EP0044991B1 - Process and device for the pyrolytic destruction of organic substances that contain halogens and/or phosphor - Google Patents

Description

The invention relates to a method and a device for the pyrolytic decomposition of halogens and / or phosphorus-containing, radioactively contaminated organic substances at 300 to 800 ° C in a reactor.

A number of radioactive contaminated organic solvents and extraction agents are generated in nuclear technology, which have to be disposed of.

The majority of the contaminated organic substances that have to be removed consist of a mixture of tributyl phosphate (TBP) and kerosene, which is used, for example, in a volume ratio of 30:70 as an extractant in the reprocessing of spent fuel using the Purex process.

At other points in nuclear technology, for example in the organic conversion of UF ₆ to UF ₄ , perchlorethylene in a mixture with partially fluorinated products accumulates as waste, which must also be removed.

But also in the conventional area there are wastes of fluorinated, chlorinated and phosphorus-containing solvents, which cannot be burned without problems or, if they are not flammable, can be pyrolyzed, because in this case - especially in the heat - they have an extremely corrosive and environmentally harmful phosphoric acid, Hydrogen chloride and hydrogen fluoride develop.

In conventional areas, these substances are generally disposed of in an environmentally harmful manner by incineration on ocean-going ships. In the case of radioactive contaminated waste, offshore incineration is still much more intolerable from an environmental policy perspective.

Waste from the Nuckleartechnik must therefore be brought into a state capable of being disposed of under much stronger safety precautions. These safety precautions include that the waste must be reduced to the lowest possible volume on the one hand and that the methods of reduction work with high decontamination factors and the lowest possible secondary waste. The waste in the categories mentioned in the nuclear technology essentially consists of TBP / kerosene.

Several processes or process variants have been described and tested specifically for the removal of TBP / kerosene.

It is disadvantageous that the combustion produces P ₂ 0 ₅ on the one hand, which is a glass former and very quickly leads to blockages in the exhaust system of the incineration system through the formation of phosphate glasses. On the other hand, phosphoric acid is formed in the medium temperature range through reaction with the resulting reducing water, which, due to its extraordinary corrosiveness, causes damage, in particular, to metallic system parts (W. Bähr, W. Hempelmann and H. Krause, KfK-2418, February 1977).

Furthermore, combustion in an Na ₂ CO _a melt was attempted to remove the TBP or TBP / kerosene mixture. Here, the P ₂ 0 ₅ or the phosphoric acid is neutralized in situ. However, high-melting metaphosphates are formed which can lead to handling problems (DL Ziegler, AJ Johnson, Proceedings 14th ERDA Air Cleaning Conference, February 1977).

GB-A-15 17 014 describes a process for the treatment of contaminated TBP / kerosene wastes, in which the TBP is first separated from kerosene using phosphoric acid and then the adduct formed is thermally decomposed at about 200 ° C. to give phosphoric acid and butene becomes. Butene is burned, the highly active phosphoric acid is precipitated with calcium hydroxide and the resulting calcium phosphate is disposed of. The additional introduction of phosphoric acid required for the adduct formation into the process generates additional secondary waste, which is a particular disadvantage of this process.

In addition, a process has been described (DE-A1-28 55 650) in which the TBP / kerosene mixture is reacted in a fluidized bed which uses the superheated steam as fluidizing gas while simultaneously presenting a basic granulate as fluidizing material. The TBP is converted to phosphoric acid and a mixture of butene and butanol. Kerosene distills off and can be burned together with butene and butanol in the presence of the excess warm steam. The phosphoric acid is bound by the basic substances presented as fluidized material - preferably CaO or Ca (OH) ₂ - to form poorly soluble phosphates, which are withdrawn from the fluidized bed reactor and sent to final storage. However, an increased amount of reaction gas is normally required in a fluidized bed, which leads to an increased amount of exhaust gas and to dust formation in the reaction space, which can have a not insignificant influence on the service life of the filter candles at the reactor outlet. In addition, radioactive water is generated, which has to be processed in an additional process step.

It was therefore an object of the present invention to find a method and a device for the pyrolytic decomposition of halogen and / or phosphorus-containing, in particular radioactive, contaminated organic substances at 300 to 800 ° C. in a reactor, so that all organic substance mixtures without the formation of corrosive secondary products , can be processed with a minimum amount of exhaust gas, low dust development in the reactor and at the same time high decontamination factors. Furthermore, an inorganic product which is as inert as possible and preferably easy to condition in cement should be obtained as a residue in the reactor.

This object was achieved according to the invention in that the substances in an over-stoichiometric ratio with basic ver bonds are mixed and introduced in the form of a suspension from above into a fixed bed reactor which is charged with mechanically moved, largely spherical ceramic structures.

• Das Flüssigabfallgemisch, z.B. TBP/Kerosin, wird in einem Mischbehälter (1) mit einer knapp überstöchiometrischen Menge an Ca(OH)₂ oder Mg(OH)₂ zu einer Suspension verrührt und die Suspension mit Hilfe eines Rührwerkes (2) aufrecht erhalten. Durch eine Dosierpumpe (3) wird die Abfallsuspension über eine Zuführungsleitung (17) in einen Festbett-Reaktor (4) von oben her eingespeist. Der Festbett-Reaktor wird über einen Widerstandsofen (5) von aussen her indirekt beheizt. Im unteren Teil des Reaktors ist ein Siebblech (6) angebracht. Auf diesem Siebblech befindet sich eine Schüttung aus kugelförmigen keramischen Gebilden (7), vorzugsweise aus Blähton. Während der Abfalleinspeisung wird die Blähtonschüttung mittels des Rührers (8) ständig leicht bewegt, und um die gasförmigen Bestandteile in Richtung Nachverbrennungskammer (11) zu fördern, wird ein Stickstoffstrom bei einer Geschwindigkeit von 3-5 cm/sec über die Zuführungsleitung (16) durch den Reaktor (4) geleitet. In und an der Blähtonschüttung (7) erfolgt die Umsetzung der phosphor- und/oder halogenhaltigen Flüssigabfälle zu Calciumphosphat, -chlorid bzw. -fluorid. Die Umsetzungsprodukte der im freien Zustand stark korrosiv wirkenden Phosphorsäure, des Chlorwasserstoffes und des Fluorwasserstoffes rieseln in pulvriger Form durch die Blähtonschüttung (7) und das Siebblech (6) hindurch und können am unteren Teil des Festbett-Reaktors (4) diskontinuierlich über die zwischen den beiden Kugelhähnen (9) liegende Schleuse (10) abgezogen werden. Die Reaktionsgase gelangen in die Nachbrennkammer (11), wo sie mit einer knapp überstöchiometrischen Menge an Luft oder Sauerstoff zu C0₂ und Wasser verbrannt werden, vorzugsweise bei etwa 1000°C.

The method according to the invention is to be explained in more detail by way of example using the figure, a preferred device being used:

• The liquid waste mixture, for example TBP / kerosene, is stirred in a mixing container (1) with a slightly overstoichiometric amount of Ca (OH) ₂ or Mg (OH) ₂ to form a suspension and the suspension is maintained with the aid of an agitator (2). The waste suspension is fed from above via a feed line (17) into a fixed-bed reactor (4) by means of a metering pump (3). The fixed bed reactor is indirectly heated from the outside via a resistance furnace (5). A sieve plate (6) is attached in the lower part of the reactor. There is a bed of spherical ceramic structures (7), preferably of expanded clay, on this screen plate. During the waste feed, the expanded clay bed is constantly moved slightly by means of the stirrer (8), and in order to convey the gaseous components in the direction of the post-combustion chamber (11), a nitrogen stream is passed through the feed line (16) at a speed of 3-5 cm / sec passed the reactor (4). The conversion of the phosphorus and / or halogen-containing liquid waste to calcium phosphate, chloride or fluoride takes place in and on the expanded clay bed (7). The reaction products of the strongly corrosive phosphoric acid, the hydrogen chloride and the hydrogen fluoride trickle in powder form through the expanded clay bed (7) and the sieve plate (6) and can be discontinuously at the bottom of the fixed bed reactor (4) between the the lock (10) located on both ball valves (9). The reaction gases enter the afterburning chamber (11), where they are burned with a slightly over-stoichiometric amount of air or oxygen to form CO ₂ and water, preferably at about 1000 ° C.

The post-combustion chamber (11) is followed by a scrubber (12) for safety, which works at a pH of about 5 in order to separate any small amounts of HCI and / or HF present in the exhaust gas, while C0 ₂ at this pH happens to the washer. In the subsequent condenser chamber (13), the remaining water vapor is condensed and the remaining water aerosols are separated on the deep-bed filter (14). The exhaust gas now to be released via an S filter (15) consists only of CO ₂ .

Calcium hydroxide, calcium oxide, magnesium hydroxide and magnesium oxide have proven particularly useful as basic compounds. The ceramic bed consists largely of spherical particles, but it is also possible to use particles which deviate from the spherical shape and are, for example, elliptical. The particles advantageously have a size of 10-30 mm. As a result of the mechanical movement of the particles, the reaction products do not settle on them, but fall through the bed and can be drawn off at the lower part of the fixed bed reactor.

Expanded clay is preferably used for the bed, but other ceramic materials can also be used.

The process according to the invention will be explained in more detail using the following examples:

example 1

In the mixing container (1), 10 l of TBP / kerosene (30/70) doped with 3.6 g of highly enriched uranium (93% U-235) in the form of a uranyl nitrate solution are added with 1000 g of Ca (OH) ₂ powder stirred in a suspension and metered in with the aid of the metering pump (3) over the course of 2 hours into the reactor (4) which is at a reaction temperature of 500 ° C. In the reactor (4) there are 2 kg of a bed of expanded clay balls (7) with a diameter of 15-30 mm, which are easily moved with the help of the agitator (8) at 1 revolution per minute. After the end of the experiment, drawn off via the lock (10), 1.5 kg of an ash primarily consisting of calcium diphosphate had accumulated in the ash collecting container.

The combustible waste gases escaping from the reactor are afterburned in the afterburning chamber (11) with a small excess of air and the afterburning gases are washed with water in the scrubber (12). After the end of the test, 0.015% of the activity fed into the reactor (4) in the form of highly enriched uranium had accumulated in the circulation water of the scrubber (12). The phosphate content in the washing circuit is 30 ppm. The radiometric activity measurements in the exhaust air duct behind the S filter (15) are in the range of the zero level. _-

Example 2

10 kg of the fluorocarbon C ₂ F ₂ C [ ₄ with 11.5 kg Ca (OH) _{2 are} stirred into a suspension in the storage container (1). This suspension is doped with 3.6 g of highly enriched uranium (93% U-235) in the form of a uranyl nitrate solution and then fed via the metering pump (3) over a period of 2 hours into the reactor (4), which is at a reaction temperature of 570 ° C . In the reactor (4) there are 2 kg of a bed of expanded clay balls (7) with a diameter of 15-30 mm, which are moved with the aid of the agitator (8). After the end of the experiment, drawn off via the lock (10), 15.5 kg primarily of CaCl ₂ and CaF ₂ as well as excess potassium oxide existing ashes accumulated in the ash collecting container.

The combustible exhaust gases escaping from the reactor are used in the afterburning Chamber (11) burned with a small excess of air and the exhaust gases escaping from the post-combustion chamber washed out with water in the scrubber (12).

After the end of the test, 0.02% of the activity fed into the reactor (4) in the form of highly enriched uranium has accumulated in the circulation water of the scrubber (12). In the exhaust air duct behind the S filter (15), the radiometric activity measurements are in the range of the zero level.

Claims (6)

1. A process for the pyrolytic decomposition of in particular radioactively contaminated organic substances containing halogens and/or phosphorus at from 300 to 800°C in a reactor, characterised in that the substances are mixed in a super stoichiometric ratio with basic compounds and are introduced from above, in the form of a suspension, into a fixed bed reactor which contains mechanically moved, substantially spherical ceramic bodies.

2. A process according to claim 1, characterised in that calcium hydroxide, calcium oxide, magnesium hydroxide or magnesium oxide is used as basic compound.

3. A process according to claims 1 and 2, characterised in that the spherical ceramic bodies consist of light expanded clay aggregate and have a diameter of from 10to 30 mm.

4. A process according to claims 1 to 3, characterised in that the waste gases from the fixed bed reactor are subjected to post-combustion at about 1,000°C and are washed with water at pH 5.

5. An apparatus for carrying out the process according to claims 1 to 4, substantially consisting of a fixed bed reactor containing a loose material substantially consisting of spherical ceramic bodies, characterised in that the ceramic bodies (7) rest on a sieve plate (6) and are moved mechanically by means of a stirring device (8), waste material (17) is supplied vertically from above and the reaction gases and the resulting solid reaction products are drawn off separately at the bottom of the fixed bed reactor (4).

6. An apparatus according to claim 5, characterised in that a mixing vessel (1) and a metering pump (3) for the waste suspension are connected upstream of the fixed bed reactor (4) and a post-combustion chamber (11), a washer (12), a condenser (13) and a filter device, consisting of an aerosol filter (14) and an S-filter (15) are connected to the fixed bed reactor (4).

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