Alkali baking and solvometallurgical leaching of NdFeB magnets

Journal Pre-proof Alkali baking and solvometallurgical leaching of NdFeB magnets

Mehmet Ali Recai Önal, Sofía Riaño, Koen Binnemans PII:

S0304-386X(19)30581-X

DOI:

https://doi.org/10.1016/j.hydromet.2019.105213

Reference:

HYDROM 105213

To appear in:

Hydrometallurgy

Received date:

28 June 2019

Revised date:

22 August 2019

Accepted date:

15 November 2019

Please cite this article as: M.A.R. Önal, S. Riaño and K. Binnemans, Alkali baking and solvometallurgical leaching of NdFeB magnets, Hydrometallurgy(2019), https://doi.org/ 10.1016/j.hydromet.2019.105213

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© 2019 Published by Elsevier.

Journal Pre-proof

Alkali baking and solvometallurgical leaching of NdFeB magnets Mehmet Ali Recai Önalⱡ, Sofía Riañoⱡ, Koen Binnemansⱡ*

ⱡ

KU Leuven, Department of Chemistry, Celestijnenlaan 200F box 2404, B-3001 Leuven,

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Belgium

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* Corresponding author. Tel.: +32 16327446

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E-mail address: [email protected]

Abstract End-of-life NdFeB magnets are an important secondary source of rare-earth elements (REEs) and cobalt. Recycling of these magnets can also mitigate the supply problems of its constituent 1

Journal Pre-proof critical REEs (mainly neodymium and dysprosium). The recycling of bonded NdFeB magnets has received much less attention than that of sintered NdFeB magnets. In this study, a novel flow sheet is presented for recycling of bonded NdFeB magnets that is applicable to sintered magnets as well. Demagnetized magnet powder was mixed with 25 or 40 wt./vol.% NaOH solution and baked at 150 to 200 °C for 30 to 540 min. In this way, REE metals were transformed into their corresponding hydroxides, whereas iron metal formed NaFeO 2 . By washing the reaction mixture

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with water, 96.5% of Na was recovered as NaOH and Na2 CO3 , whereas 90.3% of B was

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recovered as borax. The calcine containing REE hydroxides and iron oxide was then leached at

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60 or 90 °C with 20 vol.% Versatic Acid 10 diluted in an aliphatic diluent. More than 95% of the

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REEs were dissolved, with less than 1% co-dissolution of iron and less than 10% co-dissolution of cobalt. Precipitation stripping with an oxalic acid solution quantitatively regenerated the

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organic solvent with virtually the same composition of fresh solvent. After calcination of the

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oxalate precipitate, a mixed rare-earth oxide with 98.4 wt.% purity was produced. The waste oxalic acid solution (pH ≤1) containing co-dissolved sodium and minor amounts of iron and

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cobalt could be used as a leaching agent for Co in the maghemite-dominated residue.

Keywords: Alkali baking; NdFeB magnets; rare earths; recycling; solvometallurgy; Versatic Acid 10.

1. Introduction Neodymium-iron-boron (NdFeB) magnets currently dominate the rare-earth permanent magnet market due to their superior energy density and lower price compared to samarium-cobalt 2

Journal Pre-proof (SmCo) magnets (Gutfleisch et al., 2011) All NdFeB magnets contain neodymium as the main rare-earth element (REE), iron as the main transition metal and boron as the non-metal forming three equilibrium phases: the hard magnetic matrix phase (Nd2 Fe14 B), the eutectic Nd-rich grain boundary phase and the minor by-product boride phase (Nd1 Fe4 B4 ) (Rademaker et al., 2013). Depending on the manufacturer, availability and price of source materials and product specifications other REEs such as Pr, Dy, etc. and other (non-)metals such as Co, Al, Si, etc. can

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be added in minor quantities (Yang et al., 2017). Several compositions were reported in the

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literature, but the total amount of REEs in a typical sintered magnet usually varies between

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3035 wt.% with about 1 wt.% boron and the remainder being largely iron and other minor

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additions (Yang et al., 2017). NdFeB magnets can be considered as a secondary source for cobalt, another critical metal that is needed in Li-ion batteries for the rapidly growing electric

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vehicle industry (Golmohammadzadeh et al., 2017; Van Roosendael et al., 2019). The addition

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of cobalt is to increase the Curie temperature of such magnets and the cobalt content can vary from close to 0 wt.% to as high as ca. 5 wt.% (Sagawa et al., 1984; Yang et al., 2017).

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The NdFeB magnet family has two sub-classes, namely, sintered and bonded NdFeB magnets.

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Although sintered magnets are more commonly preferred because of their higher energy product, they suffer from easy oxidation and corrosion through their Nd-rich grain boundary phase. These magnets are also prone to production and machining limitations due to their high brittleness causing swarf/sludge formation and product losses. Although the mineralogy of the magnetic powder is the same, bonded magnets are nanocrystalline with an additional polymeric material as a binder (Suprapedi et al., 2016). Because of this dilution factor, they have comparatively poorer magnetic properties. However, this is compensated by other benefits such as easier machining, shaping and handling, higher production rates, lower mass, better corrosion and oxidation

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Journal Pre-proof resistance (Suprapedi et al., 2016). Hence, there is a growing trend towards bonded magnets to replace sintered magnets wherever possible (Zhang and Xiong, 2009). Several polymeric materials have been investigated as a binder in bonded magnets, but the most commonly used ones are epoxy and polyamide (PA). Epoxy resins offer a cheaper production route (i.e. compression molding) compared to PA (i.e. injection molding). If the magnet has to perform at high temperatures or in a corrosive environment,

high-performance polymers such as

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polyphenylsulfide (PPS) and polyphtalamide (PPA) can be used instead (Ferraris et al., 2010).

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Recycling of NdFeB magnets for critical REE recovery has attracted a lot of attention over the

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last decade. Several innovative flow sheets were proposed and comparatively evaluated

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elsewhere (Binnemans et al., 2013; Firdaus et al., 2016; Sethurajan et al., 2019; Yang et al., 2017). Those works mainly centered on pyrometallurgical and hydrometallurgical treatments or

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their combinations. However, the literature on this topic is largely focused on recycling of

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sintered NdFeB magnets. Although hydrometallurgical treatments are more favorable due to lower energy consumption, they are criticized due to high chemical consumption and waste

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water generation (Önal et al., 2015; Venkatesan et al., 2018). In the hydrometallurgy literature, a

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limited number of studies exist on alkali treatment of NdFeB magnets, mainly due to the nonamphoteric nature of REEs. Lee et al. (2013) studied the leaching of sintered magnet scrap in NaOH solution (Lee et al., 2013). It resulted in negligible dissolution of REEs and iron, but more than half of boron was selectively leached. Rabatho et al. (2012) studied ammonium sulfate leaching with similar results, but on a magnet sludge containing REEs as hydroxides and iron as hematite (α-Fe2 O3 ) (Rabatho et al., 2012). Yoon et al. (2015) studied the transformation of a sintered magnet into rare-earth hydroxides and metallic iron via mechanochemical grinding with NaOH and small H2 O additions (Yoon et al., 2015). However, they proceeded with oxidative

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Journal Pre-proof roasting to obtain selectivity in the subsequent acetic acid leaching step. Chung et al. (2015) studied a novel process where a sintered magnet was treated with a diluted NaOH solution at 100 °C in a closed PTFE cell at ambient pressure (Chung et al., 2015). This alkali treatment transformed neodymium into Nd(OH)3 via the reaction shown in Eq. (1): 2 Nd(s) + 6 H2 O(l)  2 Nd(OH)3(s) + 3 H2(g)

(1)

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Although there is no involvement of NaOH, it acts as a catalyst. Meanwhile, iron was portioned

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between α-Fe and magnetite (Fe3 O4 ), where higher NaOH addition favored formation of the

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magnetite. Subsequent acid leaching with 0.5 M H2 SO4 for 5 min. resulted in poor selectivity (ca. 25% Fe co-dissolution) as both forms are easily soluble in mineral acids.

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In this paper, an innovative flow sheet on epoxy-based bonded NdFeB magnets is presented that

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can be adopted to sintered NdFeB magnets as well. The process is based on three major steps: (i)

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low-temperature alkali baking of the magnet powder with aqueous NaOH addition, (ii) lowtemperature solvometallurgical leaching of washed and dried calcine by diluted Versatic Acid 10

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and (iii) precipitation stripping of REEs by contacting the organic leachate with an oxalic acid

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solution. The flowsheet is a combination of hydrometallurgical and solvometallurgical process steps (Binnemans and Jones, 2017).

2. Materials and methods 2.1. Chemicals and materials

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Journal Pre-proof Sodium hydroxide pearls (analytical reagent grade) and absolute ethanol (≥ 99.5%) were purchased from Fisher Scientific (Loughborough, United Kingdom). Oxalic acid (anhydrous, ≥ 99.0%) was obtained from Sigma-Aldrich (Diegem, Belgium). Versatic Acid 10 was obtained from Resolution Europe B.V. (Rotterdam, the Netherlands). The aliphatic diluent Shell GTL Fluid G70 was obtained from Shell Chemicals Europe (Rotterdam, the Netherlands). The silicone

solution

in

isopropanol was

purchased

from SERVA

GmbH

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(Heidelberg, Germany).

Electrophoresis

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Varying quantities of 33 different epoxy-bonded NdFeB magnets with different compositions

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were purchased from or kindly supplied by Goudsmit Magnetics (Waalre, the Netherlands), BEC

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GmbH (Moers, Germany), Kolektor GmbH (Essen, Germany), Magdev LTD (Swindon, United Kingdom), University of Birmingham (Birmingham, United Kingdom), Ningbo Lihe Magnetic

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Industries (Ningbo, China), Aichi Steel (Aichi-ken, Japan), SM Magnetics (Pelham, AL, USA),

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Bunting Magnetics Co. (Newton, KS, USA) and AliExpress AllMagnets Store. As some of these magnets were still magnetic, a demagnetization step was required. Regardless of their magnetic

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strength, a batch of ca. 200 g was prepared from all of the collected samples in different

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quantities and were demagnetized together under inert atmosphere by heating from room temperature to 450 °C at a heating rate of 10 °C min-1 . Lower temperatures resulted in incomplete demagnetization of some samples. The magnet batch was cooled back to room temperature in the furnace. The whole demagnetized magnet batch was then ground with a disc mill (Pulverisette 13, Fritsch, Germany) and sieved to get a particle size of < 250 µm. The oversized particles were further ground in a planetary ball mill (Pulverisette Premium 7, Fritsch, Germany), sieved and thoroughly mixed with undersized portion to form a final finely-sized and demagnetized magnet batch.

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Journal Pre-proof 2.2. Materials characterization Semi-quantitative chemical characterization of solid samples was performed using a Bruker S8 Tiger 4K wavelength dispersive X-ray fluorescence (WD-XRF) spectrometer. The fully quantitative composition of the solid samples was determined by an Optima 8300 inductively coupled plasma optical emission spectrometer (ICP-OES) after digestion of 50 mg sample in hot aqua regia and dilution accordingly. Mineralogical analysis of the solid samples was performed

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by a D2 Phaser X-ray diffraction (XRD) spectrometer with Cu-Kα X-ray radiation (30 kV; 10

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mA). The step size increment was 0.02 (2) with 0.06 s/step. The raw data were processed both

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with the X’pert HighScore Plus PANalytical and EVA software with the ICDD database.

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Chemical characterization of organic Versatic Acid 10 leachates was performed on a Bruker S2

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Picofox total reflection X-ray fluorescence (TXRF) spectroscopy with a molybdenum X-ray source. The raw data were analyzed via the Bruker Spectra Picofox v. 7.5.3.0 software. For the

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analysis, an aliquot of the leachate was diluted 100 times with absolute ethanol. Holmium (50

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ppm) was selected as standard for neodymium, praseodymium and high iron concentrations. Scandium (1 ppm) was selected as standard for cobalt and low iron concentrations. Europium (5

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ppm) was selected as an additional standard for praseodymium and low neodymium concentrations. After mixing, a droplet of 4 µL was transferred on a quartz carrier that was previously treated with 30 µL silicone-isopropanol solution to prevent spreading of the droplet. The quartz glass carriers were finally placed in a furnace for 30 min at 60 °C prior to the measurements. Each measurement was set for 1000 s. Each leachate was analyzed in triplicate and the average and standard deviations were calculated for each measured metal. The uncertainty in measurements were within the analytical error range of ± 10%.

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Journal Pre-proof The depleted and fresh Versatic Acid 10 solutions were comparatively analyzed by a Bruker Vertex 70 Fourier Transform Infrared-Attenuated Total Reflectance (ATR-FTIR) operating at room temperature in a wavelength range of 400-4500 cm-1 . Comparative 1 H NMR spectra of both samples were recorded with a Bruker Avance 300 spectrometer operating at 300 MHz. A small amount of the samples was separately dissolved in deuterated DMSO (DMSO-d6 ) prior to analysis. Lastly, a TA Instruments T500 thermo-gravimetric analysis (TGA) was used to

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comparatively investigate the residue analysis from fresh, loaded and depleted Versatic Acid 10

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solutions. The heating rate was 10 °C min-1 under air atmosphere and the temperature ranged

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from room temperature to 400 °C.

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2.3. Experimental set-up and procedure

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In alkali baking experiments, 0.50 g of the magnet powder was mixed with a 25 or 40 wt./vol.%

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NaOH solution in a NaOH(aq)/Magnet ratio of 1020 mL g-1 . In all experiments, stainless steel crucibles (50 mL) were used. To eliminate any contamination, they were cleaned by sand in

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between experiments. For alkali baking, a drying furnace (Hydrion Scientific Instruments,

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China) was used at 150 or 200 °C for 30540 min. under ambient atmosphere. At the end of the baking experiment, the crucible was removed from the furnace and quickly cooled to room temperature in air. The reddish, swollen and hydrophilic calcine was then washed with demineralized water. The crucible quickly heated due to an exothermic reaction while the sticky calcine formed a very fluent sludge. The sludge was then transferred to a 50 mL centrifuge tube. After decantation, the wash solution was collected in a polyethylene bottle. The washed calcine in the centrifuge tube and the crucible after washing were dried for 48 h at 80 °C in the same drying furnace. The mass of the crucible and centrifuge tube was measured in each step to track the mass changes and to calculate the final mass of the dried calcine. The collected wash solution 8

Journal Pre-proof was dried at 80 °C in a muffle furnace under ambient atmosphere and the produced crystallites were collected and analyzed by WD-XRF and XRD. For the Versatic Acid 10 leaching experiments, a stock of 20 vol.% Versatic Acid 10 solution was prepared by diluting concentrated Versatic Acid 10 in the aliphatic diluent Shellsol G70. 0.25 g of the dried calcine was added to 5 mL Versatic Acid 10 solution in a 10 mL glass vial giving 20 mL g-1 liquid-to-solid (L/S) ratio. The vials were then placed on a shaker (Turbo

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Thermo Shakers TMS-200, China) preheated to 60 or 90 °C for 1 h at 2000 rpm. After leaching,

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solid-liquid separation was performed either via filtration with a 0.45 µm syringe filter (for

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TXRF analysis of the leachate) or decantation as explained for the washing step (for XRD, WD-

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XRF and ICP-OES analysis of the solid output). Leaching efficiencies of Nd, Pr, Co and Fe were calculated based on Eq. (2). At optimum conditions, the leaching efficiencies of the other

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elements were calculated based on Eq. (3). Here, input and output solids refer to magnet powder

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and dried calcine, respectively, for calculating Na and B leaching efficiencies during water

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residue, respectively.

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washing step. For the Versatic Acid 10 leaching step, they refer to dried calcine and leach

(2)

(3)

After determination of the optimum conditions for the alkali baking and Versatic Acid 10 leaching steps, a stock leachate was prepared by repeating the experiments six more times. The standard deviation of the leaching efficiencies for Nd, Pr, Co and Fe were 1.38%, 1.81%, 1.73% and 0.54%, respectively. In the precipitation stripping experiments, 4 mL of the organic leachate was shaken with 4 mL of a 1 M oxalic acid solution in a 10 mL glass vial at 2000 rpm for 2 h at 9

Journal Pre-proof room temperature. The experiments were repeated five times in order to have enough sample for characterization purposes. After each experiment, a three-phase mixture was formed. The mixtures were transferred together to a 50 mL tube and centrifuged at 4500 rpm for 10 min. The depleted Versatic Acid 10 solution (top phase) was removed with the help of a syringe and analyzed by 1 H NMR, TGA and ATR-FTIR. The waste oxalic acid solution was collected using another syringe and stored for ICP-OES analysis. The oxalate precipitate at the bottom was

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washed with demineralized water by decantation and dried at 80 °C overnight. The oxalate

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powder was placed in an alumina crucible (50 mL) for calcination at 950 °C for 3 h. The mixed

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rare earth oxide was collected and analyzed by WD-XRF, XRD and ICP-OES. The precipitation

(4)

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(%) of any element was calculated based on Eq. (4).

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3. Results and discussion

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3.1. Characterization of the materials The chemical composition of the magnet batch prepared in Section 2.1. is given in Table 1, and the results are based on two analysis methods (ICP-OES and WD-XRF). In both analytical methods, the total composition included about 10 wt.% as others that could be attributed to the epoxy resin and associated (in)organic additives for magnet production. Individual WD-XRF analysis of some of the collected magnet samples showed that the total composition could occasionally drop to as low as 63%, but in general stayed above 85%. In addition to the expected presence of Nd, Dy and Pr, other minor REEs (Ce, La and Sm) were also present in the magnet 10

Journal Pre-proof batch. Individual analysis by ICP-OES and WD-XRF also confirmed their presence in these magnets. This implies that some bonded NdFeB magnets are produced from impure master alloys. Since bonded magnets in general have poorer magnetic properties, using such alloys might be possible when the high performance is not of great concern and/or when there is a scarcity of raw materials. The Sm concentration in particular was unusually high (4.14 wt.%) in one of the samples with 15.88 wt.% Nd content. However, because of this specific magnet type,

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complete demagnetization of the batch was only possible at 450 °C. Hence, the presence of Sm

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together with Nd is the evidence that this magnet was a hybrid NdFeB-SmFeN bonded magnet

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that exhibits high oxidation resistance and Curie temperature (Gandha et al., 2018; Hosoya et al.,

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2018).

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Table 1 Chemical composition of the magnet batch analysed by two analysis methods (wt.%).

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Element

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Al B Ce Co Dy Fe Ga La Nd Pr Sm Cu Nb Zr Others Total

Method ICP-OES WDXRF 0.39 0.30 1.09 na 0.19 0.44 1.04 0.99 0.14 nd 63.3 61.1 0.09 0.04 0.18 nd 21.7 24.3 1.34 1.59 0.47 0.33 0.11 0.05 na 0.22 na 0.76 10.0 10.6 100 100

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Journal Pre-proof The mineralogy of the magnet batch is given in Fig. 1. It is comparable to that of a sintered magnet showing the dominant hard matrix phase (Nd 2 Fe14 B) (Önal et al., 2015). However, there is an exception of new diffraction peaks between 1020° 2 that were identified as one or two organic phases based on the C, H, O content and possibly Cl such as 16,17-epoxy-5-pregnene-

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3β,21-diol-20-one 21-acetate.

Fig. 1 XRD patterns of a) epoxy NdFeB magnet batch, b) dried calcine and c) leach residue obtained under optimum conditions.

3.2. Theoretical background and proof-of-concept Versatic Acid 10, also known as neodecanoic acid, is a commercially available mixture of carboxylic acids with 10 carbon atoms. It has been investigated for the industrial solvent

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Journal Pre-proof extraction of REEs and non-ferrous metals due to its unique properties (du Preez and Preston, 1992; Gotfryd et al., 2015). It has a low solubility in water (0.025 g per 100 mL of H2 O) with a high flash point (94 °C) and high boiling range (243253 °C) (National Center for Biotechnology Information, 2019). Researchers investigated the precipitation stripping of iron and other transition metals from Versatic Acid 10 solutions by different methodologies (e.g. hydrolytic or hydrogen precipitation) (Doyle-Garner and Monhemius, 1985a, 1985b). Research

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was also performed on its use in a biphasic purification treatment through a liquid-liquid

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exchange reaction between iron and zinc (Thorsen et al., 1984; Thorsen and Grislingas, 1981) or

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yttrium/europium and zinc (Önal and Binnemans, 2019). To the best of our knowledge, there is

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no literature available on alkali baking of any NdFeB magnet type or dissolution behavior of REE hydroxides in Versatic Acid 10 solutions. As a proof-of-concept, a synthetic Nd(OH)3 was

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dissolved in concentrated Versatic Acid 10 with or without the addition of metallic iron powder.

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The results showed selective and complete leaching of Nd with < 1% Fe co-dissolution. However, the resulting leachate was viscous which is why an aliphatic diluent (Shellsol G70)

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was used in the rest of this study to eliminate this problem. Although hydroxide formation of

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REEs in NdFeB magnets was shown to be thermodynamically favorable by aqueous NaOH through Eq. (1), the behavior of iron is of great interest to obtain selectivity in the subsequent leaching step. In the literature, hydrothermal oxidation of iron powder in pure water was shown to result in hardly any reaction (Li et al., 2018). Uchida et al. (1996) studied its behavior in more detail at temperatures between 100300 °C in various molality (m) of NaOH solution. Depending on the conditions, magnetite (Fe3 O4 ), hematite (α-Fe2 O3 ) and sodium ferrite (α-NaFeO 2 ) were found to form based on Eqs. (5)-(8).

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Journal Pre-proof Fe(s) + 2 NaOH(aq)  Na2 FeO 2(aq) + H2(g)

(5)

3 Na2 FeO 2(aq) + 4 H2 O(l)  Fe3 O 4(s) + 6 NaOH(aq) + H2(g)

(6)

Na2 FeO 2(aq) + H2 O(l)  NaFeO 2(s) + NaOH(aq) + 0.5 H2(g)

(7)

2 Na2 FeO 2(aq) + 0.5 O 2(g) + 2 H2 O(l)  Fe2 O3(s) + 4 NaOH(aq)

(8)

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Oxidation of iron was only noticeable above 150 °C. In the absence of oxygen and at 10-40 m

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NaOH, magnetite and sodium ferrite were the products formed. Under 5 MPa oxygen pressure,

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hematite was also detected in the 515 m NaOH range. More importantly, the molality of NaOH required for the formation of only sodium ferrite decreased by more than half for all the studied

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temperature range. Among these possible compounds, having iron as hematite (Eq. (8)) would be

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ideal as no NaOH is consumed and selectivity over iron is guaranteed (Önal et al., 2017, 2015).

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However, hematite formation would require much higher oxygen partial pressures than in air at high temperatures (> 250 °C). During thermal demagnetization of the magnet in open

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atmosphere, intensive bluish-gray fuming was observed at ≥ 250 °C due to burning of organic

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material. Hence, a maximum baking temperature of 200 °C was selected for the initial experiments. Since the oxidation of iron would be negligible, temperatures below 150 °C were considered ineffective and were not studied. As in Eq. (6), formation of magnetite does not consume any NaOH either. However, magnetite was found to be partially soluble in 20 vol.% Versatic Acid 10 solution under certain conditions in another study performed by the authors (unreported). This was probably the result of its wüstite (FeO) portion that could dissolve even in a very weak acid such as Versatic Acid 10. Instead, NaFeO 2 (Eq. (7)) seems to be the second ideal option even though NaOH is partly consumed for its formation. However, in contact with water, it exothermically dissociates to form NaOH or Na2 CO3 along with α-Fe2 O3, as shown in 14

Journal Pre-proof Eq. (9) or Eq. (10) depending on the partial pressure of CO 2 , respectively (Yanase et al., 2019). Santirini and Boos (1979) showed that NaFeO 2 -type complex oxides are not limited to iron but also other transition metals with similar formation patterns (Santirini and Boos, 1979). Later studies on Na-ion batteries showed that transition metals like cobalt and iron can also interchange in this complex oxide due to their similar chemical properties (Kubota et al., 2014; Yoshida et al., 2013). Hence, cobalt could very likely form its own complex oxide or be

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incorporated in that of iron.

(9)

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2 NaFeO 2(s) + H2 O(l)  2 NaOH(aq) + Fe2 O3(s)

2 NaFeO 2(s) + H2 O(l) + CO 2(g)  2 Na2 CO3 .H2 O(s/aq) + Fe2 O3(s)

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(10)

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Although maghemite (γ-Fe2 O3 ) is not a direct product, there are literature reports of its formation such as after alkali fusion and water leaching of a bauxite residue (Shoppert et al., 2019).

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Maghemite can be considered as fully oxidized magnetite and an intermediate between magnetite

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and hematite (Khan et al., 2015). One of its formation routes is via oxidation of magnetite by (moist) air (Barron and Torrent, 2002; Shoppert et al., 2019). Swaddle and Oltmann (1980)

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explained the transformation of magnetite into maghemite and hematite in air and in diluted alkaline conditions at temperatures of 20287 °C (Swaddle and Oltmann, 1980). Once formed, oxidation of magnetite to maghemite was found to occur at a temperature as low as 170 °C with faster kinetics under (moist) air than in hydrothermal conditions. However, the presence of silica significantly

retarded

further

transformation

of maghemite

to

hematite

requiring

higher

temperatures. Therefore, to avoid any magnetite that might survive during alkali baking and to transform it into at least maghemite via dry oxidation, the experiments in this study were performed in an open furnace atmosphere. 15

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3.3. Alkali baking experiments – effect of added NaOH solution Based on the discussion in Section 3.2., initial experiments were performed at 200 °C for 2 h. In Fig. 2, the effect of different amounts of NaOH solution on the leaching efficiencies of Nd, Pr,

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Fe and Co is given for two NaOH concentrations and leaching temperatures. The effect of the

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added amount of NaOH solution or leaching temperature was negligible for higher NaOH

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concentrations since more than 95% of Nd and Pr were leached with less than 1% Fe codissolution, giving very high selectivity. To achieve the same efficiency with lower NaOH

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concentration, the added amount of NaOH solution needed to be at least 15 mL g-1 . These results

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indicate that the transformation of REEs in the magnet sample into their respective hydroxides

lower leaching temperature.

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was easily achievable at 200 °C, allowing more than 95% leaching efficiencies for REEs, even at

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The negligible iron leaching for most part of Fig. 2 is due to formation of a hard-to-dissolve

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oxide after the baking and washing-drying stages. This means that the residue should have been dominated by hematite and/or maghemite instead of magnetite. For both concentrations, cobalt seems to follow iron which is often the case due to their similar chemical properties. All possible iron oxides in this study have the potential to include cobalt in its structure as a replacement for iron (Liu et al., 2009). The highest cobalt leaching efficiency achieved was close to 60% with ca. 20% iron co-dissolution at 90 °C when 25 wt./vol.% NaOH concentration was used with the lowest added amount. Under these conditions, iron can be partly in the magnetite form which can partly dissolve. This could explain the better leaching efficiencies of cobalt (and iron) compared to the other studied conditions in Fig. 2. 16

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Fig. 2 Leaching efficiencies of Nd, Pr, Fe and Co after baking at 200 °C with a) 25 and b) 40

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wt./vol.% NaOH concentration (fixed conditions: tBaking=2 h, tLeaching=1 h and L/S=20 mL g-1 ).

When the baking temperature was decreased to 150 °C under the same conditions (Fig. 3), the effect of the added amount of NaOH solution on the leaching efficiencies of the REEs became more clear, especially at a leaching temperature of 60 °C. Although the amount of water varied more or less in the same range for both concentrations, the positive role of NaOH presence (e.g. catalytic effect) during REE-hydroxide formation became visible, as was also observed by others (Chung et al., 2015; Yoon et al., 2015). The leaching efficiencies for the REEs reached more than 90% only with the higher NaOH concentration and the highest added amount. The leaching

17

Journal Pre-proof efficiencies of the REEs were partly improved (5-10%) at 90 °C, due to the improved leaching kinetics. However, this also resulted in a loss of selectivity over iron as its leaching efficiency increased up to 15%.This implies that the reaction kinetics for REE hydroxide formation was

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slower at 150 °C, requiring more NaOH and water along with faster leaching kinetics.

Fig. 3 Leaching efficiencies of Nd, Pr, Fe and Co after baking at 150 °C with a) 25 and b) 40 wt./vol.% NaOH concentration (fixed conditions: tBaking=2 h, tLeaching=1 h and L/S=20 mL g-1 ).

Contrary to the REEs, the leaching efficiencies of iron and cobalt for both concentrations were not affected significantly by the amount of added NaOH, when the leaching conditions were the same. This could be explained by the fact that the region of magnetite + sodium ferrite with respect to NaOH concentration is larger at lower temperatures than at higher temperatures 18

Journal Pre-proof (Uchida et al., 1996). Therefore, addition of a much higher amount of NaOH could be necessary to improve the slow reaction kinetics at 150 °C that can cause a drastic change (e.g. form more sodium ferrite). In our synthetic work, it was shown that metallic iron cannot be dissolved in concentrated Versatic Acid 10. Since 150 °C is the borderline temperature where the pure α-Fe region meets both magnetite and sodium ferrite dominant regions, some of the iron could remain as metallic iron especially with the lowest added amount of NaOH solution that could partly be

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responsible for the low leaching efficiencies.

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Based on these results, it was possible to have more than 95% REE leaching with less than 1%

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iron co-dissolution in several occasions. However, the highest cobalt leaching efficiency (ca.

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60%) was only possible with reduced selectivity over iron (ca. 20%). To improve the cobalt leaching efficiency and selectivity over iron, a set of experiments was carried out to investigate

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the effect of the alkali baking duration. For the case of heating at 150 °C, longer durations were

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studied that could help with the selectivity by enhancing the oxidation of magnetite into maghemite, while maintaining an acceptable leaching efficiency for cobalt. Similarly, for 200

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°C, shorter durations could shift the position towards magnetite giving more cobalt leaching

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efficiency, while maintaining acceptable iron co-dissolution. For these experiments, addition of 15 mL g-1 of NaOH solution was selected for both baking temperatures and NaOH concentrations.

3.4. Alkali baking experiments – effect of baking duration Fig. 4 shows the leaching efficiencies of the investigated metals at 200 °C for various baking durations. After 0.5 h of alkali baking, there was barely any leaching of metals. This result is in agreement with the synthetic work: the diluted Versatic Acid 10 solution is not able to dissolve 19

Journal Pre-proof these metals from mostly intact magnet structure under these conditions. In order to obtain meaningful leaching efficiencies for REEs, at least 1 h was required for both concentrations. While at higher NaOH concentration, 1.5 h baking was enough for ≥ 95% leaching of REEs at both 60 and 90 °C, at least 2 h of baking duration or 90 °C of leaching temperature was needed with lower NaOH concentration for the same levels. This subtle difference between the two NaOH concentrations still shows that the catalytic effect of more NaOH has a positive effect at

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higher baking temperatures, which can shorten the required alkali baking duration.

Fig. 4 Leaching efficiencies of Nd, Pr, Fe and Co after baking for different durations at 200 °C with a) 25 and b) 40 wt./vol.% NaOH concentration (fixed conditions NaOH(aq)/Magnet=15 mL g-1 , tLeaching=1 h and L/S=20 mL g-1 ).

20

Journal Pre-proof The behavior of REEs with respect to prolonged baking duration is fairly similar in the case of a 150 °C baking temperature (Fig. 5). At higher NaOH concentration, the leaching efficiencies of the REEs reached ≥ 90% and ≥ 95%, after only 3 h and 6 h baking for both leaching temperatures. In the case of lower concentration, only 6 h of alkali baking resulted in more than 90% REE leaching. These results confirm that REE hydroxide formation at 150 °C is rather slow, even with the highest NaOH amount requiring prolonged durations.

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The analogy in the leaching behavior of cobalt and iron was clearly visible especially for lower

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baking and higher leaching temperatures. At the very early stages, a sudden increase was

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observed in their leaching efficiency similar to REEs, especially when the leaching temperature

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was 90 °C. The leaching efficiency of cobalt exceeded 60% for the first time in these experiments with close to 25% iron co-dissolution (Fig. 5). However, with longer durations, the

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reactions in alkali baking favored formation of more sodium ferrite and/or maghemite instead of

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magnetite that reduced their solubility, even when the leaching temperature was higher. Based on these results, it is clear that achieving high REEs and cobalt leaching while maintaining

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high selectivity over iron would require intensive optimization, if possible at all. Hence, alkali

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baking experiments were not investigated further and an optimum set of parameters was selected. For the experiments at 150 °C, ≥ 95% REE leaching with high selectivity over iron (i.e. ≤ 1%) was possible only when the baking duration was kept at 6 h for both leaching temperatures and concentrations (Fig. 5). On the other hand, 1.5 h was sufficient at 200 °C with 40 wt./vol.% NaOH concentration at both leaching temperatures (Fig. 4). In order to reduce the baking duration by four times, the second set was chosen where the leaching temperature was 60 °C, leaching duration was 1 h and L/S ratio was 20 mL g-1 , despite of slightly higher baking

21

Journal Pre-proof temperature,. In this way, a cobalt co-dissolution of only 7.1% was allowed that prevented its

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portioning between the leachate and residue.

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Fig. 5 Leaching efficiencies of Nd, Pr, Fe and Co after baking at 150 °C for different durations

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with a) 25 and b) 40 wt./vol.% NaOH concentration (fixed conditions NaOH(aq)/Magnet=15 mL g-1 , tLeaching=1 h and L/S=20 mL g-1 ).

3.5. Characterization of dried calcine and leach residue A set of reproducibility experiments were performed under the selected optimum conditions in order to produce a stock solution for the subsequent precipitation stripping step. The average leaching efficiencies of elements are given in Table 2, together with their composition in the

22

Journal Pre-proof loaded Versatic Acid 10 leachate. Based on these results, it is evident that the organic leachate was rather pure with negligible iron and cobalt impurities present, due to their minor leaching efficiency. In Fig. 1, the dried calcine is dominated by maghemite together with highly crystalline Nd(OH)3 although it corresponds to ca. 18 wt.%. The minor sodium ferrite peaks can explain the presence of 9.85 wt.% sodium oxide (or 7.31 wt.% sodium) in the dried calcine in Table 3. After leaching,

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only maghemite peaks remained dominant in the residue, which is very similar to that was

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obtained by (Shoppert et al., 2019) or (Liu et al., 2009). The characteristic peaks of Nd(OH)3

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mostly disappeared confirming its almost complete dissolution and negligible amount in the

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residue.

Table 2 Chemical composition of the loaded Versatic Acid 10 (VA) solution and waste oxalic

Al B Ce Co Dy Fe Ga La Na Nd Pr Sm

Loaded VA Solution (ppm) 31.8 5.28 54.4 22.6 41.0 181 18.2 54.1 na 6282 392 136

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Sample

conditions.

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acid (OA) with the leaching and precipitation stripping efficiencies obtained under optimum

Leaching Efficiency (%) 26.8 16.2 94.6 7.08 95.9 0.93 67.3 96.2 > 18.7 94.5 95.7 95.4

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Precipitation (%) 85.9 14.3 99.9 2.15 99.9 0.98 95.9 99.9 na 99.9 99.9 99.9

Waste OA Solution (ppm) 4.49 4.52 0.08 22.1 0.01 179 0.75 0.01 2343 2.46 0.18 0.03

Journal Pre-proof Based on the low-intensity peaks in Fig. 1, there is an indication that some sodium ferrite still remained in the residue, which could explain the 2.80 wt.% sodium oxide (or 2.08 wt.% sodium) existence in the residue in Table 3. After removal of the Nd(OH)3 peaks, a minor phase was also clearly detected which matched with metallic iron. This suggests that although the added NaOH amount was very likely sufficient to avoid magnetite formation, the baking temperature was not high enough to completely eliminate the minor co-existence of metallic iron with NaFeO 2

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(Uchida et al., 1996). However, the fact that metallic iron in a real material did not dissolve in 20

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vol.% Versatic Acid 10 solution can be useful information for future applications on other source

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materials containing metallic iron. Finally, the peaks at the range of 1020° 2 remained stable

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in all three diffraction patterns, suggesting that the organic portion ended up intact in the leach residue. This could explain the less than 100% total of residue composition similar to the case for

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agreement with that conclusion.

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the magnet and dried calcine. The WD-XRF analysis of the calcine and residue were in

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Table 3 Chemical compositions of the calcine, leach residue obtained under optimum conditions

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and mixed rare-earth oxide (REO) (wt.%). Method Oxide Al2 O3 B2 O3 CeO 2 CoO Dy2 O3 Fe2 O3 Ga2 O3 La2 O3 Na2 O Nd2 O3 Pr6 O11 Sm2 O3

ICP-OES Calcine Residue REO 0.45 0.50 0.62 0.21 0.27 0.03 0.14 0.01 0.81 0.81 1.15 0.01 0.10 0.01 0.57 55.5 83.8 0.03 0.07 0.04 0.28 0.13 0.01 0.77 9.85 2.80 0.57 15.5 1.31 88.6 0.99 0.07 5.72 0.33 0.02 1.91 24

Journal Pre-proof CuO Others Total

0.08 15.8 100

0.11 9.89 100

0.05 0.02 100

3.6. Precipitation stripping of REEs and regeneration of consumed Versatic Acid 10

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solution

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The final step was the precipitation stripping of the dissolved REEs with 1 M oxalic acid solution

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in a 1:1 organic-to-aqueous volume ratio at room temperature for 2 h while shaking at 2000 rpm.

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Alternatively, stripping of REEs by an aqueous mineral acid solution can also be considered (Doyle-Garner and Monhemius, 1985a, 1985b). This way, a highly pure REE-containing

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solution can be directly produced for a subsequent solvent extraction step for the separation of

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REEs. In either case, the consumed Versatic Acid 10 (VA) can be regenerated through Eq. (11). (11)

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2 REE(VA)3 + 3 H2 C2 O4  REE2 (C2 O4 )3 + 6 HVA

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The precipitation efficiencies of all elements are given in Table 3. All REEs were successfully precipitated as their oxalates from the loaded solution by > 99% efficiencies with only ca. 1% and 2% co-precipitation of minor Fe and Co impurities, respectively. After calcination of the mixed REE oxalate at 950 °C for 3 h, a mixed rare-earth oxide (REO) was produced with 98.4 wt.% total REO (TREO) content. The major impurities were found to be aluminium and sodium oxides with concentrations of 0.62 and 0.57 wt.%, respectively. The XRD pattern of the oxide is dominated only by the peaks of REO (Figure 6).

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Fig. 6 XRD patterns of a) Na-crystallites obtained from wash solution and b) mixed REO

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obtained after calcination.

The analysis of the waste oxalic acid solution shows a major presence of sodium and minor

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presence of iron and cobalt together with other elements in negligible quantities (Table 2). Based

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on these results, the depleted Versatic Acid 10 was found to be highly pure (derived from the mass balance), with the concentrations of all elements less than 1 ppm, except for Na which will be explained in following sections. In Fig. 7, the ATR-FTIR and 1 H NMR spectra of fresh and depleted 20 vol.% Versatic Acid 10 solutions are given. These are virtually the same, suggesting that no structural damage occurred in the depleted solution during the leaching or precipitation stripping steps. In order to quantify the purity of fresh, loaded and depleted solutions, TGA analysis was performed (Fig. 8). Both fresh and depleted Versatic Acid 10 solutions reached to ca. 0 mass% at around 230 °C where the loaded solution still had ca. 6.5 mass%, which gradually decreased further to ca. 2.5% at 400 °C. 26

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Fig. 7 ATR-FTIR (bottom) and 1 H NMR analysis (top) of a, c) fresh and b, d) depleted Versatic

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Acid 10 solutions, respectively.

As stated in Section 3.2., pure Versatic Acid 10 has a boiling range of 243-253 °C. The aliphatic diluent (Shell GTL Fluid G70) has an initial and a final boiling point of 179 and 343 °C, respectively, according to its technical datasheet. Theoretically, both solvents should gradually evaporate and leave the system in gaseous form during heating. Hence, any impurity present (e.g. metal dissolved in the solution) would stay in some solid form (e.g. oxide, carbide, carbonate, etc.) as in the case of the loaded solution. However, both depleted and fresh solutions did not have any noticeable residue, suggesting that they have equally negligible impurities. Based on these results, it is safe to state that the depleted Versatic Acid 10 solution has the same structural 27

Journal Pre-proof and compositional properties as the fresh solution used in this study and can be fully regenerated due to its negligible solubility in aqueous solutions. Therefore, the sodium quantity in the

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depleted Versatic Acid 10 solution should also be <1 ppm.

Fig. 8 TGA analysis of fresh, loaded and depleted Versatic Acid 10 solutions (20 vol.% diluted

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in the aliphatic diluent Shell GTL Fluid G70).

3.7. Characterization of sodium crystallites and recovery of Na and B In addition to the oxalic acid solution, the only other chemical consumed in this flow sheet is NaOH. Under the selected optimum conditions, sodium is introduced to the system by 40 wt./vol.% NaOH solution with a NaOH/Magnet ratio of 15 mL g-1 . Based on the sodium content of the dried calcine (Table 3), 96.5% of this amount was removed into the wash solution after alkali baking. 90.3% of boron was also removed into the same solution. After complete evaporation of water, white crystallites were formed. XRD analysis of these crystallites in Fig. 6 28

Journal Pre-proof shows highly crystalline anhydrous or hydrated forms of sodium hydroxide and sodium carbonate as dominant phases after the reactions in Eqs. (9)-(10). Recovered boron was found in the form of sodium tetra borate decahydrate (borax, Na2 B4 O7 ·10H2 O) with minor, but distinguishable diffraction peaks. The WD-XRF analysis of the crystallites showed 21.0 wt.% of Na2 O, 4.02 wt.% B2 O 3 and no other element was detected. Based on Table 3, 18.7% of sodium in the dried calcine reported to the residue. If the remaining

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81.3% of sodium were to dissolve in the leachate, its concentration would be 2973 ppm.

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However, the combined sodium quantities of the waste oxalic acid solution and mixed REO

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could only account for 2378 ppm while assuming completely sodium-free depleted Versatic Acid

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10 solution based on the results in Figs. 7 and 8. Hence, the difference of 595 ppm should be removed into the wash solution during washing of the leach residue. In order to investigate if the

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9.85 wt.% sodium oxide (or 7.31 wt.% sodium) in the dried calcine (Table 3) could further be

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removed, an intensive washing was performed in three steps, until no sodium was detected by WD-XRF in the calcine. Since the sodium source in the calcine was sodium ferrite (Fig. 1),

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which can still dissociate with more water washing, that portion should have ended up in the

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wash solution of the residue. This means that the actual leaching efficiency of sodium was 65.1%, while the remaining 16.2% of sodium was removed during washing of the residue. In that case, the 16.2% of sodium in the wash solution of the residue can also be recovered together with the sodium in the wash solution of the calcine. Based on these results, the precipitation efficiency of sodium in the precipitation stripping step becomes 1.47%. 3.8. Overall process flow sheet The overall process flow sheet is summarized in Fig. 9, together with a mass balance at the selected optimum conditions. Assuming 1 kg input of bonded magnet, about 0.27 kg of a mixed 29

Journal Pre-proof REO can be produced with small amounts of aluminium and sodium impurities. Based on 9.81 g of recovered boron, 86.5 g of borax can be produced containing 10.4 g of sodium. Additional improvements can be made on the flow sheet. For example, at 30 °C, the solubility of NaOH, Na2 CO3 and borax in water are 119, 50.5 and 3.90 g per 100 g of water, respectively (Poling et al., 2008). At 60 °C, these values become 174, 46.4 and 20.3 g per 100 g of water, respectively. Hence, instead of using cold water in this study, more boron and sodium can be removed from

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the residue by washing it with hot water thereby increasing their recovery. Furthermore, instead

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of complete evaporation of water from this wash solution, a fractional crystallization process

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under CO 2 -free atmosphere can selectively remove borax as solid crystals leaving an alkali

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solution containing only NaOH (Eq. (9)). Since the waste oxalic acid solution is highly acidic with the small amount of co-dissolved cobalt, it can be used as a leaching agent for dissolution

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and recovery of cobalt in the residue (Sohn et al., 2006). However, this has to be experimentally

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studied especially for the co-dissolution of iron from the maghemite-dominated residue. Another possible use of the waste oxalic acid is its circulation back to precipitation stripping step for a

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number of times. Other improvements can be reducing the L/S ratio below 20 mL g-1 in the

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leaching step and increasing the organic-to-aqueous ratio above 1:1 in the precipitation stripping step. This will increase the concentration of REEs in the leachate (and thus the production rate) while reducing the volume of consumed oxalic acid solution (and thus the waste oxalic acid solution).

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Fig. 9 Overall process flow sheet with mass balance at optimum conditions.

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In Fig. 10, the images of solid and liquid samples used or produced throughout the flow sheet are shown after the completion of the study.

The reddish-brown color is identical for

maghemite/hematite dominated calcines/residues (Yanase et al., 2019; Ӧnal, 2017). In the collected white sodium-crystallites that partly turned to a very light yellowish taint overtime, a few pale blue grains were visually detected (e.g. upper left of the weighing cap). This color matches with one of the identical colors of borax crystals (Pohanish, 2017). Pure Nd2 O 3 is the strongest REO in coloring and can appear as grayish-blue to pinky-violet depending on the source of illumination, a characteristic known as dichroism (Jha, 2014; Binnemans and WalrandGörller, C., 1995). Based on the composition in Table 3, the mixed REO assumed a somewhat 31

Journal Pre-proof green color mainly affected by the other major compounds that are dark brown Pr6 O11 and pale yellow Sm2 O 3 . Particularly Pr6 O11 , the second strongest coloring REO, can give off a green color

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when mixed with Nd2 O3 ( Jha, 2014).

Fig. 10 Images of a) milled magnet batch, b) dried calcine, c) sodium crystallites, d) residue, e) mixed REO and f) depleted, g) fresh and h) loaded Versatic Acid 10 solutions with j) waste oxalic acid solution.

The fresh 20 vol.% Versatic Acid 10 solution was transparent with a barely detectable yellowish taint (originating from concentrated Versatic Acid 10). After leaching, it became a reddish-dark to reddish-orange color with decreasing iron leaching efficiency. After precipitation stripping, it turned back to its original color, which is an indication of the ability to regenerate the Versatic 32

Journal Pre-proof Acid 10 back to its original state. The waste oxalic acid, containing predominantly sodium oxalate with minor amounts of iron and cobalt oxalate, was found to be greenish-yellow, which can be due to the presence of sodium ferrioxalate (green) and/or ferric oxalate (pale yellow) and/or ferrous oxalate (orange), depending on the oxidation state of dissolved iron. If the iron dissolution was the result of a minor magnetite leftover in the dried calcine, then this would result in ferrous oxalate while a small portion of maghemite dissolution would result in ferric

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oxalate.

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Conclusions

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In this study, a novel process is presented for the recovery of REEs from resin-bonded NdFeB magnets that can be applied to sintered NdFeB and sintered/bonded SmCo magnets as well. A

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total of 33 different epoxy-bonded NdFeB magnets were collected from several magnet

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suppliers. Chemical analysis of the individual magnets showed the presence of unusual REEs in these magnets such as samarium, lanthanum and cerium. This shows that magnet alloys of lesser

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quality are used for bonded NdFeB magnets than for sintered NdFeB magnets. After an alkali

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baking treatment at relatively low temperatures (150 to 200 °C) for short durations (e.g. 1.5 h), the magnet powder was transformed into a mixture of REE hydroxides and NaFeO 2 with the organic component left intact. After water washing, NaFeO 2 was transformed into maghemite, giving back sodium-containing compounds. 96.5% of Na was removed into a highly alkaline wash solution together with 90.3% of B. After crystallization, the analysis of sodium crystallites showed the presence of NaOH, Na2 CO 3 and borax. Hereby, boron was turned into a useful byproduct and the majority of the consumed NaOH could be recovered. The washed and dried calcine was then leached in 20 vol.% Versatic Acid 10 solution. ≥ 95% of all present REEs were dissolved with < 1% iron and < 10% cobalt co-dissolution. By applying a precipitation stripping 33

Journal Pre-proof method with 1 M oxalic acid solution, more than 99% of the REEs were selectively precipitated as oxalates. After calcination, a mixed REO was formed with 98.4 wt.% TREO and minor impurities of aluminium and sodium oxides. The co-dissolved sodium and other minor impurities were collected in the waste oxalic acid solution which was found to be very acidic. This solution could be further used to leach the majority of cobalt and remove the remaining 2.80 wt.% sodium oxide (or 2.08 wt.% sodium) from the residue, increasing its potential as a valuable by-product.

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The depleted Versatic Acid 10 solution was regenerated, with less than 1 ppm metal impurities

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present. This way, the Versatic Acid 10 could almost completely be reused in the leaching step,

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with negligible losses due to its low solubility in aqueous solutions.

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Acknowledgements

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There are no conflicts to declare.

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Conflicts of Interest

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The research leading to these results has received funding from the European Union’s Horizon

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2020 research and innovation programme under grant agreement No 720838 (NEOHIRE). The authors would like to thank Sven Dewilde for his help in collecting the magnet samples, Dzenita Avdibegovic for her help in ICP-OES and ATR-FTIR measurements, Nerea Rodríguez Rodríguez for her help in TGA measurement and Martina Orefice for her help in 1 H NMR measurements.

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Highlights

A novel flow sheet is presented with alkali baking and Versatic Acid 10 leaching.

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≥ 95% REEs with < 1% Fe and < 10% Co co-dissolution was achieved.

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90.3% B and 96.5% of Na were removed as NaOH, Na2 CO3 and borax.

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By stripping precipitation, a mixed REO was formed with 98.4 wt.% purity.

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Almost all consumed Versatic Acid 10 was regenerated.

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