WASTE MANAGEMENT, Vol. 12, pp. 337-343, 1992 0956-053X/92 $5.00 + .00 Printed in the USA. All rights reserved. Copyright c~ 1992 Pergamon Press Ltd.

I N C O R P O R A T I O N OF S P E N T ION-EXCHANGE RESINS IN T H E R M O S E T T I N G RESINS

S. V. S. Rao, K. B. Lal and R. V. Amalraj Centralized Waste Management Facility, Fuel Reproce~ing & Nuclear Waste Management Group, B.,4.R. ('.. Kalpakkam, 603 102, India

ABSTIL4CT. Urea formaldehyde and phenol formaldehyde polymers have been examined as possible candidate materials for immobilization of spent ion-exchange resin. Polymer blocks of 45 mm diameter and 44 mm height containing 30 g of resin, employing alkaline catalysts at a temperature range of 50-100°C, were prepared in the laboratory. The blocks were characterized for physical and mechanical properties and also for leach resistance. These values were compared with those of cement matrices. The compressive strengths of polymer blocks were found to be 6-9 times higher and the diffusion co- efficients lower by four orders of magnitude. The thermogravimetric analysis of the blocks showed that the polymer matrices were stable up to 250°C. In order to predict the peak center temperature in a 200 L drum operation, polymer blocks of 10 times larger volume were prepared and temperatures were measured during solidification. The theoretical peak center tem- perature was predicted and found to be less than the decomposition temperatures &the polymers.

I N T R O D U C T I O N

Spent ion-exchange resins from moderator and cool- ant water purification loops of Pressurised Heavy Water Reactors (PHWRs) are required to be condi- t ioned before disposal. The candidate materials widely used for ion-exchange resin fixation are ce- ment, asphalt, and polymers. Cement can incorpo- rate a max imum of 300 g of waste per kg of matrix ( 1 ) and hence offers little advantage in respect of reduc- tion in waste volume. The suitability of asphalt is being questioned because of its possible reaction with resin at high temperatures (1).

For immobil izat ion of ion-exchange resins and wastes containing boric acid solution, thermosetting resins like urea formaldehyde (2,3), epoxide resin (4), polyester (5), etc., were found to be good inasmuch as they are resistant to heat, radiation, and mechanical stresses, and, in addition, they have good leach resist- ance. A good deal of work has been carried out on urea formaldehyde (UF) immobilization and a con- siderable amount of information is available in the literature (2,3,6). The UF resin (2) used in radioactive waste systems is a commercially available viscous product formed by reacting urea with formaldehyde under alkaline or neutral conditions. Catalysts like phosphoric acid, sodium bisulphate, etc., are used to adjust pH 1-2 for matrix preparation. There are cer- tain primary problems reported with UF matrix (6-

RECEIVED 28 DECEMBER 1990; ACCEPTED 29 AUGUST 1991.

337

8), such as (a) exothermicity, (b) release of liquid when the material solidifies, (c) the low pH of the liq- uid released causing corrosion of the containers, and (d) requirement of enough acidity for solidification of UF/waste/catalyst mixture.

Polymerization reactions, in general, are exother- mic in nature, which needs consideration while pre- paring polymer matrices. The peak center tempera- tures (9) in 180 L drums containing 55 wt% waste for polyurethane, epoxy, vinyl ester, and polyester ma- trices were in the range of 52-93°C. Mahalingam et al. (10) have calculated peak center temperatures for styrene polymer systems and provided analysis for the prediction of temperature profiles during poly- merization. The rate of reaction of formaldehyde with urea was studied and the heat of reaction ( 11 ) for monomethano l urea formation was reported as 6 kcal/mole and for dimethylol formation as 5 kcal/ mole.

Besides exothermicity, the problem of generation of water in the U F matrix due to condensation reac- tion needs consideration. The rates (12,13) of addi- tion reaction of urea and formaldehyde and the con- densation of methanol urea with amino hydrogens are pH dependent and the rates are high at extreme pH values. The rate constants (12) for the polymer- ization reaction are of the order of 0.1 L/s. mole at pH between 1 and 2, while these values are of the order of mL/s . mole at pH between 4 and 5. Thus, an appro- priate selection of pH may control the rate of reac- tions a great deal.

An at tempt was, therefore, made in the laboratory

338 S . V . S . RAO, K. B. LAL, A N D R. V. A M A L R A J

An attempt was, therefore, made in the laboratory to overcome the above problems by altering the con- ditions of polymerization. The present paper dis- cusses the preparation and characterization of urea formaldehyde, phenol formaldehyde (PF) and ce- ment matrices. The experimental data from the lab- oratory studies in the case of UF was used for esti- mating peak center temperatures that may be attained during 200 L drum operation. The limita- tions in scaling up the operation are also briefly dis- cussed.

EXPERIMENTAL METHODS AND PROCEDURES

Preparation of Standard Size Block The details of the polymerization conditions (14) for each of the polymer matrices are given in Table 1. The preparation of UF matrix was carried out by dis- solving the catalysts in formaldehyde solution and followed by the addition of urea. The mixture was heated for 2-3 h and at the gelation point, 30 g of cat- ion exchange resin (Tulsion-42) containing 1 g of in- active cesium were added. The content were stirred well and transferred to a stainless steel mold with di- mensions of 45 mm (diameter) and 44 mm (height) and were allowed to cure for 2 days.

The preparation of PF matrix was carried out by heating the mixture of phenol and formaldehyde with barium hydroxide as a catalyst. Thirty g of cation ex- change resin, loaded with 1 g of inactive cesium, were added at the gelation point and the contents were al- lowed to cure in the mold as was done in the case of UF matrix.

The UF and PF matrices were removed from the mold after curing for further characterization.

Preparation of O. 7 L UF Matrix and Temperature Measurements In order to understand the problems associated in scaling up the operation, a block of 0.7 L waspre- pared containing 200 g of resin per kg of matrix. The procedure for the preparation of the matrix was slightly modified. In this method, UF resin prepara-

TABLE ! Conditions of Polymerization

Phenol Formaldehyde Urea Formaldehyde

1. Molar ratio 1 : 1.5 2. Catalysts Barium hydroxide

3. pH

(4) Temperature and duration

Initially 9 but adjusted to 7, at the gel point with oxalic acid.

100-110°C 3-4 hours

1:1.5 Sodium acetate +

Ammonia solution ~ 9

50-60"C for 2-3 hours and ~ hour at 110°C

tion was carried out in a separate vessel and when the solids content reached a level of 2 to 3%, the contents were transferred to another vessel containing wet ion- exchange resin with 64% water content. The contents were stirred and heated at 900C for 15 min to reach gelation point, when both heating and stirring were withdrawn. The matrix thus prepared was allowed to cure. During the period of curing, the temperatures were measured at the center and at points radially 0.037 m away.

Compressive Strength Measurement Compressive strength is an index of the extent of po- lymerization for the condensation polymers such as urea formaldehyde, phenol formaldehyde, etc. Dur- ing the formation of these polymers the released water may get trapped in the matrix, reducing the de- gree of polymerization and resulting in the reduction of compressive strength of the polymer. The mea- surement of compressive strength was carried out on an FIE compression testing machine (Fuel Instru- ments and Engineer's Pvt. Ltd., Ichalkaranji, India) conforming to Grade "A" of BS: 1610:1964.

Leach Rates and Diffusion Coefficients Leach tests were performed on the polymers in ac- cordance with International Atomic Energy Agency (IAEA) static leach test method (15) by immersing the standard size blocks in 350 mL demineralized water. The leachant was analyzed for cesium using an atomic absorption spectrophotometer. Leach rates were calculated using the following equation:

Leach rate = Rn

= (gadAo). (V /F) . (1/ET,) [1]

Where

Ea. = Cumulative radioactivity leached during the leachant removal period n;

A0 = Initial radioactivity in the block; F = Exposed surface area of the specimen, cmZ; V = Volume of the specimen, cm~;

X T n = Duration of leachant renewal period, days.

Diffusion coefficients (D) were calculated using the equation given under

D = ~r/4. (V/F) 2 " M 2 [2]

where M is the slope of the straight line obtained from a plot of [Ea,,/Ao] vs. V~.

S P E N T I O N - E X C H A N G E R E S I N S 339

Thermogravimetric Analysis Thermogravimetric analysis ofUF, PF, ion-exchange resin, UF matrix, and PF matrix was carried out using powdered A1203 as a reference material by maintain- ing the rate of heating at 60C per minute.

Microscopic Examination To understand the distribution of the ion-exchange resin in the polymer matrix, microscopic examina- tion of the cross section was performed at a hundred times magnification.

RESULTS AND DISCUSSION

The density, homogeneity, and compressive strength of the UF, PF, and cement matrices are given in Table 2. The compressive strengths of polymer matrices were 6-9 times higher than cement. Figs. 1 and 2, ob- tained from microscopic examination of PF and UF matrices respectively, show the distribution of the ion-exchange resin. The distribution of the resin in PF matrix was not uniform (ion-exchange resin clus- ters are seen in the matrix) whereas it was fairly uni- form in UF matrix. The voids observed in PF matrix may be attributed to the entrapment of air during the polymerization process.

TABLE 2 Physical Properties of the Matrices

UF PF Cement

1. Density (g/ 1.1 1.2 2.4 cm 3)

2. Compressive- 65 40 7 strength (MPa)

3. Homogenity Good Clusters Good (Microscopic found examination)

4. Decomposition 260°C 310"C - - temperature

5a. Cumulative 8.81 X 10 5 8.46 X 10 -5 8.05 X 10 3 leach rates after nine weeks (cm/ day)

5b. Diffusion 2.20 X 10 ii 2.14 × 10 -ll 1.20 X 10 7 coefficient (m2/ day)

The slopes of the plots of were computed for the matrices from Figs. 3-5 and the leach rates and dis- tribution coefficients were calculated using Eqs. [1] and [2]. The cumulative leach rates plotted against time for the matrices, are given in Fig. 6. The diffu- sion coefficients and the leach rates after 63 days are

FIGURE 1. Cross section of phenol formaldehyde matrix. FIGURE 2. Cross section of urea formaldehyde matrix.

340 S . V . S . RAO, K. B. LAL, AND R. V. A M A L R A J

Cumulative leach traction 0,8

0,7

0 . 6 -

0,5

0,4

0.3

0.2

01

0

/ I i . . . . . . T . . . . I

2 4 6 8

Squareroot of days 10

Normal ~ Regression

FIGURE 3. Cement matrix.

given in Table 2. In all the cases, the initial leach rate was expectedly higher since the leachant encounters larger surface area of the matrix. It was observed that the leach rates attained equilibrium much faster in the case of polymer matrices. The diffusion coefficients of the polymer matrices were found to be four orders of magnitude lower than cement matrices.

The Thermogravimetric Analysis (TGA) diagrams of the polymers and the polymer matrices are given in Fig. 7. Loss of weight during initial heating may be attributed to the loss of moisture, CO2 and formal- dehyde (16). From the TGA diagrams the decompo- sition temperatures for urea formaldehyde, phenol formaldehyde, and ion-exchange resin were found to be 2600C, 310°C, and 380°C respectively. When the resin was fixed in polymers, the matrix decomposi-

tion was observed to take place at a temperature be- yond 2500C.

Based on the temperature measurements in the ex- periments described previously in the section on preparation of 0.7 L UF matrix and temperature measurements, the steady state heat generation rate per unit volume of the block under the operating con- ditions during solidification was estimated. The cal- culations are shown in the Appendix. It is evident from the calculations that the peak center tempera- ture for 175 L block containing not in excess of 200 g of ion-exchange resin per kg of matrix would not ex- ceed 213°C.

The exothermic nature of the reaction cautions that an improper pH control in a larger scale opera- tion may lead to a violent reaction. This problem can be avoided by splitting the operation into two stages,

0 0 0 8

0,006

0 0 0 4

0 0 0 2

Cumulative leach fraction

i i i

2 4 6

Square root of days

Normal I Regression

FIGURE 4. Phenol formaldehyde matrix.

10

SPENT ION-EXCHANGE RESINS 341

0 0 0 8

0 006

0 004

0 002

Cumulative leach f ract ion

f

0 2

i i i

4 6 8

Squareroot of days

N o r m a l J R e g r e s s i o n

F I G U R E ft. U r e a f o r m a l d e h y d e mat r ix .

10

involving the preparation of resin and polymer ma- trix separately. The first stage can be performed in a stainless steel reactor wherein UF resin can be pre- pared as described in the experimental section. The reactor may have the provisions for heating/cooling, agitation, torque measurement, temperature mea- surement and control, bottom drain, etc. The second stage consists of in situ preparation of matrix in the waste drum containing the ion-exchange resin by pouring the UF resin in fluid state. The drum is then required to be heated to maintain a temperature around 90°C (by heating tapes or thermal chamber). The contents are to be kept under mild agitation to keep the ion-exchange resin in suspension in addition to maintaining uniform temperature. At the gelation

point, both heating and agitation could be withdrawn and the contents be allowed to cure.

The selection ofpH between 7 and 9, for the pres- ent study, not only controlled the increase in temper- ature during resin preparation but also allowed the re- moval of water formed during the reaction. Under the conditions of polymer matrix preparation described previously in the section on temperature measure- ment, there was a slow transition of pH value from around 5 to 4 due to evaporation of ammonia. As the kinetics of polymerization is relatively slow in this range in comparison with the pH range of 1 and 2, the scale up operation may provide scope for consider- able elimination of water of condensation on account of evaporation.

0 1

0 0 l

C u m u l a t i v e l e a c h r a t e ( c m / d a y )

] 0 0 0 E - 0 3

1 0 0 0 E - 0 4

1 0 0 0 E - 0 5 I I I I I I

10 20 30 40 50 60

Days

Cement ~ P h e n o l - f o r m a l d e h y d e ~ U r e a - f o r m a l d e h y d e

F I G U R E 6. C u m u l a t i v e leach rates o f the matr ices .

70

342 S . V . S . RAO, K. B. LAL, AND R. V. AMALRAJ

z

£fl (/3 0

0

10

20

50

40

50

60

70

80

90

100

[] UF + RESIN x UF '~ PF ,,- RESIN

50 100 150 200 250 500 550 400 450

TEMP. IN "C

FIGURE 7. Thermogravimetric analysis of matrices and IX resin.

5oo s~o

CONCLUSION

By employing alkaline catalysts the mechanical strength of UF matrix could be improved by a factor often compared to reported values (7). The compres- sive strength of phenol formaldehyde matrix was lower than urea formaldehyde, which may be due to the voids present in the matrix. The leach rate for each of the matrices was found to be 10 5 cm/day which compares well with other polymer matrices such as polyester, epoxy resin, etc. On extrapolation to a large scale operation, the estimated peak center temperature was lower than the decomposition tem- perature of the polymers. A choice of a larger waste to matrix ratio with convection cooling of the drum from outside will not allow the temperature to reach the estimated peak center temperature. However, temperatures beyond 100°C cannot be altogether ruled out. This may result in bulging of drum or cracks in the matrix due to escape of water in the form of steam. Studies are, therefore, required to be con- ducted on a large scale operation in order to assess the problems that may arise due to increase in tempera- ture during solidification and to observe the effect on the strength and the leachability of the matrix. Fur- thermore, use of disposable cooling coils within the polymer matrix may provide a solution to the prob- lems ofexothermicity of polymerization and low heat conduction of polymer matrix.

REFERENCES

I. Moriyama, N., Dojiri, S., Emura, S., Sugo, T., and Machi, S. Incorporation of radioactive spent ion-exchange resins in plas- tics. J. Nucl. Sci. Teehnol. 12:362 (1975).

2. Moghissi, A. A., Godbee, H. W., Ozker, M. S., and Carter, M. W. Nuclear Power Waste Technology. The American Society of Mechanical Engineers, New York (1978).

3. Carter, M. W., Moghissi, A. A., and Kahn, B. Management of Low Level Radioactive Waste, Vol. 1, Pergamon Press, New York (1979).

4. Burnay, S. G. Leaching of ~37Cs from an epoxide resin. Nucl. Chem. l~ "aste Manage. 6:139 (1986).

5. Ikaladious, N. E., Ghattas, N. K., and Eskander, S. B. Incor- poration of radioactive wastes into styrenated polyester. Nucl. Chem. Waste Manage. 6:101 (1986).

6. Moghissi, A. A., Godbee, H. W., and Hobart, S. A. Radioactive bl~ste Technology. The American Society of Mechanical En- gineers, New York (1986).

7. International Atomic Energy Agency. Conditioning oflow and intermediate level radioactive wastes. Technical Reports Series No.222, IAEA, Vienna (1983).

8. International Atomic Energy Agency. Immobilization of low and intermediate level radioactive wastes with polymers. Technical Reports Series No, 289, IAEA, Vienna (1988).

9. Haighlon, A. P., Gore, J. E., and Haigh, C. P. Developments in conditioning of nuclear power station wastes. Nuclear En- ergy 21:189 (1982).

10. Mahalingam, R., Blyanl, R. K., and Sha, J. T. Simulation of solidification temperature profiles in the polyester process for immobilization of hazardous waste. Ind. Eng. Chem. Process Des. Dev. 20:85 ( 1981).

1 I. Solomon, D. H. Kinetics and Mechanism cfPolymerization-- Step Grcmth Polymerization. Marcel Dekker, New York (1972).

12. Kirk-Othmer, (ed.). Encyclopedia of Chemical Technology, 3rd ed, Vol.2. John Wiley & Sons, New York (1978).

13. Kroschwitz, J. !. En¢3,clopedia q/Polymer Science and Engi- neering, 2nd ed, Vol. 1. John Wiley & Sons, New York (1985).

14. Sandier, S. R., and Wolf, K. Polymer Synthesis, Vol. 2. Aca- demic Press, New York (1974).

15. Hepse, E. D. Leach testing of immobilized radioactive waste solids. Atom. EnergyRev. 9:195 (1971).

16. Gitchley, J. P., Knight, G. J., and Wright, W. W. tteat Resis- tant PoO'mers. Plenum Press, New York (1983).

17. Dean, J. A. Handbook of Organic Chemisto,. McGraw Hill, New York (1987).

SPENT ION-EXCHANGE RESINS 343

A P P E N D I X - - E X T R A P O L A T I O N O F

L A B O R A T O R Y S T U D I E S I N T H E P R E D I C T I O N O F P E A K C E N T E R T E M P E R A T U R E IN A 200 L D R U M O P E R A T I O N

Estimation of Heat Generation Rate During Solidification at Steady State Conditions Assuming the heat conduct ion in one d imension and at steady state, for a uni form heat generat ion rate q in a cyl- inder of radius R, by a energy balance

q R : / 4 = k~T [1]

where q = Heat generat ion rate per unit vo lume (kcal/hr- m 3 )

R = Radial distance between the two observation points.

k = Thermal conduct iv i ty (kcal/hr . m. °C) 6T = Tempera tu re gradient between the two obser-

vat ion points.

In the laboratory study, under steady state conditions, the thermal gradient 6 T observed between two fixed points at a distance o f 0.037 m was,

6T= 2°C

k ~7~ = 0.3636 kcal /hr , m - ° C

R = 0.037 m

Using Eq. [1]

q = (0 .3636)- (2) • (4)/(3.7 X 10 ~_)2

= 2124.76 k c a l / h r . m 3

Estimation of Peak Center Temperature For operat ion in standard 200 L drum, assuming the block vo lume as 175 L,

F rom the est imated value of q, the temperature drop 6 T~ between the center of the block and outside surface can be found from Eq. [ 1 ].

6T~ = (2124. 76) • (0.2825):/(4 X 0.3636)

= 116.6°C

Total heat generat ion rate = Q = q x (vo lume of block) = q X 0.175 = 372 kcal /hr

Heat transfer coefficient ~°~ h (for laminar free convect ion of air at the outside surface) = 4.88 kcal /hr • m 2- °C

Available heat transfer area, A = ~rDL where D is the d iameter of the block = ~r X 0.565 X 0.7 = 1.2425 m 2

Considering the heat transfer rate from the surface of the d rum to the a tmosphere h A ~ = Q

6T2 = 372/(4.88 X 1.2425) = 61°C.

Where 6Tz is the tempera ture difference between surface of the d rum and ambient temperature.

Hence, outside surface temperature = Ambien t t emp + bT2 = 3 5 0 C + 6 1 ° C = 96°C

Predicted peak center temperature = Surface temp. + 6T~ = 96°C + 117°C = 213oc.

Radius of the block = 0.2825 m

Height o f the block (L) = 0.7 m