WO1992003829A1

WO1992003829A1 - Organic material oxidation process utilizing no added catalyst

Info

Publication number: WO1992003829A1
Application number: PCT/US1991/006121
Authority: WO
Inventors: Christopher J. Wood; David Bradbury
Original assignee: Electric Power Research Institute
Priority date: 1990-08-28
Filing date: 1991-08-27
Publication date: 1992-03-05

Abstract

Waste streams containing organic materials, such as radioactive waste streams such as those in which ion exchange resins are found, mixed wastes which contain both chemical and radioactive hazards, and industrial wastes containing organic materials, including solid ion exchange resins, and transition metal ions can be oxidized in the presence of hydrogen peroxide without the need for added transition metal catalysts. Contrary to the teachings of the prior art, the present invention demonstrates that the oxidation reaction proceeds efficiently across a broad range of transition metal ion concentrations typically found in nuclear plant coolant system decontamination process wastes. High hydrogen peroxide efficiency is obtained by high organic material concentration and the removal of volatile partial oxidation products. The oxidation reaction is conducted at approximately 100 °C. Temperature control and heat exchange are required to bring the reaction mass to this temperature and to maintain this reactor temperature throughout the exothermic oxidation reaction.

Description

ORGANIC MATERIAL OXIDATION PROCESS UTILIZING NO ADDED O^T YST_.

Technical Field

This invention relates generally to the treatment of waste streams containing organic materials generated at nuclear power plants and in other industrial plants, and more particularly, this invention relates to the efficient oxidation of the organic components in these waste streams, including ion exchange resins, to carbon dioxide and water uεj.ng less than εtoichiometric amounts of hydrogen peroxide as an oxidizing agent, in the absence of added transition metal catalysts.

Background of the Invention Radioactive waste requires proper treatment and conditioning prior to its disposal to ensure that no environmental damage is caused. Safe storage of the waste following disposal relies upon the containment of radioactive materials by a combination of physical and chemical barriers, with the object of confining the material for long enough to allow its radioactivity to decay to harmless levels before its release to the environment. In certain techniques volume reduction and solidification are used in combination to contain the radioactive components.

The presence of organic materials in radioactive waste is detrimental to the stability and long term confinement of such waste in the solid state for ^■a number of important reasons. For example, these organic materials can degrade to form gaseous products and thereby leave void spaces within the solid structure containing the waste. Void spaces in the solid, caused by organic materials, increase the accessibility of the radioactive components in the solid to groundwater which, in turn, can lead to the undesirable spread of radioactive materials outside of the confines of the disposal site. Potentially one of the most important mechanisms for transport of radioactive materials out of a waste site is chelation. Certain types of organic materials called chelating agents make radioactive materials more soluble in groundwater, thereby increasing the ease with which those materials can be transported by groundwater. These same chelating agents can also block the absorption of migrating radioactive materials on the natural geological materials, which is otherwise an important mechanism for retarding migration of the radioactive components. Organic materials are present in some of the radioactive waste streams generated by the operation and decontamination of nuclear power facilities. Organic chelating agents are frequently present. A typical example of such waste materials is an ion exchange resin waste which arises from the nuclear reactor decontamination process. This waste material consists of polystyrene or acrylic ion exchange resin beads which retain, among other things, chelating agents used in the decontamination formulations. Oxidation or destruction of organic compounds in radioactive waste can be accomplished by a number of different processes. Incineration, for example, is capable of converting the organic compounds to harmless by-products such as carbon dioxide and water. Wet wastes, however, are less prone to incineration. The use of low temperature and low pressure wet oxidation has been described in the prior art for treatment of wet wastes. In this prior art process, hydrogen peroxide is used with an appropriate added catalyst to secure the oxidation of the organic materials in the waste to carbon dioxide and water. The hydrogen peroxide is converted in the presence cf the catalyst to the hydrcxyl radical, which is capable of oxidizing the organic compounds almost indiscriminately to simple gaseous by- products. In descriptions of this wet oxidation process, the amount of transition metal oxidation catalyst is carefully controlled to optimize the efficiency of organic oxidation. However, in actual radioactive waste streams seen in practice, transition metal ions of the type typically used as oxidation catalysts are often present in uncontrolled amounts. This situation presents a perceived problem for traditional low temperature, low pressure oxidation processes. If the ion exchange resins in the waste are already fully loaded with transition metal ions, there is a perceived risk that further addition of the transition metal ion catalyst will make the oxidation process less efficient. Thus, the prior art consistently teaches that great care must be taken to quantify and to control the amount of transition metal catalysts added to the waste stream to be oxidized.

Summary of the Invention

The process of the present invention provides an efficient means for oxidizing the organic materials present in industrial wastes, including chelating agents, which are generated in industrial processes, including nuclear reactor facility decontamination. Waste materials, containing the organic materials to be oxidized, are placed in water suspension or solution and reacted with hydrogen peroxide at temperatures less than 110 degrees Centigrade. We have discovered that in the oxidation of organic waste materials, the oxidation process is surprisingly tolerant of a wide variation in amount of transition metal ion catalyst present in the reaction mass, contrary to the teachings of the prior art. In nearly all circumstances that would be encountered in practical radioactive waste management, there is no necessity to add to, adjust or control the amount of transition metal catalyst present in the waste stream. Therefore, efficient oxidation of organic materials in the waste stream is accomplished in the absence of added transition metal catalysts.

It is known that process economics are highly dependent on the efficient utilization of the hydrogen peroxide reagent. It has been reported, for low level radioactive waste disposal, that the hydrogen peroxide reagent may represent up to 35% of the cost of disposing of the waste without treatment. The efficient utilization of hydrogen peroxide is therefore a key factor in commercial viability.

It is therefore an object of this invention to provide a process which efficiently oxidizes organic components and chelating agents present in radioactive waste streams at low temperatures.

It is a further object of this invention to practice an oxidation process which is tolerant of the widely diverse types and quantities of transition metals typically present in the waste stream initially.

It is another object of this invention to reduce the overall volume of nuclear reactor decontamination wastes by complete oxidation of all organic components, including ion exchange resins.

It is a still further object of this invention to efficiently utilize hydrogen peroxide in the oxidation process. It is another object of this invention to use less than the apparent theoretical amount of hydrogen peroxide by the removal of partial oxidation products.

Although the invention is described herein with reference to radioactive waste management it will be readily apparent to those of ordinary skill in the art that the principles described herein are equally applicable to other types of waste that contain organic materials, and in particular to the oxidation of waste streams including ion exchange resins.

Brief Description of the Drawings

Figure 1 is a process flow diagram which shows the process equipment used in the waste stream oxidation process of the present invention.

Figure 2 is a graph which shows the Total Organic Carbon present versus time in the oxidation reaction of a waste stream containing nuclear reactor decontamination ion exchange resins.

Figure 3 is a graph which plots Total Organic Carbon versus time for the oxidation of an SG Cleaning Solvent.

Detailed Description of the Invention

The equipment shown in Figure 1 is typical of that which can be used to carry out the oxidation process described in this invention. The description of the process with reference to Figure 1 is intended to illustrate a presently preferred mode of practicing the subject invention and not to limit the present invention.

At the start of the oxidation process a portion, or all of the waste material is added through an inlet 10 to a reactor vessel 12 together with an appropriate volume of water create a suspension or solution. The transition metal content of the waste stream is monitored by random analysis to insure that adequate levels are present to catalyze the oxidation reaction. One common transition metal monitoring technique involves combusting a small sample of the ion exchange resin to ash, dissolving the ash in acid and using known atomic absorption spectrometry procedures to measure the transition metal content.

If the waste material added through inlet 10 is solid it is suspended in added water by appropriate agitation in the reactor vessel 12. The temperature of the reactor vessel is raised to 100+10°C, using conventional heating/cooling jacket 14 and heat exchanger loop 18. Hydrogen peroxide (30%-60% aqueous solution) is added to the reactor vessel 12 from peroxide supply 16. A heater (not shown) is used to bring the temperature up to 100°C. A water heater or oil-based heater can be used. If the temperature of the oxidation reactor 12 is not brought up to this temperature, the oxidation reaction proceeds slowly, causing slow consumption of hydrogen peroxide, leading to an undesirable build¬ up of hydrogen peroxide in the reaction mass which in turn can lead to a run-away exothermic reaction.

Oxidation of the organic materials in the waste materials proceeds exothermically with the liberation of heat. The heat causes the contents of the reactor vessel 12 to boil, and steam and gases exit the vessel 12 through an anti-foaming device 20. Anti- foaming device 20 is a mechanical device which is externally motor driven. The device has internal cones which rotate to spin the foam and to break bubbles causing them to form drops. We have used FUNDAFOMER anti-foaming device from Chemap, Inc. , South Plainfield, N^".J. Other anti-foaming devices did not appear to function properly in this application. Standard, commercially available anti-foaming agents are not appropriate because, as organic compounds, they are consumed by the desired oxidation process, and thus would disappear during the course of the oxidation reaction.

From anti-foaming device 20, the gases pass through a condenser 22, spray condenser 24 and demisting system 26 to remove moisture from the off- gases. This process equipment is conventional, and its selection and design based upon the volume and reaction products are well within the skill of the ordinary artisan.

During the oxidation reaction, additional waste materials can be- added to the reactor vessel 12. Liquid level control in the reactor vessel 12 is maintained by the application of heating or cooling, and/or by the recycling of condensate from spray condenser 24. When the oxidation reaction in the reactor vessel 12 is complete, as determined by measurements of the total organic carbon in the reaction solution, the reactor contents are allowed to cool. At this point the residue remaining in reactor vessel 12 which has not degraded to carbon dioxide and water can be pumped out into a separate vessel (not shown in Figure 1) , where it can be contacted with lime or other alkali to raise the pH of the residue. The residue can then be solidified in a cement based matrix. We believe the waste volume reduction which can be achieved in the cement -based case is between 25% and 50% of the original wet settled resin.

Alternatively, the reactor residue can be evaporated to dryness for placement in a high integrity container (HIC) . As another alternative, the partially solid residue could be dewatered and the supernate returned to the reactor vessel 12. In this case, we anticipate a volume reduction of 10% to 25% of the original wet settled resin. In a final alternative, the supernatant solution can be treated with suitable materials to absorb the radioactive materials present, and the remaining solution discarded in liquid form. Whichever alternative is chosen, the final volume of the residue is substantially less than the original waste, the undesirable organic materials are destroyed, and the waste can be safely stored without the penalties normally associated with wastes containing organic components.

Hydrogen peroxide is used in the oxidation process according to the following reaction:

C_εH₇S0₃H + 20H₂0₂ → 8C0₂ + H₂S0_<, + 23H₂0

Approximately 2.5 moles of hydrogen peroxide are used per mole of carbon oxidized in the resin which in turn equates to 3.6 volumes of 50% hydrogen peroxide per volume of resin oxidized. The efficiency of the utilization of hydrogen peroxide is highest in the presence of high concentrations (about l,000ppm) of unreacted organic compounds in the system. However, a competing interest is the removal of the oxidation reaction residue in an organic free condition-. This interest favors operation at the end of the reaction when the concentration of organics is lower. During this final period of the reaction there is loss of hydrogen peroxide through catalyzed decomposition of hydrogen peroxide to oxygen and water — sometimes called the reaction tail.

The present invention has been devised to overcome this problem by adding resin continuously or in a series of batches. This leads to a higher -Si- concentration of organics throughout the majority of the reaction, thus enabling efficient hydrogen peroxide use throughout the reaction. When the reaction solution cannot tolerate any more loading, the resin addition is ceased. The remaining organics are then destroyed by the conventional reaction tail. This high loading technique has increased the utilization of hydrogen peroxide from about 70% efficiency to close to 100% efficiency in terms of reaction stoichiometry.

We have further increased hydrogen peroxide efficiency by use of a second development. It has been found that oxidation can occur with less than the apparent theoretical amount of hydrogen peroxide, increasing hydrogen peroxide utilization above 100% efficiency. It is believed that since volatile "partial oxidation" products are lost from the solution, confirmed by measurements of carbon monoxide and the odor of aldehydes and ketones in the off-gases. Under ideal circumstances, the volatile partial oxidation products would be further oxidized by catalytic air oxidation (e.g., like an automobile exhaust) . The removal of volatile partial oxidation products from the reaction mass can be maximized by maintaining boiling temperature, a low degree of reflux, high organic loading and low hydrogen peroxide addition rate.

The ion exchange resins which are to be decomposed by the oxidation process of the instant invention are typically formed of aromatic polymers, usually a copolymer of styrene and divinyl benzene, or acrylics. Common ion exchange resins associated with nuclear plant coolant system decontamination processes include Amberlyte 120(H) , IRA (400) or IRA (410) , and IONAC 365 (Sybron Corp.) . All of these resins can be easily oxidized according to the process of this invention. Powdex precoat filter resins, also widely used in nuclear reactor decontamination, can also be decomposed by the process of the present invention. Prior investigators have found that transition metals, such as titanium, chromium, manganese, iron and copper are particularly good oxidation catalysts for organic materials in general when used with hydrogen peroxide. While the present invention specifically teaches that there is no need to add more transition metals to the oxidation reaction as catalysts, the waste stream to be oxidized must contain at least one of these transition metals to catalyze the oxidation reaction. Iron and copper are preferred. The waste stream, and particularly the ion exchange resin, can be easily monitored for transition metal content using analytical techniques well known in the art, including atomic absorption spectrometry. Adequate transition metal catalyst is present to conduct the desired oxidation when at least 5% of the active sites on the ion exchange resins are loaded with metal. The upper limit of transition metal catalyst concentrates is more difficult to establish. As exemplified below, the oxidation reaction proceeds adequately when the ion exchange resin and its associated supernatant liquid is fully loaded with transition metal. The amount of transition metal present in the waste stream is preferably between 0.5% and 20% by 'weight of the organic content of the waste stream.

The present invention has been found to be particularly well suited to the oxidation of organic materials found in wastes generated during the LOMI process (see U.S. Patent No. 4,705,573) and the SGOG ("Steam Generators Owners Group") process. The process of the present invention is also useful in the decomposition oi reprocesόiiig solvents (e.g., tributyl phosphate (kerosene) ) , surface decontamination solutions, and mixed wastes, i.e., wastes which are both radioactively and chemically hazardous.

The invention will be further described with reference to the following examples.

Example 1 A series of small scale tests were conducted at

90-95°C to examine the effect of different amounts of transition metal ions upon carbon conversion. The capacity of the IR120H cation resin used in these tests was 1.9 meq cm^"3. Analysis of ferrous sulfate solutions equilibrated with a batch of IR120H showed that over 98% of the iron was transferred to the resin provided that the calculated capacity was not exceeded. Accordingly, a range of iron concentrations was chosen which corresponded to 10- 160% loading of the active sites on a 10 gram sample ^• of IR120H. Free ferrous ions were present in the liquid supernate in the 160% loading case. A peroxide flowrate of 72 ml/h was used and the experiment was allowed to proceed until carbon dioxide evolution was below detectability. The total amount of carbon dioxide evolved and the time to completion are shown in Table 1. It will be seen that the reaction proceeds quite satisfactorily over a wide range of iron concentrations.

TABLE 1 - Reaction as a function of Iron Present

(Theoretical Yield of Carbon Dioxide 5100 ml)

Example 2

A 100 ml jacketed reaction vessel was thermostatted at approximately 90°C. The reaction vessel contents were stirred by a magnetic follower. Dowex 1 anion resin (1 g) was oxidized in the presence of varying amounts of ferrous sulfate, using a peroxide flow rate of lOml/h. The results, shown in Table 2 , confirm that for anion resins the reaction proceeds satisfactorily over a wide range of iron concentrations.

TABLE 2 - Anion Resin Oxidation

Example 3

200 grams of ion exchange resins (chemically loaded with the constituents in Table 3 , as would be typical of materials resulting from a nuclear reactor decontamination) were oxidized in a ten liter vessel nαer t_cnJIi .iv_.ns s_.x_.ilar '.. t oώe o__ previous examples. Total Organic Carbon analyses are shown in Figure 2, from which it is deduced that the oxidation had proceeded successfully.

TABLE 3 - Simulant Decontamination Resin Loading

Example 4

Ion exchange resin was loaded with materials typical of those which would result from clean up following reactor decontamination with the LOMI process (see U.S. Patent No. 4,705,573). To prepare this resin, a LOMI solution (vanadous formate, picolinic acid and sodium hydroxide) was exposed to a carbon steel surface for 24 hours, and the resulting solution absorbed on a mixture of 40% cation, 60% anion resin. The total quantity of resin used for the test was 1 cubic foot (28.3 liters) . The resin was placed with water (total volume 77 liters) in the reactor vessel (total volume of vessel 283 liters) and the solution was heated to 95°C. Hydrogen peroxide solution (50%) was added to the reactor vessel at an average rate of 30 liters per hour. Reaction proceeded fcr a total of four hours after which the reactor volume was 74 liters. At the end of the reaction there was virtually no solid material present in the reactor vessel and the total organic carbon content of the reactor vessel contents was 1.9 kg (of 7 kg of the original resin) . A sample of the residue was neutralized and solidified with cement. The resultant product produced a stable monolithic form.

Example 5

A simulant solution was prepared of the spent iron solvent which arises from the cleaning of secondary side steam generators by the "SGOG" (Steam Generators Owners Group) process. This solution had the composition shown in Table 4 and contained large concentrations of the chelating agent EDTA. Hydrogen peroxide was added at the rate of 5ml/min to one liter of this solution at 100°C. This led to a steady decrease in the quantity of total organic carbon present. At the end, the measured Total Organic Carbon was I20ppm, indicating 99.6% completion of oxidation. The TOC values as a function of time are shown in Figure 3.

TABLE 4 - steam Generator Cleaning Solution Simulant Example 6

This example shows the utilization of hydrogen peroxide in excess of 100% efficiency in a run with the procedures described above with the equipment as in Example 4. In this case the resin used was a simulant of material that would arise following a "Citrox" decontamination, and consisted of 3 cubic feet of resin, 75% Purolite A-600 Anion and 25% Dowex NRW-37 Mixed Bed (40% Cation, 60% Anion) loaded with. 9.24kg Citrox and 0.45kg magnetite.

At 100% efficiency the utilization of 50% hydrogen peroxide is approximately 3.6 liters (0.95) gallons per liter of resin.

TABLE 6 - Mass Balance Information

Claims

Cla ims

1. A method for decomposing organic materials present in radioactive waste streams which comprises the step of contacting a radioactive waste stream containing organic materials and transition metal ions, with hydrogen peroxide under aqueous conditions at a controlled temperature of approximately 100°C to oxidize the organic materials present in said waste stream to form carbon dioxide and water in the absence of any added transition metal catalysts, wherein said organic material are present at a high concentration throughout said contacting step, and wherein volatile partial oxidation products generated during said oxidation step are removed by maintaining the boiling temperature, a low degree of reflux, high organic material loading and low hydrogen peroxide addition rate.

2. The method of Claim 1 in which the amount of transition metal present in said radioactive waste stream is between 0.5% and 20% by weight of the organic content of said waste stream.

3. The method of Claim 1 wherein said organic materials are selected from the group comprising chelating agents, ion exchange resins and organic solvents.

4. The method of Claim 1 wherein said radioactive waste stream is Low Oxidation-state Metal Ion process wastes.

5. The method of Claim 1 wherein said radioactive waste stream is Steam Generators Owners Group process secondary side wastes.

6. The method of Claim l further comprising providing a rotating-cone mechanical foam-collapsing device to assist in the separation of liquid and gaseous phases resulting from said oxidation reaction.

7. A method for decomposing organic materials present in industrial waste streams which comprises the step of _.contacting an industrial waste stream containing organic materials and transition metal ions with hydrogen peroxide under aqueous conditions at a controlled temperature of approximately 100°C to oxidize the organic materials present in said waste stream to form carbon dioxide and water in the absence of any added transition metal catalyst, wherein said organic materials are present at a high concentration throughout said contacting step, and wherein volatile partial oxidation products generated during said oxidation step are removed by maintaining the boiling temperature, a low degree of reflux, high organic material loading and low hydrogen peroxide addition rate.