Ind. Eng. Chem. Res. 1993,32, 709-715
709
GENERAL RESEARCH Recovery of Uranium from Seawater. 14. System Arrangements for the Recovery of Uranium from Seawater by Spherical Amidoxime Chelating Resins Utilizing Natural Seawater Motions Hiroaki Egawa,’ Nalan Kabay,t Taketomi Shuto, and Akinori Jyo Department of Applied Chemistry, Faculty of Engineering, Kumamoto University, Kumamoto 860, Japan
In order to evaluate performances of lightly cross-linked highly porous amidoxime resins in uraniumadsorption systems utilizing natural seawater motions, uranium uptake by the resins from seawater was studied by different approaches, such as simulated sea current exposure tests, towing trials, and/or mooring trials. In general, the efficiency of uranium uptake became higher with a decrease in the thickness of packing layers, indicating important roles of fluidization of the resin particles. On the basis of these fundamental data, mooring tests in the natural sea current were designed and conducted. By mooring flat adsorption beds (base area 260 cm2, height 3.0 cm) packed with 780 mL of the resin for 40 h, promising uranium uptake as high as 44 mg/kg of resin (9.9 mg/L of resin) was achieved under sea conditions in which the velocity of sea currents and the vertical velocity of waves were 5.5-49.7 cm/s and 3.4-27 cm/s, respectively.
A number of processes using solid adsorbents were tested for the recovery of uranium from seawater. Although earlier works have focused on actively pumped systems, energy for pumping of seawater accounts for the major part of the cost for yellow cake production from seawater. Thus, recent attention has been paid to passive contactor systems utilizing natural seawater motions, such as wave, tide, and ocean currents (Keen, 1968; Harrington et al., 1974;Kellner and Bitte, 1982;Forberg et al., 1983;Masuda, 1983; Driscoll, 1984;Jimenez and Driscoll, 1987; Okazaki, 1988; Kobuke et al., 1990). Recently, towing and/or mooring systems were proposed for amidoxime fiber balls, and these systems gave promising results (Nobukawa, 1990). In a preceding paper (Egawa et al., 1992b), we have preliminarily reported that lightly cross-linked highly porous chelating resins containing amidoxime groups showed the highest performance in uptake of uranium from seawater among so far reported amidoxime resins. Accordingly, of interest is the potentiality of these highperformance amidoxime resins in the adsorption systems designed by Nobukawa. This paper describes system arrangements suitable for the spherical high-performance amidoxime resins utilizing natural sea currents andlor wave motions. Experimental Section Preparation of Chelating Resins. The lightly crosslinked highly porous amidoxime chelating resins were prepared according to the reported method (Egawa et al. 1991, 1992b). The resin RNH-5 was prepared from acrylonitrile-divinylbenzene copolymer beads containing a nominal 5 mol % divinylbenzene. The copolymer beads were synthesized by suspension polymerization in the presence of 120 vol % chloroform (as a porogen) per the
* To whom correspondence should be addressed.
Present address: Institute of Nuclear Sciences, Ege University, 35100 Bornova, Izmir, Turkey. +
monomeric mixture. The alternative resin RNH-2 was also derived from analogous copolymer beads containing a nominal 2 mol % divinylbenzene, resulting from a similar copolymerizationin the presence of 100vol % of adifferent porogen 1,2-dichloroethane. The resins RNH-2 and RNH-5 were characterized according to the reported methods (Egawa et al., 1991,199213). Portions of RNH-2 and RNH-5 were used after the treatment with 1M NaOH at 30 O C for 3 d (alkali treatment), since it has been recognized that the alkali treatment enhanced significantly the adsorption rate of uranium from seawater (Egawa et al., 1991,199213). Prior to adsorption tests, all resins were conditioned in aqueous NaCl(3 % by weight), which has almost the same ionic strength as that of seawater. Adsorption of Uranium by Means of Up-Flow Columnar Method. A 50-mL portion of each wet resin was taken into a plastic column (inner diameter 6 cm, length 100 cm). Seawater was continuously up-flow supplied to the column by a pump at a space velocity of 640 h-l. Here, the space velocity (SV) means the ratio of a flow rate of seawater to the volume of the packed resin. Simulated Sea Current Exposure Tests (Tests 1.1-3 and Test 11). A 1.5-mL portion of the alkali-treated RNH-2 was packed into thin square planar bags (3.5 X 3.5 cm) made of apolyethylene net (100mesh). The thickness of the packed resin layer was ca. 1.5 mm. After the bags were sealed, they were immersed in streams of seawater. Figure 1schematically showsthe experimental equipmenta of tests I and location of the bags in the streams. In test 11,a 30- or 50-mLportion of alkali-treated RNH-5 was packed into rectangular prism shells (base 4 X 4 cm, height 6 cm) made of the polyethylene net (100 mesh) and a plastic-coated wire (for the framework). Each shell packed with the resin was immersed in a separate stream of seawater. The stream used here was the same one used in test 1.1 but its depth was adjusted to be 8 cm. Each shell was located a t the 40-cm point from the top of the stream. Detailed conditions for tests I and 11, such as velocities of the streams and contact periods, are described in Tables II-IV with results.
OSSS-5SS5f 93/2632-OlO9$O4.QQ~ 0 0 1993 American Chemical Society
710 Ind. Eng. Chem. Res., Vol. 32, No. 4,1993
~~~~~~~
feature of adsorption bed
bed A 3Ikm
&m
t
Figure 1. Equipments for tests I and loeation of the bags in the streama. (A)Viewparalleltothestzeam. (B)Sideview. ( C ) h t i o n of the bags in the streams. The labeling numbers 1,2,3, and 4 mean that the bsgs were located at 30, 40-, 50-, and 60-an points, respectively,from the top of the streams. The labeling symbols R1 and R2 in test 1.1 correspond to runs 1 and 2, respectively,in Table 11. Lengths of all streams were 100 em. The depth of the stream in test 1.1 wan 7 an,and those in teste 1.2 and 1.3 were 5 cm.
Towing and/or Mooring Exposure Tests i n Imari Bay (Tests I11 and 1%’).In test 111, four kinds of the resinswereused: thenontreated RNH-2andRNHdresina and the alkali-treated ones. A 3.5-mL portion of each resin was packed into thin rectangular bags (3.5 X 5.0 cm) made of a polyethylene net (80 mesh). After the bags were sealed (thickness of the packed resin layer, ca. 2.6 mm),they were fixed onto the inner bottom of adsorption beds (acting as support of the bags). The features of the beds and location of the hags on the inner bottom of each bed are shown in Figure 2. The beds were then fastened to frameworks made of metal pipes. The thus assembled setup was called an “adsorption-bed unit”, and its configuration is shown in Figure 3. The sc-called units were towed and moored in Imari Bay. The units were towed by a small ship at a veloeity of 1m/s for 10 h in a day as shown in Figure 4, and then moored in the bay until the next towing. This towing and mooring cycle was repeated three timesuntiltotal towing andmooring periods attained 30 and 50 h, respectively (run 1). At this point, the bags labeled R1 (refer to Figure 2) were taken out from the units. The units with the remaining hags Labeled R2 were further moored for 830 h in the bay. During this mooring, the units were covered with a light-shieldingsheet (Mark Unitika Co. Ltd., Osaka) in order to prevent growth of marine plants on the units. Finally, the towing and mooring cycles were repeated again just as in run 1. Consequently, the overall towing and mooring periods in run 2 were 60 and 930 h, respectively. IntestIV,a500-or1000-mLportionofthealkali-treated RNH-5 was packed into rectangular prism shells (base 10 X 10 cm, height 15 em)made of the polyethylene net (100 mesh) and the plastic-coatedwire (for the framework).Aa showninFigure5, theseshells were placedinanadsorption bed made of a nylon net with a mesh of 5 X 5 mm, and the void space of the bed was filled with dummy adsorbent
hed H Figwe 2. Feature of the adsorption bed in teat 111 and loeation of the bags on the inner bottom of the beds. T h e beda A and B were made of a nylon net with a mesh of 2 X 2 mm and a polyethylene net with a mesh of 0.3 X 0.4 mm, respectively. Labeling symbols 2-NT and 5-NTrepwent nontreated RNH-2 and RNH-5, respeetively. Alkali-treated RNH-2 and RNH-5 are denoted by respective symbols 2-AT and SAT. Symbols R1 and R2 correspond to ~ l l1s and 2, respectively. in Table V.
‘adsorption k d
Fwre 3. T h e adsorption-bed unit for towing and mooring triala in test 111. AI dimensions are in mm.
balls (diameter ca. 1 cm) made of polyacryl fiber. Then, the adsorption bed was fixed to a metal pipe framework. The thus fabricated ‘adsorption-bed units” were towed by the ship at a velocity of 1m/s for 30 h in runs 1and
Ind. Eng. Chem. Res., Val. 32, No. 4,1993 711 cylindrical hmy
lowing
Figure 4. Towing operation in test 111. The huoy, from which the five adsorbent-hed units were suspended, was towed by a small ship a t the velocity of 1d s . The beds A and B shown in Figure 2 were contained in the units 4 and 3, respectively. The remaining units, 1,2,and 5, were used in other works. Dimensions are in mm. shell packed wilh adsorbC"1
dummy
\
Figure 6. Towing operation in test N. From the huoy, two adsorption-bed units (1 and 2) were suspended: 1, a dummy unit with the bed packed with the dummy adsorbent balls only; 2, the unit shown in Figure 5. The huoy was towed hy the ship a t the velocity of 1 mls. Dimensions are in mm.
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Fwure 5. The adsarption-bed unit in test IV. The shells packed with the resin were fired on the adsorption bed made of a nylon net with a mesh of 5 X 5 mm. For simplicity, the nets for the bed as well as fortheshebarenotdrawn here. Dimensionsareinmm. Positions 2 and 5 were used in this work (refer to Table VI), and the remaining positions 1.3, and 4 were used in other works.
2, as shown in Figure 6. In the alternative run 3, the adsorption-bed unit was moored for 40 d in the bay as shown in Figure 7. Mooring Test in the Sea outside Imari Bay (Test V). Alkali-treated RNH-5(520or 780 mL) was packed into hexagonal bags (length of each side 10 cm, height 3 cm)made of the polyethylene net (100mesh). After the bags were sealed, they were fixed in the metal pipe frameworks as shown in Figure 8. The assembled adsorption-bed units were immersed into the sea current from a ship, which was at its mooring in the sea outside Imari Bay as shown in Figure 9. In a day, the units were moored for 8 h a n d were drawn up from the sea until the next mooring. This mooring operation was repeated until the total mooring time was 40 h. During the mooring operation, velocity of the sea current and height as well
Figure 7. Mooring operation in test IV. The adsorption-bed unit was covered with thelight-shieldingsheet toprevent growthofmarine plants. Dimensions are in mm, unless otherwise noted.
as vertical velocity of waves were monitored three times in a day. Sea conditions are described in detail in a later section. Determination ofUraniumUptake. After individual adsorption tests, the resins were taken from each adsorption bag or bed and washed with deionized water. An exact portion of each resin tested was taken into glass columns (diameter 1 cm,length 29 em), and the uranium adsorbed was quantitatively eluted with 10 bed volumes of 0.5 M H2S04 a t a flow rate of SV 3 h-' (down-flow).The uranium in eluates was colorimetrically determined according to the Arsenazo I11 method (Egawa et al., 1991, 199%). After the elution, the resins were washed with deionized water until the acid was not detected in the
712 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 metal pipe framework
time (d)
time (d)
Figure 10. Uptake of uranium in up-flow column trials. (A) RNH2, temperature of seawater 19-30 O C ; (B)RNH-5, temperature of seawater 19-24 "C. Open and fiied circlesstand for the alkali-treated resins and the nontreated ones, respectively.
Figure 8. Adsorption-bed unit in natural sea current exposure trial (test V). The resin was packed into the hexagonal beds made of the 100-mesh polyethylene net. The bed was fastened onto the metal pipe framework. Dimensions are in mm. accelerometer
0
sea current
hor
Figure 9. Mooring operation utilizing natural sea current and wave motions (test V). Units 2 and 3 contained the adsorbent beds for runs 2 and 1,respectively, in Table VIII. Unit 1was used for other works.
washings. They were dried at 40 "C in vacuo and then weighed. Uptake of uranium is represented in pg of uranium/mL of wet resin (pg/mL of resin) or in pg of uranium/g of dried resin (pg/g of resin); here, the volume of the wet resins means one after adsorption tests, since the resins are shrunk by the adsorption of uranium as described in later sections.
Results and Discussion Properties of the Chelating Resins. Table I summarizes properties of the amidoxime chelating resins prepared for this work. Despite the light cross-linking, large specific surface areas of RNH-2 and RNH-5 mean that they have highly porous structures. Clearly, the alkali treatment results in high swelling. The high swellingcomes from the formation of negatively charged functional groups during the alkali treatment as judged from the increased cation exchange capacities after the alkali treatment (Egawa et al., 1991). The highly porous structures as well as the high swelling are essential for rapid diffusion of uranium in the resin particles (Egawa et al. 1992a,b),since uranium is dissolved in seawater as the large complexed anionic species UO2(CO3)&. Uranium Uptake in Up-Flow Columnar Method. Figure 10 shows the time course of uranium uptake from seawater by four kinds of resins: the nontreated and alkalitreated RNH-2 and RNH-5. The alkali treatment en-
hances greatly the rate of uranium uptake from seawater. The superior uptake as high as ca. 130 pg/mL of resin for 10 d was accomplished by the alkali-treated RNH-2 and RNH-5 even at a low linear velocity of 0.31 cm/s (upflow). These resulta suggest that the high swelling of the alkali-treated resins plays an important role in the rapid diffusion of the complexed species of uranium in the resin particles (Egawa et al., 1991). Uptake of Uranium from Simulated Sea Current (Tests I and 11). Ocean and tide currents providea rapid linear velocity up to ca. 1 m/s. However, velocities of seawater inside adsorption beds are significantly decelerated by packed resins. Taking into account this situation, the simulated sea current exposure tests were carried out in a low linear velocity range (0.63-4.7 cm/s) using the alkali-treated resin RNH-2. Results of Tests 1.1-3 are summarized in Tables I1 and I11 (refer to Figure 1). In test 1.1,the effect of exposure time was evaluated. The alkali-treated RNH-2 took up 72-87 and 99-118 pg/mL of resin for 10- and 20-d exposures, respectively. Compared with the adsorption for the first 10 d, that for the subsequent 10 d was not so highly efficient. As illustrated in Figure 10, the rate of uranium uptake decreases with an increase in the exposure time. This is ascribable to the shrinkage of the resins, which proceeds slowly with an increase in uranium uptake (refer to Tables IV, VI, and VIII). The shrinkage of the resins lowers the diffusibility of the large complexed uranium in the resin particles, resulting in deceleration of uranium uptake. In tests 1.2 and 1.3, the seawater was supplied at a linear velocity of 1.7 cm/s, which is greater than that of test 1.1. Since the contact periods were somewhat different between testa 1.2 and 1.3, values of uranium uptake for exactly 10 d were calculated. The calculated values are 121-129 and 115-125 pg/mL of resin for tests 1.2 and 1.3, respectively. These values are greater than those in test 1.1, indicating that the higher the linear velocity of seawater the greater the rate of uranium uptake. In the effect of location of the absorbents in the stream, it is likely that the uptake at the upper stream is slightly greater than that at the downstream (Table 111). This can be probably ascribed to disturbance of the smooth flow of seawater by the bags located a t the upper stream, since the adsorbents in the upper streams little affect the concentration of uranium in the streams. Test I1 was conducted in order to evaluate the effect of thickness of adsorption layer as well as of packing fraction using the alkali-treated RNH-5. In this test, the thickness of the packed resin layer in the direction of the stream is 40 mm, whereas that in tests I is 1.5-2.6 mm. Results are given in Table IV. In spite of the higher linear velocity of the stream in test I1 (2.4 or 4.7 cm/s) than that in testa
Ind. Eng. Chem. Res., Vol. 32, No. 4,1993 713 Table I. Properties of the Lightly Cross-Linked Highly Porous Amidoxime Chelating Resins
nontreatment
alkali treatment
SSAb C*' CCd Vwe UUDtakd C.' C,d V2 u UDtakd (mL/g) (mAol/g) (meqiiv/g) (meq;iv/g) (mL/g) (mLoi/g) (meqiiv/g) (meqiiv/g) (m2/g) resina 1.15 4.0 4.2 8.4 1.04 3.3 4.2 2.6 17.1 RNH-2 RNH-5 29.7 3.8 1.6 2.7 1.06 3.3 3.1 4.6 0.96 a Particle size is 32-60 mesh. Specific surface area of dry resin. Anion exchange capacity. Cation exchange capacity. e Wet volume in aqueous sodium chloride (3% by weight). f Uranium uptake was determined by equilibrating 0.125 g of each resin with 50 mL of 0.01 M aqueous uranyl nitrate for 24 h. Table 11. Results of Simulated Sea Current Test 1.1.
contact time (h)
run
temp of seawater (min-maxPC)
flow rateb (L/h)
1
uranium uptake (pg/mL of resin) location of adsorbent bags in streame 2 3
4
1 238 27.0-33.0 144 83 (440)d 87 (460) 72 (383) 74 (380) 2 479 27.0-33.0 148 104 (532) 109 (566) 118 (605) 99 (516) The alkali-treated RNH-2 (1.5 mL) was used. Linear velocity of seawater waa 0.65-0.67 cm/s. For location of the bags, refer to Figure 1. Values in parentheses are uptake in pglg of resin. Table 111. Results of Simulated Sea Current Tests 1.2 and 1.3.
uranium uDtake (unlmL of resin) temp of seawater flow rateb location of a'dsorbencbags in stre& test no. (min-max/"C) (L/h) 1 2 3 4 116 (624) 1.2 27.0-33.0 153 124 (704)d 124 (705) 119 (686) 1.3 25.0-26.0 153 138 (768) 128 (715) The alkali-treated RNH-2 (1.5 mL) waa used. Linear velocity of seawater is 1.7 cm/s. For the location of bags, refer to Figure 1. Values in parentheses are uptake in pg/g of resin. contact time (h) 231 265
Table IV. Results of Simulated Sea Current Exposure (Test 11). ~
contact time (h) 264 696 694
run 1 2 3 a
temp of seawater (min-max/"C) 18.0-20.0 18.0-25.0 23.5-28.0
flow rate (L/h) 1230 1220 621
linear vel (cm/s) 4.7 4.7 2.4
volume of resin (mL) before test after test 50.0 45.0 50.0 45.6 30.0 26.8
U uptake (pg/mL of resin) 9.0 (41)b 22.6 (98) 28.0 (117)
The alkali-treated RNH-5 waa used. Values in parentheses are uptake in pg/g of resin.
Table V. Results of Towing and Mooring Exposure in Imari Bay (Test 111) ~ _ _ _ uranium uptake (pg/mL of resin) adsorption bed B (net 100 mesh) adsorption bed A (net 3 X 3 mm)
towing mooring temp of seawater run time (hl (min-maxPC) . , time (h) .. . 1 2 a
30 60
50 930
27.0-31.6 26.6-31.6
RNH-2 RNH-5 RNH-2 RNH-5 ATQ NTQ AT NT AT NT AT NT 39 (228)b 22 (63) 50 (157) 36 (85) 40 (241) 23 (67) 23 (71) 37 (87) 255 (1324) 159 (449) 285 (851) 212 (515) 216 (1082) 100 (283) 185 (573) 150 (363) ~~
AT and N T mean alkali-treated and nontreated resins, respectively. For location of each resin in the adsorption beds, refer to Figure 2.
* Values in parentheses are uptake in pglg of resin.
I (0.63-0.67 or 1.7 cm/s), the efficiency of uranium uptake in test I1 is quite low. Since the alkali-treated RNH-2 and RNH-5 show nearly equal adsorption rates as shown in Figure 10, it can be estimated that the efficiency of uranium uptake in test I1 is much less than that in tests I. This suggests that the thicker the packed resin layer the lower the adsorption efficiency. In addition, a decrease in the packing fraction markedly enhances the adsorption efficiency; the uranium uptake in run 3 is greater than that of run 2, even though the velocity of the stream in run 3 is about a half of that in run 2. The comparison of the results in tests I and I1 with those in the up-flow column test (Figure 10) clearly indicates the significant role of fluidization of the resin particles in uptake of uranium from seawater. Towing and/or Mooring Exposure Tests in Imari Bay (Tests I11and IV). In test 111,efficienciesof uranium uptake by four kinds of the resins were compared. The results are summarized in Table V (refer to Figures 2-4). In run 1, namely in a shorter towing and mooring period,
the uranium uptake was not remarkably affected by the mesh size of the adsorption beds; each resin fixed on beds A and B took up nearly equal amounts of uranium except for the questionable result for the alkali-treated RNH-5 on bed B. In run 2, on the other hand, a clear difference in uranium uptake can be seen between beds A and B; bed A with the larger mesh size is of greater advantage than bed B with the smaller one. The smaller mesh is likely to be more easily covered with floating particles in seawater during the prolonged mooring, resulting in an incomplete supply of seawater to the inside of the adsorption bed. This is a possible reason why bed B shows lower efficiency than bed A in the prolonged mooring (run 2). In accordance with the results obtained in the up-flow column test, the alkali-treated resins take up uranium much more effectively than the nontreated ones. From the results shown in Table V, the difference in the efficiency of uranium uptake between towing and mooring operations can be roughly estimated. For example, the alkali-treated RNH-2 in bed A takes up 228 and 1324 pg/g
714 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 Table VI. Results of Towing or Mooring Trials in Imari Bay. (Test IV) run
location of the shellb
towing time (h)
mooring time (h)
1 2 3
2 5 5
30 30
0 0
0
960
temp of seawater (min-max/OC)
U uptake (pg/mL of resin)
volume of resin (mL) before test after test
23.9-26.7 23.9-26.7 24.0-26.7
910 456 333c
lo00 500 500
3.6 (16.3)d 7.2 (32.6) 20.9 (90.0)
The alkali-treated RNH-5 was used. For location of the shells in the adsorption beds, refer to Figure 5. A portion of the resin was spilt by partial breakage of the shell during the mooring. Values in parentheses denote uptake in pg/g of resin.
of resin in runs 1 and 2, respectively. In run 2, the prolonged mooring for 830 h was interposed between two “towingand mooring”operations. Accordingly,the uptake during the mooring for 830 h is evaluated to be 868 pg/g of resin under the assumption that the uptake in the second “towing and mooring” operation in run 2 is the same as that in run 1(228 pg/g of resin). Thus, the time-averaged uptake during the mooring for 830 h is calculated to be 1.05 (pg/g of resin)/h. This leads to an estimation that the time-averaged uptake by the towing is 5.83 (pg/g of resin)/ h. For the alkali-treated RNH-5on bed A, the same tendency was observed; the time-averaged values for the towing and the mooring are 4.17 and 0.647 (pg/g of resin)/ h, respectively. Accordingly, it can be reasonably estimated that the towing a t the high speed of 1m/sis much more effective than the mooring in a calm sea like Imari Bay, although the assumptions as well as the approximations made here may oversimplify the real situations. In order to obtain further evidence for the high efficiency of the towing operation, test IV was conducted. Table VI summarizes the results of test IV. In runs 1 and 2, the packed shells were towed but not moored. The uranium uptake per unit volume (or weight) of the resin in run 1 is just a half that in run 2. This means that the total uptake in run 1 is exactly the same as that in run 2, indicating that nearly equal amounts of seawater were passed through the packed shells in runs 1and 2, despite the large difference in the packing fraction. The packing fractions in runs 1and 2 are 0.667 and 0.333, respectively. In contrast, the packed shell in run 3 was moored but not towed. The resin in run 3 takes up only 20.9 pg/mL of resin (90.0 pg/g of resin) despite the long period of 960 h. The time-averaged values for the uranium uptake in runs 1, 2, and 3 are 0.54, 1.1, and 0.094 (pg/g of resin)/h, respectively. These results clearly suggest the greater advantage of towing at the high velocity and of the smaller packing fraction as well. The results from tests I-IV suggest that the fluidization of the packed resin particles is essential for the highly efficient uptake of uranium. These observations lead us to further adsorption tests utilizing natural sea current and wave motions in the sea outside Imari Bay. Mooring Test Utilizing t h e Natural Sea Current and Wave Motions (Test V). Table VI1 summarizes conditions of the sea during test V (refer to Figure 91, and the results are listed in Table VIII. During the test, the velocity of sea currents was 5.5-49.7 cm/s, and the height and the vertical velocity of waves were 0.3-1.2 m and 3.427 cm/s, respectively. Surprisingly, the bed packed with 780 mL of the resin (run 2, packing fraction = 1)showed slightly higher uptake than the other one in which 520 mL of the resin was packed (run 1, packing fraction = 0.667). As shown in Figure 9, the adsorption units were inclined at ca. 30° to the horizon. Thus, the resin in run 1 might be localized in a certain side of the bed by gravity as well as by sea current, since the packing fraction is small. This localization of the resin in the bed may yield void bypass for seawater in the bed. At the present, we speculate that passing of seawater through the void bypass may be a possible reason for the lowered efficiency in run 1.
Table VII. Conditions of the Sea during the Mooring (Test VIP
temp of seawater dateb time 6th 7th 11th 12th
20th
801 12:15 1617 7:lO 11:18 1508 810 12:05 16:12 7:20 11:21 7:25 11:24 15:26
(OC)
vel of sea current (cm/s)
height of wave (m)
23.2 24.4 24.5 24.6 24.7 25.0 23.8 24.0 24.6 26.6 23.2 21.5 22.0 22.0
22.4 5.5 18.6 6.8 21.4 19.8 24-25 8.9 49.7 27-28 40.7-43 42.3 18.5 8-12
0.5 0.5 1.0 0.3 0.4 0.7 0.5 0.3 0.5 0.5 0.6 1.2 1.0 0.8
vertical vel of wave (cm/s) 16% 5-6 10-11 3.4 7.5 4.1 11-12 7 14 13 15-20 22-27 8-13 14-26
The sea outside the Imari Bay. The adsorption-bedunite were moored for 8 h in a day. Refer to Figure 9. August 1992. Table VIII. Results of the Mooring Test Utilizing Natural Sea Current and Wave Motions (Test V)’ run
mooring time (h)
1 2
40 40
volume of resin (mL) before test after test 520 780
300 490
U uptake (mg/L of resin) 7.42 (32.6)* 9.87 (43.9)
The alkali-treatedRNH-5was used. Refer to Figure 9 and Table VII. Values in parentheses are uptake in mg/kg of resin.
The efficiency of uranium uptake in test V is nearly equal to that observed in the towing operation of test IV, whereas the linear velocity of seawater during most of test V (ca. 20-30 cm/s) is less than a half the towing velocity in test IV. Then, it is estimated that the vertical flow of seawater caused by wave motions in test V also plays an important role in fluidization of the packed resin, resulting in the effective contact of the resin particles with seawater. Uranium uptake as high as 44 mg/kg of resin (9.9 mg/L of resin) for 40 h was accomplished by only the mooring operation in natural sea current using no energy for seawater supply to the adsorbent. This value corresponds to about half the uranium uptake observed in the up-flow column test (ca. 22 mg/L of resin for 40 h), so long as simple proportional calculations can be allowed between the two different adsorption systems. However, even the highest velocity of the currents in this work (ca. 0.50 m/s) is only half that of ocean currents (1m/s). Thus, a further high efficiency in uranium uptake will be promising. As a conclusion, the spherical amidoxime resins will be successfully applicable to the recovery of uranium from seawater by mooring operations in sea currenta and/or wave motions. Although the uranium adsorption per unit amount of the resins will increase with decreasing the thickness of packing layers, it seems that the packing layers about 3-5 cm in thickness will be most suitable for high overall efficiency.
Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 716
Acknowledgment The present work was supported by a Grant-in-Aid for Scientific Research (Energy Project Research) from the Ministry of Education of Japan. We thank Aitsu Marine Biological Station, Faculty of Science, Kumamoto University, for the facility to conduct simulated sea current exposure tests (tests I and 11). We also acknowledge Marine Technology Institute Co. Ltd., Imari, for their help in the sea trials (tests 111, IV, and V). Special thanks are also given to Prof. Nobukawa, Hiroshima University, for his contribution to the sea trials.
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Received for review June 16, 1992 Revised manuscript received November 26, 1992 Accepted January 4, 1993
