Ind. Eng. Chem. Res. 1988,27, 1675-1680
1675
Kinetics of the Extraction of Uranium(V1) from Nitric Acid Solutions with Bis(2-ethylhexy1)phosphoric Acid Ting-Chia H u a n g * a n d Ching-Tsven H u a n g t Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, R.O.C.
A transfer cell of constant interfacial area was utilized to measure initial rates of the extraction and the stripping with aqueous phases containing 0.1114 mol/dm3 of nitric acid and up to 0.0267 mol/dm3 of uranium and kerosene diluted organic phases containing 0.01265-0.0894 mol/dm3 of dimeric HDEHP and up to 0.00963 mol/dm3 of uranium. The extraction rate is one-half order with respect to the concentration of H D E H P dimer in the organic phase, first order with respect to the concentration of uranium in the aqueous phase, and decreased slightly with increasing aqueous-phase acidity. The stripping rate is first order and zero to inverse one-half order with respect to the concentrations of uranium and HDEHP dimer in the organic phase, respectively, and zero to second order with respect to the concentration of nitric acid in the aqueous phase. A kinetic scheme is proposed, and the apparent rate constants are also evaluated.
Bis(2-ethylhexy1)phosphoric acid (HDEHP or HR) has found extensive use in liquid-liquid extraction, due to its chemical stability, extremely low solubility in acidic solutions, and high selectivities for metal ions. Although kinetics of the extraction of metals with HDEHP has been extensively studied by many investigators (Coleman and Roddy, 1971; Vandegrift and Horwitz, 1977; Islam and Biwas, 1979, 1980a,b;Danesi and Vandegrift, 1981; Komasawa and Otake, 1983; Ajawin et al., 1983; Huang and Juang, 1986a,b) much remains to be done in the kinetic study of the extraction of uranium(V1). Ferguson et al. (1966) reported briefly that the extraction of uranium(V1) from perchloric acid with HDEHP is controlled by a moderately fast reaction; Tarasov et al. (1977) measured the rate of the extraction of uranium(V1) from sulfuric acid with HDEHP and concluded that the rate of the extraction is determined by the interaction of the extractant with the metal cation at the interface. In the studies involving uranium, little information regarding the kinetics of those extractions has been reported. The equilibrium behaviors of the extraction of uranium(V1) with HDEHP from various media, such as sulfate (Blake et al., 1956), perchlorate (Baes et al., 1958) and chloride (Ruvarac et al., 1974), have been investigated. The reaction can be represented by - UOz2+ H2Rz = U02R2 + 2H+ (1) - U02R2 + H2R2 = UO&(HR)2 (2) where the solid upper bar denotes the species in the organic phase. The complex U02R2produced in eq 1is solvated by H2R2to form U02R2(HR)2 provided that free H2R2is available in the solution. As elaborated in our previous study (Huang and Huang, 1987), the following reactions were also observed in the extraction of uranium(V1) from nitric acid solutions with HDEHP, Le., U022++ 2H2R2+ HN03 = U02R2(HR)2.HN03+ 2H+ (3) 2UOZ2++ 2 m 2 + HN03 = (U02R2)2-HN03 + 2H+ (4) UOZ2+ + 2HN03 = U02R2.2HN03+ 2H+ (5) The extent to which these reactions proceed depends on the concentrations of the uranyl nitrate and the nitric
+
+
* To whom the correspondence should be addressed. 'Current address: Institute of Nuclear Energy Research, P.O. Box 3-22, Lung-Tan, Taiwan 32500, R.O.C. 0888-5885/88/262~-1675$01.50/0
acid in the aqueous phase. Reactions 1 and 2 are predominant for high and low uranium(V1) loadings, respectively, at low concentrations of nitric acid in the aqueous phase. At high uranium(V1) loadings, the resulting complex is primarily U02R2,which is solvated by HDEHP to form U02R,(HR), at low uranium(V1) loadings. Reactions 3 and 4 become important for low and high uranium(V1) loadings, respectively, as the concentration of nitric acid in the aqueous phase exceeds 1 mol/dm3. Equation 5 represents the reaction responsible for the complete saturation of HDEHP with uranium(V1) and nitric acid. Nitric acid is also solvated by HDEHP in this extraction system; the reaction is + 2HN03 = 2HR.HN03 (6) This reaction is insignificant when the concentration of nitric acid in the aqueous phase is below 3 mol/dm3 and becomes important above 3 mol/dm3. In the present study, initial rates of the extraction and the stripping processes were measured with a transfer cell of constant interfacial area. A kinetic scheme in agreement with the experimental result was proposed based on the equilibrium reactions expressed in eq 1-5.
m2
Experimental Section 1. Reagents and Apparatus. A glass stirred transfer cell as shown in Figure 1 was used for this study. Teflon limiting rings with different open areas were used to adjust the interfacial area to 7.81 or 13.35 cm2 alternatively. HDEHP, supplied by Daihachi Chem. Co., Japan, was purified following the procedures described by McDowell et al. (1976). Kerosene purchased from China Petroleum Co., R.O.C., was washed twice with 1/5 vol of 98% sulfuric acid and then with distilled water until it was neutral. Uranyl nitrate obtained from the Institute of Nuclear Energy Research, Taiwan, R.O.C., was purified with TBP extraction and then by recrystallization. All other reagents purchased from E. Merck Co. were GR grade and used without further treatment. Organic solutions used for the extraction process were freshly prepared by diluting HDEHP with kerosene. They were preequilibrated by mixing with the aqueous phase solutions free from uranyl nitrate and then separated by centrifugation. Appropriate amounts o f uranyl nitrate were added to the aqueous solutions which would be used for the extraction process. The kerosene diluted HDEHP solutions used for the stripping process were also preequilibrated by mixing with the aqueous solutions containing varying concentrations 0 1988 American Chemical Society
1676
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 Motor
A 7.5
-?15
I 7.5
T
.
0 0
5
10
15
20
25
30
Time, min.
Jacket
Figure 3. Typical variation of the organic-phase UV absorbance with time. Figure 1. Experimental apparatus. 0 5 r
,
I
a 0 40
60
80
100
120
140
160
Stlrrer Speed, rpm
Figure 4. Effect of the agitation speed on the initial rate of the extraction process.
0 0
2
4
6
8
[5]x 10,
1
0
1
2
g/dm3
Figure 2. Plot of UV absorbance versus uranium(V1) concentration in the organic phase.
of uranyl nitrate and nitric acid and then were also separated by centrifugation. The concentration of uranium(VI) was adjusted by diluting with organic solutions, which contained the same concentration of HDEHP and had been preequilibrated with stripping solutions. The stripping solutions were nitric acid solutions preequilibrated with HDEHP-kerosene mixtures. 2. Procedure. The aqueous phase (100 mL) was placed in the transfer cell first, and the stirrer was started. The organic phase (30-50 mL) was then carefully added so as to avoid disturbance of the interface. The timing was started upon the beginning of the addition of the organic phase. Samples were taken from the organic phase at time intervals of 2-5 min, analyzed spectrophotometrically, and then returned to the cell. All experiments were performed at 25 f 0.2 "C. 3. Analysis of Uranium. The concentration of uranium(V1) in the organic phase was analyzed with a UV spectrophotometer, Shimadzu UV-240, at 323 nm. Its absorbance at this wavelength is affected by the concentration of nitric acid in the organic phase. However, the absorbance is directly proportional to the concentration of uranium(V1) below 3 g/dm3, only if the aqueous phase in contact with the organic phase contains exactly the equal concentrations of nitric acid. The plot of absorbance versus the concentrations of uranium(V1) extracted into the organic phase is shown in Figure 2. 4. Measurements of the Initial Rates. Initial rates of both the extraction and the stripping processes were determined by measuring the variation of the absorbance ( A ) of the organic phase at 323 nm with time ( t ) . The
relationship between A and t is linear in quite a long time period as demonstrated in Figure 3. But, for some stripping processes performed at low concentration of nitric acid in the stripping solution, and the high concentration of the free HDEHP in the organic phase, the time period for linearity becomes much shorter. In those cases, samplings should be finished in several minutes. The initial fluxes of the extraction (R)and the stripping ( S )can be calculated with the equation
R (or -S) = q(p/A)(dA/dt) where Y is the volume of the organic phase, A is the interfacial area, and q is the slope of the line of absorbance versus concentration plotted for each run. Results and Discussion 1. Extraction Regime. On the basis of the rate-controlling step, the extraction regime is usually classified as diffusional, kinetic, and mixed regimes (Danesi and Chiarizia, 1980). The extraction regime in the present study was identified by measuring the variation of the extraction rate with the intensity of agitation. Figure 4 shows the effect of the agitation speed on the rate of the extraction. It will be observed that the curve of the extraction rate rises initially with increasing agitation speed from 52 to 90 rpm and then reaches a plateau region from 90 to 133 rpm. According to Danesi and Chiarizia (1980), the presence of the plateau region indicates that the extraction is located in the kinetic regime at the agitation speeds from 90 to 133 rpm. At agitation speeds above 133 rpm, the extraction rate increased again rapidly, due to the increase of the interfacial area caused by overagitation. Therefore, the agitration speed of 110 rpm was subsequently adopted in all runs.
Ind. Eng. Chem. Res., Vol. 27, NO. 9, 1988 1677
[m]
0.05 m o l / d d
0
10
5
PP
15
A,”
Figure 5. Variation of the initial rate of the extraction process with the interfacial area.
The variation of the extraction rate with the interfacial area was also observed. Figure 5 shows that the total amount of uranium(V1) transferred per unit time is directly proportional to the open area of the limiting ring, i.e., 7.81 and 13.35 cm2, respectively, indicating that the rate-controlling reaction does not take place in the bulk phases. Because of the high interfacial activity and the extremely low concentration of HDEHP in the aqueous phase, the reaction is very likely to take place a t the interfacial region. 2. Diffusional Resistance. In order to make sure that the extraction process is located in the kinetic regime, the contribution of the diffusional resistance to the overall extraction rate was examined. The rate of extraction occurring at the interface may be limited by the physical-transfer rate of the extractant from the bulk organic phase to the interface. Hence, the maximum possible extraction rate can be expressed as R = k’[H2R2]in this study, where k’is the interfacial masstransfer coefficient of HDEHP in this extraction system and can be evaluated with the empirical correlation given by Asai et al. (1983), who used an agitated vessel geometrically similar to the transfer cell of our own in evaluating the value of k’. The physical properties of this extraction system used in the evaluation of k’are the viscosity of the organic phase, 1.2 cP; the viscosity of the aqueous phase, 1cP; the density of the organic phase, 0.8 g/cm3; and the density of the aqueous phase, 1g/cm3. Since kerosene is a mixture of saturated hydrocarbons containing chiefly ClO-Cl6alkanes, the properties of n-dodecane were used to represent those of kerosene in calculating the diffusivity of HDEHP in the organic phase. The diffusivity thus estimated with the empirical equation given by Scheibel (1954) is 4.2 X lo+ cm2/s. The interfacial tension was N/m, by referring to those estimated to be 3.84 X tensions reported by Asai et al. (1983). Consequently, the value of k’evaluated is 6.18 X lo4 cm/s and the maximum extraction rate is 3.09 X lo4 mol/(cm2.s)for [ HzR2]= 0.05 mol/dm3. The observed extraction rate shown in Figure 4 is 5.83 X lo4 mol/(cm2-s),which is only about one-fifth of the estimated maximum extraction rate. Thus, the contribution of the diffusional resistance to the observed rate is negligible, and the process is located in the kinetic regime. This conclusion was further ensured by the measurement of the transfer flux of uranium(V1) across a solidsupported liquid membrane, containing kerosene-diluted HDEHP solution as a mobile carrier (Huang and Huang, 1988). The transfer flux of this solid-supported liquid membrane was found to be also limited by the rate of the reaction occurring at the feed-side interface. 3. Initial Rates of the Extraction Process. Initial rates of the extraction process were measured independently at varying concentrations of uranyl nitrate and nitric acid in the aqueous phase, and of dimeric HDEHP in the organic phase. Figure 6 shows the effect of the concen-
I
1 1
2
4
[uoH+]
20
8 1 0
6
x 103, moildm3
Figure 6. Effect of the aqueous-phase uranium(V1) concentration on the initial rate of the extraction process. .
.
.
,
,
. . 2 y c
[ H + ] : 0 . 5 mo1/dm3
9”
m
-1 1
2
4
6
[m]x 102,
810
20
mo1/dm3
Figure 7. Effect of the free HDEHP concentration on the initial rate of the extraction process. 1
I
I 1
2
4 [H+] x
6
8 1 0
io,
molldm3
20
40
Figure 8. Effect of the aqueous-phase nitric acid concentration on the initial rate of the extraction process.
tration of the aqueous-phase uranyl nitrate, the slopes of the lines being nearly 1. Figures 7 and 8 show that the initial rate of the extraction is proportional to the square root of the concentrations of the free dimeric HDEHP in the organic phase and decreased slightly with the increasing concentration of nitric acid in the aqueous phase. Also, it was observed that the variation of the ionic strength in the aqueous phase does not significantly affect the extraction rate. 4. Initial Rates of the Stripping Process. The comprises equilibrium reaction of this extraction system the solvation of UO,R, - - and UO,R,.HNO, - - by - HzRl - - to form U02R2(HR)2and U02R2(HR)2.HN03.It was observed in the equilibrium study that the solvating HDEHP in these comdexes is able to-extract uranium(V1). Based on this . . and the coexistence of U02R2and U02R2(HR),,it can be concluded that the solvating HDEHP is not firmly held
1678 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988
.
t
2 '
'
1
"
"
I
"
I
'
2
4
[U]
6
8
10
20
I 2
'
'
4
x 104, moi/am3
I
" " "
6 810
[Wltx 103,
20
4
6
8 10
[H+] x
Figure 9. Effect of the organic-phase uranium(V1) concentration on the initial rate of the stripping process.
1
2
40 60
moi/dm3
Figure 10. Effect of the effective HDEHP concentration on the initial rate of the stripping process.
by the central molecule. The kinetic mechanism reflects the formation of U02Rzand UOzR2-HN03instead of their H2R2-solvated complexes, U0zR2(HR)2 and U02R2(HR)2.HN03. The effect of the dimeric HDEHP concentration on the stripping rate was measured with the varying concentrations of the total effective HDEHP dimer, which includes the solvating and the free ones. The concentration of nitric acid in the aqueous phase was maintained below 3 mol/dm3, so that HDEHP was not appreciably complexed by nitric acid. Thus, the stripping rates were measured with the varying concentrations of the total uranium(VI), the total effective HDEHP dimer in the organic phase, and the nitric acid in the aqueous phase. The experimental results show that the initial rate of the stripping process is directly proportional to the concentration of uranium(V1) in the organic phase as indicated in Figure 9. It is inversely proportional to the square root of the total effective HDEHP dimer at high concentration but independent of it at low concentrations as indicated in Figure 10. Figure 11 shows that the stripping rate is proportional to the second power of the concentration of nitric acid in the aqueous phase below 0.5 mol/dm3, but nearly independennt of i t at 4-6 mol/dm3. The total effective HDEHP dimer, [ H2R2It,indicated in Figure 10 comprised the free and the solvating ones in different proportions. It was observed that the considerable difference of the proportions did not show a significant effect on the characteristics of the stripping rate variation with the concentration of the total effective HDEHP dimer, suggesting that the solvation reactions, including __ those between HzRzand HN03 as well as the uranium(V1)
40
20
60
10'
i o , mo1/dm3
Figure 11. Effect of the aqueous-phase nitric acid concentration on the initial rate of the stripping process.
complexes, do not affect the kinetic behavior significantly. 5. Kinetic Scheme. HDEHP is strongly dimerized in organic solvents. The dimerization constant of HDEHP is higher in weakly polar solvents and lower in more polar ones. For instance, the dimerization constant is 3.89 X lo4 m3/kmol for HDEHP in isooctane, 2.09 X lo4m3/kmol in chloroform (Ul'yanov and Sviridova, 1963), and 2.63 X lo4 m3/kmol in kerosene (Huang and Juang, 1986b). In a biphasic system, the dimeric HDEHP diffuses to the interface, where it dissociates to monomers which may even ionize. The degree of the ionization of HDEHP at the interface depends upon the aqueous-phase acidity. At the concentration range of nitric acid covered in this study, the HDEHP species would be totally unionized. The interface between the n-dodecane-diluted HDEHP solution and some acidic aqueous solutions was assumed to be saturated with HDEHP at very low concentration of HDEHP in the bulk organic phase (Vandegrift and Horwitz, 1980). But it appears that the characteristics of the interfacial adsorption depend on the properties of the aqueous phase as well as those of the organic phase. For example, the interface has never been detectably saturated when HDEHP is diluted with o-xylene (Vandegrift and Horwitz, 1980), and Komasawa and Otake (1983) reported that no interfacial saturation was ever observed when they made extraction of copper, nickel, and cobalt from acidic nitrate media using organic solutions of HDEHP in toluene or n-heptane. Later, the concentration of the HDEHP adsorbed at the interface was considered to be proportional to its concentration in the bulk organic phase by Ajawin et al. (1983) and Huang and Juang (1986a). Since the mechanism of the interfacial adsorption has not yet been clearly understood, it appears to be the principle that the consistence with the kinetic behavior is primarily considered in determining the interfacial saturation. In this study, the extraction rate has been found to be proportional to the concentration of HDEHP dimer in the bulk organic phase. For an extraction process located in the kinetic regime, this phenomenon suggests that the interface is not saturated. Considering only the dissociation of HzR2to HR at the interface, we may express the equilibrium between the dimeric HDEHP in the bulk organic phase and the monomeric HDEHP adsorbed at the interface as K1
= zHR
(fast)
(7)
where the dashed upper bar indicates the interfacially adsorbed species. Following that suggested by V_amdegrift and Horwitz (1980), we assume that the species HR.HN03 is formed at the interface in the presence of nitric acid, i.e.,
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1679 HR
- --- + 'H+ + NO3- K*= HR-HNO,
(fast)
(8)
The following mechanism is proposed such that the resulting rate expressions will be consistent with the experimental result. Uranyl cations in the aqueous phase-diffuse to the interface and react with HR and HRTHNO, concurrently as follows: ks
UO+ :
- _
-
+ HR k-3= U02R++ H+
(slow)
(9)
k , - - -- - - -
+
UOz2+ H R y H N O 3 = U02R*HN03++ H+ (slow) (10) k-4
k6
U02R+ + HR = D02Rz+ H+ (slow)
+
+
Q = k4[H+] K 1 1 / 2 k 7 [ m ] 1 /+2 K1'/2K2ka[H+][NO3-][H,R2]1/2(25)
(11)
-- --- - + H R I H N 0 3 ks UO2R2.HN03 + H+(slow)
Because [UO?+] = 0 in measuring the initial rate of the stripping process, therefore eq 23 becomes
(12)
_-___ + HR k7= UOzR2.HNO3 + H+ (Slow)
S = k-,[H+I2(k-,[ UOzRz]/K9 h-6[ UOzRz.HNO3 I /Kio) / P +
(13)
k-6
UO,R+
S = (K11/2k3k-3[H+][U0~+][~]1/2 + k - 3 k - 5 [ H + ] 2 [ m ] / K 9+ k-3k+[H+I2[UO2Rz.HN03 ]/K1o)/P (K1'/'K&4k-4[ H+] [NO3- ] [UOz2+]X [ m ] ' / 2 + k-4k-7[H+]2[U02R2*HN03]/K10 + k-&-a[H+Iz[ UOzR2.2HN03 l/Kii)/Q (23) where P = k-,[H+] + K 1 ' / 2 k 5 [ m ] 1 / 2+ K11/2Kzk6[H+][N03-] [H,R2]1/2 (24)
UO,R;HNO3+
k-7
UOzR:HNO3+
+ HRLHNO3
k8
UO2Ri-ZHNO3
+ H+ (Slow) (14)
The neutral intermediates leave the interface and go into the bulk organic phase; hence, we may write Kg
-
UOzRz = U02R2 (fast) UO2R2:HNO3
KlO
(15)
= U02R2.HN03 (fast)
(16)
2
(17)
U02Ri*ZHN03 U02R2.2HN03 (fast)
The complex UOzR2.2HN03is the resulting complex of the overallreaction indicated in eq 5; U02R2will be solvated by HzR2 following eq 2, and UO2Rz-HNO3will be solvated by H2R2or combined with UOzR2 to give the resulting complexes indicated in eq 3 and 4 by way of the following equations: UO2Rz.HN03 + U02Rz(HR)2-HNO3 (18) UO2R2 + UOZR2.HN03 = (U02R2)2*HNO, (19) The rates of the production and the disappearance of uranium(V1) in the organic phase are not affected by reactions 2, 18, and 19; therefore, they are not taken into account for the rate measurement. Regarding reactions 7,8,15,16, and 17 as fast steps and reactions 9-14 as slow steps, we have the extraction rate law, namely,
R = - ( V / A ) d[U02"]/dt
=
k3[UO22+][HR] + k4[U022+][HRyHN03] (20)
Expressing [HR] and [HRyHN03] in terms of the bulk concentrations [ H2R2], [H+], and [NO3-] according to equilibrium reactions 7 and 8, we obtain R = K 1 1 ~ 2 k 3 [ U 0 z 2 + ] [ ~+2 ] 1 ~ 2 K 1 1 / 2 K z k 4 [ U 0 2 2 +]1/2[H+][N03-] ][~ (21) The stripping rate law is S = ( V / A ) d[U02'+]/dt = k-3[UOzR+][H+]+ k-,[U02R~HN03+][H+] (22) -- Applying the steady-state approximation to [UOzR+] and [UOzRsHN03+],we obtain
+
k-4[H+]2(k_7[U0~R~~HN03]/K~~ + k-a[ UOzRz.2HNO3 1/Kii)/Q (26)
The previous study by Huang and Huang (1987) showed that the equilibrium distribution ratio of the concentration of nitric acid in the organic phase to that in the aqueous phase is very small, i.e.,
