JPS625027B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for treating flue gas desulfurization wastewater, in particular, a method for coagulating flue gas desulfurization wastewater discharged from a wet flue gas desulfurization process in which gypsum is recovered by bringing flue gas into contact with an absorbing liquid to separate solid and liquid. It is about the method. Wet flue gas desulfurization processes include methods such as the lime-gypsum method, in which flue gas is brought into contact with an absorbing liquid to recover gypsum. In such processes, the resulting gypsum slurry is dehydrated and recovered, while the wastewater discharged from the process is treated and released. Such flue gas desulfurization wastewater contains impurities such as fluorine, aluminum, heavy metals, and COD, and these are usually removed by neutralization coagulation and precipitation treatment using slaked lime. In this case, when neutralization is performed with slaked lime, sulfate ions in the wastewater, in addition to fluorine and heavy metals, precipitate as gypsum. The sludge produced in this way is difficult to dewater, so a centrifugal filter for dewatering gypsum slurry with good dewatering properties cannot
The SS recovery rate was around 78%, making dewatering impossible, requiring different types of dehydrators such as filter presses and centrifugal sedimentation dehydrators, and had drawbacks such as complicated operation management and maintenance. This invention is intended to eliminate the drawbacks of the conventional methods as described above, and by mixing the generated sludge with gypsum slurry before dewatering,
The purpose of the present invention is to provide a method for treating flue gas desulfurization wastewater that can improve the dewatering properties of sludge and efficiently perform dewatering using a gypsum dehydrator. This invention relates to a method for treating flue gas desulfurization wastewater discharged from a wet flue gas desulfurization process, in which flue gas is desulfurized by contacting with an absorbing liquid and the generated gypsum slurry is dehydrated using a dehydrator to recover gypsum. , the flue gas desulfurization wastewater is coagulated, the generated sludge is mixed with the gypsum slurry before dehydration, and the pH is 5.
This is a method for treating flue gas desulfurization wastewater, which is characterized by dehydrating it with a dehydrator under the above conditions. This invention will be explained below with reference to the drawings. FIG. 1 is a system diagram showing a flue gas desulfurization process in an embodiment of the present invention, and FIG. 2 is a system diagram showing a flue gas desulfurization wastewater treatment method. First, the flue gas desulfurization process will be explained with reference to FIG. In FIG. 1, 1 is an electric precipitator, 2 is a cooling tower, 3 is an absorption tower, 4 is an oxidation tower, 5 is a settling tank, and 6 is a dehydrator. Exhaust gas generated in a boiler, etc. enters the electric precipitator 1 through a duct 7 and is collected, cooled by contact with cooling water circulated by a pump 8 in a cooling tower 2, and absorbed by a pump 9 in an absorption tower 3. The treated gas is desulfurized by contact with the liquid and is discharged from the duct 10 to the chimney. Since the cooling water circulating in the cooling tower 2 absorbs dust components in the exhaust gas and becomes contaminated, it is discharged from the pipe 11 as cooling tower blow water. The absorption liquid in the absorption tower 3 differs depending on the desulfurization method, but in the case of the lime-gypsum method, calcium carbonate or slaked lime slurry is used, and the absorption tower 3 absorbs sulfur dioxide gas and other pollutants to become a slurry containing calcium sulfite. One part at a time is taken out to the oxidation tower 4 and oxidized to form a gypsum slurry. The generated gypsum slurry is separated into solid and liquid in the settling tank 5, and the separated water is returned to the absorption tower 3 as an absorption liquid, and if necessary, a part of it is passed through the pipe 12.
is discharged from the system. In some cases, the water may be returned to the cooling tower as cooling water (not shown). The concentrated gypsum slurry is dehydrated by a dehydrator 6 and recovered as gypsum, and the filtrate is discharged as waste water through a pipe 12'.
The dehydrator 6 used here is usually of a centrifugal filtration type in which a filter medium is placed in a perforated basket, and solid-liquid separation is performed by allowing liquid to pass through the filter and solids to remain within the filter. The wastewater discharged from the above wet flue gas desulfurization process includes cooling tower blow water discharged from pipe 11, absorption tower wastewater discharged from pipe 12 and/or pipe 12', etc., and a method for treating these wastewaters. This will be explained with reference to FIG. In FIG. 2, 13 is a settling tank, 14 is a reaction tank, 15 is a settling tank, 16 is a filtration tank, and 17 is an adsorption tower. The cooling tower blow water discharged from the pipe 11 enters the precipitation tank 13, where an organic polymer flocculant is added from the chemical injection pipe 18 as necessary to perform coagulation and precipitation. Here, fly ash and other suspended substances are sedimented and separated and discharged from the pipe 19, and are subjected to separate treatment such as dehydration. The separated water enters the reaction tank 14 together with the absorption tower waste water discharged from the pipe 12, an alkali agent is added from the chemical injection pipe 20, and a coagulation reaction mainly consisting of neutralization is performed. When the cooling tower blow water and absorption tower waste water are discharged in a pre-mixed state, the entire mixture is precipitated and separated in the precipitation tank 13, and then reacted in the same manner in the reaction tank 14. In the reaction tank 14, fluoride ions, aluminum ions, heavy metal ions and
A part of COD reacts with alkaline agents and precipitates out. As the alkaline agent added to the reaction tank 14, sodium hydroxide, potassium hydroxide, slaked lime, quicklime, magnesium hydroxide, etc. can be used.
Adjust to 6-12. The main reactions when slaked lime is added are as follows. (1) Neutralization of PH H ₂ SO ₄ +Ca(OH) ₂ →CaSO ₄ ↓+2H ₂ O Removal of fluorine 2HF+Ca(OH) ₂ →CaF ₂ ↓+2H ₂ O Removal of metals MeSO ₄ +Ca(OH) ₂ →Me(OH) ₂ ↓＋CaSO ₄ As can be seen from the above reaction, the alkaline agent preferably contains calcium, and even when other alkaline agents are used, it is desirable to use them together with slaked lime, etc. The reaction liquid in the reaction tank 14 is precipitated and separated in a settling tank 15, the settled sludge is discharged from a pipe 21, the separated water is sent to a filtration tank 16, where residual SS is removed by a filter medium such as sand or anthracite, and then passed through an adsorption tower 1.
Dissolved in 7 by weakly basic synthetic adsorbent etc.
The COD is removed and the treated water is discharged through pipe 22. The sludge discharged from the pipe 21 is difficult to dewater,
Even if it was directly dehydrated using the dehydrator 6 shown in Fig. 1, the SS recovery rate was low and dehydration could not be performed. Here, the filtrate from the dehydrator 6 is mixed with the nuclear power plant water and subjected to the same treatment again, but if the SS recovery rate is poor, the load on wastewater treatment becomes large and has a negative impact on the wastewater treatment itself, so conventionally As mentioned above, other types of dehydrators such as filter presses and centrifugal sedimentation dehydrators were used. In the present invention, when dewatering the sludge generated in the wastewater treatment as described above, the gypsum slurry before dewatering generated in the flue gas desulfurization process is transferred to the pipe 2.
3, mixed with sludge in a mixing tank 24, preferably adding a polymeric organic flocculant, and then transferred to a dehydrator 2.
Dehydration treatment is performed in step 5. In this case, the dehydrator 25
As a dehydrator, one having the same type as the dehydrator 6 shown in FIG. 1 can be used. In a normal flue gas desulfurization equipment, multiple dehydrators are installed, some of which are spares, so some of them may be used for sludge dewatering, and in some cases, some of them may be used for sludge dewatering. returned to the smoke desulfurization process;
It may be mixed with gypsum slurry and subjected to dehydration treatment. The mixing ratio of the gypsum slurry to the sludge is 0.5 parts by weight or more, preferably 1 to 10 parts by weight of the dry solids of the gypsum slurry per 1 part by weight of the dry solids of the sludge. The gypsum slurry to be mixed with the sludge may be the slurry immediately before entering the dehydrator 6 in FIG. 1, but it may be from another location. The mixing method is not particularly limited, as long as it can be mixed uniformly. The pH changes depending on the mixture of sludge and gypsum slurry, but there is no problem with dewatering without any particular adjustment. However, in order to prevent fluorine compounds and metal compounds such as iron from redissolving, the pH must be 5 or higher, preferably 6 to 8.
need to be adjusted. By mixing gypsum slurry as described above, the dewatering performance of sludge is improved, and even with the centrifugal filtration type dehydrator used to dewater gypsum slurry, dewatering can be performed efficiently with a high SS recovery rate. The dehydrator can be used in common with the flue gas desulfurization process. Since the dewatered sludge cake contains a large amount of gypsum, it can be used as recovered gypsum, and if the purity of gypsum is not an issue, it can also be used by mixing it with recovered gypsum in the flue gas desulfurization process. In the above embodiment, the case where one reaction tank 14 and one precipitation tank 15 are provided is described, but a plurality of each may be provided. When a second reaction tank and a second precipitation tank are provided, residual calcium may be further removed by precipitation in the presence of carbonate ions by adding sodium carbonate or the like in the second reaction tank, or sodium hydroxide or magnesium may be added to the second reaction tank to further remove residual calcium. Salt may be added to further precipitate fluoride ions, heavy metals, etc. The sludge produced by these treatments can also be treated in the same manner as described above. In addition, the treatment flow shown in FIG. 2 can be changed as appropriate, and treatment can be performed to suit the characteristics of the wastewater to be discharged. The present invention is not limited to the flue gas desulfurization process shown in FIG.
It can be similarly applied to other wet flue gas desulfurization processes in which gypsum is recovered by contacting flue gas with an absorbing liquid. As described above, according to the present invention, flue gas desulfurization wastewater is subjected to coagulation treatment, and gypsum slurry before dewatering is mixed with the generated sludge for dewatering, so that the dewaterability of sludge can be improved. Using a dehydrator for gypsum slurry, it can be efficiently dehydrated with high SS recovery rate and low cake moisture content. Therefore, the dewatering facility can be shared, making operation management and maintenance easier. In addition, since dehydrated cake contains a large amount of gypsum, it can be used as is as recovered gypsum.
Even if it is mixed into the recovered gypsum in the desulfurization process, the amount is about 1% of the total, so it will not significantly reduce the purity of the recovered gypsum. Next, examples of the present invention will be described. . Example Flue gas desulfurization wastewater discharged from a lime-gypsum method coal combustion flue gas desulfurization device using the flow shown in FIG. 1 was treated according to the flow shown in FIG. 2. From medicine injection pipe 18
3 mg of EDP floc (Kurita Industries product) was added to perform sedimentation separation, slaked lime was added from the chemical injection pipe 20 to neutralize it to pH 7, and the precipitated sludge was concentrated in the sedimentation tank 15 (solid concentration 2.0 to 5.2 %), concentrated gypsum slurry (solids concentration 11 to 34%) obtained from the settling tank 5 in Fig. 1 is mixed in the mixing tank 24, and the dehydrator 2
Dehydrated at step 5. Dehydrator 25 has an inner diameter of 275 mm and a height of 230 mm.
It is a centrifugal filter with a capacity of 11.7 mm and approximately 15 mm.
Table 1 shows the results when mud was fed at a constant flow rate of Kg/min, the rotation speed was fixed so that the centrifugal force on the filter cloth surface was approximately 500G, and the dewatering time was 6 min. In Table 1, the sludge mixing ratio is the solid weight percent of the settled sludge based on the total solid weight, and the solid concentration is the total solid weight percent based on the total weight of the mixed sludge. No. 1 in Table 1 shows that when wastewater sludge alone is dehydrated using a centrifugal filter, the SS recovery rate is 78.5%, and 21.5% of SS leaks into the filtrate; No. 5, when using gypsum slurry alone will leak by 0.2%. For mixed sludge, if you calculate and add the leakage amounts corresponding to settled sludge and gypsum slurry, No. 2 will have (0.36 x 21.5) + (0.64 x 0.2) = 7.9 leaks, and if you calculate in the same way, No. .3 is
4.5%, and No. 4 has a leakage amount of 2.0%. Table 2 shows the results of calculating the leakage reduction rate from these values and experimental values. From the above results, mixing gypsum slurry with settled sludge improves dewatering performance, increases SS recovery rate, and lowers cake moisture content, and the leakage reduction rate calculated from calculated and experimental values is independent. This shows that the dehydration effect is superior to that predicted from the dehydration properties when dehydrating with.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a system diagram showing a flue gas desulfurization process in an embodiment of the present invention, and FIG. 2 is a system diagram showing a flue gas desulfurization wastewater treatment method. 1... Electrostatic precipitator, 2... Cooling tower, 3... Absorption tower, 4... Oxidation tower, 5, 13, 15... Precipitation tank,
6,25... dehydrator, 14... reaction tank, 16...
Filter tank, 17...adsorption tower, 24...mixing tank.

Claims (1)

[Claims] 1 Flue gas desulfurization wastewater discharged from a wet flue gas desulfurization process in which flue gas is desulfurized by contacting with an absorbing liquid and the generated gypsum slurry is dehydrated using a dehydrator to recover gypsum. In the processing method of
A method for treating flue gas desulfurization wastewater, which comprises coagulating the flue gas desulfurization wastewater, mixing the generated sludge with gypsum slurry before dehydration, and dewatering it in a dehydrator under conditions of pH 5 or higher. 2 Claim 1 in which the mixing ratio of sludge and gypsum slurry (based on dry solids) is 1:2 or more
The treatment method for flue gas desulfurization wastewater described in Section 1. 3. The method for treating flue gas desulfurization wastewater according to claim 1 or 2, wherein the wet flue gas desulfurization process is based on a lime-gypsum method. 4. The method for treating flue gas desulfurization wastewater according to any one of claims 1 to 3, wherein the flocculation treatment of flue gas desulfurization wastewater is neutralization flocculation by adding an alkali agent. 5. The method for treating flue gas desulfurization wastewater according to claim 4, wherein the alkaline agent is calcium hydroxide.