Amounts Of Rare Earth Elements Research Articles

Rare earth elements in parasol mushroom Macrolepiota procera.

This study aimed to investigate occurrence and distribution of 16 rare earth elements (REEs) in edible saprobic mushroom Macrolepiota procera, and to estimate possible intake and risk to human consumer. Mushrooms samples were collected from sixteen geographically diverse sites in the northern regions of Poland. The results showed that for Ce as the most abundant among the RREs in edible caps, the mean concentration was at 0.18±0.29mgkg-1dry biomass. The mean concentration for Σ16 REEs determined in caps of fungus was 0.50mgkg-1dry biomass and in whole fruiting bodies was 0.75mgkg-1dry biomass. From a point of view by consumer, the amounts of REEs contained in edible caps of M. procera could be considered small. Hence, eating a tasty caps of this fungus would not result in a health risk for consumer because of exposure to the REEs.

Recycling of NdFeB Magnets from Electric Drive Motors of (Hybrid) Electric Vehicles

(Hybrid) electric vehicles are assumed to play a major role in future mobility concepts. Although sales numbers are increasing, little emphasis has been laid on the recycling of some key components such as power electronics or electric motors. Permanent magnet synchronous motors contain considerable amounts of rare earth elements that cannot be recovered in conventional recycling routes. Although their recycling could have large economic, environmental, and strategic advantages, no industrial recycling for permanent magnets is available in western countries at the moment. Regarding the essential steps, dismantling of electric vehicles as well as the extraction of magnets from the rotors, little has been published before. This paper therefore presents and discusses different recycling approaches for the recycling of NdFeB magnets from (hybrid) electric vehicles. Many results stem from the German research project “Recycling of components and strategic metals of electric drive motors.”

Separation/Preconcentration Techniques for Rare Earth Elements Analysis

Abstract The main aim of this chapter exactly characterizes the contribution. The analytical chemistry of the rare earth elements (REEs) very often is highly complicated and the determination of a specific element is impossible without a sample pre-concentration. Sample preparation can be carried out either by separation of the REEs from the matrix or by concentrating the REEs. The separation of REEs from each other is mainly made by chromatography. At the beginning of REE analysis, the method of precipitation/coprecipitation was applied for the treatment of REE mixtures. The method is not applicable for the separation of trace amounts of REEs. The majority of the methods used are based on the distribution of REEs in a two-phase system, a liquid–liquid or a liquid–solid system. Various techniques have been developed for the liquid–liquid extraction (LLE), in particular the liquid phase micro-extraction. The extraction is always combined with a pre-concentration of the REEs in a single drop of extractant or in a hollow fiber filled with the extractant. Further modified techniques for special applications and for difficult REE separation have been developed. Compared to the LLE, the solid phase micro-extraction is preferred. The method is robust and easy to handle, in which the solid phase loaded with the REEs can be used directly for subsequent determination methods. At present, very new solid materials, like nanotubes, are developed and tested for solid phase extraction.

Study on Ecological and Chemical Properties of Rare Earth Elements in Tropical Marine Organisms

Abstract A total of 30 kinds of fish samples, 5 kinds of shellfish samples and 4 kinds of crustacean samples from the Nansha sea area in China were digested using a microwave digestion system with HNO 3 -H 2 O 2 as the digestion reagent. Then, the contents (ICP-MS determination) and ecological chemical characteristics of rare earth elements (REEs) were studied. The results showed that the method of microwave digestion-ICP-MS for the determination of rare earth elements was perfect, and the linear relationship for each element was good with r ≥ 0.9997. The detection limit reached ng L −1 level with a relative standard deviation (RSD, n = 3) of less than 5.0%. The recovery of REEs was between 91.5% and 106.7%. The total amount of REEs in fishes, shellfishes, and crustaceans were (5.0–34.8) μg kg −1 , (30.4–1481) μg kg −1 , and (103–863) μg kg −1 , individually. The order of the enrichment average contents of the REEs in the three species was crustacean>shellfish>fish. The 14 kinds of REE contents in fish/shellfish/crustacean showed a significantly positive correlation ( r > 0.80). The contents of light REEs (from La to Eu) were higher than those of heavy REEs (from Gd to Lu). The light and heavy REEs demonstrated obvious fractionation, and the REE distribution pattern in fish/shellfish/crustacean was consistent with the negative anomaly of Gd. The σ Eu values had negative anomalies, which were similar to the σ Eu values in the corresponding area sediment, and σ Ce values showed positive anomalies, which showed that Ce followed a different oxidation and reduction process in the organism and sediment deposition. At the same time, the enrichment relationship of REEs in sediment and organism was also studied. In this study, the content and distribution of REEs in the tropical marine fish/shellfish/shellfish were analyzed, which could provide basic data for the study of the level, migration, and accumulation of REEs in the environment of South China Sea.

Enhanced electrical properties in Nd doped cobalt ferrite nano-particles

Spinel ferrites are important class of compounds which has variety of electrical, magnetic and catalytic applications. A small amount of rare earth element causes modification in structural, electrical and magnetic properties of ferrite materials for practical applications. Neodymium doped cobalt ferrites with composition CoNdxFe2-xO4 where x is 0.1 has been synthesized by sol-gel method. Sol-gel method was preferred because it has good control over stoichiometry, crystallite size and particle size distribution. Characterization was done by using X-Ray Diffraction (XRD) technique for structural analysis and crystal structure was found to be spinel. Particles like morphology was observed in micrographs obtained by Scanning Electron Microscopy (SEM). Thermal analysis of sample has been done which includes Thermogravimetric analysis (TGA) and Differential Scanning calorimetry (DSC). Fourier transform infra-red spectroscopy (FT-IR) of samples was also performed. DC resistivity as a function of temperature has been studied and its shows direct dependence on temperature and inverse dependence on the concentration of Nd dopant. The studied material is a potential candidate for resistive random access memory application.

Brannerite, UTi2O6: Crystal chemistry, synthesis, properties, and use for actinide waste immobilization

The host materials suggested for immobilization of actinide waste of military or civil origin often contain the secondary (U,Pu)Ti2O6 phase of brannerite structure. For example, the materials for incorporation of excess plutonium, mainly consisting of pyrochlore, contain up to 30% brannerite. This is a usual phase in titanate host materials for isolating spent nuclear fuel (SNF) and products of its reprocessing, including waste from production of 99mТс for medical purposes and other kinds of waste with high U and Pu content. Despite simple ideal stoichiometry, brannerite can contain large amounts of rare earths. This feature is due to the presence of uranium not only in the 4+ oxidation state, but also in the 5+ and 6+ states, which favors the exchange of rare earth elements (REE), e.g., in accordance with the scheme 2U4+ ↔ U5+ + REE3+. The REE amount in brannerite reaches 0.5–0.7 atom per formula unit. Therefore, brannerite is of interest as a host material for the rare earth–actinide fraction of high-level waste (HLW). To evaluate the prospects for such use of brannerite, data on the radiation resistance of brannerite and its behavior in aqueous solutions are analyzed. In these properties, brannerite is inferior to pyrochlore and zirconolite. The rate of actinide leaching from brannerite is higher by an order of magnitude than from these phases, but lower by 3–4 orders of magnitude than from glass host materials. Natural brannerite is stable in media with weakly alkaline and reducing waters. Therefore, brannerite seems suitable for immobilization of rare earth–actinide waste. This host material can be synthesized by sintering or cold crucible induction melting followed by crystallization.

CONCENTRATION AND RECYCLING OF RARE EARTH ELEMENTS (REES) FROM IRON MINE WASTE USING A COMBINATION OF PHYSICAL SEPARATION METHODS

This study aims to investigate and optimize the effects of the main parameters including the particle size, gravity and magnetic separation combination, high gradient magnetic separation, magnetic field intensity, shaking table slope, washing water flow, and electrostatic separation upon the rare earth element (REE) recoveries from iron mine waste. The electron microprobe showed that high amounts of REEs were distributed on the fluorapatite mineral, and hence, it was necessary to remove the high magnetic minerals by a low-intensity magnetic separation using a magnetic drum in an experimental procedure. A cyclic magnetic separator was used for the low-gradient magnetic separation. Moreover, a shaking table and an electrostatic separator were used to expand the recovery and grade of REEs. A combination of these methods was considered to optimize the REE recoveries based on the best combination including two steps of low magnetic separation, one step of medium intensity magnetic separation, a shaking table, and an electrostatic separator. Two low-intensity magnetic of 800 and 2000 gauss, one medium-intensity magnetic of 8000 gauss, a one-step shaking table with a water flow of 90 mL/s and a table slope of 3 degree, and one electrostatic separator of 25000 V with a blade angel of 20 degree had the best performance to separate REEs. The microscopic studies carried out showed that the monazite degree of freedom was between 75 and 105 micron. The results obtained showed that a particle size of ‒75 + 63 micron was a proper one to separate REEs. The total recovery and grade of the REE (Ce, La, Nd, Er, and Gd) concentrate obtained from the sample with a grade of 1499 ppm of REEs were 67.1 and 1.2%, respectively, at the optimum conditions. The results obtained showed that there was a direct relation between the phosphor grade and the REE recoveries, and that the REE recoveries increased by increasing the quantity of phosphor.

Trace elements in hazardous mineral fibres

Both occupational and environmental exposure to asbestos-mineral fibres can be associated with lung diseases. The pathogenic effects are related to the dimension, biopersistence and chemical composition of the fibres. In addition to the major mineral elements, mineral fibres contain trace elements and their content may play a role in fibre toxicity. To shed light on the role of trace elements in asbestos carcinogenesis, knowledge on their concentration in asbestos-mineral fibres is mandatory. It is possible that trace elements play a synergetic factor in the pathogenesis of diseases caused by the inhalation of mineral fibres. In this paper, the concentration levels of trace elements from three chrysotile samples, four amphibole asbestos samples (UICC amosite, UICC anthophyllite, UICC crocidolite and tremolite) and fibrous erionite from Jersey, Nevada (USA) were determined using inductively coupled plasma mass spectrometry (ICP-MS). For all samples, the following trace elements were measured: Li, Be, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Sb, Cs, Ba, La, Pb, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, U. Their distribution in the various mineral species is thoroughly discussed.The obtained results indicate that the amount of trace metals such as Mn, Cr, Co, Ni, Cu and Zn is higher in anthophyllite and chrysotile samples, whereas the amount of rare earth elements (REE) is higher in erionite and tremolite samples. The results of this work can be useful to the pathologists and biochemists who use asbestos minerals and fibrous erionite in-vitro studies as positive cyto- and geno-toxic standard references.

Scenario analysis for recovery of rare earth elements from end-of-life vehicles

End-of-life vehicles (ELVs) are increasingly being recognized as a possible future resource pool for rare earth elements (REEs). This study provides the amount of REEs that can be recovered from ELVs in Japan based on dismantling survey, chemical identification and substance flow analysis. The REEs were quantified from common passenger vehicles and hybrid electric vehicles. We targeted 17 REEs in estimation of REE contents in ELVs. Four scenarios were developed to explore the recovery of REEs from ELVs. In these scenarios, NiMH batteries and motors containing NdFeB magnets were identified as target components due to they are main REEs carriers; we focused on interpretation of neodymium (Nd) and dysprosium (Dy) owing to they are two of the most critical REEs. The results suggest that 2700 (±500) tons of REEs can be recovered, of which 520 (±100) tons and 31 (±7) tons will be contributed by Nd and Dy in 2030. Meanwhile, the Dy recovered from ELVs can satisfy 23 % (±6 %) of the demand for NdFeB magnets and NiMH battery cells in automobile production of Japan; the Nd recovered from ELVs can satisfy 49 % (±9 %) of the production demands.

Pre-concentration of trace levels of rare-earth elements in high purity uranium samples for nuclear forensic purposes

Abstract This paper describes the development of a method for the pre-concentration and analysis of trace-level amounts of rare-earth elements (REE) in high purity uranium (U) samples. The method comprises pre-concentration and subsequent group separation of REEs from the uranium matrix followed by their quantitative determination using inductively coupled plasma mass spectrometry. The achieved detection limit of the method (in the low pg/g range) in combination with the high chemical recovery (around 90%) enabled the investigation of REE patterns in high purity refined uranium oxides. Applying this method one could follow the REE pattern along the entire front-end of the nuclear fuel cycle.

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.

Selective recovery of heavy rare earth elements from apatite with an adsorbent bearing immobilized tridentate amido ligands

Abstract Apatite, which is used in the production of phosphorus fertilizers, contains small amounts of rare earth elements (REEs; 0.1–1%). Apatite deposits occur all over the world and thus have the potential to be a secure source of REEs. We investigated a method for recovery of REEs from an apatite sample; specifically, REEs were leached from the sample with an H2SO4 solution, the acid that is most commonly used for fertilizer production. At high H2SO4 concentrations, most of the REEs in the solution were lost to precipitation as insoluble gypsum (CaSO4·2H2O). However, at H2SO4 concentrations of

Impact of the decarboxylation reaction on rare earth elements binding to organic matter: From humic substances to crude oil

The rare earth elements (REEs) are widely used as tracers in studies of various geochemical systems. Their behavior in natural systems such as groundwater and river water is strongly influenced by organic complexation. The behavior of REE during the maturation of organic compounds (e.g., during decarboxylation reaction) is not well understood, and thus forms the topic of this study. In this study, the behavior of REEs during the maturation of organic matters was examined by heating experiments and analysis of crude oil. Heating increased the release of REEs from humic acid, and was associated with a decreased concentration of carboxyl groups in the humic acid. Ternary heating experiments (humic acid+REE+slate) led to the re-adsorption of released REEs to the slate.Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) did not show peaks corresponding to carboxyl groups in the natural crude oils examined here, meaning that the oils are more highly mature than other oils containing carboxyl groups. The abundances of REEs in the oils without carboxyl groups (this study) were about three orders of magnitude lower than those previously found in oils with carboxyl groups, suggesting that the carboxyl group is important to REE retention in crude oil. These findings suggest that a considerable amount of REEs is released during the early stage of diagenesis, or during decarboxylation, and is likely to be subsequently adsorbed to the petroleum source rock. The variation in the Y/Ho ratio among oils suggests the occurrence of a secondary distribution to the oil-hosting rock. Other metallic elements that behave similarly to REEs, such as Ca, Al, and Fe, can also be released from the organic phase, whereas those that behave differently from the REEs may be retained.

Comparative Diagenesis and Rare Earth Element Variation in Miocene Invertebrate and Vertebrate Fossils from Panama

AbstractThe incorporation of rare earth elements (REEs) into the mineral lattices of skeletonized fossils has been used, particularly in vertebrates, to understand diagenesis and other postmortem paleoenvironmental parameters. Little is known about similar processes in invertebrates. Invertebrate and vertebrate fossils were analyzed for REE concentrations and patterns from five Miocene formations in central Panama (∼20–6 m.yr. ago). These formations (Culebra, Cucaracha, Gatun, Chucunaque, and Chagres) contain sediments interpreted to range from freshwater to shallow marine environments (depth of <300 m). We also compared different compositional types of fossils, including hydroxylapatite (bone) and aragonite (otoliths) in vertebrates and calcite and aragonite in invertebrates. REEs from modern invertebrates indicate that, as previously reported for vertebrates, invertebrates do not incorporate significant amounts of REEs into their mineral lattices during life; therefore, uptake must primarily occur during diagenesis. Comparisons of REEN concentrations (REE concentrations normalized to Post-Archean Australian Shale) of the fossils from Panama indicate differences in diagenesis. Within our largest sample from the Gatun Formation, there are significance differences in REEN concentrations and patterns within different compositional types. For example, shark teeth have the highest REEN concentrations and otoliths the lowest, with these differences likely a result of physical, chemical, and mineralogical factors. Calcitic and aragonitic fossils from the Gatun Formation show REEN patterns indicating differential uptake of individual elements during diagenesis. The cerium anomaly, previously used to indicate relative paleobathymetry, did not discriminate water depth in the marine formations. Comparative analysis of REEs in vertebrates and invertebrates provides a powerful tool to understand diagenetic processes, as demonstrated in the five formations and different compositional types studied from the Miocene of Panama.

Optimized elemental analysis of fluorescence lamp shredder waste

Fluorescence lamps contain considerable amounts of rare earth elements (REE). Several recycling procedures for REE recovery from spent lamps have been established. However, despite their economic importance, the respective recycling is scarce so far, with an REE recovery rate of less than 1%. A reliable analysis of REE and other relevant metals like Yttrium is crucial for a thorough and complete recovery process. This applies both to the solid matter and aqueous phase, since most of the recycling processes include wet-chemical steps. We tested seven different reagent mixtures for microwave-assisted digestion of fluorescent lamp shredder, including hydrofluoric acid, perchloric acid, and hydrogen peroxide. We determined the concentrations of 25 of the most relevant rare earth and other trace elements (Al, P, Ti, V, Cr, Fe, Ni, Cu, Ga, Ge, As, Y, Ag, Cd, Sn, Sb, La, Ce, Eu, Gd, Tb, W, Au, Hg, and Pb) in the respective dilutions. Two independent digestions, one a mixture of perchlorid/nitric/hydrofluoric acid and the other aqua regia, showed the highest concentrations of 23 of these elements, excluding only Sn and Tb. The REE concentrations in the tested lamp shredder sample (stated in g/kg) were 10.2 (Y), 12.1 (La), 7.77 (Ce), 6.91 (Eu), 1.90 (Gd), and 4.11 (Tb).

Extraction of rare-earth elements(III) from nitric acid solutions with bis(diarylphosphorylmethyl)benzenes

Extraction of micro quantities of rare-earth elements(III) from HNO3 solutions as complexes with bis(diarylphosphorylmethyl)benzenes depending on the nature of organic diluent and HNO3 concentration in equilibrium aqueous phase was studied, the stoichiometry of extracted complexes was determined. The effect of structure of a bridge between two phosphine oxide fragments in an extractant molecule on the efficiency and selectivity of rare-earth elements(III) extraction was considered.

Global critical materials markets: An agent-based modeling approach

As part of efforts to position the United States as a leader in clean energy technology production, the U. S. Department of Energy (DOE) issued two Critical Materials Strategy reports, which assessed 16 materials on the basis of their importance to clean energy development and their supply risk (DOE, 2010, 2011). To understand the implications for clean energy of disruptions in supplies of critical materials, it is important to understand supply chain dynamics from mining to final product production. As a case study of critical material supply chains, we focus on the supply of two rare earth metals, neodymium (Nd) and dysprosium (Dy), for permanent magnets used in wind turbines, electric vehicles and other applications. We introduce GCMat, a dynamic agent-based model that includes interacting agents at five supply chain stages consisting of mining, metal refining, magnet production, final product production and demand. Agents throughout the supply chain make pricing, production and inventory management decisions. Deposit developers choose which deposits to develop based on market conditions and detailed data on 57 rare earth deposits. Wind turbine and electric vehicle producers choose from a set of possible production technologies that require different amounts of rare earths. We ran the model under a baseline scenario and four alternative scenarios with different demand and production technology inputs. Model results from 2010 to 2013 fit well with historical data. Projections through 2025 show a number of possible future price, demand, and supply trajectories. For each scenario, we highlight reasons for turning points under market conditions, for differences between Nd and Dy markets, and for differences between scenarios. Because GCMat can model causal dynamics and provide fine-grain representation of agents and their decisions, it provides explanations for turning points under market conditions that are not otherwise available from other modeling approaches. Our baseline projections show very different behaviors for Nd and Dy prices. Nd prices continue to drop and remain low even at the end of our simulation period as new capacity comes online and leads to a market in which production capacity outpaces demand. Dy price movements, on the other hand, change directions several times with several key turning points related to inventory behaviors of particular agents in the supply chain and asymmetric supply and demand trends. Scenario analyses show the impact of stronger demand growth for rare earths, and in particular finds that Nd price impacts are significantly delayed as compared to Dy. This is explained by the substantial excess production capacity for Nd in the early simulation years that keeps prices down. Scenarios that explore the impact of reducing the Dy content of magnets show the intricate interdependencies of these two markets as price trends for both rare earths reverse directions – reducing the Dy content of magnets reduces Dy demand, which drives down Dy prices and translates into lower magnet prices. This in turn raises the demand for magnets and therefore the demand for Nd and eventually drives up the Nd price.

In vivo evaluation of a magnesium-based degradable intramedullary nailing system in a sheep model

The biocompatibility and the degradation behavior of the LAE442 magnesium-based intramedullary interlocked nailing system (IM-NS) was assessed in vivo in a comparative study (stainless austenitic steel 1.4441LA) for the first time. IM-NS was implanted into the right tibia (24-week investigation period; nails/screws diameter: 9mm/3.5mm, length: 130mm/15–40mm) of 10 adult sheep (LAE442, stainless steel, n=5 each group). Clinical and radiographic examinations, in vivo computed tomography (CT), ex vivo micro-computed tomography (μCT), mechanical and histological examinations and element analyses of alloying elements in inner organs were performed. The mechanical examinations (four-point bending) revealed a significant decrease of LAE442 implant stiffness, force at 0.2% offset yield point and maximum force. Periosteal (new bone formation) and endosteal (bone decline) located bone alterations occurred in both groups (LAE442 alloy more pronounced). Moderate gas formation was observed within the LAE442 alloy group. The CT-measured implant volume decreased slightly (not significant). Histologically a predominantly direct bone-to-implant interface existed within the LAE442 alloy group. Formation of a fibrous tissue capsule around the nail occurred in the steel group. Minor inflammatory infiltration was observed in the LAE442 alloy group. Significantly increased quantities of rare earth elements were detected in the LAE442 alloy group. μCT examination showed the beginning of corrosion in dependence of the surrounding tissue. After 24weeks the local biocompatibility of LAE442 can be considered as suitable for a degradable implant material. Statement of SignificanceAn application oriented interlocked intramedullary nailing system in a comparative study (degradable magnesium-based LAE442 alloy vs. steel alloy) was examined in a sheep model for the first time. We focused in particular on the examination of implant degradation by means of (μ-)CT, mechanical properties (four-point bending), clinical compatibility, local bone reactions (X-ray and histology) and possible systemic toxicity (histology and element analyses of inner organs).A significant decrease of magnesium (LAE442 alloy) implant stiffness and maximum force occurred. Moderate not clinically relevant gas accumulation was determined. A predominantly direct bone-to-implant contact existed within the magnesium (LAE442 alloy) group compared to an indirect contact in the steel group. Rare earth element accumulation could be observed in inner organs but H&E staining was inconspicuous.

China: Tosaf/Colloids – masterbatch

Ion adsorption deposits, in which the rare earth elements (REE) occur adsorbed onto clay mineral surfaces, currently provide the world’s dominant supply of heavy REE (Gd-Lu). Concentration of REE within ion adsorption deposits has been proposed to be a dominantly supergene process, where easily degradable REE-minerals (e.g. REE-fluorcarbonates) break down and release REE that are then adsorbed onto clay mineral surfaces in the weathered material. Here we present data from the Cenozoic Ambohimirahavavy alkaline complex in Madagascar, with the aim of further constraining controls on the formation and HREE enrichment processes in ion adsorption deposits.The laterite weathering profiles described here are developed on alkaline igneous rocks, including both SiO2-undersaturated and oversaturated lithologies. The latter group includes REE mineralised peralkaline granitic pegmatites and granitic dykelets. The weathering mineralogy includes gibbsite, clay minerals and minor Fe and Mn oxyhydroxides. X-ray diffraction and infrared spectroscopy show that the clay fraction in all sites is dominated by kaolinite and halloysite (7 Å and 10 Å). Extraction with ammonium sulfate (0.5 M (NH4)2SO4, pH = 4; removes weakly adsorbed metals retained on the solid surface and dissolves carbonates) indicates that, in the different profiles and along the same profile, the leachable REE content is heterogeneous, ranging from 5 to 2300 mg/kg total REE, with 1 to 32% heavy REE. Leaching with magnesium chloride (0.5 M MgCl2, pH = 6; removes only weakly adsorbed REE) releases similar total REE concentrations, suggesting that most of the leachable REE are adsorbed onto mineral surfaces. In most of the laterite profiles, the amount of leachable REE continuously increases with depth up to the saprock. Recovery rates depend on the REE atomic number, generally decreasing from La to Lu except for Ce which is invariably low.In the Ambohimirahavavy complex, the nature of the protolith is the main factor controlling the amount of easily leachable REE in the laterite weathering profile. Hydrology and topography are secondary factors. The most favourable protoliths include SiO2-undersaturated volcanic lithologies and altered mudstone with granitic dykelets. The main primary REE minerals include agpaitic minerals (eudialyte) and allanite-(Ce). Locally in granitic pegmatite dykes, autometasomatism by late magmatic fluids inhibits formation of ion adsorption ore by transforming easily weathered agpaitic minerals into unweatherable zircon.

Recovery of rare earth elements, uranium, and thorium from monazite concentrate by supercritical fluid extraction

Quantitative recovery of rare earth elements (REEs), Th, and U by supercritical fluid extraction (SCFE) with carbon dioxide containing adducts of TBP and HDEHP with HNO3 directly from monazite concentrate (MC) powder is impossible and requires the conversion of the constituent elements into more soluble compounds. Microwave (MW) radiation can be efficiently used for MC pretreatment by sintering with Na2CO3 in the presence of coal. The resulting product consists of two phases. One of them contains REEs (∼50%) recoverable by supercritical carbon dioxide (SC-CO2) containing adducts of TBP or HDEHP with HNO3. The second phase is a solid solution of CeO2 with Th and U oxides and remaining amount of REEs. It is resistant to SCFE. Conditions were determined for quantitative dissolution of this phase in a mixture of 4 M HCl with 0.05 M HF. The use of HDEHP under the SCFE conditions allows quantitative recovery of Th and U from the hydrochloric acid solution. In the process, REEs remain in the aqueous phase and are thus separated from Th and U. A possible flowsheet was suggested for the recovery REEs from MC using SCFE with their simultaneous separation from Th and U.