I
W. E. PROUT and L. P. FERNANDEZ Savannah River Laboratory, E. 1. du Pont de Nemours &
Co., Aiken, S. C.
Performance of Anion Resins in Agitated Beds This contactor can be used when the feed contains solids
h
SOLUTIONS
that contain relatively large amounts of solids cannot be processed in fixed beds of ion exchange resin because the bed is rapidly plugged by the solids. Solutions are often filtered to remove solids prior to passage through fixed beds of resin, but with some liquidsolids mixtures this is not practical. Special ion exchange contactors have been designed to process mixtures of liquid and solids. These special contactors include a fluidized bed in which the resin is expanded by the flow of feed through the contactor. The fluidized bed permits the passage of solids through the bed. The Higgins solid-liquid contactor ( 4 ) also permits the processing of solutions containing solids. I n this contactor both the resin and liquid phases are moved; in one cycle the resin is moved through a static solution phase, in another cycle the resin is static and the liquid moves through the resin phase. During the first cycle the resin phase is expanded,
permitting solids to pass through the bed. Agitated beds of several types have been used successfully to recover uranium from ores (7, 5). These resin beds, agitated by pulsing, “jigging,” and jerking, operate at efficiencies less than that for a fixed resin bed. However, they are not sufficiently agitated to qualify for the complete mixing necessary for some of the assumptions in the calculation method presented here. The application of the calculation method is valid for any agitated resin bed, regardless of the means of agitation, provided the resin and solution are completely mixed in a short time period. Another contactor lhat permits the passage of solids through the resin bed is a vessel designed to permit agitation of the resin by stirring. This type of contactor has been used to recover streptomycin ( 2 ) . This report deals with the performance of such an agitated bed of resin, since complete mixing occurs almost instantly.
The specific objectives were io study the chemical performance of the agitated bed of resin and to develop a method to predict the performance of the agitated bed. Comparison of the performance of the miniature agitated bed with a large unit showed that data obtained with the miniature unit can be scaled up without difficulty. Calculated performance of both the large-scale and miniature-scale agitated beds was in excellent agreement with measured performance.
Chemistry of the Actinides in Nitrate Solution
The actinide elements, thorium and plutonium, were used to measure the performance of the agitated bed. These ions were selected because of the relatively large difference in their affinities for anion resins. Plutonium(1V) in concentrated nitrate solution has a high affinity, whereas thorium(1V) has a much lower affinity for anion resin.
Agitator
Schematic diagram of miniature agitated bed shows dimensions of unit and flow pattern Bed diameter, 3.8 cm.; b e d area, 11.4 b e d height, 3.8 cm.; bed volume, 4 3 cc.; resin, Dowex 1-X4, 20-50 mesh; agitator, flat blade, 0.75 X 0.25 inch
sq. cm.)
r-
Weir
Screen
/
U
VOL. 53, NO. 6
JUNE 1961
449
4
R e m Dorex I-X4,20-5mmerh Feed: 8M " 0 3 &T ResTime.min
Points Feed.mdlter
.
15iThI
75
25 25
IO(P"1
Figure 1 . Typical absorption curves obtained when plutonium and thorium are absorbed on anion resin in agitated bed
7.5
Thorium
Plutonium losses were not affected b y increasing the temperature to 55' C.
Figure 2. Absorption curves for thorium compare miniature and largesize agitated beds
b
10 Bed Volumes O f Feed
I n solutions containing a high concentration of nitrare ion, the anionic nitrate complex of the tetravalent actinide elements is formed, for example:
+
Th+C+++ 6N03-
~5
Th(N02)6--
I t is this anionic complex that is absorbed by anion exchange resin (7) : 2RN03
+ Th(N03)6--
-5
RzTh(N03)a
+ 2NO3-
T h e absorption reaches a maximum in the range of 7 to 9M " 0 3 acid ( 3 ) Desorption from the resin is favored by a low nitrate concentration which causes ionization of the absorbable anionic complex ion to form the cationic nitrate complex ion Th(N03)+-+ ( 6 ) . Experimental
T h e miniature agitated bed used in this study was constructed of borosilicate glass and was jacketed for constant temperature work. A schematic representation of the contactor is shown (p. 449). Resin was retained at the base of the bed by a 100-mesh screen and was agitated by a motor-driven paddle that rotated a t 125 r.p.m. just above the screen. Liquids were fed to the bed from the feed tank by displacement with kerosine, and the flow of displacement liquid was controlled by an adjustable Lapp Pulsafeeder pump. The resin used in these studies was Dowex 1-X4. 20 to 50 mesh. Feed
solutions were 8M " 0 3 containing thorium(1V) or pluionium(1V) as the anionic nitrate complexes. Performance of the bed was determined by measuring the Th234 or PuZ39content of the effluent stream and comparing the results with the concentration of these ions in the feed solutions. Results
Absorption of Thorium and Plutonium. T h e plutonium losses during absorption on the agitated resin were about a factor of 6 less than the thorium losses, because of the greater affinity of the anion exchange resin for plutonium. T h e plutonium losses were unaffected by increasing the temperature to 55" C. Comparison of the absorption of thorium and plutonium and the effect of temperature on the absorption of plutonium from 8M H h - 0 3 solutions by anion resin are shown in Figure 1 . T h e results of six experiments on the absorption of thorium from 8M " 0 3 solution by anion resin in a miniature bed and a large bed (1000 times the volume of the miniature bed) are shoxn in Figure 2. A comparison of these data shows that the absorption of thorium was essentially the same for both beds and that miniature beds can be successfully used to obtain data suitable for scale-up. The effect of liquid residence time (ratio of total liquid volume in contactor to volume rate of liquid flow) at various
The data presented a r e f o r dilute soluficms o f plutonium and W i t h metal nitrate systems, thorium in the "0, system. When different equilibria and rate data could b e expected. the resin is loaded nearly to capacity, different rate d a t a a n d equilibria will b e obtained. The calculation method is not applicable without modification to systems of high concentrations of absorbable species, because of change in the distribution coefficient, Kd, with concentration ( 8 )
450
INDUSTRIAL AND ENGINEERING CHEMISTRY
1
1
001)
10
100
Bed Volumas of Feed
stages in the loading of a bed with thorium is shown in Table I. The calculated losses (cumulativr) givm in Table I werc transposed from Figure 2. These results confirm that liquid residence time has an important effect on loss, but increasing the residence time beyond about 12 minutes has relatively little effect in these cases. Without aqitation the general elution of the settled resin was similar to that for a fixed bed of resin. The feed solution or wash solution of 8M " 0 3 has a higher specific gravity than the 0.3M HNO? elutriant. With the agitator off, the wash solution i s easily displaced downward, and the resulting change in acid concentration is rather abrupt in the resin column. With the agitator on, a slightly larger volume of elutriant is required to achieve the same degree of recovery as a fixed bed elution. it'ith the agitator on better mixing is achieved, and the nitric acid concentration does not change so abrupt11 KO experiments were performed to stud) the effect of stirring on elution was removed in after the 8M " 0 3 fixed bed operation. The results of the elution of plutonium and thorium are shown in Figure 3. Calculation of Losses from an Agitated Bed. I t is possible to characterize the operation of a n agitated bed of resin from a knowledge of the liquid I
Table 1. Increasing the Residence Time Beyond 12 Minutes Has Little Effect on Thorium Absorption Feed: 8 M "OB; Resin: Dowex 1-X4, 20-50 mesh; Temp.:
Bed:
Feed. Mg. Th/Liter 15 15
60 60
1.5 in. diameter,
1.5 in. high;
25' C. Liauid Residence Time, Rlin.
Cumulative Loss, % 7 . 5 BV 11.5 BV
6.6
13.3
7.5 12.0
11.4 6.8 5.3
43.0
16.0 13.9 9.3
...
ANION RESINS
4 Figure 3. Rate of elution of plutonium and thorium is influenced b y temperature and agitation
Figure 4. Rate of absorption b y anion resin is influenced b y contact time and nature of absorbable metal I on
1
b
Bed Volumes of Elution1
residence time and the distribution ratio, R, as a function of time. Measurement of the absorption of plutonium and thorium by Dowex 1-X4 resin, 20 to 50 mesh, from 8M " 0 3 solutions (Figure 4) showed that the initial absorption was rapid. After about 1 hour the absorption became much slower. The initial rapid absorption appears to correspond to the saturation of the resin surface with plutonium (or thorium) ; the slower subsequent absorption is probably controlled by the rate of diffusion of plutonium through the resin. If the slower absorption is controlled by diffusion, the rate of desorption of plutonium (or thorium) from the resin should also be time dependent; that is, the length of time the plutonium has been absorbed should affect the rate of desorgtion. These considerations provide the basis for the assumptions used in the method for calculating losses. T h e distribution ratio, R, is a practical measure of the relative affinity of thorium and plutonium for the anion resin. By determination of how R values vary with time, it is possible to establish the conditions for maximum recovery and to predict what recovery might be expected from an agitated bed of resin.
Calculation of Performance. In calculating the losses of plutonium or other elements during absorption, the agitated bed of resin is treated as a single-stage batch contactor. I n this batch contactor, liquid and resin, in the same proportions (V,/V, = 1) as i n the agitated bed, are mixed for a period equal to the liquid residence time in the agitated bed. The plutonium that remains in the liquid a t the end of the liquid residence time is considered lost. This loss can be calculated from the distribution ratio of plutonium between the resin and liquid at a time equal to the liquid residence time, according to the following equations :
of the contact time, Co is the concentration of plutonium in the feed, and RI is the distribution ratio at a time, t, equal to one liquid residence time. After that time, the first volume of feed is considered to be instantaneously displaced by a second equal volume of feed. I t is assumed that the surface exchange sites have become available for further absorption by virtue of the diffusion of the initially absorbed ions into the resin beads. A rapid surface absorption from the second volume of feed therefore occurs, and a loss equal to that from the first volume, Cl, results. I n addition, it is assumed that a fraction of the ions which have diffused into the interior of the resin bead are lost by desorption during the second contact period. T h e concentration in the second volume of effluent is thus represented by two terms:
cz If Vs and V,, the volumes of solution and resin phases, are equal Equation 2 becomes : (3)
where CI is the concentration of plutonium in the liquid phase a t the end
cz
=
=
Cl
+ c,
+
cz RI + 1
(4) (5)
where C, represents the contribution from the desorbed ions. C, is calculated from the distribution ratio, Rz, and the amount of plutonium absorbed from the first increment (C, - Cl). T h e distri-
Figure 5. Predicted absorption losses are influenced b y feed residence time in agitated bed. Left-thorium; right-plutonium R e m Uowex I-X4,20-50merh Feed 10- Pullilei EM "03
0.011
I
I 10 Bed VoIurnw of Feed
I
1
I
IM)
10
100
B e d Vaiumer of Feed
VOL. 53, NO. 6
JUNE 1961
451
Figure 6. Observed absorption performance in agitated bed agrees with predicted results. Left-thorium; right-plutonium
R e m . D w e x I-X4,20.50 mesh
Feed; TOrng.Th/liter E M " 0 3 Tempsmtura: 25%. Curves: Predicted Losses
1
001' 0
1
10
20
Pm15. Measured Losses
&&--..A
0.01
40 50 Bed V o I ~ ~ l of e i Feed
3
6C
70
I
80
io0
Bed V o l ~ m eof~Feed
bution ratio used, RP,corresponds to the total time the absorbed plutonium has been in contact with the resin-in this case, twice the liquid residence time. Thus :
each increment. T h e loss for the nth increment is given by the following equation:
that the assumptions used in the calculations predicting the performance of a n agitated bed are reasonable.
Nomenclature BV
By substitutions in Equation 5, the effluent concentration is given:
Effluent losses from the addition of a third increment of feed are given by the following equation :
(8)
where R3 is the distribution ratio a t time, t , equal to three liquid residence times. Repetition of this process for each increment of liquid passed through the resin bed provides a calculated loss for
-1
0' 0
I ~
I
I
Resin: Dowex I-X4,20-5C!mesh
I
I
I
K ) 2 0 3 0 4 0 Liquid Residence Time, minutes
Figure 7. Losses of thorium and plutonium decrease as the residence time increases, up to a limit of 1 2 minutes
452
=
bed volumes-ratio of feed volume processed to bed volume,
where C, is the concentration lost in the nth bed volume conracting the resin.
CO = concentration
Discussion
C
Distribution ratios for plutonium and thorium between 8M HNOs solution and Dowex 1-X4 resin, 20 to 50 mesh (Figure 4) Tvere used in calculating the absorption losses ar several values of liquid residence time. T h e results are shown for thorium and for plutonium (Figure 5). Comparison of these data with actual losses from miniature agitated beds is also shown for thorium and plutonium (Figure 6). T h e calculated losses are slightly lower than the measured losses, particularly from the first 10 bed volumes of feed solution, as a consequence of the simplifying assumptions made for the mathematical treatment. The actual flow of solution through the agitated bed is continuous, not batchwise as was assumed in the calculations. Therefore a portion of each increment of feed passes through the bed in a time shorter than the nominal contact time, while a portion remains in the bed for a time longer than the nominal contact time. Failure to allow for this effect accounts for part, if not all, of the deviations. Calculations of the cumulative loss, after 70 bed volumes of feed and for various periods of liquid residence, show that plutonium losses will probably be very high when the liquid residence time is less than 3 to 6 minutes and that little or no improvement in recovery is obtained above 12-minute liquid residence time. A similar patLern is obtained with thorium. These data are shown in Figure 7. T h e excellent agreement between the calculated and experimental losses shows
R
INDUSTRIAL AND ENGINEERING CHEMISTRY
V,
of plutonium or thorium in fwd solution, mg./ liter = concentration of plutonium or thorium in effluent, mg./liter = distribution ratio-ratio of plutonium (or thorium) absorbed per milliliter of resin to plutonium (or thorium) unabsorbed Der milliliter of solution
V,
bed volume-bulk volume, settled, of resin in bed, cc. V , = total volume of liquid in contactor. ml. =
literature Cited (1) Arden, T. V., Davis, J. B., Herwig, G. L., Stewart, R. M., Swinton, E. A., W'eiss. D. E.. Proc. U. N. Intern. Conf. Peaceful Uses At. Energy, 2nd, Geneva, 1958 3, 396 (1958). (2) Bartels, C. R., Kleiman, G., Korzun, 6. N.? Irish, D. B., Chem. En,
