2021 Vol. 28, No. 6
With the increasing demand of rare earth metals in functional materials, recovery of rare earth elements (REEs) from secondary resources has become important for the green economy transition. Molten salt electrolysis has the advantages of low water consumption and low hazardous waste during REE recovery. This review systematically summarizes the separation and electroextraction of REEs on various reactive electrodes in different molten salts. It also highlights the relationship between the formed alloy phases and electrodeposition parameters, including applied potential, current, and ion concentration. Moreover, the feasibility of using LiF–NaF–KF electrolyte to recover REEs is evaluated through thermodynamic analysis. Problems related to REE separation/recovery the choice of electrolyte are discussed in detail to realize the low-energy and high current efficiency of practical applications.
High-temperature oxidation is a common failure in high-temperature environments, which widely occur in aircraft engines and aerospace thrusters; as a result, the development of anti-high-temperature oxidation materials has been pursued. Ni-based alloys are a common high-temperature material; however, they are too expensive. High-entropy alloys are alternatives for the anti-oxidation property at high temperatures because of their special structure and properties. The recent achievements of high-temperature oxidation are reviewed in this paper. The high-temperature oxidation environment, temperature, phase structure, alloy elements, and preparation methods of high-entropy alloys are summarized. The reason why high-entropy alloys have anti-oxidation ability at high temperatures is illuminated. Current research, material selection, and application prospects of high-temperature oxidation are introduced.
High-entropy alloys (HEAs) have attracted increasing attention because of their unique properties, including high strength, hardness, chemical stability, and good wear resistance. Powder metallurgy is one of the most important methods used to fabricate HEA materials. This paper introduces the methods used to synthesize HEA powders and consolidate HEA bulk. The phase transformation, microstructural evolution, and mechanical properties of HEAs obtained by powder metallurgy are summarized. We also address HEA-related materials such as ceramic–HEA cermets and HEA-based composites fabricated by powder metallurgy.
As ore grades constantly decline, more copper tailings, which still contain a considerable amount of unrecovered copper, are expected to be produced as a byproduct of froth flotation. This research reveals the occurrence mechanism of copper minerals in typical copper sulfide tailings using quantitative mineral liberation analysis (MLA) integrated with scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). A comprehensive mineralogical characterization was carried out, and the results showed that almost all copper minerals were highly disseminated within coarse gangue particles, except for 9.2wt% chalcopyrite that occurred in the 160–180 μm size fraction. The predominant copper-bearing mineral was chalcopyrite, which was closely intergrown with orthoclase and muscovite rather than quartz. The flotation tailings sample still contained 3.28wt% liberated chalcopyrite and 3.13wt% liberated bornite because of their extremely fine granularity. The SEM–EDS analysis further demonstrated that copper minerals mainly occurred as fine dispersed and fully enclosed structures in gangue minerals. The information obtained from this research could offer useful references for recovering residual copper from flotation tailings.
The extraction of gold from refractory gold ores (RGOs) without side reactions is an extremely promising endeavor. However, most RGOs contain large amounts of sulfide, such as pyrite. Thus, investigation of the influence of sulfide on the gold leaching process is important to maximize the utilization of RGOs. In this work, the effects of pyrite on the stability of the thiourea system were systematically investigated under different conditions. Results showed that the decomposition rate of thiourea was accelerated sharply in the presence of pyrite. The effect of pyrite on gold recovery in thiourea leaching systems was then confirmed via a series of experiments. The decomposition efficiency of thiourea decreased by 40% and the recovery efficiency of gold increased by 56% after the removal of sulfide by roasting. Under optimal conditions, the efficiency of the gold recovery system increased to 83.69% and only 57.92% of thiourea decomposition was observed. The high consumption of thiourea by the leaching system may be attributed to not only adsorption by mineral particles but also catalytic decomposition by some impurities in the ores, such as pyrite and soluble ferric oxide.
A high-voltage pulsed discharge (HVPD) pretreatment was used to strengthen the leaching effect of Carlin-type gold ore containing arsenic. Optimal results of the pretreatment experiments were obtained at the following operating conditions: a spherical gap spacing of 20 mm, pulse number of 100, and voltage of 30 kV. The leaching rate of gold was increased by 15.65% via the HVPD pretreatment. The mass fraction of –0.5+0.35 mm and –0.35+0.1 mm was increased by 10.97% and 6.83% compared to the untreated samples, respectively, and the Au grade of –0.1 mm was increased by 22.84%. However, the superiority of the HVPD pretreatment would be weakened by prolonged grinding time. Scanning electron microscopy results indicated that the pretreated products presented as a melting state and then condensation, accompanying by some pore formation. More micro-cracks were generated at the interface of the ore and the original crack were expended via pulsed discharge pretreatment, with the contact area between the leaching reagent and ore increased, the leaching reaction rate enhanced and the leaching effect strengthened.
The fusion of the leaching and purification processes was realized by directly using microemulsion as the leaching agent. The bis-(2-ethyhexyl) phosphoric acid (DEHPA)/n-heptane/NaOH microemulsion system was established to directly leach vanadates from sodium-roasted vanadium slag. The effect of the leaching agent on the leaching efficiency was investigated, in addition to the molar ratio of H2O/NaDEHP (W), DEHPA concentration, solid/liquid ratio, stirring time, and leaching temperature. In optimal situations, the vanadium leaching efficiency reaches 79.57%. The X-ray diffraction characterization of the leaching residue and the Raman spectrum of the microemulsion before and after leaching demonstrate the successful entry of vanadates from the sodium-roasted vanadium slag into the microemulsion. The proposed method successfully realizes the leaching and purification of vanadates in one step, thereby greatly reducing production costs and environmental pollution. It also offers a new way to achieve the green recovery of valuable metals from solid resources.
The oxidation pathway and kinetics of titania slag powders in air were analyzed using differential scanning calorimetry (DSC) and thermogravimetry (TG). The oxidation pathway of titania slag powder in air was divided into three stages according to their three exothermic peaks and three corresponding mass gain stages indicated by the respective non-isothermal DSC and TG curves. The isothermal oxidation kinetics of high titania slag powders of different sizes were analyzed using the ln-ln analysis method. The results revealed that the entire isothermal oxidation process comprises two stages. The kinetic mechanism of the first stage can be described as
The reductant is a critical factor in the hydrometallurgical recycling of valuable metals from spent lithium-ion batteries (LIBs). There is limited information regarding the use of SnCl2 as a reductant with organic acid (maleic acid) for recovering valuable metals from spent LiCoO2 material. In this study, the leaching efficiencies of Li and Co with 1 mol·L−1 of maleic acid and 0.3 mol·L−1 of SnCl2 were found to be 98.67% and 97.5%, respectively, at 60°C and a reaction time of 40 min. We investigated the kinetics and thermodynamics of the leaching process in this study to better understand the mechanism of the leaching process. Based on a comparison with H2O2 with respect to leaching efficiency, the optimal leaching parameters, and the activation energy, we determined that it is feasible to replace H2O2 with SnCl2 as a leaching reductant in the leaching process. In addition, when SnCl2 is used in the acid-leaching process, Sn residue in the leachate may have a positive effect on the re-synthesis of nickel-rich cathode materials. Therefore, the results of this study provide a potential direction for the selection of reductants in the hydrometallurgical recovery of valuable metals from spent LIBs.
Blast furnace data processing is prone to problems such as outliers. To overcome these problems and identify an improved method for processing blast furnace data, we conducted an in-depth study of blast furnace data. Based on data samples from selected iron and steel companies, data types were classified according to different characteristics; then, appropriate methods were selected to process them in order to solve the deficiencies and outliers of the original blast furnace data. Linear interpolation was used to fill in the divided continuation data, the K-nearest neighbor (KNN) algorithm was used to fill in correlation data with the internal law, and periodic statistical data were filled by the average. The error rate in the filling was low, and the fitting degree was over 85%. For the screening of outliers, corresponding indicator parameters were added according to the continuity, relevance, and periodicity of different data. Also, a variety of algorithms were used for processing. Through the analysis of screening results, a large amount of efficient information in the data was retained, and ineffective outliers were eliminated. Standardized processing of blast furnace big data as the basis of applied research on blast furnace big data can serve as an important means to improve data quality and retain data value.
It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.
The effects of the welding current mode in resistance spot welding on the microstructure and mechanical properties of advanced high-strength steel dual-phase 590 (DP590) sheets were investigated. Results showed that a rough martensitic structure was formed in the weld zone of the sample welded via the single-pulsed mode, whereas the microstructure in the heat-affected zone consisted of a very rough martensitic microstructure and rough ferrite. However, using the secondary pulse mode led to the formation of tempered martensite in the weld zone. The maximum load and the energy absorption to failure of the samples with the secondary pulsed cycle were higher than those of the samples with the single-pulsed mode. Tensile shear results indicated that the secondary pulsed mode could significantly change the mode of failure upon shear tension testing. Therefore, the obtained results suggest that the use of secondary pulsed mode can improve the microstructural feature and mechanical properties of advanced high-strength steel DP590 welds.
Lead halide perovskites have received increasing attention recently as a candidate material in various optoelectronic areas because of their high performance as light absorbers. Herein, we report the growth of CsPbI3 nanobelts via a solution process. A single-crystalline CsPbI3 nanobelt with uniform morphology can be achieved by controlling the amount of PbI2. A single-crystalline CsPbI3 nanobelt possesses a mean width, length, and thickness of 100 nm, 5 µm, and 20 nm, respectively. In this work, photodetectors (PDs) based on individual CsPbI3 nanobelts are constructed and found to perform well with an external quantum efficiency and responsivity of 2.39 × 105% and 770 A/W, respectively. The PDs also show a high detectivity of up to 3.12 × 1012 Jones, which is at par with that of Si PDs. The PDs developed in this work exhibit great promise in various optoelectronic nanodevices.
We report the electrochemical performance of Ni(OH)2 on a gas diffusion layer (GDL). The Ni(OH)2 working electrode was successfully prepared via a simple method, and its electrochemical performance in 1 M NaOH electrolyte was investigated. The electrochemical results showed that the Ni(OH)2/GDL provided the maximum specific capacitance value (418.11 F·g−1) at 1 A·g−1. Furthermore, the Ni(OH)2 electrode delivered a high specific energy of 17.25 Wh·kg−1 at a specific power of 272.5 W·kg−1 and retained about 81% of the capacitance after 1000 cycles of galvanostatic charge–discharge (GCD) measurements. The results of scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed the occurrence of sodium deposition after long-time cycling, which caused the reduction in the specific capacitance. This study results suggest that the light-weight GDL, which can help overcome the problem of the oxide layer on metal–foam substrates, is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.
Utilization of novel materials, particularly high-Tc (critical temperature) superconductors, is essential to pursue the United Nations’ Sustainable Goals, as well as to meet the increasing worldwide demand for clean and carbon-free electric power technologies. Superconducting magnets are beneficial in several real-life applications including transportation, energy production, magnetic resonance imaging (MRI), and drug delivery systems. To achieve high performance, one must develop uniform, large-grain, infiltration-growth (IG) processed bulk YBa2Cu3Oy (Y-123) super-magnets. In this study, we report the magnetic and microstructural properties of a large-grain, top-seeded, IG-processed Y-123 pellet, which is 20 mm in diameter and 6 mm in height; the pellet is produced utilizing liquid Yb-123+Ba3Cu5O8 as the liquid source. All the samples cut from the top of the bulk exhibit a sharp superconducting transition (approximately 1 K wide) with the onset Tc of approximately 90 K. However, in the samples cut from the bottom surface, the onset Tc values slightly decreased to between 88 and 90 K, although still exhibiting a sharp superconducting transition. The top and bottom samples exhibited the highest remnant value of Jc (critical current density) at 77 K H//c-axis of 50 and 55 kA/cm2, respectively. The remnant Jc and irreversibility field values significantly fluctuated, being fairly low in some bottom samples. Scanning electron microscopy identified nanometer size Y-211 (Y2BaCuO5) secondary-phase particles dispersed in the Y-123 matrix. Energy-dispersive spectroscopy clarified that the decreased both Tc and Jc for the bottom samples were attributed to liquid phase dispersion within the Y-123 phase.
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.
For this study, we synthesized Aurivillius Bi5Ti3FeO15 ceramic using the generic solid-state reaction route and then performed room-temperature X-ray diffraction to confirm that the compound had a single phase with no impurities. The surface morphology of the prepared sample was observed to contain microstructural grains approximately 0.2–2 μm in size. The dielectric properties of the sample were determined as a function of frequency in a range of approximately 100 Hz to 1 MHz at various temperatures (303 K ≤ T ≤ 773 K). Nyquist plots of the impedance data were found to exhibit a semi-circular arc in the high-temperature region, which is explained by the equivalent electrical circuit (R1C1)(R2QC2), where R1 and R2 represent the resistances associated with the grains and grain boundaries, respectively, C1 and C2 are the respective capacitances, and Q is the constant phase element (CPE), which accounts for non-Debye type of behavior. Our results indicate that both the resistance and capacitance of the grain boundaries are more prominent than those of the grains. The alternating current (ac) conductivity data were analyzed based on the Jonscher universal power law, which indicated that the conduction process is dominated by the hopping mechanism. The calculated activation energies of the relaxation and conduction processes were very similar (0.32 to 0.53 eV), from which we conclude that the same type of charge carriers are involved in both processes.
A facile approach was developed to construct Fe2O3-modified ZnO micro/nanostructures with excellent superhydrophobicity and photocatalytic activities. The effects of stearic acid (SA) and Fe2O3 on the morphological characteristics, water contact angle (WCA), and photocatalytic degradation were investigated. Superhydrophobicity results showed that WCA increased from 144° ± 2° to 154° ± 2° when the weight of SA increased from 5 to 20 mg because of the formation of a hierarchical or rough structure. Furthermore, Fe2O3-modified ZnO micro/nanostructure surfaces before and after SA treatment (20 mg) were chosen to evaluate the photodegradation of methylene blue (MB) dye under the support of visible light. MB degraded after 80 min of irradiation, and its photodegradation efficiencies were 91.5% at the superhydrophobic state and 92% at the hydrophilic state. This improvement in photocatalytic activity at both states might be attributed to an increase in surface area and improvement in charge carrier separation.
In the present research, aluminum metal matrix composites were processed by the stir casting technique. The effects of TiB2 reinforcement particles, severe plastic deformation through accumulative roll bonding (ARB), and aging treatment on the microstructural characteristics and mechanical properties were also evaluated. Uniaxial tensile tests and microhardness measurements were conducted, and the microstructural characteristics were investigated. Notably, the important problems associated with cast samples, including nonuniformity of the reinforcement particles and high porosity content, were solved through the ARB process. At the initial stage, particle-free zones, as well as particle clusters, were observed on the microstructure of the composite. However, after the ARB process, fracturing phenomena occurred in brittle ceramic particles, followed by breaking down of the fragments into fine particles as the number of rolling cycles increased. Subsequently, composites with a uniform distribution of particles were produced. Moreover, the tensile strength and microhardness of the ARB-processed composites increased with the increase in the reinforcement mass fraction. However, their ductility exhibited a different trend. With post-deformation aging treatment (T6), the mechanical properties of composites were improved because of the formation of fine Mg2Si precipitates.
Copper matrix composites reinforced by in situ-formed hybrid titanium boride whiskers (TiBw) and titanium diboride particles (TiB2p) were fabricated by powder metallurgy. Microstructural observations showed competitive precipitation behavior between TiBw and TiB2p, where the relative contents of the two reinforcements varied with sintering temperature. Based on thermodynamic and kinetic assessments, the precipitation mechanisms of the hybrid reinforcements were discussed, and the formation of both TiBw and TiB2p from the local melting zone was thermodynamically favored. The precipitation kinetics were mainly controlled by a solid-state diffusion of B atoms. By forming a compact compound layer, in situ reactions were divided into two stages, where Zener growth and Dybkov growth prevailed, respectively. Accordingly, the competitive precipitation behavior was attributed to the transition of the growth model during the reaction process.