Leaching Of Rare Earth Elements Research Articles

Chelation-Assisted Ion-Exchange Leaching of Rare Earths from Clay Minerals

The effect of biodegradable chelating agents on the recovery of rare earth elements (REE) from clay minerals via ion-exchange leaching was investigated, with the aim of proposing a cost-effective, enhanced procedure that is environmentally benign and allows high REE recovery while reducing/eliminating ammonium sulfate usage. A processing route employing a lixiviant system consisting of simulated sea water (equivalent to about 0.5 mol/L NaCl) in conjunction with chelating agents was also explored, in order to offer a process alternative for situations with restricted access to fresh water (either due to remote location or to lower the operating costs). Screening criteria for the selection of chelating agents were established and experiments were conducted to assess the efficiency of selected reagents in terms of REE recovery. The results were compared to extraction levels obtained during conventional ion-exchange leaching procedures with ammonium sulfate and simulated sea water only. It was found that stoichiometric addition of N,N′-ethylenediaminedisuccinic acid (EDDS) and nitrilotriacetic acid-trisodium form (NTA-Na3) resulted in 10–20% increased REE extraction when compared to lixiviant only, while achieving moderate Al co-desorption and maintaining neutral pH values in the final solution.

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.

Selective Leaching of Rare Earth Elements from Ion-Adsorption Rare Earth Tailings: A Synergy between CeO2 Reduction and Fe/Mn Stabilization.

The increasing demand for rare earth elements (REEs) motivates the development of novel strategies for cost-effective REE recovery from secondary sources, especially rare earth tailings. The biggest challenges in recovering REEs from ion-adsorption rare earth tailings are incomplete extraction of cerium (Ce) and the coleaching of iron (Fe) and manganese (Mn). Here, a synergistic process between reduction and stabilization was proposed by innovatively using elemental sulfur (S) as reductant for converting insoluble CeO2 into soluble Ce2(SO4)3 and transforming Fe and Mn oxides into inert FeFe2O4 and MnFe2O4 spinel minerals. After the calcination at 400 °C, 97.0% of Ce can be dissolved using a diluted sulfuric acid, along with only 3.67% of Fe and 23.3% of Mn leached out. Thermodynamic analysis reveals that CeO2 was indirectly reduced by the intermediates MnSO4 and FeS in the system. Density functional theory calculations indicated that Fe(II) and Mn(II) shared similar outer electron arrangements and coordination environments, favoring Mn(II) over Ce(III) as a replacement for Fe(II) in the FeO6 octahedral structure of FeFe2O4. Further investigation on the leaching process suggested that 0.5 mol L-1 H2SO4 is sufficient for the recovery of REEs (97.0%). This research provides a promising strategy to selectively recover REEs from mining tailings or secondary sources via controlling the mineral phase transformation.

A Study on the Leaching of Rare Earth Elements from Waste Phosphor Powder

This study was carried out to obtain data to design a process to recover rare earth elements, specifically Y(Yttrium), La(Lanthanum), Ce(Cerium), Eu(Europium), Tb(Terbium) from waste phosphor powder. For this purpose, we investigated the effect of temperature, concentration, time and acids on leaching of the rare earth elements. The effect of roasting temperature, roasting time, roasting agent and its dosage on the leaching of rare earth elements were also investigated. 92% of the Yttrium, 70% of the Europium and 8% of the Cerium contained in the waste phosphor powder was leached at the condition of 50 <sup>o</sup>C and 0.3N HCl solution for 3hours. However, Terbium and Lanthanum were never leached at this condition. The leaching ratio increased to 100% of Yttrium and Europium, 98% of Cerium, 92% of Terbium and 89% of Lanthanum by leaching after soda ash roasting. In the leaching experiment with unroasted phosphor at 80 <sup>o</sup>C, the initial leaching reaction rate of Yttrium was 0.035 mol/L·s in 0.3N sulfuric acid solution, 0.033 mol/L·s in nitric acid solution and 0.028 mol/L·s in 0.3N hydrochloric acid solution. And the initial leaching reaction rate of Europium was 0.0017 mol/L·s in 0.3N sulfuric acid solution, 0.00114 mol/L·s in nitric acid solution and 0.00113 mol/L·s in 0.3N hydrochloric acid solution. For Cerium, the initial leaching reaction rate was 0.00019 mol/L·s in 0.3N sulfuric acid solution, 0.00025 mol/L·s in nitric acid solution and 0.00014 mol/L·s in 0.3N hydrochloric acid solution.

Experimental research on the laws of acidification and salinization under the conditions of Ionic rare earth leaching and water Leaching

In the ionic rare earth mining,ammonium sulfate is used as a leaching agent for in-situ leaching, the concentration of ammonium sulfate is 2%∼3%, and the pH is 4∼5, leading to soil acidification and increased salt content. In this paper,taking a rare earth mining area in Guangdong as the research object, carried out soil column simulation test research. After leaching with water for the soil column that complete leaching, in condition of using different leaching agents of pH values and different water leaching process characteristics, the variation of pH and total salt content was studied. The results show that, after leaching ,the mineral sample changed from mild acidification to non-acidification, and the the salt content of the mineral sample was effectively reduced, the salt leaching rate is above 57%. The research conclusions laid a theoretical foundation for soil acidification and salinization control in ion-type rare earth mining areas.

Carbothermal reduction followed by sulfuric acid leaching of Bayan Obo tailings for selective concentration of iron and rare earth metals

• Carbothermal reduction–H 2 SO 4 leaching of high-fluorine Bayan Obo tailings is studied. • More than 99% of Fe is reduced to the metallic state as the basicity increases to 1.87. • Approximately 97% of Fe and 83% of the REEs were separated and extracted. • Leaching temperature (>25 °C) and H 2 SO 4 concentration (>2 mol/L) hinder the leaching of rare earth elements. In this work, carbothermal reduction followed by sulfuric acid (H 2 SO 4 ) leaching of Bayan Obo tailings has been studied for the selective concentration of Fe and rare earth (RE) minerals. The effect of basicity on the metallization rate of Fe, transfer behaviors of metallic Fe and RE elements, phase composition, and element migration was investigated using XRD, SEM-EDS, EDS mapping, and FT-IR spectroscopy. At a roasting temperature of 1100–1200 °C and a basicity of 1.87, more than 99% of Fe was metallized and 97.91% of Fe and 94.94% of RE were separated magnetically. After roasting, the RE elements mainly existed in Ca 2 RE 8 (SiO 4 ) 6 O 2 and Ca 8 RE 2 (PO 4 ) 6 O 2 , 88% of which were leached out by H 2 SO 4 at 25 °C with an initial concentration of 2 mol/L and a stirring speed of 900 r/min. The precipitation of RE 2 (SO 4 ) 3 was responsible for the incomplete recovery of RE elements during leaching.

Leaching of rare earth elements and base metals from spent NiMH batteries using gluconate and its potential bio-oxidation products

Gluconate is known to mediate metal leaching. However, during bioleaching by e.g., Gluconobacter oxydans, gluconate can be oxidized to 2-ketogluconate and 5-ketogluconate. The impact of bio-oxidation of gluconate on metal leaching has not been investigated. Therefore, the aim of this study was to investigate leaching of rare earth elements (REEs) and base metals from spent nickel-metal-hydride (NiMH) batteries using gluconate, 2-ketogluconate and 5-ketogluconate. Batch leaching assays were conducted under controlled and uncontrolled pH conditions for 14 days using 60 mM of either the individual leaching agents or their various combinations. At target pH of 6.0 ± 0.1 and 9.0 ± 0.1 and without pH control, complexolysis was the dominating leaching mechanism and higher REE leaching efficiency was obtained with gluconate, while 5-ketogluconate enabled more efficient base metal leaching. At target pH of 3.0 ± 0.1, acidolysis dominated, and the base metal and REE leaching yields with all the tested leaching agents were higher than under the other studied pH conditions. The highest base metal and REE leaching yields (%) were obtained using gluconate at target pH of 3.0 ± 0.1 being 100.0 Mn, 90.3 Fe, 89.5 Co, 58.5 Ni, 24.0 Cu, 29.3 Zn and 56.1 total REEs. The obtained results are useful in optimization of heterotrophic bioleaching.

A single-step process to leach out rare earth elements from coal ash using organic carboxylic acids

The ever increasing applications of rare earth elements and their limited primary resources has compelled researchers worldwide to find alternate methods of efficient leaching from various secondary resources, in particular from end of life products. The present research work describes such a process to extract rare earth elements from coal ash using low molecular weight carboxylic acids. Different carboxylic acids, i.e. monocarboxylic (lactic acid), dicarboxylic (malonic acid, succinic acid, tartaric acid) and tricarboxylic acid (citric acid) were used to successfully leach out significant proportion of rare earth elements (up to 62%) from coal ash at the natural pH of the carboxylic acid solutions (pH 1.6–2.3). The carboxylic acid concentration, leaching duration, temperature, and pulp density were optimized for the leaching process. Best leaching efficiency (65% for LREE, 19% for HREE & 62% for total REE) was achieved using 5% tartaric acid solution having natural solution pH 1.8 at 90 °C with a leaching duration of 60 min. The leaching efficiency of the different carboxylic acids were in the order of tartaric acid>lactic acid>citric acid>malonic acid>succinic acid. The leaching kinetics of individual rare earth elements was found to be different from each other owing to their different complex formation ability with carboxylic acids. Appreciable selectivity was observed for leaching of rare earth elements over transition metal and main group elements. XRD and electron probe micro analyzer (EPMA) studies revealed that unlike mineral acids, carboxylic acids leached out rare earth elements without damaging the minerology of coal ash, thus providing the opportunity to utilize the treated coal ash for other applications like in cement and bricks industries. Thus, the novelty of the present work lies in efficient and selective leaching of lanthanides from coal ash using only organic acids, which are considered much milder and greener options compared to mineral acids.

Removal of Sulfate Ions from Calcium Oxide Precipitation Enrichment of a Rare Earth Leaching Liquor by Stirring Washing with Sodium Hydroxide

The use of calcium oxide as a precipitant can achieve a nonammonia enrichment of a rare earth leaching liquor. However, an alkaline rare earth sulfate forms during the precipitation process, thus resulting in excessive SO42– content in the mixed rare earth oxides. Therefore, a stirring washing process for precipitation enrichment, which was obtained from calcium oxide precipitation, was investigated using a sodium hydroxide solution. It was determined that the Gibbs free energy of the stirring washing reaction, which was calculated by a group contribution method, was between −60 and −300 kJ/mol, depending on the different rare earth elements. The above results indicated that the reaction was thermodynamically feasible. The optimum conditions of the washing process were obtained, namely, a feed ratio of 2.85, a liquid–solid ratio of 6.5 mL/g, a stirring washing temperature of 35 °C, and a stirring washing time of 20 min. Under the optimal conditions, the purity and the SO42– content of the mixed rare earth oxides were 94.38% and 3.48%, respectively, and the stirring washing process with the sodium hydroxide solution had good recyclability. Moreover, the washing product was tested using thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), which verified that the stirring washing process with NaOH could effectively remove SO42– from the precipitation enrichment into solution. On this basis, a new extraction process of the ion-adsorption-type rare earth ore by magnesium salt leaching–calcium oxide precipitation–sodium hydroxide stirring washing is proposed. This new process can eliminate the traditional aluminum-removal process and effectively reduces the rare earth loss in the process. It can also solve the problem of the excessive SO42– content in the mixed rare earth oxides caused by the calcium oxide precipitation process. The research in this paper can have great significance for green, efficient extraction of the ion-adsorption-type rare earth ore and the improvement of resource utilization.

Editorial for Special Issue “Leaching of Rare Earth Elements from Various Sources”

Rare earth elements (REEs) have become an important group of metals used in many high-tech industries, including high-strength magnets, plasma TVs, various military applications, and clean and efficient green energy industries [...]

Effect of potassium chloride on leaching process of residual ammonium from weathered crust elution-deposited rare earth ore tailings

Ammonium sulfate has been widely applied as a leaching agent to leach rare earth from weathered crust elution-deposited rare earth ore because of its higher selectivity. However, large amount of ammonium remained on the ore body after the completion of rare earth leaching, and it was necessary to leach it from the ore body from the environmental point of view. In this work, potassium chloride was used as a leaching agent and effects of potassium chloride concentration, leaching agent pH, liquid/solid ratio and flow rate on the leaching process of ammonium were investigated in detail. It was found that potassium chloride could effectively leach the residual ammonium from rare earth tailings, and the leaching efficiency and leaching rate of residual ammonium were significantly higher than that with deionized water. Leaching agent pH almost had no effects on the leaching process of residual ammonium in the range of 2–6, and a greater impact on it at pH 8. The flow rate could effectively enhance the leaching efficiency. The optimum conditions of leaching residual ammonium were 0.06 mol/L of potassium chloride, pH 5.5–6.5 of leaching agent, 2:1 of liquid/solid ratio and 0.6 mL/min of flow rate. At these conditions, the leaching efficiency of residual ammonium was 95.78%, and the concentration of ammonium in the leachate at the final stage was 0.89 mg/L, which could meet the requirements for environment. The leaching process of ammonium in rare earth ore tailings was controlled by the inner particle diffusing according to the kinetic analysis. The reaction order was 0.117. These results suggest a promising route for the environmental remediation of weathered crust elution-deposited rare earth ore after in-situ leaching.

Formation Mechanism of Preferential Flow Paths and Seepage Characteristics of a Novel Growable Pile for Heap Leaching of Rare Earth

In the traditional heap leaching of rare earth minerals, the top of the rare earth pile is covered with leaching liquid. This creates trouble for vegetation restoration carried out timely on the top of the pile. In order to solve this trouble, a novel pile structure into which leaching liquid is laterally injected is proposed for heap leaching of rare earth. In this study, a laboratory test is carried out to study the formation and distribution of preferential flow paths for the rare earth pile under a horizontal liquid injection condition. Furthermore, numerical simulations based on a dual‐permeability model are conducted to investigate the influence of the preferential flow paths on the seepage characteristics in the rare earth pile. The results show that, under the horizontal liquid injection condition, the fine particles of the rare earth move away from the liquid injection end and also toward the lower part of the pile. The migration of the fine particles results in the formation and connection of macropore, thereby generating preferential flow paths in the rare earth pile. The preferential flow paths are mainly distributed in the lower part of the pile near the liquid injection end. This causes the fluid in the lower part of the pile to seep faster significantly than that in the upper part. Within the region where the preferential flow paths develop, the seepage in the early stage of the horizontal liquid injection is dominated by preferential flow. The preferential flow is more significant at the locations farther away from the liquid injection end.

Effects of Ion Characteristics on the Leaching of Weathered Crust Elution-Deposited Rare Earth Ore.

To reveal the ion-exchange mechanism in the leaching process of weathered crust elution-deposited rare earth ores with different leaching agents, the effects of a variety of cations and anions at different concentrations on the leaching process were investigated, including Al3+, Fe3+, Ca2+, Mg2+, Na+, K+, and Cl−, , and . Meanwhile, the relationships between different concentrations of cations and anions and leaching efficiency were investigated, as was the relationship between different concentrations of cations and anions and zeta potential. The effect of different ions on the swelling of clay minerals during leaching process was also investigated. The results shown that was the most affected electrolyte cation in terms of rare earth leaching efficiency during the leaching process of weathered crust elution-deposited rare earth ore among three different cationic valence states, and the leaching efficiency was 86.93% at the optimal leaching concentration. The influence of the three anions on the leaching efficiency of rare earth was , and the leaching efficiency of rare earth were 83.21, 81.52, and 80.12% at the optimal leaching concentration, respectively. The had the greatest effect on the zeta potential of weathered crust elution-deposited rare earth ore, and the zeta potential was −18.1 mV at the optimal leaching concentration. Additionally, the order of the effect of three anions on zeta potential was . Combined with the effect on the rare earth leaching process, anions and cations were considered separately, and and Cl− were selected; the relationship between the rare earth leaching efficiency and zeta potential conforms to the follow equations: :Y = −0.48X2 – 13.51X – 1.58, R2 = 0.98133 and Cl−:Y= −1.22X2 – 17.64X + 23.29, R2 = 0.99010. It was also found in the swelling experiment of the weathered crust elution-deposited rare earth ore that the swelling ratio of clay minerals was the lowest when the cation and anion were and Cl− and the swelling ratios were 1.874 and 2.015%, respectively.

A safer and cleaner process for recovering thorium and rare earth elements from radioactive waste residue

Rare earth elements (REEs) have attracted widely attentions because of their excellent properties, however, radioactive waste residue generated during the REEs production has created serious environmental problems. This study aimed to develop a safer and cleaner technology, including residue leaching, thorium (Th) separating and REEs recovering, for the proper disposal of radioactive waste residue from ion-absorbed rare earth separation industry to reduce the environmental hazards. First, the chemical composition of residue was analyzed. Then, the leaching factors such as acid type, acid concentration and liquid-solid ratio were investigated and a multi-step leaching process was proposed to improve acid utilization and the leaching of REEs. After the multi-step leaching with HCl, the total leaching efficiency of REEs and Th were higher than 98.14% and 99.07%, respectively. Next, a commercial extractant of sec-octylphenoxy acetic acid (CA-12) was used to separate Th and enrich REEs from the residue leachate. The extraction factors of CA-12 toward Th were investigated in detail and a fractional extraction for separating Th and enriching lanthanides from the leachate of residue was carried out, showing that the separation efficiency of Th was higher than 99.53% and the concentration of lanthanides in the concentrated solution was 223.19 g L−1.

Leaching Process of Weathered Crust Elution-Deposited Rare Earth Ore With Formate Salts.

To strengthen the rare earth leaching process and weaken the hydration of clay minerals for preventing landslides, it is of great importance to adopt a green and sustainable leaching agent in the industry. In this work, the leaching process of weathered crust elution-deposited rare earth ores with formate salts (ammonium formate, potassium formate, and sodium formate) was investigated. The effects of formate salts on the linear swelling ratio and zeta potential of the clay minerals were studied. The experimental results showed that ammonium formate could effectively recover the rare earth elements from weathered crust elution-deposited rare earth as well as inhibit the leaching of impurity aluminum. At room temperature, when the ammonium formate concentration was 1% wt, the leaching efficiencies of rare earth and aluminum were 87 and 37%, respectively. Compared with traditional inorganic ammonium salts, the inhibition effect of impurity aluminum was obvious. In addition, the results of the linear swelling ratio in the clay minerals showed that the inhibit ability of formate salts on the hydration of clay minerals enhanced with the increase of the formate concentration, and the order of the inhabitation on the clay minerals followed: 1% ammonium formate > 1.5% potassium formate > 2.5% sodium formate > distilled water. Based on the double layer theory, ammonium formate and potassium formate could effectively compress clay mineral particles to avoid water intake, which could increase the interaction between clay mineral particles and greatly reduce the electronegative property of the clay minerals, so as to effectively reduce the surface hydration of clay minerals to decrease the swelling of rare earth ore. The results of this experiment have important and practical significance in guiding the prevention of landslides, promoting the in-situ leaching technology, and effectively protecting the ecological environment in mining areas.

Optimization of Nitric Acid Leaching of Rare Earth Elements From Moroccan Natural Phosphate

Phosphate is a very important natural resource in Morocco and one of the secondary resources of rare earth elements. Our study is particularly interested in Youssoufia phosphate, which contains 228.77 ppm of rare earth elements (ΣREEs). The purpose of our work is to study the influence of different parameters (acid concentration, solid/liquid ratio and temperature) in order to determine the optimal conditions for the leaching of rare earths. An experimental design (Doehlert matrix) has been drawn up to optimize the experimental conditions of the leaching. All tests were made with nitric acid at different concentrations varying between 1.5M and 4.5M with a solid/liquid ratio of 1/12 to 1/6; reaction temperature and duration are respectively 20°C to 80 °C and 60 min. The optimal conditions are obtained when using 69 °C as temperature, 4.1 M as acid concentration and 1/9 as solid/liquid ratio.

Geochemical and isotopic study of metasomatic apatite: Implications for gold mineralization in Xindigou, northern China

The 0.7 Moz Xindigou gold deposit lies on the north margin of North China Craton. The orebodies are controlled by NWW-trending shear zones and hosted by an Archean metavolcanosedimentary sequence. These host schist, amphibolite and marble rocks were intruded by late Paleozoic granites and mafic dikes. Two independent magmatic-hydrothermal assemblages were identified based on petrographic evidence: a pre-ore magmatic-hydrothermal magnetite – apatite ± pyrite ± zircon assemblage and an ore-related potassium feldspar – muscovite – sulfide ± minor monazite assemblage.Apatite that has undergone later alteration shows significant intracrystalline spatial variability with respect to the concentrations of rare earth elements (REEs), Si, and Sr. Scanning electron microscope backscattered images and statistical interrogation of the compositional data acquired by LA-ICP-MS indicate that apatite grains in the pre-ore magnetite – apatite assemblage are homogeneous and enriched in REEs and Si (Ap1). Where sulfide-related alteration of mineralization stage was overlapped, these apatite grains display a distinctive texture of homogeneous cores and turbid rims. This dissolution-reprecipitation process preferentially transported elements from the original apatite. REEs and Si gradually decrease from the core (Ap1) to the altered rim (Ap2 & Ap3) whereas Sr increases. Localized leaching of REEs (especially LREEs) from apatite was followed by precipitation of secondary monazite grains in the metasomatic apatite rims and the muscovite – sulfide – quartz – phosphate sequence. In situ Sr isotope analyses of the apatite show the 87Sr/86Sr ratios varying from the lowest value, 0.707874 in unaltered Ap1, to the highest, 0.714160 in altered Ap3. Zircon grains from migmatite display a peak metamorphic age at 2512 ± 9 Ma, whereas zircons from the early magnetite-apatite association formed near 1776 ± 12 Ma, and late hydrothermal monazite yields a Th-Pb age of 301.0 ± 1.7 Ma.These geochronology results show that the mineralization occurred during the late Paleozoic orogenesis, long after Neoarchean regional metamorphism and late Paleoproterozoic rift-related magmatism. The typical hydrothermal features of orogenic gold deposit could be inferred from the fluorapatite. The elemental patterns of fluorapatite grains indicate that a carbon and Sr enriched, acid, slightly oxidized and low salinity hydrothermal system was responsible for apatite metasomatism and gold mineralization. Strontium isotopes further reveal that the fluid related to gold deposition was derived from mafic lower crust, with extensive radiogenic Sr accumulation during fluid-rock interaction. This study demonstrates the potential application of apatite in exploration for orogenic gold deposits and its use in tracking hydrothermal REE re-mobilization during metasomatism.

Experimental Study on Recovering Rare Earths from Separation Slag of Ni–Metal Hydride Battery Wastes by Hydrochloric Acid Leaching

The RE-bearing slag used in the experiment was obtained from Ni–metal hydride battery wastes by H2 selective reduction and melting separation method, and the slag composition mainly consisted of 46.44 wt% REO, 26.05 wt% SiO2, 17.68 wt% Al2O3 and 6.32 wt% MnO. The influence of different reacting factors on the leaching rate of rare earth was investigated, and subsequently, the leaching dynamics was analyzed at low temperature and ordinary pressure and at high pressure and high temperature separately. The results showed that the optimal leaching parameters for the low-temperature and ordinary pressure conditions were 2.69 mol L−1 hydrochloric acid, 10:1 liquid to solid ratio at 85 °C for 75 min and the rare-earth leaching rate reaching 94.94%. According to Arrhenius formula, the apparent activation energy was 57.68 kJ mol−1 and the preexponential factor was 5.02 × 104 s−1, and the related reaction orders of the hydrochloric acid concentration and particle size were 3.63 and − 1.59, respectively. For the high-pressure and high-temperature leaching process, the optimal conditions were 2.49 mol L−1 hydrochloric acid, 10:1 liquid to solid ratio at 130 °C for 25 min and the rare-earth leaching rate 96.70%. The apparent activation energy analyzed was 53.63 kJ mol−1, and the preexponential factor was 5.32 × 103 s−1. The two leaching processes were both controlled by the interfacial mass transfer and solid film diffusion.

Efficient Recovery of End-of-Life NdFeB Permanent Magnets by Selective Leaching with Deep Eutectic Solvents

The NdFeB permanent magnet is a critical material in digital electronics and clean energy industry. Traditional recovery processes based on the solvent extraction technique would consume high energy and large amounts of chemicals as well as resulting in abundant secondary organic wastes. In this work, a green process using deep eutectic solvents (DESs) in the selective leaching technology was designed to recover NdFeB permanent magnets. Nine kinds of DESs composed of guanidine were prepared and screened as the leachants. The guanidine hydrochloride-lactic acid (GUC-LAC) combined DES achieved the highest separation factor (>1300) between neodymium and iron through simple dissolution of their corresponding oxide mixture. The mass concentration of Nd dissolved in the GUC-LAC DES could reach 6.7 × 104 ppm. The viscosity of this type of DES at 50 °C was 36 cP, which was comparable to many common organic solvents. In a practical recovery of roasted magnet powders, the Nd2O3 product with 99% purity was facilely obtained with only one dissolution step, followed by a stripping process with oxalic acid. Even after 3 cycles, the GUC-LAC DES kept the same dissolution property and chemical stability. With such superior performances in selective leaching of rare earth elements from transition metals, the GUC-LAC DES is greatly promising in the rare earth element recovery field.

Leaching tungsten and rare earth elements from scheelite through H2SO4–H3PO4 mixed acid decomposition

Rare earth elements (REEs) possess distinctivephysicalandchemical properties and are mainly extracted from several rare earth (RE) and ion-absorbing type RE ores. Traditional RE sources are unable to meet the global demand, making it necessary to explore other resources for the extraction of rare REEs. While this is possible, the recovery of REEs from the leach residue of such resources is uneconomical due to the enormous consumption of mineralacids. H2SO4–H3PO4 mixed acid decomposition is an effective method for leaching tungsten from scheelite and is evaluated in this study for its efficient extraction of tungsten and REEs from the same mineral. The scheelite used in this study contains REEs such as lanthanum, cerium, and neodymium. The properties of the REs in the leaching system are used to evaluate our method. Additionally, the effects of the reaction conditions on the leaching of tungsten and REEs are investigated. The results show that increasing the reaction temperature and H2SO4 concentration favors the leaching process. However, high H2SO4 concentration also promotesthetransformationof gypsumfromCaSO4·2H2OtoCaSO4, which affects the leaching process negatively. Further, excessive H3PO4 concentration increases the viscosity of the leaching solution. Increasing the liquid–solid ratio is beneficial for the dissolution of scheelite. The addition of an appropriate amount of CaSO4·2H2O promotes the leaching of tungsten and REEs but the addition of PEG-400 does not. In terms of the experimental conditions used in this study, the leaching efficiency of W, La, Ce, and Nd can reach approximately 100%, 96.6%, 66.6%, and 57.1%, respectively. After the circulationleaching process, the concentrations of tungsten and the REEs in the leach liquor are increased, which proves these materials can be efficiently leached from scheelite using the H2SO4–H3PO4 mixed acid.