Flotation is the most common method to recover valuable minerals by selective adsorption of collectors on target mineral surfaces. However, in subsequent hydrometallurgy of mineral flotation concentrates, the adsorbed collectors must be desorbed since it can adversely affect the efficiency of metallurgical process and produce wastewater. ZL, as a fatty acid mixture, is a typical industrially used collector for scheelite flotation in China. Sodium oleate (NaOL) has similar fatty acid group as ZL. In this study, the desorption behavior of NaOL/ZL from scheelite surface by a physical method of stirring at a low temperature was investigated. NaOL desorption tests of single mineral showed that a desorption rate of 77.75% for NaOL from scheelite surface into pulp was achieved in a stirring speed of 2500 r/min at 5°C in a neutral environment. Under the above desorption condition, in the pulp containing desorbed collector by adding extra 30% normal NaOL dosage, the scheelite recovery reached about 95% in the single mineral flotation test. Desorption and reuse of ZL collector for the flotation of real scheelite ore showed only a 75% normal dosage of ZL could produce a qualified rough concentrate. The atomic force microscope (AFM) tests showed that after desorption treatment of low temperature and strong stirring, the dense strip-like structure of NaOL on the scheelite surface was destroyed to be speck-like. Molecular dynamics simulations (MDS) demonstrated that the adsorption energy between NaOL and scheelite surface was more negative at 25°C (−13.39 kcal/mol) than at 5°C (−11.50 kcal/mol) in a neutral pH, indicating that a low temperature was beneficial for the desorption of collector from mineral surface. Due to its simplicity and economy, the method we proposed of desorption of collector from mineral surface and its reuse for flotation has a great potential for industrial application.
Hemimorphite exhibits poor floatability during sulfidization flotation. Cu2+ and Pb2+ addition enhances the reactivity of the hemimorphite surface and subsequently improves its flotation behavior. In this study, the mechanisms of Cu2+ + Pb2+ adsorption onto a hemimorphite surface were investigated. We examined the interaction mechanism of xanthate with the hemimorphite surface and observed the changes in the mineral surface hydrophobicity after the synergistic activation with Cu2+ + Pb2+. Microflotation tests indicated that individual activation with Cu2+ or Pb2+ increased the flotation recovery of hemimorphite, with Pb2+ showing greater effectiveness than Cu2+. Meanwhile, synergistic activation with Cu2+ + Pb2+ considerably boosted the flotation recovery of hemimorphite. Cu2+ and Pb2+ were both adsorbed onto the hemimorphite surface, forming an adsorption layer containing Cu or Pb. Following the synergistic activation with Cu2+ + Pb2+, the activated layer on the hemimorphite surface consisted of Cu and Pb and a larger amount of the active product compared with the surface activated by Cu2+ or Pb2+ alone. In addition, xanthate adsorption on the hemimorphite surface increased noticeably after synergistic activation with Cu2+ + Pb2+, suggesting a vigorous reaction between xanthate and the activated minerals. Therefore, synergistic activation with Cu2+ + Pb2+ effectively increased the content of active products on the hemimorphite surface, thereby enhancing mineral surface reactivity, promoting collector adsorption, and improving surface hydrophobicity.
Hydrogen-based mineral phase transformation (HMPT) technology has demonstrated its effectiveness in separating iron and enriching rare earths from Bayan Obo refractory ores. However, further research is needed to clarify the phase composition and floatability of rare earths obtained after HMPT owing to the associated phase transformations. This study explored the mineralogical characteristics and separation behavior of rare earths in HMPT-treated iron tailings. Process mineralogy studies conducted via BGRIMM process mineralogy analysis and X-ray diffraction revealed that the main valuable minerals in the tailings included rare-earth oxides (9.15wt%), monazite (5.31wt%), and fluorite (23.52wt%). The study also examined the impact of mineral liberation and gangue mineral intergrowth on flotation performance. Flotation tests achieved a rare-earth oxide (REO) grade of 74.12wt% with a recovery of 34.17% in open-circuit flotation, whereas closed-circuit flotation resulted in a REO grade of 60.27wt% with a recovery of 73%. Transmission electron microscopy and scanning electron microscopy coupled with energy-dispersive spectroscopy revealed that monazite remained stable during the HMPT process, while bastnaesite was transformed into Ce7O12 and CeF3, leading to increased collector consumption. Nonetheless, the HMPT process did not significantly affect the flotation performance of rare earths. The enrichment of fluorite in the tailings highlighted its further recovery potential. The integration of HMPT with magnetic separation and flotation presents an efficient strategy for recovering rare earths, iron, and fluorite from Bayan Obo ores.
To advance the precise regulation and high-value utilization of halloysite nanotubes (HNTs), this work systematically investigated five treatment strategies, including calcination, acid treatment, alkali treatment, acid treatment of calcined HNTs, and alkali treatment of calcined HNTs, to modulate their structural and application properties. The structural characteristics, surface properties, and methylene blue (MB) adsorption capacity of HNTs under multiple treatments were systematically analyzed. Calcination at varying temperatures modified the crystal structure, morphology, and surface properties of HNTs, with higher calcination temperatures reducing their reactivity towards MB. Moderate acid treatment expanded the lumen and decreased the surface potential of HNTs, significantly enhancing MB adsorption capacity. In contrast, alkali treatment dispersed the multilayered walls of HNTs and raised surface potential, reducing MB affinity. Acid treatment of calcined HNTs effectively increased their specific surface areas by leaching most of Al while maintaining the tubular structure, thereby maximizing MB adsorption. Alkali treatment of calcined HNTs destroyed the tubular structure and resulted in poor MB adsorption. HNTs pre-calcined at 600°C for 3 h and acid-treated at 60°C for 8 h exhibited an optimal specific surface area of 443 m2·g−1 and an MB adsorption capacity of 190 mg·g−1. Kinetic and Arrhenius equation fittings indicated that chemical reactions control interactions of acids and alkalis with HNTs. This study provides a comprehensive comparison and analysis of five treatment methods, offering insights into regulating the structures and surface properties of HNTs by controlling the treatment condition, thereby laying a foundation for their efficient utilization in practical applications.
The escalating production of industrial solid waste, combined with the dwindling availability of natural resources, has intensified the focus on waste recycling. However, the heterogeneity and complexity of waste pose significant challenges to determining process parameters. In this study, burnt coal cinder (BCC), granite powder (GP), and high-calcium fly ash (Class-C FA) were used as raw materials, and the response surface methodology (RSM) and single-factor experiments were applied to optimize the process parameters for geopolymer preparation. The optimized precursor powder composition was determined to be a mass ratio of 1.6:0.9:7.3 for BCC, GP, and Class-C FA. The NaOH-precursor powder ratio and liquid–solid ratio were adjusted to 0.084 and 0.222, respectively. The curing condition was set at 80°C for 24 h. The resulting 28 d-aged multi-solid wastes-based geopolymer exhibited a high compressive strength of 61.34 MPa. The microstructure, mineral phase, and atomic bonding of geopolymers were investigated using X-ray diffraction (XRD), thermal analysis (TA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Findings indicate that the compressive strength of geopolymer is most significantly influenced by the Class-C FA, followed by BCC. Furthermore, a minor addition of GP can optimize the structural density of the geopolymer. The Ca present in the Class-C FA participates in the geopolymerization, forming a hybrid N–(C)–A–S–H gel. RSM optimization facilitates the synergistic utilization of multi-solid wastes, ensuring an even distribution of gel and filler. This research establishes a theoretical framework for optimizing the preparation parameters of multi-solid wastes-based geopolymer and its subsequent applications; it holds significant scientific implications for the circular economy, resource transformation, and environmental conservation.
The large-scale accumulation of industrial solid waste, including red mud and coal gangue, coupled with goafs left by underground mining activities, poses significant challenges to sustainable human development. In this study, red mud, coal gangue, and other solid wastes were used to prepare underground backfilling materials. The utilization rate of the total solid waste reached 95%, with red mud accounting for approximately 40wt% of the total. The unconfined compressive strength, setting time, and slump tests were conducted to evaluate the mechanical properties of the material. At the optimal ratio, the 7- and 28-d strengths reach 4.4 and 6.9 MPa, respectively. The initial and final setting times were 200 and 250 min, respectively, whereas the initial and 1-h slump exceed 250 and 210 mm, respectively. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed to explore the microstructure, phase composition, and chemical bonding within the material. Needle-like, clustered, and granular hydration products were observed, and the primary crystalline structures were identified as ettringite, gmelinite, C–A–S–H, and C–S–H. In addition, a thorough environmental risk assessment was conducted, complemented by detailed economic cost and carbon emission calculations. During the creation of backfill material, hazardous elements from solid waste are immobilized through adsorption, precipitation, and incorporation into the crystal lattice. The immobilization efficiencies for Ni, Al, Cr6+, and As were 97.03%, 94.32%, 86.43%, and 84.22%, respectively, at a pH of 8.49. Moreover, the use of solid waste as a raw material results in considerable cost savings and marked reduction in carbon emissions. This study innovatively promotes the green cycle of alumina production in the bauxite mining industry.
The experiment explored the Fe2O3 reduction process with H2/CO mixed gas and confirmed a promoting effect from CO when its volume proportion in mixed gas is 20% at 850°C. The ReaxFF molecular dynamics (MD) simulation method was used to observe the reduction process and provide an atomic-level explanation. The accuracy of the parameters used in the simulation was verified by the density functional theory (DFT) calculation. The simulation shows that the initial reduction rate of H2 is much faster than that of CO (from 800 to 950°C). As the reduction proceeds, cementite, obtained after CO participates in the reduction at 850°C, will appear on the iron surface. Due to the active properties of C atoms in cementite, they are easy to further react with the O atoms in Fe2O3. The generation of internal CO may destroy the dense structure of the surface layer, thereby affecting the overall reduction swelling of Fe2O3. However, excess CO is detrimental to the reaction rate, mainly because of the poor thermodynamic conditions of CO in the temperature range and the molecular diffusion capacity is not as good as that of H2. Furthermore, the surface structures obtained after H2 and CO reduction have been compared, and it was found that the structure obtained by CO reduction has a larger surface area, thus promoting the subsequent reaction of H2.
The instantaneous morphological transition of triangular Al2O3 particles with various sizes in the molten Ca-treated steel was observed using a confocal scanning laser microscope at the steelmaking temperature. The composition of inclusions at different times was analyzed using scanning electron microscopy–energy dispersive spectroscopy. The shape evolution of particles was characterized by the shape parameter of overall regularity. It was found that the overall regularity of particles gradually increased with time during the calcium treatment. The geometry of particles tended to be more rounded and regular as the overall regularity increased during the modification process. An empirical formula was proposed to predict the composition of inclusion particles based on their overall regularity during the calcium treatment. When the CaO/Al2O3 mass ratio in the particle increased to 0.451, the particle was considered an ideal spherical calcium aluminate inclusion with the overall regularity of 1. Smaller particle sizes promoted the transformation of Al2O3 inclusions to spherical calcium aluminates during the calcium treatment.
The microstructural characteristics of austenite in Ti microalloyed steel during continuous casting significantly influence the thermoplasticity, thereby affecting the quality of the slab. In this work, a prediction model for two-stage austenite growth under varying cooling rates was established by incorporating the effect of second-phase pinning and high-temperature ferrite–austenite phase transformation and growth theory. The results indicate that with 0.02wt% Ti, the high-temperature ferrite growth exhibits typical parabolic growth characteristics. When the Ti content increases to 0.04wt%, the high-temperature ferrite grain boundary migration rate significantly slows during the initial solidification stage. The predicted austenite grain sizes for 0.02wt% Ti microalloyed steel at the center, quarter, and surface of the slab are 5592, 3529, and 1524 µm, respectively. For 0.04wt% Ti microalloyed steel, the austenite grain sizes are 4074, 2942, and 1179 µm at the same positions. The average error is within 5%. As the Ti content increases from 0.02wt% to 0.04wt%, the austenite grain refinement at the center is most significant, with an average grain size reduction of 27.14%.
Through thermodynamic calculations and microstructural characterization, the effect of niobium (Nb) content on the solidification characteristics of Alloy 625 Plus was systematically investigated. Subsequently, the effect of Nb content on hot deformation behavior was examined through hot compression experiments. The results indicated that increasing the Nb content lowers the liquidus temperature of the alloy by 51°C, producing a denser solidification microstructure. The secondary dendrite arm spacing (SDAS) of the alloy decreases from 39.09 to 22.61 µm. Increasing the Nb content alleviates element segregation but increases interdendritic precipitates, increasing their area fraction from 0.15% to 5.82%. These precipitates are primarily composed of large Laves, δ, η, and γ″ phases, and trace amounts of NbC. The shapes of these precipitates change from small chunks to large elongated forms. No significant change in the type or amount of inclusions within the alloy is detected. The inclusions are predominantly individual Al2O3 and TiN, as well as Al2O3/TiN composite inclusions. Samples with varying Nb contents underwent hot compression deformation at a true strain of 0.69, a strain rate of 0.5 s−1, and a deformation temperature of 1150°C. Increasing the Nb content also elevates the peak stress observed in the flow curves. However, alloys with higher Nb content exhibit more pronounced recrystallization softening effects. The Laves phase precipitates do not completely redissolve during hot deformation and are stretched to elongated shapes. The high-strain energy storage increases the recrystallization fraction from 32.4% to 95.5%, significantly enhancing the degree of recrystallization and producing a more uniform deformation microstructure. This effect is primarily attributed to the addition of Nb, which refines the initial grains of the alloy, enhances the solid solution strengthening of the matrix, and improves the induction of particle-stimulated nucleation.
Metal vanadates garner significant interest because of their exceptional potential for use in diverse practical applications, which stems from their unique framework structures, bond strength heterogeneities, and strong O2−–V5+ charge-transfer bands. However, their optoelectronic properties have not yet been sufficiently explored. In this study, we synthesized three high-purity calcium vanadate compounds (CaV2O6, Ca2V2O7, and Ca3V2O8) and comprehensively investigated their optoelectronic properties via first-principles calculations and experimental characterizations. CaV2O6, Ca2V2O7, and Ca3V2O8 are indirect band gap semiconductors with band gaps of 2.5–3.4 eV. A comparative analysis between density functional theory (DFT) and DFT + U (local Coulomb interaction, U) calculations revealed that standard DFT was sufficient to accurately describe the lattice parameters and band gaps of these vanadates. Further luminescence studies revealed significant photo- and electro-luminescence properties within the visible light spectrum. Notably, the luminescence intensity of CaV2O6 exhibited a remarkable 10-fold enhancement under a modest pressure of only 0.88 GPa, underscoring its exceptional potential for use in pressure-tunable optical applications. These findings provide deeper insight into the electronic structures and optical behaviors of vanadates and highlight their potential as strong candidates for application in phosphor materials and optoelectronic devices.
Hypoeutectoid steel, a crucial metal structural material, is characterized by the coexisting microstructure of ferrite and pearlite. Driven by multiphase competition and multicomponent characteristics, the intricate interplay among its composition, processing conditions, and microstructure substantially complicates the understanding of austenite decomposition kinetics and elemental diffusion mechanisms during phase transformations. The present study explores the effects of cooling rate, prior austenite grain size, and C content on the component distribution and microstructure evolution during the austenite decomposition of hypoeutectoid steels to address the aforementioned complexities. Results of a multiphase field model reveal that an increase in the cooling rate from 1.0 to 7.0°C/s leads to a reduction in the ferrite proportion and fine pearlite lamellae spacing from 52vol% to 22vol% at 400°C and from 1.01 to 0.67 µm at 660°C, respectively. Concurrently, a decreased prior austenite grain size from 25.23 to 8.91 µm enhances the phase transformation driving force, resulting in small average grain sizes of pearlite clusters and proeutectoid ferrite. Moreover, increasing the C content from 0.22wt% to 0.37wt% decreases the phase transition temperature from 795 to 750°C and enhances the proportion of pearlite phases from 27vol% to 61vol% at 500°C, concurrently refining the spacing of pearlite layers from 1.25 to 0.87 µm at 600°C. Overall, this work aims to elucidate the complex dynamics governing the microstructural transformations of hypoeutectoid steels, thereby facilitating their wide application across different industrial scenes.
Thermal and mechanical properties of yttrium tantalate (YTaO4), a top coat ceramic of thermal barrier coatings (TBCs) for aeroengines, are enhanced by synthesizing Y1−xTa1−xM2xO4 (M = Ti, Zr, Hf; x = 0.06, 0.12, 0.18, 0.24) medium-entropy ceramics (MECs) using a two-step sintering method. In addition, the thermal conductivity, thermal expansion coefficients (TECs), and fracture toughness of MECs were investigated. An X-ray diffraction study revealed that the Y1−xTa1−xM2xO4 MECs were monoclinic, and the Ti, Zr, and Hf doping elements replaced Y and Ta. The variations in atomic weights and ionic radii led to disturbed atomic arrangements and severe lattice distortions, resulting in improving the phonon scattering and reduced thermal conductivity, with Y1−xTa1−xM2xO4 MECs (x = 0.24) exhibiting the lowest thermal conductivity of 1.23 W·m−1·K−1 at 900°C. The introduction of MO2 increased the configurational entropy and weakened the ionic bonding energy, obtaining high TECs (10.4 × 10−6 K−1 at 1400°C). The reduction in the monoclinic angle β lowered the ferroelastic domain inversion energy barrier. Moreover, microcracks and crack extension toughening endowed Y1−xTa1−xM2xO4 MECs (x = 0.24) with the highest fracture toughness of (4.1 ± 0.5) MPa·m1/2. The simultaneous improvement of the thermal and mechanical properties of the MO2 (M = Ti, Zr, Hf) co-doped YTaO4 MECs can be extended to other materials.
Laser powder bed fusion (LPBF) is used to fabricate complex-shaped, dense, and high-performance oxide ceramics. During LPBF, bubbles form and evolve in the melt pool and ultimately remain in the printed ceramics as pores, which significantly degrade the mechanical properties. Therefore, it is essential to understand the bubble behaviors during LPBF. Herein, we conducted an in-situ investigation of the bubble dynamics in the melt pool of homogeneously mixed Al2O3–Y2O3 powders using synchrotron high-speed X-ray imaging. The formation, growth, motion, and evolution of bubbles, as well as the relationship between the instability of melt flow and bubble rupture during LPBF, were elucidated. The findings reveal that bubbles from the interstices within the powder bed grow following three distinct modes, i.e., uplift growth, gas channel attachment, and bubble coalescence. Furthermore, melt flow oscillations caused by the bursting of large bubbles can lead to local instability of the melt pool. Results from this study enhance the understanding of bubble dynamics during LPBF and may provide valuable insights for pore elimination in LPBF-processed oxide ceramics.
Along with the surging demand for energy storage devices, the cost and availability of the materials remain dominant factors in slowing down their industrial application. The repurposing of waste asphalt into high-performance electrode materials is of significant interest, as it holds the potential to circumvent energy and environmental issues. Here, we report the controllable synthesis of asphalt-derived mesoporous carbon as an active material for electrocatalytic hydrogen gas capacitor (EHGC). The hierarchically porous carbon (HPC) with a high surface area of 1943.4 m2·g−1 can operate in pH universal aqueous electrolytes in EHGC. It displays a specific energy and power density of 57 Wh·kg−1 and 554 W·kg−1 in neutral electrolyte as well as 52 Wh·kg−1 and 657 W·kg−1 in acidic electrolyte. Additionally, the charge storage mechanism of HPC–EHGC is studied with the help of Raman spectroscopy and X-ray photoelectron spectroscopy. Furthermore, the assembled HPC–EHGC device displays a discharge capacitance of 170 F·g−1 with an excellent capacitance retention rate of 100% up to 20000 cycles at 10 A·g−1 in acidic electrolyte. This work introduces a novel approach to converting waste asphalt into high-performance carbon for EHGC, achieving superior performance over commercial materials. By simultaneously addressing environmental waste issues and advancing energy storage technology, this study makes a significant contribution to sustainable materials science and next-generation battery development.