Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal

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Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal Hydride Battery Yuxiang Yao, Nina F Farac, and Gisele Azimi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03803 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Sustainable Chemistry & Engineering

Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal Hydride Battery Yuxiang Yao1, Nina F. Farac1, and Gisele Azimi1,2* 1

Laboratory for Strategic Materials, Department of Chemical Engineering and Applied Chemistry,

200 College Street, Toronto, Ontario, M5S 3E5, Canada 2

Department of Materials Science and Engineering, University of Toronto, 184 College Street,

Toronto, Ontario, M5S 3E4, Canada *corresponding: [email protected]

ABSTRACT Today’s world relies upon critical green technologies that are made of elements with unique properties, irreplaceable by other materials. Such elements are classified under strategic materials; examples include rare earth elements that are in increasingly high demand, but facing supply uncertainty and near zero recycling. To tackle the sustainability challenges associated with rare earth elements supply, new strategies have been initiated to mine these elements from secondary sources. Waste electrical and electronic equipment contain considerable amounts of rare earth elements; however, the current level of their recycling is less than 1%. Current recycling practices use either pyrometallurgy, which is energy intensive, or hydrometallurgy that rely on large volumes of acids and organic solvents, generating large volumes of environmentally unsafe residues. This study put the emphasis on developing an innovative and sustainable process for the urban mining of rare earth elements from waste electrical and electronic equipment, in particular nickel metal hydride battery. The developed process relies on supercritical fluid extraction utilizing CO2 as the solvent, which is inert, safe, and abundant. This process is very efficient in a sense that it is safe, runs at low temperature, and does not produce hazardous waste,

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while recovering about 90% of rare earth elements. Furthermore, we propose a mechanism for the supercritical fluid extraction of rare earth elements, where we considered a trivalent rare earth element state bonded with three tri-n-butyl phosphate molecules and three nitrates model for the extracted rare earth tri-n-butyl phosphate complex. The supercritical fluid extraction process has the double advantage of waste valorization without utilizing hazardous reagents, thus minimizing the negative impacts of process tailings. Key words: Rare earth elements, Supercritical fluid extraction, Recycling, Urban mining, Waste electrical and electronic equipment, Nickel metal hydride battery

Introduction Because of their unique physiochemical properties, rare earth elements (REEs) are imperative for the proper performance of many critical green technologies, including permanent magnets in wind turbines and batteries in electric and hybrid vehicles, thus they are recognized as “future materials1”. Some REEs, including neodymium (Nd), praseodymium (Pr), and dysprosium (Dy), are categorized under critical materials, i.e., essential in use and subject to supply risk2–4. According to the historic data, the annual demand for REEs has been increasing at a rate of 5.3% annually, but it is predicted that the rate could increase to 25% annually, as more green technologies for renewable power generation (in particular wind and solar) and electrified transportation will join the market to meet the target GHG emissions – i.e., to stabilize global CO2 concentrations at about 450 ppm by 20505.


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REEs are not rare, but their supply is insecure because of geologic scarcity, extraction difficulties, and dependence on sources in politically volatile countries6. Existing REE raw materials could meet many decades of increased demand, but the challenge is scaling up the supply at a rate that matches expected increases in demand. Developing new mines, including prospecting, siting, permitting and construction of refinery, can take a decade or more. Thus, to tackle their supply challenge and satisfy their demand, many countries have initiated activities to look for alternative “secondary resources”7. Examples of such sources are REE contaning postconsumer products, including lamp phosphors, permanent magnets, and hybrid electric vehicle (HEV) batteries8 as well as stocks of landfilled industrial process residues9.

Waste electrical and electronic equipment (WEEE), including permanent magnets and NiMH batteries, used at both small- and large-scale from computer hard disk drives and small tools to wind turbines and hybrid cars, contain considerable amounts of REEs. In terms of volume, Europe generates 12 million tonnes of WEEE annually and its REE recycling market is estimated to be worth at €1 billion. However, the current level of REE recycling is very limited (less than 1%), and there are only a few groups in Europe that are active in this field10.

NiMH batteries are among most efficient rechargeable batteries that are widely used in HEVs. The anode of this battery consists a mischmetal alloy of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). The active material in the anode is either of the disordered AB5(LaCePrNdNiCoMnAl), A2B7 (LaCePrNdNiCoMnAl plus Mg), or disordered AB2(VTiZr-NiCrCoMnAlSn) type, where the ABx designation refers to the ratio of the A type elements (LaCePrNd or TiZr) to that of the B type elements (VNiCrCoMnAlSn)11. The number


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of HEV NiMH batteries manufactured so far is substantial, and the global annual production is expected to grow, as Toyota plan to sell 15 million units of Prius hybrid model by 202012. Approximately 7% of a typical NiMH battery is made up of REEs, which is equal to 2 kg for a HEV battery; however, the current level of REE recycling is very limited11. Considering the great potential, it is imperative to develop a performant process for the urban mining of REEs from this class of WEEE.

There have been several studies on the recycling of REEs from postconsumer NiMH batteries utilizing

hydrometallurgical13–21(leaching

followed

by solvent

extraction

or

selective

precipitation) or pyrometallurgical technologies22,23. Table 1 presents an overview of some previous studies focused on hydrometallurgical leaching of REEs. Compared with hydrometallurgical processes, pyrometallurgical routes are less common. In 2011, Umicore and Rhodia developed a pyrometallurgical process on the basis of Umicore’s patented Ultra High Temperature (UHT) smelting technology22, and build an industrial scale pilot plant facility for the recycling of NiMH batteries. In this process, the batteries, coke, and a slag former are fed into a shaft furnace and oxygen-enriched air is injected at the bottom of the furnace. The metals are converted into an alloy of Ni-Co-Cu-Fe and a slag, which contains oxides of Ca, Al, Si, and Fe together with REEs23. The REE-containing slag is sent to Rhodia separation plant in La Rochelle (France) to recover REEs through hydrometallurgical routs. In 2013, Honda Motor Co., Ltd. and Japan Metals and Chemicals Co. Ltd., established a recycling plant to extract REEs from NiMH batteries collected from Honda hybrid vehicles; however the details of the process are not disclosed24.


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Table 1. An overview of previous studies on the leaching of REEs from spent NiMH batteries Reference

Zhang et al. (1998)13 Pietrelli et al. (2002)14 Nan et al. (2006)15 Bertuol et al. (2009)16 Rodrigues and Mansur (2010)17 Innocenzi and Veglio (2012)18 Yang et al. (2014)19 Meshram et al. (2016)20 Liu et al. (2017)21

Leachant

3 M HCl

Conditions Temperature Time (°C) (h) 95 3

S:L ratio 1:9

Leaching efficiency for Ce, La, Nd, Pr (%) 99

2 M H2SO4

20

2

1:10

90

3 M H2SO4, H2O2 2 M H2SO4

70

5

1:15

99.5

90

4

1:20

83.3, 84, 86.4, 79.7

8% H2SO4

30

1

1:10

87.3

2 M H2SO4

80

3

1:6.6

99

20% HCl

70

1.6

1:10

95.2

2 M H2SO4

75

2

1:10

89.4, -, 98.1, 95.5

4 stage leaching, H2SO4

40-80

3-5

1:101:20

94.3-98.6

Comparing hydrometallurgical with pyrometallurgical routes, the former requires lower investment costs and lower energy consumption, while the latter has the advantage of being a well-developed technology. In terms of disadvantages, pyrometallurgical routes require high investment costs for furnaces, have high energy consumption, and REEs need to be extracted from the slag using hydrometallurgical routes. Hydrometallurgical routes require several pretreatment steps to dismantle the battery and separate different components, consume large quantities of chemicals, including acids and organic solvents, and generate large volume of hazardous waste. Organic solvents consumption in solvent extraction processes can be as high as


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twice the volume of the aqueous phase25. Although, in many cases, a large portion of the organics is recycled, the organic inventory is large and there is also a loss of organics in the process.

Supercritical fluid extraction (SCFE), an emerging green separation technology, has been used for the recovery of various metals in recent years26. This is a rapidly growing field because of the desirable properties of supercritical fluids (SCFs) as solvents27. A fluid is described as supercritical, once it has been heated and compressed above its critical temperature and pressure. SCFs have a low viscosity and a high diffusivity (gas-like properties), and the ability to dissolve materials like a liquid. These properties enable them to penetrate and transport solutes from different matrices at a higher rate and more efficiently compared with that in a liquid phase. In addition, close to the critical point, small changes in pressure or temperature result in significant changes in density, enabling fine-tuning of supercritical fluid properties. Direct extraction of metal ions and organometallic compounds in SCFs heavily depends on the chemical nature of complexing (chelating) agents, because the agent is responsible for satisfying the charge neutrality and improving solvent-solute interactions, which are generally weak because of the significant polarity difference between solute and solvent. Complexation in SCFE can be achieved with the same agents employed in conventional solvent extraction, provided they have significant solubility in SCFs in both their complexed and non-complexed forms.

The most widely used SCFs is supercritical CO2 (sc-CO2), because of its moderate critical points (Tc=304.25 K (31.1 °C), Pc=7.38 MPa, ρ=471 kg/m3)26. It is also non-toxic, inexpensive, nonflammable, and inert, making it environmentally friendly and easy to recycle, as SCFE does not produce secondary waste26,27. The most advantageous property of sc-CO2 compared with


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traditional organic solvents is the excellent mass transfer that combines high diffusivity of gas with reasonably good solvation of liquid, enhancing solute extraction rates from various matrices. Furthermore, the final extraction products from SCFE are easily recoverable26,27.

Similar to any emerging technology, in addition to all potential benefits, there are also some challenges associated with SCFE, which are worth to be mentioned here. Most previous studies of SCFE are at bench scale, and there are some aspects that need further investigations to enable scale up. Furthermore, there is a lack of phase equilibrium and enthalpy data as well as fundamentally based thermodynamic models that could accurately predict the extraction behavior, which makes the process design and economic evaluation difficult. Also, similar to any high pressure process, there could be challenges with energy losses during decompression, high pressure materials selection, and difficulty of process control under supercritical conditions28.

As with any chemical process, there exist some challenges with the process scale up and transition from bench scale to pilot plant and commercial operation. There is limited knowledge of the mechanisms governing the kinetics in processes running under supercritical conditions, which limits the ability to determine the size and geometry of the reactor. Despite the challenges, SCFE has been commercialized, focusing first on extraction or production of high value products, and then to less specialized applications. As more SCFE processes are developed at bench scale, more fundamental studies are conducted, which could fill the knowledge gap in this field.

There are several chelating agents available for REE extraction, among which, four ligands including β-diketones, dithiocarbamates, organophosphorous agents, and macrocyclic compounds


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have been extensively investigated 26. Ligand chemistry and coordination geometry of REEs are major factors determining the type of agent to be utilized in SCFE. There have been several studies that investigated SCFE of REEs from solid matrices and liquid solutions, an overview of which is presented in Table 229–38.

Table 2. An overview of previous studies on SCFE of REEs Reference

Feed

Solvent/che Conditions lating agent Temperature Pressure (°C) (MPa) Lin and Oxides of La, CO2/β dike 60 15 Wai Eu, Lu tone (1994)29 Without methanol

Time (h) 0.5

Nd2O3 Gd2O3

Nd2O3 Nd2O3-ZrO2 Nd2O3-MoO3 Nd2O3-RuO2 Fox et al. Ho(NO3)3.5H2O (2005)32 Shimizu Simulated waste et al. fluorescent lamp (2005)33 (Y, Eu, La, Ce, Tb) Zhu et al. Nd2O3 (2009)34 Vincent et Nd2O3 al. (2009)35

Depending the β diketone type La: 5-14, Eu: 416, Lu: 6-20

With 5 mol% methanol Tomioka et al. 30 (1998) Tomioka et al. (2002)31

Extraction efficiency (%)

La: 54-91, Eu: 2596, Lu: 33-99

CO2/TBPHNO3*

40

12, 15

1

Nd: 51, 45 Gd: 46, 28

CO2/TBPHNO3

40

12

2

CO2/TBPHNO3 CO2/TBPHNO3

35

27.5-31

0.5

Nd: 66.3 50.87 18.69 50.26 *logKext: Ho=8.9

60

15

2

Y: 99.7 Eu: 99.8 La, Ce, Tb < 7

50 40-60 50

15-30 21 35

2.5

Nd: 96 95 Nd: 65

CO2/TBPHNO3 CO2/TBP& TTA**HNO3 Without methanol With

2


3

Nd: 67

8

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mol% methanol Wuhua et Nd2O3 al. (2010)36 Nd2O3

CO2/TBPHNO3 CO2/TRPO -HNO3*** CO2/TBPCeO2 HNO3 Baek et al. Oxides of Y, Ce, TBP(HNO3 (2016)37 Eu, Tb, Dy )1.7(H2O)0.6

50

15

2

Nd: 95

40, 50

15-40

2

Nd< 3

40-60

21

2

Ce< 1

65

34.5

1.5

Y: 99, Ce: 0.12, Eu: 99, Tb: 92.1, Dy: 98.5 Y: 92.7, Ce: 0.15, Eu: 98.7, Tb: 40, Dy: 99.9

65

34

1.5

La: 3%, Ce: 100, Pr: 99, Nd: 100

TBP(HNO3 )5.2(H2O)1.7

Sinclair et Roasted al. Bastnäsite 38 (2017) *tributyl phosphate

CO2/TBPHNO3

***thenoyl tri fluoroacetone ***Trialkylphosphine oxide

Most previous studies have focused on pure REE oxides or their mixture, and there has been no previous study on SCFE of REEs from postconsumer commercial products, such as real anode material of a HEV vehicle. To this end, here we developed an innovative process on the basis of SCFE (sc-CO2) to extract REEs from the anode materials of an end-of-life NiMH from a Toyota Prius. To systematically enhance the extraction process, two different TBP(HNO3)x(H2O)y chelating agents were synthesized and tested under various operating conditions in terms of temperature, pressure, solid to chelating agent ratio (S:CA), residence time, and agitation rate with and without cosolvent (methanol) addition, to determine optimal operating conditions. We expect the findings of this study help realize the environmentally sustainable urban mining of


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REEs from WEEE, in particular NiMH battery, while also enabling circular economy (make, use, recover) as an alternative to traditional linear economy (make, use, dispose).

Experimental Section Chemicals and materials. Tri-n-butyl phosphate (TBP, ≤ 100%), concentrated nitric acid (15.7 M, 70 w/w %), concentrated sodium hydroxide (19.4, 50 w/w %), phenolphthalein (solution 1% in alcohol), and lanthanum oxide (La2O3, 99.99%) were purchased from VWR. Cerium oxide (CeO2, 99.9%), praseodymium oxide (Pr6O11, 99.9%), and neodymium oxide (Nd2O3, 99.9%) were purchased from Sigma-Aldrich. Carbon dioxide (CO2, grade 5.0) was purchased from Linde Canada, and methanol (CH3OH, HPLC, 99.9%) was purchased from Fisher Chemical. The battery (2012 Toyota Prius model C) was donated by Carcone’s Auto Recycling and utilized as the feed. Anode samples were retrieved from the battery by removing the casing and sawing into the individual modules. Figure S1 in the Supplementary Information illustrates the dismantling process steps. All anode samples were ground using mortar and pestle prior to SCFE. The mean particle size of the anode material was determined to be 11.7 µm. Figure S2 in the Supplementary Information presents the particle size analysis (PSA) results. Safety warning about the dismantling procedure of the battery: extreme caution is advised when sawing into the modules, as this could potentially lead to short circuit of the battery, generating extreme heat and potential fire.

Preparation of TBP-HNO3 complex. TBP-HNO3 complexes were prepared by vigorously mixing TBP with two different concentrations of HNO3 (10.4 M, 50 w/w % and 15.7 M, 70 w/w %) for 5 min in a separatory funnel followed by gravity separation for 5 min. The upper


10

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organic phase was the TBP-HNO3 complex. The concentration of HNO3 in the TBP-HNO3 complex was determined by acid-base titration with 0.1 M NaOH. The water content was measured by Karl Fischer titration using a C20 instrument (Mettler Toledo International Inc.). The density of the TBP-HNO3 complex was calculated by weighing a known sample volume in triplicate runs with a TLE303E balance (Mettler Toledo International Inc.). Combining density, water content, and acid concentration data, the TBP-HNO3 complex was fully characterized.

Experimental design. The process flow diagram is presented in Figure S3 in the Supporting Information. The 100-mL high pressure rated reactor, magnetic drive mixer, reactor controller, constant flow dual piston pump, and solvent pump were manufactured by Supercritical Fluid Technology Inc., USA.

SCFE process. A known amount of anode material or synthetic anode along with TBP-HNO3 was loaded into the reactor chamber. After closing the reactor head, liquid CO2 was pumped into the system with both restrictor valve and static/dynamic valve closed until the desired pressure was reached. Heating was provided by and maintained through the electric heating jacket surrounding the reactor chamber until the desired temperature was reached. Soluble rare earth complex formed during the reaction was collected in the EPA vial by opening both restrictor valve and static/dynamic valve upon reaching the designated extraction time. Reproducibility tests (three independent experiments) showed that the experimentally measured data are accurate to within ±5%.


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In the experiments where methanol was added as the cosolvent, the amount was calculated on the basis of total number of moles of CO2 in the reactor. Considering methanol has higher critical temperature compared with liquid CO2 (240 °C vs. 31.1 °C) and on the basis of previous studies in the literature, we chose 2 mol% concentration in this study. The formula to calculate volume of methanol is as follow: = 2% ×

×

(1)

The density of supercritical CO2 was obtained from National Institute of Standards and Technology (NIST) database: 871.22 kg/m3 at 35 °C and 20.7 MPa, and 934.19 kg/m3 at 35 °C and 31.0 MPa39.

Characterization of the test specimen. Aqua Regia digestion and ICP-OES characterization To determine the concentration of REEs in the unprocessed and extracted samples, aqua regia digestion was performed at 200 °C, using an Ethos EZ Microwave Digestion System, followed by inductively coupled plasma optical emission spectroscopy (ICP-OES) (PerkinElmer Optima 8000). Four independent experiments were conducted to determine the average concentration of REEs in the unprocessed samples. Three independent dilutions were conducted to determine the average concentration of REEs in the extracted samples. The extraction efficiency (E) is defined as follows: =

% ! "#$%% &"! "#$ % ! "#$

× 100%

(2)

Morphological, mineralogical and particle size analysis


12

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Morphological characterization of the unprocessed and extracted samples was performed using scanning electron microscopy energy dispersive spectroscopy (SEM-EDS, Hitachi SU8230). The mineralogical characterization of the sample was performed using X-ray diffraction (XRD, Philips PW1830). The particle size of the anode sample was determined using a laser particle size analyzer (Horiba Partica LA-950).

Results and Discussion Characterization of the anode material. The chemical composition of the anode material was characterized by ICP-OES after aqua regia digestion of the samples. Four REEs were identified, lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). The total REE weight percent in the sample was approximately 30 wt% (Fig. 1a), consistent with the literature data11. The major impurities were nickel (Ni), manganese (Mn), cobalt (Co), and aluminum (Al) (Fig. 1a). The crystal structure of the anode material was analyzed by X-ray diffraction (XRD) and (Ce0.47La0.34Nd0.14Pr0.05)Ni3.56Co0.75Al0.29Mn0.4, a mischmetal-nickel alloy was detected as the only phase (Fig. 1b). The XRD result confirms the anode material is in its AB5 form, where A is La, Nd, Ce, and Nd, and B is Ni, Co, Al and Mn. The surface morphology and elemental mapping of the anode material was characterized using SEM (Fig. 1c). Backscattered secondary electron (BSE) image and EDS elemental mapping of an anode particle cross-section are presented in Fig. 1d-m, which are consistent with the ICP-OES results, also consistent with the literature values for AB5 type NiMH battery40.

Characterization of TBP-HNO3 complexes. Two TBP-HNO3 complexes were characterized


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using the method described in the experimental section. The one with 70% w/w nitric acid was identified to be TBP(HNO3)1.745(H2O)0.52, which has a slightly higher nitric acid content than the literature case36,37 because of the differences in the synthesis method.

Figure 1. NiMH battery anode material characterization results. (a) Aqua regia digestion - ICPOES results indicating REEs composition. (b) X-ray diffractogram. (c) SEM image of the anode particle; bright regions indicate the high atomic number REEs. (d)-(m) Backscattered secondary electron (BSE) image and EDS elemental mapping of the anode particle cross-section.

Establishing baseline operating conditions. To establish the baseline conditions, we focused on the existing literature reported in Table 2. The main parameters that affect the extraction efficiency are temperature, pressure, residence time, type of the chelating agent, sample to chelating agent ratio (S:CA), and the presence of cosolvent (methanol). The baseline conditions


14

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were established at temperature of 35 °C, pressure of 20.7 MPa, residence time of 1 h, S:CA of 1:5 (w/v, 1 g to 5 mL), agitation rate of 750 rpm, with TBP(HNO3)1.171(H2O)0.384 chelating agent and no methanol addition.

Determining optimum operating conditions. To determine the optimum operating conditions that yield maximum REE extraction efficiency, a systematic investigation was performed. First, using the baseline conditions, we obtained 49% La, 45% Ce, 58% Pr, and 45%, Nd extraction efficiency, as presented in Table 3 (run 1). Second, the effect of each individual process parameter on the REE extraction efficiency was investigated by only changing that single parameter compared with the baseline conditions (Table 3, run 2-7 and 9), where in run 9, 2 mol% methanol was added as a cosolvent to enhance the extraction process. On the basis of the extraction results in Table 3, increasing all the parameters had a positive effect on the extraction efficiency with the exception of S:CA ratio with negligible effect (run 3) and temperature with negative effect (run 2). Third, all individual parameters with positive effects on REE extraction were combined to examine the extraction efficiency with an overall enhanced extraction process with and without methanol addition (2 mol% ) (Table 2, run 8 and 10). The overall enhanced run with 2 mol% methanol addition resulted in the highest extraction efficiency (La: 79%, Ce: 77%, Pr: 83%, Nd: 69%) (Table 3, run 10). Fourth, to prevent oversaturating the system and further enhancing the extraction efficiency of all REEs, we decreased the anode sample size of the overall enhanced run by half (from 1 g to 0.5 g), which was identified as the optimized run (Table 3, run 11). Lastly, a mixture of pure rare earth oxides (REOs) were processed under the optimized conditions with and without 2 mol% methanol addition to assess the efficacy of the developed SCFE process for a synthetic anode material


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(Table 2, run 12 and 13). The results of the synthetic anode material will be discussed in the next section of the paper.

Table 3. An overview of the experimental runs with corresponding processing parameters and extraction efficiencies for La, Ce, Pr, and Nd Run

Run Name

#

Temperature

Pressure

time

S:CA

Agitation

(°C)

(MPa)

(h)

(g:mL)

Rate

Chelating agent

Methanol

Extraction

Addition

Efficiency for

(rpm)

La, Ce, Pr, Nd (%)

1

Baseline

35

20.7

1

1:5

750

TBP(HNO3)1.171

No

49, 45, 58, 45

No

48, 45, 56, 45

No

51, 48, 61, 48

No

52, 49, 62, 49

No

53, 50, 62, 49

No

53, 50, 63, 50

No

58, 57, 68, 58

No

56, 54, 65, 55

(H2O)0.384 2

Temperature

55

20.7

1

1:5

750

increase 3

TBP(HNO3)1.171 (H2O)0.384

S:CA increase

35

20.7

1

1:10

750

TBP(HNO3)1.171 (H2O)0.384

4

Agitation

35

20.7

1

1:5

1500

increase 5

(H2O)0.384

Residence

35

20.7

2

1:5

750

time increase 6

Pressure

Stock

35

31.0

1

1:5

750

Overall

TBP(HNO3)1.171 (H2O)0.384

acid

35

20.7

1

1:5

750

complex 8

TBP(HNO3)1.171 (H2O)0.384

increase 7

TBP(HNO3)1.171

TBP(HNO3)1.745 (H2O)0.52

35

31.0

2

1:5

1500

TBP(HNO3)1.745

enhanced


16

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Run


Run Name

#

Temperature

Pressure

time

S:CA

Agitation

(°C)

(MPa)

(h)

(g:mL)

Rate

Chelating agent

Methanol

Extraction

Addition

Efficiency for

(rpm)

La, Ce, Pr, Nd (%) (H2O)0.52

9

Baseline

35

20.7

1

1:5

750

methanol

10

Overall

TBP(HNO3)1.171

Yes

67, 65, 73, 64

Yes

79, 77, 83, 69

Yes

86, 86, 88, 90

No

30, 73, 61, 73

Yes

86, 61, 78, 90

(H2O)0.384

35

31.0

2

1:5

1500

enhanced

TBP(HNO3)1.745 (H2O)0.52

methanol 11

Optimized

35

31.0

2

1:10

1500

TBP(HNO3)1.745 (H2O)0.52

12

Synthetic

35

31.0

2

1:10

1500

anode 13

Synthetic

TBP(HNO3)1.745 (H2O)0.52

35

31.0

2

1:10

1500

anode

TBP(HNO3)1.745 (H2O)0.52

methanol

Effect of process parameter on REE extraction efficiency. The effect of increasing each operating parameter on REE extraction efficiency was investigated with respect to the baseline conditions. As can be seen in Figure 2 (run 2), increasing temperature resulted in lower REE extraction efficiency, whereas increasing pressure increased the REE leaching efficiency (Fig. 2, run 6). The relationship between the solubility of a solute in a SCF and the density is described by Chrastil model41 (Eq. 3): /

)*+ = ,)*- + + 1 0

(3)


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where, S (g/L) is the solubility of a solute in supercritical fluid; ρ (g/L) is the density of the supercritical fluid; k is the association number that describes the number of solvent molecules associated with the complex; T is the temperature in K; A and B are empirical parameters.

Because increasing temperature at constant pressure decreases the density ρ and A/T term in Eq. 1, it decreases the solubility of the solute, thus decreasing the extraction efficiency41. In the contrary, increasing pressure at constant temperature increases the density as more CO2 is pumped into the system, thus it increases the solubility and therefore the extraction efficiency41,42. Furthermore, increasing pressure improves the penetration of sc-CO2 into deeper pores of the anode material, which ultimately resulted in higher degree of complexation43.

Increasing the residence time from 1 h to 2 h increased the leaching efficiency (Fig. 2, run 5) because longer residence time allows for the reaction to proceed towards completion, leading to higher REE extraction efficiency. Previous studies (Table 2) have tested residence time of between 30 min and 3h, where 2h has shown to be sufficient. Increasing the agitation rate increased the leaching efficiency because it increased the turbulency of the system and the surface contact between the chelating agent and the sample (Fig. 2, run 4). Chelating agent with higher acid content, i.e., TBP(HNO3)1.745(H2O)0.52, compared with TBP(HNO3)1.171(H2O)0.384 resulted in higher extraction efficiency, because it improved the formation of REE-TBP nitrate complex, as more HNO3 was available (Fig. 2, run 7). The proposed mechanism will be discussed later in the paper. It should be mentioned that the molar acid content in the TBP(HNO3)1.745(H2O)0.52 adduct is only 1.745 times higher than TBP. Literature data indicate


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that higher acid content in the chelating agent does not necessarily produce the highest extraction efficiency36,37. The amount of the chelating agent that is dissolvable in sc-CO2 is capped under certain conditions, and once this limit is reached, further increase in volume of the chelating agent does not improve the REE extraction efficiency, as demonstrated by run 3 in Fig. 2.

Next, the effect of adding a cosolvent on the extraction efficiency was investigated. Here we chose 2 mol% methanol as the cosolvent and observed an increase in the extraction efficiency (Fig. 2, run 9-11). The main reason behind the addition of methanol is that sc-CO2 is non-polar, thus it has low solvating power for polar compounds. The dissolution of chemical compounds in SCFs depends on the relative strength of the solute/solute and solute/solvent molecular interactions. Addition of a small amount of a polar cosolvent, such as methanol, improves dipolarity/polarizability and hydrogen bond acceptor basicity of the solvent, thus significantly improving the solvation power of sc-CO2/cosolvent mixtures, enhancing the extraction efficiency44. Previous studies also reported an increase in the extraction efficiency with the addition of methanol29,35.


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Figure 2. Extraction efficiency for lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd) under various operating conditions. Error bars represent the standard error of the mean for three replicates. Run 1-baseline, run 2-temperature increase, run 3-S:CA increase, run 4-agitation increase, run 5-residence time increase, run 6-pressure increase, run 7-stock acid complex, run 8-overall enhanced, run 9-baseline with methanol, run 10-overall enhanced with methanol, run 11-optimized.

Comparison between REE extraction from synthetic and actual NiMH anode material. Here, we also investigated the extraction efficiency of REEs from a synthetic mixture of oxides of cerium, lanthanum, neodymium, and praseodymium, in the same ratio as that in the actual anode material (0.5 g sample size). To calculate the extraction efficiency, REE wt% in unprocessed synthetic anode was determined to be 51.3% La, 15.3% Ce, 9.5% Pr, and 5.9% Nd. The normalized REE wt% in the synthetic anode material were 62.6% La, 18.6% Ce, 11.6% Pr, and 7.2% Nd, which are very close to those in the actual anode material (60.4% La, 24.5% Ce,


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8.1% Pr, and 7.0 % Nd). Two separate experimental runs were performed: one without and another with 2 mol% methanol addition (Fig. 3, run 12 and 13, respectively). Extraction results from the synthetic anode were compared with those from the optimized run of the actual anode material (Fig. 3, run 11). The synthetic anode with 2 mol% methanol reached the same level of extractions for La and Nd and lower for Ce and Pr compared with the actual anode material. Furthermore, in contrary to all cases studied here, the extraction efficiency of Ce from the synthetic anode with methanol was lower than that without methanol. This discrepancy could be attributed to the +4 oxidation state of Ce versus +3 for La and Nd. At this point, the only conclusion can be drawn is that methanol has a negative effect on the extraction of cerium (IV) oxide. Future studies involving X-ray absorption spectroscopy (XAS) and small- angle X-ray scattering (SAXS) could explain the observed inconsistency.

Figure 3. REE extraction from synthetic anode using sc-CO2 with and without methanol addition


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in comparison with that of actual NiMH anode material in the optimized run. Error bars represent the standard error of the mean for three replicates. Run 11-actual anode material optimized conditions, run 12-synthetic anode without methanol, run 13-synthetic anode with methanol.

Characterization of anode material before and after SCFE. Elemental mapping identifies large amounts of REEs on the surface of the anode material (Fig. 1, d-m). After extraction, a significant evolution in the surface morphology was observed using SEM (Fig. 4, a-d). The unprocessed anode sample displayed powder-like structure with a mean particle size of 11 µm (Figure S2). The unprocessed anode material consists agglomerated particles with smooth surfaces, whereas the extracted anode particles are highly porous. Channel-like structures appear on the surface of the anode material after extraction with pores smaller than 1 µm in diameter distributed throughout the surface (Fig. 4, c-d). This suggests that REEs were able to extract out of the anode matrix.

Figure 4. (a) SEM image of unprocessed anode sample; (b) high magnification SEM image of unprocessed anode sample; (c) SEM image of the anode sample after extrcation; (d) high magnification SEM image of the anode sample after extrcation.


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Exploring the mechanism of REE extraction using sc-CO2. Informed by earlier works and by the extraction results above, we propose a molecular mechanism for the recovery of REEs from spent NiMH battery anode material using TBP-HNO3 as an extracting agent in sc-CO2. A number of recent studies have investigated the process chemistry and structural coordination of trivalent and tetravalent f-element ions in conventional solvent extractions using x-ray absorption spectroscopy (XAS)45–47. Braatz et al.45, reported an 8 oxygen (O) coordinate environment for Ln(III)-TBP complexes (Ln = Dy3+, Lu3+) when extracted by 1 M TBP from 2 M HNO3. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data indicate 3 bidentate nitrate anions and 2 TBP molecules coordinated to the central Ln(III) ion giving rise to 8 O atom coordination, as determined by Ln-O bonding data. La3+ results were not included because of extremely low concentrations upon extraction and were not detectable by the EXAFS method. This is in good agreement with the 9 O coordinate complex of Ln(III)-TBP solvates consisting of 6 O atoms from three bidentate inner-sphere nitrate anions and 3 O atoms from three TBP molecules coordinated to an Ln3+ ion45, originally proposed by Healy and McKay48,49 and further supported in later studies50,51. Ellis and Antonio47 focused on an advanced feature of solvent extraction, mainly aggregation phenomena and self-assembly of reverse micelles upon lanthanide extraction. Trivalent cerium was recovered from nitric acid solutions using a malonamide extractant in ndodecane, and insightful macroscopic phase behaviours were reported: complexes consisting of a lanthanide ion coordinated to extractant-acid molecules aggregate into higher-order structures of reverse micellar assemblies upon solvent extraction. Antonio et al.,46 made similar observations for the tetravalent cerium species in the tri-n-butyl phosphate – n-dodecane solvent extraction system, including coagulation of reverse micelles consisting of tetranuclear Ce(IV)-TBP solvates


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with (Ce4O4)8+ cores. In the present study, we propose a hypothetical mechanism for REEs extraction using sc-CO2. Future studies are underway that involves XAS and SAXS to elucidate the chemical mechanism of the present extraction process. Based on our hypothetical mechanism, bidentate nitrate anions from the TBP-HNO3 extractant chelate to the Ln3+ cation, forming neutral lanthanide nitrates Ln(NO3)3. Hydrophobic effects from aliphatic tails and dipole-dipole interactions between phosphate head-groups of TBP molecules promote the formation of reverse micelles, whereby, upon contact with the sample matrix, hydrophilic solutes, such as lanthanide salts, assimilate into their polar cores. Such coordination sites comply with the lanthanide metal geometry, and as a result, the cation is centered in an 9 O coordinate environment comprised of 3 bidentate nitrate anions and 3 TBP molecules (Figure 5a). This agrees with the originally proposed TBP-trisolvate complexes of Ln(III) ions, which assumes all lanthanides have a coordination number of 9, suggesting that three TBP molecules are required to complete the coordination sphere45. The presence of hydrated species in the organic phase could also interfere; however, 3 TBP molecules would significantly increase the solubility of the metal complex in non-polar sc-CO2. Shimizu et al.,33 considered a similar coordination chemistry for SCFE of lanthanide oxides with a TBP-HNO3-H2O system: 3 nitrate anions for charge neutralization, but the number of TBP molecules varies, as shown in Eqs. (4) and (5), where n is a constant and Ln represents the lanthanide elements.

Ln2O3+6HNO3 2 Ln3+ + 6 NO3- + 3 H2O

(4)

Ln3+ + 3 NO3- + n TBP Ln(NO3)3(TBP)n

(5)


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The following step involves the synergy between the reverse micelles that, upon incorporation of lanthanide ions and/or other hydrophilic species, attract and adhere to one another via hydrophobic interactions of aliphatic functionalities, as depicted in Figure 5b47. As this interaction exceeds a certain threshold, micellar condensation occurs representing the final step: a random aggregation of reverse micelles is possible with significant lipophilic interdigitation among the aliphatic chains, or, alternatively, there is formation of thermodynamically favorable micellar chains that are a result of stacking individual aliphatic shells into macroscopic assemblies and producing a more balanced higher-order structure (Figure 5c)47.


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Figure 5. Spherical reverse micellar mechanism proposed for REE extraction from spent NiMH battery anode material using sc-CO2 including (a) 9-oxygen coordinate chemistry of the lanthanide cation, (b) lipophilic interactions between aliphatic chains between micelles, and (c) formation of super aggregates as either stable chains from micellar stacking or randomly adhered spheres via interdigitation.

Conclusions We developed an environmental friendly supercritical fluid extraction process to extract REEs from postconsumer NiMH batteries. The effect of seven operating parameters, namely temperature, pressure, residence time, sample to chelating agent ratio, agitation rate, complex chemistry, and cosolvent (methanol) addition, on the REE extraction efficiency was investigated, and 35 °C, 31 MPa, 2 h residence time, S:CA ratio of 1:10, TBP(HNO3)1.745(H2O)0.52 chelating agent with 2 mol% methanol addition were determined to be the optimum conditions, resulting in 86% La, 86% Ce, 88% Pr, and 90% Nd recovery. Potential options to further increase the extraction efficiency include increasing pressure and/or residence time, decreasing S:CA ratio, and increasing methanol concentration. Furthermore, a synthetic anode material was prepared and processed utilizing pure REOs, which resulted in similar or lower REE extraction efficiency compared with the actual anode material, confirming the robustness of the developed process for the actual anode material. Moreover, on the basis of previous studies, here we proposed a mechanism for the extraction of REEs in sc-CO2, considering a trivalent REE state bonded with three TBP molecules and three nitrate ions model for the extracted REE TBP complex. The results of this study ascertain the feasibility of recovering technologically critical REEs from widely available end-of-life HEV NiMH batteries. From an industrial point of view, SCFE


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mainly requires a high-pressure reactor for extraction and a depressurization tank for the separation of the products from recyclable CO2. Because this process requires low content of chelating agent, it generates minimal hazardous waste. Furthermore, energy consumption of this process is significantly lower than that of a pyrometallurgical process.

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Acknowledgements The authors would like to acknowledge the financial support provided by Natural Sciences and Engineering Research Council of Canada (NSERC) (No. 498382). We thank Mr. Jason Tam for help with SEM; Dr. Reiden Acosta for help with XRD and PSA. Access to the electron microscopy facility in the Canada Foundation for Innovation (CFI) funded Ontario Centre for the Characterization of Advanced Materials is acknowledged.

Author contributions statement G.A. conceived and supervised the research. Y.Y. and N.F.F. designed and performed the experiments. Y.Y. performed XRD and PSA. Y.Y. and N.F.F. performed ICP-OES. All authors contributed to writing and revising the manuscript.

Supporting information Is provided as an attachment. Additional information Authors have no competing interests, or other interests that might be perceived to influence the results and/or discussion reported in this paper


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Abstract Graphic

Synopsis A robust environmentally friendly process on the basis of supercritical fluid extraction was developed for the urban mining of REEs from postconsumer NiMH batteries. This innovative process offers the recovery of REEs from waste electrical and electronic equipment (WEEE).


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Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal

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