Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 271-276
27 1
Stability of Ion-Exchange Resins. 1. Resistance to Osmotic Shock Williaim M. Alvino Polymtvs & Plastics Department, Westinghouse R&D Center, Pittsburgh, Pennsylvania 75235
An osmotic shock test based on the cycling of ion-exchange resin beads between hot H$04 and cold deionized H,O has been successfully used to determine the attrition resistance of anion-exchange resins. The parameter measured to detect differences among batches of resin is designated as percent whole perfect beads. This gives an indication of the resistance of the beads to fragmentation and cracking. This property has been correlated with the sphericity values of a resin determined by an independent laboratory using an attrition test that subjects the resin beads to a mechanical pumping. Increasingthe temperature of the test increases the rate of deterioration and has a greater effect with the same batch of resin than between different batches of the same resin. Resin deterioration is manifested by the formation of cracks within the beads rather than by a fragmentation process. Use was made of the appropriate rate equations and the Arrhenius equation to interpret the data: we were thus enabled to rank and predict the relative stability of the ion-exchange resins under evaluation.
Introduction Of the many properties that contribute to the usefulness of ion-exchange resins, there is one that has been the subject of investigation for many years. This property is resin stability. The economics of utilizing ion-exchange resins depends on the resin replacement rate due to loss of the exchanger through either mechanical or chemical attrition. Resin attrition is dependent on the particular environment in which the resin is used and, while there are many tests to determine resin resistance to attrition, no one test is applicable for every process. Because of the large quantity of ion-exchange resins being contemplated for use by Westinghouse in their uranium operations, we have developed a test method for determining the stability of ion-exchange resins relative to the Westinghouse uranium operation in their Acid Leach Process. Experimental Section The ion-exchange resins examined are strong base anion gel type materials including both 16-20 and 16-30 mesh particle sizes. Osmotic Shock Attrition Test. Samples consisting of 15-20 p of fully hydrated vacuum drained beads in the chloride form were cycled between hot 20 wt 5% H2S04and cold deionized H 2 0 according to the following schedule: (1)Vacuum drain resin and soak in cold deionized H 2 0 at 10-12 "C for 15 s. (2) Vacuum drain and soak in 20 wt. 70 H$04 a t 60-63 "C for 15 s. (3) Repeat steps 1 and 2 ten times. (4) Conkert resin back to chloride form by immersing in excess 10% NaCl solution. (5) Wash resin with deionized H 2 0 and vacuum drain. (6) Dry resin at 1100 "C until beads ;ire free rolling (30 min). (7) Measure attrition effect by the whole perfect bead and sphericity methods. Measurement of Bead Attrition. Resin stability was measured in terms o f sphericity and whole uncracked beads. The values obtained before and after testing are compared and the severity of each test can be ascertained. Sphericity. The ciried free-flowing ion-exchange beads after cycling are poured onto an inclined plane (a highly polished metal plane with an approximate slope of 1/12) so that the beads flow down the plane without interfering with each other. The beads that roll off, as well as those that remain on the plane, are collected and weighed separately. Sphericity is calculated as 0196-4321/80/1219-0271$01 .OO/O
(weight of beads rolling off plane) / (total weight of beads)
X
100
Perfect Beads. The beads that rolled off the inclined plane are then examined microscopically (10 to 30X magnification) to determine cracked and uncracked beads. A quantity of the beads is placed in a Il2-in.diameter ring and the number of uncracked whole beads is counted. This is repeated until from 500 to 1000 beads have been examined. The percent perfect beads is calculated as (number of perfect beads)/(total number of beads) X 100 Whole Perfect Beads. This determination is made from either the sphericity or the perfect beads as (weight of beads rolling off plane)/ (total weight of beads) x % perfect beads or (number of perfect beads)/(total number of beads) X sphericity Dow Chemical Co. Attrition Test. Another attrition test was performed on the same batches of resin used in the cyclic test above. This test was run by the Dow Chemical Co. (Dow, 1968). Briefly, the resin is pumped as a slurry (pumping rate approximately 900 mL/min) through an apparatus designed to create attrition forces similar to those encountered in general commercial use. The sphericity of the beads before and after the test is measured and the measurement serves as an indication of the stability of the resin. Results and Discussion Osmotic Shock Test. The values for sphericity and whole perfect beads for the resins as they were received from the manufacturer are reported in Table I. By subjecting each batch of resin to our osmotic shock test, we expected to be able to identify both weak and strong batches of resin. The same properties of the resin after the osmotic shock test are shown in Table 11. The sphericity values in Table I1 indicate that the majority of the resins remained essentially unchanged; i.e., little fragmentation occurred during the test. While the sphericity values are relatively high, the whole perfect bead values for about half the batches are considerably below the 90% minimum used as our pass-fail criterion. It was E 1980 American Chemical Society
272
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
Table I. Properties of “As Received” Ion-Exchange Resins before Osmotic Shock Test
-
batch no.
sphericity, %
whole perfect beads, %
Resin A, 16-20 Mesh 1 2 3 4 5 6 7 8 9 10 11 12
91 99 99 100 99 95 97 97 93 85 95 92
86 93 98 99 99 91 97 97 92 80 95 90
o Resin A
Resin B, 16-30 Mesh 99 97 99 89 99 99
1
2 3 4 5 6
99 96 97 84 99 99
- 45
sphericity, %
perfect bead values). It would appear that one cannot predict, a priori, the stability of an ion-exchange resin to osmotic shock based solely on its sphericity values. The whole perfect bead measurement seems to be a better indicator of bead stability under the present test conditions. Effect of Acid Strength and Number of Cycles. Acid strength and cycles were varied to optimize the test procedure. Tables I11 and IV present the results of these tests. No significant effect was observed as a result of changing either the acid concentration or the number of cycles. Pump Test. The same batches of resin used in the osmotic test were also examined in the Dow Chemical attrition test. The results of their tests are reported in Table V. Keep in mind that this test measures only the sphericity of the resin. For comparison, the values of sphericity and whole perfect beads after our osmotic shock are in the same table. The sphericity values indicate that the resin fragments more in the Dow test than in the osmotic shock test, and seven batches of resin have lower sphericity values in the Dow test than in our test. If we compare the whole perfect bead values under the osmotic test with the sphericity values after attrition in the Dow test, we note that the same resins performed poorly in both
whole perfect beads, %
85 98 98 99 99 95 97 98 93 86 95 93
2 3 4 5 6
7 8 9
10 11 12
84 98 98 98 98 90 84 97 71 59 82 56
Resin B, 16-30 Mesh 1 2 3 4 5 6
82
88 97 98 82 99 99
1m
80 Sphericity After Attrition. 5
Figure 1. Relationship between osmotic shock test and Dow Chemical Co. attrition test.
Resin A, 16-20 Mesh
1
w
70
Table 11. Properties of Ion-Exchange Resins after Osmotic Shock Test batch no.
Y
96 90 73 99 99
observed that a large portion of the resin beads had cracked after the osmotic shock test (indicated by whole Table 111. Effect of Acid Strengtha
4 N H2S0,
H2O
H2O
6 N H,SO,
H*O
batch 3 resin A
batch 1 resin B
batch 3 resin A
batch 1 resin B
batch 3 resin A
batch 1 resin B
99 96
86 83
98 95
86 80
99 96
86 84
sphericity % whole perfect beads a
2 N H,SO,
Cycling between acid at 60-63 “ C and H 2 0at 10-12 ’ C.
Table I V . Effect of Number of Cyclesa and Acid Strength (Resin A) ~
2 N H,SO, H, 0 ~
batch 3 sphericity % w h o l e perfect beads a
batch 1
4 N H,SO, H2 0 batch 3 batch 1
1ocy
20cy
10 cy
20cy
1 0 cy
20 cy
1ocy
20 cy
99 96
97 96
86 83
85 84
98 95
98 95
86 80
84 82
Cycling between acid at 60-63
C and H,O at 10-12 “ C for 1 0 and 2 0 cycles.
6 N H,SO,
H2O batch 3 1 o c y 20 cy 99 96
98 95
batch 1 ___-_ 10 cy
20 cy
86 84
85 83
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
Table V. Effects of Attrition o n Ion-Exchange Resins after osmotic shock test
whole sphericity sphericit,y, perfect % beads, % before
batch no.
85 98 99 99 99 95 97 98 93 86 95 93
2 3 4
5 6 7 8 9 10 11 12
84 98 98 98 98 90 84 97 71 59 82 56
75’
80
: E
97 99 99 99 99 91 97 98 94 97 95 93
96 93 93 96 93 87 87 89 83 81 89 70
60
P ._
50
f
40
L
YI
* M 20
i
63OC
w 70
sphericity after
Resin A, 16-20 Mesh 1
m
1W
Dow Chemical Test
273
‘
1I
I
i iw 2w
5w
Mo 400
tm 700 8W
900
Cycles
Figure 3. The effect of alternate cycling from hot H2S0, to cold deionized H 2 0 on the stability of ion-exchange beads (batch no. 2).
Resin B, 16-30 Mesh 88 97 98 82 99 99
82 96 90 73 99 99
99 96 99 94 99 99
81 94 87 84 98 98
4
1 ’&
7-
70
10
f
1
I
1w 2w
I
i I
“t,
,
!
I
sw
Mo 400
50
I
1
bM1
7w
I1
Bw 900
Cycler
Figure 4. The effect of alternate cycling from hot H2S04to cold deionized H 2 0 on the stability of ion-euchange beads (batch no. 1). Table VI. Kinetic Parameters for Resin Deterioration
10
% sphericity retained, 10”
1w iw m
4w
5w wo
7 w 8w Pw
Cycles
Figure 2. The effect of‘ alternate cycling from hot H2S0, to cold deionized H 2 0 on the stcibility of ion-exchange beads (batch no. 3).
tests. While the two tests appear to distinguish between weak and strong resins, the magnitudes of the measured values differ. Lower values are obtained for whole perfect beads in the osmotic shock test. An attempt was made to correlate the data from the two tests. Figure 1 is a least-squares fit to the data points. This line is represented by the equation W = -64.33 1.70s
+
The correlation factor for the two test methods is calculated to be 0.896 with a value of 1.0 being indicative of a direct correlation between W and S. Accelerated Laboratory Stability Tests. In these tests the basic osmotic shock test described in this paper was employed except that the acid was used at three different temperatures (60, 7 3 , and 85 “C). Resin life is commonly measured by the number of cycles or years of operation before replacement is necessary. By accelerating the conditions of the osmotic shock test we hoped to be able to predict the life of the beads based solely on their resistance to osmotic shock. Resin Identification. Three different batches of the same resin were tested. These resins are identified as resin A, batch 1, 2, and 3. All are 16-20 mesh, strong base anion-exchange materials. The batches of resin were further classified as being “poor”, “good”, or “excellent”,
resin A
60-63°C
72-75°C
85-87 “ C
batch 3 batch 2 batch 1
2.1 0.64 0.35
5.0
1.5
7.1 4.5
1.1
0.8
respectively. This classification was based on the measurement of their initial sphericity and whole perfect bead content. A poor resin is defined as one having a whole perfect bead content of 2 > 1. This corresponds to our initial classification of “poor”, “good”, and “excellent” resins, respectively. That is, the “poor” resin deteriorates faster than the “good” resin, etc. The effect of temperature serves only to increase the rate of deterioration. It is surprising that the sphericity of the resin does not deteriorate to a greater extent considering the accelerated conditions of the test. In fact, after 300 cycles at 85-87 “C, there is only a 20% loss in sphericity for the “poor” resin (batch 3). It should be noted that when the resin was contacted with the hot acid, the initial temperature was not maintained at a constant value and, in fact, dropped at least 10 to 1 2 “C. As a result, the temperature drop most likely reduced the resin deterio-
274
Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No. 2 , 1980
Table VII. Percent Change in Rate Constants as Affected by Temperature and Resin Batcha
-
% ' A h at
resin A
60-63 " C
batch 3
03. + I)
batch 2
-.
batch 1
-
72.1 0
853. 0
a
85-87 " C
72-75 " C 03. + lZ8 -703. +132
0.1
+ 2x4
- 3L.1
+ 603
- 7$3.
- 8%.
+ 128
+ 214
1 refers t o numbers vertically.
--f
refers t o numbers
*
2ol 10
I
,
l
,
1
1
1
1
1
1W 2W Mo 4W 500 bw 700 800 900 Cycles IO0
Figure 7. The effect of alternate cycling from hot H2S04to cold deionized H20 on the stability of ion-exchangebeads (batch no. 1).
w 80
Table VIII. Kinetic Parameters for Resin Deterioration
70
40 50
% whole perfect beads
retained,
1
resin A
60-63°C
72-75°C
85-87 " C
batch 3 batch 2 batch 1
3.4 1.1 1.4
11 2.7 2.5
20.9 15 8.1
Table IX. Percent Change in Rate Constants as Affected by Temperature and Resin Batcha % A h at
Figure 5. The effect of alternate cycling from hot H2S04to cold deionized H20 on the stability of ion-exchangebeads (batch no. 2).
batch A batch 3
60-63°C
-+O i 0
batch2
-653. 0
batch 1
-554 0
72-75°C 03.
+ az3
-754
+ lz5
-77.1
+38
a 1 refers to numbers vertically. horizontally.
Io
L
100 200
i 1
MO 4W
.
8
1
500 bw 700 8W Cycler
WO
Figure 6. The effect of alternate cycling from hot H2S04to cold deionized H20 on the stability of ion-exchangebeads (batch no. 3).
ration rate. The rate of resin deterioration is shown in Table VI. As expected, the rate increases with temperature, and the resins deteriorate in the order predicted by their initial classification, i.e., poor resin > good resin > excellent resin. If we further analyze the rate data in Table VI one can determine that the change in rate of deterioration in terms of sphericity is affected more by temperature (+) than by differences in resin batches (4) (see Table VII). This temperature effect is not uniform among the three batches of resin studied. However, within a given temperature interval, the % Ah appears to be dependent upon the resin batch as originally classified. In the preceding section, resin degradation was analyzed in terms of sphericity. In this section, resin degradation will be analyzed in terms of whole perfect beads. Figures 5-7 show resin degradation in terms of this property. Unlike sphericity, the whole perfect bead content decreases much faster than the former property. In fact, after 300 cycles for batch 3 at 85-87 "C, there is a 50% decrease in whole perfect beads. The corresponding decrease for
-+
75-87 " C
04
+ 574 .-
2 8.2.1
+1563
- 61.2
+ 478
4
refers t o numbers
sphericity is only 20%. The whole perfect bead content is a measure of both fragmented and whole cracked beads and better describes the resistance of the resin to degradation. The data describing the rate of deterioration of this property are shown in Table VIII. In Figure 5 the deterioration for batch no. 2 at 72-75 "C appears to take place in two steps. For purposes of comparing this with the other batches of resin, the rate was calculated from a composite line drawn to reflect the average between the two steps. Resin deterioration in terms of whole perfect beads behaves in a similar manner as sphericity. However, the magnitude of the change is greater for the former property (see Table IX). Sphericity vs. Whole Perfect Beads. Resin degradation has been measured by following the change in sphericity and whole perfect bead content. The deterioration of the resin by fragmentation is seen to be a slow process compared to the cracking of the beads without fragmenting. The rate constants shown in Tables V and VI for these degradation processes confirm this fact. Therefore, it would appear that a buildup of fragments in the resin is negligible compared to the amount of beads that are cracked. It would also appear that even though the resin beads developed cracks, this would not necessarily lead to a corresponding increase in fragmentation in these laboratory tests. It is clear from the data that the calculation of the whole perfect bead value constitutes a better
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980 375
I 11
I
30 *
2 7 XlG-3
1/T
!
!
2 8
29
Figure 9. Arrhenius plot of the time to reach a 10% reduction in whole perfect heads. f
1
i
1;-
B
;2 ,
D
T
X
1
7
-
-
I
6
im
no xc
bm 100 cycles
fcc i m aa
PJ)
Figure 8. First-order plot of ion-exchange resin stability under osmotic shock condition;.
method for distinguishing weak resins from strong ones. Resin Life. In Figure 8 is plotted the percent whole perfect beads vs. number of cycles using the first-order rate equation. Briefly, the rate at which a reaction proceeds will be some function of the reaction property. In this case, the rate is related to the decrease in the whole perfect bead content of the resin. This rate may be expressed as
where P is the measured property and n is defined as the order of the reaction. On integration, one obtains
For a first-order reaction In (P,/P) = k t
(3)
and a plot of log property vs. time should be linear with a slope equal to -k/:!.303. We are interested in obtaining information about the life of a material under a given set of conditions. The Arrhenius equation has been used for treating accelerated aging test data for this purpose and expresses the effect of temperature on the rate of a reaction by the expression (4)
In using this equation, a point which will be considered the end of a product’s useful life must be chosen. The time to reach this end point at a series of constant temperatures is then determined, preferably by measuring some property as a continuous function of time. The time to reach this end point is then plotted against the reciprocal of the absolute temperature on semilogarithmic paper. As long as the heat of activation is independent of temperature, the logarithm of the rate of the reaction will yield a straight line when plotted against the reciprocal of the absolute temperature at which the reaction takes place. Extrapolation of this straight line is the key to the use of the Arrhenius equation in aging studies and enables one to
1-
39
2’9
2’8
llT
2
I
d E
Figure 10. Arrhenius plot of the time to reach a 20% reduction in whole perfect beads.
predict the stability of materials at lower temperatures by extrapolating data obtained at elevated temperatures. In Figures 9 and 10 we have used the Arrhenius equation and chosen, as our end point, the time in cycles to lose 10 and 20% whole perfect beads. This is plotted against 1/T for the three different batches of the same resin. Because of the nature of the accelerated conditions of the test, Le., the dual effect of temperature and ionic strength on resin stability coupled with the fact that true isothermal conditions could not be maintained, a rigorous treatment of the data using the above mentioned equations could not be maintained. We do feel, however, that differences in resin performance could be ascertained by this method, but caution should be exercised in extrapolating the Arrhenius plots. From these curves it is seen that resin performance does differ from batch to batch and appears to be dependent on the quality of the initial resin which we have classified as “poor”, “good”, and “excellent” by measuring the whole perfect bead content after an osmotic shock test. It can also be seen that there are rather large stability differences between resins classified as “poor” and “excellent”, and much smaller differences between the “good” and “excellent” resins. Examining the plots in Figures 9 and 10 further, it is evident that the individual points do not lie exactly on a straight line and the order of the reaction may be different from unity. It does appear, however, that the same d e radation mechanism is applicable to each batch of resjn and is operable over the temperature range studied beca,use lir of the parallelism of the lines. Conclusions Differences among various batches of a given ion-exchange resin in terms of their resistance to attrition can be ascertained by subjecting the resin beads to an osmotic shock test. This osmotic shock test indicates the following: h
Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 276-281
276
(1) There are real differences in the stability of different batches of the same ion-exchange resin. (2) Different batches of the same resin can be classified in terms of their resistance to osmotic shock. (3) Their resistance to osmotic shock can be defined by measuring a property defined as percent whole perfect beads. (4)The rate of deterioration in terms of bead fragmentation and cracking can be measured by calculating sphericity and whole perfect bead values. (5) The resins tested having a whole perfect bead content of 96%. Resins having values of 90-9570 are intermediate in their resistance to osmotic shock. (6) Increasing the test temperature within the same batch of resin affects the rate of deterioration more than between different batches of
the same resin. (7)Under the conditions of this test, bead cracking takes place to a greater extent than bead fragmentation so that the buildup of fines within the resin is less than the buildup of cracked beads. (8) The deterioration of the resin appears to proceed by the same mechanism among the three batches of resin examined. Acknowledgment
The author would like to thank W. R. Koryak for his assistance in this work. Literature Cited Dow Chemical Co., "Anaiytical Methods". Dowex Resin Method, 14, Dec 1968.
Received for reuicu March 30, 1979 Accepted January 21, 1980
Stability of Ion-Exchange Resins. 2. Factors Affecting the Stability of Ion-Exchange Resins William
M. Alvino
Polymers & Plastics Department, Westinghouse R&D Center, Pittsburgh, Pennsylvania 15235
The stability of ionexchange resins relative to their moisture content and crush strength was examined to determine why some resins are more resistant than others to deterioration under the osmotic shock conditions of a hot H2S04 and cold H,O cycling test. The resins used in this study were strong base anion gel-type resins having particle sizes between 16-20 and 16-30 mesh. For individual batches of resin within a given series, no direct relationship between crush strength and/or moisture content and resin stability was found, even though some of these batches were more resistant to osmotic shock deterioration. However, a general trend was indicated by comparing the data from each type of resin. I n general, resin stability appears to be associated with a decrease in moisture content and an increase in crush strength of the resin beads. Decreasing the moisture of the resin increases the hardness of the bead and, consequently, the crush strength. I t appears that an even distribution of properties is necessary for optimum resin stability. More important is that the bead should have a reasonably high crush strength but exhibit toughness (strength with flexibility) rather than brittleness.
formed on resin in the chloride form,
Introduction
As part of a continuing program to investigate the stability of ion-exchange resins to osmotic shock, we have examined the effects of moisture content and crush strength of resin beads. It was thought that the stability of ion exchange resins might be related to the strength of the beads. In fact, several users of these resins for water treatment in the nuclear power industry have included a crush strength requirement in their resin specifications which range from 200 to 350 g minimum. Apparently, before a minimum crush strength was specified, these users had experienced severe breakage of the resin beads so that meeting this requirement was considered necessary to ensure the physical stability of the resin. The objective of this work was to examine the relationship between crush strength, moisture content, and resin stability as they pertain to the ion-exchange resins used in the Westinghouse acid leach uranium operations. E x p e r i m e n t a l Section
Six batches each of three resins were tested according to the procedures outlines below. These resins are identified as resin A and resin C, both being 16-20 mesh in particle size distribution, and resin B, a 16-30 mesh resin. These resins are anion-exchange materials and are manufactured in the chloride form. All resin tests were per0196-4321/80/1219-0276$01.OO/O
T e s t Procedures
The test procedures outlined below were used in determining the stability of the ion exchange beads. About 1G20 g of resin in the chloride form is soaked in deionized water for 2 h or more and then vacuum drained. The osmotic shock test and measurement of bead attrition are described in part 1 of this series. M o i s t u r e Content Treatment a n d Measurement. (1) Soak ion-exchange beads in deionized water for 2 h. (2) Vacuum filter on fritted glass funnel for 15 min. (3) Weigh out three portions of the ion-exchange beads (approximate weight 6-7 g total). (4)Dry ion-exchange beads in a gravity convection oven overnight a t 110 "C. ( 5 ) Calculate percent moisture content of beads. C r u s h S t r e n g t h T r e a t m e n t a n d Measurement.
Crush strength was measured on an Instron Tester at a loading rate of 0.05 in./min. The values reported represent the total loading in grams required to crush individual beads. The procedure outlined below is used to measure crush strength as a function of moisture content: (1)Soak in deionized water for 2 h. (2) Run wet screen analysis and collect mesh fractions. (3) Measure crush strength after each of the following conditions: (a) soaking in deionized H20 for 1/2 h; (b) vacuum draining for 15 min; (c) vacuum 0 1980 American Chemical Society
