JP3319053B2 - Treatment method for fluoride-containing water - Google Patents

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 fluoride-containing water, and more particularly to a method for treating fluoride-containing water such as flue gas desulfurization effluent by coagulation treatment, followed by membrane separation treatment, and further using a fluoride ion adsorbent. In the method of treating fluoride-containing water for adsorption treatment,
The present invention relates to a method for preventing a decrease in membrane performance of a separation membrane and effectively utilizing a regenerated effluent of a fluoride ion adsorbent.

[0002]

2. Description of the Related Art In a thermal power plant using fossil fuel, desulfurization treatment is performed in order to take measures against SO _{3 in} flue gas. The wastewater obtained by this desulfurization treatment cannot be discharged as it is because it contains a large amount of heavy metals, fluorine and COD components contained in the fuel.

For this reason, conventionally, the wastewater from the flue gas desulfurization unit is
After the two-stage coagulation sedimentation treatment, the mixture is filtered and then discharged. In this two-stage flocculation treatment method, most of heavy metals and fluorine are precipitated and removed as metal hydroxide or calcium fluoride in the first flocculation and precipitation process, and heavy metals and fluorine are removed in the second flocculation and precipitation process. The remaining part is treated by coprecipitation with magnesium hydroxide under strong alkalinity. However, in this treatment method, two sedimentation tanks requiring a large installation area are required, and the equipment installation area is large. In addition, there is a disadvantage that the control of the coagulant injection is complicated.

[0004] As a solution to such a drawback, concentration and removal of SS content by membrane treatment have been studied. For example, a calcium compound and / or an aluminum compound are added to the fluoride-containing water to adjust the pH to 6 to 8, and at least a part of the concentrated liquid discharged from the subsequent membrane separation step is added and stirred to obtain A reaction step of introducing the suspension into the circulation tank, a membrane separation step of subjecting the liquid introduced from the circulation tank to membrane separation and separation into a permeate and a concentrate, and a process of the concentrate discharged from the membrane separation step. The applicant of the present invention has proposed a method for treating fluoride-containing water, comprising a circulation step of circulating at least a part to the reaction step and circulating the remainder to the circulation tank. -25850
No. 8).

In the method utilizing such a membrane treatment, the removal effect by the conventional two-stage coagulation sedimentation treatment cannot be obtained, so that it is difficult to reduce the fluorine ion concentration in the treated water to the discharge regulation value or less. . For this reason, after the membrane treatment, treatment by a fluorine adsorption tower that selectively adsorbs fluorine ions is required.

In the method utilizing the membrane treatment, the suspension is concentrated and separated by a separation membrane. Therefore, after a long-term operation, the silica-based scale adheres to the membrane surface to reduce the membrane processing capacity. A decrease is observed. For this reason, membrane cleaning is required to maintain the membrane performance.

Conventionally, in a general film processing apparatus, a cleaning operation of a film is performed by the following method. The treated water of the membrane treatment device flows in the direction opposite to the membrane filtration direction. Immerse and wash in a concentrated acid solution. Immerse and wash with a concentrated alkaline solution. Immerse and wash with a surfactant. Washing treatment with an oxidizing agent. Washing treatment is performed by combining any two or more of the above washing methods.

The washing operation is carried out by selecting an appropriate method from the above methods according to the deposits on the membrane surface. In the treatment of the integrated wastewater of the power plant, oxides of heavy metals, silica, Organic substances are often attached to the film surface, and generally, a cleaning treatment with an acid and / or alkali is performed.

[0009]

However, in a membrane treatment apparatus incorporated in a thermal power plant integrated wastewater treatment,
When the film is washed with an acid and / or an alkali, actually, due to restrictions on the washing temperature, immersion time, etc., deposits remain on the film surface even after the washing treatment, and it is not possible to obtain a sufficient washing effect. There's a problem.

[0010] The present invention solves the above-mentioned conventional problems, and can efficiently carry out membrane cleaning of a membrane treatment apparatus incorporated in the treatment of fluoride-containing wastewater such as thermal power plant integrated wastewater. An object of the present invention is to provide a method for treating water.

[0011]

According to the method for treating fluoride-containing water of the present invention, a calcium compound and / or an aluminum compound is added to the fluoride-containing water, the pH is adjusted to 6 to 8, and the obtained suspension is treated. The method for treating fluoride-containing water, wherein the solution is separated into a permeate and a concentrate by membrane-separating the solution with a separation membrane, and the permeate is contacted with a fluoride ion adsorbent to remove fluoride ions. The method is characterized in that the adsorbent is regenerated by contacting with an alkali, an acid is added to the regenerated effluent to generate hydrofluoric acid, and then used for washing the separation membrane.

[0012]

[Function] Fluoric acid derived from fluorine contained in the regenerated effluent of a fluoride ion adsorbent is a scale mainly composed of silica, which is a film surface deposit that is difficult to remove by washing with ordinary acid or alkali. Dissolve and remove efficiently.

Incidentally, the composition of the effluent discharged by regeneration of the fluorine adsorption tower filled with the fluoride ion adsorbent is shown in FIG.
And the fluorine concentration reaches up to about 4 g / l. Therefore, by separating the effluent having a high fluorine concentration and adding an acid to generate hydrofluoric acid, efficient membrane cleaning can be performed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method for treating fluoride-containing water according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a system diagram showing one embodiment of the method for treating fluoride-containing water of the present invention.

In the illustrated method, first, fluoride-containing water, which is raw water, is supplied from a pipe 11 to a reaction tank 1 provided with a stirrer 1A, and a calcium compound and / or an aluminum compound, and if necessary, from a pipe 12. The pH is adjusted to 6 to 8 by adding a pH adjuster, and at least a part of the concentrate of the membrane separation treatment returned from the subsequent membrane separation device 3 via the pipes 15 and 16 is added and stirred.

Examples of the calcium compound added to the reaction tank 1 include calcium hydroxide (Ca (OH) ₂ ) and calcium oxide (CaO). As the aluminum compound, a sulfuric acid band (Al ₂ (SO ₄ ) ₃ ),
Polyaluminum chloride (PAC) and the like. These may be used alone or in combination of two or more.

In the reaction step, the pH of the reaction solution is adjusted to 6 to
In order to adjust the pH to 8, preferably 6 to 7, a pH adjuster is added as required. In other words, when Ca (OH) ₂ or the like is added, Ca (OH) ₂ itself acts as a pH adjuster, so there is no need to use a separate pH adjuster. An alkali such as NaOH or Ca (OH) ₂ is added as a pH adjuster. In the present invention, the problem of scale hindrance has been solved, and there is no risk of scale hindrance unlike the conventional sulfate band method. Therefore, it is advantageous to use Ca (OH) ₂ as a pH adjuster.

By adjusting the pH to 6 to 8 by adding a calcium compound and / or an aluminum compound,
Fluoride ions in the raw water are solidified. That is, the fluoride ion is precipitated as CaF ₂ by the calcium compound, and the fluoride ion is adsorbed and precipitated by the aluminum compound. At this time, since the concentrated solution of the subsequent membrane separation device 3 is returned to and mixed with the reaction tank 1, the SS in the concentrated solution acts as a seed crystal to prevent scale formation.

The calcium compound and / or
Or, the amount of addition of the aluminum compound is usually preferably about 0.5 to 3 equivalents to the fluoride ions in the raw water. Normally, flue gas desulfurization wastewater and the like contain calcium and aluminum compounds, and the above ranges may be included including these. The amount of the concentrate returned from the membrane separation device 3 varies depending on the SS concentration, but is preferably about 0.1 to 4 times the amount of the raw water.

The reaction time in the reaction tank 1 is as follows:
Usually, it is 30 minutes or more. That is, the time required for sufficiently generating a precipitate from the reaction solution in the reaction tank 1, particularly the time required for the gypsum formation reaction, is about 30 minutes. The time is preferably 30 minutes or more.

The suspension obtained in the reaction tank 1 is then introduced into the circulation tank 2 via a pipe 13.

Of the concentrated liquid discharged from the subsequent membrane separation device 3 through the pipe 15 into the circulation tank 2, the remaining part of the concentrated liquid returned to the reaction tank 1 through the pipe 16 is introduced through the pipe 17. I have. The suspension from the reaction tank 1 and the concentrated liquid of the membrane separation device 3 in the circulation tank 2 are introduced into the membrane separation device 3 through the pipe 14 having the pump 31 and are separated by the separation membrane 3A. To process.

It is preferable that the residence time of the liquid in the circulation tank 2 is set to be about 1 to 20 hours. Further, since SS is concentrated and accumulated in the circulation tank 2 with time, this SS is discharged from the lower part of the tank through the pipe 18 provided with the pump 32 as needed.

As the separation membrane 3A of the membrane separation device 3, it is preferable to use a microfiltration (MF) membrane or an ultrafiltration (UF) membrane. When the MF film is used, if the pore size is large and 1 μm or more, the precipitate containing fluoride may permeate. Therefore, it is preferable to use an MF film having a pore size of 0.5 μm or less. In such a membrane separation device 3, unreacted calcium compounds and aluminum compounds in the liquid remain in the concentrated water without being permeated through the membrane and are returned to the reaction tank 1. This is extremely advantageous because it exerts a function of immobilizing fluoride ions.

The reverse osmosis (RO) membrane is made of Ca ²⁺ , F ⁻ ,
It has the ability to remove SO ₄ ²⁻ and generates scale due to concentration polarization on the film surface, which is not preferred in the present invention.

As described above, a part or all of the concentrated liquid in the membrane separation device 3 is returned to the reaction tank 1 through the pipes 15 and 16, and the remainder is circulated to the circulation tank 2 through the pipe 17. .

On the other hand, the permeate of the membrane separation device 3 is
The mixture is fed to a pH adjusting tank 4 having a stirrer 4A, and if necessary, a pH adjusting agent such as NaOH or HCl is added from a pipe 20 to adjust the pH to about 2 to 9, and then a pump 33
Is introduced into the COD adsorption tower 5 through a pipe 21 equipped with
The D component is adsorbed and removed.

The effluent of the COD adsorption tower 5 is supplied to a pipe 22
The mixture is fed to a pH adjusting tank 6 equipped with a stirrer 6A, and if necessary, a pH adjusting agent such as NaOH or HCl is added from a pipe 23 to adjust the pH to about 3 to 7, and then a pump 34
Is introduced into the fluorine adsorption tower 7 through the pipe 24 provided with the above, and is subjected to the adsorption treatment, and the fluoride ions still remaining are adsorbed and removed. The effluent from the fluorine adsorption tower 7 is discharged out of the system through a pipe 25 as treated water.

The fluoride ion adsorbents include tritium, zirconium, titanium or hafnium type cation exchange resins, strong and weakly acidic cation exchange resins, haloalkylsilane-based adsorption resins, weakly basic anion exchange resins, and rare earth metal hydrates. Adsorption resins such as oxide-type chelate resins and Al-salt-type chelate resins, as well as other activated aluminas and magnesia-type adsorbents are exemplified. Examples of the COD adsorbent include a COD adsorbing resin such as a gel type or MR type weak, medium, and strong basic anion exchange resin, and other activated carbon.

In the present invention, in such treatment, the fluoride ion adsorbent in the fluorine adsorption tower is replaced with NaOH.
Among the regenerated effluent obtained when the regenerating process is performed by bonding with an alkali such as, for example, a stream having a high fluorine concentration is collected in the storage tank 8 from the pipe 26. When the performance of the separation membrane 3A of the membrane separation device 3 decreases, the operation of the processing system is stopped, and the regenerated wastewater in the storage tank 8 and the acid solution in the storage tank 9 are supplied to a pipe 26 having a pump 35, respectively. Then, the mixture is mixed from a pipe 27 provided with a pump 36 and introduced into the membrane separation device 3 through a pipe 28, and immersion cleaning is performed for a predetermined time. Thereafter, after washing with water, the alkaline solution in the storage tank 10 is introduced into the membrane separation device through the pipes 29 and 28 provided with the pump 37, immersed and washed for a predetermined time, and then washed with water.

By such a film cleaning treatment, the deposits on the film surface are efficiently cleaned and removed, and the film performance is restored. The cleaning effluent obtained by this membrane cleaning is introduced into a reaction tank via a pipe 30, and is treated together with raw water to remove silica and fluorine contained therein.

The regenerated effluent used for such membrane cleaning preferably contains fluorine at a high concentration of 500 mg / l or more. Also, the proportion of acid added to such reclaimed effluent is
It depends on the pH of the regenerated effluent, and is usually added at such a rate that the regenerated effluent is acidified to a pH of about 3.5 or less.

In the present invention, the fluoride-containing water of the raw water is not particularly limited. For example, flue gas desulfurization and / or denitrification wastewater, aluminum electrolytic refining process wastewater, phosphate fertilizer production process wastewater, and electrical components such as silicon The present invention is particularly suitable for comprehensive wastewater treatment for thermal power plants.

The example shown in FIG. 1 is an embodiment of the present invention, and the present invention is not limited to the illustrated method. In the method of the present embodiment, the permeate of the membrane separation device is adsorbed on COD, but if the permeate has sufficiently high water quality, the COD adsorption process may not be performed.
In addition, in the adsorption treatment, it goes without saying that the adsorption tower may be treated in an upward flow or in a downward flow, but generally, fluoride ion adsorption and COD adsorption are performed in a downward flow. . Furthermore, although the concentrated water of the membrane separation device does not necessarily need to be returned to the reaction tank and the circulation tank, the return of the concentrated liquid is advantageous in that scale formation can be prevented as described above.

Specific examples will be described below.

Example 1 The treatment was performed by the method shown in FIG. 1 using the actual flue gas desulfurization wastewater of a thermal power plant. As the membrane separation apparatus, an apparatus provided with a separation membrane having a pore diameter of 0.5 μm and an area of 0.2 m ² was used.

First, as a result of performing a water-passing treatment for about one month using a new membrane, the flux (permeation flux) showing the performance of the membrane is reduced from the initial 12 m / day as shown by the square in FIG. It decreased to 8 m / day.

Then, 5 liters of a 4% by weight aqueous solution of HCl was introduced into the membrane separator, left standing for 5 hours, washed with water, and then 5 liters of a 2% by weight aqueous solution of NaOH was introduced into the membrane separator, followed by 4 hours. After standing, it was washed with water (washing treatment I). After this cleaning treatment I, the second water passage was performed. As shown by + in FIG. 3, the flux was initially 9.2 m.
/ Day, it was 5.9 m / day one month later, and the performance was 25% lower than that of the new film.

After passing the water, 5 liters of a 4% by weight aqueous solution of HCl was introduced into the membrane separator, left standing for 5 hours, washed with water, and then 5 liters of a 2% by weight aqueous solution of NaOH heated to 50 ° C. After being introduced into the separation device and allowed to stand for 4 hours, it was washed with water (washing treatment II). After this cleaning treatment II, the third
When the water was passed for the first time, the flux was initially 8.0 m / day, 5.2 months after one month, as shown by ◇ in FIG.
m / day, and a decrease in performance could not be prevented.

At this time, a part of the deposit on the film surface was sampled and analyzed for composition. As a result, it was found that the main components of the deposit scale were silica and alumina.

Therefore, a separately prepared liquid (pH 2.8) obtained by mixing 3 liters of the simulated regenerated effluent of the fluorine adsorption tower having the following properties and 0.5 liter of an 8% by weight HCl aqueous solution was introduced into the membrane separation apparatus. After standing still for 5 hours, wash with water,
Five liters of a 2% by weight NaOH aqueous solution was introduced into the membrane separation device, allowed to stand for 4 hours, and then washed with water (washing treatment III).

Simulated regenerated effluent (property) Fluorine concentration: 3300 mg / l SO ₄ ^2- concentration: 300 mg / l PO ₄ ^3- -P concentration: 100 mg / l pH: 13.1 The fourth time after this washing treatment III As a result, the flux was initially 9.2 m /
From day, it became 8 m / day one month later, and almost the same performance as the new film was obtained.

[0044]

As described above in detail, according to the method for treating fluoride-containing water of the present invention, the membrane treatment incorporated in the treatment containing fluoride by utilizing the regenerated effluent of the fluoride ion adsorbent. It is possible to efficiently remove and remove the scale mainly composed of silica, which is attached to the separation membrane of the apparatus and is difficult to remove by the conventional membrane cleaning treatment. For this reason, according to the present invention, it is possible to prevent the deterioration of the membrane performance and to effectively use the regenerated effluent, which is industrially extremely advantageous.

[Brief description of the drawings]

FIG. 1 is a system diagram showing one embodiment of a method for treating fluoride-containing water of the present invention.

FIG. 2 is a graph showing properties of an effluent of a regenerated effluent of a fluorine adsorption tower.

FIG. 3 is a graph showing a temporal change of a flux at the time of each water flow test in Example 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reaction tank 2 Circulation tank 3 Membrane separation device 4,6 pH adjustment tank 5 COD adsorption tower 7 Fluorine adsorption tower 8,9,10 Storage tank

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C02F 9/00 503 C02F 9/00 503G (56) References JP-A-5-253576 (JP, A) JP-A-4-371292 (JP, A) JP-A-3-118897 (JP, A) JP-A-61-192385 (JP, A) JP-A-57-84704 (JP, A) JP-A-62-298490 (JP, A) ( 58) Field surveyed (Int.Cl. ⁷ , DB name) C02F 1/58 C02F 1/28 C02F 1/44 B01D 65/00

Claims (1)

(57) [Claims] 1. A calcium compound and / or an aluminum compound are added to fluoride-containing water to adjust the pH to 6 to 8, and the obtained suspension is subjected to membrane separation with a separation membrane to form a permeate and a concentrate. In the method for treating fluoride-containing water for separating and contacting the permeate with a fluoride ion adsorbent to remove fluoride ions, the fluoride ion adsorbent is regenerated by contacting it with an alkali, A method for treating fluoride-containing water, comprising using an acid to generate hydrofluoric acid and then washing the separation membrane.