JP2003047972A - Method for treating fluorine-containing wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating fluorine-containing wastewater, whereby the fluorine contained in the wastewater can be treated while the amount of dehydrated cake produced as waste is minimized. SOLUTION: The treatment method is one wherein wastewater discharged from a desulfurizer is subjected to pH adjustment in a pH adjustment tank, and fluorine contained in the wastewater is then adsorbed on a precipitative compound in a reactor to remove it by coagulation and sedimentation. Basic magnesium sulfate and magnesium hydroxide on both of which fluorine is adsorbed are formed by adding sodium hydroxide to the reactor. The slurry is then returned to the pH adjustment tank to allow the magnesium hydroxide in the slurry to react with an acid to dissolve and discharge it. The basic magnesium sulfate improves the degree of fluorine adsorption. The return cycle is repeated so that the slurry containing the basic magnesium sulfate on which fluorine is sufficiently adsorbed may be finally discharged from the system after dehydration.

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 fluorine-containing wastewater, which is capable of treating fluorine contained in wastewater discharged from a desulfurization unit without generating dehydrated cake which is waste. Is.

[0002]

2. Description of the Related Art For example, since fluorine is contained in the wastewater discharged from a desulfurization unit installed in a coal-fired power plant, it is necessary to remove fluorine so that it falls below a standard value. As a conventional general method for treating fluorine-containing wastewater, for example, as shown in FIG. 2, a coagulation sedimentation method is known in which a chemical is added to wastewater to cause precipitation. In this method, the pH of the wastewater to be treated is first adjusted with hydrochloric acid (HCl), then reacted with slaked lime (Ca (OH) ₂ ) in a reaction tank to produce calcium fluoride (CaF ₂ ), and then polymer aggregation is carried out. The first step of coagulation / precipitation in which an agent is added and coagulation / precipitation is performed, and fluorine remaining in the wastewater is further coprecipitated with polyaluminum chloride (PAC), and then a polymer coagulant is added to perform coagulation / precipitation processing. The second step is coagulation and sedimentation, and fluorine is taken out of the system as a dehydrated cake. However, this method has a problem that a large amount of dehydrated cake is generated and a large amount of chemicals is used.

As a method for solving the problems of the two-step type coagulation-sedimentation method, recently, as shown in FIG.
Add sodium hydroxide (NaOH) to the reaction tank to adjust the pH.
Is adjusted to 9 or more, and a one-step coagulation-sedimentation method is proposed in which magnesium hydroxide, which is an adsorbent for sulfur oxides, adsorbs fluorine ions to perform coagulation / precipitation treatment (for example, JP-A-2000-176241). (See the official gazette). However, in the case of this method, there is a problem that a large amount of sodium hydroxide is required to raise the pH, and since the slurry obtained in the settling tank is repeatedly returned to the desulfurization device, the fluorine concentration becomes high. When it is completely concentrated, it cannot be returned, and it is necessary to take it out of the system as a whole dehydrated cake and discard it. In this case, the amount of dehydrated cake discarded is enormous.
There was also the problem that the amount of disposal work would be enormous.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned conventional problems and prevents fluorine contained in wastewater discharged from a desulfurization unit from generating dehydrated cake as waste as much as possible. It has been completed with the object of providing a method for treating fluorine-containing wastewater that can be treated at low cost, and can reduce the running cost because the amount of sodium hydroxide added to the reaction tank is small. is there.

[0005]

The method for treating fluorine-containing wastewater of the present invention made to solve the above-mentioned problems is to adjust pH of wastewater discharged from a desulfurization apparatus using magnesium hydroxide as an adsorbent. A method for treating fluorine-containing wastewater, which comprises performing a pH adjustment in a tank and then adsorbing fluorine contained in the wastewater into a precipitation compound in a reaction tank to remove the fluorine-containing wastewater by coagulation precipitation. After generating basic magnesium sulfate and magnesium hydroxide that have adsorbed fluorine by adding the above, the slurry containing these is returned to the pH adjusting tank, and the magnesium hydroxide in the slurry is reacted with an acid to dissolve and treat it. While being discharged as water, the degree of adsorption of fluorine of basic magnesium sulfate is increased, and this return cycle is repeated to finally release fluorine After dehydrating the slurry containing the basic magnesium sulfate adsorbed to the minute, is characterized in that the taking out from the system as dewatered slurry.

The invention according to claim 2 made to solve the same problem is to adjust the pH of the pH adjusting tank to a neutral range, and to solve the same problem. The invention according to claim 3 made is the pH of the reaction tank.
Is adjusted to be 9 or more.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic flow chart of a method for treating fluorine-containing wastewater according to the present invention. The water to be treated (raw water) to be the subject of the present invention is, for example, waste water discharged from a desulfurization device using magnesium hydroxide as an adsorbent installed in a coal-fired power plant,
The wastewater contains fluorine. In addition, the removal treatment method is based on the general two-stage coagulation sedimentation method.

First, the wastewater discharged from the desulfurizer is p
Guided to the H adjustment tank, where the pH suitable for coagulation sedimentation treatment
Is prepared. Next, the pH-adjusted wastewater is introduced into the reaction tank, where the fluorine contained in the wastewater is adsorbed by the precipitation compound and is removed by coagulation precipitation.

The first feature of the present invention is that sodium hydroxide (NaOH) is added to this reaction vessel to produce basic magnesium sulfate and magnesium hydroxide having adsorbed fluorine. Conventionally, by adding slaked lime (Ca (OH) ₂ ) to a reaction tank, fluorine in the raw water is fixed by Ca, calcium fluoride is generated, and it is removed as sludge. In this case, the chemical reaction in the reaction tank is as shown in the following chemical formula 1.

[0010]

[Chemical 1]

That is, in the removal of fluorine by slaked lime, fluorine was precipitated as calcium fluoride and aluminum fluoride and extracted as sludge. According to the analysis result of the sludge component of the present inventor, these ratios are about 48% for Mg (OH) ₂ and about 3% for calcium fluoride and aluminum fluoride. About 22% of the sludge component is gypsum (CaSO ₄ ), and about 27% is hydroxide caused by injected chemicals such as Al and Fe. That is, the present inventor has clarified that when slaked lime is used, a large amount of gypsum is generated under the influence of sulfate ions in raw water as a sludge component, which increases the amount of sludge generated. .

Therefore, in the present invention, by adding sodium hydroxide (NaOH) to the conventional slaked lime replacing reaction tank, basic magnesium sulfate and magnesium hydroxide are produced as shown in Chemical formula 2. This aims to eliminate the generation of gypsum (CaSO ₄ ) and reduce the amount of sludge generated.

[0013]

[Chemical 2]

On the other hand, according to this method, basic magnesium sulfate and magnesium hydroxide are produced as solids instead of gypsum, but these are not a factor for increasing sludge by treating as follows. That is, in the present invention, the generated slurry is returned to the pH adjusting tank, magnesium hydroxide in the slurry is reacted with an acid to be dissolved and discharged as treated water, and the degree of fluorine adsorption of basic magnesium sulfate is increased. The second feature is that by repeating the returning cycle, the slurry containing the basic magnesium sulfate in which fluorine is sufficiently adsorbed is finally dehydrated and then taken out of the system as a dehydrated slurry.

The generated magnesium hydroxide is an insoluble substance, but as shown in Chemical formula 3, it is a highly soluble compound (MgCl ₂ or MgSO ₄ ) by reacting with an acid (for example, hydrochloric acid or sulfuric acid). In the present invention, magnesium hydroxide is dissolved by utilizing this reaction and discharged out of the system as treated water. As a result, the amount of sludge generated can be substantially reduced. In addition, basic magnesium sulfate is a substance having an excellent adsorptive power, and by returning this to the pH adjusting tank, it is reused as a fluorine adsorbent and further promotes the adsorption and removal of fluorine. Then, by repeating this returning cycle, finally a slurry containing basic magnesium sulfate in which fluorine is sufficiently adsorbed is dehydrated, and after this is dehydrated, fluorine is removed by taking it out of the system as dehydrated slurry. Of. As described above, in the present invention, since neither the generated basic magnesium sulfate nor magnesium hydroxide is a factor for increasing sludge, it is possible to reduce sludge by about 60% as compared with the conventional method.

[0016]

[Chemical 3]

In this case, the pH of the pH adjusting tank is adjusted to be in the neutral range. This is because the slurry mainly composed of magnesium hydroxide is easily redissolved, and the amount of dehydrated cake discharged to the outside of the system can be reduced. Further, the pH of the reaction tank is adjusted to 9 or higher. This is to increase the conversion rate of magnesium sulfate to basic magnesium sulfate.

Further, by returning the basic magnesium sulfate to the pH adjusting tank, the basic magnesium sulfate having an excellent adsorbing power adsorbs and removes fluorine in the raw water in advance, so that water added to the reaction tank is removed. The amount of sodium oxide can be significantly reduced, and the chemical cost can be reduced.

As described above, in the present invention, basic magnesium sulfate and magnesium hydroxide are produced by adding sodium hydroxide to the reaction tank, and the obtained slurry is returned to the pH adjusting tank to remove water in the slurry. Since sodium oxide is made to react with an acid to dissolve it, gypsum is not generated and the amount of sludge generation can be remarkably reduced as compared with a conventional reaction product containing slaked lime. Further, since the basic magnesium sulfate is returned to the pH adjusting tank, the fluorine can be removed in a high concentration state, and the amount of sodium hydroxide added can be greatly reduced.

[0020]

[Examples] Table 1 shows the results obtained by treating the wastewater discharged from the desulfurization device using magnesium hydroxide as an adsorbent installed in a coal-fired power plant according to the treatment process shown in FIG. It was as it was. The fluorine content in the final treated water was completely cleared to the target value of 10 mg / l, and it was confirmed that the sludge generation amount (DSS) could be 40% or less compared to the conventional one. In addition, DSS
Is a Dry-based SS (Suspendend So)
lids). Table 2 shows the results of processing performed by the conventional example shown in FIG.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

As is apparent from the above description, the present invention can treat the fluorine contained in the waste water discharged from the desulfurization device without generating the dehydrated cake as waste as much as possible. Moreover, since the amount of sodium hydroxide added to the reaction tank is small, the running cost can be reduced. Therefore, the present invention is extremely large in that it contributes to the development of industry as a method for treating fluorine-containing wastewater that eliminates the conventional problems.

[Brief description of drawings]

FIG. 1 is a processing flow chart showing an embodiment of the present invention.

FIG. 2 is a processing flow chart showing a conventional example.

FIG. 3 is a processing flow chart showing a conventional example.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Hidemi Watanabe             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works F-term (reference) 4D038 AA08 AB40 BA06 BB13

Claims (3)

[Claims] 1. The pH of waste water discharged from a desulfurization apparatus using magnesium hydroxide as an adsorbent is adjusted in a pH adjusting tank, and then the fluorine contained in the waste water is adsorbed by the precipitation compound in the reaction tank. A method for treating fluorine-containing wastewater, which is configured to remove by coagulation sedimentation, in which sodium hydroxide is added to a reaction tank to generate basic magnesium sulfate and magnesium hydroxide having adsorbed fluorine, and then these are included. The slurry is returned to the pH adjusting tank, the magnesium hydroxide in the slurry is reacted with an acid to dissolve it and is discharged as treated water, and the fluorine adsorption degree of basic magnesium sulfate is increased. In particular, after dehydrating the slurry containing basic magnesium sulfate with sufficient adsorption of fluorine, it is removed as a dehydrated slurry from the system A method for treating fluorine-containing wastewater, characterized by taking out. 2. The method for treating fluorine-containing wastewater according to claim 1, wherein the pH of the pH adjusting tank is adjusted to a neutral range. 3. The method for treating fluorine-containing wastewater according to claim 1, wherein the pH of the reaction tank is adjusted to 9 or higher.