Recovery Of Rare Earth Elements Research Articles

Selective recovery of Sm from Sm-Co magnet scrap by vacuum distillation

Sm-Co rare earth permanent magnet materials are widely used in special fields such as new energy and electronic information. Sm-Co permanent magnet scraps are generated during the preparation and the use of end-of-life permanent magnets, which need to be recycled for the sustainable development. It is proposed to selectively recover Sm from the Sm-Co permanent magnet scraps by vacuum distillation. The experimental investigations were carried out in a vacuum furnace with double temperature zones for the efficient condensation of Sm vapor based on the thermodynamic and kinetic calculations. The recycling feasibility of the recovery of rare earth and the evaporation mechanism of Sm were also demonstrated in this work. The results show that Sm with the purity over 99% was obtained by the molybdenum condenser used as a condensing collectors at given conditions, providing a reference for the pyrometallurgical recovery of rare earth secondary resources.

Molding the future: Optimization of bioleaching of rare earth elements from electronic waste by Penicillium expansum and insights into its mechanism

The recovery of rare earth elements (REE) from electronic waste is crucial for ensuring future demand security, as there is a high supply risk for this group of elements, and mitigating the environmental impacts of conventional mining. This research focuses on extracting REE from waste printed circuit boards through bioleaching, addressing the limited attention given to this source. A strain of Penicillium expansum demonstrated efficient bioleaching under optimal conditions of 7.5 initial pH, 0.1 mM phosphate concentration, and excluding a buffering agent. The study achieved significant improvements in La and Tb extraction and enhancements in Pr, Nd, and Gd recovery, approaching 70 % within 24 h. Fungal mechanisms involved in REE extraction included fungal pH control, organic acid biosynthesis, phosphate bioavailability, and potential fungal proton pump involvement. This approach offers a promising solution for sustainable REE recovery from e-waste, contributing to resource security and circular economy.

Recovery of rare earth elements from mine wastewater using alginate microspheres encapsulated with zeolitic imidazolate framework-8

There is increasing demand and interest in efficient methods for the recovery of rare earth elements (REEs) from wastewater because of the growing concerns associated with the negative impacts of REEs-rich waste discharged on pristine ecosystems. Here, we designed a ZIF-8@ALG composite hydrogel by encapsulating zeolitic imidazolate frameworks-8 (ZIF-8) into sodium alginate and poly (vinyl alcohol) double cross-linked networks (ALG) for the recovery of REEs from mine wastewater. ZIF-8@ALG showed exceptional REEs adsorption performance with the most superior separation factor (Ho/Mn) of 597.5. For the REEs considered, the ZIF-8@ALG composite exhibited a preference for heavy REEs with high adsorption efficiencies (65.3 ∼ 97.2%) and distribution coefficients (2045.5 ∼ 28500.0 mL·g−1). Adsorption involved a combination of electrostatic attraction, complexation and ion exchange mechanisms. REEs adsorbed on ZIF-8@ALG could also be desorbed using sodium citrate via ion-exchange and complexation, thus achieving efficient REEs recovery. In addition, ZIF-8@ALG was stable and reusable, maintaining effective adsorption in wastewater over four consecutive cycles, where the optimal adsorption efficiency reached 80.0%. Overall, this study provided an effective and feasible method for the recovery of REEs in mine wastewater, and confirmed that ZIF-8-based materials have significant potential for REEs recovery applications in wastewater engineering treatment.

Bioleaching ion-unexchangeable rare earth in ion-adsorption type rare earth waste tailing

Ion-adsorption type rare earth ores (IREO) in China serve as the primary source of medium and heavy rare earth globally. With the rapidly growing demand for rare earth elements (REEs) and the dwindling supply of premium IREO, enhancing the recovery of REEs, especially the ion-unexchangeable REEs with ultra-low content and ambiguous speciation from IREO or its tailing, has become a critical trend and challenge. This study identified the occurrences of ion-unexchangeable REEs, primarily detected in Fe-enriched minerals, xenotime, and monazite of IREO using TESCAN integrated mineral analyzer (TIMA) and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) analysis. To extract these elusive REEs, a bioleaching technique utilizing Aspergillus niger (A. niger) metabolites was proposed, achieving a leaching yield of 31.4 wt%. Complementary sequential chemical extraction methods (SCEM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations further elucidate the underlying mechanism, involving the carboxylic acid produced by the metabolic process of microbes that dissolves goethite by disrupting FeO bonds and liberating REEs, which complexed with carboxylate (RCOO), to promote further dissolution. This work offers insight into enhancing the recovery of ion-unexchangeable REEs from IREO or its tailing, paving the way for sustainable and efficient rare earth mining practices.

Cerium oxide and neodymium oxide phytoextraction by ryegrass in bioenhanced hydroponic environments

Sustainable technologies for the recovery of rare earth elements (REE) from waste need to be developed to decrease the volume of ore mining extractions and its negative environmental consequences, while simultaneously restoring previously impacted lands. This is critical due to the extensive application of REE in everyday life from electronic devices to energy and medical technologies, and the dispersed distribution of REE resources in the world. REE recovery by plants has been previously studied but the feasibility of REE phytoextraction from a poorly soluble solid phase (i.e., nanoparticles) by different plant species has been rarely investigated. In this study, the effect of biostimulation and bioaugmentation on phytorecovery of REE nanoparticles (REE-NP) was investigated by exposing ryegrass seeds to REE-NP in hydroponic environments. This was studied in two sets of experiments: bioaugmentation (using CeO2 nanoparticles and Methylobacterium extorquens AM1 pure culture), and biostimulation (using CeO2 or Nd2O3 nanoparticles and endogenous microorganisms). Addition of M. extorquens AM1 in bioaugmentation experiment including 500 mg/L CeO2 nanoparticles could not promote the nanoparticles accumulation in both natural and surface-sterilized treatments. However, it enhanced the translocation of Ce from roots to shoots in sterile samples. Moreover, another REE-utilizing bacterium, Bacillus subtilis, was enriched more than M. extorquens in control samples (no M. extorquens AM1), and associated with 52% and 14% higher Ce extraction in both natural (165 μg/gdried-plant) and surface-sterilized samples (136 μg/gdried-plant), respectively; showing the superior effect of endogenous microorganisms’ enrichment over bioaugmentation in this experiment. In the biostimulation experiments, up to 705 μg/gdried-plant Ce and 19,641 μg/gdried-plant Nd could be extracted when 500 mg/L REE-NP were added. Furthermore, SEM-EDS analysis of the surface and longitudinal cross-sections of roots in Nd2O3 treatments confirmed surface and intracellular accumulation of Nd2O3-NP. These results demonstrate stimulation of endogenous microbial community can lead to an enhanced REE phytoaccumulation.

High-performance electrosorption of lanthanum ion by Mn3O4-loaded phosphorus-doped porous carbon electrodes via capacitive deionization

Transition-metal-oxide@heteroatom doped porous carbon composites have attracted considerable research interest because of their large theoretical adsorption capacity, excellent electrical conductivity and well-developed pore structure. Herein, Mn3O4-loaded phosphorus-doped porous carbon composites (Mn3O4@PC-900) were designed and fabricated for the electrosorption of La3+ in aqueous solutions. Due to the synergistic effect between Mn3O4 and PC-900, and the active sites provided by Mn–O–Mn, C/PO, C–P–O and Mn–OH, Mn3O4@PC-900 exhibits high electrosorption performance. The electrosorption value of Mn3O4@PC-900 was 45.34% higher than that of PC-900, reaching 93.02 mg g−1. Moreover, the adsorption selectivity reached 87.93% and 89.27% in La3+/Ca2+ and La3+/Na+ coexistence system, respectively. After 15 adsorption-desorption cycles, its adsorption capacity and retention rate were 50.34 mg g−1 and 54.12%, respectively. The electrosorption process is that La3+ first accesses the pores of Mn3O4@PC-900 to generate an electric double layer (EDL), and then undergoes further Faradaic reaction with Mn3O4 and phosphorus-containing functional groups through intercalation, surface adsorption and complexation. This work is hoped to offer a new idea for exploring transition-metal-oxide @ heteroatom doped porous carbon composites for separation and recovery of rare earth elements (REEs) by capacitive deionization.

Template-free efficacious morphology of electrosynthesized polyaniline/β-cyclodextrin host-guest complex on Au/rGO modified electrode for removal and recovery of rare-earth and heavy elements from seawater

Global water pollution and scarcity of water resources are turning increasingly into serious threats to the survival of all living organisms on Earth. This study offers an influent strategy for the electrosynthesis of reduced graphene oxide/polyaniline/β-cyclodextrin (rGO/PAni/βCD) nanocomposite and its application to the removal/recovery of heavy elements (HEs) and rare-earth elements (REEs). Besides physicochemical and electrochemical studies, the surface morphological and statistical properties of fabricated nanocomposite electrode were examined. The textural and morphological characteristics of nanocomposite electrode were investigated via AFM data based on statistical, stereometric, and fractal theory. The cohesive, porous, and well-developed morphology of fabricated nanocomposite electrode has enabled the electrodeposition technique to achieve significant simultaneous removal/recovery efficiency of HE and REE ions such as Pb(II), Cu(II), Cd(II), Hg(II), Ce(IV), and Nb(V). Therefore, using rGO/PAni/βCD, considerable removal of HEs and REEs was achieved under optimized pH, 0.1 M KNO3, and 35 mg L−1 metal ion initial concentration during 20 min. Removal capacity of the nanocomposite electrode is preserved subsequent to 10 cycles of electrodeposition/desorption, according to the desorption investigation through eluted adsorbent at time intervals in deionized water and adjusted acidic pH values. Then, using rGO/PAni/CD nanocomposite, simulated seawater remediation was accomplished successfully. This interdisciplinary approach reveals that the removal/recovery efficiency enhance linearly along with the improvement of well-developed morphology for electrosynthesized composites. Thus, these results suggest how the morphological features of the polymer composites could improve remediation of water resources.

Transport Model of Rare Earth Elements in Weathering Crusts during Electrokinetic Mining

Electrokinetic mining (EKM) is a novel method for rare earth element (REE) mining that can achieve green and efficient recovery of REEs. However, as yet, there is no accurate model for describing the electrokinetic transport of REEs in weathering crusts, and this hinders the wider application of EKM. The conventional model fails to capture the microscale transport physics occurring in the nanochannels that exist ubiquitously in weathering crusts. Consequently, the existing models cannot distinguish the mobilities of different REEs. Here, we report a new model for a more faithful description of the electrokinetic transport of REEs in weathering crusts that considers the ionic size, which has previously been neglected. We reveal that the electrokinetic transport of heavy REEs (HREEs) is faster than that of light REEs (LREEs) in weathering crusts, which is contrary to the predictions of conventional models. Our model was validated experimentally by measurements of the electrokinetic transport of two LREEs (La and Sm) and an HREE (Er) in weathering crusts. The speed of electrokinetic transport follows the order Er > Sm > La. Our findings suggest that the ionic size is a non-negligible factor affecting the electrokinetic transport of REEs in weathering crusts containing nanochannels. This work offers a constitutive model to describe the electrokinetic transport of REEs in weathering crusts, which promotes both theoretical developments and practical applications of EKM.

Water-soluble rare earth elements (REEs) recovered from uranium tailings

The global energy transition from fossil fuels to renewable sources poses substantial challenges to increase metal production, including industry-critical rare earth elements (REEs). Environmental and social concerns obfuscate the production of these ‘green’ metals and only ∼1% of REE demand is met from recycling. This work highlights the potential for the ‘economic rehabilitation’ of the Mary Kathleen Mine, a relinquished uranium (U) mine with very high (∼3 wt%) total REE concentrations within the tailings storage facility (TSF). Leaching of the tailings revealed that approximately 5% of the REEs are water-soluble, suggesting the possibility for in situ extraction. Using an in situ water leach, the projected total REE recovery is >10,000 t, which would exceed the total amount of U recovered (7,532 t) during the initial mining operation. The proposed in situ recovery would offer a truck-free mine, using solar energy and water recycling technologies to recover REEs from a waste source, reducing environmental contamination.

High sedimentation efficiency and enhanced rare earth recovery in the impurity removal process of rare earth leachate by flocculation system

The main impurity ion in rare earth (RE) leachate are aluminum ions, which can be effectively removed by precipitation in the form of aluminum hydroxides. The flocculent colloidal precipitation formed by aluminum ions has a loose structure and high liquid content, resulting in lower sedimentation efficiency and higher RE loss. The addition of flocculants in the impurity removal process of RE leachate is expected to solve this problem. The cationic polyacrylamide flocculant (CPAM) was screened as the flocculant to accelerate the sedimentation of the impurity removal process as well as to effectively improve the recovery of RE. The effects of CPAM concentration, dosage, stirring rate and stirring time on the flocculation process was investigated. The optimal flocculation effect could be obtained under conditions of dosage of 0.5 mL, concentration of 0.1%, stirring speed of 700 r/min and stirring time of 2 min. Under this condition, RE recovery efficiency was 97%, Al removal efficiency was 96%, and the sedimentation time of the flocs was 20 min. Compared with absence of flocculants in this process, Al removal efficiency remained almost unchanged, while RE recovery efficiency and the sedimentation time of flocs increased by 4% and shortened by 40 min, respectively. The flocculation mechanism was discussed by the theory of two-dimensional fractal dimension, and it was found that the flocculation was mainly based on the combined effect of bridge flocculation and electrical neutralization.

Investigation of Rare Earth Element Binding to a Surface-Bound Affinity Peptide Derived from EF-Hand Loop I of Lanmodulin.

Bioinspired strategies have been given extensive attention for the recovery of rare earth elements (REEs) from waste streams because of their high selectivity, regeneration potential, and sustainability as well as low cost. Lanmodulin protein is an emerging biotechnology that is highly selective for REE binding. Mimicking lanmodulin with shorter peptides is advantageous because they are simpler and potentially easier to manipulate and optimize. Lanmodulin-derived peptides have been found to bind REEs, but their properties have not been explored when immobilized on solid substrates, which is required for many advanced separation technologies. Here, two peptides, LanM1 and scrambled LanM1, are designed from the EF-hand loop 1 of lanmodulin and investigated for their binding affinity toward different REEs when surface-bound. First, the ability of LanM1 to bind REEs was confirmed and characterized in solution using circular dichroism (CD), nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulations for Ce(III) ions. Isothermal titration calorimetry (ITC) was used to further analyze the binding of the LanM1 to Ce(III), Nd(III), Eu(III), and Y(III) ions and in low-pH conditions. The performance of the immobilized peptides on a model gold surface was examined using a quartz crystal microbalance with dissipation (QCM-D). The studies show that the LanM1 peptide has a stronger REE binding affinity than that of scrambled LanM1 when in solution and when immobilized on a gold surface. QCM-D data were fit to the Langmuir adsorption model to estimate the surface-bound dissociation constant (Kd) of LanM1 with Ce(III) and Nd(III). The results indicate that LanM1 peptides maintain a high affinity for REEs when immobilized, and surface-bound LanM1 has no affinity for potential competitor calcium and copper ions. The utility of surface-bound LanM1 peptides was further demonstrated by immobilizing them to gold nanoparticles (GNPs) and capturing REEs from solution in experiments utilizing an Arsenazo III-based colorimetric dye displacement assay and ultraviolet-visible (UV-vis) spectrophotometry. The saturated adsorption capacity of GNPs was estimated to be around 3.5 μmol REE/g for Ce(III), Nd(III), Eu(III), and Y(III) ions, with no binding of non-REE Ca(II) ions observed.

Study on the leaching behavior differences of rare earth elements from coal gangue through calcination-acid leaching

Critical metals in coal-based byproducts have been gradually regarded as one of the most important alternative resources due to the market parameters, strategic perspective, and advanced technology. In this study, the mechanism of calcination-acid leaching for rare earth elements (REY) recovery from coal gangue of Jungar coalfield was proposed based on the results of sample characteristics, extraction law, kinetic analysis, and typical mineral leaching. The micro-particles of bastnaesite and monazite were found and embedded in the kaolinite via micro-observation with SEM-EDS. Kaolinite in coal gangue calcined at 600 ℃ was transformed into highly positively reactive metakaolinite, enhancing the dissolution of aluminum (Al) and exposing the rare earth minerals encapsulated within the particles. Meanwhile, bastnaesite was thermally decomposed into rare earth oxides and rare earth fluorides. Rare earth oxides are more easily dissolved by acid, and the fluoride ions and the aluminum ions form a stable complex ion [AlF6]3-, which promotes the dissolution of rare earth fluoride. In addition, a high REY and Al recovery (78.8% of REY, and 88.5% of Al) came true from calcined coal gangue at optimal conditions. The leaching kinetics showed that REY leaching followed the hybrid control model of interfacial transfer and diffusion through solid layer, while Al leaching is controlled by chemical reaction. The leaching test of monazite shows that the leaching efficiency of LREY is higher than that of HREY indicating the leaching difference of LREY (82.3%) and HREY (61.3%) in coal gangue is mainly influenced by monazite. This may be related to the crystal structure and the radius of trivalent rare earth ions. This study provides a useful reference for the extraction of REY from coal gangue.

Recovery of Rare Earth Elements from Coal Fly and Bottom Ashes by Ultrasonic Roasting Followed by Microwave Leaching

Considering the rising demand for rare earth elements (REEs), researchers are looking for new sources for their extraction, thereby fostering economic and environmentally justified processing solutions. Among potential industrial sources, coal fly ash emerges as one of the most promising. The recovery of REEs from coal fly and bottom ashes derived from different thermal power plants was the main focus of this study. A dual-step methodology was conducted on ash samples, which involved an ultrasonic roasting process to disintegrate the silica matrix, followed by a microwave-assisted acid leaching step to extract REEs. The roasting procedure was studied using the Plackett–Burman design, and the Box–Behnken design was subsequently implemented to optimize the leaching procedure. The optimized ultrasonic roasting procedure was set up at 95 °C for 10 min with an ash-to-roasting agent (3M NaOH) ratio of 0.5:1 (m/V). For acid leaching, the optimal conditions were obtained at 174 °C for 30 min with an HCl ÷ HNO3 mixture (1:1 V/V). The standard reference material (NIST 1633c) was used in the conclusive experiments to estimate the average recovery (80%) of REEs. The green aspects of this methodology were evaluated using several metrics (atom economy, E-factor, and energy consumption). The proposed process outperforms high-temperature roasting procedures in terms of greenness; however, the REE recovery rate is lower.

Recovery of rare earths and lithium from rare earth molten salt electrolytic slag by lime transformation, co-leaching and stepwise precipitation

The rare earth molten salt electrolytic slag (REMSES) has recently attracted significant attention due to its potential environmental hazards and high content of rare earths and lithium, leading to a surge in recycling efforts. In this study, we propose and demonstrate a novel and straightforward process for the simultaneous extraction of rare earths and lithium from REMSES through lime transformation and sulfuric acid leaching at low temperatures. Firstly, during the lime transformation process, REMSES is converted into hydroxides that can be easily dissolved in acids. Secondly, REEs and Li present in the slag are co-extracted using a conditional sulfuric acid leaching method, resulting in 95.72% REEs leaching efficiency and 99.41% Li leaching efficiency under optimal conditions. Finally, REEs and Li in the solution are precipitated using oxalic acid and trisodium phosphate with precipitation efficiencies of 99.02% for REEs and 94.85% for Li respectively. This innovative process enables the conversion of REEs and lithium from REMSES into high-purity products (a mixture of REOs with 99.31% purity; Li<sub>3</sub>PO<sub>4</sub> with 98.93% purity), thereby facilitating their valuable utilization.

Utilizing Deep Eutectic Solvents in the Recycle, Recovery, Purification and Miscellaneous Uses of Rare Earth Elements.

The boosted interest in using rare earth elements (REEs) in modern technologies has also increased the necessity of their recovery from various sources, including raw materials and wastes. Though hydrometallurgy plays a key role in these recovery processes, some drawbacks (apparent or not) of these processes (including the use of aggressive mineral acids, harmful extractants, and diluents, etc.) have led to the development of an environmental friendship subclass named solvometallurgy, in which non-aqueous solvents substituted to the aqueous media of the hydrometallurgical processing. Together with ionic liquids (ILs), the non-aqueous solvents chosen for these usages are the chemicals known as deep eutectic solvents (DEEs). The utilization of DEEs included the leaching of REEs from the different sources containing them and also in the separation-purification steps necessary for yielding these elements, normally oxides or salts, in the most purified form. This work reviewed the most recent literature (2023 year) about using deep eutectic solvents to recover REEs from various sources and coupling these two (DESs and REEs) to derive compounds to be used in other fields.

Closed-loop purification process of industrial phosphoric acid: Selective recovery of heavy metals and rare earth elements via solvent extraction

Phosphate fertilizers are produced using phosphoric acid as a vital chemical component. This essential ingredient is typically derived from phosphate rock, through a wet process phosphoric acid (WPPA). However, the WPPA contains impurities, including heavy metals (HMs) like cadmium (Cd), zinc (Zn) and copper (Cu), as well as unwanted elements like iron, and other valuables elements, notably rare earth elements (REEs) such as yttrium (Y), holmium (Ho) and ytterbium (Yb). The current study aims to develop a novel process for a simultaneous and selective recovery of HMs and REEs through solvent extraction from WPPA. Single-stage extraction, cascade extraction and synergistic extraction were explored to gain a deeper understanding of the mechanisms and identify the most efficient process. 90, 80 and 99 % for Y, Ho and Yb, respectively, were extracted using D2EHPA. While 89 % for Zn, 99 % for Cu and Cd were removed using Cyanex 301. On the other hand, 60 % of Yb, 74 % of Y and 78 % of Ho were stripped using H2SO4, while 98 % of Cd and 99 % of Zn were stripped using HCl. The organic phase can be successfully reused with an effective extraction and stripping. Oxalic acid (C2H2O4) was employed to precipitate REEs. However, sodium carbonate (Na2CO3) was used to precipitate HMs. Finally, solvent extraction was carried out on WPPA solution to ensure the extraction performance under industrial conditions.

Selective separation and recovery of boron from spent Nd-Fe-B magnets leaching solution

The accumulation of boron during the recovery of rare earth elements (REEs) from spent Neodymium-Iron-Boron (Nd-Fe-B) magnets affects the extraction of REEs and causes environmental pollution through the boron-containing wastewater. Given that boron is a globally scarce strategic resource, this study proposes a method for the selective separation and recovery of boron from spent Nd-Fe-B magnets leaching solution. The synergistic extraction of boron adopting 2-ethylhexanol (EHA) and 2-ethyl-1,3-hexanediol (EHD), and the extraction mechanism were investigated. Under optimized conditions (30 % EHA-20 % EHD-50 % sulfonated kerosene, 1:1O/A phase ratio, pH 3.3, 10 min), the two-stage countercurrent extraction achieved a remarkable 99.9 % extraction efficiency with minimal loss of REEs. Slope analysis, Raman, FT-IR, and NMR combined with DFT were used to elucidate the extraction mechanism. Specifically, some of the boric acid molecules complexed with the OH groups of EHD to form stable six-membered ring complexes, while another part bound with both EHA and EHD to form linear complexes containing six-membered rings. Employing 0.4 mol/L NaOH as the stripping agent, the two-stage countercurrent stripping process achieved a stripping efficiency of 99.8 %. Subsequently, borax pentahydrate products exceeding 99 % purity were obtained via evaporative crystallization from the stripping solution. Notably, the raffinate contained only ∼ 5 mg/L of boron, which could be further reduced below the discharge standard via the REEs recovery process, allowing for safe emission. This work offers a novel approach for the selective separation and recovery of boron from spent Nd-Fe-B magnets, eliminating its adverse effects on REEs extraction and the environment, while simultaneously recovering a valuable resource.

Can rare earth elements be recovered from abandoned mine tailings by means of electrokinetic-assisted phytoextraction?

Given the high impact of traditional mining, the recovery of rare earth elements (REEs) from hazardous waste materials could become an option for the future in accordance with the principles of the circular economy. In this work, the technical feasibility of REEs recovery from metal mine tailings has been explored using electrokinetic-assisted phytoremediation with ryegrass (Lolium perenne L.). Phytoextraction combined with both AC current and DC current with reversal polarity was applied (1 V cm−1, 8 h day−1) to real mine tailings containing a total concentration of REEs (Sc, Y, La, Ce, Pr, and Nd) of around 146 mg kg−1. Changes in REEs geochemical fractionation and their concentrations in the soil pore water showed the mobilization of REEs caused by plants and electric current; REE availability was increased to a higher extent for combined electrokinetic-assisted phytoextraction treatments showing the relevant role of plants in the process. Our results demonstrated the initial hypothesis that it is feasible to recover REEs from real metal mining waste by phytoextraction and that the performance of this technology can be significantly improved by applying electric current, especially of the AC type, which increased REE accumulation in ryegrass in the range 57–68% as compared to that of the treatment without electric field application.

The distribution coefficients of Nd3+ and Y3+ between 0.1 and 2.1 M HNO3 and HDEHP at 298.15 K

Rare earth metals are considered to be some of the most critical raw materials on the planet due to their use in many products and technologies, the difficulty in isolating the individual elements from ores and each other, and the market restrictions that threaten their supply. However, despite this, the recycling rates for rare earth metals remain low. Hydrometallurgy has been proposed to hold the greatest potential for the recovery of rare earth metals from waste materials. However, the design of hydrometallurgical processes for the recovery of rare earth elements from waste requires a good understanding of the behaviour of the rare earth metals between different phases.In this study, the distribution coefficients of two rare earth metals, yttrium, and neodymium, between an organic solvent; bis(2-ethylhexyl) phosphate, diluted in n-dodecane at between 0.25 M and 1 M; and an aqueous nitric acid solution at concentrations of between 0.1 M and 2.8 M were measured. Distribution measurements were performed using a series of liquid–liquid extraction cells at a constant temperature of 298.2 K. The concentration of the rare earth metals was measured using ICP-OES analysis. The distribution coefficients and the acid concentration were found to be inversely related to each other. Conversely, the distribution coefficients were directly proportional to the concentration of the solvent in the organic phase.The data reported in this paper will be used to design a counter-current liquid–liquid extraction column for the recovery of neodymium from waste permanent magnet powder.

Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization

Phytomining is a sustainable approach that uses hyperaccumulators for critical element extraction from various substrates, such as contaminated soils, mine tailings, and aqueous solutions. In this study, grass seeds were fed with a solution containing Y, La, Ce, and Dy, resulting in around 510 mg/kg (dry basis) of total rare earth elements (TREEs) accumulated in grass leaves. Electron probe microanalyzer (EPMA) analysis showed that rare earth elements (REEs) in the grass leaves (GL) predominantly complexed with phosphorous (P). Around 95% of Y, 93% of La, 92% of Ce, and 93% of Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. The effect of temperature on the structural properties and chemical composition of the resulting hydrochar was evaluated. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destroyed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, a higher heating value (HHV), and energy densification, coupled with a decrease in the hydrogen and oxygen contents of hydrochar. The evolution of H/C and O/C ratios indicated that dehydration and decarboxylation occurred during MHTC. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbents via MHTC.