Patent Number: 045591709
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the present invention accomplishes volume reduction and makes possible the safe disposal of bead ion exchange resin wastes of various types. In particular, low-level radioactive wastes containing bead ion exchange resins having activities within the range of less than about 0.1 to about 100 .mu.Ci/cm.sup.3 can be treated in accordance with the process of this invention. Such bead ion exchange resin wastes may contain any one or several of the radioactive isotopes frequently encountered in the wastes of nuclear power plants, principally isotopes of Cs, Co, or I, especially Cs.sup.134, Cs.sup.137, Co.sup.58, or I.sup.129, as well as other commonly encountered radioactive isotopes. Substantially all of the water, both the water on the surface of the ion exchange resin beads and the water inside the porous beads, is removed in order to produce a dry product which contains substantially no water. The bead ion exchange resin waste is contacted in the form of a finely atomized spray with a hot gas to vaporize the water from the waste. The water vaporized from the waste includes interstitial water, the water settled from the waste, and any additional water which has been added for producing a slurry. The water absorbed in the porous resin beads themselves is also removed, there being an equilibrium between the water on the surface of the beads and the water inside the beads. Under the conditions of the process of the present invention, this equilibrium is disturbed by evaporating the water from the surface of the beads and causing the water inside the beads to diffuse outwardly to the surface of the beads and, in turn, to also be evaporated. A suitable apparatus in which to carry out the process of this invention is a heated gas spray dryer. A hot gas is produced, for example, by burning a suitable gaseous liquid or solid fuel with an excess of an oxygen-containing gas such as air, oxygen-enriched air, or oxygen in a suitable burner. If desired, the hot gas can be provided by means of an electrically heated gas heater or other suitable means. The resulting hot gas is then introduced into the spray dryer at a rate to produce the desired temperature in the spray dryer. Where a burner is used, any combustible gas such as natural gas or propane, liquid, such as fuel oil or kerosene, or solid fuel, such as coal or coke, can be used in the burner. Fuel oil is preferred as the fuel because of its lower cost and convenience. In any case, the hot gas which contacts the waste consists of a mixture of the oxidation products of the fuel used as well as any unreacted oxygen or air, depending upon the oxygen-containing gas selected. Where an electrically heated gas heater provides the hot gas, any gas of suitable heat capacity such as nitrogen, carbon dioxide, or air can be used. The temperature of the spray-drying zone is uniformly maintained in the range of about 200.degree. to 450.degree. C., and preferably in the range of about 300.degree. to 350.degree. C., by varying the rate of feeding the hot gas or the ion exchange resin beads into the spray dryer. Temperatures above about 450.degree. C. result in undesired oxidation and destruction of the spray-dried bead ion exchange resin waste and the production of noxious off-gases or the unwanted volatilization of radionuclides. The upper temperature limit is also constrained by the equipment used for particulate removal. At outlet temperatures below about 200.degree. C., the spray-dried ion exchange resin waste is not completely dry. It is, therefore, important that the temperature in the spray-drying zone be uniform so as to avoid the occurrence of unusually hot or unusually cold areas within the zone. Residence times of about 3 to 12 seconds are suitably used in the process of the present invention. At temperatures within the preferred range, namely about 300.degree. to 350.degree. C., residence times of about 3 to 6 seconds are preferred. If the residence time is increased to about 5 to 10 seconds, the temperature can be lowered to about 275.degree. to 325.degree. C. A finely atomized spray of the bead ion exchange resin waste being treated is introduced into the spray-drying zone by means of a suitable spray nozzle or other distribution means. The necessary degree of atomization can be achieved by varying the amount of water included in the waste, such as by slurrying the bead ion exchange resin waste in an amount of water to give the desired degree of atomization. Aqueous slurries of bead ion exchange resin wastes or wet bead ion exchange resins can be suitably treated by the process of the present invention. Spray drying of the bead ion exchange resin waste results in the production of a dry, flowable solid which contains the radioactive contaminants and bead ion exchange resin from which essentially all of the water has been removed and a nonradioactive gas which, after filtering, can be released to the atmosphere as a non-polluting gas. Substantial volume reductions can be realized according to the process of the present invention. In general, the ratio of the volume of the bead ion exchange resin waste to the volume of the spray-dried ion exchange resin beads is found to be in the range of about 1.5:1 to 3:1. In a particular embodiment of the present invention, the spray-dried ion exchange resin beads are introduced into a matrix-forming composition to provide a monolithic disposal means. For example, the bead ion exchange resin, which has been spray-dried and which consequently contains essentially no water, is introduced into a ceramic, asphaltic, polymeric or concrete matrix-forming composition in a ratio of dry ion exchange resin beads to solid matrix-forming composition of about 0.35:1 to 4:1 and preferably about 1.5:1 to 2.5:1. It is preferred to use a polymeric matrix, since the polymer of which the matrix is formed can be of a similar composition to that of the ion exchange resin beads themselves. In particular, a polymer formed from the copolymerization of a mixture of styrene and a vinyl ester, known as Dow polymer, has a composition which is similar to that of the ion exchange resin bead, which is itself composed of a polymeric styrene cross-linked with divinyl benzene and contains various ion exchanging functional groups, such as sulfonic acid or amine groups. In general, the monolithic disposal means produced in accordance with the process of the present invention display a high water impermeability resulting in the radionuclides present in the monolith being substantially unleachable. A radionuclide leachability below about 10.sup.-2 g/cm.sup.2 /day is generally obtainable. Leachabilities below about 10.sup.-4 g/cm.sup.2 /day can be realized and are preferred. The spray-dried bead ion exchange resin waste contains essentially no water. Thus, it is possible to realize extremely high loading of the various matrix materials, since free standing water is not encountered. A preferred method of practicing the invention is to use a polymeric matrix formed by stirring the spray-dried bead ion exchange resin waste with a low viscosity liquid solution of a vinyl ester and styrene and polymerizing the mixture of monomers by means of a peroxide catalyst and a tertiary amine promoter. A continuous matrix of polymer containing the spray-dried ion exchange resin waste within the polymeric matrix is thereby obtained. Loading of the matrix to the extent of about 25 to 80% by weight is readily achievable using the above-described polymeric matrix system and the spray-dried ion exchange resin waste produced in accordance with the process of this invention. Because of the essentially water-free composition, it is also possible to realize even higher waste loading than was previously possible using various other matrix materials. In general, loading in the range of about 25 to 40 wt % is achievable using ceramic, asphaltic, or concrete matrices. Referring now to the drawing, the sole figure of which illustrates the spray drying and solidification of a bead ion exchange resin waste, air is introduced into a heater 12 via a conduit 10. Heated air is conducted into spray dryer 16 via a conduit 14. Bead ion exchange resin waste in the form of wet solid or aqueous slurry is introduced into a feed tank 20 via a conduit 18. Water, if desired to form a slurry with the wet resin waste, is introduced via a conduit 22. Bead resin waste slurried with water, if desired, is conducted via a conduit 24, a metering pump 26, and a conduit 28 to spray dryer 16. Dried ion exchange beads and product gas exit spray dryer 16 via a conduit 30 and are conducted to a dry cyclone 32 wherein the gaseous product and the solid product are separated. Gaseous product exits dry cyclone 32 via a conduit 34 and is conducted to a Venturi scrubber 36. The scrubbed gas product exits via a conduit 38 and is conducted to a reheater 40. From reheater 40, the heated scrubbed gases are conducted to HEPA filters 44 via a conduit 42. The filtered gaseous product then exits HEPA filters 44 via a conduit 46, a fan 48, and a conduit 50 to the stack. Scrubbing solution for Venturi scrubber 36 is fed via a conduit 52, a pump 54, and a conduit 56 into the high velocity section of Venturi scrubber 36 where it contacts the gaseous product from dry cyclone 32. A portion of the liquid is recycled from Venturi scrubber 36 via a conduit 58, a pump 60, and a conduit 62 to feed tank 20. The solid product from dry cyclone 32 exits via a conduit 64 to a solidification system 66 wherein spray-dried ion exchange resin beads are processed to provide monolithic disposal means 68 containing the spray-dried ion exchange resin beads. The invention may be better understood by reference to the following examples which are intended to be illustrative of the process of the present invention and not in any way limitative thereof. EXAMPLE 1 A spray dryer having a diameter of 76 cm and utilizing a dry cyclone collector to collect the powder product of the spray dryer was used in this example. The cation exchange resin was Gravex-2; the anion exchange resin used was Gravex-1. The bead ion exchange resins were fed as either wet solids or slurries. In the case of the wet solids, the anion exchange resin contained approximately 65% water and the cation exchange resin approximately 35% water. Slurries consisted of approximately 30 wt % solid resin in water. The total water content of the anion exchange resin slurry was 89.5 wt %, and the total water content of the cation exchange resin slurry was 80.5 wt %. Wet solids were fed at average feed rates of 13 to 25 kilograms per hour, and slurries were fed at average feed rates of 17 to 28 liters per hour. Outlet temperatures of the spray dryer varied from 115.degree. to 370.degree. C. Residence times were in the range of 3 to 12 seconds. The results obtained in a series of 15 experiments are shown in the following table. TABLE ______________________________________ Ion Temp. Total Exchange at Water Resin Test Feed Rate Outlet Lost.sup. Type No. Feed* (kg/h) (l/h) (.degree.C.) (%) ______________________________________ Cation 1 W-S 19 25 115 32 2 W-S 15 20 140 51 3 W-S 25 33 140 62 4 W-S 17 23 150 42 5 W-S 14 19 220 79 6 S 16 25 325 98 7 S 20 19 335 100 8 S 20 19 370 100 Anion 9 W-S 16 26 120 23 10 W-S 13 21 135 30 11 W-S 15 24 160 35 12 W-S 6 10 160 49 13 W-S 9 15 200 71 14 S 17 17 290 74 15 S 29 28 310 82 ______________________________________ *W-S = wetsolid, S = slurry .sup. Amount of weight lost during spray drying compared to amount of weight lost at 125.degree. C. for 500 hours in a convection drying oven. From the foregoing results, it can be seen that over 70% of the total water present in the ion exchange resin beads can be removed according to the process of the present invention at temperatures of above about 200.degree. C. The following example illustrates the use of the process of the present invention in producing solid monoliths. EXAMPLE 2 Spray-dried ion exchange resin beads having a particle size distribution of 80 wt % greater than 300 microns were solidifed using Dow solidification binder 101. A sample of dried ion exchange resin beads was mixed with binder in a dried resin-to-binder ratio of 2:1. A 40% emulsion of benzoyl peroxide in inert diluents was added as catalyst, and a tertiary amine, N, N-dimethyl toluidine was used as promoter. Dow solidification binder 101 is a mixture of styrene and a vinyl ester. After 24 hours the spray-dried ion exchange resin beads were contained in a solidified monolithic mass of binder. The foregoing example shows that the spray-dried ion exchange resin beads prepared according to the process of the present invention can be solidified into a polymeric monolith suitable for disposal by burial. It will, of course, be realized that various modifications can be made to the design and operation of the process of this invention without departing from the spirit thereof. For example, baghouse filters can be used instead of the Venturi scrubber in order to achieve purification of the gases produced in the spray dryer. Other solidification polymers than the Dow polymer exemplifed herein, for example, urea-formaldehyde polymers, can be used in order to prepare monolithic disposal means. Thus, while the principle, preferred design and mode of operation of the invention have been explained and what is now considered to represent its best embodiment has been illustrated and described, it should be understood that within the scope of the appended claims, the invention can be practiced otherwise than as specifically illustrated and described.