Ind. Eng. Chem. Res. 2002, 41, 3669-3675
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Separation of Rare Metal Ions by a Column Packed with Microcapsules Containing an Extractant Eiji Kamio and Kazuo Kondo* Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe 610-0321, Japan
The separation of gallium and indium was investigated using a column in which microcapsules containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester were packed. First, adsorption isotherms were measured and analyzed according to the Freundlich adsorption isotherm model, from which the saturation capacities and the operation lines were determined. Then, breakthrough curves were measured at several flow rates and pH’s of the feed solution. When the flow rate decreased, the adsorption capacity of gallium also decreased, but that of indium did not change. By selecting the appropriate pH, the separation of gallium from indium was achieved. Specifically, it was possible to separate the two metals by selecting the hydrogen-ion concentration in the stripping operation. Even after the fifth cycle of adsorption and stripping, the loss of the extractant in the microcapsules was only about 0.01%. The theoretical equation for the breakthrough curve was established. The experimental data were correlated to the lines calculated using the predictive equation. Introduction In recent years, increasing interest in environmental protection and energy saving, as well as process optimization and continuous progress in fundamental chemistry, has produced the important development of new chemical separation techniques. The ecological and economical need for more specific systems for the recovery of rare metal from dilute solutions has led to developments in the synthesis of new extractants, ion exchangers, and adsorbents.1-10 These products have significantly improved the selectivity and efficiency of a large number of techniques for separation processes, such as solvent extraction, supported liquid membranes, solvent-impregnated resins, and so on.11-16 Among these new techniques, we propose the use of microcapsules as a new technological alternative method for the recovery and separation of metals. In solvent-extraction systems, mixer-settler cascades have been used to separate metals. Nishihama et al.17 studied the extraction equilibria of Ga and In with organophosphorus extractants to simulate their separation with a continuous countercurrent mixer-settler cascade. From the calculated results, it was found that 15 stage cascades are needed to carry out the effective separation of these metals. That is, the separation of the metals with the solvent-extraction technique requires multiple cascades and a wide area in which to set them. Furthermore, there is a problem of extractant loss. Under high loading conditions, a third phase is formed in the aqueous and/or organic phase. The separation of that phase from the liquid phases is very difficult. Moreover, it is also difficult to recover extractant from the third phase. As compared with a solventextraction system, a column system packed with microcapsules requires only one column. In addition, the third phase formed during the adsorption process is also * To whom correspondence should be addressed. Tel./Fax: +81-774-65-6656. E-mail:
[email protected].
adsorbed into the microcapsule, and so it is expected that extractant loss will be considerably lowered. Considering these facts, the use of microcapsules containing a selective extractant offers some advantages over solvent-extraction systems. Yoshizawa et al.18 studied the recovery of several metals using microcapsules that consist of divinylbenzene polymer containing extractants. The microcapsules we prepared are porous organic polymers with good mechanical stability. Therefore, these microcapsules have the possibility of being used in extraction chromatography at the analytical scale and are of potential application at the industrial scale. The application of this microcapsule system to industrial-scale equipment using a fixed-column technology requires knowledge of the equilibrium and kinetics of the metal extraction process. Such information is very important in the design of extraction chromatographic processes. In this study, we measure the extraction equilibria of gallium and indium with microcapsules containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (hereafter abbreviated EHPNA) and investigate the possibility of separating gallium and indium with a column operation. Furthermore, we analyze the breakthrough curve by using the operation line based on the Freundlich adsorption isotherm equation and the rate equation based on a linear driving force approximation. Several investigators have recently presented methods for calculating breakthrough curves. Miura and Hashimoto19 proposed a basic equation that was solved analytically by using the fluid-solid interface concentration as the dependent variable for the Langmuir and Freundlich isotherms and by taking into account both external and intraparticle mass-transfer resistances. Kataoka and Yoshida20 investigated the adsorption behavior of Zn(II) or Ce(II) on DIAION SK 116 or 1B, respectively, and designed breakthrough curves by the linear driving force method where simple rectangular isotherm models were considered. However, these ana-
10.1021/ie010737c CCC: $22.00 © 2002 American Chemical Society Published on Web 06/25/2002
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lytical solutions consist of very complex equations. Therefore, in this study, we propose a simple mathematical model for the breakthrough curve of a singlecomponent system, which means that only one metal is contained in a feed solution. Theoretical Section Adsorption Isotherm. In this work, the adsorption isotherm is assumed to be represented by a Freundlich type of equation
q ) kC1/n
uA(C0 - C) ) UFbA(q0 - q)
(2)
uA(C0 - 0) ) UFbA(q0 - 0)
(3)
where u and U are the linear velocity of the feed and the velocity of the adsorption band, respectively. A is the cross section of the column, and Fb is the density of the bed. From eqs 2 and 3, one can write
q0 C ) βC C0
(4)
where β is the adsorption coefficient. Equation 4 represents the operation line. The mass balance of metal in a differential height (∂Z) for a differential time period (∂t) is expressed by
u
∂q ∂C ∂C + Fb + b )0 ∂Z ∂t ∂t
(5)
Furthermore, by the linear driving force approximation, the adsorption velocity in eq 5 is written as
Fb
∂q ) Kf av(C - C*) ∂t
(6)
where C* is the equilibrium metal concentration in the aqueous phase. Kf av is the overall capacity coefficient for the aqueous phase. By substituting eq 6 into eq 5 and integrating, the differential height of the adsorption band, Za, is obtained as
u Za ) Kf av
∫C
C0-Cb b
C* )
dC C - C*
(7)
where Cb is the metal concentration in the eluent when t corresponds to the breakthrough time tb. The equilib-
β C β0
(8)
where β0 is the equilibrium adsorption coefficient. The difference in concept between β and β0 is explained in Appendix 2. By substituting eq 8 into eq 7 and integrating, Za can be expressed as
(1)
where C is the fluid-phase concentration, q is the solidphase concentration, and k and n are the Freundlich constants. Analysis of Column Adsorption Characteristics. The breakthrough curves were analyzed under the following two assumptions: (1) By treating the parameter for the diffusion phenomena of metal ions in the microcapsules as a lumped parameter, the overall mass capacity coefficient Kf av can be employed and determined. (2) After the adsorption zone, which is formed in the fixed bed in the column, has reached a transient state, it passes through the bed at a constant velocity, keeping the same form. The mass balance between the entrance and an arbitrary point in the adsorption band, or between the entrance and the exit of the adsorption band, is given by (see Appendix 1)
q)
rium metal concentration in the aqueous solution, C*, is given by
Za )
(
)
β0 C0 - Cb u ln Kf av β0 - β Cb
(9)
The breakthrough time, tb, is given by
tb )
(
)
Za 1 ZU 2
(10)
From eqs 3, 9, and 10, one can obtain
C)
C0
[
2Kf av(β0 - β)Z 2Kf av(β0 - β) 1 + exp t uβ0 β0βFb
]
(11)
In eq 11, the metal concentration at t ) tb is expressed as
Cb )
C0
[
]
2Kf av(β0 - β)Z 2Kf av(β0 - β) 1 + exp tb uβ0 β0βFb
(12)
which can be rewritten as
K f av )
(
) (
)
tb β0 C0 Z ln -1 Cb 2(β0 - β) u βFb
(13)
By substituting the relationship of the Freundlich adsorption isotherm, β0,b ) (Cbβ)1-nk, into eq 13, one obtains
Kf av )
(
(Cbβ)1-nkn
tb Z 2[(Cbβ)1-nkn - β] u βFb
) ( -1
ln
C0 -1 Cb
)
(14)
By substituting the known values tb, Cb, β, k, Z, u, Fb, C0, and n into eq 14, the overall capacity coefficient Kf av can be calculated. Furthermore, eq 11 can also be rewritten as
t)
[
(
2Kf av(β0 - β)Z C0 -1 - ln uβ0 C 2Kf av(β0 - β) β0βFb
)]
n
βFbZ ) u
x βk β C F ln(C - 1) C β C 2K a (1 β ) xk 2
1/n-1
b
n
0
(15)
1/n-1
f
v
Then, by substituting the known values Kf av, β, k, Z, u, Fb, C0, and n and an arbitrary concentration C into eq 15, the time, t, when the concentration becomes C can be obtained.
Ind. Eng. Chem. Res., Vol. 41, No. 15, 2002 3671
Experimental Section Reagents. The microcapsules used in this study were prepared by the same method as described previously.21 The monomer solution containing the extractant and an aqueous solution were mixed for over 3 h at 343 K. The aqueous solution is composed of 2 wt % gum-arabic solution. The extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA, trade name PC-88A) supplied by Daihachi Chemical Ind. Co., Ltd., Osaka, Japan, was used as the active component without further purification. The purity was more than 95%. Divinylbenzene monomer was used after distillation to remove contaminants. The special-grade chemicals toluene and 2,2′-azobis(2,4-dimethyl valeronitrile) (ADVN) were used as the diluent and polymerization initiator, respectively. The obtained polymers, that is, the microcapsules, were filtered and washed with distilled water. They were dried at room temperature and stored in a glass vessel. Reagent-grade gallium(III) and indium(III) chlorides whose purities were more than 99.0% were used. The aqueous solutions were prepared by dissolving gallium(III) chloride (GaCl3‚6H2O) or indium(III) chloride (InCl3‚ 6H2O) in a H2SO4-Na2SO4 solution at a concentration of 100 mol/m3. Extraction of Gallium and Indium. All experiments were carried out batchwise. The extraction equilibrium of the metal was measured as follows: An appropriate weight of microcapsules and 10 cm3 of an aqueous solution containing gallium or indium with a known hydrogen ion concentration were put into a test tube to be shaken using a mechanical shaker (at 303 K) and allowed to attain equilibrium. It was found in preliminary experiments that 20 h was sufficient for equilibrium in the metal adsorption to be reached, so a contact time of 24 h between the two phases was established as the experimental condition in this study. After about 24 h, the microcapsules and the aqueous solution were separated from each other, and the metal concentration in the aqueous solution was measured with an inductively coupled plasma spectrometer (ICPOES, Shimadzu ICPS-8000). The pH of the aqueous solution after equilibration was measured with a pH meter (Horiba F-23). Column Operation. A glass column of 9.6 mm inner diameter packed with 11.0 g of microcapsules containing EHPNA was used. The bed height of the column was about 280 mm. After the column had been conditioned, an aqueous solution having a known concentration of gallium and/or indium was continuously introduced onto the top of the column at a constant flow rate of 2.0 cm3/ min. The eluent samples were collected at appropriate intervals, and the metal concentration was determined by ICP-OES. Next, the metal ion adsorbed on the column was washed out with distilled water and then a stripping solution. In this study, (H,Na)2SO4 with pH ) 0.30 for gallium and 5 mol/dm3 H2SO4 for indium were used as the stripping solutions. Results and Discussion Equilibria. Figure 1 shows the effect of the equilibrium hydrogen-ion concentration in the aqueous solution on the extent of metal stripped, E′. From this result, it is suggested that column operation for the adsorption of both metals should be carried out at [H+] ) 10 mol/ m3, and then the two metals can be separated by
Figure 1. Effect of equilibrium hydrogen ion concentration on extent of metal stripped E′.
Figure 2. Experimental adsorption isotherm and theoretical line based on Freundlich isotherm at pH ) 2.17. Table 1. Summary of Adsorption Isotherm Based on Freundlich Model and Operation Line for C ) 5 mol/m3 Freundlich equation gallium indium
10-4)C1/8.31
q ) (7.76 × q ) (6.68 × 10-4)C1/59.3
operation line q ) 1.83C q ) 1.27C
selecting the hydrogen-ion concentration of the stripping solutions. The adsorption equilibria of gallium and indium were measured at pH ) 2.20 and at room temperature (303 K). Figure 2 shows the adsorption isotherm based on the Freundlich adsorption isotherm model. The experimental data are correlated with the theoretical lines. From this figure, it is clear that indium has a lower saturation capacity than gallium. In Figure 2, the operation lines for both metals at C ) 5 mol/m3 are also shown. The resulting Freundlich equations and the operation lines are listed in Table 1. Column Operation. Figure 3 shows the typical breakthrough curves for the binary system, which means that both metals are included in the feed solution. The breakthrough point of gallium appeared at the start of the operation and that of indium appeared when the volume of the introduced feed solution was about 260 cm3, that is, about 130 min after the operation started. This means that it is possible to separate these
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Figure 3. Breakthrough curves of gallium and indium.
Figure 4. Effect of flow rate on breakthrough curves for adsorption of gallium and indium: (9) Ga, 0.2 cm3/min; (0) In, 0.2 cm3/ min; (b) Ga, 1.0 cm3/min; (O) In, 1.0 cm3/min; (2) Ga, 2.0 cm3/ min; (4) In, 2.0 cm3/min.
metals for 130 min after the operation starts for the conditions of this study, that is, the eluent can be obtained as a gallium-containing solution that is free of indium until 130 min of the mixture feed solution flowed out. The breakthrough curve of indium seems sharper than that of gallium, from which it is considered that the adsorption velocity of indium is faster than that of gallium. Subsequently, we further investigated the possibility of separating the two metals by varying the flow rates or the pH of the feed solution. Figure 4 shows the effect of flow rate on the breakthrough curve. From this result, it is clear that, when the flow rate increases, the breakthrough point of gallium appears earlier. This suggests that the ratedetermining step of gallium adsorption might be a liquid-film-diffusion-controlled step. The breakthrough curve of indium is not affected by the flow rate at all. This might be because the adsorption rate of indium is controlled by intraparticle diffusion or chemical reaction. However, under these conditions, it is difficult to separate the two metals by varying the flow rates. Figure 5 shows the breakthrough curves when the pH of the feed solution is 2.0 or 1.3. From this figure, it is clear that selecting the pH is effective in separating gallium from indium. When the pH equals 1.3, the
Figure 5. Effect of pH on breakthrough curves for gallium and indium.
Figure 6. Elution curves of gallium and indium. Stripping solution ) 5 mol/dm3 H2SO4.
breakthrough point of gallium appears earlier, and that of indium appears later. This is because the adsorption efficiency of gallium at pH 1.3 is lower than that at pH 2.0 but the efficiency of indium is almost the same for both pH’s. When the volume of the feed solution reaches about 300 cm3, the concentration of gallium in the eluent becomes higher than that in the feed. Such an overshoot behavior is due to the exchange of gallium adsorbed during the initial period of operation with indium in the feed. This phenomenon would be affected by the adsorption rate and the adsorption efficiencies of the two metals. We subsequently investigated the possibility of separating the two metals with a stripping operation. Figure 6 shows the eluent curves of the two metals when 5 mol/ dm3 H2SO4 was used as the stripping solution. During the early time of operation, the two metals were stripped together; however, gallium was stripped almost completely by 200 cm3. Therefore, only indium exists in the eluent solution after an eluent volume of 200 cm3, which means that it is possible to separate only indium. As the results show, it is possible to separate the two metals with two steps; that is, the first step is the recovery of gallium free from indium with the adsorption operation until the breakthrough time, and the second step is the stripping of indium with the stripping operation. However, there is difficulty in predicting the
Ind. Eng. Chem. Res., Vol. 41, No. 15, 2002 3673
Figure 7. Elution curves of gallium and indium from the column packed with the microcapsules loaded with gallium and indium. Stripping solution ) 0.1 mol/dm3 (H,Na)2SO4 (pH ) 0.30).
Figure 8. Elution curves of gallium and indium from the column after stripping gallium in Figure 7. Stripping solution ) 5 mol/ dm3 (H,Na)2SO4.
breakthrough time for several concentrations of metal ions in the feed. Then, we investigated the separation of these metals by selecting the hydrogen-ion concentration of the stripping solution. First, the separation of gallium was examined. The stripping solution used was a H2SO4Na2SO4 solution of pH ) 0.30, because gallium was stripped almost completely at that pH but indium was not stripped at all. The results are shown in Figure 7. From Figure 7, it is apparent that gallium was stripped from the column but indium was not. Subsequently, the separation of indium was examined. The stripping solution used was 5 mol/dm3 H2SO4, because gallium and indium were stripped almost completely with this solution. The results are shown in Figure 8. It is apparent from Figure 8 that indium was stripped from the column but gallium was not. These results mean that gallium is completely stripped by a stripping solution of pH ) 0.30 and completely separated from indium. As mentioned above, we could achieve to the separation of gallium and indium by selecting the hydrogenion concentration of the stripping solutions. However, to apply this type of microcapsule to extraction chro-
Figure 9. Breakthrough curve for gallium and indium for binary system on repeated operation.
Figure 10. Breakthrough curve for gallium adsorption. Solid line shows line calculated according to eq 14.
matography on the industrial scale, it is important to research the possibility of using the microcapsules in repeated operations. Therefore, we examined the possibility of using this column repeatedly. One continuous operation to adsorb and strip gallium and indium is regarded as one cycle. The resulting breakthrough curves for gallium and indium are shown in Figure 9. The results for the first and fifth cycle indicate no decline in adsorption capacity. After the fifth cycle, the microcapsules packed in the column were collected, and the concentration of the extractant held within them was measured. Even after the fifth cycle, the microcapsules were not destroyed, and the loss of the extractant was under 0.01%. Thus, this microcapsule has a strong durability against acid and can be considered suitable for repeated use. Column Analysis. Using the prediction equation, eq 14, the volumetric overall capacity coefficients, Kf av, for gallium and indium could be obtained. Figures 10 and 11 show the experimental and calculated breakthrough curves for gallium and indium, respectively. The breakthrough curves calculated according to eq 15 correlate well with the experimental data. From these results, it is suggested that the proposed equation is useful for simulating the breakthrough curves. The parameters used are listed in Table 2.
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Figure 11. Breakthrough curve for indium adsorption. Solid line shows line calculated according to eq 14. Table 2. Parameters for Modeling of Breakthrough Curves Ga In
C0 (mol/m3)
tb (s)
β (m3/g)
Kf av (1/s)
5.00 5.35
10 500 12 750
1.83 × 10-4 1.27 × 10-4
6.70 × 10-3 3.14 × 10-2
Conclusions The adsorption and separation of gallium and indium in a column packed with microcapsules containing EHPNA were conducted in this study. According to the Freundlich model, the adsorption behavior of gallium and indium was analyzed, and the saturation capacities and the operation lines were determined. Indium has a larger adsorption efficiency and a lower saturation capacity than gallium. Both metals were adsorbed efficiently onto the column packed with the microcapsules. The selective separation of the two metals was achieved by selecting the hydrogen-ion concentration in the stripping solutions. From the results of measuring the effect of flow rate, it appears that the rate-determining step for gallium adsorption in the microcapsule is liquid-film diffusion and that for indium is chemical reaction or intraparticle diffusion. Even after five repeated uses, the loss of extractant was almost negligible. A simple model was developed for calculating the breakthrough curves based on the linear driving force approximation by taking into account the Freundlich adsorption isotherm equation and the operation line. The breakthrough curves for both gallium and indium were correlated by the proposed breakthrough model. Taking these results into consideration, this microcapsule system would be beneficial as an alternative method for the extraction and separation of metal ions from a dilute aqueous solution.
Figure I. Concentration of metal in both aqueous and microcapsule phases at the adsorption band. (a) Concentration profile, (b) stepwise description.
C0 ) initial metal concentration in the aqueous phase (mol/m3) Cb ) metal concentration at breakthrough (mol/m3) C* ) equilibrium metal concentration with q (mol/m3) k ) Freundlich constant K ) equilibrium constant (m3/mol) Kf av ) overall capacity coefficient (1/s) q ) concentration of metal adsorbed in microcapsule (g/g-extractant) q0 ) solid phase concentration in equilibrium with C0 (g/g-extractant) q∞ ) saturation capacity (g/g-extractant) n ) Freundlich constant t ) time (s) tb ) breakthrough time (s) T ) temperature (K) u ) linear velocity of fluid (m/s) U ) velocity of adsorption band (m/s) Z ) bed height (m) Za ) differential height of adsorption band (m) Greek Letters β ) adsorption coefficient (m3/g) β0 ) equilibrium adsorption coefficient (m3/g) Fb ) packing density of column (g/m3) Subscripts 0 ) initial state b ) breakthrough time Superscript * ) equilibrium state
Acknowledgment
Appendix 1
The authors express their thanks to Daihachi Chem. Co., Ltd., for providing EHPNA and to Prof. M. Matsumoto for providing valuable suggestions. This work was supported by a grant to RCAST at Doshisha University from the Ministry of Education, Science, Sports and Culture, Japan.
The adsorption band formed in the microcapsulecontaining packed bed gradually moves downward through the bed. However, it can be assumed that the feed solution and the packed bed move countercurrent to each other with the same velocity, by which the adsorption band is kept at constant form, as shown in Figure Ia. Now, the adsorption band can be considered as a series of bands with n stages, as shown in Figure Ib. In each stage, C and q are at equilibrium with each other.
Nomenclature A ) cross section of the column (m2) C ) metal concentration in the aqueous phase (mol/m3)
Ind. Eng. Chem. Res., Vol. 41, No. 15, 2002 3675
Figure II. Metal concentration profile in the aqueous solution at the end of the column.
For the mass balance of adsorbate in each stage, the following equations are presented
uC0 - uC1
)
Uq0 - Uq1
(1)
uC1 - uC2
)
Uq1 - Uq2
(2)
l Uqi-1 - Uqi
(i)
l l uCn-1 - uCn ) Uqn-1 - Uqn
(n)
l uCi-1 - uCi )
The mass balance from the first stage to the ith stage is expressed by adding the equations from eq 1 to eq i
u(C0 - Ci) ) U(q0 - qi)
(I)
In the same manner, the mass balance from the first stage to the nth stage is given by
u(C0 - Cn) ) U(q0 - qn)
(II)
As shown in Figure Ia, it is clear that Ci ) C at qi ) q and Cn ) 0 at qn ) 0 under the assumptions of this study. Therefore, by substituting Ci ) C and qi ) q into eq I above, eq 2 in the paper is obtained. In the same manner, by substituting Cn ) 0 and qn ) 0 into eq II, eq 3 in the paper is obtained. Appendix 2 β in eq 4 is the ratio of the saturation adsorption capacity to the metal concentration in the feed solution when they are at equilibrium state. That is, β is the adsorption coefficient in the first stage in Figure Ib described above. On the other hand, β0 in eq 8 is also the adsorption coefficient, but it is the adsorption coefficient for each stage in Figure Ib. That is, β0 ) q1/ C1, q2/C2, ..., qi/Ci, ..., or qn/Cn. In the calculation, β0 corresponding to q and C at the end of the column was used. For example, β0 ) qn/Cn at the breakthrough point, and β0 ) q1/C1 at the end point (see Figure II). Literature Cited (1) Matsumoto, S.; Takeshita, K.; Koga, J.; Takashima, Y. A Production Process for Uniform-Size Polymer Particles. J. Chem. Eng. Jpn. 1989, 22, 691.
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Received for review September 4, 2001 Revised manuscript received April 24, 2002 Accepted May 1, 2002 IE010737C
