Recovery of uranium from phosphoric acid by means of supported liquid membranes

HydrometaUurgy, 7 (1981) 201--212

201

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

RECOVERY OF URANIUM FROM PHOSPHORIC ACID BY MEANS OF SUPPORTED LIQUID MEMBRANES

STYLIANOS SIFNIADES, T H E O D O R E LARGMAN, ALLEN A. TUNICK and F R E D W. KOFF

Corporate Research and Development, Allied Chemical Corporation, Morristown, NJ 07960 (U.S.A.) (Received November 4, 1980; accepted January 15, 1981 )

ABSTRACT Sifniades, S., Largman, T., Tunick, A.A. and Koff, F.W., 1981. Recovery of uranium from phosphoric acid by means of supported liquid membranes. Hydrometallurgy, 7: 201-212. U(VI) was transported at 23 + I°C from 5--6 M phosphoric acid solutions through liquid membranes of kerosene solutions of di(2-ethylhexyl) phosphoric acid and trioctyl phosphine oxide (D2EHPA/TOPO) supported on porous polytetrafluoroethylene to a solution of phosphoric acid o f equal or greater molarity containing ferrous ion as a reducing agent. The ferrous ion could be omitted when the higher molarity acid was used. The uranium flux was proportional to the U(VI) concentration. The overall resistivity of the membranes to uranium flux had a diffusional c o m p o n e n t that was proportional to the membrane thickness and an interfacial c o m p o n e n t that resulted from rate-limiting uranium complexation/ decomplexation kinetics. The interfacial c o m p o n e n t accounted for over 80% of the resistivity of a membrane 75 u m thick. Increasing the temperature to 60°C only slightly diminished the interfacial resistivity. A theoretical model was constructed that accommodated data obtained from uranium transport through the membranes and through quiescent layers of phosphoric acid and D2EHPA/TOPO in kerosene. The average uranium flux from simulated solutions of wet-process phosphoric acid at 90% uranium transfer was estimated to be 1.3 × 10 -1~ mol cm -2 sec -1, or 0.09 lb ft -2 y r - ' . The flux was judged to be too low for supported liquid membranes to be competitive with liquid/liquid extraction for recovery of uranium from wet-process phosphoric acid.

INTRODUCTION

Wet-process phosphoric acid contains uranium which can be recovered by liquid/liquid extraction. Two extractant systems are used commercially for this purpose: a kerosene solution of di-(2-ethylhexyl) phosphoric acid and trioctylphosphine oxide (D2EHPA/TOPO), or a kerosene solution of octylphenyl acid phosphate (OPAP). The first system preferentially extracts U(VI) (Hurst et al., 1972), whereas the second extracts U(IV) (Hurst and Crouse, 1974). As part of a continuing study of liquid membranes for refining and concentration of metal ions from solution (Largman and Sifniades, 1978)

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we were interested in demonstrating transport of uranium through such membranes from model solutions of uranium in phosphoric acid. Removal of uranium from nitric acid solutions b y means of a supported tributyl phosphate liquid membrane has been described (Moskvin et al., 1976). Uranium removal f r o m phosphoric acid solutions, a medium o f particularly strong affinity for uranium, b y means of supported liquid membranes has not been reported as yet. Use of a water-in-oil emulsion of D2EHPA-TOPO for extraction of uranium from wet~process phosphoric acid was recently reported (Bock and Valint, 1980; Hayworth, 1980). The extractant in that system functions as a liquid membrane b u t the operations associated with the system are much closer to liquid/liquid extraction than to membrane technology. EXPERIMENTAL

Octylphenyl acid phosphate (OPAP, mixture of mono- and di-esters) was obtained from Mobil Chemical; di-(2-ethylhexyl) phosphoric acid (D2EHPA) was obtained from Union Carbide; trioctylphosphine oxide (TOPO) was obtained from Aldrich; kerosene was obtained from Aetna Chemical Corp. and contained 97% aliphatic protons by NMR analysis. These materials were used w i t h o u t prior purification. All other chemicals used were reagent grade. Solutions of U(VI) were made b y dissolving the appropriate a m o u n t of uranyl nitrate (UO2(NO3)2" 6 H 2 0 ) in phosphoric acid. In order to make a solution of U(IV), UO2 was heated with conc. phosphoric acid until it dissolved, then the mixture was filtered and diluted with phosphoric acid (or water) to the desired strength. Polytetmfluoroethylene membrane, Goretex ®, 75 p m thick, 85% porous, 0.5 p m average pore size was obtained from W.L. Gore and Associates. Transport experiments were carried out in cells fashioned from standard 2 inch pipe parts; solutions were stirred magnetically (Largman and Sifniades, 1978). Each cell c o m p a r t m e n t had 350 ml capacity and the geometric area of the membrane was 20.27 cm 2. In some experiments the membrane was clamped between t w o perforated stainless steel plates, 1.59 m thick, each with 33 matching round holes of 7/32 inch diameter. The exposed geometric surface of the membrane was 8.00 cm 2. This arrangement allowed the use of multiple layers of membrane so that the effect of membrane thickness on transport could be studied. Liquid membranes were prepared by wetting one side of the polytetrafluoroethylene membrane in a pre-assembled cell with an excess a m o u n t of a kerosene solution of a chosen extractant and then forcing the liquid through the membrane pores by applying a vacuum on the other side. Excess liquid was removed b y pipette followed by rinsing b o t h sides of the membrane with 5 M phosphoric acid. A simple Lewis cell was used in order to study the transport of uranium from aqueous phosphoric acid to an extractant solution. A 600 ml Pyrex beaker, 7.55 cm ID, was equipped with a thermometer, a 5 cm long magnetic bar and a mechanical stirrer with a 5 cm long paddle located 3 cm above the b o t t o m of the beaker. A 100 ml sample

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of 5 M phosphoric acid containing uranium (VI) (2.00 g 1-1) was placed in the beaker and covered with 200 ml of kerosene solution of D2EHPA (0.5 M) and TOPO (0.125 M). The mechanical stirrer paddle was about 1 cm above the interface of the two liquids and rotated at 60 rpm while the magnetic bar remained anchored at the b o t t o m . Consequently, the two liquid phases acquired a gentle counterrotating m o t i o n relative to each other which allowed a relatively quiescent interface to be maintained. Samples were withdrawn periodically from the aqueous phase for analysis. The same apparatus was also used to study the transport of uranium from a kerosene solution of D2EPA/ TOPO to 10 M phosphoric acid. All transport experiments were carried out at ambient temperature (23 + I°C), except for one run in the Lewis cell at 60 -+ 2°C. Uranium analysis was carried o u t spectrophotometrically after extraction from acid
20001~ L E 1000

J 0 011y$

10

Fig. 1. Transport of U(VI) from 15M to 10 M phosphoric acid through a D2EHPA/TOPO liquid membrane: zx, half-cell A; o, half-cell B. Curves were drawn by means of eqn. (2) for VA = VB = 350 em 3, A = 20.27 em ~, P = 7.50 X 10 -s em see -~.

204

creased correspondingly. Small amounts of iron (65 mg 1-1) were also transported across the membrane. A plot of the logarithm of the ratio of the initial uranium concentration, [U] o, to the uranium concentration at time t, [U] t, versus time gave a straight line (Fig. 2) indicating first order dependence of uranium flux on uranium concentration. The slope of this line was used to calculate the permeability of the membrane for uranium, PU, by means of the following equation (see also eqn. (12)). ln([U] 0/[U] t) = PuAt/VA

(1)

A is the geometric surface area of the membrane and VA the volume of halfcell A. PU was found to be equal to (7.5 -+ 0.1) × 10 -s cm sec -z by least square fitting of all data points, except the last, through a linear regression analysis (the stated error is one standard deviation). By similar treatment, the permeabilities to phosphoric acid and to iron (Fe ~+ and Fe 3÷) were found to be 1.7 × 10 -6 and 3.3 × 10 -7 cm sec -1, respectively. It is evident that the selectivity of the membrane for uranium transport is very good. Equation (1) can be solved for [U]t: [U]t = [U]0 exp

(-PuAt/VA)

(2)

Equation (2) was used for drawing the curve in Fig. 1 for half-cell A.

© 3.0

0.6

[uJ o

[Ulo

0.4

0.2

0

5

Days

Days

Fig. 2. The data of Fig. 1 plotted according to eqn. (1). In drawing the solid line by least square fitting of the data the uppermost point was ignored. Fig. 3. Transport of U(VI) from 5 M to 10 M phosphoric acid through multiple liquid membranes of D2EHPA/TOPO. o, double membrane; zx, quadruple membrane.

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Similar experiments were carried out using double and quadruple membranes of D2EHPA/TOPO clamped between perforated plates (Table 1). A control experiment using a single membrane between the plates gave essentially the same value of PU as that obtained previously, thus confirming the absence of any artifacts induced by the plates. When the data from the multiple membrane experiments were plotted according to eqn. (1) downward curving lines were obtained (Fig. 3). Inspection of the membranes after the experiment revealed t h a t the individual membrane layers had separated from each other by up to a millimeter. The space between the layers was filled with an aqueous acidic phase. Presumably membrane separation occurred ~adually during the run and caused increasing resistance to uranium flux. Since the data deviated visibly from linearity, we did n o t calculate slopes by submitting all data points to a least square linear regression analysis but rather t o o k the tangents at the origin of the curves to represent the slope under the initial conditions of the experiment. At which point the thickness was known to be 1.5 X 10 -2 cm and 3.0 X 10 -2 cm for the double and quadruple membranes respectively. The corresponding uranium permeabilities were thus calculated to be (6.7 + 0.7) × 10 -s cm sec -1 and (5.7 + 0.6) X 10 -5 cm sec -1. (The error TABLE 1 U r a n i u m transport through liquid m e m b r a n e s No.

Membrane a

Half-cell A b

Half-cell B b

UB/UA c

PU (cm sec -1 × 10 5)

1

D/T, 0 . 5 0 / 0 . 1 2 5 , 75

7.50 + 0.10

D / T d, 0 . 5 0 / 0 . 1 2 5 , 75

4.0

7.42 +-0.15

3

D / T d, 0 . 5 0 / 0 . 1 2 5 , 150

0.9

6.68 -+0.67

4

D / T d, 0 . 5 0 / 0 . 1 2 5 , 300

1.2

5.67 +-0.57

5

D/T, 0.28/0.070, 75

5.7

3.50

6

D/T, 0.56/0.14, 75

8.7

2.78

7

D/T, 0.28/0.070, 75

FeSO4, 0.09 H3PO,, 10 FeSO4, 0.09 H3PO4, 10 FeSO4, 0.09 H3PO4, 10 FeSO4, 0.09 H3PO4, 10 Fe ° , p o w d e r H3PO4, 5.5 FeSO4, 0.05 H3PO4, 5.5 H3PO4, 10

21.2

2

1.5

1.72

8

D/T, 0.50/0.125, 75

U(VI), H3PO4, U(VI), H3PO4, U(VI), H3PO4, U(VI), H3PO,, U(VI), H3PO4, U(VI), H3PO4, U(VI), H3PO4, U(VI), H3PO4,

FeSO4, 0.09 H3PO4, 10

7.1

3.00

200 5.0 2000 5.0 2000 5.0 2000 5.0 225 5.5 200 5.5 200 5.5 200 6

a Kerosene solution ( e x t r a c t a n t ( D / T - D 2 E H P A / T O P O ) , molarity, t h i c k n e s s in u m) supp o r t e d on G o r e t e x , 20.27 cm 2 unless o t h e r w i s e n o t e d ; T = 23 +-1°C. b Initial conditions. All c o n c e n t r a t i o n s are molarities, e x c e p t u r a n i u m which is m g 1-1 . V o l u m e of half-cells 350 ml each. c Uranium c o n c e n t r a t i o n ratio in half-cells B and A at end of run; n o t an equilibrium ratio. d M e m b r a n e c l a m p e d b e t w e e n p e r f o r a t e d plates, e x p o s e d surface 8.00 cm 2 .

206

limits were estimated to be approximately 10% due to the subjectivity involved in drawing the curves and tangents). A plot of the inverse of uranium permeability, i.e., the resistivity, of the various membranes showed a large membrane resistivity to uranium at zero m e m b r a n e thickness (Fig. 4, intercept of upper line). This resistivity can be accounted for by the relatively slow reaction taking place at the membrane surfaces (see Theory below). During uranium transport through the membranes small amounts of phosphoric acid were also transported in the opposite direction. The resistivity to phosphoric acid was plotted versus membrane thickness (Fig. 4, lower line) and found to fit to a straight line which extrapolated essentially to zero resistivity at zero thickness. This finding corroborates the absence of diffusional resistance due to stagnant layers at the membrane/solution interfaces. The slope of the line was equal to 8.3 × 107 sec cm -1. Assuming that the diffusivity of the phosphoric acid species being transported is between 10 -s and 10 -6 cm sec -1, it can be calculated that the distribution coefficient of phosphoric acid between the aqueous and membrane phases is between 500 and 50. One practical corollary of the different effect of membrane thickness on the transport rates of uranium and phosphoric acid is that the thicker the membrane the more selective it is in transporting uranium relative to phosphoric acid. We also carried out a few U(VI) transport experiments with D2EHPA/ TOPO membranes starting at a m u c h lower uranium concentration ( ~ 2 0 0 mg 1-z) in 5.5 M and 6 M phosphoric acid. These concentrations are close to those encountered in commercial wet-process phosphoric acid streams. The permeability to uranium was about half the permeability observed at the higher Membrane Layers 0 i

1 i

2 +

4 i

15

3O

.., lo

Z0

E "

10

=

~o 1<

.s.

z

E

i

o

5

I o,:

o a

0

,;

,'o

2'o

~o

Total Thickness, cm • 10 ]

Fig. 4. Effect of membrane thickness on resistivity, o, membrane clamped between two perforated plates, A = 8.00 cm~; l , perforated plates omitted, A ffi 20.27 cm2; A, uranium resistivity calculated from quiescent interface experiments. Upper line: eqn. (14) with kinetic parameters of Table 2. Lower Line: least square fitting of phosphoric acid data.

207

concentration, probably reflecting the effect of increased phosphoric acid concentration in half-cell A and also the non-ideality of uranium solutions over a large range of concentrations. Of interest is the fact that transport of uranium was possible at a good rate even when half-cell B contained phosphoric acid of similar concentration to half-cell A (runs 5 and 6). This could be of some practical significance in a process because it would avoid the use of 10 M acid. In liquid[liquid extraction of uranium b y D 2 E H P A / T O P O the concentration of phosphoric acid used in stripping is also similar to the concentration o f the acid being extracted (Hurst et al., 1972). Uranium, U(IV), was also transported through OPAP liquid membranes into a half-cell conraining a phosphoric acid solution of sodium chlorate. The uranium flux, although comparable with that through D 2 E H P A / T O P O membranes, was erratic and no detailed study was undertaken. All membrane experiments are summarized in Table 1. Since interfacial resistance is responsible for a large portion of the overall resistivity of D 2 E H P A / T O P O membranes, we carried o u t U (VI) transport experiments through a quiescent interface first from 5 M phosphoric acid to a D2EHPA]TOPO kerosene solution and then from D 2 E H P A / T O P O to 10 M phosphoric acid using a simple Lewis cell. In the first case the data were plotted according to the following modified form of eqn. (1) that t o o k account of the fact that the concentration of uranium reached a non-zero equilibrium value, l n ( [ C ] 0 - [ V ] e q ) / ( [ U l t - [U]eq) = [PA(1 +KA)Atl/VA

(1')

where [V]eq is the equilibrium U(VI) concentration in 5 M phosphoric acid and K A is the equilibrium distribution coefficient of uranium b e t w e e n 5 M phosphoric acid and D 2 E H P A / T O P O (Coleman and R o d d y , 1971, p. 66). KA was found to be 0.127 by determining the uranium transported after vigorously shaking the t w o phases for several minutes so that equilibrium could be established. The permeability, PA, was calculated to be (1.05 + 0.01) X 10 -4 cm sec -~. KA and iDA increased to 0.48 and 1.4 × 10 -4 cm sec -~, respectively at 60 + 2°C. In the second case U(VI) was transferred essentially quantitatively to the 10 M acid and eqn. (1) was used. The corresponding permeability, PB, was (9.20 + 0.12) X 10 -s cm sec -~ (Table 2). The interfacial permeabilities found in this work are similar in magnitude to the permeability, 8.5 × 10 -s cm sec -~ reported for extraction of U ( V I ) from simulated sulfuric acid leach liquors ( 0 . 5 " 1 M sulfate, pH ~<1) by kerosene solutions of D2EHPA (0.1--0.2 M) and tributyl phosphate (0.1 M) (Coleman and R o d d y , 1971, p. 77).

Theory We will n o w relate the observed permeability of D 2 E H P A / T O P O membranes to the interracial kinetics and equilibria and to membrane thickness. A schematic representation of the liquid membrane cell is shown in Fig. 5. The following processes describe the transport of uranium from half-cell A, through the liquid membrane, M, to half-cell B. A fiat line on t o p of a symbol

208 TABLE 2 K i n e t i c p a r a m e t e r s in u r a n i u m t r a n s p o r t a Parameter

Value

Source

PA KA PB KB

1.05 X 10 -4 c m sec -z 0.127 9.20 X 10 -5 c m see -1 ~0 6 . 0 X 1 0 -7 c m sec -1 1 . 0 9 X 104 sec cm-* 1.15 x 104 sec c m -~

A -+ D 2 E H P A / T O P O b A ~ D2EHPA/TOPO b D 2 E H P A / T O P O -+ B b D 2 E H P A / T O P O -~ B b Fig. 4, slope eqn. (14), (L = 0) Fig.4, i n t e r c e p t

D~ Rs

a F o r t r a n s p o r t f r o m A, 5 M H3PO 4 c o n t a i n i n g initially U ( V I ) , 2 g 1-t, t h r o u g h a liquid m e m b r a n e of D 2 E H P A / T O P O ( 0 . 5 / 0 . 1 2 5 M in k e r o s e n e ) t o B, 10 M H 3 P O J 0 . 0 9 M FeSO4; T = 23 -+ I°C. b T r a n s p o r t t h r o u g h q u i e s c e n t i n t e r f a c e in a Lewis cell; at 60 + 2°C, P A = 1.4 × 10 -4 c m sec -1 a n d K A = 0.48.

A

A

UA + FA -

A

EB+ UB

OA

OB L

Fig. 5. S c h e m a t i c r e p r e s e n t a t i o n o f u r a n i u m t r a n s p o r t t h r o u g h a l i q u i d m e m b r a n e . Vertical a r r o w s r e p r e s e n t c h e m i c a l r e a c t i o n s ; h o r i z o n t a l a r r o w s r e p r e s e n t d i f f n s i o n a l m o t i o n . See also eqns. ( 3 ) - - ( 6 ) a n d e x p l a n a t o r y t e x t . denotes a species in the membrane phase and a simple symbol denotes a species in an aqueous phase. Subscripts denote the half-cell closest to the species.

UA+LA~ kA. UA

(3)

P-A

CA- P~U.. UB

(4)

p-~

-

-

UB

PB - '~ " L B k-B

L B ~"

"~ L A

+ UB

(5)

(6)

209 Sequentially, these processes represent complexation of uranium from the solution in half-cell A to D 2 E H P A / T O P O ligands, L, at interface A (3), diffusion of the complexed uranium species U to interface B (4), dissociation of the complex (5) and back diffusion of the free ligand to interface A (6). k A and kB represent the rate constants for complexation of uranium at interfaces A and B respectively. P-A and PB represent the permeabilities of uranium complex at interfaces A and B, respectively. PU is the permeability in the interior of the membrane. Equations (4) and (6) represent coupled diffusion processes, therefore, only one, (4), need be considered kinetically. We begin by writing steady-state equations for UA and UB. This treatment is justified because the quantity of uranium in the membrane is very small compared with the uranium in half-cells A and B. In the following equations symbols of species denote concentrations. dUA]dA = kALAUA - P - A U A - P-uUA + P-UUB = 0

(7)

dUB/dA = P ~ U A - PUUB - PBUB + k-BLBUB = 0

(8)

In these experiments the concentration of ligand was much larger than the concentration of uranium, therefore LA -~LB = L0. The quantity kALA = kAL0 is a constant that represents the permeability PA of uranium at interface A. k_B -~ 0, since transfer of uranium from D 2 E H P A / T O P O to 10 M phosphoric acid has been shown to be essentially quantitative. Making these substitutions in eqns. (7) and (8) we find the steady-state concentrations of uranium complex at A and B UA = [PA(P-u+ PB)UA]/(P-AP-u + P-APB + PBP-uu) UB = (PAP-uuUA)/(P-AP-u + P-APB + PB~UU)

(9) (I0)

From reaction (5) the uranium flux JU is equal to PBUB. Considering also (10) we get J u = (PAPBP-uUA)/(P-AP-u + P-APB + PBP-u)

(11)

With reference to half-cell A the flux is Ju = - ( V A d U A ) / A d t = PUUA

(12)

The integrated form of this equation is eqn. (1), which was used to calculate Pu from the experimental data. Comparison of (11) and (12) gives: PU = (PAPBP-u)/(P-AI~ + P-APB + PBP-U)

(13)

A more useful form of this equation can be obtained by considering that P-~ = D-~/L, where D~ is the diffusion coefficient of U in the membrane and L is the thickness of the membrane. Also KA = P-A/PA. Making these substitutions and inverting (13) we find that the resistivity R U = 1/Pu, of the mem-

brane is

210

RU = (KA/PB) + (1]PA) + ( K A L / D ~ )

(14)

Equation (14) shows that a plot of resistivity versus membrane thickness should have an intercept equal to (KA/P B + 1/PA) and slope equal to KA/DU. The upper straight line in Fig. 4 was drawn by least square fitting of the resistivities from the multiple layer experiments and the resistivity at zero membrane thickness calculated from (14) for L=O using the data obtained from quiescent interface experiments. The match was very good. The diffusion coefficient, DU, of the uranium complex in the membrane was calculated from the slope of Fig.4 to be 6.0 × 10 -7 cm 2 sec -1. This is somewhat lower than expected for a species of relatively low molecular weight and it indicates a greater effective membrane thickness than the geometric thickness of the membrane. This may be caused by the tortuosity of the pores of the Goretex ® support. A tortuosity factor of three has been found for liquid membranes of cholanic acid supported on a composite of dialysis paper and glass fiber paper (Choy et al., 1974). If a similar factor applies in our membranes, DU = 1.8 X 10 -6 cm 2 sec -1 , a reasonable value. CONCLUSIONS We have shown t h a t uranium can be transported from 5--6 M phosphoric acid solutions (A) through a liquid membrane and concentrated into 5.5--10 M phosphoric acid solutions (B). The uranium flux is proportional to the uranium concentration in half-cell A. When a D2EHPA/TOPO liquid membrane is used, a U(VI) species is transported to a receiving cell containing a reducing agent t h a t transforms uranium to U(IV). With an OPAP membrane U(IV) is transported and is oxidized to U(VI) in the receiving cell. The D2EHPA/TOPO liquid membrane has a high interracial resistivity to uranium transport due to slow kinetics of uranium complexation/decomplexation at the solution/membrane interfaces. In fact, over 80% of the overall resistivity of a membrane 75 p m thick resides at the interfaces. From the practical point of view, the high interracial resistivity means t h a t no significant gains in uranium flux will be made by decreasing the membrane thickness. Higher temperature is n o t promising in improving the flux, because the interracial resistivity shows little temperature dependence; furthermore, higher temperatures m a y destabilize the liquid membrane. The average uranium flux through a membrane of permeability PU transporting uranium from a stock solution containing uranium at initial concentration U0 and proceeding to uranium recovery X (X = fraction transported) can be calculated with the aid of the following formula, which is derived from eqn. (1) Ju = (PUXUo)/[ln(1-X)I The uranium concentration in 5--6 M wet-process phosphoric acid is of the order of 100 mg 1-1 or 4.2 X 10 -7 mol cm -3. At that concentration the

211

permeability is a b o u t 4 × 10 -s cm sec -1. Assuming that a 90% recovery of uranium is desired (X = 0.9), we find J u = 1.3 X 10 -1' mol cm -2 sec-' or 0.09 lb ft -2 yr -1 (1 yr = 8000 h). The cost of a commercial liquid membrane is n o t established at present because the required porous supports are n o t available on the scale needed to recover uranium from a wet-process phosphoric acid plant. The least expensive commercial membranes available t o d a y are cellulose acetate hollow fibers used in dialysis. The cost of a module is $1.50 per square foot of surface area (L.G. Gilbert, personal communication, 1978). If it is assumed that porous hollow fibers of polyethylene, or other suitable material, can be made at a comparable cost, the capital investment for the membranes would be ~ $ 1 6 per p o u n d of uranium produced annually. By comparison, the extractant inventory {e.g., of OPAl )) in a liquid/liquid extraction process for uranium recovery from wet-process phosphoric acid of similar uranium content is estimated at ~ 1 lb (lb U) -1 yr-l; this corresponds to a capital investment of less than $2 per p o u n d of uranium (Rickard and Symens, 1979). Therefore, we conclude that, unless there is a major break-through in porous membrane technology, resulting in cost reduction by at least one order of magnitude, use of liquid membranes is n o t economically attractive for uranium recovery from wet-process phosphoric acid. LIST OF SYMBOLS AND ABBREVIATIONS PA P-A

PB P-B KA KB

Pu DU Ru RS A L

VA VB t

permeability of interface A for transport of uranium from aqueous to membrane phase permeability of interface A for transport of uranium from membrane to aqueous phase permeability of interface B for transport of uranium from membrane to aqueous phase permeability of interface B for transport of uranium from aqueous to membrane phase P _ A / P A = distribution coefficient at interface A P _ B / P B = distribution coefficient at interface B overall permeability of membrane to uranium permeability of membrane to uranium excluding interfaces diffusion coefficient of uranium in the membrane phase 1 / P u = overall resistivity of membrane to uranium transport surface resistivity of membrane to uranium transport surface area of the membrane, each side thickness of membrane volume of half-cell A volume of half-cell B time

212

D2EHPA TOPO OPAP D/T

di-2-ethylhexyl phosphoric acid trioctyl phosphine oxide octylphenyl acid phosphate D2EHPA/TOPO

REFERENCES Bock, J. and Valint, P.L., Jr., 1980. Uranium from wet-process phosphoric acid -- a liquid membrane approach. Second Chemical Congress of the North American Continent, Division of Fertilizerand Soil Chemistry, Paper No. 37. Choy, E.M., Evans, D.F. and Cussler, E.L., 1974. A selective membrane for transporting sodium ion against its concentration gradient. J. A m . Chem. Soc., 96: 1085--1090. Coleman, C.F. and Roddy, J.W., 1971. Kinetics of metal extraction by organophosphorus acids. In Y. Marcus (Ed.), Solvent Extraction Reviews, Vol. 1, Marcel Dekker, N e w York. Hayworth, H.C., 1980. Advantages of liquid membrane technology for the extraction of uranium from wet-process phosphoric acid. Second Chemical Congress of the North American Continent, Division of Fertilizerand Soil Chemistry, Paper No. 38. Hurst, F.J., Crouse, D.J. and Brown, K.B., 1972. Recovery of uranium from wet-process phosphoric acid. Ind. Eng. Chem. Process Res. Develop., 11: 122--128. Hurst, F.J. and Crouse, D.J., 1974. Recovery of uranium from wet-process phosphoric acid by extraction with octylphenylphosphoric acid. Ind. Eng. Chem. Process Res. Develop., 13: 286--291. Largman, T. and Sifniades, S., 1978. Recovery of copper(II) from aqueous solutions by means of supported liquid membranes. Hydrometallurgy, 3: 153--162. Moskvin, L.N., Krasnoperov, V.M., Grigoriev, G.L. and Tsaritsyna, L.G:, 1976. Separation of uranium and fission products by using a liquid extraction membrane. Radiokhimiya, 18:851-857. Rickard, R.S. and Symens, R.D., 1979. Uranium recovery from wet-process phosphoric acid. In: P.H. DeVoto and D.N. Stevens (Eds.), Technology and economics of uranium recovery from phosphate resources, United States and Free World, Vol. 2. Earth Sciences, Inc. Report GJBX-110 (79) submitted to U.S. Department of Energy.