HydrometaUurgy, 18 (1987) 117-122 117 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Technical Note

Solvent Extract ion of Uranium (VI) from Industrial and Analytical Grade Phosphoric Acid Using Organic Extractants in Kerosene with Variable Content of Aromatic Hydrocarbons

IVAN BRCIC, IVO FATOVIC, SMILJANA MELES, EUGENI0 POLLA* and MARKO RADOSEVIC

INA-Research & Development, Proleterskih brigada 78, Zagreb (Yugoslavia)

(Received June 20, 1986; accepted in revised form September 29, 1986)

ABSTRACT

Braid, I., Fatovid, I., Melee, S., Polla, E. and Rado§evid, M., 1987. Solvent extraction of ura- nium(VI) from industrial and analytical grade phosphoric acid using organic extractants in kerosene with variable content of aromatic hydrocarbons. HydrometaUurgy, 18:117-122.

The distribution ratio of U (VI) between phosphoric acid, industrial and analytical grade, and kerosene solutions of D2EHPA-TOPO extractants with variable amount of aromatic hydrocar- bons has been studied. Under these conditions it was observed that the content of aromatic hydro- carbons in the diluent as well as other ions present in industrial phosphoric acid play a significant role in the extraction of uranium (VI).

INTRODUCTION

Much attention has been devoted lately [ 1,2 ] to extraction of uranium ( VI ) from phosphoric acid using D2EHPA-TOPO extractants in kerosene. How- ever, parameters such as the content of aromatic hydrocarbons in the diluent and the influence of other ions present in industrial phosphoric acid on the extraction of uranium (VI) have not been considered [ 3,4 ].

The current study was undertaken to quantify this behaviour and to obtain further information on the extraction of uranium (VI) from phosphoric acid, of industrial and analytical grade, by D2EHPA with TOPO synergist in a ker- osene diluent with different contents of aromatics. This paper presents the findings of these investigations.

0304-386X/87/$03.50 © 1987 Elsevier Science Publishers B.V.

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TABLE 1

Analysis of the industrial phosphoric acid used in the test programme

Element Concentration, Element Concentration, M M

Fe 5.3×10 2 Cu 7.4X10 1 V 7.8×10 -:~ Cr 3.0×10 :~ B 5.5×10- 4 Pb 6.3×10 ~' Mo 8.3X10 ~

E X P E R I M E N T A L

Reagents

Crystalline uranyl nitrate hexahydrate was used for all uranium solutions. Di-2-ethylhexylphosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO) were provided as high purity products by Mobil Oil Co. Solutions of these organophosphoric extractants were prepared in kerosene. Two kerosene samples, Sample A (Solvent 190-210 from Mobil Oil Co.) and Sample B (Refinery Oil, Sisak, Yugoslavia) with aromatic hydrocarbon contents of 0.5 % and 18.3%, respectively, were used. The aromatics content was determined by the ASTM method [ 5 ].

The kerosene was pre-equilibrated with equal volumes of acid solutions in a separating funnel for 5 minutes. The D2EHPA and TOPO organic phase was prepared by diluting D2EHPA and TOPO with kerosene and then purifying the solution from possible acidic degradation products by scrubbing twice with a 10% sodium carbonate solution, followed by repeated washing with water.

The model H3PO4 solution was prepared from analytical grade phosphoric acid, and the industrial H~PO4 solution was prepared from an industrial sam- ple of phosphoric acid (green grade) containing other ionic impurities.

Some of the major impurities and their corresponding concentrations for a typical industrial 4.91 M phosphoric acid solution are presented in Table 1.

Procedure

Aqueous-organic equilibrations were performed by contacting equal vol- umes (50 ml) of aqueous and organic solution in a Lewis-cell-like apparatus. The two phases are mixed by four paddles revolving at 1310_+ 10 min-1. The apparatus was thermostated by an external thermostatic bath and the temper- ature fluctuation was within _+ 0.5 ° C. Although the equilibrium was attained in less than 2 min the contacting time for all the extractions was 10 min.

The initial aqueous uranium concentration in analytical grade samples was

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always around 4.62 m M and in industrial samples about 5.46 mM. The ura- nium concentrat ion in the equilibrium aqueous and organic phases was deter- mined by spectrophotometry [6,7] using a Pye Unicam spectrophotometer, Model SP 6-550.

The experimental results are expressed in terms of distribution ratio D:

uranium concentrat ion in the organic phase (Cu) o D = (1)

uranium concentrat ion in the aqueous phase (C~j) a q

RESULTS AND DISCUSSION

The effect of temperature on the extraction reaction

The temperature dependence for the extraction of uranium from model and industrial solutions of phosphoric acid (4.91 M) using 0.5 M D2EHPA-0.125 M TOPO in two kerosene solutions containing different amounts of aromatic hydrocarbons were measured. The enthalpy changes associated with the extraction of U (VI) were calculated by the van 't Hoff equation

~ H l o g D - - (2)

2.303 R T

The distribution ratios obtained were plotted as log D vs l I T (Fig. 1 ). The experimental results reveal that the distribution ratio for the extraction

decreases, in all cases, with increasing temperature. The same results have been observed by other authors [ 8,9 ] for the extraction of various species from different media using D 2 E H P A and TOPO.

The enthalpy changes (kJ mol 1 ) are for the model system (4.91 M H3PO4; 0.5 M D2EHPA-0.125 M TOPO ) :

- 36.97 + 1.48 for kerosene A - 43.45 _+ 1.30 for kerosene B

industrial system ( 4.91 M H3PO4; 0.5 M D2EHPA-0.125 M TOPO ): - 42.49 _+ 2.21 for kerosene A - 3 9 . 8 6 + 1.76 for kerosene B

which indicates that the extraction of U ( V I ) by D 2 E H P A - T O P O - k e r o s e n e is an exothermic reaction.

The effect of aromatic hydrocarbons in the diluent on uranium(VI) extraction

The temperature effect on the distribution ratio for D 2 E H P A - T O P O extraction of U ( V I ) from 4.91 M phosphoric acid, model and industrial sys- tems, has been examined as a function of aromatic hydrocarbon content. The experimental results are shown in Fig. 1. The behaviour of the distribution

120

1,2

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// i II

08

l/Tx 103

a

o) o

0.4

02

a 0.i

o~ o

<~0.0

-0.1

j ~ a j ~

, 1 I 20 40 60

T°C

Fig. 1. Effect of temperature on uranium (VI) extraction by 0.5 M D2EHPA-0.125 M TOPO. O: model system {4.91 M H3PO4)-kerosene A; O: model system (4.91 M H,~PO4)-kerosene B; O: industrial system ( 4.91 M H3PO4 ) -kerosene A; m: industrial system ( 4.91 M H3P04 ) -kerosene B.

Fig. 2. The plots of Alog D vs Tfor (a) model system and (b) industrial system.

ratios is different for kerosenes A and B. The AAH values reveal that in the model system the difference in enthalpies is about 6.5 kJ mol - ' and in the industrial system about - 2.7 kJ mol- 1

In Fig. 2 the plot of Alog D vs T (line a) indicates that there is a positive difference between the distribution ratios using a model system and kerosene A and B with increasing temperature. Line b represents the dependence for the industrial system in which the enthalpy difference is of the opposite sign.

Thus the content of aromatic hydrocarbons in kerosene plays a significant role in the extraction of uranium (VI).

Synergistic effect of TOPO on extraction process

The distribution ratios for the extraction of U(VI) in the H3PO4-D2EHPA-TOPO-kerosene A system, model and industrial, at 0.025-0.125 M TOPO are shown in Fig. 3. A synergistic enhancement in both systems in 4.91 M phosphoric acid at 40 ° C is obtained. The plots of log D vs log ( CTopo ) o have a slope approximately equal to 1 at constant D2EHPA and acidity concentration which is in agreement with the following expected extraction equilibrium:

U 0 2 2 + (a) + 2 (HX)n (o)+TOPO(o) ~UO2H2n_2X2nTOPO(o) +2 H + (a)

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1.6

I./+

1.2

O.B

a 0.£

0

0.0

1.0

o o C30.B

. _o 0.6

O.Z,

/ SLOPE 0.90 0.2 /

/ /

/ / o / 0.0

I I I i I I

-16 -1,4 -1.2 -to -as 3.0 s.o log [CToPO] M H 3 PO/.,

\ ~\ , \

7.0

Fig. 3. Synergistic effect of TOPO on uranium(VI) extraction: org. phase: 0.5 M D2EHPA-TOPO-kerosene A; aq. phase: 4.91 M H.~P04 (model and industrial) at 40 ± 0.5 ° C.

Fig. 4. Effect of H.~P04 concentration on uranium (VI) extraction; org. phase 0.5 M D2EHPA-0.125 M TOPO. [~: kerosene A and H3P04 model system; m: kerosene B and HaPO4 model system; O: kerosene A and H,~PO4 industrial system; O: kerosene B and H3P04 industrial system.

The experimental results show that the adduct formation increases the distri- bution ratio to the same extent in different phosphoric acids, industrial and model.

Effect of phosphoric acid concentration on the distribution ratio of U(VI)

The distribution ratios obtained for uranium (VI) as a function of H3P04 concentration, in model and industrial solutions, using a kerosene solution with two different contents of aromatics, are presented in Fig. 4. The experi- ments were performed at 40 ° C.

The distribution ratio decreases very rapidly with increasing H3PO4 concen- tration, independent of the diluent used. The values of D in both systems are

122

TABLE 2

Distribution coefficients of uranium (VI) between a model solution of phosphoric acid (4.91 M) and 0.5 M D2EHPA-0.125 M TOPO in kerosene containing different amounts of aromatic hydro- carbons at 40 °C

Aromatic Distribution hydrocarbons, coefficient, (%) D

0.5 6.53 6.0 6.04

12.0 5.28 18.3 4.64

higher us ing ke rosene wi th n o n - a r o m a t i c h y d r o c a r b o n s as d i luent ( k e r o s e n e A) . T h e s e e x p e r i m e n t a l resu l t s are cons i s t en t wi th the fac t t h a t a t lower and higher ac id i ty the chemica l species which are p r e d o m i n a n t in the aqueous solu- t ion are UO22÷ and UO2(H2PO42-" )n , r e spec t ive ly [8 ] .

CONCLUSION

In this work the e x t r a c t i o n b e h a v i o u r of u r a n i u m ( V I ) f rom model a n d indus t r ia l p h o s p h o r i c acid s y s t e m s wi th D 2 E H P A - T O P O in ke rosene wi th di f ferent con ten t s of a roma t i c hyd roca rbons was invest igated. T h e resul ts show t h a t the d i s t r ibu t ion ra t ios for the ex t r ac t i on of U (VI ) va ry apprec iab ly . T h e resul ts show how the phys ica l p rope r t i e s of the d i luent as well as the ionic impur i t i e s in indus t r i a l p h o s p h o r i c acid af fec t the ex t r ac t i on of u r a n i u m ( VI ).

REFERENCES

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2 Ting, G. and Lee, T.W., Radiochim. Acta, 28 (1981) 103. 3 Ritcey, G.M. and Lucas, B.H., Proceedings of the International Solvent Extraction Confer-

ence, ISEC '74, Lyon, 1974, Society of Chemical Industry, London, 1974, p. 2437. 4 Arnold, W.D., McKamey, D.R. and Baes, C.F., Report ORNL-TM-7182, Chemical Technology

Division, Oak Ridge National Laboratory, 1977. 5 ASTM Annual Book of Standards, Part 23, ASTM Method D 1319-70, American Society for

Testing and Materials, Philadelphia, PA, 1975, p. 672. 6 BrSid, I., Polla, E. and Rado~evid, M., Analyst, 110 (1985) 1249. 7 Braid, I., Polla, E. and Radogevid, M., Analyst, 110 (1985) 1463. 8 Bunus, F.T., Domocus, V.C. and Dumitrescu, P., J. Inorg. Nucl. Chem., 40 (1978) 117. 9 Sato, T., J. Inorg. Nucl. Chem., 24 (1962) 699.