Method of decomposing dioxins

A method for decomposing and eliminating dioxins contained in flyash at a lower temperature and in a shorter time is provided, in which dioxides or dioxins-containing materials are brought into contact with amine compounds and/or ammonium compounds at a temperature lower than 300 .degree. C. Chlorine in dioxines reacts with amine compounds and/or ammonium compounds and thus the dechlorination or replacement reaction of dioxin quickly proceeds under lower temperature. The method makes it possible to decompose dioxins under low temperature range in which dioxins have been thought to be undecomposable.

TECHNICAL FIELD
 This invention relates to a method for decomposing dioxins, and more
 particularly, to a method for effectively decomposing
 polychloro-p-dibenzodioxins (PCDD) and polychlorodibenzofurans (these
 compounds are hereinafter referred to as "dioxins") contained in flyash
 and bottom ashes (these materials are hereinafter referred to as
 "incineration ashes") discharged from various incineration plants such as
 an incineration plant for municipal solid waste, industrial waste and/or
 medical waste.
 BACKGROUND OF THE INVENTION
 In the incineration plants such as incineration plants for municipal solid
 waste, industrial waste and/or medical waste, the dioxides precursors
 including organic compounds such as phenols, benzene and acetylene, and
 chlorinated aromatic compounds such as chlorophenols and chlorobenzenes
 are formed during incineration. When flyash coexists in these precursors,
 the precursors change to dioxides under the catalytic action of flyash,
 and thus generated dioxins exist in incineration ashes.
 Conventionally proposed methods for treating such dioxides-containing
 flyash are as follows. (1) To keep the dioxides-containing flash for 1-2
 hours at a temperature between 320 and 400.degree. C. under a reductive
 atmosphere (for instance, 2 hours at 320.degree. C. or 1-1.5 hours at
 340.degree. C.) (Hagenmaier process, "ORGANOHALOGEN COMPOUNDS Vol. 27
 (1996)" p. 147-152)
 (2) A heat treatment of the dioxins-containing flash at 300-500.degree. C.
 under the existence of a dioxins formation inhibitor (JPA 4-241880). It
 has been said that dioxins do not heat-decompose at a temperature lower
 than 300.degree. C. This method is basically in accordance with the above
 mentioned theory, decomposing dioxides at above 300.degree. C., and a
 dioxins formation inhibitor is added to the flash in order to prevent the
 formation of dioxins during the heat treatment in a temperature range in
 which dioxides decompose. For particulars, flyash is heated at 400.degree.
 C. for 2 hours under the existence of pyridine vapor as the dioxins
 formation inhibitor.
 The aforementioned conventional methods have a drawback in that their high
 treatment temperature and long treatment time require much energy and high
 cost. Especially, in the above mentioned method (1) it is required to
 perform the treatment under a reductive atmosphere such as nitrogen gas,
 bringing about complexity and a high cost.
 DISCLOSURE OF THE INVENTION
 The object of the present invention is to solve the problems of the prior
 art and to provide a method which makes it possible to decompose and
 eliminate the dioxins under a low temperature region in which dioxides
 have been thought not to decompose, and to perform the treatment even
 under the existence of oxygen.
 According to the method of the present invention for decomposing dioxins,
 dioxins are decomposed by bringing dioxides or dioxides-containing
 materials into contact with at least one treating chemical selected from
 the group consisting of amine compounds and ammonium compounds at a
 temperature lower than 300.degree. C.
 According to the present invention, chlorines in dioxides react with the
 amine compound and/or the ammonium compound and this reaction causes quick
 dechlorination or replacement of chlorine atoms of dioxides and
 eventually, the detoxifications of dioxides at a temperature below
 300.degree. C., at which dioxides have been thought usually
 undecomposable.
 Such effect of the amine compound and/or ammonium compound to decompose
 dioxins at a temperature below 300.degree. C. has been hitherto unknown.
 PREFERRED EMBODIMENT OF THE INVENTION
 Dioxins to be treated according to the present invention may be contained
 in the exhaust gas discharged from various incineration plants such as
 municipal solid waste incineration plants, industrial wastes incineration
 plants, medical waste incineration plants and so on. The
 dioxins-containing materials are such as incineration ash which has
 dioxins adsorbed thereon, activated carbon powder used in the adsorption
 treatment of dioxins, and soil contaminated with dioxins.
 The amine compound as the treating chemical may be at least one of
 alkanolamines such as monoethanolamine, diethanolamine, triethanolamin,
 methanolamine, and aminomethylpropanol, lower alkyl-derived amines such as
 diethylamine, propylamine, and ethylenediamine, and cyclic amines such as
 aniline. The preferred compounds among above mentioned are
 triethanolamine, diethanolamine, monoethanolamine, aniline, propylamine,
 ethylendiamine, and aminomethylpropanol.
 The ammonium compound as the treating chemical may be at least one of
 ammonia, urea, and ammonium salts such as ammonium bicarbonate, ammonium
 carbonate, ammonium hydroxide, ammonium acetate. ammonium sulfate,
 ammonium phosphate, and ammonium hydrogen phosphate. The preferred
 compounds among above mentioned are ammonia, urea, ammonium bicarbonate,
 ammonium sulfate, and ammonium hydrogen phosphate, and the most preferred
 compound is ammonia.
 When at least one treating material of the amine compounds and the ammonium
 compounds is brought into contact with the incineration ashes to decompose
 the dioxines, it is preferred that the treating chemical is added to the
 reaction system in a ratio of 0.1-10% by weight and especially 1-5% by
 weight to the amount of incineration ashes. The chemical is preferable to
 be added in a larger amount as the reaction temperature is lower.
 The amine compounds and/or ammonium compounds to be added to the reaction
 system may be in the state of either gas, liquid, or aqueous solution. As
 the dioxins and the dioxins-containing materials are effectively
 decomposed when they are brought into contact with at least one of the
 gaseous amine compounds and the ammonium compounds, it is preferred that
 the amine compounds and the ammonium compounds have a sufficiently high
 vapor pressure at a temperature lower than 300.degree. C.
 In case that the liquid or the aqueous solution of the treating material is
 sprayed into the exhaust gas, or in case the material is preliminarily
 mixed with the incineration ashes, the material is preferable to have a
 vapor pressure that they are fully vaporized at a temperature lower than
 300.degree. C.
 As to the method in which at least one of the amine compounds and the
 ammonium compounds is brought into contact with the dioxin-containing
 materials to be treated, either of the following methods A, B and C can be
 employed in case of the dioxin-containing materials are soil or
 incineration ashes.
 A: The dioxins-containing materials and the amine compounds and /or
 ammonium compounds are mixed so that they are brought into contact at an
 ambient temperature. Otherwise, the mixture is heated to a temperature
 lower than 300.degree. C. to gasify the amine compounds and/or ammonium
 compounds so that the amine compounds and/or ammonium compounds in gaseous
 form are brought into contact with dioxins. In these cases, it is
 preferable to previously dissolve the amine compounds and/or ammonium
 compounds into water or other solvents.
 B: The amine compounds and/or ammonium compounds are heated to gasify at a
 temperature lower than 300.degree. C., and a gas flow containing this
 resultant gas is brought into contact with the dioxins-containing
 materials.
 C: The amine compounds and/or ammonium compounds are applied to a solid
 material and then, this solid material is mixed with, or placed on the
 dioxins-containing materials. After that, they are heated to a temperature
 lower than 300.degree. C. In this case, preferably a gas flow is passed
 through the reaction system.
 In a case dioxins or dioxins-containing materials exist in gaseous form or
 as floating small particles in a gas flow such as incineration exhaust
 gas, the following methods D, E, and F, for example, can be employed.
 D: Vapor of the amine compounds and/or ammonium compounds or a gas
 containing this vapor is supplied into the above mentioned gas flow.
 E: The amine compounds and/or ammonium compounds in liquid form are
 supplied into the gas flow in mist or in liquid drops.
 F: A solution containing amine compounds and/or ammonium compounds
 dissolved in it is supplied into the gas flow in mist or in liquid drops.
 In an incineration system in which a dust collector to remove flyash in the
 exhaust gas is equipped, it is preferable to supply the amine compounds
 and/or ammonium compounds in a gas, liquid, or solution state into the
 exhaust duct before the dust collector or into the dust collector itself.
 Typically, the gas temperature at the inlet of an electrostatic
 precipitator is in a range from 200 to 300.degree. C., and gas temperature
 at the inlet of bag filter is in a range form 140 to 230.degree. C.
 Accordingly, it is preferable that the amine compounds and/or ammonium
 compounds to be supplied into the dust collector or into the duct upstream
 thereof has a sufficiently high vapor pressure in a temperature range from
 140 to 230.degree. C.
 Usually, dioxins do not decompose at a temperature lower than 300.degree.
 C. According to the present invention, the temperature at which dioxins
 and the dioxins-containing materials are brought into contact with amine
 compounds and/or ammonium compounds is lower than 300.degree. C.
 Nevertheless, the reaction between chlorines in dioxins and the amine
 compounds and/or ammonium compounds causes the dechlorination reaction or
 replacement reaction leading to the detoxification of dioxins. The lowest
 temperature during the contact is determined by the vapor pressure or the
 ease of vaporization of the amine compounds and/or ammonium compounds.
 Namely, the preferred temperatures at which the contact is made are:
 ambient temperature for monoethanolamine, over 150.degree. C. for
 diethanolamine, and over 200.degree. C. for triethanolamine, aniline,
 n-propylamine, ethylenediamine, and aminomethylpropanol. When the amine
 compounds and/or ammonium compounds are brought into contact with the
 dioxins or dioxins-containing materials at a temperature over 200.degree.
 C., dioxins are generally decomposed with a sufficiently high
 decomposition ratio. Besides, ammonia gives a sufficiently high
 decomposition ratio at over ambient temperature. Little or no improvement
 in the decomposition ratio is obtained by using contact temperature higher
 than 300.degree. C.
 The longer the time of the contact between amine compounds and/or ammonium
 compounds and materials to be treated, the higher the decomposition ratio
 of dioxins, but too long a contact time makes the treatment cost too high.
 According to the method of the present invention, a sufficiently high
 decomposition ratio can be obtained with the contact time of typically
 3-60 minutes, especially 4-40 minutes and particularly 5-30 minutes. It is
 preferable that the lower the temperature during the contact, the longer
 the contact time. For example, if the contact is made at ambient
 temperature, a contact time of 20-40 minutes is preferable.
 The contact between the amine compounds and/or ammonium compounds and the
 dioxins or dioxins-containing materials can give a sufficiently high
 decomposition ratio even when the contact is made in a reductive
 atmosphere or in the presence of oxygen, i.e. in the open air or in the
 exhaust duct. Consequently, when carrying out the method of the present
 invention, the devices and the operations for the conditioning of
 atmosphere are generally not necessary.
 However, when the amine compounds and/or ammonium compounds are brought
 into contact with dioxides or dioxides-containing materials, attention
 must be paid against the explosion hazard, if the compound used has a
 flash point which is lower than the temperature during the contact. If the
 concentration of the compound in concern in the atmosphere in which the
 contact is made is lower than the explosion limit, the explosion can
 effectively be avoided. Therefore, to lower the oxygen concentration in
 the atmosphere is effective for the prevention of the explosion. Examples
 of such precaution include to supply to the atmosphere a gas containing
 little or no oxygen, such as nitrogen, carbon dioxide, water vapor, and
 combustion exhaust gas. Severer precautions must be taken if the compound
 is supplied into the exhaust duct or into the dust collector. To introduce
 a part of the combustion exhaust discharged from the dust collector to the
 exhaust duct using a fan or the like is a preferable method because it is
 a low cost method to effectively lower the oxygen concentration in the
 flue gas flowing in the exhaust duct.
 In treating the incineration ashes according to the method of the present
 invention, an immobilization treatment of the heavy metals in the ashes
 may be performed at the same time as the decomposition of the dioxides,
 using a chelating agent or a heavy metal immobilization chemicals such as
 phosphoric acid.
 The ashes to be treated may include the activated carbon powder which has
 been blown into the exhaust duct of the incineration plant in order to
 remove dioxins from the incineration flue gas.
 The incineration ashes and soil which have been treated according to the
 method of the present invention may be, either as it is or after having
 received the further treatment such as heavy metal immobilization, kept in
 drums or used as land fill.

Hereinafter, the present invention will be described in more with reference
 to examples and comparative examples.
 EXAMPLES 1 through 4
 Comparative Example 2
 A triethanolamine aqueous solution of which concentration is 5% by weight
 is added to 10 grams of flash discharged and collected from a municipal
 solid waste incinerator in such a manner as to make the proportion of
 triethanolamine to the flyash to be 5% by weight. The mixture was
 thoroughly mixed and then heated for 10 minutes at the temperatures
 specified in Table 1, respectively.
 After that, the dioxins concentrations of the treatment products were
 measured to give the results shown in Table 1.
 Comparative Examples 1, 3 through 6
 Except that water was added to the flyash instead of triethanolamine
 solution and the mixture was heated to temperatures specified in Table 1,
 identical treatment was carried out and dioxins concentrations of the
 resultant products were measured to give the results shown in Table 1.
 Moreover, the decomposition ratios in Examples 1 through 4 were calculated
 according to the following equation, from the results of Examples 1
 through 4 and Comparative Examples 3 through 6 in which the treatment was
 made at identical temperatures with the examples but without the addition
 of triethanolamine. The values of these calculations are also shown in
 Table 1.
 TABLE 1
 ##EQU1##
 Dioxins
 Concentration Dioxins
 Treatment Triethanolamine in Treatment Products Decomposition
 Example Temperature Addition (ng/g-flyash) Ratio(%)
 Comperative 180 none 1885 0
 Example 1
 Comperative 180 added 2303 --
 Example 2
 Comperative 200 none 2142 0
 Example 3
 Example 1 200 added 726 66.1
 Comperative 220 none 2535 0
 Example 4
 Example 2 220 added 314 87.6
 Comperative 250 none 2614 0
 Example 5
 Example 3 250 added 6.5 99.8
 Comperative 290 none 2655 0
 Example 6
 Example 4 290 added 6.0 99.8
 As apparent from Table 1, by adding triethanolamine to flash followed by
 heating to temperatures from 200 to 290.degree. C., dioxides are
 decomposed in high decomposition rates. Especially, when the treatment was
 performed at a temperature between 250.degree. C. and 290.degree. C., more
 than 99% of dioxides present in the materials are decomposed.
 Meanwhile, as it is evident from Comparative Example 2, dioxins does not
 decompose in spite of triethanolamine addition when the treatment is
 performed at 180.degree. C. at which triethanolamine does not vaporize.
 Example 5
 A glass column having a size of 20 mm .phi..times.250 mm was filled with 3
 g of flyash discharged and collected from a municipal solid waste
 incinerator. The flash column was then topped with glass beads on which
 150 mg of monoethanolamine had been applied. The column was heated to the
 temperatures of ambient temperature (20.degree. C.), 50.degree. C.,
 100.degree. C., 150.degree. C., 180.degree. C., 200.degree. C.,
 220.degree. C., 250.degree. C. and 290.degree. C., respectively, for 20
 minutes while passing air flow through the column at a rate of 15
 ml/minute from the glass beads side.
 Afterward, the dioxins concentrations of the treated products were measured
 with the results shown in Table 2.
 EXAMPLES 6, 7
 Instead of monoethanolamine in the Examples 1-5, diethanolamine was used in
 Example 6, and triethanolamine in Example 7. For the other respects, the
 experiment was carried out in the same manner as in Example 5. The
 measured values of dioxins concentrations are shown in Table 2.
 Comparative Example 7
 Except that nothing was applied on the glass beads, the experiment was
 carried out in the same manner as in Example 5.
 The dioxins residual ratio was calculated according to the following
 equation from the results of Examples 5 through 7 and Comparative Example
 7. The results are shown in Table 2.
 TABLE 2
 ##EQU2##
 Dioxins Concentration (ng/g) (residual ratio(%) in the parentheses)
 Examples and Comperative Examples
 Example Comperative
 5 6 7 Example 7
 Kinds of Amine Compounds
 Monoethanolamine Diethanolamine Triethanolamine --
 Treatment
 temperature (.degree. C.)
 Ambient 800 (60.1) 1337 (100) 1335 (100) --
 temperature
 (20)
 50 170 (13.0) 1337 (100) 1334 (99.9) 1335 (100)
 100 120 (9.0) 1328 (99.5) -- 1340 (100)
 150 97 (7.3) 245 (18.4) -- 1510 (113)
 180 54 (4.0) 178 (13.3) 1912 (143) 1885 (141)
 200 90 (6.7) 100 (7.5) 726 (54.4) 2142 (160)
 220 48 (3.6) 93 (7.0) 314 (23.5) 2530 (190)
 250 39 (2.9) 50 (3.7) 6.5 (0.5) 2614 (196)
 290 -- -- 6.0 (0.4) 2665 (200)
 As apparent from Table 2, in Comparative Example 7 in which no amine
 compound was used, heating of flash increases the dioxins concentration
 showing that in this temperature range, only the generation of dioxides
 occurs and decomposition reaction does not. On the other hand, in the
 Examples 5 through 7 where different amine compounds were brought into
 contact with flyash, dioxins are effectively decomposed. It is also shown
 that the lower limit of temperature for decomposition is different by
 amine compounds. Namely, monoethanolamine shows a decomposition ratio of
 over 90% at 100.degree. C., and 80% even at 50.degree. C., while
 diethanolamine and triethanolamine give high decomposition ratios at over
 150.degree. C. and over 200.degree. C., respectively.
 EXAMPLES 8, 9, 10, 11
 Experiments were carried out in the same mariner as Example 5 except that
 aniline, n-propylamine, ethylenediamine, and aninomethylpropanol were used
 as amine compounds in Examples 8, 9, 10, and 11, respectively, and that
 the treatment temperatures set at 250.degree. C. Residual concentrations
 and decomposition ratios of dioxins are shown in Table 3.
 EXAMPLE 12
 An experiment was carried out in the same manner as Example 5 except that
 aqueous solution of ammonia was used instead of monoethanolamine and
 treatment temperature was set at 250.degree. C. Residual concentrations
 and decomposition ratios of dioxins shown in Table 3. The application
 quantity of ammonia (NH.sub.3) on glass beads was 150 mg.
 TABLE 3
 Residual Concentrations and Decomposition Ratios of Dioxins
 (treatment temperature 250.degree. C.)
 Residual
 concentration decomposition
 Examples Compound (ng/g) ratio
 8 aniline 210 84.2%
 9 n-propylamine 350 73.8%
 10 ethylenediamine 100 92.5%
 11 aminomethylpropanol 300 77.5%
 12 ammonia 810 39.3%
 It is evident from Table 3 that the amine compounds used in Examples 8
 through 12 also give a high decomposition ratio of dioxins.
 INDUSTRIAL APPLICABILITY
 As described in detail in the above, according to the method of the present
 invention for the decomposition of dioxins, dioxins are decomposed and
 eliminated in a short time, and in a low temperature range in which
 usually dioxins decomposition does not occur. This brings about a decrease
 in the energy cost required for the treatment and improvement in the
 effectiveness of the treatment, resulting in the considerable decrease in
 the total cost of the treatment. Moreover, since the method of the present
 invention does not require a reductive treatment atmosphere so that the
 treatment can be performed in the open air or in the flue gas, the method
 can be conducted with simple treatment equipment and easy operation.