JPH0714514B2 - 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 specifically,
The present invention relates to a method for removing organic substances (hereinafter abbreviated as COD), heavy metals and harmful substances from a chemical cleaning waste liquid including an organic cleaning waste liquid and a rust preventive liquid when the chemical cleaning waste liquid is discharged.

[Conventional technology]

Acid corrosion inhibitor for citric acid or organic acid that is a mixture of citric acid and hydroxyacetic acid, which is oxide scale (mainly Fe ₃ O ₄ ) adhering to the metal surface of boilers of heat power plants, chemical plants, heat exchangers, etc. (Abbreviated as "inhibitor" below) Acid cleaning to remove with an acid solution added and the composition of chemical cleaning waste solution when rust preventive solution treatment to protect metal surface after rust prevention is applied to, for example, a once-through boiler of a thermal power plant Table 1 shows examples of such properties.

As can be seen from Table 1, the mixed waste liquid before treatment contains a large amount of harmful components such as COD, dissolved iron, pH (free acid) and phosphate, and the color of the waste liquid is dark brown. There is. Such chemical cleaning waste liquid cannot be discharged as it is because of pollution. As shown in Table 2, it is necessary to purify the discharge water quality up to the regulation value determined by the pollution prevention agreement with local governments.

Conventionally, as a treatment method for such a chemical cleaning waste liquid, there is incineration treatment, wet chemical treatment or a combined treatment of chemical treatment and electrolytic treatment, but in the wet chemical treatment, as a COD removal method. Oxidation reaction product produced by the above-mentioned oxidation treatment by adding hydrogen peroxide water in the acidic region and then adding caustic alkali and alkali such as calcium hydroxide to raise the pH to about 10.6 to 13 There is also known a method in which is used as a sparingly soluble compound, and at the same time, metal ions in the waste liquid are precipitated and formed as sparingly soluble hydroxides, and then precipitated and separated.

[Problems to be Solved by the Present Invention]

With the conventional method described above, metal ions and phosphates can be almost completely removed, but the oxidative decomposition of citric acid, which is a COD component, is not complete during the oxidation treatment with hydrogen peroxide solution, and part of it is converted to citric acid. There is a limit to the removal of COD because it remains, and it does not completely satisfy the COD wastewater standard values shown in Table 2. Also, by adding calcium hydroxide and caustic alkali, metal ions in the waste liquid are converted to hydroxides. When the precipitate is completely formed, its pH must be raised to about 10.6 to 13.Therefore, when discharging the supernatant after sedimentation and the treatment liquid at the time of dehydration of the precipitate, the drainage standard value pH 5.8 to
There were some drawbacks, such as the need to adjust the range to 8.6.

[Object of the Invention]

In order to solve the drawbacks of the above conventional method, the present invention is a method for treating a chemical cleaning waste liquid in which COD in the waste liquid is removed to within the drainage standard value of Table 2 and colorless and transparent treated water is obtained within the drainage standard value pH range Is to provide.

[Means for solving problems]

In the treatment of a chemical cleaning solution containing citric acid or a mixture of citric acid and hydroxyacetic acid and an organic acid cleaning solution and a rust preventive solution, the waste solution contains calcium compounds such as calcium hydroxide, calcium carbonate, calcium sulfate and calcium oxide. After adding 0.7 equivalent or more to citric acid and then adding sulfuric acid to adjust the pH to the range of 3 to 5.5, ferrous sulfate (Fe ²⁺ ) concentration should be 2,000 ppm or more. Is added to the COD in the waste liquor in an amount of 1 equivalent or more, preferably 1.2 equivalents to oxidize and decompose to precipitate calcium oxalate, and then calcium hydroxide is added to the waste liquor to adjust the pH. Was adjusted to a range of 7 to 8.6 to precipitate the heavy metal in the waste liquid as heavy metal hydroxide, and then the precipitate was separated by sedimentation, and the supernatant liquid was discharged as it was without adjusting the pH, Object is a processing method of chemical cleaning waste solution to be dehydrated by the dehydration treatment apparatus such as a press filter or a centrifuge.

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

First, a mixed waste liquid containing 1 volume of the acid cleaning waste fluid, 2 volumes of the washing water waste fluid after acid cleaning, and 1 volume of the rust-preventing waste fluid in the waste liquid treatment tank was added to the calcium waste compound, such as calcium hydroxide, calcium carbonate, calcium sulfate and calcium oxide. Add one or more equivalents to citric acid, and then add sulfuric acid to adjust the pH to 3
After adjusting to a range of 5.5, ferrous sulfate is additionally added so that the ferrous ion (Fe ²⁺ ) concentration is 2,000 ppm or more, and hydrogen peroxide solution is more than equivalent to COD in the waste liquid. Is added 1.2 equivalents to oxidize and decompose COD-loaded organic acids (citric acid, hydroxyacetic acid), inhibitors and hydrazine, and then calcium hydroxide is added to the waste liquid to adjust the pH to the range of 7 to 8.6. . As described above, in the case of adding a series of chemicals, it is preferable to forcibly stir the waste liquid with an air lacing or the like in order to make the liquid uniform and accelerate the oxidation reaction rate.

Heavy metal hydroxide (calcium phosphate) that then precipitates
And calcium oxalate is completely separated into supernatant and precipitate (hereinafter referred to as sludge) by settling, and the dehydrated solution of supernatant and sludge is discharged as it is without any pH adjustment. It is dehydrated by a dehydration treatment device, incinerated, and treated by other methods.

In addition, in the above waste liquid treatment, it is preferable to add hydrogen peroxide while stirring the waste liquid uniformly and slowly, and it is suitable to inject it in 2 to 3 hours. The standing time after addition is preferably 15 hours or more. COD loading components such as organic acid (citric acid, hydroxyacetic acid) inhibitors and hydrazine are oxidatively decomposed by hydrogen peroxide solution, and COD
Although the mechanism of removing γ is not known because the chemical components of the inhibitor are not clear, it is considered that citric acid, hydroxyacetic acid and hydrazine are due to the chemical reactions shown in the following formulas 1 to 3.

Although hydrogen peroxide alone has a weak oxidizing power, it is known that a mixed solution with ferrous ion has an excellent oxidizing effect as a Fenton's reagent in the acidic region. It is considered that Fe ^{2+ acts} strongly as a catalyst in the oxidative decomposition reaction by hydrogen.

However, in the oxidative decomposition of citric acid of the first formula, the oxidation rate is not perfect and part of it remains as citric acid.
It was confirmed by experiments that there is a limit to D removal. That is, if the oxidation rate of the first equation is almost 100%, the oxalic acid (C ₂ H ₂ O ₄ ) produced by the first equation is converted to calcium oxalate (CaC ₂ O ₄ ) by calcium hydroxide added later. Since almost 100% of the precipitate is formed, the COD of the supernatant liquid should drop to a few ppm.
About 0 to 70 ppm remains.

Therefore, in the present invention, by adding a calcium compound such as calcium hydroxide, calcium oxide, calcium carbonate, or calcium sulfate in advance in the COD oxidation treatment with hydrogen peroxide solution as shown in the chemical reaction of the following formula 4, It has been found that the oxidative decomposition rate of citric acid reaches almost 100% and effectively removes COD components.

That is, as can be seen from the chemical reaction of formula 4, by adding a calcium compound, the ionized calcium reacts instantly with oxalic acid produced by the oxidative decomposition of citric acid to form calcium oxalate (CaC ₂ O ₄ ). It is thought that the chemical equilibrium is that the reaction proceeds to the right until the citric acid is completely oxidatively decomposed because a precipitate is formed and the oxalic acid disappears instantaneously. The proper pH range during COD oxidation is 3 to 5.5
If the pH is less than 3, the precipitation of calcium oxalate will not be complete and the COD removal rate will be poor. On the other hand, if the pH exceeds 5.5, the synergistic effect of hydrogen peroxide and ferrous ion (Fe ²⁺ ) will decrease, and the oxidation efficiency as a Fenton's reagent will deteriorate.
The optimum concentration of ferrous ion (Fe ²⁺ ) during COD oxidation is 2,000 ppm or more, preferably about 3,000 ppm. Even if it is less than 2,000 ppm, COD is oxidatively decomposed, but COD does not fall below the wastewater standard value in Table 2. Even if it exceeds 3,000 ppm, its effect does not change much. 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 it within the wastewater standard values in Table 2, it is equivalent to or more than COD, preferably 1.2 equivalents. Is required. According to the treatment method of the present invention, CO
D, heavy metals (mainly iron ions), SS and other harmful substances are second
It is possible to remove to within the drainage standard value in the table, and it is also possible to almost completely remove phosphate, which is one of the factors that cause red tide generation,
Moreover, since the supernatant liquid obtained by sedimentation and separation of sludge can be made odorless and colorless and transparent, no pH is required when discharging treated water.
It can be discharged as it is without any adjustment.

〔Example〕

 Table 3 shows examples of the present invention.

The following tests were conducted in the treatment of the chemical cleaning waste liquid containing the organic acid cleaning liquid and the rust preventive liquid. This test was conducted for the chemical cleaning waste liquid having the composition and properties shown in Table 1, and each of the three kinds of mixed waste liquids (D-1), (D-2), and (D-4). Add 0.7 equivalents of calcium compound to citric acid, then change the amount of sulfuric acid and adjust the pH.
After adjusting to 3 to 5.5, ferrous sulfate is added so that the concentration of ferrous ion (Fe ²⁺ ) becomes 2,000 to 3,000 ppm,
While stirring, hydrogen peroxide solution was further added to the COD content in the waste liquid in an amount of 1 to 1.2 equivalents over about 1 hour, and stirring was continued for 2 hours, then left for about 15 hours, and then the same with stirring. Add calcium hydroxide to the waste liquid to adjust the pH to 7-8.6
To completely precipitate ferric hydroxide, (calcium phosphate) and calcium oxalate (which precipitate during the oxidation treatment), and then COD, dissolved iron, (phosphate) and SS of the treated water.
(Solid suspended matter) is measured and the test number (2) of Table 3 ~
Results such as (7), (9) to (15) and (17) to (23) were obtained. For comparison, a conventional method in which no calcium compound was added during the oxidation treatment with hydrogen peroxide solution was also conducted (test number (24 to 27)).

The reference examples of the test numbers (1), (8) and (16) show the properties of the mixed waste liquids (D-1), (D-2) and (D-4) shown in Table 1 before the waste liquid treatment.

[Effects of the Invention] The following effects are achieved by the present invention.

(1) By adding calcium compounds such as calcium hydroxide, calcium oxide, calcium carbonate, and calcium sulfate in advance during COD oxidation treatment with hydrogen peroxide solution, the oxidative decomposition reaction of citric acid is significantly promoted.
The COD component could be removed effectively.

(2) Due to the action and effect of (1) above, the appropriate pH range during COD oxidation treatment is widened to 3 to 5.5, which facilitates the treatment.

(3) Since pH adjustment after oxidation treatment gives colorless and transparent treated water within the drainage standard value pH range, it was possible to discharge the water as it was without adjusting the pH at all.

Claims (1)

[Claims] 1. In the treatment of a chemical cleaning waste solution containing citric acid or an organic acid cleaning solution containing citric acid and hydroxyacetic acid and a rust preventive solution, a calcium compound is added to the waste solution in an amount of 0.7 equivalent or more relative to citric acid. After adding sulfuric acid to adjust the pH to the range of 3 to 5.5, ferrous sulfate is added so that the ferrous ion concentration is 2,000 ppm or more, and hydrogen peroxide solution is added to COD in the waste liquid. On the other hand, 1 equivalent or more is added to oxidize and decompose to precipitate calcium oxalate, and then calcium hydroxide is added to the waste liquid to adjust the pH to the range of 7 to 8.6, and the heavy metal in the waste liquid is converted to heavy metal hydroxide. A method for treating a chemical cleaning waste liquid, which comprises depositing and then separating the precipitate.