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Extraction equilibrium of cobalt(II) from sulphate solutions by di(2-etrylhexyl)phosphoric acid dissolved in kerosene
Polyhedron Vol. 9, No. 9, pp. 1147-1153, Printed in Great Britain
1990 0
0277-5387/W $3.00+.00 1990 Pergamoo Press plc
EXTRACTION EQUILIBRIUM OF COBALT@) FROM SULPHATE SOLUTIONS BY DI(2-ETHYLHEXY’L) PHOSPHORIC ACID DISSOLVED IN KEROSENE TING-CHIA Department
I-WANG*
and TEH-HUA
TSAI
of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republic of China (Received
18 October
1989 ; accepted 8 December
1989)
Abstract-The distribution of cobalt(I1) between 0.5 kmol mW3aqueous sulphate medium and kerosene solutions of di(2-ethylhexyl)phosphoric acid (HDEHP) has been studied at 25°C. The extraction equilibrium is influenced by the total extractant concentration, equilibrium pH value and total metal concentration in the aqueous phase. The distribution data have been analysed both graphically and numerically. The results show that the species extracted into the organic phase have the composition CORED and CORED. The stability constants of HSO;, NaSO; and CoS04 in sulphate solutions have also been estimated from the extraction data.
Di(2-ethylhexyl)phosphoric acid (abbreviated as HDEHP or simply HR) has been studied extensively as an extraction reagent in hydrometallurgical processes for the separation and purification of a number of metals. It extracts first-row transition metals such as copper, cobalt, nickel and zinc, as well as
uranium and rare earths in familiar nuclear fuel reprocessing, in a wide range of operating conditions. l-7 The extraction of cobalt with HDEHP has been studied by many investigators. Madigan’ studied the distribution of cobalt between an aqueous sulphate solution and a solution of HDEHP in kerosene, and proposed that CoR, compounds * Author to whom correspondence should be addressed. are formed in the organic phase. Brisk and McManamey,’ working in sulphate media, found Nomenclature that a single dimeric acid molecule was formed by D : Distribution ratio (-) ; K. : acid dissociation conthe extracted species in aliphatic diluent. Grimm stant of monomeric HDEHP in the aqueous phase (kmol m- ‘) ; 1yd: distribution constant of monomeric HDEHP and Kolai-ik” and Komasawa et al.,” using both between the organic and aqueous phases (-) ; K2 : dimeraliphatic and aromatic diluents, explained the exization constant of HDEHP in the diluent (m’ kmol- ‘) ; traction of Co’+ from nitrate medium by assuming com- the formation of CoR2(HR)2. Sato and Nakamura” K ,,,Pq: equilibrium constant of cobalt-HDEHP plexes ; m : degree of aggregation of the extracted species proposed the extraction of CoR2(H20)2 after spectro(-) ; p : number of R group except HR molecule involved scopic studies on the solid compound obtained by in the extracted species (-) ; q : number of free monoloading the metal in the organic phase. Komasawa meric HDEHP involved in the extracted species (-); et a1.‘3 studied the effect of diluents on extraction Co” : metal species ; U : error square sum (- ) ; [ 1,: total from nitrate media and showed the existence of concentration of the species in the brackets (km01 mM3). CORED formed in alcohol diluents and nonGreek letters polar or weak polar diluents. p: Stability constants of sulphate complexes (m3 Further experimental studies seem necessary to kmol- ‘) ; c : standard deviation (-). clarify the uncertainties regarding species comSuperscripts position. In this work, the extraction of cobalt by ~ : Denotes the organic phase species or organic phase HDEHP is studied in detail to determine the comconcentration. position of the species extracted by HDEHP in kerosene. In order to obtain a complete underSubscripts min : Minimum value. standing of the system, the extraction of cobalt was 1147
TING-CHIA HUANG and TEH-HUA TSAI
1148
studied first by graphic and slope analysis, which assumed no complexes were in the aqueous phase. Then, by numerical analysis of the experimental data, values for the stability constants of the HSO;, NaSO; and CoSO, complexes in the aqueous sulphate solutions can be obtained. EXPERIMENTAL Reagents and solutions
Di(2-ethylhexyl)phosphoric acid used in this work was the product of Daihachi Chemical Ind. Co. Ltd., Osaka, Japan. A purity of 95% was found by potentiometric titration with NaOH in ethanolic medium. HDEHP was further precipitated as a copper complex from toluene and acetone solution, and then dissolved in toluene and a 4 km01 m- 3sulphuric acid solution, following the procedure of McDowell et al. I4 Kerosene (Chinese Petroleum Co., Taiwan, R.O.C.) was washed twice with l/5 volume 98% H2S04, then with distilled water until it was neutral. ’ 5 The other inorganic chemicals were analytical reagent grade supplied by Hayashi Pure Chemical Ind. Ltd., Osaka, Japan. Metal solutions were always prepared by weighing the appropriate volumes of the stock solution and making up with diluent.
titative and the mass balance for the metal was always within f 2%. RESULTS
AND DISCUSSION
Extraction equilibrium of cobalt
Metal complexes of HDEHP tend to form very large polymeric species in a fully loaded organic phase. These polymers are strongly depolymerized in the presence of free HDEHP molecules and ethylene glycol.‘6 Thus, it is necessary to assume that cobalt is extracted as an m-merized complex into the slightly loaded organic phase. The extraction of cobalt(I1) with HDEHP can be represented by the following general reaction : mCo2++m(p+q)/2H2R2-_‘ (CoR,(HW,),+
mpH+
(1)
where
(2) is the stoichiometric equilibrium constant referring to 0.5 kmol me3 (Na,H)SO, with kerosene as diluent, and the bar indicates the species in the organic phase. The distribution ratio of cobalt is defined as
Procedure
Twenty millilitres of the organic solution and an equal volume of aqueous solution were mixed in glass flasks with ground glass stoppers and shaken by a mechanical shaker for at least 30 min at 25.0 + 0.2”C. Preliminary experiments have shown the time needed to reach equilibrium to be less than 15 min. The organic solutions containd a 0.01-0.35 kmol m- 3monomeric form of HDEHP in kerosene. The aqueous phase consisted of 0.5 kmol m- 3(Na+, H+, Co*+)SO:-. The total anion concentrations were kept constant. The concentrations of cobalt ion in the initial aqueous solutions ranged from 8.5 x lop4 to 1.7 x lo-’ kmol rnp3. The two phases were separated after they had been allowed to settle for 4 h in a thermostat at 25.0 +0.2”C. After phase separation, the equilibrium hydrogen ion concentration was measured by a pH meter. The concentration of cobalt was measured with a IL-551 atomic absorption spectrophotometer (Instrumentation Laboratory Inc.) at a wavelength of 240.7 nm, coupled with background correction. Cobalt in the organic phases was stripped with 4 kmol m- 3 sulphuric acid, and the metal concentration in the acidic solution was analysed by AAS. The process had been shown to be quan-
(3) Substituting eq. (2) into (3) we obtain D
mNCoRp(HWq)mI
=
CO
[Co2f] = mK,,,pq[Co2+]“- ‘[H2R2]m(P+q~~2[H+]-“‘~.(4)
At constant concentrations of HDEHP and low distribution ratio, [Co’+]“- 1[H2R2]m@+q)‘2 will be approximately constant. Thus, a plot of log&, vs pH will give a straight line with a slope of mp. Figure 1 shows the relationships between DC0 and pH with HDEHP concentration as parameter for 0.5 kmol rnp3 (Na,H)SO, and [Co”], = 0.0017 kmol m- ’ at 25°C. The straight lines have slopes = 2, i.e. mp = 2. Thus, eq. (4) can be simplified to [Co”][H+12 = mKm,q[Co2+]m[~2R2]m(P+q)~2. (5) At constant concentrations of HDEHP, the degree of aggregation of the cobalt-HDEHP complex in the organic phase, m, is obtained from the plot of log [Co”][H ‘1’ vs log [Co 2+]. The experimental results are shown in Fig. 2 for various HDEHP concentration and 0.5 kmol mm3 (Na, H)S04 at 25°C. The straight lines have unit slopes in Fig. 2.
Extraction equilibrium of Co” from sulphate solutions
1149
-3
, 3
I
I
4
5
10 -;; _a’ ’
PH
a““”10 -Dma“‘1’1’ anBx11tJ 10 -I
l
[RZTZ]
Fig. 1. Effect of pH on the extraction of cobalt between 0.5 kmol mW3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol rne3. (0) [H,RJ = 0.005 km01 m -3, slope = 1.94; (0) [H,RJ = 0.010 km01 rnm3, slope = 1.91; (A) [H,RJ = 0.025 km01 rnm3, slope = 1.95.
Fig. 3. Effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol me3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol m-‘, slope = 2.09.
First, it could be assumed that only species of the type CoR,(HR), are formed. Thus, a plot of logDc,,[H+12 vs log[H,R,] would give a straight line with an intercept equal to Kzp and a slope equal to (2 + q)/2. Figure 3 shows the relationship between D&I-I’]2 and [H,R,] for 0.5 kmol me3 (Na, H)S04 and [Con], = 0.0017 kmol rnp3 at 25°C. In Fig. 3, a straight line with a slope of 2.09 is obtained, i.e. q = 2. Thus, the species formed in the organic phase is CORED. From Fig. 3, the intercept is - 5.42, i.e. K22 = 3.84 x 10W6. ReconjCmation of extraction equilibrium formation by slope analysis
Next, assuming two species are formed in the Fig. 2. Extraction of cobalt between 0.5 km01 mm3 and COR,(HR)~, (Na,H)SO, and kerosene at constant concentrations of organic phase, e.g. CoR,(HR), HDEHP and 25°C. (0) [H2R2] = 0.005 km01 mW3, eq. (3) could be expressed as slope = 1.01; (0) [H,R,] = 0.025 kmol rne3, slope = [CobWW + [CoRdHW,I 1.02; (A) = [H,R,] = 0.10 kmol rnp3, slope = 1.06. CO D
=
[Co”]
*
(9)
Using eq. (6), eq. (9) becomes Therefore, the extracted species is monomeric, i.e. m = 1. And then p = 2. Thus, eqs (2) and (4) can be simplified to (6) and DC0 = K,,[H,R j(2+q)‘2[H+]-2. Rearranging
(7)
eq. (7), it follows that
log DC‘,- 2 pH = log Kzq+ (2 +q)/2 log [H,R;I.
(8)
&o[H+12= K22[H2RJ2+K23[H2R215’2 (10) or
DcoW+12= K 1 22[Hgj11/2 [H&l S/2
+K23*
(11)
Now, a plot of Dc,[H+]2/[H2R2]5’2 vs [H2R2]- “2 would give a straight line with a slope equal to K,, and an intercept equal to K,,. Figure 4 shows the effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol m- 3 (Na,H)SO, and kerosene for [Co”] = 0.0017 km01 rnp3 at 25°C. In Fig. 4, a straight line with a
TING-CHIA HUANG and TEH-HUA TSAI Table 1. Equilibrium constants for the different HDEHP species in the 0.5 kmol mm3(Na,H)SO,/HDEHP-kerosene system4 Reaction HR--‘R-+H+ HReHR 2HR+H,R,
Fig. 4. Effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol rnd3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol me3, slope = 2.31, intercept = 2.37.
slope of 2.31 and an intercept of 2.37 is obtained, i.e. Kz2 = 2.31 x 10m6and Kz3 = 2.37 x 10e6 (kmol m- 3)- l/2*
Reconjirmation of extra&ion equilibrium formation by computer treatment
Numerical treatment of the data was performed by modifying the LETAGROP-DISTR programI in order to apply it to the five-component system in this work. The aim of these calculations was to confirm the results obtained with the previous graphic analysis and to refine the values of the equilibrium constants. In this calculation, the computer searches for the best set of equilibrium constants that would minimize the error squares sum defined by u = 1 (log &ilc - log Q2
Constant pK, = 1.27 f0.03 log Kd = 3.54kO.02 log K, = 4.42 f 0.04
given from Huang and Juang4 and shown in Table 1. Table 2 summarizes the results obtained with some of the models tried. Figure 5 presents the plot of Uminvs q for models I-VII given in Table 2 ; the optimal q values are between 2 and 3. And then, two species formed in organic phase (models VIII-XV) were tried, the result showed the optimal one turns out to be the set of species (p,q) equal to (2,2) and (2,3). Furthermore, several more than two species calculations were carried out to investigate the possibility of finding other complexes which could improve the fit of the experimental data (summarized as models XVI and XVII). From Table 2, the optimal fit for sulphate systems was obtained when the species were the same as those suggested by the graphic treatment, i.e. CoR,(HR), and COR~(HR)~ with logK22 = -5.56kO.04 and log K23 = - 5.65 max - 5.37. The experimental data for the above numerical calculation are given in Figs 1, 2 and 3. The mass balance equation for H2R2 in the optimal model is given by [a],
= [H,R,]+
1/2K; “2[Hx]“2
+2K22[Co2+][H,R2]2[H+]-2 + 5/2K23[Co2+][H2R2]S’2[H+]- 2. (14)
(12)
where Dexp is the distribution ratio of cobalt measured experimentally and Dcalcis the corresponding value calculated by the program obtained by solving the mass balance equations for Co*+, H2R2, SO:-, Na+, using a given set of complexes and their equilibrium constants. The optimal model is the one which minimizes the error squares sum function and gives the lowest mean standard deviation a(log D), defined by o(log D) = (U/Np) Iv2
(13)
where Np is the degree of freedom, approximately equal to the number of data points. The equilibrium constants for the different HDEHP species between 0.5 kmol me3 (Na,H)S04 and kerosene system used in the present work are
9 Fig. 5. Variation of Uminwith number of free monomeric HDEHP involved in the extracted species. (Table 2 models I-VII.)
1151
Extraction equilibrium of Co” from sulphate solutions Table 2. Results of the numerical
Model I II III IV V VI VII VIII IX X XI XII XIII XIV xv XVI XVII
calculation CoR,(HR),
for the various model species,
Species (PA
W) (291)
cm
t2,3) (2,4) (25) (2561 (2,2), (2,01* (2,2), (2,11* (232) (2,3) (2,2) (2,4) (292) (25) (292) (236) (2,3), (2,4)* (2,31, (2,5)* (2,213(2,3), (2,4)* (2,0)*, (2,1)*, (2,2), (2,3), (2,41*, (2,5)*, (2,6)*
-7.39f0.16” -6.45 + 0.09 -5.52f0.02 -4.58kO.07 -3.63f0.14 -2.67f0.21 -1.71 max -1.49*
-5.56kO.04 - 5.65 max -5.53-10.03 - 5.29 max -5.53f0.02 -4.88 max -5.52kO.02 -4.47 max
15.4 4.42 0.267 2.76 11.4 25.7 44.9 0.267 0.267 0.228
0.457 0.244 0.060 0.193 0.393 0.589 0.779 0.060 0.060 0.056
0.235
0.057
0.240
0.057
0.243
0.058
2.76 2.76 0.228 0.228
0.195 0.196 0.056 0.058
- 5.37 -4.98 -4.55 -4.12
* Rejected species. “The error given corresponds to 3 * &log K). bThe value after max corresponds to log [K+ 3a(K)].
Complexation of cobalt in sulphate solutions
B2
In the sulphate medium, the equilibrium constants obtained are a sort of “conditional” constant, since cobalt is complexed by sulphate ions. Considering also the species SO:-, eq. (9) should be written as follows :
where the stability constant of CoS04 is defined by
[CoSO41 PI = [co’+][so:-]
(16)
[SO:-] can be calculated from the mass balance expression for the sulphate concentration as given by eq. (17) [so:-],
= [so:-]+ +
[COSO,]
B2[Na+lWi-l+ B$-U[SO~-l (17)
where /I2 and /I3 are the stability constants NaSO; and HSO;, respectively, defined by
of
=
[NaSOiI l?Ja+lFW1
and
IHSW 83 =
[H+][SO$-]
.
(19)
The values of log pi, log /I2 and log /I3 were taken to be 2.4, 1.06 and 1.99 at zero ionic strength. ‘* Table 3 summarizes the results of the graphic and computer calculations. However, in the 0.5 kmol me3 (Na+, H+, Co*+)SO:- system, the ionic strength is not zero. Therefore, for minimizing error square sum calculations, the best log values of K23, K2.+ /?,, /I2 and /I3 were obtained and shown in Table 3. The published values for the equilibrium constants are compiled in Table 4. Figure 6 represents the distribution diagram of Con as a function of the total HDEHP concentration at various total concentrations of cobalt and pH values. It is worth noting the increasing predominance of the CoR2(HR)* species as the total concentration of HDEHP is increased.
1152
TING-CHIA
HUANG
Table 3. Equilibrium constants for species CoR,(HR), and CoR,(HR), calculated by different methods for sulphate solutions Method of calculation Conditional
log K,*
and TEH-HUA
TSAI
1.0
I
(a)
I
[ao(lI))t - a.oo17y DE - 4.50
log K23
constants
Graphic Numerical
- 5.64 -5.56+0&l
-5.63 -5.65 max -5.37
Corrected for CoSO, complexation at zero ion strength” Numerical
-4.31 f 0.04
Corrected for CoSO, complexation (Na,H)SO, solutions” Numerical
-4.67f0.13
-4.23 max -4.16 at 0.5 kmol mW3 -4.98 max - 4.59
“logp, = 2.4,logp, = l.O6andlog/?, = 1.99obtained from Morel. ’ * bThe optimal values of stability constants from calculation were lag/3, = 2.01+0.20,” logj, = 1.06kO.19 and logb, = 1.79max3.00. “The value after max corresponds to log[K+3a(K)] and the error given corresponds to 3 - a(log K).
CONCLUSION For the sulphate system, the cobalt distribution data could be best explained by assuming the formations of two species: CoR*(HR)* and COR*(HR)~. Graphic and computer modelling of the data excluded the existence of the species CoR,. It could also be established that the existence of these two species together gives the best fit for the distribution data, whereas the assumption that either of them exists alone gives a considerably worse fit. Grimm and KolaEik” and Komasawa et al. ” also reported the existence of the CoR,(HR), we identified, although the aqueous phase contained
nitrate solutions. However, in the sulphate solutions, CoR,(HR), existed at the same time from the above graphic and numerical analysis. On the other hand, the numerical values of the “conditional” equilibrium constants determined for these two species depend on the composition of the aqueous phase. This is due to the possible complexation of cobalt in the aqueous phase. In this work, we adopted two approaches for considering the existence of CoS04, NaSO; and HSO; complexations. In the first approach, computer calculations on all sulphate system data were performed by introducing the values of the stability constants of the sulphates in the literatures. In the second approach,
the equilibrium
constants
of Co-
,.,U .O
0.1
0.2
[lmr2]t Fig. 6. Cobalt distribution diagrams as a function of the total HDEHP concentration at two different total concentrations of cobalt and two different pH values. Lines 1, 2, 3 and 4 represent CoR,(HR),, CoR,(HR),, Co*+ and CoSO,, respectively. (a) [Co”], = 0.0017 kmol rnm3, pH = 4.50 ; (b) [Co”], = 0.05 km01 rnm3, pH = 4.50 ; (c) [Co”], = 0.0017 kmol m- 3, pH = 3.50.
HDEHP complexes and sulphates were calculated at the same time. New values of Kzz, Kz,, PI, /3* and fi3 were calculated and are given in Table 3. As can be seen in Table 4, the value of Kzz obtained agrees
1153
Extraction equilibrium of Co” from sulphate solutions Table 4. The equilibrium constants for CoR,(HR), extracted by HDEHP found in the literature log %j Aqueous phase (0.5 M IrJa,H)SOd 1.O M (Na,H)NO 3 (0.5 M lXa,H)NO, No medium 0.5 M (Na,H)NO, No medium (Na,H)NG, (Na,H)NO, (Na,H)NO, (Na,H)NG,
Temperature (“C) 253 0.2 252 0.2 25f 0.2 25+0.2 25f 0.2 -
Diluent
(292)
Kerosene n-Dodecane n-Heprane n-Heptane Toluene or benzene Toluene or benzene 2-Ethylhexyl alcohol Isodecanol Nitrobenzene Ether
-4.67_+0.33 -4.41 -4.4D -4.22 -5.35 -5.14 -4.59 -3.89 -5.20 -5.13
well with those obtained for the nitrate systems and
aliphatic diluents. It is worth mentioning that some other models were tested and the optimal fit remained the same as that obtained by assuming the same species (p,q) equal to (2,2) and (2,3). Acknowledgements-This work was performed under the auspices of the National Science Council of the Republic of China, under contract number NSC76-0402-E006-04, to which the authors wish to express their thanks.
REFERENCES 1. T. Sekine and Y. Hasegawa, Solvent Extraction Chemistry : Fundamentals and Applications, Ch. 7. Marcel Dekker, New York (1977). 2. G. M. Ritcey and A. W. Ashbrook, Solvent Extraction : Principles and Applications to Process Metallurgy, Part II. Elsevier, Amsterdam (1984). 3. T. C. Huang and R. S. Juang, Hydrometall. Symp. 1985, 79. 4. T. C. Huang and R. S. Juang, Znd. Eng. Chem. Fundam. 1986,25,752.
-4%
(2,3)
References
max - 4.59
7333swork IO 33 II 11 II 13 13 13 13
5. T. C. Huang and R. S. Juang, J. Chem. Eng. Japan 1986, 19, 379. 6. C. T. Huang and T. C. Huang, Solvent Extr. Zon Exch. 1987,5,611. 7. T. C. Huang and C. T. Huang, Znd. Eng. Chem. Res. 1988,27, 1675. 8. D. C. Madigan, Aust. J. Chem. 1960, 13, 58. 9. M. L. Brisk and W. J. McManamey, J. Appl. Chem. 1969, 19, 103. 10. R. Grimm and Z. Kolaiik, J. Znorg. Nucl. Chem. 1974,36, 189. 11. I. Komasawa, T. Otake and Y. Higaki, J. Znorg. Nucl. Chem. 1981,43,3351. 12. T. Sato and T. Nakamura, J. Znorg. Nucl. Chem. 1972,34,3721. 13. I. Komasawa, T. Otake and Y. Ogawa, J. Chem. Eng. Japan 1984,17,410. 14. W. J. McDowell, P. T. Perdue and G. N. Case, J. Znorg. Nucl. Chem. 1976,38,2127. 15. T. Sato, J. Znorg. Nucl. Chem. 1965,21, 1395. 16. Z. Kolaiik and R. Grimm, J. Znorg. Nucl. Chem. 1976,38, 1721. 17. D. H. Liem, Acta Chem. &and. 1971,25, 1521. 18. F. M. M. Morel, Principles of Aquatic Chemistry. Wiley-Interscience, New York (1983).
1990 0
0277-5387/W $3.00+.00 1990 Pergamoo Press plc
EXTRACTION EQUILIBRIUM OF COBALT@) FROM SULPHATE SOLUTIONS BY DI(2-ETHYLHEXY’L) PHOSPHORIC ACID DISSOLVED IN KEROSENE TING-CHIA Department
I-WANG*
and TEH-HUA
TSAI
of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republic of China (Received
18 October
1989 ; accepted 8 December
1989)
Abstract-The distribution of cobalt(I1) between 0.5 kmol mW3aqueous sulphate medium and kerosene solutions of di(2-ethylhexyl)phosphoric acid (HDEHP) has been studied at 25°C. The extraction equilibrium is influenced by the total extractant concentration, equilibrium pH value and total metal concentration in the aqueous phase. The distribution data have been analysed both graphically and numerically. The results show that the species extracted into the organic phase have the composition CORED and CORED. The stability constants of HSO;, NaSO; and CoS04 in sulphate solutions have also been estimated from the extraction data.
Di(2-ethylhexyl)phosphoric acid (abbreviated as HDEHP or simply HR) has been studied extensively as an extraction reagent in hydrometallurgical processes for the separation and purification of a number of metals. It extracts first-row transition metals such as copper, cobalt, nickel and zinc, as well as
uranium and rare earths in familiar nuclear fuel reprocessing, in a wide range of operating conditions. l-7 The extraction of cobalt with HDEHP has been studied by many investigators. Madigan’ studied the distribution of cobalt between an aqueous sulphate solution and a solution of HDEHP in kerosene, and proposed that CoR, compounds * Author to whom correspondence should be addressed. are formed in the organic phase. Brisk and McManamey,’ working in sulphate media, found Nomenclature that a single dimeric acid molecule was formed by D : Distribution ratio (-) ; K. : acid dissociation conthe extracted species in aliphatic diluent. Grimm stant of monomeric HDEHP in the aqueous phase (kmol m- ‘) ; 1yd: distribution constant of monomeric HDEHP and Kolai-ik” and Komasawa et al.,” using both between the organic and aqueous phases (-) ; K2 : dimeraliphatic and aromatic diluents, explained the exization constant of HDEHP in the diluent (m’ kmol- ‘) ; traction of Co’+ from nitrate medium by assuming com- the formation of CoR2(HR)2. Sato and Nakamura” K ,,,Pq: equilibrium constant of cobalt-HDEHP plexes ; m : degree of aggregation of the extracted species proposed the extraction of CoR2(H20)2 after spectro(-) ; p : number of R group except HR molecule involved scopic studies on the solid compound obtained by in the extracted species (-) ; q : number of free monoloading the metal in the organic phase. Komasawa meric HDEHP involved in the extracted species (-); et a1.‘3 studied the effect of diluents on extraction Co” : metal species ; U : error square sum (- ) ; [ 1,: total from nitrate media and showed the existence of concentration of the species in the brackets (km01 mM3). CORED formed in alcohol diluents and nonGreek letters polar or weak polar diluents. p: Stability constants of sulphate complexes (m3 Further experimental studies seem necessary to kmol- ‘) ; c : standard deviation (-). clarify the uncertainties regarding species comSuperscripts position. In this work, the extraction of cobalt by ~ : Denotes the organic phase species or organic phase HDEHP is studied in detail to determine the comconcentration. position of the species extracted by HDEHP in kerosene. In order to obtain a complete underSubscripts min : Minimum value. standing of the system, the extraction of cobalt was 1147
TING-CHIA HUANG and TEH-HUA TSAI
1148
studied first by graphic and slope analysis, which assumed no complexes were in the aqueous phase. Then, by numerical analysis of the experimental data, values for the stability constants of the HSO;, NaSO; and CoSO, complexes in the aqueous sulphate solutions can be obtained. EXPERIMENTAL Reagents and solutions
Di(2-ethylhexyl)phosphoric acid used in this work was the product of Daihachi Chemical Ind. Co. Ltd., Osaka, Japan. A purity of 95% was found by potentiometric titration with NaOH in ethanolic medium. HDEHP was further precipitated as a copper complex from toluene and acetone solution, and then dissolved in toluene and a 4 km01 m- 3sulphuric acid solution, following the procedure of McDowell et al. I4 Kerosene (Chinese Petroleum Co., Taiwan, R.O.C.) was washed twice with l/5 volume 98% H2S04, then with distilled water until it was neutral. ’ 5 The other inorganic chemicals were analytical reagent grade supplied by Hayashi Pure Chemical Ind. Ltd., Osaka, Japan. Metal solutions were always prepared by weighing the appropriate volumes of the stock solution and making up with diluent.
titative and the mass balance for the metal was always within f 2%. RESULTS
AND DISCUSSION
Extraction equilibrium of cobalt
Metal complexes of HDEHP tend to form very large polymeric species in a fully loaded organic phase. These polymers are strongly depolymerized in the presence of free HDEHP molecules and ethylene glycol.‘6 Thus, it is necessary to assume that cobalt is extracted as an m-merized complex into the slightly loaded organic phase. The extraction of cobalt(I1) with HDEHP can be represented by the following general reaction : mCo2++m(p+q)/2H2R2-_‘ (CoR,(HW,),+
mpH+
(1)
where
(2) is the stoichiometric equilibrium constant referring to 0.5 kmol me3 (Na,H)SO, with kerosene as diluent, and the bar indicates the species in the organic phase. The distribution ratio of cobalt is defined as
Procedure
Twenty millilitres of the organic solution and an equal volume of aqueous solution were mixed in glass flasks with ground glass stoppers and shaken by a mechanical shaker for at least 30 min at 25.0 + 0.2”C. Preliminary experiments have shown the time needed to reach equilibrium to be less than 15 min. The organic solutions containd a 0.01-0.35 kmol m- 3monomeric form of HDEHP in kerosene. The aqueous phase consisted of 0.5 kmol m- 3(Na+, H+, Co*+)SO:-. The total anion concentrations were kept constant. The concentrations of cobalt ion in the initial aqueous solutions ranged from 8.5 x lop4 to 1.7 x lo-’ kmol rnp3. The two phases were separated after they had been allowed to settle for 4 h in a thermostat at 25.0 +0.2”C. After phase separation, the equilibrium hydrogen ion concentration was measured by a pH meter. The concentration of cobalt was measured with a IL-551 atomic absorption spectrophotometer (Instrumentation Laboratory Inc.) at a wavelength of 240.7 nm, coupled with background correction. Cobalt in the organic phases was stripped with 4 kmol m- 3 sulphuric acid, and the metal concentration in the acidic solution was analysed by AAS. The process had been shown to be quan-
(3) Substituting eq. (2) into (3) we obtain D
mNCoRp(HWq)mI
=
CO
[Co2f] = mK,,,pq[Co2+]“- ‘[H2R2]m(P+q~~2[H+]-“‘~.(4)
At constant concentrations of HDEHP and low distribution ratio, [Co’+]“- 1[H2R2]m@+q)‘2 will be approximately constant. Thus, a plot of log&, vs pH will give a straight line with a slope of mp. Figure 1 shows the relationships between DC0 and pH with HDEHP concentration as parameter for 0.5 kmol rnp3 (Na,H)SO, and [Co”], = 0.0017 kmol m- ’ at 25°C. The straight lines have slopes = 2, i.e. mp = 2. Thus, eq. (4) can be simplified to [Co”][H+12 = mKm,q[Co2+]m[~2R2]m(P+q)~2. (5) At constant concentrations of HDEHP, the degree of aggregation of the cobalt-HDEHP complex in the organic phase, m, is obtained from the plot of log [Co”][H ‘1’ vs log [Co 2+]. The experimental results are shown in Fig. 2 for various HDEHP concentration and 0.5 kmol mm3 (Na, H)S04 at 25°C. The straight lines have unit slopes in Fig. 2.
Extraction equilibrium of Co” from sulphate solutions
1149
-3
, 3
I
I
4
5
10 -;; _a’ ’
PH
a““”10 -Dma“‘1’1’ anBx11tJ 10 -I
l
[RZTZ]
Fig. 1. Effect of pH on the extraction of cobalt between 0.5 kmol mW3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol rne3. (0) [H,RJ = 0.005 km01 m -3, slope = 1.94; (0) [H,RJ = 0.010 km01 rnm3, slope = 1.91; (A) [H,RJ = 0.025 km01 rnm3, slope = 1.95.
Fig. 3. Effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol me3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol m-‘, slope = 2.09.
First, it could be assumed that only species of the type CoR,(HR), are formed. Thus, a plot of logDc,,[H+12 vs log[H,R,] would give a straight line with an intercept equal to Kzp and a slope equal to (2 + q)/2. Figure 3 shows the relationship between D&I-I’]2 and [H,R,] for 0.5 kmol me3 (Na, H)S04 and [Con], = 0.0017 kmol rnp3 at 25°C. In Fig. 3, a straight line with a slope of 2.09 is obtained, i.e. q = 2. Thus, the species formed in the organic phase is CORED. From Fig. 3, the intercept is - 5.42, i.e. K22 = 3.84 x 10W6. ReconjCmation of extraction equilibrium formation by slope analysis
Next, assuming two species are formed in the Fig. 2. Extraction of cobalt between 0.5 km01 mm3 and COR,(HR)~, (Na,H)SO, and kerosene at constant concentrations of organic phase, e.g. CoR,(HR), HDEHP and 25°C. (0) [H2R2] = 0.005 km01 mW3, eq. (3) could be expressed as slope = 1.01; (0) [H,R,] = 0.025 kmol rne3, slope = [CobWW + [CoRdHW,I 1.02; (A) = [H,R,] = 0.10 kmol rnp3, slope = 1.06. CO D
=
[Co”]
*
(9)
Using eq. (6), eq. (9) becomes Therefore, the extracted species is monomeric, i.e. m = 1. And then p = 2. Thus, eqs (2) and (4) can be simplified to (6) and DC0 = K,,[H,R j(2+q)‘2[H+]-2. Rearranging
(7)
eq. (7), it follows that
log DC‘,- 2 pH = log Kzq+ (2 +q)/2 log [H,R;I.
(8)
&o[H+12= K22[H2RJ2+K23[H2R215’2 (10) or
DcoW+12= K 1 22[Hgj11/2 [H&l S/2
+K23*
(11)
Now, a plot of Dc,[H+]2/[H2R2]5’2 vs [H2R2]- “2 would give a straight line with a slope equal to K,, and an intercept equal to K,,. Figure 4 shows the effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol m- 3 (Na,H)SO, and kerosene for [Co”] = 0.0017 km01 rnp3 at 25°C. In Fig. 4, a straight line with a
TING-CHIA HUANG and TEH-HUA TSAI Table 1. Equilibrium constants for the different HDEHP species in the 0.5 kmol mm3(Na,H)SO,/HDEHP-kerosene system4 Reaction HR--‘R-+H+ HReHR 2HR+H,R,
Fig. 4. Effect of HDEHP concentrations on the extraction equilibrium of cobalt between 0.5 kmol rnd3 (Na,H)SO, and kerosene at 25°C. [Co”], = 0.0017 kmol me3, slope = 2.31, intercept = 2.37.
slope of 2.31 and an intercept of 2.37 is obtained, i.e. Kz2 = 2.31 x 10m6and Kz3 = 2.37 x 10e6 (kmol m- 3)- l/2*
Reconjirmation of extra&ion equilibrium formation by computer treatment
Numerical treatment of the data was performed by modifying the LETAGROP-DISTR programI in order to apply it to the five-component system in this work. The aim of these calculations was to confirm the results obtained with the previous graphic analysis and to refine the values of the equilibrium constants. In this calculation, the computer searches for the best set of equilibrium constants that would minimize the error squares sum defined by u = 1 (log &ilc - log Q2
Constant pK, = 1.27 f0.03 log Kd = 3.54kO.02 log K, = 4.42 f 0.04
given from Huang and Juang4 and shown in Table 1. Table 2 summarizes the results obtained with some of the models tried. Figure 5 presents the plot of Uminvs q for models I-VII given in Table 2 ; the optimal q values are between 2 and 3. And then, two species formed in organic phase (models VIII-XV) were tried, the result showed the optimal one turns out to be the set of species (p,q) equal to (2,2) and (2,3). Furthermore, several more than two species calculations were carried out to investigate the possibility of finding other complexes which could improve the fit of the experimental data (summarized as models XVI and XVII). From Table 2, the optimal fit for sulphate systems was obtained when the species were the same as those suggested by the graphic treatment, i.e. CoR,(HR), and COR~(HR)~ with logK22 = -5.56kO.04 and log K23 = - 5.65 max - 5.37. The experimental data for the above numerical calculation are given in Figs 1, 2 and 3. The mass balance equation for H2R2 in the optimal model is given by [a],
= [H,R,]+
1/2K; “2[Hx]“2
+2K22[Co2+][H,R2]2[H+]-2 + 5/2K23[Co2+][H2R2]S’2[H+]- 2. (14)
(12)
where Dexp is the distribution ratio of cobalt measured experimentally and Dcalcis the corresponding value calculated by the program obtained by solving the mass balance equations for Co*+, H2R2, SO:-, Na+, using a given set of complexes and their equilibrium constants. The optimal model is the one which minimizes the error squares sum function and gives the lowest mean standard deviation a(log D), defined by o(log D) = (U/Np) Iv2
(13)
where Np is the degree of freedom, approximately equal to the number of data points. The equilibrium constants for the different HDEHP species between 0.5 kmol me3 (Na,H)S04 and kerosene system used in the present work are
9 Fig. 5. Variation of Uminwith number of free monomeric HDEHP involved in the extracted species. (Table 2 models I-VII.)
1151
Extraction equilibrium of Co” from sulphate solutions Table 2. Results of the numerical
Model I II III IV V VI VII VIII IX X XI XII XIII XIV xv XVI XVII
calculation CoR,(HR),
for the various model species,
Species (PA
W) (291)
cm
t2,3) (2,4) (25) (2561 (2,2), (2,01* (2,2), (2,11* (232) (2,3) (2,2) (2,4) (292) (25) (292) (236) (2,3), (2,4)* (2,31, (2,5)* (2,213(2,3), (2,4)* (2,0)*, (2,1)*, (2,2), (2,3), (2,41*, (2,5)*, (2,6)*
-7.39f0.16” -6.45 + 0.09 -5.52f0.02 -4.58kO.07 -3.63f0.14 -2.67f0.21 -1.71 max -1.49*
-5.56kO.04 - 5.65 max -5.53-10.03 - 5.29 max -5.53f0.02 -4.88 max -5.52kO.02 -4.47 max
15.4 4.42 0.267 2.76 11.4 25.7 44.9 0.267 0.267 0.228
0.457 0.244 0.060 0.193 0.393 0.589 0.779 0.060 0.060 0.056
0.235
0.057
0.240
0.057
0.243
0.058
2.76 2.76 0.228 0.228
0.195 0.196 0.056 0.058
- 5.37 -4.98 -4.55 -4.12
* Rejected species. “The error given corresponds to 3 * &log K). bThe value after max corresponds to log [K+ 3a(K)].
Complexation of cobalt in sulphate solutions
B2
In the sulphate medium, the equilibrium constants obtained are a sort of “conditional” constant, since cobalt is complexed by sulphate ions. Considering also the species SO:-, eq. (9) should be written as follows :
where the stability constant of CoS04 is defined by
[CoSO41 PI = [co’+][so:-]
(16)
[SO:-] can be calculated from the mass balance expression for the sulphate concentration as given by eq. (17) [so:-],
= [so:-]+ +
[COSO,]
B2[Na+lWi-l+ B$-U[SO~-l (17)
where /I2 and /I3 are the stability constants NaSO; and HSO;, respectively, defined by
of
=
[NaSOiI l?Ja+lFW1
and
IHSW 83 =
[H+][SO$-]
.
(19)
The values of log pi, log /I2 and log /I3 were taken to be 2.4, 1.06 and 1.99 at zero ionic strength. ‘* Table 3 summarizes the results of the graphic and computer calculations. However, in the 0.5 kmol me3 (Na+, H+, Co*+)SO:- system, the ionic strength is not zero. Therefore, for minimizing error square sum calculations, the best log values of K23, K2.+ /?,, /I2 and /I3 were obtained and shown in Table 3. The published values for the equilibrium constants are compiled in Table 4. Figure 6 represents the distribution diagram of Con as a function of the total HDEHP concentration at various total concentrations of cobalt and pH values. It is worth noting the increasing predominance of the CoR2(HR)* species as the total concentration of HDEHP is increased.
1152
TING-CHIA
HUANG
Table 3. Equilibrium constants for species CoR,(HR), and CoR,(HR), calculated by different methods for sulphate solutions Method of calculation Conditional
log K,*
and TEH-HUA
TSAI
1.0
I
(a)
I
[ao(lI))t - a.oo17y DE - 4.50
log K23
constants
Graphic Numerical
- 5.64 -5.56+0&l
-5.63 -5.65 max -5.37
Corrected for CoSO, complexation at zero ion strength” Numerical
-4.31 f 0.04
Corrected for CoSO, complexation (Na,H)SO, solutions” Numerical
-4.67f0.13
-4.23 max -4.16 at 0.5 kmol mW3 -4.98 max - 4.59
“logp, = 2.4,logp, = l.O6andlog/?, = 1.99obtained from Morel. ’ * bThe optimal values of stability constants from calculation were lag/3, = 2.01+0.20,” logj, = 1.06kO.19 and logb, = 1.79max3.00. “The value after max corresponds to log[K+3a(K)] and the error given corresponds to 3 - a(log K).
CONCLUSION For the sulphate system, the cobalt distribution data could be best explained by assuming the formations of two species: CoR*(HR)* and COR*(HR)~. Graphic and computer modelling of the data excluded the existence of the species CoR,. It could also be established that the existence of these two species together gives the best fit for the distribution data, whereas the assumption that either of them exists alone gives a considerably worse fit. Grimm and KolaEik” and Komasawa et al. ” also reported the existence of the CoR,(HR), we identified, although the aqueous phase contained
nitrate solutions. However, in the sulphate solutions, CoR,(HR), existed at the same time from the above graphic and numerical analysis. On the other hand, the numerical values of the “conditional” equilibrium constants determined for these two species depend on the composition of the aqueous phase. This is due to the possible complexation of cobalt in the aqueous phase. In this work, we adopted two approaches for considering the existence of CoS04, NaSO; and HSO; complexations. In the first approach, computer calculations on all sulphate system data were performed by introducing the values of the stability constants of the sulphates in the literatures. In the second approach,
the equilibrium
constants
of Co-
,.,U .O
0.1
0.2
[lmr2]t Fig. 6. Cobalt distribution diagrams as a function of the total HDEHP concentration at two different total concentrations of cobalt and two different pH values. Lines 1, 2, 3 and 4 represent CoR,(HR),, CoR,(HR),, Co*+ and CoSO,, respectively. (a) [Co”], = 0.0017 kmol rnm3, pH = 4.50 ; (b) [Co”], = 0.05 km01 rnm3, pH = 4.50 ; (c) [Co”], = 0.0017 kmol m- 3, pH = 3.50.
HDEHP complexes and sulphates were calculated at the same time. New values of Kzz, Kz,, PI, /3* and fi3 were calculated and are given in Table 3. As can be seen in Table 4, the value of Kzz obtained agrees
1153
Extraction equilibrium of Co” from sulphate solutions Table 4. The equilibrium constants for CoR,(HR), extracted by HDEHP found in the literature log %j Aqueous phase (0.5 M IrJa,H)SOd 1.O M (Na,H)NO 3 (0.5 M lXa,H)NO, No medium 0.5 M (Na,H)NO, No medium (Na,H)NG, (Na,H)NO, (Na,H)NO, (Na,H)NG,
Temperature (“C) 253 0.2 252 0.2 25f 0.2 25+0.2 25f 0.2 -
Diluent
(292)
Kerosene n-Dodecane n-Heprane n-Heptane Toluene or benzene Toluene or benzene 2-Ethylhexyl alcohol Isodecanol Nitrobenzene Ether
-4.67_+0.33 -4.41 -4.4D -4.22 -5.35 -5.14 -4.59 -3.89 -5.20 -5.13
well with those obtained for the nitrate systems and
aliphatic diluents. It is worth mentioning that some other models were tested and the optimal fit remained the same as that obtained by assuming the same species (p,q) equal to (2,2) and (2,3). Acknowledgements-This work was performed under the auspices of the National Science Council of the Republic of China, under contract number NSC76-0402-E006-04, to which the authors wish to express their thanks.
REFERENCES 1. T. Sekine and Y. Hasegawa, Solvent Extraction Chemistry : Fundamentals and Applications, Ch. 7. Marcel Dekker, New York (1977). 2. G. M. Ritcey and A. W. Ashbrook, Solvent Extraction : Principles and Applications to Process Metallurgy, Part II. Elsevier, Amsterdam (1984). 3. T. C. Huang and R. S. Juang, Hydrometall. Symp. 1985, 79. 4. T. C. Huang and R. S. Juang, Znd. Eng. Chem. Fundam. 1986,25,752.
-4%
(2,3)
References
max - 4.59
7333swork IO 33 II 11 II 13 13 13 13
5. T. C. Huang and R. S. Juang, J. Chem. Eng. Japan 1986, 19, 379. 6. C. T. Huang and T. C. Huang, Solvent Extr. Zon Exch. 1987,5,611. 7. T. C. Huang and C. T. Huang, Znd. Eng. Chem. Res. 1988,27, 1675. 8. D. C. Madigan, Aust. J. Chem. 1960, 13, 58. 9. M. L. Brisk and W. J. McManamey, J. Appl. Chem. 1969, 19, 103. 10. R. Grimm and Z. Kolaiik, J. Znorg. Nucl. Chem. 1974,36, 189. 11. I. Komasawa, T. Otake and Y. Higaki, J. Znorg. Nucl. Chem. 1981,43,3351. 12. T. Sato and T. Nakamura, J. Znorg. Nucl. Chem. 1972,34,3721. 13. I. Komasawa, T. Otake and Y. Ogawa, J. Chem. Eng. Japan 1984,17,410. 14. W. J. McDowell, P. T. Perdue and G. N. Case, J. Znorg. Nucl. Chem. 1976,38,2127. 15. T. Sato, J. Znorg. Nucl. Chem. 1965,21, 1395. 16. Z. Kolaiik and R. Grimm, J. Znorg. Nucl. Chem. 1976,38, 1721. 17. D. H. Liem, Acta Chem. &and. 1971,25, 1521. 18. F. M. M. Morel, Principles of Aquatic Chemistry. Wiley-Interscience, New York (1983).