A comparative investigation into floatability of bastnaesite with three di/trialkyl phosphate surfactants

Flotation separation and recovery of rare earth minerals (REM) have returned to an important position due to the growing strategy demand for rare earth elements (REE). In this paper, a comparative investigation into the floatability of bastnaesite ((Ce,La)FCO₃) was conducted by using three di/trialkyl phosphate collectors, di(2-ethylhexyl) phosphate (DEHPA), dibutyl phosphate (DBP) and tributyl phosphate (TBP). The density functional theory (DFT) computation recommends that the chemical activity of the three phosphate collectors is in order of DEHPA ≥ DBP >> TBP, and their hydrophobization as-suggested by the lgP (oil-water partition coefficient) value is in the order of DEHPA > TBP > DBP. The micro-flotation indicates that the preferable pH values for flotation of bastnaesite with the three phosphate collectors are 7.0–8.0, and DEHPA achieves much higher flotation recovery of bastnaesite, followed by DBP, and then TBP, which coincides with their reactivity and hydrophobicity, the two prerequisites for froth flotation. The contact angle, zeta potential, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) deduce that DEHPA likely reacts with the Ce(Ⅲ) atoms of bastnaesite interface through its O atom(s) of the P(=O)–O^– group to generate the Ce–O–P bonds, and its two 2-ethylhexyl groups orient outside for attaching bubbles, resulting in flotation enrichment of bastnaesite. Furthermore, this investigation offers a novel strategy for developing collectors in selective beneficiation of REM.     

Mechanism of phthalic acid collector in flotation separation of fluorite and rare earth

The mechanism of phthalic acid,a dicarboxylic acid collector,in flotation separation of fluorite and rare earth(RE)was studied in this paper.The experimental data of flotation show that phthalic acid,as the collector,can realize highly efficient separation of fluorite and rare earth under weakly acidic conditions.The adsorption mechanism of phthalic acid on the surface of fluorite and bastnaesite was analyzed in this paper by means of the zeta potential measurement,the Fourier transform infrared(FT-IR),the X-ray photoelectron spectroscopy(XPS)and the stability constant measurement of active metal ion and phthalic acid coo rdination complex.According to the zeta potential testing results,the surfaces of fluorite adsorb the collector phthalate ion with negative charge under weakly acidic conditions which,in turn,increases its electronegativity and results in the motion of its potential.After the reaction between phthalic acid and fluorite ores under weakly acidic conditions,the peak of the fluorite ores is found to have significant changes in the FT-IR results,indicating strong chemical adsorption on the surfaces of phthalic acid and fluorite ores.According to the XPS analysis,the peak of benzene ring of phthalic acid is as high as 2%on the surface of fluorite,while no obvious characteristic peak of benzene ring is found on the surface of bastnaesite.According to the pH potentiometric titration results,the stability constant Ktotal of calcium phthalate complex within the acid range is higher than the stability constant K’total of cerium phthalate complex,indicating that the complex generated between phthalic acid and Ca²⁺is more stable than the complex generated between phthalic acid and Ce³⁺.The possible reason is that Ca²⁺,with the highest reticular density,plays a prevailing role in the octahedron structure of fluorite amidst the acidic media.As the active point of flotation,Ca²⁺works with the carboxyl groups of the collector phthalic acid(-C=O-)to form polycyclic calcium phthalate complex.     

Synergistic depression mechanism of Ca2+ ions and sodium silicate on bastnaesite flotation

Bastnaesite is an important rare earth mineral and is usually beneficiated by flotation. Sodium silicate is commonly used to depress calcium-bearing gangue minerals, however it can also depress bastnaesite when Ca²⁺ ions exist in the pulp. In this study, the effect of Ca²⁺ ions and sodium silicate individually or in combination on bastnaesite flotation was studied through micro-flotation, zeta potential, fluorescence spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements. Micro-flotation results show that the combination of Ca²⁺ ions and sodium silicate depresses bastnaesite more severely due to their synergistic effect. Zeta potential results show that the combination renders the surface potential of bastnaesite negatively shifted more significantly. Fluorescence spectroscopy shows that the combination decreases the surface hydrophobicity of bastnaesite more severely. XPS shows that the combination increases the adsorption of sodium silicate on bastnaesite by forming hydrophilic Ca-SiO₃ precipitate, which causes more serious depression on bastnaesite flotation.     

Fabrication of wide temperature lanthanum and cerium doped Cu/TNU-9 catalyst with excellent NH3-SCR performance and outstanding SO2+H2O tolerance

The effects of La on the catalytic performance,SO₂ and H2O resistance of Cu-Ce/TNU-9 catalyst were studied in the selective catalytic reduction of NO_x via ammonia(NH₃-SCR).The results show that the La doped Ce-Cu/TNU-9(CCL/T9) catalyst exhibits better SCR performance than Ce-Cu/TNU-9(CC/T9) and Cu/TNU-9(C/T9) in the wide temperature window(200-450 ℃) due to La benefiting from enhancing Cu⁺+Ce⁴⁺?Cu²⁺-+Ce³⁺ to facilitate ...     

Behavior of phase transformation of Baotou mixed rare earth concentrate during oxidation roasting

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.     

Defluorination of bastnaesite by steam roasting process

The bastnaesite was defluorinated by steam roasting process, and fluorine was removed in the form of hydrogen fluoride to realize the separation from rare earth. The effects of particle size of raw material, roasting temperature and time on defluorination rate of bastnaesite were studied. The suitable conditions of steam roasting process are that the particle size of raw material is less than 74 μm, the roasting temperature is 1000 °C and the roasting time is 4 h. The defluorination rate of bastnaesite is close to 100% under above these conditions. When the temperature rises to 1000 °C, the voids on the surface of the particles increase obviously, and there is a developed network of voids, which indicates that the fluorine oxides in bastnaesite react fully with the saturated water vapor at this stage, and a large amount of fluorine escapes in the form of hydrogen fluoride from the surface and inside of the mineral. X-ray diffraction results show that there are only rare-earth oxides in the form of Ce_0.33La_0.33Ca_0.33O_1.5 and Ce₇O₁₂ in the slag. The results of energy spectra and chemical analysis show that the fluorine in the baking sand was basically completely removed.     

Molten salt synthesis of Z-scheme CeO2/C3N4 photocatalysts with excellent properties for removal of organic pollutants: Characterization,kinetics and mechanisms

In this work,we firstly synthesized a CeO₂/C₃N₄ photocatalyst with Z-scheme heterojunction by a facile LiC-KCI molten salt method.The synthesized catalyst has an excellent quality for removing organic pollution of dyes and antibiotics in wastewater.As an example,the CeCN-1:5 prepared with a mass ratio of Ce₂(CO₃)₃·xH₂O:C₃H₃N₆=1:5 exhibits a methylene blue(MB) removal capacity of 100%with...     

Thermal decomposition and oxidation of bastnaesite concentrate in inert and oxidative atmosphere

To clearly elucidate the oxidative roasting behaviors of the bastnaesite, the thermal decomposition and oxidation of the bastnaesite concentrate in inert and oxidative atmosphere have been investigated in detail. Experimental data indicated that the initial decomposition temperature of the concentrate under N_2 atmosphere is 150 ℃ higher than that under O_2 atmosphere,most likely because the oxidation of the cerium induces the decomposition of the concentrate. For the roasted samples under N_2 atmosphere at500 ℃ and above,the oxidation efficiency of the cerium is 19.8%-26.8% because of the fact that rareearth fluorocarbonate is first decomposed to form rare-earth oxyfluoride and CO_2, and the cerium oxyfluoride is then partially oxidized by the CO_2 gas. The rest cerium in these samples can be further oxidized in air at room temperature, with the oxidation efficiency of the cerium gradually increasing to above 80% in 7 d. This can be attributed to the obvious changes in the inner morphology of the roasted samples under N_2 atmosphere at high temperatures, which largely induce the diffusion of the air and improves the oxidation activity of CeOF, and further induces the oxidation of CeOF by the air. XRD and XPS techniques were used to further verify the significant differences in the thermal decomposition behaviors of the bastnaesite concentrate under N_2 and O_2 atmosphere. Moreover, no oxidation of Pr~(3+) to Pr~(4+) in the roasted samples under both N_2 and O_2 atmosphere is observed. This gives an overall understanding of the oxidative roasting of the bastnaesite concentrate without additives.     

La_2Ce_2O_7 supported ruthenium as a robust catalyst for ammonia synthesis

La_2Ce_2O_7 nanoparticles were prepared by citric acid complexation method followed by calcination at varied temperatures. Then, supported with 4 wt% Ru, they were evaluated as the catalysts for ammonia synthesis under conditions similar with industry. With La_2Ce_2O_7 being calcinated at 700 or 800℃, the experimental results indicate that the Ru/La_2Ce_2O_7 catalyst exhibits much higher ammonia concentration or ammonia synthesis rate than that of Ru/CeO_2 and Ru/La_2O_3. In addition, Ru/La_2Ce_2O_7 possesses high stability under over-heating test. In the absence of any promotor, ammonia concentration of Ru/La_2Ce_2O_7 catalyst approaches 14% at 450℃, GHSV of 10000 h~(-1) and pressure of 10 MPa. The rate-determining step of ammonia synthesis, dissociation of N_2 is significantly facilitated by the strong metalesupport interaction(SMSI) between Ru and La_2Ce_2O_7. Due to the interaction, La_2Ce_2O_7 tends to donate electrons to Ru,resulting in the high electron density over the surface of Ru active sites which is favorable for the dissociation of N_2. Consequently, high activity is achieved.     

A comparative DFT+U study of CO oxidation on Pd- and Zr-doped ceria

Metal-doped ceria catalysts have been applied in many important catalytic processes.In this work,we performed density functional theory calculations corrected by on-site Coulomb interactions to study the Pd-and Zr-doped CeO₂(111) surfaces with the dopant at different locations.The formation of oxygen vacancies and CO oxidation were systematically calculated on the various doped surfaces.We find that both Pd and Zr doping can activate the surface lattice O and reduce the energy barrier...     

Reaction between tetravalent cerium ion and chloride ion and effect of thiourea using cyclic voltammetry

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.     

Preparation and reaction mechanism of novel CexCoyCuz oxide composite catalysts towards oxidation of o-xylene

Ce_xCo_yCu_z oxide composite catalysts were prepared by using polyethylene glycol, citrate sol–gel method combined with PMMA template for the oxidation of o-xylene. The catalysts were characterized by the X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), etc. The catalytic activity for o-xylene was investigated. The catalytic degradation pathway and mechanism of o-xylene were inferred. The results show that CeO₂ is mainly present on the surface of all catalysts. The surface area of Ce₂Co₁Cu₁ is up to 77.2 m²/g, and the average pore size is 10.62 nm. It exhibits redox and sufficient Ce⁴⁺ and Ce³⁺, and reactive oxygen species, and has maximum O–H and CO in the five catalyst samples. The catalytic activity of Ce₂Co₁Cu₁ is the best at low temperature, with the T₅₀ and T₉₀ values of 235 and 258 °C at a space velocity of 32000 h^?1, respectively. The o-xylene is oxidized to o-methyl benzaldehyde, and then further oxidized to o-methylbenzoic acid, and finally CO₂ and H₂O are formed.     

Study of K~+ doping on structure and properties of γ-Ce_2S_3

In this study,K~+-doped γ-Ce_2 S_3 was successfully prepared via a gas-solid reaction method using CeO_2,K_2 CO_3,and CS_2 as raw materials.The effects of the suitable sulfide system and different molar ratios of K to Ce(n_(K/Ce)=0-0.30) on the phase composition,crystal structure,chromaticity and thermal stability ofγ-Ce_2 S_3 were systematically investigated.Pure γ-Ce_2 S_3 was obtained by calcining the doped samples at840℃ for 150 min.After calcination at the same temperature the undoped K+samples exhibit a pure α-phase.Samples with a K/Ce molar ratio(n_(K/Ce)) of 0.10-0.25 comprise only the γ-phase;and when n_(K/Ce) exceeds 0.25,a new heterogeneous phase,KCeS_2,emerges.For values of n_(K/Ce) in the range of0-0.25,the γ-Ce_2 S_3 lattice parameters gradually increases with increasing K~+ content.When n_(K/Ce)exceedes 0.25,the lattice parameters remains unchanged.As n_(K/Ce) increased,the synthesized color gradually changes from red to orange—red and finally,to yellow.The redness value a~* reaches the maximum(L~*=33.86,a~*=36.68,b~*=38.15) when n_(K/Ce)=0.10,The n_(K/Ce)=0.10 composition continues to exhibit the y-phase after heat treatment at 420℃ for 10 min in air.The K+doping fills the internal vacancies of γ-Ce_2 S_3 and formed a solid solution,which is beneficial for the stability of its lattice,thus improving the thermal stability of γ-Ce_2 S_3(from 350 to 420℃).     

Optical and electrical conductivity properties of rare earth elements (Sm,Y, La,Er) co-doped CeO2

In this research, un-doped CeO₂ and Ce_0.85La_0.10M_0.05O₂ (M: Sm, Er, Y) compounds were synthesized by hydrothermal method and the multi-functional properties are reported. Oxygen defects were created with the additives of rare earth ions. The electrical and luminescence behaviors of the synthesized compounds were investigated in accord with the types of additives. The synthesized products were characterized by X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) measurement, UV–vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopic (EIS). All synthesized compounds are found to be nano-structured and have cubic phase. The total conductivity of all samples was calculated. Hence, the total conductivity of un-doped CeO₂, Ce_0.85La_0.10Y_0.05O₂, Ce_0.85La_0.10Er_0.05O₂ and Ce_0.85La_0.10Sm_0.05O₂ is found to be 2.07 × 10^?10, 5.70 × 10^?4, 1.0 × 10^?3 and 0.0747 S/cm, respectively. Also, bandgap energy (E_g) of these samples calculated from UV visible absorption spectra is discussed, and the optical results show variation between 3.2 and 2.15 eV. Additionally, the luminescence properties of the compounds were investigated and different emissions occur depending on the additive type. Accordingly, photoluminescent emission spectra of Ce_0.85La_0.10Y_0.05O₂, Ce_0.85La_0.10Er_0.05O₂ and Ce_0.85La_0.10Sm_0.05O₂ phosphors indicate that these phosphors have red, green and orange-red colors, respectively.     

Evaluation of surface tension of Ce-bearing slag based on calcium aluminate system

The surface tension of the Ce-bearing calcium aluminate slag was studied by the modified Butler’s equation. By taking the slag structure into account, the parameter related to AlO₄-tetrahedron has been modified, imparting the model with good reproducibility. Ce₂O₃ and Li₂O were surfactants. Ce₂O₃ can decrease the surface tension because of the decrease in the total ion moment of the slag and the weaker interactions between Ce³⁺ and O^2?. Li⁺ had the weakest ionic potential, and increasing Li₂O content decreased the total ion moment, which led to the decrease in the surface tension. The dissociated Ca²⁺ ions showed stronger absorption of the anions in the surface, so the surface tension increased with increasing the content of CaO. Increasing the value of w(CaO)/w(Al₂O₃) decreased the surface tension, because of the decrease in the total polymerised anions in the slag and the increase in the total ion moment.     

Preparation and characterization of Ce_(0.8)La_(0.2–x)Y_xO_(1.9) as electrolyte for solid oxide fuel cells

In this study, ultrafine Ce0.8La0.2–x Y x O1.9（for x=0, 0.05, 0.10, 0.15, 0.20） powders were successfully prepared by the sol-gel method.The samples were characterized by fourier transform infrared（FTIR）, thermogravimetric and differential scanning calorimetry（TG-DSC）, X-ray diffraction（XRD）, scanning electron microscopy（SEM）, AC impedance and thermal expansion measurements.Experimental results indicated that highly phase-pure cubic fluorite electrolyte Ce0.8La0.2–x Y x O1.9 powders were obtained after calcining at 600 °C.The as-synthesized powders exhibited high sintering activity, the Ce0.8La0.2–x Y x O1.9 series electrolytes which have higher relative densities over 96% could be obtained after sintered at 1400 °C for 4 h.Ce0.8La0.15Y0.05O1.9 electrolyte sintered at 1400 °C for 4 h exhibited higher oxide ionic conductivity（σ800 oC=0.057 S/cm）, lower electrical activation energy（E a=0.87 e V） and moderate thermal expansion coefficient（TEC=15.5×10-6 K-1, temperature range 25–800 °C）.     

Improved Flotation Separation of Apatite from Calcite with Benzohydroxamic Acid Collector

ABSTRACT

Apatite (Ca₁₀(PO₄)₆F₂) is the most important phosphate mineral, and flotation is the main beneficiation method to separate apatite from its major gangue mineral calcite (CaCO₃). Till date, fatty acids and their salts have been widely used as collectors in the apatite/calcite flotation separation due to their low cost and strong collecting ability, but their selectivity is limited. Therefore, screening or designing a selective collector becomes the key to the efficient separation. In this work, an attempt was made to utilize benzohydroxamic acid (BHA) as the collector for the selective separation of apatite from calcite without any depressant. The single and mixed binary mineral flotation experimental results prove the excellent selectivity of BHA in the apatite/calcite flotation separation. Zeta potential measurement results indicate a greater affinity of BHA on the apatite surface than calcite, which is also confirmed by the higher adsorption energy of BHA on the apatite surface based on the first-principle density functional theory calculations. The X-ray photoelectron spectroscopy analysis shows that the selective chemisorption of BHA on apatite over calcite is due to the stronger reactivity and the higher density of Ca²⁺ ion on the apatite surface than calcite. This work shows that surfactants of hydroxamic acid type can be an ideal collector for phosphate mineral flotation.     

Thermodynamic description of the interaction processes in the Cu–Ce–O system in the temperature range 1100–1300°C

A thermodynamic simulation and an experimental study of the interaction between cerium and oxygen in liquid copper have been performed. The thermodynamic analysis of the interaction processes in the Cu–Ce–O system is carried out using the technique of constructing the surface of solubility of components in a metal in the temperature range 1100–1300°C. As a result of simulation, data on changes in the Gibbs energy ΔG _T ° and the equilibrium constants of formation of cerium oxides Ce₂O₃ and CeO₂ from the components of a copper-based metallic melt are obtained. The first-order interaction parameters (according to Wagner) of cerium and oxygen dissolved in liquid copper, namely, e _Ce ^Ce, e _O ^Ce, and e _Ce ^O, are evaluated. Experimental studies of the Cu–Ce–O system have been performed. The morphological features, the size, and the composition of nonmetallic inclusions formed as a result of interaction in the Cu–Ce–O system are studied using scanning electron microscopy and electron-probe microanalysis.     

Synthesis,evaluation of kinetic characteristics and investigation of apoptosis of Cu2+-modified ceria nano discs

Ceria nano discs were synthesized by the stepwise thermal decomposition strategy of the oxalate precursor. A series of Ce_1–xCu_xO₂ (x = 0, 0.02, 0.1, 0.2 and 0.3) nano sized oxide systems were prepared through thermal decomposition route. Kinetic characterization of formation of solid solution was made by isoconversional strategy under non-isothermal condition. Introduction of various reactant molar ratios of Cu²⁺:Ce⁴⁺ has a pivotal role in the creation of new oxygen vacancies, decomposition strategy, particle size and shape. Cu²⁺ doping (x = 0.02 and 0.1) damages the disc shaped morphology of ceria. Homogeneous distribution of Cu²⁺ on the oxalate precursor has a significant role in the catalyzing activity for the destruction of oxalate bond to oxide. 2 mol% doped Cu²⁺ promotes breaking of oxalate bonds in nitrogen atmosphere. In vitro cell viability assay illustrates enhanced toxicity to cancer cells with 10 mol% Cu²⁺ doped ceria.