NdFeB magnet recycling: Dysprosium recovery by non-dispersive solvent extraction employing hollow fibre membrane contactor

NdFeB magnet recycling: Dysprosium recovery by non-dispersive solvent extraction employing hollow fibre membrane contactor

Separation and Purification Technology 194 (2018) 265–271 Contents lists available at ScienceDirect Separation and Purification Technology journal ho...

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Separation and Purification Technology 194 (2018) 265–271

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

NdFeB magnet recycling: Dysprosium recovery by non-dispersive solvent extraction employing hollow fibre membrane contactor

T



Kartikey K. Yadav, Mallavarapu Anitha, D.K. Singh , V. Kain Rare Earths Development Section, Material’s Processing & Corrosion Engineering Division, Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Recycling NdFeB magnets Hollow fibre membrane Dysprosium

Hollow Fibre Membrane (HFM) operation in non-dispersive solvent extraction (NDSX) mode has been successfully employed for the separation of dysprosium from NdFeB magnetic scrap material using EHEHPA as an extractant. Effect of various hydrodynamic parameters including aqueous phase acidity, extractant concentration, Dy concentration in feed, phase ratio and flow rate were investigated to optimize the condition for quantitative recovery of dysprosium from the leach liquor obtained by the dissolution of hard disk drive (HDD) in nitric acid media. 0.5 M EHEHPA, 0.3 M HNO3 as aqueous feed phase, 100 ml/min flow rate and phase ratio of 1:1 were found to be optimum for both the cycle test runs. Taking the advantage of fast extraction kinetics of dysprosium, two cycle HFM-NDSX approach was adopted to concentrate Dy from 20 to 83% in first cycle while raising its purity > 97% in the second cycle. The overall process also yielded neodymium rich by-product for its further purification.

1. Introduction Rare earth elements are critical raw material for wide range of modern age products ranging from catalysts, phosphors, magnet, lasers, fibre optics and several more [1]. There is a great research interest growing in rare-earth (RE) recycling due to increase in demand for REs and associated complexity of REs separation from primary ores [2]. Neodymium Iron Boron magnet (NdFeB) is one of the major consumer for rare earth elements which include Nd, Dy and Pr. NdFeB magnets are best available magnets due to their superior energy product (with a theoretical maximum of 512 kJ/m3) and fulfil almost 70% of permanent magnetic material demand worldwide. NdFeB permanent magnet contains about ∼32 wt% REEs (mainly 21–31 wt% (Nd/Pr), 0–10 wt% Dy and small amounts of Gd and Tb) [3]. Dysprosium added to NdFeB magnets plays a crucial role in preserving the performance of the magnet at elevated temperature which is essential for high-temperature applications [4]. Apart from NdFeB magnets, dysprosium also finds an important place in the manufacturing of laser materials, nuclear reactors, hybrid cars and colour television tubes [5]. Du and Graedel [6] have recently published very interesting fact about the global stock in use for four REs in NdFeB permanent magnets, i.e., neodymium (Nd), praseodymium (Pr), terbium (Tb), and dysprosium (Dy), which amount to a total of 97.0 kilotons (kt): 62.6 kt Nd, 15.7 kt Pr, 15.7 kt Dy, and 3.1 kt Tb. This vast resource of RE elements could augment the existing supply as they are quantitatively almost



four times the 2007 annual production of the individual elements, if recycled efficiently. RE recycling not only supplements existing demand but can also reduce the geopolitical impact on economic aspects due to supply risk. Less than 1% of the REEs were recycled by 2011, and developmental researches for new large-scale processes are scanty [7]. The scarcity of data on the current quantity of REE materials from magnets in the waste streams and the fate of the magnets after shredding has hampered the possible development of REE recycling processes [8]. There are various metallurgical processes practised to separate and recover the REEs in the NdFeB magnet scrap such as liquid metal extraction [9], hydrometallurgical processing [10,11], and pyrometallurgical slag extraction [4,12]. Hydrometallurgical, an important route, involves leaching of magnetic scraps with acids at room temperature, leading to dissolution of all the components in NdFeB magnets. Subsequently rare earth can be precipitated as double sulphate salt which could be easily converted to corresponding fluoride or oxide [13]. The resultant oxide consists majority of rare earth elements mostly Nd, Pr and Dy present in magnetic scrap. However subsequent purification of dysprosium can enhance the value as well as importance of the recycling process due to its invariably high cost and demand. There are various conventional methods available for purification of individual rare earth elements such as solvent extraction, ion exchange and fractional precipitation [14]. Solvent extraction is the industrially accepted method for purification of rare earth elements. In solvent extraction process, organophosphorous acids based organic extractants

Corresponding author. E-mail address: [email protected] (D.K. Singh).

https://doi.org/10.1016/j.seppur.2017.11.025 Received 1 September 2017; Received in revised form 23 October 2017; Accepted 10 November 2017 Available online 13 November 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.

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are employed for separating REE, due to their solvation properties as well as their chemical stability and low aqueous solubilities [15]. Acidic organophosphorous extractants, such as 2-ethylhexyl 2-ethyhexylphosphonic acid (EHEHPA also known as PC88A), di-2-ethyl hexyl phosphoric acid (D2EHPA) are widely used for the extraction and separation of individual rare earths including dysprosium [16]. However, Dy recovery from NdFeB magnet scrap is difficult with solvent extraction process owing to requirement of multiple stages and low Dy content in feed source [17]. The present scenario consisting shortfall in supply of critical rare earths elements has attracted a lot of research interest in the field of rare earth separation by alternative methods i.e. membrane based separation, solvent impregnated resins and ionic liquids from waste and secondary sources [18–21]. Membrane assisted solvent extraction employing hollow fibre membrane modules can provide suitable alternative to recover and purify dysprosium from mixed rare earths feed [22]. Membrane based techniques eliminates the disadvantages of conventional solvent extraction process such as lean source, environmental hazard associated with loss of organic extractant, loss of diluents in aqueous streams and third phase formation [23]. In the case of hollow fibre membrane operation mode, individual separation of rare earth elements is enhanced in non-equilibrium conditions due to the continuous high driving forces over extended periods of time compared to equilibrium limited conventional solvent extraction [24]. Hollow fibre membrane modules provide high surface area contact per unit volume resulting in high RE extraction rate and can be employed in two modes namely supported liquid membrane and non-dispersive solvent extraction (NDSX) mode. In NDSX mode an organic extractant phase passing through one side of the membrane wets the microporous hydrophobic membrane. A nonwetting aqueous phase is passed through other side of the membrane at a pressure higher than that of the organic phase, but lower than that needed for the aqueous phase to displace the organic phase in the pores of the membrane. The aqueous–organic (aqueous–membrane) interface is essentially immobilized at the pore mouth of the hydrophobic membrane support through which the solute mass transfer takes place. Here, the membrane support does not function as a size selective sieve, but it merely prevents the dispersion of one phase into the other. Among them only few have explored hollow fibre membrane module for rare earth separation [25]. However non dispersive solvent extraction mode which has promising potential for scale up among membrane based separation methods has not yet been explored extensively. Therefore it is desirable to develop an efficient separation method to process the leach solution obtained from NdFeB magnet scrap material to get high purity rare earth oxides. Accordingly the objective of the present study is to explore the HFM-NDSX under wide range of experimental variables to address the complexity involved in Dy separation from NdFeB magnet scrap by solvent extraction process by employing hollow fibre membrane module in non dispersive mode to recover the same.

Table 1 Characteristic properties of petrofin. Characteristics

Specifications

Density (at 15 °C)

0.75 ± 0.01

Purity

99%

Carbon distribution C11 C12 C13 C14

20–30% 25–35% 20–30% 20–25%

Flash point

70 °C

purity of 95% has been received from Heavy Water Board and used as it is. Experiments performed during the recovery of rare earth by employing hollow fibre membrane required organic extractant to be used as carrier phase. The extractants were diluted to optimize hydrodynamic conditions of the process. Petroffin (heavy normal paraffin) having specification shown in Table 1 has been used as diluent. Rare earth (such as Dy (III), Nd(III), Pr(III) etc.) solutions were prepared by dissolving their oxides (> 99% purity, received from Indian Rare Earth Limited, India) in concentrated nitric acid and individual working solution were prepared by the appropriate dilution of the stock solutions. NdFeB magnetic scraps shown in Fig. 2 were recovered from end of life Hard Disk Drive obtained from Computer Division, Bhabha Atomic Research Centre, Mumbai. The magnets were first demagnetized by heating well above the Curie temperature (30 min at 350 °C) and then fed into a jaw crusher resulting in magnetically-neutral particles ready for chemical processing. Subsequently the same was crushed in mortar and pestle to powder form for efficient leaching of metal into corresponding acids. 2.2. Non-dispersive solvent extraction studies HFM contactor (Liqui Cell 2.5 × 8) used in the present work was procured from Polypore, USA and it consists of hydrophobic microporous polypropylene hollow fibres enclosed in a polypropylene shell of 2.5″ × 8″ dimension as shown in Fig. 3. The module contained about 10,000 fibres of 40% porosity and had an effective surface area of 1.4 m2 with other specifications, listed in Table 2. The aqueous and organic phases (agitated continuously in the reservoir for uniform concentration) were contacted in counter-current mode in the hollow fibre module pores. The flow rates of aqueous and organic phases were kept at 100 ml/min with the help of gear pumps (cole-palmer) through the lumen side and shell side of the HFM module, respectively. The polypropylene membrane is hydrophobic in nature so the organic phase wets and even passes through the membrane fibres. To prevent the dispersion of the organic phase into the aqueous feed phase a small trans-membrane pressure in the lumen side is applied.

2. Experimental 2.3. Sample analysis 2.1. Materials The evaluation of rare earth elements extraction/stripping was studied by monitoring its concentration in aqueous phase by using Inductively Coupled Plasma-Atomic Emission Spectrophotometer, ICPAES (JY-Ultima 2). The experiments were performed thrice and the mean values are reported. The errors in non dispersive solvent extraction experiments and metal ion concentration analysis were less than 4%.

2-Ethylhexyl 2-ethylhexyphosphonic acid (EHEHPA) shown in Fig. 1 also known as EHEHPA having molecular weight of 306.4 and

3. Results and discussion 3.1. Batch extraction of Dy by EHEHPA Fig. 1. Structure of 2-Ethylhexyl 2-ethylhexyphosphonic acid.

Separation of heavy rare earth (Dy) by Hollow fibre membrane 266

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Fig. 2. NdFeB magnets recovered from Hard Disk Drives with casing and without casing.

while subscripts (aq) and (org) depict the aqueous and organic phase respectively. One mole dysprosium metal ion forms complex with 3 mol of dimer of EHEHPA and liberate three moles of hydrogen ion.

K ex =

[Dy (HA2)3](org ) [H+]3(aq) [Dy 3 +](aq) [H2 A2 ]3(org )

(2)

The distribution coefficient of the metal ion (D) is the ratio of Dy (III) concentration in the organic phase to that of in the aqueous phase as represented in Eq. (3).

D=

[Dy 3 +](aq)

(3)

In order to select the aqueous acidity for HFM–NDSX test, preliminary experiments on the effect of acidity on D value of dysprosium extraction were performed in single contact mode (batch) by equilibrating an aqueous solution containing fixed metal ion concentration with varying nitric acid concentration (0.1–0.75 M) with 0.5 M EHEHPA. Results are summarized in Table 3. It is evident with increase in aqueous phase acidity from 0.1 to 0.75 M HNO3 distribution ratio for Dy extraction decreases from 53 to 0.3. The decrease in D value with increase in acidity is due to protonation of EHEHPA given in Eq. (1). The distribution data indicated that the extraction of Dy(III) can only be performed at or below the acidity value of 0.3 M HNO3.

Fig. 3. Hollow fibre membrane module.

Table 2 HFM module specification. Parameter

Specification

Fibre material Number of fibres Fibre internal diameter (µm) Fibre outer diameter (µm) Fibre wall thickness (µm) Effective pore size (µm) Porosity (%) Tortuosity Effective fibre length (cm) Effective surface area (m2)

Polypropylene 9950 240 300 30 0.03 40 2.5 15 1.4

3.2. Non-dispersive solvent extraction 3.2.1. Effect of aqueous phase acidity on Dy extraction In the case of solvent extraction by organophosphorus based acidic extractants, aqueous phase acidity plays a key role in extraction as well as stripping of metal ion. The nitric acid concentration (0.3–0.5 M HNO3) was varied to investigate the behaviour on Dy extraction with 0.5 M EHEHPA using HFM in non dispersive solvent extraction mode under recirculation at organic to aqueous phase ratio of 1:1. Results illustrated in Fig. 4 indicate that an increase in acidity from 0.1 to 0.5 M HNO3 led to decrease in extraction of Dy(III) from 99.5 to 45.2%. The decrease in extraction (transport) of Dy3+ from aqueous side to organic

module and operating the system in non dispersive solvent extraction mode from end of life rare earth scrap material (which also consist Nd and Pr as major constituents in NdFeB magnets) requires understanding of two phase reaction mechanism for Dy extraction. EHEHPA an acidic extractant exists as a dimer in non polar diluents which extract dysprosium by cationic exchange mechanism from nitric acid media as per Eq. (1) [25,26].

Table 3 Distribution coefficient (D) for Dy(III) extraction by EHEHPA from different aqueous phase acidities: Organic phase: 0.5 M EHEHPA in petrofin, aqueous phase: 1089 mg/L Dy.

K ex

Dy(3aq+) + 3[H2 A2 ](org ) ⇐ ⇒ [Dy (HA2)3](org ) + 3H(+aq)

[Dy (HA2)3](org )

(1)

where H2A2 shows the dimer form of EHEHPA in non polar diluents, Kex is the equilibrium constant for the chemical reaction shown in Eq. (1) 267

[HNO3], M

D

0.1 0.3 0.5 0.75

53.48 3.91 1.15 0.35

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Fig. 5. Effect of organic phase flow rate on the percentage extraction of Dy(III) at fixed aqueous phase flow rate (100 ml/min); organic phase: 0.5 M EHEHPA; aqueous phase: 1000 mg/l of Dy(III) at 0.1 M HNO3; Phase ratio: 1:1.

Fig. 4. Effect of Aqueous phase acidity on the extraction of Dy(III) by 0.5 M EHEHPA; aqueous phase: 1000 mg/L; flow rate: 100 mL/min, Phase ratio: 1:1.

phase with increase in aqueous acidity (0.1–0.5 M HNO3) leads to increase in H+ ion concentration and equilibrium shown in chemical Eq. (1) shifts in backward direction. Increase in acidity also affects the equilibration time required from 30 to 60 min to achieve the quantitative transport of Dy(III) from aqueous phase to organic phase. The loaded organic phase of 0.5 M EHEHPA with Dy(III) resulted from this experiment was quantitatively stripped by equilibrating the organic phase with 3.5 M HNO3 in a separating funnel. The regenerated 0.5 M EHEHPA was reused for the repeat experiments and results obtained showed excellent reproducibility in terms of Dy(III) extraction and no degradation of solvent system/membrane. 3.2.2. Effect of flow rate In order to study the mechanism of mass transfer in NDSX mode of operation in HFM and also to evaluate the rate determining step of total mass transfer resistance, it is important to investigate the hydrodynamic characteristics of current system. The flow rates of chosen system (RE-HNO3-EHEHPA-Petrofin) in both phases (feed and extractant) play a pivotal role in the transport of metal ion from the feed side to the organic side. Flow rates of both organic and aqueous phases in non-dispersive solvent extraction carried out in hollow fibre membrane affects the contact time of organic and aqueous phase at the interface. In the case of Dy(III) extraction with 0.5 M EHEHPA, effect of organic phase flow rate was examined by varying them in the range of 50–200 mL/min, while keeping aqueous phase flow rate constant at 100 mL/min. Results shown in Fig. 5 indicate that though the extraction of Dy(III) in organic phase was quicker at lower flow rate in the initial stages of the experiment (< 10 min of run) it was independent of contact time beyond 30 min. The result also led to the conclusion that > 99.0% Dy(III) could be extracted in the organic phase in 35 min irrespective of flow rates. Similar kind of results were also observed when aqueous phase flow rate was varied between 50 and 200 mL/min keeping organic phase flow rate at 100 mL/min. The minimal effect of flow rates on Dy3+ extraction may be attributed to the availability of excessive extractant (EHEHPA) (1:1 organic to aqueous phase ratio) on organic side and low metal ion concentration in feed phase. Due to the insignificant effect of aqueous and organic phase flow rates on equilibrium extraction of Dy(III), all subsequent experiments were performed at 100 mL/min flow rate of both the phases.

Fig. 6. Effect of Dy(III) concentration on its extraction by 0.5 M EHEHPA organic phase and 0.1 M HNO3 aqueous phase at flow rates of 100 mL/min for both the phases.

mode with fixed concentration of 0.5 M EHEHPA and nitric acid 0.3 M. Fig. 6 summarizes the results with increase in Dy(III) concentration from 250 to 2000 mg/L increased the time required for the quantitative (99%) transport across the membrane. However, in all the cases > 98% extraction of Dy(III) from aqueous phase was observed in 60 min even with 2000 mg/L Dy (III) in the feed phase. The loaded organic phase of 0.5 M EHEHPA with Dy(III) resulted from this experiment was quantitatively stripped by equilibrating the organic phase with 3.5 M HNO3 in a separating funnel. The strippant was later analyzed for Dy(III) content by ICP-AES and subsequently stripping results were used to ascertain the mass balance of extraction by NDSX experiment. The observed behaviour is in accordance with conventional solvent extraction process where the percentage extraction of metal ion decreases with increase in metal ion concentration in aqueous phase. The decrease in extraction is explained in terms of the less anionic binding sites of free extractant in organic phase due to continuous forward reaction leading to enhanced H+ ions in the medium as shown in chemical Eq. (1).

3.2.3. Effect of Dy(III) concentration After investigating the effect of acidity and flow rates another important parameter the effect of Dy(III) concentration in aqueous solution was also monitored by carrying out the HFM test run in NDSX

3.2.4. Influence of extractant concentration on Dy(III) extraction Organic extractant plays a significant role on extraction of metal ion 268

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Fig. 7. Effect of EHEHPA concentration on Dy extraction from 0.1 M HNO3 aqueous phase at flow rates of 100 mL/min for both the phases.

Fig. 8. Extraction of Dy(III) at different organic to aqueous phase volume ratios; organic phase: 0.5 M EHEHPA; aqueous phase: 1.0 g/L Dy(III) at 0.3 M HNO3; flow rate: 100 mL/ min.

from aqueous solution under comparable extraction conditions. In the present investigation effect of EHEHPA concentration on percentage transport of Dy(III) was studied in the extractant concentration range from 0.2 M to 0.83 M. The results of percentage Dy(III) transport obtained from varying concentration of EHEHPA are shown in Fig. 7. The % transport of Dy(III) increased from 82 to 99 in 90 min of equilibration with increase in concentration of the organic phase from 0.2 M to 0.45 M, however, when EHEHPA concentration increased from 0.45 M to 0.83 M, the increase in % extraction of Dy(III) was not obvious. Within this concentration of EHEHPA range from 0.2 M to 0.45 M, the availability of EHEHPA at the feed–membrane–organic interfaces increased with the increasing of concentration of carrier. Beyond 0.5 M EHEHPA the increase in metal ion extraction is independent of extractant concentration due to the excess of extractant present at the interface [26]. When the EHEHPA concentrations were 0.2, 0.45, 0.6, 0.75 and 0.83 M, the percentage extraction values were 82.49, 99.60, 99.65, 99.81 and 99.80, respectively. The optimum concentration of EHEHPA to perform further experiments was chosen to be 0.5 M EHEHPA in petrofin.

was prepared by demagnetizing the NdFeB magnet followed by crushing and grinding in mortar and pestle and subsequent selective leaching, precipitation and re-dissolution of rare earths described elsewhere [13]. The feed solution prepared from magnetic scrap composed of RE such as Dy, Nd and Pr in varying concentration depending upon the type of source. Experiments were performed to evaluate the separation of Dy(III) from other rare earths such as Nd and Pr, found in magnetic scrap. The maximum amount of Dy found in rare earth magnet was found to be approximately 20% of total rare earths and the composition of resultant aqueous phase 200, 438, 385 mg/L of Dy(III), Nd(III) and Pr (III) respectively in HNO3 media. To maximize the recovery of Dy, aqueous feed having acidity of 0.3 M and 0.1 M respectively was processed in HFM module employing 0.5 M EHEHPA of organic extractant. Figs. 9 and 10 show that in both the feed acidities (0.1 and 0.3 M) initially % extraction of Dy(III) in organic phase increased and rose up to 70% and 95% respectively in 10 min of contact time, however competing ions Nd and Pr were extracted < 3 and < 4% in 0.3 M aqueous acidity and < 13 and < 11% in 0.1 M HNO3 media respectively in same time. Experimental results depicted in Figs. 9 and 10 clearly indicate the possibility of separation as well as purifying Dy

3.2.5. Effect of phase ratio In order to determine the loading of Dy(III) in the organic phase of EHEHPA, effect of organic to aqueous phase ratio O/A was studied in the range of 0.25–1.0. Experimental results depicted in Fig. 8 suggest that the time required to achieve quantitative recovery of Dy(III) from aqueous to organic phase increased with decrease in organic to aqueous phase volume ratio. At an organic to aqueous phase ratio of 1, about 25 min of contact time was required for the quantitative extraction of Dy (III) from a feed phase containing 1.0 g/L Dy(III) in 0.3 M HNO3. However under the comparable experimental conditions, time required for quantitative extraction (99%) of dysprosium increases from 80 to 130 min, when phase ratio decreased from 0.5 to 0.25. It is also observed that ∼96% extraction of Dy(III) could be achieved in the case O/ A of 0.25 which is relatively less than the recovery obtained in phase ratios of 0.5 or 1. 3.2.6. Dysprosium recovery from NdFeB magnetic scraps After optimization of experimental variables for quantitative recovery of Dy from aqueous nitric acid media using EHEHPA in HFMNDSX mode of operation, actual rare earth magnetic scrap (NdFeB) obtained from end of life computer hard disk drives were evaluated to recover valuable rare earth i.e. Dysprosium, Neodymium and Praseodymium by employing the developed NDSX process in hollow fibre membrane module. The feed solution for the recovery experiments

Fig. 9. Extraction of Dy(III) in presence of competing rare earths ions; organic phase: 0.5 M EHEHPA; aqueous phase: 220, 438, 385 mg/L of Dy(III), Nd(III) and Pr(III) respectively in 0.3 M HNO3; flow rate: 100 mL/min.

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Fig. 10. Extraction of Dy(III) in presence of competing rare earths ions; organic phase: 0.5 M EHEHPA; aqueous phase: 215, 416, 399 mg/L of Dy(III), Nd(III) and Pr(III) respectively in 0.1 M HNO3; flow rate: 100 mL/min.

Fig. 12. Stripping of Dy(III) from loaded 0.5 M EHEHPA phase; strip phase: 1.5 M HNO3; flow rate:100 mL/min, phase ratio 1:1, organic phase 0.5 M EHEHPA having 3.3 g/L Dy (III).

from the Nd and Pr mixture by employing hollow fibre membrane contactor. Dysprosium purification possibilities from the mixture of Nd and Pr by using EHEHPA in hollow fibre membrane were found to be time dependent. This finding leads the way to recover high pure Dy from the mixture of rare earths found in magnetic scrap material. After complete equilibration (120 min of operation), % extraction of Dy, Nd and Pr by 0.5 M EHEHPA from 0.3 M HNO3 feed in HFM contactor in NDSX mode were found to be 99.88, 13 and 8% respectively. Whereas, in the case of 0.1 M HNO3 media % extraction of Dy, Nd and Pr by HFM module under similar condition was 99.8, 78.9 and 68.57 respectively. A second cycle of non dispersive extraction in hollow fibre membrane was introduced to further purify Dysprosium from 70% to > 90% obtained from the above operation (first cycle) of NDSX in HFM module of 10 min equilibration time. The loaded organic obtained from the first cycle was stripped with 3.5 M HNO3 and this formed a feed for the second cycle. Aqueous second cycle feed consisting 1059, 226 and 184 mg/L Dy, Nd and Pr in 0.1 M HNO3 was processed in HFM-NDSX mode of operation using 0.5 M EHEHPA with flow rate of 100 mL/min each. Results summarized in Fig. 11 suggested that the Dy separation from Nd and Pr by HFM was due to kinetics of metal ion transport across the membrane. A careful operation of the system yielded Dy with > 97% purity and 94% recovery from its mixture with Nd and Pr obtained from the NdFeB magnets of Hard Disk Drives.

3.2.7. Stripping of Dy(III) from loaded EHEHPA by HFM Experiments on rare earth stripping from loaded organic phase were carried out to establish the viability of non-dispersive mode of operation by employing hollow fibre contactor to separate Dy from aqueous phase in continuous mode operation. Furthermore, the effect of contact time at a definite flow rate of strip phase i.e. 1.5 M HNO3 on the transport of Dy(III) from loaded organic phase i.e. 0.5 M EHEHPA was evaluated. Fig. 12 depicts the stripping of 0.5 M EHEHPA loaded with 3.3 g/L Dy(III) with 1.5 M HNO3 aqueous strip phase. It is evident from the results that complete stripping could be achieved with 150 min of equilibration through the pores of HFM. Lower phase ratio i.e. O/ A < 0.5 can be utilized to further concentrate the Dy product in strip phase.

4. Conclusions Extraction behaviour of rare earth elements (Dy, Nd and Pr) found in NdFeB magnet, across the hollow fibre membrane pores with EHEHPA as an extractant have been studied by operating HFM in nondispersive solvent extraction mode. The separation and purification of Dy(III) by non dispersive mode of extraction was found to be affected by process variables such as aqueous phase acidity, organic phase concentration, metal ion concentration and presence of competing ions. Successful stripping of Dysprosium from loaded organic phase in nondispersive mode of solvent extraction indicate the possibility of using this method in continuous operation mode even at large scale for rare earth separation. The recycling of the organic phase 0.5 M EHEHPA resulted in excellent reproducibility of the extraction results. Experimental findings revealed that under optimized conditions. Dysprosium could be successfully separated from the scrap rare earth magnetic material by employing NDSX method with product consisting up to 97% purity and > 94% recovery. The results obtained with the present study establish that hollow fibre membrane module not only offers a better separation method but also have the potential to be up scaled for rare earth recycling.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2017.11.025.

Fig. 11. Purification of Dy(III) from mixed rare earths feed; organic phase: 0.5 M EHEHPA; aqueous phase: 1059, 226, 184 mg/L of Dy(III), Nd(III) and Pr(III) respectively in 0.1 M HNO3; flow rate: 100 mL/min.

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