JP2564252B2 - Fluorine-containing wastewater treatment method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for treating fluorine-containing wastewater, and more particularly to a method for treating wastewater containing fluorine and coexisting with boron.

[Problems to be Solved by Prior Art and Invention] Examples of the fluorine-containing wastewater include wastewater discharged from a flue gas desulfurization apparatus that treats coal combustion exhaust gas with an absorbent containing a calcium compound or the like.

As a method for removing fluorine from such fluorine-containing wastewater, a method is known in which a calcium compound is added and precipitated as calcium fluoride (CaF ₂ ) or the like.

However, if boron is also contained in the wastewater, it reacts with fluorine to generate borofluorides such as fluoroboric acid, so that it is difficult to remove them by a normal method.

Therefore, in order to solve this problem, a method of adding an aluminum compound in the treatment of fluorine-containing wastewater (Gypsum & Lime, No.133, 1974), a Mg coprecipitation treatment method (Japanese Patent Publication No. 58-13230, etc.) , A method of using an ion exchange resin (Japanese Patent Publication No. 53-43745) has been proposed.

However, the Mg coprecipitation treatment method has a problem that the removal performance of fluorine is not sufficient, and particularly when boron is contained, the removal performance is significantly lowered.

In addition, using the ion exchange resin requires high equipment cost,
Moreover, since there is a regeneration operation, it is complicated and the drivability is low,
Furthermore, there is a drawback that the operating cost is high because the reclaimed water needs to be treated separately.

Therefore, at present, the method of adding an aluminum compound is often adopted.

However, this method requires a relatively large amount of aluminum to be added, which is problematic in terms of cost. In particular, when the concentration of fluorine or boron becomes high, the addition amount must be remarkably increased, and improvement thereof has been desired.

Therefore, a method has been proposed in which part of the aluminum-containing sludge that has been conventionally discharged is returned to the treatment process as it is and reused (Japanese Patent Publication No. 59-43237).

However, in this method, since heavy metals and fly ash that must be discharged to the outside of the system are contained in the aluminum-containing sludge, only about 10 to 20% can be reused, so the amount of aluminum compound added is large. It was difficult to reduce the width. Also, 20%
When the sludge is returned to the above treatment process, fluorine is similarly contained in the sludge, so that the entire fluorine removing effect is hardly improved.

Further, with the addition of a large amount of new aluminum compound, the amount of sludge increases, and the treatment of the sludge becomes a big problem. It is expected that the environmental regulations for sludge dumping will be strengthened in the future, and this solution is strongly required.

It is an object of the present invention to provide a method for treating fluorine-containing wastewater in which the above-mentioned conventional problems have been solved.

[Means for Solving Problems] That is, according to the present invention, calcium compounds are added to wastewater containing fluorine and boron in the presence of aluminum ions to generate precipitated sludge containing fluorine and aluminum compounds to remove fluorine. Then, when treating the wastewater, sodium hydroxide was added to the precipitated sludge containing fluorine and aluminum compounds to obtain a suspension having a pH of 7 or more, and the suspension was subjected to solid-liquid separation to obtain the water-soluble solution. The present invention provides a method for treating fluorine-containing wastewater, which comprises using an aqueous solution containing aluminum for removing fluorine.

The wastewater targeted by the present invention contains fluorine and also contains boron. As such wastewater, for example, flue gas desulfurization wastewater discharged from a flue gas desulfurization apparatus that treats coal combustion exhaust gas with an absorbent containing an appropriate absorbent, for example, a calcium compound, flue gas denitration wastewater, and from semiconductor manufacturing processes. Examples include drainage.

In the method of the present invention, to the wastewater containing fluorine and boron as described above, in the presence of aluminum ions, a calcium compound is added to generate precipitated sludge containing fluorine and an aluminum compound to remove fluorine, and the wastewater is treated. The aluminum compound in the precipitated sludge is dissolved and recovered with sodium hydroxide and reused.

Fluorine in the waste water is precipitated and removed as calcium fluoride (CaF ₂ ) by adding a calcium compound, and fluorine existing as a borofluoride such as fluoroboric acid is decomposed by the presence of aluminum ion by the reaction of fluorine and boron. It is removed by sedimentation and aggregation.

The aluminum ions to coexist here may be those eluted from fly ash or the like originally existing in the waste water, or may be newly added water-soluble aluminum compounds. The concentration of aluminum ions to be coexistent depends on the concentration of fluorine or boron in the waste water, and thus the aluminum ion concentration may be appropriately selected so that the aluminum ions suitable for the required fluorine removal performance will coexist at the start of operation. Examples of the aluminum compound to be added include water-soluble aluminum salts such as aluminum sulfate (van sulfate), polyaluminum chloride (PAC), aluminum chloride, aluminum phosphate complex, and aluminum complex salts. Examples of calcium compounds include calcium hydroxide (slaked lime), calcium carbonate and calcium chloride. Of these, slaked lime is particularly preferable in terms of pH increasing effect, dissolution rate and the like. The amount of the calcium compound added is not particularly limited, but the pH of the wastewater is adjusted to 4 or more by adding the calcium compound.

In this way, precipitated sludge containing fluorine and an aluminum compound can be generated to remove fluorine from the wastewater. In the present invention, sodium hydroxide is added to the previously sludge-containing precipitated sludge containing fluorine and aluminum compounds to dissolve and extract aluminum, and aluminum ions are coexisted during wastewater treatment to be used again for fluorine removal. It is possible to suppress the consumption of the compound as much as possible and reduce the amount of sludge discharged.

That is, in the present invention, first, sodium hydroxide is added to the precipitated sludge containing the fluorine and aluminum compounds to obtain a suspension having a pH of 7 or more. Here, the amount of sodium hydroxide added is appropriately selected depending on the amount of aluminum to be dissolved and circulated from the sludge, but the amount is usually 7 or more, preferably 9 or more. Also, when aluminum compounds are contained in the wastewater, the pH for treating the aluminum-containing sludge is adjusted according to the concentration of aluminum contained in the wastewater to dissolve the aluminum.
The concentration to be circulated can be appropriately selected. As a result, the coexisting aluminum concentration can be kept constant and the wastewater can be continuously treated. Further, the sodium hydroxide to be added may be a solid substance or an aqueous solution. Aluminum is dissolved as an aqueous solution of sodium aluminate by adding sodium hydroxide.

Then, the suspension thus obtained is subjected to solid-liquid separation. In this solid-liquid separation step, a general one such as a thickener (concentrator) or a filter press can be used. By this solid-liquid separation, heavy metals such as cadmium and arsenic; solid matters such as fly ash and CaF ₂ are separated and discharged out of the system, and an aqueous solution containing water-soluble aluminum is obtained.

The resulting aqueous solution containing water-soluble aluminum is returned to the system for removing fluorine in the waste water. In this case, part of the water-soluble aluminum may be precipitated depending on the conditions. There are no particular restrictions on the location in the system to be returned as long as it is an effective location for removing fluorine in the wastewater. In addition, when the fluorine removal performance is low or when it is necessary to further reduce the fluorine concentration in the discharged water, an aqueous solution containing water-soluble aluminum obtained by treating two or more reaction tanks and treating the sludge containing an aluminum compound. May be returned to each reaction tank. Furthermore, if necessary, a water-soluble aluminum compound may be appropriately added to each reaction tank.

In the present invention, a water-soluble chelating agent or the like can be used as a heavy metal scavenger, or hydrochloric acid or the like can be used for pH adjustment. Further, the calcium compound may be added not only once but also in plural times.
Fluorine can be removed more efficiently by adding it a plurality of times.

[Examples] Next, examples of the present invention will be described.

Example 1 Waste water (pH 2, fluorine concentration 300 ppm, boron concentration 20 ppm) was treated by flowing at a flow rate of 3 l / Hr according to the flow shown in FIG.

That is, first of all, the aluminum concentration is
After adding vanadium sulfate to 300 ppm, it was introduced into the first reaction tank 1 and slaked lime was added to adjust the pH to 7. Next, the reaction liquid in the first reaction tank 1 is subjected to solid-liquid separation in a first solid-liquid separator (thickener) 2, the supernatant aqueous solution is discharged to the outside of the system, while the aluminum-containing precipitated sludge is discharged to the second stage.
It was introduced into the reaction tank 3. In the second reaction tank 3, a 20% sodium hydroxide aqueous solution was added to adjust the pH to 11, and aluminum was dissolved. Next, the reaction liquid in the second reaction tank 3 is subjected to solid-liquid separation in a second solid-liquid separator (thickener) 4 to separate heavy metals, fly ash, CaF _{2 and} other solids, and to discharge sludge and to dissolve water. An aqueous solution containing 0.3 to 0.6 l / Hr containing 2000 to 3000 ppm of soluble aluminum was returned to the first reaction tank 1 and used for removing fluorine. After van sulphate was added at the start of the operation, when the aluminum concentration in the solution containing water-soluble aluminum became constant, new addition was not performed, and the wastewater was constantly and smoothly treated.

The pH of the discharged water discharged through the above steps was 7, and the fluorine concentration was 13 ppm.

Example 2 Wastewater discharged from a flue gas desulfurization unit for coal combustion boiler exhaust gas (pH 1.2, fluorine concentration 1500 ppm, boron concentration 120 ppm,
Aluminum concentration 100ppm, fly ash 1%) second
In the flow shown in the figure, it was processed by flowing it at a flow rate of 1 / Hr.

That is, first, the above wastewater is introduced into the first reaction tank 1,
Water-soluble chelating agent 20pp as slaked lime and heavy metal collector
m was added. Then, the reaction liquid in the first reaction tank 1 was subjected to solid-liquid separation in the first solid-liquid separator 2, and 40 to 70 g / Hr of aluminum-containing precipitation sludge obtained as a precipitate was introduced into the second reaction tank 3. In the second reaction tank 3, sodium hydroxide was added to adjust the pH to 12, and aluminum was dissolved. The reaction liquid in the second reaction tank 3 is the second solid-liquid separator 4
Solid-liquid separation at heavy metals, fly ash, C
Solid matter such as aF ₂ was separated and discharged as sludge, and an aqueous solution containing water-soluble aluminum was obtained.

On the other hand, the aqueous solution obtained in the first solid-liquid separator 2 is introduced into the third reaction tank 5, hydrochloric acid is added to adjust the pH to 3, and then introduced into the fourth reaction tank 6, and slaked lime is added to adjust the pH. Was adjusted to 7. Here, 20-40 cc / Hr of the aqueous solution containing water-soluble aluminum obtained in the second solid-liquid separator 4 was introduced into the third reaction tank 5 and the fourth reaction tank 6 and used for removing fluorine. Then, it is introduced into the third solid-liquid separator 7 to perform solid-liquid separation, the supernatant aqueous solution is discharged out of the system, while the sludge is returned to the second reaction tank 3, and the sludge from the first solid-liquid separator 2 is discharged. And aluminum dissolution treatment.

The pH of the discharged water discharged through the above steps was 7, and the fluorine concentration was 11 ppm.

Further, when 200 ppm of aluminum sulfate as an aluminum compound was continuously added to the third reaction tank 5 as an aluminum compound, the fluorine concentration of the discharged water was 6 ppm.

Example 3 A dust removal tower liquid (pH 1.5, fluorine concentration 2500 ppm, boron concentration 300 ppm, aluminum concentration 300 ppm, fly ash 2%) discharged from the dust removal tower A by introducing coal boiler exhaust gas into the dust removal tower A is shown in FIG. The flow shown in Fig. 4 was applied at a flow rate of 4 m ³ / Hr for processing. In FIG. 3, reference numeral B is an absorption tower.

That is, first, the above-mentioned dust removal tower liquid is introduced into the first reaction tank 1, and a water-soluble chelating agent as slaked lime and a heavy metal collecting agent is introduced.
The pH was adjusted to 7 by adding 30 ppm. Then, the reaction liquid in the first reaction tank 1 is introduced into the first solid-liquid separator 2 to perform solid-liquid separation, the aqueous solution is discharged to the subsequent treatment step, while the aluminum-containing precipitation sludge is introduced into the second reaction tank 3. did. In this second reaction tank 3, a 20% aqueous sodium hydroxide solution was added to adjust the pH to 12, and aluminum was dissolved. Next, the reaction liquid in the second reaction tank 3 is subjected to solid-liquid separation in the second solid-liquid separator 4 to separate solid substances such as heavy metals, fly ash, CaF _{2 and the} like, and to discharge them as sludge, while at the same time removing water-soluble aluminum. Aqueous solution containing 0.2-0.5 m ³ / Hr
Then, it was returned to the dust removing tower A.

The pH of the discharged water discharged through the above steps was 7, and the fluorine concentration was 200 ppm. Also, the Na concentration of the liquid in the dust removal tower and
The Al concentration increased, the occurrence of scaling and corrosion was suppressed, and the desulfurization rate of the total wastewater removal system was improved. Furthermore, the processing steps in the subsequent stage have been greatly simplified.

[Effect of the Invention] According to the method of the present invention, since the aluminum compound is recovered and used, the amount of the aluminum compound used can be greatly reduced, and the fluorine in the waste water can be efficiently removed.

That is, if the concentration of fluorine or boron in the wastewater is particularly high, or if the wastewater does not contain an aluminum compound, add the aluminum compound initially, or if the wastewater contains an aluminum compound. In this case, by adding a shortage of aluminum at the start, fluorine in the waste water can be effectively removed without continuously adding an aluminum compound thereafter. Therefore, according to the present invention, the amount of the aluminum compound used can be greatly reduced, and the fluorine can be removed more than the conventional method.

Moreover, in the method of the present invention, since the aluminum is recovered and used in circulation, continuous addition of the aluminum compound is not required, so that the sludge amount can be greatly reduced.

[Brief description of drawings]

1 is a flow sheet of Example 1 of the present invention, FIG. 2 is a flow sheet of Example 2 of the present invention, and FIG. 3 is a flow sheet of Example 3 of the present invention. 1 ... First reaction tank, 2 ... First solid-liquid separator, 3 ... Second reaction tank, 4 ... Second solid-liquid separator, 5 ... Third reaction tank, 6 ... Third solid Liquid separator

Claims (4)

(57) [Claims] 1. A wastewater containing fluorine and boron is added with a calcium compound in the presence of aluminum ions to form a precipitated sludge containing fluorine and an aluminum compound to remove the fluorine, and the wastewater is treated with After adding sodium hydroxide to the precipitated sludge containing an aluminum compound to obtain a suspension having a pH of 7 or more,
A method for treating fluorine-containing wastewater, characterized in that the suspension is subjected to solid-liquid separation, and the resulting aqueous solution containing water-soluble aluminum is used for removing fluorine. 2. The method according to claim 1, wherein the waste water containing fluorine and boron is flue gas desulfurization waste water or flue gas denitration waste water. 3. The method according to claim 1 or 2, wherein aluminum in the precipitated sludge containing fluorine and an aluminum compound is eluted from fly ash. 4. The aluminum in the precipitation sludge containing fluorine and an aluminum compound is an aluminum compound added as an agent having an effect of decomposing borofluoride and removing fluorine. The method described.