Role of extractants and diluents in recovery of rare earths from waste materials

Role of extractants and diluents in recovery of rare earths from waste materials

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Role of extractants and diluents in recovery of rare earths from waste materials Sanghamitra Pradhan, Nilam Swain, Susmita Prusty, Rakesh Kumar Sahu, Sujata Mishra ⇑ Department of Chemistry, Institute of Technical Education and Research (FET), Siksha ‘O’ Anusandhan Deemed to be University, Khandagiri Square, Bhubaneswar 751030, Odisha, India

a r t i c l e

i n f o

Article history: Received 11 December 2019 Received in revised form 11 January 2020 Accepted 13 January 2020 Available online xxxx Keywords: Solvent extraction Rare earths Extractants Diluents Sustainability

a b s t r a c t It is perceived that our society is heading towards an ultra-connected world with the help of emerging technologies. The swift frequency of technological development has been possible due to enhanced advances in IT, cheapness of raw materials that lead to industrial development. Rare earth elements are playing a key role in the economic progress having diverse applications in alloys, magnets, catalyst, phosphors and are utilized in equipments such as batteries, sensors, electric vehicles. The prominence of these elements has gone up due to high demand, limited supply and non-availability of appropriate substitutes. Considering the present scenario, recovery of rare earths from end of life products through economical technology have become top priorities in metallurgy. There are diverse routes to recover rare earths from secondary resources ranging from hydrometallurgical to pyrometallurgical processes. Hydrometallurgical technique such as solvent extraction has been proved beneficial in recovering rare earths from these secondary resources. This review has been framed to discuss the role of extracting agents and diluents in the extraction circuits used for rare earth extraction and separation studies taking into consideration the end of life products. The function of different extractants such as di-(2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexyl phosphonic acid mono-2-ethylhexylester (PC 88A), bis(2,4,4trimethylpentyl) phosphinic acid (Cyanex 272), trialkyl phosphine oxides (Cyanex 923) and diluents like pentane, hexane, Solvent 70, dodecane, Octanol and cyclohexanone employed for the extraction of rare earths from the waste materials, particularly magnet scraps, spent batteries and lamp phosphors have been highlighted. The recent challenges concerning the development of cost effective, eco-friendly green extractants like tricaprylmethyl ammonium chloride (Aliquat 336), DEHPA Alamine 336 IL have also been discussed. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.

1. Introduction The on-going revolution of technology and industries emphasizes on the crusade towards low-carbon technology or green technology. In this transition, rare earth metals (REMs) play an essential role as major contributors in these emerging infrastructures and are recognized as the vitamins of modern technology [1]. These are a part of several devices starting from mobile phones to military appliances. Upgradation in technologies creates an increasing demand for REMs putting them at a stage of supplyrisk. The limited supply and abruptly increasing demand of these ⇑ Corresponding author. E-mail addresses: (S. Mishra).

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elements have led to the so called ‘‘balance problem” and hence, these are labeled as ‘‘critical” elements by European Union Commission [2]. Though the market share of REMs in economy of industries is unquestionable, the production of materials creates a lot of waste laying environment at risk which is of increased global concern. The anthropogenic inputs are the master sources of rare earth accumulation in soil, water, plant and air, which in turn enter into the food chain. The rare earth chain from mining to food is shown in Fig. 1 [3]. Based on this fact, it could be concluded that there is urgent need of sustainable recycling techniques with special attention to waste products containing substantial amount of rare earths. The potential material streams of REMs are (1) pre-consumer products or scraps (2) post-consumer products or end-of-life products (EOLs) and (3) landfills rendered by pre-consumer and

https://doi.org/10.1016/j.matpr.2020.01.288 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.

Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288

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S. Pradhan et al. / Materials Today: Proceedings xxx (xxxx) xxx

waste streams studied elaborately are permanent magnets, NiMH batteries and phosphor-products, emphasizing on the up to date deliberations about material pre-treatment, leaching and solvent extraction separation of REEs. 2. Extractant activity in extraction system

Fig. 1. Rare earth chain.

post-consumer product waste streams. However, the postconsumer products are regarded as the most appreciable waste for recycling purpose considering their numbers and significant weight percentage of rare earth content [4]. EOLs like nickel metal hydride batteries (NiMHBs), permanent magnets (PMs) and REMbased phosphors (FLs) or luminescent products are the significant sources of REMs whose recycling could comply the supply-demand problem. NiMHBs are basically used in household appliances, power tools and hybrid electric vehicles. Of total, 7% of NiMHBs is comprised REMs which corresponds to an approximate of 1 g per battery, 60 g per power tool used in domestic application and 2 kg per hybrid electric vehicle battery [5,6]. Previously, it was estimated that more than 23% of rare earths are present in phosphors which is 15 times of total rare earth content in primary ores. About 15% of REMs are consumed in the production of luminescent materials and hence, recycling of these materials is an effective strategy to mitigate supply risk [7]. Over the last decades, the share of rare earths held by PMs (NdFeB, SmCo, SmFeN) is increasing substantially in smart and emerging green technologies. The growing demand of these PMs is coupled with the accelerating switch towards clean, more efficient and miniaturized infrastructures. Their applications cover a wide range starting from mobile phones to wind turbines. The estimated global demand of these PMs is increasing with an approximate compound rate of 9% per annum [8,9]. The recycling of these end-of-life products having a fair share of REMs in global market could ease the supply disruption problem. There are several ways to recover metals from waste materials, namely, supercritical fluid extraction, liquid metal extraction, molten slag extraction, solvent extraction, etc. Among these, solvent extraction is the most flexible method taking into account its cost-effectiveness and less ornamental methodology which is extensively used technique for the separation and preconcentration of elements. This approach is well-established for the recovery and separation of REMs into individual elements or groups [10]. In this process, an aqueous solution containing the solutes/ metals is brought into contact with another solvent i.e. organic phase comprising extractant and diluent essentially immiscible aqueous phase. Herein, the parting of metals is basically centred on the inequitable distribution of elements in two non-compatible liquids [11]. Thereafter, metals are recovered in purified form by shaking with acid or base or water through stripping or back extraction. This review article talks about the outlooks of reclaiming the rare earth elements (REEs) from different wastes using solvent extraction, a novel hydrometallurgical method. The three major

The extractants play a unique role in the extraction process as they involve in the dragging of metals from the aqueous phase to the organic phase by forming extractable extractant-metal complex. The presence of requisite sites of atoms like O, P, N, and S assist them to bind with metal ions [12,13]. To achieve complete or an effective separation, an extractant must have certain characteristics like (i) highly selective towards the metal of interest (ii) chemically and thermally stable (iii) moderate viscosity (iv) low surface tension [14]. Based on the extraction mechanism, these are classified mainly into three groups i.e., (i) cationic or acidic extractants (ii) neutral or solvating extractants (iii) anionic or basic extractants. Acidic extractants are the derivatives of phosphoric acid or carboxylic acids substituted by different alkyl chains and are co-ordinated to the metal ion with the exchange of H+ ion. The second category of extractants extract metal ion in neutral form by solvation. Basic extractants are amines (primary, secondary, tertiary) or ammonium salts or phosphonium salts and they extract metal ion in the anionic form and co-ordinate by simple ion-pair association [15–18]. In mineral hydrometallurgy, minerals are usually leached with high concentration of acid and the selection of extractants is based on the leaching stage. The anionic part of the aqueous phase gives the clue about the kind of extractants which is potent for the extraction process. If hydrochloric acid is used as lixiviant, the selection of extractant is based on the chloride concentration. At low concentration of chloride, solvating extractants show better efficiency. Chloride ions are very good inner-sphere ligands which make them able to solvate the metal ions easily. So, the formation of neutral complex and thereby extractable metal species become easy. However, metal ions exist as chloride metalates at high molarity of chloride anions. In this case, cationic extractants are preferred. On the other side, if leaching is carried out with sulphuric acid, anionic extractants are good candidates for extractive metallurgy [16]. This could be explained on the premise that sulphate ions are weak inner-sphere ligands as compared to chloride ions. The choice of extractants on the basis of leaching stage of minerals can also be extended to the recovery process of valuable REMs from waste materials. As the whole system deals with transportation of metal from aqueous phase to organic phase, in order to increase the mass-transfer process it is necessary to choose a diluent which would be capable of reducing the viscosity of extractant and hence, ensures the adequate contact between extractant and aqueous phase. 3. Diluent activity in extraction system Diluents play a pronounced role in solvent extraction process. These are required to dissolve extractants and also, they help to prevent the emulsification of extractant. The dispersion and coalescence properties of solvents are also controlled by diluents [12]. The extraction behaviour of extractants is regulated by the physical properties of diluents, namely, dielectric constant, polarity, viscosity, density and solubility parameter [19]. On top of all these, good diluents must have low volatility, solubilising in organic phase, high flash point and ready availability with affordable price. Predominantly, diluents are categorized into two groups on the basis of their polarity i.e. (i) active diluents (polar due to presence of functional group) and (ii) inactive diluents (non polar).

Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288

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In general, polar diluents show low distribution ratio as they interact with the extractants through dipole-ipole interaction and the extractants become less available, which in turn decreases the extraction. But in case of ion-pair (polar) complexes like acids, they increase the distribution ratio by increasing the solvation of extracted species [20]. Some of the examples of active diluents are halogenated aromatic solvents, chlorinated hydrocarbons, alcohols and ketones. Inactive diluents are good solvents and customarily smoothen the extraction procedure by enhancing the solubility of extracted species in the organic phase. On the contrary, inactive diluents show poor solvation for the polar complexes and hence, decrease the distribution ratio. They include aromatic and aliphatic hydrocarbons, alkyl substituted aromatics, etc. Comprehensively, aliphatic diluents show better extraction as compared to aromatic diluents [21]. The performance of diluents is judged by their chemical composition. The aromatic content of diluents makes them less inactive near the metal ion as well as to the extracted species. Consequently, the extraction of the metal ions declines with rise in aromatic constituents. However, it is noteworthy that the extraction behaviour cannot be fully decided by the amount of aromatic contents of diluents. One more important role played by diluent in extraction process is to modulate the thermodynamic parameter i.e. enthalpy of the extraction isotherm. The overall enthalpy involved in the extraction of metal is the sum of three enthalpy factors (i) enthalpy associated with dehydration of metal ion (DH1) (ii) enthalpy corresponding to the formation of metal-extractant complex (DH2) (iii) enthalpy of dissolution of extracted complex in diluent (DH3). For a given metal ion and fixed aqueous composition, DH1 will remain unchanged. Thus, DH2 and DH3 will determine the overall enthalpy [22]. Though, their function in extraction process is noteworthy, these are often assumed as inert or inactive in extraction equilibrium. Nevertheless, they greatly affect the distribution ratios [23]. Depending on the extraction mechanism, different diluents give different equilibrium constant values. For rare earth extraction, kerosene was found as the most desired diluents as it gives higher equilibrium constant values. The dispersion property of a diluting agent gauges the interaction between it and solute molecules which adds to various dispersion forces [14].

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Fig. 2. Schematic representation of hydrometallurgical processes for the recovery of rare earths from different wastes.

4. REMs recovery from wastes using hydrometallurgical techniques Hydrometallurgical techniques offer to be the most preferable practice in terms of separation and recovery of individual rare earths from secondary resources [24,25]. Leaching and recovery of metals of interest present in the solution are important steps of this technique. A streamlined demonstration of characteristic hydrometallurgical technique adopted for the removal of REEs from end of life wastes is presented in Fig. 2. 4.1. Recovery of rare earths from magnets in post-consumer products The procedure for the recovery of rare earth elements from magnet wastes is centered on unified hydrometallurgical path embracing leaching followed by solvent extraction. This technique has been exceedingly encouraging than other informed energy exhaustive developments using wide range of extractants [26]. D2EHPA, Cyanex 272, Cyanex 923are preferentially chosen for industrial practice on account of their high extractability and low and operational cost. The schematic representation for processing of magnet scraps to recover rare earths is shown in Fig. 3. Utilizing the procedure of solvent extraction, evaluation of two organophosphorus extractants i.e., D2EHPA and PC 88Ahas been made by Yoon et al. for the retrieval of dysprosium and neody-

Fig. 3. Representation of processing of magnetic scraps.

mium from the scraps of permanent magnets. This comparative study accomplishes that extractants like D2EHPA and PC 88A are suitable for the recovery of rare earths from the magnetic waste and PC 88A outclassed D2EHPA diluted in kerosene [26]. Case et al. have performed investigations on the extraction of REMs containing NdFeB magnets and lighting phosphor materials using tetrabutyldiglycolamide in 1-Octanol as extractant [27]. Gergoric et al. have efficaciously recovered rare earth elements from neodymium magnet wastes by leaching with completely inflammable organic lixiviants such as maleic, glycolic and ascorbic acids. This is in order to minimize the use of strong mineral acids those are normally consumed in the hydrometallurgical recovery of rare earth elements. Different organophosphorous extractants like trin-butyl phosphate (TBP) and D2EHPA were used for selectively extracting the rare earths from the leachates. In this investigation the influence of various aliphatic and aromatic diluents on the extraction system has been examined since diluents show a crucial

Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288

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role in enhancing the selectivity of the extractants towards rare earths. Investigational extraction data demonstrated that D2EHPA extraction efficiency for neodymium (Nd) and praseodymium (Pr) diminishes in the order pentane > hexane > Solvent 70 > dodecane. Octanol and cyclohexanone were judged as inappropriate diluents for REEs than other non polar solvents [28]. Using octanol and cyclohexanone as diluents, more transition metal extraction occurs, since these compounds take part in the extraction process using oxygen atoms to bind the metal ions [29]. Separation of heavier and lighter rare earth elements from neodymium magnet leachate has been studied through solvent extraction. The effect of various diluents such as hexane, octane, cyclohexanone, chloroform, 1-octanol and toluene on the extraction and separation of higher and lighter rare earth elements using D2EHPA has been investigated. From this investigation it has been concluded that non polar diluents like hexane, octane are appropriate for solvent extraction of rare earths with D2EHPA whereas polar diluents such as cyclohexanone, chloroform exhibited lower distribution ratios for the extracted rare earth elements [30]. Riano et al. have used deep-eutectic solvents as a novel substitute to aqueous phase for the retrieval of important metals from NdFeB magnets. The separation of Nd and dysprosium (Dy) from other metals like iron (Fe), boron (B) and cobalt (Co) in the deep-eutectic solvent has been carried out using ionic liquid such as Aliquat 336 in toluene. Further the separation of Nd and Dy has been evaluated employing two extractants such as Cyanex 923 and D2EHPA [31]. Orefice et al. developed a method to recover samarium (Sm), cobalt (Co), copper (Cu) and iron (Fe) from permanent magnets such as SmCo. Co, Cu and Fe were extracted efficiently by Aliquat 336 in toluene along with 37 wt% hydrochloric acid. After the separation of Sm from the transition metals, it was recovered from the leachate using Cyanex 272 in dodecane. Different commercial extractants were tested: first a mixture predominantly made up of Cyanex 923 and then a mixture mainly made up of Cyanex 272. Cyanex 923 was not proper, since the considerable quantity of water in the feed phase, reduces the extraction. Cyanex 272 efficiently works at pH above 4 and the extraction mechanism involves release of protons and complexation with samarium ion. Consequently, the pH diminishes in the aqueous feed and the extraction percentage decreases [32]. Mohammadi et al. compared the extraction efficiency of EHEPA and D2EHPA for the extraction of Y(III), Dy(III), Nd(III) from acidic chloride medium present as constituents of NdFeB magnet. The results revealed that the metal ions are extracted by both D2EHPA and EHEPA in the order: Y(III) > Dy (III) > Nd(III) [33]. The extraction trend revealed that EHEPA and D2EHPA preferentially extract heavier REMs. It has been reported that the extraction efficiency of D2EHPA was superior to EHEPA. The ionic size of rare earth metal ions decrease along the period from La to Lu (with increase in atomic number) due to the poor screening effect of electrons in the f-subshell. So, the heavier rare earths being smaller in size behave as hard acids than those of lighter rare earths. D2EHPA and EHEPA having O-atom as binding site act as hard base. As per the hard soft acid base rule (HSAB), hard prefers hard and soft prefers soft and is describing the basis for the preferential extraction of heavier rare earth metals (HREMs) by D2EPHA and EHEPA. It was previously reported that the extraction tendency of acidic extractants (PC88A, D2EHPA, Cyanex 272) for the REMs decreases in the order of D2EHPA > EHEPA > Cyanex 272. The extraction efficiency could be explained on the basis of their pKa value, an extractant with higher pKa value have lower dissociation thereby lower extraction ability [34]. Hoogerstraete et al. developed an ecofriendly hydrmetallurgical method for the separation of rare earths and transition elements from NdFeB and SmCo magnets. This process has been efficiently carried out using hydrophobic ionic liquid trihexyl(tetradecyl) phosphonium chloride without any use of diluents. The highly vis-

cous ionic liquids have unfavorable effect on the mass transport and therefore on the rate studies. This is reason for which the non-fluorinated ionic liquids were used in diluents. It was monitored that equilibrium was achieved after twenty minutes, even if the extraction of samarium somewhat increased. The extraction of Sm(III) at elevated loadings happened mostly through formation of pentakis nitrato complex [35]. Swain et al. developed a process in nitrate media, with Aliquat 336 blended in kerosene and 20% propanol to recover Sm(III) and Co(II) from simulated SmCo magnet waste liquor. They concluded that complete separation of Sm (III) and Co(II) can be achieved sinceAliquat 336 selectively extracted Sm(III) from nitrate medium [36]. Recycling of SmCo magnet was also investigated by Swain et al. in slightly acidic nitrate medium using TOPO in kerosene, whereby purified Sm2O3 and Co3O4 were obtained by precipitation with sodium oxalate followed by calcination. TOPO with O-donor atom selectively extracted the Sm(III) ion which resulted in complete separation of Sm(III) and Co(II) [37]. The efficiency of different extractants and bi-functional ionic liquids were compared for the extraction of Nd(III) and Pr(III) from the leached liquor of NdFeB magnet. It has been concluded that the extraction ability of the extractants follows the order of Cyanex 272Alamine 336 IL > DEHPA. Alamine 336 IL > Cyanex 272 > DEHPA > Aliquat 336 [38]. Kumari et al. performed solvent extraction using the NdFeB magnet leach solution present in wind turbines for extraction of lighter rare earths. The stability of the complex formation with EDTA follows the trend Dy > Nd > Pr and order of extraction of rare earths using ([Aliquat336][NO3]) has been reported as Pr > Nd > Dy [39]. Pavon et al have reported on Nd recovery from NdFeB magnetic wastes using Primene 81. Cyanex 572IL diluted in Solvesso 100. The percentage of extraction for Nd(III) has been reported 99.9%. REEs extraction enhances when the extractant molarity increases, the increase in the extraction follows the sequence: Nd(III) < Dy(III) < Tb(III). The equilibrium pH shows decreasing trend with increase in the concentration of the extractants. The equilibrium pH values using Cyanex 572 were less than those with Cyanex 272 and extraction results were well agreed with this behavior [40]. The highest separation factors of various rare earths present in NdFeB magnet scrap using Cyanex 272, Cyanex 572, Primene 81R. Cyanex 572IL and [3336At][NO3] are depicted in Table 1. The extraction reactions involved using Cyanex 572IL are illustrated as follows: þ RE3þ ðaqÞ þ 3ðHAÞ2ðorgÞ ¡ REðHA2 Þ3 þ 3H

ð1Þ

2RNH2 þ ðHAÞ2 ¡ 2RNHþ3 A org

ð2Þ





þ 3RNHþ3 AðorgÞ RE3þ ðaqÞ þ 3ClðaqÞ ¡ REA3 :3RNH3 Clðorg Þ

ð3Þ

where RE3þ ¼ Rare earth cation. 4.2. Rare earth elements recovery from spent batteries atteries are mainly used in electronic devices and spent batteries exemplify an imperative environmental pollutant within the wastes in terms of heavy metals content. The spent batteries are composed of 36–42% nickel, 3–4% cobalt and 8–10% (lanthanum, cerium, praseodymium and neodymium) [41]. Till date numerous investigations have been reported on the extraction of metals in spent batteries in order to guard the environment and preserve the mineral resources [42–44]. Aly et al. have reported on the recovery and separation of valuable metals from spent nickel metal-hydride batteries with the aid of organophosphorous extractants. The spent mobile phone batteries were subjected to manual separation followed by crushing to release the battery electrodes. The electrodes were leached with acids and then filtered. The rare

Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288

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S. Pradhan et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 List of highest separation factors achieved with different extractants at different experimental conditions for REMs. Extractants

Experimental Conditions

b (separation factor)

Extraction Trends

References

[Aliquat336][NO3]

[NO 3 ] = 6 M, pH = 3, [Aliquat336][NO3] = 0.43 M, [Nd] = 0.012 M, [Pr] = 0.003 M, [Dy] = 0.0002 M  [NO 3 ] = 6 M, pH = 3, [Aliquat336][NO3 ] = 0.43 M, EDTA = 0.003 M, [Nd] = 0.012 M , [Pr] = 0.003 M, [Dy] = 0.0002 M [Cl] = 4 M, Cyanex 272 = 0.30 M, pH = 3.50, [REM] = 1 g dm3 [Cl] = 4 M, Cyanex 572 = 0.54 M, pH = 3.50, [REM] = 1 g dm3 [Cl] = 4 M, Primene 81R. Cyanex 572 = 0.6 M, pH = 0.10, [REM] = 1 g dm3

Pr/Dy = 10.9, Nd/ Dy = 5.9, Pr/Nd = 1.8 Pr/Dy = 55.6, Nd/ Dy = 19.4, Pr/Nd = 2.9 Dy/Nd: 26.1, Tb/Nd: 11.8 Dy/Nd: 597, Tb/Nd: 227 Dy/Nd: 553, Tb/Nd: 238

Pr > Nd > Dy

[39]

Dy(III) > Tb (III)  Nd(III)

[40]

Cyanex 272 Cyanex 572 Primene 81RCyanex 572 IL

earths present in the filtrate have been precipitated with NaOH at pH = 2.5. From the dense white precipitate of rare earths hydroxides chlorides of La and Nd have been obtained using 2 M HCl. Separation of transition metals present in the filtrate have been carried out effectively using 0.6 M Cyanex 272 in kerosene. In comparison to HDEHP, Cyanex 272 diluted in kerosene proved to be effective in the separation of cobalt and nickel from the spent NiMHBs [45]. Li et al. have performed investigations on the separation and recovery of rare earths by leaching the electrode materials with 3 M H2SO4 maintaining 95 °C. Due to poor solubility of rare earth sulphates at high temperature, 94.8% of rare earths were effectively separated from the mixture of other metals. The separation of transition metals has been carried out using 20% D2EHPA and the rare earths remaining in the residue were treated with NaOH to convert the leached residue to hydroxides [46]. Talebi et al. have performed investigations on solvent extraction of rare earth metals from waste electrical and electronic equipment leaching solution using Versatic 10 as a carrier and TBP as phase modifier in kerosene. The outcomes conclude that higher percentages of Ce, Y and La have been extracted at pH 7 using neodecanoic acid (Versatic 10) concentration (200 mM) and TBP concentration, 100 mM. Oxalic acid (750 mM) performance was 100% for the recovery of rare earth metals excluding cerium [47]. Korkmaz et al. have performed investigations on the defensible hydrometallurgical recovery of rare earths from spent NiMHBs. Leaching has been done using HCl and H2SO4 and it was observed that lighter rare earths precipitated out with increase in temperature. The separation of rare earths from sulphuric acid was carried out by adding sodium ions [48]. For the separation and recovery of rare earths and transition metals from spent NiMHBs, Zhang et al. adopted hydrometallurgical route. Effective leaching has been reported using 2 M H2SO4 exhibiting leaching efficiency of 96% rare earths, 97% Ni and 100% Co. Nickel and cobalt were extracted using trioctylamine in kerosene. The recovery of rare earth values from the leachate solution was successfully done by means of a solvent extraction with 25% D2EHPA in kerosene with the help of twostage counter-current extraction at an equilibrium pH of 2.0 at an O:A ratio of 3:1, followed by single stage stripping with 2.0 M hydrochloric acid maintaining O:A ratio of 5:1 are involved. A mixed rare earth oxide of more than 99% purity and yield of 98% was obtained by oxalate precipitation and calcination [49]. Fernandez et al. have developed a process for NiMHBs recycling using TBP, tri-n-octylamine (Alamine 336), and PC-88A [6]. Above 98% of the rare earths present in the raffinate were recovered after extraction with 20% PC88A in kerosene at pH 1 followed by precipitation at pH 0.5. The selectivity prospective of ionic liquids for recycling of REMs and base metals present in slag concentrates of NiMHBs have been reported by Sahin et al. Leaching tests have been performed by varying temperature, particle size and time using methylimidazolium hydrogen sulfate (HmimHSO4) and leaching efficiencies for La and Ce have been reported as 15% and 20% respectively. The dissolution of 63–90 lm particle size slag powder were cent percent for yttrium after two hours at 80 °C. Meanwhile, leaching

efficiencies were about 15% for lanthanum and cerium and 20% for neodymium [50]. Larsson and Binnemans have reported an optimized system for the recovery of metal values from NiMHBs using ionic liquids trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) or Aliquat 336. Both the ionic liquids exhibited good extraction efficiencies for the transition metals. REMs recovery has been reported using nitrated forms of Cyphos IL 101 or Aliquat 336 along with Cyanex 923. The unique feature of ionic liquid cation is its distinctive impact on the extraction process, since ammonium-based ionic liquids preferentially extract the rare earths over transition elements. The variations in the interactions of metal ions with the ionic liquid cations are clear from the colour of complexes in the extracted phase [51]. 4.3. Rare earth elements recovery from lamp phosphors Rare earth elements recovery from waste phosphors in the endof-life fluorescent lamps represents an interesting source and is essential for sustainable and supply. Almost 15% REEs are employed in light emitting products like fluorescent lamps. Fig. 4 represents the three different phosphors present in fluorescent lamps. The production of fluorescent lamps containing phosphors has increased in the last few years due to energy saving potential and it avoids emission of CO2. These electronic wastes generated from the EOL products contain (150–220 g/t) of REEs, which is significantly higher than the REEs occur in the nature. This has attracted attention of researchers throughout the globe and extensively reported in the literatures [52,53]. Innocenzi et al. have recovered rare earths from fluorescent lamps through solvent extraction. In this investigation the efficiency of three extractants; Cyanex 272, Cyanex 572 and D2EHPA in kerosene has been examined for the recovery of REMs from the leached sulphate solutions. Out of the three extractants, D2EHPA proved to be imminent in

Fig. 4. Representation of three different phosphors contained in fluorescent lamps.

Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288

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separation of terbium (Tb) and yttrium (Y) from other rare earths at pH less than 1. This investigation revealed the positive influence of pH on the rare earth extraction procedure. The order of the extraction has been found to be Y > Tb > Gd > Eu > Ce > La [54]. Eduafo et al. have performed investigations on recycling REMs from waste fluorescent lamp phosphors. In this study three stages of leaching and separation have been reported since direct leaching of REMs carried dissolved impurities like calcium, phosphorous, silicon, iron and zinc [55]. Gijsemans et al. developed a solvometallurgical step with the use of green organic acid methanesulphonic acid as lixivant that facilitates increasing leaching efficiencies (74% Tb, 78% Ce and 95% La) within short time at low temperature without the use of concentrated sulphuric acid or fused alkali. Other rare earths such as yttrium, europium and gadolinium which are present in the form of impurities have been collected in the form of oxides [56]. Extraction of rare earth elements in lamp phosphors has been carried out using TBP with nitric acid and water under supercritical conditions and atmospheric pressure. The efficiency of extraction has been reported to be Y (37.4%), Eu (36.8%) and less than 3% for La, Ce and Tb at atmospheric condition. Under supercritical conditionsEu and Y (99%) has been extracted and less than 7% extraction has been reported for La, Ce and Tb [57]. Nakamura et al have reported the separation of rare earths in fluorescent lamps using PC 88Ain kerosene [58]. The extraction equilibrium expressions for PC-88A with trivalent (rare earths) and divalent alkaline earth metals are represented as

RE3þ þ 3ðRHÞ2 ¡ RER3 ðRHÞ3 þ 3Hþ

ð4Þ

AE3þ þ 3ðRHÞ2 ¡ AER2 ðRHÞ4 þ 2Hþ

ð5Þ

where RE, AE and (RH)2 represent rare earths, alkaline earth metals and dimeric species of PC 88A, respectively. On the basis of extraction equilibrium studies, separation of rare earths have been performed focusing primarily on Eu, Tb and Y. Tunsu et al. have studied the viability of using Cyanex 923 for the recovery of rare earth metals from fluorescent lamp waste. The extraction equilibrium for the rare earths has been achieved with contact time less than one minute in comparison to iron and mercury. Low temperature favored the extraction of rare earths into the organic phase leaving mercury and iron in the leachates. It has been reported that enhancing the Cyanex 923 concentration, the extraction of mercury and iron increases. So the optimum condition for extraction of rare earths at room temperature has been possible by using 1 M Cyanex 923 and maintaining one minute contact time between the phases [59]. Rabah has extracted Eu and Y from thiocyanate leachates of lamp phosphors using trimethyl-benzylammoniumchloride. The extraction efficiency exhibited for both the REEs is greater than 96% and stripping has been performed using TBP in 1 M nitric acid. The separation factor between the two REEs is reported to be 9.4 [60]. Yang et al. have investigated the recovery of rare earths from phosphor powders in waste fluorescent lamps by solvent extraction using N, N-dioctyldiglycolamic acid (DODGAA). This novel extractant exhibited greater attraction for REMs in comparison to conventional phosphonic extractant [61]. Pavon et al. have reported the rare earths separation by employing ionic liquids as extracting agent [62]. Extractants like Cyanex 572, D2EHPA and ionic liquids, such as Primene 81R. Cyanex 572 IL and Primene 81R. D2EHPA IL have been selected to scrutinize the recovery of Ce, Eu and Y with 99.9% purity in red phosphors. The oxides of yttrium and europium acquired from fluorescent lamps have been separated into individual components by using two undiluted thiocyanate ionic liquids as extractants. Trihexyl(tetradecyl)phosphonium thiocyanate exhibited best extraction performances using organic to aqueous

ratio as 1:10. Y and hydroxides of Eu is obtained by precipitation with ammonia [63]. 5. Conclusions and future scope Extracting agents such as organophosphorous compounds along with the recently developed green solvents, ionic liquids have been successfully employed forthe extraction and separation of rare earths with sufficient purity. Solvent extraction equilibrium thermodynamics is greatly influenced by the organic phase composition. The diluent plays significant role in determination of Gibbs energy of extraction process which is connected to its cohesive dispersive energy. More the cohesive dispersive energy less is the extraction efficiency. The parameters like association, nonspecific interaction while considering acid and water uptake by organic phase in the extraction system could be more interesting which may provide additional proof to the diluent effect investigation. This review provides baseline information about the efficiency of different extractants and diluents in the extraction of rare earths from secondary sources. The researchers can proceed with further study to use the end of life products as alternatives for critical raw materials since there is a greater need to reinforce our search for rare earth resources. Instead of opening new mining schemes, REE recycling can be considered as promising options for near future REE supply due to low environmental impact, reasonable price and operational facility. This summarized review will be beneficial for the researchers and industrialists to design the extraction circuits for sustainability in the rare earth industries. CRediT authorship contribution statement Sanghamitra Pradhan: Data curation, Writing - original draft, Visualization, Investigation. Nilam Swain: Data curation, Writing - original draft, Visualization, Investigation. Susmita Prusty: Formal analysis, resources. Rakesh Kumar Sahu: Methodology. Sujata Mishra: Conceptualization, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are gratified to the authorities of Siksha ‘O’ Anusandhan Deemed to be University, Bhubaneswar for the encouragement to prepare this review article. References [1] M. Humphries, Rare Earth Elements: The Global Supply Chain, Congressional Research Service, Washington, DC (USA), 2013, Report No.: R41347. [2] K. Binnemans, P.T. Jones, B. Blanpain, T.V. Gerven, Y. Yang, A. Walton, M. Buchert, J. Clean. Prod. 51 (2013) 1–22. [3] A. Jain, Environment, United Nation and Programme, Framework of Global Partnership on Waste Manage, 2010. [4] P.T. Jones, T.V. Gerven, K.V. Acker, D. Geysen, K. Binnemans, J. Fransaer, B. Blanpain, B. Mishra, D. Apelian, J.O.M 63 (2011) 14–15. [5] D. Reisman, R. Weber, J. Mckernan, C. Northeim, USEPA, Sci. Inv. (2012). [6] A. Fernandes, J.C. Afonso, A.J.B. Dutra, Hydrometallurgy 133 (2013) 37–43. [7] N. Swain, S. Mishra, J. Clean. Prod. 220 (2019) 884–898. [8] IMARC, Prefeasibility Report on Rare Earth Magnet Manufacturing Plant, 2014. [9] A. Kumar, Permanent Magnets: Technologies and Global Markets, Bangalore, 2017. [10] F. Xie, T.A. Zhang, D. Dreisinger, F. Doyle, Miner. Eng. 56 (2014) 10–28. [11] C. Gupta, N. Krishnamurthy, Int. Mater. Rev. 37 (1992) 197–248. [12] G.M. Ritcey, A.W. Ashbrook, Solvent Extraction Principles and Applications to Process Metallurgy, Part I. Elsevier, The University of Michigan, USA, 1984.

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Please cite this article as: S. Pradhan, N. Swain, S. Prusty et al., Role of extractants and diluents in recovery of rare earths from waste materials, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.288