Oxalic Acid Precipitation Research Articles

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.

Processing of mineralized microgranite dike for the recovery of niobium, tantalum, and rare earth elements using 1, 2-propyleneglycol extractant

ABSTRACT A number of strategic and economic rare-metal and Rare Earth Elements (REEs)-bearing minerals were recorded in mineralized microgranite dike (MMGD) from Ras Abdah area located in the Northern Eastern Desert of Egypt. Firstly, by sulphuric acid digestion at 300 °C, 3 h, 1/3 ore/Acid ratio, the RE, Nb, and Ta present in the MMGD can be efficiently extracted into the leach solution while Zr, Fe, and Th remained in the unattacked residue. The experimental digestion proved the dissolution of 98.9, 99.2, and 96 % for Nb, Ta, and REEs, respectively. In this study, a novel method was used to selectively recover Nb and Ta content from REEs in the digested concentrate sulphate solution. Solvent extraction using 1, 2-propyleneglycol was applied for the first time to achieve this secondary recovery process. From infrared and 1H-NMR data, the extraction was achieved, and the extractant bonded to Nb and Ta through glycol and alcoholic oxygen atoms. Niobium and Tantalum ions were stripped with water for Ta and 1M HF for Nb, achieving 98.6 and 98.8 % stripping efficiency, respectively. By employing an oxalic acid precipitation technique, a precipitate of extremely pure REEs was extracted from the raffinate solutions.

Selective N-monomethylation of amines using CO2/H2 catalyzed by high-activity Cu–ZrOx interface on SBA-15

N-methylation of amines using CO2 and H2 addresses the problems of expensive raw materials and harsh reaction conditions. However, designing a high-active and stable interface for the efficient preparation of monomethyl amine compounds in this tandem reaction remains a challenge to be overcome. Here, Cu–ZrOx composites are dispersed on SBA-15, silicalite-1, and ZSM-5 through the oxalic acid precipitation method for the N-methylation of amines using CO2 and H2. Results show that the dispersion of Cu–ZrOx on the surface of SBA-15 is superior to the other two molecular sieves and effectively prevents the sintering of active metals. A 20 wt% Cu–ZrOx/SBA-15 catalyst proves to be optimal for the N-methylation of aniline (AN) using CO2 and H2 to methylaniline (MA) and the catalyst performance remains stable after 5 cycle runs. The optimal catalyst exhibits an AN conversion of 97.6% and a selectivity of 96.5% for MA under the conditions of 180 °C, 4.0 MPa (H2/CO2/N2 = 72/24/4, molar ratio), and 8 h. Spectroscopic investigation and controlled experiments indicated that the preferential adsorbed AN on the catalyst surface reacts with CO2 to generate C–N bond, thereby avoiding the self-coupling side reaction of AN and achieving a high yield of 94.2% for MA.

Characterization of Nickel in Chromite Beneficiation Tailings by Mineral Liberation Analysis and Its Recovery by H2SO4 Leaching Followed by Oxalic Acid Precipitation

Characterization of Nickel in Chromite Beneficiation Tailings by Mineral Liberation Analysis and Its Recovery by H2SO4 Leaching Followed by Oxalic Acid Precipitation

Resource Utilization of Cyanide Tailings: Preparation of Ferrous Oxalate by a Combined Technique of Anaerobic Roasting–Persulfate Leaching Followed by Oxalic Acid Precipitation

Resource Utilization of Cyanide Tailings: Preparation of Ferrous Oxalate by a Combined Technique of Anaerobic Roasting–Persulfate Leaching Followed by Oxalic Acid Precipitation

Pilot Scale Testing of Lignite Adsorption Capability and the Benefits for the Recovery of Rare Earth Elements from Dilute Leach Solutions

Naturally occurring organic materials containing humic acids show a strong affinity towards rare earth elements (REE) and other critical elements. Leaching experiments on lignite coal waste produced from construction sand production revealed that the contained REEs were associated with the organic matter. Furthermore, adsorption studies revealed that the lignite waste was capable of extracting REEs from a model solution and increased the REE content of the lignite waste by more than 100%. As such, this study aimed to utilize the lignite waste to adsorb REEs from pregnant leach solutions and acid mine drainage sources having low REE concentrations and subsequently leach the lignite material to produce pregnant leach solutions containing relatively high amounts of REEs, which benefits the performance and economic viability of downstream separation and purification processes. An integrated flowsheet was developed based on this concept and tested at a pilot scale. The pregnant leachate solution (PLS) was generated from a heap leach pad containing 2000 tons of Baker seam coarse refuse. The pilot scale circuit was comprised of aluminum precipitation, adsorption using the waste lignite, and rare earth-critical metal (RE-CM) precipitation stages in succession. The results indicated that the aluminum precipitation stage removed over 88% and 99% of the Al and Fe, respectively. The adsorption stage increased the REE content associated with the waste lignite from 457 ppm to 1065 ppm on a whole mass basis. Furthermore, the heavy REE (HREE) content in the feedstock increased by approximately 250%, which raised the percentage of HREE in the REE distribution by 19 absolute percentage points. In addition to the REEs, concentrations of other critical elements such as Mn, Ni, and Zn also improved by 75%, 37%, and 250%, respectively. Bench-scale tests revealed that increasing the solids concentration in the waste lignite and PLS mix from 1% to 20% by weight enhanced the adsorption efficiency from 32.0% to 99.5%, respectively. As such, a new flowsheet was proposed which provides significantly higher REE concentrations in the PLS that can be fed directly to solvent extraction and/or oxalic acid precipitation and, thereby, enhancing process efficiency and economics.

Separation of Hydrochloric Acid and Oxalic Acid from Rare Earth Oxalic Acid Precipitation Mother Liquor by Electrodialysis.

In this study, the hydrochloric acid from rare earth oxalic acid precipitation mother liquor was separated by electrodialysis (ED) with different anion exchange membranes, including selective anion exchange membrane (SAEM), polymer alloy anion exchange membrane (PAAEM), and homogenous anion exchange membrane (HAEM). In addition to actual wastewater, nine types of simulated solutions with different concentrations of hydrochloric acid and oxalic acid were used in the experiments. The results indicated that the hydrochloric acid could be separated effectively by electrodialysis with SAEM from simulated and real rare earth oxalic acid precipitation mother liquor under the operating voltage 15 V and ampere 2.2 A, in which the hydrochloric acid obtained in the concentrate chamber of ED is of higher purity (>91.5%) generally. It was found that the separation effect of the two acids was related to the concentrations and molar ratios of hydrochloric acid and oxalic acid contained in their mixtures. The SEM images and ESD-mapping analyses indicated that membrane fouling appeared on the surface of ACS and CSE at the diluted side of the ED membrane stack when electrodialysis was used to treat the real rare earth oxalic acid precipitation mother liquor. Fe, Yb, Al, and Dy were found in the CSE membrane section, and organic compounds containing carbon and sulfur were attached to the surface of the ACS. The results also indicated that the real rare earth precipitation mother liquor needed to be pretreated before the separation of hydrochloric acid and oxalic acid by electrodialysis.

Comparison of the Preparation Process of Rare Earth Oxides from the Water Leaching Solution of Waste Nd-Fe-B Magnets’ Sulfate Roasting Products

The new process developed here consisting of sulfurization roasting transformation and water immersion can effectively realize the separation of rare earth elements (REEs) and impurities from spent Nd-Fe-B magnets. For the industrial application of the new process, it is critical to determine how to economically and efficiently prepare rare earth oxide (RExOy) products with higher purity from the obtained water leaching solution. Therefore, according to rare earth sulfate (RE2(SO4)3) solution characteristics, the oxalic acid precipitation–calcination method, sodium carbonate precipitation–calcination method, and double sulfates precipitation–alkali conversion–calcination method were optimized and compared. The results show that the recovery efficiency of REE recovery via the oxalic acid precipitation–calcination method is 99.44%, and the purity of RExOy is 99.83% under optimal technological conditions. However, the cost of oxalic acid precipitation is higher. The process consisting of the double sulfates precipitation–alkali conversion–calcination method is relatively complicated, the recovery efficiency of REEs is 97.95%, and the purity of the RExOy is 98.04%. The recovery efficiency of the REEs and the purity of the RExOy obtained from the sodium carbonate precipitation–calcination method are 99.12% and 98.33%, respectively. Moreover, the recycling cost of sodium carbonate precipitation is the lowest among the three processes for preparing RExOy, so it has industrial application potential. The obtained results for REE recovery from spent Nd-Fe-B magnets in this research can provide theoretical guidance for the innovation of the recycling process for REEs as secondary resources.

Comparative Study of Manufacturing NdFeB Magnet Wastes Recycling: Oxidative Roasting-Selective Leaching and Whole Leaching Routes

This research investigated recycling of manufacturing NdFeB magnet wastes in as-sintered and powder forms which contained high carbon via pyro-hydro metallurgy process. Effects of oxidative roasting on selective leaching of the magnet wastes were the main focus in comparison to recycling via whole leaching without oxidative roasting. The process started from oxidative roasting at 600 °C, sulfuric leaching, drying, roasting at 750 °C for powder and 800 °C for sintered wastes, water leaching, oxalic acid precipitation and calcination at 1000 °C to obtain neodymium oxides. Oxidative roasting was found to reduce carbon and resulted in neodymium and iron oxide formation with a minimum amount of neodymium iron oxide. This provided effective selective leaching of neodymium. For whole leaching, a significant loss of neodymium into leached residue was observed. Oxidative roasting-selective leaching provided significant recovery in the amount of 75.46% while whole leaching resulted in only 31.62 wt.% in the case of sintered waste. The final composition via oxidative roasting-selective leaching consisted of 68.11 wt.% neodymium, 19.83 wt.% praseodymium and 0.31 wt.% iron, while whole leaching resulted in a higher amount of iron at 1.20 wt.%. Similar results were obtained for powder magnet waste.

Study of the thermal stability and decomposition behaviour of Lu(NH 4 )(C 2 O 4 ) 2 ·2H 2 O

Lu(NH4)(C2O4)2·2H2O has been successfully fabricated by the improved oxalate precipitation method. Its thermal stability and decomposition behaviour were investigated by using scanning electron microscopy, thermalgravimetric-differantial scanning calorimetry and in situ high temperature X-ray diffraction measurements. Despite substantial removal of oxide, carbon, nitrogen, and hydrogen elements, the scanning electron microscopy images clearly revealed that the precursor calcined Lu2O3 products maintained well the morphology properties of the precursor. Moreover, TG-DSC analysis demonstrated that the peak temperatures of the precursor to Lu2O2CO3 and Lu2O2CO3 to Lu2O3 were 400°C and 630°C, respectively. Finally, and most importantly, the in situ XRD results exhibited that the ‘Lu(NH4)(C2O4)2’, ‘Lu2O2CO3’ and ‘Lu2O3’ crystal structures formed by heating the Lu (NH4) (C2O4)2·H2O at 25∼220°C, 400∼680°C and 680∼980°C, respectively. The experimental results provided an interesting fact that Lu(NH4)(C2O4)2·H2O has been completely decomposed into cubic phase Lu2O3 crystal at 680°C, which is nearly 200°C lower than that of lutetium oxalate tetrahydrate prepared by traditional oxalic acid precipitation process.

ZnCl2: A Green Brønsted Acid for Selectively Recovering Rare Earth Elements from Spent NdFeB Permanent Magnets.

The spent neodymium-iron-boron (NdFeB) magnet is a highly valuable secondary resource of rare earth elements (REEs). Hydrometallurgical processes are widely used in recovering REEs from spent NdFeB magnets, but they will consume large amounts of organic chemicals, leading to severe environmental pollution. This work developed an alternative green route to selectively recover REEs from spent NdFeB permanent magnets using a purely inorganic zinc salt. The Hammett acidity measurement showed that concentrated ZnCl2 solutions could be regarded as a strong Brønsted acid. Concentrated ZnCl2 solutions achieved a high separation factor (>1 × 105) between neodymium and iron through simple dissolution of their corresponding oxide mixture. In the simulated recovery process of spent NdFeB magnets, the Nd2O3 product was successfully recovered with a purity close to 100% after selective leaching by ZnCl2 solution, sulfate double-salt precipitation, and oxalic acid precipitation. The separation performance of the ZnCl2 solution for Nd2O3 and Fe2O3 remained almost unchanged after four cycles. The energy consumption and chemical inputs of this process are about 1/10 and half of the traditional hydrometallurgy process separately. This work provides a promising approach for the green recovery of secondary REE resources.

A Hybrid Experimental and Theoretical Approach to Optimize Recovery of Rare Earth Elements from Acid Mine Drainage Precipitates by Oxalic Acid Precipitation

The development of processing techniques for the extraction of rare earth elements and critical minerals (REE/CM) from acid mine drainage precipitates (AMDp) has attracted increased interest in recent years. Processes under development often utilize a standard hydrometallurgical approach that includes leaching and solvent extraction followed by oxalic acid precipitation and calcination to produce a final rare earth oxide product. Impurities such as Ca, Al, Mn, Fe and Mg can be detrimental in the oxalate precipitation step and a survey of the literature showed limited data pertaining to the REE precipitation efficiency in solutions with high impurity concentrations. As such, a systematic laboratory-scale precipitation study was performed on a strip solution generated by the acid leaching and solvent extraction of an AMDp feedstock to identify the optimal processing conditions that maximize REE precipitation efficiency and product purity while minimizing the oxalic acid dosage. Given the unique chemical characteristics of AMDp, the feed solution utilized in this study contained a moderate concentration of REEs (440 mg/L) as well a significant concentration (>7000 mg/L total) of non-REE contaminants such as Ca, Al, Mn, Fe and Mg. Initially, a theoretical basis for the required oxalic acid dose, optimal pH and predicted precipitation efficiency was established by solution equilibrium calculations. Following the solution chemistry calculations, bench-scale precipitation experiments were conducted and these test results indicate that a pH of 1.5 to 2, a reaction time of more than 2 h and an oxalic acid dosage of 30 to 40 g/L optimized the REEs recovery of at ~95% to nearly 100% for individual REE species. The test results validated the optimal pH predicted by the solution chemistry calculations (1.5 to 5); however, the predicted dosage needed for complete REE recovery (10 g/L) was significantly lower than the experimentally-determined dosage of 30 to 40 g/L. The reason for this discrepancy was determined to be due to the large concentration of impurities and large number of potential metal complexes that cause inaccuracies in the solution equilibrium calculations. Based on these findings, a hybrid experimental and theoretical approach is proposed for future oxalic acid precipitation optimization studies.

The selective recovery of rare earth from radioactive waste residue using improved oxamic acid for environmental and resource concerns

Because radioactive elements may migrate with rainwater and pollute the environment, the treatment of rare earth (RE) radioactive waste residue is of great significance. The process is helpful to recover RE and radioactive element, also reduce the volume of solid pollutants. In this paper, a novel oil-soluble oxamic acid extractant was synthesized to separate RE and Th from the waste residue. The low water soluble extractant 2-(didecylamino)-2-oxoacetic acid (DDOA) could be dissolved in common industrial diluent and used to remove low concentration of radioactive element Th from RE. The loading capacity of DDOA to Th is 2.81 g/L, which can meet the needs of treating ion adsorbed RE waste residue. The whole extraction process was carried out at lower acidities. After 3 times of stripping with 3 mol/L hydrochloric acid, the total stripping capacity of Th reached 98%. Slope analysis method indicates that 2 mol DDOA could extract 1 mol Th4+ in the extraction process. After five cycles, the extraction efficiency of Th could be maintained at 98%. After 4-stage extraction and 3-stage scrubbing, the concentration of Th in the RE aqueous phase outlet was lower than 0.22 mg/L, and the yield of RE was 95%. After oxalic acid precipitation, the recovery of RE could reach 80%. Overall, due to the high selectivity, better oil solubility, simplified fractional extraction, Th in radioactive slag could be efficiently removed and RE could be enriched.

Parametric study and speciation analysis of rare earth precipitation using oxalic acid in a chloride solution system

Oxalic acid precipitation is a common step in the purification of rare earth elements (REE) from a concentrated pregnant leach solution (PLS). However, the presence of contaminants such as Al, Fe, and Ca in given amounts decreases the REE precipitation efficiency and product purity while also increasing the amount of oxalic acid needed to maximize recovery. As such, a statistically designed test program was performed to identify the optimal conditions necessary for a relatively low REE content PLS containing elevated concentrations of contaminant ions. The performance objective was maximization of REE precipitation efficiency while minimizing the oxalic acid dosage. A central composite design was utilized to quantify performance impacts and identify the ultimate set of parameter values for oxalic acid dosage, Fe(III) contamination concentration, solution pH, and reaction temperature. The resultant model suggested that oxalic acid dosage and reaction pH are the most significant factors for the REE precipitation efficiency, followed by the interaction of oxalic dosage and Fe concentration. Test results indicate that increasing the oxalic acid concentration from 0 g/L to 80 g/L improved the REE precipitation efficiency from approximately 4.2% to 95.0%. Furthermore, raising the solution pH from 0.5 to 2.5 considerably enhanced the precipitation efficiency from 0.0% to 98.9%. A solution temperature elevation decreased REE recovery, which indicated an exothermic reaction between REEs and oxalate anions. Finally, a high level of Fe contamination adversely impacted REE precipitation efficiency. To further the understanding of the REE-oxalate system, a fundamental solution chemistry study was performed using the equilibrium constants of the reactions. The study resulted in the development of oxalate speciation diagrams and provided an analysis of the REE precipitation characteristics at various oxalate anion concentrations and Fe(III) contamination levels using MINTEQ software. The dominant Fe(III) species in the solution system were found to be Fe-(C2O4)33-, Fe-(C2O4)2-, and Fe-(C2O4)+, which consume the majority of the oxalate anions. The simulated model was found to be in agreement with the experimental findings and helped to explain the adverse impact of increased iron concentrations on REE precipitation efficiency.

Cobalt Recovery by Oxalic Acid and Hydroxide Precipitation from Waste Cemented Carbide Scrap Cobalt Leaching Solution

Cobalt Recovery by Oxalic Acid and Hydroxide Precipitation from Waste Cemented Carbide Scrap Cobalt Leaching Solution

Recovery of Y and Eu from waste CRT phosphor using closed-vessel microwave leaching

• Closed-vessel microwave is effective for Y and Eu leaching from CRT phosphor. • It is energy-efficient with total energy requirement of 456.7 kJ (217 W) within 30 min. • 86.67% of Y and 100% of Eu were leached out using 2 M of H 2 SO 4 at 125 °C. • The recovery efficiency was 96.9% for Y and 86.6% for Eu as (Y 0.95 Eu 0.05 ) 2 O 3. Yttrium (Y) and europium (Eu) are critical rare earth elements (REEs) due to their high supply risk and importance for clean energy technologies. Their presence in electronic waste (e-waste) provides great potential of recovery and reuse. This study focused on recovery process of Y and Eu from waste cathode ray tube (CRT) phosphor using closed-vessel microwave leaching, followed by precipitation by oxalic acid and calcination. The effects of acid concentration and energy efficiency of closed-vessel microwave leaching were investigated. Leaching efficiency of Y and Eu was 86.67% and 100%, respectively, at the optimum operation conditions (10 g/L of solid concentration, 2 mol·L −1 of H 2 SO 4 solution, 125 °C, 30 min of leaching time). When comparing with conventional heating process, closed-vessel microwave leaching process proved to be rapid, effective and efficient for Y and Eu recovery from waste CRT phosphor. Oxalic acid precipitation and calcination recovered 96.9 ± 0.26% of Y and 86.6 ± 0.76% of Eu as the crystallized product of (Y 0.95 Eu 0.05 ) 2 O 3 .

Optimization of the leaching process for uranium recovery and some associated valuable elements from low-grade uranium ore

ABSTRACT The present work study the acidic pug leaching technique for U-extraction in addition to some associated valuable metals from a collected technological sample of ferruginous shale from Wadi Um Hamed, Um Bogma Formation, Abu Thor locality, southwestern Sinai, Egypt. A representative sample of the ore material was found to assay 9000, 700 and 4000 ppm of REEs, U and Cu, respectively, in addition to some major and minor oxides. Numerous steps were performed upon the studied sample to process parametric studies such as sulphuric acid concentration (50–150 kg/ton), curing time (0.5–5.0 h) and curing temperature (50–150°C) in order to find the optimum condition. Under the optimal conditions (150 kg/ton sulphuric acid for 4.0 h at 150°C), the first step involved Fe-removal as hydroxide from the pregnant leaching solution using pH adjustment method followed by U-recovery through sorption/desorption and precipitation steps with the help of Amberlite IRA-400-Cl (strongly basic anion exchange resin) via column method. In the third step, REEs- recovery from U-free effluent was investigated using oxalic acid precipitation process while Cu-recovery by precipitation as hydroxide with ammonia solution took place. Energy dispersive X-ray analysis performed on the different obtained precipitates showed successive recovery of all above mentioned elements. Finally, U-uptake data were subjected to kinetic modelling. The kinetics of leaching reaction followed shrinking core model with chemical reaction as controlling mechanism based on activation energy value.

Extraction of Rare Earths from Red Mud Iron Nugget Slags with Oxalic Acid Precipitation

ABSTRACT Red mud contains large amounts of rare and valuable minerals. Specifically, rare earth elements are present at a concentrated amount in many red mud samples around the world. There is currently only one ore source in the United States that can produce rare earth elements. Pursuing avenues to extract rare earths from red mud is highly advantageous to reduce the amount of red mud being stockpiled, give value to red mud as a waste, and utilize a source for producing rare earths. The iron nugget process effectively increases the concentration of rare earth elements by removing iron. Slag from the iron nugget process upgraded the concentration of rare earth elements by 100%, which makes this a desirable feed for processing. Hydrochloric acid was used to dissolve the rare earth oxides present in the nugget slags, rare earths were then precipitated as a solid using oxalic acid. HCl leach can recover 170 grams of rare earths per ton of red mud nugget slag and oxalic acid precipitation can recover 45 grams per ton.

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.