Dissolution and characterization of HEV NiMH batteries

Waste Management 33 (2013) 689–698

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Dissolution and characterization of HEV NiMH batteries Kristian Larsson ⇑, Christian Ekberg, Arvid Ødegaard-Jensen Industrial Materials Recycling, Department of Chemical and Biological Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden

a r t i c l e

i n f o

Article history: Received 24 November 2011 Accepted 1 June 2012 Available online 12 July 2012 Keywords: Dissolution NiMH HEV Batteries Characterization

a b s t r a c t Metal recovery is an essential part of the recycling of hybrid electric vehicle battery waste and the first step in a hydrometallurgical treatment is dissolution of the solid material. The properties of separated battery electrode materials were investigated. Focus was put on both the solid waste and then the dissolution behaviour. The cathode contains metallic nickel that remains undissolved when utilizing non-oxidizing conditions such as hydrochloric or sulphuric acid in combination with a low oxygen atmosphere. In these conditions the cathode active electrode material is fully dissolved. Not dissolving the nickel metal saves up to 37% of the acid consumption for the cathode electrode material. In the commonly used case of oxidizing conditions the nickel metal dissolves and a cobalt-rich phase remains undissolved from the cathode active material. For the anode material a complete and rapid dissolution can be achieved at mild conditions with hydrochloric, nitric or sulphuric acid. Optimal parameters for all cases of dissolution was pH 1 with a reaction time of approximately P20,000 s. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Nickel metal hydride (NiMH) batteries are one of the most common battery types used in hybrid electric vehicles (HEV), the number of which has dramatically increased in the last decade. With a finite lifetime for the vehicles and batteries a need exists to recycle these batteries in order to reuse the materials used in their construction. The general features of the examined NiMH batteries have been described previously by Taniguchi et al. (2001). Several processes for material recovery from NiMH batteries utilizing whole battery dissolution methods have been suggested in the literature (Zhang et al., 1998, 1999; Tzanetakis and Scott, 2004a,b; Nan et al., 2006; Rabah et al., 2008; Li et al., 2009; Bertuol et al., 2009; Rodrigues and Mansur, 2010). However, from a sustainability and optimization point of view it may be more efficient to treat the different electrodes separately. A limited study on electrode dissolution in sulphuric acid was performed by Pietrelli et al. (2005) and a deeper study by Miyake and Maeda (2006) on nickel hydroxide dissolution in ammoniacal aqueous solutions. A more thorough study on the individual electrode dissolution behaviours in acidic solutions is therefore needed. The chemistry of the batteries varies between manufacturers and types of batteries (Pietrelli et al., 2005; Mantuano et al., 2006; Bertuol et al., 2006). Since batteries for HEVs and EVs are fairly uniform and spent batteries are available in large amounts a process specialized for recycling of such batteries may be preferable both economically and practically. This study examines the batter⇑ Corresponding author. Tel.: +46 (0) 317722804. E-mail address: [email protected] (K. Larsson). 0956-053X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2012.06.001

ies from a HEV in order to determine the dissolution behaviour of the electrode materials at a constant pH using nitric, hydrochloric and sulphuric acid. In addition, the physical and chemical properties of the batteries have been studied. The purpose of the constant pH dissolution experiments was to determine the consumption of acid over time to maintain a constant pH. The acid consumption was then used to evaluate the dissolution behaviour, since it is an indirect measurement proportional to the dissolved mass of metal, metal oxide and hydroxide. One objective of this study was to find a method for selective leaching of battery materials, especially avoiding the dissolution of the nickel metal backing of the cathode.

1.1. Theory The principal element in NiMH batteries is nickel with lesser amounts of cobalt, manganese, aluminium, zinc, yttrium and lanthanoids in the electrodes, an alkaline electrolyte (often 30% potassium hydroxide in water, sometimes with a lithium hydroxide addition) and iron in the support material (Pietrelli et al., 2005; Zhang et al., 1999, 1998; Miyake and Maeda, 2006; Linden and Reddy, 2002). The active material in the cathode is a nickel hydroxide material (in a discharged state) containing nickel with cobalt, zinc and yttrium additives. The anode contains a hydrogen absorbing material consisting of nickel, manganese, cobalt, aluminium and lanthanoids (La, Ce, Nd and Pr) in an AB5 type structure (B = Ni, Co, Mn, Al, A = lanthanoids) with Y2O3 or Yb2O3 added for corrosion resistance (Pietrelli et al., 2005; Linden and Reddy, 2002; Sakai et al., 1999). The anode material also contains an organic binding additive.

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The electrodes in the cells in prismatic batteries are stacked, as seen in Fig. 1. The battery reactions are presented below (Linden and Reddy, 2002) with the discharge reaction as the forward reaction. When considering the discharge reaction the positive electrode is the cathode and the negative electrode the anode. Cathode reaction

NiOOH þ H2 O þ e ! NiðOHÞ2 þ OH Anode reaction, M = hydrogen absorbing alloy

MH þ OH ! M þ H2 O þ e Overall reaction

MH þ NiOOH ! M þ NiðOHÞ2 The active materials are pasted onto two different forms of support (current collectors). The cathode requires a strong current collector as Ni(OH)2 is a weak conductor and is therefore most often made as a foam-type mesh structure of nickel metal (Linden and Reddy, 2002). The electrical conductivity can be improved by creating a cobalt-rich conductive surface coating on the active material (Sakai et al., 1999). The greater conductivity of the active anode material allows the anode charge collector to be made from a perforated steel sheet. The electrodes are separated by a permeable separator material which prevents electrical contact between the electrodes but allows ionic transport. The separators are usually made from a mixture of polypropylene and polyethylene fibers whose surface has been sulphonated or carbonated (Linden and Reddy, 2002; Sakai et al., 1999). The nickel hydroxide active material is most commonly a nickel–cobalt–zinc triprecipitate, in which cobalt and zinc content varies between 1% and 5%. The material can also contain calcium and magnesium (Linden and Reddy, 2002). Nickel metal is more resistant to dissolution by non-oxidizing acids (e.g. unaerated dilute hydrochloric acid, unaerated dilute sulphuric acid) compared to oxidizing acids (e.g. nitric acid) (Friend, 1980). The reason for this can be found, not in the oxidative potential of the solution as the thermodynamically stable species at the relevant conditions 2þ is oxidized as NiðaqÞ , but instead in the high overpotential for hydrogen evolution, since hydrogen cannot easily desorb from a nickel surface (Revie, 2000). In most common non-oxidizing acids a supply of an oxidizing species is therefore necessary for rapid dissolution of nickel metal to occur. This means that in order to prevent the disso-

lution of the nickel metal in the cathode material, oxidizing species in the dissolving solution need to be avoided. 2. Experimental 2.1. Mechanical separation Panasonic Prismatic Module 6.5 Ah NiMH 7.2 V plastic casing batteries for HEVs were used in the experiments. The battery modules, shown in Fig. 2, were discharged with a constant current below 4 V and the plastic casing was subsequently cut open and the six cells removed. The electrodes are rectangular and attached to steel side plates alternating between cathode and anode plates. Cutting away the side plate separates the individual electrode plates. The polymer separator covered the cathode plates and thereby protected the plates from contaminating each other. Experiments were performed on samples that were washed with pure water (MilliQ, Millipore, >18 MX/cm), to remove the electrolyte, and subsequently dried in air. The active anode material was removed by light scraping since the material was fairly brittle when dry and easily removed from the supporting plate. The cathode material was pasted to a mesh substrate. Removal of this material from the mesh by means of a mechanical process proved difficult. The modules weighed 1040 g and the individual cells weighed approximately 150 g. The cells were dismantled and consisted of 12 cathode plates and 13 anode plates, with an average dry weight after washing of 4 g and 3.8 g respectively (including supporting plates). The anode supporting material weighed 0.88 g, the cathode electrode supporting mesh weighed 1 g (the hydroxide was removed using dilute sulphuric acid). The possibility of magnetic and mechanical separation methods has been reported (Tsunekawa et al., 2007; Ito et al., 2009, 2010) demonstrating that the individual active electrode materials can be separated. Utilizing the fact that the electrodes are large and well separated should also make a detailed automated disassembly possible, this was confirmed in an external study (Tunell and Adås, 2011). 2.2. Dissolution During the dissolution of materials the solution was monitored in order to keep a constant potential by the automated addition

Fig. 1. A cell from a HEV battery module with the top electrode plates bent up, discharge is the solid lines, charge the dashed lines. The paste area is 32 mm wide and the cathode is covered in the separator.

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Fig. 2. Battery module containing six individual cells.

of acid using a computer controlled potentiostat (Metrohm 905 Titrando). The electrode was calibrated with buffer solutions (Reagecon) for pH 1, 4 and 7, while the measured pH-value was not corrected to compensate for changes in ionic strength. The experiments used 1 M hydrochloric and nitric and sulphuric acid (Fixanal, Sigma–Aldrich) for the titrations. The initial solid to liquid ratio was approximately 50:1 and sample sizes were approximately 1 g. The initial solution was 1 M NaCl(aq) (Scharlau, P99.8%) and 1 M NaNO3(aq) (Merck, P99%) for the hydrochloric and nitrate experiments. For the sulphate experiments 0.5 M Na2SO4(aq) (Fluka, P99%) was used. The solution was adjusted to the relevant pH before the sample was added. 2.3. Analysis A Thermo iCAP 6500 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to determine the elemental composition of the solutions. Total metal content of the electrode materials was determined by dissolving in concentrated (65%) nitric acid (Sigma–Aldrich,) and total cell dissolution was performed in 1.5 L of 8 M nitric acid. A Siemens D5000 diffractometer employing a Cu Ka source and a scintillation detector was used for X-ray Powder Diffraction (XRD) to determine the phases of the anode and cathode materials, using a step size of 0.05°. Identification of crystalline compounds was done by comparison with standards in the JCPDS-ICCD (PDF-4+,2010) database. For Scanning Electron Microscopy (SEM) a FEI Quanta 200 ESEM FEG was used with an Oxford Inca 300 Energy Dispersive X-ray (EDX) system for chemical analysis. Images were taken with 20 kV operating voltage in high vacuum mode. Two detectors were used, a solid state backscatter detector (SSD-BSD) and an Everhart Thornley secondary electron detector (ETD). The SSD detector is sensitive to the atomic number and gives an intensity difference with varying atomic number. 3. Results and discussion Dismantling of a module resulted in the mass balance presented in Table 1, note that the anode material entry does not include the supporting nickel-plated steel plate which is counted as support steel. 3.1. Chemical analysis Total cell dissolution resulted in the compositions shown in Table 2. The dissolutions of electrode materials in concentrated nitric acid gave the compositions shown in Table 3. The predominant metal in both electrode materials is nickel, with a smaller quantity of cobalt and yttrium in both materials. The differences between

Table 1 Mass Balance (wt.%) – one module (1040 g in total). Electrolyte 13% Casing 13%

Support Steel 21% Cathode plates 28%

Separator 3% Anode material 23%

the two electrode materials are that the anode electrode material has a large lanthanoid content together with manganese and aluminium whilst the cathode does not contain these but does contain zinc and magnesium. A potential recycling method could also aim towards recovering the cathode supporting mesh (Fig. 5), which was shown to consist solely of nickel, intact after leaching the active electrode material. The potassium content in Table 3 comes from the potassium hydroxide electrolyte. The lanthanoid content represents 31 wt.% of the anode material and the combination of material is a lanthanum rich misch metal (Mm). The uncertainties given in Table 3 are measurement uncertainties calculated from four duplicate samples diluted to different degrees of a single total digestion in concentrated nitric acid. Values outside the linear range of the instrument were removed.

Table 2 Total metal distribution (wt.%) in one cell. Al 0.6 Mn 2.3

Ce 2.6 Nd 0.9

Co 3.7 Ni 47

Fe 30.8 Pr 0.3

K 3.1 Y 0.6

La 7.4 Zn 0.6

Mg 0.1

Table 3 Dissolution results in concentrated nitric acid. Weightpercent of cathode plate material (including metal support), anode active material and anode metal support plate. Uncertainty given as 1r. (wt.%, nd – not detected or below detection limit).

Al Ce Co Fe K La Mg Mn Nd Ni Pr Y Zn

Cathode

Anode

Anode plate

0.1 ± 0.01 nd 5.7 ± 0.03 nd 0.2 ± 0.08 nd 0.3 ± 0.08 0.2 ± 0.06 nd 64.7 ± 0.45 nd 0.9 ± 0.01 0.5 ± 0.08

1.5 ± 0.07 7.4 ± 0.14 3.6 ± 0.14 0.1 ± 0.01 0.4 ± 0.14 20.2 ± 0.63 nd 5.6 ± 0.15 2.4 ± 0.03 52.3 ± 2.06 1.0 ± 0.07 0.7 ± 0.01 nd

nd nd nd 89 ± 1.55 nd 0.1 ± 0.01 nd 0.4 ± 0.03 nd 5.2 ± 0.15 nd nd nd

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Fig. 3. Cathode powder XRD diffractogram, arb. units versus 2h.

3.2. XRD The cathode active material was removed from the supporting mesh structure and XRD analysis is shown in Fig. 3. The structure was identified as a hexagonal Ni(OH)2 structure (01-076-8988). The broadness of the peaks is due to structural disorder (Ramesh and Kamath, 2008). No metallic nickel or nickel oxide was detected. The yttrium, cobalt and zinc content could not be detected as separate phases. This indicates that the majority of the active material is a hydroxide material. The hydrogen absorbing alloy of the anode was identified as being of the LaNi5 type with substitutions of the lanthanum with different lanthanoids and substitutions of nickel by cobalt, aluminium and manganese. The diffractogram in Fig. 4 was similar to one given by the structure Ni3.56Co0.75Mn0.4Al0.29La0.34Ce0.47Nd0.14Pr0.05 (01-076-7719) (Endo et al., 2008). This combination of substitutions did not correspond exactly to the chemical analysis, which can be used to calculate the stoichiometry of the structure. Assuming a LaNi5 type structure with complete substitutions and that yttrium is present as Y2O3, this gives the structure. Ni4.15Co0.29Mn0.44Al0.12La0.65Ce0.24Nd0.08Pr0.03. This stoichiometry is similar to such materials reported elsewhere in literature

(Yuan and Xu, 2002; Endo et al., 2008). Since there are no strong peaks at 16° or 27–28° only a small amount of Mm(OH)3 corrosion products on the anode. Both active materials are highly homogeneous with only one major phase present in each. 3.3. SEM The interaction volume of the SEM electron beam with the sample is dependent on the incident electron beam energy and material properties and can be estimated using the atomic number, atomic weight and density. For aluminium this entails an interaction volume of approximately 3–4 lm and for neodymium approximately 2 lm. This limits the resolution even though the spot size is much smaller. The depth of the primary X-ray production used to determine compositions is also dependent on material properties such as density and the characteristic energy (Ec) of the relevant X-ray emission. The investigated elements with the lowest Ec and 20 keV beam energy have a depth of up to approximately 3–4 lm (Goldstein et al., 2003). This means there is a spillover from surrounding material during spot analysis unless the investigated particle is of sufficient size.

Fig. 4. Anode powder XRD diffractogram, arb. units versus 2h.

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3.3.1. Cathode The cathode material was analysed using SEM (shown in Figs. 5 and 6) to determine morphology and surface compositions. Fig. 5 shows the support material of the cathode having been stripped of active material with dilute nitric acid. EDX and dissolution measurements show that it consists entirely of nickel metal. The shape of this mesh material is consistent with meshes created with foam techniques (Linden and Reddy, 2002). The cathode active material on the mesh support is shown in Fig. 6. A number of spherical hydroxide particles from different sites on a single electrode were examined with EDX, as seen in Table 4. The results show that there are local variations in the metal composition but that the particles are mainly nickel with a fairly constant amount of substituents. The high oxygen content is consistent with a hydroxide material and varies due to the variable amount of oxygen containing material on the particles. Between the hydroxide spheres yttrium oxide was present as elongated pins up to 10 lm long or as cubic particles with a side of approximately 15 lm. The pins were distributed throughout the hydroxide material, the larger cubic particles were primarily at the surface of the electrode material. Table 5 shows EDX results for spot analysis of the yttrium oxide pins and cubes. Between the spherical hydroxide particles were areas with high concentration of cobalt and manganese. This mate-

Fig. 5. SEM image of cathode substrate mesh, SSD detector.

Fig. 6. Cathode active material (Ni (OH)2 spheres), ETD detector.

Table 4 Spherical hydroxide particle spot analysis EDX results (wt.%, nd – not detected or below detection limit). Al

Co

Fe

K

Mg

Mn

Ni

O

Y

Zn

1 2 3 4 5 6

0.4 nd 0.3 0.2 0.3 0.2

3.5 5.1 3.7 3.6 3.2 6.0

1.2 1.3 nd 1.2 nd nd

0.2 0.2 0.4 nd 0.2 0.3

0.6 0.6 1.2 0.8 0.8 0.9

0.6 0.8 0.6 0.5 0.6 0.7

38.3 51.3 57.7 36.7 42.1 40.8

55.2 40.1 35.0 56.5 51.9 49.9

nd nd nd nd 1.1 1.1

nd 0.7 1.2 0.4 nd nd

Avg.

0.3

4.2

–

0.2

0.8

0.7

44.5

48.1

–

–

Table 5 Yttrium oxide spot analysis EDX results (wt.%, nd – not detected or below detection limit).

YO pin YO Cube

Al

Co

Fe

K

Mg

Mn

Ni

O

Y

Zn

nd nd

1.2 nd

nd nd

nd 0.2

nd nd

nd nd

16.3 2.3

33.7 43.1

48.9 54.5

nd nd

rial creates a conductive network structure between the hydroxide spheres, the structure is shown in Fig. 7. Spot analysis with EDX shown in Table 6 gives two different compositions with regard to oxygen content. 3.3.2. Anode The anode support and active material are shown in Figs. 8 and 9 respectively. The support was made of nickel-plated steel and had very little active material remaining after the physical removal was carried out. This shows that the active material is possible to remove from the surface of the support, which is positive for the recovery of the anode battery materials. The particles of the anode material have a varied size distribution ranging from very small up to approximately 50 lm. The larger particles of the active anode material shown in Fig. 9 were quite homogeneous. Primarily there was a slight variation in the lanthanoid ratios between larger particles determined by EDX analysis as shown in Table 7. The yttrium content is not part of the alloy material since the ratio of A (lanthanoids) to B (nickel and substituents) in the AB5 type structure changes significantly if the yttrium is included as A in the structure (consider points 11 and 12 in Table 7). The yttrium content was concentrated in smaller particles around the larger alloy particles, probably as yttrium oxide. The praseodym-

Fig. 7. Cobalt and manganese network structure, SSD detector.

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Table 6 Cobalt structure spot analysis EDX results (wt.%, nd – not detected or below detection limit).

Oxidized Co structure Oxidized Co structure Average Co Structure Co Structure Average

Al

Co

Fe

K

Mg

Mn

Ni

O

Y

Zn

nd nd

50.4 50.7 50.5 79.1 81.2 80.1

nd nd

0.2 0.3 0.3 nd 0.8 0.8

nd nd

1.6 1.6 1.6 1.0 3.2 2.1

5.2 3.1 4.2 6.6 3.5 5.1

41.4 43.7 42.5 13.3 10.8 12.1

0.8 0.7 0.7 nd 0.5 0.5

0.4 nd 0.4 nd nd

nd nd

nd nd

nd nd

Table 7 Anode active material spot analysis EDX results, the points are referencing points in Fig. 9 (wt.%, nd – not detected or below detection limit).

1 2 3 4 5 6 7 8 9 10 11 12

Fig. 8. Anode nickel plated steel support, ETD detector.

Al

Ce

Co

Fe

K

La

Mn

Nd

Ni

Pr

Y

1.3 0.4 1.2 1.7 1.3 1.5 1.3 1.3 1.2 0.5 1.0 0.4

8.7 9.3 7.5 9.1 8.9 8.3 8.7 8.1 8.3 7.2 7.4 6.8

5.2 4.1 6.1 4.7 5.2 5.4 4.4 4.7 5.2 10.3 4.4 3.8

nd nd nd nd nd nd 0.4 0.4 0.4 1.1 0.3 0.0

nd nd nd nd nd nd nd nd 0.3 0.4 0.2 0.2

22.7 21.4 20.2 21.9 21.2 21.0 21.3 20.7 21.3 19.4 18.2 16.7

3.9 5.3 5.5 5.6 5.3 6.2 5.4 5.4 5.1 3.4 5.1 3.9

nd 2.3 3.3 2.2 2.2 2.1 2.5 2.3 2.2 2.4 2.0 2.3

58.2 57.1 56.2 53.0 54.8 54.4 54.6 54.8 51.8 50.0 47.0 37.7

nd nd nd 0.8 nd nd nd nd nd nd nd nd

nd nd nd 0.9 1.1 1.1 1.3 2.3 4.2 5.4 14.3 28.2

surface area of the material) the figures with added volume acid have been normalized with respect to mass in order to make the experiments comparable. Thereby Figs. 10–19 show the amount of acid consumed per gram electrode material dissolved. Note that there is some uncertainty due to inhomogeneities in the samples, the normalizing procedure described above does not account for these differences. The consumption of acid (protons) to dissolve the active materials can be calculated from the total dissolution values in Table 3. Assuming that metals are present in the form of oxides, hydroxides and metals the following reactions are likely for the dissolution process.

XðOHÞnðsÞ þ nHþðaqÞ ! Xnþ ðaqÞ þ nH2 OðlÞ ð2n=mÞþ

Xm OnðsÞ þ 2nHþðaqÞ ! mXðaqÞ

þ nH2 OðlÞ

n H2ðgÞ XðsÞ þ nHþðaqÞ ! Xnþ ðaqÞ þ 2 Assuming nickel, cobalt, magnesium, manganese and zinc to be divalent after dissolution and thereby require a molar ratio of acid to metal of 2:1, and that iron, aluminium, yttrium and lanthanoids

Fig. 9. SEM image of anode active material, SSD detector.

ium content was not detected due to its concentration being below the detection limit.

3.4. Dissolution The added acid volume was used as a direct measurement of how much material had been dissolved. The dissolution rate for individual elements can be identified using concentrations measurements, this difference in rate was insufficient to allow separation of elements and the total rate described by acid consumption was deemed more relevant. Since the rate of dissolution is a function of the amount of the substance to be dissolved (related to the

Fig. 10. Nitric acid dissolution of cathode material at 30 °C, pH 1, 1.5, 2 and 2.5 (left to right).

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Fig. 11. Hydrochloric acid dissolution of cathode material at 30 °C, pH 1, 1.5 and 2 (left to right), oxidizing atmosphere (air).

Fig. 14. Sulphuric acid dissolution of cathode material at 30 °C with 1 M H2SO4, pH 1, 1.5 and 2 (left to right, before 20 ks), oxidizing atmosphere (air).

Fig. 12. Hydrochloric acid dissolution of cathode material at 30 °C, nitrogen atmosphere, pH 1, 1.5, 2 and 3 (left to right).

Fig. 15. Sulphuric acid dissolution of cathode material at 30 °C with 1 M H2SO4, nitrogen atmosphere, pH 1, 1.5 and 2 (left to right).

Fig. 13. Hydrochloric acid dissolution of cathode material at 30 °C, nitrogen atmosphere (thick line), air atmosphere (thin line), pH 1, 1.5 and 2 (left to right).

to be trivalent and require a molar ratio of 3:1. The calculations give a consumption of 24.9 mmol of protons per gram of cathode electrode material, 9.2 mmol/g of which would come from the nickel metal mesh and 15.7 mmol/g from the hydroxide material. Not dissolving the nickel metal thereby reduces the acid consumption for the cathode by approximately 37%. For the anode active material, a consumption of 29.7 mmol protons per gram is needed to dissolve the material.

3.5. Cathode dissolution The cathode electrodes were dissolved in nitric, sulphuric and hydrochloric acid in order to investigate the rate of dissolution at

Fig. 16. Sulphuric acid dissolution of cathode material at 30 °C with 1 M H2SO4, nitrogen atmosphere (thick line), air atmosphere (thin line), pH 1, 1.5 and 2 (left to right, before 20 ks).

different pH. To maintain a low pH when raising the volume, the entire content of protons in the added acid cannot be consumed by dissolution. This is seen in Figs. 10 and 12, where the final consumption of acid is higher for the pH 1 case, which are reasonable values considering only approximately 90% of the 1 M hydrochloric and nitric acid can be consumed by dissolution. Similarly, the sulphuric acid dissolution experiments carried out at a lower pH require a higher amount of final added volume. Additionally, an effect from the level of dissociation of sulphuric acid influences the consumption. The level of hydrogen sulphate formed will influence the amount of protons available for dissolution and the protons required to maintain a low pH in the added volume.

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des, since the dissolution rate of cobalt oxides in acidic media is very slow, much slower than metallic cobalt (Vu et al., 1980). A mixed valence cobalt oxide, cobalto-cobaltic oxide (Co3O4), is slower to dissolve than cobaltous oxide (CoO). However, the oxidation state of the metals in the residue was not determined.

Fig. 17. Nitric acid dissolution of anode active material with 1 M HNO3 at 30 °C, pH 3, 3.5 and 4.5 (left to right).

Fig. 18. Sulphuric acid dissolution of anode active material with 1 M H2SO4 at 30 °C, pH 1, 1.5, 2, 3 and 3.5 (left to right).

Fig. 19. Hydrochloric acid dissolution of anode active material with 1 M HCl at 30 °C, pH 1, 1.5, 2 and 3 (left to right).

3.5.1. Nitric acid dissolution As seen in Fig. 10, the interaction of nitric acid with the cathode material is such that the dissolution of material occurs in two distinct steps; dissolution of the hydroxide material followed by the dissolution of the metal mesh. If the dissolution is allowed to proceed the nickel mesh will be rapidly dissolved. The second step was confirmed by concentration measurements being due solely to the dissolution of nickel metal. At the end of the first step of dissolution of active material there was a residue rich in cobalt and manganese (determined by dissolution of the residue) in all the nitric acid experiments. The composition of the cobalt and manganese residue corresponds well to the cobalt and manganese network structures observed with SEM/EDX. The compounds are likely oxi-

3.5.2. Hydrochloric acid dissolution As seen in Fig. 11, there is no plateau in the dissolution behaviour for hydrochloric acid as there was for nitric acid. Rather, it is possible to observe one region with fast dissolution which corresponds to the dissolution of the hydroxide phase. Thereafter the metallic phase continues to dissolve at a considerably slower rate. There is no clear break-point between the hydroxide dissolution and the dissolution of the metal mesh. Figs. 11 and 12 show that low oxygen conditions (bubbled nitrogen) reduce the dissolution rate of the nickel metal. The bulk of the hydroxide material was dissolved after approximately 20 ks, see Fig. 13 at pH 1. The rate of dissolution of hydroxide material in low oxygen conditions is increased at pH 1, 1.5 and decreased at pH 2. No hydroxide residues were found and the nickel metal dissolution was extremely slow. The average metal composition in the solutions obtained at the end of the dissolution experiments are shown in Table 8. The results show that approximately 27 wt.% of the cathode total weight is in the form of metallic nickel, which means that the cobalt content in the hydroxide material is approximately 12.4%. 3.5.3. Sulphuric acid dissolution When sulphuric acid is used for dissolution the chemical behaviour is somewhat similar to that of hydrochloric acid, as seen when comparing Figs. 11 and 14. The initial slope of the curve describing the acid consumption primarily corresponds to the dissolution of active hydroxide material and, when the active material has largely been dissolved, then the dissolution of the nickel metal starts to dominate. A residue rich in cobalt and manganese (determined by dissolution of the residue) remained at the end of the initial dissolution of active material in all the sulphate experiments. This residue constitutes a relatively small mass which is difficult to appreciate from Fig. 15. This affects the dissolution by lengthening the time needed until all active material has been dissolved as compared to the hydrochloric acid dissolution. This cobalt and manganese residue corresponds well to the cobalt and manganese network structures observed with SEM/EDX. Similar to the hydrochloric acid case, the removal of oxygen reduces the dissolution of the nickel metal as shown in Fig. 16. The average remaining nickel metal at the end of dissolution in a nitrogen atmosphere was 27 wt.% of the original sample weight, which is consistent with the results obtained in dissolution experiments with hydrochloric acid. 3.6. Anode dissolution The anode dissolution experiments were performed on washed anode active material after being removed from the nickel-plated Table 8 Metal distribution of active cathode material after selective dissolution HCl in nitrogen atmosphere and calculated values for the hydroxide dissolution from the total dissolution values and the total cathode dissolution distribution (%).

Al Co Mg Mn Ni Y Zn

HCl, N2

Total diss. calc.

Total cathode

0.4 12.2 1.1 0.7 82.5 2 1

0.2 12.4 0.7 0.4 82.9 1.9 1.1

0.14 7.8 0.4 0.2 89 1.2 0.7

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steel backing material. Comparisons with the experimental results (total acid consumed) in Figs. 17–19 show that quite reasonable values were obtained. Due to the high rate of reaction of the anode material a rise above the given pH was possible when the rate of addition in the experiments exceeded 1 ml/min. 3.6.1. Nitric acid dissolution The acid consumption shown in Fig. 17 was rapid at pH values at or below 3. The bulk of the active material was dissolved at 20 ks for pH values below 3.5. The dissolutions at pH 3 and 3.5 required more than 35 ml/g of 1 M nitric acid, which was more than the expected equivalent value of 29.7 mmol/g. 3.6.2. Sulphuric acid dissolution The consumption rate of sulphuric acid in the dissolution experiments shown in Fig. 18 were rapid at pH values at or below 3, with the bulk of the active material dissolved at 10 ks for pH values at or below 2. The sulphuric acid anode dissolution rate results were similar to the results for nitric acid anode dissolution. 3.6.3. Hydrochloric acid dissolution In the hydrochloric acid dissolution experiments shown in Fig. 19, at pH 3 the rate of reaction was initially slow but increased over time. The final volume is approximately 30 ml acid per gram for all the experiments, which is 5 ml acid per gram lower than for nitric acid. 3.7. Discussion The complexation of dissolved metals by the counter-ions is likely to influence the dissolution rate as the free metal concentration in the solution is decreased by complexation. This is apparent when comparing the dissolution rate when using hydrochloric and sulphuric acid, for both the cathode and anode cases, where the rate in the sulphate case is slightly higher at equivalent pH. This is consistent with known stability constants for nickel where nitrate < chloride < sulphate (Hummel, 2002). Since the nitrate dissolution follows a different mechanism the results are not comparable with the chloride or sulphate cases. The counter-ion concentration influences this and it should be noted that the sulphate concentration is half the chloride (this was done in order to have similar ion strength). It is worth noting that the dissolution experiments were performed in dilute solutions (with regard to the metals to be dissolved) and that the dissolution behaviour might differ in more concentrated solutions. Fully dissolving all the cathode electrodes in one battery pack (28 modules) would require approximately 200 mol of protons to be consumed. Not dissolving the nickel metal reduces this to 126 mol of protons. In turn dissolving the active anode material in turn requires approximately 189 mol of protons. The reduction achieved by not dissolving the nickel metal precludes the use of nitric acid at lower pH in any feed for dissolution containing cathode plates. The dissolution behaviour resulting from a mixture of the electrode material was briefly examined and has an additive appearance. Due to the metallic nickel in the mixed material the preferable dissolution method is with hydrochloric or sulphuric acid in a nitrogen atmosphere to avoid nickel dissolution and to dissolve the cobalt/manganese material. Recovering all the metal from the modules of a single HEV battery pack (28 modules, 168 cells) means recovering approximately 9 kg of nickel, 2 kg of lanthanoids, 6 kg of iron, 0.7 kg of cobalt and smaller amounts of manganese, magnesium, potassium, zinc and aluminium. Without a selective separation, selective dissolution or washing of the material, dissolving the entire metal content

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(and neutralising the electrolyte) of all modules in a battery pack requires approximately 750 mol of protons. 4. Conclusions Nickel metal hydride batteries for hybrid electric vehicles were characterized using three main methods; XRD, chemical elemental analysis and SEM. Chemical analysis of the batteries, in combination with XRD showed that these contained nickel in three different chemical forms; nickel hydroxide, metallic nickel and as part of the hydrogen absorbing alloy of the anode. SEM studies showed the morphology of the different electrode materials. The anode active material was found to be a metallic alloy of the general form Ni4.15Co0.29Mn0.44Al0.12La0.65Ce0.24Nd0.08Pr0.03 (local variation) with additions of Y2O3. The cathode active material had a total metal distribution of 82.9% nickel, 12.4% cobalt, 1.1% zinc, 0.7% magnesium, 0.4% manganese and 1.9% yttrium. The cathode active material had four types of structures, nickel hydroxide material, two types of cobalt and manganese structures and yttrium oxide. Two specialized waste streams can be attained from these batteries, one for the cathode material and one for the anode material. This would be beneficial in reducing the acid required to dissolve the cathode material and an initial separation of elements since the cathode material lacks lanthanoids and the anode material has no magnesium or zinc. The acid consumption data can easily be compared between acids and be used to evaluate the total volume of a potential process and can e.g. be adopted in a LCA analysis in order to compare methods. The results of the dissolution experiments showed that selective leaching of the cathode material results in a residue of undissolved nickel metal, thus reducing the use of acid in the leaching part of a process. Using hydrochloric or sulphuric acid at 30 °C and pH 1 or above, in a nitrogen atmosphere (low oxygen partial pressure), to dissolve cathode material does not dissolve the nickel metal backing material and gives a relatively fast dissolution of the active material. Hydrochloric acid seems more suitable than sulphuric acid for these conditions as the dissolution of the cobalt rich phase is faster and it lacks the buffering behaviour of the sulphuric acid when adjusting the pH upwards. Nitric acid dissolves nickel metal at the tested conditions and has an incomplete dissolution of the cathode active material (cobalt and manganese phase). It is therefore unsuited for cathode dissolution. Complete dissolution of the anode active material can be achieved at the tested conditions with hydrochloric, nitric or sulphuric acid. Nitric and sulphuric acid dissolutions were more rapid than hydrochloric acid dissolutions, especially for pH 3 and above. Acknowledgements The authors would like to acknowledge the financial support of Stena Metall AB and VINNOVA (the Swedish government agency for innovation systems) under Grant 2009-04440 for financial support. References Bertuol, D.A., Bernardes, A.M., Tenorio, J.A.S., 2006. Spent NiMH batteries: Characterization and metal recovery through mechanical processing. Journal of Power Sources 160, 1465–1470. Bertuol, D.A., Bernardes, A.M., Tenorio, J.A.S., 2009. Spent NiMH batteries – the role of selective precipitations in the recovery of valuable metals. Journal of Power Sources 193, 914–923. Endo, D., Sakaki, K., Akiba, E., 2008. Effect of rare earth on lattice size and equilibrium hydrogen pressure for AB5-type MmNi3.55Co0.75Al0.30Mn0.40. Journal of Alloys and Compounds 459, 215–219. Friend, W.Z., 1980. Corrosion of Nickel and Nickel-base Alloys. The Corrosion Monograph Series. Wiley, New York, ISBN-10: 0471282855. Goldstein, J., Newbury, D., Joy, D., Lyman, C., Echlin, P., Lifshin, E., Sawyer, L., Michael, J., 2003. Scanning Electron Microscopy and X-ray Microanalysis, third ed. Kluwer Academic Publishers Group, ISBN: 978-0-306-47292-3.

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K. Larsson et al. / Waste Management 33 (2013) 689–698

Hummel, W., 2002. Nagra/PSI Chemical Thermodynamic Data Base Version 01/01. Universal-Publishers. ISBN: 1-58112-620-4 Ito, M., Kashiwaya, K., Sumiya, N., Hiroyoshi, N., Tsunekawa, M., 2009. Anode activating agent recovery by magnetic separation from the <0.075 mm fraction of crushed nickel metal hydride batteries from hybrid vehicles. Separation and Purification Technology 69, 149–152. Ito, M., Kashiwaya, K.b., Sumiya, N.c., Furuya, H.d., Hiroyoshi, N., Tsunekawa, M., 2010. Estimating the size distribution of anode and cathode activating agents in the crushed products of nickel metal hydride batteries from hybrid vehicles and its classification. International Journal of Mineral Processing 97, 92–95. Li, L., Xu, S., Ju, Z., Wu, F., 2009. Recovery of Ni, Co and rare earths from spent Ni– metal hydride batteries and preparation of spherical Ni(OH)2. Hydrometallurgy 100, 41–46. Linden, D., Reddy, T. (Eds.), 2002. Handbook of Batteries, third ed. McGraw-Hill. Mantuano, D.P., Dorella, G., Elias, R.C.A., Mansur, M.B., 2006. Analysis of a hydrometallurgical route to recover base metals from spent rechargeable batteries by liquid–liquid extraction with Cyanex 272. Journal of Power Sources 159, 1510–1518. Miyake, M., Maeda, M., 2006. Dissolution of nickel hydroxide in ammoniacal aqueous solutions. Metallurgical and Materials Transactions B 37, 181–188. Nan, J., Han, D., Yang, M., Cui, M., 2006. Dismantling, recovery, and reuse of spent nickel–metal hydride batteries. Journal of The Electrochemical Society 153, A101–A105. Pietrelli, L., Bellomo, B., Fontana, D., Montereali, M., 2005. Characterization and leaching of NiCd and NiMH spent batteries for the recovery of metals. Waste Management 25, 221–226. Rabah, M., Farghaly, F., Motaleb, M.A.E., 2008. Recovery of nickel, cobalt and some salts from spent NiMH batteries. Waste Management 28, 1159–1167. Ramesh, T., Kamath, P.V., 2008. Planar defects in layered hydroxides: simulation and structure refinement of b-nickel hydroxide. Materials Research Bulletin 43, 3227–3233.

Revie, R.W. (Ed.), 2000. Uhlig’s Corrosion Handbook, . 2nd ed.. Electrochemical Society 2nd ed. John Wiley & Sons. Rodrigues, L.E.O.C., Mansur, M.B., 2010. Hydrometallurgical separation of rare earth elements, cobalt and nickel from spent nickel–metal-hydride batteries. Journal of Power Sources 195, 3735–3741. Sakai, T., Uehara, I., Ishikawa, H., 1999. R&D on metal hydride materials and Ni–MH batteries in Japan. Journal of Alloys and Compounds, 762–769. Taniguchi, A., Fujioka, N., Ikoma, M., Ohta, A., 2001. Development of nickel/metalhydride batteries for EVS and HEVs. Journal of Power Sources 100, 117–124. Tsunekawa, M., Ito, M., Furuya, H., Hiroyoshi, N., 2007. The recovery of electrode compounds from waste nickel metal hydride batteries by physical separation techniques. Materials Transactions 48, 1089–1094. Tunell, H., Adås, C., 2011. CITRET Rapport Batteridemontering. Technical Report. Chalmers Innovation Technology Recycling Development AB. Ghalmers Teknikpark, SE-412 88 Göteborg, Sweden. Tzanetakis, N., Scott, K., 2004a. Recycling of nickel–metal hydride batteries. I: Dissolution and solvent extraction of metals. Journal of Chemical Technology and Biotechnology 79, 919–926. Tzanetakis, N., Scott, K., 2004b. Recycling of nickel–metal hydride batteries II: Electrochemical deposition of cobalt and nickel. Journal of Chemical Technology and Biotechnology 79, 927–934. Vu, C., Han, K., Lawson, F., 1980. Leaching behaviour of cobaltous and cobaltocobaltic oxides in ammonia and acid solutions. Hydrometallurgy 6, 75–87. Yuan, X., Xu, N., 2002. Electrochemical and hydrogen transport kinetic performance of MlNi3.75Co0.65Mn0.4Al0.2 metal hydride electrodes at various temperatures. Journal of The Electrochemical Society 149, A407–A413. Zhang, P., Yokoyama, T., Itabashi, O., Wakui, Y., Suzuki, T.M., Inoue, K., 1998. Hydrometallurgical process for recovery of metal values from spent nickel– metal hydride secondary batteries. Hydrometallurgy 50, 61–75. Zhang, P., Yokoyama, T., Itabashi, O., Wakui, Y., Suzuki, T.M., Inoue, K., 1999. Recovery of metal values from spent nickel–metal hydride rechargeable batteries. Journal of Power Sources 77, 116–122.