Lanthanide Oxalate Research Articles

Leaching Efficacy of Ethylenediaminetetraacetic Acid (EDTA) to Extract Rare-Earth Elements from Monazite Concentrate

Alkaline EDTA solution has been previously identified as an effective leaching agent for solubilising rare-earth oxalates. These oxalates are the product of an oxalic acid conversion leach dissolving monazite and redepositing the salt. Pervious work suggested a significant increase in recovery was observed between pH 8 and 10; we have demonstrated that, in an excess of EDTA, this is not the case, and the dissolution is similar. While demonstrating that, at a nominal solid loading of 100 g/L, 0.2 M EDTA solution produced the highest dissolution, elevated solids require an equivalent increase in lixiviant concentration driven by consumption. Very-high-solution concentrations (>50 g/L dissolved TREEs) were achieved at a high solid loading, indicating both that a solution equilibrium is yet to be reached and that a build-up of oxalate in the system (estimated at ~1 M) does not impact the leach efficiency. We have also demonstrated the recycling of EDTA to use in multiple stages as well as the ability to recover oxalate from this solution.

Chloridizing roasting studies of spent NdFeB magnets for recovery of rare earth values

The increased demand for rare earth elements in advanced technological applications and supply shortages call for metal recovery from secondary sources. Permanent magnet (Nd2Fe14B or NdFeB) may serve as a potential secondary source due to its high rare earth (Nd+Pr+Dy: ∼30 %) content and its vast application. The present study utilizes a chloridizing roasting (CaCl2.2 H2O) pre-treatment process followed by water leaching, acid leaching (0.5 M HCl, S/L =1/10 g/ml, 90 °C, 3 h), oxalic acid precipitation and calcination (850 °C, 2 h) to obtain mixed rare earth oxides. The process was optimized based on temperature (400–700 °C), dosage (CaCl2.2H2O: NdFeB=0.5:1–2.5:1), and time (30–120 min) on the rare earth dissolution. The theoretical activation energy for the chloridizing roasting process is estimated as 22.3 (OFW) and 16.7 kJ/mol (KAS), while the experimental activation energy for Nd and Dy dissolution was determined to ∼29.3 and ∼17.7 kJ/mol, respectively depicting product layer diffusion-controlled kinetics. Higher dosages of CaCl2.2H2O (1.5:1 and 2:1) favored NdOCl formation, thereby, higher dissolution; however, further higher dosage (2.5:1) leads to reduced Nd dissolution due to higher CaO formation and acid consumption by Ca during leaching. Incomplete oxidation at lower temperatures (400 °C) and iron dissolution impair the Nd dissolution and selectivity. Excessive oxidation at >700 °C favors the formation of NdFeO3, decreasing Nd dissolution. The maximum dissolution of Nd was ∼89 %, while for Dy, it was ∼88 % at optimum conditions of 600 °C, 90 min, 2:1. Water leaching post-roasting leads to ∼87 % Ca removal and the precipitation efficiency of rare earth oxalates was 99 %. The overall extraction for rare earth elements was ∼89 %, and 1 kg of NdFeB powder can yield ∼285 g of rare earth oxides (∼239 g Nd2O3, ∼14 g Dy2O3) with 96 % purity. Further, this study demonstrates that using CaCl2.2 H2O as a solid chlorinating agent in chlorination roasting enhances recovery rates of mixed rare earth oxides while providing a safer and more environment-friendly alternative for industrial applications.

Heterogeneous Acetalization of Benzaldehyde over Lanthanide Oxalate Metal-Organic Frameworks.

Lanthanides (Ln) from the f-blocks of the periodic table have gained significant interest due to their unique characteristics, including magnetism, photoluminescence, and catalysis. In this study, a series of lanthanide metal-organic frameworks [Ln-MOFs, Ln = Eu(III), Tb(III), Nd(III), Er(III), Ho(III), Gd(III), Pr(III), and Dy(III)] were constructed based on oxalic acid and lanthanide metals as the building blocks. These MOFs were comprehensively characterized using various analytical and spectroscopic techniques, including powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, nitrogen adsorption-desorption, and Raman spectroscopy. The magnetic properties of the investigated materials were examined, revealing both antiferromagnetic and ferromagnetic interactions within the Ln-Ox MOFs. The catalytic activities of Ln-Ox MOFs were evaluated through the heterogeneous acetalization of benzaldehyde with methanol. Reaction yields by the reported catalysts varied up to 90% depending on the MOF's metal center, and the product was confirmed by gas chromatography-mass spectrometry. Recycling experiments have confirmed the stable regeneration of Ln-Ox MOFs in which the product yields remained the same over four consecutive cycles. The hydrothermal synthesis of these MOFs paves the way for a diverse array of materials showcasing unique lanthanide properties, making them suitable for various applications.

Subangstrom tuning of pore size for selective separation of radioactive strontium over environmental calcium by lanthanide–oxalate frameworks

Subangstrom tuning of pore size for selective separation of radioactive strontium over environmental calcium by lanthanide–oxalate frameworks

Selective leaching and recovery of neodymium from NdFeB carbonyl residues

Neodymium iron boron (NdFeB) is a widely-used magnetic material; however, the production process generates significant amounts of rare earth element-rich waste. Refining the Fe via carbonylation leads to carbonylated slag containing 0.2–2.0 % rare earth elements. The extraction of trace rare earth elements from waste materials has not been reported, although the Nd recovery would be economically and environmentally beneficial. In this work, we calcinated NdFeB carbonylation slag (with less than 0.3 % rare earth content) and identified a green deep eutectic solvent (DES) that can selectively recover Nd. The best performing DES comprised a 2:1 M ratio of ethylene glycol to maleic acid and was highly selective for Nd and inert toward Fe, thus enabling effective separation. Specifically, the Nd/Fe selectivity factor exceeded 3000, i.e., under optimal conditions, 28.07 % of the Nd was extracted, while the Fe leaching rate was less than 0.01 %. The Nd in the DES was subsequently extracted and then enriched by reverse extraction using a di-(2-ethylhexyl)phosphoric acid (P204)-sulfonated kerosene system and hydrochloric acid. DES still had recyclability after extraction. Finally, the Nd was precipitated by adding oxalic acid to form the rare earth oxalate, which was calcined to obtain a high-purity Nd2O3. The strategy applied in this work based on DES and conventional organic extraction represents a promising and sustainable approach for the selective recovery of trace rare earth elements.

Semicontinuous Process of Rare Earths Recovery from End-of-Life NdFeB Magnets on a Large Laboratory Scale

Hydrometallurgical methods for NdFeB recycling typically consist of several unit operations and require the extensive use of energy, water and chemicals which may negatively affect the applicability of these methods on an industrial scale. Based on the data from our previous studies, a simplified process of rare earth elements (REE) recovery from spent NdFeB magnets was developed further. The possibility of regenerating the leaching agent, as well as water recovery, in the process was investigated. This study also investigates a possibility of scaling up the recycling process developed on a laboratory scale. The leaching and precipitation stages were tested on a larger scale, where about 1 kg of end-of-life magnets was used as feed to the leaching step. In this study, end-of-life magnets were obtained from the manual disassembly of computer hard disc drives. After disassembly, the magnets were demagnetized, broken into pieces and fed to the leaching process. In the following step, rare earths were precipitated in the form of oxalates. The rare earths’ precipitation efficiency reached a maximum of 95.6%. The results showed that the co-precipitation of Fe highly depends on the amount of oxalic acid used as the precipitant. Smaller losses of Fe were achieved while using a stoichiometric amount of oxalic acid in relation to the REE present in the solution. At the end of the investigated process, rare earth oxalates were calcinated to oxides and their purity was investigated. The recirculation of a solution after oxalate precipitation and acid regeneration was tested with hydrochloric and sulfuric acids used as leaching agents. Solution recirculation was found to be possible only in the case of hydrochloric acid.

Recovery of Rare Earth Elements from the Leaching Solutions of Spent NdFeB Permanent Magnets by Selective Precipitation of Rare Earth Oxalates

After mechanical pre-treatment, the typical hydrometallurgical route of NdFeB magnet recycling starts with leaching in acidic solutions. However, due to the high concentration of iron ions in the leaching solution, the selective recovery of rare earths from the solution is challenging. In our work, the selective precipitation of rare earth oxalates as a potential separation method was proposed. The precipitation of neodymium oxalate was first tested on model solutions, which was then followed by experimental tests carried out on real solutions after the leaching of NdFeB magnets. The recovery of rare earths in the form of oxalates was investigated with the use of different amounts of oxalic acid in relation to its stoichiometric amount. The most efficient separation of rare earths was observed in the case where sulfuric acid was used for leaching. The use of oxalic acid in stoichiometric amounts resulted in the precipitation of about 93% of all rare earths present in the solution, whereas the concentration of Fe and other elements (Ni, Co, and B) practically did not change. An increase in oxalic acid of 20% and 40% more than the stoichiometric amount (100%) led to the increase in the precipitation efficiency of rare earths to 96.7% and 98.1%, respectively. However, the use of oxalic acid in a 1.4 ratio caused a 7% decrease in Fe concentration, which suggests Fe co-precipitation. In order to investigate a possibility of further increasing the separation of rare earths from iron, an additional method was tested, in which iron was first oxidized from Fe2+ to Fe3+ before the precipitation of rare earth oxalates.

Hydrometallurgy two stage process for preparation of (Nd, La, Ce)2O3 from end-of-life NiMH batteries

The present work aims to investigate the recovery of light rare earth elements (LREEs) oxides from end-of-life NiMH batteries using a hydrometallurgical process followed by effective precipitation. The operational leaching parameters such as phosphoric acid concentration, temperature, and the solid–liquid ratio were first optimized by Box-Behnken design. The results reveal that under optimum conditions ([H3PO4] = 2 mol/L, T = 80 °C, and S/L = 1: 10 g/mL) the leaching efficiencies of Ni, Co reach 98.1% and 99.3%. While La, Ce, and Nd elements remain in the leaching residue as (La, Ce, Nd)PO4 with yields of 98.2%, 98.6%, and 99.6% for La, Ce, and Nd, respectively. Afterward, the (La, Ce, Nd)PO4 is leached with HCl acid, then the rare earth oxalate was precipitated using oxalic acid at a pH of 1.8 and then the product was calcined at 800 °C for 2 h in order to synthesize the (Nd, La, Ce)2O3. The analysis using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) confirms the homogeneity of (Nd, La, Ce)2O3 particles that have two morphologies, i.e., flower and sticks with a particle size between 3 and 6 μm. The unit cell parameters of (Nd, La, Ce)2O3 were calculated after Rietveld refinement of the XRD patterns, in the space group of Fm-3m are a = b = c = 0.57921 nm and the volume equal to 0.194322 nm3.

Determining Local Magnetic Susceptibility Tensors in Paramagnetic Lanthanide Crystalline Powders from Solid-State NMR Chemical Shift Anisotropies

Exploring magnetic properties at the molecular level is a challenge that has been met by developing many experimental and theoretical solutions, such as polarized neutron diffraction (PND), muon-spin rotation (μ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements, and advanced modeling on open-shell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR, etc. All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species. This method relies on a rigorous acquisition thanks to short high-power adiabatic pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modeling of this data thanks to a semiempirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modeling of the hyperfine interaction (including up to 400 paramagnetic centers) that yield local magnetic susceptibility tensor measurements that are self-consistent as well as consistent with bulk susceptibility measurements.

Sulfuric acid bioproduction and its application in rare earth extraction from phosphogypsum

Industrial production of sulfuric acid requires a high amount of energy and in remote regions such as mineral extraction pits, in situ production becomes unfeasible. This work proposes an alternative for the production of H2SO4 that uses sulfur-oxidizing microorganisms. The production capacity of this acid in a mesophilic (30 °C) and a thermophilic (65 °C) condition was studied by three collected consortia from acid mine drainage and compared with Acidithiobacillus thiooxidans. It was found that At. thiooxidans presented higher sulfuric acid production than the consortia collected and, therefore, it was selected for the bioleaching of rare earths elements (REEs) from phosphogypsum (PG). Due to the acid consumption of phosphogypsum, the two-step bioleaching condition resulted in higher REEs extraction (98% Nd, 60% Ce, 58% La, and 62% Y) when compared to one-step bioleaching (28% Nd, 17% Ce, 18% La, and 30% Y). The process was performed on a reactor scale and it was possible to extract 55.0% of the REEs contained in 300 g of waste and concentrate them into 0.922 g of rare earth oxalates, with a final yield of 52.5%, showing that the proposed bioprocess has potential application even in remote areas due to its low energy consumption when compared to traditional processes.

Conceptual Process Development for the Separation of Thorium, Uranium, and Rare Earths from Coarse Coal Refuse

ABSTRACT Increasing disruption in the rare earth supply chain creates an urgency to develop alternative resources, in which utilization of coal-based materials presents great potential. Nevertheless, environmental control is a significant challenge in rare earth extraction processes. This study was conducted to contribute to the limited information on removing thorium and uranium from rare earths while coal-based products are used as feedstock. The laboratory studies suggested that the selective precipitation and solvent extraction approach yields the most favorable separation performance. Complete thorium precipitation was achieved around a pH value of 4.8. Due to the close precipitation pH ranges of uranium and rare earths, further separation by solvent extraction was applied to achieve an enhanced separation. Based on a Box-Behnken experimental design, the effect of extractant concentration, pH, strippant concentration, and O/A ratio was investigated. Best separation performance was achieved using 50 v% TBP at a pH of 3.5 with an O/A ratio of 3 and 1 mol/L H2SO4, which resulted in 1.8% uranium and 73.4% rare earth extraction. The extraction and precipitation behavior of the elements were further assessed with the distribution ratio, separation factor, thermodynamic parameters, and species distribution diagrams to provide a thorough understanding of the separation mechanisms. The results were statistically analyzed, and a model was developed to predict uranium recovery. The developed experimental protocol was validated using a rare earth oxalate sample produced at the pilot-scale processing facility. Finally, a conceptual process flowsheet was developed to effectively separate radionuclides while producing rare earth oxide products.

Homogeneous Precipitation of Lanthanide Oxalates.

Oxalic acid is an important separation agent in the technology of lanthanides, actinides, and transition metals. Thanks to the low solubility of the oxalate salts, the metal ions can be easily precipitated into crystalline material, which is a convenient precursor for oxide preparation. However, it is difficult to obtain oxalate monocrystals due to the fast precipitation. We have developed a synthetic route for homogeneous precipitation of oxalates based on the thermal decomposition of oxamic acid. This work primarily concerns lanthanide oxalates; however, since no information was found about oxamic acid, a brief characterization was included. The precipitation method was tested on selected elements (Ce, Pr, Gd, Er, and Yb), for which the kinetics was determined at 100 °C. Several scoping tests at 90 °C or using different starting concentrations were performed on Ce and Gd. The reaction products were studied by means of solid-state analysis with focus on the structure and morphology. Well-developed microcrystals were successfully synthesized with the largest size for gadolinium oxalate.

Rare Earth Group Separation after Extraction Using Sodium Diethyldithiocarbamate/Polyvinyl Chloride from Lamprophyre Dykes Leachate.

This study presents the first application of sodium diethyldithiocarbamate/polyvinyl chloride (DdTC/PVC) as a novel adsorbent for rare earth element (REE) sorption from leach liquors. DdTC/PVC has higher adsorption properties than other sorbents, the synthesis of DdTC/PVC is more accessible than other resins, and it is considered a more affordable sorbent. The three-liquid-phase extraction technique (TLPE) was applied to separate REEs into light, middle, and heavy rare earth elements as groups. The TLPE is an excellent achievable technique in the separation of REEs. DdTC/PVC was prepared as a sorbent to sorb rare-earth ions in chloride solution. It was described by XRD, SEM, TGA, and FTIR. The factors pH, initial rare-earth ion concentration, contact time, and DdTC/PVC dose were also analyzed. The ideal pH was 5.5, and the ideal equilibration time was found to be 45 min. The rare-earth ion uptake on DdTC/PVC was 156.2 mg/g. The rare-earth ion sorption on DdTC/PVC was fitted to Langmuir and pseudo-2nd-order models. The rare-earth ions’ thermodynamic adsorption was spontaneous and exothermic. In addition, rare-earth ion desorption from the loaded DdTC/PVC was scrutinized using 1 M HCl, 45 min time of contact, and a 1:60 S:L phase ratio. The obtained rare earth oxalate concentrate was utilized after dissolving it in HCl to extract and separate the RE ions into three groups—light (La, Ce, Nd, and Sm), middle (Gd, Ho, and Er), and heavy (Yb, Lu, and Y)—via three-liquid-phase extraction (TLPE). This technique is simple and suitable for extracting REEs.

Separation and coextraction of REEs and Fe from NdFeB sludge by co-leaching and stepwise precipitation

NdFeB sludge is a waste with high economic value. NdFeB waste recycling has the disadvantage that only rare earths are recycled, but iron (Fe) is not recycled, which results in large amounts of secondary solid waste. In this paper, a process flowchart of the co-extracted rare earth elements (REEs) and Fe by co-leaching and stepwise precipitation from NdFeB sludge was developed. The results show that after the NdFeB sludge was pretreated, co-leaching with hydrochloric acid, rare earth precipitation with oxalic acid, and Fe precipitation with reduction iron powder, 98.28% of the REEs were recovered in the form of RE2(C2O4)3·10H2O, and 94.65% of the Fe was recovered in the form of FeC2O4·2H2O. Due to the low solubility product constants of the rare earth oxalates, when oxalic acid was added to the leachate, the REEs precipitated rapidly, while Fe was still present as FeCl4–, Fe(C2O4)2–, and FeC2O4+ in the precipitated REEs solution. Therefore, the selective separation and precipitation of REEs and Fe was achieved. The process has the advantages of high comprehensive utilization of resources and no secondary solid waste or wastewater pollution.

Rapid and highly selective Sr2+ uptake by 3D microporous rare earth oxalates with the facile synthesis, high water stability and radiation resistance

Radiostrontium remediation is of vital significance due to the hazard of radiostrontium to ecological environments and human health. However, it remains a challenge to effectively remove Sr2+ ion from complex water environments due to its solubility, mobility and influence of interfering ions. Herein, we report a series of rare earth oxalates with three-dimensional (3D) anionic microporous frameworks, namely [Me2NH2][Ln(C2O4)2(H2O)]·3H2O (Ln-ox; Ln = Y, Nd, Sm, Eu, Gd, Er, Tm, Yb). Ln-ox can be easily synthesized in large scale and possess high water stability and radiation resistance. The rapid and highly selective uptake of Sr2+ ion is achieved by Eu-ox with fast kinetics (3 min) and KdSr value up to 2.61 × 105 mL/g. Under high concentrations of interfering metal ions (M = Na+, Cs+, Mg2+ and Ca2+), Eu-ox exhibits high selectivity for Sr2+ with high separation coefficient SFSr/M. The Sr2+ removal mechanism has been revealed and attributed to the ion exchange between Sr2+ and [Me2NH2]+ cations. Further, by comparing a series of rare earth oxalate frameworks, it is found that the occurrence of ion exchange in 3D microporous rare earth oxalate materials is related to the channel size and the interaction of exchangeable cations with anionic framework. The strong complexation ability of oxalate groups to Sr2+, the large channel size of Eu-ox and the exchangeable [Me2NH2]+ cations are crucial factors for Sr2+ ion exchange. Rare earth oxalate frameworks as ion exchange materials are firstly used for the Sr2+ removal, confirming the potential of rare earth oxalates in radionuclide remediation.

Kinetics and mechanism of hydrochloric acid leaching of rare earths from Bayan Obo slag and recovery of rare earth oxalate and high purity oxides

Recovery of rare earth elements (REEs) from tailings is the solution to the waste of valuable resource and hazard of tailing dam. In order to lower the cost and increase the rate of REEs recovery, we investigated leaching kinetics and dissolution mechanisms of REE slag in hydrochloric acid (HCl) solution, and present a method for selectively recovering REEs directly from the leachate. Experiments were performed to elucidate the effects of the initial concentration of HCl, liquid–solid ratio, and reaction temperature on the leaching efficiency of REEs. The maximum leaching efficiency of REEs in this work exceeded 99%. The leaching kinetics and dissolution mechanism have also been analyzed by model fitting and microstructural observation. The leaching reaction was controlled by both of the interfacial transfer and diffusion across the product layer. The pitting corrosion and the colloidal silicic acid generated at the interface of REE-rich phase showed positive and negative effects on the leaching reaction rate, respectively. Finally, the selective recovery of REEs from the leachate was developed, and both the recovery of REEs and purity of the REE precipitate exceeded 99%. This study is expected to provide a feasible method for the recovery of REEs from the silicates containing slag.

Development of an innovative process involving the use of ionic liquids for the recovery and purification of rare earths from permanent magnets and NIMH batteries

Background: Nowadays, the industry trends are reflecting an increase in the consumption of products containing rare earth elements (REEs), which leads to the generation of several REE-containing residues such as spent permanent magnets (SPM), permanent magnet swarf (PMS), and nickel metal hydride (NiMH) batteries. Methods: Due to the risk of supply and to decrease the dependency of Europe in obtaining REEs, an innovative process for obtaining REEs in the form of rare earth oxalates (REOx) that can be easily transformed to an xide mixture by calcination is proposed. The proposed method includes leaching of REEs from SPM, PMS, and NiMH batteries using different solvents such as ionic liquids and/or mineral acids; precipitation of REE in the form of REOx and purification of the final products by an ionic liquid extraction (ILE) process for removing the impurities using Cyphos 101 as ionic liquid. Intensive research, based on laboratory tests, is described for each of the parts of the process with the aim of providing optimized results. Results: In this study, >99% recovery of the REE initially present in the leachates after the leaching phase is achieved, with a purity of the REOxafter the precipitation and purification steps higher than 95%. Conclusion: A novel and innovative process for the extraction of REEs from secondary sources has been investigated in this paper, demonstrating strong potential for its implementation. The REEEs recovery rate and the purity obtained together with the low environmental impact of this process compared to conventional ones can contribute to a greener future where the usage of REEs will presumably be even more relevant.

Predicting inorganic dimensionality in templated metal oxides

Amine-templated metal oxides are a class of hybrid organic-inorganic compounds with great structural diversity; by varying the compositions, 0D, 1D, 2D, and 3D inorganic dimensionalities can be achieved. In this work, we created a dataset of 3725 amine-templated metal oxides (including some metalloid oxides), their composition, amine identity, and dimensionality, extracted from the Cambridge Structure Database (CSD), which spans 71 elements, 25 main group building units, and 349 amines. We characterize the diversity of this dataset over reactants and in time. Artificial neural network models trained on this dataset can predict the most and least probable outcome dimensionalities with 71% and 95% accuracies, respectively, using only information about reactant identities, without stoichiometric information. Surprisingly, the amine identity plays only a minor role in most cases, as omitting this information only reduces the accuracy by <2%. The generality of this model is demonstrated on a time held-out test set of 36 amine-templated lanthanide oxalates, vanadium tellurites, vanadium selenites, vanadates, molybdates, and molybdenum sulfates, whose syntheses and structural characterizations are reported here for the first time, and which contain two new element combinations and four amines that are not present in the CSD.

One-step separation and recovery of rare earth and iron from NdFeB slurry via phosphoric acid leaching

The recovery of rare earth elements (REEs) from NdFeB slurry by traditional hydrometallurgy has been limited becuase a large number of REEs are lost during separation together with iron. In this paper, a simple and sustainable method is proposed to efficiently separate and recover REEs and iron from NdFeB slurry. REEs were recovered by one-step selective precipitation in phosphoric acid, and the dissolved iron was recovered by oxalic acid. Phosphoric acid leaching results show that under the conditions of 4 mol/L phosphoric acid, 80 °C, L/S of 30:1 and 90 min, the leaching efficiencies of Fe and REEs reach 98.76% and 1.09%, respectively. While the rest of REEs remained in the leaching residue in the form of REEPO4·nH2O precipitation. Subsequently, the mixed rare earth oxide (rare earth oxalate roasted at 800 °C) and FeC2O4·2H2O are obtained by oxalic acid precipitation with purities of 99.49% and 97.17% from the REEPO4·nH2O dissolving solution and the phosphoric acid leaching solution. Moreover, the phosphoric acid is regenerated while recovering iron, and it can be reused in the phosphoric acid leaching step after removing the impurity C2O42−. In summary, this work provides an efficient and environmentally friendly method for recovering REEs and iron from NdFeB slurry waste.

Effects of contaminant metal ions on precipitation recovery of rare earth elements using oxalic acid

Solution equilibrium calculations were performed in this study to understand the impact of contaminant metal ions on the precipitation efficiency of selected rare earth elements (Ce3+, Nd3+, and Y3+) using oxalic acid as a precipitant. Trivalent metal ions, Al3+ and Fe3+, are found to considerably affect the precipitation efficiency of REEs. When Al3+ and Fe3+ concentrations are increased by 1 × 10−4 mol/L, in order to achieve an acceptable cerium recovery of 93% from solutions containing 1 × 10−4 mol/L Ce3+, oxalate dosage needs to increase by 1.2 × 10−4 and 1.68 × 10−4 mol/L, respectively. Such great impacts on the required oxalate dosage are also observed for Nd3+ and Y3+, which indicates that oxalic acid consumption and cost will be largely increased when the trivalent metal ions exist in REE-concentrated solutions. Effects of the divalent metal ions on the oxalate dosage are minimal. Furthermore, solution equilibrium calculation results show that the precipitation of Fe3+ and Ca2+ (e.g., hematite and Ca(C2O4)∙H2O(s)) likely occurs during the oxalate precipitation of REEs at relatively high pH (e.g., pH 2.5), which will reduce rare earth oxalate product purity. In addition to the metal ions, anionic species, especially SO42−, are also found to negatively affect the precipitation recovery of REEs. For example, when 0.1 mol/L SO42− occurs in a solution containing 1 × 10−4 mol/L Ce3+ and 4 × 10−4 mol/L oxalate, the pH needs to be elevated from 2.0 to 3.3 to achieve the acceptable recovery. Overall, findings from this study provide guidance for the obtainment of high-purity rare earth products from solutions containing a considerable amount of contaminant metal ions by means of oxalic acid precipitation.