Catalyst for decomposing organohalogen compound

A composite catalyst for decomposing an organohalogen compound of the present invention, comprises composite particles comprising: PA1 iron compound particles having an average particle size of 0.01 to 2.0 .mu.m, a phosphorus content of not more than 0.02% by weight based on the weight of the particles, a sulfur content of not more than 0.3% by weight based on the weight of the particles, and a sodium content of not more than 0.3% by weight based on the weight of the particles; and PA1 an amine compound, PA1 said composite catalyst having a catalytic activity capable of decomposing not less than 50% by weight of monochlorobenzene when 50 mg of a mixture comprising iron oxide particles obtained by heat-treating said iron compound particles at a temperature of 300.degree. C. for 60 minutes in air, and the amine compound, is instantaneously contacted with 5.0.times.10.sup.-7 mol of monochlorobenzene at a temperature of 300.degree. C. at a space velocity of 150,000 h.sup.-1 in an inert gas atmosphere using a pulse catalytic reactor.

BACKGROUND OF THE INVENTION
 The present invention relates to a catalyst for the decomposition of an
 organohalogen compound, and more particularly, to a catalyst for the
 decomposition of an organohalogen compound for efficiently decomposing
 aromatic organohalogen compounds such as dioxins and precursors thereof,
 which are contained in a very small amount in an exhaust gas discharged
 from waste incinerators, or aliphatic organohalogen compounds such as
 trichloroethylene and dichlorometahne.
 Exhaust gases discharged from incinerators for incinerating municipal solid
 wastes or industrial wastes contain a small amount of aromatic chlorine
 compounds called "dioxins" which have an extremely strong toxicity to
 human bodies. The dioxins are a generic name of compounds formed by
 substituting hydrogen atoms of dibenzo-p-dioxine, dibenzofuran or the like
 with chlorine atoms.
 Further, aliphatic organohalogen compounds such as trichloroethylene or
 tetrachloroethylene have been widely used in various applications such as
 degreasing of metal or dry-cleaning.
 These organohalogen compounds are not only difficult to decompose, but also
 have a carcinogenic property. Therefore, there arises such a problem that
 the disposal of these compounds causes environmental pollution by
 diffusion in air and dissolving-out into ground water or soils. There have
 been proposed various methods for removing these organohalogen compounds.
 However, any of the conventional methods has failed to establish
 economical and efficient techniques for decomposing the organohalogen
 compounds and converting the compounds into unharmful ones.
 Conventionally, various techniques for removal and decomposition of the
 organohalogen compounds have been reported. For example, there are known a
 method of decomposing poly-halogenated aromatic compounds having at least
 five carbon atoms by heating at a temperature of 200 to 550.degree. C. in
 the presence of a catalyst such as iron oxide (Japanese Patent Publication
 (KOKOKU) No. 6-38863(1994)); a method of removing halogenated aromatic
 compounds or the like from an exhaust gas or reducing amounts thereof by
 heat-treating at a temperature of 300 to 700.degree. C. in the presence of
 a catalyst containing iron oxide (Japanese Patent Application Laid-Open
 (KOKAI) No. 2-280816(1990)); a method of introducing an inhibitor
 containing an activated carbon in which an amine compound is carried
 thereon, for inhibiting the generation of dioxins, into exhaust gas flues
 or the like of an incinerator (Japanese Patent Application Laid-Open
 (KOKAI) No. 11-9960(1999)); a method of decomposing an organohalogen
 compound at a temperature of from 60.degree. C. to less than 150.degree.
 C. in the presence of oxygen using a iron oxide-based and/or titanium
 dioxide-based solid catalyst (Japanese Patent Application Laid-Open
 (KOKAI) No. 11-188235(1999)); or the like.
 In addition, as the methods comprising preliminarily mixing wastes with
 iron oxide or the like and then incinerating the wastes, there are known a
 method of burning combustible wastes at a temperature of not less than
 850.degree. C. under the coexistence of an acid gas neutralizing agent,
 iron oxide particles and the like (Japanese Patent Application Laid-Open
 (KOKAI) No. 8-270924(1996)); and a method comprising burning wastes in an
 incinerator under the coexistence of iron oxide hydroxide particles or
 iron oxide particles containing sulfur and sodium in less than
 predetermined amounts (Japanese Patent Application Laid-Open (KOKAI) No.
 9-89228(1997)).
 Although it has been desired to provide a method for treating an exhaust
 gas so as to decompose and remove organohalogen compounds contained
 therein, the methods described in the above publications are still
 unsatisfactory.
 Namely, in the method described in Japanese Patent Publication (KOKOKU) No.
 6-38863(1994), poly-halogenated cycloalkyl compounds and poly-halogenated
 aromatic compounds in fly ash generated in an incinerator are decomposed
 by catalysts such as iron oxide, calcium carbonate and sodium carbonate in
 a fixed bed. However, in this method, it is difficult to sufficiently
 remove the organohalogen compounds for a short period of time, and huge
 plant and equipment investment is required to construct a facility for
 converting the fly ash into unharmful substances. Such a construction is
 almost impossible practically.
 In the method described in Japanese Patent Application Laid-Open (KOKAI)
 No. 2-280816(1990), after ammonia is added to an exhaust gas containing
 halogenated aromatic compounds, the halogenated aromatic compounds are
 decomposed in the presence of an iron oxide-containing catalyst in a fixed
 bed. Therefore, the construction of such a complicated facility after the
 waste incinerator also requires huge plant and equipment investment.
 In the method described in Japanese Patent Application Laid-Open (KOKAI)
 No. 11-9960(1999), the amine-carrying activated carbon is introduced into
 an exhaust gas containing dioxins, so that the dioxins are adsorbed into
 the activated carbon by the adsorptivity thereof, and then decomposed by
 reacting with the amine compound. The amine-carrying activated carbon has
 a high adsorptivity, but is incapable of sufficiently decomposing the
 dioxins and inhibiting the generation of dioxins. Further, the activated
 carbon has a risk of ignition at an elevated temperature, thereby causing
 problems concerning safety.
 In the method described in Japanese Patent Application Laid-Open (KOKAI)
 Nos. 11-188235(1999), the organohalogen compounds contained in the exhaust
 gas are decomposed at a temperature of from 60.degree. C. to less than
 150.degree. C. in the presence of oxygen using the iron oxide-based and/or
 titanium dioxide-based solid catalysts. In this method, although the
 organohalogen compounds are decomposed at a relatively low temperature,
 the decomposition percentage (conversion rate) is low and, therefore,
 impractical.
 In the methods described in Japanese Patent Application Laid-Open (KOKAI)
 Nos. 8-270924(1996) and 9-89228(1997), it is required to sufficiently
 premix solid wastes with iron oxide particles. Therefore, it is not easy
 to conduct this method.
 Meanwhile, in the incineration method using iron oxide hydroxide particles
 or iron oxide particles containing sulfur and sodium in not more than
 predetermined amounts (Japanese Patent Application Laid-Open (KOKAI) No.
 9-89228(1997)), the organohalogen compounds such as dioxins in exhaust
 gases cannot be sufficiently decomposed due to the low monochlorobenzene
 decomposition percentage thereof at 300.degree. C. as described in Table 1
 (Iron Compound 4) hereinafter.
 As a result of the present inventors' earnest studies for solving the above
 problems, it has been found that by contacting a combustion exhaust gas
 containing organohalogen compounds with a composite particles as an
 organohalogen compound-decomposition catalyst, which comprise iron
 compound particles having an average particle size of 0.01 to 2.0 .mu.m, a
 phosphorus content of not more than 0.02% by weight, a sulfur content of
 not more than 0.3% by weight and a sodium content of not more than 0.3% by
 weight, and an amine compound, the organohalogen compounds can be
 decomposed at a high efficiency. The present invention has been attained
 on the basis of this finding.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a catalyst for
 efficiently decomposing aromatic organohalogen compounds such as dioxins
 and precursors thereof, which are contained in an exhaust gas discharged
 from waste incinerators, or aliphatic organohalogen compounds such as
 trichloroethylene and dichlorometahne.
 It is an another object of the present invention to provide a method for
 treating an organohalogen compound by contacting a combustion exhaust gas
 containing organohalogen compounds with a catalyst.
 To accomplish the aim, in a first aspect of the present invention, there is
 provided a composite catalyst for decomposing an organohalogen compound,
 comprising composite particles comprising:
 iron compound particles having an average particle size of 0.01 to 2.0
 .mu.m, a phosphorus content of not more than 0.02% by weight based on the
 weight of the particles, a sulfur content of not more than 0.3% by weight
 based on the weight of the particles, and a sodium content of not more
 than 0.3% by weight based on the weight of the particles; and
 an amine compound,
 said composite catalyst having a catalytic activity capable of decomposing
 not less than 50% by weight of monochlorobenzene when 50 mg of a mixture
 comprising iron oxide particles obtained by heat-treating said iron
 compound particles at a temperature of 300.degree. C. for 60 minutes in
 air, and the amine compound, is instantaneously contacted with
 5.0.times.10.sup.-7 mol of monochlorobenzene at a temperature of
 300.degree. C. at a space velocity of 150,000 h.sup.-1 in an inert gas
 atmosphere using a pulse catalytic reactor.
 In a second aspect of the present invention, there is provided a method for
 treating an organohalogen compound, comprising:
 contacting a organohalogen compound-containing gas, with a composite
 catalyst for the decomposition of the organohalogen compound,
 the said composite catalyst having a catalytic activity capable of
 decomposing not less than 50% by weight of monochlorobenzene when 50 mg of
 a mixture comprising iron oxide particles obtained by heat-treating said
 iron compound particles at a temperature of 300.degree. C. for 60 minutes
 in air, and the amine compound, is instantaneously contacted with
 5.0.times.10.sup.-7 mol of monochlorobenzene at a temperature of
 300.degree. C. at a space velocity of 150,000 h.sup.-1 in an inert gas
 atmosphere using a pulse catalytic reactor, which composite catalyst
 comprise:
 iron compound particles having an average particle size of 0.01 to 2.0
 .mu.m, a phosphorus content of not more than 0.02% by weight based on the
 weight of the particles, a sulfur content of not more than 0.3% by weight
 based on the weight of the particles and a sodium content of not more than
 0.3% by weight based on the weight of the particles, and
 an amine compound.
 In a third aspect of the present invention, there is provided a method of
 using a composite catalyst for decomposing an organohalogen compound,
 which composite catalyst comprises composite particles comprising:
 iron compound particles having an average particle size of 0.01 to 2.0
 .mu.m, a phosphorus content of not more than 0.02% by weight based on the
 weight of the particles, a sulfur content of not more than 0.3% by weight
 based on the weight of the particles, and a sodium content of not more
 than 0.3% by weight based on the weight of the particles; and
 an amine compound,
 said composite catalyst having a catalytic activity capable of decomposing
 not less than 50% by weight of monochlorobenzene when 50 mg of a mixture
 comprising iron oxide particles obtained by heat-treating said iron
 compound particles at a temperature of 300.degree. C. for 60 minutes in
 air, and the amine compound, is instantaneously contacted with
 5.0.times.10.sup.-7 mol of monochlorobenzene at a temperature of
 300.degree. C. at a space velocity of 150,000 h.sup.-1 in an inert gas
 atmosphere using a pulse catalytic reactor.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will now be described in detail below.
 First, composite particles constituting a composite catalyst for the
 decomposition of an organohalogen compound (hereinafter referred to as
 "organohalogen compound-decomposition catalyst") according to the present
 invention, is described.
 The iron compound particles contained in the composite particles
 constituting the organohalogen compound-decomposition catalyst of the
 present invention, have an average particle size of usually 0.01 to 2.0
 .mu.m, preferably 0.02 to 2.0 .mu.m, more preferably 0.02 to 1.0 .mu.m.
 When the average particle size of the iron compound particles is more than
 2.0 .mu.m, a sufficient activity for decomposing the organohalogen
 compound may not be exhibited due to deterioration in efficiency of
 contact with the organohalogen compound. When the average particle size of
 the iron compound particles is less than 0.01 .mu.m, the particles may
 rather suffer from undesired agglomeration due to sintering therebetween
 or the like, resulting in deteriorated activity for decomposing the
 organohalogen compound.
 The iron compound particles used in the present invention have a BET
 specific surface area value of usually 0.2 to 200 m.sup.2 /g, preferably
 0.5 to 200 m.sup.2 /g, more preferably 0.5 to 100 m.sup.2 /g .
 The iron compound particles used in the present invention comprises at
 least one selected from the group consisting of iron oxide hydroxide
 particles such as goethite particles, akaganeite particles and
 lepidocrocite particles, and iron oxide particles such as hematite
 particles, maghemite particles and magnetite particles. Among these
 particles, goethite particles and hematite particles are preferred.
 The shape of the iron compound particles used in the present invention may
 be either a granular shape such as a spherical shape, a granular shape, an
 octahedral shape, a hexahedral shape or a polyhedral shape, or an acicular
 shape such as an acicular shape, a spindle shape or a rice grain-like
 shape. Among these particles, spindle-shaped particles or acicular
 particles are preferred.
 The iron compound particles used in the present invention have a phosphorus
 content of usually not more than 0.02% by weight, preferably not more than
 0.01% by weight, more preferably not more than 0.005% by weight. When the
 phosphorus content is more than 0.02% by weight, the catalyst poison
 ability of the phosphorus may become large, so that the activity for
 decomposing the organohalogen compound is deteriorated.
 The iron compound particles used in the present invention have a sulfur
 content of usually not more than 0.3% by weight, preferably not more than
 0.1% by weight, more preferably not more than 0.07% by weight. When the
 sulfur content is more than 0.3% by weight, the catalyst poison ability of
 the sulfur may become large, so that the activity for decomposing the
 organohalogen compound may be deteriorated.
 The iron compound particles used in the present invention have a sodium
 content of usually not more than 0.3% by weight, preferably not more than
 0.2% by weight, more preferably not more than 0.15% by weight. When the
 sulfur content is more than 0.3% by weight, the catalyst poison ability of
 the sodium may become large, so that the activity for decomposing the
 organohalogen compound may be deteriorated.
 In the iron compound particles used in the present invention, the sum of
 the phosphorus, sulfur and sodium contents is preferably not more than
 0.5% by weight, more preferably not more than 0.3% by weight, still more
 preferably not more than 0.2% by weight. When the sum of the phosphorus,
 sulfur and sodium contents is more than 0.5% by weight, the activity for
 decomposing the organohalogen compound may be deteriorated.
 The iron compound particles used in the present invention exhibit a
 catalytic activity capable of decomposing usually not less than 20% by
 weight of monochlorobenzene when 50 mg of iron oxide particles obtained by
 heat-treating the iron compound particles at a temperature of 300.degree.
 C. for 60 minutes in air, are instantaneously contacted with
 5.0.times.10.sup.-7 mol of monochlorobenzene at a temperature of
 300.degree. C. at a hourly space velocity of 150,000 h.sup.-1 in an inert
 gas atmosphere using a pulse catalytic reactor.
 When the monochlorobenzene decomposition activity of the iron compound
 particles is less than 20%, the aimed effect of decomposing the
 organohalogen compound according to the present invention may not be
 obtained. The monochlorobenzene decomposition activity of the iron
 compound particles used in the present invention is preferably not less
 than 25%, more preferably not less than 30%.
 The amine compound used in the composite particles constituting the
 organohalogen compound-decomposition catalyst of the present invention,
 comprises at least one compound selected from the group consisting of
 alkylamines such as diethylenetriamine and triethylenetetramine;
 alkanolamines such as triethanolamine and diethanolamine; cyclicamines
 such as aniline; hexamethylenetetramine and the like.
 The amine compound used in the present invention preferably has a boiling
 point of not less than 150.degree. C. When the boiling point of the amine
 compound is less than 150.degree. C., the amine compound may tend to be
 volatilized upon treating the organohalogen compound, thereby failing to
 exhibit the effect obtained by the combination with the iron compound
 particles.
 The particle shape and particle size of the composite particles
 constituting the organohalogen compound-decomposition catalyst of the
 present invention are the substantially same as those of the iron compound
 particles.
 The BET specific surface area of the composite particles constituting the
 organohalogen compound-decomposition catalyst of the present invention is
 preferably 0.2 to 200 m.sup.2 /g, more preferably 0.5 to 200 m.sup.2 /g
 still more preferably 0.5 to 100 m.sup.2 /g.
 The weight ratio between the iron compound particles and the amine compound
 in the composite particles constituting the organohalogen
 compound-decomposition catalyst of the present invention, is controlled
 such that the amount of the amine compound is preferably 0.1 to 10% by
 weight, more preferably 0.5 to 8.0% by weight, still more preferably 0.5
 to 5.0% by weight based on the weight of the iron compound particles. When
 the content of the amine compound is less than 0.1% by weight, the amine
 compound may not sufficiently show the effect for accelerating
 decomposition of the organohalogen compound. When the content of the amine
 compound is more than 10% by weight, the amine compound may tend to
 deteriorate the activity of the iron compound catalyst for decomposing the
 organohalogen compound.
 The composite particles constituting the organohalogen
 compound-decomposition catalyst of the present invention, has a catalytic
 activity capable of decomposing usually not less than 50% by weight,
 preferably not less than 55% by weight of monochlorobenzene when 50 mg of
 a mixture obtained by mixing iron oxide particles obtained by
 heat-treating the iron compound particles at a temperature of 300.degree.
 C. for 60 minutes in air, with the amine compound at a predetermined
 ratio, is instantaneously contacted with 5.0.times.10 mol of
 monochlorobenzene at a temperature of 300.degree. C. at a hourly space
 velocity of 150,000 h.sup.-1 in an inert gas atmosphere using a pulse
 catalytic reactor.
 When the decomposition activity of the composite particles constituting the
 organohalogen compound-decomposition catalyst of the present invention by
 the above method, is less than 50% by weight, it means that the
 organohalogen compounds cannot be effectively decomposed.
 In general, monochlorobenzene is a typical one of the organohalogen
 compounds and is known as a precursor of dioxins. Therefore, the catalytic
 activity for the decomposition of monochlorobenzene is regarded as an
 index of the activity for decomposition of the organohalogen compounds
 including dioxins, or the activity for inhibiting the generation thereof.
 Meanwhile, the decomposition percentage (conversion) of monochlorobenzene
 is represented by the following formula:
EQU Conversion (%)=[1-(amount of monochlorobenzene detected after
 reaction/amount of monochlorobenzene initially charged before
 reaction)].times.100
 Next, the process for producing the composite particles constituting the
 organohalogen compound-decomposition catalyst of the present invention, is
 described.
 First, the process for producing the iron compound particles used in the
 present invention is described.
 Among the iron compound particles used in the present invention, goethite
 particles may be produced, for example, by passing an oxygen-containing
 gas such as air through a suspension containing a ferrous iron-containing
 precipitate such as hydroxides of iron or iron carbonates which are
 obtained by reacting a ferrous salt with at least one compound selected
 from the group consisting of alkali hydroxides, alkali carbonates and
 ammonia.
 Among the iron compound particles used in the present invention, the
 hematite particles can be produced, for example, by heat-dehydrating or
 heat-treating the above obtained goethite particles at a temperature of
 200 to 800.degree. C. in air; the magnetite particles can be produced, for
 example, by heat-reducing the above obtained hematite particles at a
 temperature of 300 to 600.degree. C. in a reducing atmosphere; and the
 maghemite particles can be produced, for example, by heat-oxidizing the
 above obtained magnetite particles in a temperature of 200 to 600.degree.
 C. in air.
 In the production of the iron compound particles used in the present
 invention, it is necessary to lessen the contents of phosphorus, sulfur
 and sodium as catalyst poisons to not more than predetermined amounts.
 More specifically, as the ferrous salt solution, there are preferably used
 those containing less contents of phosphorus, sulfur or the like as
 catalyst poisons. Further, the contents of phosphorus, sulfur and sodium
 should be reduced by avoiding the use of sodium hexametaphosphate usually
 added as a sintering preventive upon heat-calcination step, and by
 removing sulfur ions derived from the raw ferrous materials and sodium
 ions derived from alkali hydroxides and/or the alkali carbonates by means
 of purification treatments such as washing with water or the like.
 The composite particles comprising the iron compound particles and the
 amine compound according to the present invention can be produced by
 driedly mixing the iron compound particles and the amine compound together
 in predetermined amounts, for example, in such amounts that the amine
 compound is present in an amount of preferably 0.1 to 10% by weight, more
 preferably 0.5 to 8.0% by weight, still more preferably 0.5 to 5.0% by
 weight based on the weight of the iron compound particles, using a mixer
 such as sand mill, Henschel mixer and Nauter mixer, or a grinder such as
 fine mill and pin mill.
 A solvent such as water or alcohol (e.g. ethanol, isopropyl alcohol) may be
 added to the amine compound in order to improve a wettability of the
 composite particles. When any solvent is used, it is preferred that the
 solvent be evaporated by heating or under reduced pressure.
 The above mixing process using the mixer or grinder is preferably conducted
 under the following conditions:
 (i) In case of using the sand mill, the mixing is conducted at a linear
 load of 5 to 50 Kg for 15 to 90 minutes.
 (ii) In case of using the Henschel mixer, the mixing is conducted at a
 temperature of 10 to 100.degree. C. and a stirring speed of 500 to 3000
 rpm for 5 to 30 minutes.
 (iii) In case of using the Nauter mixer, the mixing is conducted at a
 rotating velocity of 25 to 200 rpm and revolving velocity of 1 to 5 rpm
 for 15 to 60 minutes.
 (iv) In case of using the fine mill or pin mill, the milling and mixing are
 conducted at a stirring speed of 1000 to 10000 rpm while adding the amine
 compound to the iron compound particles.
 The thus obtained composite particles constituting the organohalogen
 compound-decomposition catalyst according to the present invention have
 such a configuration that the amine compound is carried on a part of the
 surface of each iron compound particle, when observed by an electron
 micrograph thereof.
 Next, the method for treating the organohalogen compounds by using the
 organohalogen compound-decomposition catalyst of the present invention,
 which comprises the above composite particles, is described.
 The incinerators to which the present invention can be applied, include
 intermittent operation-type incinerators such as mechanical batch
 incinerators or semi-continuous incinerators, and continuous
 operation-type incinerators.
 One of the intermittent operation-type incinerators usable in the present
 invention is schematically illustrated in FIG. 1. In FIG. 1, respective
 reference numerals denote the following members and portions: 1: waste
 (municipal solid waste) hopper; 2: incinerator; 3: combustion chamber; 4:
 supplementary combustion burner port; 5: gas cooling chamber; 6: air
 preheater; 7: blower for feeding organohalogen compound-decomposition
 catalyst; 8: feed tank of organohalogen compound-decomposition catalyst;
 9: dust collector; 10: induced draft fan; 11: chimney; 12: forced draft
 fan; 13 and 18: flues; 14 to 17 and 25: feed port for organohalogen
 compound-decomposition catalyst; 19: water sprayer; 20: drying stage of
 incinerator; 21: combustion stage of incinerator; 22: rear combustion
 stage of incinerator; 23: fly ash from dust collector; and 24: combustion
 air.
 In operation, a combustion air is introduced into the combustion chamber 3
 from the bottom thereof in such an amount 1.5 to 3.5 times a theoretical
 combustion air amount required for complete combustion of wastes
 (municipal solid wastes). As the combustion air, there is used intake air
 received through the forced graft fan 12 and heated by the preheater 6.
 The combustion chamber is provided with the supplementary combustion
 burner port 4. The organohalogen compound-decomposition catalyst for
 inhibiting the generation of dioxin, is fed through the respective feed
 ports 14 to 17, preferably through the feed ports 15 to 17 (namely,
 supplied into exhaust gases in flue 13, preheater 6 and flue 18 by a gas
 carrying method.
 In the method for treating the organohalogen compounds according to the
 present invention, it is preferred that the organohalogen
 compound-decomposition catalyst be contacted with a gas containing the
 organohalogen compound. The treating temperature is usually 150 to
 600.degree. C., preferably 200 to 600.degree. C. When the treating
 temperature is less than 150.degree. C., the decomposition activity of the
 organohalogen compound-decomposition catalyst may be deteriorated. When
 the treating temperature is more than 600.degree. C., the amine compound
 in the organohalogen compound-decomposition catalyst may readily undergo
 thermal degradation, resulting in deteriorated decomposition activity of
 the catalyst.
 As the method of contacting the organohalogen compounds with the
 organohalogen compound-decomposition catalyst, there may be used the
 method of adding particles, granules or a slurry of the organohalogen
 compound-decomposition catalyst to the organohalogen compound-containing
 gas by an gas carrying method; or the method of passing the organohalogen
 compound-containing gas through a catalytic reactor of a fixed-bed type
 reactor, a fluidized-bed type reactor or the like which is filled with
 pellets of the organohalogen compound-decomposition catalyst.
 In case of adding the organohalogen compound-decomposition catalyst into
 the organohalogen compound-containing gas by the gas carrying method, the
 amount of the organohalogen compound-decomposition catalyst used is
 preferably 0.01 to 0.5 g, more preferably 0.05 to 0.5 g, still more
 preferably 0.05 to 0.3 g based on 1 Nm.sup.3 of the organohalogen
 compound-containing gas.
 In case of using the catalytic reactor, a SV (space velocity) of the
 organohalogen compound-decomposition catalyst in the organohalogen
 compound-containing gas is preferably 500 to 10,000 h.sup.-1, more
 preferably 500 to 8,000 h.sup.-1, still more preferably 800 to 8,000
 h.sup.-1.
 By conducting the method for treating the organohalogen compounds using the
 organohalogen compound-decomposition catalyst of the present invention,
 the concentration of dioxin in the exhaust gas discharged, for example,
 through an outlet of an electric dust collector of waste incineration
 facilities can be reduced to usually not more than 2.0 ngTEQ/Nm.sup.3,
 preferably not more than 1.8 ngTEQ/Nm.sup.3, more preferably not more than
 1.5 ngTEQ/Nm.sup.3.
 Further, by conducting the treatment of the organohalogen compounds using
 the organohalogen compound-decomposition catalyst of the present
 invention, for example, when 50 mg of a composite (mixture) is
 instantaneously contacted with 5.0.times.10.sup.-7 mol of the
 organohalogen compound at a temperature of 300.degree. C. at a hourly
 space velocity of 150,000 h.sup.-1 in an inert gas atmosphere using a
 pulse catalytic reactor, usually not less than 50% by weight, preferably
 not less than 55% by weight of the organohalogen compound are decomposed.
 In the method for treating the organohalogen compounds according to the
 present invention, an acid gas neutralizing agent and/or activated carbon
 particles may be used jointly with the organohalogen
 compound-decomposition catalyst of the present invention.
 The acid gas neutralizing agents may include alkali earth metal compounds
 such as calcium hydroxide, calcium oxide, calcium carbonate, magnesium
 oxide, magnesium carbonate and dolomite; and alkali metal compounds such
 as lithium carbonate, sodium carbonate, sodium hydrogen carbonate and
 potassium hydrogen carbonate. Among these compounds, the calcium compounds
 are preferred, and calcium hydroxide is more preferred. The acid gas
 neutralizing agent has an average particle size (D.sub.50) (particle size
 of 50% of a total volume thereof when measured by a dry particle size
 distribution meter) of preferably not more than 20 .mu.m, and a BET
 specific surface area of preferably not less than 0.5 m.sup.2 /g.
 The activated carbon particles has an average particle size (D.sub.50)
 (particle size of 50% of a total volume thereof when measured by a dry
 particle size distribution meter) of preferably not more than 50 .mu.m,
 and a BET specific surface area of preferably not less than 600 m.sup.2
 /g.
 The important point of the present invention is that by using the
 organohalogen compound-decomposition catalyst constituted by the composite
 particles comprising the iron compound particles and the amine compound
 which is carried on a part of the surface of each iron compound particles,
 the organohalogen compound can be effectively decomposed.
 The reason why the organohalogen compound can be effectively decomposed, is
 considered as follows. That is, it is considered that due to the fact that
 the iron compound particles itself exhibits an excellent decomposition
 activity; the amine compound carried on a part of the surface of each iron
 compound particle accelerates the adsorption reaction of the organohalogen
 compound thereonto; and both the iron compound and the amine compound are
 exposed to the surface of the composite catalyst, so that the
 decomposition reaction of the organohalogen compound adsorbed can be
 accelerated. Further, it is considered that the amine compound accelerates
 not only the adsorption of the organohalogen compound but also the
 dechlorination reaction thereof.
 Thus, the organohalogen compound-decomposition catalyst of the present
 invention can effectively decompose dioxins or precursors thereof and,
 therefore, is suitable as the catalyst for the decomposition of the
 organohalogen compounds.
 EXAMPLES
 The present invention is described in more detail by Examples and
 Comparative Examples, but the Examples are only illustrative and,
 therefore, not intended to limit the scope of the present invention.
 Various properties were measured by the following methods.
 (1) The average particle size of the iron compound particles and
 organohalogen compound-decomposition catalyst was expressed by the value
 measured from an electron micrograph. The specific surface area of the
 iron compound particles was expressed by the value measured by a BET
 method.
 (2) The contents of phosphorus and sodium contained in the iron compound
 particles were expressed by the values measured by an inductively coupled
 plasma atomic emission spectrometer (SPS-4000, manufactured by Seiko
 Denshi Kogyo Co., Ltd.).
 (3) The content of sulfur contained in the iron compound particles was
 expressed by the value measured by a Carbon-Sulfur Analyzer (EMIA-2200
 Model, manufactured by Horiba Seisakusho Co., Ltd.).
 (4) The catalyst property of the organohalogen compound-decomposition
 catalyst was measured by the following method.
 That is, 50 mg of a composite material comprising iron oxide particles
 (Fe.sub.2 O.sub.3) obtained by heat-treating the iron compound particles
 at a temperature of 300.degree. C. for 60 minutes in air, and the amine
 compound, was instantaneously contacted with 5.0.times.10.sup.-7 mol of
 monochlorobenzene at a temperature of 300.degree. C. at a hourly space
 velocity of 150,000 h.sup.-1 in an inert gas atmosphere using a pulse
 catalytic reactor. The catalyst property of the composite catalyst was
 expressed by the concentration of monochlorobenzene decomposed in the
 above process.
 The pulse catalytic reactor used comprises a reactor portion and a gas
 chromatography portion which is constituted by Gas Chromatography-Mass
 Spectroscopy GC/MSQP-5050 (manufactured by Shimadzu Seisakusho Co., Ltd.).
 Meanwhile, the above evaluation method was conducted by referring to
 methods described in the literatures (e.g., R. J. Kobes, et al, "J. Am.
 Chem. Soc.", 77, 5860(1955) or "Experimental Chemistry II-Reaction and
 Velocity" edited by Chemical Society of Japan and published by Maruzen,
 Tokyo (1993)).
 Example 1
 &lt;Iron Compound Particles&gt;
 As the iron compound particles, there were used spindle-shaped goethite
 particles having an average particle size of 0.24 .mu.m, a phosphorus
 content of 0.001% by weight, a sulfur content of 0.05% by weight, a sodium
 content of 0.09% by weight, and a BET specific surface area of 90 m.sup.2
 /g.
 When measured by the above evaluation method, the goethite particles
 exhibited a monochlorobenzene decomposition percentage at a temperature of
 300.degree. C. of 32%.
 &lt;Production of Organohaloqen Compound-decomposition Catalyst&gt;
 1.5 kg of the spindle-shaped goethite particles and 75 g of triethanolamine
 (5.0% by weight based on the weight of the goethite particles) were
 dry-mixed together at a temperature of 50.degree. C. for 5 minutes in a
 Henschel mixer (nominal capacity: 10 liters) operated at 1,440 rpm,
 thereby obtaining goethite particles carrying triethanol amine thereon.
 The thus obtained triethanol amine-carrying goethite particles exhibited a
 monochlorobenzene decomposition percentage at a temperature of 300.degree.
 C. of 86% when measured by the above evaluation method.
 &lt;Decomposition Test for Dioxins&gt;
 Dry municipal solid wastes were charged into the intermittent
 operation-type incineration facility used in the decomposition test, which
 is schematically illustrated in FIG. 1, (municipal solid wastes
 incineration capacity when operated for 16 hours a day: 30 tons per day).
 Then, the above the organohalogen compound-decomposition catalyst was
 spray-added into the exhaust gas (gas temperature: 262.degree. C.) through
 a feed port 16 in an amount of 0.25% by weight based on the weight of the
 dry municipal solid wastes for 16 hours, i.e., for a period from the
 start-up to the shut-down via steady operation of the incinerator, by an
 air carrying method.
 The concentration of dioxin was expressed by the average of values obtained
 when the exhaust gas sampled at an outlet of the electric dust collector 9
 was measured for 4 hours subsequent to the elapse of 2 hours from the
 start-up of the incinerator. The measurement of the concentration of
 dioxin in the exhaust gas was conducted according to the method approved
 by Waste Matter Research Foundation (15, Kagurazaka 1-chome, Shinjuku-ku,
 Tokyo).
 As a result, it was confirmed that the concentration of dioxin in the
 exhaust gas sampled at the outlet of the electric dust collector was 1.5
 ngTEQ/Nm.sup.3.
 As a blank test, the incinerator was similarly operated without the
 addition of the organohalogen compound- decomposition catalyst, and the
 exhaust gas discharged therefrom were measured similarly.
 In the blank test, the concentration of dioxin in the exhaust gas sampled
 at the outlet of the electric dust collector was 16 ngTEQ/Nm.sup.3.
 From the above results, it was recognized that by using the organohalogen
 compound-decomposition catalyst of the present invention, the
 concentration of dioxin as one of the organohalogen compounds could be
 effectively reduced.
 &lt;Iron Compounds 1 to 5&gt;
 As the iron compound for the organohalogen compound-decomposition catalyst,
 iron compounds 1 to 5 were prepared. Various properties of the iron
 compounds are shown in Table 1.
 Examples 2 to 5 and Comparative Examples 1 to 3
 &lt;Composite Catalysts 1 to 7&gt;
 The same procedure as defined in Example 1 was conducted except that kind
 of iron compound and kind and amount of amine compound were varied,
 thereby obtaining composite catalysts. In Comparative Example 3 (composite
 catalyst 7), silica gel having no catalytic activity in itself was used
 instead of the iron compound. Various properties of the obtained composite
 catalysts are shown in Table 2.
 Examples 6 to 8 and Comparative Examples 4 to 5
 &lt;Dioxin Decomposition Test&gt;
 The same procedure as defined in Example 1 was conducted except that kind
 of organohalogen compound-decomposition catalyst was varied, thereby
 performing decomposition tests for dioxin. Various conditions of the
 dioxin decomposition tests and the results thereof are shown in Table 3.
 Examples 9 to 13 and Comparative Examples 6 to 11
 &lt;Decomposition Test for Other Organohalogen Compounds&gt;
 50 mg of the composite catalyst was instantaneously contacted with
 5.0.times.10.sup.-7 mol of the organohalogen compound at 300.degree. C. at
 an hourly space velocity of 150,000 h.sup.-1 in an inert gas atmosphere
 using a pulse catalytic reactor. The catalyst property of the composite
 catalyst was expressed by the concentration of the organohalogen compound
 decomposed upon the above contact.
 Meanwhile, the decomposition percentage (conversion) of the organohalogen
 compound is represented by the following formula:
EQU Conversion (%)=[1-(amount of organohalogen compound detected after
 reaction/amount of organohalogen compound initially charged before
 reaction)].times.100
 The results of the above decomposition tests for the
 other organohalogen compounds are shown in Table 4.
 TABLE 1
 Properties of iron compound
 Iron Average BET specific
 compound particle size surface area
 catalyst Kind (.mu.m) (m.sup.2 /g)
 Iron Acicular 0.25 85
 compound 1 goethite
 Iron Spindle-shaped 0.25 83
 compound 2 goethite
 Iron Spindle-shaped 0.30 52
 compound 3 hematite
 Iron Spindle-shaped 0.30 71
 compound 4 goethite
 Iron Acicular 0.30 54
 compound 5 hematite
 Properties of iron compound
 Catalyst property
 (Adsoprtion and
 conversion of
 Iron Phosphorus Sulfur Sodium chlorobenzene at
 compound content content content 300.degree. C.)
 catalyst (wt. %) (wt. %) (wt. %) (%)
 Iron 0.002 0.05 0.08 33
 compound 1
 Iron 0 0.01 0.05 37
 compound 2
 Iron 0.002 0.01 0.07 35
 compound 3
 Iron 0.49 0.08 0.18 11
 compound 4
 Iron 0.01 0.01 0.60 18
 compound 5
 TABLE 2
 Properties of organohalogen compound-
 decomposition catalyst
 Kind of
 Examples iron Amine compound
 and compound Boiling
 Comparative Composite and silica point
 Examples catalyst gel Kind (.degree. C.)
 Example 2 Composite Iron Triethanol- 360
 material 1 compound 1 amine
 Example 3 Composite Iron Triethylene- 278
 material 2 compound 2 tetramine
 Example 4 Composite Iron Aniline 184
 material 3 compound 3
 Example 5 Composite Iron Triethanol- 360
 material 4 compound 1 amine
 Comparative Composite Iron Triethanol- 360
 Example 1 material 5 compound 4 amine
 Comparative Composite Iron Triethylene- 278
 Example 2 material 6 compound 5 tetramine
 Comparative Composite Silica gel Triethanol- 360
 Example 3 material 7 amine
 Example 2 5.0 0.25 55 88
 Example 3 5.0 0.25 54 95
 Example 4 5.0 0.30 30 86
 Example 5 1.0 0.25 82 75
 Comparative 5.0 0.30 43 22
 Example 1
 Comparative 5.0 0.30 32 32
 Example 2
 Comparative 5.0 150 250 10
 Example 3
 TABLE 3
 Organohalogen compound-decomposition
 catalyst
 Examples and Amount added (based
 Comparative on dry waste)
 Examples Kind (%)
 Example 6 Composite catalyst 1 0.25
 Example 7 Composite catalyst 2 0.25
 Example 8 Composite catalyst 3 0.25
 Comparative Composite catalyst 5 0.25
 Example 4
 Comparative Iron compound 1 0.25
 Example 5
 Examples and
 Comparative Organohalogen compound-decomposition catalyst
 Examples Adding position
 Example 6 Before electric dust at 16 in FIG. 1
 collector
 Example 7 Before air preheater at 15 in FIG. 1
 Example 8 Before electric dust at 16 in FIG. 1
 collector
 Comparative Before electric dust at 16 in FIG. 1
 Example 4 collector
 Comparative Before electric dust at 16 in FIG. 1
 Example 5 collector
 Organohalogen
 compound- Concentration
 decomposition Amount of of dioxin in
 catalyst Organohalogen exhaust gas
 Gas temperature compound- (at outlet of
 Examples and at adding decomposition electric dust
 Comparative position catalyst collector)
 Examples (.degree. C.) (g/Nm.sup.3) (ngTEQ/Nm.sup.3)
 Example 6 261 0.10 1.1
 Example 7 350 0.10 0.82
 Example 8 262 0.10 1.3
 Comparative 260 0.10 12
 Example 4
 Comparative 260 0.10 2.1
 Example 5
 TABLE 4
 Kind of
 organohalogen Conversion of
 Examples and Kind of compound- organohalogen
 Comparative organohalogen decomposition compound
 Examples compound catalyst (%)
 Example 9 Monochloro- Composite 93
 phenol catalyst 1
 Example 10 Monochloro- Composite 97
 phenol catalyst 2
 Example 11 Monochloro- Composite 90
 phenol catalyst 3
 Example 12 Trichloro- Composite 97
 ethylene catalyst 1
 Example 13 Trichloro- Composite 86
 ethylene catalyst 4
 Comparative Monochloro- Composite 24
 Example 6 phenol catalyst 5
 Comparative Monochloro- Iron compound 35
 Example 7 phenol 1
 Comparative Monochloro- Composite 12
 Example 8 phenol catalyst 7
 Comparative Trichloro- Composite 45
 Example 9 ethylene catalyst 6
 Comparative Trichloro- Iron compound 41
 Example 10 ethylene 1
 Comparative Trichloro- Composite 16
 Example 11 ethylene catalyst 7