Ind. E n g . Chem. Res. 1990,29, 2111-2118
88-SR-111. We also thank the Chemistry Department of Lamar University for the facilities provided during the preparation of this manuscript. Registry No. DMF, 68-12-2; ethanol, 64-17-5; anisole, 100-66-3; acetophenone, 98-86-2; carbon tetrachloride, 56-23-5.
Literature Cited Aminabhavi, T. M. Polystyrene in Mixed Solvents. Ph.D. Dissertation, The University of Texas, Austin, 1979. Aminabhavi, T. M.; Manjeshwar, L. S.; Balundgi, R. H. Density and Refractive Index Incrementa and Excess Molar Volumes of Binary Liquid Mixtures. Indian J . Chem. 1986, 25A, 820. Aminabhavi, T. M.; Manjeshwar, L. S.; Balundgi, R. H. Theoretical Justification for Viscosity Models and Prediction of Excess Thermodynamic Functions for Binary Liquid Mixtures. Indian J . Chem. 1987, 26A, 641. Aminabhavi, T. M.; Manjeshwar, L. S.; Balundgi, R. H. Thermodynamic and Optical Studies on Binary Liquid Mixtures. Indian J . Chem. 1988,27A, 303. Bottcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1952.
2111
Karachewski, A. M.; McNiel, M. M.; Eckert, C. A. A Study of Hydrogen Bonding in Alcohol Solutions Using NMR Spectroscopy. Ind. Eng. Chem. Res. 1989,28, 315. Manjeshwar, L. S. Theoretical and Experimental Studies on Liquid Mixtures. Ph.D. Dissertation, Karnatak University, India, 1987. Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents. Techniques of Organic Chemistry, 4th ed.; John Wiley & Sons: New York, 1986; Vol. 11. Roux, A. H.; Desnoyers, J. E. Association Models for Alcohol-Water Mixtures. Indian Acad. Sci. (Chem. Soc.) 1987, 98, 435. Rowlinson, J. S. Liquids and Liquid Mixtures; Butterworths: London, 1959. Timmermans, J. Physicochemical Properties of Pure Organic Compounds; Elsevier: Amsterdam, 1950. Vogel, A. I. Textbook of Practical Organic Chemistry, 5th ed.; John Wiley & Sons: New York, 1989. Wilson, L.; Bicca De Alencastro, R.; Sandorfy, C. Hydrogen Bonding of n-Alcohols of Different Chain Lengths. Can. J . Chem. 1985, 63, 40.
Received for review January 16, 1990 Revised manuscript received May 30, 1990 Accepted June 12, 1990
Solvent Extraction of Palladium(I1) with Nonchelating Oximes with Different Alkyl Chain Lengths Yoshinari Baba,*” Katsutoshi Inoue,t Kazuharu Yoshizuka,t and Takashi Furusawat Joint Research and Development Center and Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi I, Saga 840, J a p a n
By using four kinds of nonchelating oximes with alkyl chains of different lengths (dodecanal oxime, decanal oxime, octanal oxime, and hexanal oxime), the effects of the hydrophobicity of these extractants on the extraction mechanism of palladium(I1) were investigated from the equilibria and kinetic aspects along with the aqueous solubilities and the interfacial adsorption equilibria of the extractants. The loading test showed that palladium(I1) was extracted as 1:2 Pd:extractant complexes with all kinds of oximes. The extraction kinetics with dodecanal oxime, which is practically insoluble in the aqueous solution and interfacially active, was reasonably interpreted in terms of the interfacial reaction model. The rate is limited by the formation of the intermediate complex a t the interface between the aqueous and organic phases. The extraction kinetics with hexanal oxime with the shortest alkyl chain length, which is of high aqueous solubility and of low interfacial activity, was explained in terms of the classical heterophase homogeneous reaction mechanism controlled by the formation of the intermediate complex in the aqueous phase. The extraction kinetics with two other kinds of oximes was reasonably explained by taking into account the formation of intermediate complexes, which takes place simultaneously at the interface and in the aqueous phase as the rate-determining steps.
Introduction Recently, we have investigated the solvent extraction of palladium(I1) from aqueous chloride media with several kinds of extracting reagents, such as sulfur-containing extractants (Baba and Inoue, 1988) and a hydroxy oxime (Inoue et al., 1985). From these studies, it was found that extraction mechanisms were significantly affected by the interfacial activities of the extracting reagents as well as by their aqueous solubilities. In the extraction with dihexyl sulfide, triisobutylphosphine sulfide, and 2-hydroxy-5nonylacetophenone oxime, all of which have low aqueous solubilities and relatively high interfacial activities, the kinetic data were reasonably interpreted in terms of the heterogeneous interfacial reaction model, where the ratedetermining complex formation reaction takes place at the interface between the aqueous and organic phases. On the Joint Research and Development Center.
* Department of Applied Chemistry.
0888-5885/90/2629-2111$02.50/0
other hand, in the extraction with 1,2-bis(tert-hexylthio)ethane, which has almost no interfacial activity and a relatively high aqueous solubility, the data were interpreted in terms of the classical homogeneous heterophase reaction model of complex formation, which takes place in the aqueous phase. Solvent extracting reagents usually possess both hydrophilic functional groups and hydrophobic alkyl or aryl radicals. The hydrophilic polar head groups of the extractants play the role of interacting with the metal ions in the aqueous solution to give rise to the metal complexes that are soluble in organic solution. The hydrophobic part of an extractant is required to maximize the solubility of the formed metal complexes in the organic solution and also to minimize the solubility losses of the extractant in the aqueous solution. Such compounds clearly exhibit interfacial activity; that is, the interfacial concentration of the extractants is in large excess compared with that in the bulk phase of the organic solution owing to adsorption at the interface between the aqueous phase and 0 1990 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
organic phases. Recently, Szymanowski (1985) attempted to experimentally obtain the relationship between the rate constants of copper extraction with homologues of hydroxy oximes with different alkyl chain lengths and the values of hydrophilic-lipophilic balance (HLB), which provides a criterion for the interfacial activity. The correlation is useful for the molecular design of metal extractants and for the qualitative interpretation of the extraction mechanism, but the HLB value is not always satisfactory for quantitatively predicting the rate constants of copper extraction. The most important factor in the solvent extraction of metals is the selectivity of the extractant for a specified metal ion to be recovered. We found that nonchelating oximes can strongly and selectively extract palladium(I1) in a previous paper (Inoue et al., 1988). Although several studies have been conducted to date on the solvent extraction kinetics of palladium(I1) with chelating extractants such as hydroxy oximes (Inoue and Maruuchi, 1986; Ma and Freiser, 1983) and an alkylated 8-quinolinol (Ma and Freiser, 1984; Al-Bazi and Freiser, 19861, no quantitative investigations have been conducted with nonchelating oximes in spite of the importance of this system. The objectives of the present work were to make a comparative investigation of the kinetics of palladium(I1) extraction from ammonium chloride media by four kinds of aldoximes with different alkyl radical lengths, which gives rise to different aqueous solubilities and interfacial properties (dodecanal oxime, decanal oxime, octanal oxime, and hexanal oxime, abbreviated C12NOH,C,,NOH, C8NOH, and C,NOH, respectively, hereafter), to elucidate the extraction mechanisms, and to find some correlations with the above-mentioned properties of the extractants. Prior to the study of the extraction kinetics, the distribution equilibrium of palladium(I1) and the aqueous solubility and the interfacial adsorption equilibrium of the extractant were investigated to obtain fundamental information necessary for analysis of the kinetic data of the extraction. Experimental Section (i) Reagents. The alkylaldoximes were synthesized from the corresponding alkylaldehyde in ethanol and an aqueous mixture of hydroxylamine hydrochloride and sodium acetate by a conventional method according to the following reaction: RCHO
+ NH,OHHCl + CH,COONa RCH=NOH
-
+ H 2 0 + CH,COOH + NaCl
After washing with distilled water and phase separation, the product was purified by means of recrystallization from ethanol. The purified products were identified by the IR and NMR spectroscopies; the purity was confirmed by elemental analysis. Found for C12H2,NO: C, 72.2%; H, 12.7%; N, 7.0%. Calculated: C, 72.3%; H, 12.6%; N, 7.0%. Found for C,J&NO: C, 70.0%; H, 12.4%; N, 8.0%. Calculated: C, 70.1%; H, 12.4%; N, 8.2%. Found for C8H,,NO: C, 67.1%; H, 11.9%; N, 9.6%. Calculated: C, 67.1%; H, 12.0%; N, 9.8%. Found for C6H13NO: C, 62.5%; H, 11.2%; N, 12.2%. Calculated: C, 62.6%; H, 11.3%; N, 12.2%. The organic phase was prepared on a gravimetric basis by diluting the purified reagent with analytical grade toluene. The aqueous phase was prepared by dissolving palladium(I1) chloride in aqueous ammonium chloride solutions containing a small amount of hydrochloric acid (about 0.01 mol/dm3). (ii) Measurement of the Distribution Equilibria of Palladium(I1). Under the condition of an excess con-
centration of oximes over the palladium(11) concentration in the aqueous phase, the palladium(I1) was almost completely extracted over the whole concentration region of chloride ion in the present experiment (0.01-6 mol/dm3). It was impossible to make a precise quantitative analysis of palladium(I1) in the aqueous phase. Accordingly, in order to determine the mole ratio of palladium(I1) to the extractant of the extracted complex, only a loading test was carried out under the condition of a large excess of the palladium(I1) concentration as follows. Equal volumes (0.015 dm3) of the two phases of known concentrations were vigorously shaken for 6 h in a separatory funnel using a IWAKI V-DV-type mechanical shaker in a thermostated air bath maintained a t 303 K. It had been confirmed by a preliminary experiment that the equilibrium was attained within about 4 h. After the separation of the two phases, the palladium(I1) concentration in the aqueous phase was determined by atomic absorption spectroscopy using a Nippon Jarrell-Ash Model AA-782 spectrophotometer. The palladium(I1) concentration in the organic phase was determined by a similar manner as mentioned above after stripping with 0.5 mol/dm3 aqueous thiourea solution. (iii) Measurement of t h e Aqueous Distribution of the Extractant. The distribution equilibrium of each extractant between the organic and aqueous phases was measured spectrophotometrically at 303 K by the same method described in our previous paper (Inoue et al.,1988). A toluene solution of each extractant of a known concentration and a 1 mol/dm3 aqueous ammonium chloride solution were shaken vigorously for 24 h using the mechanical shaker with a volume ratio of the aqueous phase to the organic phase of 1O:l. After phase separation, a sample of the aqueous phase (0.19 dm3) was transferred to a separatory funnel. In order to completely convert any extractant dissolved in the aqueous phase to its palladium(I1) complexes, 5 X dm3 of a 1 X mol/dm3 aqueous palladium(I1) chloride solution containing 0.01 mol/dm3 hydrochloric acid and 1 x low2dm3 of toluene was added to the aqueous sample, and the mixture was shaken for 24 h by using a mechanical shaker. After phase separation, palladium(I1) was stripped from the toluene solution with a 0.5 mol/dm3 thiourea solution and analyzed by the same method as above. (iv) Measurement of t h e Interfacial Adsorption Equilibrium. The interfacial tension between the organic solution and the 1 mol/dm3 aqueous ammonium chloride solution containing 0.01 mol/dm3 hydrochloric acid was measured at 303 K by the pendant-drop method to examine the interfacial adsorption equilibria of each extractant. (v) Measurement of t h e Extraction Rate. As is shown in Figure 1,the extraction rate of palladium(I1) was measured by using a batch-type stirred glass cell with an inner diameter of 10 cm and a depth of 15 cm; it was fit with four baffles made of poly(tetrafluoroethylene), each of which was 1 cm wide and 15 cm long. Stirring was carried out at a constant stirring speed by using a turbine impeller with six flat blades connected to a speed controller. Equal volumes (0.3 dm3) of aqueous ammonium chloride solution containing 0.01 mol/dm3 hydrochloric acid and a toluene solution of the extractant were carefully introduced into the cell so as not to disturb the interface. After the initiation of stirring, a small amount of the samples was taken out at definite time intervals in order to measure the time variation of the palladium(I1) concentration in the aqueous phase. The concentration was determined by means of atomic absorption spectrophotomet r y .
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2113 /
A/
-7' -3
/
A'
, -2
I
-1
Logtml, [molldm31
Figure 3. Aqueous distribution of CBNOH,and CloNOH to 1 mol/dm3 aqueous ammonium chloride solution.
J
5
1
M Figure 1. Experimental apparatus. 1, stirred glass cell; 2, baffle; 3, impeller; 4, reservoir with jacket; 5, thermostated bath.
Table I. Physical Properties of Alkylaldoximes oximes KD Kad,dm3/mol Sm, cm2/mol 8.5 X lo4 4.1 31.7 CeNOH CBNOH 8.0 X lo-' 24.8 70.8 CloNOH 2.5 X 10" 24.8 70.8 ClzNOH 24.8 70.8
of aldoximes between the organic and aqueous phases is expressed as follows:
HR
Go=5~10-~1molldm3 3 2.5
0'
- 2.0
Log Cw.0,
-1.5
1
Cmolldm3 1
Figure 2. Typical experimental result of the loading test with CIZNOH.
The initial palladium(I1) concentration was 1 X mol/dm3 in all kinetic runs except the extraction with hexanal oxime (1 X mol/dm3). The concentrations of the extractant and chloride ion were varied over the ranges 2 X 10-5-1 X lo-' mol/dm3 and 1X W3-5 mol/dm3, respectively. Since the extraction rate was found to be independent of the stirring speed over a speed range greater than 1500 rpm, subsequent experiments were carried out at a constant stirring speed of 1500 rpm.
Experimental Results (i) Stoichiometry of the Extracted Species. Figure 2 shows a typical result of the loading test with CI2NOH. The mole ratio of the extractant to palladium(I1) in the organic phase asymptotically approaches 2 with an increase in the aqueous palladium(I1) concentration. This suggests that palladium(I1) is extracted as a 1:2 meta1:reagent complex in the organic phase. Identical experimental results were obtained for the loading test with all other alkylaldoximes. Taking into account the coordination number of palladium(I1) (=4),the stoichiometry of these extraction reactions may be expressed as follows:
+
PdCli(i-2)- 2 a PdC12.2HR + (i - 2)Clwhere HR and the overhead bar denote the extractant species and the species existing in the organic phase, respectively. (ii) Aqueous Distribution of the Extractants. From the measurement of the apparent molecular weight of the aldoximes by means of using vapor-phaseosmometry using a Hitachi Model 117 osmometer, it was found that all of the aldoximes employed in this study exist as monomeric species in the organic phase. Therefore, the distribution
D.HR
KD = [HR] / [HR]
(1)
(2)
where KD is the partition coefficient of aldoxime. From eq 2, we obtain log [HR] = log
[HR] + log KD
(3)
The partition coefficient is so small that the concentration is nearly equal to of aldoxime in the organic phase, its initial concentration, CBO. As may be expressed by eq 3, the plot of the logarithm of the concentration of aldoxime in the aqueous phase against that in the organic phase gives a linear relationship with a slope of unity, as is shown in Figure 3. From the intercept of the straight line with the ordinate in Figure 3, the partition coefficient was evaluated according to eq 3 as is shown in Table I. On the other hand, the aqueous solubility of CI2NOH was too small to measure. (iii) Interfacial Tension. The adsorption equilibria of the extractants are expressed as follows:
[m],
(4)
where Kad is the adsorption equilibrium constant defined by eq 5 on the basis of the Langmuir monolayer adsorption model (5)
and the subscript ad denotes the adsorbed species at the interface. In eq 5, OHR denotes the fractional coverage of the interface by the extractant molecules. The combination of Gibb's adsorption isotherm and the interfacial adsorption equilibrium relationship expressed by eq 5 gives the following relationship (Inoue et al., 1974) between the interfacial tension, y, and the extractant concentration, [HR]:
Y = yo - (RT/SHR)In (1 + K a d [ m l ) (6) where yodenotes the interfacial tension between toluene
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Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
.
-- -2.5I
”VI
.
1’
zoi 0 39
-3-
[si
AI
,,,to,::‘”
,,,,,,,,
0001
, ,
001
(,,,(
01
, , , , , , , , I
1
-3.51
1
Cmol/dm3
C30,
Figure 4. Relationship between interfacial tension and each extractant concentration.
-4
00
CEO’ 0 , o 1 [mol/dm31
I 8
-2
K~~CNHLCII Iinolldn;l31 A 0 A
O
and the aqueous phase and Sm denotes the interfacial area occupied by a unit mole of the extractant. From the experimental results shown in Figure 4,the values of ISadand SHR were evaluated by the nonlinear least-squares method based on eq 6 as is shown in Table I. The solid curves in Figure 4 are those calculated from eq 6 using the evaluated values. The calculated curves are in good agreement with the experimental results. (iv) Extraction Rate of Palladium(I1). By assuming that the forward and reverse reactions are pseudo first order with respect to palladium(I1) in the aqueous and organic phases, respectively, the relationship between the palladium(I1) concentration in the aqueous phase (a) and the contact time ( t ) may be expressed as follows: ai
In
7:
-3.5-
[SI
Figure 5. Typical plot of the experimental results based on the pseudo-first-order rate expression a t constant CI2NOHconcentration (0.01 mol/dm3!.
ai - a,
ai - a, = kFt at - a,
0 Cmol/dm31
Figure 6. Effect of chloride ion concentration, at constant C&OH mol/dm3!, on the observed extraction rate concentration (1.0 x constant
i
t ,
-1 LogCcI-I,
\
O a T C12NOH
/” -LI
#
-3
[NHL,CII = 0.2 CHCII=O.Ol Cmolldd
-2 -1 L o g [C~ZNOHI.Cmol/dm’I
Figure 7. Effect of ClzNOH concentration, at constant chloride ion concentration (0.2 mol/dm3), on the observed extraction rate constant.
(7)
where kf represents the observed reaction rate constant for the forward reaction and the quantities with the subscripts t , i, and e denote the concentrations after time t , at t = 0, and at equilibrium, respectively. Figure 5 shows a typical set of the experimental result in the extraction with CI2NOHplotted based on eq 7 . The plots in this figure give straight lines passing through the point of origin, as expected from eq 7 . Similar results were obtained in the extraction with the other extractants. The apparent pseudo-first-order reaction rate constants (kf) were calculated from the slopes of these straight lines. Figures 6 and 7 show the effects of chloride ion and extractant concentrations, respectively, on the observed reaction rate constants in the extraction with CI2NOH. In
1’
CEO- 0.0 1
-41
-2
Cmol/dm33 I
-1 LogCCI-I,
0 Cmol/dm3:
Figure 8. Effect of chloride ion concentration, at constant extractant concentration (0.01 mol/dm3), on the observed extraction rate constant in the extraction with CsNOH and C,,NOH.
Figure 6, the plotted points lie on a straight line with a slope of -1 in the intermediate concentration region of chloride ion and appear to approach constant values in its high- and low-concentration regions. Hence, it can be concluded that the extraction rates are inverse first order with respect to chloride ion in its intermediate-concentration range and zeroth order in its high- and low-con-
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2115
I
'
-41
-3
-2
LOg[C6NOH], [molldm31
Ih
I
-1
I
I
-2
Figure 11. Effect of C6NOH concentration, at constant chloride ion concentration (1 mol/dm3), on the observed extraction rate constant.
Figure 9. Effect of each extraction concentration, at constant chloride ion concentration (1 mol/dm3 for CBNOHand 0.2 mol/dm3 for C,,NOH), on the observed extraction rate constant in the extraction with C8NOH and CloNOH.
I
I
-3
I
-1
L o g r w l , h"o/dm31
-2
1
-4
I
]
1 Log [CI-I , [mol/dm31
0
Figure 10. Effect of chloride ion concentration, at constant CsNOH concentration (0.01 mol/dm3), on the observed extraction rate constant.
centration range. In Figure 7, the points cluster on a straight line with a slope of 1 in the low-concentration region of the extractant and appear to approach a constant value in its high-concentration region. Figures 8 and 9 show the effects of chloride ion concentration and the extractant concentration respectively on the observed reaction rate constants in the extraction with C8NOH and CloNOH. Figures 10 and 11 show the effects of the chloride ion concentration and the extractant concentration, respectively, on the observed extraction rate constants in the extraction with C6NOH. As is clear from these results, the experimental results for C8NOH and CloNOH show similar tendencies to that of CI2NOH in the whole concentration regions of the extractant and chloride ion concentrations. However, the dependence of the observed extraction rate constant on C6NOH concentration appears to be first order over its whole concentration region. On the other hand, as shown in Figures 7 and 9, the downward deviation from the straight line with a slope of 1 increases with an increase of the number of carbon atoms in the alkyl radicals. This suggests that the contribution of the interfacial reaction increases with increasing the hydrophobicity of the extractant.
Analysis and Discussion of the Experimental Results The extraction mechanism with C12NOHand C6NOH were analyzed as follows. Since it was found in the preceding section that C12NOH has a very low aqueous solubility and a high interfacial activity while C6NOH has a high aqueous solubility and a relatively low interfacial activity, the kinetic data for C12NOHwere analyzed based on the heterogeneous interfacial reaction model while those with C6NOH were analyzed based on the classical heterophase homogeneous reaction model of complexation. Over the whole concentration region of chloride ion under the present experimental conditions, the majority of palladium(I1) exists as a tetrachloro complex, PdC1,2-, and a small amount of aquatrichloro complex, PdC13(H20)-, coexists, while the concentrations of the species Pd2+, PdC1(H20)3+,and PdC12(H20),can be negligible compared with those of PdC13(H20)-or PdC1,2-. Taking into account the complexation of palladium(I1) in the aqueous chloride media expressed by eq 8, the concentrations of the aquatrichloro complex, PdCl,(H,O)-, and the tetrachloro complex, PdC1,2-, are expressed by eqs 10 and 11, respectively: Pd2+ + iC1Pi
=
& pdCli(i-2)-
(8)
[ [PdC1i](i-2)-]
[Pd2+][Cl-Ii
where Pi denotes the stability constants of the ith chloro complex of palladium(I1) [PdC13(HzO)-] =
P3[C1-] 3at/a
[PdC1d2-] = P4[C1-I4a,/a
(10) (11)
where a is defined by the following equation: A
a=1
+ iCp;[Cl-]' =l
Here, as mentioned earlier, a may be approximated by eq 13 over the whole concentration range of chloride ion studied. a
= &[C1-]3 + /34[C1-]4
(13)
In the present study, the following values (Gel'fman and Kiseleva, 1969) were used as the stability constants of the chloro complexes of palladium(I1): PI = 5.01 X lo4,p2 =
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Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
5.01 X lo7,p3 = 2.00 X lolo, p4 = 7.94 X 10”. Among these chloro complexes, the aquatrichloro complex, PdC13(H20)-,is considered to be much more labile than other complexes as Rund (1971) pointed out in his study on the mechanism of substitution reactions of chloro complexes of palladium(I1) with 1,lO-phenanthroline and 2,2’-bipyridyl. (i) Extraction Rate of Palladium(I1) with C12NOH. As mentioned above, it is considered that a certain elementary process taking place a t the interface plays an important role in the extraction mechanism judging from the following results obtained in the preceding section: (1) the aqueous solubility of C12NOHis very low; (2) C12NOH is adsorbed at the interface; and (3) the reaction order with respect to the extractant is less than unity in its highconcentration region. Taking into account these facts, the following reaction scheme was proposed to explain the extraction kinetics of palladium(I1) with C12NOH:
HR
-
2PdC&HR-,d + H2O
(16) (17)
(18) Here, the extractant molecule adsorbed at the interface undergoes the complex-formationreaction with tetrachloro and aquatrichloro complexes of palladium(I1) to form the intermediate 1:1 metdreagent complex, PdC13HR-,at the interface, which is subsequently attacked by extractant molecules in the aqueous phase to form the final complex, PdC12(HR)2,in the organic phase. From the fact that the reaction order with respect to C12NOHis less than unity in its high-concentration region, the simultaneous interfacial steps expressed by eqs 16 and 17 may be considered to be the rate-determining steps, which gives the rate expression described as follows:
where k l and k, are the forward reaction rate constants of elementary reactions described by eqs 16 and 17, respectively. In eq 19, the fractional coverage of the interface by the extractant molecule, BHR, is expressed by eq 20 from the equilibrium relation described by eq 5.
Substitution of eq 20 into eq 19 gives the following rate expression:
Ultimately, eq 21 is approximately expressed by eq 22 using the relations of eqs 10, 11, and 13. cv
Kad[ml 1 + Kad[ml
5 PdC13HR- + H 2 0
+ HR 2PdC13HR- + ClPdClZ(HR)* + C1PdC13HR- + HR
(15)
+ HR,d -% PdC&HR-,d + c1PdC&HR-,d + HR + PdC12(HR)2 + C1-
dr
PdC13(H20)-+ HR PdC12-
Kd
PdC1,’-
--dat
HR E HR
SHR
HR eHR,d PdC13(H20)-+ HR,d
The optimum values of kl and k2 in eq 22 were evaluated by the nonlinear least-squares method from the data shown in Figures 6 and 7, using the Kad value evaluated earlier and the stability constants of the palladium(I1) chloro complexes mentioned earlier as follows: k, = 6.7 X lo-, s-l, k 2 = 7.0 X s-l. The solid lines shown in Figures 6 and 7 were calculated from eq 22 using these values in eq 22. The calculated lines are in fairly good agreement with the experimental results. (ii) Extraction Rate of Palladium(I1) with C,NOH. Since C,NOH has a relatively low interfacial activity and a high aqueous solubility as mentioned earlier, complexation with palladium(I1) was assumed to take place in the aqueous phase as proposed by the following reaction scheme:
(klP3 p3
+ k2p4[cl-I
+ p4[c1-]
at (22)
The effects of varying the concentrations of each reactant species on the extraction rate, shown in Figures 6 and 7, can be qualitatively interpreted in terms of eq 22, derived from the proposed interfacial reaction scheme.
(23) (24)
(25) (26)
Assuming that the parallel reactions of PdC13(H20)-and PdC142-with C,NOH are the rate-determining steps, the rate expression can be described as follows: -da,/dt = k’1[PdCl,(H2O)-][HR] + k’2[PdC1,2-][HR] (28) -da,/dt = (k’#3[Cl-I3/ct
+ k’2@4[C1-]4/cu)~t[HR] (29)
From eqs 2 and 10-13, eq 29 is approximately expressed as
The proposed rate expression, eq 30, suggests that the extraction rate is first order with respect to C6NOH in the organic phase and is inversely first order with respect to chloride ion in its low-concentration region, which is qualitatively in accordance with the experimental results shown in Figures 10 and 11. From the experimental results shown in Figures 10 and 11, the apparent reaction rate constants, k’, and kh, were evaluated by the least-squares method based on eq 30 as k’l = 7.0 X lo4 dm3/(mol s) and k’, = 1.2 X lo2 dm3/(mol s) using the partition coefficients of C6NOH evaluated earlier and the stability constants of palladium(I1)-chloro complexes. These evaluated values are in good agreement for either k’, or k’, with those evaluated in the extraction with 1,2-bis(hexylthio)ethane(k’, = 6.4 X lo4,k\ = 2.6 X lo2). The large difference between k’, and k’, indicates that the aquatrichloro complex, PdC13(H20)-,is much more labile than the tetrachloro complex, PdC1,2-, as mentioned earlier. The solid lines in Figures 10 and 11 are the calculated results from eq 30 using the evaluated values of k; and k $, along with the partition coefficient of C6NOH evaluated earlier. These lines are in fairly good agreement with the experimental results. (iii) Extraction Rate of Pd(I1) with C8NOH and CloNOH. For the kinetic data of the extraction with C8NOH and CloNOH,kl and k2 were evaluated according to eq 22 on the basis of interfacial reaction model. The solid curves in Figures 8 and 9 are the calculated lines according to eq 22 using the values of kl and k 2 thus
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2117
I
0'51 0
I
P
-
t
0
6
6
I
10 12 Carbon number
Figure 12. Relationship between the contribution of the aqueous bulk reaction (8) to the observed extraction rate constant and the number of carbon atoms in the alkyl radicals.
evaluated as well as those of Kad evaluated earlier for CBNOH and C,,NOH, respectively. These curves deviate downward from the plotted points in the high-concentration region of the extractant. This suggests that the aqueous bulk reaction as in the extraction with C6NOH contributes to the overall extraction kinetics to some extent in these extraction systems owing to the relatively high aqueous distribution of C8NOH and CloNOH compared with that of C12NOH. That is, the extraction reaction of palladium(I1) with CBNOH and CloNOH takes place through the simultaneous reaction mechanisms in the aqueous bulk phase and at the interface. In order to estimate the contribution of the aqueous bulk reaction to the overall extraction rate by aldoximes with different numbers of carbon atoms in the alkyl radicals, the contribution of the aqueous bulk reaction (0)to the measured overall reaction rate (V) was defined as follows: = VbP + v,(l - 0) (31) where V, and v b represent the extraction rate via aqueous bulk reaction and interfacial reaction, respectively, as follows:
v
Here, ki and k: (i = 1, 2) represent the rate constants of the interfacial and aqueous bulk reactions, respectively. By using the values of kl and k, obtained for Cl,NOH, as well as k', and k', obtained for C6NOH and Kad for CBNOH and CloNOH, the values of /3 were evaluated so as to suit the experimental kinetic data for these extractants. The results are shown in Figure 12 as the relationship between @ and the number of carbon atoms in the alkyl radicals. The broken curves in Figures 8 and 9 are the calculated results based on eq 31 using the values evaluated for CBNOH and C,,NOH, respectively. These curves are in relatively good agreement with the experimental results for both aldoximes.
Conclusion From the experimental results of the aqueous solubilities and the interfacial adsorption equilibria of four kinds of aldoximes, it was found that ClzNOH has a very low aqueous solubility and a fairly high interfacial activity,
suggesting the possibility that the extraction undergoes the interfacial reaction mechanism with this extractant. On the other hand, was found to have a relatively high aqueous solubility and a low interfacial activity, suggesting the possibility of going through the homogeneous reaction in the aqueous phase with this extractant. The experimental results on the ultimate loading of palladium(I1) suggested that palladium(I1) is extracted as a 1:2 Pdextractant complex, PdC12(HR)2,with all of aldoximes used in the present study. From the experimental results of the extraction kinetics, the following conclusions were obtained. (1)The extraction with C6NOH can be interpreted based on a homogeneous reaction where the parallel reactions of C6NOH with PdC13(H20)-and PdC1,2- to form the 1:l intermediate complex in an aqueous phase are the ratedetermining steps. (2) The extraction reaction with ClzNOH can be reasonably interpreted based on the interfacial reaction mechanism where the elementary reactions between the extractant molecules adsorbed at the interface and PdC13(H20)-and PdC1,2- to form the intermediate complex at the interface are the rate-determining steps. (3) The extraction reaction with CBNOH or CIJOH was found to proceed via rate-determining elementary reactions both in the aqueous bulk phase and at the interface. Consequently, the extraction mechanism of palladium(11) by nonchelating oximes with different alkyl chain lengths can be related to the aqueous solubility rather than the interfacial activity of the aldoximes.
Nomenclature a, = total palladium concentration at time t, mol/dm3 Kad = interfacial adsorption equilibrium constant of the extractant, dm3/mol KD = partition coefficient of aldoxime to the aqueous phase kf = apparent extraction rate constant of the pseudo-fmt-order reaction with respect to palladium(II), s-l kl = forward reaction rate constant of the elementary step k2 = forward reaction rate constant of the elementary step kfl = forward reaction rate constant of the elementary step k', = forward reaction rate constant of the elementary step R = gas constant, N m/(mol K) Sm = interfacial area occupied by unit mole of the extractant, m2/mol T = temperature, K pi = stability constant of the ith chloro complex of palladium, (mol/dm3)-' y = interfacial tension, N/m yo = interfacial tension of the diluent, N/m t9HR = fraction of interfacial area occupied by HR
Superscript
- = organic phase Subscript ad = adsorbed Registry No. Pd, 7440-05-3; C,,NOH, 13372-76-4;CloNOH, 13372-74-2; CBNOH, 929-55-5; CBNOH, 6033-61-0.
Literature Cited Al-Bazi, S. J.; Freiser, H. Trans Effect in Extraction Kinetics: Palladium Extraction by 7-(Vinyl-3,3,5,5-tetramethylhexyl)-8quinolinol. Solu. Extr. Ion Exch. 1986,4, 1121-1138. Baba, Y.; Inoue, K. The Kinetics of Solvent Extraction of Palladium(I1) from Acidic Chloride Media with Sulfur-Containing Extractanta. Znd. Eng. Chem. Res. 1988,27,1613-1620. Gel'fman, M. I.; Kiseleva, N. V. Stability Constants of Palladium Chloride Complexes. Zh.Neorg. Khim. 1969,14, 502-504. Inoue, K.; Maruuchi, T. Solvent Extraction of Palladium with SME 529 Equilibria and Kinetics. Hydrometallurgy 1986,16,93-104.
Ind. Eng. Chem. Res. 1990,29, 2118-2123
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Inoue, K.; Kawano, Y.; Nakashio, F.; Sakai, W. Extraction Equilibrium of Hydrochloric Acid by Long-chain Alkylamine Solution. Kagaku Kogaku 1974,38, 41-46. Inoue, K.; Tomita, S.; Maruuchi, T. Extraction Kinetics of Nickel with a Hydroxyoxime Extractaat. J . Chem. Eng. Jpn. 1985,18, 445-449. Inoue, K.; Baba, Y.; Yoshizuka, K.; Oka, T. The Solvent Extraction of Palladium(I1) from Aqueous Chloride Media with 7-Tridecanone Oxime. Bull. Chem. SOC.J p n . 1988, 61, 803-807. Ma, E.; Freiser, H. Mechanistic Studies on the Extraction of PallaOxime (LIX65N). dium(I1) with 2-Hydroxy-5-nonylbenzophenone Extr. Ion Exch. 1983, 1, 485-496.
Ma, E.; Freiser, H. Solvent Extraction Equilibria and Kinetics in the Palladium(I1)-Hydrochloric Acid-7-( l-Vinyl-3,3,5,5-tetramethylhexyl)-8-quinolinol System. Inorg. Chem. 1984,23, 3344-3347. Rund, J. V. Kinetics of the Reactions of Tetrahalo Complexes of Palladium(I1) and Platinum(I1) with 1,lO-Phenanthroline. Inorg. Chem. 1971, 13, 738-740. Szymanowski, J. The Hydrophilic Lipophilic Balance of Hydroxyoximes and the Mechanism of Copper Extraction. Polyhedron 1985, 4 , 269.
Received for review December 15, 1989 Accepted May 22, 1990
Sintering and Sulfation of Calcium Silicate-Calcium Aluminate R. H. Borgwardt Air and Energy Engineering Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
G. T. Rochelle* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
The reactivity of a solid at high temperature can be greatly retarded by physical changes occurring in its pore structure due to sintering. The nature of that effect was studied with calcium silicatecalcium aluminate reacting with SO2 between 665 and 800 "C, where sintering of this material progresses rapidly. The isothermal kinetics of the sintering and sulfation processes were measured independently as a function of the specific surface area of the solid. The rates of both processes were increased by the presence of water vapor in the gas as well as by higher temperatures. A combined sinter/sulfation model, based on the parameters evaluated independently, is in qualitative agreement with sulfation rates observed when both processes occur simultaneously.
Introduction An earlier study that evaluated the sulfation kinetics of CaO (Borgwardt and Bruce, 1986) showed the rate to increase rapidly with both the temperature and the specific surface area of the solid. At high temperatures, the surface area begins to decline because of sintering, a process caused by coalescence of the CaO micrograins that comprise the porous particles. The rate of surface reduction of CaO was shown (Borgwardt, 1989a) to follow the German-Munir (1976) sintering model given by
where So is the nascent surface area of the CaO existing immediately after thermal decomposition of the precursor compound (e.g., Ca(OH),], S is the specific surface area at time t , y is a mechanism-dependent constant, and k is a function of temperature. In many practical applications, H 2 0 or C 0 2 will be present during sintering, both of which catalyze the surface area reduction of CaO (Anderson et al., 1965; Beruto et al., 1984). Equation 1 can be modified to include the catalytic effects of HzO and C02 on the singering of CaO by empirical correlations of y and k in terms of partial pressure and temperature (Borgwardt, 1989b). The sulfation behavior of small CaO particles at high temperatures is explained by the combined sintering and sulfation kinetics when the surface area effects are accounted for (Borgwardt, 1989b). Prior investigations of the sulfation of calcium silicate (Rochelle et al., 1987) have shown it to be an effective sorbent for flue gas desulfurization at 50-100 "C, especially at high relative humidity. It, might also be used in con-
junction with bag filters at 300-500 "C (Chu and Downs, 1989) and in fluidized beds at 800-950 OC (Yang and Shen, 1979). The study reported here is aimed at an evaluation of the sintering of calcium silicate-calcium aluminate (CSA) as it affects the reactivity of that material with SOz when both processes occur simultaneously. The approach follows the method previously used for evaluating these effects on the CaO reaction. That method assumes three important steps are involved, each of which can be evaluated by independent measurement: (i) thermal decomposition of the solid to produce a nascent surface of maximum specific area, (ii) reduction of the surface area, by sintering, at a rate dependent on temperature and the gas-phase concentrations of HzO and COP,and (iii) reaction of the solid with SO2 at a rate dependent on its specific surface area and temperature.
Experimental Section The CSA consisted of the product formed by the reaction of calcium hydroxide with the silica and alumina in fly ash when 1 part Ca(OH), is slurried with 3 parts fly ash (Clinch River) for 12 h at 90 "C as described by Jozewicz and Chang (1989). Scanning electron micrographs suggest that the hydrated CSA is deposited as a porous layer on the surface of the excess fly ash. Analysis of the dry product showed 13.1 wt 70calcium, a BET surface area of 16.0 m2/g, a pore volume of 0.103 cm3/g, and a true density of 2.62 g/cm3. Additional work with related CSA reagents suggests that silicate, rather than aluminate, is responsible for the surface area and reactivity to SOp (Peterson and Rochelle, 1990). X-ray diffraction of CSA was inconclusive but demonstrated the absence of Ca(OH), and other crystalline phases, suggesting that CSA is mostly amorphous. Se-
0888-5885/90/2629-2118$02.50/0 0 1990 American Chemical Society
