Treatment of Uranium Effluent by Reverse Osmosis Membrane

Treatment of Uranium Effluent by Reverse Osmosis Membrane

Desalination, 71 (1989) 35-44 35 Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands T r e a t m e n t of U r a n i u m Effl...

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Desalination, 71 (1989) 35-44

35

Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands

T r e a t m e n t of U r a n i u m Effluent by R e v e r s e Osmosis M e m b r a n e GING-HO HSIUE* and LIEH-SHENG PUNG

Department of Chemical Engineering, National Tsing Hua University, Hsin Chu, Taiwan (China), Tel. (035) 719956 MIN-LIN CHU and MU-CHANG SHIEH

Institute of Nuclear Energy Research, Lung Tan, Taiwan (China), Tel. (034) 712201 ext. 2464 (Received December 25, 1987; in revised form July 24, 1988)

SUMMARY

Membrane separation processes play a very important role in wastewater treatment. Uranium conversion process effluent, which contains many toxic, corrosive and radioactive compounds, needs further treatment and recovery of uranium. In this research the reverse osmosis membrane separation process was applied to treat uranium conversion process effluent. The uranium content was lowered to less than 1 rag/l, the rejection of uranium was 99.5% or higher, and the overall decontamination factor was higher than 100. After repeated treatments in a batch-type concentration process of the uranium conversion process effluent, the uranium content in the concentrated solution was higher than 2.1 g/l, and the volume was reduced to 30% of its original value. These results show that uranium conversion process effluent can be successfully treated to meet the standards of waste disposal. Because of the variations in the composition, chemical properties and p H value of uranium conversion process effluent, it is necessary to study the effect of these parameters on the membrane separation process. The results show that the most important factor is the feed p H value. The effect of pressure was also important. It is shown that proper control of these three factors is essential for successful treatment of uranium conversion process effluent. Keywords: membranes, reverse osmosis, uranium effluent, ammonium fluoride.

INTRODUCTION

The development of the reverse osmosis (RO) process was started in 1953. A homogeneous cellulose acetate membrane was first successfully tested by *To whom all correspondence should be addressed.

0011-9164/89/$03.50

© 1989 Elsevier Science Publishers B.V.

36 Reid and Breton [ 1 ]. Thereafter, a high water-flux asymmetric cellulose acetate membrane was synthesized by Loeb and Sourirajan in 1960 [2]. Hence, the prospect of using a polymer membrane in separation processes was effectively raised. However, the effect of compaction occurring during practical operation may cause a decrease in water flux, and the properties of the dense layer in the cast film were difficult to control in these membranes. Therefore, Merten [3] prepared a composite membrane by casting a film on a porous membrane support. Compared to the asymmetric membrane, the composite membrane shows a small compaction effect. Due to better mechanical properties, the composite membrane is more suitable for operation at high pressure. Because of these advantages, the composite membrane has become a major product in the membrane industry. So far, many commercialized products are available, among which those produced from cellulose acetate, polyamide-polysulfone, and polyacrylonitrile [4]. The uranium conversion process effluent (UCPE) can be classified into two different types [5]: (1) Uranium nitrate effluent (UNE), such as yellow cake or ammonium uranyl tricarbonate (AUT), which comes from the extraction and conversion process of natural uranium. The basic precipitation reaction is: 2 UO2(NO3)2 (aq) + 6 NH4OH (aq)

' (NH4)zU207 (s) + 4 NH4NO3 (aq) + 3 H20 (~)

(2) Uranium fluoride effluent (UFE), which comes from the ammonium diurate (ADU) or ammonium uranyl carbonate (AUC) conversion process of UF6. The basic precipitation reaction is: 2 UO2F2 ( s ) + 4 HF ( a q ) + 1 0 N H 4 0 H (aq)

, ( N H 4 ) z U 2 0 7 (s)

+ 8 N H 4 F ( a q ) + 7 HzO (~) The composition and concentrations of UCPE are very complicated and depend on the process conditions. The major components in UCPE are uranium compounds, nitrate, ammonia, ammonium ions and fluoride ions. The UCPE is radioactive, toxic and corrosive. If it is released without effective treatment, not only will the environment be polluted, but also the valuable uranium will be wasted. So far there are no reports in the literature concerning the use of RO membranes in the treatment of UCPE. However, there are a few reports [6-9] on the use of RO membranes in the treatment of uranium leaching liquor and nuclear power plant wastewater. Several studies done by the Institute of Nuclear Energy Research (INER) on the solute rejection of Filmtec's FT-30 membrane module show that the solute rejection of uranium can be higher than 98% and the radioactivity of permeate water can be reduced to the lower limit detection (LLD) value. Based

37

on the above results, this research is focused on the study of the separation characteristics of UNE and NH4F (aq), using the uranium recovery membrane process developed by INER. By changing the operation parameters such as concentration, flow rate, pH value and pressure, and analyzing the separation effectiveness of each solute in the Filmtec's FT-30 membrane module, the relationship between all these factors was elucidated. Furthermore, the degree of concentration of uranium ion was also studied in order to achieve better uranium recovery and pollution control. EXPERIMENTAL

Equipment A schematic diagram of the apparatus is shown in Fig. 1. The RO membrane was obtained from Filmtec Corporation (Model FT-30). UNE and UFE were provided by INER. All of the other chemicals were reagent grade. The permeate water flux of the overall RO separation process can be expressed as

~

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~-~ oncentrate

Fig. 1. Schematic diagram of the apparatus used for uranium recovery by membrane separation process.

38

Jw =A (,4p-,4~) where A is the permeability of water in membrane (mZ/mZd atm), ziP is the statis pressure gauge of liquid (atm), and z]~ is the osmotic pressure (atm). The permeate solute flux can be written as

J, = BACI were B is the permeability of solute in the membrane (m3/m 2 d) and Cl is the concentration difference of solute ( m o l / m 3 ). The solute rejection Rl is defined as R, = ( 1 - - ~ )

× 100%

where C, and Cf are the solute concentration in permeate and feed, respectively.

Experimental procedure (1) Effect of the feed pH The RO membrane separation system was set up. To evaluate the effect of the pH value of the feed stream on the separation performance, the pH values of the feed were adjusted and varied from 3.0 to 10.0. Other conditions, such as concentration of the feed, flow rate, temperature and pressure, were kept constant. The pH affects the separation via its effect on the hydration and absorption capacity of the solutes in the membrane, and the conformation of the solutes in the solutions.

(2) Effect of the operating pressure The operating pressure was varied while other conditions were kept constant. Variation of the operating pressure would affect the permeate flow rate as well as the rejection ratios of the solutes. The extent of compaction effect can also be estimated from the measured changes in solute concentrations.

(3) Effect of feed concentration The concentrations of the feed stream were varied while other conditions were kept constant. The feed concentrations would affect the permeate flow rate and rejection ratios of the various solutes, and thus affect the extent of concentrate achieved.

(4) Effect of the permeate water recovery The feed rate, temperature, and operating pressure were kept constant while the feed concentrations, permeate flow rate and rejection ratios were measured in a batch-type operation. The recovery of the permeate water was thus demonstrated and related to the operating conditions.

39 RESULTS A N D DISCUSSION

Studies on the separation characteristics of ammonium fluoride using RO membrane (1) Effect of feed pH The influence of the pH on the separation behavior under the conditions of 54.44 atm, 22°C and 0.45 g/1 o f F - in the feed is shown in Fig. 2. The rejection ratios of ammonium fluoride depended on the feed pH value. At pH = 6-8, the rejection ratios were higher than 95%. At pH = 7.5, the separation efficiency was the best and the rejection ratio was higher than 98%. With pH > 8, large amounts of undissociated ammonia molecules were in the solution, and because of the strong hydrogen bonding between the ammonia molecules and the membrane [10], they were easily dissolved in the membrane. This resulted in a decrease in the separation efficiency. At pH = 9.65, the rejection ratio of NH3NH4 + was only 45%, but the rejection ratio for the fluoride ion remained higher than 99.5% because of the solvating effect of the fluoride ion in basic solution. With pH < 4, the strong hydrogen bonding of the fluoride ion in acidic solution resulted in an increase in the solubility of the fluoride ion in the membrane and hence the permeability of the fluoride ion was increased. Therefore, the rejection ratio of the fluoride ion was only about 45% at pH = 3.25. At the same time, the rejection ratio of ammonia ions also decreased because of their weak hydrogen bonding. The ratio decreased to 89.4% which, however, was not as low as for the fluoride ion. The permeate water flow, which was maintained at 15.8 gal/h, did not depend on the pH value. This means that the pH variation did not affect the property of the membrane.

(2) Effect of pressure The permeate water flux increased linearly as the pressure increased when the properties of feed water were kept constant. As shown in Fig. 3, the average I00

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Fig. 2. Rejection vs. pH for the RO membrane. Operating conditions: feed flow rate = 636.0 l/h; T=20°C; [F- ]f~ed=450 g/1. ( I ) NH + ; (o) F - ; ([]) permeate rate Qw.

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Fig. 3. Permeate rate vs. operating pressure for the RO membrane. Operating conditions: pH = 9.0 ___0.2; feed flow rate = 636.0 l/h; T = 22 ° C. [ F - ] in the feed, in g/l: ( • ) 0.60; ( o ) 2.81; ( 0 ) 4.46; ( A ) 6.84.

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Fig. 4. Rejection vs. operation pressure for the RO membrane. Operating conditions: p H = 8 . 8 ; feed flow rate=636.0 l/h; T = 2 2 ° C . ( e ) [NH4F]feed=14.76 g/l; (o) [F-]feed=6-84 g/l; ([~) [ NH4 ] feed= 7.92 g/1.

increase constant A, obtained by using the least squares method with the pressure varying from 13.6 to 54.4 atm, is 2.08X10 -2 m3/m 2 d atm. The result shows that the permeate coefficient of Filmtec's FT-30 is independent of the pressure when the pressure ranges between 13.6 and 54.4 atm. Fig. 4 shows that the rejection ratio of each component increased as the pressure increased. When the pressure was raised from 13.6 to 54.4 atm, the rejection ratio of ammonium fluoride increased by about 2%. Although at higher pressure, higher capacity can be obtained, there is a limit to which the operating pressure can be raised. If the pressure is too high, much more energy will be consumed, the compact effect will be increased, the permeate flux will be lowered and the lifetime of the membrane will decrease. Therefore it is very important to optimize these factors to obtain the most economic operation conditions in designing the RO system.

41

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Fig. 5. NH4F(aq) on permeate and rejection vs. feed concentration for the R O membrane. Operating conditions:pH--9.0 +_0.2;feed flow rate= 636.0 I/h; T=22°C; operating pressure = 54.4 arm. (• ) F-; (o ) NH4F; ([]) N H + • Fig. 6. Permeate rate vs. feed concentration for the R O membrane. Operating conditions: pH=9.0_-L-0.2; feed flow rate=636.0 I/h; T--22°C. (e) 54.4 atm; (o) 47.6 atm; ([7) 27.2 atm; (A ) 13.6 attn.

(3) Effect of feed concentration The influence of the feed concentration on permeate flow and rejection ratio under different pressures, i.e., 13.5, 27.2, 40.8 and 54.4 atm, is shown in Figs. 5 and 6. Fig. 5 shows that the rejection of the ammonium ion increased as the feed concentration increased. As has been shown in Section 1, at 54.4 atm pressure, when the operation conditions were changed from [ F - ] =0.6 g/l, pH = 9.2 to [ F - ] = 6.81 g/l, pH = 8.8, the rejection ratio of the fluoride ion increased from 70% to 89%, and therefore the average rejection ratio of ammonium fluoride increased by about 10%. Fig. 6 shows that the permeate flow rate decreased linearly as the concentration increased, at four different pressures, i.e., 1.36, 27.2, 40.8 and 54.4 atm, all the four lines being parallel to each other. Therefore, the concentration polarization effect in the above concentration range can be neglected.

Studies on separation characteristics of UNE by using RO membrane (1) Effect of operating pressure The relation between the operating pressure and the permeate flux of U N E is shown in Fig. 7. The permeate flux increased linearly as the pressure increased. The permeability was independent of the pressure and its value, which was calculated by using the least squares method, was 2.05 X 10 -2 mS/m 2 d atm. Fig. 8 shows that the rejection ratio of the uranium ion was constant, i.e., 99.6%, and that of the nitrate ion was kept between 90 and 95%. The rejection ratio of the nitrate ion increased slightly when the pressure increased. For

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P , otto

Fig. 7. Permeate rate vs. operating pressure for the RO membrane. Operating conditions: p H = 3.2 +_0.4; feed flow rate = 636.0 l/h; T = 22.5 ° C. [U s+ ] in the feed, in g/l: ( • ) 0.92; ( o ) 1.63; ( [ ] ) 3.29; ( A ) 7.30. I00 90 80 70 *

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Fig. 8. Rejection vs. operating pressure for the RO membrane. Operating conditions: p H = 2.7; feed flow rate = 636.0 l/h; T = 22.5 ° C. ( • ) [U 6+ ] teed= 7.30 g/l; ( O ) [ NO~- ] ~e~d= 5.76 g/1.

example, when the pressure increased from 27.2 to 54.4 atm, its value increased by 3%.

(2) Effect of feed concentration The effect of the feed concentration is shown in Figs. 9 and 10. The permeate flow decreased linearly as the feed concentration increased because the osmotic pressure increased and the net static driving force decreased (see Fig. 10). The lines obtained at four different pressures, i.e., 27.2, 40.8, 47.6 and 54.4 atm were parallel to each other. This means that the concentration polarization effect in this concentration range can be neglected. On the other hand, the rejection ratios of the uranium ion and the nitrate ion remained constant when the feed concentration increased.

43 I00

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Fig. 9. Permeate rate vs. feed concentration for the RO membrane. Operating conditions: pH = 3.2 ___0.4; feed flow rate = 636.0 l/h; T= 22.5 ° C. ( • ) 54.4 atm; ( o ) 47.6 atm; ([]) 27.2 atm; ( A ) 13.6 atm. Fig. 10. Rejection and [U 6+ ] in permeate vs. feed concentration for the RO membrane. Operating conditions: pH = 3.2 ___0.4; feed flow rate = 636.0 l/h; T= 22.5 ° C, operating pressure = 54.4 atm.

(e) U6+; (o) N0~-. -j ~1oo,

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~ 4o

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Fig. 11. Effect of % conversion on various parameters for the RO membrane. Operating conditions: pH=4.00; feed flow rate=636.0 l/h; [U e+ ]f~d=9.81 g/l; T=22.5°C; operating pressure=54.4 a t m . ( e ) Rj; ( [ 7 ) Qw; ( O ) U 6+.

(3) Recovery efficiency of uranium ion by batch-type process The UCPE was sequentially concentrated by using a batch-type process and t h e c o n c e n t r a t i o n o f t h e u r a n i u m ion i n c r e a s e d gradually u n t i l t h e p e r m e a t e flux r e a c h e d a l o w limit. A s s h o w n in Fig. 11, t h e c o n c e n t r a t i o n o f t h e u r a n i u m i o n c o u l d r e a c h a v a l u e o f 2.1 g/1 or higher. In t h e c a s e o f s o l u b l e u r a n i u m , t h e a v e r a g e rejection ratio w a s still h i g h e r t h a n 99.5% w h i l e t h e v o l u m e w a s re-

44 duced to 30% of its original value, and the quality of perm eat e water was still acceptable. CONCLUSION

p H effect. T h e influence of the p H on the rejection ratio of a m m o n i u m fluoride is significant. W h e n the p H value is higher t h a n 9, the rejection ratio of the a m m o n i u m ion is below 50%. T h e rejection ratio of the fluoride ion is below 50% when the p H value is lower t h a n 4. T hi s behavior is due to the strong hydrogen bondings formed by the fluoride ion in the acidic solution and the a m m o n i u m ion in the basic one. Pressure effect. B o t h the rejection ratios of U N E and a m m o n i u m fluoride increase with the operating pressure. C o n cen tr atio n effect. T h e rejection ratios of bot h U N E and a m m o n i u m fluoride increase as the feed c o n c e n t r a t i o n increases, and the concent rat i on p er meate fluxes also increase. B y using the m e m b r a n e separation process to t reat the U C P E , the valuable water-soluble u ra ni um can be recovered and its concent rat i on can reach 2.1 g/ 1. At the same time the quality of the permeate water obtained from two batchtype t r e a t m e n t s satisfies the disposal standards for effluent.

REFERENCES 1 C.E. Reid and E.J. Breton, Water and ion flow across cellulosemembrane, J. Appl. Polym. Sci., 1 (1959) 133-143. 2 S. Sourirajan, Reverse Osmosis, Academic Press, New York, N.Y., 1970. 3 U. Merten, Desalination by Reverse Osmosis, M.I.T. Press, Cambridge, 1966. 4 M.E.Mattson and M. Lew,Recent advances in reverseosmosis and electrodialysismembrane desalting technology,Desalination, 41 (1982) 1-24. 5 M.L.Chu, C.P. Wang and M.C. Shieh, Application of reverse osmosis to uranium recovery process, HydrometallurgicalSymposium, Taipei, 3 (1985) 280-299. 6 J.E. Craver and I. Nusbaum, Applicationof reverseosmosisto waste-watertreatment, JWPC, 46(2) (1974) 70-76. 7 W.G.Light, An application of ultra-filtration to tad waste, Electrical Power Research Institute, Final Report, NP 2335, April, 1982. 8 R.R.Stana and E.W. Tiepel, Use of a semipermeablemembraneto concentrate uranium from a solution, U.S. Patent 4,316,800, 3 February 1982. 9 Tam. v. Tran, Advanced membrane filtration process treated industrial waste-water efficiency, Chemical Engineering Progress, 81 (3) (1985) 29-33. 10 E.C. Kaup, Design factors in reverse osmosis, Chem. Eng., April 80 (8) (1973) 46-55.