JPH0829319B2 - Chemical cleaning waste liquid treatment method - Google Patents

Description

The present invention relates to a method for treating a chemical cleaning waste liquid containing a hydrochloric acid cleaning liquid and a neutralizing rust preventive liquid used for chemical cleaning of a boiler of a thermal power plant, a heat exchanger, etc. Regarding

[Conventional technology]

Various metal oxide scales containing copper deposited on metal surfaces of operating boilers and heat exchangers (main component is iron oxide)
Is cleaned with acid solution containing acid corrosion inhibitor and thiourea that dissolves and seals copper in hydrochloric acid and a corrosion inhibitor aid for steel materials (thiocyanate). Table 1 shows an example of the composition and properties of these waste liquids in the chemical cleaning in the neutralization and rust preventive treatment in which the rust-preventing liquid is used for rust-proofing.

In addition, the standard values for wastewater are determined by pollution control agreements with local governments, and there are some items that differ slightly from region to region, but an example is shown in Table 2.

As you can see from Table 1, free acid, COD, which is a harmful component,
Since a large amount of dissolved iron, copper and phosphate (neutralization rust preventive waste liquid is phosphoric acid type chemical cleaning waste liquid only) are contained, discharge of such chemical cleaning waste liquid is shown in Table 2. It is necessary to purify up to the emission standard value.

Conventionally, as a method of treating such chemical cleaning waste liquid,
After adding hydrogen peroxide water and stirring for several minutes, adjust the pH to 3.5 with sodium hydroxide and leave it for one day and night, and then adjust the pH to 5.0 with sodium hydroxide and maintain it for about two days and night to oxidize COD. There is also known a method of decomposing and then adding calcium hydroxide to adjust the pH of the liquid to 10 or more, and separating and separating heavy metals in the waste liquid as heavy metal hydroxides (JP-A-52-105573).

[Problems to be Solved by the Invention]

However, the above-mentioned conventional method has a problem in that it has a limited oxidative decomposition ability with respect to COD and cannot satisfy the standard value of wastewater as shown in Table 2. In view of the above technical level, the present invention aims to provide a method for treating a chemical cleaning waste liquid capable of maintaining increasingly strict wastewater standards.

[Means for solving the problem]

According to the present invention, in the treatment of a chemical cleaning waste liquid containing a hydrochloric acid cleaning liquid and a neutralizing anticorrosive liquid, calcium hydroxide is added to the above-mentioned waste liquid to adjust the pH within the range of 1 to 5, activated carbon is added, and then peroxide is added. 1 to 1 for hydrogen water to COD in the above waste liquid
Add 1.5 equivalents to oxidize and decompose COD, then add calcium hydroxide to adjust the pH to 11 or more to precipitate the heavy metals in the waste liquid as heavy metal hydroxides, and then add hydrogen peroxide solution. Add 1 to 1.5 equivalents to the undecomposed COD in the oxidation treatment to oxidize and decompose the COD, and then add sulfuric acid or hydrochloric acid to the waste liquid to adjust the pH to a wastewater standard value and perform precipitation separation. And a method for treating a chemical cleaning waste liquid.

That is, the present invention will be described in more detail. In the treatment of a chemical cleaning waste liquid containing a hydrochloric acid cleaning liquid and a neutralizing rust preventive liquid shown in Table 1, the present invention adds calcium hydroxide to the waste liquid to adjust the pH to 1 After adjusting to within the range of ~ 5, activated carbon 1m waste liquid
³ to 0.3 kg or more and hydrogen peroxide water to the COD in the waste liquid in an equivalent amount or more, preferably 1.5 equivalents to oxidize and decompose the COD, and further add calcium hydroxide to adjust the pH to 11 or more. After the heavy metal in the waste liquid is precipitated as a heavy metal hydroxide, hydrogen peroxide solution is further added in an equivalent amount or more, preferably 1.5 equivalent amount, relative to the undecomposed COD in the oxidation treatment.
This is a method for treating chemical cleaning waste liquid, in which COD is oxidatively decomposed, and then sulfuric acid or hydrochloric acid is added to the waste liquid to adjust the pH to the standard value of waste water and precipitate separation.

 Next, an embodiment of the present invention will be described.

After successively receiving the hydrochloric acid cleaning solution, the washing water after acid cleaning and the neutralizing anticorrosion solution shown in Table 1 above in the treatment tank, calcium hydroxide was added while forcibly stirring the solution by blowing air. The pH is adjusted, but the pH value at this time is preferably within the range of 1 to 5, and if it is out of this pH range, it is performed after that.
The oxidation effect in COD oxidation treatment deteriorates. When pH adjustment is completed, add 0.3 kg or more of activated carbon to 1 m ³ of waste liquid,
The activated carbon may be powdery or granular. On the other hand, if the amount of activated carbon added is 0.3 kg or less, the oxidation efficiency in COD oxidation treatment will deteriorate, which is not preferable.

Next, hydrogen peroxide water is added in an amount equivalent to or more than COD in the waste liquid, preferably 1.5 equivalents to oxidatively decompose the COD load component, but during the oxidation treatment, the liquid is appropriately forced by blowing air or the like. It is preferable to stir, and the end point of the oxidation reaction is confirmed by measuring the concentration of the oxidizing agent in the liquid.

After the oxidation process is completed, add calcium hydroxide to the waste liquid to adjust the pH to 11 or higher, and dissolve heavy metals and phosphates (neutralizing rust preventive solution is phosphoric acid-based chemical cleaning). Waste liquid) and oxalic acid (when the neutralizing rust preventive liquid is citric acid and amine chemical cleaning waste liquid, citric acid is oxidized and produced in the above-mentioned oxidation treatment) as a heavy metal hydroxide,
Precipitation is generated as calcium phosphate and calcium oxalate, but in this case, it is preferable to always forcefully stir the liquid by blowing air. Further, the above-mentioned heavy metals, phosphates and oxalic acid are sufficiently capable of forming the hardly soluble precipitates if the pH is adjusted to a neutral level with calcium hydroxide.
The reason why the pH is adjusted to 11 or higher with calcium hydroxide is to increase the oxidation efficiency in the COD oxidation treatment that is performed next.

Next, hydrogen peroxide solution is added in an amount equivalent to or more than the undecomposed COD in the oxidation treatment, preferably 1.5 equivalents to oxidize and decompose the COD, but during the oxidation treatment, the liquid is forcibly stirred at all times by blowing air or the like. It is preferable to accelerate the oxidative decomposition reaction rate, and the end point of the oxidation reaction is confirmed by measuring the concentration of the oxidizing agent in the liquid.

When the oxidation process is completed, add sulfuric acid or hydrochloric acid to the waste liquid to adjust the pH to the standard value for waste water. Next, settling is performed to completely decompose the supernatant liquid and the precipitate (hereinafter referred to as sludge), and the supernatant liquid can be discharged as it is through an underwater pump or the like. Dehydrate and incinerate or otherwise process.

In the case of separating the supernatant liquid and the sludge in the above waste liquid treatment, an effective polymer flocculant may be added to accelerate the sedimentation of the sludge.

As described above, the mechanism of effectively oxidizing and removing various COD components by the hydrogen peroxide solution, which is a feature of the present invention, is considered to be due to the following chemical reaction.

COD components with a high oxidative decomposition rate on the acidic side are thiocyanates, acid corrosion inhibitors (inhibitors), citric acid and hydrazine.

The oxidative decomposition of ammonium thiocyanate (NH ₄ SCN) is the first formula, 2NH ₄ SCN ＋ 11H ₂ O ₂ → 2NH ₄ HSO ₄ ＋ 2CO ₂ ＋ N ₂ ＋ 10H ₂ O… The first formula is the oxidation of citric acid (C ₆ H ₈ O ₇ ). Decomposition is the second formula, C ₆ H ₈ O ₇ ＋ 8H ₂ O ₂ → C ₂ H ₂ O ₄ ＋ 4CO ₂ ＋ 11H ₂ O… The second formula The oxalic acid generated by the chemical reaction of the second formula (C ₂ H ₂ O ₄ ) is removed by precipitation of insoluble calcium oxalate with calcium hydroxide as shown in the following third formula.

C ₂ H ₂ O ₄ + Ca (OH) ₂ → CaC ₂ O ₄ ↓ + 2H ₂ O ... 3rd formula Also oxidative decomposition of hydrazine (N ₂ H ₄ ) is the 4th formula N ₂ H ₄ + 2H ₂ O ₂ → N ₂ + 4H ₂ O ... shown by the fourth equation.

In order to effectively promote the reactions of the above-mentioned first, second and third equations, ferrous iron in the waste liquid and activated carbon used in the waste liquid treatment are indispensable.

The COD component having a high oxidative decomposition rate on the alkaline side is thiourea. Thiourea is oxidatively decomposed even on the acidic side, but its oxidizing power is weaker than that on the alkaline side. The oxidative decomposition of thiourea [(NH ₂ ) ₂ CS] is represented by Formula 5.

(NH ₂ ) ₂ CS + 7H ₂ O ₂ → N ₂ + CO ₂ + H ₂ SO ₄ + 8H ₂ O ... Fifth Formula Incidentally, the role of the activated carbon used in the present invention is the above-mentioned first formula, second formula, fourth formula and In addition to the oxidation catalytic action that promotes the reaction of the formula 5, it also has the action of adsorbing the inhibitor and reducing the COD by the inhibitor.

Next, a comparative example of the conventional method, which was carried out for comparison with one embodiment of the present invention, will be shown.

The following tests were conducted in the treatment of the chemical cleaning waste liquid containing the hydrochloric acid cleaning liquid and the neutralizing rust preventive liquid. This test was carried out on two types of mixed waste liquids having the compositions and properties shown in Table 1 (the neutralization rust preventive liquids were phosphoric acid type and citric acid and amine type).

〔Example〕

The above two kinds of mixed waste liquid 1 were placed in separate containers, calcium hydroxide was added while stirring the liquid with a stirrer to adjust the pH to 3, and 0.3 g of activated carbon was added. Next, 19.5 ml (corresponding to 1.5 equivalents of COD) of 35% hydrogen peroxide solution to the mixed waste liquid of rust preventive solution, phosphoric acid type, and mixed waste liquid when rust preventive solution uses citric acid and amine type For 22 ml
(Equivalent to 1.5 equivalents of COD) is added and stirred with a stirrer for about 5 hours. Then, while stirring, calcium hydroxide is added to adjust the pH to 11 or more, and 2% each of 35% hydrogen peroxide solution is added. The solution was added and the solution was forced to stir by bubbling air for about 16 hours. Then add the sulfuric acid
The pH was adjusted to 8 and allowed to stand for about 15 hours to completely separate the supernatant and the precipitate.

[Comparative example]

The same two kinds of mixed waste liquid 1 as in the embodiment of the present invention were placed in separate containers, and 35% hydrogen peroxide solution was added to the mixed waste liquid in which the rust preventive solution was phosphoric acid type was 19.5 ml, and the rust preventive solution was citric acid. 22 ml was added to the mixed waste liquid in the case of using the amine and amine system, and after stirring for about 5 minutes, adjust the pH to 3.5 with sodium hydroxide, leave for 24 hours, and then add the pH of the solution with sodium hydroxide. Was adjusted to 5.0 and left to stand for about 48 hours. Next, calcium hydroxide was added to adjust the pH of the solution to 10 or more, the solution was stirred by blowing air for 12 hours, the pH was adjusted to 8 with sulfuric acid, and the solution was allowed to stand for 24 hours for precipitation separation.

In the above-described chemical washing waste liquid treatment between the method of the present invention (Example) and the conventional method (Comparative Example), the pH and CO
D, dissolved iron, phosphoric acid and SS (solid suspension) were measured, and the results shown in Tables 3 and 4 were obtained.

As is clear from Tables 3 and 4, according to the method of the present invention, the quality of the treated water satisfies all the drainage standard values in Table 2 above in all the items, so that no treatment is required at the time of discharge. It can be discharged as it is without applying.

On the other hand, in the conventional method, the COD of the treated water exceeds the discharge water standard value in Table 2 so much that it cannot be discharged as it is in order to prevent pollution.

[Advantages of the Invention] (1) Various COD load components can be effectively oxidatively removed to within the wastewater standard value by separately performing COD oxidation treatment with hydrogen peroxide water on both the acidic side and the alkaline side.

(2) Hazardous substances such as COD, heavy metals, and SS can be treated up to the wastewater standard values in Table 2, and phosphate, which is one of the factors contributing to red tide generation, can be removed almost completely, and sludge settles. Since the separated supernatant can be made colorless and transparent, the treated water can be discharged as it is without the need to dilute the treated water with seawater or the like.

Front page continuation (72) Inventor Ryoichi Noguchi 5-7 Atsunoura-cho, Nagasaki-shi, Nagasaki Ryoko Building Annex 5F, Nagahishi Engineering Co., Ltd. (72) Inventor Koichiro Bando 5-7 Atsunoura-cho, Nagasaki-shi, Nagasaki Ryoko Building Annex 5th floor Choryo Engineering Co., Ltd.

Claims (1)

[Claims] 1. In the treatment of a chemical cleaning waste liquid containing a hydrochloric acid cleaning liquid and a neutralizing rust preventive liquid, calcium hydroxide is added to the waste liquid to adjust the pH within the range of 1 to 5, and then activated carbon is added. Hydrogen peroxide water is 1 to COD in the above waste liquid
Add 1.5 equivalents to oxidize and decompose COD, then add calcium hydroxide to adjust the pH to 11 or more to precipitate the heavy metals in the waste liquid as heavy metal hydroxides, and then add hydrogen peroxide solution. Add 1 to 1.5 equivalents to the undecomposed COD in the oxidation treatment to oxidize and decompose the COD, and then add sulfuric acid or hydrochloric acid to the waste liquid to adjust the pH to a wastewater standard value and perform precipitation separation. A method for treating a chemical cleaning waste liquid, characterized by: