Precipitation Of Rare Earths Research Articles

Preparation of large specific surface rare earth oxides from calcium-containing rare earth solution by carbon dioxide carbonization method☆

The calcium-containing rare earth solution is generated during the recovery processes of NdFeB waste, which is treated as wastewater by enterprises. In this paper, the carbon dioxide carbonization method was applied to the separation of rare earths and calcium in the solution, as well as the preparation of rare earth oxides with a large specific surface. It is shown that the process of CO2 carbonization of solution includes reactions such as the dissolution, diffusion and ionization of CO2, the carbonate precipitation of rare earth ions, and the neutralization of hydrogen ions. At a pH of 4.5, the carbonization precipitation rate is effectively controlled, enabling homogeneous precipitation and ensuring both high precipitation yield and rare earth oxides purity. In this way, the crystallization of carbonization products is a process dominated by the oriented attachment theory and coexisting with the Ostwald ripening theory, resulting in abundant pores formed by multiple layers of stacking in the products. With the optimal carbonization conditions, the rare earth precipitation yield solution reaches 99.32%. The obtained carbonization products are crystalline (LaCe)(CO3)3·8H2O, and the purity of the rare earth oxides is as high as 99.22 wt%. The specific surface area of the rare earth oxides reaches 94.7 m2/g, and its adsorption efficiency for tetracycline hydrochloride in solution can reach 92.6% in a short time. The rare earth oxides are expected to be used as an adsorption material for wastewater treatment and other adsorption environments.

Recovery of Rare Earth Elements from the Leaching Solutions of Spent NdFeB Permanent Magnets by Selective Precipitation of Rare Earth Oxalates

After mechanical pre-treatment, the typical hydrometallurgical route of NdFeB magnet recycling starts with leaching in acidic solutions. However, due to the high concentration of iron ions in the leaching solution, the selective recovery of rare earths from the solution is challenging. In our work, the selective precipitation of rare earth oxalates as a potential separation method was proposed. The precipitation of neodymium oxalate was first tested on model solutions, which was then followed by experimental tests carried out on real solutions after the leaching of NdFeB magnets. The recovery of rare earths in the form of oxalates was investigated with the use of different amounts of oxalic acid in relation to its stoichiometric amount. The most efficient separation of rare earths was observed in the case where sulfuric acid was used for leaching. The use of oxalic acid in stoichiometric amounts resulted in the precipitation of about 93% of all rare earths present in the solution, whereas the concentration of Fe and other elements (Ni, Co, and B) practically did not change. An increase in oxalic acid of 20% and 40% more than the stoichiometric amount (100%) led to the increase in the precipitation efficiency of rare earths to 96.7% and 98.1%, respectively. However, the use of oxalic acid in a 1.4 ratio caused a 7% decrease in Fe concentration, which suggests Fe co-precipitation. In order to investigate a possibility of further increasing the separation of rare earths from iron, an additional method was tested, in which iron was first oxidized from Fe2+ to Fe3+ before the precipitation of rare earth oxalates.

Separation of Hydrochloric Acid and Oxalic Acid from Rare Earth Oxalic Acid Precipitation Mother Liquor by Electrodialysis.

In this study, the hydrochloric acid from rare earth oxalic acid precipitation mother liquor was separated by electrodialysis (ED) with different anion exchange membranes, including selective anion exchange membrane (SAEM), polymer alloy anion exchange membrane (PAAEM), and homogenous anion exchange membrane (HAEM). In addition to actual wastewater, nine types of simulated solutions with different concentrations of hydrochloric acid and oxalic acid were used in the experiments. The results indicated that the hydrochloric acid could be separated effectively by electrodialysis with SAEM from simulated and real rare earth oxalic acid precipitation mother liquor under the operating voltage 15 V and ampere 2.2 A, in which the hydrochloric acid obtained in the concentrate chamber of ED is of higher purity (>91.5%) generally. It was found that the separation effect of the two acids was related to the concentrations and molar ratios of hydrochloric acid and oxalic acid contained in their mixtures. The SEM images and ESD-mapping analyses indicated that membrane fouling appeared on the surface of ACS and CSE at the diluted side of the ED membrane stack when electrodialysis was used to treat the real rare earth oxalic acid precipitation mother liquor. Fe, Yb, Al, and Dy were found in the CSE membrane section, and organic compounds containing carbon and sulfur were attached to the surface of the ACS. The results also indicated that the real rare earth precipitation mother liquor needed to be pretreated before the separation of hydrochloric acid and oxalic acid by electrodialysis.

Precipitation process for combined impurity removal from a magnesium sulphate-based leachate of ionic rare earth ore

Impurity metal ions in ionic rare earth ores are similar to rare earth ions in terms of occurrences and similarities in properties. During the leaching operation, a large number of impurity ions enter the rare earth leaching solution, which makes the crystal form of the rare earth unstable or forms an amorphous flocculent precipitate. The commonly used ammonium bicarbonate (NH4HCO3) precipitation crystallization process has low rare earth precipitation rate, long precipitation crystallization period, and high ammonia nitrogen content in precipitation wastewater. In this paper, the leaching solution obtained by ionic rare earth ore magnesium salt leaching-combined impurity removal process was taken as the research object, and the effects of precipitants, such as NH4HCO3, calcium oxide, sodium carbonate, and oxalic acid, on the preparation of crystalline rare earth precipitation were investigated through comparison. On the premise of determining the best rare earth precipitant, the effects of dosage, stirring speed, stirring time, and aging time on the preparation of crystalline rare earth precipitate were investigated further. The precipitation effects of different precipitation processes were studied systematically combined with SEM-EDS, laser particle size analysis, and other means to characterize the morphology of crystalline products. Results showed that when calcium oxide was used as a precipitant, the molar ratio was 2.5. After stirring at a constant speed of 200 r/min for 4 h and aging for 4 h, precipitates with good crystal appearance, smooth crystal surface and easy filtration were obtained. This process promoted crystal growth of rare earth precipitates, and the crystal grain size, which was beneficial to the improvement of the quality of rare earth products, increases significantly.

Selective precipitation of rare earth and critical elements from acid mine drainage - Part I: Kinetics and thermodynamics of staged precipitation process

Critical elements (CEs) have been in the spotlight recently due to their promising role in green energy transition and high-tech developments. Secondary resources, such as acid mine drainage (AMD) are of great potential source for these elements. Selective recovery of CEs such as Al, rare earth elements (REEs), Co, and Mn from AMD is deemed viable. To design and scale up the CEs recovery process, parameters such as kinetics and other thermodynamics parameters are vital. This work determined the reaction rates for precipitation of the Al, REEs, Co, and Mn in a three-staged precipitation process. The experimental data were examined with Avrami and second-order kinetics models. Analyzing the parameters driving and controlling the precipitation showed that the mechanism in the precipitation of elements from the solution is the supersaturation of species. Furthermore, the condensation and polymerization of the metal ions with ligand molecules results in large polycation complexation and growth.

Selective precipitation of rare earth and critical elements from acid mine drainage - Part II: Mechanistic effect of ligands in staged precipitation process

Recovery of critical elements from secondary resources, such as acid mine drainage (AMD), has been deemed a potential path to support the raw materials needed to support the increasing high-tech and green energy developments. The precipitation patterns of critical elements in AMD treatment ponds differ when NaOH and Na2CO3 are used. In order to determine the reason for the difference in precipitation pattern of REEs and critical elements in these two conditions as well as the prospects of selective recovery of these elements, surface charge, particle size, and formation of the precipitates obtained from the treatment of AMD using a 3-stage precipitation process were studied. It was shown that the REEs surface complexation and co-precipitation with Al precipitates are unlikely. Particle size analysis concluded that the Al and Co-Mn precipitates tend to agglomerate after the initial precipitation of very fine particles. However, no agglomeration of the REE precipitates was observed.

Review on the Development and Utilization of Ionic Rare Earth Ore

Rare earth, with the reputation of “industrial vitamins”, has become a strategic key metal for industrial powers with increasingly significant industrial application value. As a unique rare earth resource, ionic rare earth ore (IREO) has the outstanding advantages of complete composition, rich resource reserves, low radioactivity, and high comprehensive utilization value. IREO is the main source of medium and heavy rare earth raw materials, which are in great demand all over the world. Since the discovery of IREO, it has attracted extensive attention. Scientists in China and the around world have carried out a lot of research and practical work and achieved a series of important breakthroughs. This paper introduces the discovery process, metallogenic causes, deposit characteristics, and the prospecting research progress of IREO, so as to deepen the understanding of the global distribution of ionic rare earth resources and the prospecting direction of ionic rare earth deposits. The leaching principle of IREO, the innovation of leaching process, the influencing factors and technological development of in situ leaching process, and the technical adaptability of in situ leaching process are reviewed. The development of leachate purification and rare earth extraction technology is summarized. We aim to provide guidance for the industrial development of IREO through the above review analysis. Additionally, the problems existing in the development of IREO are pointed out from the aspects of technology, economy, and the environment. Ultimately, a series of suggestions are put forward, such as the development of ammonium free extraction technology in the whole exploitation process of in situ leaching and leachate purification and rare earth precipitation, research on enhancing of seepage and mass transfer process, and research on the development of new technologies for impurity removal of leachate and extraction of rare earth, so as to promote the development of green and efficient exploitation new technologies and sustainable development of ionic rare earth ore.

Separation and coextraction of REEs and Fe from NdFeB sludge by co-leaching and stepwise precipitation

NdFeB sludge is a waste with high economic value. NdFeB waste recycling has the disadvantage that only rare earths are recycled, but iron (Fe) is not recycled, which results in large amounts of secondary solid waste. In this paper, a process flowchart of the co-extracted rare earth elements (REEs) and Fe by co-leaching and stepwise precipitation from NdFeB sludge was developed. The results show that after the NdFeB sludge was pretreated, co-leaching with hydrochloric acid, rare earth precipitation with oxalic acid, and Fe precipitation with reduction iron powder, 98.28% of the REEs were recovered in the form of RE2(C2O4)3·10H2O, and 94.65% of the Fe was recovered in the form of FeC2O4·2H2O. Due to the low solubility product constants of the rare earth oxalates, when oxalic acid was added to the leachate, the REEs precipitated rapidly, while Fe was still present as FeCl4–, Fe(C2O4)2–, and FeC2O4+ in the precipitated REEs solution. Therefore, the selective separation and precipitation of REEs and Fe was achieved. The process has the advantages of high comprehensive utilization of resources and no secondary solid waste or wastewater pollution.

Parametric study and speciation analysis of rare earth precipitation using oxalic acid in a chloride solution system

Oxalic acid precipitation is a common step in the purification of rare earth elements (REE) from a concentrated pregnant leach solution (PLS). However, the presence of contaminants such as Al, Fe, and Ca in given amounts decreases the REE precipitation efficiency and product purity while also increasing the amount of oxalic acid needed to maximize recovery. As such, a statistically designed test program was performed to identify the optimal conditions necessary for a relatively low REE content PLS containing elevated concentrations of contaminant ions. The performance objective was maximization of REE precipitation efficiency while minimizing the oxalic acid dosage. A central composite design was utilized to quantify performance impacts and identify the ultimate set of parameter values for oxalic acid dosage, Fe(III) contamination concentration, solution pH, and reaction temperature. The resultant model suggested that oxalic acid dosage and reaction pH are the most significant factors for the REE precipitation efficiency, followed by the interaction of oxalic dosage and Fe concentration. Test results indicate that increasing the oxalic acid concentration from 0 g/L to 80 g/L improved the REE precipitation efficiency from approximately 4.2% to 95.0%. Furthermore, raising the solution pH from 0.5 to 2.5 considerably enhanced the precipitation efficiency from 0.0% to 98.9%. A solution temperature elevation decreased REE recovery, which indicated an exothermic reaction between REEs and oxalate anions. Finally, a high level of Fe contamination adversely impacted REE precipitation efficiency. To further the understanding of the REE-oxalate system, a fundamental solution chemistry study was performed using the equilibrium constants of the reactions. The study resulted in the development of oxalate speciation diagrams and provided an analysis of the REE precipitation characteristics at various oxalate anion concentrations and Fe(III) contamination levels using MINTEQ software. The dominant Fe(III) species in the solution system were found to be Fe-(C2O4)33-, Fe-(C2O4)2-, and Fe-(C2O4)+, which consume the majority of the oxalate anions. The simulated model was found to be in agreement with the experimental findings and helped to explain the adverse impact of increased iron concentrations on REE precipitation efficiency.

Neodymium naphthenate-loaded organic phase stripping using sodium oxalate

Neodymium naphthenate-loaded organic phase stripping using sodium oxalate solution was studied to explore the feasibility of synchronous rare earth-loaded organic phase stripping, rare earth precipitation, and blank organic phase saponification. Experimental results show that loaded organic phase stripping, rare earth precipitation, and blank organic phase saponification can be realized simultaneously. When using 20% excess of sodium oxalate over the stoichiometry with the volume ratio of organic phase to aqueous phase of 1:1 at 25 °C for 40 min, the single stage stripping rate and saponification value are about 40% and 0.29 mol/L, respectively. After 16 stages of countercurrent continuous stripping, the stripping rate of neodymium can reach 99%, the saponification value is 0.42 mol/L, the Nd3+ concentration in saponified organic phase is less than 0.0020 mol/L, and the main phase in precipitation is Nd2(C2O4)3·10H2O. Afterwards, this saponified organic phase can be used in the extraction of NdCl3 solution, and then the loaded organic phases (neodymium naphthenate) with 0.16 mol/L Nd3+ can be retrieved. The morphology, particle size distribution, and composition of the Nd2(C2O4)3·10H2O products are similar to those of the current direct precipitation products. The neodymium oxide prepared by continuous calcination of neodymium oxalate meets the national standard of China (GB/T 5240−2015). These results prove the feasibility of stripping neodymium naphthenate-loaded organic phase by using sodium oxalate solution. Sodium oxalate can serve as a stripping agent, a saponifier, and a precipitator, thereby simplifying rare earth extraction and separation. This study provides theoretical and technical support for the development of a novel method for rare earth extraction and separation.

Simulation-based life cycle assessment for hydrometallurgical recycling of mixed LIB and NiMH waste

The sustainable and competitive recovery of materials from spent batteries is becoming an urgent issue due to the increasing popularity of hybrid and fully electric vehicles. In this present study, flowsheet simulation was combined with life cycle assessment to investigate the environmental impacts of a conceptual, experimentally proven hydrometallurgical battery recycling process, where nickel metal hydride (NiMH) battery waste is used as reductant for lithium-ion battery (LIB) waste, synergistically improving the extraction of valuable metals from both waste types. Various options for sodium circulation in the precipitation of rare earths were considered in different scenarios. The results showed that the main benefit of the process was the reduced consumption of leaching chemicals. Of the investigated sodium management scenarios, crystallization of sodium sulfate was found to be the most environmentally feasible option, allowing the use of Na as a precipitation chemical for rare earth recovery. Significant potential reductions in climate change, acidification, freshwater eutrophication, and human toxicity were also achievable when compared to the life cycle impacts of the primary production of battery metals, mainly due to the high environmental footprint of primary nickel and cobalt sulfate production. Although further improvements to the process were found to be possible, the future availability of waste NiMH batteries may limit the application of the described process concept on an industrial scale.

Thermodynamics and Kinetics of Sulfuric Acid Leaching Transformation of Rare Earth Fluoride Molten Salt Electrolysis Slag.

Rare earth element recovery in molten salt electrolysis is approximately between 91 and 93%, whereof 8% is lost in waste molten salt slag. Presently, minimal research has been conducted on the technology for recycling waste rare earth molten salt slag, which is either discarded as industrial garbage or mixed with waste slag into qualified molten salt. The development of a new approach toward the effective treatment of rare earth fluoride molten salt electrolytic slag, which can recycle the remaining rare earth and improve the utilization rate, is essential. Herein, weak magnetic iron separation, sulfuric acid leaching transformation, water leaching, hydrogen fluoride water absorption, and cycle precipitation of rare earth are used to recover rare earth from their fluoride molten salt electrolytic slag, wherein the thermodynamic and kinetic processes of sulfuric acid leaching transformation are emphatically studied. Thermodynamic results show that temperature has a great influence on sulfuric acid leaching. With rising temperature, the equilibrium constant of the reaction gradually increases, and the stable interval of NdF3 decreases, while that of Nd3+ increases, indicating that high temperature is conducive to the sulfuric acid leaching process, whereof the kinetic results reveal that the activation energy E of Nd transformation is 41.57 kJ/mol, which indicates that the sulfuric acid leaching process is controlled by interfacial chemical reaction. According to the Nd transformation rate equation in the sulfuric acid leaching process of rare earth fluoride molten salt electrolytic slag under different particle size conditions, it is determinable that with the decrease of particle size, the reaction rate increases accordingly, while strengthening the leaching kinetic process. According to the equation of Nd transformation rate in the sulfuric acid leaching process under different sulfuric acid concentration conditions, the reaction series of sulfuric acid concentration K = 6.4, which is greater than 1, indicating that increasing sulfuric acid concentration can change the kinetic-control region and strengthen the kinetic process.

Recycling of ultrafine NdFeB waste by the selective precipitation of rare earth and the electrodeposition of iron in hydrofluoric acid

In this work, a simple and sustainable approach is proposed to recover rare earths and iron from an ultrafine NdFeB waste. Rare earths are directly recovered by one-step precipitation in hydrofluoric acid and the dissolved iron by electrodeposition. The rate constant and activation energy of the reaction of rare earth hydroxide with hydrofluoric acid are investigated to understand the kinetics of iron leaching and rare earth precipitation. The rare earth precipitation is much faster than the iron leaching, enabling the rare earths separation from iron after a complete iron dissolution. The cathodic deposits of iron were characterized by X-ray photoelectron spectrometer (XPS) and scanning electron microscope (SEM). The nano-scale FeF2(s) and Fe(s) were deposited on Pt electrode in hydrofluoric acid solution with pH 2.3 and pH 2.89, respectively. The reduction of Fe(III) to Fe(0) was investigated by cyclic voltammetry, chronopotentiometry and open circuit chronopotentiometry. This reduction was a quasi-reversible process which was controlled by diffusion. The temperature dependence of the diffusion coefficient was established as lnD = −9.84 − 1805.43/T and the activation energy value of the diffusion process was calculated to be −15.01 kJ mol−1. The relationship between the Gibbs energy of intermetallic Fe-Pt formation and the temperature was established to be ΔG = −487.53 + 0.936T. The hydrofluoric acid after the electrodeposition could be recycled for the leaching of the ultrafine NdFeB waste again.

Selective precipitation of rare earth from non-purified and purified sulfate liquors using sodium sulfate and disodium hydrogen phosphate

The conventional hydrometallurgical process routes for the recovery of rare earth elements (REEs) from ores employs sulfuric acid. After being leached, the rare earth sulfate liquor presents impurities that need to be removed before they can be used in the production of marketable products. In this sense, a non-purified rare earth sulfate liquor obtained from a rare earth mineral processing and containing low rare earth oxides (REOs) concentration (∼4.6 g/L) and high Ca2+, Mg2+, Mn2+, Fe3+, Al3+, Th4+, UO22+, PO43− and SO42− content were used in this study. In order to remove all the Fe3+, Th4+ and PO4−3 ions and reduce Al3+, Ca2+, UO22+ and SO42− concentrations, the liquor was purified in two consecutive steps: first by addition of limestone to pH of 3.5 and then with a dosage of lime until it reached a pH of 5.0. Only about 7% of the REEs mass feed in the liquor was lost in the purification step. The REEs precipitation experiments were carried out with two liquors, non-purified and purified, using Na2SO4 and Na2HPO4 as reagents. The rare earth products with the highest purity were obtained with purified rare earth sulfate liquor. In the best condition, the consumption ratio was 0.31 ton of Na per ton of REEs and it resulted in the precipitation of 100% of the REEs feed in the liquor by addition of Na2HPO4. In the experiments in which Na2SO4 was used, the consumption was 0.82 ton of Na per ton of REEs and only 88% of the REEs feed was precipitated. The temperature to precipitate the REEs using Na2HPO4 ranged from 20 °C to 70 °C, whereas precipitation with Na2SO4, only occurred at 70 °C. The rare earth products obtained with Na2SO4 (sodium and rare earth double sulfate) and with Na2HPO4 (a mixture of rare earth phosphate and sodium and rare earth double sulfate), containing respectively 41.4% w/w and 42.9% w/w of REOs, seemed to have enough purity to be processed in downstream steps. As a result, a flowsheet for a process route to recover the rare earth elements from a non-purified rare earth liquor was proposed.

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 ion-adsorption 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, SO3 and Al2O3 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 TG-DSC, 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 °C and the thermal decomposition of RE2O2SO4 at 900–1300 °C. Therefore, a high-temperature calcinations method for removing SO3 from precipitates is proposed. When the precipitates are calcined at 1300 °C for 2 h, the rare earth concentrates with a purity of 92.03 wt% can be acquired. Moreover, the content of SO3 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 non-ammonia 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.

Enhanced characteristics and mechanism of static magnetic field on ion-absorbed rare earth precipitation

Separation of ion-absorbed rare earth elements (REE) by oxalate used as a precipitator is the principal extraction process employed in the mining industry. The oxalic acid precipitator plays the key role in the precipitation process. In this paper, the enhanced effect of a static magnetic treatment applied to the precipitation process of REs was investigated. Compared to the non-magnetization, the results of magnetic processes appeared to be better and its optimal precipitation conditions was obtained at pH value of the precipitation solution at 2,RE2O3: H2C2O4=2.5:1, a magnetic strength 400kAm−1, a 20min magnetization time in a magnetized oxalate solution. The dosage of oxalate precipitator is less 5% to that employed in conventional processes conditions with non-magnetization while the purity of the rare earth still keeps stable. The electrical conductivity of the oxalate precipitator was increased by magnetization. A possible change in the electron spin of the magnetized precipitator ultimately affected the pace of rare earth precipitation crystallization and growth direction of the rare earth-bearing precipitated crystals.

Removing aluminum from a low-concentration lixivium of weathered crust elution-deposited rare earth ore with neutralizing hydrolysis

Aluminum, the main impurity in the lixivium of weathered crust elution deposited rare earth ore, not only results in an increasing consumption of precipitant in the rare earth precipitation process, but also lowers the purity of final rare earth product. Aluminum in the weathered crust elution-deposited rare earth ore lixivium should be removed. Neutralizing hydrolysis method was employed to remove aluminum from the lixivium. Hexamethylenetetramine was found to be the optimum pH regulator for the removal of aluminum in the low concentration. When used to adjust the pH value of the lixivium to 5.0, aluminum in the lixivium can be effectively removed in the form of aluminum hydroxide precipitation with removal rate of 97.60 %. It shows that hexamethylenetetramine has a good effect on the removing of aluminum ions from the low-concentration lixivium. Moreover, hexamethylenetetramine in removing aluminum from lixivium has little adverse effect on the RE precipitation process.

Inhibitive effect of Ce(III) and La(III) cations for AA2219 aluminium alloy corrosion in sodium chloride medium

In this paper, the rare earth chlorides were used to mitigate the dealloying of Al2Cu intermetallic particles in aluminium alloy 2219-T87 plate exposed to 0.6 M NaCl solution. The negative shift of cathodic branches of potentiodynamic polarisation curves revealed the cathodic inhibition towards intermetallic dealloying. The low and high frequency time constants obtained from electrochemical impedance spectra revealed an increase in the corrosion resistance due to precipitation of rare earth oxides/hydroxides on the porous surface of native oxide film of aluminium alloy. Scanning electron microscopy coupled with energy dispersive X-ray analysis and atomic force microscopy revealed that intermetallic inclusions are the sites for the formation of cerium and lanthanum deposits which suppress copper redeposition and minimise galvanic interactions between the matrix and Al2Cu particles.

(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

In this invited talk, we will report our latest advances in the field of nanostructured silicon-based light emitters with special emphasis in those that are rare earth doped. We have recently shown that it is possible to push external quantum efficiencies over the 10% by fine tuning the material composition and by using a new device concept that we have called hot electron engineering.Most of the rare earth amount introduced in the host material must be optically active. This can be accomplished by using a deposition method that introduces the rare earth during film growth and by a suitable passivating annealing procedure that: i) does not promote rare earth precipitation and clustering; ii) brings the rare earth to the 3+ oxidation state and iii) reduces non radiative defects. Additionally, for electroluminescent devices the matrix composition must be engineered so that it allows conduction-band transport in contrast with defective systems in which transport is hopping-like.The hot electron engineering concept relies on the fact that the rare earth excitation process in Si-based dielectric matrices proceeds mainly via impact excitation by hot electrons. Thus, devising suitable structures for which electrons can be accelerated to average energies that are resonant to the rare earth excitation energies, will improve both efficiency and maximum power emission. Additionally, driving the device with pulsed polarization and short duty cycle will improve device efficiency and lifetime. Finally, the structures must allow at the same time significant light extraction efficiency. This can be accomplished by improving the antireflective properties of the whole stack and/or by nanostructuring the ITO or polysilicon electrode.We will be show silicon oxides, nitrides and oxynitrides as hosts for the rare-earth species and, additionally, we will display the advantages and drawbacks of introducing small percentages of carbon and aluminum. The rare-earth species incorporated in those matrices are Er, Ce, Tb, Eu and Nd and their light emission efficiency depend in a complex way on matrix composition, material processing, device structure and device polarization. The simplest hot electron engineering structures are bilayers and trilayers with silicon oxide as accelerator. More complex structures like multilayers and superlattices will be also introduced and optimal designs will be proposed. Some reliability issues and potential solutions for them will be also presented.We acknowledge existing collaborations in this field with teams of the McMaster University at Hamilton (Canada), Institut de Microelectrònica de Barcelona (Spain), Nanophotonics Technology Center of Valencia (Spain), Instituto de Óptica of Madrid (Spain), Ion Beam Institute at Rossendorf (Germany), IMTEK at the University of Freiburg (Germany) and CIMAP-CNRS at Caen (France).

Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths

A process is described for the recovery of rare earths from xenotime concentrate by digestion with sulphuric acid. A low grade xenotime concentrate (typically assaying 57% light rare earths, 27% Y 2O 3 and 15.6% heavy rare earth) and a high grade xenotime (assaying 38% light rare earths, 41.8% Y 2O 3 and 20.55% heavy rare earths) is routinely being produced in India. Both types of materials have been tested and >98% solubilisation of metal values has been achieved. Xenotime sulphate leach liquor has been processed for removal of thorium by selective precipitation with ammonia and sodium pyrophosphate. Although in both systems thorium removal was almost complete, the ammonia precipitation was found suitable since it facilitates the preferential precipitation of lighter rare earths along with thorium. Consequently, the heavy rare earths have been enriched in the filtrate. Other impurities such as iron, uranium, sulphate and phosphate have been effectively removed by precipitation of rare earths with oxalic acid. It has been shown possible to recover rare earths along with yttrium (∼99% recovery) by these processes.