Hualing Yang,Ji Chen*,Wei Wang,Hongmin CuiDongli ZhangYu Liu

Research Notes

ExtractionKineticsof Lanthanum inChlorideMedium by BifunctionalIonic Liquid[A336][CA-12]Using a Constant InterfacialCell with Laminar Flow☆

Hualing Yang1,2,Ji Chen1,*,Wei Wang3,Hongmin Cui1,Dongli Zhang1,Yu Liu1

1State Key Laboratory of Rare Earth Resources Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,China2School of Chemistry and Chemical Engineering,Nantong University,Nantong 226019,China3Zhengzhou Institute of Multipurpose Utilization of Mineral Resources,China Academy of Geological Sciences,Zhengzhou 450006,China

A R T I C L EI N F O

Article history:

The extraction kinetics of La(III)from aqueous chloride solutions into n-heptane solutions of bifunctional ionic liquid extractant[A336][CA-12](tricaprylmethylammonium sec-octylphenoxy acetic acid)was investigated using a constant interfacial cell with laminar f l ow.The effects of stirring speed,temperature and specif i c interfacial area on the extraction rate were examined.The results indicate that mass transfer kinetics of La(III)is a mixed-controlledprocessinf l uencedbyinterfacialreaction.Onthebasisofmasstransferkineticresultsintheextraction of La(III)by[A336][CA-12],the extraction rate equation of La(III)is proposed in terms of pseudo-f i rstorder constants,which is supported by the measured thermodynamic equations.The mass-transfer kinetic model deduced from the rate controlling step is adequate to interpret the experimental data qualitatively.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

As an eff i cient and classical analytical method to separate rare earth elements,solventextraction method plays an important role in production industry of rare earths.Ionic liquids(ILs)have been widely investigated in liquid-liquid extraction as solvent or extractant,since the f i rst report by Rogers and co-workers[1].The booming“tailor-made”ILs, whichistheconceptoftask-specif i cionicliquids(TSILs),revealsconsiderablepotentialitiesforsomeindustrialprocesses.ThismeansthatTSILs themselves could extract metal ions from aqueous phase in terms of functional groups on the cation or anion.In recent two decades,many TSILs incorporatingfunctional groups have been reported for extraction and separation of metal ions as the extractants[2-8].

It is reported that TSILs have high extraction eff i ciency for specif i c metal ions,especially TSIL bearing urea,thiourea,and thioether.In the extraction process the distribution ratio for Hg2+could reach 710 with a TSIL bearing thiourea used as extractant[7].Ouadi et al.have reported a TSILbearingphosphoryl groups,whichpresents higher extraction ability for UO22+than conventional extractant TBP[9].Recently, quaternary ammonium ILs have attracted wide attention due to their lower cost and toxicity,such as imidazolium-type and pyrrolidiniumtype[3,4,10,11].Our group has reported a class of bifunctional ionic liquid extractants(Bif-ILEs)composed of quaternary ammonium A336 cation and phosphonic acid or carboxylic acid anions[12],in which the anions are obtained from conventional phosphonic acid and carboxylic acid extractants for extraction of rare earth ions.Several researchers have studied the extraction properties of Bif-ILEs for metal ions[10,13-15]and indicated that Bif-ILEs may be better extractant than their precursor.The extraction process using Bif-ILEs shows some advantages,such as avoiding saponif i cation wastewater from the application of acidic extractants,which is a remarkable advantage of the environment-friendly extraction protocol,low acidy for extraction and good interfacial phenomena[16,17].The extraction thermodynamics of Bif-ILE[A336][CA-12](tricaprylmethylammonium secoctylphenoxy acetic acid)for the extraction of rare earth ions has been investigated in detail[13],and the results imply that Bif-ILE[A336][CA-12]may have potential industrial applications for La(III)separation.For a new extraction system applied to industrial practices,the kinetic study is necessarytoidentifytherate-determiningstepandproposemasstransfer models[18].Hence,it is necessary to investigate the lanthanum extraction kinetics with Bif-ILE[A336][CA-12].Some experimental equipment and techniques have been developed to investigate the extraction kinetics[19-22],such as single drop technique,constant interfacial area stirred cell,and constant interfacial cell with laminar f l ow, among which the constant interfacial cell with laminar f l ow is used widely for its stability and reproducibility of data[18,23-28].As a part of our group's work,the extraction kinetics of La(III)with Bif-ILE [A336][CA-12]has been investigated using a constant interfacial cellwith laminar f l ow.In our present work,the extraction controlling regimeisevaluatedand themass transfermodelis determined byconsidering the effects of various conditions on the extraction process.The purpose is to understand the extraction kinetics of La(III)with Bif-ILE [A336][CA-12],which would provide useful information for separation and purif i cation of La(III).

2.Experimental

2.1.Materials

Tricaprylmethylammonium chloride(A336)and sec-octylphenoxy acetic acid(CA-12)were obtained from Sigma-Aldrich and Tianjin Xiandai Chemical Plant(China),respectively.[A336][CA-12](the structure in Fig.1)was synthesized as previous report[12]and dissolved in n-heptane.Stocksolution of La(III)waspreparedbydissolvingtheir oxides(99.9%)inconcentratedhydrochloricacid.Allotherchemicalswere of analytical grade.

Fig.1.The structures of[A336]+[CA-12].

2.2.Apparatus and experimental procedure

Allkineticexperimentsintheconstantinterfacialarea cellwithlaminar f l ow were performed at 298 K,except temperature experiments. Thevolumesofaqueousandorganicphaseswereboth95mL.Theinterfacialarea betweenaqueousandorganic phaseswasf i xed at18cm2except for interfacial area experiments.In the mass transfer process, 0.4 ml aqueous phase was obtained every 2 min,concentrations of La(III)in aqueous phase after extracting were determined spectrophotometrically using Shimadzu(Kyoto,Japan)UVmini-1240 UV-visible spectrophotometer,and concentrations of La(III)in organic phase were calculated by mass balance.1H NMR spectra were recorded on an AV-400 NMR spectrometer(Bruker,F?llanden,Switzerland)in CDCl3to characterize the purity of[A336][CA-12].

2.3.Data treatment

The experimental data are processed following the theoretical formulas deduced by Danesi and Vandegrift[29].The pseudo-f i rst-order reversible reaction is introduced into the mass transfer process with respect to a metal cation:

where Kaoand Koaare the forward and backward pseudo- fi rst-order rate constants(cm·s?1),respectively,[La](a)and[La](o)stand for the concentrations of La(III)in aqueous and organic phases at time t, respectively,A is the specif i c interfacial area(cm2),and V is the volume of aqueous or organic phase(ml).

The mass transfer rate is zero at equilibrium,and Eq.(2)leads to

where superscripts ini and e represent the initial and equilibrium values,respectively.

Integrating Eq.(2)with Eq.(3),we have

The left side of Eq.(4)versus time t is plotted for each experiment, and the slopes are used to calculate Kaoand Koa.

3.Results and Discussion

Fig.2.Dependence of lgKaoon stirring speed.

3.1.Dependence of lgKaoon stirring speed

In extraction kinetic experiments,one of the criteria generally used to identify the extraction regime is the dependence of extraction rate onstirringspeed intheconstantinterfacialareacell.Fig.2shows theeffect of stirring speed on the extraction rate of La(III).A‘kinetics plateau’[23,29]appears in the stirring speed range from 250 to 350 r·min?1. The extraction rate increases due to the waving observed at the interface.Therelationshipofextractionrateversusstirringspeedisastraight lineatlowstirringspeed.Thepresenceof“plateauregion”indicatesthat the extraction rate may be kinetic controlled,but it may be resulted from otherphenomena.It is possible that theextractionrate is diffusion controlled or at least not fully kinetic controlled.Therefore,it is necessary to further identify the extraction regime.

3.2.Dependence of lgKaoon temperature

The activation energy(Ea)of extraction process is a further criterion that can distinguish between diffusion controlled and kinetic controlled processes.Generally,when Ea of an extraction is more than 42 kJ·mol?1,the mass transfer process is controlled by chemical reaction;when Ea is lower than 20 kJ·mol?1,species diffusion is the ratelimitingstep;themasstransferrateisdeterminedbybothchemicalreaction and diffusion when Ea is in the range from 20 to 42 kJ·mol?1.Theeffect of temperature on the La(III)extraction rate by[A336][CA-12]is studied in the temperature range 293 to 313 K,and the results are shown in Fig.3.The extraction rate f i rst increases with temperature sharply and then slightly.The extraction activation energy is calculated by the Arrhenius Equation

Fig.3.Dependence of lgKaoon temperature.

where C is the rate constant and Ea is the activation energy.Based on experimental data,the apparent activationenergy(Ea)for La(III)extraction is calculated to be 3.31 kJ·mol?1at temperatures higher than 303 K, while it is 23.9 kJ·mol?1at temperatures lower than 303 K.It suggests a mixed kinetic control regime in the temperature range from 293 to 303 K and a diffusion controlled regime in the range from 303 to 313 K. All other kinetic experiments are measured at 250 r·min?1and 303 K in order to maintain the same hydrodynamic conditions.

3.3.Dependence of lgKaoon specif i c interfacial area

The dependence of specif i c interfacial area(A/V)on the extraction rate is one of the criteria for determination of rate-controlling step in thekinetic study.Fig.4 shows that the extraction rate decreases slightly with the increase of specif i c interfacial area for La(III)systems,indicating that the chemical reaction possibly occurs in the aqueous homogeneous phases.

3.4.Extraction rate equation and kinetic model

Fig.5 presents the rate constant Kaoas a function of concentration of Bif-ILE[A336][CA-12].Theslope of the lineis 1.01,which is the reaction order of Bif-ILE[A336][CA-12]in the rate equation.Fig.6 shows the effectof Cl?concentration ontheextractionrate,and theslope of theline or reaction order of Cl-in the rate equation is 0.51.According to these results,the rate equation for the extraction of La(III)with Bif-ILE [A336][CA-12]can be written as

Fig.4.Dependence of lgKaoon specif i c interfacial area.

Fig.5.The effect of[A336][CA-12]concentration on the extraction rate.

Bif-ILE[A336][CA-12]used for extraction of rare earths has been investigated by our group,and the thermodynamic equilibrium equation is written as[13]

Fig.6.The effect of[NaCl]concentration on the extraction rate.

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2 August 2013

☆Supported by the National Natural Science Foundation of China(51174184),the National Basic Research Program of China(2012CBA01202),the Key Research Program of the Chinese Academy of Sciences(KGZD-EW-201-1),the Science and Technology Plan of Nantong City(BK2013030),the University Science Research Project of Jiangsu Province(14KJB150019),Open Subject of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences(RERU2014016).

*Corresponding author.

E-mail address:jchen@ciac.ac.cn(J.Chen).

http://dx.doi.org/10.1016/j.cjche.2014.09.010

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 26 November 2013

Accepted 18 December 2013

Available online 18 September 2014

Extraction kinetics

Bifunctional ionic liquid

Constant interfacial cell with laminar f l ow