Sustainable Hydrometallurgical Recovery of Valuable Elements from Spent Nickel–Metal Hydride HEV Batteries

metals

Article Sustainable Hydrometallurgical Recovery of Valuable Elements from Spent Nickel–Metal Hydride HEV Batteries

Kivanc Korkmaz , Mahmood Alemrajabi, Åke C. Rasmuson and Kerstin M. Forsberg *

Department of Chemical Engineering, KTH Royal Institute of Technology, Stockholm 100 44, Sweden; [email protected] (K.K.); [email protected] (M.A.); [email protected] (Å.C.R.) * Correspondence: [email protected]; Tel.: +46-8-790-6404

Received: 8 November 2018; Accepted: 6 December 2018; Published: 14 December 2018

Abstract: In the present study, the recovery of valuable metals from a Panasonic Prismatic Module 6.5 Ah NiMH 7.2 V plastic casing hybrid electric vehicle (HEV) battery has been investigated, processing the anode and cathode electrodes separately. The study focuses on the recovery of the most valuable compounds, i.e., nickel, cobalt and rare earth elements (REE). Most of the REE (La, Ce, Nd, Pr and Y) were found in the anode active material (33% by mass), whereas only a small amount of Y was found in the cathode material. The electrodes were leached in sulfuric acid and in hydrochloric acid, respectively, under different conditions. The results indicated that the dissolution kinetics of nickel could be slow as a result of slow dissolution kinetics of nickel oxide. At leaching in sulfuric acid, light rare earths were found to reprecipitate increasingly with increasing temperature and sulfuric acid concentration. Following the leaching, the separation of REE from the sulfuric acid leach liquor by precipitation as NaREE (SO4)2·H2O and from the hydrochloric acid leach solution as REE2(C2O4)3·xH2O were investigated. By adding sodium ions, the REE could be precipitated as NaREE (SO4)2·H2O with little loss of Co and Ni. By using a stoichiometric oxalic acid excess of 300%, the REE could be precipitated as oxalates while avoiding nickel and cobalt co-precipitation. By using nanoﬁltration it was possible to recover hydrochloric acid after leaching the anode material.

Keywords: Ni-MH battery recycling; hydrometallurgy; precipitation; nanoﬁltration; rare earth elements; nickel

1. Introduction It is well known that the hybrid electric vehicle (HEV) is becoming more common due to economic and environmental reasons. Batteries are widely used in HEVs such as automobiles or larger trucks for transportation purposes. The most common type of battery in these vehicles is nickel metal hydride (NiMH), mainly due to Toyota’s decision to use them in their popular Prius. More than two million hybrid cars worldwide are running with NiMH batteries, such as the Prius, Lexus (Toyota), Civic, Insight (Honda), and Fusion (Ford). Panasonic manufactures HEV prismatic NiMH batteries that have been used by Toyota and GM (among others), while Sanyo manufactures HEV cylindrical cell NiMH batteries, which have been used by Honda and Ford. Considering the limited life span of these batteries and their valuable components such as nickel, cobalt, and rare earth elements (REE), it is very important to be able to recycle battery materials. The REE have been declared as high supply risk materials by the European Commission [1,2]. Guyonnet et al. (2015) showed an imbalance between the upstream and downstream parts of the value chain for Nd in NiMH battery applications within Europe in 2010, where Europe largely relied on the import of batteries [3]. The results underlined the potential of recycling the batteries in Europe,

Metals 2018, 8, 1062; doi:10.3390/met8121062 www.mdpi.com/journal/metals Metals 2018, 8, 1062 2 of 17 considering the large amounts of in-use stocks and spent batteries (waste). Rare earth elements have a vital role in many technological advances, including green and sustainable applications. Praseodymium, neodymium and dysprosium drive the demand for light and heavy REE and their demand is expected to remain high in coming years due to their use in magnets [3–5]. The recovery of REE from magnets has been investigated, e.g., using organic acids for leaching and applying solvent extraction to recover the REE from the leach liquor [6]. Spent NiMH batteries contain about 8–10% REEs by weight, including praseodymium and neodymium [7]. After magnets, these batteries are the largest alternative REE source available for recycling [8,9]. There are many scientific and industry cooperative studies for the urban mining and recycling of batteries at different scales [8,10–14]. This address both the physical separation techniques based on gravity, magnetic, and electrostatic methods and subsequent costlier hydrometallurgical and/or pyrometallurgical processes. Pyrometallurgical processes are often simple but the value of the resulting alloys is not particularly high and the REE ends up in a slag [12,15]. Umicore and Rhodia have developed a process to recover metals from NiMH batteries by pyrometallurgical processing. The end product is an alloy containing Ni, Co, Cu, Fe and a slag containing the REE; the REE can be recovered from the slag and purified by hydrometallurgical processing [15]. Benefits of hydrometallurgical processes compared to alternative pyrometallurgical processes are that a full recovery of the metal content with high purity can be achieved, they require less energy, and they generate less waste water and air emissions. Pyro/hydro hybrid methods have been studied as a complement to established techniques [16]. Many hydrometallurgical processes start with an almost complete leaching of the batteries [12,17–24]. For example, leaching by sulfuric acid and hydrochloric acid has been investigated in different studies [17,18,20–22,25–27]. The rare earth elements present in metallic form are oxidized into trivalent ions in contact with hydrochloric acid and sulfuric acid according to the following reactions: +3 −2 REE (s) + 3H2SO4 + 1.5O2 (g) = REE + 3SO4 + 3H2O (1) +3 - REE (s) + 3HCl = REE + 3Cl + 3/2H2 (g) (2) Ni, Co, and Mn can be assumed to be divalent and Fe, Al and the REEs can be assumed to be trivalent in the acid leach liquor. Larsson [22] studied the minimum required amount of different types of acid to completely dissolve the electrode materials. In this study the temperature was kept constant and acid was dosed during the leaching to also keep the pH constant. For further processing, precipitation and solvent extraction can be used to recover Co, Ni, and the rare earth elements [17,25,27–33]. The recovery of nickel and cobalt has an important role for the economics of the recycling processes. There are many proposed processes for the recovery of nickel and cobalt ions from different leach liquors in the literature [34]. The present project aims at developing an environmentally and economically sustainable hydrometallurgical process for recovery of the valuable elements from spent NiMH HEV batteries. The focus is on investigating new pathways where acid can be recycled, undesired side products are avoided, and separation systems are designed for maximum recovery and minimum energy usage. In the present study, the cathode and anode materials have been properly characterized by powder XRD and SEM–EDS. Two different leaching routes have been studied and compared, using either HCl or H2SO4 for dissolving the active anode material. The main objective behind the leaching stage is to obtain maximum recovery of the rare earth elements, nickel, and cobalt. The two electrode materials are dissolved separately. The leaching of individual elements is recorded with time and the optimum conditions for leaching are determined. In previous studies, the leaching outcomes under similar conditions have been reported [12,17,23,31], but the fate of the elements as a function of time is seldom reported for leaching of NiMH HEV batteries nor for the leaching of anode and cathode materials separately. In the present study, the separation of REE from the leach liquors by precipitation and an integrated recovery of acid by nanofiltration have been investigated and assessed. Nanofiltration can be applied to recycle used waste pickling acid [35], to separate REE from acid mine drainage and to Metals 2018, 8, 1062 3 of 17 deal with environmentally problematic wastes [36], to recover nickel [37] and to separate Nd from waste water [38].

2. Experimental Procedures

2.1. Mechanical Separation A Panasonic Prismatic NiMH HEV battery with 6.5 Ah and 7.2 V was used in the experiments. The modules had plastic casings and were discharged below 4 V. First the plastic case was carefully cut open and then the cells were removed. There were six cells and each was covered with pink polymeric sheets to keep them apart. Each cell was washed with deionized water to remove the residual electrolyte and then air-dried. Each cell consisted of 12 cathode and 13 anode plates. The active material of the anode was effortlessly scraped off from the supporting mesh, while the cathode active material was nearly impossible to completely separate from its mesh. Thus, the cathode active material was mixed with the cathode mesh and will hereafter be referred to as the cathode material.

2.2. Material Characterization Each module (1050 g) consisted of six separate cells (about 150 g each), a plastic casing, metal connections, and electrolyte (KOH). Characterizations of the anode and cathode materials, their supporting mesh, leach residues, leach liquors, and precipitates were done by ICP-OES (Thermo Fisher iCAP 7000), SEM EDS (Hitachi S-3700N) and powder XRD (Brukar D8 Discover) employing a Cu K-alpha source with a step size of 0.01 degree. The cathode electrode was soaked in dilute nitric acid solution to wash away the active material so the examination of the supporting mesh by SEM would be possible. Total dissolution of the separate active materials was performed in 8 mol/L nitric acid using a solid to liquid ratio of 1/200 (g/mL) at 90 ◦C for 6 h. This procedure was repeated 3 times and mean values are reported.

2.3. Acid Leaching The leaching experiments were performed in partly sealed Erlenmeyer glass flasks placed on magnetic stirring plates in a thermostatic water bath. A condenser unit was used to preserve atmospheric pressure. Suspensions were continuously stirred at 500 rpm using Teflon-coated stirring bars, which was sufficient to fully disperse and suspend the solid material in the solutions. Deionized water and acids of analytical grade (PA) were used in the experiments. The anode and cathode materials were separately dissolved in H2SO4 or HCl of different concentration at atmospheric pressure and at different temperatures (see Table1). The aim was to study the rate of dissolution of the different materials and the equilibrium compositions under different conditions. The acid concentration ranges were selected based on the estimated acid consumption during leaching. In a full-scale process one option could be to mechanically grind the anode active material together with its nickel-covered stainless-steel mesh, even though the anode active material separates easily from the mesh. To investigate this processing condition, in separate experiments the anode active material was leached together with the mesh (see Table2). The mass ratio of the anode active material to mesh was chosen in accordance to the composition of the battery cells. The dissolution was recorded as a function of time by sampling using syringes. The solid material was immediately separated from the liquid phase using syringe filters (0.2 µm). The liquid samples were then diluted 100 times and stored in high density polyethylene (HDPE) bottles before ICP-OES analysis to determine the REE concentration and other minor elements such as zinc, aluminum, iron, and potassium. Some samples were further diluted to appropriate concentrations for ICP-OES measurements of elements present at high concentration (Ni, La). When the dissolution process had ceased, pH was measured using an Orion ROSS electrode, which was calibrated by Thermo Fisher Scientific standard solutions at pH 2, 4, and 7. The leach residues were separated from the leach Metals 2018, 8, 1062 4 of 17 liquor by using filter paper (0.2 µm) and the leach liquor was stored in plastic bottles prior to further processing. The leach residue was washed with dilute acidic water (5 vol% with same type of acid used in leaching).

Table 1. Leaching of anode (A) and cathode (B) active materials. The experiments were performed under atmospheric pressure, s/L = 1/20 (g/mL), stirring rate = 500 rpm. Cathode and anode were leached separately.

Experiment Acid Type Acid Concentration (mol/L) Temperature (◦C)

1 (A, B) H2SO4 1 25 2 (A, B) H2SO4 2 25 3 (A, B) H2SO4 2 90 4 (A, B) H2SO4 4 90 5 (A, B) HCl 1 25 6 (A, B) HCl 2 25 7 (A, B) HCl 2 90 8 (A, B) HCl 4 90 9 (A, B) HCl 8 90 10 (A&B) HCl Permeate from nanoﬁltration (NF) 25

Table 2. Leaching of anode active material including the supporting mesh. The experiments were performed at 25 ◦C under atmospheric pressure, s/L = 1/20 (g/mL), stirring rate = 500 rpm.

Acid Concentration Anode Active Anode Mesh Experiment Acid Type (mol/L) Material (g) Material (g)

C1 H2SO4 2 0.78 0.22 C2 H2SO4 256 0,78 0.22 C3 H2SO4 2 1 0.28 C4 HCl 2 0.78 0.22 C5 HCl 2.56 0.78 0.22 C6 HCl 2 1 0.28

2.4. Acid Recovery by Nanofiltration The application of nanofiltration to recover part of the acid from the hydrochloric acid leach liquor was investigated. The aim is to recycle back the hydrochloric acid to the leaching stage. The purpose of the experiments was to investigate the capacity of the chosen membrane in terms of flux (permeate flow rate) and retention of the metal ions. First anode and cathode material were leached together using 15 L of 2 mol/L hydrochloric acid and a solid to liquid ratio of 1/200 (Sample L1). In addition, 20L of synthetic leach liquor containing Ni, Co and Mn was prepared (Sample L2) with a composition chosen to resemble the leach liquor composition obtained under optimal hydrochloric acid leaching conditions of 2 mol/L hydrochloric acid and a solid to liquid ratio of 1/20. The compositions of the resulting leach liquors are presented in Table3. The nanofiltration experiments performed are presented in Table4. The metals will be retained in the reject stream while acid with low concentration of metal ions is recovered in the permeate stream. The membrane used was a pH-stable spiral-wound nanofiltration membrane (MPS-34 2540 A2X, Koch) with an effective membrane area of 1.6 m2 and a typical operating pressure of 15–35 bar. The small pilot scale setup used consists of the membrane unit, two holding tanks and a pump. Manometers are installed before and after the nanofiltration module to record the pressure and a valve, placed on the outlet side of the membrane, was used to adjust the operating pressure. Before the experiment, the membrane unit was washed with P3 Ultrasil ca. 1%, at 40 ◦C for 30 min and then rinsed 3 times. The water flow during the wash test was 52 L/h at 20 bar and 25 ◦C. The leach liquor was first repeatedly circulated through the nanofiltration unit with full recycle of both the retentate and permeate streams for 30 min until a steady flow was obtained, recorded as the initial flux. Then the Metals 2018, 8, 1062 5 of 17 permeate stream was separated while the retentate stream was re-circulated until a volume reduction of 50% was reached (i.e., 7.5 L of permeate in experiment N1). This can be expressed in terms of a volume reduction factor (VRF):

Initial volume of leach liquor VRF = Volume of retentate at time t

Initially, when all permeate and retentate is re-circulated the VRF equals 1 (time 0). When half of the initial leachate volume can be found in the permeate tank the VRF equals 2. Under the applied conditions, with a ﬂow rate of 26.4 L/h, this occurs after about 30 min. Samples of 100 mL were collected from both the retentate and permeate streams for VRF = 1 and VRF = 2. The samples were diluted by 5% nitric acid and the total concentration of Ni, Co, La, Ce, Nd, Pr, Y, Mn, Al and Zn were measured by ICP-OES. The residual acidity of the permeate was determined by titration. The permeate from experiment N2 was collected and then used to leach anode material at 25 ◦C, presented as experiment 10A in Table1.

Table 3. Composition of the leach liquors prepared for the nanoﬁltration experiments.

Total Elemental Concentration Leach liq. Ni Co La Ce Nd Pr Y Mn Al Zn K Fe L1 (g/L): 4.18 0.48 0.94 0.29 0.13 0.12 0.05 0.22 0.10 0.18 0.05 0.01 L2 (g/L): 26.0 2.59 - - - - - 0.9 - - - -

Table 4. Acid recovery by nanoﬁltration performed at room temperature.

Experiment Leach Liquor Initial Volume (L) Flow Rate (L/h) Initial Flux (L/m2, h) N1 L1 15 26.4 15.5 N2 L2 20 26.4 15.5

2.5. Separation of Rare Earth Elements by Precipitation The REE were separated from the sulfuric anode leach liquors by precipitation as sodium REE sulfate monohydrate through addition of sodium hydroxide. The REE was separated from the hydrochloric acid anode leach liquors by precipitation as oxalates by the addition of oxalic acid. The experiments performed are presented in Table5.

Table 5. Precipitation experiments.

Experiment Agent Type (Excess %) * P1 Oxalic Acid (100%) P2 Oxalic Acid (200%) P3 Oxalic Acid (300%) P4 Oxalic Acid (600%) P5 Oxalic Acid (24 h stirring version of P4) P6 NaOH * 100% excess represents double the amount of the stoichiometric needed for complete precipitation of the rare earth elements.

The precipitation experiments were performed in sealed Erlenmeyer glass flasks placed in a water bath with controlled temperature (25 ◦C). The solutions/suspensions were constantly stirred at 500 rpm using a magnetic stir bar. Samples were extracted by a syringe, the first 15 min after reagent addition. The apparent pH was also measured. The solid phase was separated from the liquid phase using syringe filters (0.2 µm) for sampling purposes. The elemental composition of the liquid phase Metals 2018, 8, 1062 6 of 17 was determined by ICP-OES after dilution of the samples, while the solid phase was filtered off using filter paper (0.2 µm), washed with ethanol, and dried at 80 ◦C for 12 h. The solid samples were then stored at room temperature prior to analysis by powder-XRD and SEM–EDS. For the oxalic acid experiments, 10 g of oxalic acid (Sigma Aldrich, 99.0% purity) was dissolved in 100 mL of water and this solution was added to the leach liquor. The theoretically needed amount to precipitate all REE, without the presence of other complexing cations, was calculated and the amount of added oxalic acid is reported as percentage excess of this amount. Experiment P5 was left for 24 h with constant stirring and at a temperature of 25 ◦C before filtering off the precipitates. In experiment P6, NaOH (Sigma Aldrich, 4 mol/L) was added at constant amount within certain time intervals under constant stirring. The pH was noted before and after each sampling. In evaluating the results, the measured ion concentrations were corrected for the dilution of the solution due to the addition of oxalic acid or sodium hydroxide.

3. Results and Discussion

3.1. Material Characterization The individual cathode plates weighed 4.1 g and the anode plates with support mesh weighed 3.9 g. The ratio per gram for anode active material to its mesh is 0.28. The cathode material was completely dissolved but the anode dissolution resulted in a small amount of solid leach residue. The chemical composition of the materials is given in Table6. The mass of the measured elements present in the cathode material amounts to 89% of the total mass, and the rest of the material mainly consists of hydroxide ions.

Table 6. Chemical composition of cathode and anode materials in mass percentage (nd- not detected).

Element Ce Co La Mn Nd Ni Pr Y Al Fe Zn K Cathode nd 6.1 nd 0.5 nd 75.5 nd 0.7 0.3 0.1 3.5 1.9 (mass %) Anode 6.4 4.7 20.4 4.1 2.9 51.7 2.5 0.7 1.9 0.1 0.4 1.4 (mass %)

The rare earths Ce, La, Nd, Pr and Y were identified. Most of the rare earth elements are concentrated in the anode active material. Lanthanum is present in the highest concentration (located in the anode). Yttrium is found in both the cathode and anode materials. Nickel is the element with the highest concentration in both materials and has high economic value to recover and recycle back into the market. The cobalt content in both materials is also high enough to be considered as recoverable from an economic point of view. The rare earth alloy Ce0.47 La0.34 Nd0.14 Pr0.05 Ni3.56 Co0.75 Al0.29 Mn0.4 (01-076-7719) and the LaNi5 crystal structure were identified in the anode material by powder-XRD. Pure rare earth elements or other metals, such as Fe, Zn, and Al, could not be detected. These elements can be assumed to exist in the crystal structure of the nickel hydroxide, and rare earth’s alloy as substitute or interstitial atoms. The cathode material consists of nickel hydroxide with a substitution of cobalt appearing as nickel-cobalt hydroxide and metallic nickel and cobalt. The metallic nickel and cobalt may come from the supporting mesh of the cathode plate. No yttrium related peaks were found. The strongest peaks belong to LaNi5 and nickel–cobalt hydroxide in the anode and cathode respectively. These results are consistent with the characterization of the same battery [22] and with other studies on the characterization of similar materials [39,40]. The cathode material consists of a nickel supporting mesh and nickel hydroxide, as can be seen in Figure1. The skeleton-like structure on the left-hand side in Figure1 was visible after washing off the active material by dilute nitric acid. The supporting mesh is made entirely of pure nickel, as confirmed by EDS. The active material covering the mesh is shown on the right-hand side in Figure1. It consists of Metals 2018, 8, 1062 7 of 17 spherical (10–20 µm) nickel hydroxide particles. The material surrounding the spheres was identified asMetals the 2018 same, 8, x nickel FOR PEER hydroxide REVIEW material as the spheres were made of. The results are consistent with7 of the 17 XRD results, which showed large nickel hydroxide peaks with possible cobalt substitution. Nickel and cobaltpossible tend cobalt to substitute substitution. for eachNickel other and in cobalt different tend structures to substitute due for to theireach other similar in ionic different radii. structures The EDS analysisdue to their results similar also ionic identified radii. yttrium The EDS in theanalysis cathode results active also material identified and yttrium other minor in the elements cathode such active as zinc,material aluminum and other and minor potassium. elements such as zinc, aluminum and potassium.

(a) (b)

FigureFigure 1. SEM 1. SEM images images of cathode of cathode material; material; (a) supporting (a) supporting mesh, mesh, (b) nickel (b) nickel hydroxide hydroxide spheres. spheres.

The supportingsupporting mesh mesh in in the the anode anode is made is made from from nickel-coated nickel-coated stainless stainless steel. EDS steel. results EDS showedresults thatshowed the rarethat earth the elementsrare earth (Ce, elements Nd, Pr, and(Ce, Y)Nd, are Pr, found and together Y) are withfound much together higher with amounts much of higher nickel, indicating that the REE are hosted in the LaNi alloys’ crystal structure. Yttrium was not one of the amounts of nickel, indicating that the REE are5 hosted in the LaNi5 alloys’ crystal structure. Yttrium rarewas earthnot one elements of the rare found earth with elemen nickelts withinfound with LaNi nickel5 particles. within Yttrium LaNi5 particles. was found Yttrium within was relatively found smaller-sizedwithin relatively particles, smaller-sized which could particles, be yttrium which oxide.could be yttrium oxide. 3.2. Acid Leaching 3.2. Acid Leaching The ﬁnal measured pH values of the leach solutions were in the range of −2.2 to −2.4. In all of The final measured pH values of the leach solutions were in the range of −2.2 to −2.4. In all of the leaching experiments (exp.1–10), approximately 4–7% of the initial weight remained undissolved the leaching experiments (exp.1–10), approximately 4–7% of the initial weight remained undissolved as leach residue. The amount of leach residue decreased slightly with increasing acid concentration as leach residue. The amount of leach residue decreased slightly with increasing acid concentration and temperature. In the low temperature cathode material experiments (exp. 1, 2, 5 and 6), there were and temperature. In the low temperature cathode material experiments (exp. 1, 2, 5 and 6), there negligible quantities of leach residue. No leach residue was left in the other experiments. were negligible quantities of leach residue. No leach residue was left in the other experiments. 3.2.1. Anode Sulfuric Acid Leaching 3.2.1. Anode Sulfuric Acid Leaching The increase in concentration of the REE (La, Ce, Pr, Nd, and Y), Co, and Ni in the aqueous phase The increase in concentration of the REE (La, Ce, Pr, Nd, and Y), Co, and Ni in the aqueous is presented as a function of time for experiments 1A and 2A (see Table1) in Figures2 and3 below. phase is presented as a function of time for experiments 1A and 2A (see Table 1) in Figures 2 and 3 The anode material is slowly leached in 1 mol/L sulfuric acid solution at 25 ◦C as shown in Figure2. below. The anode material is slowly leached in 1 mol/L sulfuric acid solution at 25 °C as shown in Recovery percentages are calculated based on concentrations of elements given in Figures2 and3 and Figure 2. Recovery percentages are calculated based on concentrations of elements given in Figures data from the total dissolution. After 240 min the recovery of nickel is 72.5% by mass in experiment 1A. 2 and 3 and data from the total dissolution. After 240 min the recovery of nickel is 72.5% by mass in In experiment 2A, leaching with 2 mol/L, 69.4% recovery of Ni was reached. A 98% recovery of nickel experiment 1A. In experiment 2A, leaching with 2 mol/L, 69.4% recovery of Ni was reached. A 98% is obtained after 240 min by using the same initial acid concentration but a higher temperature in recovery of nickel is obtained after 240 min by using the same initial acid concentration but a higher experiment 3. It is possible that the recovery is lower at lower temperature due to the etch kinetics temperature in experiment 3. It is possible that the recovery is lower at lower temperature due to the of nickel. When using oxidizing acids (such as H2SO4) nickel metal may passivate and remain in the etch kinetics of nickel. When using oxidizing acids (such as H2SO4) nickel metal may passivate and solid phase, i.e., form a surface layer of nickel oxide/hydroxide that is not easily dissolved and that remain in the solid phase, i.e., form a surface layer of nickel oxide/hydroxide that is not easily prevents further dissolution of the alloy. The formation of a nickel compound, possibly nickel oxide, dissolved and that prevents further dissolution of the alloy. The formation of a nickel compound, could also occur before the leaching. The SEM–EDS study partly conﬁrms the theory since nickel possibly nickel oxide, could also occur before the leaching. The SEM–EDS study partly confirms the oxide traces are found in the leach residue of the low temperature and low acid concentrated acid theory since nickel oxide traces are found in the leach residue of the low temperature and low acid leaching experiments, which does not exist in the initial XRD patterns. Nickel oxide/hydroxide has a concentrated acid leaching experiments, which does not exist in the initial XRD patterns. Nickel low dissolution rate in sulfuric acid and the rate is increased by an increase in temperature. However, oxide/hydroxide has a low dissolution rate in sulfuric acid and the rate is increased by an increase in temperature. However, the results reported in literature for Ni metal passivation are not directly comparable to the present alloy. Acid concentration and temperature are important variables when considering corrosion of alloys. In addition, the presence of impurities can substantially affect the corrosion rate.

Metals 2018, 8, 1062 8 of 17

Metals 2018, 8, x FOR PEER REVIEW 8 of 17 the results reported in literature for Ni metal passivation are not directly comparable to the present Metalsalloy.The 2018 Acid leaching, 8, x concentration FOR PEERrecovery REVIEW andof rare temperature earth elements are important and cobalt variables are high even when at considering low temperatures corrosion8 (93% of of17 foralloys. Ce, 95% In addition, for La, 95.4 the % presence for Nd, of94.3% impurities for Pr, can91.1% substantially for Y, and 94% affect for the Co corrosion by mass rate.in 1 mol/L H2SO4 The leaching recovery of rare earth elements and cobalt are high even at low temperatures (93% at 25 °TheC after leaching 240 min, recovery experiment of rare earth1A). elementsThe small and decrease cobalt arein the high rare even earth at low concentration temperatures with (93% time for for Ce, 95% for La, 95.4 % for Nd, 94.3% for Pr, 91.1% for Y, and 94% for Co by mass in 1 mol/L H2SO4 inCe, 2 95%mol/L for H La,2SO 95.44 at %25 for°C, Nd, seen 94.3% in Figure for Pr, 3, 91.1% could for be Y,due and to 94% precipitation for Co by massof rare in earth 1 mol/L sulfates, H2SO but4 at at 25◦ °C after 240 min, experiment 1A). The small decrease in the rare earth concentration with time there25 C was after no 240 visible min, experiment precipitation 1A). in The the small leach decrease solution in in the this rare experiment. earth concentration There are with many time REE in 2 in 2 mol/L H2SO4 at ◦25 °C, seen in Figure 3, could be due to precipitation of rare earth sulfates, but sulfatemol/L Hhydrates2SO4 at 25andC, also seen polymorphs in Figure3, couldreported be due in tothe precipitation literature, however, of rare earth at sulfates,25 °C the but binary there there was no visible precipitation in the leach solution in this experiment. There are many REE nonahydrateswas no visible (La precipitation and Ce) or octahydr in the leachates solution(heavier REE) in this are experiment. expected to Therebe the aremost many stable REE phases sulfate in sulfate hydrates and also polymorphs reported in the literature, however,◦ at 25 °C the binary saturatedhydrates andwater also solution polymorphs [41,42]. reported The solubility in the literature,of rare earth however, sulfates at (REE 25 C2(SO the4) binary3.xH2O) nonahydrates decreases as nonahydrates (La and Ce) or octahydrates (heavier REE) are expected to be the most stable phases in the(La andtemperature Ce) or octahydrates increases (heavierand as REE)the sulfuric are expected acid toconcentration be the most stable increases phases [43,44]. in saturated At higher water saturated water solution [41,42]. The solubility of rare earth sulfates (REE2(SO4)3.xH2O) decreases as temperatures,solution [41,42 the]. The decrease solubility in REE of rare concentration earth sulfates with (REE time2(SO is 4more)3·xH obvious.2O) decreases The same as the phenomenon temperature the temperature increases and as the sulfuric acid concentration increases [43,44]. At higher wasincreases observed and asfor the an sulfuric initial sulfuric acid concentration acid concentration increases of [43 4,44 mol/L]. At higherand at temperatures,90 °C (exp. 4) the where decrease the temperatures, the decrease in REE concentration with time is more obvious. The same phenomenon precipitationin REE concentration could be withclearly time seen is moreby the obvious. naked eye The during same phenomenonthe experiment. was The observed solid material for an initial was was observed for an initial sulfuric acid concentration◦ of 4 mol/L and at 90 °C (exp. 4) where the collectedsulfuric acidat the concentration end of the experiment. of 4 mol/L andSEM–EDS at 90 C me (exp.asurements 4) where showed the precipitation that it consisted could be of clearly leach precipitation could be clearly seen by the naked eye during the experiment. The solid material was residueseen by of the the naked anode eye material during and the indicated experiment. the presence The solid of materialREE2(SO4 was)3.xH collected2O. at the end of the collectedexperiment. at the SEM–EDS end of measurementsthe experiment. showed SEM–EDS that me it consistedasurements of leachshowed residue that ofit theconsisted anode materialof leach residue of the anode material and indicated· the presence of REE2(SO4)3.xH2O. and indicated25 the presence of REE2(SO4)3 xH2O.

25 20

20 Ni 15 Co Ni La 15 Co 10 Ce La 10 Nd Concentration(g/L) Ce 5 Pr Nd Concentration(g/L) Y 5 Pr 0 0 50 100 150 200 250 Y 0 Time (minutes) 0 50 100 150 200 250 Time (minutes) Figure 2. Leaching of anode material with 1 mol/L sulfuric acid at 25 °C (Exp. 1A). Figure 2. Leaching of anode material with 1 mol/L sulfuric acid at 25 ◦C (Exp. 1A). Figure 2. Leaching of anode material with 1 mol/L sulfuric acid at 25 °C (Exp. 1A). 4

4 3 Co 3 Mn 2 Co Ce Mn 2 Nd Ce Concentration(g/L) 1 Pr Nd

Concentration(g/L) Y 1 Pr 0 0 50 100 150 200 250 300 Y 0 Time (minutes) 0 50 100 150 200 250 300 Figure 3. Leaching of anode materialTime (minutes) with 2 mol/L sulfuric acid at 25 ◦C (Exp. 2A).

The resultsFigure from 3. Leaching leaching of the anode active material anode with material 2 mol/L together sulfuric with acid theat 25 nickel-coated °C (Exp. 2A). stainless-steel mesh is presentedFigure in3. Leaching Figure4. of As anode can be material seen inwith Figure 2 mol/L4, the sulfuric recovery acid at of 25 Ni, °C Co (Exp. and 2A). REE are lower The results from leaching the active anode material together with the nickel-coated stainless-steel mesh is presented in Figure 4. As can be seen in Figure 4, the recovery of Ni, Co and The results from leaching the active anode material together with the nickel-coated REE are lower than in the experiments using only the active anode materials. The Fe concentration stainless-steel mesh is presented in Figure 4. As can be seen in Figure 4, the recovery of Ni, Co and REE are lower than in the experiments using only the active anode materials. The Fe concentration

Metals 2018, 8, 1062 9 of 17

Metals 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 9 of 17 than in the experiments using only the active anode materials. The Fe concentration in the ﬁnal leach inliquor the final after leach leaching liquor of theafter active leaching anode of the material active was anode 46 mg/Lmaterial in was experiment 46 mg/L 2A. in experiment When leaching 2A. Whenthe active leaching anode the material active togetheranode material with the together mesh, thewith concentration the mesh, the of ironconcentration in the ﬁnal of leach iron liquorin the finalincreased leach up liquor to 1.6 increased g/L in experiment up to 1.6 C1. g/L The in decreased experiment leaching C1. The recovery decreased of nickel, leaching cobalt recovery and REE of is nickel,caused cobalt by the and consumption REE is caused of acid by tothe leach consumption the supporting of acid mesh. to leach the supporting mesh.

100

80 Ce 60 Co 40 40 La Recovery % Recovery Recovery % Recovery 20 Ni Y 0 C1 C2 C3 C4 C5 C6 experiments (C1-C6)

FigureFigure 4.4. 222 mol/Lmol/L acidacid acid anodeanode anode leachingleaching leaching withwith with meshmesh mesh material material (C (C series series experiment). experiment).

A SEM image of the leach residue and EDS resultsresults measured as a point analysis are shown in Figure5 5 and and Table Table7, respectively.7, respectively. The The rare rare earth earth sulfate sulfate precipitates precipitates have have a smaller a smaller particle particle size than size thanthe anode the anode leach leach residue. residue. The phase The phase contains contains a mixture a mixture of the of light the light REE (La,REE Ce,(La, Pr, Ce, Nd) Pr, togetherNd) together with withsome nickelsome andnickel sulfur. and The sulfur. atomic The % ratio atomic of REE % toratio S is 0.8,of whichREE to is similarS is 0.8, to REE which2(SO 4is)3 ·xHsimilar2O (0.7). to TheREE heavy2(SO4)3.xH REE,.xH2O yttrium, (0.7). The was heavy not found REE, yttrium, to precipitate was not as sulfate found hydrateto precipitate with the as sulfate rest of thehydrate REE. with That thecould rest be theof the result REE. of heavier That could rare earth be the elements result creating of heavier aqueous rare complexesearth elements in the solutioncreating to aqueous a larger complexesextent compared in the tosolution light rare to aearth larger elements extent compar due toed increased to light strengthrare earth of elements cation-anion due to interactions increased withstrength the of decrease cation-anion in the interactions ionic radii in with the lanthanidethe decrease series in the [45 ionic]. In radii the light in the of lanthanide this information, series [45]. it is Inclear the that light high of this concentration information, of sulfuricit is clear acid that cannot high concentration be used in the of leaching sulfuric stage; acid otherwise,cannot be theused loss in theof rare leaching earth elementsstage; otherwise, and nickel the as loss precipitates of rare earth during elements the leaching and ni stageckel wouldas precipitates affect the during economics the leachingof the process. stage would affect the economics of the process.

Figure 5. SEMSEM imageimage ofof leachleach residue residue with with precipitate precipitate of experiment 4.

Table 7. EDSEDS resultsresults ofof pointpoint analysisanalysis ofof precipitprecipitate on the red mark on the left figure.

Element Unnorm. wt% Norm. wt% Atom. wt% Nickel 0.20 0.20 0.13 Sulfur 15.13 15.42 17.66 Oxygen 28.96 29.52 67.78

Metals 2018, 8, 1062 10 of 17

Table 7. EDS results of point analysis of precipitate on the red mark on the left ﬁgure.

Element Unnorm. wt% Norm. wt% Atom. wt% Metals 2018, 8, x FOR PEER REVIEWNickel 0.20 0.20 0.13 10 of 17 Sulfur 15.13 15.42 17.66 OxygenCerium 28.9615.96 16.27 29.52 4.26 67.78 PraseodymiumCerium 15.961.46 1.4916.27 0.39 4.26 Praseodymium 1.46 1.49 0.39 Neodymium 3.66 3.73 0.95 Neodymium 3.66 3.73 0.95 LanthanumLanthanum 32.7432.74 33.37 33.37 8.83 8.83 TotalTotal 98.1098.10 100.00 100.00 100.00 100.00

3.2.2.3.2.2. Anode Anode Hydrochloric Hydrochloric Acid Acid Leaching Leaching TheThe leachingleaching ofof the the anode anode active active material material at 25 ◦Cat with25 an°C initialwith hydrochlorican initial hydrochloric acid concentration acid concentrationof 1 mol/L (exp. of 1 5A, mol/L see Table(exp. 15A,), is see shown Table in Figure1), is shown6. Recovery in Figure percentages 6. Recovery are calculated percentages based are calculatedon concentrations based on of concentrations elements given of in elements Figure6 andgiven data in fromFigure total 6 and dissolution. data from The total mass dissolution. recovery Theof nickel mass andrecovery cobalt of afternickel 240 and min cobalt is 44.2% after and240 min 80.7%, is 44.2% respectively. and 80.7%, The respectively. recovery of theThe rarerecovery earth ofelements the rare is 99%earth by elements mass after is 240 99% min. by The mass rare after earth 240 elements min. areThe almost rare earth completely elements dissolved are almost within completely60 min at low dissolved concentration within of60 hydrochloric min at low concentration acid (1 mol/L) of and hydrochloric at low temperature acid (1 mol/L) (25 ◦C), and while at low the temperaturerecovery of Ni(25 is °C), poor. while the recovery of Ni is poor.

15 14 13 12 11 10 Ni 9 Co 8 7 La 6 Ce 5 4 Nd Concentration(g/L) 3 Pr 2 1 Y 0 0 50 100 150 200 250 Time (minutes)

Figure 6. Leaching of anode material with 1 mol/L hydrochloric acid at 25 ◦C. Figure 6. Leaching of anode material with 1 mol/L hydrochloric acid at 25 °C. At higher temperature (90 ◦C) in experiments 7–9A, the recovery of nickel is improved. After 1 h, nickelAt has higher reached temperature 99% recovery (90 °C) and in Coexperiments has reached 7–9A, 90%, the whereas recovery the of rare nickel earth is element improved. dissolution After 1 h,is stillnickel up has to 99%reached by mass 99% inrecovery experiment and 7A.Co Withhas reached continued 90%, leaching. whereas however, the rare nickel earth beginselement to dissolutionprecipitate (presumablyis still up to 99% as a nickelby mass chloride in experiment phase) and 7A. the With recovery continued decreases leaching. to 95%, however, 94%, and nickel 87% beginsfor an initialto precipitate concentration (presumably of 2 mol/L, as a 4nickel mol/L chlo andride 8 mol/L phase) of and HCl, the respectively. recovery decreases The same decreaseto 95%, 94%,in concentration and 87% for byan timeinitial could concentration be observed of 2 formol/ theL, rare4 mol/L earth and elements. 8 mol/L However,of HCl, respectively. the precipitates The samedissolve decrease again in at concentration the end of the by 4-h time experiments. could be observed The best for conditions the rare earth for HCl elements. leaching However, for recovery the precipitatesof the valuable dissolve elements again in at the the anode end of material, the 4-h wasexperiments. an initial The concentration best conditions of 2 mol/L for HCl of leaching HCl and forleaching recovery for of 1 hthe at valuable 90 ◦C (experiment elements in 7A). the anode material, was an initial concentration of 2 mol/L of HCl andLeaching leaching the for active 1 h anodeat 90 °C material (experiment together 7A). with the nickel coated stainless steel mesh (4C) gave lowerLeaching recoveries the of active Ni, Co anode and REE materi comparedal together to the with experiments the nickel with coated only activestainless anode steel material mesh (4C) (6A). gaveThe Felower concentration recoveries in of the Ni, ﬁnal Co leach and liquorREE compar after leachinged to the of theexperiments active anode with material only active was 43 anode mg/L materialin experiment (6A). 6A.The WhenFe concentration leaching the activein the anodefinal materialleach liquor together after with leaching the mesh of the the active concentration anode materialof iron in was the 43 ﬁnal mg/L leach in liquorexperiment increased 6A. upWhen to 4 leaching g/L in experiment the active C4.anode Less material iron is leachedtogether using with HClthe meshcompared the concentration to using H2SO of4 ironof corresponding in the final leach initial liquor molarity. increased Similarly, up to the 4 g/L chromium in experiment concentration C4. Less is iron is leached using HCl compared to using H2SO4 of corresponding initial molarity. Similarly, the chromium concentration is 5 mg/L in experiment 6A, but when leached together with the mesh it became 129 mg/L in experiment C4, which was expected since the characterization of anodic mesh revealed that it was stainless steel. The decreased leaching recovery of nickel, cobalt and REE is caused by the consumption of acid to leach the supporting mesh.

Metals 2018, 8, 1062 11 of 17

5 mg/L in experiment 6A, but when leached together with the mesh it became 129 mg/L in experiment C4, which was expected since the characterization of anodic mesh revealed that it was stainless steel. The decreased leaching recovery of nickel, cobalt and REE is caused by the consumption of acid to Metals 2018, 8, x FOR PEER REVIEW 11 of 17 leach the supporting mesh. 3.2.3. Cathode Sulfuric Acid Leaching 3.2.3. Cathode Sulfuric Acid Leaching The cathode consists of nickel hydroxide with substitutions of cobalt, as reported above, and The cathode consists of nickel hydroxide with substitutions of cobalt, as reported above, and presumably of zinc [24,39] and Y2O3 and nickel metal. At 25 ◦°C in 2 mol/L H2SO4, the recovery of presumably of zinc [24,39] and Y2O3 and nickel metal. At 25 C in 2 mol/L H2SO4, the recovery of nickel was 38.7%, measured after 15 min and with no measurable increase in concentration for up to nickel was 38.7%, measured after 15 min and with no measurable increase in concentration for up to 240 min leaching time (exp. 2B, see Table 1). However, at a higher temperature (exp. 3B) the 240 min leaching time (exp. 2B, see Table1). However, at a higher temperature (exp. 3B) the recovery recovery reaches up to 99.9% after 4 h. The cobalt and yttrium solid phases leach quickly and with reaches up to 99.9% after 4 h. The cobalt and yttrium solid phases leach quickly and with high recovery high recovery rates, even at low sulfuric acid concentrations and low temperatures. Their recoveries rates, even at low sulfuric acid concentrations and low temperatures. Their recoveries were 87.9% and were 87.9% and 100%, respectively, at 25 °C and 1 mol/L initial sulfuric acid concentration after 240 100%, respectively, at 25 ◦C and 1 mol/L initial sulfuric acid concentration after 240 min in experiment min in experiment 1B. This shows that the spherical nickel hydroxide particles dissolve while the 1B. This shows that the spherical nickel hydroxide particles dissolve while the nickel metal part of the nickel metal part of the cathode material remains largely as a solid phase at lower temperature. Even cathode material remains largely as a solid phase at lower temperature. Even at high concentrations of at high concentrations of sulfuric acid (4 mol/L), yttrium did not precipitate. sulfuric acid (4 mol/L), yttrium did not precipitate.

3.2.4.3.2.4. Cathode Cathode Hydrochloric Hydrochloric Acid Acid Leaching Leaching ° TheThe recovery recovery of of Ni Ni in in experiments experiments 5B 5B and and 6B 6B after after 4 4 h h leaching leaching time time at at 25 25 ◦CC by by mass mass was was 41.3% 41.3% andand 43.8% 43.8% using using an an initial concentration of 1 mol/L and and 2 2 mol/L mol/L of of HCl, HCl, respectively respectively (see (see Figure Figure 7).7). ° AtAt higher temperature (90 C),◦C), 99.9% 99.9% recovery recovery of of nickel nickel was was obtained obtained after after 4 4 h h using using an an initial initial concentrationconcentration of of 2 2 mol/L mol/L of of HCl. HCl. By By using using an an initia initiall concentration concentration of of4 mol/L 4 mol/L and and 8 mol/L 8 mol/L of HCl, of HCl, the maximumthe maximum recovery recovery was wasreached reached after after only only 15 min 15 minin experiments in experiments 8B and 8B and9B. 9B. ° TheThe massmass recoveryrecovery of of yttrium yttrium is 100% is 100% after 15after min 15 of leachingmin of atleaching 25 ◦C using at 25 an initialC using concentration an initial ° concentrationof 1 mol/L. This of 1 suggests mol/L. This the possibility suggests the of partialpossibility selective of partial leaching selective of yttrium leaching at 25 of◦ Cyttrium using anat initial25 C usingconcentration an initial of concentration 1 mol/L, as only of 1 41.3%mol/L, of as the only nickel 41.3% and of 83.6%the nickel of cobalt and 83.6% is leached of cobalt in the is ﬁrst leached 15 min. in theThe first remaining 15 min. solid The remaining phase is mainly solid phase pure metallic is mainly nickel. pure metallic nickel.

12 Ni 1M 10 Ni 2M

Concentration(g/L) 8

6 0 50 100 150 200 250 Time (minutes)

Figure 7. Leaching of nickel from cathode material with 1 mol/L and 2 mol/L hydrochloric acid at 25 Figure◦C. 7. Leaching of nickel from cathode material with 1 mol/L and 2 mol/L hydrochloric acid at 25 °C. 3.2.5. Acid Recovery via Nanoﬁltration 3.2.5. TheAcid conditioning Recovery via of Nanofiltration the membrane, by recirculating the retentate and permeate for 30 min prior to startingThe theconditioning experiments, of the did membrane, not show any by signsrecirculat of concentrationing the retentate buildup and causingpermeate membrane for 30 min scaling. prior to startingThe measured the experiments, concentrations did not of theshow main any metal signs ions of presentconcentration in the leachbuildup liquors causing at the membrane beginning scaling.of the experiment (VRF = 1) and after a 50% volume reduction (VRF = 2) are presented in Table8. 2 The testThe wasmeasured conducted concentrations at 20 bar and of the the ﬂux main at VRF2 metal was ions 18 L/(mpresent, h). in the leach liquors at the beginning of the experiment (VRF = 1) and after a 50% volume reduction (VRF = 2) are presented in Table 8. The test was conducted at 20 bar and the flux at VRF2 was 18 L/(m2, h).

Table 8. Concentration of individual elements after separation of acid by nanofiltration in experiment N1.

Total Concentration (mg/L) Nanofiltration Stages Ce Co La Mn Nd Ni Pr Y Al Fe K Zn

Metals 2018, 8, 1062 12 of 17

Table 8. Concentration of individual elements after separation of acid by nanofiltration in experiment N1.

Total Concentration (mg/L) Nanofiltration Stages Ce Co La Mn Nd Ni Pr Y Al Fe K Zn Metals 2018, 8, x FOR PEER REVIEW 12 of 17 Initial composition 286 483 942 221 130 4178 119 53 101 8 51 183 Permeate (VRF = 1) 2 6 8 3 1 47 1 0 1 1 21 52 PermeateInitial composition (VRF = 2) 4 286 10483 14 942 6 221 2130 794178 2119 1 53 1 101 2 8 32 51 83183 RetentatePermeate (VRF (VRF = 1) = 1) 2702 4856 931 8 217 3 1271 405347 1201 590 1091 331 5221 16452 RetentatePermeate (VRF (VRF = 2) = 2) 5314 93610 180014 4216 2472 836479 2362 1171 2131 1832 8332 26083 Retentate (VRF = 1) 270 485 931 217 127 4053 120 59 109 33 52 164 TheRetentate results (VRF show = that 2) there531 is no936 accumulation 1800 421 of247 elements 8364 in the236 system,117 e.g.,213 due183 to precipitation.83 260 The permeateThe results stream show contains that lowthere concentration is no accumulati of metalon ions,of elements both at the in beginningthe system, of thee.g., experiment due to (VRFprecipitation. = 1) and after The apermeate 50% volume stream reduction contains (VRF low = concentration 2). The concentrations of metal ofions, metal both ions at inthe the beginning permeate streamof the areexperiment higher after(VRF a = 50% 1) and volume after a reduction. 50% volume This reduction is expected (VRF since= 2). The the concentrations feed liquor at thisof metal point containsions in the about permeate double stream the concentration are higher after of metal a 50% ions. volume The pH reduction. of the permeate This is expected at a VRF since of 1 andthe feed 2 was measuredliquor at tothis be point−0.19 contains and −0.17, about respectively, double the conc whichentration is very closeof metal to theions. initial The pH of the permeate leach liquors. at Thea VRF free of hydrochloric 1 and 2 was acid measured concentration to be −0.19 of the and permeate −0.17, respectively, after 50% volume which reduction is very close was to calculated the initial to bepH 1.7 of mol/L, the leach which liquors. is very The close free tohydrochloric the initial acid acid concentration concentration used of the in permeate the leaching after stage 50% (2volume mol/L) andreduction the existing was calculated metal ion concentrationto be 1.7 mol/L, is which very low, is very thus close the permeateto the initial stream acid isconcentration suitable for recyclingused in tothe the leaching leaching stage stage. (2 mol/L) and the existing metal ion concentration is very low, thus the permeate streamThe is results suitable with for respectrecycling to to the the concentration leaching stage. of metal ions in the feed, retentate and permeate The results with respect to the concentration of metal ions in the feed, retentate and permeate from the experiment with synthetic leach liquor (N2) are presented in Figure8. The results show that from the experiment with synthetic leach liquor (N2) are presented in Figure 8. The results show when using approximately six times higher initial metal ion concentrations in experiment N2 than in that when using approximately six times higher initial metal ion concentrations in experiment N2 experiment N1, the metal ion concentrations in the permeate are higher than in experiment N1, but the than in experiment N1, the metal ion concentrations in the permeate are higher than in experiment metal ion concentrations are still low enough to be able to re-cycle the permeate stream to the leaching N1, but the metal ion concentrations are still low enough to be able to re-cycle the permeate stream stage. However, the high metal ion concentration in N2 resulted in a higher pressure drop. The flux to the leaching stage. However, the high metal ion concentration in N2 resulted in a higher pressure at VRF2 was approximately 2 L/(m2, h) with a pressure of 25 bar. The permeate after VFR = 3 was drop. The flux at VRF2 was approximately 2 L/(m2, h) with a pressure of 25 bar. The permeate after used to leach the anode active material under optimum conditions (experiment 10A). The results show VFR = 3 was used to leach the anode active material under optimum conditions (experiment 10A). that the leaching recovery after 240 min of leaching was 99% for the REE, 94.3% for Ni and 93% for The results show that the leaching recovery after 240 min of leaching was 99% for the REE, 94.3% for CoNi when and 93% compared for Co withwhen using compared fresh 2with mol/L using HCl fresh under 2 mol/L corresponding HCl under conditions corresponding (experiment conditions 6A). In(experiment addition, no 6A). precipitation In addition, could no precipitation be seen during coul thed be nanofiltration seen during the experiment. nanofiltration experiment.

(a)

5 Co Initial 4 Co Permeate

3 Co Retentate Mn Initial

Concentration(g/L) 2 Mn Retentate Mn Permeate 1

0 1 1.5 2 2.5 3 VRF

Figure 8. Cont.

Metals 2018, 8, x FOR PEER REVIEW 13 of 17

MetalsMetals2018 2018, 8, ,8 1062, x FOR PEER REVIEW 13 13of of17 17 (b) 60(b) 60 50

50 40

40 30 Ni Initial 30 Ni RetentateNi Initial 20 Ni Permeate

Concentration(g/L) Ni Retentate 20 Ni Permeate 10 Concentration(g/L) 10 0 0 11.522.53 11.522.53VRF VRF Figure 8. Concentration of metal ions in the retentate and permeate streams in experiment N2: (a) Co and MnFigureFigure and 8. (8.bConcentration )Concentration Ni. of of metalmetal ionsions inin the retentateretentate and permeate streams streams in in experiment experiment N2: N2: (a ()a Co) Co andand Mn Mn and and ( b(b)) Ni. Ni. 3.2.6. Separation of Rare Earth Elements by Precipitation 3.2.6.3.2.6. Separation Separation of of Rare Rare EarthEarth ElementsElements byby PrecipitationPrecipitation After leaching using an initial concentration of 2 mol/L of H2SO4 at 90 °C, the liquor was used ◦ for the firstAfterAfter series leaching leaching of usingprecipitation using an an initial initial experiments. concentration concentration In of ofexperiment 2 mol/L2 mol/L of of H P6,H2SO2 SOthe4 4at at 90REE 90 C,° C,were the the liquor separatedliquor was was used byused for precipitationthefor ﬁrst the series firstas NaREE(SO ofseries precipitation of 4precipitation)2.H2O experiments. by adding experiments. InNaOH experiment Into theexperiment P6,leach the REEliquor. P6, were the All separatedREE the werelight by separatedrare precipitation earth by elementsasprecipitation NaREE(SO were found 4as)2· HinNaREE(SO2 theO by solid adding4 )phase2.H2O NaOH atby around adding to the pH NaOH leach −0.1 liquor.andto the 0.7 Allleachmol/L the liquor. sodium light rareAll ion the earth concentrations; light elements rare earth were see foundFigureelements in9. the wereThe solid heavyfound phase inREE, the at around solidY, however, phase pH − at0.1 presaround andent 0.7 pHat mol/L −comparably0.1 and sodium 0.7 mol/Llow ion concentration, concentrations;sodium ion concentrations; does see Figurenot 9 . precipitate.Thesee heavy Figure There REE, 9. was The Y, however,no heavy significant REE, present nickelY, athowever, comparably and coba preslt loss lowent atoccurring concentration, comparably during doeslow the notconcentration, precipitation precipitate. ofdoes There rare not was earthno precipitate.elements. signiﬁcant There nickel was and no cobalt significant loss occurring nickel and during cobaltthe loss precipitation occurring during of rare the earth precipitation elements. of rare earth elements. 4 4

3 3 Ce Ce 2 Nd 2 Nd Pr Pr 1 Concentration(g/L) 1 Y

Concentration(g/L) Y Co Co 0 0 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 pH pH Figure 9. Concentration of Co, Ce, Nd, Pr and Y vs pH after addition of NaOH. FigureFigure 9. Concentration 9. Concentration of Co, of Ce, Co, Nd, Ce, PrNd, and Pr Yand vs YpH vs after pH after addition addition of NaOH. of NaOH. In the second series of experiments, hydrochloric acid leach liquor was used (from experiment 8A, In the second series of experiments, hydrochloric acid leach liquor was used (from experiment see TableIn the1). second The results series showof experiments, that about hydrochlor 95% by massic acid of leach the rareliquor earth was elementsused (from precipitate experiment as 8A, see8A, Table see Table 1). The 1). resultsThe results show show that thatabout about 95% 95% by mass by mass of the of therare rare earth earth elements elements precipitate precipitate as as REE2(C2O4)3·nH2O at a 300% excess addition of oxalic acid, see Figure 10. This value increases up REE2(C2O4)3.nH2O at a 300% excess addition of oxalic acid. This value increases up to 100% at 600% toREE 100%2(C at2O 600%4)3.nH2 excessO at a 300% oxalic excess acid addition addition in of experiment oxalic acid. P1–P5.This value At higherincreases concentrations up to 100% at of 600% oxalic excess oxalic acid addition in experiment P1–P5. At higher concentrations of oxalic acid, all of the acid,excess all oxalic of the acid nickel addition precipitates in experiment as well. TheP1–P5. optimum At higher point concentrations seems to be of 300% oxalic excess acid,addition all of the of nickel precipitates as well. The optimum point seems to be 300% excess addition of the reagent. The thenickel reagent. precipitates The relatively as well. highThe optimum dose of oxalic point acidseem neededs to be 300% to precipitate excess addition the REE of isthe due reagent. to the The high

Metals 2018, 8, x FOR PEER REVIEW 14 of 17 Metals 2018, 8, 1062 14 of 17 relatively high dose of oxalic acid needed to precipitate the REE is due to the high concentration of otherconcentration metal ions of otherin the metal solution ions forming in the solution complexe formings with complexes the oxalate with ions. the oxalateThe purities ions. Theof the purities REE oxalateof the REE precipitates oxalate precipitates are 99.9% are(100% 99.9% excess), (100% 90 excess),.1% (300% 90.1% excess) (300% excess)and 39 and.1% 39.1%(600% (600% excess). excess). See FigureBased on10. aBased mass balance,on a mass the balance, overall recoverythe overall of REErecovery after of leaching REE after with leaching sulfuric with acid (2sulfuric mol/L, acid 25 ◦ (2C) mol/L,was calculated 25 °C) was to becalculated 90%. The to overall be 90%. recovery The overall of REE recovery after leaching of REE after with leaching hydrochloric with acidhydrochloric (2 mol/L, acid90 ◦ C)(2 mol/L, was calculated 90 °C) was to becalculated 92%. to be 92%.

10 9 8 7 6 La 5 Ce 4 Nd 3 Y Concentration(g/L) 2 1 Pr 0 0 100 200 300 400 500 600 700 % Excess oxalate amount of stoichiometric needed

Figure 10. Concentration of La, Ce, Nd, Pr and Y after addition of oxalate. Figure 10. Concentration of La, Ce, Nd, Pr and Y after addition of oxalate. 4. Conclusions 4. Conclusions In the anode, the rare earth elements were found to be in in the form of LaNi5 and Ce0.47 La0.34

Nd0.14In Prthe0.05 anode,Ni3.56 theCo 0.75rareAl earth0.29Mn elements0.4 phases. were In thefound cathode to be activein in the material, form of the LaNi5 nickel and hydroxide Ce0.47 La was0.34 Ndthe0.14 main Pr0.05 phase Ni3.56 withCo0.75 theAl0.29 existenceMn0.4 phases. of metallic In the cobalt cathode and active substitutional material, cobaltthe nickel in the hydroxide nickel hydroxide was the maintogether phase with with yttrium the existence (III) oxide. of metallic The supporting cobalt and mesh substitutional of the cathode cobalt was in made the nickel of metallic hydroxide nickel. togetherThe chemical with yttrium composition (III) oxide. showed The that supporting nickel is themesh dominant of the cathod metale inwas both made of the of electrodemetallic nickel. active Thematerials. chemical The composition cathode material’s showed metalthat nickel composition is the dominant was 75.5% metal Ni, 4.7%in both Co, of 0.7% the electrode Y, and 3.5% active Zn, materials.with smaller The amounts cathode of material’s Fe, Al, and metal K. composition was 75.5% Ni, 4.7% Co, 0.7% Y, and 3.5% Zn, ◦ with smallerIt was found amounts that of the Fe, acid Al, leachingand K. of nickel is kinetically limited at 25 C under the conditions appliedIt was and found higher that temperature the acid leaching is required of nickel to is achieve kinetically an industriallylimited at 25 reasonable °C under the leaching conditions rate. appliedThe best and REE, higher Ni, and temperature Co recoveries is required were obtained to achieve when an industrially using an initial reasonable concentration leaching of rate. 2 mol/L The ◦ bestof hydrochloric REE, Ni, and or Co sulfuric recoveries acid and were high obtained temperature when (90usingC). an The initial recoveries concentration of rare earthof 2 mol/L elements of hydrochloricin all experiments or sulfuric done acid under and these high conditions temperature were (90 95% °C). or The more, recoveries except whenof rare leaching earth elements the anode in allmaterial experiments with sulfuric done under acid. In these that specificconditions experiment, were 95% the or recoveries more, except were when reduced leaching to 60–80% the anode due to materialsulfate precipitation with sulfuric during acid. In the that leaching. specific For expe leachingriment, the the anode recoveries material were with reduced sulfuric to acid,60–80% the due best ◦ toconditions sulfate precipitation for rare earth during elements the recovery leaching. were Forusing leaching an initial the anode acid concentration material with of sulfuric 2 mol/L acid, at 25 theC. bestWhen conditions the anode for active rare earth material elements is leached recovery with were its supporting using an initial mesh, acid the concentration anode and cathode of 2 mol/L active at 25materials °C. When were the dissolved, anode active while material the mesh is wasleached partially with dissolvedits supporting leading mesh, to a highthe anode concentration and cathode of Fe activeand Cr materials in the leach were liquor—around dissolved, 4mg/Lwhile andthe 129me mg/L,sh was respectively, partially dissolved depending leading on given to conditions. a high concentrationBy using of nanofiltration Fe and Cr itin was the possible leach liquor—around to recover hydrochloric 4mg/L and acid 129 after mg/L, leaching respectively, the anode dependingmaterial. No on precipitation given conditions. was observed during the nanofiltration operations. The acid can be re-used 2 in leachingBy using of nanofiltration the anode materials. it was However,possible to the recover flux was hydrochloric low (2 L/(m acid, h)), after even leaching when applyingthe anode a material.pressure ofNo 25 precipitation bar. was observed during the nanofiltration operations. The acid can be re-usedBy in addition leaching of NaOH,of the anode the light materials. rare earth However, elements the precipitated flux was low as NaREE(SO (2 L/(m2, 4h)),)2·H even2O around when applyingpH −0.1 a and pressure 0.7 mol/L of 25 sodiumbar. ion concentration with no significant loss of Co and Ni. However, the heavyBy addition REE, Y,of present NaOH, at the a concentrationlight rare earth of elements 0.4 g/L, didprecipitated not precipitate as NaREE(SO under these4)2.H2 conditions.O around pHFor − the0.1 precipitationand 0.7 mol/L of sodium REE2(C ion2O 4concentration)3·nH2O, the optimumwith no significant conditions loss for of precipitating Co and Ni. However, all rare earths the heavywhile avoidingREE, Y, present nickel or at cobalta concentration co-precipitation of 0.4 wasg/L, obtaineddid not precipitate using a stoichiometric under these oxalic conditions. acid excess For ◦ theof 300%. precipitation The overall of REE recovery2(C2O4 of)3.nH REE2O, after the leaching optimum with conditions sulfuric acidfor precipitating (2 mol/L, 25 allC) rare and earths precipitation while avoiding nickel or cobalt co-precipitation was obtained using a stoichiometric oxalic acid excess of

Metals 2018, 8, 1062 15 of 17

of NaREE(SO4)2·H2O was 90%. The overall recovery of REE after leaching with hydrochloric acid (2 mol/L, 90 ◦C) and precipitation of REE oxalates was 92%.

Author Contributions: Conceptualization, K.K. and K.M.F.; Methodology, K.K.; Validation, K.K.; Investigation, K.K.; Resources, K.K. and M.A.; Writing-Original Draft Preparation, K.K.; Writing-Review & Editing, K.K., M.A., Å.C.R. and K.M.F.; Supervision, K.M.F. and Å.C.R.; Project Administration, K.M.F.; Funding Acquisition, K.M.F. Funding: This research was funded by Swedish Energy Agency (Energimyndigheten) with the project number 37724-1. Acknowledgments: The authors would like to acknowledge the Swedish Energy Agency for funding the project (project nr 37724-1) and Chalmers University of Technology, Department of Chemistry and Chemical Engineering for supplying the battery modules and Swedish Environmental Research Institute (IVL) for instrumentation and expertise in connection to conducting the nanofiltration experiments. Part of the work was also supported by the Swedish Foundation for Strategic Research SSF with the grant number of IRT 11-0026. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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