Temperature Enhancement of Zinc and Iron Separation from

Article pubs.acs.org/IECR

Temperature Enhancement of Zinc and Iron Separation from Chromium(III) Passivation Baths by Emulsion Pertraction Technology Nazely Diban,† Verónica 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 a chromium(III) passivation bath by emulsion pertraction technology using Cyanex 272 as extractant and hollow fiber membrane contactors. 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 the results were fitted to the Riedel and Grunberg and Nissan correlations. The improvement observed from 20 to 40 °C was explained 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 were attributed to a shift in the driving force due to an endothermic change of the interfacial extraction reaction. The equilibrium parameters 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 reactive membrane systems. liquid extraction with hollow fiber membrane contactors to extract and back-extract targeted compounds from an aqueous solution in one operational step.7−10 The nondispersive contact between the passivation fluid and the liquid membrane allows the definition of a hybrid and stable11 process in which the EPT is integrated into the continuous operation of the passivation process (Figure 1). The values of certain variables of the EPT operation are determined by the passivation process, such as metals concentration, pH, and temperature, while others are set independently such as flow rates and the concentrations of the extractant and stripping solutions.6 Also, the influence of the

1. INTRODUCTION Zinc electrodeposition is widely used as a galvanic protection for metallic surfaces in decorative and industrial applications. However, the corrosion rate of the electroplated surfaces is high as a consequence of the Zn electrochemical reactivity.1 The active corrosion may be changed to passive state by immersing the piece in trivalent chromium passivation baths.2 The key working parameters in the conversion process are the concentration of Cr(III) and the pH and the temperature of the passivation bath. The pH of the formulation must be approximately 2 in order to partially dissolve the Zn layer of the electroplated piece. Temperature affects the weight of the conversion coating, and hence the corrosion-inhibiting effect. Increasing temperature implies a thicker passivation layer; however, high temperatures may cause undesired yellowish color in the coatings.3,4 According to the suppliers, the recommended temperature range is 15−50 °C depending on the type of commercial bath. During the conversion process, the Zn that is not incorporated into the coating layer remains dissolved in the bath. The existing iron in the uncovered areas of the plated pieces is also partially transferred to the formulation. Both metals contaminate the chemical formulation affecting the quality of the passivation. The passivation bath is replaced when it does not fulfill its purpose, generating a significant amount of liquid hazardous waste. These wastewaters are treated by conventional physical−chemical processes, which produce considerable quantities of metallic sludge and consume chemicals and natural resources.5 A newly developed process, based on the emulsion pertraction technology (EPT), enables the selective removal of Zn and Fe impurities during the passivation process, preventing the loss in passivation efficiency and promoting waste prevention.6 This separation technique combines liquid− © 2012 American Chemical Society

Figure 1. Diagram of the integration of the EPT into the passivation process. Received: Revised: Accepted: Published: 9867

May 15, 2012 June 19, 2012 June 29, 2012 June 29, 2012 dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−9874

Industrial & Engineering Chemistry Research

Article

mentioned parameters was mathematically described.12,13 The authors found that the rate-limiting step of the process was the diffusion of the metallic species through the impregnated liquid membrane. The reported EPT studies were conducted at room temperature, and the developed model did not include the influence of the temperature. However, this variable may largely affect the mass transport phenomena of the EPT process through both kinetic and equilibrium parameters. First, the diffusivity of the permeating compounds through the impregnated liquid membrane is higher with increasing temperatures. On the other hand, the temperature dependence of the interfacial extraction reaction may affect the concentration gradient of the permeating metallic species between the feed 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) passivation baths by means of EPT in hollow fiber contactors using the organic solution Cyanex 272/Shellsol D70 as liquid membrane and sulfuric acid as stripping agent. At the acidic pH of the passivation bath, Cyanex 272 reacts selectively with Zn2+ and Fe3+, while Cr3+ remains in the aqueous passivation fluid.6 The aim of this study was to evaluate the temperature dependence of the mass transport parameters that affect the kinetics of the process. The considered kinetic parameters were the diffusion coefficients of Zn and Fe organometallic complexes and organic extractant species in the organic solution. Further, the equilibrium parameters under study were the apparent equilibrium constants for Zn and Fe extraction. In order to attain the goal, the viscosity of the Cyanex 272/Shellsol D70 mixtures was determined at different temperatures and Cyanex 272 concentrations and the apparent equilibrium constants were estimated mathematically. Finally, this work presents a model able to describe the extraction of Zn and Fe from Cr(III) passivation baths at different operational conditions including temperature.

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

and Fe cations were then released into the acidic media and the free extractant was regenerated as follows. MeR n(org) + nH+(s) ↔ Men +(s) + nHR (org)

2. THEORETICAL BACKGROUND 2.1. Mass Transport. In the emulsion pertraction process, the aqueous passivation bath containing the species to be removed (Zn and Fe) was circulated through the shell side of a hydrophobic microporous membrane contactor. The emulsion phase flowed inside the hollow fibers. The emulsion phase was formed by the organic solution consisting of Cyanex 272/ Shellsol D70 and the stripping agent, sulfuric acid, which was dispersed by vigorous stirring in an external tank. The cross section of the hollow fiber and the concentration profiles of the permeating species through the liquid membrane are shown in Figure 2. The concentration profile of Fe was similar to the one presented for Zn. According to previous studies, the resistance to the mass transfer at the liquid boundary layers was considered negligible.12 The organic phase was embedded inside the pores of the membrane and was in contact with the aqueous feed phase at the outer wall of the membrane. In this contacting interface, the following interfacial reactions between the Fe and Zn cations (Men+) and the cationic extractant Cyanex 272 (HR) took place: Men +(aq) + nHR (org) ↔ MeR n(org) + nH+(aq)

(2)

The free extractant HR was counterdiffused to the feed− organic interface. The flux of the diffusing species through the liquid membrane, ji, was described by means of the following expression. ji = αk m, i(cio * − cio)

∀ i = ZnR 2, FeR3, HR

(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 the permeating species within the membrane phase, (coi * − coi ), and (ii) the diffusional mass transfer coefficient km,i. At the feed side, the concentrations coi * of the ZnR2 and FeR3 species in the membrane phase were described by the following simplified equilibrium expression: Keq,Zn =

o* (c Ha+)2 c ZnR 2 a (c o * )2 c Zn 2+ HR

Keq,Fe =

o* c FeR (c Ha+)3 3 a (c o * )3 c Fe 3+ HR

(4)

Due to the low pH of the stripping agent, the concentration of the organometallic species in the organic−stripping interface during the back-extraction reaction was neglected assuming a maximum concentration gradient through the membrane, that is, coi = 0. Further, the values of km,i for ZnR2, FeR3, and HR through the liquid membrane were calculated considering their diffusion coefficient Do,i values and the geometric and structural

(1)

The organometallic complexes MeRn were transported across the organic phase inside the pores toward the droplets of the stripping phase located at the inner side of the membrane. Zn 9868

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


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characteristics of the porous hollow fibers that were impregnated by the organic liquid membrane. k m, i =

Do, iε


τδ

The mass balances in the feed and emulsion stirred tanks were described as follows. d(VTj cij,T)

(5)

dt

where ε is the membrane porosity, δ is the membrane thickness, and τ is the membrane tortuosity. The organometallic complex and free extractant diffusivity Do,i values were calculated using the Wilke−Chang correlation: Do, i =

∀ i = Zn, Fe, H;

7.4 × 10−8(φoMo)1/2 T

∀ z;

∀ t;

(7)

∀ z;

∀ t;

∀ i = Zn, Fe, H

(8)

Assuming pseudo steady state 2jZnR + 3jFeR = jHR 2

(9)

3

with the following initial and boundary conditions: cij(t = 0) = cij,initial

∀ z;

∀ i = Zn, Fe, H;

∀ j = a, o, s

(10)

cia(z = 0) = cia,T

∀ t;

∀ i = Zn, Fe, H

cij(z = L) = cij,T

∀ t;

∀ i = Zn, Fe, H;

∀ j = o, s

∀j=a

(14)

zout = 0

∀ j = o, s

(15)

∀ i = Zn, Fe, H;

∀ j = a, o, s

3. EXPERIMENTAL SECTION 3.1. Chemicals. The passivating bath under study was provided by a local plating industry. The bath contained 5681 mg 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 the formulation was nitrate with a concentration of 67 g L−1. The commercial extractant Cyanex 272 was kindly supplied by Cytec Industries, France. The active component of Cyanex 272 is bis(2,4,4-trimethylpenthyl)phosphinic acid and has a molecular weight of 290 g mol−1, a density of 920 kg/m3 at 24 °C, and a purity of 83%. The purity was determined by titration according to the procedure recommended by the manufacturer.14 At acidic pH ranges below 3, Cyanex 272 extracts selectively Zn and Fe, while Cr(III) cations are retained in the aqueous phase.6 Shellsol D70 was purchased from Kremer Pigmente GmbH & Co. KG. This chemical is a low aromatic hydrocarbon solvent and was employed as a diluent of Cyanex 272. Solutions of sulfuric acid (reagent grade ISO, Panreac) and sodium hydroxide (pro analysi, Merck) were prepared using deionized Milli-Q water and used as back-extraction agent and to set the value of the pH of the feed solutions, respectively. 3.2. Procedure. The Zn and Fe extraction experiments were conducted by mixing equal volumes of the passivation bath (A) and the extractant solution (O) (A/O = 1) at 10, 20, and 50 °C in several thermostatized mixer−settler units arranged in series (Figure 3). The extractant consisted of 10% v/v Cyanex 272 diluted in Shellsol D70. Prior to each experiment, the initial pH of the passivating bath was modified by adding a solution of sodium hydroxide in order to attain different pHeq’s. The evaluation of the influence of the contact time on the extraction percentage proved that 10 min was enough to reach chemical equilibrium. The solutions were under continuous stirring until equilibrium was attained. The phases were led to settle, and samples of the aqueous solution were taken for further analysis. The viscosity of the organic phase was quantified using a thermostatized Brookfield rotational viscometer (Model Alpha Series L, Fungilab S.A., Spain). The viscosity values were obtained at different Cyanex 272/Shellsol D70 compositions (0/100, 10/90, 15/85, 20/80, and 100/0% v/v) in the temperature range 5−45 °C. The viscosities of the mixtures containing Shellsol D70 were measured at a rotational speed of 100 rpm using a LCP spindle, whereas 50−100 rpm and a TL5 spindle were used for the Cyanex 272 100% v/v samples.

where α = +1 ∀ i = Zn, Fe, and α = −1 ∀ i = H. dcio =0 dz

(13)

In this model, the influence of temperature was given through the flux of species ji, as described previously in eqs 3−6.

∀ i = Zn, Fe, H;

∀ j = a, s

∀ j = a, o, s

(16)

(6)

where φo is the association factor (φo = 1), Mo is the average molecular weight of the organic solution (g mol−1), T is the temperature (K), μo is the organic solution viscosity (cP), and Vo,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 the chemical system. Moreover, the values of km,i were temperature dependent as Do,i and μo were also affected by the temperature. Therefore, the temperature is a variable that may strongly influence the EPT performance and the effect of this variable must be assessed. 2.2. Mass Balances. The mathematical model that described the removal of Zn and Fe impurities from the passivation bath consisted of a set of mass balances applied to the metallic species, proton, and organic extractant within the three fluid phases that participated in the EPT process.12 Under the high flow rate conditions employed in the present system, ideal plug flow was considered and therefore the radial variation in the fluid properties was neglected. The pseudostationary state inside the module is also assumed. The mass balances inside the membrane module were given by dcij = −αA m ji dz

∀ t;

zout = L

cij,T(t = 0) = cij,initial

μo Vo, i 0.6


F jL

= F j(cij(z = zout) − cij,T)

(11)

(12)

where “a”, “s”, and “o” refer to the feed, stripping, and organic phases, respectively. 9869



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acid and 0.8 L of an organic mixture of 15% v/v Cyanex 272 in Shellsol D70. Both the aqueous and emulsion phases were thermostatized, 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. Table 1. EPT Experimental Working Conditions variable

aqueous feed

emulsion

operation mode circulation flow rate volume inlet pressure outlet pressure

recirculation shell side 220 L h−1 2L 0.85 bar 0.65 bar

recirculation lumen side 70 L h−1 1L 0.5 bar 0 bar

Samples of the passivation bath and stripping acid were collected at regular intervals to follow the development of the concentration of the metals during the EPT process. The samples were quantitatively analyzed by atomic absorption spectroscopy (Perkin-Elmer, AAnalyst 3110). The obtained results were described mathematically by the model included in section 2 that was solved using Aspen Custom Modeler, version 2004.1.

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

The kinetic EPT study was conducted utilizing the experimental setup illustrated in Figure 4. The experiments were performed in countercurrent mode using a membrane contactor (Liqui-Cel Extra-Flow 2.5×8, Celgard) with a total effective mass transfer area of 1.4 m2. The membrane contactor enclosed hydrophobic polypropylene microporous X-50 hollow fibers with the following characteristics: porosity ε = 0.4, membrane thickness δ = 40 μm, and tortuosity τ = 6.4. The feed tank contained 2 L of the passivation bath with the average composition detailed in section 3.1. During the EPT process the pH of the bath was maintained constant at 1.8 by adding a solution of 4 mol L−1 sodium hydroxide. The emulsion tank had a capacity of 1 L and contained 0.2 L of 4 mol L−1 sulfuric

4. RESULTS AND DISCUSSION 4.1. Effect of Temperature on Extraction Equilibrium. The effect of the temperature on Zn and Fe extraction equilibria using a 10% v/v Cyanex 272 solution as extractant is presented in Figure 5. In the present system, the equilibrium isotherms at 20 and 50 °C were overlapped. However, the extraction isotherm shifted to higher equilibrium pH values when the temperature decreased to 10 °C. The extraction of Zn at 10 °C was approximately 20% at a pHeq of 1.8. At 20 or 50 °C the extraction rose to 40%. At these temperatures and equilibrium pH values, the Fe extraction always exceeded 90%,

Figure 4. Experimental emulsion pertraction setup. 9870



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

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

whereas the extraction of Cr(III) was by far lower than that of Zn. 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 the one observed at temperatures above 20 °C. This change was endothermic in nature, increasing the extraction performance at higher working temperatures. Several authors have also observed endothermic thermodynamics with different levels of temperature influence in the Zn(II) and Fe(III) extractions with Cyanex 272 organic solutions from different sulfate and chloride media at several temperature ranges. When conducting the Zn extraction from chloride media, Wang et al.15 reported a mild temperature dependence in the range 17−45 °C and an enthalpy change, ΔH, of 10.19 kJ mol−1, and Baba and Adekola16 found a more significant temperature influence in the range 27−50 °C, with ΔH = 26.81 kJ mol−1. Similarly, Biswas and Singha17 found a ΔH of 13 kJ mol−1 in the extraction of Fe from sulfate media in the range 25−45 °C, whereas Deep et al.18 observed a ΔH value of 159 kJ mol−1 in the temperature range 27−50 °C. On the contrary, Ali et al.19 found an exothermic nature in the extraction of Zn from a complex nitrate−sulfate−chloride medium with 2% v/v Cyanex 272 in kerosene in the temperature range 15−45 °C. Naik and Dhadke20 also obtained an exothermic tendency in the extraction of Fe(III) from nitrate media in the temperature range 30−55 °C. It is worth noting that in the present work a change in the thermodynamic behavior from endothermic to isothermal was observed above 20 °C. Sarangi et al.21 also reported a thermodynamic shift in the Co(II) extraction from chloride media using Cyanex 272. In this case the process shifted from endothermic to exothermic at 30 °C. This was attributed to a sharp decrease of the stability of the extracted organometallic complex at temperatures above 30 °C. The comparison of the results obtained in this study with the works reported in the literature indicated that the thermodynamic parameters determined experimentally might vary widely depending on the complexity of the matrix composition of the aqueous solution employed and the equilibration characteristics of the experimental systems employed. 4.2. Effect of Temperature on Overall Kinetics of the EPT Process. The development of the concentrations of Zn and Fe in the passivation bath during the EPT experiments at different temperatures is illustrated in Figures 6 and 7,

Figure 7. Development of concentration of Fe in passivation bath at 10, 20, and 40 °C. The solid lines represent model simulation results.

respectively. As observed, the temperature enhanced the extraction of both metals. Bey et al.22 reported the same trend when conducting a kinetic study on the extraction of Cr(VI) by nondispersive solvent extraction using Aliquat 336 as extractant and a modified polyether ether ketone (PEEK-WC) membrane contactor in the temperature range 20−50 °C. Our results showed a significant improvement of the process kinetics when the temperature was doubled from 10 to 20 °C, while the growth was less intense for a temperature rise from 20 to 40 °C. The effect of temperature on the diffusion of permeating species through the impregnated liquid membrane was evaluated measuring the viscosities of Cyanex 272, Shellsol D70, and their mixtures at different temperatures (Figure 8). As expected, the viscosities of the organic samples decreased with increasing temperature. According to the results presented in Figure 8A, the viscosity of pure Cyanex 272 decreased from 300 to 52 cP as the temperature increased from 5 to 41.2 °C. This suggested that the effect of temperature on the viscosities of the organic solutions would be stronger at high Cyanex 272 concentration. However, Figure 8B indicates that, in the experimental conditions utilized in the EPT and equilibrium tests, with Cyanex 272 concentration range of 10−20% v/v, the effect 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 Grunberg and Nissan correlation:23 9871



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Further, G12 was assumed to be proportional to the difference between the structural summations σ of the group contributions of each pure component 1 and 2. G12 =

∑ σ1 − ∑ σ2 + W

(20)

where W is a correction factor that is zero when any of the components of the mixture contains atoms other than hydrogen or carbon. The value of G12 for the Cyanex 272/Shellsol D70 mixture (Table 2) was hence calculated as follows: G12 =

∑ σ1 − ∑ σ2 + W

= (8σCH3 + 4σCH2 + 2σCH + 2σC + σP) − (2σCH3 + 10σCH2)

(21)

where σ values were obtained from Isdale et al. and σP was estimated to minimize the standard error between the experimental and calculated viscosity values according to the Grunberg and Nissan model. The comparison between the experimental and predicted values of the viscosity using the Riedel model for the pure organics and the Grunberg and Nissan mixing rule for the Cyanex 272/Shellsol D70 mixtures, is plotted in the parity graph of Figure 9. As observed, 86% of the predicted μοmodel 24

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

ln μo = x1 ln μ1 + x 2 ln μ2 + x1x 2G12(T )

(17)

where μo (cP) is the viscosity of the organic mixture, the subscripts “1” and “2” refer to the pure components Cyanex 272 and Shellsol D70, respectively, x is the molar fraction, and G12(T) is a constant that measures the deviation of the system from Arrhenius behavior.24 The viscosities of the pure components μ1 and μ2 were given by the empirical modified Riedel equation:25 ln(μi ) = A +

B + C ln(T ) T

Figure 9. Parity graph: comparison of experimental and predicted viscosity values for the organic phase at different Cyanex 272/Shellsol D70 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 and Grunberg and Nissan equations for pure components and mixtures, respectively.

(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 Shellsol D70 were obtained by fitting the experimental data plotted in Figure 8A to eq 18, and the obtained values are given in Table 2.G12(T) was described as follows:23,24 G12(T ) = 1 − (1 − G12)

573 − T 275

viscosity values fall within μοexp ± 10%μοexp. Therefore, the proposed viscosity model predicted adequately the experimental data and can be implemented in the mathematical model in order to consider the temperature dependence of the mass transfer 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 solution containing 15% v/v Cyanex 272 at 10, 20, and 40 °C was determined 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 Zn and Fe organometallic complexes increased about 20% when the temperature rose from 10 to 20 °C; i.e., km,Zn changed from

(19)

where T is the temperature in kelvin and G12 is G12(T) for the mixture at 298 K. Table 2. Riedel and Grunberg and Nissan Model Parameters Riedel

Grunberg and Nissan

component

A

B

C

G12

Cyanex 272 Shellsol D70

−72.9 −44.8

6670 2823

9.69 6.29

3.61 9872



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

viscosity, μo (cP)

zinc (ZnR2) diffusion coeff, Do,Zn (m2 s−1)

10 20 40

2.80 2.41 1.89

5.31 × 10−10 6.42 × 10−10 8.78 × 10−10

zinc mass transfer const, km,Zn (m s−1)

iron (FeR3) diffusion coeff, Do,Fe (m2 s−1)

iron mass transfer const, km,Fe (m s−1)

free extractant (HR) diffusion coeff, Do,H (m2 s−1)

HR mass transfer const, km,H (m s−1)

Zn equilib coeff, ln Keq,Zn

Fe equilib coeff, ln Keq,Fe

3.00 × 10−7 3.60 × 10−7 4.94 × 10−7

4.19 × 10−10 5.03 × 10−10 6.92 × 10−10

2.35 × 10−7 2.83 × 10−7 3.88 × 10−7

8.03 × 10−10 9.67 × 10−10 13.3 × 10−10

4.53 × 10−7 5.44 × 10−7 7.44 × 10−7

−10.04 ± 0.22 −8.19 ± 0.10 −7.98 ± 0.16

−12.80 ± 0.19 −11.57 ± 0.33 −10.86 ± 0.34

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 extraction observed experimentally in the EPT process (Figures 6 and 7) when changing the operation temperature from 10 to 20 °C was significantly higher and was not justified only by the increment in the diffusivity. This behavior was easily explained in terms of the improvement of the percentage of extraction equilibrium previously observed in Figure 5. The interfacial concentrations of the metallic species ZnR2 and FeR3 at the feed side of the liquid membrane at 20 °C was higher than the concentrations obtained at 10 °C. The increment in the driving force for mass transfer made the flux of the metallic species higher. The values of the equilibrium constants (Keq,Zn and Keq,Fe) defined in eq 4 were estimated from the best fitting (minimum weighted standard deviation) of the predicted curves to the experimental data shown in Figures 6 and 7. This was done using the estimation tool of the Aspen Custom Modeler software package. The results of the estimated parameters at 10, 20, and 40 °C are presented in Table 3. As observed, the small difference in the average values of the equilibrium parameters for Zn and Fe extraction reactions at 20 and 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 standard deviation error. A comparable apparent equilibrium constant for the extraction of Zn was experimentally obtained by Bringas et al.12 under analogous working conditions at room temperature: Keq,Zn = 2.74 × 10−4. This result indicated that the enhancement of the Zn and Fe extraction from 20 to 40 °C (Figures 6 and 7) was caused only by the effect of temperature on the diffusion coefficients of the targeted heavy metals. Table 3 also shows that lower values of the equilibrium constants were obtained at 10 °C: Keq,Zn = 0.44 × 10−4 ± 0.09 × 10−4 and Keq,Fe = 0.28 × 10−5 ± 0.05 × 10−5. These results were in agreement with those obtained in the equilibrium experiments discussed in section 4.1. The model and the mass transfer and equilibrium parameters were employed to simulate the EPT separation of the targeted heavy metals at the temperatures under study. Figures 6 and 7 show the comparison between simulated and experimental data for the extraction of Zn and Fe, respectively. As illustrated, the simulated curves predicted adequately the experimental data for the development of the concentrations of Zn and Fe in the passivation bath during the EPT process.

This study concludes that a significant influence of the temperature on the kinetics of Zn and Fe removal from the passivation bath by EPT existed in the temperature range 10− 40 °C. The higher velocity of Zn and Fe separation at higher temperatures was partially explained by the increase of the diffusion coefficients of the organometallic complexes through the liquid membrane. This enhancement was largely due to the reduction of the viscosity of the organic solution, μo. The extraction equilibrium for Zn and Fe presented a different behavior when the temperature was 10 °C in comparison with the results in the range between 20 and 40 °C that showed an endothermic change. The influence of the temperature observed on the mass transport parameters was included in a mathematical model that enabled the accurate description of the Zn and Fe removal kinetics from the passivating bath by means of an EPT system. The dependence of the viscosity of the organic solution with the temperature and the concentration of extractant Cyanex 272 was described accurately by the combination of the Riedel and Grunberg and Nissan correlations. The diffusion coefficient was described by the widely employed Wilke−Chang equation. This work finally concludes that the proposed model is a useful tool for design purposes: it enables an accurate prediction of the minimum membrane area of the membrane contactor required to remove the incoming metallic impurities into the passivation process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by Projects CTQ2008-00690 (MCI, Spain) and TIGI (European Comission, Grant agreement 218390). Componentes y Conjuntos S.A. is acknowledged for the kind supply of passivating bath samples.

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5. SUMMARY AND CONCLUSIONS During the conversion of electroplated Zn surfaces using Cr(III)-based passivation baths, the temperature is a critical working variable affecting the thickness and corrosive resistance of the passivation layer. An emulsion pertraction unit may be integrated into the conversion process to remove the impurities of Zn and Fe that shorten the lifetime of the passivation bath. In the present work, the influence of the temperature on the performance of EPT is evaluated. 9873

NOMENCLATURE A = parameter of the Riedel equation Am = effective membrane area (m2) B = parameter of the Riedel equation c = liquid concentration of the species (mol m−3) C = parameter of the Riedel equation Do = diffusion coefficient of diffusion species in the organic phase of the liquid membrane (m2 h−1) F = flow rate (m3 h−1) G = parameter of the Grunberg and Nissan correlation H = enthalpy (kJ mol−1) j = flux of the species through the liquid membrane (mol h−1 m−2) dx.doi.org/10.1021/ie301251q | Ind. Eng. Chem. Res. 2012, 51, 9867−9874


Article

(9) Ho, W. S. W. Combined Supported Liquid Membrane/Strip Dispersion Process for the Removal and Recovery of Penicillin and Organic Acids. U.S. Patent 6,433,163, 2002. (10) Urtiaga, A.; Abellán, M. J.; Irabien, J. A.; Ortiz, I. Membrane contactors for the recovery of metallic compounds: Modelling of copper recovery from WPO processes. J. Membr. Sci. 2005, 257, 161. (11) Sonawane, J. V.; Pabby, A. K.; Sastre, A. M. Pseudo-emulsion based hollow fiber strip dispersion: A novel methodology for gold recovery. AIChE J. 2008, 54, 453. (12) Bringas, E.; Mediavilla, R.; Urtiaga, A. M.; Ortiz, I. Development and validation of a dynamic model for regeneration of passivating baths using membrane contactors. Comput. Chem. Eng. 2011, 35, 918. (13) Bringas, E.; San Román, M. F.; Irabien, J. A.; Ortiz, I. An overview of the mathematical modelling of liquid membrane separation 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 of Zinc(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 sphalerite mineral: 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 interfacial adsorption and extraction characteristics towards iron(III). Hydrometallurgy 2006, 82, 63. (18) Deep, A.; Correia, P. F. M.; de Carvalho, J. M. R. Liquid−Liquid Extraction 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 the extraction and recovery of zinc from aqueous waste solution using a mixer-settler unit. Sep. Purif. Technol. 2006, 47, 135. (20) Naik, M. T.; Dhadke, P. M. Extraction of iron(III) with bis(2ethylhexyl)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 of cobalt(II) and nickel(II) from chloride solutions using Na-Cyanex 272: 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 contactors for chromium (VI) removal. Desalination 2011, 283, 16. (23) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids; McGraw-Hill: New York, 2001. (24) Isdale, J. D.; MacGillivray, J. C.; Cartwright, G. Prediction of Viscosity 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.

Keq = apparent equilibrium coefficient of the extraction reaction km = diffusional mass transfer coefficient of species through the liquid membrane (m h−1) Mo = average molecular weight of the organic solution (g mol−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 Nissan correlation x = molar fraction z = axial coordinate (m) Greek Symbols

α = parameter attributing the positive/negative character ε = membrane porosity δ = membrane thickness (m) Δ = thermodynamic property change φo = association factor in the Wilke−Chang correlation μ = viscosity (cP) σ = structural group contribution term of the Grunberg and Nissan correlation τ = membrane tortuosity Subscripts

i = diffusing species present on the system initial = values at operation time = 0 o = organic phase (Cyanex 272/Shellsol D70 mixture) out = outside the membrane module T = feed and/or emulsion tank 1 = first component of the organic mixture 2 = second component of the organic mixture Superscripts

a = aqueous feed phase j = fluid phases o = organic phase (Cyanex 272/Shellsol D70 mixture) s = stripping phase * = values of the concentrations in equilibrium after the extraction reaction

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REFERENCES

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