JPH1133351A - Waste gas desulfurization equipment wastewater treatment method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To treat wastewater from a flue gas desulfurization unit, simplify the treatment process to facilitate operation management, and reduce the sludge disposal cost and chemicals cost. SOLUTION: After removing fly ash from the effluent of a dust removal tower, the ash is mixed with the effluent of an absorption tower at a specific ratio, and subjected to coagulation treatment and solid-liquid separation treatment. [Effect] Separation of high-purity gypsum-containing sludge by mixing a specific ratio of the absorption tower effluent before separating gypsum into the dust tower effluent after removing fly ash and coagulating the mixture. In addition, separated water having a low fluorine concentration can be obtained. The number of precipitation separation steps, the amount of chemicals used, and the amount of sludge generated are reduced.

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 unit wastewater, and more particularly, to a treatment process for treating flue gas desulfurization unit wastewater generated in a suit-separated desulfurization unit in a coal-fired power plant. The present invention relates to a method for treating wastewater from a flue gas desulfurization apparatus that simplifies and reduces the amount of chemicals used and the amount of sludge disposal.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional treatment process of flue gas desulfurization unit wastewater generated in a suit separated type desulfurization unit in a coal-fired power plant.

That is, gypsum is separated from the effluent of the absorption tower 1 (absorption tower slurry) by a gypsum thickener 4 through a storage tank 3, and the separated water is circulated to a desulfurization device.

On the other hand, the effluent from the dust removal tower 2 (dust removal tower drainage)
Is passed through the intermediate tank 5 and then the fly ash removal sedimentation tank 6
The fly ash is removed by allowing to stand still.
Then, after passing through the raw water tank 7, the slaked lime (Ca
An alkali such as (OH) ₂ ) and a polymer flocculant for increasing the sedimentation rate are added to carry out flocculation treatment in a neutral pH range, and sludge containing heavy metal ions and fluorine is separated in the first sedimentation tank 9. Is done. Part of this sludge is returned to the reaction tank 8, and the remaining part is discharged out of the system. The separated water in the first precipitation tank 9 is then sent to a decalcification step, where an alkali agent such as sodium hydroxide (NaOH) and sodium carbonate (Na ₂ CO ₃ ) are added in a reaction tank 10, and further in a coagulation tank 11. After a small amount of a polymer flocculant is added and subjected to a flocculation treatment in a pH alkaline range, sludge containing fluorine, a calcium compound, and the like is separated in the second precipitation tank 12 at a higher level. The sludge in the second settling tank 12 is returned to the reaction tanks 8 and / or 10, and the separated water is discharged outside the system as treated water.

[0005]

The above-mentioned conventional processing process has the following problems. Since the effluent from the dust removal tower and the effluent from the absorption tower are treated in different systems, four sedimentation / separation steps are required, and the process is complicated and the operation management is difficult. A large amount of sludge to be disposed of is generated from the first settling tank, and the sludge disposal cost is high. High chemical usage and high chemical costs.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems and to provide a method for treating wastewater from a flue gas desulfurization apparatus capable of simplifying the treatment process and reducing the amount of chemicals used and the amount of sludge disposal. .

[0007]

According to a first aspect of the present invention, there is provided a method for treating wastewater from a flue gas desulfurization unit, which is obtained when treating flue gas with a dust removal tower and a sulfur oxide absorption tower. After the effluent of the dust tower is removed by fly ash, the effluent of the dust tower is mixed with the effluent of the absorption tower such that the effluent of the dust tower: the effluent of the absorption tower = 1: 0.5 to 1:10 (volume ratio), and then the alkaline agent Is added to the mixture to adjust the pH to 6 to 9, and then subjected to coagulation treatment, followed by solid-liquid separation treatment to recover high-purity gypsum.

[0008] In a second aspect of the present invention, there is provided a method for treating wastewater from a flue gas desulfurization apparatus, wherein the supernatant obtained after the solid-liquid separation is provided with a reaction layer to which a carbonate is added, a coagulation tank and a sedimentation tank. And then returning the precipitate obtained in the precipitation tank to the alkali agent addition step and / or the reaction tank to which carbonate is added.

That is, the inventors of the present invention have conducted an ingenious study to solve the above-mentioned problems. As a result, when the effluent of the dust removal tower and the effluent of the absorption tower are mixed at a specific ratio, they are combined in one system. Even if treated, it hardly reduces the purity of the recovered gypsum,
The inventors have found that the treatment process can be simplified to facilitate operation management, and that the sludge disposal cost and the chemical use cost can be reduced, and the present invention has been completed.

The effluent of the absorption tower contains a large amount of high-purity gypsum. Even if fly ash in the effluent of the dust removal tower is removed in advance and both are mixed and treated at a specific ratio, only the addition of an alkali agent is required. High purity gypsum can be deposited. The purity of the gypsum is almost the same as the purity of the gypsum obtained by the conventional method in which the effluent of the absorption tower and the effluent of the dust removal tower are respectively treated in different systems.

Further, by supplying the supernatant water obtained by the solid-liquid separation to the decalcification step, treated water having better water quality can be obtained, and the precipitate obtained here is returned. The calcium compound contained can be effectively reused as an alkaline agent.

[0012]

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

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

In the method of the present embodiment, the sludge mainly composed of fly ash is introduced by leaving the effluent from the dust removal tower 2 (dust removal tower drainage) through the intermediate tank 5 into the fly ash removal sedimentation tank 6 and leaving it to stand. Is separated and then introduced into the mixing tank 13. On the other hand, the effluent (absorption tower slurry) of the absorption tower 1 is directly introduced into the mixing tank 13 and mixed with the dust removal tower waste water after removing the fly ash.

The mixing ratio of the wastewater from the dust removal tower to the slurry in the absorption tower in the mixing tank 13 is important because it greatly affects the purity of the gypsum obtained in the first settling tank 15 in the subsequent step. In the case, the dust removal tower wastewater from which fly ash was removed: absorption tower slurry = 1: 0.5 to 1:10,
Preferably, it is 1: 1 to 1: 5 (capacity ratio).

The mixed water obtained in the mixing tank 13 may be used together with a part of the separated sludge in the second settling tank 12 in the subsequent step, if necessary.
After being introduced into the reaction tank 14 and adding an alkaline agent,
-9, preferably in the range of pH 6-8.

Conventionally, in such a neutralization reaction tank, a polymer flocculant was always added in order to quickly settle the generated solid matter. However, in the present invention, the slurry in the absorption tower originally had a sedimentation property. Because it contains an excellent amount of plaster, plaster with good sedimentation can be obtained only by neutralization treatment.
There is no need to add a polymeric flocculant.

The sludge mainly composed of gypsum is aggregated from the mixed water by the pH adjustment in the reaction tank 14, and the sludge is separated in the first settling tank 15. This first settling tank 15
In the above, a high-purity gypsum having a gypsum content of about 97% can be obtained by employing the above mixing ratio. This purity can be shipped for industrial use.

As the alkali agent to be added to the reaction tank 14, alkali such as caustic soda and slaked lime can be used, but slaked lime is advantageous because it is inexpensive.

A part of the separated water from the first settling tank is returned to the desulfurization step, and the remaining part is preferably sent to the calcium removal step, as in the conventional method. That is, the sludge is fed to the reaction tank 10, the coagulation tank 11 and the second sedimentation tank 12 for processing, and the separated sludge in the second sedimentation tank 12 is returned to the reaction tank 14 and / or the reaction tank 10 for scale prevention. At the same time, the supernatant water is discharged as treated water.

In the decalcification step, a small amount of a polymer flocculant of about 1 mg / l is added in order to rapidly settle the calcium carbonate particles.
4, the polymer flocculant is returned to the reaction tank 14 together with the returned sludge. In this case, too, the amount of returned sludge is overwhelming compared to the amount of sludge precipitated in the reaction tank 14. It has been confirmed that it has little effect on the purity of the recovered gypsum because it is relatively small.

According to the method of this embodiment, the number of sedimentation tanks can be reduced from four to three as compared with the conventional method shown in FIG. 2, and the treatment process can be simplified. The only sludge other than gypsum generated in the entire process is fly ash sludge separated in the fly ash removal sedimentation tank 6, and the amount of generated sludge is small and the sludge disposal cost is low.

Further, in the reaction tank 14, efficient coagulation treatment can be performed only by adding a neutralizing agent, and the amount of chemicals used and the cost of chemicals used can be reduced.

According to the present invention, as described above, the effects of simplifying the treatment process, reducing the sludge disposal cost and the cost of chemicals used are achieved while maintaining the same purity of the recovered gypsum and the quality of the treated water as in the conventional method. can do.

[0025]

The present invention will be described more specifically below with reference to examples and comparative examples.

Examples 1 to 3 Wastewater from a flue gas desulfurization unit was treated according to the method of the present invention shown in FIG.

The mixing ratio of the wastewater from the dust removal tower after the fly ash was removed in the mixing tank 13 to the slurry in the absorption tower was as shown in Table 1, and the alkali in the reaction tank 14 was C
The pH was adjusted to 7 with a (OH) ₂ in a reaction time of 30 minutes. In the reaction tank 10, NaOH and Na ₂
CO ₃ was added to adjust the pH to 11 in a reaction time of 30 minutes.

The fluorine concentration before and after the first-stage treatment (the fluorine concentration of the inflow water and the effluent of the first sedimentation tank 15) and the fluorine concentration of the treated water (the fluorine concentration of the effluent water of the second sedimentation tank) were measured.
The results are shown in Table 1.

Comparative Example 1 The procedure of Example 1 was repeated, except that only the wastewater from the dust tower was treated alone without mixing the slurry in the absorber tower with the wastewater from the dust tower. The fluorine concentration and the fluorine concentration of the treated water were measured, and the results are shown in Table 1.

[0030]

[Table 1]

From Table 1, it is clear that high quality treated water can be obtained by mixing the absorption tower slurry with the dust removal tower wastewater according to the method of the present invention.

In the methods of Examples 1 to 3, the gypsum purity of the sludge obtained in the first stage treatment is 97.0 to 97.5.
%, Which was almost equivalent to the gypsum purity of 97.6% obtained in the gypsum thickener 4 by the conventional method shown in FIG.

In the conventional method shown in FIG. 2, 6.7 t / day of sludge is generated in the first settling tank 9 with respect to 220 m ³ / day of drainage. No sludge is generated in the method of the present invention shown in FIG.

Further, the cost of chemicals used in the first-stage treatment and the second-stage treatment is 173 yen / m ³ per 1 m ^{3 of the} amount of water to be treated in the conventional method, but 141 yen in the method of this embodiment. /
m ³ , and the cost of chemicals used is reduced by about 18%.

[0035]

As described above in detail, according to the method for treating flue gas desulfurization unit wastewater of the present invention, one settling and separation step is omitted in the treatment of flue gas desulfurization unit wastewater as compared with the conventional method. Simplification of the processing process and easy operation management. Cost of chemicals used is reduced. Sludge other than fly ash is not generated, and sludge disposal costs can be reduced. The gypsum purity and treated water quality obtained are the same as in the conventional method. Thus, the wastewater from the flue gas desulfurization device can be treated easily, efficiently, and at low cost.

[Brief description of the drawings]

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

FIG. 2 is a system diagram showing a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Absorption tower 2 Dust tower 6 Fly ash removal sedimentation tank 10, 14 Reaction tank 11 Coagulation tank 12 Second sedimentation tank 13 Mixing tank 15 First sedimentation tank

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mikio Nihei 3-7-1, Ichibancho, Aoba-ku, Sendai-shi, Miyagi Tohoku Electric Power Co., Inc. (72) Inventor Takafumi Murakami 3-4 Nishishinjuku, Shinjuku-ku, Tokyo No. 7 Kurita Industrial Co., Ltd.

Claims (2)

[Claims] 1. A method for treating wastewater of a flue gas desulfurization unit obtained when treating flue gas with a dust removing tower and a sulfur oxide absorbing tower, comprising: removing fly ash from the dust removing tower effluent; Outflow water = 1: 0.5 to 1:10 (volume ratio)
Mixing with the effluent of the absorption tower so as to obtain an aqueous solution, and then adjusting the pH to 6 to 9 by adding an alkali agent, performing coagulation treatment, and then performing solid-liquid separation treatment to recover high-purity gypsum. Treatment method for wastewater from desulfurization equipment. 2. The method according to claim 1, wherein the supernatant water obtained after the solid-liquid separation is sent to a decalcification step comprising a reaction layer to which a carbonate is added, a flocculation tank and a sedimentation tank for treatment. The precipitate obtained in the above step of adding an alkali agent and / or
Alternatively, a method for treating wastewater from a flue gas desulfurization unit, wherein the wastewater is returned to a reaction tank to which a carbonate is added.