Adsorption of rare earth elements on organic matter in coal

The occurrence modes of rare earth elements(lanthanide and yttrium,abbreviated as REY) in coal are impo rtant both for coal geochemistry studies and the application potential of REY as a by-product of coalbased resources.In this study,the adsorption behaviors of REY on organic matter in coal were investigated by leaching tests using REY solution and ultra-low ash coal samples.On this basis,the adsorption mechanism of REY on organic matter in coal was also studied by molecular simulation calculat...     

Leaching recovery of rare earth elements from the calcination product of a coal coarse refuse using organic acids

Due to the increasing criticality of rare earth elements(REEs),it has become essential to recover REEs from alternative resources.In this study,systematic REEs leaching tests were performed on the calcination product of a coal coarse refuse using hydrochloric acid and different types of organic acid as lixiviants.Experimental results show that the recovery of REEs,especially heavy REEs(HREEs)and scandium(Sc),is improved by using selected organic acids.Citric acid and DL-malic acid afford the best leaching performances;whereas,malonic acid,oxalic acid,and DL-tartaric acid are inferior to hydrochloric acid.Results of zeta potential measurements and solution chemical equilibrium calculations show that malonic acid is more likely adsorbed on the surface of the calcined material compared with citric acid and DL-malic acid.The adsorption may reduce the effective concentration of malonic species in solution and/or increase the amount of REEs adsorbed on the surface,thereby impairing the leaching recovery.Compared with light REEs(LREEs),a stronger adsorption of the HREEs on the surface is observed from electro-kinetic test results.This finding explains why organic acids impose a more positive impact on the leaching recovery of HREEs,By complexing with the HREEs,organic acids can keep the metal ions in solution and improve the leaching recovery.The adsorption of Sc³⁺on the surface is the lowest compared with other REEs.Therefore,rather than complexing,the organic anionic species likely play a function of solubilizing Sc from the solid,which is similar to that of hydrogen ions.     

Modes of occurrence and pre-concentration of rare earth elements in No. 17 coal in Liupanshui coalfield,China

Rare earth elements(lanthanide and yttrium,abbreviated as REY) provide important support for national security,energy production,environmental protection and economic growth.In recent years,many studies have shown that the content of REY in some coal or coal by-products is close to or even higher than that in traditional REY ore,which makes coal a potential source of REY.This study took the No.17 coal of Panzhou mining area in Liupanshui coal field as the research object,and the content and occu...     

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.     

硝酸法生产磷酸过程中稀土的浸出研究

研究了采用硝酸法生产磷酸工艺中稀土的浸出情况。详细探讨了温度、硝酸浓度、时间、粒度、液固比等因素对磷精矿中稀土浸出率的影响。结果表明,浸出稀土的适宜工艺条件是:40%HNO3、60℃、液固比2.5、搅拌转速300 r/min、-0.038 mm占82%、浸出90 min。此条件下稀土浸出率与磷矿分解率均大于98%。     

Rare earth elements of fly ash from Wyoming's Powder River Basin coal

The advancement and increasing interests in green energy production and environmental protection technologies have spurred the demand for rare earth elements.On the other hand,the U.S.has to rely 100% on import of these materials as the industry was crashed because of the environmental issues associated with mining and processing,and more importantly lack competitiveness.As rare earth elements have many applications in defense and national security sectors,some of the elements were listed as strategic materials or critical materials because of the uncertainties in supply and prices.Therefore,it has become imperative in searching for alternative resources for supplying rare earth elements including unconventional resources.With the strong incentive,coal fly ash is identified as one of the candidates among them.In this study,we present rare earth element data for the 42 ash samples(all derived from Powder River Basin coal) that we collected from seven states(UT,WY,10,WI,ND,CO,and MI) representing 158 million tons of fly ash.The results indicate scattered distributions of rare earth elements with concentration ranging from 156 to 590 ppm although all the ash samples originated from the same coal basin(Powder River Basin).The rare earth element resource in these ashes is estimated between 74000 and 106000 t.The ash samples were also characterized by elemental analysis,XRD,SEM-EDS and BET surface area.The characterizations of the ashes were discussed as they might have implications in the subsequent rare earth element extraction processes.In summary,fly ash may represent a potential resource for rare earth element production.     

Study on the Formation of Crystalline Rare Earth Carbonates from Rare Earth Ore Leaching Liquors

The crystalline precipitate of rare earth carbonates has been produced by the ammonium bicar-bonate precipitation method in the recovery of RE from the ammonium sulfate leaching liquor of the weath-ered leach-deposited rare earth ore.Various influencing factors such as time,temperature and reagent con-centration have been studied.And chemical analysis,XRD,SEM,IR spectrography,TGA and DTA havebeen used to analyze and examine the crystalline RE carbonates.Good results were obtained.     

Extraction of selected rare earth elements from anthracite acid mine drainage using supercritical CO_2 via coagulation and complexation

Rare earth elements(REEs) are valuable raw materials which are in great demand in modern high technology industries.Developing methods to produce/recover REEs from waste is significant to the national security of any developed country.This study was focused on investigating the use of supercritical CO2(sCO_2) to extract REEs from anthracite acid mine drainage(AMD).Four different mine drainage water source locations at Blaschak Coal Corp.in Pennsylvania,USA were selected for sample collection.An extraction process was developed and demonstrated for two of those water sources containing the highest concentration of REEs.A method involving metal ion coagulation,their dissolution from the sludge into a concentrated aqueous HNO_3 solution,complexation with organic ligands and sCO_2 extraction was developed to recover REEs from AMD.Specifically,sodium aluminate(NaAlO_2) was used as the coagulant to concentrate REEs from the AMD into a solid precipitate.Consequently,over 99%of the REEs in AMD is concentrated in the remaining sludge.During the coagulation process,the effects of pH and NaAlO_2 concentration on REE precipitation were investigated.Fuming nitric acid(HNO_3) was used to digest the pre-concentrated sludge and tributyl phosphate(TBP) was used to form REE/TBP/HNO_3,a non-polar complex with selected REEs,specifically,cerium(Ce),lanthanum(La) and neodymium(Nd).HNO_3 concentration and organic/aqueous phase ratio were considered as the variables to improve complexation efficiency.Dynamic extraction experiments using sCO_2 and REE/TBP/HNO_3 solutions were then conducted at optimal conditions of 60℃ and 20 MPa.The overall REE extraction efficiencies are found to increase with the atomic number of the REE.As a result,the average overall REE extraction efficiencies of 41.8%,40.1% and 58.2% for Ce,La and Nd,respectively,are obtained.The potential improvements in the overall extraction efficiency are also discussed.     

Review on catalytic roles of rare earth elements in ammonia synthesis: Development and perspective

Ammonia(NH3)is mainly produced via the Haber-Bosch process.It was discovered that the performance of a wide variety of catalysts in NH3 synthesis could be considerably enhanced by the addition of rare earth elements(REEs).As a result,catalysts promoted by REEs,especially the Ru-based ones have been extensively investigated.In this review,we summarize the progress of utilizing REEs for ammonia synthesis and outline the prospects of using them in the design and development of highly efficient and stable catalysts for ammonia synthesis.     

Recovery of rare earths from ion-absorbed rare earths ore with MgSO4-ascorbic acid compound leaching agent

The magnesium sulfate leaching technology for the ion-absorbed rare earths ore can solve the ammonia pollution problem existing in ammonium sulfate leaching process. However, the leaching capacity of magnesium sulfate is slightly weaker than that of ammonium sulfate, resulting in a bigger consumption of magnesium sulfate. In this paper, the MgSO_4-ascorbic acid compound leaching agent had been demonstrated to deal with the ion-absorbed rare earths ore. The ascorbic acid could form a stable coordination with rare earth ions, so that it can strengthen the leaching of ion-exchangeable phase.Moreover, ascorbic acid has a strong reductive property, it can leach the colloidal sediment phase rare earth as well. The present study investigates the effect of the initial pH and the composition of leaching agent on the rare earth leaching. It is determined that the rare earth leaching efficiency is 107.5% under the condition of pH 2.00,0.15 mol/L magnesium sulfate and 1.0 g/L ascorbic acid in leaching agent. In this case, the content of the ion-exchangeable phase and colloidal sediment phase rare earth in the leaching residues are both only 0.02‰. The leaching efficiency of colloid sediment phase rare earth can be 85.7%,so that the Ce partition in the leaching liquor increases to be 5.77%. The magnesium-ascorbic acid compound leaching agent is proposed to be a promising choice to deal with the ion-absorbed rare earths ore, which can realize the efficient leaching, low consumption of MgSO_4 and environmentally friendly leaching.     

Dissolved rare earth elements estimation of ion-absorption rare earth ores using reflectance spectroscopy in south Jiangxi province,China

Ion-absorption rare earth ores are an important mineral resource in China. Nowadays, the unauthorized mining has become a serious problem, resulting in severe water pollution and the wastage of rare earth elements (REEs). Being able to estimate the concentration of dissolved REEs in water bodies near mines is essential for tackling this environmental problem. Conventionally, quantitative analyses of the contents of dissolved REEs are performed using laboratory-based techniques, which can be time consuming and costly. Spectral reflectance is a rapid and cost-effective means of characterizing the chemical compositions of light-absorbing materials. In this study, reflectance spectroscopy was performed on dissolved REEs, and the correlation between their reflectance characteristics and REE content was determined. A total of 50 aqueous media samples collected in south Jiangxi Province and 25 laboratory-produced aqueous media samples were tested, and their reflectance spectra and REE contents were measured using reflectance spectroscopy and inductively coupled plasma mass spectrometry, respectively. Next, the reflectance, differential reflectance, and absorption depth were analysed based on the REE content. Six diagnostic absorption features related to REEs are recognised in the visible and near-infrared wavelength regions, along with several smaller peaks. It indicates that the results of the absorption depth analysis are in accordance with the absorption spectra characteristics of the REEs, with the R² value being higher than 0.97. The intensity of each of the six absorption bands exhibits a linear correlation to the total REE content. Therefore, linear regression models can be derived for estimating the total concentration of REEs in aqueous media samples. What's more, the detection limit for REEs is determined to be about 30 μg/L. Thus, it can be concluded that reflectance spectroscopy is a suitable technique for estimating the concentration of dissolved REEs.     

Rare earth elements in flotation products of coals from East Kootenay coalfields,British Columbia

With the identification of coal deposits that are enriched in rare earth elements (REEs) across the world, coal and coal by-products are considered as an alternative source for REEs due to the combined supply and demand restrictions for these elements. Studies are being conducted to recover these elements from various coal sources. In this study, coal samples from the East Kootenay coalfields in British Columbia, Canada were examined for the presence of REEs in run-of-mine coal samples and flotation products.Results reveal that on an ash basis, the concentration of REEs varies from 174 to 1038 mg/kg in the samples. To understand the REE as sociation, which is essential for extraction purposes, a three-step sequential extraction process along with a mineralogical evaluation using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and quantitative X-ray diffraction (XRD) data was used. It is found that a significant portion of REEs in the feed samples were reported to middling and tailings streams. The mineralogical analyses reveal REE association with aluminosilicate mineral in the samples.     

Estimating REY content of eastern Kentucky coal samples based on their associated ash elements

Coal and coal combustion byproducts can be considered as an alternative source of rare earth elements and yttrium (REY). The study of relationships between REY and other main coal properties could have several advantages such as estimating REY content of coal particles and designing beneficial extraction method. In this investigation, inter-correlations between REY content with coal parameters (proximate and ash elements) for a wide range of eastern Kentucky coal samples (708 records) were explored. Results demonstrate that zircon and monazite are the main source of heavy and light rare earth elements (HREE and LREE), respectively. Zr has the highest correlation with Y and Th has the strength relationship with Ce and La. In general, LREE have higher interaction with coal ash content in comparison with HREE. Results indicated that REY can be estimated quite satisfactorily by using their associated elements in coal ash.     

Evaluation of toxicity profiles of rare earth elements salts (lanthanides)

The presented study aims to extend the knowledge of toxicological profile of rare earth elements salts(REEs).The basal toxicity test performed comprised assessment of cytotoxicity(3 T3 Balb/c Neutral Red Uptake Test)that allows for calculation of LD50(rats)on the basis of the concentration which leads to a50%reduction in cell growth(IC50).Environmental toxicity was addressed by the Tubifex tubifex(T.t.)express test.The in vitro skin irritation(OECD TG 439)and skin corrosion tests(OECD TG 431)utilizing the 3 D in vitro reconstructed human epidermal model EpiDerm(MatTek IVSL,SK)were used for assessment of skin irritation and corrosion potential hazard of REEs.Mutagenic effects were determined using the bacterial reverse mutation assay(Ames Test)on 5 Salmonella typhimurium strains with and without metabolic activation(OECD TG 471).Endocrine disruption was evaluated by means of a yeastbased assay YES/YAS(Xenometrix,CH).Skin sensitization was assessed using the LuSens assay,based on a genetically modified human keratinocyte cell line(OECD TG 442 D).The tested REEs have no potential of mutagenicity or skin sensitization,exhibit very weak endocrine disruption potential and only exceptional local irritation/corrosion effects for thulium(Ⅲ)chloride anhydrous,but have acute and chronic toxic effects on the aquatic environment.     

Simultaneous recovery of rare earth elements and phosphorus from phosphate rock by phosphoric acid leaching and selective precipitation:Towards green process

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%P_2 O_5 acid with an L/S ratio of 10:1 and a stirring speed of 250 r/min at 25 ℃ 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 ℃ and keeping for4 h. Thereafter, more than 95% phosphoric acid is recovered by H_2 SO_4 and high purity gypsum, more than95% CaSO_4(tested by XRF), is also produced at the same time. Ultimately, a green process that leaches phosphate rock with H_3 PO_4, selectively precipitates REEs from leaching solution by heating up, recovers H_3 PO_4 with H2 SO4 is proposed. Compared with REE recovery in traditional processes, this process owns the merits of simple operation, energy saving and minimum wastes.     

Kinetics model for leaching of ion-adsorption type rare earth ores

This paper aims to establish a mathematical model that can analyze the whole leaching kinetics process of ion-adsorption type rare earth ores. This leaching process is composed of three steps: (1) ammonium ions arrive at the ore particle surface through the diffusion layer; (2) ammonium ions exchange with rare earth ions; and (3) rare earth ions enter into the external solution through the diffusion layer. In the leaching process, it is hypothesized that the ore particle size remains constant. The process of ammonium ions and rare earth ions passing through the diffusion layer was described by the Fick law, and the reversible ion exchange process between ammonium ions and rare earth ions was described by the Kerr model. A leaching kinetics model of rare earth ions by ammonium ions was constructed. Accuracy of this kinetics model was verified with laboratory tests. It is found that the correlation coefficients of all data are greater than 0.9000. The proposed kinetics model is therefore feasible for kinetics analysis throughout the leaching process.     

Effects of contaminant metal ions on precipitation recovery of rare earth elements using oxalic acid

Solution equilibrium calculations were performed in this study to understand the impact of contaminant metal ions on the precipitation efficiency of selected ra...     

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.     

Progress in green and efficient enrichment of rare earth from leaching liquor of ion adsorption type rare earth ores

Ion adsorption type rare earth ores (IATREOs) are a valuable strategic mineral resource in China, which feature a complete composition of fifteen rare earth elements and are rich in medium and heavy rare earth (RE) elements. In the leaching process for recovering rare earth elements from IATREOs, many impurities will be leached together with rare earth elements and enter the leaching liquor. An impurity removal-precipitation enrichment technique is currently applied to selectively recovery rare earth elements from the leaching liquor with the high content of impurities and low concentration of rare earth elements by using ammonium bicarbonate in the industry. However, a high loss of rare earth elements and severe ammonia nitrogen pollution are caused by this process. Therefore, more beneficial impurities removal technologies, mainly for aluminum, and green enrichment technologies with lower pollution are now urgently needed. For this purpose, this paper analyzed two aspects of research progress in recent decades: the green separation of rare earth elements and aluminum from leaching liquor and the green and efficient enrichment of rare earth elements. Finally, an approach for the high-efficiency and green enrichment of rare earth elements from leaching liquor of the IATREOs is proposed in several aspects, including impurity inhibition leaching, neutralization and impurity removal, alkaline calcium and magnesium salt precipitation enrichment, and centrifugal extraction enrichment.     

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.