Precipitation Of Uranium Research Articles

Investigation of nuclide inventory of cladding material irradiated in the Goesgen PWR core

A characterization of the spent nuclear fuel (SNF) for its radionuclide (RN) inventory is vital for various back-end stages of the nuclear fuel cycle. It concerns both the fuel and the metallic (i.e., cladding and structural material) components of the spent fuel assemblies, where different calculation approaches and methods should be deployed for their characterization. This study concentrates on fuel traces and other impurities within the cladding. During the operating cycles, the Zircaloy cladding is exposed to a considerable amount of irradiation. The impact of the exposure should be checked to assure the integrity of the cladding and thus the safety of the stored spent fuel. Within the work package “Spent Nuclear Fuel Characterization and Evolution until Disposal” (SFC) of the EURAD project, dedicated samples were produced, irradiated and the radionuclide inventory of the cladding was analysed and compared. In parallel a blind test was performed, in which different partners used different codes to simulate the irradiation quantity. The blind test showed good agreement between most of the codes, in particular in view of the small amount of the evolved fuel traces. Furthermore, the presence of actinides, caused by precipitation of uranium on the inner surface of the cladding during manufacturing, was found to be negligible in comparison to precipitation of traces of fuel pellets on the cladding during operation. The good agreement between the simulating codes enables to depict further the initial amount of alloying elements of the cladding material itself in a better manner. In particular specific isotopes of cobalt, nickel and iron, which are directly connected to the unique properties of each cladding material can be better identified based on the accurate measuring techniques used in this study.

Microbial production of methyl-uranium via the Wood-Ljungdahl pathway

The misuse of uranium is a major threat to human health and the environment. In microbial ecosystems, microbes deploy various strategies to cope with uranium-induced stress. However, the exact ecological strategies and mechanisms underlying uranium tolerance in microbes remain unclear. Therefore, this study aimed to investigate the survival strategies and tolerance mechanisms of microbial communities in uranium-contaminated soil and groundwater. Microbial co-occurrence networks and molecular biology techniques were used to analyze the properties of microbes in groundwater and soil samples from various depths of uranium-contaminated areas in Northwest China. Uranium pollution altered microbial ecological strategies. Uranium stress facilitated the formation of microbial community structures, leading to symbiosis. Furthermore, microbes primarily resisted uranium hazards by producing polysaccharides and phosphate groups that chelate uranium, releasing phosphate substances that precipitate uranium, and reducing U(VI) through sulfate- and iron-reducing processes. The relative abundance of metal-methylation genes in soil microorganisms positively correlated with uranium concentration, indicating that soil microorganisms can produce methyl uranium via the Wood-Ljungdahl pathway. Furthermore, soil and groundwater microorganisms demonstrated different responses to uranium stress. This study provides new insights into microbial responses to uranium stress and novel approaches for the bioremediation of uranium-contaminated sites.

Hydrothermal alteration and elemental mass balance for the Surda Copper Deposit, Singhbhum Shear Zone, Eastern India: implications for both copper and magnetite–apatite mineralization

This paper explores the geological relationships, mineralogical and geochemical characteristics, and quantitative elemental mass balance constraints of the hydrothermal alteration zones within the uraniferous Surda copper deposit in the Singhbhum Shear Zone in eastern India, which is affiliated with an iron oxide–copper–gold system. Five distinct alteration zones are identified: a proximal potassic (biotitization) zone containing magnetite–apatite; a central chloritization zone containing copper, uranium, gold and magnetite; distal silicification (quartz) and albitization zones; and a transitional chlorite–sericitization zone. Evidence suggests these zones originated from mafic precursors linked to a subduction zone. Magnetite mineralization shows both magmatic ( δ 18 O + 2.0 to +4.8‰) and hydrothermal ( δ 18 O + 0.4 to +0.6‰) signatures. Hydrothermal magnetite–apatite formed at temperatures of 440 to 550°C, with continued crystallization at lower temperatures (280 to 360°C) concurrent with copper sulfide and uranium precipitation from an Fe-rich, isotopically heavy fluid ( δ 34 S + 4.55 to +9.66‰). Major elements (Ca, Na, Mg) and some minor and rare earth elements decrease from weakly altered mafic rocks to late alteration zones, reflecting the breakdown of Ca-bearing hornblende, biotite and plagioclase. Potassium decreases from early to late alteration zones due to mineral transformations. Mass-balanced geochemical data align with hydrothermal alteration mineral assemblages, indicating processes such as mineral dissolution, element absorption and sulfide behaviour. Overall, the study highlights the complex hydrothermal alteration processes shaping the Surda copper deposit and provides insights into ore formation mechanisms.

A suggested eco-friendly procedure for the recovery of uranium from Egyptian Abu Tartur phosphorites before industrial processing

The manufacture of phosphate fertilizers without prior uranium extraction leads to widespread dispersal of uranium compounds across agricultural fields, posing significant cumulative environmental and health risks. Furthermore, this represents a wasteful loss of a critical global energy resource. Against this backdrop, we propose an environmentally friendly method for extracting uranium from Egyptian Abu Tartur phosphate rocks using humic acid (HA). Almost complete dissolution of uranium with a 3% HA concentration, solid-to-liquid ratio of 1/2, and a leaching temperature of 60°C. The uranium was then effectively adsorbed from the HA solution, with a concentration of 0.026 g/L, onto activated carbon, influenced by variables such as: pH, contact time, and adsorbent amount per leach solution volume. Subsequent uranium regeneration from the activated carbon was accomplished using 0.5 M ammonium bicarbonate, achieving a desorption efficiency of 98.7%. This process culminated in the production of a sodium diuranate concentrate following uranium precipitation with NaOH solution.

Assessment of water supply to the east European arctic agglomeration from groundwater, taking into account their quality and health risks

The purpose of the study is to assess the possibilities of using groundwater for water supply in the East European Arctic agglomeration based on an assessment of their quality and health risks. For this purpose, high-precision determinations of the complete macro- and microcomponent composition were carried out in sixty-six water samples taken from wells up to 180 m deep. It was found that in some samples the concentrations of Na+, Fe, B, Ba, Mn and U exceeded WHO standards. The least mineralized young waters are characterized by the processes of dissolution of carbonates with the transition of Ca, Mg, Ba, Sr into water, and the processes of leaching of Fe and Mn by acidic swamp waters from near-surface sediments. Waters of high mineralization, enriched in Na+, Cl-, B, Mo, Cd, Pb, were formed as a result of the dissolution of aluminosilicate rocks over thousands of years and mixing with relics of ancient and modern marine transgressions. An assessment of the average Water Quality Index value of the studied aquifer showed that, in general, the water is of excellent quality. Non-carcinogenic risks were determined primarily by uranium concentrations. The average danger index values for this element for children were 1.22. In adults it was slightly lower and amounted to 0.83. Carcinogenic risks are associated primarily with arsenic concentrations. The average total carcinogenic risk associated with this element was 3.8.10−5, which is acceptable, but samples from two wells showed total carcinogenic risk values above 10−4, which is in the high-risk area. For drinking purposes, it is preferable to use low-mineralized water with a minimum content of toxic elements. If necessary, preliminary aeration of the water is possible, during which precipitation of iron, arsenic and uranium occurs. Due to the typical nature of the problem under consideration for the Arctic regions, the results obtained can be used at other sites in the Subpolar zone.

The origin and migration laws of hydrocarbons in uranium‐bearing Luohe Formation, Pengyang area, SW Ordos Basin

The hydrocarbon activity in Pengyang area, situated in the southwestern Ordos Basin, is notably prominent. Investigation on the migration laws of hydrocarbons is imperative for comprehending the involvement in uranium mineralization. Based on the analysis of spatial distribution of hydrocarbon containing fluid and hydrocarbon generation conditions of sandstone in the Luohe Formation, the organic geochemical characteristics including hydrocarbon components, carbon isotopes and biomarker compounds were analysed. The research results indicate that: (1) hydrocarbon fluid activities in the Luohe Formation are predominantly observed in layers exhibiting higher uranium mineralization. The mudstone of the Luohe Formation had low organic matter content and low thermal maturity, which was not conducive to hydrocarbon generation. (2) Hydrocarbon‐containing fluid in the sandstone of Luohe Formation not only contained reducing gases such as methane and hydrogen but also chloroform asphalt components. The carbon isotopes of hydrocarbon in sandstone inform Luohe Formation resemble oil and gas in the Mesozoic. The biomarker parameter inferred that the parent rock of hydrocarbons in the Luohe Formation was formed under reducing and freshwater conditions, and hydrocarbon generation occurred at the mature stage. As above mentioned, a comparison was carried out between the affinity of hydrocarbon‐containing fluid in the Luohe Formation and different layers of hydrocarbon source rocks. The migration behaviour of hydrocarbon‐containing fluid in the Pengyang area has been summarized, and the involvement of hydrocarbon‐containing fluid in uranium mineralization has been discussed. The main concepts are as follows: the sedimentary environment and thermal evolution conditions of hydrocarbons in the sandstone of Luohe Formation resemble those of the primary hydrocarbon source rocks in the Yanchang Formation. The main hydrocarbon charging events in the Luohe Formation occurred before the Late Cretaceous period, which is primarily related to two hydrocarbon generation events from 130 to 100 Ma in the Yanchang Formation and fault conduits connecting the Triassic to the Cretaceous Strata. The hydrocarbon‐containing fluid released from Yanchang Formation migrating to the Luohe Formation provides reducing conditions for the precipitation of uranium in oxygen‐bearing water bodies.

Hydrogeochemical characteristics and enrichment regularities of groundwater uranium in the Erlian basin, China

Groundwater is a key medium for the migration and enrichment of elements into ore bodies in sandstone-type uranium deposits, with anomalies of uranium elements and their associated components in water serving as indicators for exploring such deposits. The Erlian Basin, located in central-northern Inner Mongolia, is a crucial production area for sandstone-type uranium deposits in China. The groundwater flow system in the basin plays a significant role in the formation of uranium deposits. To study the hydrogeochemical characteristics and enrichment regularities of groundwater uranium in the Erlian Basin, 269 groundwater samples were collected. Hydrogeochemical analysis methods, such as Piper diagrams, Gibbs diagrams, and contour maps, were employed to determine the distribution characteristics and occurrence forms of groundwater uranium in the study area. The primary water chemistry types in the study area were HCO3–Ca•Na, HCO3–Na, HCO3•SO4–Na, and SO4•HCO3–Na•Ca. The distribution range of uranium content in groundwater in the study area was 0.1–453 μg/L (average of 53.08 μg/L). A comprehensive analysis, based on the direction of groundwater flow and changes in the redox environment, suggested that the higher uranium values in groundwater were distributed in the runoff and discharge areas, with uranium anomaly points existing in areas with alternating oxidation and reduction zones. These uranium anomaly points were in good agreement with the known large uranium deposits of Nuheting, Qiharigetu, Daoersu, and others, thereby predicting six other uranium hydrochemical anomaly points as prospective areas for uranium mineralization. The pH value distribution range in the study area's groundwater was 6.8–9.1, and the redox potential (Eh) value range was -126–52 mV, indicating that uranium exists in groundwater in the form of UO2(CO3)34− and UO2(CO3)22−. An increase in bicarbonate (HCO3−) is conducive to uranium dissolution. The enrichment and occurrence forms of uranium in groundwater are influenced by the concentrations of Fe and (Ca2++Mg2+), with uranium precipitation and enrichment occurring as the groundwater evolves and changes in the redox environment. The uranium content in deep wells is higher than in nearby shallow wells, indicating that deep water circulation is beneficial for mineralization. The pH, Eh, HCO3−, Fe, and other hydrogeochemical indicators are indicative of uranium enrichment; thus, they can be considered as reference bases when exploring for potential uranium resources.

A Supramolecular Organic Framework‐Mediated Electrochemical Strategy Achieves Highly Selective and Continuous Uranium Extraction

AbstractThe integration of selectivity and electron transfer ability remains a primary challenge in developing electrode materials for uranium electroextraction. Herein, a phenanthroline‐based supramolecular organic framework (MPSOF) is elaborately constructed as a pioneering cathode material through the hydrogen bond‐driven self‐assembly of melamine and 1,10‐phenanthroline 2,9‐dicarboxylic acid (PDA) for selective and continuous electrochemical uranium extraction (EUE). PDA moieties selectively capture UO22+, while the hydrogen bond‐supporting frameworks provide an efficient electron transfer channel for the redox of UO22+. These structural features enable the rapid formation and spontaneous shedding of uranium precipitate from MPSOF, allowing for the regeneration of the selective adsorption sites. As a result, MPSOF‐mediated EUE exhibits a high extraction capacity of 7311 mg U g−1 at a low voltage of −3.5 V but does not reach equilibrium. Cyclic EUE is employed to uranium extraction from simulated high‐salt radioactive effluents and attains high selectivity for uranium. The electroextraction mechanism is confirmed, wherein uranium species transform into (UO2)O2·4H2O. This work not only provides an efficient electrode material for uranium electroextraction, but also presents a novel electrochemical strategy for separation and adsorption of other radionuclides and contaminant ions.

Recovery of uranium from phosphate ore of Iran mine: Part II- solvent extraction of uranium from wet-process phosphoric acid

The current study in laboratory scale was aimed at developing an optimal two-stage extraction process for production of yellow cake from the phosphoric acid solution that was extracted from the Sheikh Habil phosphate rocks in central Iran. To attain an optimized extraction process, different mixtures of D2EHPA + TOPO + TBP were examined. A total extraction cycle including extraction, stripping and yellow cake precipitation was run to realistically evaluate the performances of the selected mixtures-conditions. As for optimization of the operational conditions, we exploited central composite design (CCD) method, that is a branch of the surface response methodology (RSM), and analyzed the results with standard statistical methods. The O/A (organic to aqueous ratio) = 2, 11 min extraction time, 5 wt% TOPO concentration, 10–15 wt% TBP concentration, 20 wt% D2EHPA concentration, 800 mv oxidation-reduction potential, and 25 °C temperature were obtained as optimum extraction parameters. The O/A = 15, 11 min stripping time, 50 wt% phosphoric acid concentration, 250 mv oxidation-reduction potential, and 40 °C temperature were obtained as optimum stripping parameters. Finally on the last uranium extraction from the H3PO4 strippant on the optimum conditions, the concentration of uranium was more than 4500 ppm with extraction efficiency near to 90 %. Scrubbing of organic phase was done by 2 wt% of sulfuric acid solution at 40 °C and precipitation of ammonium uranyl carbonate (AUC, (NH4)4UO2(CO3)3) was done with 2 M ammonium carbonate solution at an O/A ratio of 1/2 with efficiency more than 95 % in the uranium precipitation process.

Geology and geochronology of the Yingqian granite-type uranium deposit in the Taoshan-Zhuguang Belt, South China

The Yingqian uranium deposit in the Taoshan-Zhuguang belt, South China is a typical granite-type uranium deposit. The geological characteristics and timing of mineralization are not clear, which hinders the understanding of the ore genesis of the Yingqian deposit as well as regional uranium metallogeny in the Taoshan-Zhuguang belt. These scientific problems are examined in this study through detailed field work and petrographic study, as well as geochemical and geochronological analyses on apatite, rutile and zircon. The orebodies of the Yingqian uranium deposit are mostly present in the medium–coarse grained biotite granite and fine grained two-mica granite, and controlled by the NE-trending faults. The principle U-bearing minerals are pitchblende and brannerite, with the presence of metal minerals like pyrite, hematite and minor galena and sphalerite. The predominant hydrothermal alterations are hematitization, fluoritization, illitization, silicification, chloritization and carbonatization. Gangue minerals are mainly quartz, fluorite, illite and calcite. Moreover, four hydrothermal stages can be recognized in the Yingqian deposit, and the second and third stages are the uraniferous ones. The alterations and mineral assemblages indicate ore fluids are acidic, oxidized, and rich in F, P, CO2, which is favorable for dissolving uranium. Abundant apatite and rutile are discovered in the uraniferous third-stage veins. Geochemical analyses indicate that rutile is plotted in the hydrothermal area in the Ti-100 (Fe + Cr + V)-1000 (W) ternary diagram, and apatite is classified as fluorapatite. In-situ LA-ICP-MS dating indicates that the ore-hosting medium–coarse grained biotite granite and fine grained two-mica granite obtain the zircon U-Pb ages of 160.0 ± 0.3 Ma and 164.1 ± 0.6 Ma, which are consistent with many Jurassic granites in the South China Block. The hydrothermal apatite and rutile yield the U-Pb ages of 96.4 ± 11.6 Ma and 85.6 ± 10.8 Ma, respectively, indicating that uranium mineralization is significantly younger than the ore-hosting granites and generally coeval with the regional mafic magmatism. Consequently, the uraniferous fluids in the Yingqian deposit are independent from the magmatic system. Combined with previous research, it is concluded that fluid convection promoted by mantle–crust interaction would leach uranium from various lithologies, and fluid-rock interactions, fluid boiling and cooling are the causes for uranium precipitation.

Research on the Formation Conditions and Preventive Measures of Uranium Precipitates during the Service Process of Medical Isotope Production Reactors.

This study focuses on the Medical Isotope Production Reactor (MIPR), an aqueous homogeneous reactor utilized for synthesizing medical isotopes like 99Mo. A pivotal aspect of MIPR's functionality involves the fuel solution's complex chemical interactions, particularly during reactor operation. These interactions result in the formation of precipitates, notably water filamentous uranium ore and columnar uranium ore, which can impact reactor performance. The research presented here delves into the reactions between liquid fuel uranyl nitrate and key radiolytic products, employing simulation calculations complemented by experimental validation. This approach facilitates the identification of uranium precipitate types and their formation conditions under operational reactor settings. Additionally, the article explores strategies to mitigate the formation of specific uranium precipitates, thereby contributing to the efficient and stable operation of MIPR.

Uranium biomineralization by immobilized Chryseobacterium sp. strain PMSZPI cells for efficient uranium removal

Uranium (U) contamination is hazardous to human health and the environment owing to its radiotoxicity and chemical toxicity and needs immediate attention. In this study, the immobilized biomass of Chryseobacterium sp. strain PMSZPI isolated from U enriched site, was investigated for U(VI) biomineralization in batch and column set-up. Under batch mode, the fresh or lyophilized cells successfully entrapped in calcium alginate beads demonstrated effectual U precipitation under acid and alkaline conditions. The maximum removal was detected at pH 7 wherein ∼98–99% of uranium was precipitated from 1 mM uranyl carbonate solution loading ∼350 mg U/g of biomass within 24 h in the presence of organic phosphate substrate. The resulting uranyl phosphate precipitates within immobilized biomass loaded beads were observed by SEM-EDX and TEM while the formation of U biomineral was confirmed by FTIR and XRD. Retention of phosphatase activity without any loss of uranium precipitation ability was observed for alginate beads with lyophilized biomass stored for 90 d at 4 °C. Continuous flow through experiment with PMSZPI biomass immobilized in polyacrylamide gel exhibited U loading of 0.8 g U/g of biomass at pH 7 using 1 l of 1 mM uranyl solution. This investigation established the feasibility for the application of immobilized PMSZPI biomass for field studies. Environmental implicationUranium contamination is currently a serious environmental concern owing to anthropogenic activities and needs immediate attention. We have developed here a biotechnological method for successful uranium removal using immobilized cells of a uranium tolerant environmental bacterium, Chryseobacterium sp. strain PMSZPI isolated from U ore deposit via phosphatase enzyme mediated uranium precipitation. The ability of immobilized PMSZPI cells to precipitate U(VI) as long-term stable U phosphates under environmental conditions relevant for contaminated waters containing high concentrations of U that exerts toxicity for biological systems is explored here. The long term stability of the immobilized biomass without compromising its U removal capacity shows the relevance of the bioremediation strategy for uranium contamination proposed in this work.

Reductive hydrothermal conversion of uranyl oxalates into UO2+x monitored by in situ XANES analyses.

Hydrothermal conversion of actinide oxalates has recently gained attention as an innovative fabrication route for nuclear fuels but has remained mainly limited to tetra- or tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalate ions into UO2+x oxides under mild hydrothermal conditions (T = 250 °C). A multi-parametric study first led to specifying the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with pH = 0.8; R = noxalate/nU = 3, and t = 72 hours. Particularly, pH was evidenced as a key parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO2+x was then investigated thanks to an in situ XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or their decomposition products in solution, following first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO2+x as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution.

Solids, colloids, and the hydrolysis of tetravalent uranium in chloride media

Understanding of the properties of dissolution and precipitation of Uranium under reducing geochemical conditions is important in radioactive waste management and assessments of natural uranium deposits. The mechanism of forming UO2+y from U(VI) and U(IV) containing aqueous solution (1 M NaCl) and the solubilities of the precipitates were studied under well-controlled reducing conditions as a function of pH, particle size, and supersaturation. The results show that tetramer and colloid formation are critical initial steps. Precipitation is not growth-controlled but appears to be nucleation-controlled, with critical nuclei dimensions of one unit cell of UO2. The precipitates were always crystalline, and amorphous UO2 was not observed.

Uranium photo-precipitation coupled with fulvic acid oxidation under anoxic and oxic conditions

Uranium (U) is of vital importance for the nuclear power industry. Its mining, processing and disposal impose significant risks to the environment. Precipitation and fate of uranium in water bodies are of interest for environmental protection. In this work, we propose that fulvic acid (FA), a natural organic matter, can photo-precipitate ∼ 93.2% U under anoxic conditions within 3 h and ∼ 88.3% under oxic conditions within 48 h, respectively. Based on HRTEM, SAED and XPS analyses of the anoxic precipitate, U2O5 and UO2(OH)2 nanocrystals precipitate with FA and its decomposition intermediates. XPS analysis of C1s of the anoxic precipitate further reveals that FA is partly oxidized and has lost its conjugated benzoic ring and aldehyde groups, forming more carboxylic groups. On the other hand, XRD and XPS analyses on the oxic precipitate indicates the formation of UIV(C2O4)2·2H2O and UVIO3·0.75CO2·0.68H2O, demonstrating much deeper oxidation of FA under oxic conditions. In contrast to numerous previous studies focusing on heterogeneous photocatalytic reduction of U using solid catalyst, the uranium removal technique here spared the cost of catalyst synthesis, the necessity of sacrificer for uranium photo-reduction and U retrieval from the catalyst’s surface. The results also shine a light on new pathways for uranium fates in uranium-bearing water bodies such as rivers and lakes.

Reaction of Oxygen with Uranium (IV) Chloride in Fused Alkali Chlorides

Reaction of oxygen with solutions of uranium(IV) chloride in fused LiCl and three alkali chloride eutectic mixtures (LiCl–KCl, NaCl–KCl–CsCl, NaCl–CsCl) was investigated at 550–750 °C. Bubbling oxygen or oxygen-containing gas mixtures (O2–H2O, O2–Ar, O2–H2O–Ar) through LiCl–UCl4 melts resulted in significant precipitation of uranium (up to 87%) in the form of oxides and alkali uranates. Increasing mean radius of the solvent melt cations decreased the degree of uranium precipitation and uranyl chloride (soluble in the melt) became the main product of the reaction. High temperature spectroscopy measurements were employed to determine the kinetic parameters of the reaction in LiCl–KCl, NaCl–KCl–CsCl and NaCl–CsCl melts. Reaction rates, order, rate constants and activation energy values were estimated. Increasing temperature led to increased reaction rates but the effect of uranium chloride concentration depended on the cationic melt composition. Oxygen reacts with uranium(IV) containing melts much faster than with the melts containing rare earth chlorides and oxygen sparging can be implemented for separating uranium and rare earth fission products in pyrochemical reprocessing of spent nuclear fuels.

Steady state population balance modelling of precipitation processes: Nucleation, growth and size-dependent agglomeration

In this work a numerical methodology to solve the steady state Population Balance Equation (PBE) is developed. Three crystallisation mechanisms are included, namely: nucleation, size-independent growth and size-dependent loose agglomeration. The numerical method is based on the discretisation of the crystal size as distributed variable. In order to describe the loose agglomeration, the numerical methodology solves two PBE: one including the nucleation and growth mechanisms and one accounting for the agglomeration process. From the first PBE, liquid phase composition, supersaturation, developed crystal surface and Crystallite Size Distribution (CSD) are obtained. Similarly, the second PBE leads to the Agglomerate Size Distribution (ASD). The study of the size-dependant agglomeration kernel induces an additional numerical difficulty due to the dependency of both PBE and agglomeration kernel on the particle size. An accelerated fixed point algorithm based on the crossed secant method is adapted to overcome the difficulty and accurately solve the agglomeration PBE. The oxalic precipitation of uranium is simulated using this numerical methodology. First, the experimental results of a reference case are compared with the numerical predictions in terms of particle size distribution, mean size, mass fraction and moments. Then, the operating conditions are varied in order to test the algorithm robustness and performances. In all cases, the crossed secant method ensures the size-dependent agglomeration PBE solution and properly predicts the ASD. The developed numerical methodology predicts the mean particle size under the experimental uncertainty in a reasonable computation time and number of iterations.

The role of uranyl complex decomposition and redox conditions in the precipitation of hydrothermal uranium deposits: Insights from chlorite mineralogy and geochemistry in the Shazhou uranium deposit, Xiangshan, SE China

Uranium precipitation involves the decomposition of uranyl complexes and the reduction of hexavalent uranium, which can occur sequentially or simultaneously within one redox reaction. The redox condition of hydrothermal fluid plays a vital role in controlling the migration and precipitation of uranium in hydrothermal uranium deposits. However, little attention has been paid to the role of uranyl complex decomposition in uranium precipitation. In this study, chlorite mineralogy and geochemistry were examined to clarify the process of uranium precipitation in the Shazhou deposit, Xiangshan uranium orefield, Southeast China. Based on comprehensive petrographic and mineral chemistry studies of chlorites obtained from altered granite porphyries and uranium ores, five types of chlorites were identified: (1) chlorite present in the form of pseudomorphous biotite, which was produced by the hydrothermal alteration of biotite in rocks that underwent hematitization and chloritization (Chl-I); (2) chlorite filling the cleavage cracks in biotite in rocks that underwent hematitization and chloritization (Chl-II); (3) chlorite occurring in pyrite veins (Chl-III); (4) chlorite intergrown with pitchblende in ore veins (Chl-IV); and (5) chlorite occurring in calcite veins (Chl-V). Chlorite geothermometry revealed that the formation temperatures of the five types of chlorites ranged from 219 °C to 254 °C. Mineral chemistry analyses revealed that the five types of chlorites formed in a reductive fluid environment, where the oxygen fugacity at different stages is similar, with log fO2 values ranging from −41.6 to −39.1. Uranium precipitation started only in stage Chl-IV. The examination of the mineral assemblage revealed that the ore-forming fluid was rich in F−, HPO32−, and CO32−. Comprehensive investigation of chemistry and physicochemical conditions of the ore-forming fluid revealed that oxidized uranium (UO22+) could be complexed with HPO42− and F−, and uranyl phosphate and the uranyl fluoride complexes were the main uranium species when uranium precipitation and the decomposition occurred at stage Chl-IV. However, the assessment of oxygen fugacity of the solution equilibria between the UO2(s) and the uranyl phosphate and uranyl fluoride complexes revealed that the reducibility of the fluid favoring the reduction of uranyl ions (UO22+) to U4+ was insufficient to reduce the uranyl phosphate and uranyl fluoride complexes. This indicates that the breakup of uranyl phosphate and the uranyl fluoride complexes to release uranyl ions should occur first at stage Chl-IV, reducing uranyl ions to U4+, and leading to uranium precipitation. Hence, the decomposition of uranyl complexes played an important role in uranium precipitation. With the increase in pH, uranyl phosphate and the uranyl fluoride complexes gradually decomposed and became reducible. Moreover, the decrease in ligand concentration was conducive to the decomposition of uranyl phosphate and the uranyl fluoride complexes. During the formation of the Shazhou deposit, the fluid boiling process induced the loss of volatiles, such as CO2, CH4, HF, and H2S, leading to an increase in pH and decrease in HPO42−, F−, and CO32− concentrations in the fluid. These factors also led to the decomposition of uranyl fluoride and the uranyl phosphate complexes and the precipitation of fluorapatite and calcite. Uranium was then reduced by the action of Fe2+ and S− (pyrite) from a hexavalent to a tetravalent, and finally, uranium was precipitated to form uranium ores.

Enrichment mechanism of REE in rhyolite and its relationship with uranium mineralization in the Dazhou uranium ore-field, Southeast China

The Dazhou rhyolites in the Gan-Hang Belt host uranium deposits and show high rare earth element (REE) concentrations. However, the occurrence, source, and enrichment process of REE and its association with uranium mineralization in rhyolite remain unclear. In this study, fresh and altered rhyolites were selected, and the petrography, elemental and isotope geochemistry were investigated to explore the genetic mechanism of REE-rich rhyolites and its relationship with uranium mineralization. Based on the alteration and mineral paragenesis, the Dazhou rhyolites can be classified into four types: fresh rhyolite (Fr-rhyolite), hematitic rhyolite (Hem-rhyolite), clayified rhyolite (Cla-rhyolite), and hydromica rhyolite (Hyd-rhyolite). The altered rhyolites share similar average εNd(t) values with the fresh rhyolites (Fr-rhyolites = −5.8, Hem-rhyolites = −6.1, Hyd-rhyolites = −6.2), indicating their same REE sources, that means there are no external REE input for the REE-enriched altered rhyolites. In the Fr-rhyolites, the REE contents (average 341 ppm) and high field strength element (HFSE) contents increase with a decrease in the Al2O3/TiO2 ratio, indicating that magmatic crystallization differentiation leads to the initial enrichment of both REE and HFSE. In the altered rhyolites, the REE concentrations (average 655 ppm, 1045 ppm, 999 ppm for Hem-rhyolites, Cla-rhyolites, Hyd-rhyolites, respectively) are elevated compared to those in the Fr-rhyolites, indicating a significant hydrothermal enrichment for the REE. Mineralogical and petrographic studies have shown that REE are mainly hosted in parisite and monazite; therefore, the ore-forming fluid should be enriched in CO32– and F−. During the hydrothermal alteration, hematitization occurred early, followed by clayization and hydromicazation in the late stage, recording a fluid property change from an early higher-temperature, oxidizing fluid to a late lower-temperature, more reducing and acidic fluid. The early-stage fluid was responsible for the REE migration, while the decrease of temperature led to a decrease in the stability of the REE-complex that promote the precipitation of REE minerals. Combined with previous studies, REE mineralization has similar mineralization age and alteration characteristics to uranium mineralization in the Dazhou uranium ore-field, indicating the two mineralization types have derived from the same source and activated, migrated, and precipitated under the same mineralization event. The factors that controlled the precipitation of REE and uranium ore minerals are different, with the former mainly controlled by temperature, whereas the latter controlled by redox conditions.

Magnetite‐fluorapatite geochemistry and monazite U–Pb geochronology of the Mohuldih uranium deposit, Singhbhum Shear Zone, eastern India

In this study, textural relations coupled with mineral chemistry of hydrothermal magnetite, fluorapatite, monazite and allanite from the Mohuldih uranium deposit in the Singhbhum Shear Zone (SSZ) is used to characterize the nature of the fluid during various events of alteration and uranium precipitation. These minerals have two distinct textural types, the earlier of which are coeval with the uraninite mineralization. The later type formed during subsequent hydrothermal overprint as a result of coupled dissolution‐reprecipitation of earlier fluorapatite and mobilization of light rare earth elements. Uranium‐lead dating of texturally‐constrained hydrothermal monazite grains yields two major concordant age clusters at 1855 ± 7 Ma and 963 ± 10 Ma and several discordant analyses. Two spot analyses furnish concordant ages of 1656 ± 33 Ma and 1438 ± 35 Ma that are identical within error of the 207Pb/206Pb ages (1628–1643 Ma and ca. 1392 Ma) of several discordant data points. Similar ages have been reported by other studies from the rocks of the SSZ and are therefore considered to be geologically meaningful. The oldest age of ca. 1855 Ma obtained from the texturally early core regions of monazite corresponds to the earliest stage of uraninite mineralization. The two main alteration types can be interpreted based on the known metamorphic events in the SSZ. A combination of high‐temperature calcic iron ± sodic and high‐temperature potassic iron alteration accompanied the M1 metamorphic event and the low‐temperature silicification/K‐Al alteration during the M2 metamorphic event. A comparative study between our data on magnetite and fluorapatite compositions with those from global iron oxide‐Cu‐gold (IOCG) and iron oxide‐apatite deposits classifies Mohuldih as an IOCG‐type deposit.