JP2575878B2 - Chemical cleaning waste liquid treatment method - Google Patents

Description

The present invention relates to a method for treating a chemical cleaning waste liquid, and more particularly, to discharging a chemical cleaning waste liquid containing an organic acid cleaning waste liquid and a rust-preventive liquid from the same. It relates to a method for removing organic substances (hereinafter abbreviated as COD), heavy metals and harmful substances.

[Conventional technology]

Oxide scale (mainly Fe ₃ O ₄ ) adhering to metal surfaces in boilers, heat exchangers, etc. of thermal power plants, chemical plants, etc. is converted to citric acid or an organic acid obtained by mixing citric acid and hydroxyacetic acid into an acid corrosion inhibitor ( In the following, the composition of chemical cleaning waste liquid in the case of applying acid washing to remove with an acid solution to which an acid solution containing added acid has been added and temporary rust prevention treatment for preventing rust on the metal surface after pickling, for example, when applied to a once-through boiler of a thermal power plant. An example of the properties is shown in Table 1.

As can be seen from Table 1, the mixed effluent before treatment contains a large amount of harmful components such as COD, dissolved iron and phosphate. Such a chemical cleaning waste liquid cannot be discharged as it is due to pollution. As shown in Table 2, it is necessary to purify effluent standard values to the regulation values determined by a pollution prevention agreement with a local government as shown in Table 2.

Conventionally, as a treatment method of such a chemical cleaning waste liquid, there is an incineration treatment or a chemical treatment by a wet method. As a wet chemical treatment method, the present inventors first added a calcium compound to a waste solution containing an organic acid washing solution and a rust preventive solution at 0.7 equivalent or more with respect to citric acid, and then added sulfuric acid to adjust the pH to 3. ~
After adjusting to the range of 5.5, as ferrous ion concentration, 2
Ferrous sulfate is added so that the concentration becomes 000 ppm or more. Further, hydrogen peroxide is added at least one equivalent to COD in the waste liquid to oxidize and decompose to form calcium oxalate, and then calcium hydroxide is added to the waste liquid. A method is proposed in which the pH is adjusted to a range of 7 to 8.6 to precipitate heavy metals in the waste liquid as heavy metal hydroxides, and then the precipitates are precipitated and separated.
(Problem to be Solved by the Invention) In the above-mentioned method as proposed above, an organic acid cleaning liquid and a rust preventive liquid are treated with a waste liquid treatment tank. When the acid solution is received, the acid solution is neutralized by a rust preventive solution (about pH 9.5) and the pH rises to 6 or more, so that ferrous ions dissolved in the acid solution (almost (Present as 100% Fe ²⁺ )
% Ferric ion (Fe ³⁺ ), which is advantageous when oxidizing COD with hydrogen peroxide.
The role of e ²⁺ ) is completely lost. Also, in the procedure for adding hydrogen peroxide, since the entire amount is added at once in a few hours, the concentration of the oxidizing aid (Fe ²⁺ ) must be 2000 ppm or more. The necessity is that, when an alkaline agent is added to the waste liquid and then neutralized, the amount of iron hydroxide precipitate generated increases in proportion to the iron ion concentration, so the treatment of the precipitate (press filter method or centrifugation) However, there are problems that it takes a long time to perform the dehydration by the separation method, and the processing cost is increased.

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

[Means for solving the problem]

The present invention relates to the treatment of an organic acid cleaning solution containing citric acid or a mixture of citric acid and hydroxyacetic acid and a chemical cleaning waste solution containing a rust preventive solution, after reducing the pH of the organic acid washing waste solution to 1.5 to 2 with sulfuric acid or hydrochloric acid. Mix with the above rust preventive liquid, and while air bubbling the mixed waste liquid, the waste liquid has a calcium concentration of calcium, 0.7
At least, and then add sulfuric acid or hydrochloric acid to adjust the pH to 1.5.
~ 3.5 range, and further adjust the ferrous ion concentration to 100
After adding ferrous sulfate or ferrous chloride so as to be 0 ppm or more, hydrogen peroxide was first added to the organic substances in the waste liquid to 1.1 ppm.
Add more than equivalent over 1.5 to 2 hours and let it stand for about 2 days, then add more than 0.1 equivalent of hydrogen peroxide to reduce the total amount of hydrogen peroxide to organic substances in the waste liquid. The organic substance in the waste liquid is oxidatively decomposed by adding 1.2 equivalents or more, and then an alkaline agent is added to the waste liquid to adjust the pH.
This is a method for treating a chemical cleaning waste liquid, comprising adjusting to 5.8 to 8.6 to precipitate heavy metals in the waste liquid as heavy metal hydroxide, and then sedimenting and separating the precipitate.

The present invention will be described in more detail.In the present invention, in the treatment of an organic acid cleaning solution containing citric acid or an organic acid cleaning solution obtained by mixing citric acid and hydroxyacetic acid and a rust preventive solution, the organic acid cleaning solution is transferred to a waste liquid treatment tank. At the time of receiving, add sulfuric acid or hydrochloric acid at the same time to lower the pH of the organic acid washing waste liquid to about 1.5 to 2, then receive the rust preventive liquid and air-bubble the waste liquid into calcium hydroxide, calcium chloride, calcium carbonate, calcium oxide, etc. The calcium concentration of at least one of the calcium compounds is 0.1 to citric acid.
7 equivalents or more, and further add sulfuric acid or hydrochloric acid to adjust the pH to 1.5.
~ 3.5, then add ferrous sulfate or ferrous chloride so that the concentration of ferrous ion (Fe ²⁺ ) becomes 1000ppm or more, and then add hydrogen peroxide to COD in waste liquid. When adding 1.2 equivalents or more, first add 1.1 equivalents or more over about 1.5 to 2 hours, and leave the remaining 0.1 equivalents or more after leaving the waste liquid to stand for about 2 days, and then oxidize and decompose COD, Next, at least one or more alkali agents such as sodium hydroxide, potassium hydroxide, calcium hydroxide and the like are added to the waste liquid to adjust the pH to the range of 5.8 to 8.6, and heavy metals in the waste liquid are precipitated as heavy metal hydroxide. After the sedimentation, the sediment is separated by sedimentation, the supernatant liquid is discharged without any pH adjustment, and the sediment is subjected to chemical washing by a dehydration treatment device such as a press filter or a centrifugal separator. This is a method for treating waste liquid.

Next, in order to more specifically explain the method of the present invention, the chemical cleaning waste liquid having the composition and properties shown in Table 1 will be described.

First, when receiving one volume of acid washing waste liquid in the waste liquid treatment tank,
By injecting sulfuric acid or hydrochloric acid at the same time, or adding sulfuric acid or hydrochloric acid to the waste liquid treatment tank in advance, the pH of the acid washing waste liquid received in the waste liquid treatment tank is lowered to about 1.5 to 2 to reduce the pH of the waste liquid. After suppressing the air oxidation of ferrous ion (Fe ²⁺ ), calcium hydroxide, calcium chloride, calcium carbonate and At least one kind of calcium compounds such as calcium oxide is added as a calcium concentration to 0.7 equivalent or more to citric acid, and after confirming the pH, if necessary, sulfuric acid or hydrochloric acid is added to adjust the pH to a range of 1.5 to 3.5. Adjust and then add ferrous sulfate or ferrous chloride so that the ferrous ion (Fe ²⁺ ) concentration is 1000 ppm or more, preferably 1500 ppm (measure the Fe ²⁺ concentration in the mixed waste liquid,
m), then add 1.2 equivalents or more of hydrogen peroxide to the COD in the waste liquid. First, add 1.1 equivalents or more over 1.5 to 2 hours.
Oxidatively decompose to about 99% of the load components, and for the remaining 0.1 equivalent or more, the waste liquid is allowed to stand for about two days, and then added to oxidatively decompose COD to almost 100%, and then sodium hydroxide is added to the waste liquid. , Potassium hydroxide, calcium hydroxide or the like is added to at least one kind of alkali agent to adjust the pH to the range of 5.8 to 8.6. For such a series of chemical additions, it is preferable to forcibly agitate the waste liquid by air bubbling in order to homogenize the liquid and increase the oxidation reaction rate.

Then, the heavy metal hydroxide (mainly ferric hydroxide), calcium oxalate and calcium phosphate (when the mixed waste liquid in Table 1 is D-1) which precipitates are settled and settled with the supernatant. (Hereinafter referred to as sludge), the supernatant and the dehydrated sludge are discharged without any pH adjustment, and the sludge is dehydrated by an appropriate dehydration treatment device and incinerated. To process.

[Action]

In the present invention, when receiving the acid washing waste liquid into the waste liquid treatment tank, sulfuric acid or hydrochloric acid is added to the waste liquid to adjust the pH of the waste liquid to 1.5.
By adjusting the degree to 2, ferrous ion in the effluent (Fe ²⁺⁾ is effective oxidizing aid in the oxidation process of COD with hydrogen peroxide can be maintained without being air oxidation (Fe ²⁺ ) As well as its Fe ²⁺
When the concentration is about 1500 ppm, there is no need to add COD again during the oxidation treatment of COD. Further, it is considered that oxidative decomposition by hydrogen peroxide is the most difficult of the COD load components and the oxidative decomposition effect of hydrogen peroxide is cited as an example of citric acid because of the following chemical reaction of the first formula.

Hydrogen peroxide alone has almost no oxidative decomposition power, but acidic or mixed solutions with ferrous ion are known to have a high oxidizing effect as a Fenton's reagent. By adding a calcium compound such as calcium hydroxide, calcium oxide or calcium chloride in advance as shown by the chemical reaction of the first formula, in addition to the addition of an oxidation aid (Fe ²⁺ ) in order to increase the concentration, The oxidative decomposition rate of the acid is significantly improved, and the COD component is effectively removed.

That is, by adding a calcium compound as can be seen from the chemical reaction of the first formula, ionized calcium reacts instantaneously with oxalic acid generated by oxidative decomposition of citric acid to form calcium oxalate (CaC ₂ O ₄ ). Since a precipitate is formed and oxalic acid disappears instantaneously, it is considered that the chemical equilibrium is that the reaction proceeds to the right until citric acid is almost completely oxidatively decomposed. As described above, the oxidative decomposition and removal of the COD component can be effectively exerted by the combined use of hydrogen peroxide, ferrous ion, and a calcium compound. Is added to the COD in an amount of 1.2 equivalents or more, preferably 1.3 equivalents, but if the entire amount is added at once, the COD removal effect according to the addition concentration is small, but when adding hydrogen peroxide, for example, First
Add 1.1 equivalents or more over 1.5 to 2 hours and add enough time to oxidize and decompose COD. For the remaining 0.1 or 0.2 equivalents or more, allow the waste liquid to stand for about 2 days and add after a while According to the so-called double addition method, almost 100% COD component
It has been confirmed that it is effectively removed up to a nearby area. It is considered that the double addition method of hydrogen peroxide enhances the COD removal effect as follows.

That is, in order to oxidize and decompose the COD component with hydrogen peroxide (H ₂ O ₂ ), active species (HO radical, HO ₂
Radical) is required, and ferrous ion (Fe ²⁺ ) is required for the generation of the active species as shown by the second to fourth reactions.
Since the presence of is indispensable, in order to oxidatively decompose the undecomposed COD component and further increase the COD removal rate, the oxidized ferric ion (Fe ³⁺ ) is converted into ferrous ion (Fe ²⁺ ).

^{_{Fe 2+ + H 2 O 2 →}} Fe 3+ + OH - + HO · ...... second equation _{HO · + H 2 O 2 →} H 2 O + HO 2 · ...... third equation HO · or HO ₂ · + COD components → COD oxidative decomposition ... ... Formula 4 It has been found that this reduction reaction (Fe ³⁺ → Fe ²⁺ ) occurs when the waste liquid is allowed to stand, and it has been confirmed that about 85% reduction is achieved by allowing the waste liquid to stand for two days. Further, the appropriate pH range during the oxidation treatment of the COD component is preferably 1.5 to 3.5, and if the pH is out of the appropriate pH range, the efficiency of COD removal becomes poor. The appropriate concentration of ferrous ion (Fe ²⁺ ) at the time of the COD oxidation treatment is 1000 ppm or more, preferably about 1500 ppm. Even if the concentration is less than 1000 ppm, the oxidative decomposition of COD proceeds, but the COD does not decrease to the effluent standard value in Table 2.

The amount of hydrogen peroxide added for practical use may be determined by measuring the COD of the waste liquid, but in order to treat the wastewater within the effluent standard values shown in Table 2, it should be at least 1.2 equivalents, preferably 1.3 equivalents, relative to the COD. Need to be added.
After adding 1.1 equivalents or more over a period of about 1.5 to 2 hours, it is necessary to add the remaining 0.1 equivalents or more after allowing the waste liquid to stand for about 2 days with a delay.

According to the treatment method of the present invention, it is possible to remove the control items such as COD, heavy metals, and SS in the chemical cleaning waste liquid to within the effluent standard values shown in Table 2, and also to remove the phosphate called plankton fertilizer substance almost completely. Completely, and the supernatant from which the sludge was settled and separated can also be made odorless and colorless and transparent, so no pH adjustment is required when discharging the treated water.
It can be released as it is.

〔Example〕

 Table 3 shows examples of the present invention.

The following tests were conducted in the treatment of a chemical cleaning waste liquid containing an organic acid cleaning liquid and a rust preventive liquid. This test was conducted on chemical cleaning waste liquids having the composition and properties shown in Table 1 before preparing the mixed waste liquid shown in Table 1 (A
Sulfuric acid is added to the acid washing waste solution (about pH 5) of (A-1) and (A-2) to adjust the pH to 2 or less (D-1).
(D-2) and (D-3) each of the three kinds of mixed effluents prepared so as to have a pH of 3 or less, a calcium compound was added to each 1 in an amount of 0.7 equivalent to citric acid, and then sulfuric acid was added.
Was adjusted to 1.5 to 3.5, and then ferrous sulfate was added so that the ferrous ion (Fe ²⁺ ) concentration became 1000 to 1500 ppm (Fe 2 ⁺ 1
In the case of 000 ppm, it is already contained in the waste liquid, so there is no need to add it again), and when adding hydrogen peroxide with stirring at least 1.2 equivalents to the COD content in the waste liquid, first add 1.1 equivalents.
The equivalent was added over 1.5 to 2 hours, and the remaining 0.1 equivalent or more was added after the waste liquid was allowed to stand for 2 days, followed by stirring for 4 hours, and calcium chloride was added to the waste liquid {Example (2).
To (6), (12) to (16)} or calcium hydroxide {Examples (8) to (10) and Comparative Examples (17) to (22)} to adjust the pH to 5.8 to 8.6. To completely precipitate ferric hydroxide, calcium oxalate and calcium phosphate,
The COD, dissolved iron, phosphate groups and SS (solid suspended solids) of the treated water were measured, and the test numbers (2) to (6) and (8) in Table 3 were measured.
To (10) and (12) to (16). For comparison, the addition of hydrogen peroxide during the oxidation treatment with hydrogen peroxide was 1
The conventional method, which can be performed once over a period of time, was also performed (test numbers ((17) to (22)).

The reference examples of the test numbers (1), (7) and (11) are the mixed waste liquids (D-1), (D-2) and (D-3) shown in Table 1.
Indicates the properties before waste liquid treatment.

[Effects of the Invention] The present invention has the following effects.

(1) When acid washing waste liquid having a pH of about 5 or more is received into the waste liquid treatment tank, sulfuric acid or hydrochloric acid is added to the waste liquid to adjust the pH of the waste liquid to about 1.5 to 2,
Even if the subsequent washing water and neutralized rust preventive liquid are received in the waste liquid treatment tank and sufficiently stirred by air bubbling, the ferrous ions (Fe ²⁺ ) in the waste liquid can be maintained without undergoing air oxidation. Useful oxidation aids (F
Not only does it play a role as e ²⁺ ), but when its Fe ²⁺ concentration is about 1500 ppm, it is not necessary to add it again during the COD oxidation treatment.

(2) The effect of the above (1) not only saves the amount of Fe ²⁺ added during the COD oxidation treatment, but also saves heavy metals (mainly iron The amount of hydroxide precipitation in (1) is also reduced by that amount, so that the cost of precipitate treatment (dehydration by press filter method or centrifugal separation method) is reduced.

(3) In the COD oxidation treatment with hydrogen peroxide, the procedure for adding hydrogen peroxide, that is, when adding hydrogen peroxide to COD at 1.2 equivalents or more, first add 1.1 equivalents or more over 1.5 to 2 hours. The remaining 0.1 equivalent or more could effectively remove the COD component by a so-called double addition method in which the waste liquid was allowed to stand for about two days and then added after a while.

(4) In the pH adjustment after the oxidation treatment, colorless and transparent treated water can be obtained within the pH range of the wastewater standard value, so that the water can be discharged without any pH adjustment at the time of discharge.

Claims (1)

(57) [Claims] In the treatment of an organic acid cleaning solution containing citric acid or a mixture of citric acid and hydroxyacetic acid and a chemical cleaning solution containing a rust preventive solution, the organic acid cleaning solution is treated with sulfuric acid or hydrochloric acid.
After lowering the pH to 1.5 to 2, the mixture is mixed with the above rust preventive liquid, and while the mixed waste liquid is air-bubbled, a calcium compound is added to the waste liquid at a calcium concentration of 0.7 equivalent or more based on citric acid, and then sulfuric acid or hydrochloric acid is added. PH to 1.5
Adjusted to the range of 3.5, and the ferrous ion concentration as 1000p
pm or more, ferrous sulfate or ferrous chloride is added thereto, and then hydrogen peroxide is added to the organic substance in the waste liquid at a rate of 1.1 equivalent or more over 1.5 to 2 hours, and the mixture is allowed to stand for about 2 days. After that, further add hydrogen equivalent of at least 1.2 equivalents to the organic substance in the waste liquid by adding more than 0.1 equivalent of hydrogen peroxide to oxidatively decompose the organic substance in the waste liquid, Next, an alkaline agent was added to the waste liquid to adjust the pH to 5.
A method for treating a chemical cleaning waste liquid, comprising adjusting the amount to 8 to 8.6 to precipitate heavy metals in the waste liquid as heavy metal hydroxide, and then sedimenting and separating the precipitate.