Ind. Eng. Chem. Process Des. Dev. 1882, 21, 781-784 Klttrell, J. R. A&. Chem. fng. 1070, 8 , 101. Krlshnaswamy, S.; Klttrell, J. R. Ind. fng. Chem. Process D e s . D e v . 1078, 17, 200. Krishnaswamy,S.; Kittrell, J. R. I d . Eng. Chem. Process Des. Dev. 1070, 18. 399. Romero. A.: Bilbao. J.: Gondlez-Velasco, J. R. I d . EnQ. Chem. Process Des.'Dev.106la, 20, 570. Rome, A.; Bllbao, J.; Gondbz-Velasco, J. R. Chem. f n g . Sci. 1981b, 36,
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781
Weller, S . AIChf J . 1056, 2 , 59.
Chemical Engineering Department University of Massachusetts Amherst, Massachusetts 01003
In-Sik Nam* J. R. Kittrell
Received for reuielo November 30, 1981 Accepted May 12, 1982
797.
New Aspects of Uranium Recovery from Seawater The properties of various adsorbents for uranium extraction from seawater are measured under standardized experimental conditions. It turns out that fractionated humic acids have exceptionally fast loading kinetics. This property leads to a substantial reduction of capital investments in conventional adsorbent bed techniques as well as in a procedure designed to avoid large adsorbent bed constructions by using carrier bodies in the open sea.
Introduction For several countries the long-term availability of uranium is a crucial point in their nuclear energy concepts, because the known reserves are limited and restricted to few parts of the world. This situation would change considerably if plenty of uranium were available throughout the world, and this for a long time to come. It seems most likely, however, that this can only come true if there will be an economically feasible recovery of uranium from seawater. The oceans contain 4.5 X lo9 tons of uranium. For comparison, terrestrial deposita of reasonable concentrations are estimated to several lo6 tons of U (Duret et al. 1978). Something of the order of lo8 tons in the upper 100 m of the oceans can be considered immediately accessable for recovery. For the exchange rate of water masses between this layer and the deep sea reservoir, presently only an estimate can be given. The real process is complex and variable from place to place. Convective transport down is partly done by currents; examples for the opposite are upwelling areas (see, e.g., Broecker, 1974). On the global average, the vertical exchange can well be described by diffusion kinetics using an apparent eddy diffusion coefficient of the order of 1 cm2/s as has been demonstrated for the uptake of carbon dioxide from the atmosphere (see, e.g., de Luca Rebello and Wagener, 1976; Oeschger et al., 1976). This means an annual exchange layer of about 80 m in thickness containing about 7 X lo7 tons of uranium, a certainly sufficiently intensive source to compensate for the human recovery rate. A recent study of Exxon Nuclear Co., Oregon State University, and Vitro Engineering Corp. (Campbell et al., 1979) analyzes the feasibility of large-scale recovery of uranium from seawater. Judging from the present state of the art as it was represented by the known investigator g-roups, it is concluded that the most reasonable chemical process is an adsorption process using hydrous titanium oxide as adsorbent (Davies et al., 1964) in a continuous fluidized bed with a pumped flow delivery of seawater from the nearby ocean. For the assumed reference design case, a plant of 500 tons of U308per year capacity located in southeastern Puerto Rico, it turns out that 87% of the production costa are capital-related charges. The total production costs in 1978 prices would be 1436 $/lb-U30B. Their conclusion is that without several major technical breakthroughs, a pumped seawater plant is not economically feasible. In general terms without looking into details of the chemistry, it can be expected that the volume of con0 196-4305/82/112 1-078 1$0 1.2510
structions necessary for loading and eluting the adsorbent is proportional to the amount of adsorbent, and thus it is the capital investment for this part. The necessary amount of adsorbent to obtain the wanted uranium fixation rate depends largely on its physicochemical properties. So far, main efforts in uranium adsorbent research have primarily been devoted to obtaining a maximum enrichment factor, rather than to optimize thw parameters which are directly responsible for the uranium production costs. This is the reason why we propose a process b a d on particularly fast reacting adsorbents.
Cost-Related Properties of Uranium Adsorbents The accumulation may be defined as A = (g of uranium/g of adsorbent)/(g of uranium/g of seawater) (1) Independent of the chemical nature of the binding process, the fixation kinetics is that of an exchange process characterized by the equilibrium loading, A,, and the relaxation time, 7 Measured data of A, and 7 are dependent on the particle size and the hydrodynamic conditions of the loading process and partly also on the preparations of the adsorbents. For this reason we compared various adsorbent materials under equal conditions: about 1 g of particles with diameters between 0.1 and 0.2 mm were suspended in 50 L of natural seawater (mean uranium concentration c, = 3.2 pg/L; pH = 8.2) and stirred in a standardized way. In all laboratory experiments care was taken to keep the seawater in equilibrium with the atmosphere by a continuous flow of fine air bubbles through it. In this way, variables such as pH and pcoI which have a strong influence on the adsorption process of uranium via the marine carbonate system were kept constant at the conditions of natural surface seawater of a given temperature. The humic acid fractions used as adsorbents were extracted from peat by a pH 13-14 solution of sodium hydroxide after pretreatment with pH 10-12 NaOH. They were fixed on anion exchangers (Sephadex A50 and Dowex 1 X 2) as carriers. The uranium uptake of all sorbers was measured by analyzing the uranium content of both seawater samples taken during the adsorption process and the sorbent material itself using fluorimetric methods. All adsorbents show uniform loading kinetics according to eq 2, with the exception of humic acids. In the case of 0 1982 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982
Table I. Properties of Some Adsorbents Tested for the Recovery of Uranium from Seawater sp elution
adsorbent hydrous titanium oxide Duolite ES 346 (amidoxime gr0UPS) Hyphan (2,2'dihydroxyazo groups) humic acids (fractionated, upon carriers)
temp, "C init accum rate:h-'
equilib loading, A,
capacity, relaxation C, Mval/ton time, T , h of U
from Harwell, UK
10 28
7 00 1940
7 . o x 104 1.4 x 105
100 70
20 10
from Dia-Prosim Co. France
21 28
350 580
> 2 . 0 x 105 > 2 . 0 x 105
>570 >340
< 10 < 10
from Riedel de Haen Co. Germany
22
450
x 104
100
4
fast process
10-25
slow process
40 000-50 000 (10000-12500)
10-25
Maximum loading of the fast process ( n o equilibrium value). (dA/dt)+, = A,/T. a
humic acids, there are two processes: a fast one dominant up to about 2 h and giving about 50% of A,, followed by a slow one with a relaxation time which is about one order of magnitude higher. Data are collected in Table I. The A values are based on the dry weight of the reactive substance. In the case of humic acids, adsorbent particles may comprise as much as 75% carrier material. Values including such carrier weights are given in brackets. The linear slope of the loading process at the beginning, (dAldt),,,, was taken directly from the measurements and was cross-checked by the ratio A,/?. It is a direct measure for the effectivenessof the adsorbent material. The inverse of it, ? / A , , determines the needed amount of adsorbent for a given fixation (1production) rate, P. In this way Table I gives the theoretical minimum amount of adsorbent necessary to obtain a fixation rate of 500 tons of U/ year using 0.15-mm particles, under the simplifying assumption that there is no depletion of uranium in the seawater when passing through the adsorption process: (MA)min = Pr/A,c,. Real amounts are higher, depending on the adsorption/elution efficiency and on dead times. In processes based on fixed and fluidized beds, the hardware constructions mean large capital investments which represent the biggest fraction of the total uranium production costs (Campbell et al., 1979). Since the necessary bed volume is proportional to the amount of adsorbent, it turns out that the major part of the capital investment is inversely proportional to the rate of loading kinetics, dA/dt. As the data in Table I demonstrate, the differences in 7 of the investigated adsorbents are much larger than those in A,. Therefore, a considerable cost reduction can be expected when using the adsorbent with the fastest loading kinetics rather than that one with the highest end concentration. This applies to all sorbent techniques providing a sufficiently high throughput of seawater such as fluidized sorber beds or static beds with a low flow resistance. The last figure in Table I characterizing the chemical properties of an adsorbent is the elution capacity, C, which determines the amount of chemicals consumed in the elution process to obtain 1 ton of uranium (Mval/ton of U). This accounts for the major part of the non-capitalrelated costs. A Uranium Recovery Process Based on Humic Acids In addition to their economic advantages in conventional bed techniques as discussed above, fasbreacting adsorbents such as humic acids (HA) offer the unique chance for an
4.5
-LOX
(-1.2
-(-2.5 105
104~
x 104)~ x 104)
- 1.5b
20
-12
Time to reach the maximum loading of the fast process.
extremely simple loading process which needs neither adsorbent beds nor a pumped flow delivery. Humic acids are ecologically acceptable, even in large quantities. Extracted from peat or other sources, they may be used as a surface coating on suitable carrier bodies which are hydrodynamically well designed for an effective exchange, big enough for handling, and protected against abrasion. The carrier density has to be a little smaller than that of seawater such that they move slowly upward when submerged in the ocean. The idea is to complete the uranium adsorption process during their way up to the surface. For HA having a relaxation time of adsorption of about 1h, this can be realized by pumping the carriers down to several hundred meters depth. They are collected at the surface by well-known fiihery techniques. Spreading of the particle beam by eddy diffusion, local currents, and wave actions is easily controlled by enclosing the whole area with fishing nets. The adsorption area can be moved if natural currents do not provide fresh seawater in sufficient amounts. Since the adsorption kinetics of HA is almost independent of temperature, the plant site can be chosen exclusively under weather and seawater quality criteria. For elution, the carriers are taken up by a special shi: . Subsequently, they are transported back to the point of release to start the next loading cycle. The scheme of the procedure is illustrated in Figure 1. Suitable sorbent carriers are structures which have large surface areas per unit of volume with good access for the penetrating seawater when rising to the surface of the sea. Structures of this kind are spongelike lattices or units of nets carrying thin threads like a tuft (Figure 2). The carriers are to keep the HA without significant losses in seawater or elution media. (A natural example of a strong fixation of especially those insoluble HA which are most effective in uranium adsorption is black peat: no decrease of its U-loading properties could be observed after 40 adsorption-elution cycles.) Some rough figures may exemplify a technical recovery plant. A production of 250 tons of uranium per year and an adsorption/elution efficiency of 0.76 would require approximately lo00 tons of HA fixed on 5 X 10s or 4 X 10s carriers of type A or B in Figure 2, respectively. The depth of their release into the ocean is 400 m at a beam width of 100 m. They rise to the surface in 1-2 hours, penetrate almost 109 m3 fresh seawater per day if the discharging ship has a relative velocity of 0.4 cm/s, and accumulate 100 pg of uranium per g of HA while the processed seawater is
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4, 1982 783
Table 11. Estimate of Production Costs production costsa per year in lo6 $
per lb of U,O, in $
cost component
carrier A
carrier B
carrier A
carrier B
uranium recove from seawater capital recovery maintenance adsorber replacement chemicals including transportd power labor other costs including administration, insurance and tax 11. uranium recovery from eluate total costs
60 12 91 56 9 9 11 17 265
43 10 58 56 8 9 11 17 212
92 18 140 86 14 14 17 26 407
66 15 89 86 12 14 17 26 325
I.
B
a Estimates in DM, converted to $ (exchange rate 1$ = 2.30 DM). Capital investment for use of type A carriers: $300 million for ships, pumps, pipes, and similar installations (mean depreciation time 20 years) and $200 million as the initial adsorber investment (estimated on the basis of polyurethane as carrier material). The corresponding figures for type B car4% per year of the installation costs. d Price riers are $250 and $110 million, respectively. Interest lo%, annuity 12%. estimate: $170 per ton of HCl including transport charges.
me0
Figure 2. Sorbent carriers: seawater resistant matrix of polymerized material with functional groups to fix humic acids. Examples: type A, 10 cm3 lattice cube like a open cell polyurethane foam, 5 cells/in., coated with an ion-exchange layer (100 pm), effective surface 2 X lo3 cm2,HA fixing capacity ca. 2 g each; type B: 10 cm2 nylon network with 3 X lo3 ion-exchange fibers (4 0.3 mm, length 10 cm), effective surface: 2.8 X lo3 cm2,HA fixing capacity ca. 2.5 g each.
I &ad&
hydraulic transportation through pipes
::", ,
I4:W7 elution
7
restoration of pH 0.6
I
eluate > 30 mg U I I
mqFl
Figure 1. Uranium extraction from seawater. Scheme of the procedure: (1) ships anchoring in a natural ocean current or moving slowly ahead (1seamile/h); (2) fishing nets for collecting the floating carriers; (3) collecting area for loading the carrier into the ship 7; (4) ships anchoring or moving like ships 1 discharging the eluted carriers through the outlet 5; (5) outlet of carriers 300-400 m below sea level; (6) upwelling area where the carriers reach the surface; (7) ship with the elution unit; (8) transport system (ship or tube) for eluted carriers.
Figure 3. Flow diagram. Adsorbent flow in the adsorption-elution cycle: 1.5 X lo8 carriers (B)/h. From the eluate (uranium concentration > 30 ppm) the uranium is recovered by conventional techniques (precipitation or ion exchange), finally producing yellow cake.
depleted by 1pg U/L. Using countercurrent techniques, elution and regeneration of HA is done effectively in mineral acid of pH 0.6 or, simply, in seawater acidified to pH 0.6 by hydrochloric or sulfuric acid. The necessary elution time is 20 min, transportation and other dead times take 40 min. This gives 9 cycles per day. At any time, 60% of the carriers are involved in the adsorption process; 40% in elution and transportation. Carrier losses which can be kept low by extending the bordering nets (Figure 1)to a sufficiently large depth and, if necessary, by covering the whole driving up area are anticipated not to exceed
per day. To account for degradation, the carriers are supposed to be recoated with HA every 10 days (90cycles). Figure 3 shows a flow diagram of the whole recovery process. The uranium production costs are estimated in Table 11. As expected, in this concept investment costs play a less significant role. More important are adsorbent replacement and chemical costs due to more frequent recyclings in fast processes. Chemical costs may be lowered by using sulfuric acid as eluant in place of hydrochloric acid. Adsorbent costs strongly depend on the type of carrier material. Use of less expensive carriers with a large
from the eluate
final product
Ind. Eng. Chem. Process Des. Dev. 1982, 27, 784-785
784
effective surface per unit of volume leads to substantial cost reductions. Further savings would result if humic acids fixed on these carriers allowed more than 90 recyclings. All considerations concerning adsorption kinetics and their cost consequences are certainly not restricted to humic acids but, in principle, also apply to any suitable adsorbent with a sufficiently short relaxation time independent of the special recovery technique. Acknowledgment We wish to thank Ing. grad. G. Putral, Ing. grad. F. Ringelmann, and Mrs.E. Borchardt for carrying out the experiments. Nomenclature A = uranium accumulation of an adsorbent defined by eq 1 A , = equilibrium accumulation t = time T = relaxation time of the accumulation process c, = uranium concentration in seawater P = uranium production per unit of time
C = elution capacity of an adsorbent Literature Cited Broecker, W. “Chemical Oceanography”; brcourt Brace Jovanovich Inc.: New York, 1974. Campbell, M. H.;Frame, J. M.; Dudey, N. D.; Kid, G. R.; Mesec. V.; Woodfield, F. W.; Bhney, s. E.; Jante, M. R.; Anderson, R. C.; Clark, G. T. Exxon Nuclear Company Report, 1979, XN-RT-15 I. Davles R. V.; Kennedy, J.; McIlroy, R. W.; Spence, R.; Hill, K. M. Nature (London) 1964, 203, 1110. de Luca Rebello, A.; Wagener, K. I n “Envkonmental Biochemistry”; Neriagu, I. O., Ed.; Vol. I; Ann Arbor Scientific Publication Ann Arbor, MI, 1976; p 13. Duret, M. F.; Phillips, G. J.; Veeder, J. I.; Wolfe, W. A.; Williams, R. M. “Nuclear Resources. The Contribution of Nuclear Power to World Energy Supply. 1975 to 2020”; publlshed for the World Energy Conference by 1% Science and Technology Press, 1978. Oeschger, H; Slegenthaler, U Schotterer, U; Qugelmann, A. reelelus 1975, 27, 168.
Institut fur Chemie Nuclear Research Center Julich 0-5170 Julich 1 Federal Republic of Germany Technical University Aachen 0-5100 Aachen Federal Republic of Germany
Dieter Heitkamp*
Klaus Wagener
Received for review May 27, 1980 Revised manuscript received November 9, 1981 Accepted June 25, 1982
M A = amount of adsorbent in contact with seawater to accomplish the uranium production P
CORRESPONDENCE Comments on “Mlnlmum Fluldlzatlon Velocity at Hlgh Temperatures” Sir: From our own measurements (Botterill et al., 1982), we would agree with Pattipati and Wen (1981) that the Wen and Yu (1965) correlation fita the data for minimum fluidization velocity with an accuracy of about *34% over a range of operating temperatures up to -950 “C and at atmospheric pressure. However, we find a tendency for the correlation to underpredict the minimum fluidizing velocity at elevated temperatures. Thus, in Figure 1 a direct comparison is made between prediction and measured values over a range of operating temperatures, the lower values for a particular particle size corresponding to experiments at the higher temperatures. We disagree with them in their conclusion, largely drawn from backfitting values from some of our reported measured minimum fluidizing velocities, that the voidage at minimum fluidization, emf, does not vary with temperature. With particles of Geldart‘s Group “B” (1973), those of intermediate size and density forming beds which tend to bubble as soon as the minimum fluidizing velocity is exceeded, there is a complicated and, as yet, unexplained variation in emf with change in temperature (Batterill et al., 1982). It is this factor which largely accounts for the underprediction of the Wen and Yu correlation and we began our study of bed behavior over a range of operating temperatures because of the accumulating evidence of consistent discrepancies between measured values and predictions based on correlations derived from ambient condition testa. As we point out in our paper (Batterill et al., 1982),the reason the Goroshko equation (1968) gives a reasonable fit over a range of operating temperatures if a fictitious voidage is chosen to fit it to an ambient temperature 0196-4305/82/1121-0784$01.25/0
0.05
0.x)
0.9 1.0 U,f , (measured),m/s
5.0
10.0
Figure 1. Plots of predicted vs. measured values of CJ,.
measurement is fortuitous. It comes about because a term was omitted to simplify the solution of a quadratic equation. This has a very pronounced effect through the important transitional flow regime: a region covered by the operating conditions of many commercial beds. With larger and/or denser particles falling within Geldart’s we find no change in emf with operating temGroup “Dn, perature. The transition between the two typea of behavior is associated with change in operating conditions through Remf 12.5 at Ar 26000 and there is then a marked
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0 1982 American Chemical Society
