JPH0326096B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for removing mercury and mercury compounds contained in exhaust gas emitted from a garbage incinerator, etc., and in particular, to improve the removal efficiency and reduce the removal cost. It is about reduction.

[Conventional technology]

Currently, in Japan, there are no legal regulations regarding mercury emitted into the atmosphere. However, the exhaust gas emitted from garbage incinerators contains a considerable amount of mercury and mercury compounds (amount exceeding the working environment standard of 0.05 mg/m ³ in the special regulations).
It has become a social problem.

By the way, currently, the exhaust gas emitted from the garbage incinerator is treated by a harmful gas removal device, which mainly removes hydrogen chloride and sulfur dioxide gas from the exhaust gas. There are three types of methods for removing harmful gases using a harmful gas removal device: dry method, semi-dry method, and wet method. In this current harmful gas removal equipment,
Dry and semi-dry harmful gas removal equipment can hardly remove mercury and mercury compounds, but wet harmful gas removal equipment can remove mercury and mercury compounds to some extent.

[Problem that the invention seeks to solve]

In the method for removing mercury and mercury compounds using the conventional harmful gas removal equipment as described above, sodium hypochlorite must be added to the cleaning solution to increase the removal efficiency of mercury and mercury compounds in the exhaust gas. However, this requires a large amount of sodium hypochlorite, which raises the cost of the drug.

In addition, with the conventional methods for removing mercury and mercury compounds as described above, there is a problem that the removal rate of mercury and mercury compounds varies, and the removal rate also differs between each incineration plant or depending on the operating conditions. .

[Means and actions for solving problems]

The method for removing mercury and its compounds from exhaust gas according to the present invention involves electrolyzing the cleaning water of a wet hydrogen chloride removal device that removes hydrogen chloride from exhaust gas from a waste incinerator, and producing hydrogen chloride on an anode through this electrolysis. The chlorine produced at the cathode is reacted with the hydroxide of an alkali metal or alkaline earth metal produced at the cathode, and the hypochlorite ions produced by this reaction are mixed with the washing water to remove mercury and/or alkaline earth metal in the exhaust gas. Or, it is characterized by dissolving a mercury compound in the washing water.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG.

In the figure, the unwashed exhaust gas from the waste incinerator enters the cleaning tower 4 through the duct 2, is cleaned with cleaning liquid, and is discharged from the upper part of the cleaning tower 4 as cleaning gas. Mercury and mercury compounds are removed. At this time, the cleaning liquid is treated as follows.

That is, the cleaning liquid is extracted from the cleaning tower 4, poured into the electrolytic cell 6, and electrolyzed. In order to control pH, alkaline metal hydroxide such as caustic soda is usually added to the cleaning liquid in the cleaning tower 4, and when this reacts with hydrogen chloride in the exhaust gas before cleaning, sodium chloride etc. Salts are formed. As this pH control agent, alkaline earth metal hydroxides such as magnesium hydroxide may be used. When the cleaning solution is electrolyzed, chlorine gas is mainly produced in the anode chamber 8 and cationic metal hydroxide is produced in the cathode chamber 10. A diaphragm 12 is provided in the electrolytic cell 6.
is provided, and the aqueous solution containing the cationic metal hydroxide generated in the cathode chamber 10 is extracted from the cathode chamber 10 and guided to the reaction tank 14. Further, chlorine gas generated from the anode chamber 8 is also led to the reaction tank 14 and blown into the liquid. The inside of the reaction tank 14 is stirred by a stirrer 16 and other devices so that the two are well contacted and mixed and reacted. In the reaction tank 14, cationic metal hypochlorite is mainly produced.

Since the pre-cleaning exhaust gas contains reducing substances, the reducing substances are taken into the cleaning liquid. This reducing substance has the problem of converting mercuric chloride dissolved in the cleaning liquid into metallic mercury and volatilizing it, but in the present invention, these harmful reducing substances are easily removed. That is, in the anode chamber 8 in the electrolytic cell 6, some of the reducing substances are oxidized and removed, and in the reaction tank 14, most of the reducing substances are oxidized and removed by the electrolytically generated hypochlorite. Excess hypochlorite in the reaction tank 14 is returned to the cleaning tower 4 to oxidize and remove reducing substances in the cleaning solution.

Since the present invention has such a mechanism,
The amount of cleaning liquid extracted from the cleaning tower 4 to the electrolytic cell 6 and the amount of cleaning liquid extracted from the cleaning tower 4 to the wastewater treatment equipment are adjusted to match the harmful substances in the exhaust gas before cleaning.
By setting it appropriately, it is possible to dramatically improve the removal rate of mercury and mercury compounds in the exhaust gas before cleaning.

In addition, although the reaction tank 14 was used in the above example,
The present invention can also be implemented by the method shown in FIG. 2 as a modification of the above embodiment. The method shown in Figure 2 is
The role of the reaction tank 14 is performed by the electrolytic tank 6, and although the energy efficiency is lower than that shown in FIG. 1, the removal efficiency of mercury and mercury compounds is almost the same, and there is no problem in practical use. That is,
The methods are essentially the same.

Moreover, these electrolytic cell 6 and reaction tank 14 can also be integrated into the cleaning tower 4.

Note that since a considerable amount of alkaline metal hydroxide still remains in the aqueous solution returned to the cleaning tower 4 from the reaction tank 14 or the electrolytic tank 6, the cleaning tower 4
It is also possible to reduce the amount of PH control chemicals added to the water.

〔Effect of the invention〕

As explained above, this invention extracts the cleaning liquid from the cleaning tower, guides it to the electrolytic cell, and generates chlorine and hypochlorous acid through electrolysis, thereby removing mercury and mercury compounds from the exhaust gas. can be self-sufficient, thereby reducing the cost of removing it.

Many of the waste incineration plants that have wet smoke scrubbing equipment are large-scale factories, and most of them also have power generation equipment, and there are many examples of surplus electricity being sold. However, since the unit price of selling electricity is low, it is cheaper to consume it at home than to purchase medicine. In addition, the electricity cost for manufacturing the chemicals necessary to remove mercury and its compounds is small, which is also advantageous over purchasing chemicals.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing one embodiment of this invention, and FIG. 2 is an explanatory diagram showing another embodiment of this invention. In the figure, 4 is a washing tower, 6 is an electrolytic cell, 8 is an anode chamber, 10 is a cathode chamber, 12 is a diaphragm, 14 is a reaction tank, and 16 is a stirrer. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Scope of Claims] 1. Washing water of a wet hydrogen chloride removal device that removes hydrogen chloride from waste gas from a garbage incinerator is electrolyzed, and by this electrolysis, chlorine is generated at the anode and alkali is generated at the cathode. React with metal or alkaline earth metal hydroxide, mix hypochlorite ions produced by this reaction with the cleaning water, and dissolve mercury and/or mercury compounds in the exhaust gas into the cleaning water. 1. A method for removing mercury and mercury compounds from exhaust gas. 2. During electrolysis, a diaphragm method is used to separate chlorine produced at the anode and at least one of alkali metal hydroxides and alkaline earth metal hydroxides produced at the cathode in a separate reaction tank. The method according to claim 1, characterized in that the reaction is carried out by: 3 Chlorine generated at the anode during electrolysis,
A patent claim characterized in that at least one of an alkali metal hydroxide and an alkaline earth metal hydroxide produced at the cathode is stirred and mixed as is in the electrolytic cell used for this electrolysis to react. The method described in Scope No. 1.