Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2000-06-20
2001-11-27
Lambkin, Deborah C. (Department: 1626)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C588S253000, C588S253000
Reexamination Certificate
active
06323385
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for the wet decomposition of dioxins into harmless substances and to a process for the wet processing of a fly ash-containing gas discharged from a combustion furnace into a harmless substance.
BACKGROUND ART
Dioxins, the typical example of which is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), are extremely harmful to human bodies and the discharge thereof to an atmosphere is strongly prohibited. Ministry of Health and Welfare of Japan published “Guideline for Prevention of Generation of Dioxins in Waste Treatment” in January, 1997, in which the concentration of dioxins in flue gases discharged from any newly constructed furnace is instructed to be suppressed to 0.1 ng-TEQ/Nm
3
or less. Environment Agency of Japan designates dioxins as being harmful substances by an amendment of the Atmospheric Pollution Prevention Law of December, 1997 and defines a regulating value for dioxins generated by combustion of industrial wastes as well as municipal solid wastes.
Various methods have been hitherto proposed for decomposing dioxins into harmless substances. Examples of these methods include a combustion method, a melting method, a thermal decomposition method, a photo-decomposition method, an ozone decomposition method, an oxidation decomposition method using hydrogen peroxide, a hydrothermal decomposition method and an alkali decomposition method.
The conventional methods, however, involve problems, because they encounter great difficulties in practicing and because they are not satisfactory from the standpoint of economy.
JP-A-H10-146574 proposes a method of decomposing dioxins, wherein a dioxin-containing fly ash is mixed with an oxidative acid such as sulfuric acid to form a slurry which is then heated at a temperature higher than 100° C.
With the above method, dioxins are efficiently converted into harmless substances. However, the method has problems that energy consumption is large and high apparatus costs are required, because the treatment is performed at a temperature of at least 100° C. which is above the boiling point of water (at atmospheric pressure, and so forth), preferably at least 200° C., while evaporating water.
It is the prime object of the present invention to provide a process for the wet decomposition of dioxins into harmless substances, which process is performed at a temperature lower than the boiling point of water and which can convert dioxins into harmless substances at a low cost.
Another object of the present invention is to provide a process for the wet processing of a flue gas from a combustion furnace containing dioxin-containing fly ash, which process is performed at a temperature lower than the boiling point of water and which can convert dioxins into harmless substances at a low cost.
The present inventors have made an intensive study for accomplishing the above objects and unexpectedly found that dioxins can be converted into harmless substances when contacted at a temperature lower than 100° C. with an aqueous solution acidified with hydrochloric acid and containing a catalyst dissolved therein and have completed the present invention.
In accordance with the present invention, there is provided a process for the wet decomposition of dioxins into harmless substances, characterized in that the dioxins are contacted with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, at a temperature lower than 100° C. to decompose the dioxins into harmless substances with a decomposition rate of at least 60%.
The present invention also provides a process for the wet processing of a flue gas from a combustion furnace containing a dioxin-containing fly ash into a harmless substance, characterized in that said flue gas is contacted with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, at a temperature lower than 100° C. to cause the fly ash contained in said flue gas to migrate into said aqueous solution and to decompose the dioxins deposited on the fly ash into a harmless substance with a decomposition rate of at least 60%.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace, having a temperature higher than 100° C. and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a cooling step of bringing the flue gas with gas-liquid contact with a cooling liquid to reduce the temperature of the flue gas to below 100° C., (ii) a gas-liquid contacting step of bringing said flue gas obtained in said cooling step into gas-liquid contact with an aqueous solution acidified with hydrochloric acid, and (iii) a dioxin decomposing step of subjecting the fly ash-containing cooling liquid “A” obtained in said cooling step and the fly ash-containing aqueous solution “B” obtained in said contacting step, separately or jointly, to treatment conditions including a chlorine ion concentration of at least 10 mmol/liter, a copper ion concentration of at least 20 mg/liter and a treatment temperature of lower than 100° C. to decompose the dioxins contained in the fly ash into harmless substances.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a gas-liquid contacting step of bringing said flue gas cooled to a temperature lower than 100° C. into gas-liquid contact with an aqueous solution acidified with hydrochloric acid, (ii) a fly ash concentrating step of increasing a fly ash content of the aqueous solution obtained in said gas-liquid contacting step and containing fly ash, and (iii) a dioxin decomposing step of maintaining said aqueous solution, obtained in said fly ash concentrating step and containing an increased amount of the fly ash, at a temperature lower than 100° C. in the presence of a catalyst in a dissolved state, thereby decomposing the dioxins contained in the fly ash into harmless substances.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a first gas-liquid contacting step of bringing said flue gas with a first treating liquid, (ii) a second gas-liquid contacting step of bringing the treated flue gas obtained in said first gas-liquid contacting step with a second treating liquid, and (iii) a dioxin decomposing step of contacting fly ash “A” captured by said first treating liquid in said first gas-liquid contacting step and fly ash “B” captured by said second treating liquid in said second gas-liquid contacting step, separately or jointly, into contact with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, to decompose the dioxins contained in the fly ash into harmless substances.
The term “dioxins” used in the present specification is intended to refer to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and similar compounds thereof and to include polychlorodibenzo-p-dioxins (PCDDs) having 1-8 chlorine atoms in the dibenzo-p-dioxin structure and polychlorodibenzofurans (PCDFs) having 1-8 chlorine atoms in the dibenzofuran nucleus.
As described above, dioxins include various chlorinated compounds. Since the toxicity of dioxins varies with the kind thereof, it is necessary to establish a standard based on which toxicity of individual dioxins can be evaluated in order to evaluate a mixture of dioxins as a whole. For this reason, a factor (toxicity equivalent factor (TEF)) for calculating an amount of a dioxin in terms of an amount of 2,3,7,8-TCDD providing the same toxicity as that of the dioxin has been determined on the basis of short time toxicity evaluation of respective dioxins. By multiplying amounts of respective dioxins by the toxicity equivalent
Asaoka Sachio
Iwasaki Mamoru
Kanai Ryuichi
Kawamura Kazushige
Sugiyama Hideki
Chiyoda Corporation
Lambkin Deborah C.
Lorusso & Loud
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