Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 79:927934 (online: 2004)DOI: 10.1002/jctb.1082

Recycling of nickelmetal hydride batteries.II: Electrochemical deposition of cobalt andnickelN Tzanetakis and K ScottSchool of Chemical Engineering and Advanced Materials, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK

Abstract: A combination of hydrometallurgical and electrochemical processes has been developed forthe separation and recovery of nickel and cobalt from cylindrical nickelmetal hydride rechargeablebatteries. Leaching tests revealed that a 4 mol dm3 hydrochloric acid solution at 95 C was suitable todissolve all metals from the battery after 3 h dissolution. The rare earths were separated from the leachingsolution by solvent extraction with 25% bis(2-ethylhexyl)phosphoric acid (D2EHPA) in kerosene. Thenickel and cobalt present in the aqueous phase were subjected to electrowinning. Galvanostatic tests onsimulated aqueous solutions investigated the effect of current density, pH, and temperature with regard tocurrent efficiency and deposit composition and morphology. The results indicated that achieving an NiCocomposition with desirable properties was possible by varying the applied current density. Preferentialcobalt deposition was observed at low current densities. Galvanostatic tests using solutions obtained fromtreatment of batteries revealed that the aqueous chloride phase, obtained from the extraction, was suitablefor recovery of nickel and cobalt through simultaneous electrodeposition. Scanning electron micrographyand X-ray diffraction analysis gave detailed information of the morphology and the crystallographicorientation of the obtained deposits. 2004 Society of Chemical Industry

Keywords: nickelmetal hydride batteries; rare earth elements; liquid extraction; recycling; electrodeposition;nickel

1 INTRODUCTIONThe worldwide market of rechargeable batteries isgrowing at a record pace due to increased con-sumer demand for portable devices. The nickelmetalhydride (NiMH) battery is one of the most pop-ular rechargeable batteries for use in portabledevices, power tools, phones, camcorders and portablecomputers.13 The high energy density and long cyclelife of this battery make it a leading technology for thepower source for electric vehicles.47

Commercial production of NiMH began in 1993with 100 million cells. By the year 1999 more than900 million cells, corresponding to 20 000 tonnes ofbatteries were manufactured world-wide.8,9 In the year2000, the one billion mark was passed, representing4% of the world-wide rechargeable battery production,with a market of 720 million. In the year 2010, NiMHbatteries are expected to comprise 29% of the totalbattery market, with a world-wide battery market of9 billion.10

Since the use of NiMH batteries is large, theexpected amount of scrap at the end of their life is

very large, representing an important source of envi-ronmental pollution if spent batteries are thrown away.Recycling of batteries is necessary both from an envi-ronmental point of view and from the fact that NiMHcells have a valuable metal content of nickel, cobalt,and rare earth elements. Various investigations forrecovering valuable metals from NiMH rechargeablebatteries have been reported,1118 most of which usea combination of mechanical and hydrometallurgicalprocesses including dissolution in inorganic acids,11

solvent extraction, and precipitation.14,15

There are many cell types of nickelmetal hydridecells available on the market (ie cylindrical, prismatic,and button cell). Since battery composition andmaterials differ between manufacturers, the workreported here describes what can be expected froma hydrometallurgical and electrochemical treatmentof NiMH batteries. The present study evaluatesthe recycling of NiMH rechargeable batteries usinghydrometallurgical processes with electrowinning forthe recovery of nickel and cobalt. An aim of this work isto recover a nickelcobalt alloy, from spent batteries,

Correspondence to: K Scott, School of Chemical Engineering and Advanced Materials, University of Newcastle upon Tyne, Newcastleupon Tyne, NE1 7RU, UKE-mail: k.scott@ncl.ac.ukContract/grant sponsor: EPSRC/HEFCE; contract/grant number: JIF4NESCEQ(Received 2 February 2004; revised version received 25 March 2004; accepted 15 April 2004)Published online 28 July 2004

2004 Society of Chemical Industry. J Chem Technol Biotechnol 02682575/2004/$30.00 927

N Tzanetakis, K Scott

with good properties, suitable for various technologicalapplications.

Electrodeposition of nickelcobalt alloys has beenwidely studied due to their excellent properties, suchas hardness and strength, and various applications inthe electronic, mechanical, magnetic, and aerospaceindustries.1923

A study of the influence of deposition parameterswas carried out to assess the effect of theseparameters both on the electrochemical responseand energy consumption, and morphology andcomposition of the obtained deposit. Optimumoperating conditions for alloy electrodeposition wereexperimentally established.

2 EXPERIMENTAL2.1 Dissolution testsCylindrical AA-size NiMH batteries, manufacturedin Japan (Toshiba) were used in this work. Thebatteries, which weighed an average of 23 g, werecut in half longitudinally and after removal of theexternal stainless steel case, were leached in aqueousHC1 solutions. Hydrochloric acid was employed asa leaching agent due to the feasibility of extractingnickel ions from chloride solutions. After dissolution,the leachate and insoluble materials, such as thepolypropylene separator and the iron grid, wereseparated by filtration.

2.2 Solvent extraction testsSolvent extraction tests were conducted on thechloride solutions obtained from leaching. Theextractions were performed by mechanical shakingof 500 cm3 conical flasks each containing 100 cm3

of the aqueous and organic solutions. A solution of25% bis-(2-ethylhexyl)phosphoric acid (D2EHPA) inkerosene was chosen as a solvent for the feasibility ofselective complexation and extraction of rare earths, atlow pH, but not of nickel and cobalt.2427 The pH ofthe aqueous and organic solution was adjusted to thedesired value by drop-wise addition of concentratedsodium hydroxide solution. After shaking, the twophases were allowed to disengage completely and thenseparated into raffinate and organic phases.

2.3 Electrodeposition testsElectrochemical experiments were performed in arectangular acrylic flow cell with removable electrodes.A platinum oxide-based coated titanium anode and astainless steel cathode, each of 35 cm2 area, were usedwith a saturated calomel electrode as a reference.The cell was divided into two compartments by aNafionR 117 membrane. Figure 1 shows the designcharacteristics of the flow cell used for the tests.

Prior to each experiment the stainless steel cathodewas abraded with silicon carbide paper (P600), rinsedwith acetone and then distilled water. Electrochemi-cal experiments were accomplished with a potentio-stat/galvanostat (Sycopel Scientific) controlled with

Anolyte Outlet

Working ElectrodeCation-Exchange Membrane

10 cm

22 cm

Catholyte Inlet

Counter Electrode

Anolyte Inlet

2 cm

Catholyte Outlet

3 cm

Saturated Calomel Electrode

Flow Distribution

Figure 1. Design characteristics of the two-compartment acrylicelectrodeposition cell.

Sycopel Scientific electrochemistry software. Duringthe test, cell voltages were continuously recorded witha multimeter.

The simulated solutions were prepared fromanalytical grade metal sulfate salts (Aldrich). Boricacid, concentration of 0.15 g dm3, was used as abuffer and 1 mol dm3 Na2SO4 as an anolyte. Eachexperiment was carried out using a fresh solutionwhich was deoxygenated with nitrogen for 30 minprior to electrolysis.

The solutions were circulated from the two 1 dm3

glass reservoirs to the cell by a peristaltic pump at aflow rate of 0.12 dm3 min1. Throughout the tests thesolutions in the reservoirs were stirred with magneticstirrers. Samples were taken from the catholyte everyhour and the amount of deposited metal was calculatedby difference from the initial value. Small amountsof metal ions migrated into the anolyte after longterm electrolysis and these amounts were allowed forin determining the current efficiencies and depositcompositions. Depending on the charge passed, up toapproximately 300 and 35 ppm of nickel and cobaltrespectively, diffused into the anolyte.

The electrochemical equivalent of the deposit, iethe charge used for total metal deposition (practicalcharge), was determined from its composition. Thevalue was then compared with the electric chargepassed through the system (theoretical charge) todetermine the current efficiency. The metal ioncontent of various elements was determined byatomic absorption analysis, using a Unicam 929 AAspectrometer.

The morphology and the crystal structure of thedeposits were examined using a Hitachi S-2400scanning electron microscope and a Philips X PertPro diffractometer respectively.

3 RESULTS AND DISCUSSIONS3.1 Leaching testsThe Parameters investigated during the dissolutiontests were acid concentration, dissolution time, and

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Recycling of nickelmetal hydride batteries. II

temperature. Detailed description of this work isgiven in another communication.28 It is importantto mention that dissolution of all metals was found todepend significantly on leaching temperature. Nearlycomplete dissolution occurred for cobalt, nickel, rareearths, zinc and manganese after 3 h using 3 mol dm3HC1 at a temperature of 95 C. Low iron dissolutionof the electrode grids was desirable for minimalcontamination of Ni and Co during recovery. Howeversome extraction of Fe was unavoidable due to the needfor high nickel recovery. After filtration of the leachingsolution the resulting green chloride solution had a pHclose to zero and a composition of (in g dm3): 55.7Ni, 5.7 Co, 2.2 Mn, 0.99 Zn, 2.8 Fe, 1.1 Al, 6.4 La,6.2 Ce and 2.5 Nd.

3.2 Solvent extraction experimentsSolvent extraction using 25% D2EHPA (0.75 moldm3) as an extractant, demonstrated the feasibilityof recovering the rare earths, while leaving Ni, Coand Mn in the aqueous phase to later undergoelectrowinning. Extraction tests over a wide rangeof aqueous pH (03) indicated that, at a pH close to2.5, all the rare earths and most of the iron could beextracted into the organic phase and hence separatedfrom nickel and cobalt.28 Extraction of manganese,aluminium and zinc was relatively low over the wholerange of acidity possibly as a result of preferentialextraction of rare earths and iron.14

3.3 Electrochemical depositionThis is a preliminary study to illustrate the feasibilityof using electrodeposition for the recovery of nickeland cobalt from the leach solutions from spentNiMH batteries and initially investigation of thevoltammetric characteristics of the systems wascarried out.

Studies using simulated solutions were performedto evaluate the effect of current density, pH, andtemperature on the composition and morphology ofthe electrodeposits. The results are then comparedwith those obtained using solutions derived from thehydrometallurgical treatment of batteries.

3.3.1 Voltammetric studiesFigure 2 shows typical linear sweep voltammograms(LSVs) obtained for the nickel, cobalt, and simulatedbattery solutions containing Ni, Co and Mn, andfor the solution obtained from hydrometallurgicaltreatment of the batteries.

The curves for the simulated solutions revealed clearnucleation processes followed by hydrogen evolutionat more negative potentials. The currentpotentialcurves showed only a small difference in the nucleationpotential for nickel and cobalt.

Electrochemical reduction of nickel and cobaltbegan at a potential of approximately 850 mV vsSCE, while hydrogen reduction occurred at potentialsmore negative than 1100 mV. The limiting currentdensities for nickel, cobalt, and simulated battery

-30

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0-1500-1300-1100-900-700-500

E/mV vs. SCE

J/mA

cm

-2

NickelCobaltSimulated solutionBattery solution

Figure 2. Linear sweep voltammograms of 0.09mol dm3 NiCl2 +1mol dm3 NaCl, 0.009mol dm3 CoCl2 + 1mol dm3 NaCl,simulated battery solution containing 0.09mol dm3 NiCl2,0.009mol dm3 CoCl2, 0.004mol dm3 MnCl2 + 1mol dm3 NaCl andactual battery solution. All solutions contained 0.25mol dm3 H3BO3,pH 3, and scan rate 20mVs1.

solutions were approximately 13, 1.5, and 17 mA cm2respectively. For the potential range of approximately,970 to 1050 mV, the different systems operatedunder mass transport control. The LSV of thesimulated battery solution shifts to a potential morepositive than that observed in the pure nickel, whichindicates that the formation of a nickelcobalt alloyfrom the simulated solutions was thermodynamicallyfavourable.

The voltammogram for the solution, from leachingof a NiMH battery revealed that deposition in thiscase occurred less readily than from simple chloridesolutions and without a clear nucleation overpotential.

The average difference in nucleation potentialbetween the simulated and the battery solution wasaround 200 mV. The low limiting current and theshift in the reduction potential towards more negativevalues in the case of the battery solution was due tothe complexity of this solution and the presence ofimpurities from the previous leaching and extractionsteps.

Figure 3 compares cyclic voltammograms from asolution of only nickel ions, and from simulated,and actual battery solutions. In all cases thevoltammograms exhibited a single reduction wave andsuggested the presence of a nucleation process whichwas less pronounced in the case of the actual batterysolution. The tests with the simulated solution showeda greater deposition rate, ie larger current densities,than that of the battery solution. The voltammetricresponse associated with the deposition from thebattery solution showed a lower current intensity and ashift of the hydrogen evolution reaction, towards morenegative potentials.

It is worth re-stating at this point the impuritiesthat might be present in the case of the actualbattery solution due to the previous hydrometallurgicaltreatment. The solution from the solvent extractioncycle contained, in addition to nickel, cobalt, and

J Chem Technol Biotechnol 79:927934 (online: 2004) 929

N Tzanetakis, K Scott

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-5

0

5

10

-1400 -1200 -1000 -800 -600 -400 -200 0 200E/mV vs. SCE

J/mA

cm

-2

NickelSimulated solutionBattery solution

Figure 3. Cyclic voltammograms of 0.09mol dm3 NiCl2 + 1moldm3 NaCl, simulated battery solution containing 0.09mol dm3

NiCl2, 0.009mol dm3 CoCl2, 0.004mol dm3 MnCl2 + 1mol dm3

NaCl and actual battery solution. All solutions contained0.25mol dm3 H3BO3, pH 3. The potential was first scannedcathodically from the rest potential with a scan rate of 20mVs1.Arrows indicate the potential scan direction.

manganese, small quantities of iron (0.36 g dm3),aluminium, zinc (0.66 g dm3), and possibly tracesof rare earth elements (0.10.2 g dm3) and theorganic solvent. Due to these impurities, a muchhigher overpotential was necessary to begin thenucleation process in the actual battery solution.Despite the fact that both the simulated and theactual battery solutions contained nickel and cobaltas the major constituents, the presence in the latterof the above metals was responsible for the differentelectrochemical behaviour.

The anodic processes revealed a clear oxidationwave in the case of the nickel ion and the simulatedsolutions, and a smaller one for the battery solution.It can be observed that the simulated solution showedsimilar characteristics to the nickel curve as nickelwas the major component of that solution. In thecase of nickel ion and the simulated solutions,the voltammetric charge of the cathodic peak wasapproximately equal to that of its correspondinganodic peak, which indicated good electrochemicalreversibility for these systems.

Overall from the voltammetric study it is expectedthat, in the case of the solution from hydrometallur-gical treatment of the NiMH batteries, the depositionprocess will be inhibited. This inhibition was causedby substrates, other than the metal ion of interest,present either at the surface of the electrode or in thediffusion layer, which hindered the cathodic processand partially covered the surface of the cathode. Theyalso affected the overpotential and the texture of thesurface.

3.3.2 Simulated solutionsGalvanostatic electrodeposition tests were conductedwith solutions containing nickel, cobalt, and man-ganese ions, the predominant elements expected in

the aqueous phase after extraction. The compositionof the solution (g dm3): 5.57 Ni, 0.57 Co, and 0.22 Mn, was similar to that obtained after diluting theleaching solution by a factor of 10.

Electrowinning experiments from dilute solutionsshould generally be operated at the maximum use-ful current density at which efficient deposition takesplace. Figure 4 shows the typical effect of current den-sity on current efficiency and composition of nickeland cobalt in the deposit. For these experiments thecharge passed (3250 C) was identical. The thickness ofthe deposit obtained was approximately 1.7 mm. Thedata confirmed co-deposition of nickel and cobalt,while in all cases the deposit contained no morethan 2% of manganese (Table 1). A greater propor-tion of cobalt was deposited at low current densitiesalthough the amount was approximately one tenth thatof nickel. Higher cobalt content potentially results inhigh strength and hardness of the obtained alloy.19

Overpotentials of 8201100 mV were observed at thebeginning of the deposition process at current den-sities of 10100 A m2 (Table 1). The overpotentialsdecreased by approximately 50100 mV during theruns. It is also apparent that the cobalt content in thedeposit decreased when the applied current densitywas increased.

The current efficiencies decreased at higher currentdensities due to metal ion depletion, which resulted

0

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0 10 20 30 40 50 60 70 80 90 100 110Current density/A m-2

Curre

nt e

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ency

/%

0

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100

Am

ount

in d

epos

it/%

Ni

Co

Figure 4. Effect of current density on alloy current efficiency andcomposition. Experiments were carried out in baths containing0.09mol dm3 NiCl2, 0.009mol dm3 CoCl2, 0.004mol dm3 MnCl2,1mol dm3 NaCl and 0.25mol dm3 H3BO3 at 22 C and at pH 3.Charge passed, 3250C.

Table 1. Effect of current density on the composition of the deposit,the cell voltage and the energy consumption for tests using simulated

battery solutions at pH 3

Current(Am2)

Ni(%)

Co(%)

Mn(%)

InitialOverpotential

(mV)

CellVoltage

(V)

EnergyConsumption(kWhmol1)

10 70 30 0 820 2.1 0.10925 83 15 2 850 2.6 0.13350 87 12 1 900 2.9 0.1575 87 11 2 1050 3.15 0.248100 88 10 2 1100 3.44 0.296

930 J Chem Technol Biotechnol 79:927934 (online: 2004)

Recycling of nickelmetal hydride batteries. II

in operating conditions above the limiting current andgreater hydrogen evolution.

When experiments were performed at currentdensities higher than 100 A m2 the deposits includedgreen nickel hydroxides which would detract fromthe properties of the deposit. The nickel hydroxidewas possibly formed due to operation above thelimiting current at which the local pH at the electrodeincreased, due to localised formation of OH ions.

Table 1 shows the cell voltages and energy con-sumptions obtained at the termination of the elec-trodeposition experiments. Higher current densitiesresulted in significantly greater cell voltages. Duringall electrodeposition tests the cell voltage decreasedslightly with time and hence so did the energy con-sumption.

Experiments carried out to optimise the operatingpH indicated that at a pH between 2 and 4 highermetal removal at greater current efficiencies andbetter quality deposits were obtained (Fig 5). Thecomposition of the deposit was similar regardless ofthe solution pH. However at low pH, reduction ofhydrogen ions was high which led to low currentefficiencies. Since cogeneration of hydrogen withnickelcobalt electrodeposition would tend to increasepH locally at the electrode surface, formation andinclusion of metal hydroxides into the deposit is likely.Thus maintaining the pH in the appropriate rangeduring these experiments was necessary.

Precipitation of metals and occlusion of hydroxidesin the growing deposit was observed during the runs atpH values above 4.5. The formation of hydroxidesinhibited the deposition process and induced theformation of cracked non-homogeneous deposits.During all runs an increase in the catholyte pH wasobserved, which was adjusted to the appropriate valueby addition of hydrochloric acid solution.

Figure 6 shows scanning electron microscopic(SEM) images of the surface morphology of thefinal deposits obtained under various electrodepositionconditions. A constant alloy composition throughout

0

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0 1 2 3 4 5

pH

Curr

ent

effi

cien

cy/%

0

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Am

ount

in d

eposi

t/%

Ni

Co

Figure 5. Effect of pH on alloy current efficiency and composition.Experiments were carried out in baths containing 0.09mol dm3

NiCl2, 0.009mol dm3 CoCl2, 0.004mol dm3 MnCl2, 1mol dm3

NaCl and 0.25mol dm3 H3BO3 at 22 C and at a currentdensity = 50Am2. Charge passed, 6120C.

the thickness of the deposit was formed. In all casesdeposits adhered well to the stainless steel substrate.It is clear that the morphology of the structures variedsignificantly with the experimental conditions used.

The grain size became gradually refined ondecreasing the current density and by ensuringthat the nucleation took place slowly. The highercobalt content at low current densities increased thecrystalline nature of the alloy and gave a compact,homogeneous, dense, and fine-grained structure.

At high current densities, the alloy structurewas characterised by large, randomly distributedgrains without visible porosity. Black inclusions weredistinguished in the micrographs while the colour ofthe alloy changed from silver to black.

The deposit obtained at higher temperatureswas globular, with variable grain sizes, without auniform distribution over the surface. Deposition testsperformed at 40 and 60 C did not give a significantimprovement in current efficiency.

The structure of the alloy and the size distributionof the crystallites became more homogeneous as thepH increased and a metallic deposit was obtained. Thedeposit at high pH exhibited a silver foil appearance.Low pH (

N Tzanetakis, K Scott

(a) (b)

(c) (d)

Figure 6. Scanning electron micrographs of deposits obtained under different experimental conditions. (a) CD = 10Am2, pH = 3, T = 25 C,(b) CD = 100Am2, pH = 3, T = 25 C, (c) CD = 50Am2, pH = 3, T = 40 C, (d) CD = 50Am2, pH = 1.5, T = 25 C.

Table 2. Effect of current density on the composition of the depositand the current efficiency for tests using solutions obtained from

treatment of batteries at pH 3 for all runs

Currentdensity(Am2)

(%)Ni(%)

Co(%)

Mn(%)

Al(%)

Fe(%)

Zn(%)

Rare earths(%)

10 100 83 14.1 0 1 1.2 0.7 050 82 76.2 12 7 4 0 0 0.8

100 39 82 11.2 3 0.7 0.5 1.4 1.2

= current efficiency.

because of re-dissolution of metals, due to their pooradhesion to the substrate caused by excessive hydrogenevolution. At such higher current densities the surfaceof the electrode was covered with metal oxides orhydroxides which appeared as black or green deposits.However runs at low current densities were moresuccessful since deposits adhered well to the electrodeand contained lower amounts of impurities (Table 2).In this case though, a high electrode area is necessaryfor efficient metal recovery. Hence a high surfacearea (three dimensional) electrode is recommended tomaximise deposition rate and the space time yield.

Figure 8 shows the X-ray diffraction patternsof deposits obtained at three current densities.

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0 500 1000 1500 2000 2500 3000 3500 4000Charge passed/Coulombs

Ni c

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1050

1250

Co c

once

ntra

tion/

ppm

, Ni, 20 A m-2 , Co, 20 A m-2

, Ni, 50 A m-2 , Co, 50 A m-2

Figure 7. Variation in nickel and cobalt concentrations in thecatholyte with charge passed. Experiments were performed at twocurrent densities at pH 3.

The crystal structure of the deposits was affectedby variations in current densities as were themorphologies shown by the SEM photographs(see Fig 6). The deposits obtained from batterysolutions showed amorphous structures with irregularmorphology and thus the SEMs are not shown.

The deposit obtained at 10 A m2 had a face-centredcubic (fcc) structure with sharp reflections from nickel

932 J Chem Technol Biotechnol 79:927934 (online: 2004)

Recycling of nickelmetal hydride batteries. II

0 20 40 60 80 100 1202-Theta (Degrees)

Inte

nsity

(Arbi

trary

units

)(111)

(200)(220) (311)

10 Am-2

50 A

100 Am-2

m-2

Figure 8. X-ray diffraction patterns of deposits obtained fromsolutions obtained from battery treatment, at three current densities,10, 50 and 100Am2. All runs performed at pH 3.

and cobalt crystalline grains, with very strong (111)growth orientation (2 = 44) with significant butweaker (200), (220) and (311) reflections at 2 valuesof 52, 76 and 93 respectively. This crystallographicorientation is similar to that observed from pure statenickel crystallites.29 Moreover the sharp reflectionsthroughout the spectrum indicated the presence oflarge-size crystallites. Single peaks were observed fornickel and cobalt due to their indistinguishable X-rayscattering powers. Similarly the spectrum obtained at acurrent density of 50 A m2 exhibited the same (111),(200), (220) and (311) oriented planes although at alower intensity when compared with those at 10 A m2.The two spectrams clearly proved the crystallinestructure of the deposit at low current density.

At higher current densities an increase in the fcclattice parameter and an absence of clear diffractionpeaks was observed, which indicated the presenceof inclusions in the deposit. The particle size of thedeposit obtained, especially at higher current densities,was irregular due to the formation of dendrites.

In addition to some nickel crystallites formed duringelectrodeposition, nickel oxideshydroxides were alsoembedded in the deposit. The absence of diffractionpeaks is also partly attributed to the presence ofother metal oxides in the deposit. This speculation issupported by the amorphous nature of these deposits.

The presence of metal impurities and the adsorptionof oxides into the electrode surface inhibited, to agreat extent, cobalt and nickel reduction by loweringthe limiting current. Future work will focus onpurification of the battery solution which will enableefficient operations at higher current densities. Ironand aluminium removal using precipitation stepsprior to the aforementioned deposition tests andanodic deposition of manganese as an oxide will beinvestigated.

4 CONCLUSIONSA combination of hydrometallurgical and electro-chemical processes can potentially provide an efficientprocess for recycling valuable metal materials from

spent rechargeable nickelmetal hydride batteries.The work revealed that appropriate leaching can beperformed with 4 mol dm3 HC1 at a temperature of95 C and a leaching time of 3 h or less. Rare earthelements were separated from nickel and cobalt bysolvent extraction using D2EHPA as an extractant ata pH close to 2.5.

The feasibility of recovering nickel and cobaltas an alloy by electrowinning from the raffinatewas demonstrated using simulated and real batterysolutions.

Galvanostatic tests on simulated solutions showedthat composition and surface morphology of the alloyis largely influenced by modifying the depositionparameters (mainly pH and current density). Thecrystalline nature of the alloy increased as the cobaltcontent of the deposit increased. Electrodepositiontests using solutions obtained after hydrometallurgicaltreatment of NiMH batteries showed that recoveryof nickel and cobalt was feasible. Operations at lowcurrent densities gave a deposit with a crystallinestructure at high current efficiencies, while highercurrent densities gave amorphous materials withsignificant reduction in current efficiencies.

Overall the data reported can be used for thedevelopment of a process which will allow efficientelectrochemical recovery of Ni and Co from spentNiMH batteries.

ACKNOWLEDGEMENTSCumberland Electrochem (Bootle) are thanked forproviding the electrode materials for the project. Thework was performed in research facilities providedthrough an EPSRC/HEFCE Joint Infrastructure Fundaward, no JIF4NESCEQ.

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