JPS5943236B2 - Treatment method for flue gas desulfurization wastewater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating flue gas desulfurization wastewater, particularly a method for treating flue gas desulfurization wastewater containing fluoride ions.

Flue gas desulfurization wastewater, especially the waste water discharged from coal-fired flue gas desulfurization equipment, contains harmful components such as fluoride ions, heavy metal ions, and COD, and therefore requires treatment.

Conventional methods for treating flue gas desulfurization wastewater include adding lime to recover gypsum, adding sodium carbonate to remove excess calcium, and then removing COD using activated carbon and ion exchange resin.

However, when attempting to recover purified gypsum using this method, in order to prevent the formation of precipitates of coexisting metal ions,
It is unavoidable to set the conditions to a considerably low pH range, and from the viewpoint of removing fluoride ions, the amount of residual fluoride ions increases, which is undesirable.

Further, this method has the problem that the total amount of sludge generated is large and that it requires labor to dispose of the sludge.

The purpose of the present invention is to eliminate the drawbacks of the conventional methods as described above, and to propose a method for treating flue gas desulfurization wastewater that removes harmful substances at a high removal rate and generates a small amount of sludge.

This invention consists of a first step in which flue gas desulfurization wastewater is adjusted to pH 5 to 8.5 in the presence of aluminum ions to separate precipitates, and an effluent from the first step is adjusted to pH 9 or higher and powdered activated carbon is added. a second step of adding and separating the precipitate; a third step of contacting the effluent from the second step with a basic anion exchange resin; and a fourth step of returning the precipitate in the second step to the first step. This is a processing power method for flue gas desulfurization wastewater, including

This invention will be explained below with reference to the drawings.

The drawing is a system diagram showing an embodiment of the present invention, in which 1 is a first reaction tank, 2 is a precipitation tank, 3 is a second reaction tank, 4 is a precipitation tank,
5 is an ion exchange tank.

In the first step, raw water is introduced into the first reaction tank 1 from the raw water pipe 6, and the pH is adjusted to 5 to 8.5 in the presence of aluminum ions to precipitate most of the fluoride ions and heavy metal ions in the raw water. It is desirable that aluminum ions be present in a weight ratio of 1 or more to the fluoride ions in the raw water. If a considerable amount is already present in the raw water, there is no need to add it externally, but if it is insufficient, chemical injection may be necessary. Add aluminum salt such as aluminum sulfate through tube 7.

The place where the aluminum salt is added is not limited to the first reaction tank 1, and the aluminum salt may be added to the raw water in advance.

If the raw water containing the returned precipitate and a predetermined amount of aluminum ions, which will be described later, has a pH in the range of 5 to 8.5, there is no need to adjust the pH, but if aluminum salt is added to the raw water and the pH falls outside of this range, then A potash agent or mineral acid is injected from the chemical injection tube 8 and adjusted to the above range.

As the alkali agent, monovalent alkali agents such as sodium hydroxide, sodium carbonate, potassium hydroxide, and ammonium hydroxide are preferred.

Calcium compounds such as calcium hydroxide are not preferred because they require a decalcification step by adding carbonate, increasing the number of steps and the amount of sludge.

When the pH is adjusted in the presence of aluminum ions in this manner, a precipitate is generated, and most of the fluoride ions and heavy metal ions are transferred to the precipitate side.

The precipitate is presumed to be Al(OH)3 containing fluoride and heavy metals, but the details are unknown.

The pH range in the first step is a range in which the solubility of these precipitates is low and magnesium hydroxide to be returned in the fourth step described below is dissolved, that is, pH 5 to 8.5, and pH 6 to 7 is particularly preferable.

The reaction liquid in the first reaction tank 1 undergoes solid-liquid separation in the settling tank 2, the precipitate is discharged from the system through the drain pipe 9, and the supernatant liquid is transferred to the second tank.
It is made to flow out into reaction tank 3.

In the second step, an alkaline agent is injected into the effluent from the first step that has entered the second reaction tank 3 through the chemical injection pipe 10 to adjust the pH to 9 or more, and powdered activated carbon is supplied from the feeder 11 to remove the remaining liquid. Performs advanced treatment of fluoride ions and heavy metal ions, and removes COD.

As the alkali agent, the same one as in the first step can be used.

If a considerable amount of fluoride ions remain in the effluent of the first step, the reaction in the second reaction tank is carried out in the presence of magnesium ions and the pH is adjusted to 9.5 or higher.
A precipitate of g(OH)2 is formed, in which fluoride and heavy metals are also entrained and precipitated.

In this case, a magnesium salt such as magnesium chloride is added from the chemical injection tube 12 as necessary.

Magnesium salt is circulated between the first and second steps, so it is only necessary to replenish it to compensate for the decrease in concentration.However, if it is contained in the raw water, it will be concentrated during circulation, so it may not be necessary to add it. There is.

If the amount of magnesium ions present in the reaction solution is at least 20 times the weight of fluoride ions, the amount of residual fluoride ions can be reduced to 1/l or less.

If the amount of fluoride ions in the effluent from the first step is small and does not need to be removed in the second step, it is not necessary to add magnesium salt, and the pH should be adjusted to 9 or higher (particularly 9 to 9.5), and heavy metals should be added. The ions are precipitated as hydroxides.

In this case, the removal efficiency will be improved if iron salts such as ferric chloride and ferric sulfate are added from the chemical injection pipe 13 to form iron hydroxide flocs.

Iron salt may also be added when removing fluoride ions in the second step.

The powdered activated carbon adsorbs and removes other forms of COD that cannot be removed by the ion exchange resin in the third step, and serves as a nucleus for precipitation in the second reaction tank 3, increasing sedimentation.

The particle size of activated carbon should be 40 mesh or less, and the amount added should be COD.
It varies depending on the amount, but it is 50 to 500 m9/l! Use the degree as a guide.

Note that the reactions in the first reaction tank 1 and the second reaction tank 3 are performed with stirring, and preferably a polymer flocculant is added.

The alkaline agent used in the second step is the same as that explained in the first step.

The reaction liquid in the second reaction tank 3 undergoes solid-liquid separation in a precipitation tank 4, and the supernatant liquid is filtered if necessary, and then flows out into an ion exchange tank 5.

In the third step, the effluent from the second step is transferred to an ion exchange tank 5.
In step 1, the sample is brought into contact with a basic anion exchange resin to remove residual COD.

As the basic anion exchange resin, any strong, medium or weakly basic resin can be used, but medium or weakly basic ones are particularly desirable.

Those with a quaternary ammonium group as an active group in the base have strong basicity, and those with an amine group have tertiary,
Basicity decreases in the order of secondary and primary amine groups.

Among these, medium or weakly basic anion exchange resins that mainly contain tertiary amine groups, contain ammonium groups or various amine groups, or have amine groups below tertiary amine are used in the third step. suitable for

In other words, strong basics are sulfate ions that coexist in water,
Since chlorine ions and the like are exchange-adsorbed, it is difficult to selectively adsorb COD, but selective adsorption is possible with medium or weakly basic resins.

These resins may be of gel type), but porous types are preferable, and salt types such as SO4 type or CI type are more preferable than OH type.

Such an anion exchange resin is packed into the ion exchange column 5 and brought into contact with the effluent of the second step in a fixed bed type or fluidized bed type.

The effluent from the second step is alkaline, so if necessary, acid is injected from the chemical injection pipe 14 to adjust the pH to 3 to 8, which is suitable for adsorption of the resin, and the water is passed through the water to remove COD to the resin. Let it absorb.

Since this adsorption is mainly based on ion exchange, it differs from the physical adsorption of activated carbon, in which anionic COD that is not adsorbed by the activated carbon is adsorbed and removed.

The treated water in the ion exchange tank 5 is discharged from the treated water pipe 15 to the outside of the system.

Further, the anion exchange resin is regenerated by a conventional method and used repeatedly.

The fourth step has no temporal relationship with the third step, and the sediment in the settling tank 4 in the second step is transferred from the return pipe 16 to the first reaction tank 1 in the first step or the raw water tank (not shown), etc. It will be sent back to.

Precipitates include powdered activated carbon, heavy metal salts, fluorides, etc.

When returning these to the first step, if the raw water in the first step is acidic, the precipitate can be directly mixed with the raw water to dissolve soluble components such as metal salts, but if the raw water is alkaline or neutral In the case of acid, it is desirable to dissolve it with acid in the dissolution tank 17 and then return it.

Note that in the second step, heavy metals such as manganese and iron are also precipitated, so when returning the precipitate, it is desirable to carry out aeration to ensure that these are removed in the first step.

Among the precipitates returned in the fourth step, the powdered activated carbon still has sufficient COD adsorption capacity, so it adsorbs COD in the raw water in the first reaction tank 1.

On the other hand, the soluble components are almost completely dissolved in the first reaction tank 1, and the released fluoride ions and heavy metal ions, together with those in the raw water, undergo the treatment in the first step described above, become precipitates, and form a system together with powdered activated carbon. It is discharged outside.

The dissolved magnesium ions flow directly to the second step and are recycled.

Therefore, the amount of magnesium ions added in the second step only needs to be the amount corresponding to the magnesium precipitate discharged from the first step, but when adjusting the pH to 7 or less in the first step, almost no magnesium precipitates. ,
This means that it is only necessary to add magnesium ions at the beginning.

In addition, if the raw water contains magnesium ions, by adjusting the amount of pH adjuster added in the second step to an appropriate amount, the magnesium necessary for removing fluoride ions will be circulated within the system, resulting in excess will be discharged into the treated water.

Also, if you want to reduce the amount of magnesium ions in the treated water, add a pH adjuster corresponding to the amount in the second step.
The excess magnesium precipitate may be removed during the third step.

Next, experimental examples, examples, and comparative examples will be explained.

Experimental example: Waste water discharged from a coal combustion flue gas removal equipment (pH 1,7
, F-223 Immediate/CAI”-6s2■/121Mg2+
In the first step, sulfur-IJium hydroxide was added to 151~/l), stirred for 30 minutes to react, and then filtered. The resulting water quality is shown in Cases 1 to 5 in Table 1.

As a result, when the pH of the first step is between 5 and 8.5, fluoride ions are significantly reduced, but Mg2+ is hardly reduced, and when the pH exceeds 8.5, the residual amount of F- and the precipitation of magnesium occur. Things start to increase and I realize that I don't like them.

Next, in the second step (however, the addition of powdered activated carbon was omitted, the treated water of Case 3 was adjusted to pf (pf) with sodium hydroxide, stirred for 30 minutes, reacted, and filtered.

The treated water quality is shown in cases 6 to 9. (Note that since there is a difference in coexisting ions in the first and second steps, even at the same pH, M
The amount of g2+ remaining will vary.

) From this, it can be seen that when the pH becomes 9.5 or higher, fluoride ions are significantly reduced, and Mg2+ is also significantly reduced.

Furthermore, in the second step, MgCl2.6H2 is added from the outside.
Case 1 shows the treated water quality when supplementing 0 to 120~/IJ (as M g) and adjusting the pH with sodium hydroxide.
Shown from 0 to 11.

In this way, the weight ratio M of F- and Mg2+ in the second step is
It can be seen that when g/F is increased by at least 20 times, the amount of F- becomes about 1 m9/l or less.

In Case 6, most of the fluoride ions were removed, so Table 2 shows the amount of metal ions when 200~/l of iron chloride was added and the pH was adjusted to 9.0 with sodium hydroxide.

Next, in the fourth step, sodium hydroxide was added to the raw water (pH 1.9, Mg" 229~/l) from which the settled sludge of Case 8 was returned to adjust the pH to 6.7, and the water quality was F -10,7719/l, AI”<2mg
/CMg2+2261n9/13.

The amount of sludge generated at this time was 3.9 kg/tri"-effluent.

Next, add sodium hydroxide to the treated water, pH 10,
As a result of adjusting to 8, the treated water quality was F-1.5m9/l, M
g2+40.1 m9/13 and Natsuta.

On the other hand, the settled sludge was returned to the first step.

For comparison, sodium hydroxide was added to raw water and p
When adjusted to H10.8, the water quality was F 1031n.
It can be seen that the water quality is not very good at 9/l, and the amount of sludge generated is 4.7 kg7 m-effluent, which shows a large difference in quantity.

EXAMPLE Wastewater discharged from a coal combustion flue gas desulfurization equipment was placed in the first reaction tank 1 with 500 m of sulfuric acid band according to the treatment flow shown in the drawing.
# and sulfuric acid 1100Tn9/l added to pH 7.2
Powdered activated carbon Kuraray Coal PF was added to the second reaction tank 3.
K (■Kuraray registered trademark) 2001n9/l and sodium hydroxide 15001n-9/l were added to pH 10.
Table 3 shows the water quality of the raw water and the effluents of the first and second steps when the water was adjusted to .

The reason why sulfuric acid is added to the first reaction tank 1 is to dissolve the retransfer sludge, and the reason why the first step effluent has a large amount of magnesium in Table 3 is because it was concentrated by circulation.

Next, add 1000~/l of sulfuric acid to the effluent of the second step and
When the water was adjusted to H4-5 and filled with SO4 type weak basic anion exchange resin Diaion WA30 (Mitsubishi Chemical Industries, Ltd. registered trademark), water was passed through the ion exchange tank 5 in a downward flow at SV 8. The pH of the treated water is 3.5 to 5.5,
The treated water CODMn has a flow rate of 50 B V (Bed V
<2.5 rru? /l.

At 100 BV, it was 3.7 m9/L, and at 150 BV, it was 5.6~/L.

Comparative Example 1 In the above example, if the sludge from the second step is not returned to the first step, 4 times more powdered activated carbon is required to obtain the same treatment result.
00■/l was required.

It can be seen that the amount added is reduced to about 1/2 by returning the sample.

In addition, 1.1 kg/1.1 kg of sludge was collected from the settling tank 4 in the second step.
m - Extra waste water will be discharged.

Comparative Example 2 When powdered activated carbon is not added in the second step, CODMn after ion exchange reaches a water flow rate of 7.0 m up to a water flow rate of 50 BV.
9.2 m9/l at 9/11100BV.

150BV and e 1.1. , 3~/l.

It can be seen that CODMn cannot be reduced to 5m9711 or less by ion exchange alone.

In the above embodiments, a reaction tank and a precipitation tank are provided separately for both the first step and the second step, but the reaction and precipitation may be carried out in one tank.

As described above, according to the present invention, fluoride ions, heavy metal ions, and COD can be efficiently removed from flue gas desulfurization wastewater, and a large amount of high-quality treated water can be obtained.
The amount of sludge generated is small, and the ion exchange resin is less contaminated.

[Brief explanation of the drawing]

The drawing is a system diagram showing an embodiment of the present invention, in which 1 is a first reaction tank, 2 is a precipitation tank, 3 is a second reaction tank, 4 is a precipitation tank,
5 is an ion exchange tank.

Claims (1)

[Claims] 1. pH of flue gas desulfurization wastewater in the presence of aluminum ions.
A first step in which the pH is adjusted to 5 to 8.5 and the precipitate is separated; a second step in which the effluent from the first step is adjusted to pH 9 or higher and powdered activated carbon is added to separate the precipitate; 2
A method for treating flue gas desulfurization wastewater, comprising a third step of contacting the effluent from the step with a basic anion exchange resin, and a step of returning the precipitate in the second step to the first step. 2. The method for treating flue gas desulfurization wastewater according to claim 1, wherein an aluminum salt and a pH adjuster are added and reacted in the first step. 3. A method for treating flue gas desulfurization wastewater according to claim 1 or 2, wherein iron salt is added and reacted in the second step. 4. The method for treating flue gas desulfurization wastewater according to any one of claims 1 to 3, wherein the anion exchange resin is medium- or weakly basic.