Method for treating fluoride-containing water

Abstract

The present invention provides a treatment method that can sufficiently reduce the fluorine concentration in treated water. [Solution] A method for treating fluorine-containing water, comprising: a first reaction step of receiving fluorine-containing wastewater and calcium compound-containing sludge and reacting the fluorine and calcium compound at a pH of 7 to 11; a second reaction step of adding an aluminum-based coagulant to the first reaction solution from the first reaction step and reacting at a pH of 6 to 8; a coagulation step of adding a polymer flocculant to the second reaction solution from the second reaction step and performing coagulation; a sedimentation step of separating the coagulated liquid from the coagulation step into solid and liquid; and a return step of returning a portion of the sludge separated in the sedimentation step to the first reaction step, wherein a calcium compound is added to at least one of the first reaction step and the return step. In the second reaction step, the aluminum-based coagulant is added as aluminum at a concentration of 7 to 30 mg / L.

Description

[Technical Field]

[0001] The present invention relates to a method for treating fluorine-containing water to remove fluorine from it. [Background technology]

[0002] Wastewater from semiconductor manufacturing processes using hydrofluoric acid-containing cleaning solutions or buffered hydrofluoric acid-containing etching agents, as well as wastewater from flue gas desulfurization processes, aluminum electrolytic refining processes, phosphate fertilizer manufacturing processes, uranium refining processes, and surface treatment cleaning processes, contains fluorine in the form of fluoride ions.

[0003] As a method for removing fluorine from such fluorine-containing wastewater, the High Density Solid (HDS) method is used, in which calcium compounds such as calcium hydroxide are added to the fluorine-containing wastewater, the fluorine reacts with the calcium compound, the resulting calcium fluoride is flocculated with a polymer flocculant, and the flocculated material is separated by sedimentation.

[0004] Patent Document 1 describes a method for improving the quality of treated water in this high-density sludge process by adding a calcium compound, followed by an aluminum-based inorganic flocculant, and then a polymer flocculant, and further by performing solid-liquid separation treatment in a single step (Figure 3 of Patent Document 1, etc.).

[0005] As shown in Example 2 (paragraph 0080) of Patent Document 1, the method of processing in a single sedimentation tank was not sufficient to lower the fluorine concentration. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2008 / 120704 [Overview of the Initiative]

Problems to be Solved by the Invention

[0007] The present invention provides a method for treating fluorine-containing water capable of sufficiently reducing the fluorine concentration in treated water.

Means for Solving the Problems

[0008] The gist of the present invention is as follows.

[0009] [1] A first reaction step of receiving fluorine-containing drainage and calcium compound-containing sludge and reacting fluorine with the calcium compound at pH 7 to 11, a second reaction step of adding an aluminum-based coagulant to the first reaction liquid from the first reaction step and reacting at pH 6 to 8, a coagulation step of adding a polymer coagulant to the second reaction liquid from the second reaction step to perform coagulation, a precipitation step of solid-liquid separating the coagulation treatment liquid from the coagulation step, a return step of returning a part of the sludge separated in the precipitation step to the first reaction step and a method for treating fluorine-containing water in which a calcium compound is added to at least one of the first reaction step and the return step, wherein in the second reaction step, 7 to 40 mg / L of the aluminum-based coagulant is added as aluminum, and a method for treating fluorine-containing water, characterized in that the seed crystal concentration in the first reaction tank defined by the following formula (1) is 1 to 10%.

[0010] Seed crystal concentration in the first reaction tank (%) = [SS r ×Q r / (Q f +Q r )] …(1) SS r : Return sludge concentration (mass%) Q f : Raw water flow rate (m 3 / h) Q r : Return sludge flow rate (m 3 / h)

[0011] [2] The method for treating fluorine-containing water according to [1], wherein the addition amount of aluminum in the aluminum coagulant is 10 to 40 mg / L.

[0012] [3] The method for treating fluorine-containing water according to [1] or [2], wherein the calcium compound and the aluminum coagulant are added such that the ratio Al / Ca of the addition amount in terms of aluminum of the aluminum coagulant to the addition amount in terms of calcium of the calcium compound is 1 / 5 or less. [Effect of the Invention]

[0013] The present invention is a method for treating fluorine-containing water using the high-density sludge method. In the method of adding an aluminum coagulant after adding a calcium compound, the aluminum coagulant is added at 7 mg (as Al) / L or more with respect to raw water. By setting the addition amount of the aluminum coagulant to 7 mg / L or more in terms of Al in this way, the aluminum coagulant and fluoride ions in the raw water react sufficiently. Further, the aluminum coagulant reacts with fine calcium fluoride (including colloidal form) present in the liquid to be treated to form a condensate, and flocs are efficiently formed in the subsequent coagulation step.

[0014] Further, in the present invention, the seed crystal concentration in the first reaction tank defined by the above formula (1) is set to 2 to 10% by mass. This seed crystal concentration in the first reaction tank is the concentration of sludge in the first reaction tank when the sludge returned to the first reaction tank is regarded as seed crystals.

[0015] By setting the seed crystal concentration in the first reaction tank to 2 to 10% by mass in this way, fluoride ions, calcium fluoride, etc. adhere to the sludge particles efficiently.

[0016] Furthermore, since Al sludge (coagulated material from aluminum-based coagulants) is lighter (bulkier and less likely to settle) than calcium fluoride (CaF2) sludge, the settling rate of the sludge decreases (settling performance decreases) as the ratio of Al addition to Ca addition increases. Therefore, in one aspect of the present invention, the ratio Al / Ca, which is the amount of aluminum-based coagulant added in terms of aluminum equivalent to the amount of calcium compound added in terms of calcium equivalent, is set to 1 / 5 or less. This further improves the settling performance of the sludge. [Brief explanation of the drawing]

[0017] [Figure 1] This is a flowchart of a fluorine-containing water treatment apparatus to which the fluorine-containing water treatment method according to the embodiment is applied. [Figure 2] This graph shows the results of the example. [Modes for carrying out the invention]

[0018] The present invention will be described in more detail below.

[0019] The fluorine-containing water (raw water) to be treated in this invention is not particularly limited, but examples include wastewater discharged from electronics industries such as electronic displays and semiconductors, thermal power plants, and the aluminum industry.

[0020] The fluorine concentration in fluorine-containing water is typically 20-3000 mg-F / L, and especially around 150-1000 mg-F / L.

[0021] Figure 1 is a flow diagram of a fluorine-containing water treatment apparatus to which the fluorine-containing water treatment method according to the embodiment is applied. This fluorine-containing water treatment apparatus includes a first reaction tank 1, a second reaction tank 2, a coagulation tank 3, a sedimentation tank 4, and a chemical reaction tank 6, etc.

[0022] Raw water (fluorine-containing water) is introduced into the first reaction tank 1 via the raw water piping 11, and the first reaction process is carried out. The raw water piping 11 is equipped with a flow meter 12 for detecting the flow rate of the raw water.

[0023] In the first reaction tank 1, calcium compounds such as calcium hydroxide and calcium chloride are added from the calcium compound adding means 13, and a pH adjuster is added from the pH adjuster adding means 14. Return sludge from the chemical reaction tank 6 is introduced into the first reaction tank 1 via the piping 7.

[0024] The pH in the first reaction vessel 1 is preferably 7 to 11, particularly 8 to 10, and especially 8.5 to 9.5.

[0025] The liquid in the first reaction tank 1 is stirred by a stirrer, and fluoride ions in the raw water react with calcium compounds added from the calcium compound adding means 13 and calcium compounds in the returned sludge from the piping 7 to precipitate as calcium fluoride. Some of the precipitated calcium fluoride precipitates using sludge particles in the returned sludge as seed crystals, and the sludge particles grow.

[0026] The first reaction liquid in the first reaction tank 1, which contains these precipitates and sludge particles, is transferred to the second reaction tank 2 via piping 1a, where the second reaction process takes place.

[0027] In the second reaction vessel 2, an aluminum-based coagulant such as PAC or aluminum sulfate is added, and a pH adjuster from an acid or alkali column is added via a pH adjuster addition means to adjust the pH to preferably 6-8, more preferably 6.5-8.0, and even more preferably 7.0-7.5. The liquid in the second reaction vessel 2 is stirred by a stirrer (not shown), and the aluminum hydroxide generated from the aluminum-based coagulant coagulates with the particles and fine components in the first reaction solution to form the second reaction solution.

[0028] This second reaction liquid is transferred to the coagulation tank 3 via piping 2a, where an organic polymer flocculant is added by the polymer flocculant adding means 17 for coagulation treatment, and then the solid-liquid is separated in the sedimentation tank 4. A portion of the polymer flocculant may also be added to the water supply piping 3a from the coagulation tank 3 to the sedimentation tank 4.

[0029] The supernatant water from the sedimentation tank 4 is discharged outside the system as treated water. A portion of the separated sludge that settles in the sedimentation tank 4 is returned to the chemical reaction tank 6 via the sludge return pipe 5, and the remainder is discharged outside the system via the discharge line 5a.

[0030] The sludge return piping 5 is equipped with a flow meter 18 and an SS meter 19 for detecting the SS (suspended solids) concentration. In the chemical reaction tank 6, a calcium compound adding means 20 adds a liquid (solution or dispersion) containing calcium chloride and slaked lime, which is mixed with the sludge returned from piping 5. This increases the density of the sludge particles. The sludge-containing liquid in the chemical reaction tank 6 is supplied to the first reaction tank 1 via piping 7.

[0031] Suitable calcium compounds to be added in the first reaction vessel 1 and the chemical reaction vessel 6 include calcium hydroxide (slaked lime) and calcium chloride.

[0032] The amount of calcium compound added is preferably equal to or greater than the reaction equivalent of fluoride ions contained in the raw water (fluorine-containing water), and is particularly preferably 1.2 equivalents or more, and more preferably 1.3 equivalents or more. Furthermore, the amount of calcium compound added is preferably 15 equivalents or less, particularly 4 equivalents or less, and more preferably 2.8 equivalents or less. The calcium compound may be added to both chemical reaction vessel 6 and the first reaction vessel 1, or it may be added only to chemical reaction vessel 6.

[0033] Suitable aluminum-based coagulants to be added to the second reaction vessel 2 include aluminum sulfate (aluminum sulfate), PAC, and aluminum chloride (LAC).

[0034] In Figure 1, the aluminum-based coagulant is added to the second reaction vessel 2, but it may also be added to the piping 1a used for transferring the liquid from the first reaction vessel 1 to the second reaction vessel 2.

[0035] The amount of aluminum-based coagulant added is 7 mg / L or more in terms of Al (140 mg / L or more as PAC), preferably 10 mg / L or more, and more preferably 15 mg / L or more. The upper limit of the amount of aluminum-based coagulant added is preferably set according to the set water surface load of the sedimentation tank 4. Normally, it is preferably 40 mg / L or less as Al, and particularly preferably 25 mg / L or less.

[0036] The water surface load of sedimentation tank 4 is 1-20 m². 3 / m 2 A rate of / h is preferred, and when a high-speed sedimentation tank is used, it is 10 to 12 m 3 / m 2 A range of / h is preferred. In the case of this water surface load, it is preferable that the amount of aluminum-based coagulant added is such that the ratio Al / Ca, which is the amount of aluminum-based coagulant added in terms of aluminum and the amount of calcium-based compound added in terms of calcium, is 1 / 5 or less, particularly 1 / 12 or less, and especially 1 / 15 or less.

[0037] By keeping the Al / Ca ratio at 1 / 5 or less, excessive addition of aluminum-based coagulants such as PAC is prevented, resulting in sludge with good settling properties. Ideally, the Al / Ca ratio should be 1 / 60 or higher, and particularly 1 / 50 or higher.

[0038] As described above, the pH of the first reaction tank 1 is preferably 7 to 11, particularly 8 to 10, and especially 8.5 to 9.5. If the pH of the first reaction tank 1 is 8.5 or higher, the aluminum hydroxide contained in the returned sludge dissolves, and the fluoride ions that were trapped in the aluminum hydroxide are released. The released fluoride ions react with calcium compounds in the first reaction tank 1 to produce calcium fluoride. Fluoride ions that do not react with calcium compounds are trapped in PAC in the second reaction tank 2.

[0039] The pH of the second reaction vessel 2 is preferably 6 to 8, particularly 6.5 to 8.0, and especially 7.0 to 7.5. By setting the pH of the second reaction vessel 2 within this range, aluminum hydroxide is efficiently generated from aluminum-based coagulants such as PAC, and the coagulation reaction is carried out sufficiently.

[0040] When pH adjustment is performed in the first reaction vessel 1 and the second reaction vessel 2, an acid or alkali such as sulfuric acid, hydrochloric acid, nitric acid, or sodium hydroxide is used as a pH adjusting agent.

[0041] The organic polymer flocculant added to the flocculation tank 3 is for flocculating insoluble substances generated in the first reaction tank 1 and the second reaction tank 2 by the addition of calcium compounds and aluminum-based coagulants. Examples of this organic polymer flocculant include anionic polymer organic flocculants, nonionic polymer organic flocculants, and polymer organic flocculants having cationic groups. The amount of organic polymer flocculant added to the second reaction tank 2 is not particularly limited, but generally, 0.1 to 50 mg / L, particularly 1 to 10 mg / L, and especially 2 to 5 mg / L is preferred.

[0042] Examples of anionic polymeric organic flocculants include, but are not limited to, alginic acid or its salts, carboxymethylcellulose, polymers of acrylic acid or its salts, copolymers of acrylic acid or its salts and acrylamide, copolymers of acrylamide and 2-acrylamide-2-methylpropanesulfonate, terpolymers of acrylic acid or its salts, acrylamide and 2-acrylamide-2-methylpropanesulfonate, and partially hydrolyzed polyacrylamide. The weight-average molecular weight range of the anionic polymeric organic flocculant is not particularly limited, but a range of 5 million to 20 million is preferred. These anionic polymeric flocculants can be used alone or in combination of two or more.

[0043] In the present invention, it is preferable that the seed crystal concentration of the first reactor, defined by the following formula (1), be 1% (mass%) or more, more preferably 2% or more, more preferably 3% or more, and especially preferably 5% or more, and also 10% or less, more preferably 8% or less, and especially preferably 6% or less.

[0044] First reaction vessel seed crystal concentration (%) = [SS r ×Q r / (Q f +Q r)] …(1) SS r : Return sludge concentration (mass%) detected by SS meter 19 Q f : Raw water flow rate (m 3 / h) Q r : Return sludge flow rate detected by flow meter 18 (m³ 3 / h)]

[0045] By setting the seed crystal concentration in the first reaction tank to 1-10%, fluoride ions and calcium fluoride efficiently adhere to sludge particles within the first reaction tank 1. Fluoride ions are efficiently incorporated into the sludge. As mentioned above, this seed crystal concentration is the concentration in the first reaction tank 1 when the sludge in the liquid flowing in from pipe 7 is considered as seed crystals. If the seed crystal concentration in the first reaction tank is less than 1%, there will be a shortage of seed crystals, resulting in a high F concentration in the treated water. Conversely, if the seed crystal concentration in the first reaction tank exceeds 10%, there will be an excess of seed crystals, leading to increased turbidity in the treated water due to seed crystal leakage, and further increasing the F concentration.

[0046] The seed crystal concentration in the first reaction tank can be adjusted by changing the return sludge concentration (SSr) and the return sludge flow rate (Qr). The return sludge concentration (SSr) can be adjusted by the amount of sludge withdrawn from the sedimentation tank.

[0047] The preferred range for the return sludge flow rate varies depending on the treatment volume of the equipment, but a return sludge concentration of 3-20% by mass, particularly 5-15%, is preferable. If the concentration is lower than this range, the return sludge flow rate will be large relative to the raw water flow rate, requiring a larger equipment. If the concentration is higher than this range, the fluidity of the return sludge will deteriorate, potentially causing the return pipe to clog. [Examples]

[0048] Using a fluorine-containing water treatment apparatus with the specifications shown in Figure 1, the seed crystal concentration (%) in the first reaction tank was changed by adjusting the return sludge flow rate and return sludge concentration under the following water quality and operating conditions, thereby treating fluorine-containing water (raw water). Figure 2 shows the relationship between the seed crystal sludge concentration in the first reaction tank and the fluorine concentration of the treated water at this time.

[0049] <Water quality of raw water> F concentration: 200~500mg / L H2SO4: 200 mg / L as SO4

[0050] <Equipment Specifications> First reaction vessel volume: 40 m³ 3 Second reaction vessel volume: 30 m³ 3 Coagulation tank volume: 10m 3 Sedimentation tank volume: 44 m³ 3 (Water area 8m 2 )

[0051] <Processing conditions> Raw water flow rate: 14~37m 3 / Hr Reactor No. 1 pH: 6~8 Reactor No. 2 pH: 6-8 pH adjusters: NaOH aqueous solution (48% by mass), hydrochloric acid (35% by mass) CaCl2 added: 2800 mg / L (1011 mg / L as Ca) PAC addition amount: 200 mg / L (10.6 mg / L as Al) Water area load: 12m / hr or more

[0052] <Consideration> As shown in Figure 2, by setting the seed crystal sludge concentration in the first reaction tank to 1% by mass or more, the fluorine concentration in the treated water remains stable at less than 8 mg / L, and in particular, by setting it to 5% by mass or more, the fluorine concentration in the treated water remains stable at less than 6 mg / L. [Explanation of Symbols]

[0053] 1. First reaction vessel 2. Second Reactor 3 Coagulation tank 4. Sedimentation tank 6. Chemical reaction vessel

Claims

1. A first reaction step involves receiving fluorine-containing wastewater and calcium compound-containing sludge and reacting the fluorine and calcium compound at a pH of 7 to 11. A second reaction step involves adding an aluminum-based coagulant to the first reaction solution from the first reaction step and reacting it at a pH of 6 to 8. A flocculation step is performed by adding a polymer flocculant to the second reaction solution from the second reaction step and carrying out flocculation. A sedimentation step is performed to separate the solid and liquid from the coagulation treatment liquid obtained from the coagulation step, A return step in which a portion of the sludge separated in the sedimentation step is returned to the first reaction step. It has, A method for treating fluorine-containing water, comprising adding a calcium compound to at least one of the first reaction step and the return step, In the second reaction step, the aluminum-based coagulant is added as aluminum at a concentration of 7 to 40 mg / L, and A method for treating fluorine-containing water, characterized in that the seed crystal concentration in the first reaction vessel, as defined by the following formula (1), is 1 to 10%. First reaction vessel seed crystal concentration (%) = [SS ｒ ×Q ｒ / (Q ｆ +Q ｒ ) ] ... (1) SS ｒ Return sludge concentration (mass%) Q ｆ Raw water flow rate (m³) ３ / h) Q ｒ : Return sludge flow rate (m ３ / h)

2. The method for treating fluorine-containing water according to claim 1, wherein the amount of aluminum added to the aluminum-based coagulant is 10 to 30 mg / L.

3. A method for treating fluorine-containing water according to claim 1 or 2, wherein a calcium compound and an aluminum-based coagulant are added such that the ratio Al / Ca of the amount of aluminum-based coagulant added in terms of aluminum to the amount of calcium-based coagulant added in terms of calcium is 1 / 5 or less.

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