Ind. Eng. Chem. Fundam. 1982, 21, 417-422
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Uranium Extraction from Wet Process Phosphoric Acid. A Liquid Membrane Approach Jan Bock and Paul L. Vallnt, Jr.' Corporate Research Laboratoty, Exxon Research and Engineering Company, Linden, New Jersey 07036
Liquid membrane (LM) technology offers a new approach for extracting mineral values from aqueous streams, such as uranium from Wet Process Phosphoric Acid (WPPA). This approach utilizes a water-in-oil emulsion as the vehicle to simultaneously remove uranium from a WPPA feed and concentrate it in the internal aqueous phase of the LM emulsion. The simultaneous extraction and stripping operation of the LM process resulted in a kinetically controlled uranium separation, removing some of the limitations of the thermodynamically dominated solvent extraction process. This difference in basic mechanism manifested itself in a unique response to variation of the physicochemical parameters affecting the transport and concentration of uranium. I n contrast to solvent extraction, LM extraction exhibited an inverse temperature response, insensitivity to phosphoric acid strength of 5 to 8 M, and insensitivity to the concentration of complexing agent in the range of 0.07 to 0.14 M.
Introduction Development of a solvent extraction process for the recovery of uranium from WPPA was conducted in the 1950's (Cronan, 1959; Greek, 1957; Long, 1955). The alkyl pyrophosphoric acids employed as the extraction agent in these studies exhibited hydrolytic instability (Long, 1955). This,coupled with the discovery of relatively low cost large deposits of high grade uranium ores in the western US., resulted in termination of commercial application of uranium extraction from WPPA. In the late 1960's, Oak Ridge National Laboratory (ORNL) screened a broad class of potential uranium extractants (Ferguson, 1968,1969,1970; Hurst et al., 1969). The synergistic extractant combination of di-2-ethylhexyl phosphoric acid (DEHPA) and trioctyl phosphine oxide (TOPO) was identified as a promising system. Utilization of this extractant requires oxidation of the WPPA feed because of selectivity toward uranyl ion. Subsequent stripping of the uranium-loaded organic phase was accomplished with a reductive phosphoric acid solution. Isolation of uranium of satisfactory purity required a second solvent extraction cycle which necessitated reoxidation of the uranium in the strip solution of the first cycle. Bench scale demonstration of a two-cycle process flow sheet, using this extractant, was reported (Hurst et al., 1972). An alternative extractant comprised a commercial mixture of mono- and dioctylphenyl phosphoric acid (OPAP) for recovering uranium from WPPA (Murthy et al., 1970). This extractant was selective to uranium as U(IV) ion in reduced WPPA and was found to exhibit a significantly higher distribution coefficient. Stripping of the organic phase, in this case, was done with an oxidant in phosphoric acid. A process of this type offered potential cost advantages in terms of minimized chemical control of the WPPA oxidation state throughout the extraction sequence. Use of OPAP in the first cycle of the two-cycle DEHPA-TOP0 process was demonstrated in bench scale continuous mixer-settler tests at ORNL (Hurst and Crouse, 1974). In the mid 1970's, research on a liquid membrane approach for separation of uranium from WPPA was initiated at Exxon Research and Engineering Co. Utilizing the chemistry developed for solvent extraction processes, bench-scale studies indicated the critical compositional and physical variables controlling the extraction in liquid 0196-4313/82/1021-0417$01,25/0
membrane separation. This paper will illustrate the unique response to the variables affecting transport and concentration of uranium in the liquid membrane system.
Liquid Membrane Separation Concepts Extractive LM technology was considered for removal of organic and inorganic pollutants such as phenol and ammonia, respectively, as well as for heavy metals removal including Hg, Cd, Cr, etc. (Bock, 1980; Frankenfeld and Li, 1977; Kitagawa et al., 1977). This extractive technology was based on a water-in-oil emulsion as the vehicle to effect separation. The process sequence involved emulsifying the aqueous strip phase into an organic phase containing a surfactant and the complexing agents. The emulsion was then dispersed in the aqueous feed phase, which contained the species to be removed. During this complexing step, the desired species was transferred to and concentrated in the emulsified aqueous internal phase. Upon completion of contracting, the emulsion and raffinate (lean feed) phases were disengaged in a gravity settler. The last step in the LM process was the isolation of the concentrated aqueous internal phase. The unique aspect of this technique was that extraction and stripping were effected simultaneously rather than sequentially as in conventional solvent extraction. The general physical state of the LM system during contacting is shown in Figure 1. Several water-in-oil emulsion globules are shown dispersed in the continuous feed phase. Each emulsion globule contains thousands of aqueous internal phase (IP) droplets stabilized by a surfactant in the continuous organic phase, referred to as the membrane phase. Along with the surfactant, the membrane phase generally contains a complexing agent, CA, to facilitate transport of the desired species from the feed phase to the aqueous internal phase. The IP contains appropriate reagents to strip and concentrate the desired species within that phase. The degree of concentration of the extracted species in the internal phase can be controlled by adjustment of the ratio of membrane liquid to internal phase, M/IP, and the ratio of feed to emulsion phase, FIE. Separation Chemistry A convenient means of visualizing the LM separation is to consider the many internal phase droplets in each emulsion globule as a single drop, as shown in Figure 2. Utilizing the separation chemistry developed by ORNL 0 1982 American Chemical Society
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IM. Eng. Chern. Fundarn.. Vol. 21. No. 4, 1982 EXTRACTIONBY LM NULSlON
,
0.l-lrn
INTERNAL PHASE
A Figure 1. Extraction hy LM emulsion.
Figure 3. LM uranium extraction chemistry far reduced WPPA. CA, OPAP; IP, oxidant in H,PO,.
Figure 2. LM uranium extraction chemistry for oxidized WPPA. CA, DEHPA-TOP0 IP, redudant in H,PO,.
(Hurst et d,1972), uranyl ion in the oxidized WPPA phase is complexed by the DEHPA-TOP0 present in the membrane phase and designated CA for complexing agent. The resultant complex is transferred across the membrane to the internal phase (IP). The reductant in the internal phase of phosphoric acid strips the uranyl complex and forms the acid-soluble U(1V) species. Since DEHPATOPO does not effectively complex the U(IV) ion, it is efficiently trapped and concentrated in the internal phase. This conversion of uranyl into U(IV) ion provides a negligible concentration of uranyl ion in the internal phase. The resultant concentration gradient a c r m the membrane thus provides an effective driving force for removal of uranyl ion from the WPPA. An alternative chemistry studied by ORNL (Hurst and C r o w , 1974) removed uranium from reduced WPPA hy utilizing octylphenyl phosphoric acid (OPAP) to complex U(IV) ion. The chemistry was explored in the liquid membrane system as illustrated in Figure 3. The complexing agent, CA, in this membrane phase was OPAP. The internal phase of the emulsion employed an oxidant in phosphoric acid to convert the stripped U(IV) species to uranyl ion. Since OPAP preferentially complexed with U(IV) ion, conversion to uranyl ion in the internal phase provided both the driving force and trapping mechanism for uranium. This was analogous to the extraction of oxidized WPPA as shown in Figure 2. This paper will focus on the extraction of oxidized WPPA with the DEHPATOPO system.
LM Uranium Transfer Kinetics Liquid membrane extraction of uranium from phosphoric acid involves a sequence of five steps to transport
uranyl ion from the feed to the internal phase. The first step is convective transport of uranyl ion from the bulk feed to the external interface of an emulsion globule. Next, the uranyl ion is complexed by the DEHPA-TOP0 near this interface. The resultant complex is oil-soluble, enabling diffusive transfer, the third step, through the hydrocarbon membrane. The fourth step involves decomposition of the DEHPA-TOP0 uranyl ion complex a t the interfacial region between the membrane and reductive internal phase. The fifth and last step is transfer of the uranium, as U(IV), into the hulk internal phase. Several other transport events were found to occur in liquid membrane separation experiments. These involved the reverse transport of uranium from the internal phase of the emulsion to the feed phase. A straightforward technique found useful for describing the liquid membrane uranium transfer kinetics involved the rate equation for a simple first-order reversible reaction (Walss, 1959). The rate constantsR, and Rz, were required to describe the kinetics completely as shown in eq 1. R
U(V1)
2 U(IV) R*
(1)
Designating the uranyl ion as species A and the U(IV) ion as species B, the rate of extraction of uranyl ion can be described by the equation
CA represents concentration of uranyl ion in the WPPA feed phase and CRrepresents the concentration of U W ) . The solution to this equation, coupled with the specification of zero initial concentration of U(IV) and no initial extraction of uranyl ion, results in the simplified form
where the concentrations, C, refer to uranyl ion in the feed phase. The degree of extraction, given by the term in brackets in eq 3, is the ratio of the total uranyl ion concentration extracted a t steady state (Ci - C,) to the concentration yet to be extracted (C, - CJ. Based on this equation, a semilogarithmic plot of the degree of extraction vs. contact time, t, should he linear. Figure 4 illustrates this linear relationship for a typical hatch extraction experiment for contact times up to 30 min. The slope of the line is proportional to the extraction rate constant, R,, given in eq 3. The extraction response
Ind. Eng. Chem. Fundam., Vol. 21, No. 4, 1982 419
R1,obtained from the slope of this plot was 0.061 min-'.
5
10
15
20
25
30
Contact lime, min.
Figure 4. LM extraction rate.
of the liquid membrane system to changes in a particular independent variable under investigation can be measured in terms of this normalized extraction rate constant. Experimental Section Reagents. The surfactant was a PIBSA-polyamine of 1500 molecular weight available from Exxon Chemical Co., U.S.A. DEHPA was used as obtained from Union Carbide Co. T O P 0 was used as obtained from American Cyanamid Co. "Low Odor Paraffinic Solvent" (LOPS), a distillate fraction with low ( for uranyl ion in solvent extraction. For extraction, twofold improvement in the distribution coefficient was observed by reducing the WPPA stream temperature from 60 to 40 "C (Hurst et al., 1972). The observed inverse relationship was the result of a combination of simultaneously occurring events. Complexing of uranyl ion by phosphoric acid in the aqueous phase provided solubilization of uranyl ion. The stability of these complex(es) decreases with increasing temperature. In the absence of other events, this would shift the equilibrium in favor of the organic phase, resulting in an increased E,". On the other hand, decreased stability with temperature of the organic soluble DEHPA-TOP0 uranyl ion complex would shift the equilibrium toward the aqueous phosphoric acid phase, resulting in a reduced E,". Thus, the latter phenomenon appears to dominate and offers a plausible explanation for the observed temperature effect of solvent extraction. Liquid-membrane extraction was shown to exhibit the opposite response to temperature as compared to solvent extraction. The extraction rate constant of 0.056 mi& obtained at 60 "C under standard conditions was used as the normalizing value. Shown in Figure 9 is an Arrhenius type plot of relative extraction rate constant and temperature over the range of 30 to 80 "C. The observed response of extraction rate to temperature changes tends to confirm that the complexing of uranyl ion by DEHPAT O P 0 at the WPPA-membrane interface is not rate controlling since complex formation is less favorable at elevated temperatures. Both diffusion or uranyl complex through the membrane and stripping should occur faster
with increasing temperature. The data suggest that the diffusion and/or the complex decomposition events are rate controlling. Conclusions The liquid membrane separation of uranium from phosphoric acid can be described in terms of kinetically controlled extraction as contrasted to the equilibrium limited solvent extraction process. This mechanistic difference is manifested in altered responses of extraction rate constant to changes in physicochemical parameters such as phosphoric acid strength, complexing agent concentrations, extraction temperature, and membrane viscosity. A simple first-order reversible kinetic expression was utilized to describe LM transport of uranyl ion from the bulk WPPA phase to the internal aqueous phase. Analysis of the response of the extraction rate constant to the variation of individual parameters indicated the critical rate-controlling steps within a set of experimental conditions. For uranyl ion separation with DEHPA-TOP0 complexing agent, the rate response to phosphoric acid strength, complexing agent concentration, and temperature indicated that transfer from the bulk WPPA phase to the external LM interface and complexing of uranyl ion by DEHPA-TOP0 at the external interface were not rate determining. The response to membrane viscosity and temperature indicated that membrane diffusion was, at least, a part of the rate-controlling mechanism. The variables explored still leave open to question the roles of stripping at the internal interface and bulk transfer of uranium in the internal phase. Studies to address the role of these transport events would involve the response to changes in surfactant concentration, internal phase droplet interfacial area, and membrane to internal phase ratio. The application of liquid membrane separation or uranium from wet process phosphoric acid offers several potential advantages over solvent extraction. By combining the extraction and stripping operations utilizing an emulsion, the organic loading limitations of solvent extraction are circumvented. In addition, reduced levels of expensive complexing agents can be employed in LM. Relative insensitivity to variations of WPPA acid strength and favorable response to temperature (60-70 "C) reduce processing limitations. The demonstration of these advantages in terms of a process flowsheet form the subject of another publication (Bock et al., 1982). Acknowledgment We wish to thank the management of Exxon Research and Engineering Company for support and permission to publish this paper. In addition, we wish to thank Steven Zushma for his excellent laboratory work. Finally, the many informative and stimulating discussions with W. S. Ho are gratefully acknowledged.
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Ind. Eng. Chem. Fundam. 1982, 21, 422-427
Nomenclature C = concentration t = contact time E: = equilibrium distribution coefficient T = temperature A = extracted species B = stored species R , = uranium extraction rate constant Subscripts i = initial f = final t = at a given time Literature Cited Bock, J. Proceedings of UMIST Seminar, April 1980, p 29. Bock, J.; Kleln, R. R.: Valint, P. L., Jr.; Ho,W. S. I n "Sulfuric/Phosphoric Acid Plant Operations", AIChE: New York, 1982. Cronan, C. S.Chem. €ng. 1959, 66, 108. Ferguson, D. E. U.S. At. Energy Cornm., Rept. ORNL-4272, 1966. Ferguson, D. E. U.S. At. Energy Comm., Rept. ORNL-4572. 1970. Frankenfeid, J. W.; Li, N. N. I n "Recent Developments in Separation Science", CRC Press: Cleveland, 1977; Vol. 3, p 285.
Greek, B. F.; Allen, 0. W.; Tynan, D. E. Ind. Eng. Chem. 1957, 49, 629. Hurst, F. J.; Crouse, D. J.; Brown, K. 8. U S . At. Energy Comm., Rept. ORNL-TM-2522, 1969. Hurst, F. S.: Crouse. D. S.;Brown, K. B. Ind. Eng. Chem. Process Des. Dev. 1972, 11, 122. Hurst, F. S.; Crouse, D. J. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 286. Kitagawa, T.; Nishikawa, Y.; Frankenfeid, J. W.; Li, N. N. Environ. Sci. Technol. 1977, 71, 602. Long, R . S.;Ellis, D. A.; Balles, R . H. Proc. I n t . Cong. Peaceful Uses A t . Energy 1955, 8, 77. Murthy, T. K. S.;Pai, V. M.; Wagle, R. A. "Proceedings, Symposium on Recovery of Uranium from Its Ores and Other Sources", International Atomic Energy Agency, 1970, p 341. Walas, S.M. "Reaction Kinetics for Chemical Engineers"; McGraw-Hill: New York, 1959; p 39.
Received for review August 13, 1981 Revised manuscript received June 1, 1982 Accepted June 24, 1982 Presented at the Division of Fertilizer and Soil Chemistry Symposium on Uranium Recovery from Wet Process Phosphoric Acid, 2nd Chemical Congress of the North American Continent, Las Vegas, NV, Aug 1980.
Approximate Solutions for Distillation Rating and Operating Problems Using the Smoker Equations Terry L. Tolllver' and Raymond C. Waggoner Department of Chemical Engineering, University of Missouri-Rolla.
Rolla, Missouri 6540 1
Algorithms for rapid solution of implicit variables in the Smoker design equations are presented. The accuracy of these solutions in the context of distillation rating and operating problems is evaluated for distillation systems where more rigorous physical property assumptions are known to apply. Alternative methods of initiilly fitting the Smoker equations to rigorous tray-by-tray material and energy balance calculations as well as methods extending the procedure to handle multicomponent systems are discussed. These methods are well suited for many optimization, reliability, and control studies as well as for the fast real-time calculations such as interactive computations or on-line control.
Introduction Recently there has been a great deal of interest in short-cut or approximate solution techniques for distillation calculations. These short-cut methods are useful for preliminary evaluation studies, optimization problems, design reliability calculations, process control studies, real-time control algorithms, interactive computations (CAD), and programmable calculator programs. One such method receiving interest anew is the analytical solution to the McCabe-Thiele diagram first presented by Smoker (1938). A paper by Strangio and Treybal(l974) discussed the advantages of using an analytical approach over the various popular empirical methods most of which are based upon correlations of Gilliland's (1940) data. Their paper presented a design algorithm which simultaneously satisfied the minimum number of stages, N, = &&,a) (Fenske, 1932); the minimum reflux ratio, R, = f(q,xd,Xf,LY)(Underwood, 1948); and the number of stages at a specified reflux ratio, N = f(q,Xf,Xd,Xb,RP) (Smoker, 1938).
* Correspondence concerning this paper should be addressed to this author at Monsanto Company, 800 North Lindbergh, St. Louis, MO 63166. 0196-4313/82/1021-0422$01.25/0
In addition to the design problem, a series of papers by McAvoy (1977), Jafary et al. (1979a), and Douglas et al. (1979) discuss the use of the Smoker equations for control and operability studies. As they point out, these equations are explicit in the design variables (N, and Ns),but must be solved iteratively for the operating or controlled variB, L,, v,, x),, or x,+)# They proceed to present ables (D, various alternate approximating equations that are valid under certain regions of column operation. These approximations are shown to be somewhat better than the empirical estimation procedures used by Shinskey (1977) for column control analysis. However, even these approximate solutions have been found unsatisfactory for columns operating close to their minimum reflux ratio and also for high-purity columns where the product impurities at one or both ends of the column are less than 1%, Efficient iterative algorithms for rapid solution of these implicit operating variables will now be presented. The use of iterative calculations is entirely satisfactory as long as convergence is guaranteed and rapidly achieved. These new algorithms have been applied to binary columns where the product compositions ranged from equal to the feed, down to impurity levels as low as one part per million. Computational accuracy of one part per million requires between 5 and 10 iterations, and intermediate calculations 0 1982 American
Chemical Society
