Znd. Eng. Chem. Res. 1993,32,207-213
207
Kinetics and Mechanism for Copper(I1) Extraction from Sulfate Solutions with Bis(2-ethylhexyl)phosphoric Acid Ruey-Shin Juang* and Yaw-Tsong Chang Department of Chemical Engineering, Yuan-Ze Institute of Technology, Nei-Li, Taoyuan, 32026, Taiwan, R.O.C.
The rate of copper(I1) extraction from 500 mol/m3 (Na,H,Cu)SO, aqueous solution with bis(2ethylhexy1)phosphoric acid (DBEHPA, HR) dissolved in kerosene were studied a t 25 "C, using a constant interfacial area cell. It was found that, under the conditions studied, the extraction rate could be satisfactorily described by the interfacial chemical reaction model accompanied by the complex diffusion in the organic stagnant film,and the stripping rate, mainly by the diffusion process of the complex. In addition, it was deduced that the rate of complex formation of copper(I1) with D2EHPA was controlled by the reaction of the adsorbed 1:2 copper-D2EHPA complex a t the interface, CuR2, and the dimeric DZEHPA, (HR),, in the organic phase adjacent to the interface.
Introduction Bis(2-ethylhexy1)phosphoric acid (abbreviated as D2EHPA or simply HR) has been widely used in the hydrometallurgical processes for the separation and purification of a number of metals. It has been reported that D2EHPA and its metal complexes are interfacially active (Ihm et al., 1988; Miyake et al., 1990; Vandergrift and Horwitz, 1977, 1980). As has been stated in these independent papers, the formation of metal complexes with DBEHPA proceeds mainly at the interfaces, since the aqueous solubility of DSEHPA is expected to be extremely low. As a result, the rate of metal extraction depends on the interfacial concentration of DSEHPA molecules equivalent to the surface exchange (I') or on its reciprocal, i.e., the molecular area occupied by a statistical D2EHPA molecule at the interface. The kinetics of the extraction of several divalent metals with D2EHPA have extensively been investigated using a constant interfacial area cell, especially from aqueous nitrate or perchlorate solutions. However, there are some discrepancies among these results For example, Cianetti and Danesi (1983) and Vandegrift and Horwitz (1977) separately indicated that the rate is mainly controlled by the diffusion of the metal ion through a structured layer of an interfacial water layer adjacent to the interface in the extraction of Zn(II), Co(II), Ni(II), and Ca(I1) from nitrate solutions, respectively. Miyake et al. (1990) also showed that the rate-determining step of the extraction of Cu(I1) and Co(I1) from perchlorate solutions is the diffusion of the metal complex in the organic film. In addition, Ihm et al. (1988) indicated that the resistance of the interfacial reaction is less than 50% of the total resistance for the extraction of Cu(I1) from nitrate solution. The remainder of the resistance is ascribed to diffusion near the diffusion. However, KOmasawa and Otake (1983) found that the rate is controlled by the interfacial chemical reaction in the extraction of Cu(II), Co(II), and Ni(I1) from nitrate solutions. Dreisinger and Cooper (1989) have studied the kinetics of Zn(II), Co(II), and Ni(I1) extraction from perchlorate solutione using the rotating diffusion cell. They found that, however, the kinetic data of Co(I1) can be analyzed in terms of a mass transfer (across both films adjacent to the interface) with an aqueous chemical reaction mechanism. But for Zn(II), the extraction rate is so fast that mass transfer alone is rate-controlling. For Ni(II), it can be analyzed in terms of a slow reaction in the aqueous film *Towhom all correspondence should be addressed.
and the aqueous bulk phase. On the other hand, fewer works were carried out in such an extraction from sulfate Solutions, especially for Cu(I1). Brisk and McManamey (1969) studied the extraction kinetics of Cu(1I) and Co(I1) by an equilibrium extraction technique. They found the interfacial reaction resistance being between 30% and 75% of the total mass-transfer resistance. For the kinetic studies of Co(I1) and Ni(I1) extraction using a steady-state stirred cell, Golding and Pushparajah (1985) suggested that the overall masstransfer rate can be considered diffusion controlled, since there is no overall increase in the extraction rate when the DZEHPA concentration is increased from 10 to 20 vol %. It should be noted that the interfacial properties of D2EHPA were not considered in the above two papers. It is believed that a stronger aqueous complexation of divalent metals or even DBEHPA molecules in sulfate solutions does exist (Svendsen et al., 1990), which can affect the extraction mechanism and kinetics or the interfacial characteristics. In this study, the interfacial characteristics between aqueous sulfate media and kerosene solutions of DZEHPA were examined. In addition, the kinetics of the extraction of Cu(I1) from sulfate solutions with D2EHPA were investigated using a constant interfacial area cell, and a reaction mechanism considering both resistances of the interfacial chemical reaction and the diffusion of the complex was proposed.
Experimental Section Reagents and Solutions. DSEHPA was the product of Merck Co., with a purity of approximately 98.5%determined by potentiometric titration of an 80 vol 90ethanol solution of the acid with 100 mol/m3 NaOH in ethanol. It was further purified by following the method of McDowell et al. (1976). The diluent kerosene, offered from Union Chemical Works Ltd., Hsinchu, Taiwan, was washed twice with 20 vol % H2S04to remove aromatics and then with distilled water several times. The other inorganic chemicals were supplied by RDH AG, as analytical reagent grade. The aqueous composition was 500 mol/m3 (Na,H,Cu)SO4,which means that the total sulfate content was kept constant. In the aqueous phase, the concentration of Cu(I1) ranged from 1.57 to 1.10 X lo2 mol/m3, while the initial pH value varied from 2.34 to 4.75. In the organic phase, the initial concentration of monomeric D2EHPA was from 10 to 2.0 X lo2 mol/m3, except for the cases where the interfacial tension was measured. Procedure and Apparatus. (a) Extraction Equilibrium. The procedures for the measurement of distri-
0888-5885/93/2632-0207$04.00/0 0 1993 American Chemical Society
208 Ind. Eng. Chem. Res., Vol. 32, No. 1,1993 A
1
n
+
'
""""
'
/
""""
'1
10
X
Y
El
10
[021,
, - , ' 1-: * I
,%
1
---*
1
Figure 1. Constant interfacial area cell used in this study: (1) aqueous-organic interface; (2,3,and 6)sampling and feeding ports; (4)water jacket; (5) two-paddle stirrer; (7) PTFE gasket. (Dimensions given in millimeters.)
bution data were similar to those described in our previous paper (Juang and Chang, 1991). The equilibrium pH value of the aqueous phase was measured with a pH meter (Radiometer Model PHM82). The concentration of Cu(II) in the aqueous phase was determined with a Perkin-Elmer atomic absorption spectrophotometer (Model 5100 PC) at a wavelength of 324.8 nm. The content of Cu(I1) in the loaded organic phase was also measured similarly after it was completely stripped with 20 wt % "OB. The mass balance for metal was proved to be fulfilled in the extraction-stripping procedure to within f2%. (b) Reaction Kinetics. Kinetic studies were carried out in this work by a constant interfacial area cell, as shown in Figure 1. It was made of Pyrex glass and had an interfacial area of about 33.2 cm2. The two stirrer blades were symmetrically located with respect to the interface, and the stirrers were driven in opposite directions in the range of 30-110 rpm by using a Cole-Parmer Servodyne motor. The initial extraction and stripping rates were separately measured by the following procedures. An aqueous solution having a volume about 130 cm3was first placed in the lower cell, and then an equal volume of the organic solution was poured into the cell in order to minimize any disturbance a t the interface. The timing was started upon addition of the organic solution. Samples (1-2 cm3) were taken at certain time intervals from the appropriate phase. The concentration of Cu(I1) was analyzed by the same method as that stated above. The solution temperature was maintained at 25 OC. The interfacial tension was measured at 25 OC by a FACE surface tensiometer (Kyowa Model CBVP-A3), which has been constructed along the lines of the Whilhelmy method. Measurements were made for the following system: DZEHPA/kerosene-500 mol/m3 (Na,H)S04 aqueous solution (free of Cu(II)),in which the aqueous pH was fixed at about 2.20 by adjusting the fraction of the combination of H2S04and Na2S04.
Results and Discussion Extraction Equilibrium. As Cu(I1) present in the aqueous phase is at tracer level, it is reasonable to assume
(mo1/m3)
Figure 2. Effect of D2EHPA concentrations on the extraction of Cu(I1) from 500 mol/m3 (Na,H,Cu)S04at 25 OC: diluent, kerosene; slope = 1.99. Table I. Extraction Equilibrium Constant Reported i n t h e Literature at 25 O C aaueous Dhase diluent loe K..O _____ -3.92 500 mol/m3 kerosene (Na,H)S04 lo00 mol/m3 n-dodecane -2.85 (Na,H)N03 -2.69 500 mol/m3 kerosene (Na,H)N03 -3.05 500 mol/m3 n-heptane (Na,H)N03 500 mol/m3 toluene or benzene -4.22 (Na,H)N03 500 mol/m3 benzene -3.39 (Na,H)C104 500 mol/m3 n-heptane -3.44 (Na,H)ClO, ~~
a
for C U R ~ ( H R ) ~ source this work Grimm and Kolarik (1974) Ihm e t al. (1988) Komasawa et al. (1981) Komasawa et al. (1981) Kojima et al. (1969) Qiu et al. (1989)
Unit dimensionless.
that no polynuclear complexes are formed (Grimm and Kolarik, 1974; Ihm et al., 1988; Kojima et al., 1969; Komasawa et al., 1981; Miyake et al., 1990). Thus, the extraction of Cu(I1) with D2EHPA dissolved in kerosene can be represented by the following reaction: Cu2++ [(n + 2)/2](HR), ~t CuR,(HR), + 2 H+ (1) and K,, = [ C U R , ( H R ) . ] [ H + ] ~ / ( [ C U ~ + ] [ ~ ~(2) J(~+~)/~) where K,, is the extraction equilibrium constant and the overbar indicates the species in the organic phase. The distribution ratio of Cu(I1) is defined as D = [CUR~(HR),]/[CU~+] = K,,[(HR)2]("+2)'2[H+]-2(3) Rearranging, it follows that log D[H+I2= log K,, + ((n + 2)/2] log [(HR),] (4) Thus,a log-log plot of D[H+I2versus [(HR),] would give a straight line with an intercept of log K,,and a slope of (n + 2)/2. In Figure 2, a straight line with a slope of 1.99 was obtained; i.e., n = 1.98. This implies that the extracted species is CuR2(HR)*. The equilibrium constant, K,,, obtained in this work and those reported in the literature are compiled in Table I. It is evident that the value of K,, changes with the aqueous composition or ionic strength and with the organic diluent. Generally, the constant obtained in the extraction from sulfate solutions is found to be somewhat less than those from nitrate or perchlorate solutions.
-
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 209
k.
v
Table 11. Value of Constants for Copper(I1) Extraction with DIEHPA constant source ~m = 3.47 x 103 Huang and Juang (1986a) K. = 53.7 m0i/m3 Huana and Juana (1986a) K; = 26.3 m3/m01 Huang and Juang (1986a) k~ = 1.86 X 10" m/s Asai et al. (1983) ~m = 67.86 m3/m01 this work K,, = 1.20 x 10-4 this work this work kf = 7.54 X lo-' m/s this work kb = 1.06 X lo-* m/s
'id
0
lo-'
lo-'
10-2
lo-'
1
10
[REI,, (mo1/m3) Figure 3. Relationship between the interfacial pressure and the initial concentration of D2EHPA. The solid curve was calculated by eq 12. pH = 2.20. Interfacial Characteristics. The relationship between the interfacial pressure, yo - y, and the DBEHPA concentration in the organic phase for the DSEHPA/kerosene-600 mol/m3 (Na,H)S04system (pH = 2.20) is shown in Figure 3. It is found that DZEHPA molecules are interfacially active. Generally, such acidic organophosphorus extractants exist in the organic bulk phase mainly as the dimer due to intermolecular hydrogen bonding. However, at the organie-aqueous interface, it is deduced that the extractants exist mainly as the monomer because the intermolecular hydrogen bonding between the extractants is destroyed due to preferential hydrogen bonding with water molecules in the interfacial zones (Miyake et al., 1990). Accordingly, it was assumed that the adsorptive species of DPEHPA were both HR and R-. The interfacial tension can be related to the amount of chemical species adsorbed at the interface under the constant pH by the Gibbs adsorption equation (Miyake et al., 1990; Kataoka et al., 1986): -dr = (rm + r R ) dwin (5) The interfacial excess quantities are expressed by the Langmuir adsorption isotherm of a two component system (Miyake et al., 1990): rHR
r m K ~ ~ [ H R ]= i erv" K m m ] i 6 v = I;kR (6) rR
= r"K~[R-]ie,
(7)
The relation KHR and Km can be derived from the following equation: (9) KHR= hi6~1 where & is the distribution ratio of monomeric DBEHPA. If the activity coefficient of HR in the organic phase is equal to unity, then eq 5 can be integrated and the interfacial pressure is expressed by yo - y = P R T In (1+ a K m [ m ] ) (10) a
1 + (KR/KHR)(Ka/[H+l)
(11)
For simplicity, KHR is assumed to be equal to KR because the nonpolar part of these species is common (Miyake et al., 1990; Sato et al., 1989a). Then, eqs 10 and 11can be combined and written as yo - y = P R T In (1+ KHR(1 + K,/[H+]) X ((1 8 K d [ m 1 0 ) " ~- 1)/(md)} (12) where K, and Kd are the acidic dissociation constant and dimerization constant of DZEHPA, respectively. These values are listed in Table 11.
t
P I
t, (min) Figure 4. Variation of the concentration of the Cu(II)-D2EHPA complex in the organic phase with contact time: [Cu2+l0= 7.16 mol/m3; [ m ] , = lo0 mol/m3; pH = 2.34 ( O ) ,2.53 ( O ) , and 2.75 ( 0 ) .
The saturated surface excess, r", and adsorption equilibrium constant of D2EHPA, Km,can be calculated from eq 12 by nonlinear regression method and are found to be 2.26 X lo4 mol/m2 and 67.86 m3/mol, respectively. The slower rate of divalent metals extraction with D2EHPA from sulfate solutions than those observed from nitrate or perchlorate solutions, as reported by Golding and Pushparajan (1985), is probably in part attributed to the larger Km value, because in this case the vacancy at the interface that is to be occupied by the interfacially active metal complex is relatively less. The covered interfacial area occupied per mole of D2EHPA, h,which corresponds to l / P ,is 4.42 X lo5 m2/ mol. This result agrees with those of 4.76 X lo5and 5.88 X lo5 m2/mol in D2EHPAln-heptane and DBEHPAI benzene200 mol/m3 (Na,H)C104 systems, respectively (Miyake et al., 1990), and of 3.97 X lo6 m2/mol in 2(ethylhexy1)phosphonic acid mono-2-ethylhexyl ester (HEHEHP)/n-heptane-lOO mol/m3 (Na,H)OAc system (Sato et al., 1989a). Reaction Kinetics. (a) Determination of the Initial Extraction and Stripping Rates. m i c a l values of the amount of Cu(I1) in the initially Cu(II)-freephase and the contact time for the extraction and stripping processes are shown in Figures 4 and 5, respectively. It is found that all data points lie on respective straight lines passing through the origin. Initial extraction and stripping rates, Rf and Rb,were calculated from the slope of each straight line according to Rf = (Vo/A)(d[CuR2(HR)21/dt) (13)
Rb = (V,/A)(d[Cu2+]/dt) (14) The effect of stirring speed on the extraction rate is shown in Figure 6. It was evident that the extraction rate does depend on the stirring speed, indicating that the
210 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
X
N
3
u
u
0
.
I
I
I
20
40
60
t , (min)
i
Figure 5. Variation of the concentration of Cu(I1) in the aqueous phase with contact time: pH = 2.50; = 100 mol/m3; [ G I o = 0.31 (O),0.63 ( O ) , and 1.26 mol/m3 ( 0 ) .
[m],
t 10
->
10
1
[ CU'+],
(moi/m3)
Figure 7. Effect of Cu(I1) concentration in the aqueous phase on = 100 (0,1-3) and 50 mol/m3 the extraction rate: pH = 3.02; [=I0 (0,4-6). The curves were calculated according to the complex diffusion control (1,4), interfacial reaction control (2,5), and combined control (3, 6).
20---0
1
*i 2
5
0 0
0
0
0
30
0
60
I
I
90
120
Stirring Speed, (rpm)
?-d
li
Figure 6. Effect of the stirring speed on the extraction rate: [Cu2+], = 4.72 mol/m3; [=I, = 100 mol/m3; pH = 3.02.
extraction rate is affected by the diffusion process (Cianetti and Danesi, 1983; Kondo et al., 1990). Since the interface was highly disturbed at stirring speeds of larger than 110 rpm, the experiment thereafter was made at 90 rpm. (b) Initial Extraction and Stripping Rates. The effect of the concentration of Cu(I1) in the aqueous phase on the extraction rate is shown in Figure 7. It is found that the extraction rate is proportional to the Cu(I1) concentration. Figure 8 shows the effect of the concentration of dimeric DBEHPA in the organic phase on the extraction rate, which also indicates that the extraction rate is proportional to the dimeric D2EHPA concentration. The effect of the concentration of hydrogen ion in the aqueous phase on the extraction rate is shown in Figure 9. It is evident that the extraction rate decreases with an increase in the hydrogen ion concentration. The effect of the concentration of the Cu(I1)-DBEHPA complex in the organic phase on the stripping rate is shown in Figure 10. It is found that the stripping rate is proportional to the Cu(II)-D2EHPA complex concentration. Figures 11 and 1 2 show that both the concentrations of hydrogen ion in the aqueous phase and of free dimeric D2EHPA in the organic phase have no significant effect on the stripping rate under the experimental ranges studied. Proposed Reaction Mechanism. From the nature of DBEHPA such as interfacial activity and extremely low aqueous solubility, it is inferred that the formation reaction
10'
10
[(HR),I, (mol/m3) Figure 8. Effect of D2EHPA concentration in the organic phase on the extraction rate: [Cu2+l0= 9.44 mol/m3; pH = 3.02 (0, 1-3) and 2.53 (0,441.The meaning of each curve was the same as that in Figure 7.
IO
1
[H'],
(mol/m3)
Figure 9. Effect of hydrogen ion concentration in the aqueous phase on the extraction rate: [CU*+]~ = 7.16; [MI,= 100 mol/m3 (0,1-3); [Cu2+],= 4.75, = 50 mol/m3 (0,443).The meaning of each curve was the same as that in Figure 7.
[m],
Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 211 I 10 2
I
' " I
I
k
c
10
i 6
4
1
)
1
[CU], (m01/m3) Figure 10. Effect of the Cu(II)-D2EHPA complex concentration in the organic phase on the stripping r a t e pH 2.50; [XI,= 100 mol/m3. The meaning of each curve was the same as that in Figure 7. !-
I
E
3 10
10'
[@%I,
(m01/m3)
Figure 11. Effect of free D2EHPA concentration in the organic phase on the stripping r a t e pH = 2.50; [&I0 = 1.57 (0, 1-3) and 0.63 mol/m3 (Q4-6). The meaning of each curve was the same as that in Figure 7.
of metal complexes is very likely to be interfacial (Aptuicio and Muhammed, 1989; Cianetti and Danesi, 1983; Huang and J u g , 1986b; Ihm et al., 1988; Komasawa and Otake, 1983; Miyake et al., 1990; Svendsen et al., 1990). On the basis of the experimental results of the extraction equilibrium, adsorption equilibrium, and extraction kinetics, the following interfacial reaction mechanism was proposed.
(K)z
HR
+ 0,
2 ( m ) , 1/Kd
(154
HR(ad), Km
(15b)
Cu2++ HR(ad) + CuR+(ad) + H+, K1 (15c) CuR+(ad) + HR(ad) + CuR,(ad) CuR,(ad)
+ H++ e,
+ (m),+ CuR2(HR), + e,
k,/k-,
Kz (15d) (15e)
where 'ad" denotes the species adsorbed at the interface. If the reaction by eq 15e is rate-determining, the rate of complex formation per unit area at the interface could be derived as Rf = ( k 3 K , K & d - 1 ( K ~ / S ~CUZ+] ) 2 [ i[ (HR),] i2[ H+]r2k-3[CuR2(HR)zliPv (16) where 0, can be essentially expressed by eq 8 since the fractions of the active site at the interface occupied by the complex species such as CUR+and CuRz are negligibly
10'
[H+],(m01/m3) Figure 12. Effect of hydrogen ion concentration in the aqueous phase on the stripping rate: = 100 mol/m3; [GI0= 1.57 (0, 1-3) and 0.63 mol/m3 (Q4-6). The meaning of each curve was the same as that in Figure 7.
[m],
small compared with those of HR and R- at the early stage of the extraction. If k & l K & d - ' ( K ~ / s ~ ~and z k-3 are replaced by kf and k b , respectively, thus, eq 16 can be written as follows: Rf = (kf[C~~+]i[(HR)z]i2[H+]{~ - kb[CUR2(HR)z]ilBv (17) In this interfacial reaction model (eq 15), a reaction of the adsorbed 1:2 Cu(II)-D2EHPA complex at the interface, CuR2, and the dimeric DBEHPA, (HR),, in the organic phase adjacent to the interface was suggested to be ratedetermining. This point is the same as those proposed in the extraction of divalent metals with D2EHPA (Aparicio and Muhammed, 1989; Cianetti and Danesi, 1983; Ihm et al., 1988; Komasawa and Otake, 1983; Vandergrift and Horwitz, 1977) or with HEHEHP (Sato et al., 1989a,b) but also different from those in several previous works (Dreisinger and Copper, 1989; Huang and Juang, 1986b; Svendsen et al., 1990). The diffusion equations of Cu2+,H+, CUR,(HR)~,and (HR), for the extraction process are expressed as follows: Jf = kc,([Cu2'] - [CU~']~) (18a)
= (k~/2)([H+li- [H+]) (18b) = kc,([C~Rz(HR)z]i- [CUR~(HR)~]) ( 1 8 ~ ) = (~,,R,,/~)([(HR)ZI- [(HR)zli) (18d)
where k~ represents the mass-transfer coefficient of the Cu(II)-D2EHPA complex, C U R ~ ( H R )in~ , the organic stagnant layer. The concentration of each species adjacent to the interface is obtained from the experimental results of Jf,in which the interfacial reaction rate equals the mass-transfer rate at steady state (Jf= Rf). Hence, eq 17 becomes Rf
k&J[Cu2+1- Rf/kc,)([(HR)zI
- 2Rf/k(~~,,)' X
(W+I + 2&/k~)-' - kb&([CuRz(HR)zI + Rf/kz) (19)
Now it is assumed that the restricting mass transfer is related to the diffusion of the Cu(II)-D2EHPA complex in the organic stagnant film (Miyake et al., 1990; Svendsen et al., 1990). The extraction rate at the initial stage ([CuR2(HR),] = 0) is derived as follows from eq 19: (20) Rf = ( k G k # , / ( k E + kbev))[CU2+Ii[(HR)21i2[H+Ii-2 Similarly, from eq 17, the rate of complex destruction per unit area at the interface is given by Rb = (kb[CuR2(HR)Ji - ~ ~ [ C U ~ ' ] ~ [ ( H R ) ~ ] ~(21) [H'~~-~)~~ Considering the diffusion equations of different species for
212 Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993
the stripping process and again assuming that only the diffusional resistance of complex is anticipated, then the stripping rate at the initial stage ( [Cu2+]= 0) can be derived from eq 21. Rb = {k&ov/(kE 4- kbov)I[CuR2(HR)zI (22) The expressions by eqs 20 and 22 show that both the extraction and stripping rates are explained by the interfacial reaction model accompanied by the diffusion of the complex. The unknown constants used here to calculate Rf or Rb are listed in Table 11. It should be noted that the intrinsic forward and backward rate constants, kf and kb, are evaluated from eqs 20 and 22 by the nonlinear regression method from the experimental resulta and m/s, reare obtained to be 7.54 X lo-' and 1.06 X spectively. The mass-action constant, kf/kb, ought to correspond to the extraction equilibrium constant, K,,,as indicated in eq 17. The values of (kf/kJ and K,,are found to be 7.11 X and 1.20 X lo4, respectively in this work, which is still considered to be in good agreement (Huang and Juang, 1986b; Komasawa and Otake, 1983; Miyake et al., 1990). This discrepancy may be due to some changes of the interfacial properties such as the loading ratio, in which it is higher at the interface than that in the bulk phase (Miyake et al., 1990). Exactly, it is believed that the extraction equilibrium constants are strongly influenced by the loading ratio. The solid curves shown in Figures 7-12 are calculated from eq 20 or 22 for the initial extraction or stripping rate. Close agreement between the experimental and the calculated values was obtained. The calculated resulta when the extraction or stripping rate is controlled by the diffusion of the complex or the interfacial chemical reaction are also presented in these figures. It is evident that the extraction rate could be satisfactorily described by the combined processes of the interfacial reaction and the complex diffusion in the organic stagnant film, depending on the experimental conditions. On the other hand, the stripping rate is mainly controlled by the diffusion process of the complex under the conditions studied. This can apparently be reflected by the similar values of kbev and kzsj (lo* order), as listed in Table 11, under the conditions studied. Evidently, the mechanism of Cu(I1) extraction with D2EHPA deduced here, especially for the forward extraction process, is somewhat different from those found from nitrate or perchlorate solutions as stated above (Komasawa and Otake, 1983; Miyake et al., 1990). Since in the extraction of divalent metals with acidic organophosphorus extractants the apparent backward rate constant kbdv is reported to be roughly identical to the mass-transfer coefficient of complex in the organic phase (Miyake et al., 1990), a smaller extraction equilibrium constant will give a smaller apparent forward rate constant kbVand hence a slower reaction rate of complex formation. Accordingly, the discrepancies in the reaction mechanism among various salt solutions may be attributed to the relatively smaller extraction equilibrium constant with sulfate media compared with those with nitrate or perchlorate media, as seen in Table I.
Conclusions Kinetic studies were made at 25 OC in the extraction of Cu(I1) from 500 mol/m3 (Na,H,Cu)SO, aqueous media with kerosene solutions of D2EHPA (HR) using a constant interfacial area cell. The following results were obtained. (1)The species extracted into the organic phase is found to be CuR2(HR),. The extraction equilibrium constant
obtained graphically is 1.20 X lo-,. (2) The values of K m and Sm for this system are calculated to be 67.86 m3/mol and 4.42 X lo5 m2/mol, respectively. Also, the intrinsic forward and backward rate constants, kf and kb, are found to be 7.54 X lo-' and 1.06 X m/s, respectively, at 25 "C. (3) Based on the interfacial characteristics and the kinetic data, the rate of complex formation of Cu(I1) and D2EHPA is controlled by the interfacial reaction of the adsorbed 1:2 Cu(WD2EHPA complex at the interface, in the organic CuR2, and the dimeric DSEHPA, (HR),, phase adjacent to the interface. (4) Under the conditions studied, the extraction rate can be successfully described by the combined proceses of the interfacial reaction and the complex diffusion in the organic stagnant film,and the stripping rate, mainly by the diffusion process of the complex.
Acknowledgment This work was supported by the ROC National Science Council under Grant No. NSC80-0402-E155-04, which is greatly appreciated.
Nomenclature A = interfacial area between two phases, m2 D = distribution ratio of Cu(I1) HR = monomeric form of DZEHPA (HR), = dimeric form of D2EHPA J = mass-transfer rate, mol/(m2-s) K, = acidic dissociation constant of DZEHPA, mol/m3 K,, = extraction equilibrium constants defined in eq 2 Kd = dimerization constant of DZEHPA, m3/mol K, = adsorption equilibrium constant of species j , m3/mol kf = intrinsic forward reaction rate constant defied in eq 17, m/s k b = intrinsic backward reaction rate constant defined in eq 17, m/s = distribution ratio of monomeric DZEHPA R = universal gas constant (=8.314J/mol.K) Rf= initial forward reaction rate, mol/(m2.s) Rb = initial backward reaction rate, mol/(m2.s) S, = interfacial area occupied per mole of the adsorbed species j , m2/mol t = time, s T = absolute temperature, K V = volume of organic or aqueous phase, m3 [ ] = molar concentration of species in the bracket, mol/m3
eR
Greek Letters
interfacial tension, N/m interfacial tension between water and kerosene, N/m r" = saturated surface excess, mol/m2 r, = surface excess of adsorbed species j , mol/m2 Bv = fraction of active vacant site in the interface y =
yo =
Subscripts i = adjacent to the interface log = logarithm of base 10 o = organic phase w = aqueous phase
0 = initial
Superscript -=
species in the organic phase
Literature Cited Aparicio, J.; Muhammed, M. Extraction Kinetics of Zinc from Aqueous Perchlorate Solution by DPEHPA Diesolved in Isopar-H. Hydrometallurgy 1989, 21, 385-399. Asai, S.; Hatanaka, J.; Uekawa, Y. Liquid-Liquid Mass Transfer in an Agitated Vessel with a Flat Interface. J. Chem. Eng. Jpn.
Ind. Eng. Chem. Res. 1993, 32, 213-221 1983,16 (6), 463-469. Brisk, M. L.; McManamey, W. J. Liquid Extraction of Metals from Sulfate Solutions by Alkylphosphoric Acids. 11. Kinetics of the Extraction of Copper and Cobalt with Di(2-ethylhexy1)phosphoric Acid. J. Appl. Chem. 1969, 19 (4), 103-108. Cianetti, C.; Daneai, P. R. Kinetica and Mechanism of the Interfacial Mass Transfer of Zinc, Cobalt, and Nickel in the System: Bis(2ethylhexy1)phosphoric Acid, n-Dodecane-KN03, Water. Solvent Extr. Zon Exch. 1983, 1 (l), 9-26. Dreisinger, D. B.; Copper, W. C. The Kinetics of Zinc, Cobalt and Nickel Extraction in the D2EHPA-Heptane-HC104 System Using the Rotating Diffusion Cell Technique. Solvent Extr. Zon Exch. 1989, 7 (21, 335-360. Golding, J. A.; Puehparajah. Maes Transfer Characteristics of Cobalt and Nickel in Di(2-ethylhexy1)phosphoric Acid under SteadyState Extraction Conditions. Hydrometallurgy 1985,14,295-307. Grimm, R.; Kolarik, Z. Acidic Organophosphorus Extractants-XIX. Extraction of Cu(II), Ni(II), Zn(1I) and Cd(I1) by Di(2-ethylhexy1)phosphoric Acid. J. Znorg. Nucl. Chem. 1974,36,189-192. Huang, T. C.; Juang, R. S. Extraction Equilibrium of Zinc from Sulfate Media with Bis(2-ethylhexy1)phosphoric Acid. Znd. Eng. Chem. Fundam. 1986a, 25 (4), 752-757. Huang, T. C.; Juang, R. S. Kinetics and Mechanism of Zinc Extraction from Sulfate Medium with Di(2-ethylhexy1)phosphoric Acid. J. Chem. Eng. Jpn. 1986b, 19 (9, 379-386. Ihm, S. K.; Lee, H. Y.; Lee, D. H. Kinetic Study of the Extraction of Copper(I1) by Di(2-ethylhexy1)phosphoricAcid in a Lewis-type Cell. J . Membr. Sci. 1988, 37, 181-191. Juang, R. S.; Chang, Y. T. Extraction of Zinc from Sulfate Solutions with Bis(2-ethylhexy1)phosphoric Acid in the Presence of Tri-noctylphosphine Oxide. Znd. Eng. Chem. Res. 1991, 30 (ll), 2444-2449. Kataoka, T.; Nishiki, T.; Yamauchi, M. Interfacial Adsorption Equilibria of Aqueous Mercury Solution and Xylene Solution of Dicyclohexyl-24-Crow-8. J. Chem. Eng. Jpn. 1986, 19 (2), 147-149. Kojima, I.; Fukuta, J.; Tanaka, M. Extraction of Copper(I1) and Sodium with Di(2-ethylhexy1)phosphoricAcid. J. Znorg. Nucl. Chem. 1969,31, 1815-1820. Komasawa, I.; Otake, T. Kinetic Studies of the Extraction of Diva-
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lent Metals from Nitrate Media with Bis(2-ethylhexy1)phosphoric Acid. Znd. Eng. Chem. Fundam. 1983,22,367-371. Komasawa, I.; Otake, T.; Higaki, Y. Equilibrium Studies of the Extraction of Divalent Metals from Nitrate Media with Di(2-ethylhexy1)phosphoric Acid. J. Znorg. Nucl. Chem. 1981, 43, 3351-3356. Kondo, K.; Momota, K.; Nakashio, F. Equilibrium and Kinetics of Solvent Extraction of Europium with Didodecylmonothiophosphoric Acid. J. Chem. Eng. Jpn. 1990,23 (l),3Ck35. McDowell, W. J.; Perdue, P. T.; Case, G. N. Purification of Di(2ethylhexy1)phosphoric Acid. J. Znorg. Nucl. Chem. 1976, 38, 2127-2129. Miyake, Y.; Matsuyama, H.; Nishida, M.; Nakai, M.; Nagase, N.; Teramoto, M. Kinetics and Mechanism of Metal Extraction with Acidic Organophosphorus Extractants (I): Extraction Rate Limited by Diffusion Process. Hydrometallurgy 1990, 23, 19-35. Qiu, D. Y.; Zheng, L. G.; Ma, R. J. The Behavior-Structure Relations in the Extraction of Cobalt(II), Nickel(II), Copper(I1) and Calcium(I1) by Monoacidic Organophosphorus Extractants. Solvent Extr. Zon Exch. 1989, 7 (6), 937-950. Sato, Y.; Akiyoshi, Y.; Kondo, K.; Nakashio, F. Extraction Kinetics of Copper with 2-Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester. J. Chem. Eng. Jpn. 1989a, 22 (2), 182-189. Sato, Y.; Kondon, K.; Nakashio, F. Extraction Kinetics of Zinc with 2-Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester Using a Hollow-Fiber Membrane Extractor. J. Chem. Eng. Jpn. 198913, 22 (6), 686-689. Svendsen, H. F.; Schei, G.; Osman, M. Kinetics of Extraction of Zinc by Di(2-ethylhexy1)phosphoric Acid in Cumene. Hydrometallurgy 1990,25, 197-212. Vandergrift, G. F.; Horwitz, E. P. The Mechanism of Interfacial Mass Transfer of Calcium in the System: Di(2-ethylhexy1)phosphoric Acid in Dodecane-Dilute Nitric Acid. J.Znorg. Nucl. Chem. 1977,39, 1425-1432. Vandergrift, G. F.; Horwitz, E. P. Interfacial Activity of LiquidLiquid Extraction Reagents-I. Dialkyl Phosphorus Based Acids. J. Znorg. Nucl. Chem. 1980, 42, 119-125. Received for review June 29, 1992 Accepted October 9, 1992
Trapping of Large Coal Molecules in a Coal Matrix Norman E. Cooke,* Javeed Razi, and Rajendra P. Gaikwad Department of Chemical Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 2A7, Canada
Fourteen sets of experimental data from literature and this investigation have been correlated by a model which postulates that there are three different fractions of molecules in coal. Fraction S can be extracted by an appropriate solvent independent of the size of the particle. Fraction L can only be extracted from a thin shell at the surface of the particle. Fraction I is insoluble at all particle sizes. The thickness of the shell varies from coal to coal, but is of the order of 1pm. Two possible mechanisms for this trapping are suggested. One possibility is a molecular sieve model in which the large insoluble molecules are so tightly packed that the molecules of Fraction L can only move a few times their own molecular length. This model is rejected because the molecules of Fraction L can move many times their own length. The other possibility is that a molecular cage, or clathrate, traps molecules. Evidence for this case is presented.
Introduction The fact that considerably more solvent extract can be obtained by grinding coal particles down to the size of 1 pm in diameter has been known since Fischer et al. (1932a) reported the results of extracting a banded coal from Seam 16 of the Mathias Stinnes Mining Company. They used trichloroethylene as the solvent. When the average particle size was 5 mm, the yield was only 0.55%. However, when the particle size was 1pm, the yield increased to 9.98%. This was followed by the work of Peters and Cremer (1934), who extended the work, extracting five different coala wing two different solvents.
Asbury (1934) and Landau and Asbury (1938) did similar work, and devised two methods of extrapolating the extraction data to predict what the yield would be after an infinite extraction time. They confirmed that there was a real and significant difference in the yields. Their method ruled out any possibility that the observed difference in yields was due to material being extracted more quickly from the smaller particles because the diffusional path was shorter. To explain their findings, Fischer et al. (1932a) advanced a theory of celllike structure for the coal, caused by the cell structure existing in the original coal-forming planta.
O888-5885f 93 f 2632-O213$OUM f 0 0 1993 American Chemical Society
