JPH07124575A - Treatment process for exhaust desulfurizing drainage - Google Patents

Abstract

PURPOSE:To prepare efficiently treated water of high quality by adjusting pH of exhaust desulfurizing drainage containing iodinic acid ions to a specified value and adding a reducer to decompose the iodinic acid ions. CONSTITUTION:In the case of exhaust desulfurizing drainage containing iodinic acid ions discharged from a desulfurizing device for exhaust gas generated when quarts and petroleum are burnt in a thermoelectric power station, is treated first drainage is fed to a cohesive precipitation separation tank 2 through a drainage storage tank 1, and calcium salt, for example, slaked lime is added in the process to treat fluorine, and pH is adjusted to turn heavy metallic ions into a hydroxide, which is precipitation separated in a cohesive precipitation separation tank 2. Supernatant liquid in the cohesive precipitation separation tank 2 is fed into a two-layer filter column 3, filtered therein, and the filtered water is fed into a pH adjusting layer 4, in which acid like chloric acid, sulfuric acid or the like is added to adjust to pH3-5. The pH adjusted water is then fed into a reduction reaction layer 5, and the reducer is added therein to reduction decompose iodinic acid ions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating flue gas desulfurization wastewater, and more particularly to a method for efficiently treating flue gas desulfurization wastewater containing iodate ions to obtain high quality treated water.

[0002]

2. Description of the Related Art Exhaust gas discharged from a desulfurization device for exhaust gas generated when coal or oil is burned in a thermal power plant, so-called flue gas desulfurization waste water is fluorine, heavy metals, COD
Since it contains the components, it is necessary to remove them.

Conventionally, as a method for treating such flue gas desulfurization effluent, first, calcium salt is added to treat heavy metals and fluorine to neutralize the acidic effluent, precipitate and separate, and then adjust pH. The method of adsorbing and removing the COD component is adopted (Japanese Patent Publication Nos. 55-43732 and 55-12).
No. 312).

More specifically, flue gas desulfurization effluent contains calcium salts such as slaked lime and calcium chloride and caustic soda,
Alkali chemicals such as sodium carbonate are added to remove most of heavy metals and fluorine harmful substances by a treatment method such as coagulation sedimentation and filtration, and then the pH is adjusted to remove COD components, preferably pH 7 to In the neutral or weakly acidic region of 3, the COD component (main component: dithionic acid (S ₂
O ₆ )) is selectively adsorbed and separated.

The treated water contains 10 to 30 fluorine.
Since a residual amount of about mg / l remains, a method of further removing residual fluorine by contacting it with a fluorine-adsorbing resin that selectively adsorbs fluorine is used.

[0006]

Recently, the quality of flue gas desulfurization wastewater has changed due to the efficiency of coal and petroleum fuels and the improvement of flue gas desulfurization system in thermal power plants. And
Due to the change in the water quality of the flue gas desulfurization wastewater, it was found that harmful substances could not be removed sufficiently by the conventional treatment method, and that the amount of treated water of the COD adsorption resin and the fluorine adsorption resin decreased and the treated water quality deteriorated. did.

[0007] The present inventors have found that such a process water reduction of COD adsorption resin or fluorine adsorption resin, cause the treated water deteriorates in the waste water of flue gas desulfurization iodate ^- that due to the influence of (IO ₃ ions), It was also found that the iodate ion was not sufficiently removed by the conventional method and remained in the treated water.

The present invention has been made on the basis of the above findings, and in the treatment of flue gas desulfurization effluent, it cannot be treated with a COD adsorbing resin and is a harmful substance which causes a decrease in the performance of the COD component and the fluorine adsorbing resin. It is an object of the present invention to provide a method for efficiently obtaining high-quality treated water by efficiently removing a certain iodate ion.

[0009]

The method of treating flue gas desulfurization wastewater according to the present invention is a method for treating flue gas desulfurization wastewater containing iodate ions.
Is adjusted to 5 or less, and a reducing agent is added to decompose the iodate ion.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a system diagram showing an embodiment of a method for treating flue gas desulfurization wastewater according to the present invention.

In FIG. 1, 1 is a waste water storage tank, 2 is a coagulation sedimentation separation tank, 3 is a two-layer filtration tower, 4 is a pH adjusting tank, 5 is a reduction reaction tank, 6 is a COD adsorption tower, and 7 is a fluorine adsorption tower. Yes,
Each symbol of 11 to 19 is piping.

In the method of this embodiment, since the flue gas desulfurization wastewater usually contains fluorine, the flue gas desulfurization wastewater is first passed through the pipe 11, the wastewater storage tank 1 and the piping 12 to the coagulation sedimentation separation tank 2. To send. In this process, a calcium salt, for example, slaked lime is added to treat the fluorine, and the pH is adjusted so that heavy metal ions are used as hydroxides to cause aggregation and separation tank 2.
And precipitate and separate. In this treatment, a heavy metal scavenger may be additionally added in order to reduce the residual amount of the heavy metal, if necessary. In this case, examples of the heavy metal scavenger include high molecular weight heavy metal chelating agents such as dithiocarbamic acid.

The supernatant water of the coagulation sedimentation separation tank 2 is sent from a pipe 13 to a filter such as a two-layer filtration tower 3 to be filtered, whereby SS (heavy metals and fluorine) contained in the flue gas desulfurization wastewater After separating and removing the water, the filtered water is sent to the pH adjusting tank 4 through the pipe 14, and an acid such as hydrochloric acid or sulfuric acid is added to adjust the pH to 5
Hereinafter, the pH is preferably adjusted to 3 to 5.

The pH-adjusted water is sent to the reduction reaction tank 5 through the pipe 15, a reducing agent is added, and iodate ions are reduced and decomposed.

As the reducing agent used here, sulfurous acid (H ₂ SO ₃ ), sodium hydrogen sulfite (NaHSO 3
₃ ) and sulfites such as sodium sulfite (Na ₂ SO ₃ ). The amount of these reducing agents added is appropriately determined according to the content of iodic acid in the flue gas desulfurization wastewater, which is raw water, but is usually about 10 to 100 mg / l.

Since iodate ion is decomposed by this treatment, it may be discharged as it is if the content of other pollutants is below the emission standard.

However, the flue gas desulfurization effluent usually contains CO
Since the D component may be contained or the residual fluorine ion may still be contained in the amount of the reference value or more, the following treatment is further performed as necessary. That is, the outflow water of the reduction reaction tank 5 is sent to the COD adsorption tower 6 through the pipe 16 to adsorb and remove the COD component. The COD adsorption tower 6 is a COD adsorption resin filled with a weakly basic or medium basic anion exchange resin. The COD adsorbing resin may be adsorbed by a reaction tank, but it is preferable that the COD adsorbing resin is passed through a column filled with water. The water flow condition of the COD adsorption tower 6 is SV = 5 to 20 hr ^−1.
It is preferably about the same.

Next, if necessary, the outflow water of the COD adsorption tower 6 is adjusted to a pH of about 3 to 7 by adding a pH adjusting agent as necessary, and then fed to the fluorine adsorption tower 7 through a pipe 17.
It is contacted with a fluorine-adsorbing resin to remove fluorine to a higher degree.

In this case, as the fluorine adsorbing resin, for example, cerium, hafnium, titanium, zirconium,
Examples thereof include resins that adsorb metal ions forming complex compounds with fluorine ions such as iron, aluminum, and lanthanides, activated carbon, activated alumina, hydrous titanium oxide, zeolite, and magnesia adsorbents. Further, the water passing conditions to the fluorine adsorption tower 7 filled with such fluorine adsorption resin are SV.
= 0.5 to 30 hr ⁻¹ is preferable. In addition,
The COD component is not so much contained in the reductive decomposition treated water, and when it is desired to remove only the residual fluorine, the above-mentioned COD
The adsorption treatment step can be omitted.

Thus, when water is passed through the COD-adsorbing resin 6 and the fluorine-adsorbing tower 7, the iodate ion, which is a harmful substance causing the performance deterioration of the COD-adsorbing resin and the fluorine-adsorbing resin, is removed. The resin and the fluorine-adsorbing resin can efficiently adsorb and remove the COD component and the fluorine without causing performance deterioration.

Outflow water from the fluorine adsorption tower 7 is sent to the treated water tank 8 through the pipe 18 and discharged out of the system through the pipe 19.

In this example, the example in which the processing is performed based on FIG. 1 has been described, but the present invention is not limited to this. That is, the reducing agent may be first added to the flue gas desulfurization wastewater after the filtration treatment, and then the pH may be adjusted. Also, the place where the reducing agent is added is not limited to that shown in FIG.
The reducing agent may be added to the waste water storage tank 1, or in some cases, the reducing agent may be directly added to the flue gas desulfurization device (not shown). In these cases, the pH of the flue gas desulfurization effluent is usually 5 or less, so that no particular pH adjustment is required.

[0024]

[Function] According to the research conducted by the present inventor, iodate ion becomes CO
It was found that the D-adsorption resin and the fluorine-adsorption resin were eluted, and the fluorine-adsorption resin supporting cerium exhibited a selective adsorption property, which caused a decrease in the amount of adsorbed fluorine. That is, the selective adsorption property as a coexisting salt with respect to the fluorine adsorption resin is fluorine ion >> iodate ion> sulfuric acid ion> chlorine ion, and the selective adsorption property of iodate ion is relatively large, which inhibits the fluorine adsorption treatment. Become a factor. Therefore, if iodate ions in flue gas desulfurization wastewater can be removed,
It is possible to prevent the COD adsorption resin and the fluorine adsorption resin from being eluted, and it is possible to greatly improve the fluorine adsorption performance of the fluorine adsorption resin.

In the present invention, from the flue gas desulfurization wastewater,
Prior to the COD adsorption treatment or the fluorine adsorption treatment, the iodate ion which deteriorates the performance of the COD adsorption resin or the fluorine adsorption resin is reduced and decomposed in advance, and then the COD adsorption treatment or the fluorine adsorption treatment is performed to obtain the COD component or the fluorine. Can be efficiently removed.

In the present invention, for example, an iodate such as potassium iodate (KIO ₃ ) undergoes the following reduction reaction under acidic conditions of pH 5 or less with a reducing agent such as H ₂ SO _3. By proceeding, it is efficiently decomposed and removed.

KIO ₃ + 2H ₂ SO ₃ + 2HCl → KCl + 2H ₂ SO ₄ + ICl + H ₂ O

[0028]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples below.

Example 1 Flue gas desulfurization wastewater was treated by the method shown in FIG.

First, slaked lime 1500 mg was added to the flue gas desulfurization wastewater.
/ L was added and well stirred to adjust to pH 7, and then polyacrylamide polymer flocculant 1 mg / l was injected to perform coagulation sedimentation treatment in the coagulation sedimentation separation tank 2. The filtration treatment was carried out in the two-layer filtration tower 3. The water quality of the filtered water was as follows.

Filtration water Water quality (mg / l) Iodate ion (IO ₃ ⁻ ): 17.0 COD: 19.0 Fluorate ion (F ⁻ ): 33.0 Next, a pH adjusting tank 4 was added to this filtered water. At sulfuric acid (H ₂ SO
₄ ) was injected at 20 mg / l and adjusted to pH 3 and then sent to the reduction reaction tank 5, where sodium bisulfite (NaH
SO ₃ ) 100 mg / l was injected, and the reaction was stirred for about 30 minutes. As a result, the iodate ion concentration in the treated water of this reduction treatment was reduced to 0.5 mg / l or less.

Then, the effluent water of the reduction reaction tank 5 is subjected to COD.
COD adsorption tower (weak (medium) basic anion exchange resin: "CA-200" Kurita Industry Co., Ltd.)
(20 ml of the product was filled) 6 was subjected to a water flow treatment at a water flow rate SV = 10 hr ⁻¹ .

After that, the effluent of the COD adsorption tower 6 is treated with a fluorine adsorption tower (cerium oxide (CeO ₂ · nH ₂ O) -supported fluorine adsorption resin (“RFAD-F”) to adsorb fluorine ions. 2 from Shin Nippon Chemical Industry Co., Ltd.
(0 ml filling) 7 was subjected to water flow treatment at a water flow rate SV = 10 hr ⁻¹ .

Table 1 shows the amount of treated water in the fluorine adsorption tower 7 and respective average values of the concentration of fluorine ion and iodine ion in the obtained treated water (outflow water).

Comparative Example 1 The amount of treated water in the fluorine adsorption tower 7 and the treated water obtained when the treatment was carried out in the same manner as in Example 1 except that the reduction reaction was not carried out and the COD adsorption treatment was carried out after the pH adjustment. (Runoff water)
The average values of the fluorine ion and iodine ion concentrations therein were determined, and the results are shown in Table 1.

[0036]

[Table 1]

As is clear from Table 1, according to the present invention, by removing the iodate ion which is the causative substance of the performance deterioration of the COD adsorption resin or the fluorine adsorption resin, CO
The performance of the D-adsorption resin and the fluorine-adsorption resin can be maintained high, and the COD component and fluorine can be efficiently adsorbed for a long period of time.

[0038]

As described in detail above, according to the method for treating flue gas desulfurization effluent of the present invention, in the treatment of flue gas desulfurization effluent containing iodate ions, the addition of a reducing agent enables the efficient removal of iodate ions. Can be removed. Also, as a result,
COD components and fluorine can be efficiently adsorbed and removed, and high-quality treated water can be stably obtained over a long period of time.

[Brief description of drawings]

FIG. 1 is a system diagram showing a method of an embodiment of a method for treating flue gas desulfurization wastewater according to the present invention.

[Explanation of symbols]

 1 Waste water storage tank 2 Coagulation sedimentation separation tank 3 Two-layer filtration tower 4 pH adjusting tank 5 Reduction reaction tank 6 COD adsorption tower 7 Fluorine adsorption tower 8 Treated water tank

Claims (1)

[Claims] 1. The p of flue gas desulfurization wastewater containing iodate ions
A method for treating flue gas desulfurization wastewater, which comprises adjusting H to 5 or less and adding a reducing agent to decompose iodate ions.