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Industrial rare earth element (REE) separations predominantly utilize solvent extraction processes tailored toward conventional resources such as bastn��site, monazite, and ion adsorption clays. Advances in diglycolamide (DGA) chemistry have shown effective extraction characteristics for REE separations. However, limitations associated with traditional DGA solvent extraction techniques, such as third-phase formation and gelling, have hindered commercial viability. By supporting DGA extractants on porous resins such as polystyrene divinyl benzene (PS-DVB), the desirable combination of solvent extraction selectivity and ease of operation of sorbent columns can be achieved. To design a low-cost model for such solid-supported DGAs, extraction characteristics as influenced by the underlying coordination chemistry must be explored to achieve efficient functional systems. Within this study, we report novel DGA resin materials, each incorporating one of the DGAs N,N,N���,N���-tetra-(1-octyl)-3-oxapentane-1,5-diamide (TODGA), N,N���-dimethyl-N,N���-dioctyl-3-oxapentane-1,5-diamide (DMDODGA), and 2,2��-oxybis(1-(3-(((2-ethylhexyl)thio)methyl)-4-methylpyrrolidin-1-yl)ethan-1-one) (DEHPDGA). The affinity of DGAs across the lanthanide (Ln) series was evaluated for both hydrochloric acid and nitric acid media with varying Ln feed concentrations to study distribution ratios and loading characteristics. Focusing on dysprosium, extended X-Ray Absorption Fine Structure (EXAFS) and density functional theory (DFT) calculations were also utilized to explore coordination chemistry and their effects on ligand performance. The general trend for both acid media resulted in DMDODGA having the highest extraction strength of all three DGAs at varying acid concentrations. Coordination-chemistry analysis supported by loading data, DFT calculations, and EXAFS results under forced loading conditions posited less than the expected 3:1 ligand-to-metal coordination.

Common collectors for rare earth mineral (REM) flotation, which include carboxylates and hydroxamates, face problems such as being non-selective and sensitive to impurity ions. A type of ionic liquid (IL), tetraethylammonium mono-(2-ethylhexyl)2-ethylhexyl phosphonate ([N2222][EHEHP]), has been investigated previously for rare earth elements (REE) solvent extraction, and was proven to be selective and effective. In this work, [N2222][EHEHP] was evaluated as a collector in bastnasite (a primary REM source for REE production) flotation for the first time. The results were compared with quartz and hematite, two common gangue minerals in REM deposits. Zeta potential measurements and Fourier-transform infrared spectroscopy (FT-IR) were completed to investigate the surface chemical properties involved in the flotation of these minerals using this collector. The findings were compared with microflotation results. FT-IR and zeta potential measurements suggest adsorption of the collector's phosphonate group onto bastnasite and hematite, likely through chemisorption; whereas for quartz, the minimum microflotation recovery is likely due to no adsorption of IL on its surface. Microflotation results show higher collectability of [N2222][EHEHP] for hematite than bastnasite, the latter only shows appreciable recovery at pH 5 with elevated dosage of IL (500 g/t). To achieve better separation, a two-stage flotation scheme was designed and evaluated by bench scale flotation on a synthetic mineral mixture. The concentrates and tails were analyzed by magnetic separation, and it is found that bastnasite recovery over 90% with maximum upgrade ratio 1.7 can be achieved with elevated collector dosage.

This study presented the first in a series of investigations currently underway to develop efficacious, cost-effective, and benign processing opportunities to produce rare earth elements (REE)-rich concentrate from an Australian complex low-grade saprolite ore [1.14% total rare earth oxides (TREO) grade], which is primarily exploited for its gold and copper values. This work specifically presented a preliminary flotation investigation carried out on the ore using sodium oleate as a collector. The relative effects of pulp pH, desliming, and depressants were investigated to ascertain any chance of recovering and upgrading REE minerals in saprolite ore using three different processing configurations. Based on the experimental results, flotation processes carried out on raw feed allowed the recovery of the majority of REE minerals (>50%), but the process was unselective, where clay and silicate gangue minerals reported into the flotation concentrate along with the REE minerals. However, desliming before flotation in the presence of depressants (starch and sodium silicate) improved REE minerals flotation selectivity, which produced concentrates assaying 5.87% and 4.22% TREO grades, with corresponding recoveries of 45% and 50% at pulp pH 9 and 10.5, respectively. Mineralogical analysis conducted on selected flotation concentrate indicated that silicate and clay gauge minerals were recovered via the synergistic act of surface activation and entrainment due to their fine to ultrafine nature. A comparison of all the test results revealed a haphazard grade-recovery relationship suggesting that there is an opportunity to further maximize both REE recovery and grade through further flotation studies where other process parameters may be investigated and optimized. The prospect of using magnetic separation has also been suggested. Refereed/Peer-reviewed

Refereed/Peer-reviewed The rapid depletion of high-grade rare earth elements (REE) resources implies that future supplies may be augmented with low-grade ores, mining tailings and other unconventional resources to meet cutoff grades and subsequently supply demands. In this work, detailed characterization of tailings from a copper processing plant was carried out to expand the knowledge of their chemical and mineralogical characteristics for REE recovery. The results showed that the REE are mainly mineralized as bastnasite, monazite, and stetindite, along with uncommon minerals, such as brannerite, stillwellite, and florencite, for which there is limited processing data. In a preliminary effort to recover the REE, the tailings were subjected to gravity, magnetic, and froth flotation separation. Over 90% REE recovery was achieved via froth flotation, whereas magnetic and gravity separation tests recovered up to 86% and 54% REE, respectively. The results from this study suggest that, with systematic optimization studies, REE recovery and upgrade could be enhanced, with significant efforts of minimizing the recovery of key gangue minerals, such as goethite, hematite, and quartz. Overall, the study confirms that the tailings used in this work could be regarded as unconventional REE resources subject to tactical development and optimization of beneficiation methods.

Isotopic dating studies have revealed that many carbonatite-related REE ore systems have protracted and complex temporal evolution histories. The mechanisms that cause these complex dating results remain poorly understood. Here, we present an in-situ Sm-Nd and Th-Pb isotopic investigation of monazite and bastnasite from a geologically-complicated deposit, the world-class Bayan Obo REE deposit of China, with the aim of defining the general mechanisms accounting for the protracted age spread observed in this and many other REE deposits. Monazite and bastnasite are two principle REE ore minerals in the Bayan Obo deposit. Most of the studied monazite and bastnasite grains have experienced fluid-aided modification. The modified grains have generally homogeneous Sm and Nd contents and Sm-Nd isotopic compositions, implying low mobility of Sm and Nd during the modification process. Thus, the Sm-Nd chronometer can be used to constrain the primary crystallization timing of the REE minerals at 1293 ± 48 Ma. However, altered REE minerals have apparent 208Pb-232Th ages (1248–261 Ma) that are variably younger than the primary crystallization age. In most samples, Th contents have no correlations with 208Pb-232Th ages, while Pb contents tend to decrease along with the younging trend of 208Pb-232Th ages. NanoSIMS mapping shows that 208Pb and 208Pb/232Th are heterogeneous, and irregularly distributed in the altered grains. Such results are indicative of variable Pb loss during the fluid-aided modification process. Thus, many Th-Pb ages represent partial isotopic resetting and, hence, are geologically meaningless. Completely reset ages cluster at 450–400 Ma or 280–260 Ma, representing two discrete episodes of post-ore hydrothermal activity. Small amounts of monazite and bastnasite that were newly crystallized during these two overprinting episodes have initial Nd isotopic signatures approaching the time-evolved isotopic compositions of primary ores, which suggests insignificant REE input to the primary ores during these events. This study provides critical evidence to resolve complex dating results and disparate ore genesis models for the giant Bayan Obo deposit. Moreover, our results indicate the resistance of Sm-Nd isotopes, but the susceptibility of Th-(U)-Pb isotopes, to fluid-aided modification. Therefore, younger Th-U-Pb dates in geologically-complicated deposits should be treated with caution as they may represent partial resetting results and/or may not represent events of increased metal tenor.

Bastnasite is widespread in alkali granite, carbonatite and REE ore deposits. The U-Th-Pb ages of bastnasite can constrain the mineralization age. Currently, the lack of suitable in situ analysis reference material is the major obstacle of bastnasite U-Th-Pb geochronology. In this study, the matrix effects of U-Pb and Th-Pb age determination in bastnasite using glass NIST 610 as the external calibration reference were evaluated with the 193-nm excimer LA-ICP-MS analysis in normal and additional gases laser ablation modes. The obtained U-Th-Pb ages of bastnasite were approximately 7–11% lower than their reference values in the normal analytical mode (without additional gas). Fortunately, the determined systematic age biases in U-Th-Pb dating were significantly reduced (less than 1–2%) with the addition of water vapor within the ablation chamber. This reduction may be attributed to the observed significant suppression of 206Pb/238U and 208Pb/232Th downhole fractionation in both NIST 610 glass and bastnasite by introducing a small quantity of water vapor in the ablation chamber. In addition, the obtained 206Pb/238U and 208Pb/232Th ages of bastnasite K-9, LZ1384, and MAD809 showed great consistency with their corresponding recommended values. The obtained results show that accurate and precise U-Th-Pb ages of bastnasite can be simultaneously obtained by using glass NIST 610 as the primary calibration standard in 193-nm LA-ICP-MS with the water vapor-assisted method. This simple and effective water vapor-assisted non-matrix-matched laser ablation method helps to promote the geological application of bastnasite U-Th-Pb geochronology.

Ca−REE fluorocarbonates occur in various geological environments, including A-type granite systems. Here we investigate the character and paragenesis of fluorocarbonates in the A-type Rödö rapakivi complex, central Sweden, and their importance for the late stages of felsic magma evolution. Magma evolution is largely controlled by fractionation of the major phases quartz, plagioclase and K-feldspar, but these do not account for the decrease in LREE in the evolved magmas. Allanite or monazite, the major LREE-fractionating accessory minerals in common silicic igneous rocks, have not been encountered. In the Rödö suite, Ca−REE fluorocarbonates occur in rocks of various evolutionary level, typically as interstitial fibrous masses or filling small vugs. In the more evolved rocks, they are generally associated with fluorite and xenotime. In part, they are rich in Fe, Th, and Y. In composition, they range between synchysite and bastnäsite (but not pure bastnäsite) and seem to involve a number of less well-known intermediate phases. Ca−REE fluorocarbonate chemistry covaries with whole rock chemistry such that they become depleted in LREE and enriched in Y+HREE with magma evolution. This suggests that fluorocarbonates have crystallized in fluid ‘bubbles’ from fluids expelled from the evolving high-level, F- and CO2-rich magma system at various stages, which resulted in a progressive depletion in LREE coupled with enrichment in HREE in the residual magmas.

The interaction of rare earth bearing (La, Nd, Dy) aqueous solutions with calcite crystals at ambient and hydrothermal conditions (25–220 °C) results in the solvent-mediated surface precipitation a...

—This paper presents the results of the first geological, isotope, geochemical, and mineralogical study of carbonatite veins that were previously unknown in the Soustov pluton. The studied veins are similar in the Sm–Nd isotope composition and model age to the host rocks, which implies a common formation processs. High contents of light lanthanides, Sr, and Nb in carbonatite veins were measured. These elements are concentrated in bastnasite, strontianite, monazite, and pyrochlore. These data significantly enlarge our concepts of the geochemical and ore specialization of the massif.

Herein, a new clean extraction technology for the decomposition of bastnasite concentrate by utilizing the microwave radiation is proposed, which prevented Ce(III) from being oxidized to its tetravalent form. The process includes microwave radiation roasting to nonoxidatively decompose the bastnasite concentrate, mechanism analysis of Ce(III) not being oxidized to Ce(IV), hydrochloric acid leaching of the nonoxidative roasted ore, and kinetics analysis of the leaching process. The experiments were carried out concentrating on the effect of roasting temperature and holding time on the decomposition rate of the bastnasite concentrate and the oxidation rate of cerium and the effect of acidity, liquid-solid ratio, leaching temperature, and stirring rate on the leaching kinetics of the nonoxidative roasting ore. When the roasting temperature is 1100 °C, the holding time is 20 min, and the m(C)/m(REFCO3) ratio is 0.2, the results show that the leaching efficiency of rare earths can reach 85.45% under the conditions 3 mol/L HCl, 90 °C, 60 min, 9 mL/g liquid-solid ratio, and 300 rpm stirring rate. The X-ray diffraction and scanning electron microscopy analyses of the samples before and after acid leaching show that the rare earth oxides were completely leached and Ce(III) was not oxidized to its tetravalent form. The apparent activation energies of leaching rare earths were calculated as 14.326 kJ/mol, and the HCl leaching process can be described by a new variant of the shrinking-core model, in which both the interfacial transfer and the diffusion through the product layer influenced the reaction rate. Furthermore, a semiempirical rate equation was created to describe the leaching process of the nonoxidative roasted ore.

The extraction of lanthanum (La) and cerium (Ce) from a bastnasite ore by direct acidic leaching was investigated. The effects of acid concentration and leaching temperature on the extraction of La and Ce from the ore were tested. Using nitric (NHO3 ), more than 85% of the La and Ce were simultaneously extracted into leach solution whereas the La and Ce dissolutions were determined as less than 85% by using sulfuric acid (H2 SO4 ). The La dissolution exceeded 90% by using hydrochloric acid (HCl); however, the Ce dissolution remained below 85% under the following conditions: solid-to-liquid ratio of 20% (w/v), the acid concentration of 20%, leaching temperature of 25 °C and leaching time of 1 h. The result revealed that HNO3 could be used as a solvent for the maximum simultaneous extraction of the La and Ce from the bastnasite ore. The leaching temperature had no crucial effect on the dissolution of La and Ce when HNO3 or HCl solutions were preferred as a solvating agent. However, the leaching temperature had a slight positive effect on the dissolutions of La and Ce when H2 SO4 was used as a solvent., Direkt asit liçi ile bir bastnazit cevherinden lantanyum (La) ve seryum (Ce) ekstraksiyonu araştırılmıştır. Bastnazit cevherinden La ve Ce ekstraksiyonuna asit konsantrasyonu ve liç sıcaklığının etkileri test edilmiştir. Nitrik asit (NHO3 ) kullanarak La ve Ce’un %85’ten fazlası liç çözeltisine alınmıştır ancak çözünme değerleri sülfürik asit (H2 SO4 ) kullanıldığında %85’ten az olarak belirlenmiştir. La çözünümü hidroklorik asit (HCl) kullanımıyla %90’ı geçmiştir fakat Ce çözünümü %20 (g/L) katı-sıvı oranı, %20 asit konsantrasyonu, 25 °C liç sıcaklığı ve 1 saat liç süresi şartları altında %85’in altında kalmıştır. Sonuçlar bastnazit cevherinden eş zamanlı olarak maksimum La ve Ce çözünümüne ulaşılabilmek için çözücü olarak HNO3 kullanılabileceğini ortaya koymuştur. Çözücü olarak HNO3 ve HCl tercih edildiğinde liç sıcaklığının La ve Ce çözünümüne önemli bir etki yapmadığı belirlenmiştir. Fakat çözücü olarak H2 SO4 kullanılmasıyla liç sıcaklığının La ve Ce çözünmeleri üzerine hafif bir pozitif etkisi saptanmıştır.

This study deals with the calcination of a rare earth bearing ore to produce rare earth oxides in a fluidized bed reactor, with the aim of demonstrating the advantage of calcining bigger particles to decompose stable minerals such as monazite by its mineral association within a particle. Rare earth oxides were produced by the decomposition of its bearing minerals, bastnasite and monazite along with the gangues. During the calcination, bastnasite decomposed at relatively low temperature while monazite reacted CaO at high temperature. The formation of cracks, revealed by BET and SEM analysis, allowed the particle’s degassing while the size remained constant, thus describing a crackling core behavior. The association of monazite to calcite for bigger particles (−60 mesh) allowed its decomposition to produce rare earth oxides with a monazite conversion of 50 %. Also, an extraction process comprised of calcination and mild acid leaching enriched the rare earth oxides in the ore by 3.34 and improved leaching efficiency of the gangues compared to smaller size (enrichment ration of 2.35). Using bigger particle sizes in the upstream process of a mining industry, i.e. comminution, can also reduce the energy consumption.

The stability relations of cerium and its species in the hydrometallurgical processing of bastnasite were studied using Pourbaix diagrams. Different systems of Ce-(F)-(CO3)-(SO4)-(C2O4)-H2O were co...

The performance of the Pickering emulsification process to selectively separate rare earth-bearing minerals from the associated gangue and to concentrate them in a novel environmentally friendly approach was enhanced by adjusting the interfacial conditions and the formulation parameters. The present study investigated the effects of interfacial tension, oil polarity and viscosity on the selective separation. Modifying the emulsion formulation and selecting an appropriate oil, oil to water volume ratio, and solid mass fraction can enhance the recovery and grade of rare earth bearing minerals in the product when compared with conventional flotation. Various oils with different viscosities and chemical compositions (silicone oil, oleic acid, paraffin, and kerosene) supplemented with an additive (1% 1-hexanol) to modify the interfacial tension, and a continuous phase other than water (i.e., 90% ethylene glycol) were tested. An oil viscosity of 200 cSt, oil-to-water ratio of 10%v, and a solid particle concentration of 5 wt% provided the best selectivity. Since silicone oil is not economically feasible at industrial scale, paraffin oil, which is less expensive, was tested and provided better selectivity. We employed a Niobec ore containing bastnasite and monazite with total rare earth elements of 3.3 wt%. The recovery of a one-stage emulsification was improved significantly by up to 56% using paraffin oil. It also resulted in a product enrichment ratio of approximately 2.7.

Detailed electron probe microanalyses (EPMA) of accessory minerals from Wadi Rod Elsayalla granitoids (SE Desert, Egypt) furnished insights into U, Th, Zr, and REE mineralization. Thorite and uranothorite are the main radioactive minerals, in addition to zircon and monazite. Thorite occurs in syenogranite and quartz syenite rocks with marked enrichment in UO2 up to 6.89 wt% in the former rock type. High Y and Zr elements content in thorite indicates marked replacement of Th by Y and Zr. Uranothorite in syenogranite contains up to 51.68 wt% ThO2, 19.12 wt% SiO2, and 20.62 wt% UO2, in addition to minor amount of Y2O3 up to 1.94 wt%. Zoned zircon shows thorium-rich core, with marked fluctuation in UO2 content, owing to extensive hydrothermal alterations. Total REE content as REE2O3 reaches to about 6 wt% in zircon rims from the syenogranite. Zircon from quartz syenite shows U, Th, and REEs below the detection limit of the electron microprobe. Identified REE prospects in the study area contain (i) REE fluorocarbonates and fluoride (bastnasite, synchysite, and flocerite), (ii) REE phosphates (monazite-Ce, xenotime, and florencite), and (iii) REE silicates (allanite-Ce and zircon), with a clear predominance of LREE minerals, in which Ce is the main REE. The average REE contents of bastnasite-Ce, flocerite, synchysite-Ce, and synchysite-Y are 77.28, 74.01, 52.89, and 49.09 wt%, respectively. Xenotime from syenogranite and quartz syenite has average HREE contents (Gd2O3–Lu2O3) of 20.45 and 23.72 wt%, respectively.

Rare earth element (REE) production is limited in part by inefficient strategies for beneficiation. Hydroxamic acid ligands are promising reagents for the selective flotation of bastnasite [(Ce,La)FCO3], a major REE ore mineral, but the mechanism and energetics of adsorption are not understood, interfering with the design of new, more efficient reagents. Here, the adsorption of octyl hydroxamic acid onto bastnasite was measured using a combination of experimental and computational methods. In-situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed changes in the hydroxamate functional group vibrational frequencies, corresponding to chelation with cerium cations at the bastnasite surface. The results indicate a monodentate chemisorption mechanism at low surface loading that changes to bidentate chemisorption at higher concentrations. This interpretation is supported by molecular vibrational frequency shifts calculated using density functional theory (DFT), and orientation of the hydrocarbon chain measured by sum frequency generation (SFG) vibrational spectroscopy. The binding enthalpies of octyl hydroxamic acid interacting with La and Ce-bastnasite surfaces were measured using isothermal titration calorimetry (ITC) revealing a stronger coordinating ability with bastnasite than with a common gangue mineral, calcite (CaCO3). Because octyl hydroxamate favors monodentate adsorption at low surface coverages, the relative chelating strength of metal ions could be a poor predictor for selectivity under monolayer adsorption conditions. At higher surface loadings, where the bidentate mode of adsorption is active, selectivity is likely to be limited by increased flotation of gangue ores.

Bastnäsite-(Ce), a rare earth element (REE) bearing carbonate (Ce,La,Y,Nd,Pr)CO3F, is one of the most common REE-bearing minerals and has importance from both economic and geologic perspectives due to its large REE concentration. It also provides an example of the structural interplay between carbonate groups and fluorine ions, as well as the complex bonding properties of rare earth elements. We report Raman vibrational and Nd3+ luminescence (4F3/2→4I9/2, 4F3/2→4I11/2, and 4F5/2+2H9/2→4I9/2) spectra of natural bastnäsite-(Ce) to 50 GPa at 300 K. Two phase transitions are observed under compression. Bastnäsite-I remains the stable phase up to 25 GPa, where it undergoes a subtle phase transition to bastnäsite-II. This is likely produced by a change in symmetry of the carbonate ion. Bastnäsite-II transforms to bastnäsite-III at ~38 GPa, as demonstrated by changes in the luminescence spectra. This second transition is particularly evident within the 4F3/2→4I9/2 luminescent transitions, and it appears that a new rare earth element site is generated at this phase change. This transition is also accompanied by modest changes in both the Raman spectra and two sets of luminescent transitions. Despite these transformations, the carbonate unit remains a stable, threefold-coordinated unit throughout this pressure range, with a possible increase in its distortion. Correspondingly, the rare-earth element site(s) appears to persist in quasi-ninefold coordination as well, implying that the general bonding configuration in bastnäsite is at least metastable over a ~30% compression range. All pressure-induced transitions are reversible, with some hysteresis, reverting to its ambient pressure phase on decompression.

Rare earths are critical to numerous materials and applications underpinning modern civilisation. The majority of the world's rare earth reserves are hosted in the three minerals bastnasite, monazite and xenotime. A key step in the processing of rare earth mineral concentrates is the chemical decomposition of the mineral structure to release the constituent elements. The sulfuric acid bake has historically been, and is also currently, one of the major processes used for this step. Current sulfuric acid bake processes for the Bayan Obo deposit in China and the Mt. Weld deposit in Australia together account for more than half of the world's rare earth production. In the sulfuric acid bake, the rare earth elements are converted to rare earth sulfates which are dissolved in a subsequent water leach. The conditions required to achieve mineral decomposition vary widely for different rare earth minerals. Adjustment of process conditions may often be used to achieve some degree of impurity rejection which is beneficial to downstream processing. This paper reviews the application of the sulfuric acid bake process to ores/concentrates containing mainly monazite, xenotime and bastnasite, and other less common rare earth minerals including euxenite, samarskite, fergusonite, loparite, allanite, eudialyte and pyrochlore. The reported effects of feed mineralogy and process variables such as reaction temperature, bake duration, acid to concentrate ratio and particle size are presented along with a brief review of current understanding of the bake chemistry and water leach results.

This high-resolution transmission electron microscopy (HRTEM) investigation of (Ca, REE)-fluorcarbonates from Mount Malosa (Malawi), previously characterized by bulk techniques, is aimed at elucidating the pervasive syntactic intergrowths that commonly affect these minerals. The studied samples are dominated by bastnäsite-(Ce)–parisite-(Ce) intergrowth with rhythmic repetitions along the c axis from the microscale to the nanoscale. Whereas bastnäsite is always structurally and compositionally homogeneous, parisite-(Ce) may be either ordered or disordered, in the latter case showing polysomatic faults (compositional faults) that locally change the composition towards bastnäsite-(Ce). The HRTEM image contrast of ordered and disordered intergrowths is interpreted and discussed in terms of layer shifts on (001) of ~2.37 Å of the bastnäsite portions (B) above the Ca-layers (V) along stacking vectors rotated by n ✕ 60° (n = odd) with respect to each other. In a few cases, sequential faults (conjugate polysomatic faults that locally alter the layer sequence without altering the overall composition) occur periodically, leading to a new parisite-(Ce) polymorph, described in terms of V and B stacking sequences. HRTEM images and associated simulations show that the new parisite-(Ce) polymorph has C1 symmetry and cell parameters similar to those of normal parisite. The detected nanostructure is consistent with a primary growth mechanism in which bastnäsite and parisite crystallize directly from a fluid whose thermodynamic (close to) equilibrium conditions quickly and repeatedly crossed their stability boundary. Polysomatic disorder in the compositional interval between parisite and bastnäsite, as occurred in the studied samples, is indicative of growth rates faster than those leading to the formation of long-range polysomes detected in other occurrences. Finally, the present study demonstrates how HRTEM techniques can elucidate polysomatic and polytypic disorder in a comprehensive way, since the contrast is sensitive to the different stacking of CO3 layers that appears with different slants in HR images, revealing the actual crystal system symmetry and any polytypic disorder.

The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (> 0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnasite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr = 0.70292–0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr = 0.70295–0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.

Dissolution in choline-chloride based deep eutectic solvents (DES) of rare earth elements (REE) from high-purity single carbonate salts of Y, La, Ce, Nd and Sm and from their multicomponent mixtures and also bastnasite mineral was assessed with a prospect to gain an understanding on the leaching behavior of bastnasite ores as REE source from DES suspensions. Urea, malonic acid and citric acid were used in different proportions with choline chloride in order to form deep eutectic solvents with desired viscosity and functionality. The results obtained prove promising for use insolvato-metallurgical processes in terms of selective dissolution of the higher-atomic-number (Z) rare earth elements at the expense of the lower-ZREEs and the gangue-originating metal impurities, thus initiating separation among the different REE members from the early hydrometallurgical steps.

Bastnasite is the main ore mineral in many carbonatite-related rare earth element (REE) deposits, which account for ∼51% of rare-earth oxide reserves worldwide. However, the occurrence, geochemistry, and genetic significance of bastnasite has not been methodically investigated. The Cenozoic Mianning–Dechang (MD) REE belt in Sichuan Province, SW China, contains the Maoniuping, Dalucao, Lizhuang, and Muluozhai deposits as well as numerous smaller REE occurrences. Individual deposits within the belt contain different types of bastnasite-bearing ore, which provides a unique opportunity to explore in detail the common mechanisms controlling the formation of bastnasite-rich REE deposits. Here, we present detailed results from field observations and petrographic, geochemical, and fluid inclusion studies of bastnasite from the main MD deposits. Calcite, fluorite, and barite form stable mineral assemblages that are commonly overprinted by bastnasite. Homogenization temperatures of fluid inclusions in bastnasite of ∼150–270 °C (Dalucao and Lizhuang deposits) and 155–210 °C (Maoniuping deposit) are systematically lower than those of fluid inclusions in gangue minerals. Therefore, the petrographic studies and homogenization temperatures both show that large-scale crystallization of bastnasite took place during the later stage of the hydrothermal system. The bastnasite, relatively geochemically homogeneous within all of the MD deposits, is enriched in Ba (293–8425 ppm), Th (16.4–2527 ppm), and U (4.19–92.7 ppm), and relatively depleted in high field strength elements such as Nb (0.15–17.4 ppm), Ta (0.06–6.48 ppm), Zr (0.71–31.1 ppm), Hf (0.62–5.65 ppm), and Ti ( carbonatite , and ore veins. In comparison, the samples from the study area show an increase in average REE contents from syenites to carbonatites to ore veins (i.e., bastnasite-bearing ores) and finally to bastnasite. Lanthanum and Ce were commonly substituted by Th, U, Sc, Ba, and Sr supplied by more evolved hydrothermal fluids. Combining the present results with existing data, we present a three-stage model for the formation of carbonatite-related REE deposits. First, partial melting of metasomatized sub-continental lithospheric mantle, fluxed by REE- and CO2-rich fluids, forms the parental carbonatite–syenite magma. Second, Sr, Ba, and REEs are strongly partitioned into carbonatite melts during liquid immiscibility in the carbonatite–syenite magmatic system. Third, hydrothermal fluids exsolved from the crystalizing syenite and carbonatite magmas form ore veins with early gangue minerals and later bastnasite overgrowths. Consequently, barite, calcite, and fluorite assemblages are a valuable guide in REE exploration.

The Taipingzhen deposit is a newly discovered REE deposit in the North Qinling Orogen (NQO), Central China. The REE ores are hosted in the altered fracture zones of Paleozoic strata and a granitic intrusion. The ore minerals mainly include bastnasite-(Ce)[(Ce(CO3)F], parisite-(Ce)[CaCe2(CO3)3F2], and synchysite-(Ce)[CaCe(CO3)2F], and this deposit also contains some uncommon minerals, such as tornebohmite-(Ce)[Ce2Al(SiO4)2(OH)], gatelite-(Ce) [CaCe3AlAl2Mg(Si2O7)(SiO4)3O(OH)2], fluocerite-(La)[LaF3] and F-rich cerite-(Ce)[(Ce,Ca)9 Mg(SiO4)3[SiO3(OH)]4F3], which are identified here for the first time in China. To determine the timing of REE mineralization and explore possible genetic relationships between mineralization and granitic magmatism, the U-Pb ages of bastnasite-(Ce) from ore and zircon from ore-hosting granite are obtained. The in situ LA-MC-ICP-MS U-Pb age of the bastnasite-(Ce) provides a reliable 238U/206Pb-207Pb/206Pb lower intersection point age of 409 ± 16 Ma, and ID-TIMS analyses yield an U-Pb age of 407.8 ± 3.3 Ma; the U-Pb dating of the magmatic zircon reveals that the wall rock was emplaced at 457.1 ± 4.9 Ma, and the hydrothermal zircon age of 399.4 ± 3.5 Ma records the time of hydrothermal alteration, thus indicating that the REE mineralization in the Taipingzhen deposit occurred at approximately 409–400 Ma. This age coincides well with the U-Pb ages of the bastnasite-(Ce) and zircon, revealing a genetic association between the hydrothermal fluid and the REE mineralization. The Hf isotopic compositions imply that the partial melting of Paleoproterozoic HP and UHP eclogite from the lower crust provided ore materials for the mineralization, and the deposit was generated in a retrograde metamorphic setting during the late Paleozoic postcollisional extension event in the NQO. Furthermore, combined with the geochronological data published for other U-REE metallic deposits in the NQO, our study indicates that the late Silurian-early Devonian granite-related U-REE metallic mineralization throughout the NQO mainly occurred during 421–395 Ma.

Carbonatite-related rare-earth element (REE) deposits are the most important source of the world’s REE resources. Hydrothermal fluids have been proposed to play a significant role in the transport and precipitation of REEs, but fluid inclusion data on the hydrothermal processes in carbonatitic settings are relatively sparse. The Dalucao deposit, located in the Mianning–Dechang (MD) REE belt, Sichuan, China, is a Cenozoic carbonatite-related REE deposit (c. 12 Ma) that offers an excellent opportunity to investigate the evolution of ore-forming fluids. Brecciated and weathered ores are common in this deposit. The former are characterized by mineral assemblages comprising fluorite + barite + celestite + calcite + quartz + bastnasite (No. 1 orebody) or fluorite + celestite + pyrite + muscovite + calcite + quartz + bastnasite (No. 3 orebody), whereas the latter contain REE minerals, clay minerals, and minor gangue minerals. We present a comprehensive study of fluid inclusions from the Dalucao deposit to constrain its hydrothermal evolution. Magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized. During the pegmatitic stage, the main minerals that formed were coarse-grained fluorite, barite, celestite, calcite, and quartz, which host melt inclusions, melt–fluid inclusions, and minor high-salinity fluid inclusions. The presence of melt and melt–fluid inclusions suggests a magmatic origin for the ore-forming fluids. Hydrothermal processes included at least two stages, characterized by hydrothermal veins that are developed in fractures within the carbonatite–syenite complex: (1) Fluid inclusions during the formation of the fluorite–quartz–barite veins in the pre-REE stage were trapped under immiscible conditions, as evidenced by the presence of CO2-bearing inclusions coexisting with aqueous ones. These immiscible CO2-bearing inclusions recorded a range of pressures from 1050 to 1600 bar. All of fluid inclusions in this stage exhibited homogenization temperatures varying from 278 to 442 °C, with salinities ranging from 3.2 to 45.1 wt% NaCl equivalent (equiv.). (2) The REE-stage fluids were represented by abundant aqueous inclusions, characterized by homogenization temperatures ranging from 147 to 323 °C and salinities between 1.1 and 9.5 wt% NaCl equiv. These data suggest that the ore-forming fluids forming the Dalucao deposit evolved from high-temperature, high-pressure, high-salinity, CO2-rich to low-temperature, low-pressure, low-salinity, CO2-poor. Gas- and ion-chromatographic analyses combined with mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6). H–O isotope analyses of quartz suggest that the hydrothermal fluids had a dominantly magmatic signature and were gradually diluted by meteoric waters. Hydrothermal REE transport was probably controlled by F−, (SO4)2−, and Cl− as complexing ligands. We propose that fluid cooling and mixing rather than immiscibility led to the precipitation of bastnasite during the waning stage of hydrothermal activity. Taken together, the inclusion data and observations of alteration, paragenesis and mineralization have provided insights into the development of REE mineralization and the further exploration of carbonatite-related REE resources.

Bastnasite is an important mineral resource in the production of rare earth materials and is usually beneficiated by flotation. The flotation of bastnasite is problematic due to the competitive adsorption of hydroxamate collector on bastnasite and its associated calcium-bearing gangue minerals such as fluorite. One strategy to solve this problem is to use effective depressants to depress the gangue minerals. However, the current depressants all have some drawbacks. In this study, the effect of ethylenediamine tetraacetic acid (EDTA) as a depressant was tested in the flotation of single mineral of bastnasite and fluorite and their mixture. The mechanism underpinning the role of EDTA was investigated through theoretical thermodynamic calculation, zeta potential and X-ray photoelectron spectroscopy (XPS) measurements. The results show that fluorite was significantly depressed, while the flotation of bastnasite was almost unaffected when EDTA was present. The separation index between bastnasite and fluorite increased from 1.18 to 12.66 with the increase of EDTA concentration from 0 to 7.79 kg/t. It was found that EDTA could dissolve the chemically adsorbed octyl hydroxamic acid (OHA) on fluorite through the formation of soluble Ca-EDTA complexes, whereas the chemically adsorbed OHA on bastnasite was more stable and could not be transformed into Ce-EDTA spontaneously. Therefore, the flotation of fluorite was selectively depressed by EDTA. The results show that EDTA was a promising depressant for fluorite gangue mineral in bastnasite flotation.

Recent exploration revealed a number of rare earth element (REE) deposits that are distributed along the Qinling Orogenic Belt, Central China. These deposits have an estimated total reserve of about 2 Mt. REE2O3, thus making this belt a world-class REE metallogenic province. Understanding the metallogenesis of the belt requires direct dating of REE minerals. In this study, LA-ICP-MS U-Th-Pb dating on bastnasite and monazite from the Huangshuian, Taipingzhen, and Miaoya deposits in different units of the belt was used to precisely determine the timing of the REE mineralization. The Huangshuian deposit in the north is a carbonatite-related Mo-(REE) deposit in which the REE minerals are closely associated with molybdenite. After correction for common Pb and excessed 206Pb decayed from 230Th, a weighted average 206Pb/238U age of 207 ± 4 Ma (n = 17; MSWD = 1.9) is obtained for bastnasite grains from this deposit. Such an age is slightly younger than that of the final collision (peak at 230–220 Ma) of the Qinling Orogenic Belt, thus indicating that the Mo-REE mineralization is likely related to a post-collisional extension setting. The Taipingzhen REE deposit in the middle contains sheet-like ore bodies composed of veins where bastnasite, the dominant REE mineral, is closely associated with quartz, fluorite, and barite. In situ bastnasite U-Th-Pb dating shows that the REE mineralization in this deposit has formed at 421 ± 7 Ma (n = 17; MSWD = 1.5), synchronous with extension-related magmatism in the region. The Miaoya deposit in the south is the largest one in the belt, and it is essentially a REE-mineralized syenite-carbonatite complex. In this deposit, the monazite grains are closely associated with major minerals of the syenites or carbonatites (e.g., K-feldspar or calcite), but commonly exhibit complex internal textures. Different domains of monazite yield two groups of U-Pb ages at 414 ± 11 Ma (n = 5; MSWD = 0.91) and 231 ± 2 Ma (n = 21; MSWD = 3.1), whereas the bastnasite has an age of 206 ± 4 Ma (n = 14; MSWD = 1.5). The early age of 414 Ma is obtained from homogenous monazite grains and in good agreement with zircon U-Pb ages of the Miaoya syenite-carbonatite complex, and thus is considered to represent the timing of the major REE mineralization in the deposit. The younger ages of 231–206 Ma are obtained from monazite grains with complex internal textures and bastnasite in late veinlets, thus recording secondary, consecutive REE remobilization events likely related to the compression process during formation of the Qinling Orogenic Belt. Our new U-Th-Pb ages, in combination with previously geochronological data, demonstrate that there are two episodes of REE mineralization at 440–410 Ma and 220–200 Ma in the Qinling Orogenic Belt.

A number of gravity separation techniques for rare earth elements (REE) minerals have been investigated and reported in the literature. These include the use of dense/heavy media, Falcon concentrators, Knelson concentrators, spiral concentrators, laboratory scale jigs, and shaking tables. Most of these methods have provided considerable REE minerals recovery and upgrade; but remain at the laboratory testing stage. There are vast quantities of iron-oxide-silicate rich tailings (IST) containing low grade REE in Australia. This work is part of research and development studies currently underway at the University of South Australia to develop economically viable methods to exploit selected IST for their REE contents. In this investigation, the feasibility of exploiting the differences in specific gravity to concentrate REE minerals from a typical IST sample was studied using two gravity separation units, a Knelson concentrator (KC) and Wilfley shaking table. The feed sample and gravity separation products were characterised using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) to study and compare the performance of the KC and shaking table as gravity preconcentration methods. The results indicated that the performance of both the KC and shaking table are significantly dependent on the mineralogical and particle size characteristics of the feed, however, the tabling was found to be the preferred choice over the KC, due to its greater selectivity. Tabling of >38 μm feed samples produced the desired split between the iron oxides and silicate minerals. The results suggest that desliming of the feed to remove fine/ultrafine particles (

The standard Gibbs free energy of formation (ΔGof, 298) of bastnasite, (Ce, La, Nd, Pr)FCO3, was estimated using four different methods. The obtained average values were − 1589.5 kJ/mol for CeFCO3, − 1599.5 kJ/mol for LaFCO3, − 1589.1 kJ/mol for NdFCO3, and − 1595.4 kJ/mol for PrFCO3. The value for cerium bastnasite, CeFCO3, was used to construct a potential vs. pH (Eh-pH) diagram to investigate the stability relations of cerium bastnasite. The diagram shows that CeFCO3 is stable in neutral to basic conditions (pH ~ 6.5–11).

Florencite, [REEAl3(PO4)2(OH)6], is the most abundant REE-phosphate mineral in the giant Olympic Dam Cu-U-Au-Ag deposit, South Australia. Florencite typically occurs as fine-grained crystals and occasional aggregates in the matrix of the granite-dominant breccia that hosts the majority of the copper mineralization. Olympic Dam florencite, with the compositional range and extended formula (Ca0.01–0.24Sr0.03–0.40La0.14–0.49Ce0.20–0.47Pr0.00–0.03Nd0.00–0.05)Σ0.43–0.96Al2.89–3.33(P1.42–1.96S0.05–0.34As0.0–0.20)Σ1.77–2.21O4(OH)6], is LREE-enriched, typically La-dominant, while HREEs are minor. There is also compositional variability with respect to Sr, Ca, SO4, and AsO4 components. Chondrite-normalized fractionation trends are steeply downwards-sloping with a relatively low and flat HREE segment. Such a fractionation trend is markedly different from that seen for REE-fluorocarbonates, the dominant REE host at Olympic Dam, which contain relatively higher MREE and HREE components. Xenotime is relatively rare at Olympic Dam and is most commonly seen as overgrowths on zircon. Compositional data for xenotime show HREE concentrations in which Y > Yb > Er > Dy > Ho. Thorium, U, and Pb are minor components in both xenotime and florencite. Monazite is a minor phase and co-existing monazite and florencite are never observed. Texturally, florencite appears to belong to a later stage of LREE mineralization at Olympic Dam, following and less prominent than the main REE-fluorocarbonate stage. Olympic Dam florencite is more La-rich and Ca-poor than it is in most of the localities worldwide for which compositional data have been published. The REE mineralization trends of florencite are compared with published REE trends for other mineral groups from Olympic Dam. Uraninite, brannerite, coffinite, and apatite show patterns of progressive LREE depletion from generation to generation, suggesting enhanced solubility of LREE over geological time. All LREE minerals (fluorocarbonates and phosphates), on the other hand, display progressive LREE enrichment correlating with a paragenetic sequence from REE-fluorocarbonates with essential Ca, through bastnasite, to florencite. The late-stage florencite reported here is the most La-rich of all REE-bearing phases and is interpreted to record either the end of a continuous fluid evolution, or a later separate event at changed physicochemical conditions under which REE-aluminum phosphate minerals are stable.

Common rare earth (RE) minerals, such as bastnasite and monazite, may be formed in deposits associated with carbonate gangue, such as calcite and dolomite. Sodium oleate is a widely used collector for the flotation of both RE and gangue minerals, which might, therefore, be an inefficient process due to the lack of selectivity of this collector. Since these minerals are also sparingly soluble in solution, they could release their constituent ions into the solution, which could affect the floatability of other minerals. In this study, the interactions of sodium oleate with bastnasite and monazite in the presence of dissolved dolomite species have been investigated. Microflotation tests were carried out to explore the effects of these dissolved species on the floatability of the RE minerals. Zeta potential measurements and XPS characterization were carried out to understand how the species affect the collector adsorption. To complement these characterizations, density functional theory (DFT) simulations were conducted to investigate the collector-mineral and collector-adsorbed species (on the mineral surface) interactions. The results show that collector-dolomite interaction energy is greater than that of collector-adsorbed species, but lower than collector-monazite interaction energy, explaining the decrease in the minerals' recovery upon exposure to the dissolved mineral species. It is also shown that oleate ions (Ol–) have the strongest interaction with the minerals compared to other oleate species such as acid soap (HOl2–) and oleate dimer (Ol22−). The behavior (strength and selectivity) of sodium oleate towards RE minerals and dolomite, as compared to other RE mineral collectors (such as aromatic hydroxamate), is attributed mainly to the collector's and the minerals' structure. The long hydrocarbon chain of sodium oleate which imparts hydrophobic characteristic to the minerals, makes it stronger collector than benzohydroxamate. Moreover, sodium oleate (with linear structure), unlike the aromatic hydroxamate, can approach the mineral easier due to lesser steric hindrance effect and higher reactivity of O involved in the interaction, making it less selective. In addition, it can interact easily with dolomite due to the presence of more exposed active sites than RE minerals.

The potentiality of the alkali breakdown of the poly mineralized ore material south Gabal El-A’urf area, central Eastern Desert, Egypt is investigated. It includes the multiple oxide mineral tanteuxenite, monazite, bastnasite, anatase, as well as zircon. This ore material assaying lanthanides (RE2O3 1.9%), niobium (Nb2O5 1.25%) as well as titanium and zirconium (TiO2 and ZrO2 1.23% and 1.12%) respectively besides tantalum (Ta2O5 0.13%), were used for the present work. This sample has thus been subjected to two alkali breakdown techniques using potash; namely agitation leaching and fusion. The latter technique was found to be preferred as it gives higher leaching efficiency. The optimum leaching conditions are weight ratio of the ore/potassium hydroxide 1/2 at 400°C for 1 h. The realized leaching efficiencies attained 95.0% and 93.7% for Nb and Ta respectively as well as 96% for lanthanides and 75% for Ti while that for Zr did not exceed 10%.

The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late-stage hydrothermal fluids that added Sr, Ba, S, F, and REE, analogous to the replacement processes described for some carbonatite complexes of Russia's Kola Peninsula. Carbon and oxygen stable isotope ratios indicate that the primary carbonatite mineralogy experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO2/H2O ratios, and/or meteoric water content. Isotopic differences of matrix calcite between Group 1 carbonatites (avg. δ13C = −7.3‰; δ18O = 9.1‰) and Group 2 carbonatites (avg. δ13C = −9.9‰; δ18O = 10.2‰) are consistent with loss of CO2 during degassing. The open-system alteration of burbankite caused a pronounced positive δ18O-shift in bulk ancylite pseudomorphs (δ18O: 14.3–25.7‰) relative to matrix calcite (δ18O: 8.7–11.2‰). Oxygen isotope compositions of biotite (δ18O: 4.5–5.9‰) and K-feldspar (δ18O: 7.3–7.9‰) in unoxidized carbonatite are typical of primary magmatic silicates and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in 13C and 18O, oxidation, matrix carbonate dissolution, and the replacement of REE carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (bastnasite, parisite, synchysite) suggest interaction with late-stage, low temperature (

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky complex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.

Stability relations of the REE-bearing accessory phases and alteration processes in the cancrinite-bearing nepheline syenite from the Čistá pluton (the center of the upper-crustal Tepla–Barrandian unit, Bohemian Massif, Czech Republic) were studied. Observations of rock microtextures, quantitative analyses of minerals and compositional X-ray mapping were performed using electron probe microanalysis (EPMA). The primary REE-bearing accessory minerals assemblage includes monazite-(Ce) associated with gadolinite-group minerals (i.e. gadolinite-(Ce) and gadolinite-(Y)), which were partially replaced by britholite-(Ce), bastnäsite-(Ce), aggregates of fine-grained REE-bearing phases (possibly fluorapatite and/or britholite-(Ce)) and, rarely, cerianite. K-feldspar and albite form intergrowths or symplectites with REE-phases in the investigated reaction microtextures. Furthermore, the zircon crystals demonstrate oscillatory zoning and/or extensive patchy zoning due to alteration processes. The alteration of accessory minerals are interpreted as driven by K- and Na-bearing alkali fluids with high CO2 activity during late- to post-magmatic processes.Acknowledgements: This work was supported by the National Science Centre research grant no. 2017/27/B/ST10/00813.

The production of mixed rare earth oxide powder from a thorium containing bastnasite ore by sulfuric acid bake-water leaching followed by precipitation with oxalic acid and thermal decomposition of the oxalates was investigated. The sulfuric acid baking was performed at 250 °C and the optimum baking time was found to be 3 h. Using deionized water as lixiviant, 92.6% La, 86.8% Ce, 86.9% Pr, 82.3% Nd, 95.4% Th and 31% Y were dissolved from the baked ore at 25 °C after 30 min of leaching. The effect of solid-to-liquid ratio on the dissolution of the rare earth elements and thorium shows that when the solid ratio in the water increased from 1:10 to 1:3, the dissolution percentage decreased. The final mixed rare earth oxide powder contained 88.54% REO and 6% ThO2 together with small amounts of other impurities. The SEM mapping results revealed that the produced REO has an irregular crystal shape. Based on the experimental results obtained from the current study, a flowsheet was proposed for the production of mixed rare earth oxide powder from a specific complex bastnasite ore.

Rare earth elements are strategic elements, extensively used in hybrid cars, power generators, powerful magnets, electronic gadgets, super alloys, etc., to enhance their performance. They are associated with minerals like bastnasite, monazite, zircon, allanite, xenotime, florencite, and rhabdophane. REEs are present in copious amount than valuable metals like (platinum, gold, and silver) in earth’s crust. China has maximum reserve of REEs in the world and has a greater influence over global market of REEs. China covers more than 90% of global supply therefore has monopoly over REEs trade. These factors led the researcher around the globe to find alternative options for recovery of REEs from secondary sources. Coal fly ash (CFA) is one such resource having 300–500 mg/kg of REEs globally. It is very essential to figure out the distribution of REEs, in coal fly ash in order to develop effective methods to recover REEs. This study is beneficial for enhancing the knowledge about the presence and abundance of REEs in different coals, coal fly ash, and various extraction procedures to recover them from coal fly ash.