Leaching Of Rare Earth Elements Research Articles

Bioleaching of rare earth elements from waste phosphors and cracking catalysts

Microbial cultures were evaluated for organic acid production and their potential utility for leaching of rare earth elements (REE) from retorted phosphor powder (RPP) and spent fluid catalytic cracking (FCC) catalyst. Two bacterial and one fungal strain were isolated from environmental and industrial materials known to contain REE and compared to the industrially important bacterium Gluconobacter oxydans. Gluconic acid was the predominant organic acid product identified in all of the cultures. Maximum REE leaching (49% of the total REE) from the FCC material was observed using cell-free culture supernatants of G. oxydans, with preferential recovery of lanthanum over cerium. The phosphor powder was more difficult to leach; only about 2% of the total REE was leached with G. oxydans. Leaching experiments with the RPP material indicated that the extent of REE solubilization was similar whether whole cell cultures or cell-free supernatants were used. Abiotic control experiments showed that increasing gluconic acid concentrations increased leaching efficiency; for example, total REE leaching from FCC catalyst increased from 24% to 45% when gluconic acid was increased from 10mM to 90mM. However, G. oxydans cell-free culture supernatants containing 10–15mM gluconic acid were more effective than abiotically prepared leaching solutions with higher gluconic acid concentrations, suggesting that other exudate components were important too. Our results indicate that microorganisms producing gluconic and other organic acids can induce effective leaching of REE from waste materials, and that increasing organic acid production will improve recovery.

Purification of rare earth bis(trifluoromethyl-sulfonyl)amide salts by hydrometallurgy and electrodeposition of neodymium metal using potassium bis(trifluoromethyl-sulfonyl)amide melts

This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using potassium bis(trifluoromethyl-sulfonyl)amide (KTFSA) melts for the recovery of rare earth (RE) elements from Nd-Fe-B magnet waste. The investigations were performed at bench scale to assess the potential of a process based on leaching, deironization, and purification of RE amide salts. In the leaching process using 3.4 kg of oxidized Nd-Fe-B and 14.2 L of an aqueous solution of 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (HTFSA), 83.0% Nd and 0.98% Fe were leached in 13 h, indicating that selective leaching of RE elements was performed at bench scale. Then, KOH or oxidized Nd-Fe-B was used as a precipitation agent in the deironization process and 100.0% Fe was successfully separated from RE components. Moreover, 4.07 kg of purified amide salts (M(TFSA)3, M = Pr, Nd, Dy, B, Al, and trace elements) were recovered from a spray dryer. The electrochemical behavior of Nd(III) in KTFSA melts containing M(TFSA)3 (molar fraction of RE components: xRE = 0.1) was investigated in this study. Electrochemical analysis revealed that the reduction peak of Nd(III) at around +1.0 V vs. K/K+ was due to the following reaction: Nd(III) + 3e− → Nd(0). The diffusion coefficient of Nd(III) was estimated to be 3.14 × 10−10 m2 s−1 at 483 K by semi-differential analysis, which is similar to that of Nd(III) in KTFSA melts containing pure Nd(TFSA)3 salts (xNd = 0.1). The electrodeposition of Nd was performed under potentiostatic conditions of +0.8 V vs. K/K+ at 483 K. The electrodeposits had a fine surface morphology with small metal particles. The electrodeposits were confirmed to be Nd metal in the middle layer analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Finally, we demonstrated the effectiveness of the novel recovery process for practical use by estimating whole material flow.

Optimization of Acid Leaching of Rare-Earth Elements from Mongolian Apatite-Based Ore

Optimization of the acid leaching process for Mongolian apatite-based ore containing rare-earth elements (REEs) was studied. The ore contained approximately 10% of REEs as total rare earth oxides, and the major impurities were Ca (33% as CaO) and Fe (23% as Fe2O3). Fe bearing minerals could be removed by passing the sample through a wet high-intensity magnetic separator before leaching. After magnetic separation, basic leaching tests were conducted to investigate the influence of the acid type and concentration, temperature, and the pH on the REE leaching level and kinetics. Hydrochloric acid was found to be the most effective leaching agent, leaching more than 90% of REEs in an hour. However, the concentrations of Ca ions in the leachate were also high, which would complicate recovery of the REEs. Therefore, to reduce the amount of Ca ions in the leachate, a two-stage leaching procedure was attempted. In stage 1, the sample was preleached using 1.0 M hydrochloric acid to dissolve Ca. In stage 2, the solid residue of stage 1 was leached using 2.0 M hydrochloric acid to dissolve REEs. Consequently, this two-stage leaching significantly reduced the Ca concentration in the final leachate without affecting the leaching levels of REEs.

Process investigation of the acid leaching of rare earth elements from phosphogypsum using HCl, HNO3, and H2SO4

The increasing demand for some critical rare earth elements (REEs), which are crucial in the production of green and technologically advanced products, has sparked the development of novel processes to recover REEs from secondary resources, such as mine tailings, metallurgical slags, and phosphogypsum. These resources have low concentration of REEs, but are available in large volumes. Phosphogypsum (PG) is the main by-product of the production of phosphoric acid by sulphuric acid digestion of phosphate rock. The current study put the emphasis on the thorough characterization of phosphogypsum as well as investigation of the kinetics of the leaching process. The results show the leaching efficiency is controlled by the solubility limit of PG. Chemical modelling indicated that the systems are at the saturation level with respect to Ca. The instantaneous and saturation concentrations of Ca were used to model the dissolution kinetics. The apparent activation energy was determined to be 10.4kJ/mol. Furthermore; systematic leaching processes using three acids under various operating conditions were performed. The results indicate the optimal operation conditions are 1.5M, 80°C and solid-to-liquid ratio (S/L) of 1/8 and 20min residence time for all three acid, but the leaching efficiency using nitric and hydrochloric acids are higher than the sulfuric acid. This novel work investigates the feasibility of acid leaching of REEs from PG from both theoretical and practical standpoints.

Novel solution injection technology for in-situ leaching of weathered crust elution-deposited rare earth ores

Weathered crust elution-deposited rare earth ores are rich in the middle and heavy rare earths. The present study investigated the content of exchangeable aluminum and the exchangeable rare earth in the different layer of orebody and the partitioning of rare earth was determined as well to understand the mode of occurrence of rare earth. Furthermore, the swelling of clay minerals from different layers was investigated. The results shown that, in completely weathered layer and partly weathered layer, the content of exchangeable rare earth was more than 0.06% and the content of exchangeable aluminum was below 0.02%. However, in humic layer, the content of exchangeable rare earth was below 0.015% and the exchangeable aluminum was more than 0.03%. In the weathered crust elution deposited rare earth ore, the exchangeable aluminum is mainly found in the humic layer, whereas the rare earth elements are abundant in the completely weathered layer and partly weathered layer and the rare earth in humic layer belong to light rare earth group. Additionally, the swelling of clay minerals in humic layer was ranged between 2.5% and 2.7%, which were higher than the swelling of clay minerals in completely weathered layer and partly weathered layer. Based on these findings, a novel in-situ leaching technology of weathered crust elution-deposited rare earth ores is proposed. The leaching agents were directly injected into the completely weathered layer without infiltrating through the humic layer. Avoiding the humic layer, only the completely weathered layer and partly weathered layer were leached. According to this method, the exchangeable aluminum in the rare earth leaching solution can be decreased and the purity of rare earth product can be improved. Meanwhile, the ammonium chloride was alternatively applied to leach rare earth and the choices of available leaching reagents were broadened.

Selective leaching of rare earth elements from bauxite residue (red mud), using a functionalized hydrophobic ionic liquid

In this study the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide (HbetTf2N) has been applied directly on leaching bauxite residue, for achieving selective dissolution of rare earth elements (REE) against iron. Several factors have been examined such as leaching temperature, retention time, pulp density and water addition into the ionic liquid. It was found that moderate water addition, elevated temperatures and increased retention time provide up to 70–85% REE extraction yields with respective Fe extraction yield <3%. Sc recovery showed a different leaching behavior from the rest of the REE with its extraction yield not exceeding 45%. Furthermore, no dissolution occurs for Ti and Si whereas 30% maximum extraction of Al was obtained, followed by almost total dissolution of Ca and Na content. This work shows that most REE and a fraction of Sc are associated with Calcium- and Aluminium-bearing phases in the bauxite residue, while the majority of Sc is associated probably with the remaining Fe and Ti-bearing phases. The metals dissolved into hydrophobic ionic liquid can be extracted with an acidic medium while the IL leaching agent is regenerated and after separation can be reused to a new portion of bauxite residue. Enabling consecutive scrubbing-stripping on the ionic liquid pregnant solution an enhanced acidic Sc solution with reduced level of impurities is obtained.

Kinetics of column leaching of rare earth and aluminum from weathered crust elution-deposited rare earth ore with ammonium salt solutions

In this study, a mixture of ammonium chloride and ammonium nitrate was used as a leaching agent to improve the leaching efficiency and leaching rate of rare earth (RE) from weathered crust elution-deposited RE ore. By performing the column leaching experiments, effects on the leaching efficiency and leaching rate of RE, such as the molar ratio of the two components, concentration of ammonium (NH4+), liquid/solid ratio and pH of the leaching agent, were investigated. In addition, the effects of above parameters on the leaching performance of aluminum (Al), which is the main impurity in RE leachate, were also studied. It was found that the optimal conditions of column leaching were a 1:1 (molar ratio) of NH4Cl and NH4NO3, 0.2mol/L of ammonium concentrate, 2:1 liquid:solid (mL/g) and a leaching agent with a pH between 4.0 and 8.0. The maximum leaching efficiency of RE could be 91%. Moreover, the leaching equilibrium time for RE was 25min shorter than for Al, which is of benefit to reduce the leaching of Al. The leaching process was analyzed using the shrinking-core model to examine the kinetic parameters and rate controlling step. The leaching rate was controlled by the inter-diffusion of the reactants and leaching products through the mineral porous layer. An empirical equation based on the apparent rate constant and ammonium concentration was established for process optimization, and the reaction orders were 1.92 for RE and 1.48 for Al. The results of this study may play a vital role in developing a high efficient leaching technology for weathered crust elution-deposited RE ore.

A hydrometallurgical process for the recovery of rare earth elements from fluorescent lamp waste fractions

Rare earth elements (REEs) are presently regarded to be some of the most critical elements, being essential in future sustainable applications. One of the many examples is fluorescent lighting, which relies on REEs such as cerium, europium, gadolinium, lanthanum, terbium and yttrium. The high demand for REEs, low supply, price fluctuations and geopolitical factors have contributed to an increased interest in reclaiming these elements from end-of-life products. This publication reports on a new potential hydrometallurgical route to reclaim REEs in fluorescent lamp waste. The investigations were carried out at bench scale and laboratory pilot scale to assess the potential of a process based on leaching and solvent extraction. Real waste fractions of various compositions, originating from several companies that use different mechanical, physical and chemical processes to treat lamps, were studied. A multi-step leaching process that selectively dissolves different groups of metals in the waste was investigated. Removal of mercury, one of the vital constituents of fluorescent lamps and an undesired contaminant in the waste, was carried out using leaching with iodine in potassium iodide solutions. Selective leaching of impurity metals e.g. calcium and selective leaching of REEs was performed with nitric acid, taking advantage of the significant differences in their leaching kinetics. Separation of REEs was achieved with Cyanex 923, a commercial mix of trialkylphosphine oxides, in a counter-current mixer setter system comprising three extraction and four stripping stages, respectively. The product, a yttrium/europium concentrate, was treated with oxalic acid, followed by thermal treatment, to produce an oxide containing 99.96% REEs, of which 94.61% was yttrium and 5.09% was europium.

An overview of rare-earth recovery by ion-exchange leaching from ion-adsorption clays of various origins

AbstractContinuous development of advanced technologies has created increasing demand for rare-earth elements (REE), with global emphasis on identifying new alternate sources to ensure adequate supply. Ore deposits containing physically adsorbed lanthanides are substantially lower grade than other REE deposit types; however, the low mining and processing costs make them economically attractive as sources of REE. To evaluate the commercial potential for the recovery of REEs from ion-adsorption deposits in a systematic manner, a standardized procedure for REE leaching was developed previously. Using this procudure it was found that, regardless of variations in ore origin and REE content, all REE consistently reached peak extraction levels under ambient conditions with fast kinetics. Various techniques to improve the REE extraction through process variations were also investigated: it was found that decreasing the L:S ratio, re-using leachate on fresh ores and counter-current leaching were all capable of increasing REE concentrations in the resultant leachate, albeit at the expense of REE extraction levels. In addition, the water content trapped in the leached material was found to contain significant amounts of REE and residual lixiviant requiring thorough washing of the solid residue.

In-Situ Recovery of Critical Technology Elements

G.U.B. Ingenieur AG in Dresden, Germany, is developing optimized in-situ extraction technologies for critical technology elements, including Rare Earth Elements (REE) Nd, Eu, Tb, Dy, and Y and Cu. We discuss some selected examples of ongoing GUB R&D projects including: (1) In-situ bioleaching of Cu shale-sandstone in an active ∼1km deep shaft mine in Poland, (2) In-situ (bio)leaching and near-surface extraction of REE from ion-adsorption clays (IAC) in Madagascar, and a new project scope (3) REE extraction from secondary phosphate streams. In-situ bioleaching of Cu from deep Cu-shale deposits in Europe allows to retrieve metals like Cu (among other metals) from deep European ore deposits at lower costs. In-situ recovery of REE from IAC deposits is important as such deposits in southern China are the main source of critical technology elements Nd, Eu, Tb, Dy and Y. REE-recovery from secondary phosphate streams is a potential major value-adding step in optimizing the phosphate recovery processing chain, yet it is essential to understand the partitioning of REEs (and NORM including U, Th and their progenies) between phosphogypsum and phosphoric acid in the fertilizer production process. We identified three main REE sinks based on existing phosphate processing chains: (1) Sand, clay and phosphoric tailings hosting ∼40-50% of total REE, (2) phosphogypsum ponds with ∼40% of total REE, and (3) phosphoric acid with dissolved ∼10-20% of total REE. In-situ recovery may potentially recover ∼80-90% of total REE from secondary streams (phosphate tailings and phosphogypsum ponds) whereas ∼10-20% of total REE may potentially be recovered from phosphoric acid and/or tailings eluate by biocomplexation of REE. Modification of the phosphate processing chain may provide additional REE extraction points. The most suitable REE extraction points may also be controlled by NORM separation. We propose a customized R&D program to develop an in-situ recovery technology for in-situ REE recovery from secondary (ex-phosphate) streams, in the framework of a joint company and/or EU-funded R&D project.

Immobilization of Rare Earth Elements of the Mine Tailings Using Phosphates and Lime

Excessive exploitation of rare earth mines produced a great deal of mine tailings which needs proper treatment. High concentration of the total rare earth elements (REEs) (685.8 ± 48.7mg/kg) still existed in the mine tailings, which was more than 3 times as much as the average background concentration of China. More importantly, 88.8% of the REEs fractionated in exchangeable fraction under sequential extraction analysis, and 91.7% were leached under TCLP procedure, which indicated a high risk of leaching of REEs into the surrounding environment. Stabilization method was adopted to control the REEs leachability in the mine tailings. Soil incubation experiments were conducted to evaluate the effectiveness of soluble phosphates and lime in immobilizing REEs in the mine tailings. The results revealed that, with the addition of lime, the leachability of REEs reduced by 0.4%-18.3% with the addition of lime at 0.1%-1% (by weight). A more effective immobilization result was obtained with the addition of phosphates, as the REEs leachability reduced by 71.1%-98.4%. The sequential extraction analysis revealed that the lime treatment converted REEs in F1 into F2. In contrast, phosphate treatment converted the REEs into more stable fractions. The mechanism of the efficient immobilization of REEs with the soluble phosphates was described as follow: (1) Cation exchange between free proton or potassium ion and the soil bounded REEs; (2) Complexation and precipitation reactions between phosphate and REEs. In addition, the final pH value after soil incubation also influenced the effect of the immobilization of REEs.

Chemical leaching of rare earth elements from highly REY-rich mud

Seafloor sediment rich in rare earth elements and Y (REY-rich mud) has received attention as a new resource for REY. During research cruise KR13-02 of R/V Kairei, mud containing more than 5,000 ppm total REY was collected near Minamitorishima Island, northwestern Pacific Ocean. We conducted a series of chemical leaching experiments on this material, varying the acid concentrations, leaching times, and temperature to determine the optimum conditions of REY leaching from REY-rich mud. The highest extraction efficiency of REY other than Ce was 95.1% using hydrochloric acid and 81.3% using sulfuric acid. Extraction efficiency of REY was highest under conditions of relatively low acid concentrations (0.25‐0.5 mol/L), short leaching times (2‐5 min), and room temperature (25∞C). REY extraction amount decreased with increasing acid concentration, leaching time (only in the case of sulfuric acid), and temperature, apparently because of precipitation of calcium sulfate, rare earth phosphate, and rare earth-Na double sulfate. Although further research is needed, the leaching properties we determined are generally favorable for industrial exploitation of REY-rich mud.

Leaching characteristics of ion-adsorption type rare earths ore with magnesium sulfate

Magnesium sulfate was proposed to be leaching agent to deal with the ion-adsorption type rare earths ore to reduce or even eliminate ammonia–nitrogen emissions. The effects of temperature, particle size and stirring speed on rare earth leaching process and the leaching behaviors of the single rare earth element were investigated in order to reveal the rare earth leaching characteristics. Besides, the comparison of leaching effects between magnesium sulfate and ammonium sulfate was also studied. The results showed that the rare earth leaching process could be well described with inner diffusion control model and the apparent activation energy was 9.48 kJ/mol. The leaching behaviors of the single rare earth element were brought into correspondence with rare earths. Moreover, when the concentration of leaching agent was 0.20 mol/L, the rare earth leaching efficiency could all reach above 95% and the leaching efficiency of aluminum impurities could be restrained by 10% using magnesium sulfate compared with ammonium sulfate.

Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing, Southwestern China: Their mobilization, redistribution, and fractionation

In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO2) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)N and (La/Sm)N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.

Reclaiming rare earth elements from end-of-life products: A review of the perspectives for urban mining using hydrometallurgical unit operations

Due to their large variety of applications, their low supply and high demand, the rare earth elements (REEs) are presently viewed as being among the most critical chemical elements. Because of this, their potential recovery from end-of-life waste products has been extensively discussed both in society and in the scientific literature. This concept of recovering elements contained in end-of-life products, known as urban mining, is regarded as an important step in achieving a sustainable, circular society. This review article discusses the perspectives of reclaiming the REEs from various waste streams using hydrometallurgical methods. Three main streams are discussed in detail (phosphor-containing products, NiMH batteries and permanent magnets), touching on the state-of-the-art of material pre-treatment, leaching and separation of REEs and refining. Comparisons with the extraction of REEs from ores are drawn, bringing forth both the advantages and some of the disadvantages of urban mining.

Role of minerals properties on leaching process of weathered crust elution-deposited rare earth ore

Granite belonged to intrusive rock and volcanic was extrusive rock. There may be many differences in their degree of weathering and mineral chemical composition. The present study investigated the minerals properties and the leaching mechanism of the granitic weathered crust elution-deposited rare earth ore from Longnan Rare Earth Mine area (LN ores) and volcanic weathered crust elution-deposited rare earth ore from Liutang Rare Earth Mine area (LT ores). The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) were used to characterize the phase of rare earth ores. The particle size distributions and main composition of the ore were also presented in this paper. The leaching mechanisms of two kinds of rare earth ores were analyzed with different kinetics models and could be described by the shrinking-core model. They were all inner diffusion-controlled leaching processes. The leaching equation of the kinetics of the LN ores could be expressed as: 1-23ηLN-(1-ηLN)2/3=0.106r0-0.377e-1.096×1048.314Tt, leaching equation of kinetics of LT ores was 1-23ηLN-(1-ηLN)2/3=8.33×10-3r0-0.411e-4.640×1038.314Tt. The rare earth leaching rate of LT ores was always lower in the same condition, and it would need more time and more (NH4)2SO4 consumption to achieve the same rare earth leaching efficiency, which would lead to more serious ammonia-nitrogen pollution. Therefore, magnesium salt was proposed as the leaching agent to eliminate ammonia-nitrogen pollution and further studies would be taken in the future.

Ruling out accumulation of thorium in sulfuric acid solutions for leaching of phosphogypsum

It is shown that introduction of fluoride ions into recycled sulfuric acid solutions used for leaching of rare-earth elements from phosphogypsum precludes accumulation of thorium in these solutions, thereby making it possible to develop a technology for obtaining concentrates of rare-earth elements that rules out formation of intermediate and waste radioactive products.

Use of Phosphate Solubilizing Bacteria to Leach Rare Earth Elements from Monazite-Bearing Ore

In the present study, the feasibility to use phosphate solubilizing bacteria (PSB) to develop a biological leaching process of rare earth elements (REE) from monazite-bearing ore was determined. To predict the REE leaching capacity of bacteria, the phosphate solubilizing abilities of 10 species of PSB were determined by halo zone formation on Reyes minimal agar media supplemented with bromo cresol green together with a phosphate solubilization test in Reyes minimal liquid media as the screening studies. Calcium phosphate was used as a model mineral phosphate. Among the test PSB strains, Pseudomonas fluorescens, P. putida, P. rhizosphaerae, Mesorhizobium ciceri, Bacillus megaterium, and Acetobacter aceti formed halo zones, with the zone of A. aceti being the widest. In the phosphate solubilization test in liquid media, Azospirillum lipoferum, P. rhizosphaerae, B. megaterium, and A. aceti caused the leaching of 6.4%, 6.9%, 7.5%, and 32.5% of calcium, respectively. When PSB were used to leach REE from monazite-bearing ore, ~5.7 mg/L of cerium (0.13% of leaching efficiency) and ~2.8 mg/L of lanthanum (0.11%) were leached by A. aceti, and Azospirillum brasilense, A. lipoferum, P. rhizosphaerae and M. ciceri leached 0.5–1 mg/L of both cerium and lanthanum (0.005%–0.01%), as measured by concentrations in the leaching liquor. These results indicate that determination of halo zone formation was found as a useful method to select high-capacity bacteria in REE leaching. However, as the leaching efficiency determined in our experiments was low, even in the presence of A. aceti, further studies are now underway to enhance leaching efficiency by selecting other microorganisms based on halo zone formation.

Recovery of rare earth from ion-adsorption rare earth ores with a compound lixiviant

The present research investigates the use of ammonium nitrate and ammonium sulfate as a compound lixiviant, and ammonium acetate as an inhibitor for aluminum in the recovery of rare earth elements from ion-adsorption rare earth ores. It is previously shown that rare earth ions, adsorbed physically on ion-adsorption rare earth ores, could be easily recovered via an ion exchange mechanism with inorganic monovalent salt solutions. A standardized desorption procedure was established to systematically investigate the effects of elution conditions such as compound lixiviant concentration, ammonium nitrate/ammonium sulfate molar ratio, particle size, elution flow rate, pH and inhibitor concentration on the leaching of rare earth and aluminum. It was determined that the leaching rate of rare earth exceeded 80% under the optimum elution conditions: particle size <74μm, compound lixiviant concentration 2.0%, ammonium nitrate/ammonium sulfate molar ratio 4:1, pH 5.5, ammonium acetate concentration 0.05%, elution flow-rate 0.5ml/min and room temperature. At these conditions, the inhibition rate of aluminum exceeded 80%.

Leaching kinetics of ionic rare-earth in ammonia-nitrogen wastewater system added with impurity inhibitors

Ammonia-nitrogen wastewater is produced during the dressing and smelting process of rare-earth ores. Such wastewater includes a very high concentration of NH4+, as well as other ions (e.g., NH4+, RE3+, Al3+, Fe3+, Ca2+, Cl−, and SiO32–) with a pH of 5.4–5.6. Its direct discharge will pollute, yet it can be recycled and used as a leaching reagent for ionic rare-earth ores. In this study, leaching kinetics studies of both rare earth ions and impurity ion Al3+ were conducted in the ammonia-nitrogen wastewater system with the aid of impurity inhibitors. Results showed that the leaching process of rare-earth followed the internal diffusion kinetic model. When the temperature was 298 K and the concentration of NH4+ was 0.3 mol/L, the leaching reaction rate constant of ionic rare-earth was 1.72 and the apparent activation energy was 9.619 kJ/mol. The leaching rate was higher than that of conventional leaching system with ammonium sulfate, which indicated that ammonia-nitrogen wastewater system and the addition of impurity inhibitors could promote ionic rare-earth leaching. The leaching kinetic process of impurity Al3+ did not follow either internal diffusion kinetic model or chemical reaction control, but the hybrid control model which was affected by a number of process factors. Thus, during the industrial production the leaching of impurity ions could be reduced by increasing the concentration of impurity inhibitors, reducing the leaching temperature to a proper range, accelerating the seepage velocity of leaching solution, or increasing the leaching rate of rare earths.