Temperature Enhancement of Zinc and Iron Separation fromChromium(III) Passivation Baths by Emulsion Pertraction TechnologyNazely Diban,† Veronica García,† Francisco Alguacil,‡ Inmaculada Ortiz,† and Ane Urtiaga*,†

†Department of Chemical Engineering, University of Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain‡Department of Primary Metallurgy and Materials Recycling, CENIM (CSIC), Avenida Gregorio del Amo, 8, 28040 Madrid, Spain

ABSTRACT: This work reports the influence of the temperature on the selective removal of zinc and iron from achromium(III) passivation bath by emulsion pertraction technology using Cyanex 272 as extractant and hollow fiber membranecontactors. The results indicate that the kinetics of the separation was largely influenced by the temperature in the range 10−40°C. The viscosity of the organic liquid phase was measured at different temperatures and extractant concentrations, and theresults were fitted to the Riedel and Grunberg and Nissan correlations. The improvement observed from 20 to 40 °C wasexplained by the increase in the diffusion coefficient of the zinc and iron organometallic complexes through the liquid membrane.However, the remarkably slower zinc and iron separation rates observed at 10 °C in comparison with those at 20−40 °C wereattributed to a shift in the driving force due to an endothermic change of the interfacial extraction reaction. The equilibriumparameters at 10 and 20−40 °C were estimated by fitting the experimental kinetic results to the proposed mathematical model.Thus, this work addresses the thermal character of equilibrium and its relevant influence on the separation kinetics of reactivemembrane systems.

1. INTRODUCTION

Zinc electrodeposition is widely used as a galvanic protectionfor metallic surfaces in decorative and industrial applications.However, the corrosion rate of the electroplated surfaces is highas a consequence of the Zn electrochemical reactivity.1 Theactive corrosion may be changed to passive state by immersingthe piece in trivalent chromium passivation baths.2 The keyworking parameters in the conversion process are theconcentration of Cr(III) and the pH and the temperature ofthe passivation bath. The pH of the formulation must beapproximately 2 in order to partially dissolve the Zn layer of theelectroplated piece. Temperature affects the weight of theconversion coating, and hence the corrosion-inhibiting effect.Increasing temperature implies a thicker passivation layer;however, high temperatures may cause undesired yellowishcolor in the coatings.3,4 According to the suppliers, therecommended temperature range is 15−50 °C depending onthe type of commercial bath.During the conversion process, the Zn that is not

incorporated into the coating layer remains dissolved in thebath. The existing iron in the uncovered areas of the platedpieces is also partially transferred to the formulation. Bothmetals contaminate the chemical formulation affecting thequality of the passivation. The passivation bath is replacedwhen it does not fulfill its purpose, generating a significantamount of liquid hazardous waste. These wastewaters aretreated by conventional physical−chemical processes, whichproduce considerable quantities of metallic sludge and consumechemicals and natural resources.5

A newly developed process, based on the emulsionpertraction technology (EPT), enables the selective removalof Zn and Fe impurities during the passivation process,preventing the loss in passivation efficiency and promotingwaste prevention.6 This separation technique combines liquid−

liquid extraction with hollow fiber membrane contactors toextract and back-extract targeted compounds from an aqueoussolution in one operational step.7−10 The nondispersive contactbetween the passivation fluid and the liquid membrane allowsthe definition of a hybrid and stable11 process in which the EPTis integrated into the continuous operation of the passivationprocess (Figure 1). The values of certain variables of the EPToperation are determined by the passivation process, such asmetals concentration, pH, and temperature, while others are setindependently such as flow rates and the concentrations of theextractant and stripping solutions.6 Also, the influence of the

Received: May 15, 2012Revised: June 19, 2012Accepted: June 29, 2012Published: June 29, 2012

Figure 1. Diagram of the integration of the EPT into the passivationprocess.

Article

pubs.acs.org/IECR

© 2012 American Chemical Society 9867 dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−9874

mentioned parameters was mathematically described.12,13 Theauthors found that the rate-limiting step of the process was thediffusion of the metallic species through the impregnated liquidmembrane. The reported EPT studies were conducted at roomtemperature, and the developed model did not include theinfluence of the temperature. However, this variable may largelyaffect the mass transport phenomena of the EPT processthrough both kinetic and equilibrium parameters. First, thediffusivity of the permeating compounds through theimpregnated liquid membrane is higher with increasingtemperatures. On the other hand, the temperature dependenceof the interfacial extraction reaction may affect the concen-tration gradient of the permeating metallic species between thefeed side and the permeate side of the membrane.This article reports the influence of temperature on the

selective separation of Zn and Fe from real Cr(III) passivationbaths by means of EPT in hollow fiber contactors using theorganic solution Cyanex 272/Shellsol D70 as liquid membraneand sulfuric acid as stripping agent. At the acidic pH of thepassivation bath, Cyanex 272 reacts selectively with Zn2+ andFe3+, while Cr3+ remains in the aqueous passivation fluid.6 Theaim of this study was to evaluate the temperature dependenceof the mass transport parameters that affect the kinetics of theprocess. The considered kinetic parameters were the diffusioncoefficients of Zn and Fe organometallic complexes and organicextractant species in the organic solution. Further, theequilibrium parameters under study were the apparentequilibrium constants for Zn and Fe extraction. In order toattain the goal, the viscosity of the Cyanex 272/Shellsol D70mixtures was determined at different temperatures and Cyanex272 concentrations and the apparent equilibrium constantswere estimated mathematically. Finally, this work presents amodel able to describe the extraction of Zn and Fe from Cr(III)passivation baths at different operational conditions includingtemperature.

2. THEORETICAL BACKGROUND

2.1. Mass Transport. In the emulsion pertraction process,the aqueous passivation bath containing the species to beremoved (Zn and Fe) was circulated through the shell side of ahydrophobic microporous membrane contactor. The emulsionphase flowed inside the hollow fibers. The emulsion phase wasformed by the organic solution consisting of Cyanex 272/Shellsol D70 and the stripping agent, sulfuric acid, which wasdispersed by vigorous stirring in an external tank. The crosssection of the hollow fiber and the concentration profiles of thepermeating species through the liquid membrane are shown inFigure 2. The concentration profile of Fe was similar to the onepresented for Zn. According to previous studies, the resistanceto the mass transfer at the liquid boundary layers wasconsidered negligible.12 The organic phase was embeddedinside the pores of the membrane and was in contact with theaqueous feed phase at the outer wall of the membrane. In thiscontacting interface, the following interfacial reactions betweenthe Fe and Zn cations (Men+) and the cationic extractantCyanex 272 (HR) took place:

+ ↔ ++ +n nMe HR MeR Hnn(aq) (org) (org) (aq) (1)

The organometallic complexes MeRn were transported acrossthe organic phase inside the pores toward the droplets of thestripping phase located at the inner side of the membrane. Zn

and Fe cations were then released into the acidic media and thefree extractant was regenerated as follows.

+ ↔ ++ +n nMeR H Me HRnn

(org) (s) (s) (org) (2)

The free extractant HR was counterdiffused to the feed−organic interface. The flux of the diffusing species through theliquid membrane, ji, was described by means of the followingexpression.

α= − ∀ =*j k c c i( ) ZnR , FeR , HRi i i im,o o

2 3 (3)

where α = +1 ∀ i = ZnR2, FeR3, and α = −1 ∀ i = HR.In eq 3 the fluxes of the different species were determined by

(i) the concentration gradient between the concentration of thepermeating species within the membrane phase, (ci

o* − cio), and

(ii) the diffusional mass transfer coefficient km,i.At the feed side, the concentrations ci

o* of the ZnR2 and FeR3species in the membrane phase were described by the followingsimplified equilibrium expression:

= =*

*

*

*

+

+

+

+

Kc c

c cK

c c

c c

( )

( )

( )

( )eq,ZnZnRo

Ha 2

Zna

HRo 2 eq,Fe

FeRo

Ha 3

Fea

HRo 3

2

2

3

3 (4)

Due to the low pH of the stripping agent, the concentrationof the organometallic species in the organic−stripping interfaceduring the back-extraction reaction was neglected assuming amaximum concentration gradient through the membrane, thatis, ci

o = 0.Further, the values of km,i for ZnR2, FeR3, and HR through

the liquid membrane were calculated considering their diffusioncoefficient Do,i values and the geometric and structural

Figure 2. Schematic detail of the hollow fiber cross section andconcentration profile of the transported species (Zn and H) across theliquid membrane of the EPT process. The concentration profile for Feis similar to the one presented for Zn.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749868

characteristics of the porous hollow fibers that wereimpregnated by the organic liquid membrane.

ετδ

= ∀ =kD

i ZnR , FeR , HRii

m,o,

2 3 (5)

where ε is the membrane porosity, δ is the membranethickness, and τ is the membrane tortuosity. The organo-metallic complex and free extractant diffusivity Do,i values werecalculated using the Wilke−Chang correlation:

φ

μ=

×

∀ =

−

DM T

V

i

7.4 10 ( )

ZnR , FeR , HR

ii

o,

8o o

1/2

o o,0.6

2 3 (6)

where φo is the association factor (φo = 1), Mo is the averagemolecular weight of the organic solution (g mol−1), T is thetemperature (K), μo is the organic solution viscosity (cP), andVo,i is the molar volume of the organometallic complex (cm3

mol−1).The equilibrium constants, Keq,Zn and Keq,Fe, depended on the

temperature due to the equilibrium thermodynamics of thechemical system. Moreover, the values of km,i were temperaturedependent as Do,i and μo were also affected by the temperature.Therefore, the temperature is a variable that may stronglyinfluence the EPT performance and the effect of this variablemust be assessed.2.2. Mass Balances. The mathematical model that

described the removal of Zn and Fe impurities from thepassivation bath consisted of a set of mass balances applied tothe metallic species, proton, and organic extractant within thethree fluid phases that participated in the EPT process.12 Underthe high flow rate conditions employed in the present system,ideal plug flow was considered and therefore the radial variationin the fluid properties was neglected. The pseudostationarystate inside the module is also assumed. The mass balancesinside the membrane module were given by

α= − ∀ ∀ ∀ =

∀ =

F Lcz

A j z t i

j

dd

; ; Zn, Fe, H;

a, s

j ij

im

(7)

where α = +1 ∀ i = Zn, Fe, and α = −1 ∀ i = H.

= ∀ ∀ ∀ =cz

z t idd

0 ; ; Zn, Fe, Hio

(8)

Assuming pseudo steady state

+ =j j j2 3ZnR FeR HR2 3 (9)

with the following initial and boundary conditions:

= = ∀ ∀ =

∀ =

c t c z i

j

( 0) ; Zn, Fe, H;

a, o, s

ij

ij,initial

(10)

= = ∀ ∀ =c z c t i( 0) ; Zn, Fe, Hi ia

,Ta

(11)

= = ∀ ∀ =

∀ =

c z L c t i

j

( ) ; Zn, Fe, H;

o, s

ij

ij,T

(12)

where “a”, “s”, and “o” refer to the feed, stripping, and organicphases, respectively.

The mass balances in the feed and emulsion stirred tankswere described as follows.

= = − ∀

∀ = ∀ =

V ct

F c z z c t

i j

d( )

d( ( ) ) ;

Zn, Fe, H; a, o, s

jij

jij

ijT ,T

out ,T

(13)

= ∀ =z L j aout (14)

= ∀ =z j0 o, sout (15)

= = ∀ = ∀ =c t c i j( 0) Zn, Fe, H; a, o, sij

ij

,T ,initial

(16)

In this model, the influence of temperature was giventhrough the flux of species ji, as described previously in eqs3−6.

3. EXPERIMENTAL SECTION3.1. Chemicals. The passivating bath under study was

provided by a local plating industry. The bath contained 5681mg L−1 Zn(II), 450 mg L−1 total Fe, and 5928 mg L−1 Cr(III)and exhibited a pH value of 1.8. The major anion found in theformulation was nitrate with a concentration of 67 g L−1. Thecommercial extractant Cyanex 272 was kindly supplied byCytec Industries, France. The active component of Cyanex 272is bis(2,4,4-trimethylpenthyl)phosphinic acid and has amolecular weight of 290 g mol−1, a density of 920 kg/m3 at24 °C, and a purity of 83%. The purity was determined bytitration according to the procedure recommended by themanufacturer.14 At acidic pH ranges below 3, Cyanex 272extracts selectively Zn and Fe, while Cr(III) cations are retainedin the aqueous phase.6

Shellsol D70 was purchased from Kremer Pigmente GmbH& Co. KG. This chemical is a low aromatic hydrocarbonsolvent and was employed as a diluent of Cyanex 272. Solutionsof sulfuric acid (reagent grade ISO, Panreac) and sodiumhydroxide (pro analysi, Merck) were prepared using deionizedMilli-Q water and used as back-extraction agent and to set thevalue of the pH of the feed solutions, respectively.

3.2. Procedure. The Zn and Fe extraction experimentswere conducted by mixing equal volumes of the passivationbath (A) and the extractant solution (O) (A/O = 1) at 10, 20,and 50 °C in several thermostatized mixer−settler unitsarranged in series (Figure 3). The extractant consisted of10% v/v Cyanex 272 diluted in Shellsol D70. Prior to eachexperiment, the initial pH of the passivating bath was modifiedby adding a solution of sodium hydroxide in order to attaindifferent pHeq’s. The evaluation of the influence of the contacttime on the extraction percentage proved that 10 min wasenough to reach chemical equilibrium. The solutions wereunder continuous stirring until equilibrium was attained. Thephases were led to settle, and samples of the aqueous solutionwere taken for further analysis.The viscosity of the organic phase was quantified using a

thermostatized Brookfield rotational viscometer (Model AlphaSeries L, Fungilab S.A., Spain). The viscosity values wereobtained at different Cyanex 272/Shellsol D70 compositions(0/100, 10/90, 15/85, 20/80, and 100/0% v/v) in thetemperature range 5−45 °C. The viscosities of the mixturescontaining Shellsol D70 were measured at a rotational speed of100 rpm using a LCP spindle, whereas 50−100 rpm and a TL5spindle were used for the Cyanex 272 100% v/v samples.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749869

The kinetic EPT study was conducted utilizing theexperimental setup illustrated in Figure 4. The experimentswere performed in countercurrent mode using a membranecontactor (Liqui-Cel Extra-Flow 2.5×8, Celgard) with a totaleffective mass transfer area of 1.4 m2. The membrane contactorenclosed hydrophobic polypropylene microporous X-50 hollowfibers with the following characteristics: porosity ε = 0.4,membrane thickness δ = 40 μm, and tortuosity τ = 6.4. Thefeed tank contained 2 L of the passivation bath with the averagecomposition detailed in section 3.1. During the EPT processthe pH of the bath was maintained constant at 1.8 by adding asolution of 4 mol L−1 sodium hydroxide. The emulsion tankhad a capacity of 1 L and contained 0.2 L of 4 mol L−1 sulfuric

acid and 0.8 L of an organic mixture of 15% v/v Cyanex 272 inShellsol D70. Both the aqueous and emulsion phases werethermostatized, and the temperatures were set at 10, 20, and 40°C. Two replicates of each experiment were conducted.Additional experimental conditions are listed in Table 1.

Samples of the passivation bath and stripping acid werecollected at regular intervals to follow the development of theconcentration of the metals during the EPT process. Thesamples were quantitatively analyzed by atomic absorptionspectroscopy (Perkin-Elmer, AAnalyst 3110). The obtainedresults were described mathematically by the model included insection 2 that was solved using Aspen Custom Modeler, version2004.1.

4. RESULTS AND DISCUSSION4.1. Effect of Temperature on Extraction Equilibrium.

The effect of the temperature on Zn and Fe extractionequilibria using a 10% v/v Cyanex 272 solution as extractant ispresented in Figure 5. In the present system, the equilibriumisotherms at 20 and 50 °C were overlapped. However, theextraction isotherm shifted to higher equilibrium pH valueswhen the temperature decreased to 10 °C. The extraction of Znat 10 °C was approximately 20% at a pHeq of 1.8. At 20 or 50°C the extraction rose to 40%. At these temperatures andequilibrium pH values, the Fe extraction always exceeded 90%,

Figure 3. Experimental setup of in-series thermostatized mixer−settlers for equilibrium tests.

Figure 4. Experimental emulsion pertraction setup.

Table 1. EPT Experimental Working Conditions

variable aqueous feed emulsion

operation mode recirculation recirculationcirculation shell side lumen sideflow rate 220 L h−1 70 L h−1

volume 2 L 1 Linlet pressure 0.85 bar 0.5 baroutlet pressure 0.65 bar 0 bar

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749870

whereas the extraction of Cr(III) was by far lower than that ofZn. An experimental extraction order was thus obtained as Fe >Zn > Cr.According to the presented Zn equilibrium isotherms, the

system presented a different behavior at 10 °C compared to theone observed at temperatures above 20 °C. This change wasendothermic in nature, increasing the extraction performance athigher working temperatures. Several authors have alsoobserved endothermic thermodynamics with different levelsof temperature influence in the Zn(II) and Fe(III) extractionswith Cyanex 272 organic solutions from different sulfate andchloride media at several temperature ranges. When conductingthe Zn extraction from chloride media, Wang et al.15 reported amild temperature dependence in the range 17−45 °C and anenthalpy change, ΔH, of 10.19 kJ mol−1, and Baba andAdekola16 found a more significant temperature influence in therange 27−50 °C, with ΔH = 26.81 kJ mol−1. Similarly, Biswasand Singha17 found a ΔH of 13 kJ mol−1 in the extraction of Fefrom sulfate media in the range 25−45 °C, whereas Deep etal.18 observed a ΔH value of 159 kJ mol−1 in the temperaturerange 27−50 °C. On the contrary, Ali et al.19 found anexothermic nature in the extraction of Zn from a complexnitrate−sulfate−chloride medium with 2% v/v Cyanex 272 inkerosene in the temperature range 15−45 °C. Naik andDhadke20 also obtained an exothermic tendency in theextraction of Fe(III) from nitrate media in the temperaturerange 30−55 °C.It is worth noting that in the present work a change in the

thermodynamic behavior from endothermic to isothermal wasobserved above 20 °C. Sarangi et al.21 also reported athermodynamic shift in the Co(II) extraction from chloridemedia using Cyanex 272. In this case the process shifted fromendothermic to exothermic at 30 °C. This was attributed to asharp decrease of the stability of the extracted organometalliccomplex at temperatures above 30 °C.The comparison of the results obtained in this study with the

works reported in the literature indicated that the thermody-namic parameters determined experimentally might vary widelydepending on the complexity of the matrix composition of theaqueous solution employed and the equilibration characteristicsof the experimental systems employed.4.2. Effect of Temperature on Overall Kinetics of the

EPT Process. The development of the concentrations of Znand Fe in the passivation bath during the EPT experiments atdifferent temperatures is illustrated in Figures 6 and 7,

respectively. As observed, the temperature enhanced theextraction of both metals. Bey et al.22 reported the sametrend when conducting a kinetic study on the extraction ofCr(VI) by nondispersive solvent extraction using Aliquat 336 asextractant and a modified polyether ether ketone (PEEK-WC)membrane contactor in the temperature range 20−50 °C. Ourresults showed a significant improvement of the processkinetics when the temperature was doubled from 10 to 20°C, while the growth was less intense for a temperature risefrom 20 to 40 °C.The effect of temperature on the diffusion of permeating

species through the impregnated liquid membrane wasevaluated measuring the viscosities of Cyanex 272, ShellsolD70, and their mixtures at different temperatures (Figure 8). Asexpected, the viscosities of the organic samples decreased withincreasing temperature. According to the results presented inFigure 8A, the viscosity of pure Cyanex 272 decreased from 300to 52 cP as the temperature increased from 5 to 41.2 °C. Thissuggested that the effect of temperature on the viscosities of theorganic solutions would be stronger at high Cyanex 272concentration. However, Figure 8B indicates that, in theexperimental conditions utilized in the EPT and equilibriumtests, with Cyanex 272 concentration range of 10−20% v/v, theeffect of temperature on the viscosity was minor.The effect of temperature on the viscosities of the Cyanex

272/Shellsol D70 mixtures was described using the Grunbergand Nissan correlation:23

Figure 5. Effect of temperature on zinc extraction with Cyanex 272 at10% v/v extractant concentration. Equilibration time 10 min. O/Aratio = 1.

Figure 6. Development of concentration of Zn in passivation bathduring EPT experiment at 10, 20, and 40 °C. The solid lines representmodel simulation results.

Figure 7. Development of concentration of Fe in passivation bath at10, 20, and 40 °C. The solid lines represent model simulation results.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749871

μ μ μ= + +x x x x G Tln ln ln ( )o 1 1 2 2 1 2 12 (17)

where μo (cP) is the viscosity of the organic mixture, thesubscripts “1” and “2” refer to the pure components Cyanex272 and Shellsol D70, respectively, x is the molar fraction, andG12(T) is a constant that measures the deviation of the systemfrom Arrhenius behavior.24

The viscosities of the pure components μ1 and μ2 were givenby the empirical modified Riedel equation:25

μ = + +ABT

C Tln( ) ln( )i (18)

where i refers to component 1 or 2 and T is expressed in kelvin.The parameters A, B, and C that describe the temperature

dependence of the viscosity of pure Cyanex 272 and ShellsolD70 were obtained by fitting the experimental data plotted inFigure 8A to eq 18, and the obtained values are given in Table2.G12(T) was described as follows:23,24

= − − −G T G

T( ) 1 (1 )

57327512 12 (19)

where T is the temperature in kelvin and G12 is G12(T) for themixture at 298 K.

Further, G12 was assumed to be proportional to thedifference between the structural summations σ of the groupcontributions of each pure component 1 and 2.

∑ ∑σ σ= − +G W12 1 2 (20)

where W is a correction factor that is zero when any of thecomponents of the mixture contains atoms other thanhydrogen or carbon.The value of G12 for the Cyanex 272/Shellsol D70 mixture

(Table 2) was hence calculated as follows:

∑ ∑σ σ

σ σ σ σ σ

σ σ

= − +

= + + + +

− +

G W

(8 4 2 2 )

(2 10 )

12 1 2

CH CH CH C P

CH CH

3 2

3 2 (21)

where σ values were obtained from Isdale et al.24 and σP wasestimated to minimize the standard error between theexperimental and calculated viscosity values according to theGrunberg and Nissan model.The comparison between the experimental and predicted

values of the viscosity using the Riedel model for the pureorganics and the Grunberg and Nissan mixing rule for theCyanex 272/Shellsol D70 mixtures, is plotted in the paritygraph of Figure 9. As observed, 86% of the predicted μο

model

viscosity values fall within μοexp ± 10%μο

exp. Therefore, theproposed viscosity model predicted adequately the experimen-tal data and can be implemented in the mathematical model inorder to consider the temperature dependence of the masstransfer parameters used to describe the EPT process.The values of km,i and Do,i for ZnR2, FeR3, and HR species at

10, 20, and 40 °C were calculated according to eqs 5 and 6,respectively. Additionally, the viscosity of the organic solutioncontaining 15% v/v Cyanex 272 at 10, 20, and 40 °C wasdetermined by means of eqs 17−21.The obtained values of the mass transfer parameters and the

viscosity of the organic mixture are summarized in Table 3.According to these results the values of km,Zn and km,Fe of the Znand Fe organometallic complexes increased about 20% whenthe temperature rose from 10 to 20 °C; i.e., km,Zn changed from

Figure 8. Influence of temperature on viscosity of (A) purecomponents Shellsol D70 and Cyanex 272 and (B) Cyanex 272/Shellsol D70 mixtures: 10/90, 15/85, and 20/80% v/v.

Table 2. Riedel and Grunberg and Nissan Model Parameters

Riedel Grunberg and Nissan

component A B C G12

Cyanex 272 −72.9 6670 9.693.61

Shellsol D70 −44.8 2823 6.29

Figure 9. Parity graph: comparison of experimental and predictedviscosity values for the organic phase at different Cyanex 272/ShellsolD70 compositions (0/100, 10/90, 15/85, 20/80, and 0/100% v/v)and temperatures between 5 and 45 °C according to the Riedel andGrunberg and Nissan equations for pure components and mixtures,respectively.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749872

3.0 × 10−7 m s−1 at 10 °C to 3.6 × 10−7 m s−1 at 20 °C.However, the promotion of the velocity of Zn and Fe extractionobserved experimentally in the EPT process (Figures 6 and 7)when changing the operation temperature from 10 to 20 °Cwas significantly higher and was not justified only by theincrement in the diffusivity. This behavior was easily explainedin terms of the improvement of the percentage of extractionequilibrium previously observed in Figure 5. The interfacialconcentrations of the metallic species ZnR2 and FeR3 at thefeed side of the liquid membrane at 20 °C was higher than theconcentrations obtained at 10 °C. The increment in the drivingforce for mass transfer made the flux of the metallic specieshigher. The values of the equilibrium constants (Keq,Zn andKeq,Fe) defined in eq 4 were estimated from the best fitting(minimum weighted standard deviation) of the predictedcurves to the experimental data shown in Figures 6 and 7. Thiswas done using the estimation tool of the Aspen CustomModeler software package. The results of the estimatedparameters at 10, 20, and 40 °C are presented in Table 3. Asobserved, the small difference in the average values of theequilibrium parameters for Zn and Fe extraction reactions at 20and 40 °C (Keq,Zn = 2.94 × 10−4 ± 0.05 × 10−4 and Keq,Fe =1.15 × 10−5 ± 0.34 × 10−5) may be attributed to the standarddeviation error. A comparable apparent equilibrium constantfor the extraction of Zn was experimentally obtained by Bringaset al.12 under analogous working conditions at room temper-ature: Keq,Zn = 2.74 × 10−4. This result indicated that theenhancement of the Zn and Fe extraction from 20 to 40 °C(Figures 6 and 7) was caused only by the effect of temperatureon the diffusion coefficients of the targeted heavy metals. Table3 also shows that lower values of the equilibrium constantswere obtained at 10 °C: Keq,Zn = 0.44 × 10−4 ± 0.09 × 10−4 andKeq,Fe = 0.28 × 10−5 ± 0.05 × 10−5. These results were inagreement with those obtained in the equilibrium experimentsdiscussed in section 4.1.The model and the mass transfer and equilibrium parameters

were employed to simulate the EPT separation of the targetedheavy metals at the temperatures under study. Figures 6 and 7show the comparison between simulated and experimental datafor the extraction of Zn and Fe, respectively. As illustrated, thesimulated curves predicted adequately the experimental data forthe development of the concentrations of Zn and Fe in thepassivation bath during the EPT process.

5. SUMMARY AND CONCLUSIONSDuring the conversion of electroplated Zn surfaces usingCr(III)-based passivation baths, the temperature is a criticalworking variable affecting the thickness and corrosive resistanceof the passivation layer. An emulsion pertraction unit may beintegrated into the conversion process to remove the impuritiesof Zn and Fe that shorten the lifetime of the passivation bath.In the present work, the influence of the temperature on theperformance of EPT is evaluated.

This study concludes that a significant influence of thetemperature on the kinetics of Zn and Fe removal from thepassivation bath by EPT existed in the temperature range 10−40 °C. The higher velocity of Zn and Fe separation at highertemperatures was partially explained by the increase of thediffusion coefficients of the organometallic complexes throughthe liquid membrane. This enhancement was largely due to thereduction of the viscosity of the organic solution, μo. Theextraction equilibrium for Zn and Fe presented a differentbehavior when the temperature was 10 °C in comparison withthe results in the range between 20 and 40 °C that showed anendothermic change.The influence of the temperature observed on the mass

transport parameters was included in a mathematical modelthat enabled the accurate description of the Zn and Fe removalkinetics from the passivating bath by means of an EPT system.The dependence of the viscosity of the organic solution withthe temperature and the concentration of extractant Cyanex272 was described accurately by the combination of the Riedeland Grunberg and Nissan correlations. The diffusion coefficientwas described by the widely employed Wilke−Chang equation.This work finally concludes that the proposed model is a usefultool for design purposes: it enables an accurate prediction ofthe minimum membrane area of the membrane contactorrequired to remove the incoming metallic impurities into thepassivation process.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: urtiaga@unican.es.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been funded by Projects CTQ2008-00690(MCI, Spain) and TIGI (European Comission, Grant agree-ment 218390). Componentes y Conjuntos S.A. is acknowl-edged for the kind supply of passivating bath samples.

■ NOMENCLATUREA = parameter of the Riedel equationAm = effective membrane area (m2)B = parameter of the Riedel equationc = liquid concentration of the species (mol m−3)C = parameter of the Riedel equationDo = diffusion coefficient of diffusion species in the organicphase of the liquid membrane (m2 h−1)F = flow rate (m3 h−1)G = parameter of the Grunberg and Nissan correlationH = enthalpy (kJ mol−1)j = flux of the species through the liquid membrane (mol h−1

m−2)

Table 3. Viscosity and Kinetic and Equilibrium Parameters of Species Transported in the Organic Phase at 15% v/v Cyanex 272at Different Working Temperatures

temp(°C)

viscosity,μo (cP)

zinc (ZnR2)diffusion coeff,Do,Zn (m

2 s−1)

zinc masstransfer const,km,Zn (m s−1)

iron (FeR3)diffusion coeff,Do,Fe (m

2 s−1)

iron masstransfer const,km,Fe (m s−1)

free extractant(HR) diffusion

coeff,Do,H (m2 s−1)

HR masstransfer const,km,H (m s−1)

Zn equilib coeff,ln Keq,Zn

Fe equilib coeff,ln Keq,Fe

10 2.80 5.31 × 10−10 3.00 × 10−7 4.19 × 10−10 2.35 × 10−7 8.03 × 10−10 4.53 × 10−7 −10.04 ± 0.22 −12.80 ± 0.1920 2.41 6.42 × 10−10 3.60 × 10−7 5.03 × 10−10 2.83 × 10−7 9.67 × 10−10 5.44 × 10−7 −8.19 ± 0.10 −11.57 ± 0.3340 1.89 8.78 × 10−10 4.94 × 10−7 6.92 × 10−10 3.88 × 10−7 13.3 × 10−10 7.44 × 10−7 −7.98 ± 0.16 −10.86 ± 0.34

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749873

Keq = apparent equilibrium coefficient of the extractionreactionkm = diffusional mass transfer coefficient of species throughthe liquid membrane (m h−1)Mo = average molecular weight of the organic solution (gmol−1)L = effective fiber membrane length (m)t = time (h)T = temperature (K)Vo = molar volume (cm3 mol−1)V = volume of the tank (m3)W = correction factor in the Grunberg and Nissancorrelationx = molar fractionz = axial coordinate (m)

Greek Symbolsα = parameter attributing the positive/negative characterε = membrane porosityδ = membrane thickness (m)Δ = thermodynamic property changeφo = association factor in the Wilke−Chang correlationμ = viscosity (cP)σ = structural group contribution term of the Grunberg andNissan correlationτ = membrane tortuosity

Subscriptsi = diffusing species present on the systeminitial = values at operation time = 0o = organic phase (Cyanex 272/Shellsol D70 mixture)out = outside the membrane moduleT = feed and/or emulsion tank1 = first component of the organic mixture2 = second component of the organic mixture

Superscriptsa = aqueous feed phasej = fluid phaseso = organic phase (Cyanex 272/Shellsol D70 mixture)s = stripping phase* = values of the concentrations in equilibrium after theextraction reaction

■ REFERENCES(1) Tomachuk, C. R.; Elsner, C. I.; Di Sarli, A. R.; Ferraz, O. B.Morphology and corrosion resistance of Cr(III)-based conversiontreatments for electrogalvanized steel. J. Coat. Technol. Res. 2010, 7,493.(2) Weber, T. J. Wastewater treatment. Met. Finish. 2007, 105, 699.(3) Jansen, R. Processing of Trivalent Blue Passivations to Reach LongerService Lifes; Surtec Technical Letter; 1995.(4) Thiery, L.; Pommier, N. Hexavalent Chromium-Free PassivationTreatments in the Automotive Industry; Coventya SAS Report; 2004.(5) Diban, N.; Mediavilla, R.; Urtiaga, A.; Ortiz, I. Zinc recovery andwaste sludge minimization from chromium passivation baths. J.Hazard. Mater. 2011, 192, 801.(6) Urtiaga, A.; Bringas, E.; Mediavilla, R.; Ortiz, I. The role ofmembranes in the selective separation and recovery of zinc for theregeneration of Cr(III) passivation baths. J. Membr. Sci. 2010, 356, 88.(7) Klaassen, R.; Jansen, A. E. The membrane contactor: environ-mental applications and possibilities. Environ. Prog. 2001, 20, 37.(8) Ho, W. S. W.; Poddar, T. K. New membrane technology forremoval and recovery of chromium from wastewaters. Environ. Prog.2001, 20, 44.

(9) Ho, W. S. W. Combined Supported Liquid Membrane/StripDispersion Process for the Removal and Recovery of Penicillin and OrganicAcids. U.S. Patent 6,433,163, 2002.(10) Urtiaga, A.; Abellan, M. J.; Irabien, J. A.; Ortiz, I. Membranecontactors for the recovery of metallic compounds: Modelling ofcopper recovery from WPO processes. J. Membr. Sci. 2005, 257, 161.(11) Sonawane, J. V.; Pabby, A. K.; Sastre, A. M. Pseudo-emulsionbased hollow fiber strip dispersion: A novel methodology for goldrecovery. AIChE J. 2008, 54, 453.(12) Bringas, E.; Mediavilla, R.; Urtiaga, A. M.; Ortiz, I. Developmentand validation of a dynamic model for regeneration of passivatingbaths using membrane contactors. Comput. Chem. Eng. 2011, 35, 918.(13) Bringas, E.; San Roman, M. F.; Irabien, J. A.; Ortiz, I. Anoverview of the mathematical modelling of liquid membraneseparation processes in hollow fibre contactors. J. Chem. Technol.Biotechnol. 2009, 84, 1583.(14) Cyanex 272 Extractant Technical Brochure; Cytec Industries:Woodland Park, NJ, 1995.(15) Wang, Y. G.; Wang, L. G.; Li, D. Q. Synergistic Extraction ofZinc(II) with Mixtures of CA-100 and Cyanex 272. Sep. Sci. Technol.2003, 38, 2291.(16) Baba, A. A.; Adekola, F. A. Beneficiation of a Nigerian sphaleritemineral: Solvent extraction of zinc by Cyanex 272 in hydrochloric acid.Hydrometallurgy 2011, 109, 187.(17) Biswas, R. K.; Singha, H. P. Purified Cyanex 272: Its interfacialadsorption and extraction characteristics towards iron(III). Hydro-metallurgy 2006, 82, 63.(18) Deep, A.; Correia, P. F. M.; de Carvalho, J. M. R. Liquid−LiquidExtraction and Separation of a Macro Concentration of Fe3+. Ind. Eng.Chem. Res. 2007, 46, 5707.(19) Ali, A. M. I.; Ahmad, I. M.; Daoud, J. A. CYANEX 272 for theextraction and recovery of zinc from aqueous waste solution using amixer-settler unit. Sep. Purif. Technol. 2006, 47, 135.(20) Naik, M. T.; Dhadke, P. M. Extraction of iron(III) with bis(2-ethylhexyl)phosphinic acid and bis(2-ethylhexyl)phosphoric acid:Experimental equilibrium study. J. Chem. Eng. Data 1999, 44, 1037.(21) Sarangi, K.; Reddy, B. R.; Das, R. P. Extraction studies ofcobalt(II) and nickel(II) from chloride solutions using Na-Cyanex272: Separation of Co(II)/Ni(II) by the sodium salts of D2EHPA,PC88A and Cyanex 272 and their mixtures. Hydrometallurgy 1999, 52,253.(22) Bey, S.; Criscuoli, A.; Simone, S.; Figoli, A.; Benamor, M.;Drioli, E. Hydrophilic PEEK-WC hollow fibre membrane contactorsfor chromium (VI) removal. Desalination 2011, 283, 16.(23) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties ofGases and Liquids; McGraw-Hill: New York, 2001.(24) Isdale, J. D.; MacGillivray, J. C.; Cartwright, G. Prediction ofViscosity of Organic Liquid Mixtures by a Group Contribution Method;National Engineering Laboratory Report: East Kilbride, Glasgow, U.K.,1981.(25) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook,7th ed.; McGraw-Hill: New York, 1997.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−98749874