monothiophosphoric Acid in a Hollow Fiber Membrane Extractor

A kinetic study on palladium extraction from chloride media with bis(2-ethylhexyl)monothiophosphoric acid in EXXSOL D-80 was carried out at 303 K usin...
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Ind. Eng. Chem. Res. 1996, 35, 3899-3906

3899

Kinetics of Palladium Extraction with Bis(2-ethylhexyl)monothiophosphoric Acid in a Hollow Fiber Membrane Extractor Mohammed S. Alam, Katsutoshi Inoue,* and Kazuharu Yoshizuka Department of Applied Chemistry, Saga University, 1 Honjo-machi, Saga 840, Japan

A kinetic study on palladium extraction from chloride media with bis(2-ethylhexyl)monothiophosphoric acid in EXXSOL D-80 was carried out at 303 K using a hollow fiber membrane extractor, together with equilibrium studies on the interfacial adsorption and aqueous distribution of the extractant as well as the loading test of palladium. It was found that bis(2-ethylhexyl)monothiophosphoric acid exists as a dimeric species in organic diluent, which is hardly soluble in the aqueous phase but adsorbs at the interface. Palladium is extracted as a 1:2 metal/reagent complex. The order of the extraction rate was found to be 1, 0, and 1 with respect to the concentrations of palladium ion, hydrogen ion, and dimeric extractant, respectively, while it was found to be 0 and -n (n < 1) with respect to chloride ion in its low and high concentration region, respectively. The apparent reaction rate constants of the extraction reaction were determined experimentally. The extraction rates were reasonably explained in terms of a combination of a diffusion model and an interfacial reaction model. Introduction In recent years, one of the successful applications of solvent extraction in the field of hydrometallurgy has been its acceptance into the processing of precious metals. Palladium is widely used in various fields of industry such as manufacturing industrial catalysts, petrochemicals, automobiles, electronic devices, corrosion-resistant and high-temperature withstanding materials, fuel cells, etc. Therefore, in order to develop the solvent extraction process of palladium, it is important to elucidate the extraction mechanism. At this time, a considerable amount of work has been conducted on solvent extraction of palladium with various kinds of extractants. However, most of this work is limited to extraction equilibrium. Only a few studies are concerned with extraction kinetics. The extractants containing sulfur as the donor atom have a high selectivity for the palladium (Kakoi et al., 1994; Antico et al., 1994; Baba et al., 1986a). Very few studies on the extraction kinetics of palladium have been conducted with sulfur-containing extractants, especially commercial ones (Al-Bazi and Freiser, 1987; Baba et al., 1986b). In a previous study (Yoshizuka et al., 1995), solvent extraction of palladium with 3,3-diethylthietane (DETE) in toluene was carried out using a hollow fiber membrane extractor. In this study, the extraction rate was explained by a combination of a diffusion model and the aqueous homogeneous reaction model. However, DETE may not be commercially suitable because of its high aqueous solubility and hence the loss of extractant. Therefore, more commercially suitable reagents are desired from the practical point of view. Diluents play an important role in extraction kinetics. Since the activity of the extractant differs in each diluent (Komasawa and Otake, 1983), it was found that aliphatic diluents greatly enhance the extraction rate from that in aromatic systems. However, only a little attention has been paid so far to the commercial aliphatic diluents for kinetic studies. Kondo et al. (1989) studied the kinetics of solvent extraction of palladium * To whom correspondence should be addressed. E-mail address: [email protected].

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with didodecylmonothiophosphoric acid in toluene in a highly stirred tank and a stirred transfer cell. Similar works have been carried out using heptane as a diluent (Kakoi et al., 1993), where it was found that the extraction rate of palladium is much faster than that of toluene. They proposed an interfacial reaction model in which four kinds of chloropalladium complexes take part. Recently, the application of a hollow fiber membrane extractor has become popular for kinetic studies on solvent extraction and liquid membrane extraction (Yoshizuka et al., 1986, 1992; Goto et al., 1992), because the flow pattern of fluids which governs the diffusion process is an ideal laminar flow in the hollow fiber. Consequently, it is easy to exactly evaluate the diffusion effect on the observed extraction rate, which differs from a highly stirred tank and a stirred transfer cell. In a previous study (Alam et al.), it was found that bis(2ethylhexyl)monothiophosphoric acid in EXXSOL D-80 strongly extracts palladium from chloride media. Therefore, in the present work, the kinetic aspect of the same extraction system was investigated at 303 K using a hollow fiber membrane extractor, together with equilibrium studies on the interfacial adsorption and aqueous distribution of the extractant as well as the loading test of palladium. This study was undertaken with the following specific objectives: (i) to elucidate the extraction mechanism of palladium with a more selective commercial extractant in a hollow fiber contactor and (ii) to measure the extraction kinetics in an aliphatic commercial diluent considering its great influence on the extraction rate over the aromatics one as well as from the viewpoint of the industrial application. Experimental Section Reagents. Bis(2-ethylhexyl)monothiophosphoric acid was purified from a commercial extraction reagent, MSP-8 (denoted as S), kindly donated by Daihachi Chemical Industry Co. Ltd., Osaka, Japan, and was employed as an extractant. Figure 1 shows the chemical structure of this reagent. The organic solution was prepared by diluting the extractant in EXXSOL D-80, which is a commercial aliphatic kerosene produced and © 1996 American Chemical Society

3900 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

Figure 1. Chemical structure of MSP-8.

marketed by EXXON Chemical Co., having an aromatic content of about 0.8 wt %. This diluent was used from the practical point of view. The aqueous solution was prepared by dissolving reagent-grade palladium dichloride (Tanaka Kikinzoku Kogyo Co. Ltd., Tokyo, Japan) in a 4000 mol m-3 hydrochloric acid solution. Lithium chloride was used to adjust the chloride ion concentration in the aqueous phase. An aqueous solution of thiourea in hydrochloric acid was used as a stripping agent. Purification of MSP-8. MSP-8 was purified as cobalt complex precipitation by slightly modifying the method of Bart and Reidetschlager (1991). A 500 mL solution of 500 mol m-3 MSP-8 in n-heptane was added to 250 mL of a saturated sodium sulfate solution containing an amount of NaOH equivalent to the 20% excess amount of MSP-8. The phases were mixed vigorously for 10 min and then aqueous phase was separated to obtain the organic solution of sodium salt of MSP-8. A volume of 500 mol m-3 cobalt sulfate aqueous solution (sufficient to provide a 10% excess amount of cobalt) was added to the organic phase and mixed vigorously for 15 min. After separating the aqueous phase, the organic phase was evaporated to half volume and kept in a freezer overnight. Acetone was added slowly to this frozen organic phase to obtain the precipitate of the Co(MSP-8)2 complex which was separated by filtration. After air drying, the fine Co(MSP8)2 complex was diluted with diethyl ether and then 2000 mol m-3 H2SO4 was added and mixed vigorously to remove cobalt. Then, the regenerated MSP-8 solution was washed with water, and finally organic diluent was removed in vacuo. Yield of pure MSP-8 was 60.5%. The product was identified by 1H-NMR and elementary analysis. The result of elementary analysis was as follows. Found: C, 55.70; H, 10.37%. Calcd for C16H35O3PS: C, 56.81; H, 10.36%. Purity of MSP-8 was found to be more than 98% by means of neutralization titration with NaOH using phenolphthalein as an indicator. Aggregation of MSP-8. Since EXXSOL D-80, employed in this study, is a mixture of aliphatic hydrocarbons including n-heptane as a major constituent, it is therefore impossible to measure the molecular weight of a solute existing in it, i.e., its aggregation, by means of vapor pressure osmometry (VPO). Consequently, the degree of the aggregation of MSP-8 was measured in n-heptane at 313 K using a Hitachi Model 117 molecular weight apparatus, where benzil was used as a standard material. Aqueous Distribution of MSP-8. Aqueous distribution of MSP-8 in EXXSOL D-80 was measured at 303 K for various concentrations of hydrochloric acid. In this measurement, equal volumes of aqueous and organic phases were shaken by a mechanical shaker for over 24 h in a flask immersed in a thermostat water bath maintained at 303 K to attain equilibrium. After separation of the two phases, both sulfur and phosphorus contents in the aqueous phase were measured by a

Shimadzu Model ICP-1000III inductively coupled plasma atomic emission spectrometer. From their values, the concentration of MSP-8 in the aqueous phase was determined. Interfacial Tension of a MSP-8 and Palladium Complex. Interfacial tension between the organic and aqueous phases was measured at 303 K by the pendant drop method to examine the interfacial adsorption equilibria of a MSP-8 and palladium complex. Extraction Equilibrium. MSP-8 so strongly extracted palladium (Alam et al.) that it was impossible to measure the trace concentration of palladium remaining in the raffinate after extraction. Therefore, it was impossible to determine the stoichiometric relation of the extraction reaction by measuring the concentration dependencies of the reactant species on the distribution ratio of palladium. Consequently, the extraction equilibria of palladium were measured by a loading test as described in a previous paper (Yoshizuka et al., 1995). Extraction Rate. The extraction rate of palladium with MSP-8 was measured at 303 K using a hollow fiber membrane extractor. Parts a-c of Figure 2 show the experimental setup, schematic diagram, and internal view of the hollow fiber membrane extractor, respectively. This extractor consists of a cylindrical glass tube in which a single hollow fiber is inserted. This fiber is made of hydrophobic tetrafluoroethylene. The aqueous solution was pumped into the lumen of the fiber, while the organic solution was fed into the annulus of the extractor. Both aqueous and organic phases flow cocurrently. The temperature was maintained at 303 K by circulating water from a thermostat water bath. At the steady state, raffinates of the aqueous phase and loaded extracts of the organic phase were sampled. Palladium concentrations in the aqueous feed, CPd0, and that in the aqueous phase raffinates, CPd, were measured by using a Seiko Model SAS-7500 atomic absorption spectrophotometer, while the palladium concentration loaded in the organic phase, CPd, was determined in a similar manner after stripping with a 1000 mol m-3 thiourea solution in 1000 mol m-3 hydrochloric acid. The extraction rate of palladium, JM, was calculated according to the following equation:

JM ) (CPd0 - CPd)(Qaq/2πr1L) ) CPd(Qorg/2πr1L) (1) Constants used for numerical analysis are given in Table 1. Results and Discussion Aggregation of MSP-8 in an Organic Diluent. Acidic organophosphorus extractants are known to dimerize (Ferraro et al., 1961) in nonpolar organic diluents such as n-heptane, benzene, and carbon tetrachloride and to exist as monomeric species (Ferraro et al., 1961; Mason et al., 1964) in polar organic diluents such as methyl alcohol and n-decyl alcohol. Therefore, it is very important to know the aggregation states of the extractant in the organic diluents in order to clarify the extraction mechanism. Cote and Bauer (1986) and Levin et al. (1974) observed that dithiophosphoric acids are monomeric, while monothiophosphoric acids are usually dimeric. Because of the similarities between sulfur and oxygen, dialkylmonothiophosphinic acids often exhibit structural tautomerism, with an equilibrium existing between the thiono (-P(dS)OH) and thiol (-P(dO)SH) resonance

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3901

Figure 2. (a) Experimental setup. (b) Schematic diagram of the hollow fiber membrane extractor. (c) Internal view of the hollow fiber membrane extractor. Table 1. Constants Used for Numerical Analysis Kinetic and Equilibrium Constants Kad ) 2.43 m3 mol-1, Sad ) 5.45 × 105 m2 mol-1 DPda ) 1.0 × 10-9 m2 s-1, DSh 2a ) 2.875 × 10-6 m2 s-1, DPdCl2S2a ) 2.687 × 10-6 m2 s-1 b β3 ) 5.37 m9 mol-3, β4b ) 0.1288 m12 mol-4 Physical Properties of Membrane Extractor hollow fiber material: polytetrafluoroethylene (Japan Goretex Co. Ltd.) r1 ) 5.3 × 10-4 m, r2 ) 9.0 × 10-4 m, r3 ) 1.2 × 10-3 m L ) 0.495 m, τ ) 1.60,  ) 0.70 Qaq ) 1.027 × 10-9-1.74 × 10-9 m3 s-1 Qorg ) 1.202 × 10-9-1.692 × 10-9 m3 s-1 a Estimated value by the Wilke-Chang correlation (Sherwood et al., 1975). b Literature data (Shlenskaya and Biryukov, 1966).

forms (Sedvic, 1974), which results in dimerization of these acids in the organic phase. Since no literature concerning the aggregation of MSP-8 has been found so far, the molecular weight of this extractant was measured in this study using vapor pressure osmometry (VPO). Since, as mentioned earlier, EXXSOL D-80, employed in this study as a diluent, is an aliphatic kerosene which is a mixture of various aliphatic hydrocarbons, it was impossible to measure the molecular weight of the extractant in this mixed diluent by means of VPO. Therefore, to examine the degree of aggregation of the reagent in the organic phase, the molecular weight of MSP-8 was measured

Figure 3. Aggregation state of MSP-8 in n-heptane.

in n-heptane, because of its similarity to kerosene (Almela and Elizalde, 1995). Figure 3 shows the experimental results of the aggregation state of MSP-8 in n-heptane by means of VPO. Here, the total analytical concentration of the extractant in the organic phase, Ct, is expressed as follows:

Ct ) CSh + 2CSh 2

(2)

where CSh and CSh 2 are the concentrations of monomeric and dimeric species, respectively. The total concentra-

3902 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

Figure 4. Distribution of MSP-8 between EXXSOL D-80 and HCl.

tion of monomeric and dimeric species in the diluent obtained experimentally, C*, is expressed as follows:

C* ) CSh + CSh 2

(3)

Therefore, the aggregation number, m, is expressed as follows:

m ) Ct/C*

(4)

From the slope of the straight line in Figure 3 the aggregation number of MSP-8 was evaluated to be 2. Therefore, all of MSP-8 exists as dimeric species in n-heptane. As mentioned earlier, it can be considered that MSP-8 exists as dimeric species also in EXXSOL D-80 employed in this study from the practical point of view. Aqueous Distribution of MSP-8. The aqueous distribution of MSP-8 between EXXSOL D-80 and a hydrochloric acid solution is shown in Figure 4. Since MSP-8 exists as a dimeric species in the organic phase and as a monomeric species in the aqueous phase, the partition of extractant between two phases is expressed as follows:

S h 2 a 2S;

KD

(5)

The partition coefficient of MSP-8, KD, is expressed as follows:

KD ) (CS)2/CSh 2

(6)

The logarithm of eq 6 gives

log CS ) 0.5 log CSh 2 + 0.5 log KD

(7)

The plotted points in Figure 4 lie on a straight line of slope 0.5, as expected from eq 7. From the intercept of the straight line with the ordinate, the partition coefficient, KD, was evaluated to be 2.51 × 10-5 mol m-3. This value is much smaller than that of N,N-dioctylglycine (Baba et al., 1993) and Versatic 10 (Inoue et al., 1980), which have been evaluated as 2.2 × 10-2 and 9.6 × 10-4 mol m-3, respectively. Therefore, it can be considered that MSP-8 is hardly soluble in the aqueous phase. Interfacial Tension of MSP-8. The relation between interfacial tension, γ, and the concentration of dimeric MSP-8, CSh 2, and that of a palladium complex, CPdCl2‚S2, are shown in Figure 5, where it was found that the interfacial tension of dimeric MSP-8 abruptly decreases with increasing MSP-8 concentration in its

Figure 5. Interfacial tension of MSP-8 and palladium complex.

high-concentration region, while the interfacial tension of the palladium complex is not affected by its concentration. Therefore, it can be evaluated that MSP-8 has the interfacial activity, while the adsorption of a palladium complex at the interface is negligible. The adsorption equilibrium of dimeric MSP-8 is expressed as follows:

S h 2 + θv a S2(ad);

Kad

(8)

where Kad and θv are the adsorption equilibrium constant of the dimeric species of MSP-8 and the vacant site of adsorption at the interface, respectively. The relation between γ and CSh 2 is derived from the Gibbs’ adsorption as well as the Langmuir adsorption isotherm for expressing the relation between the amount of extractant adsorbed and CSh 2 as follows (Inoue et al., 1974):

γ ) γ0 - (RT/Sad) ln(1 + KadCSh 2)

(9)

where γ0 is the interfacial tension between EXXSOL D-80 and the aqueous phase, and Sad is the interfacial area occupied by unit mole of dimeric species of extractant. From the experimental result, the values of Kad and Sad were evaluated by nonlinear regression and are listed in Table 1. The Kad value is nearly 1.5 times greater than that of dimeric (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester measured in the n-heptane-aqueous acetate buffer solution system (Sato et al., 1989). Loading Test of Palladium with MSP-8. Figure 6 shows the result of the loading test of palladium with MSP-8. The ratio of the concentration of dimeric MSP-8 to that of palladium extracted in the organic phase, CSh 2/CPd, approaches unity. Since palladium is extracted as a 1:2 metal/reagent complex, the following stoichiometric relation can be derived for palladium extraction:

PdCli(i-2)- + S h 2 a PdCl2‚S2 + (i-2)Cl-

(10)

where PdCli(i-2)- is the ith chloro complex of palladium in the aqueous chloride media. Figure 7 shows the structure of extracted species of palladium with MSP8, where 1 mol of palladium is associated with two molecules of MSP-8. Extraction Rate. Figure 8 shows the relation between the extraction rate of palladium, JM, and the

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3903

Figure 6. Loading test of palladium with MSP-8. Figure 9. Relations between extraction rates, JM, and concentration of dimeric MSP-8, CSh 2, and chloride ion concentration, CCl.

Figure 7. Structure of extracted species of palladium with MSP8.

relationship, which is considered to be attributable to the adsorption of extractant as well as the diffusional effect of palladium. Concerning the concentration dependency on the extraction rate, it is independent of the chloride ion concentration in the low concentration region, while it decreases in the high concentration region with a slope of -n (where n < 1). It is well-known that palladium gives rise to four kinds of chloro complexes in aqueous chloride media (Shlenskaya and Biryukov, 1966). Under the present experimental condition of high chloride ion concentration, the majority of the palladium ion exists as tetrachloro complex and a small portion is trichloro complex (Kondo et al., 1989). However, since the trichloro complex is much more labile than other chloro complexes, both tri- and tetrachloro complexes simultaneously take part in the extraction reaction. As it was found that MSP-8 has low aqueous solubility but high interfacial activity and the loading test suggested that palladium is extracted as a 1:2 metal/reagent complex, therefore the following interfacial reaction model for the palladium extraction may be proposed:

Pd(H2O)Cl3- + S2(ad) f PdCl2‚S2 + H2O +

Figure 8. Relations between extraction rates, JM, and hydrogen ion concentration, CH, and palladium ion concentration, CPd.

hydrogen ion concentration in the aqueous phase, CH, and that between JM and the palladium concentration in the aqueous phase, CPd. It is obvious that the hydrogen ion concentration does not affect the extraction rate. The extraction rate is proportional to the palladium concentration at the low concentration region, while, at the high concentration region of palladium, the extraction rate deviates downward from the linear relationship, which is due to the diffusional effect (Yoshizuka et al., 1995). Figure 9 shows the relation between the extraction rate of palladium, JM, and the extractant concentration in the organic phase, CSh 2, and that between JM and the chloride ion concentration in the aqueous phase, CCl. It was found that the extraction rate is proportional to the extractant concentration at the low concentration region. At the high concentration region of the extractant, the extraction rate deviates downward from the linear

Cl- + θv;

k3 (11)

PdCl42- + S2(ad) f PdCl2‚S2 + 2Cl- + θv;

k4 (12)

According to the experimental results, the extraction rate is of zeroth order with respect to hydrogen ion concentration, first order with respect to palladium and extractant concentrations, and zeroth and negative order with respect to chloride ion in its low and high concentration regions, respectively. Considering the above-mentioned reaction mechanism and using the stability constants, βi, of the palladium chloro complex together with a Langmuir adsorption isotherm, the apparent reaction rate of palladium extraction, RM, is derived as follows:

RM )

[

]

(Kad/Sad)CSh 2 k3β3 + k4β4CCl CPd β3 + β4CCl 1 + KadCSh 2

(13)

This rate equation gives a first-order dependency with respect to palladium, and first- and zeroth-order dependency with respect to the extractant in its low and high concentration regions, respectively, as well as

3904 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

zeroth and first-order dependency with respect to chloride ion in its low (k3β3 . k4β4CCl, β3 . β4CCl) and high (k3β3 . k4β4CCl, β3 , β4CCl) concentration regions, respectively. The experimental extraction rate was analyzed on the basis of the diffusion model accompanied by the abovementioned interfacial reaction, taking into account the velocity distributions of the laminar flows in aqueous and organic phases in the hollow fiber extractor as shown in Figure 2c. The steady-state diffusional equations for palladium and chloride ion in the aqueous phase (0 < r < r1) are expressed as follows:

( (

) )

∂2CPd 1 ∂CPd ∂CPd ) DPd uaq(r) + ∂l r ∂r ∂r2

(14)

∂2CCl 1 ∂CCl ∂CCl ) DCl + ∂l r ∂r ∂r2

(15)

uaq(r)

-DPd

∂CPd 1 ∂CCl 1  ∂CSh 2 ) DCl ) DSh 2 ) ∂r 2 ∂r 2 τ ∂r  ∂CPdCl2S2 -DPdCl2S2 ) RM (23) τ ∂r when l > 0, on the outer surface of the fiber (r ) r2):

DSh 2

|

∂CSh 2 ∂r

r2-0

)

|

uorg(r)

uorg(r)

∂CPdCl2S2 ∂l

∂l

) DSh 2

( (

) DPdCl2S2

∂2CSh 2 ∂r2

)

1 ∂CSh 2 +

r ∂r

∂2CPdCl2S2 ∂r2

DSh 2 (16)

)

(

)

2  ∂ CSh 2 1 ∂CSh 2 DSh 2 + )0 τ ∂r2 r ∂r

(

)

2  ∂ CPdCl2S2 1 ∂CPdCl2S2 + )0 DPdCl2S2 τ r ∂r ∂r2

(19)

The initial and boundary conditions for the abovedescribed diffusional equations are expressed as follows:

I.C.:

when l ) 0, in the aqueous phase (0 e r e r1): CPd ) CPd0,

CCl ) CCl0

(20)

when l ) 0, in the organic phase (r2 e r e r3): CSh 2 ) CSh 2 , 0

CPdCl2S2 ) 0

(21)

B.C.: when l > 0, at the center of the fiber (r ) 0): DPd

∂CPd ∂CCl ) DCl )0 ∂r ∂r

(22)

when l > 0, at the interface on the inner surface of the fiber (r ) r1) where extraction reactions take place:

∂CPdCl2S2 ∂r

)0

(25)

[

2Qaq [1 - (r/r1)2] πr12

(26)

uorg(r) ) 2Qorg πr12

(18)

∂r

) DPdCl2S2

uaq(r) )

(17) Those for dimeric MSP-8 and palladium complex in the porous membrane of the hollow fiber (r1 < r < r2) are expressed as follows:

∂CSh 2

In laminar flow, the linear velocities of the aqueous and organic phases, uaq(r) and uorg(r), are expressed as follows:

1 ∂CPdCl2S2 r ∂r

+

|

when l > 0, on the inner surface of the extractor (r ) r3):

Those for dimeric MSP-8 and palladium complex in the organic phase (r2 < r < r3) are expressed as follows:

∂CSh 2

|

∂CPdCl2S2  ∂CSh 2 DSh 2 DPdCl2S2 r2+0, r2-0 ) τ ∂r ∂r  ∂CPdCl2S2 DPdCl2S2 CSh 2|r2-0 ) r2+0, τ ∂r CPdCl2S2|r2-0 ) CPdCl2S2|r2+0 (24) CSh 2|r2+0,

(r3/r1)2 - (r/r1)2 +

(r3/r1)2 - (r2/r1)2

(r3/r1)4 - (r2/r1)4 -

ln(r3/r2)

(r3/r1)2 - (r2/r1)2 ln(r3/r2)

]

ln(r/r3)

(27)

The above-described diffusional equations and the initial and boundary conditions were solved by numerical analysis by means of the implicit finite difference approximations, to obtain the extraction rate of palladium, JM, derived from eq 1. The values of constants used for the calculation are listed in Table 1. The unknown parameters k3 and k4 in eq 13 are the reaction rate constants of eqs 11 and 12, which were evaluated by the trial and error method so as to minimize the standard deviation between the experimental and calculated results. The values of k3 and k4 were evaluated to be 7.5 × 10-1 and 2.61 × 10-3 m3 mol-1 s-1, respectively. The reaction rate of the trichloro complex is much higher than that of the tetrachloro complex because of its lability. Reaction rates of triand tetrachloropalladium, found in this study, are nearly 40 and 2 times greater than those of Kondo et al. (1989) which were measured in a toluene system. Such a higher rate constant may be attributable to the influence of aliphatic diluent, which also follows the literature data (Kakoi et al., 1993). The solid lines in Figures 8 and 9 are the results calculated by using the rate constants. The calculated results satisfactorily agree with the experimental results. Figure 10 shows the fit between Ecalc and Eobs in all experiments.

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 3905

Figure 10. Comparison of experimental results with calculated results of the extent of palladium extraction.

Conclusions The extraction kinetics of palladium from chloride media with bis(2-ethylhexyl)monothiophosphoric acid (MSP-8) were studied at 303 K in a membrane extractor using a hollow fiber. Equilibrium studies on the interfacial adsorption and aqueous distribution equilibria of extractant as well as the loading test of palladium were carried out. Conclusions of the study can be summarized as follows: (1) MSP-8 exists as dimeric species in an aliphatic organic diluent. (2) MSP-8 is hardly soluble in the aqueous phase. (3) MSP-8 is adsorbed at the interface between the organic and aqueous phases, while its metal complex is not adsorbed. (4) Palladium is extracted as a 1:2 metal/reagent complex. (5) The order of the extraction rate was found to be 1, 0, and 1 with respect to the concentrations of palladium ion, hydrogen ion, and dimeric extractant, respectively, while 0 and -n (n < 1) with respect to chloride ion in its low and high concentration regions, respectively. (6) The apparent reaction rate constants of the extraction reaction were determined experimentally, and the extraction rates were reasonably explained in terms of a combination of a diffusion model and an interfacial reaction model, taking into account the velocity distribution of the aqueous and organic phases through the inner and outer sides of the hollow fiber. Acknowledgment The authors gratefully acknowledge the kind supply of MSP-8 from Daihachi Chemical Industry Co. Ltd., Osaka, Japan. Nomenclature Ck ) concentration of species k [mol m-3] JM ) rate of extraction of palladium [mol m-2 s-1] E ) extent of palladium extraction Q ) volumetric flow rate [m3 s-1] r ) distance in radial direction of extractor [m] r1 ) inner radius of hollow fiber [m] r2 ) outer radius of hollow fiber [m] r3 ) inner radius of membrane extractor [m] u ) linear velocity [m s-1] L ) length of membrane extractor [m] l ) horizontal distance from contact point in extractor [m] Ct ) total analytical concentration [mol m-3] C * ) experimentally found total concentration [mol m-3]

m ) aggregation number of extractant S ) extractant: bis(2-ethylhexyl)monothiophosphoric acid (MSP-8) S2 ) dimeric form of extractant KD ) distribution coefficient of extractant [mol m-3] Kad ) adsorption equilibrium constant of extractant [m3 mol-1] k3 ) reaction rate constant of eq 11 [m3 mol-1 s-1] k4 ) reaction rate constant of eq 12 [m3 mol-1 s-1] Sad ) interfacial area occupied by unit mole of extractant [m2 mol-1] RM ) reaction rate in extraction [mol m-2 s-1] R ) universal gas constant [N m mol-1 K-1] T ) temperature [K] Dk ) diffusivity of species k [m2 s-1] θv ) fraction of active vacant site at interface γ ) interfacial tension [N m-1]  ) porosity of hollow fiber τ ) tortuosity of hollow fiber β ) stability constant of ith chloro complex of palladium [(m3 mol-1)i] Superscript ) organic phase Subscripts aq ) aqueous phase org ) organic phase ad ) adsorption state 0 ) initial state k ) species ()Pd2+, S, S2, PdCl2‚S2, H+, Cl-)

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Received for review February 29, 1996 Revised manuscript received June 11, 1996 Accepted July 17, 1996X IE9601209

X Abstract published in Advance ACS Abstracts, October 1, 1996.