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To monitor the reaction between Ce4+ ion and Cl– ion at the electron level, an electrochemical experiment was designed in this work. Herein, the intermediate and final products that may be produced during the redox reaction are directly tracked by using cyclic voltammetry, and the influences of Ce4+ ion concentration, temperature and F– ion on the reduction peak potential of Ce4+ ion were investigated. The results show that Ce4+ ion reacts with Cl– ion through an irreversible reaction without any intermediate products, and the rate-determining step of the reaction is diffusion during the electrode reaction. The effects of temperature (20–40 °C) and Ce4+ ion concentration (0.04–0.12 mol/L) on the reduction peak potential of Ce4+ ion can be ignored, but the higher the molar ratio of F– to Ce4+ (0–3 mol/mol), the more easily the reduction of Ce4+ ion to Ce3+ ion occurs. Additionally, the Ce4+ ions are preferentially reduced by thiourea when thiourea is added in the HCl solution, and thiourea inhibits the oxidation of Cl– ions to Cl2 by forming a complex with Cl– ions. This work provides a theoretical basis for the role of thiourea in inhibiting Cl2 production and offers a new way to find new reductants.

Ceria-zirconia mixed oxides (CZMO) is widely used in many important catalysis fields. However, pure CZMO is known to have poor thermal stability. In this paper, a strategy was proposed to design high thermal stability Ce0.475Zr0.475M0.05O2 (M=La, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Er, Lu, and, Yb) oxide surface by using first-principles molecular dynamics (FPMD) simulation and experiment method. Through the structure stability analysis at different temperatures, the surface energy γ as a function of Rion/Dave is identified as a quantitative structure descriptor for analyzing the doping effect of rare earth (RE) elements on the thermal stability of Ce0.475Zr0.475M0.05O2. By doping the suitable RE, the γ can be adjusted to the optimal range to enhance the thermal stability of Ce0.475Zr0.475M0.05O2. With this strategy, it can be predicted that the sequence of thermal stability improvement is Y > La > Gd > Nd > Pr > Pm > Sm > Eu > Tb > Er > Yb > Lu, which was further verified by our experiment results. After thermal treatment at 1100 °C for 10 h, the specific surface area (SSA) of aged Y-CZ and La-CZ samples can reach 21.34 m2/g and 19.51 m2/g, which is 63.02% and 49.04% higher than the CZMO sample without doping because the surface doping of Y and La is in favor of inhibiting the surface atoms thermal displacement. In a word, the strategy proposed in this work can be expected to provide a viable way for designing the highly efficient CZMO materials in extensive applications and promoting the usages of the high-abundance rare-earth elements Y and La.

Based on the new process named “Combination Method” for metallurgy and separation of Baotou mixed rare earth concentrate (BMREC), the aim of this paper is to clearly elucidate the phase change behavior of BMREC without additives during oxidative roasting at 450–800 °C. The results indicate that the bastnaesite in BMREC is decomposed at 450–550 °C, the weight loss is about 10.3 wt%, and the activation energy (E) is 144 kJ/mol. The bastnaesite in BMREC is decomposed into rare earth fluoride, rare earth oxides (La2O3, Ce7O12, Pr6O11 and Nd2O3), and CO2, particularly, with the increase of roasting temperature, bastnaesite in BMREC is more completely decomposed into LaF3, which causes a decrease in leaching rate of La during the HCl leaching process. Additionally, the maximum cerium oxidation efficiency reaches about 60 wt% when the roasting temperature is equal to or above 500 °C, and the oxidation reaction rate of cerium increases with the increasing roasting temperature.

In order to clarify the solvent extraction and separation behaviors of rare earths and impurity of Al during the extraction and enrichment of low-concentration leach solution of ion-adsorption rare earth ore, the extraction mechanism and separation behaviors of Nd3+ and Al3+ in the Nd2(SO4)3–Al2(SO4)3 mixed solution using P507 were studied in this work. The extraction of Nd3+ and Al3+ follows the cation exchange mechanism. With the increase of the equilibrium pH, βNd/Al in the extraction of the Nd2(SO4)3–Al2(SO4)3 mixed solution using P507 is always higher than that in the extraction of single Nd2(SO4)3 and Al2(SO4)3 solutions. It can be attributed to the fact that the extraction of Nd3+ using P507 is much faster than that of Al3+, and Al3+ is more prone to be hydrolyzed at lower pH. βNd/Al in the extraction of the Nd2(SO4)3–Al2(SO4)3 mixed solution decreases gradually with the increase of mixing time within the equilibrium pH range of 1.5–1.9. The extraction of Nd3+ using P507 is much faster than that of Al3+, but the stability of Al3+-loaded organic phase is better than that of Nd3+-loaded organic phase, thus Nd3+ in the Nd3+-loaded organic phase is gradually replaced by Al3+ in the aqueous phase with the increase of mixing time.

As the oxygen redox ability shows great effects on the catalytic performances of ceria-zirconia based materials, many strategies have been utilized to improve the oxygen storage capacity. Here in this study, we report a simple and facile approach to prepare a SO42–-modified La, Y-doped ceria-zirconia material (SO/CZLY-f) with high oxygen storage capacity. Due to the additional redox process between SO42− and S2−, oxygen storage capacity of SO/CZLY-f (745.3 μmol O2/g) is about 1.6 times higher than that of La, Y-doped ceria-zirconia material without SO42– modification. Moreover, the catalytic activities and stability of the corresponding Pd-only three-way catalyst were measured. Compared to that of Pd@CZLY-f, the operation window of CO, full conversion temperature of HC and NO over Pd@SO/CZLY-f are obviously widened and lowered, respectively. After aging treatment at 1100 °C for 4 h, the superiority of aged Pd-loading composite is still maintained.

The precipitation of the water-leaching solution of Baotou mixed rare earth (RE) concentrate roasted with sulfuric acid using ammonium bicarbonate for producing RE carbonate produces a mass of ammonia-nitrogen wastewater because of the relatively low solubility of rare earth sulfate. To solve the serious problem of ammonia-nitrogen pollution, new precipitators need to be developed urgently so as to meet the requirements of environmental protection and impurities content of the product (SO42−

The filtration efficiency deterioration over operating time due to exposure to particles, organic solvents and other factors is an obvious defect of electret filter, especially during the coronavirus pandemic. Electrostatic enhanced method has been proposed as a promising way to improve the filtration efficiency and service time of electret filters without increasing the pressure drop. In this work, the effect of discharge electrode structure and operation mode of the electrostatic enhanced structure(EES) on the filtration efficiency of commercial pleated electret filters were studied firstly, then the EES was installed in a special designed prototype air purifier, two key indicators of air purifiers(clean air delivery rate (CADR) and cumulate clean mass (CCM)) were tested to evaluate the actual performance improvement of electret filters by the electrostatic enhanced method. It was found that the discharge electrode structure had significant influence on the filtration efficiency and multi-wire array electrode was more suitable for the discharge electrode to pleated filter to ground mesh structure used in this paper. The decayed electrostatic charges of pleated electret filter cannot be recharged again through the EES in actual operation condition. The filtration efficiency improvement of the test pleated electret filters was dominantly contributed by particle charging. The filter polarizing induced by the external electric field was helpful in increasing the filtration efficiency when the particles were charged while its effect on uncharged particles was almost negligible. Besides, the actual performance of the prototype air purifier indicated that the EES can alleviate the filtration efficiency deterioration of test electret filters and extend the service life of H11 and H13 filters by more than 3 and 1.5 times respectively according to the CCM test results.

The leaching behaviors of calcium and magnesium in the rare earth tailings leached with magnesium sulfate using deionized water, CaCl2 solution and lime water were investigated. Experimental data indicated that magnesium in the tailings was easy to be leached out since most of the magnesium was in the form of water-soluble phase. Most of calcium in the lime water was electrostatically adsorbed on the clay mineral of the tailings, and the water-soluble magnesium was also gradually converted into exchangeable phase because of back-adsorption of Mg2+ on the clay mineral with increasing the pH values. When the liquid-to-solid ratio was 0.80, the contents of readily-available magnesium and calcium were 104.4−207.6 and 201.7−1426.3 mg/kg, respectively, which could meet the requirements for plants. These results suggest a promising route for the environmental remediation of ion-adsorption rare earth ore after in-situ leaching.

Numerous light rare earth elements (LREE) minerals containing Fe and P were processed by sulfuric acid roasting method, and the leaching solution mainly comprises LREE sulfate, Fe2(SO4)3, H3PO4, and H2SO4, however, the solubility data of LREE sulfates in this system is few. This work studies the solubility of LREE sulfates in independent LREE sulfate system RE2(SO4)3-Fe2(SO4)3-H3PO4-H2SO4 (RE = La, Ce, Pr or Nd) and mixed LREE sulfates system (La,Ce,Pr,Nd)2(SO4)3-Fe2(SO4)3-H3PO4-H2SO4 at different temperature (25–65 °C) and concentrations of Fe2(SO4)3 (Fe2O3, 0–50.13 g/L), H2SO4 (0.5 mol/L), and H3PO4 (P2O5, 20.34 g/L) based on the industrial operating condition at low liquid and solid ratio 2:1. The solubility of each LREE sulfate in the independent system (La2O3, 12.25–20.88 g/L; CeO2, 41.93–62.35 g/L; Pr6O11, 37.34–56.69 g/L; Nd2O3, 26.60–37.63 g/L) is much higher than that of the mixed system (La2O3, 6.95–11.03 g/L; CeO2, 10.63–21.51 g/L; Pr6O11, 11.56–20.36 g/L; Nd2O3, 12.36–19.79 g/L) under the same other conditions. The results also indicate that, in the two systems, both Fe and the temperature have negative effects on the solubility of LREE sulfates. That may occur due to the complication reactions between the complexes of RESO4+ and Fe(SO4)2–. However, the influence degree of temperature and iron concentration on the LREE sulfates solubility varies in the two systems and among different LREE species. This research is of theoretical significance for optimizing the conditions of the sulfuric acid process for recovering the LREE from the mixed LREE bearing minerals as well as the single LREE containing secondary rare earth scraps.

In acid treatment technology of Baotou mixed rare earth ore, large quantities of ammonia-nitrogen wastewater are produced in the step of ammonium bicarbonate precipitation to transform rare earth sulfate. In this paper, we adopted a green precipitant magnesium bicarbonate (Mg(HCO3)2) to substitute ammonium bicarbonate to eliminate ammonia-nitrogen pollution. The effects of n(HCO3−):n(RE3+), aging temperature and aging time on the crystallization using Mg(HCO3)2 precipitation method were investigated. The results indicate that the rare earths could be completely recovered when n(HCO3−):n(RE3+) is higher than 3.15:1. The crystal water content of rare earth carbonates is affected by the aging temperature. The precipitate has a bad filterability when the aging temperature is over 40 °C. This can be attributed to the less crystallized water molecules of the hydrated rare earth carbonate precipitation. The mixed rare earth carbonates are prone to be crystalline, and have a good filterability at aging temperatures below 40 °C. Meanwhile, the evolution mechanism of crystalline mixed rare earth carbonates is reasonably deduced, the amorphous rare earth carbonates are first dissolute and then recrystallized. Under the optimized aging conditions, the purity of the crystalline precipitate meets the requirements of the fine product standard (GB/T 16479-2008). The filtrated could be used to produce Mg(HCO3)2, thus to realize the recycling of magnesium sulfate.

Solid oxide fuel cell is a promising energy conversion system which converts chemical energy into electrical energy directly. Electrolyte is the key component and determines the working temperature. In this paper, ceria and scandia co-doped zirconia electrolytes sintered from 1300 to 1550 °C were chosen as research objects. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy were performed to characterize the ceramic samples. The effects of grain size and grain boundary element segregation on the electrical conductivity were focused. Electrochemical impedance spectroscopy was used to calculate the bulk, grain boundary and specific grain boundary conductivity. Results show that the bulk and grain boundary ionic conductivity increases with the increasing grain size. However, the specific grain boundary conductivity decreases with the increasing grain size. This is explained by the fact that Sc3+ is segregated at the grain boundary, which leads to higher oxygen vacancy concentration when sintered at lower temperature.

Phosphate rock has been considered as one of the potential promising resources for rare earth elements (REEs). But the cost issues and the technical challenges caused by the low content of REEs in ores did hinder the further development of REEs recovery technologies. In order to explore a green process for the recovery of REEs from phosphate rock, this study investigates the effects of phosphoric acid concentration, liquid-to-solid ratio (L/S ratio), leaching time and temperature on the leaching efficiencies of the major components from phosphate rock. A REEs recovery of 94.3% and a phosphorus recovery of 95.3% are achieved under the optimal conditions of attacking phosphate rock using 30%P2O5 acid with an L/S ratio of 10:1 and a stirring speed of 250 r/min at 25 °C for 4 h. Then, the selective precipitation of REEs with 81.3% REEs recovery is realized by heating up the leaching solution from 25 to 90 °C and keeping for 4 h. Thereafter, more than 95% phosphoric acid is recovered by H2SO4 and high purity gypsum, more than 95% CaSO4 (tested by XRF), is also produced at the same time. Ultimately, a green process that leaches phosphate rock with H3PO4, selectively precipitates REEs from leaching solution by heating up, recovers H3PO4 with H2SO4 is proposed. Compared with REE recovery in traditional processes, this process owns the merits of simple operation, energy saving and minimum wastes.

In order to achieve deeper understanding of rare earth elements (REEs) behaviors during phosphate rock processing with H3PO4. The solubility of REEs in Ca(H2PO4)2 H3PO4 solutions with various concentrations of Ca(H2PO4)2 at different temperatures were tested. The results demonstrate that REEs solubility decreases sharply with the increasing concentration of Ca(H2PO4)2. Equations between [REE3+] and [H+], [H+] and [Ca2+] in Ca(H2PO4)2 H3PO4 solutions were built based on the precipitation-dissolution equilibrium of rare earth phosphates and the ionization equilibrium of H3PO4. According to the equations, the decreasing mechanism of REEs solubility caused by elevated concentration of Ca(H2PO4)2 was determined. The mechanism can be illustrated as that the elevated concentration of [H2PO4–] decreases the concentration of hydrogen ion by retarding the ionization process of H3PO4 and directly promotes the precipitation of rare earth phosphates. Furthermore, it can be easy deduced that similar effect would be caused by the other cation impurities (Fe3+, Al3+, etc.) on REEs solubility based on the mechanism. In addition, superimposed reduction effect on REEs solubility caused by the elevated concentration of Ca(H2PO4)2 and the elevated temperature is found. This superimposed effect leads to a super low solubility of REEs in Ca(H2PO4)2 H3PO4 solution. On the basis of the experimental study, outlooks and suggestions for further development of REEs recovery method are given.

Rare earths (REs) are of vital importance for the development of new materials and green energy. Magnesium bicarbonate is one of the most recyclable and environmental-friendly precipitant for REs recovery from leaching solutions. Nd2(CO3)3 has difficulties in industrial production. So in this study, the precipitation of neodymium from chloride solution by magnesium bicarbonate are investigated. The effects of feeding method, [ HC O 3 − ]/[Nd3+] mole ratio, feeding speed and reaction temperature on yield and impurity (magnesia) content are systematically studied. Results show that the impurity (magnesia) content decreases to 0.010 wt% with a yield approaching to 100% obtained under the conditions of [ HC O 3 − ]/[Nd3+] = 3.00 by parallel flow addition at 50 °C. The major impurity (magnesia) in rare earth carbonates mainly presents in the form of physical absorption, which can be easily removed by scrubbing. Therefore, it offers a promising green process that uses magnesium bicarbonate to produce neodymium carbonate due to its cycling of carbon dioxide, magnesium salt and waste water.

Magnesium bicarbonate solution is considered as an environmentally friendly extractant saponification agent for the solvent extraction of rare earth elements due to its advantage of minimum water pollution. In order to reveal the extraction regularity, optimize production-process and guide the use of this new extraction system, the extraction of Nd(Ⅲ) in chloride medium with HEH/EHP saponified by magnesium bicarbonate solution was investigated with the self-designed constant interfacial area cell. Besides, the effects of stirring rate, temperature, specific interfacial area and concentration of Mg-HEH/EHP on the extraction rate of Nd(Ⅲ) were systematically investigated. Results show that, the rate of extraction is governed by both diffusion and chemical reaction, and the extraction reaction takes place at the interface. The apparent activation energy of the extraction reaction is 16.88 kJ/mol. The corresponding rate equation is deduced. The mechanisms and rate-determining step are speculated based on interfacial reaction models, which is consistent with the experimental results.

In order to provide practical fundamental data for rare-earth elements (REEs) recovery from phosphoric acid and to better understand REEs behavior during the phosphoric acid evaporation process, the solubilities of REEs in phosphoric acid with various concentrations of phosphorus at different temperatures were measured. A simple linear model between REEs solubility and phosphoric acid concentration is built and the experimental data are found to fit it very well (R2>0.94). Hydrogen-ion concentration is found to be the predominant factor controlling the solubility of REEs in phosphoric acid. In addition, the solubility of REEs in phosphoric acid is found to sharply decrease with increasing temperature, which can be attributed to the increase of the Gibbs energy of the REEPO4 dissolution reaction or the restraint of the disassociation of phosphoric acid molecules owing to the elevated temperature.

In order to improve the application values of Ce element, in this paper, rare earth chloride solution was used as raw material, the pH value was controlled by inorganic alkali, the ceria powders with special physical properties were prepared by carbon dioxide carbonization method. According to characterization of SEM, XRD, and TG-DSC, Ce(OH)3 prepared at pH = 7.5 exhibits smaller particle size than that prepared at other conditions. CeO2 precursor obtained by direct carbonization of Ce(OH)3 shows smaller particle size and narrow size distribution, CeO2 precursor forms at first by carbonization of Ce(OH)3 with the continuous addition of CO2 gas, and the chemical component is indicated to be Ce2O(CO3)2·6H2O. Cubic phase CeO2 powders are obtained by calcined at 750 °C for 4 h. The mean particle size D50 is 0.941 μm, and particle size distribution is smaller than 1. The microscopic appearance is homogeneous, with a spherical-like shape and a grain size of 200–500 nm. The light quality characteristics of sedimentation volume and accumulation density are obviously better than those of carbonate precipitation products. The carbonization method can be used not only to obtain ultra-fine rare earth oxides with fine particle size, narrow distribution and high dispersion properties, but also to achieve the reuse of carbon dioxide greenhouse gas.

In most cases, wet-process phosphoric acid (WPA) contains trace amount of rare earth elements and uranium, which ultimately enter and accumulate in soil and may cause environmental damages. In order to remove and recover these elements, studies about solvent extraction of heavy rare earth elements (HREEs) and uranium (U) with D2EHPA, selective stripping of HREEs from U, and evaporation precipitation of light rare earth elements (LREEs) were investigated. Extraction efficiency of U from WPA was greatly enhanced, but HREEs extraction were significantly retarded when using the solvent mixture of di- (2-ethylhexyl) phosphoric acid (D2EHPA) and tri-butyl-phosphate (TBP) or tri-octyl-phosphine oxide (TOPO) compared with D2EHPA alone. 89.4% and 94.2% accumulative two-stage extraction efficiencies of HREEs and U, respectively, were achieved with 1.0 M D2EHPA alone. And more than 98.1% HREEs were selectively stripped from loaded D2EHPA with 6.0 M HCl via three stages. In addition, LREEs were enriched to 1204.3 μg/g in evaporation sludge during the concentration process of phosphoric acid from 30% to 50% P2O5, and the filtration performance was greatly improved. Based on the studies, an alternative process was developed to recover and group LREEs, HREEs and U during the phosphoric acid production.

Rare earth elements (REEs) play a critical role in numerous high-tech applications owing to their unique magnetic, optical, and electrical properties. China is widely recognized by its abundant rare earth resources with the annual output of 100,000–150,000 tonnes, which accounts for over 90% of the world’s total production. Bastnaesite is one of the four rare earth minerals in China. With the rapid development of rare earth industry, resources and environmental issues have attracted more and more public attention around the world. The Chinese government, together with the Chinese researchers, have been devoted to establish a high standard by proactively addressing resources and environmental issues. This review gives an overview of the recent progresses in the environmentally friendly rare earth separation and industrial application of bastnaesite in China, including oxidation roasting-acid leaching process, sulfuric acid bake-water leaching process, caustic soda decomposition process, chlorination process, NH4Cl roasting technique, hyperlink extraction process, equilibrium acidity controlling technique, and some other improved processes. These progresses have exerted important impacts on the wolrd’s rare earth industry and futher pointed out the future directions for the technical development of the rare earth industry in China. Moreover, the recent policies on the cleaner production of rare earth industry issued by the Chinese government have been briefly discussed, which provides sufficient policy support for the generalization and application of the advanced cleaner production technologies in the rare earth industry.

This paper investigated the recovery of rare earth elements (REEs) and aluminum (Al) from the waste slag discharged by FCC catalyst factory (FCC waste slag) via acid leaching and selective precipitation. Analysis methods such as ICP-AES, XRF and XRD were applied to obtain experiment data. The maximum leaching efficiency of REEs and Al was achieved at pH value of 1 and with liquid to solid ratio of 4:1. Under such conditions, 91.01%, 92.24% and 94.77% of La, Ce and Al were extracted at 20 °C for 2 h from the FCC waste slag, respectively. The SiO2 content in the leaching residue was 88.3%, which can be used as an available silicon resource. The REEs can be precipitated in the form of REEs and sodium double sulfate (NaRE(SO4)2·xH2O) by adding Na2SO4 to the leaching solution, while Al remained in the solution. Afterwards, the pH value of the filtrate was adjusted to 4.5, and Al was precipitated as Al(OH)3. Finally, NaRE(SO4)2·xH2O and Al(OH)3 were converted into RECl3 and Al2(SO4)3 solution, respectively, which were recycled to manufacture zeolite. This process recovered REEs and Al from the FCC waste slag and reduced the emissions of waste slag simultaneously, which has an important economic and environment significance.

Weathered crust elution-deposited rare earth ore is an important resource of rare earths, including grantic weathered crust elution-deposited rare earth ore and volcanic weathered crust elution-deposited rare earth ore. The development condition of the weathered crust, weathering degree and mineral composition of these ores will be different because of the differences between their parent rocks and weathered crust causes of mineralogy path. Therefore, mineral properties and leaching characteristics of volcanic weathered crust elution-deposited rare earth ore from Chongzuo (CZ), Guangxi province were investigated. It was found that the CZ rare earth ore was a typical mid-yttrium and rich-europium ore, with the overall rare earth (REE) grade in ion-exchangeable phase of 0.15%. Partide size classification showed that finer particle had a higher REE grade. Column leaching tests showed that the leaching efficiency of REE was above 94% with leaching agent concentration of 0.20 mol/L, liquid-solid ratio of 4:3, flow rate of 0.60 mL/min, and initial pH value around 5.67. Compared to ammonium sulfate leaching, magnesium sulfate leaching was advantaged by nearly zero ammonia nitrogen emission while their REE leaching was almost equivalent.

During the industrial production of FCC catalyst, a large amount of FCC catalyst waste slag (FCC waste slag) containing rare earths and aluminum was generated. Landfill was commonly used to deal with the slag, which may cause serious environmental degradation and resources depletion. A novel hydrometallurgical process consisting of hydrochloric acid leaching and subsequent neutralization is proposed to recover rare earths and aluminum from the FCC waste slag. The effect of leaching temperature, hydrochloric acid concentration and leaching time on the metal extraction are studied. The leaching kinetics of aluminum and rare earths are also investigated. It is shown that 91.0% of La, 92.2% of Ce, and 94.2% of Al are extracted under optimum leaching conditions of hydrochloric acid concentration 9.00 mol/L, solid to liquid ratio (S/L ratio) 0.05 g/mL, leaching temperature 293 K and leaching time 30 min. The leaching behaviors of aluminum and rare earths follow a shrinking core model which can be expressed as 1 − 1 − x 1 3 = k ap t , and the overall leaching process is controlled by the surface chemical reaction. The activation energies for the leaching of Al, La, and Ce are 6.24, 9.71, and 9.10 kJ/mol, respectively. Zeolite Na-Y and FCC catalyst presented in the FCC waste slag is leached with HCl individually. Only the FCC catalyst has a negative effect on the leaching of rare earths and aluminum. Finally, selective precipitation is employed to produce Al(OH)3 and RE(OH)3, which can be reused for the industrial production of FCC catalyst.

Ion-adsorption rare earth element (REE) ores are strategic mineral resources, particularly for heavy REEs. To reduce the amount of nitrogen in the form of ammonia in waste waters and to increase the low recovery efficiency of the current extraction process for REEs, we developed a novel environmentally friendly stepwise solvent extraction method for the separation and enrichment of REEs. Based on their different extraction abilities and a cation-exchange mechanism, two acidic phosphorus extractants (HEH(EHP) and HDEHP) were selected to sequentially separate and enrich REEs. Using this novel technique, ammonia nitrogen emissions were completely avoided and the recovery efficiency of REEs was significantly improved. From the McCabe–Thiele plots, it was found that a three-stage HEH(EHP) extraction at VA/VO = 10/1 and two-stage HCl stripping at VA/VO = 1/16 were required to enrich heavy REEs, whereas a three-stage HDEHP extraction at VA/VO = 25/1 and three-stage HCl stripping at VA/VO = 1/20 were required to enrich light REEs. The total recovery efficiency realized for REEs was >99% and the heavy and light REE chloride concentrations in solution were up to 240 and 200 g L−1, respectively. These results suggest an efficient way of recovering REEs from a leaching solution with low concentrations.

To improve the electrical properties of an 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte, nanocrystalline (10–100 nm) 8YSZ powders were synthesized via a hydrothermal method followed by heat treatment at temperatures ranging from 800 °C to 1200 °C. Note that all the powders maintained the single-cubic phase structure, as observed via the results of the XRD and Raman spectra. Moreover, the annealing procedure of the powders caused the yttrium concentration of the surface to slightly increase, as observed via X-ray photoelectron spectroscopy (XPS). Moreover, according to the morphology shown in the SEM images and the electrochemical impedance spectroscopy (EIS), the powder grain size had an effect on the microstructure of the electrolytes fabricated via the conventional sintering method. In addition, it further influenced the relative density and conductivity of the electrolytes, which both first increased and then decreased with the increase in the powder grain size in the range of 10–100 nm. This showed that the powders with 50–80 nm grain size exhibited the best performance since it was not easy to induce grain coarsening in ceramics. The electrolyte sintered by these powders showed uniform and hexagonal grains with a mean size of 0.97 μm and a relative density of over 99%. Because of a large sum of grain boundaries and point defects, the electrolyte had a good conductivity of 174.1 mS cm−1 at 1000 °C measured by the four-probe DC method. In addition, the activation energy (0.955 eV) is lower than that for other samples; this suggests that 8 mol% Y2O3-stabilized ZrO2 has more advantages at lower working temperatures for SOFCs.

The practice of in-situ leaching of the ion-adsorption type rare earths ore with ammonium sulfate could only leach most of rare earth in ion-exchangeable phase, but not the colloidal sediment phase. Therefore, the reduction leaching of rare earth from the ion-adsorption type rare earths ore with ferrous sulfate was innovatively put forward. The soak leaching process and the column leaching process were investigated in the present study. It was determined that ion-exchangeable phase could be released, and part of colloidal sediment phase rare earth could be reduction leached by the cations with reduction properties. The mechanism of reduction leaching was discussed with the Eh-pH diagram of cerium. Moreover, the stronger reduction of reductive ions, the greater acidity of leaching agent solution, and the higher reductive ion concentration, could result in the higher rare earth efficiency and the bigger cerium partition in the leaching liquor. In the ferrous sulfate column leaching process, the rare earth leaching rate and the rare earth efficiency were a little higher than with (NH4)2SO4 agent, and the rare earth efficiency and the partitioning of cerium in leaching liquor could be about 102% and 5.31%, respectively. However, the ferrous sulfate leaching process revealed some problems, so compound leaching with magnesium sulfate and a small amount of ferrous sulfate was proposed to an excellent alternative leaching agent for further studies, which may realize efficiency extraction and be environment-friendly.

Oriented attachment and Ostwald ripening are two aging mechanisms of precipitation particles which may result in different crystallization mechanism of precipitates during the aging process. In this work, the effects of different aging process on the structure and properties of cerium zirconium mixed oxides were investigated. The results indicated that the mixed structure of 11.48% CeO 2 phase and 88.52% Ce 0.26 Zr 0.62 (LaPr) 0.12 O 2 solid solution phase were obtained under oriented attachment aging process. The rod-like CeO 2 phase coexisted with spherical Ce 0.26 Zr 0.62 (LaPr) 0.12 O 2 solid solution phase, which improved the surface area (64 m 2 /g) and pore volume (0.32 mL/g) of cerium zirconium mixed oxides after 1000 °C 4 h thermal treatment. However, through controlling the aging process, the Ce 0.35 Zr 0.55 (LaPr) 0.10 O 2 solid solution with homogenous phase structure was generated by Ostwald ripening aging process, exhibiting higher oxygen storage capacity (501 µmol O 2 /g) and H 2 consumption per gram (1378.3 µmol H 2 /g).

The adsorption behaviors of rare earth elements on clay minerals would have great influence on the mineralization process and the leaching process of the ion-adsorption type rare earths ore. In this work, the adsorption thermodynamics of REEs on kaolin were investigated thoroughly and systematically. The experimental results showed that the adsorption characteristics of La, Nd, Y on kaolin did fit well with the Langmuir isotherm model and their saturated adsorption capacities were 1.731, 1.587 and 0.971 mg/g, respectively. The free energy change (ΔG) values were −16.91 kJ/mol (La), −16.05 kJ/mol (Nd) and −15.58 kJ/mol (Y), respectively. The negative values of ΔG demonstrated that the adsorption of rare earth on kaolin was a spontaneously physisorption process. The deposit characteristic of the volcanic ion-adsorption type rare earths ore and the behavior of the rare earth in the column leaching process were also developed here. With the increase of the ore body depth, the distribution of the LREEs decreased and the HREEs increased. And the slight differences in the adsorption ability of REEs on clay minerals led to the fractionation effect in the column leaching process. These developed more evidences and better understanding of metallogenic regularity, and provided a theoretical basis and scientific approach to separation of the HREEs and LREEs in the leaching process.

Ultrafine powders of scandia-stabilized zirconia (ScSZ) were prepared by the co-precipitation method, using ZrOCl2 and ScO2 as raw materials and NH3·H2O as a precipitant. In this paper, the optimum process parameters were investigated. The pH of the reaction solution directly impacted the precursor structure, which further affected the obtained crystal forming. Many experiment methods of thermogravimetric analysis and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy (Raman), and nitrogen adsorption were employed to characterize the ScSZ powder. The structure transition mechanism from cubic to rhombohedral was discussed. In addition, the electrical conductivity of the powders was also studied after dry-pressing and calcining. The results showed that the structure of ScSZ with complete crystal surface belonged to the cubic phase. The crystallite sizes of the powders prepared are about 60-80 nm, meet the conditions of (D90-D10)/2D50≤1, and exhibited the good flow properties. The electrical conductivity was more than 190 mS/cm in air measured at 850 °C.

The largest rare earth (RE) reserves minerals of China is the bastnaesite (RE[CO3]F) and monazite ((RE, Th)[PO4]) mixed type RE ore in Bayan Obo, and 90% of this concentrate is processed by the sulfuric acid roasting method. However, during purification of RE sulfate leach solution with solid MgO, the RE loss is noticeable and accompanied by a mass of acid and alkali consumption and discharge of radioactive solid waste, which also causes environmental pollution. In this paper, the behavior of RE, Fe, and P during purification were investigated focusing on the effect of pH, concentration ratio of iron and phosphorus, RE concentration, and step-wise purification (based on different pH). The characterization of precipitated impurities by BET specific surface area, XRD, SEM and FTIR reveal the reaction sequence of RE loss and the formation of the precipitate. Two new factors, neglected previously, were found to be the main contributors to the production of large solid-waste:(1) sulfate which exists in the form of Fe(OH)x(SO4)(3-x)/2 accounts for 9 to 12% of the total weight of the precipitate; (2) higher solid content in the purification slurry at a relatively high pH range (pH = 2.5–4.5) significantly promotes the production of precipitate and higher loss of RE during purification. This paper proposes a two-step purification method (based on different middle pH point), step (i): a low pH purification stage in which most P, Fe(III) and H+ were removed with minimum loss of RE followed by filtration; step (ii), high pH purification stage in which the rest of Fe(III) was completely removed with less than 2% RE lost caused by low solid-to-liquid ratio consuming less MgO. Through two-step purification under the most suitable conditions, the RE recovery increased by 6% while MgO consumption and solid residue production decreased by 12% and 16%, respectively. It can greatly alleviate the environmental problems with just a minimal modification of the operation conditions and equipment in the proposed process.

The Baotou bastnaesite and monazite mixed rare earth ore is one of the most significant rare earth ore in the world and is mainly processed by the sulfuric acid roasting method. However, the high content of Fe and P in the leaching solution contributes to a large amount of radioactive precipitate waste produced in the purification process, and that causes the primary loss of rare-earth elements (REEs) and the environmental issue and safety concern. The goal of this paper is to reduce the solid-waste by improving the selective leaching efficiency of REEs based on a leaching kinetics study of REEs, Fe and P in the H2SO4 system. The experiments were carried out focusing on the effect of acidity, roasted concentrate particle size, temperature, and liquid-solid ratio on the leaching kinetics. The results indicate that the equilibrium leaching time of REEs (

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

Scandia stabilized zirconia powder (ScSZ) was first synthesized by a microwave-hydrothermal method. The crystalline and aggregated particle sizes were investigated by means of X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Weakly agglomerated and well crystallized ScSZ powder was prepared by microwave-heating to 150 °C and 2.5 h. The structure of the ScSZ powder changed from a tetragonal to a cubic phase, and accordingly, the powder conductivity was increased from 90.55 to 120.56 ms/cm by the introduction of the mineralizer solutions (KOH+K 2 CO 3 ) during the microwave-hydrothermal processing.

Granite belonged to intrusive rock and volcanic was extrusive rock. There may be many differences in their degree of weathering and mineral chemical composition. The present study investigated the minerals properties and the leaching mechanism of the granitic weathered crust elution-deposited rare earth ore from Longnan Rare Earth Mine area (LN ores) and volcanic weathered crust elution-deposited rare earth ore from Liutang Rare Earth Mine area (LT ores). The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) were used to characterize the phase of rare earth ores. The particle size distributions and main composition of the ore were also presented in this paper. The leaching mechanisms of two kinds of rare earth ores were analyzed with different kinetics models and could be described by the shrinking-core model. They were all inner diffusion-controlled leaching processes. The leaching equation of the kinetics of the LN ores could be expressed as: 1 - 2 3 η LN - ( 1 - η LN ) 2 / 3 = 0.106 r 0 - 0.377 e - 1.096 × 10 4 8.314 T t , leaching equation of kinetics of LT ores was 1 - 2 3 η LN - ( 1 - η LN ) 2 / 3 = 8.33 × 10 - 3 r 0 - 0.411 e - 4.640 × 10 3 8.314 T t . The rare earth leaching rate of LT ores was always lower in the same condition, and it would need more time and more (NH4)2SO4 consumption to achieve the same rare earth leaching efficiency, which would lead to more serious ammonia-nitrogen pollution. Therefore, magnesium salt was proposed as the leaching agent to eliminate ammonia-nitrogen pollution and further studies would be taken in the future.

The differences in the leaching and mass transfer characteristics of ion-adsorption type rare earth ore between rare earth and non-rare earth impurities were investigated. It is determined that the leaching kinetics of rare earths (REs), Mg and Ca could be described by the shrinking-core model. The leaching rate is controlled by inner diffusion and is in the order of Mg > Ca > RE. The apparent activation energy is 8.48, 6.79 and 6.32 kJ·mol−1, respectively. In leaching system of Al, the leaching efficiency decreases with the increase in temperature because of hydrolysis reactions. The leaching efficiency of aluminum increases firstly and then decreases with the passage of time when the temperature is higher than 35 °C. Besides, there is a good consistency in leaching characteristics and mass transfer, and the order of the time-to-peak is Mg < Ca < RE < Al. Therefore, rare earth and non-rare earth may be partly separated by expanding the separation coefficient. The research could provide a theoretic basis for the separation of rare earth and non-rare earth impurities. Furthermore, compound leaching with magnesium salt and calcium salt is innovatively proposed to solve the problem of calcium and magnesium nutrient loss in the soil as well as eliminate ammonia-nitrogen emissions from the source.

China is the global leader in rare earth (RE) production using hydrometallurgical processes. Advantageous extraction techniques of rare earths from Baotou mixed rare earth minerals, bastnaesite, and ion-adsorption clays of rare earth deposits have been developed in China. The separation and purification technologies have also achieved rapid development. The industrial application processes for rare earth hydrometallurgy were summarized in the present paper. With the large demands and rapid development of rare earths, the issues of resources and environment are more prominent. This review gives an overview of the main processes that were developed in the past toward greener hydrometallurgy of rare earths in China. Based on the development of the rare earth industry, comprehensive utilization and cleaner production should still be focused on in the future, to support the sustainable development.

Two-dimensional metal oxide nanostructures, such as nanoplates, nanowalls, and nanosheets, have received further attentions in recent years, due to their outstanding properties resulted from the small thickness and quantum size effects. In this work, lanthanum oxide nanosheets with the thickness of 56 nm had been successfully prepared by an originally simple method of carbon dioxide carbonation; the preparation process was green without using surfactants and detergent. The X-ray diffraction, transmission electron microscopy, and scanning electron microscopy were used to characterize the products. Thermogravimetric-differential thermal analysis and FT-IR were introduced to prove that the lanthanum oxide precursor with lamellar morphology was La2(CO3)3·3H2O. The carbon dioxide carbonation method would provide significant benefits containing mild reaction conditions, high efficiency, low cost, and easy to realize large-scale production.

Magnesium bicarbonate, prepared by the carbonation of magnesium hydroxide slurry, was used as a novel saponifier to eliminate the ammonia nitrogen pollution in the rare earth extraction separation process. The effect of impurity ions introduced by system on the carbonation reaction of magnesium hydroxide was studied in the work. The results showed that the presence of Ca2+ could lead to side reactions so as to reduce the conversion rate of magnesium hydroxide, and a small number of rare earth ions would have great influence on the carbonation reaction. What's more, there was no influence on carbonation reaction with the low concentration of Na+ or Mg2+, the conversion rate of magnesium hydroxide could reach above 96%. This paper showed a practical theory which could provide scientific guidance for the preparation of novel saponifier in rare earth extraction separation process.

The purpose of this study was to overcome the disadvantages of ammonia-nitrogen wastewater pollution and high cost of sodium saponification in rare earth separation process. The study focused on the non-saponification extraction technology with magnesia. The influences of the content and particle size of magnesia, reaction time, reaction temperature, and O/A on cerium extraction rate were also discussed. The results showed that the hydrogen ions of extractant were exchanged by rare earth ions when organic extractant and rare earth solution were mixed with magnesia powder, and then the exchanged hydrogen dissolved magnesia to make the acidity of the system stable. The magnesium ions were not participated in the extraction reaction. Non-saponification extraction process of rare earth had been realized. The cerium extraction rate could reach up to 99% in single stage within the optimal reaction conditions.

Novel precipitant prepared through carbonation with MgCl2 wastewater generated from rare earth extraction separation process and low-price dolomite as raw materials was studied in this paper. The purification methods of novel precipitant by adding appropriate oxidizing agent were studied. It was found that optimal purification result could be achieved with sodium hypochlorite as iron removal reagent and the iron removal rate could reach up to 90% when the adding amount was 0.1 vol.%. During the preparation, the particle size and distribution of ceria-based polishing powder were affected obviously by the parameters such as concentration, reaction temperature and feeding rate. The results showed that ceria-based polishing powder with D50=2.5–3.5 μm and the particle size distribution of 0.65–0.75 μm could be prepared when the concentration of CeCl3 was 0.6 mol/L, the reaction temperature was maintained at 50 °C and the feeding speed was controlled at 25 ml/min. Compared with commercial powder, the self-made polishing powder had roughly the same cutting amount, but the surface finish of polished glass was better than that of commercial polishing powder.

In view of the problem of ammonia-nitrogen wastewater pollution in rare earths extraction and separation, the novel saponification agent of organic phase, which is magnesium bicarbonate solution, was prepared with the natural rich and cheap dolomite as raw material through carbonation process. The behavior and purification of main impurities ions in the carbonation process as well as the application effect of the novel saponification agent in the extraction and separation was researched. The results showed that the concentration of Fe, Al, Si impurities ions was less than 5 ppm in the saponification agent through the development of effective removal technology, respectively. When the novel saponification agent was used in the extraction and separation, magnesium utilization rate was more than 95%, and rare earths extraction rate above 99.5% has achieved. Therefore, the technology could replace ammonia-water to saponify the organic phase in rare earth extraction and separation process.

The green-emitting (Ce,Tb)MgAl 11 O 19 (CTMA) phosphor was prepared by the conventional high temperature solid-state reaction method. The effect of fluxes on the crystal structure, particle morphology, size distribution and photoluminescence properties of CTMA phosphor was investigated by means of the X-ray powder diffraction, scanning electron microscopy and photoluminescence spectrum. The results showed that the addition of appropriate amount of MgF 2 -H 3 BO 3 flux improved the phase purity of CTMA phosphor, and influenced the growth of different crystal plane; furthermore, the particle morphology of CTMA phosphor became regular and size distribution narrowed. In addition, the CTMA phosphor prepared with MgF 2 -H 3 BO 3 flux displayed higher photoluminescence intensity than that of the phosphor prepared with MgF 2 , H 3 BO 3 or without flux. As a result, the optimum content of fluxes was 0.10 mol/mol MgF 2 and 0.04 mol/mol H 3 BO 3 , and the maximum photoluminescence intensity of the CTMA phosphor prepared with MgF 2 -H 3 BO 3 flux was 167% of that of the phosphor prepared without flux.