metals

Article Leaching and Recovery of Rare-Earth Elements from Waste Using Organic Acids

Marino Gergoric 1,* , Christophe Ravaux 2, Britt-Marie Steenari 1, Fredrik Espegren 1 and Teodora Retegan 1

1 Nuclear Chemistry and Industrial Materials Recycling, Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden; [email protected] (B.-M.S.); [email protected] (F.E.); [email protected] (T.R.) 2 Decontamination and Environmental Management, National School of Chemistry Montpellier, 34090 Montpellier, France; [email protected] * Correspondence: [email protected]; Tel.: +46-(0)31-772-28-42

 Received: 20 August 2018; Accepted: 11 September 2018; Published: 13 September 2018 

Abstract: Over the last decade, rare-earth elements (REEs) have become critical in the European Union (EU) in terms of supply risk, and they remain critical to this day. End-of-life electronic scrap (e-scrap) recycling can provide a partial solution to the supply of REEs in the EU. One such product is end-of-life neodymium (NdFeB) , which can be a feasible source of Nd, Dy, and Pr. REEs are normally leached out of NdFeB magnet waste using strong mineral acids, which can have an adverse impact on the environment in case of accidental release. Organic acids can be a solution to this problem due to easier handling, degradability, and less poisonous gas evolution during leaching. However, the literature on leaching NdFeB magnets waste with organic acids is very scarce and poorly investigated. This paper investigates the recovery of Nd, Pr, and Dy from NdFeB magnets waste powder using leaching and solvent extraction. The goal was to determine potential selectivity between the recovery of REEs and other impurities in the material. Citric acid and acetic acid were used as leaching agents, while di-(2-ethylhexyl) phosphoric acid (D2EHPA) was used for preliminary solvent extraction tests. The highest leaching efficiencies were achieved with 1 mol/L citric acid (where almost 100% of the REEs were leached after 24 h) and 1 mol/L acetic acid (where >95% of the REEs were leached). Fe and Co—two major impurities—were co-leached into the solution, and no leaching selectivity was achieved between the impurities and the REEs. The solvent extraction experiments with D2EHPA in Solvent 70 on 1 mol/L leachates of both acetic acid and citric acid showed much higher affinity for Nd than Fe, with better extraction properties observed in acetic acid leachate. The results showed that acetic acid and citric acid are feasible for the recovery of REEs out of NdFeB waste under certain conditions.

Keywords: rare earths; neodymium magnets; solvent extraction; leaching; citric acid; acetic acid

1. Introduction Rare-earth elements (REEs) possess excellent physical and chemical properties, which is why they are used on a large scale in permanent magnets, lamp phosphors, NiMH batteries, catalysts, various mechanical and electric devices as well as electric motors and generators, which are important for transition to a greener economy. Their demand in industry is thus growing rapidly today [1]. Mined mostly in China (over 95% of global supply), REEs are still considered the most critical raw materials in the European Union (EU) in terms of supply risk [2–4]. Due to the geopolitical situation, even the most optimistic predictions cannot rule out a REE market crisis on a global level, similar to the one that occurred in 2011 [5]. However, an imminent crisis in REEs supply in the EU is not expected.

Metals 2018, 8, 721; doi:10.3390/met8090721 www.mdpi.com/journal/metals Metals 2018, 8, 721 2 of 17

The processes for the production of pure REEs out of ores use highly concentrated mineral acids, which take a toll on the environment, including acids leakage and release of toxic gasses generated during leaching [6]. Furthermore, large amounts of solvents are used for the recovery of REEs out of the obtained leachates. Despite the environmental issues, there are very few ongoing large-scale processes for recycling REEs from end-of-life products, mainly due to price stabilization since the crisis in 2011 and due to varying environmental legislation across the globe [7]. Development of a sustainable and economically feasible recycling process for the recovery of the REEs out of secondary sources is needed in order to decrease the dependence of the industry on mining. As of today, the recycling of REEs in the EU is still in its early stages, with only 7% of the light REEs and 6% of the heavy REEs recovered from secondary sources to meet the EU’s demand for REEs [2]. NdFeB scrap magnets are a viable source of some REEs. They consist mainly of Nd (around 30%) and Fe (around 64%), with trace amounts of B (around 0.5%) [6]. Small admixtures of Pr and Dy are also often present [3], with Dy being especially important because it is added to increase the temperature stability against demagnetization. Furthermore, according to predicted growth [8], REE magnets and are expected to be the most in-demand materials (in tons) due to the development of green technologies, with an average annual growth of 5.3% between 2010 and 2035 [7]. Electronic devices, such as speakers, mobile devices, and hard disk drives, are available as waste today and can be readily recycled unlike large magnets found in wind turbines and electric cars that have a long life span [3]. Hydrometallurgical methods, such as leaching, solvent extraction, and precipitation, are attractive and efficient methods for recovery of REEs out of NdFeB magnets [3]. The elements contained in the magnet are normally dissolved/leached into an aqueous solution, followed by solvent extraction (occasionally ion-exchange) from the leachate and back-extraction into a new aqueous solution for further reprocessing. Various leaching processes for recovery of REEs out of NdFeB magnets have been developed. In 2016, Önal et al. [9] developed a selective leaching process for REEs by leaching Fe in the solid residue. The NdFeB magnet powder sample was turned into a sulfate mixture by mixing with concentrated H2SO4 (12–16 mol/L) in crucibles. After drying and high-temperature treatment, the powder was leached in demineralized water for 15 min to 24 h, which led to >95% recovery of REEs while Fe remained in the solid residue in the form of a sulfate. In 2014, Yoon et al. [10] studied the leaching of NdFeB magnet scrap using H2SO4 at different temperatures and concentrations. ◦ The optimal leaching conditions were determined to be 70 C and 3 mol/L H2SO4 with a leaching time of 4 h. In 2013, Lee et al. [11] carried out an investigation on leaching of NdFeB magnets using H2SO4, HCl, HNO3, and NaOH. Out of the four leaching agents, HCl and H2SO4 showed the best leaching performance. Optimum conditions for this process were S:L (solid-to-liquid ratio) ratio of 20 g/L, leaching time of 15 min, and concentrations of 3 mol/L HCl and 1.5 mol/L H2SO4. The vast majority of research so far has been conducted using strong mineral acids for leaching [12–15]. The extraction of the metals of interest after leaching is most commonly done using solvent extraction agents like di-(2-ethylhexyl) phosphoric acid (D2EHPA), tributyl phosphate (TBP) and 2-ethylhexyl phosphonic acid (HEHEHP) [16], although a lot of research in recent years has been focused on the investigation of solvent extraction using ionic liquids as promising new, environmentally friendly extractants [3,17]. Even though efficient, hydrometallurgical methods have drawbacks. As already mentioned, large amount of waste is generated during the processes and toxic chemicals are often used. This has to be mitigated using more environmentally friendly chemicals and decreasing the quantities of both the aqueous and the organic waste produced. Despite possessing excellent properties for leaching REEs out of NdFeB magnets, strong mineral acids such as HCl, HNO3 and H2SO4 pose the risk of adverse impact on the environment [6,18,19] because of issues with regeneration of the used acids, while handling can also pose a great challenge in some situations. In cases where highly concentrated acids are released to the environment, they could acidify the soil and would require further soil treatment to neutralize the acidified soil [20]. Moreover, when using strong mineral acids, evolution of poisonous gasses is often a problem. Using organic acids or significantly more diluted mineral acids [21] could have advantages in this regard because of Metals 2018, 8, 721 3 of 17 easier handling, less poisonous gas evolution due to lower acidities, and much easier degradability. Metals 2018, 8, x FOR PEER REVIEW 3 of 17 Some researchers even claim that a more sustainable REE-leaching process could be achieved using someachieved bacteria using like Shewanella some bacteria putrefaciens like Shewanella[22]. putrefaciens [22]. Citric acid is an organic acid that contains three carboxylic groups, with the three hydrogen Citric acid is an organic acid that contains three carboxylic groups,◦ with the three hydrogen dissociationdissociation constants constants being being pKa1 pKa1 = 3.13, = 3.13, pKa2 pKa2 = 4.76, = 4.76, and and pKa3 pKa3 = 6.40 = 6.40 at 25 at 25C[ °C23]. [23] It is. It naturally is naturally foundfound in citrus in citrus fruits fruits but isbut usually is usually produced produced through through a cheaper a cheaper process process by fermentation by fermentation of sugars of sugars usingusingAspergilus Aspergilus niger bacterianiger bacteria [24]. As [24] citric. As acid citric is in acid high is demand in high both demand in chemical both in and chemical food industry, and food the priceindustry, is comparable the price is to comparable that of mineral to that acids. of mineral Two possible acids. mechanismsTwo possible of mechanisms dissolution withof dissolution citric acidwith include citric metal acid include ion displacement metal ion displacement with hydronium with ionshydronium and formation ions and of formation soluble metal–ligandof soluble metal– 3+ complexesligand complexes by metal chelation by metal [25 chelation]. In a study [25]. by In Browna study et by al. Brown [26], it waset al. shown [26], it that was Nd showncomplex that Nd3+ withcomplex citrate counterion with citrate is counterion formed according is formed to theaccording following to the equilibrium: following equilibrium:

3+ 3+ + + 3−3− (3n+j(3n+j−3k)−3k) nNdnNd+ + jH jH+ + kCit kCit ⇆ NdkHjjCitCitkk (1)(1) The existence of NdCit, NdHCit, NdHCit2, and NdCit2 was confirmed by the solution in the pH The existence of NdCit, NdHCit, NdHCit2, and NdCit2 was confirmed by the solution in the pH range 2–5, which proved that citric acid could be used for the leaching of Nd and other REEs out of range 2–5, which proved that citric acid could be used for the leaching of Nd and other REEs out of e‐scrap. Acetic acid is an organic acid containing one carboxylic group with pKa = 4.76. It is largely e-scrap. Acetic acid is an organic acid containing one carboxylic group with pKa = 4.76. It is largely produced by methanol carbonylation [27], and the price is comparable with that of mineral acids, produced by methanol carbonylation [27], and the price is comparable with that of mineral acids, making it attractive for usage as a leaching agent. In the work of Zanonato et al. [28], it was shown making it attractive for usage as a leaching agent. In the work of Zanonato et al. [28], it was shown that Nd3+ forms at least successive mononuclear complexes according to the following equilibrium: that Nd3+ forms at least successive mononuclear complexes according to the following equilibrium: Nd3+ + jCH3COO− ⇆ Nd (OOCCH3)j(3−j)+, j = 1, 2, 3 (2) 3+ − (3−j)+ Nd + jCH3COO Nd (OOCCH3)j , j = 1, 2, 3 (2) This indicates that acetic acid could be used for the recovery of Nd and possibly other REEs from electronicThis indicates waste. that acetic acid could be used for the recovery of Nd and possibly other REEs from electronicCarboxylic waste. acids have been previously used for actinide and separations [29,30] and couldCarboxylic be suitable acids havefor the been recovery previously of REEs used out for actinideof NdFeB and magnet lanthanide waste separations as they can [29 form,30] andsoluble couldcomplexes be suitable with for REEs. the recovery However, of the REEs leaching out of NdFeBof neodymium magnet magnets waste as wit theyh organic can form acids soluble is very complexesscarcely with investigated. REEs. However, In 2016, Behera the leaching et al. [6] of neodymiuminvestigated in magnets detail the with kinetics organic of acidsextraction is very of Nd scarcelyout of investigated. NdFeB waste In using 2016, acetic Behera acid. et al.The [6 study] investigated showed inthat detail acetic the acid kinetics in concentrations of extraction higher of Nd than out0.4 of NdFeBmol/L wasteat 800 usingrpm, 1% acetic (w/v) acid. and The 308 study K was showed effective that at acetic leaching acid Nd in concentrationsand Fe out of higherthe waste. thanHowever, 0.4 mol/L in at this 800 study rpm,— 1%as ( wwell/v) as and in 308other K works was effective—little atattention leaching was Nd paid and to Fe the out recovery of the waste. of other However,REEs, inmeaning this study—as no detailed well kinetic as in other or temperature works—little gradient attention study was was paid performed to the recovery for vital of REEs other like REEs,Dy meaning and Pr in no the detailed waste; kinetic the behavior or temperature of other gradient impurities study such was as performed Fe, B and for Co vital during REEs the like leaching Dy andprocess Pr in the was waste; not the completely behavior ofaddressed other impurities either. The such feasibility as Fe, B and of Cofurther during extraction the leaching of REEs process out of wasleachates not completely of organic addressed acids was either. also The notfeasibility addressed. of further extraction of REEs out of leachates of organic acidsIn this was study, also not leaching addressed. of a high‐temperature‐treated neodymium magnet powder was investigatedIn this study, using leaching diluted of a high-temperature-treatedcitric acid and acetic acid. neodymium The leaching magnet process powder was optimized was investigated in order to usingachieve diluted the citric maximum acid and REEs acetic recovery acid. The by leaching varying processleaching was time, optimized acids concentration, in order to achieve and S:L the ratio. maximumBefore the REEs leaching recovery experiments, by varying the leaching powder time, before acids andconcentration, after roasting treatment and S:L ratio. was analyzed Before the using leachingscanning experiments, electron themicroscopy powder before coupled and with after roastingenergy dispersive treatment wasX‐ray analyzed spectroscopy using scanning (SEM/EDX) electronanalysis, microscopy x‐ray diffraction coupled with (XRD), energy and dispersive total dissolution X-ray spectroscopy using aqua (SEM/EDX)regia. Solvent analysis, extraction X-ray with diffractionD2EHPA (XRD), was performed and total dissolutionon the obtained using leachates aqua regia. to test Solvent the feasibility extraction of REEs with recovery D2EHPA from was such performedleachates on using the obtained conventional leachates methods. to test the feasibility of REEs recovery from such leachates using conventional methods. 2. Materials and Methods 2. Materials and Methods 2.1. Material Characterization 2.1. Material Characterization The hydrogen decrepitated (HD) NdFeB powder was used in both original and pretreated form. PretreatingThe hydrogen was decrepitated done by roasting (HD) NdFeBat 400 ° powderC for 1.5 was h in used an Entech in both muffle original furnace and pretreated (LF2) in a form. ceramic Pretreatingcrucible. was After done roasting, by roasting the powder at 400 ◦wasC for sieved 1.5 h to in a an particle Entech size muffle of <355 furnace µm using (LF2) ina Retsch a ceramic AS 200 crucible.(Retsch, After Haan, roasting, Germany) the powder vibratory was sieve sieved shaker. to a particle Throughout size of this <355 research,µm using these a Retsch two forms AS of powder used will be referred to as HD NdfeB powder and roasted NdFeB powder. To determine the elemental composition, 0.5 g of the powder was dissolved in 25 mL aqua regia (V (conc. HCl): V (conc. HNO3) = 3:1) at 23 ± 1 °C for 6 hours and filtered with Whatman microfiber filters (GF/A 125 and 65 mm diameter). After filtration, the solution was diluted using 0.5 mol/L HNO3 (suprapur®

Metals 2018, 8, 721 4 of 17

200 (Retsch, Haan, Germany) vibratory sieve shaker. Throughout this research, these two forms of powder used will be referred to as HD NdfeB powder and roasted NdFeB powder. To determine the elemental composition, 0.5 g of the powder was dissolved in 25 mL aqua regia (V (conc. HCl): V (conc. HNO3) = 3:1) at 23 ± 1 ◦C for 6 hours and filtered with Whatman microfiber filters (GF/A ® 125 and 65 mm diameter). After filtration, the solution was diluted using 0.5 mol/L HNO3 (suprapur 65%, Merck, Kenilworth, NJ, USA). No residues were observed on the filter paper after filtration. The composition of the prepared solution was measured using ICP-OES (inductively coupled plasma optical emission spectroscopy, Thermo Scientific iCAP 600 Series ICP Spectrometer, Thermo Fisher, Cambridge, UK) and recalculated into weight percentages of the elements in the roasted powder. The same procedure was performed on the nonroasted HD NdFeB powder. The same measuring method was used throughout this paper. All experiments were done in triplicate to ensure statistical reliability of the results. SEM/EDX analysis was carried out on the HD NdFeB before and after roasting using a Phenom ProX (Thermo Fisher Scientific, Phenom-World B.V., Eindhoven, The Netherlands) scanning electron microscope with Phenom ProSuite (Thermo Fisher Scientific, Phenom-World B.V., Eindhoven, The Netherlands) software and Element Identification module in order to determine whether any surface modifications might have occurred during roasting or if there was any elemental redistribution in the particles. The chemical composition of both the HD and roasted NdFeB powders were further ascertained using XRD Bruker D8 Advance (Bruker Corporation, Billerica, MA, USA) with Cu Kα used to examine the phase content. The 2θ range was 20–80◦ and step size 0.04◦/s.

2.2. Leaching Kinetics Acetic acid (Sigma-Aldrich, ≥99.5%) and citric acid (Sigma-Aldrich, ≥99.5%) were used as leaching agents, prepared by diluting the concentrated acids with MiliQ water (Merck Millipore Q-POD) to a desired concentration. In order to choose the optimal leaching conditions, the first variables investigated were time and concentration. Each sample was reacting in a closed 100 mL PP plastic cup. Solid to liquid ratio was kept at 0.5 g of magnet powder per 25 mL of acid, the temperature at 23 ± 1 ◦C, and stirring speed at 400 rpm. The samples were leached for 100, 200, 300 min, and 24 h. The starting concentrations of the organic acids used were 0.1, 0.2, 0.4, 0.8, and 1 mol/L. Each experiment was done in triplicate. The heating and stirring plate used was an IKA® RT15. Powder was weighed on a Fisher MH-214 Analytical scale before the reaction and was used in leaching efficiency calculation. ® The sampled leachates were diluted in 1 mol/L HNO3 (suprapur 65%, Merck, Kenilworth, NJ, USA), and the metal concentrations were measured using ICP-OES. The pH values of the acids before leaching and leachates after 24 h leaching time was measured using MeterLab™ PHM 240 pH/ion Meter pH electrode (Radiometer, Copenhagen, Denmark).

2.3. Solid to Liquid Ratio Study After the determination of the optimal leaching time and acid concentration, the solid to liquid ratio for leaching was investigated. For this study 1/30, 1/50, and 1/80 g/mL S:L ratio was chosen. The volume of the leaching agent was kept at 25 mL in all cases, and the mass of the NdFeB powder was varied. The samples were reacted with 1 mol/L citric and 1 mol/L acetic acid for 300 min in PP plastic cups. The rotation speed was kept constant at 400 rpm, and the temperature was kept at 23 ± 1 ◦C. After the reaction, the leachates were sampled, diluted in 1 mol/L nitric acid, and the metal concentrations were measured using ICP-OES.

2.4. Solvent Extraction To extract metals from the aqueous leachate, D2EHPA (Sigma-Aldrich 97%) in Solvent 70 was used. D2EHPA is a widely used industrial extracting agent, which was used in the case of 1 mol/L citric acid and acetic acid leachates to determine the possible use for the recovery of REEs and other Metals 2018, 8, x FOR PEER REVIEW 3 of 17

achieved using some bacteria like Shewanella putrefaciens [22]. Citric acid is an organic acid that contains three carboxylic groups, with the three hydrogen dissociation constants being pKa1 = 3.13, pKa2 = 4.76, and pKa3 = 6.40 at 25 °C [23]. It is naturally found in citrus fruits but is usually produced through a cheaper process by fermentation of sugars Metalsusing2018 Aspergilus, 8, 721 niger bacteria [24]. As citric acid is in high demand both in chemical and5 offood 17 industry, the price is comparable to that of mineral acids. Two possible mechanisms of dissolution with citric acid include metal ion displacement with hydronium ions and formation of soluble metal– elementsligand complexes out of organic by metal acids chelation leachates [25] using. In a solvent study by extraction. Brown et The al. [26] usual, it extractionwas shown reaction that Nd of3+ lanthanidescomplex with using citrate acidic counterion extractants is formed like D2EHPA according [31] to is the of ion-exchange following equilibrium: character:

3+ 3+ + 3− (3n+j−3k) + nNdLn + jH+ +3 (kCitHX)2 ⇆ LnXNdkH3(jHXCitk)3 + 3H (1) (3) The existence of NdCit, NdHCit, NdHCit2, and NdCit2 was confirmed by the solution in the pH where Ln represents the lanthanide ion and HX represents a molecule of D2EHPA. The experiments range 2–5, which proved that citric acid could be used for the leaching of Nd and other REEs out of were performed on the Ika Vibrax Vxr basic shaking machine in 3.5 mL glass shaking vials, for 1 h e‐scrap. Acetic acid is an organic acid containing one carboxylic group with pKa = 4.76. It is largely and the Vorg: Vaq = 1. After the extraction, the aqueous phase was sampled, diluted in 1 mol/L HNO3, produced by methanol carbonylation [27], and the price is comparable with that of mineral acids, and measured using the ICP-OES. The distribution ratios (D) were calculated as the mass balance in making it attractive for usage as a leaching agent. In the work of Zanonato et al. [28], it was shown the aqueous phase before and after the extraction. Distribution ratios were calculated according to the that Nd3+ forms at least successive mononuclear complexes according to the following equilibrium: following equation: Nd3+ + jCH3COO− ⇆ Nd [(OOCCHA]org 3)j(3−j)+, j = 1, 2, 3 (2) D = (4) A [A] This indicates that acetic acid could be used for theaq recovery of Nd and possibly other REEs from whereelectronic [A]org waste.and [A] aq are the equilibrium concentration of the metal of interest in all its existing species in theCarboxylic organic and acids aqueous have phase,been previously respectively. used Separation for actinide factor and represents lanthanide the separations degree of separation[29,30] and betweencould be species suitable A andfor the B in recovery the extraction of REEs system out and of NdFeB is calculated magnet according waste as to thethey following can form equation. soluble complexes with REEs. However, the leaching of neodymium magnets with organic acids is very D scarcely investigated. In 2016, Behera et al. [6]SF investigated= A in detail the kinetics of extraction of Nd(5) out of NdFeB waste using acetic acid. The study showedDB that acetic acid in concentrations higher than 0.4 mol/LThe equilibrium at 800 rpm,pH 1% values(w/v) and inthe 308 aqueous K was effective phase were at leaching measured Nd usingand Fe MeterLab out of the™ waste.PHM 240However, pH/ion in Meter this study pH electrode.—as well as in other works—little attention was paid to the recovery of other REEs, meaning no detailed kinetic or temperature gradient study was performed for vital REEs like 3.Dy Results and Pr and in the Discussion waste; the behavior of other impurities such as Fe, B and Co during the leaching process was not completely addressed either. The feasibility of further extraction of REEs out of 3.1.leachates Material of Characterizationorganic acids was also not addressed. TheIn this composition study, leaching of the HD of NdFeB a high powder‐temperature used in‐treated this research neodymium was determined magnet inpowder a previous was workinvestigated performed using by diluted our group citric [13 acid], where and HNOacetic3 acid.was usedThe leaching to leach theprocess HD NdFeB was optimized and TODGA in order (N,N, to Nachieve0,N0-tetraoctyl the maximum diglycolamide) REEs recovery was used by tovarying extract leaching the REEs time, from acids the acquired concentration, leachate. and The S:L results ratio. wereBefore in the accordance leaching experiments, with the composition the powder of before neodymium and after magnets roastingfound treatment on thewas market analyzed [3 ,using7,32]. Preliminaryscanning electron leaching microscopy tests were performedcoupled with on the energy HD NdFeB dispersive powder X‐ usingray spectroscopy 0.4–1.6 mol/L (SEM/EDX) citric acid andanalysis, acetic x acid‐rayat diffraction S:L ratio 1/50 (XRD), g/mL and and total 25 ±dissolution1 ◦C. REEs using exhibited aqua very regia. slow Solvent leaching extraction kinetics (lesswith thanD2EHPA 20% REEswas performed leached after on 8the hours, obtained even leachates at concentrations to test the above feasibility 1.2 mol/L, of REEs while recovery it took from roughly such fiveleachates days tousing leach conventional 80% of Nd methods. and 60% of Pr and a mere 20% of Dy). Thus, the HD NdFeB powder was roasted and demagnetized to potentially increase its leaching properties. Roasting was used to change2. Materials the powder and Methods chemically and demagnetization to minimize the amount of powder clinging to the stirring magnet. After roasting, the composition of the roasted powder was determined by dissolving 2.1. Material Characterization it in aqua regia (results shown in Table1). The hydrogen decrepitated (HD) NdFeB powder was used in both original and pretreated form. PretreatingTable 1. Elementalwas done composition, by roasting in at wt 400 %, of°C the for (a) 1.5 hydrogen h in an decrepitated Entech muffle (HD) NdFeBfurnace totally (LF2) dissolved in a ceramic ◦ crucible.in aqua After regia roasting, for 1 h at the 80 ±powder1 C and was S:L sieved = 1/10 to g/mL a particle and (b) size roasted of <355 NdFeB µm powder using dissolveda Retsch inAS 200 ◦ (Retsch,aqua Haan,regia for Germany) approximately vibratory six hours sieve at temperature shaker. Throughout 23 ± 1 C and this S:L research, = 0.1/5 g/mL. these No two residues forms of powderwere used left after will filtration. be referred to as HD NdfeB powder and roasted NdFeB powder. To determine the elemental composition, 0.5(a) g Mass of the Fraction powder in HD was NdFeB dissolved(b) in Mass25 mL Fraction aqua inregia Roasted (V (conc. HCl): V Element (conc. HNO3) = 3:1) at 23 ± 1Magnet °C for Powder/wt6 hours and % [filtered13] withNdFeB Whatman Magnet microfiber Powder/wt filters % (GF/A 125 ® and 65 mm diameter).Fe After filtration, 61.1 ±the1.0 solution was diluted using 56.4 0.5± 0.3mol/L HNO3 (suprapur Nd 25.3 ± 0.7 22.7 ± 0.0 Pr 2.62 ± 0.17 2.73 ± 0.05 Dy 1.08 ± 0.27 0.77 ± 0.01 Co 1.42 ± 0.07 1.25 ± 0.03 B 1.00 ± 0.02 0.74 ± 0.01 Metals 2018, 8, 721 6 of 17

Over one quarter of the roasted NdFeB powder weight fraction consists of REEs, which makes it a feasible source for REEs recovery. As expected, the two major components of the roasted neodymium magnet powder were Fe and Nd, making up 56.4 ± 0.3% and 22.7 ± 0.0% of the powder, respectively. Pr made up 2.73 ± 0.05% and Dy 0.77 ± 0.01% of the overall composition—percentages that are similar to the ones in the HD NdFeB powder. Some impurities present in the NdFeB magnet structure, Metals 2018, 8, x FOR PEER REVIEW 6 of 17 such as Co and B, were also present, but in percentages lower than 2%. Fe and Nd were represented in slightlythat lower are similar percentages to the ones than in expected,the HD NdFeB which powder. can be Some attributed impurities to thepresent reaction in the ofNdFeB oxygen magnet and other atmosphericstructure, gases such withas Co theandoriginal B, were also powder present, during but in roasting percentages as thelower oven than did 2%. not Fe and have Nd a were controlled atmosphere.represented Due in to slightly the inability lower topercentages measure than thecontent expected, of which oxygen can and be attributed other air componentsto the reaction using of the ICP-OES,oxygen the and composition other atmospheric of these gases could with not the be original reported powder by aqua during regia roasting dissolution. as the Someoven did percentages not have a controlled atmosphere. Due to the inability to measure the content of oxygen and other air of Fe and Ni (normally present in the magnet coating) might have been lost during the sieving of the components using the ICP‐OES, the composition of these could not be reported by aqua regia roasted powder as larger chunk-like chips from the protective coating of the original magnet were dissolution. Some percentages of Fe and Ni (normally present in the magnet coating) might have removed.been Thelost protectiveduring the coatingsieving of is the not, roasted however, powder of interest as larger here chunk from‐like arecycling chips from standpoint the protective as it does not containcoating REEs. of the original Lastly, themagnet difference were removed. from 100% The protective can be attributed coating is tonot, the however, experimental of interest errors here while performingfrom a therecycling experiments. standpoint as it does not contain REEs. Lastly, the difference from 100% can be Toattributed further to characterize the experimental the waste, errors SEM/EDXwhile performing analysis the experiments. was performed. This was done in order to determineTo the further surface characterize changesthat the waste, occurred SEM/EDX in the analysis HD NdFeB was performed. powder during This was roasting done in and order to examineto possibledetermine redistributions the surface of elementschanges that during occurred roasting in the and HD sieving. NdFeB Figurepowder1 showsduring theroasting SEM and to EDX of examine possible redistributions of elements during roasting and sieving. Figure 1 shows the SEM the HD NdFeB powder (a and b) and the roasted NdFeB powder (c and d). and EDX of the HD NdFeB powder (a and b) and the roasted NdFeB powder (c and d).

(a) (b)

(c) (d)

FigureFigure 1. Four 1. Four different different SEM SEM magnifications magnifications (4100 (4100×,× ,10 10 kV kV-Map)‐Map) HD HD NdFeB NdFeB powder powder in two in different two different locationslocations (a,b) and(a,b) roastedand roasted neodymium neodymium magnet magnet powder powder sample sample inin twotwo locationslocations ( (cc,d,d).). The The crosses crosses and and lines on the SEM pictures show the locations where the EDX analysis was performed. lines on the SEM pictures show the locations where the EDX analysis was performed. In Figure 1a,b, it can be observed that the morphology of the powder looked even except for the

Metals 2018, 8, 721 7 of 17

In Figure1a,b, it can be observed that the morphology of the powder looked even except for the particle size, which varied and ranged from 2–20 µm. EDX analysis was also performed on the same powder shown in Figure1. The EDX results suggested that the HD NdFeB powder elemental composition correlated to the composition of NdFeB magnets found in the market [7]. No clusters with higher concentration of certain elements than in the bulk material were found. Some locations on the powder—like the one represented by Line 1 in Figure1a and point 2 in Figure1b—showed higher amounts of oxygen, which can be attributed to the fact that the REE-rich phase oxidizes fast when exposed to air during the HD process [33]. The results of the SEM/EDX analysis of the roasted NdFeB powder showed different results. In Figure1c,d, it can be observed that the morphology of the powder was significantly more diverse now than it was before the roasting treatment. Three types of particles could be observed: the flat surface particles sized around 10 µm in diameter, the bulkier particles with a distinct rugged morphology with the diameter size of around 10–20 µm, and the small white specs less than 5 µm in diameter. EDX was performed in order to determine the elemental composition of the various particle types. The flat surface particles were largely composed of and oxygen (spots 1 and 2 in Figure1d). EDX analysis of the white specs showed that these consisted mainly of Nd, Pr, and O, pointing to the creation of clusters of REE oxides during the roasting treatment. The large particles with the rugged surface consisted mostly of Fe and O and smaller amounts of REEs. The weight percentage fluctuations were probably due to variations in the penetration depth of the electrons during the EDX analysis, which can vary depending on the surface topography and composition. Because the electrons penetrate at an approximately constant mass, spatial resolution is a function of density. From the SEM images, it can be observed that the morphology of the hydrogen decrepitated and roasted powder differed significantly, which was expected. In the case of the HD NdFeB powder, the surfaces of the particles were relatively similar, with most of the particles looking homogeneous and having sharp edges. On the other hand, the roasted powder was heterogeneous. It was confirmed using the EDX analysis that the heterogeneity was also present from a particle composition point of view as it was shown that the composition varied significantly across the particle size and shape in the roasted NdFeB powder. To determine the chemical species that can be found in the powder before and after treatment, as well as the changes that occurred in the HD NdFeB power during the roasting process, both powders were analyzed using XRD. The XRD pattern of the HD NdFeB powder (Figure2a) showed the presence of Nd2Fe14BH3, a hydride formed during the hydrogen decrepitation process. Apart from the main Nd2Fe14BH3, the presence of Fe2B and NdFe1.14Co0.76 was observed. The XRD pattern also showed the presence of traces of Ni. The XRD pattern of the roasted NdFeB powder (Figure2b) showed the presence of Fe2O3 and Nd2O3, which were formed during the roasting process in the presence of air atmosphere. Ni was also present in higher amounts in elemental form. Apart from these main components there were also present traces of Pr2O3 and Fe in elemental form. The results of the XRD correlated with the results of the SEM-EDX analysis. Metals 2018, 8, 721 8 of 17

MetalsMetals 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 17 17

((aa))

((bb))

FigureFigureFigure 2. XRD 2. 2. XRD patternXRD pattern pattern of the ofof ( a the the) hydrogen ( (aa)) hydrogen hydrogen decrepitated decrepitated decrepitated NdFeB NdFeB NdFeB powder powder powder and and ( andb) roasted ( (bb)) roasted roasted NdFeB NdFeB NdFeB powder. powder.powder. 3.2. Leaching Kinetics 33.2..2. LeachingLeaching KineticsKinetics Apart from the REEs, Fe and Co leaching efficiencies were also monitored as they make up a sufficientApartApart percentage fromfrom thethe of REEs, theREEs, material FeFe andand Co toCo compromise leachingleaching efficienciesefficiencies the leaching werewere alsoalso process. monitoredmonitored Citric asas acidtheythey andmakemake acetic upup aa acid sufficient percentage of the material to compromise the leaching process. Citric acid and acetic acid showedsufficient good leachingpercentage properties of the material for REEs to compromise out of the roastedthe leaching magnet process. powder Citric waste. acid and The acetic results acidfrom showedshowed good good leaching leaching properties properties for for REEs REEs out out of of the the roasted roasted magnetmagnet powder powder waste. waste. The The results results from from leaching of roasted neodymium magnet powder with acetic acid and citric acid are shown in Figures3 leachingleaching ofof roastedroasted neodymiumneodymium magnetmagnet powderpowder withwith aceticacetic acidacid andand citriccitric acidacid areare shownshown inin FiguresFigures and43,3 respectively. andand 4,4, rerespectively.spectively. The leaching TheThe leachingleaching kinetics kineticskinetics of the ofof thethe REEs, REEs,REEs, Fe, Fe,Fe, and andand Co CoCo are areare representedrepresentedrepresented asas as anan an averageaverage average ofof of a triplicateaa triplicatetriplicate experiment. experiment.experiment.

((aa))

Figure 3. Cont. Metals 2018, 8, 721 9 of 17 Metals 2018, 8, x FOR PEER REVIEW 9 of 17

(b)

(c)

(d)

(e)

Figure 3. Effect of the concentration of acetic acid (0.1, 0.2, 0.4, 0.8 and 1 mol/L) and leaching time (100 min, 200 min, 300 min and 24 h) on the leaching efficiency of (a) Nd, (b) Pr, (c) Dy, (d) Fe, and (e) Co. The temperature was kept at 25 ± 1 ◦C, magnetic stirring at 400 rpm, and S:L ratio at 1/50 g/mL. Metals 2018, 8, x FOR PEER REVIEW 10 of 17

Metals 2018, 8, 721 10 of 17 Figure 3. Effect of the concentration of acetic acid (0.1, 0.2, 0.4, 0.8 and 1 mol/L) and leaching time (100 min, 200 min, 300 min and 24 h) on the leaching efficiency of (a) Nd, (b) Pr, (c) Dy, (d) Fe, and (e) LeachingCo. The temperature of Nd, Pr, was Dy, kept Fe, at Co, 25 ± and1 °C, magnetic B at varying stirring concentrationsat 400 rpm, and S:L of ratio acetic at 1/50 acid g/mL. were studied. LeachingLeaching time varied of Nd, from Pr, 0–24Dy, Fe, h, whileCo, and the B temperature at varying conc andentrations S:L ratio of were acetic kept acid constant. were studied. The leaching efficiencyLeaching was time found varied to from be significantly 0–24 h, while dependent the temperature on the and acetic S:L ratio acid were concentration kept constant. as well The as the leachingleaching time. efficiency It can was be observedfound to be that significantly changing dependent the acetic on acid the concentrationacetic acid concentration from 0.05 as towell 1 mol/L andas using the leaching a 24 h time. leaching It can time be observed increased that the changing leaching the efficiency acetic acid from concentration <20% to from >95% 0.05 for to all 1 REEs. Thus,mol/L acetic and acid using in aconcentrations 24 h leaching time higher increased than the 1 mol/L leaching is recommendedefficiency from <20% for the to >95% efficient for all recovery of REEsREEs. fromThus, roasted acetic acid neodymium in concentrations magnet higher powder. than After 1 mol/L 24 h is of recommended leaching with for 1 mol/Lthe efficient acetic acid, almostrecovery all of of Dy, REEs Pr, from Fe, Co, roasted and neodymium B were leached magnet into powder. the solution, After 24 while h of leaching over 95% with of 1Nd mol/L was acetic recovered. Dilutingacid, almost acetic all acid of Dy, from Pr,1 Fe, mol/L Co, and to B 0.1 were mol/L leached decreased into the solution, the pH valuewhile over from 95% 2.2 of to Nd 2.8. was The pH recovered. Diluting acetic acid from 1 mol/L to 0.1 mol/L decreased the pH value from 2.2 to 2.8. The values of the leachates after 24 h leaching were 4.1, 4.2, 4.6, 5.2, 5.6, and 5.9 for 1 mol/L, 0.8 mol/L, pH values of the leachates after 24 h leaching were 4.1, 4.2, 4.6, 5.2, 5.6, and 5.9 for 1 mol/L, 0.8 mol/L, 0.60.6 mol/L, mol/L, 0.4 0.4 mol/L, mol/L, 0.20.2 mol/L,mol/L, and and 0.1 0.1 mol/L mol/L leachates, leachates, respectively, respectively, pointing pointing at proto atn proton starvation starvation duringduring leaching. leaching. Fe,Fe, B, andB, and Co Co showed showedsimilar similar trends and and no no selectivity selectivity for forREEs REEs was wasobserved, observed, meaning meaning that the that the increaseincrease in leachingin leaching of theof the REEs REEs was was also also followed followed by by the the co-leaching co‐leaching of of Fe, Fe, B, B, and and Co Co into into the the solution. As 1solution. mol/L As acetic 1 mol/L acid acetic was neededacid was forneeded quantitative for quantitative leaching leaching of REEs, of REEs, this concentrationthis concentration was was used for furtherused experiments.for further experiments.

(a)

(b)

Figure 4. Cont.

Metals 2018, 8, 721 11 of 17 Metals 2018, 8, x FOR PEER REVIEW 11 of 17

(c)

(d)

(e)

Figure 4. Effect of the concentration of citric acid (0.1, 0.2, 0.4, 0.8 and 1 mol/L) and leaching time Figure 4. Effect of the concentration of citric acid (0.1, 0.2, 0.4, 0.8 and 1 mol/L) and leaching time (100 (100min, min, 200 200 min, min, 300 300 min min and and24 h) 24 on h) the on leaching the leaching efficiency efficiency of (a) Nd, of ( (ba)) Pr, Nd, (c) ( bDy,) Pr, (d) ( cFe,) Dy, and ( d(e)) Fe,Co. and (e) ◦ Co.The The temperature temperature was was kept kept at 25 at ± 25 1 ±°C1 by,C magnetic by, magnetic stirring stirring at 400 atrpm, 400 and rpm, S:L and ratio S:L at ratio1/50 g/mL. at 1/50 g/mL.

AsA ins thein the case case of of acetic aceticacid, acid, the effect effect of of variation variation in inthe the concentration concentration of citric ofcitric acid was acid observed was observed to influenceto influence the the leaching leaching efficiency efficiency of the the components components of ofthe the roasted roasted neodymium neodymium powder powder (Figure (Figure 4). 4). IncreasingIncreasing the the concentration concentration of of citric citric acidacid fr fromom 0.1 0.1 mol/L mol/L to to1 mol/L 1 mol/L increased increased the leaching the leaching efficiency efficiency forfor the the REEs REEs from from <20% <20% to to >95%, >95%, reaching the the highest highest leaching leaching efficiency efficiency afterafter 24 h. 24The h. increase The increase in in the leaching efficiency of REEs was accompanied by the co-leaching of Fe, B, and Co into the solution. The pH value of the leaching acid measured increased from 1.23 to 2.5 from 1 mol/L to 0.1 mol/L, while the values after 24 h leaching were 2.3, 2.6, 3.4, 4.5, and 5.3 for 1 mol/L, 0.8 mol/L, 0.6 mol/L, Metals 2018, 8, x FOR PEER REVIEW 12 of 17 the leaching efficiency of REEs was accompanied by the co‐leaching of Fe, B, and Co into the solution. The pH value of the leaching acid measured increased from 1.23 to 2.5 from 1 mol/L to 0.1 mol/L, Metalswhile2018 the, values8, 721 after 24 h leaching were 2.3, 2.6, 3.4, 4.5, and 5.3 for 1 mol/L, 0.8 mol/L, 0.6 mol/L,12 of 0.4 17 mol/L, 0.2 mol/L, and 0.1 mol/L of citric acid, respectively. An interesting observation here is the decrease in leaching efficiency of the Nd, Pr, and Dy that occurred after 24 h leaching with 0.1 mol/L 0.4 mol/L, 0.2 mol/L, and 0.1 mol/L of citric acid, respectively. An interesting observation here citric acid. Fe and Co did not exhibit the same behavior—the leaching efficiency reached equilibrium is the decrease in leaching efficiency of the Nd, Pr, and Dy that occurred after 24 h leaching with after 300 min for Fe, while the leaching efficiency for Co increased another 10% after 24 h. This can 0.1 mol/L citric acid. Fe and Co did not exhibit the same behavior—the leaching efficiency reached be explained by the pH increase up to 5.3 in the 0.1 mol/L, which led to the precipitation of the REEs, equilibrium after 300 min for Fe, while the leaching efficiency for Co increased another 10% after 24 h. most likely as citrates as citrates of REEs are known to be soluble in aqueous solutions around pH 2– This can be explained by the pH increase up to 5.3 in the 0.1 mol/L, which led to the precipitation 5 [26]. of the REEs, most likely as citrates as citrates of REEs are known to be soluble in aqueous solutions Both acetic acid and citric acid showed much better leaching properties when the roasted and around pH 2–5 [26]. sieved NdFeB powder was used than was the case for the HD NdFeB magnet. It was shown that Both acetic acid and citric acid showed much better leaching properties when the roasted and roasting and the elemental redistributions that occurred during the process (molar volume changes sieved NdFeB powder was used than was the case for the HD NdFeB magnet. It was shown that during the oxidation and pore formation that make the inner surfaces of the powder more available roasting and the elemental redistributions that occurred during the process (molar volume changes for leaching) benefited the leaching properties. Sieving the powder into smaller particles also during the oxidation and pore formation that make the inner surfaces of the powder more available for improved the leaching process, which was also shown in a previous work by Behera et al. [6]. The leaching) benefited the leaching properties. Sieving the powder into smaller particles also improved the ideal conditions for the leaching of Nd were determined to be 1 mol/L of either citric or acetic acid, leaching process, which was also shown in a previous work by Behera et al. [6]. The ideal conditions and further experiments were conducted with these concentrations. for the leaching of Nd were determined to be 1 mol/L of either citric or acetic acid, and further experiments3.3. Solid to Liquid were Ratio conducted Study with these concentrations. 3.3. SolidThe toleaching Liquid Ratioefficiencies Study of Nr, Pr, Dy, Fe, and Co in 1 mol/L citric acid and acetic acid as a function of solid to liquid ratio are shown in Figure 5a,b. The S:L ratios used were 1/30, 1/50, and 1/80 The leaching efficiencies of Nr, Pr, Dy, Fe, and Co in 1 mol/L citric acid and acetic acid as a function g/mL. In the case of 1 mol/L citric acid, changing the S:L ratio from 1/30 to 1/80 g/mL had negligible of solid to liquid ratio are shown in Figure5a,b. The S:L ratios used were 1/30, 1/50, and 1/80 g/mL. effects on the leaching efficiencies of all the elements present. Dy showed slightly higher leaching In the case of 1 mol/L citric acid, changing the S:L ratio from 1/30 to 1/80 g/mL had negligible effects efficiency at 1/80 S:L ratio, but all within the statistical error. On the other hand, in the case of 1 mol/L on the leaching efficiencies of all the elements present. Dy showed slightly higher leaching efficiency at acetic acid, the change of S:L ratio from 1/30 to 1/80 g/mL had a significant effect on the leaching of 1/80 S:L ratio, but all within the statistical error. On the other hand, in the case of 1 mol/L acetic acid, Fe, which was leached at 61.69 ± 6.58% at 1/30 g/mL and 79.30 ± 2.28% at 1/80 g/mL. This can be the change of S:L ratio from 1/30 to 1/80 g/mL had a significant effect on the leaching of Fe, which was attributed to the strength of the acids. Acetic acid is a monoprotic acid with pKa = 4.76, while citric leached at 61.69 ± 6.58% at 1/30 g/mL and 79.30 ± 2.28% at 1/80 g/mL. This can be attributed to the acid is a triprotic acid with pKa1 = 3.13, pKa2 = 4.76 and pKa3 = 6.39, which can explain why the strength of the acids. Acetic acid is a monoprotic acid with pKa = 4.76, while citric acid is a triprotic acid leaching efficiency of Fe was lower due to proton starvation in the leachate. Exact amounts of H+ were with pKa1 = 3.13, pKa2 = 4.76 and pKa3 = 6.39, which can explain why the leaching efficiency of Fe was not recalculated from the pKa values as activity coefficients would need to be determined in that case. lower due to proton starvation in the leachate. Exact amounts of H+ were not recalculated from the pKa The leaching efficiency of Co was also slightly affected by increasing the S:L ratio; it increased from values as activity coefficients would need to be determined in that case. The leaching efficiency of Co 86.11 ± 0.38% at 1/30 g/mL to 93.92 ± 1.70% at 1/80 g/mL. For industrial purposes, it can be extracted was also slightly affected by increasing the S:L ratio; it increased from 86.11 ± 0.38% at 1/30 g/mL to from the leachate using Cyanex 272 (di‐(2,4,4‐trimethylpentyl) phosphinic acid) at equilibrium pH 4– 93.92 ± 1.70% at 1/80 g/mL. For industrial purposes, it can be extracted from the leachate using Cyanex 5 [34]. Changing the S:L ratio from 1/30 to 1/80 g/mL did not, however, affect the leaching efficiency 272 (di-(2,4,4-trimethylpentyl) phosphinic acid) at equilibrium pH 4–5 [34]. Changing the S:L ratio from of Nd, Pr, and Dy, which stayed the same across the S:L ratio. B was found to be leached 1/30 to 1/80 g/mL did not, however, affect the leaching efficiency of Nd, Pr, and Dy, which stayed quantitatively at all S/L ratios investigated. For complete extraction of B from the leachate, aliphatic the same across the S:L ratio. B was found to be leached quantitatively at all S/L ratios investigated. 1,3‐diols can be used [35]. For complete extraction of B from the leachate, aliphatic 1,3-diols can be used [35].

(a)

Figure 5. Cont. Metals 2018, 8, 721 13 of 17 Metals 2018, 8, x FOR PEER REVIEW 13 of 17

(b)

Figure 5. The dependence of the amount of Nd leached on the S:L ratio at 25 ± 1 ◦C using (a) 1 mol/L Figure 5. The dependence of the amount of Nd leached on the S:L ratio at 25 ± 1 °C using (a) 1 mol/L citric acid and (b) 1 mol/L acetic acid with 400 rpm stirring for 24 h in both cases. citric acid and (b) 1 mol/L acetic acid with 400 rpm stirring for 24 h in both cases. 3.4. Solvent Extraction 3.4. Solvent Extraction A screening investigation regarding the possible separation of the leached metal ions by solvent extractionA screening was made investigation on the leachates regarding obtained the possible after 24 separation h leaching ofof thethe roastedleachedmagnet metal ions powder by solvent using 1extraction mol/L citric was acid made and on acetic the leachates acid. D2EHPA obtained dissolved after 24 in Solventh leaching 70 inof concentrationsthe roasted magnet of 0.2, powder 0.4, 0.6, 0.8,using and 1 mol/L 1 mol/L citric was acid used and as acetic the organic acid. D2EHPA phase for dissolved the extraction in Solvent of the 70 REEs in concentrations out of the leachates. of 0.2, Figure0.4, 0.6,6 represents0.8, and 1 themol/L D valueswas used for as Fe the and organic Nd as theyphase are for the the major extraction components of the REEs of the out leachate. of the Theleachates. distribution Figure ratios 6 represents of other the REEs D werevalues shown for Fe to and behave Nd veryas they similarly are the to major the distribution components ratios of the of Nd,leachate. andthese The distribution metals were ratios completely of other extracted REEs were at highershown D2EHPAto behave concentrations.very similarly to The the distributiondistribution ratiosratios ofof REEsNd, and increased these metals with increasing were completely D2EHPA extracted concentration at higher in the D2EHPA organic concentrations. phase, as expected. The Thedistribution distribution ratios ratios of REEs of Feincreased were much with increasing lower compared D2EHPA to Nd,concentration so high separation in the organic factors phase, were as expectedexpected.to The be distribution found, the highest ratios of being Fe were the onesmuch at lower 0.2 acetic compared acid extraction to Nd, so with high 1 separation mol/L D2EHPA, factors whichwere expected reached theto valuebe found, of 161.9 the ±highest21.0.The being distribution the ones ratiosat 0.2 wereacetic one acid order extraction of magnitude with 1 highermol/L inD2EHPA, the acetic which acid mediareached than the in value the citric of 161.9 acid solutions.± 21.0. The This distribution can be attributed ratios were to the one equilibrium order of pHmagnitude values, whichhigherwere in the around acetic 3.5acid after media the than extraction in the fromcitric 1acid mol/L solutions. acetic acidThis leachatecan be attributed and slowly to increasedthe equilibrium with the pH decreasing values, which D2EHPA were concentration around 3.5 butafter stayed the extraction around 1.5 from after 1 the mol/L extraction acetic fromacid 1leachate mol/L and citric slowly acid leachate. increased As with shown the decreasing in a previous D2EHPA study [concentration36], higher equilibrium but stayed pHaround values 1.5 gaveafter higherthe extraction distribution from ratios 1 mol/L for citric REEs acid due leachate. to REE–D2EHPA As shown complex in a previous formation, study which [36], ishigher favored equilibrium in higher equilibriumpH values gave pH mediahigher asdistribution D2EHPA isratios an acidic for REEs extractant due to that REE exhibits–D2EHPA the complex ion-exchange formation, mechanism, which withis favored release in of higher H+ ions equilibrium after the complex pH media has beenas D2EHPA formed (Equationis an acidic (3)). extractant The behavior that exhibits of B and the Co wasion‐ + alsoexchange monitored mechanism, in the extraction with release process. of H It ions was after found the that complex at investigated has been conditions formed (Equation (0.2–1 M D2EHPA(3)). The inbehavior Solvent of 70), B and less Co than was 1% also of both monitored the B and in the Co wereextraction extracted process. from It the was leachates, found that which at investigated makes this systemconditions even (0.2 more–1 M attractive D2EHPA in in terms Solvent of selective70), less than REEs 1% recovery. of both the According B and Co to were Table extracted2, separation from factorsthe leachates, between which Nd and makes Fe were this generally system highereven more in the attractive case of 1 Min aceticterms acid of selective leachate butREEs had recovery. reached aAccording value of 129to Table± 55 2 in, separation extraction factors from 1 between M citric Nd acid and leachate, Fe were making generally these higher systems in the promising case of 1 inM separatingacetic acid valuableleachate elementsbut had reached like Nd froma value large of amounts129 ± 55 in of extraction Fe usually from contained 1 M citric in NdFeB acid magnets.leachate, making these systems promising in separating valuable elements like Nd from large amounts of Fe usually contained in NdFeB magnets.

Metals 2018, 8, 721 14 of 17 Metals 2018, 8, x FOR PEER REVIEW 14 of 17

Figure 6. Distribution ratioratio ofof Nd Nd and and Fe Fe as as a functiona function of theof the concentration concentration of D2EHPAof D2EHPA in a small-scalein a small‐ solventscale solvent extraction extraction set-up set (‐Vuporg (/VVorgaq/V=aq 1;= 1 1; 1 h h shaking shaking at at 1500 1500rpm rpm onon anan adaptableadaptable vortex mixer mixer).). ◦ The aqueous aqueous phase phase was was a aleachate leachate (citric (citric acid acid and and acetic acetic acid acid1 M; 1S:L M; 1g S:L /50 1gmL; /50 25 mL;°C; 24 25 h; 400C; 24rpm h; 400stirring) rpm stirring)using D2EHPA using D2EHPA in Solvent in Solvent 70 solutions 70 solutions as extracting as extracting agents, agents, while while temperature temperature was ◦ wasthermostatically thermostatically kept keptat 25 at ± 251 °C.± 1 C. Table 2. Separation factors between Nd and Fe after extraction with 0.2, 0.4, 0.6, 0.8, and 1 mol/L Table 2. Separation factors between Nd and Fe after extraction with 0.2, 0.4, 0.6, 0.8, and 1 mol/L D2EHPA in Solvent 70. The aqueous phases used were 1 mol/L citric acid and acetic acid leachates D2EHPA in Solvent 70. The aqueous phases used were 1 mol/L citric acid and acetic acid leachates previously obtained. Phase ratios were kept at Vorg/Vaq = 1 during 30 min shaking at 1500 rpm on the previously obtained. Phase ratios were kept at Vorg/Vaq = 1 during 30 min shaking at 1500 rpm on the custom-made shaking machine. Temperature was thermostatically kept at 25 ± 1 ◦C. custom‐made shaking machine. Temperature was thermostatically kept at 25 ± 1 °C.

c (D2EHPA)/mol/L SFNd/Fe (1 mol/L Acetic Acid) SFNd/Fe (1 mol/L Citric Acid) c (D2EHPA)/mol/L SFNd/Fe (1 mol/L Acetic Acid) SFNd/Fe (1 mol/L Citric Acid) 0.20.2 161.9161.9± 21.0± 21.0 0.390.39± ±0.10 0.10 ± ± 0.40.4 94.994.9 9.4± 9.4 2.072.07 ±0.86 0.86 0.6 64.5 ± 13.4 17.0 ± 2.4 0.6 64.5 ± 13.4 17.0 ± 2.4 0.8 64.1 ± 16.0 76.7 ± 5.9 0.8 64.1 ± 16.0 76.7 ± 5.9 1 82.2 ± 21.0 129 ± 55 1 82.2 ± 21.0 129 ± 55

4. Conclusion Conclusionss In the present work, the leaching of REEs, Fe, and Co from roasted NdFeB magnet powder was studied using citric acid and acetic acid. The treated NdFeB powder was characterized prior to the leaching experiments by totaltotal dissolutiondissolution withwith aquaaqua regiaregia andand SEM/EDXSEM/EDX analysis. Kinetics of the leaching process at different concentrations of acids and S:L ratiosratios were studied.studied. After the leaching experiments, solventsolvent extractionextraction was was performed performed using using D2EHPA D2EHPA in Solventin Solvent 70 as70 theas the organic organic phase phase and theand leachatesthe leachates of citric of citric acid andacid aceticand acetic acid toacid determine to determine the feasibility the feasibility of the of extraction the extraction of REEs of REEs from suchfrom leachatessuch leachates as well as aswell possible as possible further further developments developments of the of process. the process. By total dissolution of the roasted NdFeB magnet powder in aqua regia, the composition of the magnet was determined to be consistent with that found in the market. Lower percentages of some elements, such such as as Fe, Fe, pointed pointed towards towards the the presence presence of oxygen of oxygen due dueto air to contact air contact during during roasting. roasting. SEM SEManalysis analysis of the of neodymium the neodymium magnet magnet powder powder before before and andafter after roasting roasting sho showedwed significant significant changes changes in inthe the morphology morphology of the of powder. the powder. EDX EDX showed showed the composition the composition of the ofpowder the powder was composed was composed of NdFeB of NdFeBmatrix and matrix further and furtherconfirmed confirmed the redistribution the redistribution of the ofelemental the elemental composition composition that ensued that ensued after afterroasting. roasting. In the In HD the NdFeB HD NdFeB magnet magnet powder, powder, the elemental the elemental composition composition was waseven even throughout throughout the theparticles particles observed observed before before roasting. roasting. By Bycontrast, contrast, in inthe the roasted roasted NdFeB NdFeB magnet magnet powder, powder, elemental composition was significantly significantly more uneven, with some parts containing significantly significantly higher amounts of REEs, whilewhile othersothers containedcontained higher higher amounts amounts of of Fe. Fe. Faster Faster leaching leaching kinetics kinetics observed observed in in the the case case of aceticof acetic acid acid and and citric citric acid acid can becan attributed be attributed to the to elemental the elemental redistribution redistribution during during roasting roasting and sieving. and sieving.Acetic acid and citric acid showed good leaching properties for REEs, Fe, and Co. Of all the concentrationsAcetic acid studied, and citric 1 mol/Lacid showed citric acidgood and leaching 1 mol/L propertie acetics acidfor REEs, exhibited Fe, and the Co. best Of leaching all the concentrations studied, 1 mol/L citric acid and 1 mol/L acetic acid exhibited the best leaching

Metals 2018, 8, 721 15 of 17 properties. Over 95% of REEs were leached after 24 h with 1 mol/L acetic acid, while REEs were leached almost quantitatively after 24 h with 1 mol/L citric acid. Co-leaching of Fe and Co into the solution could not be avoided due to similar dissolution properties as the REEs, and both were leached >95% in both 1 mol/L citric acid and 1 mol/L acetic acid after 24 h. Changing the S:L ratio from 1/30 to 1/80 g/mL in 1 mol/L citric acid after 24 h showed no difference in leaching efficiency, while 1 mol/L acetic acid after 24 h showed significantly less Fe leached with a S:L ratio 1/30 g/mL. When making preliminary solvent extraction tests with 1 mol/L D2EHPA in Solvent 70 on the leachates in 1 mol/L citric acid and acetic acid, the REEs distribution ratios increased with increasing D2EHPA concentration from 0.2 mol/L to 1 mol/L. Higher distribution ratios were achieved in acetic acid with higher SF between Nd and Fe, but good separation between Nd and Fe was achieved in extraction from 1 M citric acid leachate. The study showed significant results with respect to organic acids leaching and further extraction of REEs from the leachate using D2EHPA. A further development in this direction would be beneficial, especially using mixer-settler bench scale systems, to further determine the process parameters for efficient separation of REEs from Fe.

Author Contributions: Conceptualization: M.G., C.R.; Methodology: M.G., C.R.; Software: F.E., B.-M.S.; Validation: M.G., T.R.; Formal analysis: M.G., C.R., F.E.; Investigation: M.G., C.R.; Resources: T.R., B.-M.S.; Data curation: M.G., C.R., F.E.; Writing—original draft preparation: M.G.; Writing—review and editing: M.G., B.-M.S., T.R.; Visualization: M.G., C.R.; Supervision: T.R.; Project administration: B.-M.S.; Funding acquisition: T.R., B.-M.S. Funding: The research leading to these results has received funding from the European Community’s Seventh Framework Programme ([FP7/2007–2013]) under grant number No. 607411 (MC-ITN EREAN: European Rare Earth Magnet Recycling Network). This publication reflects only the authors’ views, exempting the Community from any liability. Project website: http://www.erean.eu. The authors would like to extend special thanks to the ENVIREE project (Environmentally friendly and efficient methods for extraction of rare-earth elements (REE) from secondary sources-ERA-MIN project 2015–2017) contribution with additional funding. Acknowledgments: The authors would like to extend special thanks to Burçak Ebin for the help with XRD measurements. Conflicts of Interest: The authors declare no conflict of interest.

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