Heavy rare earths (HREs), namely Ho3+, Er3+, Tm3+, Yb3+ and Lu3+, are rarer and more exceptional than light rare earths, due to the stronger extraction capacity for 100 000 extractions. Therefore, their incomplete stripping and high acidity of stripping become problems for HRE separation by organophosphoric extractants. However, the theories of extractant structure–performance relationship and molecular design method of novel HRE extractants are still not perfect. Beyond the coordination chemistry of the HRE-extracted complex, the extractant dimer dissociation, acid ionization, and complexation behaviors can be crucial to HRE extraction and reactivity of ionic species for understanding and further improving the extraction performance. To address the above issues, three primary fundamental processes, including extractant dimer dissociation, acid ionization, and HRE complexation, were identified and investigated systematically. The intrinsic extraction performances of HRE cations with four acidic organophosphoric extractants (P507, P204, P227 and Cyanex 272) were studied by using relativistic energy-consistent 4f core pseudopotentials, combined with density functional theory and a solvation model. Four acidic organophosphoric extractants have been qualified quantitatively from microscopic structures to chemical properties. It has been found that the Gibbs free energy changes of the overall extraction process (sequence: P204 > P227 > P507 > Cyanex 272) and their differences as a function of HREs (sequence: Ho/Er > Er/Tm > Tm/Yb > Yb/Lu) are in good agreement with the experimental maximum extraction capacities and separation factors. These results could provide an important approach to evaluate HRE extractants by the comprehensive consideration of dimer dissociation, acid ionization, and complexation processes. This paper also demonstrates the importance of the P–O bond, the P–C bond, isomer substituent, and solvation effects on the structure–performance relationship that can be used to guide molecular designs of HRE extraction in future.

Hydrometallurgy is a widely studied recovery process to recover NiMH battery, but the large chemical consumption restricted its application in industry. To achieve a low chemical consumption recovery process of NiMH battery anode alloy, an integrated process with selective leaching and multistage extraction was designed. In selective leaching, the leaching procedure was divided into four stages. The acid used could be reacted with the battery anode alloy totally except in the last stage. The metal components of the alloy have different reaction activities with acid, and they were leached into liquor by the sequence La > Pr > Nd > Ce > Al > Mn > Co > Ni. Hence, the selectivity of metals was achieved to make the following extraction much easier. The total chemical consumption was calculated by the ratio of UMAC/UMACmin and S, which in this integrated process was 60% less than in traditional recovery process. Through this recovery process, the recovery rate of rare earth elements (REEs) reached 90.5%, and the purity of the rare earth oxide (REO) product exceeded 99%. This integrated process was considered as a practical approach to recover the waste alloy.

The aqueous partition mechanism of traditional extractants and ionic liquid for the extraction of rare earths (REs) is explored in this work. To investigate the aqueous partition of extractants, the aqueous solubility of di(2-ethylhexyl) phosphoric acid (P204), 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (P507), di(2-ethylhexyl) phosphinic acid (P227), bis (2,4,4-trimethylpentyl) phosphinic acid (C272), and [trialkylmethylammonium][di(2-ethylhexyl)orthophosphinate] ([A336][P507]) was analyzed systemically. The results demonstrated that the solubility of extractants decreased with increase of aqueous acidity, RE loading, and electrolyte concentration. Especially, the solubility of P204, P507, P227, and C272 decreased with the increase of RE complex, indicating that aqueous partition of the extractants was accompanied by the RE coordination reaction. The electrical double layer theory and the Pitzer equation was used to explain the inhibition of electrolyte on the aqueous partition of extractants. In addition, a solubility of [A336][P507] lower than that of saponified P507 illustrated that the partition behavior was related to extractant property.

A novel polymer inclusion membrane (PIM) was prepared for the preconcentration and separation of heavy rare earth element (HREE) Lutetium (Lu) from dilute solution. The membrane was composed of poly(vinylidene fluoride) (PVDF) as a matrix and bifunctional ionic liquid extractant [tricaprylmethylammonium][di(2-ethylhexyl)orthophosphinate] ([A336][P507]) as carrier, without any plasticizer. A weak physical effect existed between PVDF chain segment and [A336][P507], but no chemical interaction. Structure characterization and transport experiment both proved the membrane to be an asymmetric structure and LuCl3 would transport faster from surface with small pores to large pores. YbCl3 was transported much faster than LuCl3 which was different in liquid–liquid extraction. Preconcentration experiment realized a great acceleration on the transport of LuCl3. And the PIM was proved stable and durable. All these would provide a novel approach to the preconcentration and separation of Lu from dilute solution like leachate of southern ion adsorbed rare earth deposit.Keywords: Heavy rare earth; Ionic liquid; Polymer inclusion membrane; Preconcentration; Separation

A series of ammonium-bifunctionalized ionic liquid extractants (ammonium-Bif-ILEs) combined with a number of anions, including carboxylic acids and 1,3-diketonates, have been prepared and studied in this report. Their extraction behavior and properties toward rare earth ions (REs(III)) are systematically investigated in chloride media as a function of important parameters such as aqueous phase pH, salting-out agent concentrations, and extraction temperature. The separation performance of ammonium-Bif-ILEs toward REs(III) are systematically discussed. The results demonstrate that ammonium-Bif-ILEs have a synergistic effect between the cation and anions in separation of REs(III). The influences of different anions on separation factor (β) values are further studied. By comparison, ammonium-Bif-ILEs containing 1,3-diketonates have more potential applications in La(III)/RE(III) separation than those containing carboxylic acids.

The extraction and separation of rare earths (REs) from nitrate medium or chloride medium using bifunctional ionic liquid extractants (Bif-ILEs) [trialkylmethylammonium][di(2-ethylhexyl)orthophosphinate] ([A336][P507]) and [trialkylmethylammonium][di-2-ethylhexylphosphate] ([A336][P204]) in n-heptane were investigated in this report. The separation factor (β) values indicated that [A336][P507] and [A336][P204] could be suitable for the separation of heavy REs(III) in nitrate medium and suitable for the separation of light REs(III) in chloride medium. Especially, in nitrate medium, the β values using [A336][P204] as the extractant were Tm/Er (3.36), Yb/Tm (7.92), and Lu/Yb (8.55), respectively, and in chloride medium, the β values using [A336][P507] as the extractant were Nd/Pr (9.52) and Sm/Nd (4.70), respectively. The β̅z+1/z values of REs(III) extracted by [A336][P507] and [A336][P204] in nitrate medium were 3.61 and 3.67, respectively, and in chloride medium, they were 2.75 and 2.59, respectively.Keywords: Bifunctional ionic liquid extractants; Extraction; Rare earths; Separation;

The investigation of wet air oxidation of cerium(III) (Ce(III)) of rare earth (RE) hydroxides was carried out in this work. The effects of air flow rate, reaction temperature, reaction time, solution pH, and anions of F– and PO43– on the oxidation rate were investigated. Under the conditions of air flow rate of 3 L min–1, reaction temperature of 80 °C, reaction time of 4 h, and pH 13, the oxidation rate of Ce(III) could reach nearly 97%. According to the kinetics study of oxidation, the reaction order for wet air oxidation was found to be 1 with respect to the concentration of Ce(III) in the suspension at a temperature range of 40–80 °C, and the apparent activation energy was calculated to be 10.49 kJ mol–1. The investigation of oxidation processes in practical systems indicated that RE-hydroxides, which contained fluorine and phosphor and were derived from the treatment of Bayan Obo mixed RE concentrate, could be successfully oxidized by air.

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.Diagram of oxidation and separation of Ce(IV)

•For the first time, 3D-hierarchical nanostructures are self-assembled at the aqueous–organic interfaces.•The strategy was energy-saving and time-efficient as it was conducted at room temperature within 30 min.•A relationship between the products and the aqueous–organic interfaces was proposed.This is the first time that uniform self-assembled 3D-hierarchical nanostructures were prepared at an aqueous–organic interface. Besides, the strategy is time and energy efficient, as it could be conducted at room temperature within 30 min. In order to investigate the influence of the nature of the interface to the nanostructure, a relationship between the morphologies and crystallinities of the products and the interfacial tensions of different aqueous–organic interfaces was investigated, and it was found that the flower-like crystalline nanostructures preferred generating at the interface with lower tension, and the spherical amorphous products were likely to form at higher-tension interface area, and fractal networks tended to occur at the interface with its tension between the above two.

The extraction kinetics of La(III) from aqueous chloride solutions into n-heptane solutions of bifunctional ionic liquid extractant [A336][CA-12] (tricaprylmethylammonium sec-octylphenoxy acetic acid) was investigated using a constant interfacial cell with laminar flow. The effects of stirring speed, temperature and specific interfacial area on the extraction rate were examined. The results indicate that mass transfer kinetics of La(III) is a mixed-controlled process influenced by interfacial reaction. On the basis of mass transfer kinetic results in the extraction of La(III) by [A336][CA-12], the extraction rate equation of La(III) is proposed in terms of pseudo-first-order constants, which is supported by the measured thermodynamic equations. The mass-transfer kinetic model deduced from the rate controlling step is adequate to interpret the experimental data qualitatively.