JP4668628B2 - Method for decomposing halogenated aromatic compounds contained in a medium with highly reactive hydrogen - Google Patents

Description

  The present invention relates to a method for safely chemically decomposing harmful halogenated aromatic compounds represented by polychlorobiphenyls (hereinafter referred to as PCB). In particular, it decomposes halogenated aromatic compounds without abrupt reaction heat, and almost completely suppresses the generation of harmful substances that can be generated after the compound is decomposed. The present invention relates to a novel method for decomposing a halogenated aromatic compound that makes it possible to recycle an alkali-resistant polar solvent.

  Safely chemically decomposing and detoxifying halogenated aromatic compounds, which are harmful substances, is the most important issue from the viewpoint of environmental protection. Conventionally, various proposals have been made regarding the decomposition of halogenated aromatic compounds.

  For example, decomposition treatment with metallic sodium (Patent Literature 1), decomposition treatment with electron beam irradiation (Patent Literatures 2, 3 and 4), mechanochemical treatment with a ball mill added with calcium oxide (Patent Literature 5), etc. are disclosed. .

However, these methods have the following problems.
In the method of decomposing a halogenated aromatic compound with metallic sodium, since ignitable metallic sodium is used, this handling is dangerous and there is a problem in safety. Moreover, the decomposition process by electron beam irradiation has a problem that the processing cost becomes high because an apparatus to be used is expensive. Further, in the mechanochemical treatment, an impact pulverizing apparatus such as a ball mill is used, but there is a problem that the apparatus used at this time is expensive. That is, none of the methods are satisfactory from the viewpoint of safety and cost.

In order to solve the problems existing in the conventional methods as described above, a chemical extraction decomposition method has been proposed as a method for chemically decomposing halogenated aromatic compounds. The chemical extraction decomposition method is disclosed as a treatment method for polychlorinated biphenyls and PCB-containing insulating oil (Patent Documents 6, 7 and 8). Specifically, a halogenated aromatic compound (PCB) is mixed with a strongly alkaline substance (potassium hydroxide or potassium hydroxide) in a heat-resistant alkali-resistant polar solvent (mainly 1,3-dimethyl-2-imidazolidinone) having excellent solubility. , Sodium hydroxide). These methods are epoch-making methods in that the PCB can be decomposed and the concentration can be reduced to a reference value (0.5 mg / kg or less). However, there is a problem that hydroxychlorinated biphenyls remain as decomposition byproducts accompanying the decomposition reaction. As will be described in detail later, the PCB decomposition reaction mechanism by this method is a nucleophilic substitution reaction to PCB by hydroxide ions (OH ⁻ ). However, there is a possibility that a form in which all of the plurality of substituted chlorines present on the PCB are not replaced by hydroxyl groups (—OH) (that is, hydroxychlorinated biphenyls) may remain. Hydroxychlorinated biphenyls, which can be said to be PCB decomposition intermediates, are substances that are considered to be endocrine disrupting substances. Therefore, a separate step for removing these by-products after the decomposition reaction is required. Therefore, while the PCB concentration can be reduced to a reference value or less by the methods described in Patent Documents 6, 7 and 8, other harmful substances are generated, and a separate process for removing these is further essential. It is.

The prior art document information relating to the invention of this application includes the following.
JP 2000-246002 JP 2001-9408 A JP 2001-9409 A JP 2001-70913 A JP 2001-47026 A JP-A-6-25691 JP-A-7-8572 JP-A-7-289656

  From the above, there is no need to treat decomposition by-products, that is, halogenated aromatic compounds containing PCB etc. are almost completely decomposed without generating harmful substances (by-products) and the main solvent is decomposed. The establishment of a chemically safe and cost-effective method for decomposing halogenated aromatic compounds that can reduce the rate is desired. Accordingly, an object of the present invention is to completely and safely decompose halogenated aromatic compounds including PCB and completely suppress the generation of decomposition byproducts such as various hydroxychlorinated biphenyls produced by the decomposition treatment. Another object of the present invention is to provide a novel treatment method that enables recycling of the heat-resistant and alkali-resistant polar solvent with high efficiency from the decomposition product after decomposition. In the method according to the present invention, a PCB decomposition method based on a completely new reaction mechanism is established, so that a separate step for removing such by-products is required without generating by-product hydroxychlorinated PCBs. An innovative PCB disassembly processing method is provided.

  The method for decomposing a halogenated aromatic compound according to the present invention comprises a catalyst comprising a medium containing a halogenated aromatic compound, a strong alkaline substance, and a substance that is decomposed by the strong alkaline substance to produce hydride ions. The halogenated aromatic compound is decomposed by bringing it into contact with a heat-resistant alkali-resistant polar solvent at room temperature to 250 ° C.

  Halogenated aromatic compounds are compounds that are highly toxic to humans, animals and plants. Many of them are specified by laws on waste treatment and cleaning as harmful substances, especially because of teratogenicity. . When these compounds are present in soil, groundwater, incineration ash, washing water, machine oil, etc., it is strictly determined that some treatment must be applied to reduce their concentration below the reference value. In the present invention, the “halogenated aromatic compound” refers to all compounds in which one or more of fluorine, chlorine, bromine and iodine are substituted on the aromatic compound. In the present invention, for example, polychlorobiphenyls (PCB), dioxins, chlorofluorocarbons, polychlorobenzenes and the like are indicated. PCB is a general term for compounds in which several chlorine atoms are substituted on the biphenyl skeleton, and there are many isomers depending on the substitution position and the number of substitutions of chlorine atoms. Dioxins refer to specific compounds specified by the Special Measures for Countermeasures against Dioxins in a narrow sense, but in the present invention, all halogenated compounds suspected as so-called endocrine disrupting substances (environmental hormones) are included.

  “Decomposing” the halogenated aromatic compound means that the halogenated aromatic compound is converted into another harmless compound by a chemical reaction described in detail below. It means that another compound is formed by dehalogenation reaction.

  In the present invention, the “medium containing a halogenated aromatic compound” means an insulating oil generally used for a high-voltage transformer, a high-voltage condenser, a ballast, etc .; central heating, panel heater, and heating / cooling used in various industries Heating medium: Lubricating oil including hydraulic oil, vacuum pump oil, cutting oil, extreme pressure additive, etc .; Plasticizer for insulation tape for wire coating, plasticizer for resin mixing, adhesive, varnish / wax, plastic for mixing asphalt, etc. Agent: Non-carbon paper and pressure-sensitive copying paper paints, various heat-resistant oils such as flame retardants, corrosion resistance, chemical resistance, water resistance, and printing inks. is there. In some of these media, halogenated aromatic compounds have been used in the past. In addition, for example, insulating oil used for transformers for heavy electrical equipment, underground transmission lines using insulating oil, etc., even if a small amount of PCB is mixed even if polychlorinated biphenyls (PCB) are not used as insulating oil There is a big problem of PCBs mixed in a small amount in such a medium. If a halogenated aromatic compound (PCB) is dissolved in such a medium, it cannot be discarded no matter how low it is, and must be stored and disposed of within a certain period of time. Don't be. Therefore, it is necessary to decompose the halogenated aromatic compound by the method of the present invention and reduce it to a prescribed concentration.

  In the present invention, the term “strong alkaline substance” refers to all substances that serve as a proton acceptor or an electron pair donor. For example, alkali metal elements and alkaline earth metal hydroxides, alkali metal carbonates, ammonia, amines And the like. Particularly, in the method of the present invention, sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium alkoxide, potassium alkoxide, and a mixture thereof can be suitably used.

  The strong alkaline substance is preferably a powdered alkaline substance, and the powdered alkaline substance can be obtained by pulverizing and pulverizing an alkaline substance. Here, the pulverization may be a physical operation that uses a mechanical force to break the solid into fine pieces and increase the surface area. Specifically, there is a method of filling the pulverizer into a pulverizer and continuing the pulverization operation until the desired fineness is maintained with the discharge port closed, and there is a concern of moisture absorption when pulverizing in a dry type. Sometimes the inside of the grinder can be evacuated or filled with an inert gas.

The particle diameter of the powdered alkaline substance is preferably 10 to 1000 μm.
In the present invention, the “substance that is decomposed by a strong alkaline substance to generate hydride ions” means a hydride ion (H ⁻ that is decomposed by the action of the above strong alkaline substance and is very important for the reaction mechanism of the method of the present invention. ). In the present specification, “amide compounds” and “aldehyde compounds” can be mentioned as examples of “substances that are decomposed by a strong alkaline substance to generate hydride ions”. The “substance that is decomposed by a strong alkaline substance to generate hydride ions” may be referred to as “hydride ion supply source”, “hydride ion supply substance”, or the like in this specification.

In the present invention, the “amide compound” refers to any cyclic or non-cyclic compound having a structure in which hydrogen of ammonia (NH ₃ ) or amine (RNH ₂ ) is substituted with a metal atom or an acyl atom group. Examples include acid amides, amino acids, and amino acid derivatives. The amide compound preferably used in the method of the present invention has the following formula:

Wherein R ₁ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl, and salts thereof, and the following formula:

Wherein R ₂ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl; R ₃ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl; R ₄ is hydrogen, C 3 _{1 to} C ₃ alkyl, and selected from the group consisting of ═O; R ₅ is selected from the group consisting of hydrogen and C _{1 to} C ₃ alkyl) and salts thereof . Particularly preferred acyclic amide compounds include, for example, N- (2-dimethylaminoethyl) formamide and ethylenebis (formamide), and suitable cyclic amide compounds include N- (2-dimethylaminoethyl) -N-methylformamide, -Methyl-2-imidazolidinone, 1,3-dimethylhydantoin, 1,3,5-trimethylhydantoin can be mentioned, and these can be used alone or in combination.

    The “aldehyde compound” used in the method of the present invention is a general term for compounds having an aldehyde group —CHO. In the method of the present invention, an aldehyde compound can be used in place of the amide compound described above. By adding an aldehyde compound, it is possible to suppress the production of hydroxychlorinated biphenyls as a decomposition byproduct of the halogenated aromatic compound. The aldehyde undergoes a Cannizzaro reaction in the presence of the strong alkaline substance described above to produce the corresponding alcohol and carboxylic acid. Particularly preferably, two aldehyde compounds are used in combination and subjected to a crossing Cannizzaro reaction. The effect of the aldehyde compound on the reaction of the present invention will be described in detail later.

    In the present invention, “catalyst” refers to all catalyst compounds that can promote dehalogenation of a halogenated aromatic compound. In particular, in the method of the present invention, nickel, aluminum, iron, molybdenum, palladium, chromium, cobalt, manganese, alloys thereof, and mixtures thereof can be suitably used.

  In the present invention, the “heat-resistant and alkali-resistant polar solvent” means that it hardly decomposes or disappears even when the reaction is carried out at a high temperature (at least 100 ° C., preferably 160 ° C. or more, particularly preferably 200 ° C. or more). The term “solvent” refers to all polar solvents that hardly decompose or disappear when the strongly alkaline substance is dissolved in the solvent. As the heat-resistant and alkali-resistant polar solvent, either an inorganic solvent or an organic solvent can be used. From the viewpoint of the solubility of the halogenated aromatic compound, an organic solvent, particularly an aliphatic hydrocarbon solvent, an alicyclic hydrocarbon solvent. Etc. are preferably used. Examples of the heat-resistant and alkali-resistant polar solvent suitably used in the present invention include 1,3-dimethyl-2-imidazolidinone (hereinafter referred to as DMI), sulfolane, ethylene glycol, triethylene glycol, lower alkyl ethers, trimethylene. And glycol, butylene glycol, and mixtures thereof. Of these, it is preferable to use 1,3-dimethyl-2-imidazolidinone, which dissolves a halogenated aromatic compound well, has high heat resistance, and does not have a bad odor when heated to a high temperature.

  In the present invention, a halogenated aromatic compound, a strong alkaline substance, and a substance that is decomposed by the strong alkaline substance to generate hydride ions (hydride ion supply source) are contacted with a catalyst and a heat-resistant alkali-resistant solvent. It is preferable that the catalyst is dispersed in the heat-resistant alkaline solvent. The order of addition is not particularly limited. For example, in a mode in which the catalyst is first added to and dispersed in the heat and alkali resistant solvent, and then the halogenated aromatic compound, the hydride ion source, and the strong alkaline substance are added, or A mode in which a halogenated aromatic compound, a hydride ion source, and a strong alkaline substance are added to a heat-resistant and alkali-resistant solvent and then the catalyst is dispersed may be used. If the halogenated aromatic compound, hydride ion source, and strong alkaline substance are added after the catalyst has been added and dispersed in the heat and alkali resistant solvent, the halogenated aromatic compound is finally added to the reactor. It is particularly preferable to add them. In any case, it is preferable to add the halogenated aromatic compound to the reactor as late as possible.

  Dispersing the catalyst in a heat-resistant alkali-resistant solvent refers to dispersing the catalyst in the heat-resistant alkali-resistant polar solvent by physical means. When the reaction is performed in the presence of a high-boiling hydrocarbon solvent, the catalyst can be dispersed in the high-boiling hydrocarbon solvent. The catalyst does not need to be dissolved in the heat-resistant alkali-resistant polar solvent as long as it can be dispersed. Examples of physical means for dispersing the catalyst include a stirrer and various industrially used stirrers.

  The method of the present invention is characterized in that a halogenated aromatic compound, a hydride ion supply substance, and a strong alkaline substance are brought into contact in a heat-resistant alkali-resistant polar solvent in which a catalyst is dispersed. The temperature when contacting is from room temperature to 250 ° C, preferably from room temperature to 210 ° C. By setting such a temperature, it becomes possible to decompose the halogenated aromatic compound substantially completely.

  The method for decomposing a halogenated aromatic compound contained in a medium by the method of the present invention with highly reactive hydrogen is a novel method in which the chemical reaction mechanism is completely different from that of the conventional method. At this time, hydroxychlorinated biphenyls and the like are not generated as a by-product, so that a separate process for removing them is not necessary. According to the method of the present invention, the decomposition of the heat-resistant and alkali-resistant solvent can be kept low, whereby the heat-resistant and alkali-resistant solvent can be recovered by distillation and recycled.

  In addition, by using a powdered alkaline substance in the present invention, the processing time can be shortened from the speed dependency in the reaction system due to the increase in the wetted surface area, so that the halogenated aromatic compound is more economical and simple. Can be disassembled.

  Specific embodiments of the method of the present invention will be described below. First, a chemical reaction mechanism for decomposing a halogenated aromatic compound by the method of the present invention will be described in detail in comparison with a conventional method (chemical extraction decomposition method).

  The chemical reaction mechanism in the method described in the conventional chemical extraction decomposition method (see Patent Documents 6, 7 and 8) (reaction in which PCB is decomposed with sodium hydroxide using DMI as a solvent) is mainly derived from sodium hydroxide. It is a nucleophilic substitution reaction to PCB by hydroxide ions. That is, in this reaction, an attempt is made to convert PCB to another compound by replacing a plurality of chlorines in the PCB with hydroxyl groups (—OH). However, in this conventional method, there is a very high possibility that hydroxychlorinated biphenyls having a high suspicion of endocrine disrupting substances remain. This is because the reactivity of hydroxide ions to an aryl group substituted with chlorine (herein, the so-called aromatic ring structure in general, including all benzene ring structures, biphenyl ring structures, etc.) is very low. Because it is. Even if one of the chlorines substituted for the aryl group is replaced with a hydroxyl group, the electron density of this hydroxyl group is large, so the electron density of the whole aryl group increases, and the nucleophilic substitution reaction with further hydroxide ions It becomes harder to progress. That is, if one of the PCBs having a plurality of substituted chlorines is substituted with a hydroxyl group, the substitution with the second hydroxyl group becomes more difficult. Therefore, the result is that hydroxychlorinated biphenyls in a form in which all of the plurality of substituted chlorines are not replaced by hydroxyl groups remain. Therefore, even if the concentration of PCB itself is reduced below the reference value, hydroxychlorinated biphenyls that are considered as other harmful substances are produced.

On the other hand, the chemical reaction mechanism of the method for decomposing a halogenated aromatic compound by the method of the present invention is completely different from that of the conventional method. In the method according to the present invention, a halogenated aromatic compound (for example, PCB), a hydride ion supply substance, and a strong alkaline substance (for example, sodium hydroxide) are dispersed in a heat-resistant alkali-resistant polar solvent in which a catalyst (for example, nickel) is dispersed ( Preferably, DMI in which hydrogen is dissolved is contacted at room temperature to 250 ° C. In this case, for example, an amide compound (for example, N- (2-dimethylaminoethyl) formamide) is used as a hydride ion supply substance. When added, the amide compound to be added, the catalyst, and hydrogen dissolved in the heat-resistant alkali-resistant solvent play a major role. That is, the chemical reaction mechanism of the method according to the present invention consists of a nucleophilic substitution reaction to PCB by hydride ion (H ⁻ ), which is a very strong nucleophilic species compared to hydroxide ion, and a reduction reaction by a catalytic hydrogenation catalyst. There is a very big feature in using together.

The hydride ion (H ⁻ ) generated by the decomposition of the amide compound added in the method of the present invention with a strong alkaline substance is known to be a very strong nucleophilic species, as described above, and hydroxylated in the reaction system. Even when physical ions coexist, it has a very high reactivity. Thereby, even if one of the plurality of substituted chlorines in the PCB is substituted with hydrogen (—H), the substitution of the second chlorine does not become difficult. Therefore, substitution of multiple chlorines in the PCB proceeds sequentially. Further, as described above, since the method of the present invention uses the catalytic hydrogen reduction reaction at the same time, all of the chlorines in the PCB are more smoothly replaced with hydrogen. Therefore, the production of hydroxychlorinated biphenyls, which is a concern with conventional methods, can be substantially suppressed, and only compounds in which chlorine is almost completely substituted with hydrogen, hydroxyl groups, and the like are produced.

  Further, in the method of the present invention, hydride ions, hydrogen radicals, and the like that are much more reactive than hydroxide ions are present in the reaction system, so that the hydrogen addition reaction occurs prior to the PCB polymerization reaction. Therefore, in the method of the present invention, decomposition of the heat-resistant and alkali-resistant solvent (for example, DMI) can be suppressed by suppressing the polymerization reaction of PCB. Furthermore, according to the method of the present invention, an alkaline substance (for example, sodium hydroxide) present in the reaction system works as a neutralizing agent for HCl generated by dechlorination, so that it is possible to suppress generation of HCl gas. is there.

  The advantages of the chemical reaction mechanism according to the method of the present invention are as follows. That is, the halogenated aromatic compound can be substantially reliably decomposed without generating by-products such as hydroxychlorinated biphenyls and PCB polymers and without generating HCl gas. Therefore, according to the method of the present invention, the decomposition rate of the heat-resistant alkali-resistant solvent (for example, DMI) becomes very low.

  Here, the mechanism by which the decomposition rate of the heat and alkali resistant solvent is lowered is due to the addition of an amide compound. In the alkali decomposition reaction of PCB according to the conventional method, the nucleophilic substitution reaction by the alkali hydroxide ion has been described above. At this time, since the alkali is excessively added, the solvent is removed by the excess alkali. DMI could be attacked.

  However, when the reactivity when the amide compound, PCB, and DMI used in the method of the present invention are each reacted with an alkali is compared, the relationship PCB> amide compound >> DMI is established. That is, the amide compound used in the method of the present invention is molecularly designed as described above. Therefore, even if excess hydroxide ions are present in the reaction system, they attack the added amide compound before attacking DMI. By this reaction, hydride ions are generated from the amide compound, and further contribute to the decomposition of PCB. From this mechanism, the decomposition rate of DMI, which is a solvent, can be suppressed.

  In order to achieve the chemical reaction mechanism by the method according to the present invention, a catalyst that can be suitably used is a catalytic hydrogen reduction catalyst, for example, nickel, aluminum, iron, molybdenum, palladium, chromium, cobalt, Manganese, alloys thereof, and mixtures thereof can be used. The catalyst is desirably used in an amount of 0.01 to 25% by weight, preferably 0.1 to 10% by weight, more preferably 0.5 to 5% by weight, based on the weight of the heat and alkali resistant solvent. The catalyst does not need to be dissolved in the heat-resistant alkali-resistant solvent as long as it can be dispersed by a stirring rod, a stirrer, or the like. In addition, when a dispersant is necessary for dispersing the catalyst in the heat-resistant and alkali-resistant solvent, various dispersants can be added as appropriate.

  Any heat-resistant and alkali-resistant solvent can be used as the solvent in the method of the present invention as long as it can withstand a reaction at a high temperature (for example, 100 ° C. or higher). For example, 1,3-dimethyl-2-imidazolide Non, sulfolane, ethylene glycol, triethylene glycol, lower alkyl ethers, trimethylene glycol, butylene glycol, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane , Naphthenic solvents and mixtures thereof. Of these, it is very preferable to use 1,3-dimethyl-2-imidazolidinone (DMI), which dissolves a halogenated aromatic compound well, has high heat resistance, and does not have a bad odor when heated to a high temperature. . It is also possible to use DMI as the main solvent and other solvents as supplements. The method of the present invention is characterized by the combined use of catalytic hydrogen reduction in which the halogen of the halogenated aromatic compound is replaced with hydrogen. For this purpose, it is particularly preferable to dissolve hydrogen in a solvent. The heat and alkali resistant solvent is used, for example, in an amount of 10 to 200% by weight, preferably 50 to 150% by weight, more preferably 80 to 120% by weight, based on the weight of the medium containing the halogenated aromatic compound to be treated. be able to.

In the method of the present invention, the amide compound that is the source of hydride ions can be any cyclic or non-cyclic compound having a structure in which hydrogen of ammonia (NH ₃ ) or amine (RNH ₂ ) is substituted with a metal atom or an acyl group. Examples thereof include metal amides, acid amides, amino acids, and amino acid derivatives. Amide compounds particularly preferably used in the method of the present invention are represented by the following formula:

Wherein R ₁ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl, and salts thereof, and the following formula:

Wherein R ₂ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl; R ₃ is selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl; R ₄ is hydrogen, C 3 _{1 to} C ₃ alkyl, and selected from the group consisting of ═O; R ₅ is selected from the group consisting of hydrogen and C _{1 to} C ₃ alkyl) and salts thereof . Further, as a particularly suitable acyclic amide compound, for example, N- (2-dimethylaminoethyl) formamide, ethylenebis (formamide), as a suitable cyclic amide compound, N- (2-dimethylaminoethyl) -N-methylformamide, Examples thereof include 1-methyl-2-imidazolidinone, 1,3-dimethylhydantoin, and 1,3,5-trimethylhydantoin, and these can be used alone or in combination. The amide compound can be used in an amount of 0.01 to 25% by weight, preferably 0.1 to 10% by weight, more preferably 0.5 to 5% by weight, based on the weight of the heat and alkali resistant solvent.

  In the method of the present invention, a strongly alkaline substance that performs an alkaline decomposition of a hydride ion supplying substance (an amide compound in the above description) or neutralizes a released chlorine ion is a halogenated aromatic compound. There is no particular limitation as long as it does not adversely affect the decomposition reaction, and examples thereof include alkali metal elements, alkaline earth metal hydroxides, alkali metal carbonates, ammonia, and amines. Particularly, in the method of the present invention, sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium alkoxide, potassium alkoxide, and a mixture thereof can be suitably used. The strongly alkaline substance is 0.1 to 50% by weight, preferably 0.5 to 10% by weight, more preferably 1 to 5% by weight, when using DMI as the solvent, based on the weight of the heat and alkali resistant solvent. % Can be used. However, the amount of the strong alkaline substance used can be varied depending on the type of the heat-resistant alkali-resistant solvent used.

  The shape of the strongly alkaline substance is more preferably a powdered alkaline substance different from that of the conventional flake shape. When a conventional flake-shaped alkaline substance is used, the specific surface area is smaller than that of the powdered alkaline substance, and the reactivity is significantly lowered due to the difference in the contact area.

  On the other hand, when the alkaline substance in the form of powder is applied to the decomposition treatment of the halogenated aromatic compound, the specific surface area is very large, so the contact area with the halogenated aromatic compound is large and the reactivity is improved. . In other words, the use of a powdery alkaline substance has a very significant feature in that the economics can be improved by shortening the processing time without lowering the reactivity of the strong alkaline substance. The particle size of the powdered alkaline substance is preferably 10 to 1000 μm. In the case of using a conventional flake-shaped alkaline substance, it can be used after being pulverized to a particle size as described above using mechanical means.

  In the method of the present invention, another compound that can be preferably used as a hydride ion source is an aldehyde compound. When an aldehyde compound is present together with a strong alkaline substance as described above, a cannizzaro reaction occurs to produce a corresponding alcohol and carboxylic acid. A general Cannizzaro reaction is represented by the following formula:

  Here, when two kinds of aldehyde compounds are used, a cross Cannizzaro reaction that can obtain a mixture of two kinds of carboxylic acids and two kinds of alcohols occurs. In this reaction, a hydroxide ion derived from a strong alkali substance attacks an aldehyde carbon (—C of —CHO) of one aldehyde compound to decompose an intermediate to produce a hydride ion. This hydride ion replaces the halogen of the halogenated aromatic compound as described above. Particularly, when formaldehyde is used as one aldehyde, hydride ions are efficiently generated from this formaldehyde, which is convenient. The generated hydride ion attacks the other aldehyde in addition to the reaction of substituting the halogen of the halogenated aromatic compound as described above, thereby generating an alcohol corresponding to the other aldehyde (cross cannizzaro). reaction).

  In the method of the present invention, hydride ions having a strong reducing power in the reaction system can be generated by simultaneously causing the cross Cannizzaro reaction. Since hydride ions are much more reactive than hydroxide ions, hydroxychlorinated biphenyls are rarely produced during PCB decomposition reactions.

  One aldehyde compound that undergoes a cross Cannizzaro reaction is formaldehyde. Formaldehyde is a gas at room temperature, and it is preferable to use paraformaldehyde which is a formaldehyde polymer in consideration of convenience in experimental operation. When paraformaldehyde is dissolved in the reaction system and the temperature of the reaction system is raised, the paraformaldehyde is decomposed to be in the same state as the formaldehyde is dissolved. Another aldehyde compound includes an aldehyde having no α hydrogen in the alkyl group R in the above formula. For example, benzaldehyde and the like can be mentioned, and it is particularly preferable to use benzaldehyde in consideration of the harmfulness of the generated alcohol. The use of an aldehyde compound as a hydride ion supply substance has the same advantages as when an amide compound is used. That is, the halogenated aromatic compound can be substantially reliably decomposed without generating by-products such as hydroxychlorinated biphenyls and PCB polymers, and without generating HCl gas, The decomposition rate of the alkaline solvent (for example, DMI) is lowered.

  The halogenated aromatic compound to be decomposed by the method of the present invention is usually often contained in groundwater, incinerated fly ash, soil and the like. In treating these, it is necessary to reduce the halogenated aromatic compound to below the reference concentration. In such a case, the contaminated groundwater, incineration fly ash, soil, and the like are extracted and transferred by contacting them with a solvent that dissolves the halogenated aromatic compound well. In the method of the present invention, the dissolved halogenated aromatic compound can be decomposed using the solvent in which the halogenated aromatic compound dissolved in the reaction is generated in the reaction. That is, the halogenated aromatic compound itself can be used for the reaction of the present invention, and the detergents that contain the halogenated aromatic compound by extracting the substance containing the halogenated aromatic compound. It is also possible to apply various oils to the present invention.

  The decomposition reaction of a halogenated aromatic compound by the method of the present invention will be described below as an example. This example describes a part of the embodiment of the present invention, and should not be considered as limiting the idea of the present invention.

Using the reaction mechanism according to the method of the present invention, polychlorobiphenyls were decomposed.
1 L four-necked reaction flask equipped with a thermometer for measuring the temperature of the reaction liquid, a thermometer for measuring the temperature of the gas phase part, a stirring rod and a stirrer (the gas phase part of the reaction system is Insulating oil (enclosed by nitrogen)) Insulating oil (polyester biphenyl (Kanekuro KC-1000, manufactured by Kaneka Chemical Co., Ltd.) 25 ppm containing 200 ppm, JIS C2320 equivalent) 1,3-dimethyl- 200 g of 2-imidazolidinone (1,3-dimethyl-2-imidazolidinone, Neos), 3 g of powdered sodium hydroxide (Asahi Glass), N- (2-dimethylaminoethyl) formamide (Neos) 2 g of Q, manufactured by Neos) and 2 g of sponge nickel catalyst (manufactured by Nikko Rica) were mixed, and the temperature of the oil bath was raised while stirring the reaction solution with a stirrer. The oil bath temperature was maintained at 210 ° C. and the reaction was continued for 4 hours. After the reaction, the reaction solution was cooled to room temperature, and the concentrations of PCBs and hydroxychlorinated biphenyls contained in the reaction solution were measured. The measurement of PCB content is based on the “Test method for standards concerning specially controlled industrial waste” (Ministry of Health, Labor and Welfare Notification No. 192 revised 1998) (High Resolution Gas Chromatograph / High Resolution Mass Spectrometer ( HRGC / HRMS). The content of hydroxychlorinated biphenyls was determined using gas chromatography-mass spectrometry / selected ion detection (GC-MS / SIM). The results of this example are listed in Table 1.

Polychlorobiphenyls were decomposed by another method based on the mechanism of the present invention using an aldehyde compound instead of an amide compound as a hydride ion source.
1 L four-necked reaction flask equipped with a thermometer for measuring the temperature of the reaction liquid, a thermometer for measuring the temperature of the gas phase part, a stirring rod and a stirrer (the gas phase part of the reaction system is Insulating oil (enclosed by nitrogen)) Insulating oil (polyester biphenyl (Kanekuro KC-1000, manufactured by Kaneka Chemical Co., Ltd.) 25 ppm containing 200 ppm, JIS C2320 equivalent) 1,3-dimethyl- 200 g of 2-imidazolidinone (1,3-dimethyl-2-imidazolidinone, manufactured by Neos), 3 g of powdered sodium hydroxide (produced by Asahi Glass), and 2 g of sponge nickel catalyst (produced by Nikko Rica) are mixed. To this, 1.5 g of benzaldehyde (manufactured by Kanto Chemical Co.) and 0.5 g of paraformaldehyde (manufactured by Aldrich) were added as additives, and the oil bath was stirred while stirring the reaction solution. The temperature of was raised. The oil bath temperature was maintained at 210 ° C. and the reaction was continued for 4 hours. After the reaction, the reaction solution was cooled to room temperature, and the concentrations of PCBs and hydroxychlorinated biphenyls contained in the reaction solution were measured in the same manner as in the above examples. The results of this example are listed in Table 1.

When a decomposition reaction of PCBs is performed by adding an aldehyde additive to the method of the present invention, not only the PCB is decomposed to 0.5 ppm or less (that is, the reference value or less), but the concentration of hydroxychlorinated biphenyls is also reduced. It is suppressed to a very low value below the lower limit of quantification (less than 0.1 ppm for each isomer). That is, even with this method, it is possible to reliably decompose the PCB while suppressing the production of by-product hydroxychlorinated biphenyls.

Claims (6)

N, which is a medium containing a halogenated aromatic compound selected from polychlorinated biphenyls, a strong alkaline substance selected from sodium hydroxide, and a substance that is decomposed by the strong alkaline substance to generate hydride ions. By contacting an amide compound selected from-(2-dimethylaminoethyl) formamide or an aldehyde compound selected from benzaldehyde and paraformaldehyde with a catalyst and a heat-resistant alkali-resistant polar solvent at room temperature to 250 ° C. Decomposing the halogenated aromatic compound. The method according to claim 1 , wherein the catalyst is dispersed in a heat-resistant alkali-resistant polar solvent. 3. The medium containing a halogenated aromatic compound selected from polychlorinated biphenyls is selected from insulating oil, heat medium, lubricating oil, plasticizer, paint and ink. The method described. The method according to any one of claims 1 to 3 , wherein the catalyst is selected from nickel, aluminum, iron, molybdenum, palladium, chromium, cobalt, manganese, alloys thereof, and mixtures thereof. The process according to any one of claims 1 to 4 , wherein the heat-resistant alkali-resistant polar solvent is selected from 1,3-dimethyl-2-imidazolidinone, sulfolane and mixtures thereof. 6. The method according to claim 5 , wherein hydrogen is dissolved in the heat and alkali resistant solvent.