Ind. Eng. Chem. Fundam. 1986, 25, 752-757
752
forms used for the breakage and coalescence functions are g(.) = K8U(P+” B(U,U’)
(1-1)
= l/u’
X(u,u’) = K,(u
+ u’)P
The steady-state number density is given by
where d is the drop diameter and d is the diameter corresponding to the mean volume 0. When the independent variable is transformed from diameter to volume, the steady-state number density is also expressed by the exponential distribution
The mean diameter based on number density is given by
d ’ = Jmd3(
$)
exp(-d3/d3) d(d) = d f ( 4 / 3 )
(1-4)
Literature Cited Bapat, P. M.; Tavlarides. L. L. AIChE J . 1985, 31,659. Bapat, P. M.; Tavlarides, L. L.; Smith, G. W. Chem. Eng. Sci. 1983, 38, 2003. Hsia, M. A.; Tavlarides, L. L. Chem. Eng. J . (Lausanne) 1980, 20, 225. Hsia, M. A.; Tavlarides, L. L. Chem. Eng. J . (Lausanne) 1983, 26, 189. Hulburt. H. M.; Katz, S. Chem. Eng. Sci. 1984, 19,555. Karlin, S.; Taylor, H. M. A first Course in Stochastic Processes, 2nd ed.; Academic: New York, 1975. Kendall, D. G. J . R . Stat. Soc., Ser. 6 1950, 72, 116. Mackelprang, R. Ph.D. Dissertation, University of Utah, Salt Lake City, UT,
1979. Rod, V.; Misek, T. Trans. I n s t . Chem. Eng. 1982, 6 0 , 48. Sastry, K. V. S.; Fuerstenau, D. W. I n d . Eng. Chem. Fundam. 1970, 9 , 145. Shah, B. H.; Ramkrishna. D.; Borwanker, J. D. AIChE J . 1977, 2 3 , 897.
Received for review November 14, 1984 Revised manuscript received December 9, 1985 Accepted February 13, 1986
Extraction Equilibrium of Zinc from Sulfate Media with Bis( 2-ethylhexyl) Phosphoric Acid Tlng-Chla Huang” and Ruey-Shin Juang Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, 7010 I, Republic of China
The distribution of bis(2-ethylhexyl)phosphoric acid (DPEHPA or RH) between 0.5 M (Na,H)SO, and kerosene has been measured spectrophotometrically. Based on the distribution data, the equilibrium constants of DPEHPA such as the dimerization, the distribution, and the acid dissociation constants were determined. The extraction equilibrium formulation was found for the distribution of zinc between D2EHPA/kerosene and 0.5 M (Na,H)SO, and was reconfirmed by using the slope analysis technique. The composition of the extracted species in the organic phase is ZnR,.RH. The monomeric extracted species exists when less than 26% of D2EHPA has been converted. I n the presence of 0.5 M (Na,H)SO,, the extraction of zinc sulfate by 2DEHPA in kerosene has the overall stoichiometry Zn2+ -t 1 . 5 0 , -+ZnR,.RH 2H’. The extraction equilibrium constant of this reaction was also found at 25 0,-
+
Introduction Bis(2-ethylhexyl) phosphoric acid (abbreviated as D2EHPA or simply RH) has been shown to be an effective extractant in hydrometallurgical processes for the separation and purification of divalent transition metals such as copper, cobalt, nickel, manganese, and zine (Flett and Spink, 1976; Sekine and Hasegawa, 1977; Sato et al., 1978). Dialkyl phosphoric acids exist as the dimeric form in most diluents of low polarity, which do not form strong hydrogen bonds with these acids (Dyrssen, 1957; Peppard et al., 1958; Baes et al., 1958; Ulyanov and Sviridova, 1963; Yagodin and Tarasov, 1969). The state of DBEHPA, in both organic and aqueous phases, is characterized by the dimerization, distribution, and acid dissociation constants, the values of which are determined by using the phase distribution data for D2EHPA. As a rule, the distribution data are obtained by analysis of D2EHPA content in the aqueous phase, which can be measured spectrophotometrically (Komasawa et al., 1981), colorimetrically (Ulyanov and Sviridova, 1963), or radiometrically with 32P-labeledD2EHPA (Kolarik, 1967; Smelov and Lanin, 1969; Ulyanov and Sviridova, 1970; Liem, 1972). In this
study, the spectrophotometrical merthod was proposed since it possessed high sensitivity (Bhattacharyya and Murthy, 1974). Many papers dealing with the equilibrium constants of D2EHPA have been published, but we found that considerable discrepancies remain among the values given by the various authors. These discrepancies may have been caused by using D2EH32PAcontaining some radioactive impurity (Liem, 1972), by phase separations, particularly with similar densities of the phases and the tendency of the system to form stable emulsion, or by the impurity of D2EHPA (Yagodin and Tarasov, 1969). However, Liem (1972) reexamined his original data using purified D2EH32PA and found that the disagreement between the values reported was still present and could hardly be explained. This is the other reason why a spectrophotometrical method was selected in this work. The equilibrium constants of D2EHPA were determined in the nitrate (Komasawa et al., 1981) and perchlorate media (Kolarik, 1967; Ulyanov and Sviridova, 1963, 1970; Liem, 1972). It was found that the effects of the type of salts were not negligible. Until now, no information for
0196-4313/86/1025-0752$01.50/00 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 4, 1986
sulfate media has been found. In spite of the considerable amount of work done on the extraction of zinc by D2EHPA and its application in practice, very few papers concerning the composition of the extracted species have been published. These species of metal include some free D2EHPA molecules when the loading of D2EHPA remains low in the organic phase. As for the number of the free dimeric DBEHPA molecules associated with zinc, there are 1.5 and 2 for normal hydrocarbon diluents and for the others, respectively, in the nitrate and perchlorate media (Smelov et al., 1972). Also, Ajawin et al. (1983) found 1.5 molecules in a sulfate medium at ionic strength 1.0 M. However, Grimm and KOlarik (1974) obtained the values of 1.5 and 1.7 for 1.0 M (Na,H)N03medium when the DPEHPA concentration in n-dodecane was either greater or less than 0.1 M, respectively. Apparently, the chemistry of this extraction system has not been fully determined. In the present work, the experiments were made to measure the equilibrium constants of D2EHPA between 0.5 M (Na,H)SO, and kerosene, following the method of Ulyanov and Sviridova (1970), and to determine the extraction equilibrium formulation for zinc in the organic phase. The notation of 0.5 M (Na,H)SO, means that the total sulfate concentration in the aqueous phase is fixed at 0.5 M, which is contributed to by adding various proportions of NaZSO4and HzS04. Brisk and McManamey (1969) had pointed out that the addition of an alkali-metal salt could adjust the hydrogen ion concentration and keep the ionic strength approximately constant for any initial metal and D2EHPA concentrations. Thus, the total ionic strength of the aqueous phase was maintained at a constant value by the addition of sodium sulfate in this work, regardless of the zinc concentration.
Experimental Section Reagents. DBEHPA was obtained from Daihachi Chemicals Ind. Co., Ltd., Osaka, Japan, with a purity of approximately 99%. It was further purified by precipitation as a copper complex from toluene and acetone solution and then was dissolved in toluene and 8 M sulfuric acid solution, following the procedure of McDowell et al. (1976). Kerosene, supplied by Kokusan Chemical Works, Ltd., Tokyo, Japan, was purified by washing several times successively with concentrated sulfuric acid, diluted sodium hydroxide solutions, and water (Sato, 1965). The other inorganic chemicals were supplied by Hayashi Pure Chemicals Ind., Ltd., Osaka, Japan, as analytical reagent grade. Procedure. (i) Distribution of DBEHPA between the Organic and Aqueous Phases. Equal volumes (25 mL) of organic and aqueous phases were shaken for at least 12 h in a constant-temperature water bath controlled a t 25 f 0.2 "C. The organic phase was D2EHPA in kerosene (10-3-10-1 M); the aqueous phase was 0.5 M (Na,H)SO,. The two phases were separated after they had been allowed to settle for more than 1 2 h. The concentration of D2EHPA in the aqueous phase was measured, following the method of Bhattacharyya and Murthy (1974); Le., anionic and molecular D2EHPA in the aqueous phase were completely extracted into 1,2-dichloroethane, and this organic layer was contacted with an aqueous solution containing rhodamine B and a phosphate buffer at pH 4. Thus, the concentration of D2EHPA, based on the formation of a strongly colored ion pair between the rhodamine B cation and the D2EHPA anion in 1,2-dichloroethane, was determined at 560 nm against a blank similarly treated with a Hitachi 330 spectrophotometer. The hydrogen ion concentration in the aqueous phase was determined with
753
a TOA Model HM-7B pH meter equipped with a glass electrode. (ii) Extraction Equilibrium of Zinc. Equal volumes (20 mL) of organic and aqueous phases were shaken by a two-armed mechanical shaker for at least 30 min at 25 f 0.2 "C. A preliminary experiment had shown that the reaction was complete after 10 min. The organic phases consisted of kerosene solutions of DBEHPA in the monomeric form in the concentration range 2.5 X to 2.0 X 10-1M; the aqueous phases contained zinc sulfate (8.1 X lo-, to 1.1 X lo-, M) in 0.5 M (Na,H)SO, media. The two phases were separated after they had been allowed to settle for 2 h. Then the pH value of the aqueous phase was measured. The concentration of zinc was determined for the phase having the lower zinc content, and the concentration in the other phase was found by the difference. If necessary, the weighed organic phase was completely stripped with 20 wt 70 nitric acid, and the resulting aqueous phase was analyzed. In all cases, the zinc concentration was measured with an I-L 551 atomic absorption spectrophotometer at a wavelength of 213.9 nm, coupled with a background correction. The concentration of unbound DBEHPA in the organic phase at equilibrium was determined by a mass balance. The concentration change of D2EHPA was negligible in most of the experiments, since a large excess of D2EHPA was used compared to the amount of zinc extracted.
Results and Discussion Equilibrium Constants of D2EHPA. The overall equilibrium during the distribution of DBEHPA between organic and aqueous phases involves dimerization in both phases, distribution of both monomeric and dimeric D2EHPA, and acid dissociation in the aqueous phase (Dyrssen, 1957). For small concentrations of D2EHPA (10-3-10-1 M in monomeric form) in the organic phase of this study, we can assume that only the monomeric D2EHPA can enter the aqueous phase (Ulyanov and Sviridova, 1970). This assumption is indirectly supported by the observation that the saturated aqueous solutions of di-namyl phosphoric acid and diisoamyl phosphoric acid in 1 M NaN03 do not contain a detectable quantity of the dimer (Kalarik, 1967). Hence RH =+ RH, Kd = [RH]/[RH] (1)
-(RH),, Kz = [(RH),]/[RHlZ RH + R- + H+, K, = [R-l[H+l/[RHl 2=+
(2)
(3)
where the bars and brackets denote organic-phase species and concentrations, respectively. The net distribution ratio of DBEHPA is defined as
If CRH >> l / K z , the concentration of the monomeric D2EHPA in an organic phase is far less than that of the dimer, i.e., [ m ]
