JP2912237B2 - Treatment method for fluorine-containing wastewater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for treating a fluorine-containing wastewater, and more particularly, to a coagulation sedimentation method using a calcium compound as a primary treatment and then a coagulation sedimentation method using an aluminum compound as a secondary treatment. The present invention relates to a method for treating fluorine-containing wastewater.

[0002]

2. Description of the Related Art Fluorine-containing effluent includes effluent from a flue gas desulfurization apparatus for treating glass etching process water or coal combustion exhaust gas with an absorbent containing a calcium compound or the like.

As a method of removing fluorine from such a fluorine-containing wastewater, a calcium compound is added as a primary treatment and precipitated and removed as calcium fluoride (CaF ₂ ), and then an aluminum compound is added and coagulated and precipitated as a secondary treatment. A method (referred to as a two-stage coagulation sedimentation method) is known.

[0004] However, this method requires a relatively large amount of aluminum to be added in the secondary treatment, which is problematic in terms of cost. In particular, when the concentration of fluorine increases, the amount of addition must be significantly increased, and improvement is desired.

[0005] Therefore, there has been proposed a method of dissolving the aluminum of the aluminum-containing sludge which has been conventionally discharged, returning it to the treatment step, and reusing it (Japanese Patent Publication No. 1-1).
No. 07890).

[0006]

However, in this method of reusing aluminum, the coexisting calcium compound forms insoluble calcium aluminate, which is discharged into sludge. Therefore, the dewaterability of the sludge containing calcium aluminate is significantly reduced. In addition, since aluminum is consumed, there is a problem that the concentration of aluminum used circulatingly is reduced and the fluoridation property is reduced.

[0007] In addition, disposal is difficult because sludge generated in the secondary treatment is alkaline sludge.

It is an object of the present invention to solve the problems of the prior art and to provide a method for treating fluorine-containing wastewater which can reduce the amount of aluminum used and can efficiently remove fluorine.

[0009]

According to the method for treating fluorine-containing wastewater of the present invention, a calcium compound is added to wastewater containing fluorine to cause coagulation and sedimentation, followed by solid-liquid separation;
A second step of adding an aluminum compound to the wastewater containing low-concentration fluorine separated from the first step to cause coagulation and sedimentation to perform solid-liquid separation; and a sedimentation sludge containing fluorine and an aluminum compound separated from the second step. After adding a predetermined amount of a calcium compound and a carbonate, sodium hydroxide is added to make the mixture alkaline, a suspension is formed, and this suspension is subjected to coagulation sedimentation, followed by solid-liquid separation. A fourth step of circulating and recycling the aqueous solution containing the water-soluble aluminum separated from the third step in the treatment of fluorine in the second step; and a wet sludge separated from the third step. Is reused as a flocculant in the treatment of fluorine in the first step, and the pH by addition of sodium hydroxide in the third step is set to 10 or more.

The wastewater to be used in the present invention contains fluorine. As such wastewater, for example, glass etching wastewater discharged from a semiconductor manufacturing plant or the like, or flue gas desulfurization wastewater and flue gas denitration discharged from a flue gas desulfurization device that treats coal combustion exhaust gas with an absorbent containing a calcium compound Drainage and the like.

[0011] The primary treatment of fluorine in the first step of the present invention is to treat wastewater containing high-concentration fluorine by a coagulation sedimentation method by adding a calcium compound. In this case, the fluorine precipitates as CaF ₂ . At this time, it is necessary to add a small amount of an aluminum compound as a coagulant. The quality of the treated water separated in solid and liquid in the first step has a fluorine concentration of 20 to 5
Approximately 0 mg / 1, several hundred mg / l of calcium compound
It remains.

The primary treatment water of fluorine separated in the first step is subjected to coagulation precipitation with aluminum and solid-liquid separation as a secondary treatment in a second step. The treatment pH at this time is around pH 7 at which aluminum forms hydroxide precipitates and can adsorb and coprecipitate fluorine, but is effective within a range of about pH 6 to 8.

In the third step, a predetermined amount of a calcium compound is added to the precipitated sludge containing the fluorine and aluminum compounds separated in the second step, and then a carbonate is added. After obtaining a suspension, the suspension is subjected to coagulation precipitation and solid-liquid separation.
At this time, in the secondary treatment in the second step, if the water to be treated already contains a considerable amount of a calcium compound, the addition of the calcium compound in the third step can be reduced or eliminated. The predetermined amount of calcium must be examined each time according to the properties of the water to be treated.

In the fourth step, the aqueous solution containing the water-soluble aluminum separated in the third step is circulated and reused in the secondary treatment of fluorine in the second step.

The wet sludge separated in the third step can be reused as a coagulant in the primary treatment of fluorine in the first step. Therefore, there is no need to perform advanced dewatering of the wet sludge.

That is, in the first step, fluorine in the high-concentration fluorine wastewater is solid-liquid separated as CaF ₂ precipitate. Fluorine remaining in the primary treatment water is adsorbed and co-precipitated on the aluminum hydroxide in the second step, and is separated into solid and liquid as precipitated sludge containing fluorine and an aluminum compound. The fluorine concentration of this secondary water is reduced to 5 mg / l or less and discharged. In the third step, a predetermined amount of a calcium compound is added to the precipitated sludge containing the fluorine and aluminum compounds separated in the second step, and the fluorine is CaF ₂
Precipitated and fixed as In addition, the excess calcium here is made up of calcium carbonate (Ca
After precipitation as CO ₃ ), the pH is adjusted to 10 or more with sodium hydroxide to dissolve the aluminum compound, aggregate and precipitate, and solid-liquid separated to form CaF ₂ and CaC.
It is separated into sludge containing O ₃ and aqueous solution containing water-soluble aluminum. In the fourth step, the separated aqueous solution containing water-soluble aluminum is circulated and reused as a coagulant for the secondary fluorine treatment in the second step.

The aluminum source used in the second step may be a newly added water-soluble aluminum salt or a material eluted from fly ash derived from coal combustion exhaust gas originally present in wastewater. Second
The concentration of aluminum ions in the process may be appropriately selected such that aluminum ions suitable for the required fluorine removal performance coexist at the start of operation because the fluorine removal performance varies depending on the fluorine concentration in the wastewater. Aluminum compounds to be added include aluminum sulfate (sulfuric acid band),
Poly aluminum chloride (PAC), aluminum chloride,
Water-soluble aluminum salts such as aluminum phosphate complexes and aluminum complex salts are exemplified.

The calcium compound used in the third step includes calcium hydroxide (slaked lime), calcium sulfate, calcium carbonate, calcium chloride and the like. Of these, the addition amount of the calcium compound is determined by the insoluble precipitation of aluminum ion and calcium aluminate
(Al (OH) ₄ ) ₂形成, which has the effect of promoting a decrease in the aluminum concentration. Therefore, there is no need to add a large amount, and it can be reduced up to about three times equivalent to the assumption that fluorine in the wastewater forms CaF _2. Is desirable. If the calcium compound already exists sufficiently in the water to be treated in the second step, the addition can be reduced or eliminated. Since the appropriateness of the amount of the calcium compound is significantly different depending on the required concentration of the treated water for fluorine and the properties of the wastewater, it is desirable to study and use an appropriate amount of the calcium compound suitable for the purpose in advance.

In the third step, a carbonate is added in order to prevent calcium not bound to fluorine from forming calcium aluminate. Examples of the carbonate added at this time include sodium bicarbonate and sodium carbonate, and sodium bicarbonate with less fluctuation in pH is more preferable. By adding a carbonate, calcium not bound to fluorine is precipitated and fixed as calcium carbonate (CaCO ₃ ).

Further, according to the present invention, the second
Sodium hydroxide is added to the precipitated sludge containing the fluorine and aluminum compound separated in the step to dissolve and extract aluminum (third step), and as aluminum ions to be re-existed in the secondary treatment of fluorine in the second step By circulating and reusing (the fourth step), consumption of the aluminum compound is suppressed as much as possible. At the same time, the wet sludge separated from the third step is used as a flocculant in the first step of the fluorine treatment as a flocculant, thereby improving the primary treatment of the fluorine and generating sludge in the second treatment of the fluorine after the second step. Can be eliminated.

That is, in the third step, sodium hydroxide is added to the precipitated sludge containing the fluorine and aluminum compounds separated in the second step to make the pH more than 10 and the aluminum is dissolved as an aqueous solution of sodium aluminate. Obtain a suspension. The sodium hydroxide used here may be a solid substance or an aqueous solution. The suspension thus obtained is aggregated and precipitated,
Solid-liquid separation. In this step, a general one such as a sedimentation tank or a filter press can be used.

The aqueous solution containing water-soluble aluminum separated in the third step is circulated and reused in the second fluorine treatment in the second step (fourth step). The circulation amount of the water-soluble aluminum at this time can be appropriately adjusted by calculating from the water content of the sludge separated in the second step. The charging location is not particularly limited as long as it is a location effective for removing fluorine in the second step. When the fluorine removal performance is low or when it is necessary to further reduce the fluorine concentration of the discharged water, two or more reaction vessels are provided in the second step, and the aqueous solution containing the water-soluble aluminum is supplied to each reaction vessel. It may be returned to. Further, if necessary, a water-soluble aluminum compound may be appropriately added to each reaction tank.

In the third step, the solid-liquid separated CaF ₂ or Ca
Since the wet sludge containing CO ₃ contains water-soluble aluminum, it is expected to have a flocculant effect of the fluorine treatment, and can be reused in the primary fluorine treatment in the first step. In addition, CaF
_Since the precipitates of ₂ and CaCO ₃ are collectively treated as sludge in the primary treatment step, there is an advantage that sludge is not generated in the fluorine secondary treatment in the second and subsequent steps.

In the present invention, the first, second, and third
In the process, a polymer flocculant or the like can be separately used as a flocculant, or sulfuric acid, hydrochloric acid, or the like can be used for pH adjustment. Further, the addition of the calcium compound in the third step is not limited to one time, and may be performed in plural times as necessary. In some cases, fluorine can be more efficiently removed by multiple additions.

[0025]

Next, the present invention will be described with reference to the drawings. FIG. 1 and FIG. 2 are diagrams showing a flow of a first step and second to fourth steps for explaining an embodiment of the present invention.

First, as shown in FIG. 1, waste water (the raw water here is glass etching waste water having a pH of 2 and a fluorine concentration of 50%).
0 mg / l) was introduced into the first reaction tank 1 and calcium hydroxide {Ca (OH) ₂ } was added thereto to adjust the pH to 12 and the first flocculation tank 2
To be introduced. Here, after adding PAC as a coagulant, it is sent to the first settling tank 3 to settle and separate into solid and liquid. The separated settled sludge is a fluorine-containing sludge containing CaF ₂ as a main component, and is discarded. The primary treated water A separated in the first step has a fluorine content of about 20 to 50 mg / l and a calcium number of 10
It contains 0 mg / l. This primary treatment water A is sent to the second step (secondary treatment step) shown in FIG.

Primary treated water A separated through the first step
(PH 12, fluorine 20 to 50 mg / l, Ca number 100
mg / l) was neutralized to pH 7 using sulfuric acid in the second reaction tank 4 and the Al concentration at the start of operation was 500 mg / l.
Then, a sulfuric acid band was added so that
After the addition of the polymer flocculant, the sedimentation and solid-liquid separation are performed in the second sedimentation tank 6 (second step). Since the separated secondary treated water B has been treated to a fluorine concentration of 5 mg / l or less, it is discharged out of the system.

On the other hand, the sludge containing fluorine and aluminum separated in the second step is drawn into the sludge thickening tank 7. Here, the moisture content is measured by the moisture content measuring device 8,
An appropriate amount of sludge (98% water content) is sent to the next Ca addition tank 10 by the sludge charging device 9. In the Ca addition tank 10, a calcium sulfate (CaSO ₄ .2H ₂ O) solution is appropriately added, and the fluorine and fluorine in the aluminum-containing sludge are solidified as CaF ₂ . Thereafter, it sent to sodium bicarbonate addition tank 11, CaCO ₃ excess Ca
And immobilize the precipitate. Thereafter, the solution is sent to the Al dissolving tank 12 and NaOH is added to adjust the pH to 12.5 to completely dissolve the aluminum. Al in the sludge dissolves at a pH of 10 or more, but for complete dissolution, the pH is preferably 12 or more. However, since it is not necessary to dissolve all the Al in the sludge, the pH may be 11 to 12. Next, this suspension is sent to the third coagulation tank 13, and after adding the polymer coagulant,
It is sent to the third settling tank 14 where it is settled and solid-liquid separated (above third step).

The aqueous solution containing water-soluble aluminum (about 300 mg / l as Al) separated in the third step is directly circulated and reused in the second reaction tank 4 (fourth step). Here, in order to adjust the pH (12.5) of the aqueous solution containing the water-soluble aluminum to pH 7, the second reaction tank 4 contains 20% H.
₂ SO ₄ is added as appropriate. The solution containing the water-soluble aluminum is adjusted to pH 7 so that Al (OH) ₃
Precipitates and is used for fluorine adsorption. After the sulfuric acid band is added to the second reaction tank 4 at the start of the operation, it is desirable that the operation is performed with the aluminum concentration in the second reaction tank 4 being constant. The additional aluminum was operated at 4% (20 mg / l) of the aluminum concentration in the second reactor 4,
It is desirable to increase or decrease as appropriate in view of the processability.

CaF ₂ and CaC separated in the third step
Moisture sludge containing O ₃ as a main component (99% water content)
The aluminum compound contained in the first coagulation tank 2 of the first step (primary treatment) shown in FIG. 1 exhibits a coagulant effect.

FIG. 3 shows the transition of the concentration of fluorine in the actual secondary treated water B thus treated.

In the first operation period O in FIG. 3, additional aluminum was deleted to start operation. After the second operation period P, additional aluminum was added. The operation of adding calcium sulfate in the Ca addition tank 10 was performed after the third operation period Q. On the other hand, in periods O to Q, wastewater containing no calcium in the water to be treated was used, but in the fourth operation period R, the water to be treated having a calcium concentration of 500 mg / l, and in the fifth operation period S, the water to be treated having a calcium concentration of 100 mg / l was used. Using.

Secondary treated water B discharged through the above steps
The fluorine concentration of 3 to 10 mg /
1 and a good processing effect was obtained.

[0034]

According to the method of the present invention, in the second treatment of fluorine (second step), the aluminum compound is recovered and used, so that the amount of aluminum used is greatly reduced and the fluorine in the wastewater is further reduced. Removal can be performed efficiently.

That is, when the fluorine concentration in the primary treated water (the treated water separated from the first step) is particularly high or when no aluminum compound is contained, the aluminum compound is added in the second step, or the primary treatment is performed. When water contains an aluminum compound, the shortage of aluminum is added in the second step, and then only a small amount of the aluminum compound is continuously added to the second step to reduce fluorine in the wastewater. It can be effectively removed.
Therefore, according to the present invention, the amount of the aluminum compound used can be significantly reduced, and the fluorine can be removed more effectively than the conventional method.

The wet sludge mainly composed of CaF ₂ and CaCO ₃ separated from the third step is reused as a coagulant in the first step, thereby increasing the fluoridation property of the primary treatment and increasing the fluoridation property. Sludge generation from the secondary treatment can be eliminated, aluminum can be completely recycled, and the amount of the aluminum compound used can be significantly reduced.

On the other hand, when the wastewater is not subjected to the first step (coagulation precipitation method using a calcium salt as a primary treatment of fluorine) and does not contain a calcium compound, the calcium compound is added in the second step. When the calcium compound is already sufficiently contained, the addition of the calcium compound is reduced or eliminated, and then the fluorine is added to CaF.
₂ and further add carbonate to remove excess calcium
By precipitating and fixing as O ₃ , precipitation of calcium aluminate in the sludge can be prevented. Therefore, aluminum can be efficiently recycled.

[Brief description of the drawings]

FIG. 1 is a flowchart of a first step for describing an embodiment of the present invention.

FIG. 2 shows second to fourth embodiments for describing the embodiment of the present invention.
It is a flowchart of a process.

FIG. 3 is a diagram showing a transition of a fluorine concentration in secondary treatment water treated in the embodiment of the present invention.

[Description of Signs] 1 First reaction tank 2 First coagulation tank 3 First sedimentation tank 4 Second reaction tank 5 Second coagulation tank 6 Second sedimentation tank 7 Sludge concentration tank 8 Water content measuring device 9 Sludge input device 10 Ca Addition tank 11 Sodium bicarbonate addition tank 12 Al dissolution tank 13 Third coagulation tank 14 Third settling tank

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yuichi Asai 1933-10 Shimonumabe, Nakahara-ku, Kawasaki-shi, Kanagawa Japan Electric Engineering Co., Ltd. In-house (56) References JP 7-36911 (JP, B2) JP JP 8-11232 (JP, B2) (58) Field surveyed (Int. Cl. ⁶ , DB name) C02F 1/58

Claims (3)

(57) [Claims] A first step of adding a calcium compound to waste water containing fluorine to cause coagulation and sedimentation, followed by solid-liquid separation;
A second step of adding an aluminum compound to the wastewater containing low-concentration fluorine separated from the first step to cause coagulation and sedimentation to perform solid-liquid separation; and a sedimentation sludge containing fluorine and an aluminum compound separated from the second step. After adding a predetermined amount of a calcium compound and a carbonate, sodium hydroxide is added to make the mixture alkaline, a suspension is formed, and the suspension is subjected to coagulation and sedimentation, followed by solid-liquid separation. A fourth step of circulating and recycling the aqueous solution containing water-soluble aluminum separated from the three steps in the fluorine treatment of the second step. 2. The method for treating fluorine-containing wastewater according to claim 1, wherein the wet sludge separated from the third step is reused as a coagulant in the treatment of fluorine in the first step. 3. The method for treating a fluorine-containing wastewater according to claim 1, wherein the pH for coagulation and sedimentation in the third step is set to 10 or more.