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Solvometallurgical process for the recovery of rare-earth elements from Nd–Fe–B magnets
Journal Pre-proofs Solvometallurgical process for the recovery of rare-earth elements from Nd– Fe–B magnets Martina Orefice, Koen Binnemans PII: DOI: Reference:
S1383-5866(20)32274-7 https://doi.org/10.1016/j.seppur.2020.117800 SEPPUR 117800
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Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 November 2019 14 August 2020 22 September 2020
Please cite this article as: M. Orefice, K. Binnemans, Solvometallurgical process for the recovery of rare-earth elements from Nd–Fe–B magnets, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/ j.seppur.2020.117800
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Manuscript for: Separation and Purification Technology
Solvometallurgical process for the recovery of rare-earth elements from Nd‒Fe‒B magnets Martina Orefice,†,* Koen Binnemans,†
†KU
Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. Box 2404,
B-3001 Heverlee (Belgium)
* Corresponding author Email: [email protected]
ORCID Martina Orefice: 0000-0003-3854-9974 Koen Binnemans: 0000-0003-4768-3606
Abstract The protic ionic liquid pyridinium chloride is known to be a non-aqueous solvent for metal oxides, including rare-earth oxides. However, its application in extractive metallurgy and especially in solvent extraction has been so far limited by its miscibility with the aqueous phase. In this paper, molten pyridinium chloride (165 °C) was used to dissolve production scrap of Nd‒Fe‒B permanent magnets to recover the valuable metals neodymium and dysprosium. The Nd‒Fe‒B scrap powder completely dissolved in just 10 min with a liquid‒to‒solid ratio of 10 g g‒1. Afterwards, non-aqueous solvent extraction was performed at high temperature (165 °C) by using molecular extractants (PC-88A, Cyanex 923) or ionic liquids (Cyphos IL 101). The temperature affects the equilibrium constants and, hence, the distribution of the metals between the two phases. Moreover, high temperatures lower the viscosity of the solvents, so that they can be used in undiluted form. In the first stage, 30 vol.% PC-88A in p-cymene was used to extract dysprosium(III), whereas in the second stage 100 vol.% PC-88A was used to extract most of the neodymium(III). Finally, the synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.) was shown to efficiently extract iron(II,III) from the concentrated leachate. A conceptual flowsheet was designed, which included the recycling of the pyridinium chloride to lower the costs and the environmental impact of the process.
Keywords: solvometallurgy; Nd‒Fe‒B permanent magnets; pyridinium chloride; non-aqueous solvent extraction; high-temperature solvent extraction
2
1. INTRODUCTION The supply of rare-earth elements is largely dependent on imports from China and at the same time these elements are crucial for many applications such as wind turbines, electronic devices and (hybrid) electric vehicles [1,2]. Urban mining of metals allows to limit the import quota and the environmental impact of primary production [3]. End-of-life consumer goods and production scrap are both resources for urban mining, but production scrap is easier to exploit because, unlike the end-of-life consumer goods, it is easy to collect and does not require any dismantling operation. Pyridinium chloride (PyHCl) is a protic ionic liquid with unique properties, such as a wide molten temperature range and the Brønsted acid character [4,5]. It is being used in organic synthesis, in catalysis and even in medicine [5–11] .A few authors have tested this protic ionic liquid as a solvent for metal oxides and metal salts [4,12,13]. Hopkins and Audrieth reported a good solubility of rare-earth oxides in PyHCl.[14] Hollebone et al. studied dissolution of ores of niobium and other refractory metals in molten PyHCl [15] , whereas Pilarczyk et al. [16] and Grzybkowski et al. [17] successfully leached copper ores in a mixture of Py and PyHCl. In more recent metal recovery applications, (highly) substituted pyridinium rings have been preferred as cations of chloride or bis(trifluoromethylsulfonyl)imide, [Tf2N]-, ionic liquid [18–22]. To the best of our knowledge, no urban mining process based on the use of molten PyHCl has been reported yet. In particular, the application of protic ionic liquids in conventional solvent extraction has been limited due to their miscibility with the aqueous phase, unless [Tf2N]- is used as anion [6]. Non-aqueous solvent extraction with protic ionic liquids has been reported to recover organic solutes from n-heptane, whereas metal extraction with protic ionic liquids has been studied only from aqueous feed [23–26]. The melting point of PyHCl (144145 °C) would seem detrimental for its application as lixiviant, compared to conventionally used mineral acids, since it cannot be used undilute below 100 °C. However, 3
the higher the temperature, the lower the viscosity, the faster the mass transfer and the higher the dissolution rates of the metals in the solvent. Furthermore, the maximum temperature for hydrometallurgical processes at atmospheric pressure is 80 °C, due to the increasing evaporation of water. Temperature is an important parameter to regulate the separation factors of different solute in commercial (aqueous) solvent extraction systems and non-aqueous solvent extraction could offer an even wider range of working temperatures. In fact, the temperature affects the speciation of the metal complexes and the equilibrium constants of the occurring reactions. Above 100 °C and in solvometallurgical conditions, differences in metal solvation are possible compared to aqueous solutions at ambient conditions [27]. Nonaqueous solvent extraction from molten salts has been studied by several authors in the 1960s and the 1970s [28,29,38–41,30–37]. Most of these works focused on actinides or on heavy metals such as zinc, but Isaac et al. studied the extraction of trivalent lanthanides [28]. Distribution ratios about 102103 times higher were observed for solvent extraction from molten salts (molten KNO3LiNO3 eutectic) than from aqueous nitrate feed solutions. The most often used extractants for separation of rare earths from aqueous leachates of primary and secondary resources are organophosphorous extractants such as 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (PC-88A), di(2-ethylhexyl)-phosphoric acid (D2EHPA), Cyanex 923 (a commercial mixture of four trialkylphosphine oxides), or bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) [42,43]. PC-88A has been applied also for large-scale (mixer-settler plant) lanthanides separation from an aqueous leachate of Nd‒Fe‒B permanent magnets, after iron precipitation with H2O2 and NaOH [44]. In this paper, the dissolution of Nd‒Fe‒B permanent magnet production scrap in molten pyridinium chloride and the consequent non-aqueous solvent extraction of neodymium and dysprosium with PC-88A as extractant were investigated. A conceptual process flowsheet is proposed. 4
2. EXPERIMENTAL 2.1 Chemicals Pyridinium chloride (≥98.0 wt%) and di(2-ethylhexyl)-phosphoric acid (D2EHPA), were purchased from J&K Scientific GmbH (Zedelgem, Belgium). Iron, neodymium, dysprosium, holmium standards (1000 μg mL-1), nitric acid, HNO3 (65 wt%) and tri-n-butyl phosphate (≥99.0 wt%) were obtained from Chem-Lab (Zedelgem, Belgium). Cyanex 923®, a mixture of four trialkylphosphine oxides, was purchased from Cytec Industry (Niagara Falls, Ontario, Canada), while 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (PC-88A®), was ordered from Hallochem (Chongqing, China). Trihexyl(tetradecyl)phosphonium chloride (Cyphos® IL 101) was purchased from Cytec (Vlaardingen, The Netherlands). Deuterated chloroform, CDCl3 (99.8% atomic pure), p-cymene (99.0%) and oxalic acid (≥99.0 wt%), were supplied by Sigma-Aldrich (Diegem, Belgium). Glacial acetic acid (≥99.7%) and sodium hydroxide (pearls, ≥99.0%) were obtained from Fisher Scientific (Merelbeke, Belgium). Ammonia solution (25 wt% NH3 solution in water) was ordered from VWR Chemicals (Haasrode, Belgium), while 1,10-phenanthroline (≥99.0 wt%) from Acros Organics (Geel, Belgium). Silicone solution in isopropanol was purchased from SERVA Electrophoresis GmbH (Heidelberg, Germany). A Millipore Milli-Q® water purification system was used for the ultrapure water with a nominal resistivity of 18.2 MΩ cm at 25 °C. The chemicals were used without further purification. Demagnetized, non-coated Nd‒Fe‒B production scrap with a high dysprosium content (6-8 wt%) was kindly provided by Magneti Ljubljana d.d. (Ljubljana, Slovenia). 2.2 Pre-treatment of the solid Nd‒Fe‒B production scrap Nd‒Fe‒B production scrap was first crushed to pieces smaller than 5 mm by a hydraulic press. Next, they were reduced to powder by milling in a planetary ball mill (Fritsch Planetary 5
Mill Pulverisette 7, bowls capacity = 80 mL, stainless steel balls Ø 10 and 1 mm) at 600 rpm, with a charge ratio balls-to-powder = 4 g g-1 in 3 cycles of 5 minutes milling and 5 minutes pause. The solid and the balls were covered with ethanol to prevent milled powder sticking to the milling pot. The pots were left to cool at room temperature, since there is the risk that fine Nd‒Fe‒B powder will catch fire if handled in atmospheric air when it is still hot. Afterwards, the Nd‒Fe‒B powder was recovered from the pots and dried on a rotary evaporator under vacuum at 50 °C for an hour. Experimental details on how the elemental composition of the Nd‒Fe‒B scrap was determined have been described in a previous paper.[45] The analysis was performed via inductively coupled plasma optical emission spectrometer (ICP-OES), i.e. a Perkin Elmer model Optima 8300, equipped with an axial/radial dual plasma view, a GemTip Cross-Flow II nebulizer, a Scott double pass with inert Ryton® spray chamber and a demountable one-piece Hybrid XLT ceramic torch with a 2.0 mm internal diameter sapphire injector. Data were processed by the software Syngistix 1.0.1.1275. 2.3 Leaching and solvent extraction Nd‒Fe‒B powder (250 mg) was leached with about 2.5 g of pyridinium chloride, at 165 °C in 20 mL glass vials heated with a heating block equipped with a system of condensers to reflux the gases back into the system. Leaching times, tl, of 10, 30 and 60 min and liquid-to-solid ratio, L‒S (g g-1), of 10, 50 and 100 were tested. The leaching experiment did not require any stirring. Possible decomposition of pyridinium chloride during leaching was evaluated by Fourier transform infrared–attenuated total reflectance (FTIR–ATR) spectroscopy on the leachate at the harshest conditions (T = 165 °C, tl = 60 min and L‒S = 10), at the mildest conditions (T = 165 °C, tl = 10 min and L‒S = 100) and on a sample of only pyridinium chloride heated at 165 °C for 60 min, with a Vertex 70 by Bruker, operating at room temperature in transmission mode in a wavelength range of 500 – 4500 cm-1. The spectra were processed using the OPUS software. Detection of iron speciation in the leachate was carried out with Agilent Cary 6000i 6
UV-VIS-NIR spectrometer. Samples and standards were prepared as follows: about 0.150 g of leachate was dissolved in ethanol and 0.1 mL of the resulting solution was diluted with 7.9 mL of ultrapure water, 1 mL of buffer solution (6 mol L-1 acetic acid―5 mol L-1 sodium hydroxide, pH = 5.49) and 1 mL of 1,10-phenanthroline (0.12 wt% in ultrapure water). Standard solutions of 0.1-0.5-1.0 g L-1 of ferrous chloride in ethanol were prepared as described for the sample. If required, the solution of leachate in ethanol was further diluted two to five times in ethanol. For solvent extraction, firstly, a screening was carried out among different undiluted phosphate solvents: Cyanex 923, D2EHPA and PC-88A. The components of the process were defined as: feed (F) = the pyridinium chloride leachate; raffinate (R) = the purified pyridinium chloride solution, the solvent (E) = the mixture extractant‒diluent (and eventually phase modifier) unloaded or loaded. The following working conditions were applied: feed-solvent ratio, F/E = 1/1 in volume; temperature, T = 165 °C; stirring time, tmix = 30 min; stirring speed, r = 150 rpm. Pyridinium chloride is solid below 144 °C, therefore the two phases could not be separated by centrifugation: the mixture was left to settle at 165 °C for 10 min and subsequently left to cool down. The upper phase, the loaded solvent E, was separated and analyzed. The percentage of extraction, %E, was calculated via Equation (1):
𝑛E
%E = 𝑛F ∙ 100
(1)
where nF and nE are the moles of metal in the feed and in the loaded solvent, respectively. The distribution ratio, D, which compares the affinity of a metal M in each phase, was calculated as in Equation (2):
𝑐E
D M = 𝑐R
(2)
7
where cE and cR are the moles of metal in the loaded solvent and in the raffinate, respectively. The separation factor, α is defined as in Equation (3):
𝐷M1
α = 𝐷M2
(3)
where M1 and M2 are two metals in the system and DM1>DM2. The solvents were treated in a microwave digester (Berghof Speedwave Xpert), prior to the elemental analysis. An aliquot of 7 mL of 65 wt% HNO3 was added to 50 μL of loaded E in DAK-100 vessels and the mixture was digested for 75 min in a temperature range of 50 to 175 °C and a pressure range from atmospheric up to 20 bars. When the samples were cooled at room temperature, they were diluted to 50.0 mL with ultrapure water and then 100 times with 2 vol.% HNO3. The samples were analyzed via a Perkin-Elmer Optima 8300 ICP-OES. Data were processed with the Syngistix software, version 1.0.1.1275. Saponified solvents were prepared too, by adding 10 vol.% of NaOH 0.1 mol L-1 to the extractant-diluent mixtures. The extraction rate was studied from 5 to 60 min, under the conditions (F/E, T, tmix, r) listed above. Extraction tests with Cyphos IL 101 were performed for 30 min at T = 165 °C with F/E = 1/1 or 1/2 and r = 150 or 300 rpm. Different Cyphos IL 101-based solvents were prepared: undiluted Cyphos IL 101; 30 or 80 vol% Cyphos IL 101 in p-cymene, a mixture of 70:30 (wt.:wt.) Cyphos IL 101―Cyanex 923. The Cyphos IL 101-based solvents were diluted in ethanol and measured via total reflection X-ray fluorescence spectroscopy (TXRF) with a Bruker S2 Picofox TXRF spectrometer, equipped with a molybdenum X-ray source. Samples were prepared according to protocols in the literature and holmium was selected as standard.[46,47] The data were analyzed via the Bruker Spectra PICOFOX v. 7.5.3.0 software.
8
A 0.2 mol L-1 solution of oxalic acid was used, in molar ratio = 1.5 for rare-earth elements precipitation stripping, whereas a 5 wt% ammonia solution was used for precipitation stripping of iron. The precipitation yields %P were calculated as in Equation (4):
(
𝑛ES
)
%P = 1 ― 𝑛FS ∙ 100
(4)
where nFS and nES are the moles of metal in the stripping feed and in the stripped solvent E, respectively.
9
3. RESULTS AND DISCUSSION 3.1 Leaching The uncoated, bulk Nd‒Fe‒B scrap was first crushed and milled. All the collected powder had a particle size below 45 μm, determined by sieve fractioning. The composition of the solid was already analyzed in a previous work, and it is reported here in Table 1 for completeness and clarity to the reader [45]. Table 1: Elemental composition of the Nd‒Fe‒B permanent magnet scrap (in wt.%) determined by ICP-OES Element
Composition* (wt.%)
Fe
62.52
Nd
24.15
Dy
6.89
Pr
1.45
Co
3.46
Cu
0.04
Ga
0.32
Al
0.11
B
1.05
*compositions normalized to 100%
Leaching tests were carried out at 165 °C for 10, 30 and 60 min with L‒S ratios (g g-1) of 10, 50 and 100. Complete dissolution of the Nd‒Fe‒B powder was achieved even after the shortest leaching duration (10 min) with the lowest L-S ratio (10). The composition of the leachate, in major elements, was 84.7 g L-1 of iron, 31.7 g L-1 of neodymium, 11.5 g L-1 of dysprosium. The leaching occurs according to the reaction in Equation (5):
10
2 M(s) + 2n [C5H5NH]Cl(l) → 2 MCln,(solvo) + 2n [C5H5N](l) + n H2,(g)
(5)
where M is any metal in the Nd‒Fe‒B powder, solvo is the solvated form in non-aqueous solutions and the stoichiometric factor n = 2 or n = 3. The cation [C5H5NH]+ delivers the proton for the oxidation of the metal from M0 to M2+ or to M3+. Hydrogen gas is released during the reaction, whereas chlorides are consumed to form the metal chloride salts. Therefore, HCl needs to be added to the system to regenerate the lixiviant PyHCl, as already reported in the literature [15]. In case of roasted magnets, water will be produced instead of hydrogen gas, and it could be condensed back to the system. The dissolution of the same powder in 37 wt% HCl took about 1 hour at 70 °C and with a L‒S ratio of 20. This shows the process intensification offered by PyHCl as lixiviant, since it allows to work above 100 °C at atmospheric pressure, which is not possible in hydrometallurgical systems. Mitchner specified that the high thermal stability of PyHCl is due to the strong bond between the nitrogen atom on the ring and the proton of the acid [13]. The PyHCl should be stable at the working temperature of 165 °C, since its liquidus range is between 144 °C and 218 °C. Nevertheless, the stability of PyHCl at high temperature and in presence of metals, with potential catalytic effect on de-hydrogenation and hydrogenation reactions, was tested via FTIR–ATR. Spectra of the leachate after the mildest (T = 165 °C, t = 10 min, L‒S = 100) and the harshest conditions (T = 165 °C, t = 60 min, L‒S = 10) are compared in Figure 1 with the FTIR spectrum of PyHCl just heated at 165 °C for 60 min.
11
Figure 1: FTIR-AR spectra of (i - green) the leachate after the test at T = 165 °C, t = 10 min, L‒S = 100; (ii - red) the leachate after the test at T = 165 °C, t = 60 min, L‒S = 10; (iii blue) PyHCl heated at 165 °C for 60 min
3.2 Rare-earths non-aqueous solvent extraction and separation Dissolution of Nd-Fe-B powder in PyHCl is very fast and consumes relatively small amounts of lixiviant per gram of treated solid, but the process is not selective. Therefore, a nonaqueous solvent extraction is required to selectively separate the metals. Extraction efficiency in conventional solvent extraction largely depends on the speciation of the metal ions in both organic and aqueous phases and and in conventional aqueous feeds the metals are present as hydrated complexes. In this work, the feed PyHCl is anhydrous and water in the first coordination sphere is replaced by chloride ions [48]. However, not any anion of the ionic liquids can enter the first coordination sphere of a metal cation. For instance, also at low water content ( ≤10 wt%), the weakly coordinating [Tf2N]- is bridged to the metal cation via water molecules [49]. In fact, Nd‒Fe‒B powder could not be leached in anhydrous [Hbet][Tf2N], because the anion [Tf2N]-, differently from the chloride, cannot replace water in the first 12
coordination sphere.[45] The speciation equilibrium of lanthanides in PyHCl is described by Equation (6):
LnCl3 + 3 [C5H5NH]Cl ⇌ [C5H5NH]3LnCl6
(6)
Such [𝑐𝑎𝑡]3LnCl6 species, where cat is a general representation for the cation of the ionic liquid, have been reported as moieties of lanthanides in protic ionic liquids for example for [bmim]Cl [50]. First studies of non-aqueous solvent extraction were carried out with a screening of undiluted D2EHPA, PC-88A and Cyanex 923 at 165 °C (Figure 2). A general good selectivity in rareearth elements extraction over iron is achieved with all the studied extractants.
Figure 2: Extraction yields, %E, of iron, neodymium and dysprosium in Cyanex 923 (), PC88A () and D2EHPA() from PyHCl leachate. Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Extraction conditions: T = 165 °C; t = 30 min; r = 150 rpm; F/E = 1/1 PC-88A and D2EHPA can both selectively extract the trivalent rare-earth ions over iron whereas Cyanex 923 extracts also about 21% of iron together with about 62% of 13
neodymium(III) and 82% of dysprosium(III). PC-88A, in particular, offers the highest selectivity since it gives a separation factor α, (Equation (3)), (rare-earth elements)/iron of 241.1, against the α of 63.1 for D2EHPA and 21.2 for Cyanex 923. Furthermore, D2EHPA phase forms a gel after the extraction, probably due to the high metal loading and the consequent polymerisation of the metal-solvent complexes [51]. Consequently, extraction of rare.earth elements from the PyHCl at high temperature (>100 °C) was optimized using PC88A. In aqueous solvent extraction, organophosphorous extractants as D2EHPA have been reported to have high affinity for iron(III), but much less for iron(II): the high α values from Figure 2 might be explained by the presence of iron as iron(II) instead of iron(III) [52]. Also considering that Nd‒Fe‒B powder was not roasted before leaching, iron is expected to be present as iron(II) forming FeCl2 complexes,. Speciation of iron in the leachate was studied via UV-VIS absorption spectroscopy and 1,10-phenanthroline (0.12 wt% in ultrapure water) was used as complexing agent [53]. The resulting spectra is reported in Figure 3, where the characteristic absorbance peak of the Fe(II)―1,10-phenanthroline complex at 510 nm is present [53].
Figure 3: UV-VIS absorption spectrum of the leachate-1,10-phenanthroline solution
14
In concentrated aqueous chloride solution, the peak at 363 nm has been reported to correspond to FeCl4‒, i.e. iron(III) complex, while the small peak at 510 nm correspond to the complex FeCl42‒, i.e. iron(II) complex [54–57]. Therefore, in the PyHCl leachate, iron is present both as iron(II) and as iron(III). The selectivity of the organophosphorous extractants cannot be ascribed to the speciation of iron, but might rather be due to the positive effect of the high temperature (≥100 °C). on the speciation and distribution of the metals between the two phases. The next step of the process was the separation of neodymium(III) and dysprosium(III). Although their concentrations in the leachate are different (31.7 g L-1 and 11.5 g L-1, respectively), neodymium and dysprosium are both significantly extracted by undiluted PC88A. The extraction of neodymium(III) from chloride solutions is strongly affected by D2EHPA concentration [51]. In this work, the effect of PC-88A concentration on neodymium(III) extraction from PyHCl leachate was investigated. The choice of the diluent was crucial since most of the commonly used diluents, such as toluene, boil below the working temperature (165 °C). Therefore, PC-88A was diluted to 30 vol.% in p-cymene which boils above 165 °C and also mitigates possible degradation of the pyridinium ring by the acidic extractant during the mixing of the feed and the solvent phases. On the other hand, the purpose of diluents in extraction process is to control the concentration of the extractants or to lower the viscosity of the extractants, which even increases after solute loading. However, decreasing the viscosity by diluents might not be necessary in solvent extractions above 100 °C since increasing the temperature has the same effect on the viscosity as the diluent. In this specific case, the diluent was used to regulate the concentration of PC-88A and, thus, to enhance the selectivity and not the mass transfer of the extraction process. At the same time, saponification of PC-88A with NaOH has been proven to increase the extraction yields of rare earths from acidic media, since it counteracts the protons released by the PC-
15
88A [58–61]. Therefore, a solution of 30 vol% PC-88A in p-cymene with 10 vol.% NaOH 0.1 mol L-1 was prepared, but it was unstable probably because of the chosen diluent pcymene (Figure 4 (a)). Noticeably, an equivalent solution of saponified D2EHPA was stable, but it again resulted in gel formation at the end of the solvent extraction test and it was discarded (Figure 4 (b)). In Table 2, %E for iron(II), neodymium(III) and dysprosium(III) with 30 vol.% PC-88A in p-cymene and with 100 vol.% PC-88A from PyHCl leachate are displayed.
(a)
(b)
Figure 4: (a) Unstable solution of 30 vol% PC-88A in p-cymene with 10 vol.% NaOH 0.1 mol L-1; (b) jelled 30 vol% D2EHPA in p-cymene with 10 vol.% NaOH 0.1 mol L-1 after solvent extraction of rare earths from PyHCl leachate
Table 2: %E for iron(II), neodymium(III) and dysprosium(III) with 30 vol.% PC-88A in pcymene and with 100 vol.% PC-88A from PyHCl feed %E
D
[PC-88A]
30 vol.%
100 vol.%
30 vol.%
100 vol.%
Fe(II,III)
1
2
0.0
0.0
Nd(III)
22
51
0.3
2.1
Dy(III)
100
71
171
4.9
Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Solvent extraction conditions: T = 165 °C, F/E = 1/1, t = 30 min, r = 150 rpm
16
By reducing the PC-88A concentration, the extraction of dysprosium(III) is favored over that of neodymium(III) in two simultaneous ways: the %E of dysprosium(III) is increased and, especially, the %E of neodymium(III) is decreased. The latter might be due to the reduced concentration of PC-88A, i.e. there are no enough extractant molecules for the neodymium(III) extraction. The fact that, at the same time, more dysprosium(III) is extracted might suggest that the two rare-earth ions compete for the transfer to PC-88A. Afterwards, the extraction rate of metals in 30 vol.% PC-88A in p-cymene was studied as function of the mixing time (Figure 5). The large error bars (≤ 9.1%) are to attribute to the experimental errors associated with the microwave digestion of the samples prior to ICP-OES analysis.
Figure 5: Extraction rate of iron(II,III) (), neodymium(III) () and dysprosium(III) () in 30 vol.% PC-88A in p-cymene. Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Extraction conditions: T = 165 °C; r = 150 rpm; F/E = 1/1 Extraction of neodymium(III) slightly increases with the time meaning that the solvent is not saturated and the slow extraction rate might be correlated to the concentration of the protons released by the PC-88A. In fact, acidic extractants such as PC-88A operate by exchanging
17
their protons with the metal cations from the other phase. In aqueous solvent extraction (Equation (7), where (HA)2 is the dimer of the acidic extractant), the longer the extraction proceeds, the higher the protons and the lower has been reported the %E [61–63].
Ln3+(aq) + 3 (HA)2 ⇌ Ln(HA2)3,(𝑎𝑞) + 3 H+(aq)
(7)
Non-aqueous solvent extraction of lanthanides in acidic organophosphorous extractants (Cyanex 272, PC-88A, D2EHPA) from chloride media has not been reported yet. The possibility of the different solvation, with chlorides substituting water in the first coordination sphere, affecting the extraction mechanism cannot be excluded here. Although more detailed speciation studies are needed, a first hypothesis is that the protons from the PC-88A participate in the reaction (Equation (8)) by taking the chlorides from the LnCl63‒ complex before such complex is extracted.
LnCl63‒ + 3 (HA)2 + 3 H+ ⇌ (LnHA2)3 + 6 HCl
(8)
The mechanism in Equation (8) does not occur in conventional aqueous solvent extraction where the lanthanides occur as positively charged hydrated complexes (Equation (7)). Dysprosium(III) was quantitatively extracted within 10 min also in diluted PC-88A, since its concentration in the PyHCl feed is about three times lower than that of neodymium(III). In the next extraction stage, neodymium(III) was extracted with undiluted PC-88A. The cumulative %ETOT after one stage of 10 min with 30 vol.% PC-88A in p-cymene and after one stage of 30 min with 100 vol.% PC-88A are reported in Table 3. The large error on the %ETOT of the iron(II) is due to the low %ETOT, i.e. the low concentrations read at the ICP-OES.
18
Table 3: Cumulative %ETOT for iron(II), neodymium(III) and dysprosium(III) after stage I with 30 vol.% PC-88A in p-cymene and stage II with 100 vol.% PC-88A from PyHCl feed.a [PC-88A]
%ETOT
%RSD
Fe(II,III)
3
14
Nd(III)
74
3
Dy(III)
102
2
aMetal
concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1,Dy = 11.5 g L-1. Solvent extraction conditions: stage(I) T = 165 °C, F/E = 1/1, t = 10 min, r = 150 rpm; stage(II) T = 165 °C, F/E = 1/1, t = 30 min, r = 150 rpm
The process has not been optimized yet, but neodymium(III) can be totally extracted with a higher F/E ratio in stage II or adding another stage of 100 vol.% PC-88A. Recovery of rareearth elements from the loaded solvents was investigated by precipitation stripping with a 0.2 mol L-1 oxalic acid solution added in a stoichiometric amount. The results are shown in Table 4. The concentration of neodymium(III) and dysprosium(III) in extract I were comparable due to the different concentrations in the PyHCl feed and to the %E(Nd(III)) of about 20%. Nevertheless, neodymium(III) precipitated after 10 min shaking at room temperature, while dysprosium(III) did not precipitate: probably higher temperature, longer time and higher molar ratios (oxalic acid)/(dysprosium(III)) are necessary.
19
Table 4: Precipitation yields (%P, %) and purity (x, wt%) of neodymium(III) and dysprosium(III) from the extract I (30 vol.% PC-88A) and from the extract II (100 vol.% PC88A) %PNd
xNd (wt%)
%PDy
xDy (wt%)
extract I
78
100
0.0
‒
extract II
50
97
‒
‒
Metal concentrations: in the extract (I) Nd = 7.7 g L-1, Dy = 11.2 g L-1; in the extract (II) Nd = 15.5 g L-1, Dy = 0.3 g L-1
3.3 Iron non-aqueous solvent extraction and conceptual flowsheet Finally, to regenerate the lixiviant, iron(II,III) had to be extracted from the PyHCl leachate. The ionic liquid Cyphos IL 101 was tested undiluted and diluted to 30 vol.% in p-cymene (Table 5). The PyHCl feed still contained about 8.2 g L-1 of neodymium(III), but this was not extracted and, thus, did not interfere with the iron(II,III) extraction. The %E increases with the concentration of Cyphos IL 101, as expected, but even with undiluted Cyphos IL 101, the %E of iron(II,III) from the PyHCl feed is <50%. In fact, it has been calculated that a concentration of 3.0 mol L-1 Cyphos IL 101 would be necessary to extract all the iron(II,III) from the feed, whereas the concentration of undiluted Cyphos IL 101 is 1.7 mol L-1.
20
Table 5: %E for iron(II,III) with 30 vol.% Cyphos IL 101 in p-cymene and with 100 vol.% Cyphos IL 101 from PyHCl leachate refined from the rare-earth elements Cyphos IL 101
%E
%RSD
D
%RSD
30 vol.%
19
3
0.2
3.0
100 vol.%
32
6
0.4
9.4
Metal concentration in the feed: Fe = 84.7 g L-1, Nd = 8.2 g L-1, Dy = 0.0 g L-1. Solvent extraction conditions: T = 165 °C, F/E = 1/1, t = 30 min, r = 300 rpm
The extraction occurs according to Equations (9) and (10), although extraction by solvation (Equation (11)) cannot be excluded. The Equations are reported for iron(II) but they can be adapted for iron(III) just by adjusting the stoichiometry, P66614 is the phosphonium cation of the Cyphos IL 101 ionic liquid:
FeCl2 + 2 Cl‒ ⇌ FeCl42‒
(9)
FeCl42‒ + 2 [P66614][Cl] ⇌ [P66614]2[FeCl4] + 2 Cl‒
(10)
FeCl2 + 2 [P66614][Cl] ⇌ [P66614]2[FeCl4]
(11)
It was calculated that, with 30 vol.% Cyphos IL 101 in p-cymene, more moles of iron(II,III) were extracted than those stoichiometrically possible with the chlorides provided by the extractant: at diluted Cyphos IL 101 concentrations, chlorides from the leachate feed are coextracted with iron(II,III). Contrarily, in case of undiluted Cyphos IL 101 with 1.7 mol L-1 of chlorides, only about 50% of the maximum iron(II,III) loading is achieved. This might be due to the viscosity of the solvent which significantly increases with the loading, until the solvent 21
appears as solid at room temperature. Therefore, in the next experiment Cyphos IL 101 was diluted 80 vol.% in p-cymene to lower the viscosity, while still keeping a moderate chloride concentration (1.4 mol L-1), and the F/E ratio was raised to 1/2 to increase the %E (Table 6). Nevertheless, still a %E of around 60% was obtained. A synergistic mixture Cyphos IL 101 with Cyanex 923 70:30 (wt.:wt.) was tested since the results in Figure 2 showed that Cyanex 923 also extracts iron(II) from the PyHCl feed and because of similar experimental evidence reported in the literature [64]. The results are displayed in Table 6 together with those obtained with the 80 vol.% Cyphos 101 and with 100 vol.% Cyanex 923 (at F/E = 1/1) to highlight the synergistic effect of the mixture. Differently from the diluent p-cymene, Cyanex 923 also participates to the extraction mechanism (following a solvation mechanism as that in Equation (11)) while lowering the viscosity of the solvent.
Table 6: %E for iron(II) with 80 vol.% Cyphos IL 101 in p-cymene, with 100 vol.% Cyanex 923 and with a synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.). from PyHCl leachate refined from the rare earths Solvent
F/E
%E
%RSD
D
%RSD
80 vol.% Cyphos IL 101 in p-cymenea
1/2
62
6
0.8
14.5
100 vol.% Cyanex 923b
1/1
21
1
0.6
11.5
70:30 (wt.:wt.) Cyphos IL 101:Cyanex 923a
1/2
70
4
1.2
12.5
a
Metal concentration in the feed: (a) Fe = 84.7 g L-1, Nd = 8.2 g L-1, Dy = 0.0 g L-1. Solvent extraction conditions: T = 165 °C, t = 30 min, r = 300 rpm b refer to caption of Figure 2 for experimental details
It is worth to mention that the leachate is highly loaded with iron(II) (84.7 g L-1) due to the excellent properties of PyHCl as lixiviant (L‒S ratio = 10) and this affects the mass transfer of
22
the metal to the solvent. However, the synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt:wt) shows good extracting properties and can be further optimized, especially in terms of mixing time and mutual ratio of the extractants. Iron(II,III) was recovered from the synergistic solvent by stripping precipitation with a 5 vol% NH3 solution. Once purified from the metals, the lixiviant PyHCl can be recycled at the head of the process. Addition of HCl might be necessary, but it has to be highlighted in the perspective of environmental impact of the process that most HCl has not been emitted as it is, but rather it has been lost as hydrogen gas in the leaching step (Equation (5)) and as chlorides in the extraction of iron(II,III). A conceptual flowsheet is suggested in Figure 6, where the blue lines represent the hydrometallurgical operations and the green lines the solvometallurgical operations. Most of the operations, dissolution of the Nd‒Fe‒B powder and solvent extraction of the metals were successfully carried out in non-aqueous conditions and the use of aqueous solutions is confined only to the recovery of metals via precipitation stripping. Precipitation of the rareearth elements from the PC-88A solutions via direct addition of oxalic acid was tested but was not successful: this suggests that first stripping to water occurs, followed by complexation and precipitation of the rare-earth oxalates.
23
Figure 6: Conceptual flowsheet for the solvometallurgical recycling of Nd‒Fe‒B scrap. The main operations are: leaching of non-roasted Nd‒Fe‒B scrap at 165 °C in pyridinium chloride; non-aqueous solvent extraction at 165 °C of (a) dysprosium(III) with 30 vol.% PC88A in p-cymene, (b) neodymium(III) with 100 vol.% PC-88A and (c) of iron(II) with a synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.).
24
4. CONCLUSIONS The protic ionic liquid pyridinium chloride has been used for the leaching of production scrap of Nd‒Fe‒B permanent magnets for the recovery of rare-earth elements. The Nd‒Fe‒B scrap powder completely dissolved at high temperatures (165 °C) after just 10 min with a liquid-tosolid ratio of 10 g g-1. Although iron was present as both iron(II) and iron(III), a selective extraction of neodymium(III) and dysprosium(III) with organophosphorous extractants was still possible, due to the effect of the high temperature (>100 °C) on the separation factors in the consequent solvent extraction step. A screening of undiluted Cyanex 923, PC-88A and D2EHPA extractants was performed at 165 °C and PC-88A was the best extractant with the highest separation factor (rare-earth elements(III))/iron(II,III): 241 against 63 in D2EHPA and 21 in Cyanex 923. At high temperatures (≥100 °C), the use of a diluent to lower the extractant viscosity is not necessary. However, dysprosium(III) and neodymium(III) could be separated by diluting the PC-88A 30 vol.% in p-cymene. In fact, only 20% of neodymium(III) was extracted at low PC-88A concentration, while %E of dysprosium(III) increased to 100%. Afterwards, the remaining neodymium(III) was extracted in one stage with undiluted PC-88A. The rare-earth elements were recovered from the loaded solvents by precipitation stripping with an aqueous solution of 0.2 mol L-1 oxalic acid. Finally, the regeneration of PyHCl required two steps: (i) extraction of iron(II,III) and (ii) addition of HCl into the remaining pyridine. About 70% of iron(II) was extracted from the PyHCl by a synergistic mixture Cyphos IL 101―Cyanex 923, in a ratio 70:30 (wt:wt.) and it was then precipitated with a 5 vol% NH3 solution. Both the extractants gave lower %E when used alone or diluted, for instance, in p-cymene. This process is relevant for the recycling of Nd‒Fe‒B permanent magnets: the use of PyHCl significantly intensifies the leaching step, whereas dysprosium(III) and neodymium(III) are extracted at different concentrations of PC-88A.
25
Appendix A. Supplementary material Supplementary data to this article can be found online at: (link to be added).
Declaration of interest There are no conflicts to declare. Acknowledgements The research leading to these results received funding from the European Commission’s Horizon 2020 Programme ([H2020/2014-2019]) under Grant Agreement no. 674973 (MSCAETN DEMETER) and under Grant Agreement 694078—Solvometallurgy for critical metals (SOLCRIMET). This publication reflects only the authors’ view, exempting the Commission from any liability. The authors also want to thank Magneti Ljubljana for providing the magnets and the production scrap samples for the carried out research, Tony Debecker and Kevin Wierinckx for crushing the magnets. Thanks to Dr. Bieke Onghena for reviewing the manuscript.
26
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35
Graphical abstract
36
Highlights • Solvometallurgical process for the recovery of rare-earth elements from Nd‒Fe‒B magnets
Nd‒Fe‒B magnets completely dissolved in pyridinium chloride (PyHCl) within 10 min
Preliminary roasting of the Nd‒Fe‒B magnets is not required, just milling
Leaching in pure PyHCl at 165 °C avoided the consumption of other solvents
Neodymium and dysprosium were extracted by different concentrations of PC-88A
High-temperature solvent extraction clearly affects the distribution ratio of iron
37
S1383-5866(20)32274-7 https://doi.org/10.1016/j.seppur.2020.117800 SEPPUR 117800
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
19 November 2019 14 August 2020 22 September 2020
Please cite this article as: M. Orefice, K. Binnemans, Solvometallurgical process for the recovery of rare-earth elements from Nd–Fe–B magnets, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/ j.seppur.2020.117800
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© 2020 Published by Elsevier B.V.
Manuscript for: Separation and Purification Technology
Solvometallurgical process for the recovery of rare-earth elements from Nd‒Fe‒B magnets Martina Orefice,†,* Koen Binnemans,†
†KU
Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. Box 2404,
B-3001 Heverlee (Belgium)
* Corresponding author Email: [email protected]
ORCID Martina Orefice: 0000-0003-3854-9974 Koen Binnemans: 0000-0003-4768-3606
Abstract The protic ionic liquid pyridinium chloride is known to be a non-aqueous solvent for metal oxides, including rare-earth oxides. However, its application in extractive metallurgy and especially in solvent extraction has been so far limited by its miscibility with the aqueous phase. In this paper, molten pyridinium chloride (165 °C) was used to dissolve production scrap of Nd‒Fe‒B permanent magnets to recover the valuable metals neodymium and dysprosium. The Nd‒Fe‒B scrap powder completely dissolved in just 10 min with a liquid‒to‒solid ratio of 10 g g‒1. Afterwards, non-aqueous solvent extraction was performed at high temperature (165 °C) by using molecular extractants (PC-88A, Cyanex 923) or ionic liquids (Cyphos IL 101). The temperature affects the equilibrium constants and, hence, the distribution of the metals between the two phases. Moreover, high temperatures lower the viscosity of the solvents, so that they can be used in undiluted form. In the first stage, 30 vol.% PC-88A in p-cymene was used to extract dysprosium(III), whereas in the second stage 100 vol.% PC-88A was used to extract most of the neodymium(III). Finally, the synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.) was shown to efficiently extract iron(II,III) from the concentrated leachate. A conceptual flowsheet was designed, which included the recycling of the pyridinium chloride to lower the costs and the environmental impact of the process.
Keywords: solvometallurgy; Nd‒Fe‒B permanent magnets; pyridinium chloride; non-aqueous solvent extraction; high-temperature solvent extraction
2
1. INTRODUCTION The supply of rare-earth elements is largely dependent on imports from China and at the same time these elements are crucial for many applications such as wind turbines, electronic devices and (hybrid) electric vehicles [1,2]. Urban mining of metals allows to limit the import quota and the environmental impact of primary production [3]. End-of-life consumer goods and production scrap are both resources for urban mining, but production scrap is easier to exploit because, unlike the end-of-life consumer goods, it is easy to collect and does not require any dismantling operation. Pyridinium chloride (PyHCl) is a protic ionic liquid with unique properties, such as a wide molten temperature range and the Brønsted acid character [4,5]. It is being used in organic synthesis, in catalysis and even in medicine [5–11] .A few authors have tested this protic ionic liquid as a solvent for metal oxides and metal salts [4,12,13]. Hopkins and Audrieth reported a good solubility of rare-earth oxides in PyHCl.[14] Hollebone et al. studied dissolution of ores of niobium and other refractory metals in molten PyHCl [15] , whereas Pilarczyk et al. [16] and Grzybkowski et al. [17] successfully leached copper ores in a mixture of Py and PyHCl. In more recent metal recovery applications, (highly) substituted pyridinium rings have been preferred as cations of chloride or bis(trifluoromethylsulfonyl)imide, [Tf2N]-, ionic liquid [18–22]. To the best of our knowledge, no urban mining process based on the use of molten PyHCl has been reported yet. In particular, the application of protic ionic liquids in conventional solvent extraction has been limited due to their miscibility with the aqueous phase, unless [Tf2N]- is used as anion [6]. Non-aqueous solvent extraction with protic ionic liquids has been reported to recover organic solutes from n-heptane, whereas metal extraction with protic ionic liquids has been studied only from aqueous feed [23–26]. The melting point of PyHCl (144145 °C) would seem detrimental for its application as lixiviant, compared to conventionally used mineral acids, since it cannot be used undilute below 100 °C. However, 3
the higher the temperature, the lower the viscosity, the faster the mass transfer and the higher the dissolution rates of the metals in the solvent. Furthermore, the maximum temperature for hydrometallurgical processes at atmospheric pressure is 80 °C, due to the increasing evaporation of water. Temperature is an important parameter to regulate the separation factors of different solute in commercial (aqueous) solvent extraction systems and non-aqueous solvent extraction could offer an even wider range of working temperatures. In fact, the temperature affects the speciation of the metal complexes and the equilibrium constants of the occurring reactions. Above 100 °C and in solvometallurgical conditions, differences in metal solvation are possible compared to aqueous solutions at ambient conditions [27]. Nonaqueous solvent extraction from molten salts has been studied by several authors in the 1960s and the 1970s [28,29,38–41,30–37]. Most of these works focused on actinides or on heavy metals such as zinc, but Isaac et al. studied the extraction of trivalent lanthanides [28]. Distribution ratios about 102103 times higher were observed for solvent extraction from molten salts (molten KNO3LiNO3 eutectic) than from aqueous nitrate feed solutions. The most often used extractants for separation of rare earths from aqueous leachates of primary and secondary resources are organophosphorous extractants such as 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (PC-88A), di(2-ethylhexyl)-phosphoric acid (D2EHPA), Cyanex 923 (a commercial mixture of four trialkylphosphine oxides), or bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) [42,43]. PC-88A has been applied also for large-scale (mixer-settler plant) lanthanides separation from an aqueous leachate of Nd‒Fe‒B permanent magnets, after iron precipitation with H2O2 and NaOH [44]. In this paper, the dissolution of Nd‒Fe‒B permanent magnet production scrap in molten pyridinium chloride and the consequent non-aqueous solvent extraction of neodymium and dysprosium with PC-88A as extractant were investigated. A conceptual process flowsheet is proposed. 4
2. EXPERIMENTAL 2.1 Chemicals Pyridinium chloride (≥98.0 wt%) and di(2-ethylhexyl)-phosphoric acid (D2EHPA), were purchased from J&K Scientific GmbH (Zedelgem, Belgium). Iron, neodymium, dysprosium, holmium standards (1000 μg mL-1), nitric acid, HNO3 (65 wt%) and tri-n-butyl phosphate (≥99.0 wt%) were obtained from Chem-Lab (Zedelgem, Belgium). Cyanex 923®, a mixture of four trialkylphosphine oxides, was purchased from Cytec Industry (Niagara Falls, Ontario, Canada), while 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (PC-88A®), was ordered from Hallochem (Chongqing, China). Trihexyl(tetradecyl)phosphonium chloride (Cyphos® IL 101) was purchased from Cytec (Vlaardingen, The Netherlands). Deuterated chloroform, CDCl3 (99.8% atomic pure), p-cymene (99.0%) and oxalic acid (≥99.0 wt%), were supplied by Sigma-Aldrich (Diegem, Belgium). Glacial acetic acid (≥99.7%) and sodium hydroxide (pearls, ≥99.0%) were obtained from Fisher Scientific (Merelbeke, Belgium). Ammonia solution (25 wt% NH3 solution in water) was ordered from VWR Chemicals (Haasrode, Belgium), while 1,10-phenanthroline (≥99.0 wt%) from Acros Organics (Geel, Belgium). Silicone solution in isopropanol was purchased from SERVA Electrophoresis GmbH (Heidelberg, Germany). A Millipore Milli-Q® water purification system was used for the ultrapure water with a nominal resistivity of 18.2 MΩ cm at 25 °C. The chemicals were used without further purification. Demagnetized, non-coated Nd‒Fe‒B production scrap with a high dysprosium content (6-8 wt%) was kindly provided by Magneti Ljubljana d.d. (Ljubljana, Slovenia). 2.2 Pre-treatment of the solid Nd‒Fe‒B production scrap Nd‒Fe‒B production scrap was first crushed to pieces smaller than 5 mm by a hydraulic press. Next, they were reduced to powder by milling in a planetary ball mill (Fritsch Planetary 5
Mill Pulverisette 7, bowls capacity = 80 mL, stainless steel balls Ø 10 and 1 mm) at 600 rpm, with a charge ratio balls-to-powder = 4 g g-1 in 3 cycles of 5 minutes milling and 5 minutes pause. The solid and the balls were covered with ethanol to prevent milled powder sticking to the milling pot. The pots were left to cool at room temperature, since there is the risk that fine Nd‒Fe‒B powder will catch fire if handled in atmospheric air when it is still hot. Afterwards, the Nd‒Fe‒B powder was recovered from the pots and dried on a rotary evaporator under vacuum at 50 °C for an hour. Experimental details on how the elemental composition of the Nd‒Fe‒B scrap was determined have been described in a previous paper.[45] The analysis was performed via inductively coupled plasma optical emission spectrometer (ICP-OES), i.e. a Perkin Elmer model Optima 8300, equipped with an axial/radial dual plasma view, a GemTip Cross-Flow II nebulizer, a Scott double pass with inert Ryton® spray chamber and a demountable one-piece Hybrid XLT ceramic torch with a 2.0 mm internal diameter sapphire injector. Data were processed by the software Syngistix 1.0.1.1275. 2.3 Leaching and solvent extraction Nd‒Fe‒B powder (250 mg) was leached with about 2.5 g of pyridinium chloride, at 165 °C in 20 mL glass vials heated with a heating block equipped with a system of condensers to reflux the gases back into the system. Leaching times, tl, of 10, 30 and 60 min and liquid-to-solid ratio, L‒S (g g-1), of 10, 50 and 100 were tested. The leaching experiment did not require any stirring. Possible decomposition of pyridinium chloride during leaching was evaluated by Fourier transform infrared–attenuated total reflectance (FTIR–ATR) spectroscopy on the leachate at the harshest conditions (T = 165 °C, tl = 60 min and L‒S = 10), at the mildest conditions (T = 165 °C, tl = 10 min and L‒S = 100) and on a sample of only pyridinium chloride heated at 165 °C for 60 min, with a Vertex 70 by Bruker, operating at room temperature in transmission mode in a wavelength range of 500 – 4500 cm-1. The spectra were processed using the OPUS software. Detection of iron speciation in the leachate was carried out with Agilent Cary 6000i 6
UV-VIS-NIR spectrometer. Samples and standards were prepared as follows: about 0.150 g of leachate was dissolved in ethanol and 0.1 mL of the resulting solution was diluted with 7.9 mL of ultrapure water, 1 mL of buffer solution (6 mol L-1 acetic acid―5 mol L-1 sodium hydroxide, pH = 5.49) and 1 mL of 1,10-phenanthroline (0.12 wt% in ultrapure water). Standard solutions of 0.1-0.5-1.0 g L-1 of ferrous chloride in ethanol were prepared as described for the sample. If required, the solution of leachate in ethanol was further diluted two to five times in ethanol. For solvent extraction, firstly, a screening was carried out among different undiluted phosphate solvents: Cyanex 923, D2EHPA and PC-88A. The components of the process were defined as: feed (F) = the pyridinium chloride leachate; raffinate (R) = the purified pyridinium chloride solution, the solvent (E) = the mixture extractant‒diluent (and eventually phase modifier) unloaded or loaded. The following working conditions were applied: feed-solvent ratio, F/E = 1/1 in volume; temperature, T = 165 °C; stirring time, tmix = 30 min; stirring speed, r = 150 rpm. Pyridinium chloride is solid below 144 °C, therefore the two phases could not be separated by centrifugation: the mixture was left to settle at 165 °C for 10 min and subsequently left to cool down. The upper phase, the loaded solvent E, was separated and analyzed. The percentage of extraction, %E, was calculated via Equation (1):
𝑛E
%E = 𝑛F ∙ 100
(1)
where nF and nE are the moles of metal in the feed and in the loaded solvent, respectively. The distribution ratio, D, which compares the affinity of a metal M in each phase, was calculated as in Equation (2):
𝑐E
D M = 𝑐R
(2)
7
where cE and cR are the moles of metal in the loaded solvent and in the raffinate, respectively. The separation factor, α is defined as in Equation (3):
𝐷M1
α = 𝐷M2
(3)
where M1 and M2 are two metals in the system and DM1>DM2. The solvents were treated in a microwave digester (Berghof Speedwave Xpert), prior to the elemental analysis. An aliquot of 7 mL of 65 wt% HNO3 was added to 50 μL of loaded E in DAK-100 vessels and the mixture was digested for 75 min in a temperature range of 50 to 175 °C and a pressure range from atmospheric up to 20 bars. When the samples were cooled at room temperature, they were diluted to 50.0 mL with ultrapure water and then 100 times with 2 vol.% HNO3. The samples were analyzed via a Perkin-Elmer Optima 8300 ICP-OES. Data were processed with the Syngistix software, version 1.0.1.1275. Saponified solvents were prepared too, by adding 10 vol.% of NaOH 0.1 mol L-1 to the extractant-diluent mixtures. The extraction rate was studied from 5 to 60 min, under the conditions (F/E, T, tmix, r) listed above. Extraction tests with Cyphos IL 101 were performed for 30 min at T = 165 °C with F/E = 1/1 or 1/2 and r = 150 or 300 rpm. Different Cyphos IL 101-based solvents were prepared: undiluted Cyphos IL 101; 30 or 80 vol% Cyphos IL 101 in p-cymene, a mixture of 70:30 (wt.:wt.) Cyphos IL 101―Cyanex 923. The Cyphos IL 101-based solvents were diluted in ethanol and measured via total reflection X-ray fluorescence spectroscopy (TXRF) with a Bruker S2 Picofox TXRF spectrometer, equipped with a molybdenum X-ray source. Samples were prepared according to protocols in the literature and holmium was selected as standard.[46,47] The data were analyzed via the Bruker Spectra PICOFOX v. 7.5.3.0 software.
8
A 0.2 mol L-1 solution of oxalic acid was used, in molar ratio = 1.5 for rare-earth elements precipitation stripping, whereas a 5 wt% ammonia solution was used for precipitation stripping of iron. The precipitation yields %P were calculated as in Equation (4):
(
𝑛ES
)
%P = 1 ― 𝑛FS ∙ 100
(4)
where nFS and nES are the moles of metal in the stripping feed and in the stripped solvent E, respectively.
9
3. RESULTS AND DISCUSSION 3.1 Leaching The uncoated, bulk Nd‒Fe‒B scrap was first crushed and milled. All the collected powder had a particle size below 45 μm, determined by sieve fractioning. The composition of the solid was already analyzed in a previous work, and it is reported here in Table 1 for completeness and clarity to the reader [45]. Table 1: Elemental composition of the Nd‒Fe‒B permanent magnet scrap (in wt.%) determined by ICP-OES Element
Composition* (wt.%)
Fe
62.52
Nd
24.15
Dy
6.89
Pr
1.45
Co
3.46
Cu
0.04
Ga
0.32
Al
0.11
B
1.05
*compositions normalized to 100%
Leaching tests were carried out at 165 °C for 10, 30 and 60 min with L‒S ratios (g g-1) of 10, 50 and 100. Complete dissolution of the Nd‒Fe‒B powder was achieved even after the shortest leaching duration (10 min) with the lowest L-S ratio (10). The composition of the leachate, in major elements, was 84.7 g L-1 of iron, 31.7 g L-1 of neodymium, 11.5 g L-1 of dysprosium. The leaching occurs according to the reaction in Equation (5):
10
2 M(s) + 2n [C5H5NH]Cl(l) → 2 MCln,(solvo) + 2n [C5H5N](l) + n H2,(g)
(5)
where M is any metal in the Nd‒Fe‒B powder, solvo is the solvated form in non-aqueous solutions and the stoichiometric factor n = 2 or n = 3. The cation [C5H5NH]+ delivers the proton for the oxidation of the metal from M0 to M2+ or to M3+. Hydrogen gas is released during the reaction, whereas chlorides are consumed to form the metal chloride salts. Therefore, HCl needs to be added to the system to regenerate the lixiviant PyHCl, as already reported in the literature [15]. In case of roasted magnets, water will be produced instead of hydrogen gas, and it could be condensed back to the system. The dissolution of the same powder in 37 wt% HCl took about 1 hour at 70 °C and with a L‒S ratio of 20. This shows the process intensification offered by PyHCl as lixiviant, since it allows to work above 100 °C at atmospheric pressure, which is not possible in hydrometallurgical systems. Mitchner specified that the high thermal stability of PyHCl is due to the strong bond between the nitrogen atom on the ring and the proton of the acid [13]. The PyHCl should be stable at the working temperature of 165 °C, since its liquidus range is between 144 °C and 218 °C. Nevertheless, the stability of PyHCl at high temperature and in presence of metals, with potential catalytic effect on de-hydrogenation and hydrogenation reactions, was tested via FTIR–ATR. Spectra of the leachate after the mildest (T = 165 °C, t = 10 min, L‒S = 100) and the harshest conditions (T = 165 °C, t = 60 min, L‒S = 10) are compared in Figure 1 with the FTIR spectrum of PyHCl just heated at 165 °C for 60 min.
11
Figure 1: FTIR-AR spectra of (i - green) the leachate after the test at T = 165 °C, t = 10 min, L‒S = 100; (ii - red) the leachate after the test at T = 165 °C, t = 60 min, L‒S = 10; (iii blue) PyHCl heated at 165 °C for 60 min
3.2 Rare-earths non-aqueous solvent extraction and separation Dissolution of Nd-Fe-B powder in PyHCl is very fast and consumes relatively small amounts of lixiviant per gram of treated solid, but the process is not selective. Therefore, a nonaqueous solvent extraction is required to selectively separate the metals. Extraction efficiency in conventional solvent extraction largely depends on the speciation of the metal ions in both organic and aqueous phases and and in conventional aqueous feeds the metals are present as hydrated complexes. In this work, the feed PyHCl is anhydrous and water in the first coordination sphere is replaced by chloride ions [48]. However, not any anion of the ionic liquids can enter the first coordination sphere of a metal cation. For instance, also at low water content ( ≤10 wt%), the weakly coordinating [Tf2N]- is bridged to the metal cation via water molecules [49]. In fact, Nd‒Fe‒B powder could not be leached in anhydrous [Hbet][Tf2N], because the anion [Tf2N]-, differently from the chloride, cannot replace water in the first 12
coordination sphere.[45] The speciation equilibrium of lanthanides in PyHCl is described by Equation (6):
LnCl3 + 3 [C5H5NH]Cl ⇌ [C5H5NH]3LnCl6
(6)
Such [𝑐𝑎𝑡]3LnCl6 species, where cat is a general representation for the cation of the ionic liquid, have been reported as moieties of lanthanides in protic ionic liquids for example for [bmim]Cl [50]. First studies of non-aqueous solvent extraction were carried out with a screening of undiluted D2EHPA, PC-88A and Cyanex 923 at 165 °C (Figure 2). A general good selectivity in rareearth elements extraction over iron is achieved with all the studied extractants.
Figure 2: Extraction yields, %E, of iron, neodymium and dysprosium in Cyanex 923 (), PC88A () and D2EHPA() from PyHCl leachate. Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Extraction conditions: T = 165 °C; t = 30 min; r = 150 rpm; F/E = 1/1 PC-88A and D2EHPA can both selectively extract the trivalent rare-earth ions over iron whereas Cyanex 923 extracts also about 21% of iron together with about 62% of 13
neodymium(III) and 82% of dysprosium(III). PC-88A, in particular, offers the highest selectivity since it gives a separation factor α, (Equation (3)), (rare-earth elements)/iron of 241.1, against the α of 63.1 for D2EHPA and 21.2 for Cyanex 923. Furthermore, D2EHPA phase forms a gel after the extraction, probably due to the high metal loading and the consequent polymerisation of the metal-solvent complexes [51]. Consequently, extraction of rare.earth elements from the PyHCl at high temperature (>100 °C) was optimized using PC88A. In aqueous solvent extraction, organophosphorous extractants as D2EHPA have been reported to have high affinity for iron(III), but much less for iron(II): the high α values from Figure 2 might be explained by the presence of iron as iron(II) instead of iron(III) [52]. Also considering that Nd‒Fe‒B powder was not roasted before leaching, iron is expected to be present as iron(II) forming FeCl2 complexes,. Speciation of iron in the leachate was studied via UV-VIS absorption spectroscopy and 1,10-phenanthroline (0.12 wt% in ultrapure water) was used as complexing agent [53]. The resulting spectra is reported in Figure 3, where the characteristic absorbance peak of the Fe(II)―1,10-phenanthroline complex at 510 nm is present [53].
Figure 3: UV-VIS absorption spectrum of the leachate-1,10-phenanthroline solution
14
In concentrated aqueous chloride solution, the peak at 363 nm has been reported to correspond to FeCl4‒, i.e. iron(III) complex, while the small peak at 510 nm correspond to the complex FeCl42‒, i.e. iron(II) complex [54–57]. Therefore, in the PyHCl leachate, iron is present both as iron(II) and as iron(III). The selectivity of the organophosphorous extractants cannot be ascribed to the speciation of iron, but might rather be due to the positive effect of the high temperature (≥100 °C). on the speciation and distribution of the metals between the two phases. The next step of the process was the separation of neodymium(III) and dysprosium(III). Although their concentrations in the leachate are different (31.7 g L-1 and 11.5 g L-1, respectively), neodymium and dysprosium are both significantly extracted by undiluted PC88A. The extraction of neodymium(III) from chloride solutions is strongly affected by D2EHPA concentration [51]. In this work, the effect of PC-88A concentration on neodymium(III) extraction from PyHCl leachate was investigated. The choice of the diluent was crucial since most of the commonly used diluents, such as toluene, boil below the working temperature (165 °C). Therefore, PC-88A was diluted to 30 vol.% in p-cymene which boils above 165 °C and also mitigates possible degradation of the pyridinium ring by the acidic extractant during the mixing of the feed and the solvent phases. On the other hand, the purpose of diluents in extraction process is to control the concentration of the extractants or to lower the viscosity of the extractants, which even increases after solute loading. However, decreasing the viscosity by diluents might not be necessary in solvent extractions above 100 °C since increasing the temperature has the same effect on the viscosity as the diluent. In this specific case, the diluent was used to regulate the concentration of PC-88A and, thus, to enhance the selectivity and not the mass transfer of the extraction process. At the same time, saponification of PC-88A with NaOH has been proven to increase the extraction yields of rare earths from acidic media, since it counteracts the protons released by the PC-
15
88A [58–61]. Therefore, a solution of 30 vol% PC-88A in p-cymene with 10 vol.% NaOH 0.1 mol L-1 was prepared, but it was unstable probably because of the chosen diluent pcymene (Figure 4 (a)). Noticeably, an equivalent solution of saponified D2EHPA was stable, but it again resulted in gel formation at the end of the solvent extraction test and it was discarded (Figure 4 (b)). In Table 2, %E for iron(II), neodymium(III) and dysprosium(III) with 30 vol.% PC-88A in p-cymene and with 100 vol.% PC-88A from PyHCl leachate are displayed.
(a)
(b)
Figure 4: (a) Unstable solution of 30 vol% PC-88A in p-cymene with 10 vol.% NaOH 0.1 mol L-1; (b) jelled 30 vol% D2EHPA in p-cymene with 10 vol.% NaOH 0.1 mol L-1 after solvent extraction of rare earths from PyHCl leachate
Table 2: %E for iron(II), neodymium(III) and dysprosium(III) with 30 vol.% PC-88A in pcymene and with 100 vol.% PC-88A from PyHCl feed %E
D
[PC-88A]
30 vol.%
100 vol.%
30 vol.%
100 vol.%
Fe(II,III)
1
2
0.0
0.0
Nd(III)
22
51
0.3
2.1
Dy(III)
100
71
171
4.9
Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Solvent extraction conditions: T = 165 °C, F/E = 1/1, t = 30 min, r = 150 rpm
16
By reducing the PC-88A concentration, the extraction of dysprosium(III) is favored over that of neodymium(III) in two simultaneous ways: the %E of dysprosium(III) is increased and, especially, the %E of neodymium(III) is decreased. The latter might be due to the reduced concentration of PC-88A, i.e. there are no enough extractant molecules for the neodymium(III) extraction. The fact that, at the same time, more dysprosium(III) is extracted might suggest that the two rare-earth ions compete for the transfer to PC-88A. Afterwards, the extraction rate of metals in 30 vol.% PC-88A in p-cymene was studied as function of the mixing time (Figure 5). The large error bars (≤ 9.1%) are to attribute to the experimental errors associated with the microwave digestion of the samples prior to ICP-OES analysis.
Figure 5: Extraction rate of iron(II,III) (), neodymium(III) () and dysprosium(III) () in 30 vol.% PC-88A in p-cymene. Metal concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1, Dy = 11.5 g L-1. Extraction conditions: T = 165 °C; r = 150 rpm; F/E = 1/1 Extraction of neodymium(III) slightly increases with the time meaning that the solvent is not saturated and the slow extraction rate might be correlated to the concentration of the protons released by the PC-88A. In fact, acidic extractants such as PC-88A operate by exchanging
17
their protons with the metal cations from the other phase. In aqueous solvent extraction (Equation (7), where (HA)2 is the dimer of the acidic extractant), the longer the extraction proceeds, the higher the protons and the lower has been reported the %E [61–63].
Ln3+(aq) + 3 (HA)2 ⇌ Ln(HA2)3,(𝑎𝑞) + 3 H+(aq)
(7)
Non-aqueous solvent extraction of lanthanides in acidic organophosphorous extractants (Cyanex 272, PC-88A, D2EHPA) from chloride media has not been reported yet. The possibility of the different solvation, with chlorides substituting water in the first coordination sphere, affecting the extraction mechanism cannot be excluded here. Although more detailed speciation studies are needed, a first hypothesis is that the protons from the PC-88A participate in the reaction (Equation (8)) by taking the chlorides from the LnCl63‒ complex before such complex is extracted.
LnCl63‒ + 3 (HA)2 + 3 H+ ⇌ (LnHA2)3 + 6 HCl
(8)
The mechanism in Equation (8) does not occur in conventional aqueous solvent extraction where the lanthanides occur as positively charged hydrated complexes (Equation (7)). Dysprosium(III) was quantitatively extracted within 10 min also in diluted PC-88A, since its concentration in the PyHCl feed is about three times lower than that of neodymium(III). In the next extraction stage, neodymium(III) was extracted with undiluted PC-88A. The cumulative %ETOT after one stage of 10 min with 30 vol.% PC-88A in p-cymene and after one stage of 30 min with 100 vol.% PC-88A are reported in Table 3. The large error on the %ETOT of the iron(II) is due to the low %ETOT, i.e. the low concentrations read at the ICP-OES.
18
Table 3: Cumulative %ETOT for iron(II), neodymium(III) and dysprosium(III) after stage I with 30 vol.% PC-88A in p-cymene and stage II with 100 vol.% PC-88A from PyHCl feed.a [PC-88A]
%ETOT
%RSD
Fe(II,III)
3
14
Nd(III)
74
3
Dy(III)
102
2
aMetal
concentrations in the feed: Fe = 84.7 g L-1, Nd = 31.7 g L-1,Dy = 11.5 g L-1. Solvent extraction conditions: stage(I) T = 165 °C, F/E = 1/1, t = 10 min, r = 150 rpm; stage(II) T = 165 °C, F/E = 1/1, t = 30 min, r = 150 rpm
The process has not been optimized yet, but neodymium(III) can be totally extracted with a higher F/E ratio in stage II or adding another stage of 100 vol.% PC-88A. Recovery of rareearth elements from the loaded solvents was investigated by precipitation stripping with a 0.2 mol L-1 oxalic acid solution added in a stoichiometric amount. The results are shown in Table 4. The concentration of neodymium(III) and dysprosium(III) in extract I were comparable due to the different concentrations in the PyHCl feed and to the %E(Nd(III)) of about 20%. Nevertheless, neodymium(III) precipitated after 10 min shaking at room temperature, while dysprosium(III) did not precipitate: probably higher temperature, longer time and higher molar ratios (oxalic acid)/(dysprosium(III)) are necessary.
19
Table 4: Precipitation yields (%P, %) and purity (x, wt%) of neodymium(III) and dysprosium(III) from the extract I (30 vol.% PC-88A) and from the extract II (100 vol.% PC88A) %PNd
xNd (wt%)
%PDy
xDy (wt%)
extract I
78
100
0.0
‒
extract II
50
97
‒
‒
Metal concentrations: in the extract (I) Nd = 7.7 g L-1, Dy = 11.2 g L-1; in the extract (II) Nd = 15.5 g L-1, Dy = 0.3 g L-1
3.3 Iron non-aqueous solvent extraction and conceptual flowsheet Finally, to regenerate the lixiviant, iron(II,III) had to be extracted from the PyHCl leachate. The ionic liquid Cyphos IL 101 was tested undiluted and diluted to 30 vol.% in p-cymene (Table 5). The PyHCl feed still contained about 8.2 g L-1 of neodymium(III), but this was not extracted and, thus, did not interfere with the iron(II,III) extraction. The %E increases with the concentration of Cyphos IL 101, as expected, but even with undiluted Cyphos IL 101, the %E of iron(II,III) from the PyHCl feed is <50%. In fact, it has been calculated that a concentration of 3.0 mol L-1 Cyphos IL 101 would be necessary to extract all the iron(II,III) from the feed, whereas the concentration of undiluted Cyphos IL 101 is 1.7 mol L-1.
20
Table 5: %E for iron(II,III) with 30 vol.% Cyphos IL 101 in p-cymene and with 100 vol.% Cyphos IL 101 from PyHCl leachate refined from the rare-earth elements Cyphos IL 101
%E
%RSD
D
%RSD
30 vol.%
19
3
0.2
3.0
100 vol.%
32
6
0.4
9.4
Metal concentration in the feed: Fe = 84.7 g L-1, Nd = 8.2 g L-1, Dy = 0.0 g L-1. Solvent extraction conditions: T = 165 °C, F/E = 1/1, t = 30 min, r = 300 rpm
The extraction occurs according to Equations (9) and (10), although extraction by solvation (Equation (11)) cannot be excluded. The Equations are reported for iron(II) but they can be adapted for iron(III) just by adjusting the stoichiometry, P66614 is the phosphonium cation of the Cyphos IL 101 ionic liquid:
FeCl2 + 2 Cl‒ ⇌ FeCl42‒
(9)
FeCl42‒ + 2 [P66614][Cl] ⇌ [P66614]2[FeCl4] + 2 Cl‒
(10)
FeCl2 + 2 [P66614][Cl] ⇌ [P66614]2[FeCl4]
(11)
It was calculated that, with 30 vol.% Cyphos IL 101 in p-cymene, more moles of iron(II,III) were extracted than those stoichiometrically possible with the chlorides provided by the extractant: at diluted Cyphos IL 101 concentrations, chlorides from the leachate feed are coextracted with iron(II,III). Contrarily, in case of undiluted Cyphos IL 101 with 1.7 mol L-1 of chlorides, only about 50% of the maximum iron(II,III) loading is achieved. This might be due to the viscosity of the solvent which significantly increases with the loading, until the solvent 21
appears as solid at room temperature. Therefore, in the next experiment Cyphos IL 101 was diluted 80 vol.% in p-cymene to lower the viscosity, while still keeping a moderate chloride concentration (1.4 mol L-1), and the F/E ratio was raised to 1/2 to increase the %E (Table 6). Nevertheless, still a %E of around 60% was obtained. A synergistic mixture Cyphos IL 101 with Cyanex 923 70:30 (wt.:wt.) was tested since the results in Figure 2 showed that Cyanex 923 also extracts iron(II) from the PyHCl feed and because of similar experimental evidence reported in the literature [64]. The results are displayed in Table 6 together with those obtained with the 80 vol.% Cyphos 101 and with 100 vol.% Cyanex 923 (at F/E = 1/1) to highlight the synergistic effect of the mixture. Differently from the diluent p-cymene, Cyanex 923 also participates to the extraction mechanism (following a solvation mechanism as that in Equation (11)) while lowering the viscosity of the solvent.
Table 6: %E for iron(II) with 80 vol.% Cyphos IL 101 in p-cymene, with 100 vol.% Cyanex 923 and with a synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.). from PyHCl leachate refined from the rare earths Solvent
F/E
%E
%RSD
D
%RSD
80 vol.% Cyphos IL 101 in p-cymenea
1/2
62
6
0.8
14.5
100 vol.% Cyanex 923b
1/1
21
1
0.6
11.5
70:30 (wt.:wt.) Cyphos IL 101:Cyanex 923a
1/2
70
4
1.2
12.5
a
Metal concentration in the feed: (a) Fe = 84.7 g L-1, Nd = 8.2 g L-1, Dy = 0.0 g L-1. Solvent extraction conditions: T = 165 °C, t = 30 min, r = 300 rpm b refer to caption of Figure 2 for experimental details
It is worth to mention that the leachate is highly loaded with iron(II) (84.7 g L-1) due to the excellent properties of PyHCl as lixiviant (L‒S ratio = 10) and this affects the mass transfer of
22
the metal to the solvent. However, the synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt:wt) shows good extracting properties and can be further optimized, especially in terms of mixing time and mutual ratio of the extractants. Iron(II,III) was recovered from the synergistic solvent by stripping precipitation with a 5 vol% NH3 solution. Once purified from the metals, the lixiviant PyHCl can be recycled at the head of the process. Addition of HCl might be necessary, but it has to be highlighted in the perspective of environmental impact of the process that most HCl has not been emitted as it is, but rather it has been lost as hydrogen gas in the leaching step (Equation (5)) and as chlorides in the extraction of iron(II,III). A conceptual flowsheet is suggested in Figure 6, where the blue lines represent the hydrometallurgical operations and the green lines the solvometallurgical operations. Most of the operations, dissolution of the Nd‒Fe‒B powder and solvent extraction of the metals were successfully carried out in non-aqueous conditions and the use of aqueous solutions is confined only to the recovery of metals via precipitation stripping. Precipitation of the rareearth elements from the PC-88A solutions via direct addition of oxalic acid was tested but was not successful: this suggests that first stripping to water occurs, followed by complexation and precipitation of the rare-earth oxalates.
23
Figure 6: Conceptual flowsheet for the solvometallurgical recycling of Nd‒Fe‒B scrap. The main operations are: leaching of non-roasted Nd‒Fe‒B scrap at 165 °C in pyridinium chloride; non-aqueous solvent extraction at 165 °C of (a) dysprosium(III) with 30 vol.% PC88A in p-cymene, (b) neodymium(III) with 100 vol.% PC-88A and (c) of iron(II) with a synergistic mixture Cyphos IL 101―Cyanex 923 70:30 (wt.:wt.).
24
4. CONCLUSIONS The protic ionic liquid pyridinium chloride has been used for the leaching of production scrap of Nd‒Fe‒B permanent magnets for the recovery of rare-earth elements. The Nd‒Fe‒B scrap powder completely dissolved at high temperatures (165 °C) after just 10 min with a liquid-tosolid ratio of 10 g g-1. Although iron was present as both iron(II) and iron(III), a selective extraction of neodymium(III) and dysprosium(III) with organophosphorous extractants was still possible, due to the effect of the high temperature (>100 °C) on the separation factors in the consequent solvent extraction step. A screening of undiluted Cyanex 923, PC-88A and D2EHPA extractants was performed at 165 °C and PC-88A was the best extractant with the highest separation factor (rare-earth elements(III))/iron(II,III): 241 against 63 in D2EHPA and 21 in Cyanex 923. At high temperatures (≥100 °C), the use of a diluent to lower the extractant viscosity is not necessary. However, dysprosium(III) and neodymium(III) could be separated by diluting the PC-88A 30 vol.% in p-cymene. In fact, only 20% of neodymium(III) was extracted at low PC-88A concentration, while %E of dysprosium(III) increased to 100%. Afterwards, the remaining neodymium(III) was extracted in one stage with undiluted PC-88A. The rare-earth elements were recovered from the loaded solvents by precipitation stripping with an aqueous solution of 0.2 mol L-1 oxalic acid. Finally, the regeneration of PyHCl required two steps: (i) extraction of iron(II,III) and (ii) addition of HCl into the remaining pyridine. About 70% of iron(II) was extracted from the PyHCl by a synergistic mixture Cyphos IL 101―Cyanex 923, in a ratio 70:30 (wt:wt.) and it was then precipitated with a 5 vol% NH3 solution. Both the extractants gave lower %E when used alone or diluted, for instance, in p-cymene. This process is relevant for the recycling of Nd‒Fe‒B permanent magnets: the use of PyHCl significantly intensifies the leaching step, whereas dysprosium(III) and neodymium(III) are extracted at different concentrations of PC-88A.
25
Appendix A. Supplementary material Supplementary data to this article can be found online at: (link to be added).
Declaration of interest There are no conflicts to declare. Acknowledgements The research leading to these results received funding from the European Commission’s Horizon 2020 Programme ([H2020/2014-2019]) under Grant Agreement no. 674973 (MSCAETN DEMETER) and under Grant Agreement 694078—Solvometallurgy for critical metals (SOLCRIMET). This publication reflects only the authors’ view, exempting the Commission from any liability. The authors also want to thank Magneti Ljubljana for providing the magnets and the production scrap samples for the carried out research, Tony Debecker and Kevin Wierinckx for crushing the magnets. Thanks to Dr. Bieke Onghena for reviewing the manuscript.
26
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Graphical abstract
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Highlights • Solvometallurgical process for the recovery of rare-earth elements from Nd‒Fe‒B magnets
Nd‒Fe‒B magnets completely dissolved in pyridinium chloride (PyHCl) within 10 min
Preliminary roasting of the Nd‒Fe‒B magnets is not required, just milling
Leaching in pure PyHCl at 165 °C avoided the consumption of other solvents
Neodymium and dysprosium were extracted by different concentrations of PC-88A
High-temperature solvent extraction clearly affects the distribution ratio of iron
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