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.     

Thermal decomposition mechanism of low-content-fluorite Bayan Obo rare earth concentrate roasted with sodium carbonate and its consequent separation study

Thermal decomposition and phase transformation for the mixture of Bayan Obo rare earth concentrate(BORC) and sodium carbonate(Na_2CO_3) roasted at different temperatures with weight ratio of 100:20 were studied in detail in our study.The aim of our study is to reveal the nature of roasting reaction between BORC and Na_2CO_3 and thus providing a new method for processing BORC.The results indicate that BORC can be decomposed completely with Na_2CO_3 at around 600℃ after 3 h.During the calcination process,Ce_(0.5)Nd_(0.5)O_(1.75),NaF,Na_3PO_4,and a rare earth double phosphate phase Na_3RE(PO_4)_2 are formed after the decomposition of BORC with Na_2CO_3.In addition,the thermal decomposition mechanism is determined in the paper.Based on these facts,a clean technique processing BORC was developed.And a CeF_3 powder,whose composition was measured and stability was also evaluated,was obtained for some potential application from the new technique.This research is of significance in terms of the Na_2CO_3-roasting BORC solid reaction study and sheds a light on a potential clean technique for BORC.     

Kinetics of roasting of chromium oxide with sodium nitrate flux

Low temperature isothermal kinetic studies were performed on alkali roasting of Cr₂O₃ with progressive replacement of Na₂CO₃ in the reaction mixture by NaNO₃ in the temperature range of 573 to 873 K. The influence of the nitrate on the course of the reactions was studied. Statistical design of experiments was employed to asses the relative influence of different process variables. Kinetic analysis of the experimental data shows that the reaction with NaNO₃ follows a simple first-order model, and the activation energy of the reaction is 22.7 kJmol_-1, as compared to 72 kJmol⁻¹ for Na₂CO₃ flux.     

Selective separation of iron and scandium from Bayer Sc-bearing red mud

In this study, we investigated the separation of iron and scandium from Sc-bearing red mud. The red mud object of our study contained 31.11 wt% total iron (TFe), 0.0045 wt% Sc, hematite (Fe₂O₃) and ferrosilite (FeO·SiO₂) as the main Fe-bearing minerals. The Sc-bearing red mud was treated by a novel deep reduction roasting and magnetic separation process that includes the addition of coke and CaO to extract Fe and enriching Sc from the Sc-bearing red mud. The addition of coke and CaO enhances the transformation of hematite (Fe₂O₃) to metallic iron (Fe⁰) and magnetite (Fe₃O₄) as well as the transformation of ferrosilite into metallic iron (Fe⁰). The test results show that utilizing the new process a Fe concentrate with a TFe content of 81.22 wt% and Fe recovery of 92.96% was obtained. Furthermore, magnetic separation tailings with Sc content of 0.0062 wt% and Sc recovery of 98.65% were also obtained. The test results were achieved under the following process conditions: roasting temperature of 1373 K, roasting time of 45 min, calcium oxide dosage of 20 wt%, coke dosage of 25 wt%, grinding fineness of 90% < 0.04 mm, and magnetic field intensity of 0.24 T. The major minerals in the Fe concentrate are metallic iron (Fe⁰) and magnetite (Fe₃O₄). The main minerals in the magnetic separation tailings with a low TFe content of 2.62% are CaO·SiO₂, Na₂O·SiO₂, FeO·SiO₂, Ca₃Fe₂Si₃O₁₂, CaAl₂SiO₆ and CaFe(SiO₃)₂.     

The standard gibbs energy of formation of CeF3 and its activity coefficient in cryolite

Literature data are analyzed to give the activity coefficient (γ_Ce) of Ce in dilute solution in Al as log₁₀γ_Ce = −11 356/T + 4.261 referred to liquid Ce as standard state. Measurements were made in the range of 977 to 1288 K of the equilibrium Al (1) + CeF₃ (s) = AlF₃ (s) + Ce(Al) and give, by a third-law calculation, °G ^o = 183 360 + 19.456T joules, and Δ _{fH 298} ^o of CeF₃ = −1701 kJ mol⁻¹. Values of the partition coefficient of Ce between Al and molten cryolite then give activity coefficients of CeF₃ in solution. These activity coefficients decrease as the NaF/AlF₃ ratio is raised, showing acid behavior of CeF₃. It appears to dissolve mainly in the form of Na₂CeF₅.     

The soda-ash roasting of chromite minerals: Kinetics considerations

Soda-ash roasting of the chromite mineral is commonly used worldwide for the production of watersoluble sodium chromate. The formation of sodium chromate during the soda-ash roasting reaction depends on the oxygen partial pressure and availability of oxygen at the reaction front. The effects of temperature, oxygen partial pressure, charge composition, and their roles on the overall roasting reaction were studied in order to analyze the reaction mechanism. The influence of process parameters such as the addition of alkali and process residue as the filler material on the overall reaction rate is discussed. The rate-determining steps for the soda-ash roasting reaction are analyzed. The importance of the binary Na₂CO₃-Na₂CrO₄ liquid phase during the reaction in determining its speed is also examined. It is shown that the experimental results for the roasting reaction can be best described by the Ginstling and Brounshtein (GB) equation for diffusion-controlled kinetics. From the measured kinetics data, the apparent activation energy for the roasting reaction was calculated to be between 180 and 190 kJ · mol⁻¹ in the temperature range from 1023 to 1210 K and between 35 and 40 kJ · mol⁻¹ above 1210 K.     

Realization of warm white light emitting in single phase Gd(PxV1−x)O4:y at% Sm3+,1 at% Bi3+ phosphor

A series of single phase, warm white light emitting phosphors, Gd(P_xV_1–x)O₄:y at% Sm³⁺, with 1 at% Bi³⁺ doping concentration were synthesized by high temperature solid state method in this work. The experimental results indicate broadband cyan emission of Bi³⁺ and characteristic orange-red emission of Sm³⁺ can be effectively tuned by changing the ratios of PO₄^3?/VO₄^3? in Gd(P_xV_1–x)O₄:1 at% Sm³⁺,1 at% Bi³⁺, and the energy transfer process among VO₄^3?, Sm³⁺, Bi³⁺ also can be adjusted. Based on this, warm white light emitting can be realized by further optimizing the doping concentration of Sm³⁺ in the phosphors. At 423 K, the PL intensity of Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ remains ～84.3% of the initial value at 293 K, while the measured quantum efficiency is 67.8%. EL spectrum analysis results of the fabricated white light emitting diode (wLED) based on a 310 nm UV-chip and Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ phosphors imply low correlated color temperature (3132 K) and appropriate color-rending index (R_a = 82.7). These results demonstrate that Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ is a good candidate for manufacturing UV-activated warm white light emitting diodes.     

The soda-ash roasting of chromite ore processing residue for the reclamation of chromium

Sodium chromate is produced via the soda-ash roasting of chromite ore with sodium carbonate. After the reaction, nearly 15 pct of the chromium oxide remains unreacted and ends up in the waste stream, for landfills. In recent years, the concern over environmental pollution from hexavalent chromium (Cr⁶⁺) from the waste residue has become a major problem for the chromium chemical industry. The main purpose of this investigation is to recover chromium oxide present in the waste residue as sodium chromate. Cr₂O₃ in the residue is distributed between the two spinel solid solutions, Mg(Al,Cr)₂O₄ and γ-Fe₂O₃. The residue from the sodium chromate production process was analyzed both physically and chemically. The compositions of the mineral phases were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The influence of alkali addition on the overall reaction rate is examined. The kinetics of the chromium extraction reaction resulting from the residue of the soda-ash roasting process under an oxidizing atmosphere is also investigated. It is shown that the experimental results for the roasting reaction can be best described by the Ginstling and Brounshtein (GB) equation for diffusion-controlled kinetics. The apparent activation energy for the roasting reaction was calculated to be between 85 and 90 kJ·mol⁻¹ in the temperature range 1223 to 1473 K. The kinetics of leaching of Cr³⁺ ions using the aqueous phase from the process residue is also studied by treating the waste into acid solutions with different concentrations.     

Single-phase white-emitting and tunable color phosphor Na3Sc2(PO4)3:Eu2+,Dy3+: Synthesis,luminescence and energy transfer

A series of Eu²⁺/Dy³⁺ single doped and co-doped Na₃Sc₂(PO₄)₃ phosphors were synthesized by the high-temperature solid-state method, and their phase, morphology, and luminescence properties were characterized. Under the excitation of 370 nm, the Na₃Sc₂(PO₄)₃:Eu²⁺,Dy³⁺ phosphor can emit white light whose spectrum is composed of a broad emission band centered at 460 nm and the other three peaks at 483, 577, and 672 nm, respectively. There is energy transfer from Eu²⁺ to Dy³⁺ ion in Na₃Sc₂(PO₄)₃:Eu²⁺,Dy³⁺ phosphor due to the good overlap between the emission spectrum of Na₃Sc₂(PO₄)₃:Eu²⁺ and the excitation spectrum of Na₃Sc₂(PO₄)₃:Dy³⁺, which is further confirmed by the fluorescence lifetime decrease of Eu²⁺ ion with the increase of Dy³⁺ concentration. The process of energy transfer is via dipole–quadruple interaction which is confirmed by applying Dexter's theory. By increasing the Dy³⁺ concentration, the color coordinates of the Na₃Sc₂(PO₄)₃:0.01Eu²⁺,xDy³⁺ phosphors can be adjusted from blue to white, and then to yellow. The optimized concentration of Dy³⁺ ions is 4.0 mol%, beyond which the concentration quenching will take place. The Na₃Sc₂(PO₄)₃:Eu²⁺,Dy³⁺ phosphor shows fairly good resistance to thermal quenching behavior, of which the emission intensity at 423 K can maintain 90.3% of the initial value (298 K). These results suggest that the Na₃Sc₂(PO₄)₃:0.01Eu²⁺,xDy³⁺ phosphors have potential applications as the color-tunable or a single-phase white emitting phosphor in white LEDs.     

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.     

The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores

The formation of a liquid phase during the early stages of the roasting reaction is a common problem in the sodium chromate manufacturing process. The molten salt phase, which is primarily constituted of a binary mixture of Na₂CrO₄ and Na₂CO₃, creates major operational problems such as the granulation and blocking of the kilns. In addition to the operational problems, it was observed that the molten salt also affects the transport of oxygen toward the reaction interface. The mechanism of the soda-ash roasting reaction has been analyzed for improving the yield of sodium chromate. It was observed that the conversion efficiency of the roasting process changed dramatically, depending on the origin and the type of the chromite ores used. Thermal and scanning electron microscopic analyses of the products of roasting were carried out to establish the reaction mechanism. It was observed that the presence of silicates in the chromite ores interferes with the formation of sodium chromate involving the binary Na₂CO₃-Na₂CrO₄ liquid. The roasting reaction proceeds in a certain crystallographic direction in the chromite spinel in the presence of a nonsilicate molten salt, whereas a complete dissolution of chromite appears to take place in the binary liquid containing silicate phases present in the ore. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Tunable emission,energy transfer and thermal stability of Ce3+, Tb3+ co-doped Na2BaCa(PO4)2 phosphors

A series of single Ce³⁺ doped and Ce³⁺ and Tb³⁺ co-doped Na₂BaCa(PO₄)₂ (NBCP) phosphors was synthesized by conventional solid-stated reaction method. The crystal structure, luminescence properties, thermal stability and energy transfer were carefully investigated. Ce³⁺ is inferred to substitute the Ba²⁺ site in NBCP lattice. The color-tunable emission from blue to green is observed by adjusting Tb³⁺ concentration among NBCP:0.03Ce³⁺,yTb³⁺ phosphors. The energy transfer behavior from Ce³⁺ to Tb³⁺ ions is both illustrated by co-doped PL spectra and decay curves. The energy transfer efficiency is as high as 91.5%. The mechanism of energy transfer is resonance type of dipole-dipole transition. In this work, the optimal phosphor exhibits the excellent thermal stability which keeps at 94.9% of that initial value at room temperature when temperature reaches to 150 °C. The Ce³⁺ and Tb³⁺ co-doped NBCP phosphor is a promising candidate for the application in the general lighting and display fields.     

Effect of excitation wavelength (blue vs near UV) and dopant concentrations on afterglow and fast decay of persistent phosphor SrAl2O4:Eu2+,Dy3+

The persistent phosphor SrAl₂O₄:Eu²⁺,Dy³⁺ is the subject of numerous investigations. One often neglected aspect is that in this phosphor, as well as in Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺, there are two different Sr²⁺ sites which can be occupied by the dopant Eu²⁺ ions. We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions. This scheme is used as a generic starting point to study experimentally specific pathways. We illustrate this application with the study of the effect of excitation wavelength (444 and 382 nm) on the afterglow of differently doped SrAl₂O₄:Eu²⁺,Dy³⁺ samples, as well as on the emission decay curves. With the same excitation intensity under 444 nm excitation, the resulting afterglow intensity is stronger than under near UV excitation. At 382 nm, Eu²⁺ ions on both Sr²⁺ sites in SrAl₂O₄ are excited, but at room temperature the blue emission is quenched, leading to a loss of photons. The observed effects can further be associated with the ratio of Eu²⁺ ions and trap states which are modulated by the concentrations of Eu²⁺ and Dy³⁺ in SrAl₂O₄, as well as by temperature. Increasing the nominal Dy³⁺ content from 0.1 mol% to 0.5 mol% with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy³⁺ ions are indeed involved in the trapping process. The concentration of trap states is much lower than the concentration of Eu²⁺ ions, as even with low excitation densities, a plateau of integrated afterglow intensity (corresponding to the total number of accessible traps) is reached. We postulate that an important fraction of excited Eu²⁺ ions can potentially transfer their energy to trap states. Once that all traps are filled or in a dynamical filling-depletion process under illumination (with thermal and/or optical depletion processes), for the remaining Eu²⁺ a “normal” steady-state emission is observed. The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation, while under 375 nm excitation, a feed process originating from the energy transfer between Eu²⁺ ions is demonstrated. Under 375 nm excitation, the non-exponential decay observed at 440 nm can be quantitatively associated to a Förster energy transfer process with R₀ = 1.58 (8) nm. For the overall understanding of the afterglow processes, it appears that one has to consider the individual contributions of all active ions on different lattice sites.     

Molten salt synthesis of neodynium oxyfluoride in various fluoride media with different fluoride ion activities

Neodymium oxyfluoride has received much attention in the fields of anionic solid electrolytes, luminescent, catalytic and magnetic materials because of its structure combined advantages of rare-earth cations with F^– and O^2– anions. In this work, neodynium oxyfluoride was synthesized by the reaction between neodymium oxide and four fluoride media with different fluoride ion activities. The synthesis processes in molten LiF–CaF₂–NdF₃, LiF–NdF₃, NaF–CaF₂-NdF₃ and NaF–KF–NdF₃ are observed in situ by a confocal scanning laser microscope. The expansion of neodymium oxide particle is observed in the LiF–CaF₂–NdF₃, LiF–NdF₃, and NaF–KF–NdF₃ melts, and the growth of needle crystals on neodymium oxide particle is clearly observed in molten NaF–CaF₂–NdF₃. Based on scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses of products, neodynium oxyfluoride was successfully synthesized in the four fluoride media. The neodynium oxyfluoride generated in the LiF–CaF₂–NdF₃, LiF–NdF₃, and NaF–KF–NdF₃ melts is a tetragonal structure. However, in molten NaF–CaF₂–NdF₃, neodynium oxyfluoride with a rhombohedral structure is formed. It is suggested that the substitution of Na(I) and Ca(II) for Nd(III) can transform NdOF from tetragonal structure to rhombohedral structure. The growth rate of needle crystals generated in molten NaF–CaF₂–NdF₃ was calculated based on the result of a confocal scanning laser microscope, and it is found that the reaction kinetics of crystal formation is zero-order reaction. The effect of fluoride media on the structure and morphology of formed NdOF were evaluated by XRD, X-ray photoelectron spectroscopy (XPS) and SEM. The neodymium oxyfluoride prepared in the fluoride media with high fluoride ion activity has low binding energy of F 1s. The ratio of adsorbed oxygen to lattice oxygen for neodymium oxyfluoride prepared in molten LiF-NdF₃ is larger than those in the other three fluoride media, so it can have better catalytic performance.     

On the solubility of aluminum carbide in cryolitic melts

The solubility of aluminum carbide in cryolitic melts was determined as a function of NaF/A1F₃ molar ratio (CR), temperature, and the concentrations of A1₂O₃, CaF₂, MgF₂, and LiF. At 1020 °C a maximum concentration of 2.1 wt pct aluminum carbide was found at CR = 1.80. The following model for the aluminum carbide dissolution reaction based on activity data for NaF and A1F₃ was found to fit the experimental solubility data: Al₄C₃(s) + 5AlF₃(diss) + 9NaF(l) = 3Na₃Al₃CF₈(diss). From the solubility data for aluminum carbide an empirical equation giving the equilibrium carbide concentration was derived for CR > 1.80.     

Energy transfer and luminescent properties of Tb3+ and Tb3+, Yb3+ doped CNGG phosphors

In this work, calcium niobium gallium garnet (Ca₃Nb_1.6875Ga_3.1875O₁₂ - CNGG) ceramic samples single-doped with Tb³⁺ and co-doped with Tb³⁺ and Yb³⁺ ions were sintered by the solid-state reaction method. The structural characterization of the samples was carried out by X-ray diffraction measurements. The optimal concentration of Tb³⁺ ions corresponding to the maximum luminescence in the green spectral range in CNGG:x at% Tb (x = 0.1, 0.5, 1, 2, 3, 4, and 5) was determined to be 4 at%. The time-resolved luminescence of the ⁵D₄ level (Tb³⁺) in the CNGG:x at% Tb samples was analysed to explore the quenching mechanisms involved in the Tb³⁺ green emission. Co-doped CNGG:4 at% Tb,y at% Yb (y = 0.5, 2, 4, 6, 8, and 10) ceramics were prepared and investigated. It is shown that CNGG:4 at% Tb,y at% Yb phosphors exhibit intense green luminescence under ultra-violet (UV), visible (VIS), and near-infrared (NIR) excitation, thus demonstrating the presence of simultaneous down-conversion (DC) and up-conversion (UC) processes. The dependence of the UC luminescence intensity on the diode laser pumping power was measured and the results indicate a two-photon process based on cooperative energy transfer (CET). Under UV excitation, the lifetime of the ⁵D₄ (Tb³⁺) level slowly increases with increase of Yb³⁺ concentration, suggesting the energy transfer from Yb³⁺ to Tb³⁺ ions, while under NIR excitation, the lifetime of the ⁵D₄ (Tb³⁺) level decreases with increase of Yb³⁺ ions concentration, indicating the presence of a strong energy transfer from Tb³⁺ to Yb³⁺ ions. The highest energy transfer efficiency of ηET ≈42% was determined for the CNGG:4 at% Tb,10 at% Yb sample. The obtained results indicate that CNGG:(Tb³⁺, Yb³⁺) could be efficient new yellowish-green-emitting phosphors.     

Synthesis and enhanced photoluminescence properties of red emitting divalent ion (Ca2+) doped Eu:Y2O3 nanophosphors for optoelectronic applications

This work presents the synthesis of Y₂O₃:Eu³⁺,xCa²⁺ (x = 0 mol%, 1 mol%, 3 mol%, 5 mol%, 7 mol%, 9 mol%, 11 mol%) nanophosphors with enhanced photoluminescence properties through a facile solution combustion method for optoelectronic, display, and lighting applications. The X-ray diffraction (XRD) patterns of the proposed nanophosphor reveal its structural properties and crystalline nature. The transmission electron microscope (TEM) results confirm the change in the shape of the particle and aggregation of particles after co-doping with Ca²⁺. Fourier transform infrared spectroscopy (FTIR) and Raman vibrations also confirm the presence of Y–O vibration and subsequently explain the crystalline nature, structural properties, and purity of the samples. All the synthesized nanophosphors samples emit intense red emission at 613 nm (⁵D₀→⁷F₂) under excitation with 235, 394 and 466 nm wavelengths of Eu³⁺ ions. The photoluminescence (PL) emission spectra excited with 235 nm illustrate the highest emission peak with two other emission peaks excited with 466 and 394 nm that is 1.4 times higher than 466 nm and 1.9 times enhanced by 394 nm wavelength, respectively. The emission intensity of Y₂O₃:Eu³⁺,xCa²⁺ (5 mol%) is increased 8-fold as compared to Eu:Y₂O₃. Doping with Ca²⁺ ions enhances the emission intensity of Eu:Y₂O₃ nanophosphors due to an increase in energy transfer in Ca²⁺→Eu³⁺ through asymmetry in the crystal field and by introduction of radiative defect centers through oxygen vacancies in the yttria matrix. It is also observed that the optical band gap and the lifetime of the ⁵D₀ level of Eu³⁺ ions in Y₂O₃:Eu³⁺,xCa²⁺ nanophosphor sample gets changed with a doping concentration of Ca²⁺ ions. Nanophosphor also reveals high thermal stability and quantum yield as estimating activation energy of 0.25 eV and 81%, respectively. CIE, CCT, and color purity values (>98%) show an improved red-emitting nanophosphor in the warm region of light, which makes this material superior with a specific potential application for UV-based white LEDs with security ink, display devices, and various other optoelectronics devices.