Recovering of copperwithmetallicaluminum

NizamettinDEMIRKRAN,AsimKNKL

ChemicalEngineeringDepartment,FacultyofEngineering,InonuUniversity,Malatya44280,Turkey

Received27January 2011accepted5May 2011

Abstract:Thecementationofcopperionsfromaqueouscoppersulfatesolutionsbyusingsphericalaluminummetalparticleswasexamined. The effects of the experimental parameters on copper cementation were investigated and evaluated. Reaction rateincreases with increasing copper concentration, reaction temperature, stirring speed and decreasing pH. It was observed that thereactionfollows the firstorderkinetics,andprogresses accordingtothediffusioncontrollingstep.Key words: recoveringmetalcementationcopperaluminumkinetics

1Introduction

With rapid industrial development and greater useofchemicalproducts,highgradeoreshavedepleted.Asa result, the hydrometallurgicalmethods have gained agreat importance in recovering metallic values fromindustrialwastesandlowgradeores[12].

Industrial waste solutions obtained after thechemical and hydrometallurgical processing containtoxic and/or precious metal ions. These solutions arefrequentlydischargeddirectlytotheenvironment.Whenthelevelsofhazardousmetalionsinindustrialeffluentsexceed the permissible levels, it will causeenvironmental pollutions, and natural life is negativelyaffected. Therefore, from both environmental andeconomical perspectives, recovering valuable and/ortoxic metal from waste solutions and purification ofelectrolytesolutionsareimportantprocess[26].

Different methods are employed to recover themetalorremovetheundesiredimpuritiesfromtheleachorindustrialwaste solutions, suchasneutralizationwithacid or base solutions, chemical precipitation,crystallization,solventextraction,flotation,ionexchange,adsorption onto different adsorbents, reverse osmosis,chelating ligands, electrodialysis and electrowinning[1,3,57].

Thesemethodshavesomedrawbacks.Forexample,chemical precipitation requires extremely long settlingtimeionexchangeandadsorptionprocessareslowand

expensive, and may require frequent regeneration foradequate performance reverse osmosis, electrodialysis,electrowinning and solvent extractionmethods are veryexpensive,andhavehighoperatingcosts[36].

Cementationisanimportantchemicalprocessusedin industry to precipitate and recover valuable metalsfrom leach orwaste solutions.Cementationmethodhassomeadvantage,suchasrecoveryofmetalsinessentiallypure metallic form, simple control requirements, lowenergyconsumptionandingenerallowcostprocess.Themain disadvantages of the technique are excesssacrificial metal consumption and redox potential ofsacrificingmetal[3,5, 6].

Cementation reactions, known as metaldisplacementreactionsorcontactreductionreactions,areprocesses where a metal ion presented in a solution ormelt is reduced to themetallic statewith amoreactivemetalplacedinthesolutionormelt[3,6,810].

Acementationreactioncanbedescribedas

nAa++mB0nA0+mBb+ (1)

wheren,m,A,B,a+,andb+representthestoichiometriccoefficients,thenobleandreductantmetals,andvalencesofthenobleandreductantmetal,respectively[3,11].

Copper is one of the most prevalent and valuablemetals used in industry. Copper cementation has beenperformed to removeCu2+ ions from electrowinning orelectroplating solutions, or recover copper from leachsolutions[3,9,12].

Copper cementation has been studied by various

Foundationitem:Project(2008/55)supportedbyInonuUniversityResearchFund,TurkeyCorrespondingauthor:NizamettinDEMIRKRAN Tel:+904223774760Email:nizamettin.demirkiran@inonu.edu.trDOI:10.1016/S10036326(11)611230

NizamettinDEMIRKRAN,etal/Trans. NonferrousMet.Soc.China21(2011) 27782782 2779

researchers.Thissubjectisstillwidelyinvestigated.Iron,nickel, zinc, and aluminum have been used as thereductantmetalincoppercementationreactions[2,46,9,1219].

2Experimental

The cementation reactions were conducted in atemperaturecontrolledandmechanicallystirredjacketedglass reactor of 500 mL in volume. The mechanicalstirrerhadadigitalcontrollerunit, and itsagitator shaftand blade (45 mm in diameter) were made of Teflon.Temperature control unit had a constanttemperaturecirculator. The solution pH was adjusted by addingH2SO4 andNaOH solutions. The cementation solutionscontaining Cu2+ ions were prepared by usingCuSO45H2O.Sphericalaluminumparticles(1.52.0mmof average diameter) used in the experiments werepreparedbyusingaluminumfoil.

After 300mL of Cu2+ containing solutionwas putintotheglassreactorandbroughttothedesiredreactiontemperature, 1.5 times of stoichiometrically requiredmetallic aluminum was added into the reactor, and thereactor content was stirred at predetermined stirringspeed. The progress of the cementation reaction wasfollowed by measuring the amounts of unprecipitatedcopper ions in the solution. Aliquots of 5 mL solutionwerewithdrawnatregularintervalsduringcementation,and were immediately filtered using filter paper. Thefilteredsampleswereanalyzedforcopperioncontentbyethylene diamine tetraacetic acid sodium salt (EDTA)titrationusingmurexideas theindicator[9,20].

Theamountsofrecoveredordepositedcopperwerecalculatedaccordingtodifferencebetweentheinitial(C1)andfinal(C2)copperconcentrationsofthesolution.Thefractionofcementedcopperisx(Cu)=C2/C1.

3Resultsanddiscussion

3.1 EffectofparametersoncoppercementationThe effect of initial copper ion concentration on

copper cementationwas studied at 0.010, 0.025, 0.050,0.100 mol/L, respectively. The temperature, stirringspeed,andpHwere343K,450r/min,and1, respectively.The results are given in Fig. 1. It is observed that theprecipitation ratio of metallic copper increases as theconcentrationofcopperionisincreased.

The effect of pH on copper cementation wasinvestigated at pH value of 1.0, 1.5, 2.0, respectively.The concentration of solution, stirring speed, andtemperatureintheseexperimentswere0.025mol/L,450r/min, and 343 K, respectively. Figure 2 shows theresultsof theeffectofpH. It isclear thatthemaximumcementation yield is obtained at pHof 1.AthigherpH

values,adecreaseintheyieldisobserved.Inordertodeterminetheeffectofstirringspeedon

cementation reactions, the experimentswere carried outat theconcentrationof0.025mol/L,temperatureof343K, and pHvalue of 1.The effect of stirring speedwastestedat250,350,450and550r/min,respectively.Theresultsare given inFig. 3.The results indicate that thecemented fraction of copper increases as the stirringspeedisincreased.

Fig. 1 Effect of initial concentration of copper on coppercementation

Fig.2 EffectofpHvalueoncoppercementation

Fig.3 Effectofstirringspeedoncoppercementation

NizamettinDEMIRKRAN,etal/Trans. NonferrousMet.Soc.China21(2011) 277827822780

Theeffectoftemperaturewasexaminedat313,323,333, 343, and 353 K, respectively. In these tests, theotherparametersarechosenattheconcentrationof0.025mol/L, stirring speed of 450 r/min, and pHvalue of 1.TheeffectoftemperatureisgiveninFig.4.Itshowsthattemperaturehasasignificanteffectontheaccelerationofcoppercementation.

Fig.4 Effectoftemperatureoncoppercementation

3.2 KineticanalysisCementation reactions are heterogeneous

electrochemical reactions, and require a transfer ofelectronsbetweenthedissolvingandprecipitatingmetals.Ionsinthesolutionarereducedtozerovalence onasolidmetallic surface.Reactionstepsare:diffusionof ionsofthe depositing metal to the depositsolution interfacefrom the bulk of the solution conduction of electronsfrom the dissolving metal through the depositincorporationofthedepositedmetalatomsintoacrystallatticereleaseofthedissolvingmetalionsintosolutionstransfer of the dissolving metal ions to thedepositsolution vicinity through the deposit layerdiffusionofthedissolvingmetalionsintothebulkofthesolution[3].

The cementation reaction between copper ionscontaining solution and metallic aluminum particlesoccursaccordingtothefollowingreaction:

3Cu2++2Al03Cu0+2Al3+ (2)

Due to the difference between the electrodepotentials of the two metals, copper ions are easilyreducedto itsmetallic stateonaluminummetal surface.The standard reduction potentials of copper andaluminumare0.34 and 1.67V,respectively.

Thehalfcellreactionsare

Cu2++2eCu0 (0=0.34 V) (3)Al0 Al3++3e (0=1.67V) (4)

0 of the reaction is positive (+2.01 V), and thestandard Gibbs free energy G0 is to be negative(G0=nF0).Therefore,a spontaneousheterogeneous

reactiontakesplacethroughthegalvaniccell.Cementation occurs through a series of shorted

electrochemicalcells,inwhichelectronsforreductionofCu2+ aretransferredfromthecementationagentthroughthe growing copper deposit. Aluminum,which suppliestheelectrons,isoxidizedatanodicsitesontheirsurface[12].

It is reported that cementation reactions follow thefirstorderkinetics[4,9],andtheratecontrollingstepisthediffusionofthedepositingmetalionstothereductantmetalsurface.

Kinetic analysis was performed according to thefirstorder kinetics in the present study. Themodel forthefirstorderreactionis

ln[1x(Cu)]=kt (5)

wherex(Cu)isthecementedcopperfractionattimetkandt aretheapparentrateconstantandtime,respectively.Cementation data obtained from the experiments wereused to perform ln[1x(Cu)] versus time graphs. Theresultsobtainedfromthegraphs(apparentrateconstant kand R2values)are giveninTable1.

Table 1 Rate constants (k) and correlation coefficients (R2)values

ParameterParametervalue

Rateconstant(k)

Correlationcoefficient(R2)

0.010 0.025 0 0.995

0.025 0.038 8 0.994

0.050 0.050 3 0.995

Concentration/(molL1)

0.100 0.066 5 0.997

1.0 0.038 8 0.994

1.5 0.032 4 0.994pH

2.0 0.027 0 0.991

250 0.022 4 0.990

350 0.027 8 0.993

450 0.038 8 0.994

Stirring speed/(rmin1)

550 0.043 4 0.998

313 0.011 8 0.990

323 0.018 5 0.986

333 0.027 5 0.992

343 0.038 8 0.994

Temperature/K

353 0.060 4 0.997

To determine the activation energy of cementationreaction,usingtheapparentrateconstantsobtainedfromthe plot of ln[1x(Cu)] versus time for differenttemperatures(Table1), anArrheniusplotwasdrawn,andthe activation energy of this process was calculated as36.8kJ/mol.Thisvalueoftheactivationenergyindicatesthattheratecontrollingstepofthereactionexaminedis

NizamettinDEMIRKRAN,etal/Trans. NonferrousMet.Soc.China21(2011) 27782782 2781

the diffusion. In diffusioncontrolled processes, theactivation energy is mostly below 40 kJ/mol. The factthat the cementation rate is dependent on the stirringspeed supports that the diffusion is the ratecontrollingstep.

3.3 DiscussiononeffectsofparametersOne of the most important events in cementation

reactions is whether deposit formed on the reductantmetal surfaceiscoherent.Acoherentdeposit formedonmetal surface affects the reaction rate and controllingstepofreaction.Thevaluesofthe parameterssuchastheconcentration, temperature, stirring speed and pH affectthedepositformationandprogressionofreaction.

The removal of copper from solution involves twomain processes, including adsorption of copper ions onthemetallicaluminum surfaceandcementationofcopperions onto the metal. The presence of a passive oxidelayeronaluminumisknown,anditinhibitsthereactionrate by creatinga resistance to diffusion of copper ionson themetal surface.This oxide layer can be destroyedby acidity ofmedium. In thisway, it is ensured a goodcontact between copper ions andmetal surface,and thecementationrateenhances.Also,pHvalueofthe processis an important economical issue for any cementationplant:corrosiondamageofreactors,excessdissolutionofthe reducing metal and hydroxide precipitation [10].When thepHvalue of solution isabove a certain limit,hydroxidesofcopper ionsanddissolvedaluminumionscanform,andthesespeciesmaycauseadecreasingyieldand grade of the precipitated copper. Strong acidicsolutionspreventprecipitationofhydroxides.Becauseofabovereasons,wehaveusedstrongacidicsolutioninourwork.The followingsidereaction,which representsthedissolution of aluminum, occurs in acidic reactionmedium. Therefore, we have used excess aluminum intheexperiments.

2Al0(s)+6H+(aq) 2Al3+(aq)+3H2(g) (6)

In practice, the amount of aluminum consumed isalwayshigherthanthetheoreticalamount.

Therateofcementationreactionsisproportionaltothe ion concentration in solutionand the exposedmetalarea. The amount of metallic aluminum used in theexperiments has been proportionally increased withincreasing initial copper ion concentration, and it wasobserved that the cementation rate increased.When theamount ofmetal is increased, the effective surface areawill behigher andas a result, theamount of cementedcopper will increase. Since the cemented copperaccumulates on the surface of aluminum particles, aproductlayeroccurson themetal surface.Thismetalliclayerdepictsaresistancetothediffusionofcopperionsthrough it. If the metallic layer formed is a coherent

deposit, then the reaction rate can excessively decreasewith increasing depositmass. If the deposit isa porouslayer, then the copper ions can diffuse towards thereducingmetal surface through theporousdeposit layerwhen theconcentrationof ions ishigh.In this situation,the thickness of the deposit layer increases, and thereaction rate can decrease. Since the cementationreactions are mostly diffusion controlled, the reactionratecanbeacceleratedbystirring.Iftheagitationofthesolution is strong enough, the deposit formed can peeloff from themetal surface.Also,agitationcan facilitatethediffusionof ions throughthedeposit layer,and thusthecementationrateincreases.Itwasdeterminedthatthereactionrate did notchangeatstirringspeedshigherthan450 r/min. Examining the temperature effect on thecementationrate,itwasobservedthatthecopperdepositwas not coherent on the reducing metal surface attemperatures higher than 333 K while was coherent atlowertemperatures.

4Conclusions

Recovering copper with aluminum particles bycementationreactionwas investigated in thiswork.Theeffects of the parameters on cementation rate wereexamined and evaluated. It is determined that theactivationenergyis36.8kJ/mol,andthereactionrateiscontrolledbydiffusion.

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NizamettinDEMIRKRAN,AsmKNKL

ChemicalEngineeringDepartment,FacultyofEngineering,InonuUniversity,Malatya44280,Turkey

pH pH

(Edited by LI Xiangqun)