ro

a,ndep

Keywords:ManganeseZincBatteryRecyclingElectrolytic depositionCurrent efciency

ang

produced MnO2 has started to increase to satisfy the demand.

angane

for batteries containing Mn are eager to identify new sources ofEMD. Standards for high quality EMD are exclusively commercialinformation for the battery suppliers and cannot be easily ob-tained. A 1984 Japanese standard (Table 1) exists for EMD (JIS1467) is currently considered a minimum standard.

tion.e or Pb an

the electrolysis cell at 9096 C and a current density2 A/dm2.

Removal, milling, washing and drying the resulting depo

The idea of simultaneous deposit (Anton et al., 2011) of MnO2and Zn initiated from the conventional method of obtaining elec-trolytic zinc using of electrolysis cell with ZnSO4 solution as elec-trolyte. It is common practice to add MnO2 to zinc sulphatesolution resulted from of roasted zinc concentrates leaching to oxi-dize divalent iron to trivalent iron. The nal precipitated asFe(OH)3 is easy lterable. Therefore, the zinc sulphate solutions

Corresponding author. Tel.: +40 0729050042.

Waste Management 33 (2013) 17641769

Contents lists available at

Waste Man

elsE-mail addresses: iacob_gh@yahoo.com, gh.iacob84@gmail.com (G. Iacob).production of manganese dioxide exists, exclude pyro metallurgi-cal production expansion. The exception is South Africa whichholds a modest capacity expansion of 11,000 tons.

Battery manufacturers which predict strong growth in demand

MnO2 to MnO. MnO leaching in diluted H2SO4. Filtering and purication of Mn sulphate solu Electrolytic depositing of MnO2 on Ti, graphit0956-053X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.wasman.2013.04.010ode, inof 0.7

sit.produced annually. To produce them, around 270,000 tons ofEMD are consumed. The portable battery industry expects the de-mand for EMD to grow at a rate of 10% per year. To meet growingdemand, the producers of EMD must develop sufcient productioncapacity. Environmental problems in developed countries, where

There are several processes for producing EMD (Innocenzi andVeglio, 2012; Bernardes et al., 2004) which generally unfold inthe following stages:

Calcinations of available manganese concentrate to convert1. Introduction

More than 60 billion alkaline mThis paper describes an electrolytic process for the simultaneous production of the following compo-nents:

MnO2, with the same quality as the one used in portable batteries; commercial quality zinc, resulted from Zn and Mn sulphate solution obtained from reductive acid

leaching (see Buzatu et al., 2012 paper). The sulphate solution is a by-product of used portable bat-teries crushing process.

The electrolysis process was conducted in a specialized laboratory facility. The study was particularly fo-cused on the following electrolysis process parameters:

evolution of MnO2 deposit efciency based anodic current density; sulphuric acid concentration inuence on zinc metal and MnO2 deposit. The results have allowed theidentication of optimal conditions for electrolysis.

2013 Elsevier Ltd. All rights reserved.

se based batteries are

1.1. Background information concerning simultaneous production ofZn and MnO2 by electrolysisAccepted 24 April 2013Available online 2 June 2013

zinc and manganese based portable batteries (alkaline with manganese AlMn and zinc carbon ZnC).Lately, due to the progressive depletion of MnO2 natural resources, the quantity of articially electrolyticSimultaneous recovery of Zn and MnO2 fraw materials, by electrolysis

Mihai Buzatu, Simona Saceanu, Valeriu Gabriel GhicPolitehnica University of Bucharest, Materials Science and Engineering Faculty, Splaiul I

a r t i c l e i n f o

Article history:Received 4 January 2013

a b s t r a c t

High purity electrolytic m

journal homepage: www.m used batteries, as

Gheorghe Iacob , Traian Buzatuendentei Blvd., No. 313, 060042 Bucharest, Romania

anese dioxide (EMD) is the main raw material used for manufacturing of

SciVerse ScienceDirect

agement

evier .com/ locate/wasman

agemused as electrolyte for obtaining electrolytic zinc contain a smallamount of manganese sulphate.

During the electrolytic zinc conventional process, zinc is depos-ited on the Al cathode, while MnO2 is deposited as anode slurry atthe bottom of the electrolysis cell by using a Pb anode. The indus-try tried to produce batteries using MnO2 obtained as a byproductof metallic zinc production by conventional process. However,those efforts have failed due to the following reasons:

This material (MnO2) produced during electrolysis of zinc wasusually slightly contaminated with Pb (up to 2.0%). Batteriesmanufactured from this material have deteriorated rapidlyafter, we can assume this was caused by to the formation ofdendritic Pb inside the battery causing them to short.

MnO2 structure is rather cryptomelane than the gamma phaseused for portable batteries.

Material density and particle sizes were low.

Presence of the alkaline metals, Na and K in electrolytic MnO2results in obtaining a MnO2 with cryptomelane structure(Kx(Mn4+)8x(Mn3+)O16 where x = 0.251), which is unsuitable foruse in batteries.

Therefore, it is possible to simultaneously produce MnO2 andmetallic zinc in an electrolytic cell; this represents the main objec-tive of this paper.

1.2. Theoretical bases concerning simultaneous electrolytic extractionof Zn and MnO2

Zinc and manganese dioxide extraction is carried out by ahydrometallurgical process from a ZnSO4 and MnSO4 solution acid-ulated with H2SO4 (Buzatu et al., 2012), with an Al cathode and aPb anode.

Electrode reactions are as follows:

at cathode takes place the discharge of zinc:

Zn2 2e ! Zn 1

Table 1Standard composition for electrolytic manganese dioxide (JIS 1467).

Product Quality

Manganese dioxide 90% min.Water content 3.0% max.Insoluble matter in hydrochloric acid 0.5% max.Iron 0.03% max.Lead 0.1% max.Copper 0.001% max.Iron sulphate 1.5% max.

M. Buzatu et al. /Waste ManSimultaneous downloading of hydrogen, however, has the ef-fect of alkalization of the area in the immediate vicinity of the cath-ode, according to the reaction:

2H2O 2e ! H2 2OH 2Simultaneous formation of zinc hydroxide and basic zinc salts

prevents the normal process of electrocrystallization and decreasecurrent efciency.

With the electrolysis advancement (Racz et al., 2011) increasingthe acidity of the electrolyte solution (composition presented inTable 2) and the hydrogen discharge potential ee,H, moving towardsmore positive values, ee,Zn, is moving to positive values due to thedischarge of overvoltage of Zn on Al anode. At the same time, withdecreasing zinc ion concentration, zinc discharge potential ee,Znmoving toward more negative values and also reducing the valueof current limit, both changes have the effect of lowering the cur-rent efciency. Therefore, electrolysis cannot be conducted untilthe complete depletion of electrolyte, but only up to a certain levelof depletion, when the continuation of electrolysis could becomeuneconomic due to low current yields. At this point electrolysisis stopped and the electrolyte is recycled in the leaching processof electrodes paste. Therefore it can be stipulated that parallel toincreasing acidity of the electrolyte solution and with displace-ment of hydrogen discharge potential to more positive values,the maximum current efciency is achieved at higher values ofcurrent density. That is why if more sulfuric acid concentratedsolutions are subjected to electrolysis, Zn2+ ion concentration beingidentical, electrolysis current density must be considerably higher.

We consider that to deposit zinc at low electrolysis current den-sities the electrolyte acidity must to be very small (Fig. 2 andTable 4).

Formation process of EMD can be expressed by the followingoverall equation:

MnSO4 2H2O!MnO2 H2SO4 H2 3The reaction (3) happens, probably due to precipitation of insol-

uble MnO2.In aqueous solution of zinc sulphate and manganese, between

manganese ions the following equilibrium is established:

2Mn3 $Mn2 Mn4 4at anode all the following electrode processes can occur:

Mn2 !Mn3 ee01 1:488 V 5

Mn2 !Mn4 2ee02 1:570 V 6

Mn3 !Mn4 ee03 1:652 V 7It is likely for an H2O reaction to occur with e = 1.229 V.Standard potential values show that the (5) process must pro-

ceed the easiest and only after most of Mn2+ ions were oxidizedat Mn3+ (90%) the processes (6) and (7) can start. As a fact, thiswould happen if Mn4+ ions would remain in solution. But they eas-ily hydrolyze, even in acidic solutions, according to the reaction:

Table 2Parameters used in experimental electrolysis tests.

Sample Da (A/dm2) Temperature(C)

Time(h)

Currentefciency (%)

DME 0.5 0.5 96 C 4 85.88DME 1 1 94.02DME 1.5 1.4 87.76DME 2 2 82.58DME 2.5 2.5 69.53DME 3 3 33.63

ent 33 (2013) 17641769 1765Mn4 2H2O$MnO2 4H 8Therefore hard soluble manganese dioxide are forming by a

slow process, the Mn4+ ions are removed from the reaction sphereand the equilibrium potential of the electrode is according to reac-tion (6):

e 1:570 RT2F

lnaMn4aMn2

9

moving more towards negative values and (10) becoming themain process:

Mn2 2H2O!MnO2 4H 2e 10whose potential is given by the relation:

iglas cell with a volume of 3 l immersed in a thermostat,continuous current source (LM338 K 1.2 to 30 Volt 5 Amp Regula-tor), a Plexiglas container for solution recirculation with the vol-ume of 2 l, and an electrolyte recirculation pump with a ow rateof 20 l/h (Wilo Star Z 20/1).

Six samples of electrolyte solution (Table 2) obtained from usedbatteries paste (43.12 g/l Zn2+, 55.94 g/l Mn2+) with pH range from4 to 4.5 (pHs were measured with a digital pH meter Mettler To-

Table 3 shows the result of electrolysis experimental tests con-ducted under the conditions presented earlier in the material.

agement 33 (2013) 17641769e e0 RT2F

lna4HaMn2

with e0 1:23 V 11

Current efciency in obtaining EMD is below 1 (100%) and canvary between very wide limits, depending on the conditions ofelectrolysis. The causes are multiple.

First, the standard potential for MnO2 forming process accordingto reaction (10) is identicalwith the potential of oxygen discharge inacidic environment. Therefore, only because of the high oxygenovervoltage it is expected that at quite high concentration of Mn2+

and moderate acidity to achieve satisfactory current efciencies.With decreasing Mn2+ ion concentration the current efciency

decreases due to the occurrence at the anode of mass transfer over-voltage which favors the oxygen discharge. Mass transfer overvolt-age is emphasized even further by the fact that on the anode aloose crust of MnO2 is formed which hinders Mn2+ ions enteringat the anode. That is why stirring, respectively a movement ofthe solution, as well as a high temperature, improves current ef-ciency. Another parameter that can be considered for increasingthe current efciency is proximity between the electrodes.

According to reaction (3) with the formation of EMD, an equiv-alent quantity of H2SO4 is also produced. Along with increasingacidity, the potential of process (10) moves towards more positivevalues, favoring the oxygen discharge, but also the formation ofMn3+ ions according to electrode process (5).

Along with increasing acidity, reaction (8) of Mn4+ ions hydroly-sis is also left displaced and their concentration in solution in-creases. Therefore, in strong acidic solutions, when workingwithout a diaphragm, a new source of current leakage appears,namely cathode reduction of Mn3+ and Mn4+, whose concentrationis much higher than in weak acidic solutions.

For the reasons presented above, the electrolysis can be con-ducted only up to a certain stage, when the decrease of Mn2+ ionconcentration and the increase of sulfuric acid concentrationwould result in obtaining of too low current efciencies.

Current efciency is reduced by the presence of foreign ions inthe electrolyte, primarily Fe ions. They are oxidized to Fe3+ at an-ode, then reduced to Fe2+ at cathode and thereby parasitically con-sume the current. For this reason, solutions subjected toelectrolysis must be puried before using them, primarily of iron.

Of course, the current efciency could be improved by using adiaphragm, which would completely avoid cathode reduction ofMn3+, Mn4+ and Fe3+ ions. Using diaphragms is not economical fea-sible due to their clogging with MnO2 and to the considerable in-crease of the cell voltage.

2. Experimental researches

2.1. Materials and methods

This paper describes an electrolytic process for the simulta-neous production of the following components: MnO2, with thesame quality as the one used in portable batteries and commercialquality zinc Buzatu et al., 2012.

Previous researches explored the industrial usage of electrodepaste obtained from crushed used batteries in order to simulta-neously recover MnO2 and Zn. The two main directions of the re-search study were:

1. Current density effect on MnO2 deposits during electrolysisprocess,

2. Sulfuric acid concentration effect on MnO2 and metallic Zndeposits.

1766 M. Buzatu et al. /Waste ManThe electrolysis process was conducted (potentiostatic condi-tions) in a laboratory facility which consists of an electrolytic Plex-Fig. 1 shows the variation of current efciency for different ano-dic current densities.

Results indicate that current efciency for EMD deposits ini-tially grows when increasing anodic current density passesthrough maximum (94.02%), then decreases. Maximum currentefciency occurs at a current density of 1 A/dm2 which is a normalvalue in the production process of EMD. A pronounced decrease upto 34% for current efciency is observed when current density isincreased to 3 A/dm2. This variation of current density is due to thefact that high current densities enable the oxygen release reaction;this could be a competitive reaction in the case of EMD deposit,reducing the current efciency.

Another aspect that could be taken into account is the fact thatfor high current density the deposit velocity is greater than thetransport speed of Mn4+ ions to anode, therefore their concentra-tion near anode is reduced. Setting the current densities in the0.52.5 A/dm2 range does not substantially affect the MnO2 con-tent in the nished product. The process overall efciency was over90%.

Table 3Parameters used in electrolysis tests and the results achieved.

Sample H2SO4,moles/l

pH Temperature,C

Time,h

Currentefciency,% (MnO)

Currentefciency,% (Zn)

DME pH 0.00 5 101 0.00 96 C 6 91.72 18.37DME pH 0.25 2.8 101 0.25 97.80 21.58

2ledo, MX 300) and were subjected to the electrolysis process for4 h at 96 C. For each sample different anodic current densitieswere established, ranging from 0.5 to 3 A/dm2. Electrolyte densityat room temperature was: q = 1.348 g/cm3 (measured with aBench Top Digital Density Meter DM 4500).

Optimumworking conditions were: titanium anodes 2 pieces,aluminum cathodes 1 piece, solution pHnal 4, electrolyte tem-perature 96 C, anodic current density 1.2 A/dm2, cathode cur-rent density 2.4 A/dm2 (with cell voltage 3.5 V), currentintensity 3.6 A, anodecathode distance 30 mm, electrolyterecirculation speed 20 l/h. Extraction processes of Zn and MnO2occurred continuously over 24 h.

After electrolysis the anode was removed from the electrolyticbath and the EMD lm was mechanically removed from the anodeand washed with water and with an ammonium hydroxide solu-tion to a neutral pH 67, then dried in an oven at 100 C untilthe weight remained constant. Dry EMD was then chemically char-acterized (losses are higher in the process of weighing of dry elec-trodes before mechanically removing MnO2).

2.2. Current density effect

Current efciency evolution during the electrolysis process forMnO2 deposit was analyzed varying the anodic current density.DME pH 2 10 2 93.71 49.06DME pH 3 103 3 93.70 64.98DME pH 4 104 4 93.71 85.87

agemBased on our experimental results, the recommended currentdensity shall be in the 11.5 A/dm2 range.

Fig. 1. Effect of anodic current density on current efciency at MnO2 deposit.

Table 4Chemical analysis of electrolytic zinc.

Sample Zn Mn Pb Fe Ti Al Mo

1 Basis 0.01 0.1

age1768 M. Buzatu et al. /Waste ManCu Kamonochromatized by a Ni lter). Powder form samples werecollected after milling and washing with acid the anodic deposit.

The X-ray diffraction for EMD1 and EMD 2 samples (electrolyticmanganese dioxide) are presented in Fig. 7. From the X-ray diffrac-tion we notice the majority presence of manganese oxides in bothsamples which have a similar composition. We could not clearlyidentify Nsutite by diffraction because the peaks overlap withRamsdellite; these peaks could not emerge clearly from the back-ground noise.

Cathodic deposits of metallic zinc with approx. 2 mm in thick-ness is colored gray open white, with ne grains that was easilyremoved from the aluminum support.

Fig. 3. Al cathode image with deposited zinc after 6 h of extraction and cathodiccurrent density 2.4 A/dm2.

Fig. 4. Ti anode image with MnO2 deposited after 6 h of extraction.

Fig. 5. Image of MnO2 extracted from Ti anode.

Fig. 6. Image of MnO2 obtained by electrolysis process after washing and grinding.Fig. 7. X-ray diffractions for EMD1 and EMD 2 samples. R Ramsdellite, c Nsutite,e Akhtenskite.

Table 5Chemical analysis of electrolytic MnO2.ment 33 (2013) 17641769Chemical analysis for zinc cathode (X-ray uorescence spec-trometer type S8 Tiger) is presented in Table 5. Samples were col-lected in the form of zinc scrap from various points on the cathodesurface.

Purity of deposited zinc obtained by us is similar to the one pro-duced by large zinc manufacturers using manganese uncontami-nated Zinc. Both procedures, ours and the industrial ones, arebased on electrolysis of zinc sulphate solutions.

The resulted deposit has a smooth, hard surface, with negrains, as can be seen in Fig. 3. The deposit was achieved as den-dritic crystals form having the growth axis perpendicular to thesupport. Towards the electrode edge a downward trend of the crys-tals number is observed, decrease accompanied by the growth ofdeposit thickness. At the edges and tops of cathode a current den-sity higher than on the remaining surface always exist, whichdetermine the obtaining of thicker deposits in these places, Fig. 3.

4. Conclusions

The main advantage of hydrometallurgical processes is the sim-plicity of leaching process Buzatu et al., 2012 and the easy mea-surement and control of reaction parameters.

Sample Mn MnO2 Zn Pb Fe Ti Sb

DME1 57.50 90.85 0.04 0.1 0.05 0.002

Hydrometallurgical process is not only environmentallyfriendly but also has signicant cost advantages and offers the pos-sibility for using ores with low Mn content.

Product quality and coherence are key factors in the productionof EMD. Prices are directly related to product quality, with a rapiddecrease in value directly proportional to the quality. Energy ef-ciency for the presented process does not vary rigorous parallelto current efciency.

For neutral or very weak acidic solutions because of poor con-ductivity of the electrolyte solution, the cell voltage is high and de-creases with increasing acidity, stronger in the beginning thenweaker. Therefore, energy efciency, depending on the solutionacidity, passes through a maximum at a certain H2SO4/MnSO4 mo-lar ratio.

Based on our studies and experiments presented above, the fol-lowing process parameters are required for obtaining high MnO2deposit efciency:

Avoiding mass transfer overvoltage, which leads to simulta-neous discharge of oxygen and reducing the current efciencyby: using high concentrations of MnSO4 and not very advanced

solution depletion; using moderate current densities, about 0.51.2 A/dm2; electrolysis control at higher-temperature (9598 C); solution recirculation;

Use weak acid solutions to prevent losses by reducing Mn3+ andMn4+ ions.

Using iron-free solutions. Control the solution PH in the 44.5 range by adding NH4OH.

Acknowledgments

The work has been funded by the Sectorial Operational Pro-gramme Human Resources Development 20072013 of the Roma-nian Ministry of Labour, Family and Social Protection through theFinancial Agreement POSDRU/107/1.5/S/76903.

References

Anton, M., Manciulea, A., Ilea, P., 2011. Comparative study of solubilization methodsfor zinc and manganese recovery from spent batteries. Studia UniversitatisBabes-Bolyai Chemia 4, 223233.

Bernardes, A.M. et al., 2004. Recycling of batteries: a review of current process andtechnologies. Journal of Powder Sources 130, 291298.

Buzatu, T., Popescu, G., Birloaga, I., Saceanu, S., 2012. Study concerning the recoveryof zinc and manganese from spent batteries by hydrometallurgical processes.Waste Management 33, 699705.

Innocenzi, V., Veglio, F., 2012. Separation of manganese, zinc and nickel fromleaching solution of nickel-metal hydride based spent batteries by solventextraction. Hydrometallurgy 129130, 5058.

Racz, R., Manciulea, A., Ilea, P., 2011. Electrochemical behaviour of metallic titaniumin MnO2 electrosynthesis from synthetic solutions simulating spent batteryleach liquors. Studia Universitatis Babes-Bolyai Chemia 4, 211222.

M. Buzatu et al. /Waste Management 33 (2013) 17641769 1769

Simultaneous recovery of Zn and MnO2 from used batteries, as raw materials, by electrolysis1 Introduction1.1 Background information concerning simultaneous production of Zn and MnO2 by electrolysis1.2 Theoretical bases concerning simultaneous electrolytic extraction of Zn and MnO2

2 Experimental researches2.1 Materials and methods2.2 Current density effect2.3 Sulfuric acid concentration effect

3 Results and discussions4 ConclusionsAcknowledgmentsReferences