Search Results (81)

This study presents research on optimizing the parameters of the passivation process for precipitation-hardening stainless steels (PHSS) to improve the corrosion resistance of AM350 and CUSTOM 450 alloys, which are extensively utilized in the aerospace and aviation sectors, since, as this is a complex process, it requires the implementation of a robust methodological approach that allows for multi-response optimization. Experiments were designed using the Taguchi method, which offered a strong framework for examining the impact of material, type of passivation solution, concentration, temperature, and passivation process time on the corrosion resistance of both PHSS alloys. To confirm the ideal PHSS passivation process parameters and measure the significance of each component, gray relational analysis (GRA) and analysis of variance (ANOVA) were also employed. The combined use of the Taguchi/GRA represents a robust and efficient methodological approach to the multi-response optimization of complex processes, overcoming the limitations inherent in the individual application of each technique. It was determined that the optimized parameters were a PHSS AM 350, a solution composed of a combination of citric acid and oxalic acid, acid concentration of 25% v/v, temperature of 50 °C, and time of 120 min. This combination of parameters resulted in significant improvements of up to 55% in corrosion resistance in the H₂SO₄ and NaCl evaluation solutions, demonstrating the effectiveness of the optimized conditions. This work emphasizes the efficacy of integrating Taguchi, GRA, and ANOVA techniques to significantly reduce the corrosion rate of PHSS undergoing the passivation process using alternatives to nitric acid. The integration of the Taguchi methodology with GRA enables the normalization and combination of responses with different scales and performance criteria into a single gray relational index, facilitating the overall evaluation of the system. Full article

To address the issues of high wastewater treatment costs and the lack of recycling associated with conventional precipitants such as oxalic acid and ammonium bicarbonate in rare earth precipitation processes, this study proposes a novel gradient alkali conversion–carbonation method based on a green process coupling “rare earth chloride alkali conversion-carbonation with sodium chloride electrolysis.” The primary scientific objective is to elucidate the crystallization mechanism and to achieve controlled preparation of high-quality lanthanum carbonate. By gradient-controlling the addition sequence and rate of alkali liquor and CO₂, lanthanum carbonate tetrahydrate was successfully synthesized. Characterization by XRD, SEM, ICP, and laser particle size analysis indicates that the product prepared by the gradient alkali conversion–carbonation method exhibits a single phase with high crystallinity, as evidenced by sharp and clear XRD diffraction peaks. Furthermore, the median particle size of the product obtained via this method is relatively large, reaching approximately 10 μm, while the particle size distribution Span value remains around 1.0. Mechanistic studies suggest that this method effectively regulates the crystallization process by precisely controlling the introduction and slow dissolution of the La(OH)₃ precursor, thereby reducing the supersaturation of the system during carbonation and facilitating the dissolution–reprecipitation of La³⁺. This work provides a theoretical basis for the preparation of high-quality rare earth carbonates and a process reference for the green recycling route. Full article

The hematite phase decorated with iron-doped cerium oxide nanoparticles (F@FC) was precipitated from cerium and iron oxalate intermediate products. The photocatalytic composite of graphitic carbon nitride (gCN) and F@FC was prepared by a simple method involving mixing the two components, followed by thermal treatment at 400 °C. According to electron microscopy, F@FC is composed of a submicron iron oxide (hematite) phase decorated with iron-doped cerium oxide nanoparticles deposited on gCN substrate. A hierarchically structured composite was observed instead of a simple mechanical mixture of α-Fe₂O₃, Fe-CeO₂, and gCN. To observe two types of degradation activity, photocatalytic and Photo-Fenton degradation activity, Rhodamine B (RhB) was applied as the model water pollutant. The influence of the amount of photocatalyst, the RhB concentration, the presence of cations and anions, the pH, and the effect of e⁻, h⁺, •OH, and •O₂⁻ scavenging reactants were studied. The Photo-Fenton degradation exhibited high efficiency across the entire tested pH range, whereas photocatalytic degradation showed comparable activity only at acidic pH. The F@FC-gCN composite catalyst exhibited a high degree of recyclability. The degradation pathways of photocatalytic and Photo-Fenton reactions were suggested by HPLC-MS analysis of the reaction products. A notable finding of this study was the observation that the green-yellow, fluorescent intermediate Rhodamine 110 was formed during the photocatalytic degradation of RhB. However, the high reactivity of the generated •OH radicals during Photo-Fenton degradation has been demonstrated to inhibit the formation of intermediate Rhodamine 110. Full article

The NdFeB magnetic material is widely used in modern industry and electronics. Recycling spent magnets containing 30–40 wt.% Rare Earth Elements (REEs) and 60–70 wt.% iron is essential for resource recovery. This study focuses on treating wastewater from hydrometallurgical REE extraction to enable water reuse and iron recovery. Experiments investigated the influence of temperature (25–60 °C) and oxalic acid concentration (50–120% stoichiometric amount) on iron precipitation as the main focus. Optimal iron recovery of 85.3% and 86.8% was achieved at 25 °C using 80% and 100% stoichiometric oxalic acid, respectively. Simultaneously, praseodymium removal exceeded 94%. The Differential Scanning Calorimetry (DSC) analysis identified a peak between 168.8 °C and 245 °C, with the peak temperature recorded at 214.5 °C, indicating the optimal calcination temperature for iron oxalate. This was confirmed through oven testing and X-ray Diffraction (XRD) analysis. The treated effluent exhibited a Chemical Oxygen Demand (COD) of 418 mg/L, with residual Fe and Pr concentrations significantly reduced to 5.7 mg/L and 4.3 mg/L, respectively. This approach demonstrates an efficient method for wastewater treatment and resource valorization, promoting sustainability in the recycling sector. Full article

The rapid increase in end-of-life lithium-ion batteries demands sustainable recycling routes for lithium recovery. This work presents a novel integrated hydrometallurgical–electrodialysis process designed specifically for recovering lithium from off-specification NMC cathode materials while enabling full reagent recyclability. Selective leaching with oxalic acid was optimised by setting the water-to-oxalic acid dihydrate ratio (H₂O/OA·2H₂O) to 7.3:1 w/w, achieving 81% lithium extraction at room temperature within 2 h while limiting the co-dissolution of Ni, Co and Mn to 0.2%, 1.6% and 1.7% by weight, respectively. The resulting leachate was processed in a four-chamber electrodialysis cell equipped with two Nafion 117 cation-exchange membranes and one Neosepta AMX-fmg anion-exchange membrane operating at −1.6 V versus Ag/AgCl, enabling 96% lithium recovery and 98% oxalic acid recovery. The regenerated oxalic acid stream (41.8 g L⁻¹) was fully restored to its initial concentration and reused in successive cycles without performance loss. Subsequent precipitation of lithium with Na₂CO₃ yielded 99.3%-pure Li₂CO₃. This combined leaching–electrodialysis–precipitation presents a high selectivity, low-waste, circular recovery system, offering a scientifically original approach that integrates reagent regeneration with high-purity lithium production. Full article

Fungal transformation is increasingly recognized as an important process influencing metal solubilization and immobilization in soil environments. In this study, a fungal strain (PTW4) isolated from mining-contaminated soil was molecularly identified as Aspergillus aculeatus. The strain was evaluated for its ability to solubilize and transform several heavy metal oxides, including ZnO, Pb₃O₄, Cu₂O, and MoO₃. PTW4 produced consistent halo formation across all tested oxides, accompanied by progressive acidification of the culture medium, suggesting organic acid-mediated solubilization. Characterization of extracellular precipitates by SEM-EDS and XRD indicated mineral phases consistent with oxalate-associated biominerals, including zinc oxalate dihydrate (ZnC₂O₄·2H₂O), lead oxalate (PbC₂O₄), and copper oxalate hydrate (CuC₂O₄·xH₂O). These minerals represent low-solubility phases that may reduce metal mobility in the surrounding environment. In contrast, molybdenum did not precipitate under the experimental conditions, suggesting metal-specific constraints in fungal biomineralization processes. Although organic acid production was not directly quantified, identification of oxalate mineral phases supports an oxalate-associated mineralization mechanism. Overall, the results provide evidence for heavy metal solubilization and selective extracellular precipitation consistent with oxalate biomineral formation by A. aculeatus PTW4, highlighting its potential relevance to fungal-mediated bioremediation and selective bioleaching processes. Full article

The growing demand for higher-energy lithium-ion batteries, encompassing consumer electronics, stationary grid storage, and electric mobility to specialized sectors like aerospace, medical devices, and industrial robotics, requires cathode materials that offer higher capacity while remaining cost-effective. This trend has intensified the development of nickel-rich LiNi_1−x−yMn_xCo_yO₂ (NMC) systems. However, high-Ni NMCs such as LiNi_0.9Mn_0.05Co_0.05O₂ (NMC90) suffer from limited thermal and cycling stability. Core–shell architectures using LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) as a shell can partially alleviate these drawbacks, but structural degradation caused by interdiffusion between the core and shell persists as a major challenge. This study investigates whether a tungsten oxide interlayer can act as a protective barrier that suppresses interdiffusion, stabilizes the crystal structure, and improves long-term electrochemical performance. In this work, NMC cathode powders were synthesized via a one-pot oxalate co-precipitation route, followed by structural characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ion scattering spectroscopy (ISS). Electrochemical performance, including capacity retention, cycling stability, and internal resistance, was evaluated through galvanostatic charge–discharge (GCD) testing and electrochemical impedance spectroscopy (EIS). The core–shell configuration delivered higher specific discharge capacity compared to the individually synthesized core-only and shell-only reference materials, and the incorporation of a tungsten oxide interlayer resulted in a twofold increase in cycle life. These results demonstrate that tungsten oxide effectively enhances cycling stability by inhibiting core–shell interdiffusion, offering a promising pathway toward more durable high-Ni NMC cathodes. Full article

Neodymium–iron–boron (NdFeB) waste represents a valuable secondary source of rare earth elements (REEs). However, existing recovery technologies face several challenges, such as the difficulty of selectively recovering REEs, the generation of large volumes of secondary iron-rich slag, and an overall low level of comprehensive resource utilization. In this study, Aliquat 336 was applied for the selective extraction and separation of REEs and iron (Fe) from hydrochloric acid leachate derived from NdFeB waste. Experimental results showed that under optimized conditions—specifically, a 15% Aliquat 336 concentration, an organic-to-aqueous phase ratio of 1:2, and a 2 min extraction time—Fe extraction efficiency reached 99.93% after three-stage countercurrent extraction, while REEs were predominantly retained in the aqueous phase. Subsequent oxalic acid precipitation of the raffinate yielded RE₂(C₂O₄)₃·10H₂O with a purity of 99.60%. Moreover, under stripping conditions of 2 mol/L NaOH, a phase ratio of 2:1 (aqueous to organic), and a 2 min contact time, over 99.21% of Fe was stripped after three-stage countercurrent stripping, resulting in Fe(OH)₃ with a purity of 99.26%. The extraction mechanism followed an anion-exchange process: under high chloride ion concentrations, Fe³⁺ formed anionic FeCl₄⁻ complexes, which were exchanged with Cl⁻ ions in Aliquat 336 and transferred into the organic phase, whereas RE³⁺ cations remained in the aqueous phase, enabling efficient separation of Fe and REEs. These findings provide important insights for improving the comprehensive utilization of NdFeB waste and promoting the green and sustainable development of secondary rare earth resource recycling. Full article

Conventional hydrometallurgical processes typically employ inorganic acids as leaching agents; however, these processes are frequently associated with significant environmental pollution and suffer from poor metal selectivity. Oxalic acid, as a green alternative leaching agent, demonstrates considerable application potential owing to its mild acidity, strong reducing capability, and superior complexing properties. This paper presents a systematic review of recent advances in the application of oxalic acid in hydrometallurgy, encompassing the coordination chemistry between oxalic acid and metal ions, its role as a selective leaching agent, and strategies for handling multicomponent oxalate-rich solutions. Furthermore, the industrial prospects of oxalic acid-based leaching technologies are discussed. Research indicates that oxalic acid exhibits high selectivity and efficient leaching performance for critical metals—including vanadium, lithium, cobalt, nickel, and gallium—from both primary ores and solid secondary resources. The underlying leaching mechanism primarily involves the formation of stable chelation complexes between oxalate anions and high charge-density metal ions, or valence state modulation via reduction, enabling selective dissolution and separation of target metals. In multicomponent oxalate systems, where metals predominantly exist as anionic complexes, established enrichment and purification approaches include anion exchange extraction, as well as precipitation techniques based on valence adjustment and double salt crystallization. To advance the industrial implementation of oxalic acid leaching technologies, further in-depth investigation is required into the recycling mechanisms of oxalic acid and the fundamental reaction pathways governing leaching and metal recovery processes. Full article

In this work, a method for leaching black mass from spent Li batteries using oxalic acid was developed and experimentally verified with the objective of selectively separating lithium and cobalt. Oxalic acid proved to be an efficient and selective leaching agent. Under 1 M C₂H₂O₄, 120 min, L:S = 20, 80 °C and 300 rpm, a lithium yield of 90% was achieved, while cobalt dissolution remained low at 1.57%. Subsequently, cobalt spontaneously precipitated from the leachate within several hours, and the solid phase was fully separated after 24 h. The leachate contained minor amounts of accompanying metals, with dissolution yields of 0.5% Mn, 8% Fe and 1.4% Cu. These impurities were removed from the leachate by controlled pH adjustment using NaOH at ambient temperature and 450 rpm, with complete precipitation at pH 12. This procedure generated a purified lithium-rich leachate, which was converted into lithium oxalate by crystallisation at 105 °C. Subsequent calcination of the resulting solid at 450 °C for 30 min produced Li₂CO₃ with a purity of 91%. Based on the experimental findings, a conceptual technological process for selective lithium leaching using oxalic acid was proposed, demonstrating the potential of this method for sustainable lithium recovery. Full article

Thorium carbide (${\mathrm{ThC}}_{2\pm x}$) nano-structured thin disc-like pellets were produced from thoria nanoparticles (${\mathrm{ThO}}_2$-NP) and multi-walled carbon nanotubes (MWCNT). These composites are to be studied as a target material candidate for radioactive ion beam (RIB) production via nuclear reactions upon impact with high-energy proton beams on a stack of solid pellets. The ${\mathrm{ThO}}_2$-NP precursor was produced via precipitation of thorium oxalate from a thorium nitrate solution with oxalic acid and subsequent hydrothermal oxidation of the oxalate, creating the thoria nanoparticles. The ${\mathrm{ThO}}_2$-NP were then mixed with MWCNT in isopropyl alcohol and sonicated by two different methods to create a nanoparticle dispersion. This dispersion was then heated under medium vacuum to evaporate the solvent; the resulting powder was pressed into pellets and taken to an inert-atmosphere oven, where it was heated to 1650 °C and carbothermally reduced to ${\mathrm{ThC}}_{2\pm x}$. The resulting pellets were characterized via XRD, SEM-EDS, and Raman spectroscopy. The resulting thorium pellets exhibited, at most, trace levels of the oxide precursor. Furthermore, the nanotube structures were still present in the final product and are expected to contribute positively towards faster radioisotope release times by lowering isotope diffusion times, which is required for the efficient extraction of the shortest-lived (<1 s half-life) radioisotopes. Full article

In the nuclear industry, the decontamination of nuclear metallic structures is an essential process to reduce radiation exposure during maintenance or dismantling. The oxide layer, such as chromium (III) oxide (Cr₂O₃), formed on stainless steel and nickel-based alloys, contributes significantly to surface radioactivity by trapping radioactive contaminants. To address this, permanganic acid (HMnO₄) has proven to be a promising oxidizing agent for dissolving these oxide layers—particularly chromium oxide—on stainless steel and nickel-based alloys. In this study, HMnO₄ was synthesized via ion exchange using AmberLite IRN97 H resin and potassium permanganate (KMnO₄). The optimized process yielded a highly acidic solution (pH~1.6) with potassium concentrations below 0.1 ppm, indicating near-complete exchange efficiency. Dissolution kinetics were investigated at HMnO₄ concentrations ranging from 240 to 1920 ppm and temperatures from 30 °C to 80 °C. At a constant temperature, increasing HMnO₄ concentration significantly improved Cr dissolution, with up to 31% of total chromium solubilized after 33 h. Lower temperatures favored higher dissolution efficiency, likely due to improved thermal stability of HMnO₄. For durations shorter than 4 h, the influence of temperature was limited compared to the effect of acid concentration. To assess post-treatment options, HMnO₄ decomposition was studied using oxalic acid (H₂C₂O₄) at 80 °C. Results showed that a minimum H₂C₂O₄/HMnO₄ molar ratio above 2.75 was necessary to achieve effective reduction while preventing MnO₂ precipitation. However, even under strongly acidic conditions and with a large excess of reductant, Mn²⁺ yields remained below 55%, suggesting that thermal degradation of oxalic acid and possible formation of undetected manganese species limited the reduction process. Full article

Based on the physicochemical properties of waste phosphate-based rare earth phosphors containing glass, this paper proposes a novel recovery method for rare earth elements (REEs) that integrates pre-enrichment, alkali roasting, and enhanced leaching. Initially, preliminary enrichment of REEs was achieved through sieving to remove silicon (from glass components) and pickling to reduce calcium content (originating from calcium phosphate compounds). The enriched material was then subjected to alkaline roasting, followed by washing for impurity removal, hydrochloric acid leaching, and finally oxalic acid precipitation to extract the rare earth elements. Experimental results demonstrate that the overall recovery rate of rare earth oxides (REO) reached 96.6%, indicating highly efficient extraction and separation of REEs from the waste phosphors. Furthermore, the mechanism of the alkali roasting process was investigated via differential thermal analysis (TG-DSC). Microstructural and phase changes in the waste phosphors before and after roasting were systematically characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that green phosphor (REPO₄) was converted into rare earth oxides and water-soluble sodium phosphate under alkaline roasting conditions. The Na₃PO₄ could be effectively removed through washing, while the rare earth elements were retained in the form of oxides within the washed residue. This study provides an important theoretical foundation and technical approach for the efficient recovery of rare earth resources from waste phosphate-based phosphors. Full article

The sustainable recycling of valuable metals from spent lithium-ion batteries (LIBs) is critical for resource conservation and environmental protection but remains challenging due to the complex coexistence of target and impurity metals. This study systematically investigates the selective leaching behaviors of metals (Co, Li, Cu, Fe, Al) in phosphoric acid media, revealing that lithium could be preferentially extracted in mild acidic conditions (0.8 mol/L H₃PO₄), while complete dissolution of both Li and Co was achieved in concentrated acid (2.0 mol/L H₃PO₄). Kinetic analysis demonstrated that metal leaching followed a chemically controlled mechanism, with distinct extraction sequences: Li > Cu~Co > Fe > Al in dilute acid and Cu > Al~Li > Fe > Co in concentrated acid. Furthermore, we developed a closed-loop process wherein oxalic acid simultaneously precipitates Co/Li while regenerating H₃PO₄, enabling acid reuse with minimal efficiency loss during cyclic leaching. These findings establish a single-step phosphoric acid leaching strategy for selective metal recovery, governed by tunable acid concentration and reaction kinetics, offering a sustainable pathway for LIBs recycling. Full article

This article presents an innovative method for phosphate(V) removal from industrial wastewater using cerium(III) chloride as a coagulant, integrated with reagent recovery. The process combines coagulation, acid extraction, and multistage recovery of cerium and phosphorus, enabling partial reagent loop closure. Based on our previously published studies, at an optimised dose (81.9 mg Ce³⁺/L), phosphate(V) removal reached 99.86% and total phosphorus (sum of all phosphorus forms as elemental P), 99.56%, and 99.94% of the added cerium was retained in sludge. Reductions were also observed for TSS (96.67%), turbidity (98.18%), and COD (81.86%). The sludge (101.5 g Ce/kg, 22.2 g P/kg) was extracted with HCl, transferring 99.6% of cerium and 97.5% of phosphorus to the solution. Cerium was recovered as cerium(III) oxalate and thermally decomposed to cerium(IV) oxide. Redissolution in HCl and H₂O₂ yielded cerium(III) chloride (97.0% recovery and 98.6% purity). The HCl used for extraction can be regenerated on-site from chlorine and hydrogen obtained from gas streams, improving material efficiency. Life cycle assessment (LCA) showed environmental benefits related to eutrophication reduction but burdens from reagent use (notably HCl and oxalic acid). Although costlier than conventional precipitation, this method may suit large-scale applications requiring high phosphorus removal, low sludge, and alignment with circular economy goals. Full article