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.     

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.     

Kinetics of nitric acid leaching of cerium from oxidation roasted Baotou mixed rare earth concentrate

The kinetics of nitric acid leaching of cerium was investigated for the oxidation roasted Baotou mixed rare earth concentrate. The effects of leaching temperature, HNO₃ concentration, liquid–solid ratio (L/S) and stirring rate on rare earth extraction were studied. The XRD and SEM mapping analysis of the samples before and after acid leaching shows that the roasted bastnaesite is completely leached. Besides, the decomposition process of oxidizing roasting was also obtained by TG–MS and XRD. Different kinetics models were applied in this leaching process. The results of dynamic fitting show that the leaching process can be described by a new variant of the shrinking-core model. And the leaching rate is controlled by both the interfacial transfer and diffusion through the product layer. The apparent activation energy is calculated as 76.78 kJ/mol and the reaction orders with respect to HNO₃ concentrations and liquid–solid ratio are determined to be 7.609 and 2.516, respectively. Besides, an empirical rate equation is obtained to describe the process.     

Decomposition of mixed rare earth concentrate by NaOH roasting and kinetics of hydrochloric acid leaching process

A new clean extraction technology for the decomposition of Bayan Obo mixed rare earth concentrate by NaOH roasting is proposed.The process mainly includes NaOH roasting to decompose rare earth concentrate and HCl leaching roasted ore.The effects of roasting temperature,roasting time,NaOH addition amount on the extraction of rare earth and factors such as HCl concentration,liquid-solid ratio,leaching temperature and leaching time on the dissolution kinetics of roasted ore were studied.The experimental results show that when the roasting temperature is 550℃ and the roasting time is 60 min,the mass ratio of NaOH:rare earth concentrate is 0.60:1,the concentration of HCl is 6.0 mol/L,the ratio of liquid to solid(L/S) 6.0:1.0,and the leaching temperature 90℃,leaching time 45 min,stirring speed 200 r/min,and the extraction of rare earth can reach 92.5%.The relevant experimental data show that the process of HCl leaching roasted ore conforms to the shrinking core model,but the control mechanism of the che mical reaction process is different when the leaching temperature is different.When the leaching temperature is between 40 and 70℃,the chemical reaction process is controlled by the diffusion of the product through the residual layer of the inert material.The average surface activation energy of the rare earth element is E_a=9.96 kJ/mol.When the leaching temperature is 75-90℃,the chemical reaction process is controlled by the interface transfer across the product layer(product layer interface mass transfer) and diffusion.The average surface activation energy of rare earth elements is E_a=41.65 kJ/mol.The results of this study have certain significance for the green extraction of mixed rare earth ore.     

Catalytic action of rare earth oxide (La2O3, CeO2, Pr6O11) on electrochemical oxidation of activated carbon in molten KOH–NaOH

The oxidation of anode carbon fuel directly affects the electrochemical performance of molten hydroxide direct carbon fuel cell (MHDCFC). In general, the anode carbon fuel can be oxidized at high temperature, thus the direct carbon fuel cell (DCFC) can show great electrochemical performance. In this study, rare earth oxides (La₂O₃, CeO₂, Pr₆O₁₁) were prepared by the method of precipitation. Activated carbon was prepared by pretreatment of lignite. Rare earth oxides and activated carbon were mixed as anode carbon fuel, and rare earth oxides were used to catalyze the electrochemical oxidation of anode carbon fuel. The results show that CeO₂ has better electrocatalytic activity compared with La₂O₃ and Pr₆O₁₁ in the MHDCFC. The electrochemical test results show that the current density (at 0.4 V) increases from 81.02 to 112.90 mA/cm² and the maximum power density increases from 34.78 to 47.05 mW/cm² at 450 °C, when the mass fraction of CeO₂ is increased from 0 to 40%. When the mass fraction of CeO₂ is 30%, the current density (82.55 mA/cm² at 0.4 V) at 400 °C is higher than that (81.02 mA/cm² at 0.4 V) without CeO₂ at 450 °C. The electrochemical oxidation mechanism of CeO₂ catalyzed anode carbon fuel is discussed.     

Rare earth recovery from fluoride molten-salt electrolytic slag by sodium carbonate roasting-hydrochloric acid leaching

Fluorinated rare earth molten-salt electrolytic slag contains a considerable amount of rare earth elements,as well as a variety of heavy metals and fluorides that cause environmental pollution.Therefore,it is of great importance to fully utilise this resource.In this study,the transformation mechanism of fluorinated rare earth molten-salt electrolytic slag roasted with sodium carbonate,and the regulation mechanism of rare earth leaching under different roasting conditions were investigated with ...     

Non-ammonia enrichment of rare earth by magnesium oxide from rare earth leaching liquor in magnesium salt system

In order to solve the problem of ammonia-nitrogen pollution in the enrichment process of the ionadsorption type rare earth ore,the technology of non-ammonia precipitation with magnesium oxide precipitant was carried out.It is determined that the rare earth precipitation efficiency is 99.6% and the purity of rare earth concentrates is only 85.89 wt%under the optimum precipitation conditions.And the contents of MgO,SO_3 and Al_2O_3 in the rare earth concentrates are 5.12 wt%,6.77 wt%and 1.78 wt%,respectively.Furthermore,the thermo-decomposition process of precipitates was investigated by TGDSC,XRD and FI-IR.The thermal decomposition process consists of two stages:the dehydration of rare earth hydroxide and alkaline rare earth sulfate within 900 ℃ and the thermal decomposition of RE_2O_2SO_4 at 900-1300 ℃.Therefore,a high-temperature calcinations method for removing SO_3 from precipitates is proposed.When the precipitates are calcined at 1300 ℃ for 2 h,the rare earth concentrates with a purity of 92.03 wt%can be acquired.Moreover,the content of SO_3 in the concentrate is only 0.46 wt%.In the MgO precipitation and high-temperature calcinations process,the raw material cost is low and the quality of rare earth concentrates is acceptable.It could have great significance for nonammonia enrichment of rare earth from the rare earth leaching liquor,and finally solve the problem of ammonia nitrogen in the extraction process of the ion-adsorption type rare earth ore within magnesium salt system.     

Rare earth extraction from bastnaesite concentrate by stepwise carbochlorination-chemical vapor transport-oxidation

A stepwise carbochlorination-chemical vapor transport-oxidation process is developed for the green rare earth extraction from a bastnaesite concentrate using carbon as reductant, chlorine gas as chlorination agent, SiCl₄ gas as defluorination agent, AlCl₃ as vapor complex former, and (O₂+H₂O) mixed gas as oxidant. Between 500 °C and 800 °C, the apparent activation energy of the carbochlorination within 2 hours changed from 17 to 10 kJ/mole roughly for the initial 20 minutes and final 1.5 hours, respectively, in the absence of SiCl₄, but these values reduced to 15 and 5.9 kJ/mole under 10 kPa of SiCl₄ gas, while the rare earth chloride conversion for 2 hours was 43 to 81 mol pct in the absence of SiCl₄ and 55 to 99 mol pct under 10 kPa of SiCl₄ gas. After carbochlorination at 550 °C for 2 hours in the (Cl₂+SiCl₄) atmosphere for efficient rare earth extraction and thorium-free volatile by-product release, throium was removed by chemical vapor transport at 800 °C for 0.5 hours in the (Cl₂+SiCl₄+AlCl₃) atmosphere and alkaline earths were separated from rare earths by oxidation at 700 °C to 1000 °C in the (O₂+H₂O) atmosphere for 0.5 hours, followed by water leaching at room temperature. Their combination allows a clean and efficient rare earth extraction from the concentrate.     

Effect of roasting activation of rare earth molten salt slag on extraction of rare earth,lithium and fluorine

A new process was proposed to extract rare earth elements(REEs),Li and F from electrolytic slag of rare earth molten salt by synergistic roasting and acid leaching.Firstly,the thermodynamic analysis of roasting reaction was carried out,then the effects of roasting factors on leaching REEs,Li and F in slag were investigated.In additions,the mineral phase and morphology of molten salt slag,roasting slag and acid leaching slag were characterized,and the migration mechanism of REES,Li and F minerals...     

High-efficiency simultaneous extraction of rare earth elements and iron from NdFeB waste by oxalic acid leaching

Iron can not be recovered at high value because only rare earth elements are effectively recovered from NdFeB waste via oxidation roasting-hydrochloric acid leaching process.In this study,a new method for leaching NdFeB waste with oxalic acid was developed.The high-efficiency,simultaneous and high-value recovery of rare earth elements and iron was realized to simplify the process and improve the economic benefit.Results of the oxalic acid leaching experiments show that under the optimum leaching conditions at 90℃ for 6 h in the aqueous solution of oxalic acid(2 mol/L) with a liquid-solid ratio of60 mL/g,the iron leaching efficiency and precipitation rate of rare earth oxalate reach 93.89% and 93.17%,respectively.Rare earth oxalate and Fe(C₂O₄)3^3- were left in the residue and the leaching solution,respectively.The leaching mechanism was further analyzed by characterising the leach residues obtained through X-ray powder diffraction(XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDS).Results of the leaching kinetics study indicate that the process of oxalic acid leaching follows the shrinking nucleus model,and the leaching kinetics model is controlled by the mixed factors of diffusion and chemical reaction.The leaching residue was calcined at 850℃ for 3 h and then decomposed into rare earth oxide,which can be directly used to prepare rare earth alloy via molten salt electrolysis.For the leaching solution,ferric oxalate solution was reduced using Fe powder to prepare the ferrous oxalate(FeC₂O₄-2H₂O).     

Preparation of enriched cerium oxide from bastnasite with hydrochloric acid by two-step leaching

Roasted with sodium carbonate, bastnasite (Ln(Ce)CO₃F) was converted to calcine containing rare earth oxides (REO), among them cerium, which existed mainly as CeO₂. The calcine was first leached with diluted hydrochloric acid, which resulted in a sludge with the enriched cerium (IV) dioxide. The sludge was further leached with a concentrated hydrochloric acid, adding hydrogen peroxide as a reducing agent; in this manner, the enriched cerium tri-chloride (CeCl₃) was prepared. The optimal technological parameters are suggested as follows: first, the hydrochloric acid concentration, the leaching temperature, the ratio of solid to liquid, and the leaching time are 1 mol/L, 60 °C, 1:20, and 90 minutes, respectively; second, the hydrochloric acid concentration, the dosage of hydrogen peroxide in every 5 g of the sludge, the ratio of solid to liquid, the leaching temperature, and the leaching time are 6 mol/L, 6 mL, 1:20, 50 °C, and 90 minutes, respectively. As a result, the cerium-enriched rare earth (RE) solution, containing over 95 pct cerium oxide, is obtained, which is in turn available for use in preparing a kind of polishing powder containing high cerium. The total recovery of cerium was 91 pct (85.3 pct, in the second step).     

Preparation of enriched cerium oxide from bastnasite with hydrochloric acid by two-step leaching

Roasted with sodium carbonate, bastnasite (Ln(Ce)CO₃F) was converted to calcine containing rare earth oxides (REO), among them cerium, which existed mainly as CeO₂. The calcine was first leached with diluted hydrochloric acid, which resulted in a sludge with the enriched cerium (IV) dioxide. The sludge was further leached with a concentrated hydrochloric acid, adding hydrogen peroxide as a reducing agent; in this manner, the enriched cerium tri-chloride (CeCl₃) was prepared. The optimal technological parameters are suggested as follows: first, the hydrochloric acid concentration, the leaching temperature, the ratio of solid to liquid, and the leaching time are 1 mol/L, 60°C, 1:20, and 90 minutes, respectively; second, the hydrochloric acid concentration, the dosage of hydrogen peroxide in every 5 g of the sludge, the ratio of solid to liquid, the leaching temperature, and the leaching time are 6 mol/L, 6 mL, 1:20, 50°C, and 90 minutes, respectively. As a result, the cerium-enriched rare earth (RE) solution, containing over 95 pct cerium oxide, is obtained, which is in turn available for use in preparing a kind of polishing powder containing high cerium. The total recovery of cerium was 91 pct (85.3 pct, in the second step).     

Evaluating organic acids as alternative leaching reagents for rare earth elements recovery from NdFeB magnets

This study proposes an advanced leaching method using organic acids to recover rare earth elements (REEs) from NdFeB permanent magnets from end-of-life computers hard disk drives (HDDs). The end-of-life HDDs were first dismantled in order to recover NdFeB magnets, which were then thermally demagnetized at 350 °C during 30 min before crushing in a ball mill under inert atmosphere. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) analyses performed on the NdFeB magnets show the heterogeneous structure containing the major matric phase Nd₂Fe₁₄B and the REEs-rich phase containing Nd and Pr oxides. Additionally, X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) analyses on the ground NdFeB magnet show that grinding NdFeB magnets under inert atmosphere helps to minimize its oxidation. Chemical analysis shows that the composition of the ground sample is Nd: 22.8 wt%, Pr: 3.3 wt%, Dy: 1.2 wt%, Fe: 62.6 wt%, Co: 1.5 wt%, B: 0.9 wt%, Ni: 0.6 wt%. Diagrams of speciation and equilibrium phases (E_h vs. pH) were calculated to determine the predominance of the formed species in the REEs–organic acids systems. The influence of the organic acid type (acetic acid, formic acid, citric acid and tartaric acid), the acid concentration (10 vol%, up to saturation), and the solid/liquid (S/L) ratio (0.5%–10%) on NdFeB magnets leaching was investigated employing an optimal experimental design conceived by the statistical software JMP. Acetic acid (CH₃COOH) shows the highest leaching performance of REEs, allowing leaching yields over 90% for Nd, Dy and Pr in the acid concentration range of 1.6–10 mol/L and the S/L ratio range of 0.5%–5% at a temperature of 60 °C. The results presented in this investigation suggest that REEs can be recovered from magnets of end-of-life HDDs using an eco-friendly method assisted by organic acids.     

Comparative studies on promotional effect of Pr6O11,Nd2O3 and Sm2O3 on Ni-SiO2 for pressurized carbon dioxide reforming of methane

The 7 wt% rare earth metal oxide promoted Ni-SiO₂ catalysts of Ni-7Pr₆O₁₁-SiO₂,Ni-7Nd₂O₃-SiO₂,and Ni-7Sm₂O₃-SiO₂ were prepared by the complex-decomposition method,and were comparatively evaluated for pressurized carbon dioxide reforming of methane(CRM) under severe conditions of 750℃,1.0 MPa,CH₄/CO₂=1,and gas hourly space velocity of 53200 mL/(g·h).The addition of r...     

In-situ measurement of mineral phase transition and kinetics in roasting process of Bayan Obo mixed rare earth concentrate by sodium carbonate

In this study, the Bayan Obo rare earth concentrates mixed with Na₂CO₃ were used for roasting research. The phase change process of each firing stage was analyzed. The kinetic mechanism model of the continuous heating process was calculated. This study aims to recover valuable elements and optimize the production process to provide a certain theoretical basis. Using X-ray diffraction (XRD), Fourier infrared spectroscopy, scanning electron microscopy with energy dispersive spectrometry, the reaction process and the existence of mineral phases were analyzed. The variable temperature XRD and thermogravimetric method were used to calculate the roasting kinetics. The phase transition results show that carbonate-like substances first decompose into fine mineral particles, and CaO, MgO, and SiO₂ react to form silicates, causing hardening. Further, REPO₄ and NaF can directly generate CeF₃ and CeF₄ at high temperatures, and a part of CeF₄ and NaF forms a solid solution substance Na₃CeF₇. Rare earth oxides calcined at a high temperature of 750 °C were separated to produce Ce_0.6Nd_0.4O_1.8, Ce₄O₇, and LaPrO_3+x. Then, BaSO₄, Na₂CO₃, and Fe₂O₃ react to form barium ferrite BaFe₁₂O₁₉; the kinetic calculation results show that during the continuous heating process, the apparent activation energy E reaches the minimum in the entire reaction stage in the temperature range of 440–524 °C, and the reaction order n reaches the maximum, which indicates that the decomposition product REFO significantly impacts the reaction system and reduces the activation energy. The mechanism function is F(α) = [?ln (1?α)]^1/3. The reaction order n reaches the minimum in the temperature range of 680–757 °C, and the apparent activation energy E is large. The difficulty of the reaction increases during the final stage. The reaction mechanism function is F(α) = [1?(1?α)^1/3]². Observing the entire reaction stage, the step of controlling the reaction rate changes from random nucleation to three-dimensional diffusion (spherical symmetry).     

Study on Roasting Decomposition of Mixed Rare Earth Concentrate in CaO-NaCl-CaCl2

The decomposed process of bastnaesite, monazite and mixed rare earth concentrate in CaO-CaCl-CaCl2 was studied by means of TG-DTA method. The relationship among decomposition ratio, roasting temperature, and CaO-NaCl addition was studied by the quadratic regression orthogonal analysis, and then the regression equation was obtained. Through analysis, the optimum process conditions of mixed rare earth concentrate decomposed by CaO-CaCl-CaCl2 were obtained as follows: roasting temperature: 700 ℃, CaO addition: 15%, NaCl-CaCl2 addition: 10%, roasting time: 60 min, the decomposition ratio: 91.3%.     

碳酸钠焙烧盐酸浸出分解氟碳铈矿精矿工艺的研究

系统地研究了焙烧温度、焙烧时间和碳酸钠的加入量对氟碳铈矿精矿焙烧分解的影响及浸出时间、浸出温度、盐酸浓度和盐酸用量对三价稀土和铈的浸出率的影响。针对氟碳铈矿精矿的分解,提出了碳酸钠焙烧、水洗除杂、稀盐酸浸出处理氟碳铈矿精矿的简单工艺。在实验室内扩大实验的条件下,首次由焙烧、浸出两步制得了高质量的高铈抛光粉,同时也得到了制取少铈富镧氯化稀土的原料。     

Corrosion behavior of rare earth cerium based conversion coating on aluminum alloy

The present paper focused on the use of the salt of rare earth cerium as corrosion inhibitor of aluminum by using cathodic electrolytic passivation method. The corrosion resistance and the microphology of the cerium passivation film were studied by the methods of electrochemical method, scanning electron microscopy (SEM), and energy dispersive spectroscopic analysis. From the results, it was shown that good corrosion resistance of cerium-based passive coating was obtained when the compositions were as follows: CeCl₃·7H₂O, 0.05 mol/L; H₂O₂, 30 ml/L; current density, 1.1 mA/cm²; temperature, 40 °C; time, 9 min. SEM and EDS revealed that the cerium conversion coatings formed on the surface of aluminum alloy were related to cerium hydroxide/hydrated oxide depositions.     

Separation chemistry and clean technique of cerium（IV）： A review

The separation method of changeable valence RE element of cerium(Ce) was reviewed in this paper. Solvent extraction is the most effective and efficient method to separate Ce(IV) from RE(Ⅲ), usually accompanied with fluorine(F) and phosphor(P) from bastnaesite and monazite etc. By roast or wet-air oxidation, Ce(Ⅲ) of bastnaesite and monazite was oxidized into Ce(IV), and Cyanex923 and [A336][P507] have been investigated to co-extract and recover Ce(IV), F and P from H2SO4 leaching liquor, leading to favorable conditions for the subsequent separation of Th(IV) and RE(Ⅲ). The interaction of Ce(IV) and F and/or P enhances the roasting, leaching and extraction of Ce(IV) due to increasing of the stability of Ce(IV), and the formation of CeF3 and CePO4 after reductive stripping will benefit the utilization of F and P. For dealing with RE ores of high-content Ce, the clean process of oxidation roasting and Ce(IV)-F separation for Sichuan bastnaesite highlights the advantages of Ce(IV) based clean technique, which firstly demonstrates the comprehensive utilization of Ce(IV), Th(IV), F and RE(Ⅲ) and prevention of environmental pollution from fountainhead. A preliminary flowsheet of two-step oxidation and extraction of Ce(IV) for Bayan Obo mixed ores was further proposed to process the oxidation and extraction of Ce(IV) in presence of both F and P, indicating the possibility of similar effects with clean process of Sichuan bastnaesite. Ce(IV) separation chemistry and clean technique will open up new realms for light RE resources utilization, meeting "Emission Standards of Pollutants from Rare Earths Industry" promulgated by China's Ministry of Environment Protection(MOP) in 2011.