evolution and sintering behavior of MO-SiO2-Al2O3-B2O3 (M=Ca,Ba) glass-ceramics by low-temperature liquid-phase sintering

In this work, network former SiO₂ and network intermediate Al₂O₃ were introduced into typical low-melting binary compositions CaO·B₂O₃, CaO·2B₂O₃, and BaO·B₂O₃ via an aqueous solid-state suspension milling route. Accordingly, multiple-phase aluminosilicate glass-ceramics were directly obtained via liquid-phase sintering at temperatures below 950℃. On the basis of liquid-phase sintering theory, mineral-phase evolutions and glass-phase formations were systematically investigated in a wide MO-SiO₂-Al₂O₃-B₂O₃ (M=Ca, Ba) composition range. The results indicate that major mineral phases of the aluminosilicate glass-ceramics are Al₂₀B₄O₃₆, CaAl₂Si₂O₈, and BaAl₂Si₂O₈ and that the glass-ceramic materials are characterized by dense microstructures and excellent dielectric properties.     

Activity coefficients of NiO and CoO in CaO-Al2O3-SiO2 slag and their application to the recycling of Ni-Co-Fe-based end-of-life superalloys via remelting

To design optimal pyrometallurgical processes for nickel and cobalt recycling, and more particularly for the end-of-life process of Ni-Co-Fe-based end-of-life (EoL) superalloys, knowledge of their activity coefficients in slags is essential. In this study, the activity coefficients of NiO and CoO in CaO-Al₂O₃-SiO₂ slag, a candidate slag used for the EoL superalloy remelting process, were measured using gas/slag/metal equilibrium experiments. These activity coefficients were then used to consider the recycling efficiency of nickel and cobalt by remelting EoL superalloys using CaO-Al₂O₃-SiO₂ slag. The activity coefficients of NiO and CoO in CaO-Al₂O₃-SiO₂ slag both show a positive deviation from Raoult's law, with values that vary from 1 to 5 depending on the change in basicity. The activity coefficients of NiO and CoO peak in the slag with a composition near B=(%CaO)/(%SiO₂)=1, where B is the basicity. We observed that controlling the slag composition at approximately B=1 effectively reduces the cobalt and nickel oxidation losses and promotes the oxidation removal of iron during the remelting process of EoL superalloys.     

单斜相SrAl2Si2O8陶瓷的烧结及其对Sr的固化

Monoclinic SrAl₂Si₂O₈ ceramics for Sr immobilization were prepared by a liquid-phase sintering method. The sintering temperature, mineral phase composition, microstructure, flexural strength, bulk density, and Sr ion leaching characteristics of the SrAl₂Si₂O₈ ceramics were investigated. A crystalline monoclinic SrAl₂Si₂O₈ phase formed through liquid-phase sintering at 1223 K. The introduction of four flux agents (B₂O₃, CaO·2B₂O₃, SrO·2B₂O₃, and BaO·2B₂O₃) to the SrAl₂Si₂O₈ ceramics not only reduced the densification temperature and decreased the volatilization of Sr during high-temperature sintering but also impacted the mechanical properties of the ceramics. Product consistency tests showed that the leaching concentration of Sr ions in the sample with flux agent B₂O₃ was the lowest, whereas that of Sr ions in the sample with flux agent BaO·2B₂O₃ was the highest. These results show that the leaching concentration of Sr ions depends largely on the amorphous phase in the ceramics. Meanwhile, the formation of mineral analog ceramics containing Sr is an important factor to improve Sr immobilization.     

Shear-thinning behavior of the CaO-SiO2-CaF2-Si3N4 system mold flux and its practical application

Satisfying the mold-flux performance requirements for high-speed continuous casting necessitates the development of a new non-Newtonian-fluid mold flux with shear-thinning behavior, i.e., a mold flux whose viscosity is relatively high under lower shear rates and relatively low under higher shear rates. In this work, a mold flux that exhibits shear-thinning behavior was developed by adding different amounts of Si₃N₄ to the CaO-SiO₂-CaF₂ mold flux. The shear-thinning behavior was investigated using a rotational viscometer. In addition, the microstructure of the newly prepared slags was studied by high-temperature Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that the mechanism of shear-thinning was attributable to a temporary viscosity loss caused by the one-way shear stress, whereas the corresponding magnitude of shear-thinning was closely related to the degree of polymerization (DP). Finally, the non-Newtonian fluid mold flux was used for laboratory casting tests, which revealed that the mold flux could reduce slag entrapment and positively affect the continuous casting optimization.     

Microwave absorption properties of SiC@SiO2@Fe3O4 hybrids in the 2-18 GHz range

To enhance the microwave absorption performance of silicon carbide nanowires (SiCNWs), SiO₂ nanoshells with a thickness of approximately 2 nm and Fe₃O₄ nanoparticles were grown on the surface of SiCNWs to form SiC@SiO₂@Fe₃O₄ hybrids. The microwave absorption performance of the SiC@SiO₂@Fe₃O₄ hybrids with different thicknesses was investigated in the frequency range from 2 to 18 GHz using a free-space antenna-based system. The results indicate that SiC@SiO₂@Fe₃O₄ hybrids exhibit improved microwave absorption. In particular, in the case of an SiC@SiO₂ to iron(Ⅲ) acetylacetonate mass ratio of 1:3, the microwave absorption with an absorber of 2-mm thickness exhibited a minimum reflection loss of -39.58 dB at 12.24 GHz. With respect to the enhanced microwave absorption mechanism, the Fe₃O₄ nanoparticles coated on SiC@SiO₂ nanowires are proposed to balance the permeability and permittivity of the materials, contributing to the microwave attenuation.     

功能梯度Ni?W/Er2O3稀土纳米颗粒复合膜的电沉积

Multi-layered functionally graded (FG) structure Ni?W/Er₂O₃ nanocomposite films were prepared by continuously changing the deposition parameters, in which the Er₂O₃ and W contents varied with thickness. The microstructure and chemical composition of the electrodeposited Ni?W/Er₂O₃ films were determined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The anti-corrosion and wear properties of the electrodeposition films were investigated by electrochemical measurement and ball-on-disk friction test. The microhardness distribution of the cross section of nanocomposites was measured by nanoindentation. The results showed that with decreasing agitation rate or increasing average current density, the contents of Er₂O₃ nanoparticles and tungsten were distributed in a gradient along the thickness, and the contents on the surface were larger. By comparison, FG Ni?W/Er₂O₃ films had better anti-corrosion and wear properties than the uniform Ni?W/Er₂O₃ films. Atomic force microscopy (AFM) and profilometry measurements indicated that Er₂O₃ nanoparticles had an effect on the surface roughness.     

Feasibility of aluminum recovery and MgAl2O4 spinel synthesis from secondary aluminum dross

The feasibility of aluminum recovery from secondary aluminum dross by extraction with NaOH solution and the subsequent synthesis of MgAl₂O₄ spinel by sintering the extracted slag were studied. The extraction percentage of soluble aluminum from the dross reached 80% at a temperature of 353 K, liquid-to-solid ratio of 12 mL·g-1, stirring speed of 300 r·min-1, and an extraction time of 15 min; the hydrolysis percentage of AlN reached 40% with an extraction time of 30 min. The activation energies of the soluble aluminum and AlN extracted from the dross were 7.15 and 8.98 kJ·mol-1, respectively, indicating that their kinetics were controlled by outer diffusion without a product layer. The extracted slag was sintered in the temperature range 1373-1773 K; MgAl₂O₄ spinel with a compressive strength as high as 69.4 MPa was produced in the sample sintered at 1673 K for 3 h. This value exceeds the threshold (40 MPa) prescribed by the National Standard for the Magnesia and Magnesia-alumina Refractory Bricks of China (GB/T 2275-2007). These results establish the effectiveness of aluminum recovery from secondary aluminum dross and subsequent MgAl₂O₄ spinel synthesis.     

碳包覆Ti3AlC2增强银基复合材料的制备与性能

Ti₃AlC₂-reinforced Ag-based composites, which are used as sliding current collectors, electrical contacts, and electrode materials, exhibit remarkable performances. However, the interfacial reactions between Ag and Ti₃AlC₂ significantly degrade the electrical and thermal properties of these composites. To diminish these interfacial reactions, we fabricated carbon-coated Ti₃AlC₂ particles (C@Ti₃AlC₂) as reinforcement and prepared Ag–10wt%C@Ti₃AlC₂ composites with carbon-layer thicknesses ranging from 50–200 nm. Compared with the uncoated Ag–Ti₃AlC₂ composite, Ag–C@Ti₃AlC₂ was found to have a better distribution of Ti₃AlC₂ particles. With increases in the carbon-layer thickness, the Vickers hardness value and relative density of Ag–C@Ti₃AlC₂ gradually decreases. With a carbon-layer thickness of 150 nm, we obtained the lowest resistivity of Ag–C@Ti₃AlC₂ of 29.4 135.5×10?9 Ω·m, which is half that of Ag–Ti₃AlC₂ (66.7 × 10?9 Ω·m). The thermal conductivity of Ag–C@Ti₃AlC₂ reached a maximum value of 135.5 W·m?1·K?1 with a 200-nm carbon coating (~1.8 times that of Ag–Ti₃AlC₂). These results indicate that the carbon-coating method is a feasible strategy for improving the performance of Ag–C@Ti₃AlC₂ composites.     

Structural and electrical properties of HCl-polyaniline-Ag composites synthesized by polymerization using Ag-coated (NH4)2S2O8 powder

Ag nanoparticles were sputter-deposited on ammonium persulfate ((NH₄)₂S₂O₈) powder to obtain (NH₄)₂S₂O₈-Ag powder, which was used to synthesize the HCl-doped polyaniline-Ag (HCl-PANI-Ag) composite via a polymerization procedure. The Ag nanoparticles were dispersed in the HCl-PANI matrix, and their sizes mainly ranged from 3 to 6 nm. The Ag nanoparticles did not affect the structure of emeraldine salt in the composite, and they increased the ordered crystalline regions in the HCl-PANI matrix. The HCl-PANI-Ag composite had a conductivity of (6.8 ±0.1) S/cm, which is about four times larger than that of the HCl-PANI. The charge transport mechanism in the composite is explained by the three-dimensional Mott variable-range hopping (3D-Mott-VRH).     

Porous α-Al2O3 thermal barrier coatings with dispersed Pt particles prepared by cathode plasma electrolytic deposition

Porous α-Al₂O₃ thermal barrier coatings (TBCs) containing dispersed Pt particles were prepared by cathode plasma electrolytic deposition (CPED). The influence of the Pt particles on the microstructure of the coatings and the CPED process were studied. The prepared coatings were mainly composed of α-Al₂O₃. The average thickness of the coatings was approximately 100 μm. Such single-layer TBCs exhibited not only excellent high-temperature cyclic oxidation and spallation resistance, but also good thermal insulation properties. Porous α-Al₂O₃ TBCs inhibit further oxidation of alloy substrates because of their extremely low oxygen diffusion rate, provide good thermal insulation because of their porous structure, and exhibit excellent mechanical properties because of the toughening effect of the Pt particles and because of stress relaxation induced by deformation of the porous structure.     

Corrosion resistance evaluation of highly dispersed MgO-MgAl2O4-ZrO2 composite and analysis of its corrosion mechanism: A chromium-free refractory for RH refining kilns

The corrosion resistance behavior of a highly dispersed MgO-MgAl₂O₄-ZrO₂ composite refractory material is examined by testing with high-basicity and low-basicity RH (Ruhrstahl-Hereaeus) slags. The composite material exhibits greater resistance to the RH slags than the traditional MgO-Cr₂O₃ composite, MgO-ZrO₂ composite, and MgO-MgAl₂O₄-ZrO₂ composite. On the basis of the microstructural analysis and mechanisms calculations, the corrosion resistance behavior of the MgO-MgAl₂O₄-ZrO₂ composite is attributable to its highly dispersed structure, which helps protect the high activity of ZrO₂. When in contact with the slag, ZrO₂ reacts with CaO to form the stable phase CaZrO₃, which protects MgAl₂O₄ against corrosion, thereby enhancing the corrosion resistance of the composite.     

Diffusion behavior and distribution regulation of MgO in MgO-bearing pellets

In this paper, the diffusion behavior between MgO and Fe₂O₃ (the main iron oxide in pellets) is investigated using a diffusion couple method. In addition, the distribution regulation of MgO in MgO-bearing pellets is analyzed via pelletizing experiments. The results illustrate that MgO is prone to diffuse into Fe₂O₃ in the form of solid solution; the diffusion rate considered here is 13.64 μm·min-1. Most MgO content distributes in the iron phase instead of the slag phase. The MF phase {(Mg_1-x Fe_x)O·Fe₂O₃, x ≤ 1} is generated in the MgO-bearing pellets. However, the distribution of MgO in the radial direction of the pellets is inconsistent. The solid solution portion of MgO in the MF phase is larger in the outer layer of the pellets than in the inner layer. In this work, the approximate chemical composition of the MF phase in the outer layer of the pellets is {(Mg_0.35-0.77·Fe_0.65-0.23) O·Fe₂O₃} and in the inner layer is {(Mg_0.13-0.45·Fe_0.87-0.55) O·Fe₂O₃}.