Accepted Manuscript Highly efficient and eco-friendly extraction of neodymium using, undiluted and non-fluorinated ionic liquids. Direct electrochemical metal separation Ramzi Zarrougui, Raouf Mdimagh, Nourreddine Raouafi PII: DOI: Reference:
S1383-5866(16)30705-5 http://dx.doi.org/10.1016/j.seppur.2016.11.017 SEPPUR 13350
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 June 2016 26 October 2016 10 November 2016
Please cite this article as: R. Zarrougui, R. Mdimagh, N. Raouafi, Highly efficient and eco-friendly extraction of neodymium using, undiluted and non-fluorinated ionic liquids. Direct electrochemical metal separation, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.11.017
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Highly efficient and eco-friendly extraction of neodymium using, undiluted and nonfluorinated ionic liquids. Direct electrochemical metal separation.
RamziZarrougui,,a, RaoufMdimagh b,and NourreddineRaouafic
a. b. c.
Laboratoire des Matériaux Utiles (LR10INRAP01), Institut national de recherche et d’analyse physico -chimique, Biotechpole Sidi Thabet, 2020, Ariana, Tunisia. Laboratoire des Substances Naturelles (LR10INRAP02), Institut national de recherche et d’analyse physico -chimique, Sidi Thabet Biotechpole, 2020, Ariana, Tunisia. Laboratoire de chimie analytique et Electrochimie (LR99ES15), faculté des sciences de Tunis, Université de Tunis El-Manar, 2092, Tunisia.
Corresponding author. Tel.: (+216) 71 537 677; fax: (+216) 71 537 767. E-mail:
[email protected] ( r. zarrougui )
Abstract A new approach for highly efficient extraction and recovery of neodymium from acidic medium by new undiluted ionic liquids(ILs) as extractants.The distribution ratio DNd and the extraction efficiency %E was mesured as a function of nitric acid concentration, extraction time, nature of the IL, concentration of the aqueous feed and molar quantity of complexing agent. DNd is higher than 10 5 and %E is close to 100% when the HNO3 concentration is greater than 6M.The extraction process follows solvation mechanism, suggesting the involvement of three molecules in the extraction of one (Nd 3+)-nitrate complex. The nature of bonding in the extracted complexes was investigated by various spectroscopic techniques. The recovery process, of metal was performed by direct electrodeposition from the IL phase. The electrodeposition of metal was realized by potentiostatic electrolysis at −2V. The current efficiency evaluated from the mass change of the anode was more than 83%. The particuleswere obtained as small crystallite rods with various size approximately 3–70m and diameters range of 0.5–30m. The liquid-liquid extraction and the direct electrodeposition of neodymium by these H-phosphonate based ILs, are new efficient and eco-friendly process.
Keywords: Green separation , Liquid-liquid extraction, Rares earths, Neodymium, Ionic liquids,Electrodeposition .
1
1. Introduction The rare-earth elements (REE) play an important role in many fields of advanced materials science due to their particular spectroscopic and magnetic properties and the industrial demand for them has increased by 10% per year, between 2001 to 2014[1,2]. They are important for wide variety of products such as rechargeable batteries, disk drives, mobile phones, plasma and LCD flat-screen televisions and catalytic converters[3–6]. Although global supply estimates to exceed total world demand, shortages are expected for some heavy elements such as neodymium, dysprosium, europium and terbium[7–11]. Neodymium is one of the most important rare earths. Indeed, it is the primary source material to produce powerful permanent magnets that are used in computers, cell phones, medical equipment, and motors. In addition, neodymium is used in lasers medical instruments, and cutting edge tools apex. The recovery of this metal from End-of-Life goods is crucial in term of resources recycling and to guarantee sufficient supply of this critical element[12–14]. Nowadays, with increasing research activities, a very important topic concern sustainable liquid-liquid extraction process of neodymium from waste streams using so-called "green solvents": ionic liquids (ILs). They are composed of bulky organic cations and organic or inorganic anions. They exhibit high chemical, thermal and electrochemical stability, negligible vapour pressure, nonflammability, ability to dissolve a wide range of organic, inorganic and polymer compounds[15–17]. These properties make ILs attractive media, for applications in solvent extraction processes: they could replace the volatile organic solvents or the corrosive melted salts used generally in hydrometallurgical and liquid-liquid extraction processes[15,18–21]. Indeed, their unique physicochemical properties allow the investigation of many f-elements that have a hard organic solvation. Actually, useful large electrochemical window and stability can lead to rare earth cations elctro-reduction giving metallic form deposit[22–24]. These alternative ionic solvents, have opened horizon to new green separation processes[15]. In the last decade, several authors were interested in using ILs for extraction of neodymium cations Nd(III)[25–30]. ILs were developed as diluents with classical extracting agents or other extracting functionalized ILs[19,21]. Usually, hydrophobic ILs based on fluorinated anions such as − bis(trifluoromethylsulfonyl)imide NT2f orhexafluorophosphate PF6 were tested[17,18,31]. However, the major drawback of these biphasic systems is their higher viscosity compared to conventional organic solvents and their expensive preparation[32,33]. The Binnemans group has extensively studied the separation and extraction of neodymium from acidic aqueous solutions using ILs. They synthesized nonfluorinated ILs based on the dialkylphosphate anion as extracting agents in the liquid-liquid retrieval of neodymium (III) from nitric acid medium. Despite significant results, use of fluorinated ILs as diluent, remains a major drawback[26]. In recent work, extraction process improvement of neodymium was done using functionalized non-fluorinated IL (trioctylmethylammoniumdioctyldiglycolamate: A336-DGA) diluted with non-fluorinated IL (trioctylmethylammonium nitrate: A336-NO3). This IL system is valuable regarding simple synthesis, non-expensive and easy recycling[28]. More recently, new, practical, efficient and eco-friendly separation methodology to recover, neodymium(III) and dysprosium(III) was achieved from end-life NdFeB magnet with non-fluorinated undiluted IL: trihexyl(tetradecyl)phosphonium nitrate[34]. The latter has high viscosity at room temperature (1440 mPa.s at 295K), which is arduous for extraction operation (treatment, centrifugation and recovery) therefore subsequent high temperature working operation is necessary in order to decrease viscosity. There are very few studies reporting nonfluorinated ILs, as undiluted extractant agents, in separation and recovery of neodymium from acidic solutions due to their high viscosity[4,34–38]. Up to date, there are no studies involving moderate viscous non-fluorinated ILs, as undiluted extracting agents and electrolytic medium for direct neodymium recovery by electrodeposition. Matsumiya et al. studied the extraction and direct electrodeposition of neodymium
from HTFSA aqueous solution using the conventional neutral extractant (tributylphosphate) diluted with fluorinated IL: triethyl-pentyl-phosphoniumbis(trifluoromethyl-sulfonyl)amide P2225-NT2f[27].However, the direct electrodeposition of neodymium in P2225-NT2f was performed by potentiostatic electrolysis at −2.7V at 373K,this recovery process is expensive and requires heating at medium temperature. In this paper, new family of ILs based on octylphosphite anion, synthesized and characterized previously, were used simultaneously as agents and organic phase extraction of neodymium(III) from concentrated nitric acid medium without requirement of diluting organic solvent[39]. We present for the first time a new practical and efficient eco-friendly methodology for the extraction and neodymium recovery by direct electrodeposition from organic phase using undiluted and slightly viscous, non-fluorinated ILs (1-octyl-1methylpiperidinium octylphosphitePip18-OP, 1-octyl-1 methylmorpholiniumoctylphosphite Mor18-OP and 1-octyl-1-methylpyrrolidinium octylphosphite Pyr18-OP). Extraction methods are optimized considering foremost concerned parameters (nitric acid concentration, extraction time, nature of the IL, the concentration of the aqueous feed and the molar ratio of complexing agent and extraction mechanism). The neodymium recovery from loaded IL phase was investigated according two methods. Regarding neodymium main industrial use especially from neodymium oxide or metal, its recovery will be studied by chemical precipitation and direct metal electrodeposition from the IL phase. Finally, recyclability of developped IL will be also examined.
2. Experimental 2.1 Chemicals and reagents All the chemicals and reagents used were of analytical grade. Neodymium(III) oxide(99.99%), anhydrous sodium sulphate(99%), oxalic acid(99%), copper rod(99.99%), neodymium rod(99%), Nitric acid(70%), 1-methylpiperidine(98%),1-methylpyrrolidine(97%),4-methylmorpholine(99%),arsenazoIII, diethylphosphite(94%), and 1-octanol(99%) were purchased fromSigma-Aldrich.1-octyl-1methylpiperidinium bis(trifluoromethanesulfonyl)-imide (99.9%)Pip18-NT2fwaspurchased from Solvionic (France).
2.1.1 Choice of the extractant system 1-octyl-1-methylpiperidinium octylphosphite Pip 18-OP , 1-octyl-1methylmorpholinium octylphosphite
Mor 18-OP
and 1-octyl-1-methylpyrrolidinium octylphosphitePyr 18-OP are new ILs whose physicochemical characteristics, including their density, dynamic viscosity, ionic conductivity, and electrochemical stability have been determined in a previous publication[39]. Specifically, 1-octyl-1-methylpiperidinium octylphosphite was selected for the extraction experiments of neodymium from acidic aqueous solution, because of its room temperature ionic liquid (RTIL) nature and low (moderate) viscosity ( =36mPa.s at 298K) compared to most of the usual ILs used in the liquid-liquid extraction of metals cations. On the other hand, this IL do not have surfactant properties in the acidic media in addition to its immiscible character with water.The structure (scheme2), hydrophobicity and the chelating nature of octylphosphite anion, allow to be investigated of liquid-liquid extraction behaviour for metal cations and especially, rare earths, by these green compounds. Heating is essential in order to reduce the IL phase viscosity. Usually the majority of the ILs (e.g.phosphonium, imidazolium, pyridinium and quaternary ammonium) used in the liquid-liquid extraction are known to be very viscous at room temperature such as: [1-Octyl-3methylimidazolium hexafluorophosphate : =682mPa.s at 298K][40] and [N,N,N-trioctyl-Nmethylammoniumtrifluoroacetate: = 740mPa.s at 298K][41]. In our case the extraction experiments were performed at room temperature (300K) which represent an additional advantage in terms of energy saving and safety. 3
2.1.2 Synthesis, purification and characterization - Synthesis of dioctylphosphite Dioctylphosphite were prepared by transesterification of diethylphosphite with 1-octanol according to the literature[42]. Diethylphosphite (0.2mol) and 1-octanol (0.4mol) were introduced in a round-bottom flask connected to a Claisen distillation fitted with a downward condenser and a receiver for vacuum distillation. The mixture was heated to 413K at 120 mm to 160 mm. Ethanol evolution ceased after about 2h of heating. The remaining crude product was purified by distillation under reduced pressure. - Synthetic of octylphosphite-based ILs The procedure synthesis of the ILs and the structure of their precursors are outlined in scheme1. All the ILs were obtained by a one-step synthesis involving the quarternisation of the cyclic ternary amine (1methylpiperidine, 1-methylpyrrolidine, 4-methylmorpholine)(1.1eq) with dioctylphosphite(C 8H17O)2PH(═O) (1eq)at 433K, under nitrogen atmosphere in the absence of organic solvent[39]. Scheme1: Synthetic reaction of octylphosphite anion-based ILs.
The excess of amine was removed from the oily mixture under reduced pressure and the impure product waswashed three times with water to remove residue and unreacted compounds. The obtained IL is further dried at 343K under reduced pressure for one week. The water content of dried ILs was determinated by a Karl–Fischer moisture titrator (Metrohm 73KF coulometer) and the values were less than 0.015wt% in all cases.The structures (scheme2) and purity of all ILs were confirmed by 500-MHz 1 HNMR (Chemical shifts were reported downfield in parts per million (ppm,δ) from a tetramethylsilane(see Figs. 1-4 in the Supporting Data). The characterization data are as follows: Scheme 2: Structures of octylphosphite anion based-ILs.
- Dioctylphosphite DOP: 1 H NMR (500-MHz; CDCl 3): 4.10 (q, 1H), 1.66 (p, 1H), 1.45 (p, 1H), 1.22–1.34 (m, 4H), 0.90 (s, 1H), 0.85–0.95 (m, 1H). - 1-octyl-1-methylpiperidinium octylphosphite Pip18-OP: 1 H NMR (500-MHz; CDCl 3): 0.91 (t, 3H); 1.4 (m,2H); 1.32 (m, 2H); 1.31 (q, 2H); 1.73 (q, 2H); 3.29 (t, 2H); 1.81 (m; 2H); 1.65 (m, 2H); 3.52 (m, 2H); 1.6(m, 2H);1.54 (m, 2H); 1.39 (m, 2H); 1.32 (m, 2H) and 0.96(t, 3H). - 1-octyl-1-methylmorpholinium octylphosphite Mor 18-OP: 1 H NMR (500-MHz; CDCl3): 1.02 (t, 3H); 1.23 (m,2H); 1.32 (m, 2H); 1.77 (m, 2H); 3.30 (t, 2H); 3.49(t, 2H); 3.82(m; 2H); 3.42(s, 3H); 3.69 (t, 2H); 1.48 (m, 2H);1.54 (m, 2H); 1.33 (m, 2H); 1.28 (m, 2H) and 0.93 (t, 3H). - 1-octyl-1-methylpyrrolidinium octylphosphite Pyr18-OP: 1 H NMR (500-MHz; CDCl3): 0.89 (t, 3H); 1.24 (m,2H); 1.28 (m, 2H); 1.71 (q, 2H); 3.27 (t, 2H); 3.43 (s; 3H); 3.29(m, 2H);3.50 (m, 2H); 1.56 (m, 2H);1.46 (m, 2H); 1.28(m, 2H); 1.28 (m, 2H) and 0.83 (t, 3H).
2.2 Experiments 2.2.1 Liquid-liquid extraction All experiments were performed by shaking 0.5ml of the organic phase with 5 ml of aqueous acidic phase for 20min at ambient temperature at 1000 rpm in vials of 15mL and in a horizontal position. The organic phase was initially pre-equilibrated with the desired concentration of nitric acid in order to fix the equilibrium acidity. The extraction of neodymium(III) as a function of nitric acid concentration was studied by equilibrating the undiluted Pip18-OP; Mor18-OP or Pyr18-OPIL phase (0.5mL) with the aqueous phase (5mL) containing the neodymium (III) solution (10 -2 M). The initial HNO 3concentration in the aqueous solution was varied from 0 to 11M, using a commercial solution of nitric acid (69%). After the extraction, separation of the phases was assisted by centrifugation for 30 min at 3000 rpm. The concentration of Nd(III) distributed between aqueous and IL phases was determinated using total reflection
X-ray fluorescence TXRF. The loading test was performed by varying the initial neodymium concentration in the aqueous phase between 5.10 -4 M and 5.10-2 M. The variation of the IL initial molar quantity was studied from 0.5 to 7mmol.The variation of the concentration of the metal cation, IL, loading tests, equilibration time were all carried out using a fixed concentration [HNO3]= 6M. The distribution ratio (DM) was determined using the following equation: (1) where, [Nd3+] is the concentration of the metal ion in the organic phase and the aqueous phase at equilibrium. CiandCf are the concentrations of the neodymium (III) in the aqueous phase before and after extraction , respectively. Vaq and VIL are the volumes of the aqueous and IL phase, respectively. The extraction efficiency (%E) was determined by using the following equation (2): (2) 2.2.2 Stripping and Reusability Studies Neodymium (III) was recovered from the organic phase by precipitation with pure (solid) oxalic acid, added directly to the IL phase. The organic phase (after scrubbing) was mixed with 100mg of oxalic acid and shaken at ambient temperature. After a few minutes, the formation of a white precipitate was observed. The samples were centrifuged and the organic phase was carefully removed. The precipitate was filtered, washed with deionized water and acetone, dried at room temperature, and stored in a desiccator. In order to obtain the metal oxide, the obtained neodymium oxalate was calcined at1173K for 12h. A light blue powder of neodymium oxide was obtained.The IL was recycled and reused,according to the following process: the organic phase was washed several times with water and extracted twice with ethyl acetate. The combined organic phases were dried with anhydrous MgSO4.The precipitate was filtered off and the solvent was evaporated in a high vacuum. Finally, the IL was placed in a vacuum drying oven to remove the remaining ethyl acetate. The purity of IL was verified after the recycling process. 2.2.3 Electrodeposition of Nd metal The electrodeposition of neodymium was performed at–2V, under a dry argon atmosphere to avoid the presence of oxygen for a total of 2h. The experiment was carried out at 313Kin order to stimulate the mass transfer and reduce much more the electrolyte viscosity. A copper disk electrode with 1.07cm 2surface area and a neodymium rod (99%) with 6.35mm inside diameterwere used as a cathode and an anode, respectively. Before electrodeposition, the surface of each electrode material was polished several times by a sandpapers (size: 400 and 1200), washed with anhydrous acetone and dried under a dry argon atmosphere. The electrodeposits on the copper electrode were rinsed with ethanol in order to thoroughly remove the electrolyte. Finally, the copper disc was quickly dried at ambient temperature, under a dry argon atmosphere to avoid the presence of oxygen and humidity. The surface morphology and the composition of the electrodeposits were evaluated by using SEM/EDX (JEOL-JSM-6510LA).
2.3 Analysis 2.3.1 Electrochemical measurements Cyclic voltammetry (CV) measurement was carried out in a conventional three-electrode cell with amultipotentiostat(VMP Multichannel Potentiostat, Perkin Elmer, equipped with eight channels)at 313K. The Pt disk electrode (2.0mm in diameter) was selected as a working electrode and a glassy carbon disk (2.0 mm 5
in diameter) was used as a counter electrode. Ag/AgCl, KCl sat(IL) was used as a reference electrode. KClsat(IL) is a saturedKCl solution in Pip18-NT 2f.The reference Ag/AgCl, KCl sat(IL) system is calibrated versus the SHE electrode using the ferrocenium/ferrocene couple at 313K (Eref= +310mV/SHE). Before use, the electrode was immersed in 1:1 nitric acid solution and handled in ultrasound bath for few minutes, and then washed with distilled water and acetone. All electrochemical experiments were performed under a dry argon atmosphere to avoid the presence of oxygen.
2.3.2 Viscosity measurement The viscosity of 1-octyl-1-methylpiperidinium octylphosphite (Pip18-OP) before and after extraction experiments was determinate by using Anton-Paar SVM 3001 viscometer. The cell temperature was regulated within ± 0.05K. Repeatability given by producer was 0.1%, whereas reproducibility was 0.35%. The viscosity was determinated for the temperatures ranging from 298 to 353K at the ambient pressure. The uncertainty of viscositymeasurements is less than ±1%.
3. Results and discussion 3.1 Liquid-liquid extraction 3.1.1 Equilibrium time The extraction kinetics using undiluted ILs (e.g., phosphonium, ammonium and imidazolium ILs) depend on physico-chemical parameters, such as density, micellar character, hydrophobicity and especially the viscosity[19,43,44]. Indeed, these compounds invariably display very higher viscosities as compared to the molecular diluents. For this reason, the experiments have to be performed usually at higher temperatures, in order to reduce viscosity of organic phase and thus improve the extraction kinetics. In our study, after pre-equilibration with nitric acid solution ([HNO 3] = 6Mat 300K), the viscosity of the IL decreased from 33 to 16.5mPa.s. This decrease compared to pure IL is attributed to a larger co-extraction of acidified water (2.7wt%). The viscosity of the IL phase did not vary remarkably as a function of the nitric acid concentration of the aqueous solution used for pre-equilibration. This is due maybe, to the good hydrophobic character of IL. On the other hand, some series of tests show that this IL does not have surfactant properties in concentrated nitric acid medium. These results favouring a rapid kinetics of extraction experiments. In order to gain more information on the extraction kinetics, we investigated the effect of the equilibration time on the extraction process. Fig.1 presents the extraction kinetics data up to a period of 1h. The distribution ratio value increases gradually with the increase of time and remain almost constant with a quantitative extraction (%E> 99) yield after 30min. The studies show that 30min is sufficient to achieve and ensurethe complete extraction. In the following, all the studies have been carried out with an equilibration time of 30min. Fig.1.Distribution coefficient DNdand extraction efficiency%E of Nd(III) as a function of equilibration time.
3.1.2 Acidity influence A partial extraction of neodymium ionswas observed at low nitric acid concentrations ([HNO 3] < 1M) with Pip18OP. This suggests that IL is very little or no extraction ability, at low nitric acid concentrations. From these preliminary results, it is interesting to examine the effect of acidity on the extraction of Nd(III) in ILs at higher nitric acid concentration.In a first series of experiments, the optimal nitric acid concentration was determined by varying the HNO3 concentration of the feed solution.The influence of nitric acid concentration on the extraction of Nd(III) aqueous phase was investigated in the range 0–11M.Fig.2gives the distribution ratiovariations of neodymium versus equilibrated nitric acid concentration in the aqueous phase. DNdincreased with increase in nitric
acid concentration acrossoverall acidity range. The plot ofthe distribution ratio DNd showed a strong dependence on the acidity of the aqueous phase. During the extraction process ([HNO 3]< 6M), for every Nd3+ cation extracted to the ILs phase, several nitrate anion NO3- are transferred from the aqueous phase to the IL phase. The increase of DNd values can be assigned to the salting-out effect of nitric acid and NO3- anions contribution in Nd3+ solvation state. Nevertheless, from [HNO 3]> 6M, the DNd values are almost unchanged. Probably,the solvation environment, of the all extracted cations in IL phase seems to be the same. Fig.2.Distribution coefficient DNdas a function of [HNO3]. The extractant: Pip18-OP () and DOP ().
Additionally, from Fig.3, we can observe that the extraction efficiency increased sharply between nitric acid concentrations 0- 6M. At [HNO3] ≥ 6M, Nd(III) ions were extracted nearly quantitatively from the aqueous phase (%E≥98%). The results suggested that, this undiluted ILmight serve as extracting phase for the extraction of Nd (III) from concentrated nitric acid medium. The scheme 3 shows a representation of liquid–liquid extraction of an aqueous solution of Nd(III) (0.15M; 25mL) with Pip18-OP (20mL) as both extracting agent and organic phase for[HNO3] = 6M,at 300K. We can perceive that all the Nd(III) ions migrated from the aqueous media to the IL phase after extraction. Fig.3.Extraction efficiency%E of Nd(III) as a function of [HNO3]. The extractant: Pip18-OP () and DOP (). Scheme 3: Visual representation of extraction of Nd (III) from aqueous phase with Pip18-OP as both the extracting agent and organic phase. The IL phase brought on top of the aqueous phase. [HNO 3]=6M; T= 300K. (a):before extraction and (b):after extraction.
3.1.3 Extraction mechanism In fig.4, we presented the variation of the distribution ratio of Nd (III) DNdas a function ofPip18-OP concentration in the Pip18-NT2fdiluent, for a nitric acid concentration equal to 6M at 300K.The ionic liquid Pip18-NT2f act solely as diluent and is not involved in the extraction mechanism. The extraction of Nd (III) by this IL, without added extractant, shows as expected the D Nd values are very small (< 10-2). It can be concluded that the extraction is solely due to the presence of the functional H-phosphonate moiety in the extractant. We observed that the distribution ratio DNdincreased with increasing the concentration of Pip18-OP. A linear regression analysis of the extraction data resulted in a straight line with a slope of 2.91(3.00), suggesting the involvement of three molecules of Pip18-OP during the extraction process. This is in good agreement with the result obtained from the class of ILs based on the dialkylphosphate anions[26,45].The following mechanism is proposed for the extraction stoichiometry: (3) Fig.4. Variation of the coefficient DNdas a function of the concentration of Pip18-OP in Pip18-TFSI as diluent.
In order to determine, if an ion-exchange or solvation mechanisms was occurring, we investigated the extraction behaviour of Nd(III) from the nitric acid medium by dioctylphosphite DOP, without any diluent. This dialkyl Hphosphonateextractant is insoluble in water and slightly viscous liquid[46]. It is a neutral donor ligand (P=O), working according to solvation mechanism, as for organophosphorus compounds (e.g. trialkylphosphate and phosphine oxide: tributylphosphate TBP, trioctylphosphine oxide TOPO etc.…). The latter were, extensively used for the liquid-liquid extraction of actinides and lanthanides[47]. As DOP does not ionize, when neodymium cation extraction is occurring, three nitrate anions must be extracted along with the metal cation Nd(III) to maintain electro-neutrality. Hence, the equation for extraction can be written as follows (4): (4)
7
Fig.2shows, the variation of distribution ratio of neodymium versus equilibrated concentration of nitric acid in aqueous phase from 0 to 11M at 300K using DOP as undiluted extractant. It is observed that the distribution ratio increases with nitric acid concentration increasing. This is the characteristic behaviour of extractions with molecular, neutral extractant such as DOP, wherein Nd (III) is coordinated to Pip18-OPIL mainly by solvation mode[48,49].Fig.2 also shows that the extraction efficiency of Nd (III) using DOP, increases sharply if nitric acid concentrations is lower than 6M. At [HNO 3] ≥ 6M, 88% Nd (III) ions were extractedfrom the aqueous phase. It is interesting to compare, this result with that observed forPip18-OPIL. Indeed, the extraction trend is similar to that of the neutral DOP extractant.Taking this assumption into consideration, the extraction of Nd(III) by Pip18-OP was carried out by solvation mechanism. In order to confirm this hypothesis, further spectroscopic investigations about the extraction behaviour of metal ions by pure IL should be made.Fig.5 shows the absorption spectrum of Nd (III) in aqueous phase(before extraction) and IL phase (after extraction) respectively. In the wavelength region of 450-900nm, several transitions within 4f3 shell of Nd(III) were visible. By comparison, of the spectra of Nd(III) cation in the two phases, we can conclude that two neodymium species were present in each phases as shown in Fig.5. The trends in the spectra features are similar. However, a shift in position of the absorption (of the order of 5nm) was observed, between the aqueous and the organic phase. The molar absorption coefficients of Nd (III) in aqueous phase before extraction and IL phase after extraction correspond to 12.5 and 27.3 L.cm -1.mol−1 respectively at 300K. The absorbance value of Nd (III) is two times higher of that of IL, than aqueous acidic phase. This could be attributed to the difference in ion-solvent interactions, and nature of the local coordination of the rare earth cation in aqueous and organic phases respectively[50].The most intense transition around 550-620nm (in aqueous or organic phase) correspond to the hypersensitive 4I9/2(4G5/2, 2G7/2) transition of Nd(III), that is sensitive to the coordination environment[51]. Interestingly, a change of the solvation state of neodymium in aqueous phase was observed (Fig.6).Indeed, a significant changes in position (from 574nm to 580nm) and intensity of these bands were observed as nitric acid concentration increases. However, below 6M, no changes in position of these bands, ( = 580 nm) could be observed. Fig.5.Absorption spectra of Nd (III) in aqueous phase (before extraction) and IL phase (after extraction) at [HNO 3 ] = 6M. Fig.6.Absorption spectra of Nd (III) in aqueous phase (before extraction) from 550-620 nm at [HNO3 ] = 1, 2, 4, 6, 8 and 10M.
This shows that, if the nitric acid concentration increases, it is likely that , the Nd(III) cation form successively, several complexes with the NO 3- anion, such as [Nd(NO3)]2+ , [Nd(NO3)2]+ and [Nd(NO3)3][52]. Nevertheless, for an acid concentration higher than 6M, the predominant form of Nd (III) is probably the neutral complex [Nd(NO 3)3]. In the organic phase after extraction, no changes in position of these bands, ( = 582 nm) were observed in a range of nitric acid concentration, from 0 to 10M (Fig.7).This indicates that, during the extraction of Nd (III), the environment solvation, of the cation in IL seems to be the same; it is expected to be in the form (scheme 4). Thus, if the concentration of nitric acid in the aqueous phase increases, the coordination of Nd (III) in IL phase does not change. These results are consistent with the assumption of a solvation mechanism extraction of Nd(III). Fig.7.Absorption spectra of Nd(III) in IL phase (after extraction) from 450-900nm at [HNO3] = 2, 4, 6, 8 and 10M.
To obtain further insight into nature of the extraction mechanism, FTIR spectra of the pure Pip18-OPIL, preequilibrated with aqueous solution of nitric acid (6M) and Pip18-OP loaded with Nd(III) and the same aqueous solution of nitric acid, were compared in Fig.8. A comparison of spectra (a) and (b) reveals the stretching vibration of (P=O) from (O=P-O-) (1247cm−1) is weakened in the pure IL. However, the vibration mentioned above appear completely in Pip18-OP loaded with Nd (III) as described in spectrum (b). As a result, the coordination ability of (P=O), can be full displayed in the extraction mechanism. The shift of (P=O) stretching vibrations from 1247cm−1 ; 1163cm-1 (a) to 1239cm−1 and 1137cm-1 (b) respectively, reveals strong interaction between (P=O) and Nd(III) in the IL phase.
Fig.8.FTIR transmittance spectra of different systems. (a): Pip18-OP pre-equilibrated with aqueous solution of nitric acid (6M) and (b):Pip18-OPloaded with Nd (III) with aqueous solution of nitric acid (6M). Scheme 4: The proposed structure of Nd(III) nitrate complex and its coordination environment with Pip18-OP.
3.1.4 Loading test of aqueous and organic phase The loading of the IL phase by neodymium wascarried out at different initial Nd (III) concentration in the aqueous phase from 5.10-5M to 2.10-2M with 0.5ml of IL phase. The obtained results are shown in Fig.9. It is observed that the percentage extraction of neodymium is almost equal to 99.8%, if the initial concentration of Nd(III) increases up to 1.210-3M. Nevertheless, the extraction efficiency decreases gradually from 99.8% to 33% with increase in Nd (III) loading in the aqueous phase. This decrease is significant from a certain threshold concentration of Nd (III) in the aqueous phase (about 1.2.10-3M). Presumably, this could be due to the unavailability of coordinating sites of the phosphonate based IL to form complexes with Nd (III) at high feed concentration. Fig.9.Variation of the %E as a function of Nd (III) concentration in the aqueous phase from 5.10 -5M to 2.10-2M ([HNO3] = 6M) with 0.5 ml of Pip18-OP IL phase at 300K.
In the same way as above, the extraction of neodymium by the IL was carried for different initial molar quantity of Pip18-OP from 5.10-4mol to 7.10-3mol, with 5ml of the initial aqueous phase of Nd (III). The extraction efficiently increases gradually from 50% to 99.8% with the increase in molar quantity of Pip18-OP (Fig.10). It is observed that the extraction efficiency of neodymium is almost equal to 99.8 %, if the IL molar quantity exceeds 2.5mmol. This shows that a very small amount of IL is sufficient to extract more than 99% of Nd (III) from 5ml of Nd (III) aqueous solution ( 10-3M). Fig.10.Variation of extraction efficiency%E of Nd (III) as a function of Pip18-OP molar quantity (organic phase). The initial concentration of Nd (III) in aqueous phase is 10 -2 M and [HNO3] = 6M.
3.1.5 Effect of the nature of ammonium ring In order to study the effect of the nature of the cation ring in the IL on the extraction efficiency of Nd (III), we have determined the variation of %E as a function of nitric acid concentration (Fig.11).Regardless of the nature of cation ring (Pip18+, Mor18+ or Pyr18+), the extraction trend is the same. It is observed also that the extraction efficiency increased sharply with increase in nitric acid concentration from 0 to11M.However, only Pip18-OP allows to quantitatively extracted Nd (III). At [HNO 3] = 6M, the extraction efficiency is equal to 99.8%; 82% and 77%, by using Pip18-OP; Mor18-OPand Pyr18-OP ILs respectively. The difference in hydrophobic character, ionic strength and dielectric constant due to change of the cation ring in IL, could explain this unexpected result. The extraction capacity of these ILs varies in the following order:(Pip18+)> (Mor18+)> (Pyr18+). Furthermore, a quantum calculation related to this topic is underway by using density functional theory (DFT/B3LYP/6-311+G**) with the aim of explain this result. Fig.11.Distribution coefficient DNd as a function of [HNO3 ] for different extracting agents: Pip18-OP (); Mor18-OP () and Pyr18-OP ().
3.1.6 Recovery of ILs The IL was recycled and reused, according to the process described in stripping and reusability studies part. Using the ICP-AES technique, neodymium content in the recovered IL is 62 g.L-1. However, the water content of the dried ILs is equal to about 0.05wt% forPip18-OP; 0.08% forMor18-OP and 0.03wt%forPyr18-OP. The physical properties of the recovered Pip18-OP IL are compared to those obtained before and after extraction. From Table 1, we can be conclude that, water and nitric acid are partly coextracted to the IL phase. A solvation phenomenon of IL ions, by few water molecules and HNO 3 could explain this result. During the extraction process, the absorption of water and nitric acid by the IL is 9
responsible for the sharp decrease in viscosity (nearly twice) and the increase of ionic conductivity. This makes the separation much easier. The physical properties of the recovered IL are very close to obtained values for the pure IL. Pip18-OP, can be reused as the extraction and separation phase (see Fig.S5 in the Supporting Data), which makes it possible to do the extraction in a continuous process. The ionic liquid used is recycled between six and nine times according to the quality of its purification. Table 1: Physical properties of thePip18-OPILbefore, after extraction and after recovery.
3.2 Neodymium recovery 3.2.1 Oxalate precipitation In order to recover almost all of neodymium (III) from (organic and aqueous phase) and avoided the problem of the loss of IL, the stripping with solid oxalic acid, directly in the IL phase is very efficient. In fact, oxalic acid forms an insoluble oxalate precipitate with neodymium cation, meaning that there is no loss of IL to the water phase[53,54]. (5)
The obtained neodymium oxalate precipitate (scheme5-a) can then be separated from the IL and was calcined at 1173.15K during 12h. A light blue powder of Nd2O3was obtained and stored in a glass vial (scheme5-b). (6)
Scheme 5: (a) Visual representation of precipitation of Nd (III) with oxalic acid from IL phase, after centrifugation for 15 min at 3000 rpm. The IL phase brought on top of the aqueous phase. [HNO 3]=6M at ambient temperature. (b) The obtained Nd2 O3 after calcination of Nd2(C 2 O4)3 at 1173K.
3.2.2 Direct electrodeposition Several studies about the electrodeposition of varioustransition and rare-earth metals have been already reported in ILs as electrolytic medium[27,55,56]. However, there has been no report about the recovery of neodymium by non-fluorinated and undilutedILs, as bothcomplexing agent and electrolytic medium for direct electrodeposition of metal. The recovery method, i.e. extraction-electrodeposition detailed in this paper differs from classical methods that employ other reagents for stripping the metal from organic phase. This advantageous process, permit to eliminate several complex processing steps involved in the extraction and recovery of metallic form of neodymium. Toward this goal, the electrochemical behaviour of the pure Pip18-OPIL before extractionwas recorded to establish its electrochemical window and to ensure the reference electrode reproducibility in the phosphonate-based IL medium, at 313K. The potential window of Pip18-OP was determined by a cyclic voltammetry method and its value was defined as the potential range where the limiting current density is less than or equal to 0.1mA.cm -2. A wide electrochemical window between −2.11V and +1.51V vs. (Ag/AgClsat,KClsat(IL)) reference,was observed for the reduction–oxidation processes of Pip18-OP (Fig.12-a). The values of reduction and oxidation limit of Pip18-OP in this study are relatively different compared to the results of Zarrougui et al.(−1.17V and +2.09 V vs. (Ag/AgClsat,KCl sat(IL)) reference[39]. Probably this result is causedby the decrease of anodic and cathodic overvoltage in this IL, by changing the platinum work electrode by vitreous carbon (catalytic activity of the platinum metal or vitreous carbon). However, the difference in electrochemical windows of Pip18-OP isvery low. The cyclic voltammograms of Nd(III) complex in IL phase after extraction at 313K were represented in Fig. 12-b. Two well-defined successive reduction waves were observed (EC1 = + 0.29 and EC2 = -1.46V vs. (Ag/AgClsat,KClsat(IL)). The first reduction wave corresponds,most likely to the reduction of NO3 - (which was extracted in organic phase) to nitrogen dioxide (NO2)[57]. The second reduction, whose height is very greater the first wave appeared only in the presence of Nd(III) at about 1.46V vs. (Ag/AgClsat,KClsat(IL)).According to the literature on the electrochemical behaviour of Nd(III) in ILs, this cathodic reaction would be assigned to the following reaction[58–60]:
(7)
This result indicates that the neodymium (III) complex present in IL phase can be reduced and deposited as Nd metal. The electrodeposition of neodymium from IL phase was conducted using an ultra-pure cathodic copper (Cu) substrate, an anodic Nd rod (99.95%). The applied over-potential on the cathode for the potentiostatic electrolysis was set at -2V with an average current density of -1.8A.m−2. Fig.12.Cyclic voltammogram (I=f (E vs. Ag/Ag+))of pure Pip18-OP IL (a) and Nd(III) complex in Pip18-OP after extraction (b).
The reduction process of Nd(III) was carried out at 313K in order to increase the ionic mobility and the diffusion coefficient of Nd(III). After two hours, a blackish-brown layer covers uniformly the copper electrode in a part of immersing the electrolyte solution. The dissolution of the Nd rod in the IL solution confirms the electrodeposition process who is the current efficiency was near 83%. AtEapplied = -2V, the anodic electrode was dissolved in to the IL solution according to the reaction: Nd(rod) = Nd3++ 3e-. Nonetheless, the extracted Nd3+was electrodeposited at the cathodic electrode according to equation (7). The choice of a neodymium rod as anode electrode was crucial. Indeed, the oxidation of Nd rod avoids the contamination of the solution and allows the determination of the current efficiency from the decrease of theanodic electrode masse. It is clear that a little decomposition of ILhas been occurred, even though the colour of the electrolytic solution after electrodeposition was almost not changed. Despite this drawback, the Pip18-OPelectrolyte remains a good candidate for extraction and the direct electrodeposition of the rare earths.The microscopic morphology of the neodymium deposited on copper substrate was examined by SEM. The microscopic morphology of the neodymium metal obtained from the potentiostatic electrodeposition was presented in Fig.13. Fig.13.The microscopic morphology of the Nd deposited on a copper substrate at −2V and 313K.
The electrodeposition process with neodymium particle has been achieved from the nucleation growth of the neodymium by fixing the bath temperature at 313K and the electrodeposition time to 2 hours. From the SEM image, it is seen that, the particle size distribution of the electrodeposits was variable. They are obtained as small crystallite rods with varying lengths approximately 3–70m, with diameters in the range of 0.5–30m. The semiquantitative analysis of the neodymium deposit by EDX was performed and the results are archived in Fig.14. From the EDX spectrum, the three related peaks with (NdL), (NdL) and (NdL2) were confirmed. The major surface elements of the crystallites were neodymium, phosphorus and oxygen present at 52.67, 16.09 and 13.23 corresponding to atomic percentages of 14.07, 20.02 and 31.86%. The presence of phosphorus and oxygen could explain the Nd (III) reduction reaction irreversibility that could be due to Pip18-OP adsorption on electrode surface or to the formation of some phosphorus neodymium-based compounds or maybe the metal oxidation reaction on the top layer of electrodeposits would partially occur in electrodeposition process. Fig.14.EDX spectrum of the electrodeposited Nd metal on a copper substrate.
4. Conclusions The ionic liquids 1-octyl-1-methylpiperidinium octylphosphitePip18-OP, 1-octyl-1 methylmorpholiniumoctylphosphiteMor18-OPand 1-octyl-1-methylpyrrolidinium octylphosphitePyr18OPhave been readily prepared, by a green process and studied for the liquid–liquid extraction of Nd(III) from concentrated aqueous nitric acid solutions (0-11M). A distribution ratio DNd higher than 10000 and an extraction yield, higher than 98%, were obtained for neodymium extraction, from [HNO 3]> 6M and after an equilibration time of 30 min. Furthermore, the extraction of neodymium is quantitative, if the initial concentration of Nd (III) and the IL molar quantity were less than 1.2mM and 2.5mmol respectively. The extraction capacity of these ILsis efficient with (Pip18 +) cation compared to those (Mor18 +) and(Pyr18+) cations.The recovery of neodymium by direct electrodeposition from the IL phase was readily realized by potentiostatic electrolysis at −2V at 313K. The current efficiency was more than 83% and the contents of metallic neodymium in electrodeposits are about 48%. The particles of depositedmetalwere obtained as small crystallite rods with varying lengths approximately 3–70m, with diameters in the range of 0.5– 11
30m. The liquid-liquid extraction and the direct electrodeposition of neodymium from the organic phase using, these undiluted and slightly viscous, non-fluorinated ILs is a new practical, efficient and environmentally friendly for the hydrometallurgical processes.
Acknowledgements The authors want to thank all staff of the useful materials laboratory (U.M.L), especially,Mr. MoomenMarzouki and Mr.RiadhHamdi for analysis and the helpful discussions.
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Figures
Scheme 1: Synthetic reaction of octylphosphite anion-based ILs.
Scheme 2: Structures of octylphosphite anion based-ILs..
1e+7
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120
1e+5
DNd
100
%E
80
1e+4
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Equilibratrion time / min
1e+2 0
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100
Equilibratrion time / min
Fig.1. Distribution coefficient DNd and extraction efficiency %E of Nd(III) as a function of equilibration time.
Pip18-OP
10000
DOP
DNd
1000
100
10
1 0
2
4
6
8
10
12
[HNO3] /M
Fig.2. Distribution coefficient DNd as a function of [HNO3]. The extractant: Pip18-OP () and DOP ().
120
100
80
%E
[HNO 3]=6M
60 Pip18-OP DOP
40
20
0 0
2
4
6
8
10
12
[HNO3] /M
Fig.3. Extraction efficiency %E of Nd(III) as a function of [HNO3]. The extractant: Pip18-OP () and DOP ().
(a)
(b)
Scheme 3: Visual representation of extraction of Nd (III) from aqueous phase with Pip 18-OP as both the extracting agent and organic phase. The IL phase brought on top of the aqueous phase. [HNO 3]=6M; T= 300K. (a):before extraction and (b):after extraction.
3
slope = 2.92 ± 0.02 r2 = 0.9874
log D Nd
2
1
0
-1
-2 -3.4
-3.2
-3.0
-2.8
-2.6
-2.4
-2.2
-2.0
log ([IL]/mM)
Fig.4. Variation of the coefficient DNd as a function of the concentration of Pip18-OP in Pip18-TFSI as diluent.
Nd(III) in IL phase after extraction
30
582 nm 799 nm
20
/ L.cm
-1
.mol
-1
745 nm
Nd(III) in aqueous phase before extraction
578 nm
794 nm
10
740 nm
0 450
500
550
600
650
700
750
800
850
900
/ nm
Fig.5. Absorption spectra of Nd (III) in aqueous phase (before extraction) and IL phase (after extraction) at [HNO3] = 6M.
25 580 nm
20 [HNO3]= 10M
/
-1 -1 L.cm .mol
[HNO3]= 8M
15
[HNO3]= 6M [HNO3]= 4M [HNO3]= 2M [HNO3]= 1M
10
5
576 nm
0 550
560
570
580
590
600
610
620
/ nm
Fig.6. Absorption spectra of Nd (III) in aqueous phase (before extraction) from 550-620 nm at [HNO3] = 1, 2, 4, 6, 8 and 10M.
582 nm
799 nm
742 nm
/ L.cm-1.mol -1
40
[HNO3]=10M
30 [HNO3]= 8M
[HNO3]= 6M [HNO3]= 4M
20
[HNO3]= 2M
10 450
500
550
600
650
700 / nm
750
800
850
900
Fig.7. Absorption spectra of Nd(III) in IL phase (after extraction) from 450-900nm at [HNO3]= 2, 4, 6, 8 and 10M.
(a)
(b)
Transmittance [%]
1247 cm
1163 cm
1239 cm
3500
3000
2500
2000
1500
-1
-1
1137 cm
4000
-1
-1
1000
500
Wavenumber cm-1
Fig.8 FTIR transmittance spectra of different systems. (a): Pip18-OP pre-equilibrated with aqueous solution of nitric acid (6M) and (b):Pip18-OPloaded with Nd (III) with aqueous solution of nitric acid (6M).
Scheme 4: The proposed structure of Nd(III) nitrate complex and its coordination environment with Pip 18-OP.
120
100
[Nd(III)]aq= 1.2 mM
%E
80
60
40
20
0 1,00e-4
1,00e-3
1,00e-2
[Nd(III)]aq/M
Fig.9. Variation of the %E as a function of Nd (III) concentration in the aqueous phase from 5.10-5M to 2.10-2M ([HNO3] = 6M) with 0.5 ml of Pip 18-OP IL phase at 300K. .
120
100
nIL= 2.5 mmol
%E
80
60
40
20
0 0
1
2
3
4
5
6
7
8
nIL/mmol
Fig.10. Variation of extraction efficiency %E of Nd (III) as a function of Pip 18-OP molar quantity (organic phase). The initial concentration of Nd (III) in aqueous phase is 10 -2M and [HNO3] = 6M. .
120
100
%E
80
60
40 Pip18-OP Mor18-OP 20
Pyr18-OP
0 0
2
4
6
8
10
12
[HNO3] / M
Fig.11. Distribution coefficient DNd as a function of [HNO3] for different extracting agents: Pip 18-OP (); Mor18-OP () and Pyr18-OP ().
Scheme 5: (a) Visual representation of precipitation of Nd (III) with oxalic acid from IL phase, after centrifugation for 15 min at 3000 rpm. The IL phase brought on top of the aqueous phase. [HNO 3]=6M at ambient temperature. (b) The obtained Nd 2O3 after calcination of Nd 2(C2O4)3 at 1173K. 0.2
I / mA
0.1
0.0
-0.1
(a)
(NO 3- ) IL + e- + 2H +
H2O + NO 2-
(b)
Ndsd
(Nd3+)IL+ 3 e-
-0.2 -3
-2
-1
0
E / V vs.Ag/Ag+
1
2
Fig.12. Cyclic voltammogram (I=f (E vs. Ag/Ag+)) of pure Pip18-OP IL (a) and Nd(III) complex in Pip 18-OP after extraction (b).
Fig.13. The microscopic morphology of the Nd deposited on a copper substrate at −2V and 313K
Fig.14. EDX spectrum of the electrodeposited Nd metal on a copper substrate.
Graphical abstract
New practical efficient and environmentally friendly methodology for the extraction and recovery by direct electrodeposition of neodymium from the organic phase using undiluted and slightly viscous, non-fluorinated ionic liquids. Compared with the traditional methods, this approach is much easy, efficacious and environmentally friendly for the hydrometallurgical processes.
15
Highlights
•
The article provides a new approach for extraction and recycling of neodymium metal from aqueous nitrate acidic media.
•
The extraction and the recovery of neodymium is highly efficient by the use of new undiluted and nonfluorinated ionic liquids as extractants.
•
When compared with the traditional methods, this approach is much easy and efficacious.
•
The direct electrodeposition of neodymium metal from ionic liquids is a new practical, efficient and environmentally friendly for the hydrometallurgical processes.