CA2066741A1

CA2066741A1 - Process and device for disintegrating spent ion exchange resins

Info

Publication number: CA2066741A1
Application number: CA 2066741
Authority: CA
Inventors: Klara-Maria Ghattas; Nader K. Ghattas
Original assignee: Individual
Current assignee: GHATTAS KLARA MARIA
Priority date: 1989-08-09
Filing date: 1990-08-08
Publication date: 1991-02-10
Also published as: DE3926252C2; WO1991002362A1; EP0486568A1; DE3926252A1; EG19766A; AU6163990A

Abstract

PROCESS AND DEVICE FOR DECOMPOSITION OF SPENT
ION EXCHANGE RESINS

ABSTRACT

A process for disintegrating special organic waste, in particular spent ion exchange resins, is carried out with hydrogen peroxide and a catalyst at temperatures between 105 and 135°C in a thermally insulated funnel reactor (1). The residual liquid is continuously filtered into a terminal reservoir (5) from where it is returned to the reactor (1) for evaporation purposes. The course of the reaction is controlled by means of a control device (ST) which controls the dosing means (13, 14, 15, 64) and regulating means (12A, 63, 16) in accordance with signals from temperature, level and activity indicators (7, N1, N2, A1, A2). The waste gas is condensed and filtered.

Description

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PROCESS AND DEVICE FOR DECOMPOSITION OF SPENT
ION EXCHANGE RESINS

The invention relates to a method for the decomposition of solid harmful substances, for example spent radioactive ion exchange resins, which are continuously delivered together with a reaction aid to a reactor chamber maintained under normal pressure, and decomposed by means of hydrogen peroxide, which is continuously delivered to the reactor chamber in metered amounts.

Such a method is known from US-A 4,624,7~2, in accordance with which the reactions react ill an aqueous medium in which the decomposition products gradually accumulate. As a reaction aid, a catalyst, for e~ample an iron salt, is continuously supplied to the reaction zone, such that it provides a catalytical effect at a temperature below the boiling point of the liquid. The reaction speed of the catalytic process is comparatively slow; and it is only possible to increase it by using increase~ pressure which, however, should be avoided for reasons of safety. Because it is necessary to supply fresh catalyst material continuously in order to maintain the reaction, the increasing amounts of mineral salts remain as mineral waste, which must be disposed of. The process must be performed in batches, because the reaction chamber must be emptied every time it is filled with decomposition products.

It is known to utilize ion exchange resins for processing radioactive materials. Following their use, these resins represent a lar~e portion of low- to medium-contaminated atomic was~e, depending on the content of radioactive materials. Most of the commercially employed ion exchange materials are synthetic resins. In view of the growing demand for capacity for special waste and because of its limited availability, a pre-treatment of the spent ion exchange resins to concentrate their volume and weight is necessary and Pconomical prior to further storage processing for safe disposal of the waste. The known pre-treatmant processes, whether dry or we~, result in completely different radioactive , .

20~7~1 materials in respect to their chemical ~nd physical properties, which are commonly advantageous for their further safe processing.

As is known, the dry processing of spent ion exchange resins is performed either by incineration or pyrolysis. In spite of considerable advances in these methods, technical safety problems are still present during dry incineration of radioactive waste, as mentioned in:
Valkiainen, M., et al., Nucl. Technol., 1982, S8, 248-255;
Johnson, T.C., et al., Trans. Am. Nuc. Soc. 1979, 32, 19-28.

The disadvantages of the incineration process in particular are:
- a complicated waste-gas treatment system is required due to large volumes of radioactively contaminated high temperature;
- flying ashes are easily dispersible materials and must be carefully treated;
- the handling of wastes and residue~s can impart harmful doses of radioactivity to the operation personnel;0 - the costs of construction, maintenance and operation of radioactively contaminated incineration systems are relatively high.

Among the wet processes, acid digestion is the most extensively studied method to reduce the volume of radioactive combustible waste, according to:
U.S. Patent 4,313,845 U.K. Patent 2 050 682 B
Although this process seems to be attractive compared with incineration, it still has some disadvantages:0 - The process operates under drastic conditions, namel~
hi~hl~ concentrated acids and temperatures between 230C
and 270C;
- a large amount of energy is consumed in order to maintain the required reaction temperature;

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- a complicated waste-gas treatment system is necessary due to the evolution of corrosive oxides of both s~llfur and nitrogen;
- recycling of the used concentrated nitric and sulfuric acids is required.

Furthermore, it is known from K. Lohs, D. Martinez, Entgiftungsmitel-Entgiftungsmethoden, Vieweg, Braunschweig, 1978, pp. g4-108, to decompose toxic organic solid waste chemicals by means of hydrogen peroxide, in which iron sulfate is used as the catalyst. The reaction temperature in this case lies below 100C in order to pravent decomposition of the peroxides. The reaction is performed in batches in the liquid medium, so that ~here is a large volume of waste.

It is the object of the invention to improve the reaction speed of the previously mentioned method of decomposition for the purpose of reducing the volume of organic harmful materials, in particular of spent, radioactively contaminated ion exchange resin, under safe operatillg condition~, and to provide a device for its simple, safe and reliable execution under easily controlled temperature and pressure conditions.

The ob;ect is attained in that starter energy, generated by means of a heating wire energized at the start of the reaction or a catalyst material s~pplied only at the start of the reaction and which exothermally decomposes the hydrogen pero~ide, is used as a reaction aid, and in that the residual liquid generated during the decomposition of the harmful materials is continuously filtered at the bottom of the reactor chamber and conducted into a terminal reæervoir, and that the metering of the hydrogen peroxide is controlled in

3~ such a way that the decomposition of the harmful matarials takes place in a temperature range between 105C and 140C.
Advantageous embodiments of the method and of the device are recited in the dependent claims.

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2~5~7~1 The reaction temperature is preferably maintained at a level between 105C and 135C, depending on the desired decomposition speed, the size of the installation, the foam level and the preset volume o~ supply of reaction components and the metering of the residual solution.

The advantage of the invention consists in particular in that the cost of operation, the cost of construction and the cost of maintenance of such equipment are low and the gas discharged can ~e prccessed quite simply in a closed emission processing apparatus.

It is particularly advantageous that no external energy input is required and under certain operating conditions free energy for external use can be made available.

In the process it is advantageous that, when the oxidizing agent is used sufficiently sparingly, only decomposition and not complete oxidation of the ion exchange resin can be achieved.

It is advantageously possible, by this process and apparatus, to achieve reliable and certain decomposition of other types of combustible non-radioactive waste an~ other radioactive, particularly alpha-contaminatedl waste without harm to the environment and thereby to achieve energy consexvation.

In order to be able to carry out the process in a simple and secure manner, an apparatus for the chemical decomposition of radioactively contaminated ion exchange resins is described, which permits a fully automatic control without the need for continual intervention in the radioactive control region.

Fig. 1 shows schematically a decomposition apparatus, with a vertical cross-section of the reactor.0 Fig. 2 shows a detailed cross-section having a replaceable filter.

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Figure l shows schematically the functionally essential combinations of parts and a reactor; their dimensions and proportions are typical but variable, provided the function is not adversely affected.

The system consists of a shaft funnel reactor vessel (l), which is surrounded by an outer protective jacket (2). A
hydrogen peroxide tank (3) is positioned outside the controlled radioactive security zone (17), and the ion exchage resin container (~), provided with a metering device (14) and a return pump (15) for the spent fluid, is positioned in this security zone (17).

The reactor (1) and the protective jacket (2) are connected at their bases via a releasable connector (120) to a terminal reservoir (5) which receives both fluid residues during the operation and also solid residue after a certain period of operation. The bottom exit (12) of the reactox (1) is advantageously in the form of a filter layer made of sintered glass, which allows only the fluid resiclue to pass through until, when opened, it allows the solid residue to pass into the terminal reservoir (5).

Various metering devices are provided in order to provide a continuous charging of the reactor (1) from the tank (3) by means of a first pump (13), from the container (4) by means of a controllable ~eed device (14) and from the terminal 2S reservoir (5) by means of a second pump (15) in a controlled and regulated manner.

In order to start the reaction, a catalytic material is introduced. This is preferably iron sulfate. The catalyst (61) is advantageously introduced into the reactor (1) via a long handled spatula (6) right under the exit (23~ of the oxidant, which is advantageously in the form of a nozzle. ~he spatula (6) is movable to various heights on the spatula shaft (62) by an adjustable drive (63); the reaction speed can thus :~ , : .

2~7~1 be influenced. The spatula shaft (63) is tubular, terminating at its lower end in the spoon-like concave spatula (6), and carrying a funnel arrangement or a metering device (64) for the introduction of the catalytic material at its upper end.

Both the r~actor vessel (1) and the protective jacket (2) are surrounded with a thermally isolating layer (20) so that, due to available energy from the decomposition reaction, heating of the feed material to the required reaction temperature takes place. Process control is achieved by one or more temperature probes (7~ whose signal or signals are fed to control e~uipment (ST), which so controls the metering equipment (13, 14, 15), the catalyst-feed equipment (64) and the catalyst positioning equipment (63) that a suitable predetermined temperature range for the decomposition reaction is quickly reached and then maintained.

The waste gas is fed via outlet (11) in the jacket (2) to a cooled condensor (8) and thereafter through an HEPA-filter (9). The condensate produced in the condensor (8) is collected in a receiver (10) and from ~here discharged.

The decomposition process is started by means of solid iron sulfate producing an exothermic decomposition reaction with preferably 60% hydrogen peroxide. A physical energy input is therefore not necessary, which simplifies the apparatus. The reactive hydrogen peroxide attacks the introduced resin, and the resul~ing decomposition of the resin proceeds exothermically, so that more heat is liberated into the reaction medium. This combined heat in the reaction medium further decomposes introduced hydrogen peroxide solution,which thereby attacks more ion exchange resin. This reaction proceeds further according to the feed of both components, which is ultimately controlled by the measurements made in the reactor vessels. The reaction produc~s are solid residues, liquid residue solutions and waste gases such as steam.

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The solid residues, which collect at the bottom of the shaft reactor, contain most of the radioa~tive materials. If present, cesium, which is highly soluble, is contained mainly in the residue solution.

The residue solution, which is collected in the terminal reservoir (5), is preferably returned to the reactor (1) for the purpose of evaporation utilizing the heat of reaction, so that its volume is reduced.

The handling of the residual solution depends on its quantity and degree of activity, which in turn depends on the type and combination of waste to be decomposed and on the type and quantity of radioactive substances therein, such as in the following ways:
- If the resiclual solution has a relatively low radioactivity, then it is fed, irrespective of its amount, to condensate container (10) outside the controlled security zone (17) via a first route through adjustable two-way valve (16).
- If the residual solution contains a high specifi~
radioactivity and is produced in a large quantity, it is chemically treated in the terminal reservoir (5) in order to precipitate the radioactive material and then returned via a second route through the two-way valve (16) to the reactor (1), where the precipitate is filtered out.5 - I~ the residual solution has a high spe~ific activity, but only a small quantity is produced, it is mixed with the solid residue and then solidified in the terminal reservoir (5) by additives and then removed.

For the control of the above procedure, temperature, level and activity sensors (7, Nl, N2, A1 and A2) are provided in the xeactor (l) and in the terminal reservoir (5), the signals from these sensors being fed to the control e~uipment (ST).
The latter controls the two-way valve (16) and tha feed of precipitant (F) and the additives (V), which produce the 2~7~

solidification, to the terminal reservoir (5), on an ongoing basis according to preset values. Continuing operation following the catalytic start is maintained by continuous metering of resin and controlled additions of oxidant. Upon the formation of foam, the feed of reactants are additionally regulated when the foam reaches a predetermined level in the reactor. Even an occasional excessive foaming is not critical because the foam can overflow through the annular gap (R) between the reactor (1) and and the protective jacket (2) into the terminal reservoir (5). Moreover, it is possible to add a known antifoaming agent~

Further, the filter layer (1~) is movable by the control device (ST) via a controlled drive (12A), so that evacuation of the solid residue into the terminal reservoir (5) can be brought about by a sideways sliding the filter layer, after the reaction chamber (1) has been filled to a predetermined level and after a succeeding completion of the decomposition reaction and, if necessary, after a precipitation reaction and the completion of the following filtering of the precipitated material.

An ~xample of the precipitating agent (]?) is, when radioactive cesium is contained in the radioactive residue solution, a concentrated solution of tri-potassium fe~rohexacyanide, whereby the cesium is bound and can be filtered and 2S concentrated as solid residue in the terminal reservoir.

An example of solidifying additives (V) for the residue is water-compatible polyester, which is added to the terminal reservoir (5) for binding purposes.

The waste gas and waste steam are fed to the condensor (8) and then through an HEPA-filter. This section of the process depends principally on the decomposition of organic ion exchange rPsins. Advantageously, it is not necessary to carry out complete oxidation in the radiation controlled zone. If , ~$74~l the concentration of the hydrocarbon in the condensate in the condensate container (10) is relatively high, it can there, or even later if necessary, be oxidiz~d further, e.g. if necessary with hydrogen peroxide. For such a secondary reaction, thè addition of hydrogen peroxide is provided at a controlled dosage (D) and an additional heater (H) may ~e built into the condensor (8). This is purposely insulated.
The resulting decomposition products of the additional oxidation, namely carbon dioxide and steam, leave the container (10) through the vent (8A) of the cooler (8) and further through the filter (9). The main reactor in the controlled zone is then not overloaded.

A part of the apparatus is shown in Fig. 2 provided with an alternative filter arrangement. In this case, a replacement 15 of the filter layer (12) during the operation is possible, in case precipitated fine residues clog the filter (12) and a sliding operation as shown in Fig. l is not sufficient to clean the filter. For the filter replacement, a horizontal transport track (21) is provided which leads directly over the filter through the protective jacket (2) and the wall of the reactor (1). A filter carrier (22), closed from above, containing a replacement filter (12') is provided on the trans~ort track and is insertable into the reactor (l). The used filter (12) is positioned in a holder (28) closable to the outside. It lets itself ~e guided to fall into the terminal reservoir (5) and is disposed of with the solid residue. After closing the holder (28), the replacement filter (12 t ) is guided freely out of a holder (24) in filter carrier (22), so that it is positioned in the holder (28) and taken up there. Then the filter carrier (22) is drawn back to its laterally outermost position, where engaging edges (25, 26) of the reactor chamber walls and the protective jacket (2) hold back the reactor contents. Electromagnets and a motor drive~
or pneumatic or h~draulic motors, are provided for the 35 guidance of the holders ~28, 24) and the carrier (22).

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The embodiments shown above are preferred, very simple and easily controllable constructions which can be modified by persons skilled in the art. In particular, instead of the catalyst feed apparatus for starting the reaction, another source of energy introduction can be used, e.g. in the form of an electrical heating apparatus, or a heating wire.
Furthermore, a filter exchange may be carried out from below in the region of the terminal reservoir instead of from the side, which obviates the need for the sluice valve and the transport apparatus. In this case, several filters pivotable at the edge of the exit of the reactor chamber may be provided, which are controllably pivoted and removed from their holdars. Upon such an exchange of filters, however, the solid contents of the reactor chamber are deposited into the terminal reservoir. From there, the fluid can be pumped back into the reactor chamber, if this is necessary.

For the purpose of providing a solidifying agent it is possible to arrange a tubular device in the terminal reservoir, so that an homogeneous solid product can be produced.

The above invention provides a simple and comprehensive syst~m for the decomposition of organic ion exchange resins and has various advantages, as follows:

- The apparatus is very simple and compact since it essentially consists of a reactor vessel and a protective ~acket, and requires low construction and maintenance costs~
- The operational costs are very low since the single active chemical which is consumed is commercially available hydrogen peroxide and its use is minimized.
- No continuing external source of heat is required since the reaction energy is sufficient due to insulation.
- The handling of the waste gas and the equipment therefor are extremely simple since the production of highly .
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aggresive s~llfur and nitrogen oxide gases is avoided, which was a disadvantage of the prior acid digestion procedure.
- The process is extremely advantageous since it is carried out at normal temperature and pressure and requires no external heat and concentrated acids. This results in guaranteed degrees of safety and reliability through known preventive measures.
- Noticeably large reductions of volumes and weights of waste to be handled are achieved, which minimizes the cost of suhsequent handling and storage of quantities of wastes.
- The process lends itself to the treatment of other kinds of wastes, e.g. combustible, non-radioactive material, and other nuclear, particularly highly toxic long-lived alpha-waste produced in the various steps of the fuel cycle.
The process can be adapted and used as an enerc3y source if corresponding technical improvements are made.
20 - Radioactive materials are not blown away on account of the low operational temperature and therefore they are not propagated, which in contrast t:hereto is a disadvantage of the prior art incineration process.
- The concentration of most of the radioactive nuclides in a small fraction of inorganic resiclue which ~athers at the base of the reactor facilitateæ the subse~u nt solidification, if the residue is delivered directly into the terminal storage container, where it is mixed with binding matarial.
30 - The process is easily controlled by the regulation of the hydrogen peroxide feed~
- The apparatus is so simple and compact that it can be constructed at the site of the waste production, thereby eliminating signi~icant handling, storage and transport costs ~or the waste material.

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Claims

Patent claims

1. A method for the decomposition of solid harmful substances, for example spent ion-exchange resins, which are continuously delivered with the use of a reaction aid to a reactor chamber (1) under normal pressure, by means of hydrogen peroxide which is continuously delivered to the reactor chamber (1) in a metered manner, characterized in that starter energy, generated by means of a heating wire charged at the start or a catalyst material only supplied at the start and which exothermally decomposes the hydrogen peroxide, is used as a reaction aid, and that the residual liquid generated during the decomposition of the harmful materials is continuously filtered out at the bottom of the reactor chamber (1) and conducted into a terminal reservoir (5), and that the metering of the hydrogen peroxide is control led in such a way that the decomposition of the harmful materials takes place in a temperature range between 105°C and 140°C.

2. A method in accordance with claim 1, characterized in that the hydrogen peroxide is at a concentration of 60% and that the catalyst material is iron sulfate.

3. A method in accordance with one of claims 1 to 3, characterized in that the reactor chamber (1) is thermally insulated and metering of the harmful material and of the hydrogen peroxide delivered to the reactor chamber (1) takes place in a controlled or regulated manner in such a way that the reacting materials do not exceed a preset level (N1).

4. A method in accordance with one of the preceding claims, characterized in that the metering of the hydrogen peroxide takes place in such a way that the organic ion- exchange resins are only decomposed, but not completely oxidized, so that liquid hydrocarbons are discharged into the terminal reservoir (5), together with the residual liquid.

5. A method in accordance with claim 4, characterized in that, if the residual liquid with the hydrocarbons shows low radioactivity, it is transferred into a container (10) to which, if required, an oxidation medium (D), for example hydrogen peroxide, and heat energy is supplied for the purpose of reoxidation.

6. A method in accordance with one of the preceding claims, characterized in that the generated exhaust gas and the generated exhaust steam are condensed by cooling outside of the radioactive control area, and that the remaining exhaust gas is HEPA-filtered.

7. A method in accordance with one of the preceding claims, characterized in that the harmful material is radioactively contaminated and the residual liquid discharged after filtration, if it shows radioactivity above a preset activity value and its amount is above a preset limit amount, is then returned in a metered way to the reactor chamber (1), so that the excess reaction heat evaporates the residual liquid.

8. A method in accordance with claim 7, characterized in that a precipitation medium for the radioactive components is added to the residual liquid and that the residual liquid is then returned to the reactor chamber (1) for the purpose of filtering.

9. A method in accordance with claim 8, characterized in that in case there is easily dissolved radioactive cesium present in the residual liquid, a concentrate of a tri-potassium ferrohexacyanide solution is used as precipitation medium.

10. A method in accordance with claims 8 or 9, characterized in that the solid reaction and precipitation residues, together with the remaining residual liquid, if the latter falls below a preset limit amount, are solidified by means of an additive, for example water-compatible polyester, in the terminal reservoir (5) and are then disposed with the terminal reservoir (5).

11. A device for executing the method in accordance with one of claims 1 to 10, with a reaction chamber (1), to which hydrogen peroxide can be delivered via a first metering device (13) and, via a second metering device (14), solid harmful materials can be delivered in a metered manner for their decomposition, characterized in that the reaction chamber (1) is a funnel-shaped shaft chamber closed at the bottom with a filter (12) which can be opened, underneath which a removable terminal reservoir (5) is disposed, and that, leaving open a space (R) leading to the terminal reservoir, the reactor chamber (I), together with the terminal reservoir (5), is surrounded by a protective casing (2), from which an exhaust gas and exhaust steam connector (11) leads to a condenser (8), the condensate from which is conducted into a receiving container (10), and that at least one temperature sensor (7) is disposed in the reactor chamber (1), the temperature signal from which controls the metering devices; (13, 14) via a control device (ST), so that the reaction temperature is maintained between 105°C and 140°C.

12. A device in accordance with claim 11, characterized in that the reactor chamber (1) and/or the protective casing (2) is/are surrounded at least partially by a thermal insulation (20).

13. A device in accordance with claims 11 or 12, characterized in that the protective casing (2) and the terminal reservoir (5), as well as a container (4) for harmful material and a harmful material metering device (14) and, if required, a residual liquid pump (15) are surrounded by a radioactivity control zone (17).

14. A device in accordance with one of claims 11 to 13, characterized in that a scoop-like catalyst mounting (6) extends into the reactor chamber (1), on which the oxidation medium (61) is disposed in the area of an outlet (23) of a feed (3) for an oxidation medium, and that the catalyst mounting (6) has a pipe-shaped support arm (62) through which the catalyst medium is conducted to a metering device (64) and which is displaceably and/or adjustably supported in the protective casing (2) and is controlled by a control drive (63), and a blast connection is disposed at the outlet (23) of the oxidation medium, in front of which the catalyst mounting (61) is placed.

15. A device in accordance with one of claims 11 to 14, characterized in that in the terminal reservoir (5) a suction nozzle of a feed pump (15) for the residual liquid is disposed, downstream of which on the pressure side a controllable two-way valve is placed, the one outlet of which terminates in the receiving container (10) and the other outlet of which is returned into the reaction chamber (1).

16. A device in accordance with claim 15, characterized in that the terminal reservoir (5) has a controllable feed for a precipitation medium (F) and a controllable feed for a solidification medium (V), and that an agitator device is disposed inside of it.

17. A device in accordance with one of claims 11 to 16, characterized in that the filter (12) is a porous sintered glass disk which can be controllably pivoted.

18. A device in accordance with one of claims 11 to 17, characterized in that the filter (12) is supported controllably removable above the terminal reservoir (5) and that a horizontal transport conveyor (21) is disposed above the filter (12) and is sealingly conducted through the protective casing (2) and the reactor wall, and that a mounting (22, 24) for a replacement filter (12') is controllably displaceable seated in the transport conveyor (21) and can be controllably removed from the mounting (24).

19. A device in accordance with one of claims 11 to 17, characterized in that a plurality of filters (12) is each controllably pivotable disposed in a mounting underneath the reactor chamber (1), from which they can each be controllably removed.

20. A device in accordance with one of claims 11 to 19, characterized in that a level sensor (N1, N2) is disposed in the reactor chamber (1) and/or the terminal reservoir (5), and/or a radioactivity sensor (A1, A2) is disposed in the reactor chamber (1) and/or the terminal reservoir (5), and that the signals of all said sensors (N1, N2, A1, A2) are provided to the control device (ST), which controls in a programmed manner and in accordance with the method and preset limit values the metering device (13) for the oxidation medium, the metering device (14) for the harmful materials, the feed device (64) for the catalyst material, the feed pump (15), the two-way valve (16), an opening and closing drive (12A) for the filter (12) and the control drive (63) for the catalyst mounting (6) as well as the transport conveyor (21) and the filter mountings (24, 28) and the metering devices for the precipitation medium (F) and the solidification medium (V) and the agitator device.