Recovery Of Rare Earth Elements Research Articles

Highly selective recovery of rare earth elements from mining wastewater using phyto-synthesized biochar dispersed iron nanoparticles

Recovery of rare earth elements (REEs) from secondary sources such as mining wastewater has attracted much recent attention due to the rising global demand for REEs and the need to environmentally protect potentially contaminated mine sites. One significant issue it that most adsorbents proposed for removal and/or recovery of REEs are not generally selective for REEs. In this study, biochar dispersed iron nanoparticles synthesized using a plant extract (BC-FeNPs) were successfully used for the selective recovery of REEs from mining wastewater, where the Kd values were Nd(III) (918 mL∙g−1), Eu(III) (1253 mL∙g−1), Tb(III) (1119 mL∙g−1), Dy(III) (1032 mL∙g−1), Lu(III) (1898 mL∙g−1), compared to only 23.9 mL∙g−1 for Zn(II). The maximum adsorption efficiency of REEs for the composite was higher than the constituent parts being 88.4 % for BC-FeNPs, but only 8.72 % for biochar and 82.4 % for FeNPs, respectively. FTIR, XPS and Zeta potential analysis suggested that the observed selective adsorption of REEs, involved interactions of REEs with O and N, as well as ion-exchange with H+ from the biochar and capping layer, as well as electrostatic interactions. The desorption efficiency of bound REEs from BC-FeNPs was > 85 % when using acetic acid, and was attributed to efficient competitive ion exchange. Pearson correlation analysis further demonstrated that the adsorption mechanism included ion complexation, ion exchange and electrostatic adsorption, while the desorption mechanism was mainly via ion exchange. Overall, BC-FeNPs has significant potential to practically recover REEs from mining wastewater having demonstrated excellent reusability after five adsorption–desorption cycles.

Rare earth elements recovery and mechanisms from coal fly ash by column leaching using citric acid

Rare earth elements (REEs) play an irreplaceable role in supporting the advancement of the global economy and technology, but their limited supply drives researchers to devise effective strategies for the recovery of REEs from alternative sources, including coal fly ash (CFA). In this work, a new column leaching method using citric acid as the lixiviant was proposed to enhance the recovery of REEs from CFA at room temperature. The mechanism of column leaching was investigated based on sample properties, leaching results, and the adsorption characteristics and complexation of citric acid. REEs in the CFA were found to be hosted into the amorphous aluminosilicate matrix in the form of oxides and fluorides using SEM-EDS. Compared to the maximum recovery of total REEs (TREEs, < 30 %) in the conventional agitation leaching tests, the recovery of TREEs in the column leaching tests increased significantly to 64.8 %, while the leaching rate of impurities (Al, Ti, and Fe) was less than 13.7 %. Results of the Zeta potential measurements showed that the citric acid and rare earth cations were adsorbed on the surface of CFA particles. These adsorption phenomena may make REEs bind to the CFA surface, thereby resulting in low leaching recoveries of REEs. Rare earth-citrate complexes in the lixivium were isolated by antisolvent precipitation, demonstrating that rare earth cations can be complexed by citrate. During the column leaching process, the leached rare earth ions were complexed with citrate and then carried away with the flowing liquid phase, which improved the recovery of REEs.

Review of Methods for Obtaining Rare Earth Elements from Recycling and Their Impact on the Environment and Human Health

Rare earth elements (REEs) are at the forefront of discussions, given their crucial role in cutting-edge and eco-friendly innovations that propel the industrial revolution towards a green economy. These elements have become indispensable to various modern technologies, such as smartphones, electronic devices, and renewable energy sources. Many different concepts and analyses have been introduced, such as the chemical similarities among REEs, health risks and ecological damages, the negative environmental impacts of current recovery processes, and strategies for advancing REE recovery towards a circular economy. Although these elements have been widely used in various applications over the last 20 years, the literature on these aspects is fragmented and spread across different research areas, shared by multiple branches and application fields. These fields include safety concerns, economic challenges, and technology. Summarizing and classifying this literature is challenging due to its fragmented nature, the variety of topics, and the different approaches used. The quest for cleaner recycling strategies necessitates a comprehensive assessment covering economic, technological, and environmental aspects. The primary goal of this review is to provide a holistic perspective on REEs, with a central focus on their economic, technological, and environmental dimensions, particularly emphasizing reuse, recycling, and occupational safety. The review begins by addressing complexities of REEs, highlighting the associated technologies, environmental concerns, and economic considerations. It further explores the aspects of reuse and recycling of REEs, shedding light on the advantages, drawbacks, hazards, and costs associated with recycling technologies for REE recovery. Additionally, the review summarizes occupational exposure and safety considerations related to REEs.

Improvements in recovery of rare earth elements (REEs) from coal via fluidized-bed combustion: Thermal alteration of REE mineralogy and its impact on element extractability

The extraction of rare earth elements (REE) from coal ash has attracted significant attention, but satisfactory solutions have not yet been achieved. Indeed, the REE extractability is largely determined by the transformation behavior of REE during coal combustion. This study found that the controlled combustion conditions of circulated fluidized-bed boiler (CFB) can both utilize the heating value and enhance the recovery of REE from coal. Tests were conducted using a bituminous coal with REE concentration of approximately 149 ppm in an industrial 35 t/h CFB boiler with controlled boiler loads and coal sources. Advanced automated image analysis by scanning electron microscopy (AIA-SEM) and sequential leaching procedure were employed to clarify the speciation transformation behavior of REE in boilers and investigate the effects of these transformations on the leachability of REE. Approximately 75 % of REE partitioned into fly ash and enriched in particles < 45 μm (∼710 ppm) after combustion. Acid leaching tests under mild conditions (1 M HCl at 60 ℃ and S/L = 1:25) performed on coal, bulk ashes, and sieved ashes revealed an improvement in REE leaching ratio from coal (21.34 %) to fly ash (71.07 %). Particle size played a crucial role in acid extractability, particularly for middle and heavy REE in cyclone ash but had no influence on those in fly ash. The dominant forms of REE in coal including Ca-REE fluorocarbonates (36.7 %), aluminophosphates (12.0 %), and organic-bound REE (approximately 20 %) significantly decreased, contributing to the formation of leachable rare earth oxides (∼65 %) in ash. Furthermore, the small size of REE mineral grains in coal (over 50 % below 5 μm) and extensive mineral fragmentation occurring in boiler resulted in the enrichment and enhanced REY leachability in fine ash.

Recovery of neodymium and iron from NdFeB waste by combined electrochemical–hydrometallurgical process

The recovery of rare-earth elements and Fe from NdFeB waste has significant potential. However, conventional hydrometallurgical and pyrometallurgical processes are energy intensive, chemical consuming, and accompanied by waste emissions, resulting in low-value iron products. This study introduced a combined electrochemical–hydrometallurgical process for NdFeB waste recovery. Electrochemical analysis revealed the active dissolution of NdFeB waste under 370 mA/cm2, enabling Fe2+ reduction to Fe even in the presence of Nd3+. Electrolysis at 60 °C using Nd2(SO4)3 and FeSO4 as electrolytes, along with H3BO3 as a pH buffer, resulted in the uniform dissolution of waste, releasing Nd3+ and Fe2+ into the electrolyte, whereas Fe2+ underwent electrodeposition onto Fe at the cathode. Investigation of FeSO4 concentration revealed its influence on current efficiency, cathode-product properties, and energy consumption. When the concentration of FeSO4 was 0.5 M, a cathode product with 87.94 wt% Fe and 0.03 wt% Nd was obtained. Finally, Nd was recovered by hydrometallurgical high-temperature crystallization at 90 °C under N2 atmosphere, yielding Nd2(SO4)3·nH2O mixed crystals with 0.1 wt% Fe. The process formed a closed loop, consuming only FeSO4 and a minimal amount of H2SO4, with an energy consumption of 0.73–1.53 kW·h/kg NdFeB.

Overview of Functionalized Porous Materials for Rare-Earth Element Separation and Recovery.

The exceptional photoelectromagnetic characteristics of rare-earth elements contribute significantly to their indispensable position in the high-tech industry. The exponential expansion of the demand for high-purity rare earth and related compounds can be attributed to the swift advancement of contemporary technology. Nevertheless, rare-earth elements are finite and limited resources, and their excessive mining unavoidably results in resource depletion and environmental degradation. Hence, it is crucial to establish a highly effective approach for the extraction and reclamation of rare-earth elements. Adsorption is regarded as a promising technique for the recovery of rare-earth elements owing to its simplicity, environmentally friendly nature, and cost-effectiveness. The efficacy of adsorption is contingent upon the performance characteristics of the adsorbent material. Presently, there is a prevalent utilization of porous adsorbent materials with substantial specific surface areas and plentiful surface functional groups in the realm of selectively separating and recovering rare-earth elements. This paper presents a thorough examination of porous inorganic carbon materials, porous inorganic silicon materials, porous organic polymers, and metal-organic framework materials. The adsorption performance and processes for rare-earth elements are the focal points of discussion about these materials. Furthermore, this work investigates the potential applications of porous materials in the domain of the adsorption of rare-earth elements.

G-C3N4 modified natural low-grade dolomite-palygorskite: Removal capacity and adsorption mechanism for Gd3+

Rare earth elements (REEs) pose significant environmental challenges due to the wastewater generated during their extraction. Developing efficient adsorbents with simple, economical and eco-friendly methods for removing and recovering REEs from wastewater is highly demanded but full of challenges. This study creates a novel adsorbent (g-C3N4/0.5DPal) for efficient REEs removal and recovery by integrating the low-grade mineral dolomite-palygorskite with g-C3N4 through a "one-pot" calcination method. Characterization techniques including SEM, XRD, FT-IR, XPS, etc., were employed to analyze the structure of the g-C3N4/0.5DPal composite. Batch adsorption experiments focusing on Gd3+ from among the REEs were conducted to evaluate the adsorption performance. The results reveal a remarkable 3.34 times increase in Gd3+ adsorption capacity of g-C3N4/0.5DPal (192.46 mg/g) compared to raw dolomite-palygorskite (57.62 mg/g) at 298 K, highlighting the effectiveness of the modification. The adsorption mechanism involves electrostatic interactions, surface complexation, and cation-π interactions. It is worth noting that g-C3N4 facilitates the conversion of dolomite to calcite during the preparation process, enhancing the Gd3+ adsorption of g-C3N4/0.5DPal. This work offers a promising solution for the removal and recovery of REEs and the high-value utilization of low-grade minerals, addressing both environmental concerns and resource sustainability.

Recovery of rare earth elements from rare earth molten salt electrolytic slag via fluorine fixation by MgCl2 roasting

Rare earth fluoride molten-salt electrolytic slag (REFES) is a precious rare earth element (REE) secondary resource, and considerable amounts of REEs exist in REFES as REF3; they are difficult to dissolve in acid or water and impede efficient REE extraction. In REFES recovery, the REF3 species in REFES are usually transformed into acid-soluble rare earth compounds by NaOH roasting or sulfating roasting and then extracted by acid leaching. Moreover, the fluorides in REFES are released as HF gas in the roasting process or enter the liquid phase during the water washing process; both of these processes cause fluorine pollution. Fixing the fluorine into the solid slag provides a way to avoid fluorine pollution. In this study, a novel method was proposed to extract REEs from REFES via MgCl2 roasting followed by HCl leaching. Thermodynamics calculations and thermogravimetry‒differential thermal analyses (TG-DTA) were conducted to investigate the reactions occurring in the roasting process. First, MgCl2 reacts with the REF3 and RE2O3 to form RECl3 and REOCl, respectively. Second, the RECl3 absorbs water and forms RE(OH)3. Third, MgCl2·6H2O is gradually dehydrated to MgCl2·2H2O and reacts with REF3 and RE(OH)3, and REOCl, MgF2 and MgO are formed. Through HCl leaching, the REOCl in the roasting products is leached by HCl acid, while fluoride remains in the solid slag as MgF2. The optimum experimental conditions are as follows: mass ratio of MgCl2 to REFES of 30%, roasting temperature of 700 °C, roasting time of 2 h, hydrochloride acid concentration of 4 mol/L, leaching time of 2 h, leaching temperature of 90 °C and leaching L/S ratio of 20:1. The efficiencies for total leaching of the REEs, La, Ce, Pr, and Nd are 99.13%, 99.20%, 98.42%, 99.38%, and 99.08%, respectively. Moreover, the concentration of fluoride in the leaching solution is 2.191 × 10−6 mol/L. This method has a short process flow with low reagent costs, and the problem of fluoride pollution from REFES recovery is solved; thus, our study has great industrial application potential.

Utilization of Eggshell Waste Calcite as a Sorbent for Rare Earth Element Recovery.

The green energy transition requires rare earth elements (REE) for the permanent magnets used in electric cars and wind turbines. REE extraction and beneficiation are chemically intensive and highly damaging to the environment. We investigated the use of eggshell waste as a sustainable alternative sorbent for the capture and separation of REE from aqueous solutions. Hen eggshell calcite was placed in multi-REE (La, Nd, Dy) solutions at 25 to 205 °C for up to 3 months. A pervasive diffusion of the REE inside the eggshell calcite was observed along pathways formed by the intracrystalline organic matrix and calcite crystal boundaries. At 90 °C, kozoite (REECO3OH, orthorhombic) spherulites precipitate on the surface of the dissolving calcite. At 165 and 205 °C, an interface-coupled dissolution-precipitation mechanism is observed, resulting in the complete dissolution of the calcite shell and its pseudomorphic replacement by polycrystalline kozoite. At 205 °C, kozoite is slowly replaced by hydroxylbastnäsite (REECO3OH, hexagonal), the stable form of the rare earth hydroxycarbonate polymorphs. Our results demonstrate two potential applications of eggshell waste for the uptake of rare earth elements in solution: at low temperatures, as a mixed organic-inorganic adsorbent and absorbent, given sufficient sorption time; and at higher temperatures, as an efficient sacrificial template for the precipitation of rare earth hydroxycarbonates.

Recovery of terbium by Lysinibacillus sp. DW018 isolated from ionic rare earth tailings based on microbial induced calcium carbonate precipitation.

Microbial induced calcium carbonate precipitation (MICP) is considered as an environmentally friendly microbial-based technique to remove heavy metals. However, its application in removal and recovery of rare earth from wastewaters remains limited and the process is still less understood. In this study, a urease-producing bacterial strain DW018 was isolated from the ionic rare earth tailings and identified as Lysinibacillus based on 16S rRNA gene sequencing. Its ability and possible mechanism to recover terbium was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and fourier transform infrared spectroscopy (FTIR). The results showed that the urease activity of DW018 could meet the biomineralization requirements for the recovery of Tb3+ from wastewaters. The recovery rate was as high as 98.28% after 10 min of treatment. The optimal conditions for mineralization and recovery were determined as a bacterial concentration of OD600 = 1.0, a temperature range of 35 to 40°C, and a urea concentration of 0.5%. Notably, irrespective of CaCO3 precipitation, the strain DW018 was able to utilize MICP to promote the attachment of Tb3+ to its cell surface. Initially, Tb3+ existed in amorphous form on the bacterial surface; however, upon the addition of a calcium source, Tb3+ was encapsulated in calcite with the growth of CaCO3 at the late stage of the MICP. The recovery effect of the strain DW018 was related to the amino, hydroxyl, carboxyl, and phosphate groups on the cell surface. Overall, the MICP system is promising for the green and efficient recovery of rare earth ions from wastewaters.

Recovery of rare earth elements from sedimentary rare earth ore via sulfuric acid roasting and water leaching

Rare earth elements were extracted using a sulfuric acid roasting-water leaching process. The effect of acid roasting on a new type of low-grade sedimentary rare earth ore found in Guizhou Province, China was analyzed using X-ray diffraction and scanning electron microscopy. A systematic study was conducted on process parameters such as amount of acid, roasting temperature, roasting time, water leaching temperature, and leaching time. The results reveal that the total recovery of rare earth elements reaches 81.37%, which is 3.1 times higher than that achieved through direct acid leaching, under the optimal conditions. In addition, the leaching rate of heavy rare earth elements reaches 72.53%. Rare earth elements and some other valuable metals are transformed into soluble sulfate through the local decomposition of clay minerals under the action of the sulfuric acid attack. The dissolution rates of aluminum, iron, and titanium ions are 34.94%, 17.05%, and 62.77%, respectively. The precipitation rate of Ti reaches 99%, and the loss of rare earth ions in the solution is less than 1%. Meanwhile, the results of a leaching kinetics analysis indicate that the leaching process of rare ions is controlled by diffusion. Precious metal ions such as iron and aluminum in the leaching solution can reduce the adsorption of rare earth ions by kaolinite. This study efficiently recovered rare earth ions under conditions of low calcination temperature and direct water leaching, resulting in reduced energy consumption of the extraction process and acidity of the leaching solution. These findings provide a solid foundation for the further separation and extraction of rare earth ions.

Recovery of rare earth elements from weathering crust soils using electrokinetic mining technology☆

Ion-absorption deposits (IADs) of rare earth elements (REEs) constitute the main economic resources of the essential heavy rare earth elements (HREE). Nonetheless, the existing leaching methods for extracting REEs from IADs face limitations stemming from significant environmental consequences and a poor REE recovery rate. In a recent development, electrokinetic mining (EKM) has emerged as a novel technology for extracting REEs from ion-adsorption deposits, demonstrating the potential for environmentally friendly and efficient recovery of REEs from weathering crusts. However, the transport mechanism of REE in weathering crust soil in an applied electric field remains poorly understood, and the influence of the EKM process has yet to be studied. In this study, we systematically investigated the transport characteristics of REE in weathering crust soil under the influence of an electric field. We demonstrate that the transport of REE is simultaneous with the transport of water, H+, and OH– in the applied electric field, where the EKM process is influenced by the voltage gradient, initial soil water content, electrode numbers, and power-on time. Under the optimal EKM conditions, 78.8% of REE is recovered from a 45 kg-scale weathering crust soil. Additionally, we identify the transport diversity between light REE (LREE) and HREE, which assists in the separation and pre-enrichment of LREE and HREE. Furthermore, we demonstrate that the mechanism for REE transport under an electric field involves a combination of electromigration, electroosmosis, and electrolysis working synergistically. This study provides new perspectives on the transport behavior and mechanism of REEs in weathering crust soil under an electric field. These findings pave the way for the practical implementation of EKM technology.

Efficient separation and comprehensive recovery of rare earth, bismuth and iron by one-step extraction and two-step stripping

A route was proposed to separate bismuth, iron, and rare earth elements from a solution containing nickel and magnesium through one-step extraction and two-step stripping. Firstly, saponified di-(2-ethylhexyl) phosphoric acid (D2EHPA) was used as the extractant to separate target elements. RE(III), Bi(III), and Fe(III) could be separated from Ni(II) and Mg(II), with extraction efficiencies of more than 99 %. Secondly, RE(III), Bi(III), and Fe(III) were stripped selectively by nitric acid and oxalic acid, respectively. Subsequently, the iron present in the oxalic acid stripping solution was treated under UV irradiation for 15 min to recover oxalic acid and obtain the ferrous oxalate product. The stripping efficiencies of RE(III), Bi(III), and Fe(III) were up to 100 % for four-stage countercurrent stripping, and the recovery rates of RE(III) and Bi(III) were 97.3 % and 99.2 %. The purities of nitrate rare earth enrichment solution, bismuth oxalate and ferrous oxalate were 99.7 %, 98.4 % and 99.5 %, respectively. In addition, the extraction dynamic demonstrated that the double salt precipitation was not observed during the extraction when the extraction time exceeded 7 min and the extraction mechanism was analyzed using the slope method. This process shortens steps, and is more efficient to separate RE(III), Bi(III), and Fe(III) from the mixed solution containing Ni(II) and Mg(II), which has a prospective industrial application.

Extraction of rare earth Eu from waste blue phosphor strengthened by microwave alkali roasting

Spent phosphor is an important secondary resource for extracting rare earth elements. Microwave absorption properties and enhanced extraction of Eu from blue phosphor by microwave alkali roasting were studied. Dielectric properties of alkali roasting system were measured by resonator perturbation method. Dielectric constant increases linearly from 250 °C until it reaches a peak at 400 °C. The dielectric loss reaches a higher value at 400–550 °C, due to the strong microwave absorption properties of molten alkali and roasted products. Effects of roasting temperature, roasting time and alkali addition amount on Eu leaching were investigated. The phosphor was completely decomposed into Eu2O3, BaCO3 and MgO at 400 °C. The alkaline decomposition process of phosphor is more consistent with diffusion control model with Eα being 28.9 kJ/mol. Effects of the main leaching conditions on Eu leaching were investigated. The leaching kinetic of Eu was in line with diffusion control model with Eα being 5.74 kJ/mol. The leaching rules of rare earths in the mixed phosphor were studied. The results showed that the presence of red and green phosphor affected the recovery of blue phosphor. The optimum process parameters of rare earth recovery in single blue phosphor and mixed phosphor were obtained, and the recovery of Eu were 97.81% and 94.80%, respectively. Microwave alkali roasting promoted the dissociation of phosphor and leaching of rare earths. The results can provide reference for the efficient and selective recovery of rare earths in phosphors.

In Situ Modification of Activated Carbon Made from Camellia oleifera Shell with Na2EDTA for Enhanced La3+ Recovery

It is important to recover La3+ from metallurgical solutions or wastewater. However, the recovery rate of La3+ is usually less than 1% and the recovery methods are not environmentally friendly or user-friendly. Therefore, a straightforward, efficient, clean, and economically friendly method is needed. In this investigation, a modified adsorbent, COSAC-Na2EDTA-15, which was made from the Camellia oleifera shell (COS) and disodium ethylenediaminetetraacetic acid (Na2EDTA), was invented. In addition, characterization of the COSAC-Na2EDTA-15 adsorbent was conducted using SEM and XPS, and the principle of adsorption was revealed. The adsorption kinetics followed P-S-O KM, while the isotherm of COS-activated carbon (COSAC) aligned more closely with the Langmuir model. Compared to COSAC, the maximum La3+ adsorption capacity of COSAC-Na2EDTA-15 increased from 50 to 162.43 mg/g, and the content of O and N changed from 7.31% and 1.48% to 12.64% and 4.15%, respectively. The surface of the COSAC-Na2EDTA-15 exhibited abundant C, N, and O elements, and La3+ was detected on the sample surface after adsorption. The test and analysis results fully indicate that La3+ can be successfully adsorbed on the surface of COSAC-Na2EDTA-15. Because of its easy preparation, low cost, and superior performance, activated carbon made from COS finds extensive applications in the adsorption and recovery of rare earth elements.

Recovery of Rare Earths from Rare-Earth Melt Electrolysis Slag by Mineral Phase Reconstruction

Recovery of Rare Earths from Rare-Earth Melt Electrolysis Slag by Mineral Phase Reconstruction

Greening the supply chain: Sustainable approaches for rare earth element recovery from neodymium iron boron magnet waste

The increasing demand for modern technologies has led to a growing reliance on rare earth elements (REEs). To address this issue, recycling used products such as permanent magnet waste containing REEs. However, this approach necessitates the development of advanced extraction and separation techniques to ensure high yields and purity of the REEs extracted. This review provides an overview of the latest extraction and separation technologies, with a particular focus on the hydrometallurgical approach for extracting REEs from secondary sources, notably permanent magnets. Hydrometallurgy, which involves leaching followed by solvent extraction for purification, has gained widespread use for obtaining REEs from secondary sources, as evidenced by its reported high recovery efficiencies. We found that the recovery of REEs using chemical leaching varied between 80% and 99%, influenced by factors such as type of source material, leaching solvent, leaching conditions, impurities, reaction kinetics and solid-liquid ratio. However, effectively employing this process on a larger scale still faces certain challenges due to the excessive use of corrosive solvents for leaching magnet waste and the generation of toxic chemicals as end products in the form of leachate. Additionally, the current process exhibits deficiencies in the targeted recovery of REEs from magnet waste, specifically in achieving purity and selectivity by eliminating iron impurities. This article concludes that the future prospects for the selective recovery of REEs from neodymium iron boron magnet waste lie in green and environment-friendly solvents. One such approach revolves around the utilization of biodegradable organic acids and salt aqua regia for leaching. These solvents are less corrosive and have high dissolution efficiency, which results in less chemical consumption and an overall environment-friendly and cost-effective recovery process.

Sustainable recovery of rare earth elements from Ni-MH batteries: Flux-free thermal isolation and Subsequent hydrometallurgical refinement

This study details a sustainable approach for efficient recovery of highly pure rare earth elements oxides (REOs) from Ni-MH batteries. REOs (i.e., La, Ce, Sm, Nd, and Pr oxides) were thermally isolated from spent Ni-MH batteries into an oxide phase with the REO concentration of 67.4 wt%. Subsequently, separate leaching processes were conducted using sulfuric acid and hydrochloric acid solutions, and the results were compared in terms of efficiency and feasibility. Sodium REEs sulfate hydrate (NaRE(SO4)2.xH2O) was precipitated from the sulfuric acid leachate by adjusting the pH to 1.5 using sodium hydroxide, while the addition of oxalic acid to the hydrochloric acid leachate enabled the recovery of REEs as oxalate hydrate (RE2(C2O4)3.xH2O). The resulting sulfate salt was mixed with a sodium hydroxide solution, leading to the formation of REEs hydroxide. The REEs hydroxide and oxalate were calcined at 1000 °C, resulting in the production of REOs with a purity exceeding 99.7%. This research demonstrates the feasibility of developing an efficient, environmentally friendly, and sustainable approach for the recovery of highly pure REOs from Ni-MH batteries which potentially can minimize the environmental impact associated with the extraction of REOs from virgin sources, while addressing the challenge of effective recycling of end-of-life Ni-MH batteries.

ZIF-8 Used for the Selective Recovery of Heavy Rare Earth Elements from Mining Wastewater.

In this study, a sample of 2-methylimidazole zinc salt (ZIF-8) demonstrated high selectivity for the recovery of heavy rare earth elements (REEs) from real rare earth mining wastewater. Results show that the distribution coefficient values of Y3+ (4.02 × 104 mL·g-1), Gd3+ (7.8 × 104 mL·g-1), and Dy3+ (6.8 × 104 mL·g-1) are orders of magnitude higher than those of K+ (359.51 mL·g-1), Mn2+ (266.67 mL·g-1), Ca2+ (396.42 mL·g-1), and Mg2+ (239.48 mL·g-1). Moreover, the desorption efficiency of heavy REEs exceeded 40%. Advanced characterizations and density functional theory (DFT) calculations were utilized to elucidate that the heavy REEs were more likely to bind to the nitrogen atoms of imidazole groups on ZIF-8 compared to non-REEs. Furthermore, the adsorption and desorption of heavy REEs primarily depend on the chemical interaction confirmed by adsorption kinetics, isotherm model, and thermodynamic analysis, which involves the dissociation of water and the formation of REE-O bonds. Finally, the ZIF-8 exhibits a remarkable recovery efficiency of over 40% for heavy REEs in column tests conducted over 7h. The findings reported here provide new insights into the selective recovery of heavy REEs from real mining wastewater.

The fractional extraction for sustainable recovery of rare earth and thorium in radioactive waste with oxoacetate functionalized ionic liquid

Rare earth (RE) are critical elements in many technologies, their clean production has attracted widespread attentions. To develop clean separation technology for RE, a novel oxoacetate ionic liquid (IL) extractant was developed to recovery RE elements and Thorium (Th) in the radioactive waste residue from ion-absorbed RE separation industry. The IL [N-methyl-N,N-dioctyloctan-1-aminium][2-(bis(2-ethylhexyl)amino)-2-oxoacetate] ([N1888][DEAO]) had good selectivity to RE and Th, which helps to remove radioactive Th and recover RE from waste residue leachate. RE and Th could be effectively separated with 0.02 mol/L [N1888][DEAO]. Maximum extraction capacity of Th reached 1.02 g/L, which could meet the needs of Th removal in the waste residue. A combination of saturated NaC2O4 solution and NaOH solution was used for the stripping of Th. After five cycles, the extraction efficiency of Th remained above 90 %. After four-stage extraction with [N1888][DEAO] and three-stage scrubbing, a RE solution with Th content less than 0.6 mg/L was obtained. It is fully achieved that there is no excess wastewater and waste salt in the fractional process. The RE elements in the waste residue can be effectively recycled, also the collection of the radioactive element Th can be realized.