JP2721740B2 - Chemical cleaning waste liquid treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for treating a chemical cleaning waste liquid, and when discharging a chemical cleaning liquid and a washing water waste liquid after the chemical cleaning, the heavy metals, COD, SS ( The present invention relates to a method for removing harmful substances such as solid suspended solids) and color tones.

[Conventional technology]

Steam oxidation scale (main component: Fe ₃ O ₄ , minor components: Cr ₂ O ₃ , MoO _2, etc.) generated on the inner surface of high-temperature steam pipes such as boiler superheater pipes, main steam pipes, and high-temperature reheat steam pipes of thermal power plants In a chemical cleaning to remove ethylenediaminetetraacetic acid (hereinafter abbreviated as EDTA) with a solution obtained by adding ammonia, hydrazine and a corrosion inhibitor (hereinafter abbreviated as an inhibitor),
For example, when applied to a main steam pipe of a thermal power plant, the type of chemical cleaning waste liquid is, as shown in Table 1, water washing for completely discharging and removing the acid liquid remaining in the pipe after acid cleaning in addition to the acid cleaning waste liquid. There is also water waste. The amount of the washing water waste liquid is 8 to 10 volumes per 1 volume of the acid washing waste liquid. Since the amount of contamination is very small, it is accepted in another processing tank.

As can be seen from Table 1, the mixed effluent before treatment contains a large amount of pH, COD, Fe ion, Cr ion, SS, etc., which are wastewater standard items. It cannot be discharged. As shown in Table 2, it is necessary to purify effluent standard values to the standard values determined by a pollution prevention agreement with a local government as shown in Table 2.

Conventionally, as such a chemical cleaning waste liquid treatment method, there is an incineration treatment or a wet chemical treatment. The incineration is outsourced to an industrial waste contractor, and the incineration is performed off-premises. However, from the viewpoint of high disposal costs and prevention of trouble when transferring waste liquid to off-premises, wet chemical treatment is preferable.

On the other hand, as a wet chemical treatment method, in a treatment method of a chemical cleaning solution containing heavy metal ions (Fe, Cu, Ni) combined with EDTA, an alkaline agent (caustic soda, slaked lime, etc.) is added to the waste solution to adjust the pH to 10 or more. A first step treatment of precipitating and separating heavy metal ions, a second step treatment of adding an acid (hydrochloric acid or sulfuric acid) to the separated solution obtained in the first step to crystallize EDTA, separating and collecting the crystals, and There is known a method comprising a third step treatment for reducing COD by adding hydrogen peroxide to a separated liquid from which crystals have been separated (Japanese Patent Laid-Open No. 59-76593). In the method as described above, in the first step, there is almost no effect of removing the hydrazine and the inhibitor which are the COD load components in the chemical cleaning solution shown in Table 1;
Oxidative decomposition of COD is not sufficient in COD oxidation treatment.
Insufficient oxidative decomposition of EDTA due to problems such as the procedure for adding hydrogen peroxide for COD oxidation, and undecomposed EDTA
There is a problem that chromium ions (Cr ³⁺ ) chelated cannot be removed.

The present invention has been made in view of the above-mentioned state of the art, and has as its object to provide a method for treating a chemical cleaning waste liquid which does not have a problem as in the conventional method.

[Means for solving the problem]

The present invention relates to a metal ion reactively bonded to EDTA (main component:
In the chemical cleaning wastewater treatment method of the acid washing wastewater containing the iron ion, the small amount component: chromium ion), ammonia, hydrazine, and the inhibitor and the washing water wastewater after the acid washing, the total amount of the acid washing wastewater and twice the amount of the acid washing wastewater. After receiving the washing water wastewater into the wastewater storage tank, hydrazine is added at pH 11 or higher by adding an alkali agent and activated carbon together while air bubbling.
After the inhibitor is removed and the iron ions are precipitated as hydroxides, the precipitate is separated into a first step, and the supernatant obtained in the first step is transferred to another waste liquid storage tank. Then, sulfuric acid or hydrochloric acid is added while air bubbling to adjust the pH to a range of 1 to 2.5 to crystallize and precipitate EDTA, and the supernatant obtained in the second step is subjected to acid treatment in advance. After transferring to a further waste liquid storage tank which has received a washing water waste liquid equivalent to 6 to 8 times the washing waste liquid amount, an alkali agent is added while air bubbling, and the pH is adjusted to a range of 2 to 3.5. As ferrous ion (Fe ²⁺ ) concentration
Ferrous sulfate or ferrous chloride is added so as to have a concentration of 100 to 200 ppm, and then 2.5 equivalents or more of hydrogen peroxide is added to COD in the waste liquid in four divided portions. Every three days, air bubbling is stopped during that time, COD is oxidized and decomposed, and then an alkaline agent is added to the waste solution while air bubbling to adjust the pH to a range of 6 to 8.6 to adjust metal ions (iron ions, chromium ions). (Ion etc.) as a hydroxide in a third step of a process for treating and separating a chemical cleaning waste liquid.

 Hereinafter, the method of the present invention will be described more specifically.

First, in the first step, sodium hydroxide, potassium hydroxide, and sodium hydroxide, potassium hydroxide, and the like are mixed into the storage tank for the first step treatment while the entire amount (1 volume) of the acid washing waste liquid and 2 volumes of the washing water waste liquid after the acid washing are received. One or more alkaline agents such as calcium hydroxide and activated carbon (preferably in powder form) are added in combination to adjust the pH to 11 or more, preferably in the range of 11.5 to 12,
Stirring by air bubbling is performed for about 4 hours to remove hydrazine and the inhibitor in the waste liquid, and to completely precipitate iron ions. Then, air bubbling is stopped to precipitate and separate the sediment.

Next, after transferring the supernatant obtained in the first step treatment to another storage tank for the second step treatment using a submersible pump or the like, sulfuric acid or hydrochloric acid is added while air bubbling, and the pH is 1 to 2.5, preferably After adjusting to around 1.7 and stirring by air bubbling for about 1 hour to promote the crystallization of EDTA, the air bubbling is stopped and the crystal precipitate is settled and separated.

Next, the supernatant obtained in the second step treatment is transferred to another storage tank for the third step treatment, which has previously received a washing water waste liquid for acid washing (6 to 8 times the amount of the acid washing waste liquid) by an underwater pump or the like. After transfer and mixing, one or more alkali agents such as sodium hydroxide, potassium hydroxide, calcium hydroxide and the like are added while air bubbling, and the pH is 2 to 3.5, preferably 2.5 to 3
After adjusting to the range, the concentration of ferrous ion (Fe ²⁺ )
Add ferrous sulfate or ferrous chloride to 100-200 ppm, then add 2.5 equivalents or more, preferably 3 equivalents of hydrogen peroxide to the COD in the waste liquid in four equal portions. However, the second and subsequent additions are made every three days, during which time air bubbling is stopped except for stirring when hydrogen peroxide is added.
Oxidatively decomposes COD in a stationary state. Next, sodium hydroxide, potassium hydroxide,
Add at least one alkali agent such as calcium hydroxide
Adjusted within the range of 6 to 8.6, stirring by air bubbling for about 2 hours to precipitate metal ions (iron ions, chromium ions) as hydroxide, and then stop air bubbling to remove precipitates. Settle and separate.

The supernatant obtained in the third step treatment can be discharged as it is, and the precipitate formed and settled during the first to third step treatment and the EDTA crystal precipitate are subjected to dehydration treatment by an appropriate dehydration treatment device. .

[Action] It is considered that the mechanism or action by which hydrazine (N ₂ H ₄ ) and the inhibitor, which are COD loading components, are effectively removed in the first step treatment of the present invention is as follows.

That, N ₂ H ₄ N ₂ and H ₂ by a chemical reaction shown in the first equation by oxygen by air bubbling with ferric hydroxide [Fe (OH) _3] to produce precipitated by addition alkali agent for
It is considered that the oxidative decomposition reaction is oxidatively decomposed into O, and the oxidative decomposition reaction is promoted by activated carbon added in combination with the alkali agent as a reaction catalyst.

Fe (OH) ₃ is produced by reduction with hydrazine
Fe (OH) ₂ is re-emitted by oxygen by air bubbling.
It is oxidized to (OH) ₃ .

On the other hand, the inhibitor removing effect is an adsorption effect by activated carbon, but it is considered that there is also a synergistic effect of ferric hydroxide generated by precipitation by addition of an alkali agent. The addition amount of the activated carbon is sufficient to waste 1 m ³ in the range of 1～1.5Kg.

Further, in order to add an alkaline agent to the waste liquid to completely precipitate the iron that has been chelated with EDTA-Fe as a hydroxide, it is necessary to adjust the pH to 11 or more, preferably to about 11.5 to 12. At a pH of 11 or more, the EDTA-Fe ³⁺ chelate formation constant is larger than the solubility of iron hydroxide [Fe (OH) ₃ ], so that iron ions (Fe ³⁺ ) cannot be completely precipitated and separated.

In the second step treatment of the present invention, it is extremely important to effectively remove EDTA occupying 95% or more of the COD load component.
Since the solubility of EDTA greatly depends on the pH, the optimum pH was examined. As a result, the minimum solubility of EDTA was found at pH 1 to 2.5, preferably around 1.7, and the insoluble matter was crystallized and all precipitated.
Sulfuric acid or hydrochloric acid may be used as the pH adjuster.
The settling volume of the crystalline precipitate of EDTA changes greatly depending on the method of adding the modifier.

That is, the method in which the pH adjuster is added in a short period of time has a sedimentation volume of the crystalline precipitate of about 1/2 compared to the method in which the pH adjuster is added over a long period of time, which is preferable in the dehydration treatment of the precipitate.

As described above, about 2000 ppm of COD remains in the waste liquid obtained by the first and second steps of the present invention. Most of this residual COD component is EDTA of EDTA-Cr3 ⁺ chelate compound.
A minutes and free EDTA.

Therefore, in order to effectively remove the residual COD component EDTA (C ₁₀ H ₁₆ N ₂ O ₈ ) in the third step treatment of the present invention, it is necessary to oxidatively decompose with hydrogen peroxide (H ₂ O ₂ ). It is considered that the oxidative decomposition action is due to the chemical reaction of the second formula shown below.

It is known that a mixed solution of hydrogen peroxide and ferrous ion (Fe ²⁺ ) in the acidic region has a high oxidizing effect as a Fenton's reagent. A major factor for effective promotion is found in the way of adding hydrogen peroxide.

In other words, if hydrogen peroxide is added in an amount of 2.5 equivalents or more to COD, but the entire amount is added at once, the COD removal effect is small and greatly deviates from the wastewater standard value, but as an optimal addition point of hydrogen peroxide, for example, When adding more than 2.5 equivalents of hydrogen peroxide to COD, add 4 times in almost equal parts,
The second and subsequent additions are made every three days, during which time the CO
According to the method of oxidatively decomposing D, EDTA, which is a COD component, can be effectively removed. It is conceivable that the method of adding the amount of hydrogen peroxide to be added after several times and then adding the hydrogen peroxide to enhance the COD removal effect is as follows.

That is, in order to oxidatively decompose the COD component with hydrogen peroxide, active species (HO radical, HO ₂ radical) involved in the oxidative decomposition mechanism are required. Third generation of this active species
Ferrous ion (Fe ²⁺ )
Is essential, the COD component of the present invention is oxidized and decomposed, and in order to further increase the COD removal rate, the oxidized ferric ion (Fe ³⁺ ) is converted to the ferrous ion (F
e ²⁺ ).

Fe ²⁺ + H ₂ O ₂ → Fe ²⁺ + OH ⁻ + HO ..... the third formula HO ・ + H ₂ O ₂ → H ₂ O + HO .... the fourth formula The COD component remains in the reduction reaction (Fe ³⁺ → Fe ²⁺ ) It was found that this occurred when the waste liquid was allowed to stand for a while, and it was confirmed that 85 to 100% reduction was achieved by standing for a few days.

By reducing the ferrous ion (Fe ²⁺ ) and adding hydrogen peroxide in this way, the COD component that has not been decomposed by the previous addition is effectively oxidatively decomposed and removed by the reaction of the second formula. Will be done.

The appropriate pH range during the oxidative decomposition treatment of the COD component is 2 to 3.5.
If the ratio is outside the range, the COD removal rate deteriorates. A ferrous ion (Fe ²⁺ ) concentration of about 100 to 200 ppm is sufficient, and the addition of more than that does not change the COD removal effect.

〔Example〕

The following tests were conducted in the chemical cleaning wastewater treatment method for the acid washing wastewater containing the metal ions (Fe3 ⁺ , Cr3 ⁺ ), ammonia, hydrazine, and the inhibitor that had reacted with EDTA, and the washing water wastewater after the acid washing. . This test was conducted on the chemical cleaning waste liquid having the composition and properties shown in Table 1. (C) While receiving the mixed waste liquid 1 in a beaker, adding 1 g of powdered activated carbon and sodium hydroxide while air bubbling. After adjusting the pH to 11.5 to 12 by stirring, the mixture was stirred for 4 hours to stop air bubbling and sedimentation and separation of iron hydroxide (precipitate). The supernatant (alkaline) after separation of the precipitate is EDTA-
It was green due to Cr ³⁺ chelation.

Take the supernatant as a waste liquid for the second step in a separate beaker and add 97% sulfuric acid while bubbling air to adjust the pH.
After adjusting to 1.9, the mixture was stirred for 1 hour to stop air bubbling, and the crystal precipitate of EDTA was separated by settling. The supernatant (acid) after sedimentation and separation of the crystalline precipitate (white) was purple due to EDTA-Cr ³⁺ chelation.

As shown in the supernatant after the second step treatment in Table 3 (D), the properties of this treatment solution were COD2200 mg / Fe0.2m
g / or less, and Cr ³⁺ 93 mg /. Also for comparison
COD2650mg /
, Fe 1 mg / or less, and Cr ³⁺ 94 mg /.

The above (D) 150 ml of the waste liquid was mixed with 300 ml of the (E) waste liquid (equivalent to 6 times the amount of the waste liquid in Table 1 (A) with washing water after pickling). (F) 450 ml of the mixed waste liquid was used as the waste liquid for the third step. Further, in a beaker, add 48% sodium hydroxide solution while adjusting the pH to 2 to 3.5 while air bubbling, and then ferrous sulfate to a ferrous ion (Fe ²⁺ ) concentration of 100 to 200 ppm. Then, 2.5 to 3 equivalents of 35% hydrogen peroxide based on COD in the waste liquid were added in four equal portions. However, the second and subsequent additions were made every three days, during which time air bubbling was stopped except for stirring during the addition of hydrogen peroxide, and COD was oxidatively decomposed in a stationary state. Then, 48% sodium hydroxide solution was added to the waste liquid while air bubbling to adjust the pH.
The mixture was adjusted to 6 to 8.5 to precipitate metal ions (iron ions and chromium ions) as hydroxides. After stirring by air bubbling for about 2 hours, the air bubbling was stopped and the precipitates were separated by settling. After the sedimentation and separation, the supernatant was measured for COD, dissolved iron, total chromium, and SS, and the results as shown in Test Nos. (2) to (10) in Table 4 were obtained. For comparison, a conventional method in which hydrogen peroxide was added at once during the COD oxidation treatment in the third step, and then subjected to static oxidation treatment was also performed, and the results such as test numbers (12) to (15) were obtained. .

Test Nos. (1) and (11) are mixed waste liquids (F) shown in Table 3, and the properties before the third step treatment in the method of the present invention and the conventional method are shown as reference examples.

[Effects of the Invention] (1) In the first step treatment, not only the iron agent in the waste liquid is precipitated and removed as ferric hydroxide, but also by adding an alkali agent and activated carbon to the waste liquid at the same time and performing air bubbling at the same time. Hydrazine and inhibitors can also be removed effectively, so CO
D Easily removed by oxidative decomposition.

(2) In the COD oxidative decomposition treatment of the third step, when the COD component (EDTA) is oxidatively decomposed by hydrogen peroxide,
When adding more than 2.5 equivalents of hydrogen peroxide to COD,
Add 4 times almost equally and add 3 times after the first
According to the method in which COD is oxidized and decomposed in a stationary state during the day, EDTA is chelated with chromium ion (Cr ³⁺ ) (ED
TA-Cr ³⁺ ) is also effectively oxidatively decomposed and removed, so chromium that has been chelated becomes free chromium ions (Cr ³⁺ ), and is neutralized with an alkaline agent after COD oxidation. The chromium ion is changed to chromium hydroxide [Cr (O
H) ₃ ].

(3) The treated water after the third step treatment, which is the final treatment step, sufficiently satisfies the drainage standard values for all items in Table 2 and can be discharged without any adjustment at the time of discharge.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location C02F 1/72 ZAB C02F 1/72 ZABZ 9/00 502 9/00 502P 502R 503 503C

Claims (1)

(57) [Claims] 1. A method for treating an acid washing waste liquid containing metal ions, ammonia, hydrazine, and a corrosion inhibitor, which are reactively bound to ethylenediaminetetraacetic acid, and a washing water waste liquid after acid washing, wherein the total amount of the acid washing waste liquid and the acid After receiving the washing water waste liquid equivalent to twice the amount of the washing waste liquid into the waste liquid storage tank, add the alkaline agent and activated carbon together while air bubbling.
After removing hydrazine and the corrosion inhibitor at pH 11 or more and causing iron ions to precipitate as hydroxides, a first step treatment in which the precipitate is separated by sedimentation, and the supernatant obtained in the first step treatment is separated. After transfer to the waste liquid storage tank, sulfuric acid or hydrochloric acid is added while air bubbling to adjust the pH to within the range of 1 to 2.5 to crystallize ethylenediaminetetraacetic acid and precipitate and separate the second step processing and the second step processing. The obtained supernatant is transferred to another waste liquid storage tank that has previously received a washing water waste liquid equivalent to 6 to 8 times the amount of the acid washing waste liquid, and then an alkaline agent is added while air bubbling to adjust the pH to 2 to 10. After adjusting to a range of 3.5, ferrous sulfate or ferrous chloride is added so that the ferrous ion concentration becomes 100 to 200 ppm, and then 2.5 equivalents or more of hydrogen peroxide to COD in the waste liquid is added. Add four times, but add after the second Every 3 days, during which time air bubbling is stopped and COD is oxidized and decomposed, and then an alkaline agent is added to the waste liquid while air bubbling to adjust the pH.
3. A method for treating a chemical cleaning waste liquid, comprising a third step of treating the metal ions as hydroxide by adjusting the concentration within a range of 6 to 8.6.