Fluorine-containing wastewater treatment method and device

Abstract

To provide a fluorine-containing wastewater method and device, which can reduce an amount of chemicals such as slaked lime and magnesium oxide added and can also reduce an amount of sludge generated.SOLUTION: There is provided a method for treating fluorine-containing wastewater, including a chemical addition step of adding a calcium compound and / or a magnesium compound to fluorine-containing wastewater, such as flue gas desulfurization wastewater; and a decarbonation treatment step of decarbonating the fluoride-containing wastewater before the chemical addition step, in which the decarbonation treatment step is performed by aerating the fluorine-containing wastewater under an acidic condition.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method and apparatus for treating fluorine-containing wastewater, and particularly to a method and apparatus for treating fluorine-containing wastewater containing carbonic acid.

Background Art

[0002] As a method for removing fluorine in the flue gas desulfurization wastewater (hereinafter referred to as desulfurization wastewater) of a thermal power plant, there are methods such as adding slaked lime (calcium hydroxide) and adding magnesium oxide (Patent Documents 1 and 2).

[0003] The method of Patent Document 1 includes a first step of adding a calcium compound to wastewater containing fluorine and performing solid-liquid separation, a second step of adding an aluminum compound to the separated drainage liquid and performing solid-liquid separation, a third step of adding a calcium compound and a carbonate to the separated precipitate sludge and then making it alkaline and performing solid-liquid separation, and a fourth step of using the separated aqueous solution containing water-soluble aluminum for the treatment in the second step.

[0004] The method of Patent Document 2 is a method of separating calcium fluoride generated by adding a calcium compound to fluorine-containing water and then removing the remaining calcium ions with a cation exchange resin.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Carbonic acid derived from carbon dioxide in the exhaust gas is dissolved in the desulfurized wastewater. Since the carbonic acid in the wastewater reacts with chemicals such as slaked lime and magnesium oxide, it inhibits the removal of fluorine, and thus the amount of chemical added to reduce fluorine to the target value tends to increase.

[0007] In addition, fluorine and carbonic acid react with the chemical to generate sludge. Therefore, in wastewater with a high carbonic acid concentration, the amount of chemical added for fluorine removal increases, and the amount of generated sludge increases.

[0008] An object of the present invention is to provide a method and an apparatus for treating fluorine-containing wastewater that can reduce the amount of chemicals such as slaked lime and magnesium oxide added and also reduce the amount of generated sludge.

Means for Solving the Problems

[0009] The present invention solves the above problems as follows. [1] In a method for treating fluorine-containing wastewater having a chemical addition step of adding a calcium compound and / or a magnesium compound to the fluorine-containing wastewater, the method for treating fluorine-containing wastewater is characterized by having a decarbonation treatment step of subjecting the fluorine-containing wastewater to decarbonation treatment before the chemical addition step.

[0010] [2] The method for treating fluorine-containing wastewater according to [1], wherein the decarbonation treatment step is performed by aerating the fluorine-containing wastewater under acidic conditions.

[0011] [3] The method for treating fluorine-containing wastewater according to [1], wherein slaked lime and / or magnesium oxide is added as the chemical.

[0012] [4] The method for treating fluorine-containing wastewater according to any one of [1] to [3], wherein the fluorine-containing wastewater is flue gas desulfurized wastewater.

[0013] [5] In an apparatus for treating fluorine-containing wastewater having a chemical addition means for adding a calcium compound and / or a magnesium compound to the fluorine-containing wastewater, A treatment device for fluorine-containing wastewater, characterized in that it has a decarbonation treatment means for decarbonating the fluorine-containing wastewater on the upstream side of the chemical addition means.

[0014] [6] The decarbonation means of the treatment device for fluorine-containing wastewater according to [5] has a pH adjustment means for making the fluorine-containing wastewater acidic and an aeration means.

Effect of the Invention

[0015] In the present invention, decarbonation treatment is performed before adding chemicals such as slaked lime and magnesium oxide to the fluorine-containing wastewater. By removing the carbonic acid in the fluorine-containing wastewater in advance in this way, the addition amount of slaked lime, magnesium oxide, etc. required for fluorine removal can be reduced. Thereby, the amount of sludge generated and the amount of chemicals used can be reduced.

Brief Description of the Drawings

[0016]

Figure 1

Embodiments for Carrying Out the Invention

[0017] Hereinafter, the present invention will be described in more detail.

[0018] As the fluorine-containing wastewater to be treated by the present invention, desulfurization wastewater from a thermal power plant or the like is suitable. The fluorine concentration in the desulfurization wastewater is usually 30 to 300 mg / L, particularly about 50 to 200 mg / L, and the carbonic acid concentration is about 500 to 2000 mg / L, particularly about 500 to 1500 mg / L in terms of calcium carbonate conversion value. Also, the pH of the desulfurization wastewater is usually 7 to 10, particularly about 8 to 9.

[0019] In the present invention, before adding chemicals such as slaked lime and magnesium oxide, the fluorine-containing wastewater is decarbonated. As the decarbonation treatment, a treatment of aerating the fluorine-containing wastewater in a state where the pH is adjusted to 4 to 6, particularly about 4 to 5, is suitable. As the acid for pH adjustment, hydrochloric acid, sulfuric acid, etc. are suitable.

[0020] The aeration air volume is 0.5 to 10 Nm 3 per hour per 1 m of fluorine-containing wastewater 3 , particularly preferably about 1 to 5 Nm 3 . Note that Nm 3 represents m under standard conditions 3 .

[0021] The residence time in the aeration tank for aeration is 0.2 to 2 hours, particularly preferably about 0.5 to 1 hour. The carbonic acid concentration in the water after aeration is preferably 10 to 200 mg / L, particularly preferably about 50 to 100 mg / L, in terms of calcium carbonate equivalent value.

[0022] After decarbonation by aeration, chemicals such as slaked lime and magnesium oxide are added. As the chemicals, calcium compounds such as slaked lime and magnesium compounds such as magnesium oxide can be used.

[0023] The chemical addition amount is preferably 15 to 100 times, particularly preferably about 10 to 40 times, the fluorine equivalent.

[0024] After adding the chemical to precipitate calcium fluoride, magnesium fluoride, etc., preferably a flocculant is added for flocculation treatment. As the flocculant, generally an anionic polymer flocculant is suitable.

[0025] This flocculation treatment liquid is subjected to solid-liquid separation treatment to be separated into sludge and treated water. For the sludge, it is preferably sent to a dehydrator for dehydration. Note that a part of it may be returned to the chemical addition step or the flocculation reaction step to act as seed sludge.

[0026] Figure 1 shows a preferred example of a treatment apparatus for fluorine-containing wastewater.

[0027] The fluorine-containing wastewater (raw water) is introduced into a pH adjustment tank 1 equipped with a pH adjuster addition means 1a (comprising a tank containing a pH adjuster solution, a chemical injection pump, etc.) and a pH meter, etc. The pH adjuster is added by the pH adjuster addition means 1a, mixed by a stirrer or air blowing, adjusted to pH 4 to 6, particularly 4 to 5, and then introduced into an aeration tank 2.

[0028] In the aeration tank 2, aeration treatment is performed by ejecting air from the blower 2B through the air diffuser pipe 2A. The water that has undergone aeration treatment and decarburization treatment is introduced into the reaction tank 3, and a chemical agent (in this embodiment, a dispersion of slaked lime) is added by the chemical agent adding means 3a, and mixing is performed by a stirrer or air blowing to precipitate calcium fluoride.

[0029] The reaction liquid in the reaction tank 3 is introduced into the coagulation tank 4, a coagulant is added by the coagulant adding means 4a, and mixing is performed by a stirrer or air blowing to perform coagulation treatment. The coagulation reaction liquid is introduced into the sedimentation tank 5 as a solid-liquid separation means and separated into sludge and treated water.

[0030] The sedimentation tank 5 has a function of sedimenting the coagulation flocs and separating the supernatant as treated water. The volume of the sedimentation tank 5 is determined by the sedimentation rate of the coagulation flocs. The sedimented sludge is drawn out from the sedimentation tank 5 and dehydrated into a dehydrated cake by a dehydrator. In one aspect of the present invention, a part of the sedimented sludge is returned to the reaction tank 3.

[0031] Thus, according to this embodiment, by removing carbonic acid in the fluorine-containing wastewater in advance, the addition amounts of slaked lime, magnesium oxide, etc. required for fluorine removal can be reduced, and the amount of generated sludge and the usage amount of chemical agents can be reduced. Although a chemical (acid) is required for pH adjustment for decarburization, the obtained effect far exceeds the pH adjustment cost.

Example

[0032] [Example 1] <Decarburization treatment> Decarburization tests were conducted on the desulfurized wastewaters A, B, and C from three types of coal-fired power plants. The fluorine concentration and carbonic acid concentration of each desulfurized wastewater are shown in Table 1. In the following, the carbonic acid concentration is expressed as a calcium carbonate conversion value.

[0033] 1 L of each of the desulfurized wastewaters A, B, and C was collected, and hydrochloric acid was added to adjust the pH to 4. Aeration was carried out at 40 °C for 60 minutes using an aerator (air volume 1 NL / hr). The measurement results of the carbonic acid concentration after aeration are shown in Table 1.

[0034]

Table 1

[0035] As shown in Table 1, it was confirmed that decarbonation occurred by aeration.

[0036] <Fluoride removal treatment by adding slaked lime> As described above, 35,000 mg / L, 30,000 mg / L, or 25,000 mg / L of slaked lime was added to each of the decarbonated wastewaters A, B, and C, and the mixture was stirred at 40 °C for 1 hour to cause a reaction. The mixture was filtered through a filter, and the fluoride concentration in the filtrate was measured. The fluoride concentrations in each filtrate are shown in Table 2.

[0037] [Example 2] A test similar to Example 1 was conducted except that 35,000 mg / L, 30,000 mg / L, or 25,000 mg / L of magnesium oxide was added instead of slaked lime. The results are shown in Table 3.

[0038] [Comparative Example 1] A test similar to Example 1 was conducted except that 35,000 mg / L, 30,000 mg / L, or 25,000 mg / L of slaked lime was added without decarbonation. The results are shown in Table 4.

[0039] [Comparative Example 2] A test was conducted in the same manner as Comparative Example 1 except that 35,000 mg / L, 30,000 mg / L, or 25,000 mg / L of magnesium oxide was added instead of slaked lime. The results are shown in Table 5.

[0040]

Table 2

[0041]

Table 3

[0042]

Table 4

[0043]

Table 5

[0044] <Results and Discussion> The target fluorine concentration after the fluorine removal treatment of the desulfurized wastewater is 20 mg / L or less, and in Examples 1 and 2, the target value was satisfied at all chemical addition amounts.

[0045] In contrast, in Comparative Example 1, the target value was not reached at all slaked lime addition amounts. In order to achieve the target value (F ≤ 20 mg / L), an addition of 45,000 mg / L or more of slaked lime was necessary.

[0046] Also, in Comparative Example 2, the target value was not reached at all magnesium oxide addition amounts. In order to achieve the target value (F ≤ 20 mg / L), an addition of 60,000 mg / L or more of magnesium oxide was necessary.

Explanation of Signs

[0047] 1 pH adjustment tank 2 Aeration tank 3 Reaction tank 4 Coagulation tank 5 Sedimentation tank

Claims

1. In a method for treating fluorine-containing wastewater having a chemical addition step of adding a calcium compound and / or a magnesium compound to the fluorine-containing wastewater, the method for treating fluorine-containing wastewater is characterized by having a decarbonation treatment step of subjecting the fluorine-containing wastewater to decarbonation treatment before the chemical addition step.

2. The method for treating fluorine-containing wastewater according to Claim 1, wherein the decarbonation treatment step is performed by aerating the fluorine-containing wastewater under acidic conditions.

3. The method for treating fluorine-containing wastewater according to Claim 1, wherein slaked lime and / or magnesium oxide is added as the chemical.

4. The method for treating fluorine-containing wastewater according to any one of Claims 1 to 3, wherein the fluorine-containing wastewater is flue gas desulfurization wastewater.

5. In a treatment apparatus for fluorine-containing wastewater having a chemical addition means for adding a calcium compound and / or a magnesium compound to the fluorine-containing wastewater, the treatment apparatus for fluorine-containing wastewater is characterized by having a decarbonation treatment means for subjecting the fluorine-containing wastewater to decarbonation treatment on the upstream side of the chemical addition means.

6. The treatment apparatus for fluorine-containing wastewater according to Claim 5, wherein the decarbonation means includes a pH adjustment means for making the fluorine-containing wastewater acidic and an aeration means.

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