JP2000325968A - Treatment method of sludge from a wastewater treatment system for fluegas desulfurization - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformize sludge and gypsum and to obtain a dehydrated cake high in commodity value as recovered gypsum by introducing a gypsum slurry and a sludge slurry into a stirring tank and adding a polymeric flocculant to the stirring tank under mixing and stirring before dehydrating the slurries by a dehydrator. SOLUTION: The gypsum slurry discharged from the gypsum thickener 1 of a lime gypsum method exhaust gas desulfurizer is introduced into a stirring tank 2 by a pump P1 while the sludge slurry discharged from the concn. tank 3 of the exhaust gas desulfurizing waste water treatment apparatus is also introduced into a stirring tank 2 by a pump P2 and the gypsum slurry and the sludge slurry are mixed and stirred in the stirring tank 2. A polymeric flocculant is added to the stirring tank 2 from a storage tank 4 by a pump P3 to perform flocculation treatment. The flocculation treatment slurry of the stirring tank 2 is fed to a dehydrator 5 by a pump P4 to be subjected to dehydration treatment and a dehydrated cake and a dehydrated filtrate are respectively discharged out of the system. As the polymeric flocculant, an anonic or nonionic polymeric flocculant is used.

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 sludge, and more particularly to a method for coagulating flue gas desulfurization wastewater discharged from a lime-gypsum method flue gas desulfurization apparatus having a gypsum recovery process. The present invention relates to a method for treating flue gas desulfurization wastewater sludge that is mixed with a gypsum slurry and dewatered to improve the dewatering efficiency of the hardly dewaterable sludge slurry that is generated.

[0002]

2. Description of the Related Art In a flue gas desulfurization process using the lime-gypsum method, exhaust gas generated in a boiler or the like is subjected to dust removal treatment, cooled in a cooling tower, and then brought into contact with an absorption solution of calcium carbonate or slaked lime slurry in an absorption tower. After desulfurization, the processing gas is discharged. In the absorption tower, a slurry containing calcium sulfite obtained by absorbing sulfur dioxide gas and other pollutants in the exhaust gas is oxidized in the oxidation tower to form a gypsum slurry. This gypsum slurry is solid-liquid separated in a sedimentation tank,
The separated water is returned to the absorption tower again as an absorbing liquid, and a part of the separated gypsum slurry is discharged out of the system as necessary, and the concentrated gypsum slurry is dehydrated by a dehydrator such as a centrifugal filter to collect gypsum.

[0003] In such a flue gas desulfurization process, the flue gas desulfurization effluent discharged from the dust removal step, the cooling step, the absorption step, and the dehydration step contains SS, fluorine and heavy metals. For this reason, these wastewaters are usually discharged out of the system after being subjected to neutralization coagulation sedimentation with slaked lime or caustic soda, or coagulation sedimentation by addition of a sulfate band or iron salt. Sludge generated by such coagulation and sedimentation treatment is concentrated in a concentration tank, dehydrated by a dehydrator, and disposed of as waste.

However, the sludge slurry generated by the coagulation and sedimentation treatment of flue gas desulfurization effluent is difficult to dewater, so that it can be sufficiently dewatered by a dehydrator such as the centrifugal filter for dewatering gypsum slurry. In addition, since a dedicated dehydrator such as a filter press is required separately, there is a disadvantage that complicated operations are required in terms of operation management and maintenance.

[0005] In order to solve such a problem, sludge slurry generated in the coagulation and sedimentation treatment of flue gas desulfurization effluent includes:
There has been proposed a method of mixing and drying gypsum slurry before dehydration in a gypsum recovery process (JP-B-62-50).
No. 27). By mixing the gypsum slurry with the sludge slurry in this manner, the dewatering property of the sludge is improved, and the centrifugal filtration type dehydrator used for dewatering the gypsum slurry has a high S value.
Dehydration can be performed efficiently at the S recovery rate. Moreover, since the dewatered cake obtained by dehydration contains a large amount of gypsum, it can be reused as recovered gypsum as it is, eliminating the need to dispose of sludge as waste and reducing waste disposal costs. The effect is also achieved.

[0006]

As described above, when mixing a gypsum slurry with a sludge slurry generated by coagulation and sedimentation of flue gas desulfurization effluent, metal hydroxide and calcium fluoride in the sludge slurry are mainly used. Sludge and plaster
Since the specific gravities differ greatly, simply mixing both slurries cannot achieve uniform mixing. When the mixed slurry in a non-uniform mixed state is subjected to a dehydration treatment, the obtained dewatered cake becomes a multi-layer non-uniform cake such as a sludge cake adhered on a gypsum cake. The cake recovered in such a non-uniform state has a low commercial value as recovered gypsum.

Japanese Patent Publication No. Sho 62-5027 discloses that gypsum slurry is mixed with sludge slurry, and a polymer coagulant is preferably added, followed by dehydration with a dehydrator. There is no description. In addition, no effect of the polymer flocculant is described.

The present invention solves the above-mentioned conventional problems, and mixes a gypsum slurry generated in a flue gas desulfurization process with a sludge slurry generated in a coagulation sedimentation process of flue gas desulfurization effluent to perform a specific gravity. Effective mixing of different types of sludge and gypsum to obtain a uniform mixed slurry, and by dewatering this uniform mixed slurry, the sludge and the gypsum are homogenized, and a dewatered cake with high commercial value as recovered gypsum It is an object of the present invention to provide a method for treating flue gas desulfurization wastewater sludge which can obtain a wastewater.

[0009]

SUMMARY OF THE INVENTION According to the present invention, there is provided a method for treating flue gas desulfurization wastewater, wherein the flue gas desulfurization wastewater discharged from a lime-gypsum method flue gas desulfurization apparatus equipped with a gypsum recovery process is subjected to a coagulation treatment. In the method of treating flue gas desulfurization wastewater sludge that mixes and dewaters the sludge slurry to be mixed with the gypsum slurry from the gypsum recovery process, the gypsum slurry and the sludge slurry are introduced into a stirring tank, and mixed and stirred in the stirring tank. After adding the polymer flocculant, dehydration is performed by a dehydrator.

As described above, since the specific gravities of gypsum and sludge are greatly different from each other, simply mixing gypsum slurry and sludge slurry cannot achieve uniform mixing. For example, when the gypsum slurry and the sludge slurry are mixed at a volume ratio of 8: 2 and allowed to stand, the gypsum slurry and the sludge layer are separated into two layers and settle. Then, if such a non-uniform mixed slurry is dewatered as it is, a non-uniform dewatered cake such as a sludge cake deposited on a gypsum cake will result. That is, in order to obtain a uniform dewatered cake, it is necessary to obtain a uniform mixed slurry and to dewater the slurry.

The present inventors have conducted the following studies in order to obtain a uniform mixed slurry by mixing a gypsum slurry and a sludge slurry.

When a polymer flocculant (anionic or nonionic polymer flocculant) was added to each of the gypsum and sludge slurries, effective flocculation was performed and good flocs were formed. Therefore, when a polymer flocculant (anionic or nonionic) was added to the mixed slurry of gypsum and sludge under stirring, effective flocculation was also performed.
A good floc was formed and, upon standing, a homogeneous mixed sediment of gypsum and sludge was obtained.

The present invention has been completed based on these findings, finding that a uniform mixed slurry can be obtained by adding a polymer flocculant while mixing and stirring a gypsum slurry and a sludge slurry.

[0014] By adding the polymer flocculant under the mixing and stirring of such a slurry, the details of the action mechanism of the uniform mixing of the gypsum and the sludge having different specific gravities to obtain a uniform mixed slurry are not clear. No, but it is estimated as follows.

That is, in order to obtain a uniform mixed slurry, it is necessary that both the gypsum and the sludge coagulate sufficiently. In this way, when both coagulate, both flocs are combined to achieve uniformity. . When the polymer flocculant is added to the slurry while mixing and stirring, the gypsum and the sludge coagulate sufficiently and uniformity can be achieved by bonding both flocs.

In the present invention, since sufficient coagulation is performed as described above, the SS recovery rate by dehydration is improved, and the SS of the dehydrated filtrate can be reduced.

In the present invention, the polymer flocculant includes:
An anionic polymer flocculant or a nonionic polymer flocculant is preferable, and the amount of the polymer flocculant is 0.5 to 50 mg based on a mixed slurry of a gypsum slurry and a sludge slurry.
/ L is preferable.

In the present invention, the mixing ratio of the gypsum slurry and the sludge slurry is such that the gypsum slurry concentration is 10 to 30%.
(Wt / vol.%), Sludge slurry concentration is 1-5%
(Wt / vol.%), The gypsum slurry: sludge slurry = 1: 1 or less (slurry volume ratio) is preferable.

[0019]

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 the method for treating flue gas desulfurization wastewater sludge of the present invention.

[0021] Gypsum slurry discharged from the gypsum thickener 1 lime gypsum method flue gas desulfurization apparatus is introduced into the stirring tank 2 by a pump P _1. On the other hand, the sludge slurry discharged from the thickening tank 3 of flue gas desulfurization waste water treatment apparatus is also introduced into the stirring tank 2 by a pump P _2, gypsum slurry and sludge slurry,
Mixing and stirring are performed in the stirring tank 2. This is stirred vessel 2, the polymeric flocculant from the reservoir 4 by the pump P ₃ is added and the aggregation treatment.

Usually, in the flue gas desulfurization treatment of the lime-gypsum method, the gypsum slurry concentration is 10 to 30% (wt / vol.
%), And the sludge slurry concentration is 1 to 5% (wt / vol.
%), The mixing ratio of the gypsum slurry and the sludge slurry in the stirring tank 2 is gypsum slurry:
Sludge slurry = 1: 1 or less, preferably 1: 0.1 to
0.4, the amount of sludge slurry is preferably small in terms of cohesion, dehydration, and uniformity of dewatered cake.

The slurry for coagulation treatment in the stirring tank 2 is pump P
_The water is fed to the dehydrator 5 by ₄ to be dewatered, and the dewatered cake and the dewatered filtrate are respectively discharged out of the system.

In the present invention, the polymer flocculant includes:
An anionic polymer coagulant or a nonionic polymer coagulant is used. Specifically, the following can be used.

[Anionic Polymer Flocculant] "Cryflock (registered trademark) PA33" manufactured by Kurita Water Industries Ltd.
1 ": 20% partial hydrolyzate of polyacrylamide (hereinafter simply referred to as" PA331 ")" Kurifloc (registered trademark) PA37 "manufactured by Kurita Water Industries Ltd.
2 ": 43% partial hydrolyzate of polyacrylamide (hereinafter, simply referred to as" PA372 ") [Nonionic polymer flocculant]" Kurifloc (registered trademark) PN10 "manufactured by Kurita Water Industries Ltd.
1 ": 5% or less partial hydrolyzate of polyacrylamide (hereinafter simply referred to as" PN101 ") The polymer flocculant is not limited to the above-mentioned ones, and may be, for example, polyacrylic acid or polyacrylic acid. Methacrylic acid, or an anionic or nonionic polymer flocculant generally used as a flocculant for sludge may be used.

If the amount of the polymer flocculant is too small, a sufficient homogenizing effect cannot be obtained by the addition. However, if the addition amount of the polymer flocculant is excessively large, the treatment cost will increase. Therefore, the addition amount of the polymer flocculant is 0.5 to 50 m with respect to the mixed slurry of the gypsum slurry and the sludge slurry.
g / L, particularly preferably in the range of 2 to 10 mg / L.

In the present invention, there is no particular limitation on the method of adding the polymer flocculant, mixing, stirring and dehydrating.
Any of the following batch system and continuous system may be used.

(1) Batch method A gypsum slurry and a sludge slurry are put into a stirring tank, and a required amount of a polymer flocculant is added with stirring. The addition of the polymer flocculant is preferably performed within a short time. After addition of the polymer flocculant, flocculation occurs in 5 minutes or less, and flocs are generated.
The coagulated slurry is fed from a stirring tank to a dehydrator to be dehydrated. The next gypsum slurry and sludge slurry are put into the empty stirring tank, and the same operation is repeated.

(2) Continuous system The gypsum slurry and the sludge slurry are continuously supplied to the stirring tank, and the polymer flocculant is continuously added under stirring. The agglomerated slurry is continuously fed from the stirring tank to a dehydrator to be dehydrated.

In addition, the polymer flocculant may be continuously added to the stirring tank and the polymer flocculant may be continuously added to the flocculated slurry supply pipe before the dehydrator to perform dehydration.

The stirring conditions in the stirring tank are as follows:
About 50 to 200 rpm is appropriate.

According to the treatment method of the present invention, a uniform flocculated slurry can be obtained by adding a polymer flocculant under mixing and stirring of a gypsum slurry and a sludge slurry. By dewatering, a uniform dewatered cake without unevenness can be obtained, the SS recovery rate can be improved, and good treatment can be performed.

[0033]

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

Examples 1 to 5 and Comparative Example 1 A gypsum slurry and a sludge slurry produced by neutralization of flue gas desulfurization effluent with slaked lime were mixed at a volume ratio of 8: 2 and stirred at a rotation speed of 150 rpm. Under stirring, PA
As shown in Table 1, 331 was added in different amounts. After leaving the obtained agglomerated slurry for 30 minutes, SV30 (S
ludge Volume: the volume% of the sludge deposited based on the total amount of the slurry after standing for 30 minutes), and the results are shown in Table 1. In addition, this coagulated slurry was rotated at 30 rpm.
The resulting dehydrated cake was subjected to centrifugal dehydration treatment at 00 rpm, and the moisture content and uniformity of the dehydrated cake and the SS of the filtrate were examined. The results are shown in Table 1.

[0035]

[Table 1]

Examples 6 to 9 and Comparative Example 2 In Example 1, coagulation and dehydration treatments were carried out in the same manner as in Example 1, except that PN101 was used in place of PA331 as a polymer coagulant and the addition amount was as shown in Table 2. The water content and the uniformity of the obtained dehydrated cake and the SS of the filtrate were examined. The results are shown in Table 2.

[0037]

[Table 2]

As is clear from the results in Tables 1 and 2, the addition of the polymer flocculant resulted in the formation of a sedimentary layer without two-layer separation, and the dehydration resulted in a uniform dewatered cake. In particular, when the polymer coagulant was added in an amount of 5 mg / L or more, a good addition effect was obtained, and the obtained dehydrated cake was uniform and the peelability was also improved. The water content of the dehydrated cake is almost the same between when no polymer coagulant is added and when a polymer coagulant is added, but when a polymer coagulant is added, the SS of the filtrate is It has dropped significantly. These tendencies are in PA3
There was no significant difference between 31 and PN101.

Examples 10 to 21, Comparative Examples 3 to 5 Gypsum slurry was mixed with sludge slurry produced by coagulation treatment of flue gas desulfurization wastewater with a sulfuric acid band (aluminum sulfate) at a volume ratio of 9: 1 and rotated. The mixture was stirred at several 150 rpm, and PA331 and PN101 were mixed under stirring.
Alternatively, PA372 was added in different amounts as shown in Tables 3, 4 and 5, respectively, followed by dehydration treatment in the same manner as in Example 1 to determine the water content and uniformity of the obtained dehydrated cake and SS of the filtrate. Inspection and the results are shown in Tables 3, 4, and 5, respectively.

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

As is clear from the results of Tables 3 to 5, a uniform dehydrated cake was obtained by adding the polymer flocculant. In particular,
By adding the polymer flocculant in an amount of 5 mg / L or more, a good addition effect was obtained, and the obtained dehydrated cake was also uniform and the peelability was also improved. The water content of the dehydrated cake is almost the same between when no polymer coagulant is added and when a polymer coagulant is added, but when a polymer coagulant is added, the SS of the filtrate is It has dropped significantly. These tendencies did not differ significantly between PA331, PN101 and PN372.

[0044]

As described in detail above, according to the method for treating flue gas desulfurization wastewater sludge of the present invention, the gypsum produced in the flue gas desulfurization process is added to the sludge slurry produced in the coagulation and sedimentation of the flue gas desulfurization wastewater. In mixing and dehydrating the slurry,
Sludge and gypsum with different specific gravities are effectively mixed to obtain a uniform mixed slurry, and by dewatering the uniform mixed slurry, the sludge and the gypsum are homogenized, and the commercial value of the recovered gypsum is high. Dehydrated cake can be obtained. Therefore, according to the present invention, in the lime gypsum flue gas desulfurization process, gypsum can be recovered without deteriorating its commercial value, and sludge generated by the coagulation and sedimentation treatment of the flue gas desulfurization effluent can be disposed of. And the complicated operation management of the sludge dewatering machine is eliminated, and the cost for dewatering can be reduced. Further, since the recovery rate of SS is also improved, the treatment of the dehydrated filtrate is reduced.

[Brief description of the drawings]

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

[Explanation of symbols]

 Reference Signs List 1 gypsum thickener 2 stirring tank 3 concentration tank 5 dehydrator

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C02F 11/14 B01D 53/34 125E (72) Inventor Takafumi Murakami 3-4 Nishishinjuku, Shinjuku-ku, Tokyo No. Kurita Kogyo Co., Ltd. (72) Takeshi Hatta 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Kogyo Co., Ltd. Ground Tohoku Electric Power Co., Inc. 3-20 Mibayashi, Higashi-Miyahama, Shichigahama-cho (72) Inventor Nobuki Fujiwara 22-2, Sawajiri, Yoshidahama-cho, Shichigahama-cho, Miyagi-gun, Miyagi F-term (reference) 4D002 AA02 AB01 AC10 BA02 DA05 FA03 4D015 BA21 BB05 CA11 DC02 DC06 DC08 EA04 EA06 EA31 4D059 AA11 AA18 BE00 BE58 BE59 BJ00 EB11 EB20 4D062 BA21 BB05 CA11 DC02 DC06 DC08 EA04 EA06 EA31

Claims (4)

[Claims] Claims: 1. Coagulation treatment of flue gas desulfurization effluent discharged from a lime gypsum method flue gas desulfurization apparatus provided with a gypsum recovery process,
In a method for treating flue gas desulfurization wastewater sludge in which the generated sludge slurry is mixed with the gypsum slurry from the gypsum recovery process and dewatered, the gypsum slurry and the sludge slurry are introduced into a stirring tank and mixed and stirred in the stirring tank. After adding the polymer flocculant,
A method for treating flue gas desulfurization wastewater sludge, comprising dewatering with a dehydrator. 2. The method according to claim 1, wherein said polymer flocculant is an anionic polymer flocculant or a nonionic polymer flocculant. 3. The amount of the polymer flocculant added to the mixed slurry of the gypsum slurry and the sludge slurry is 0.5 to 0.5.
The method for treating flue gas desulfurization wastewater treatment sludge according to claim 1 or 2, wherein the amount is 50 mg / L. 4. A gypsum slurry concentration of 10 to 30% (wt.
/ Vol. %), And the sludge slurry concentration is 1 to 5% (wt /
vol. %), The mixing ratio of the gypsum slurry and the sludge slurry is 1: 1 or less (slurry volume ratio), and the treatment of the flue gas desulfurization wastewater treatment sludge according to any one of claims 1 to 3, Method.