JPS5923875B2 - Processing method for chemical cleaning waste liquid - Google Patents

Description

[Detailed description of the invention] The present invention uses chemical cleaning waste liquid (hereinafter abbreviated as chemical cleaning waste liquid).

), and more specifically, it relates to a method for treating chemical washing waste liquid that removes organic substances and heavy metals from chemical washing waste liquid containing organic substances and heavy metals.

To remove deposits such as metal oxide scale on the metal surfaces of boilers, heat exchangers, and other plants.
Usually, chemical cleaning is performed using inorganic acids such as hydrochloric acid, sulfuric acid, and sulfamino acid, organic acids such as citric acid, formic acid, glycolic acid, ethylenediaminetetraacetic acid, and phosphonic acid, or water-soluble salts thereof.

In addition, organic copper dissolving agents (sequestering agents) such as thiourea are used to remove metallic copper scale that adheres to high-pressure boilers, etc.
is added to the above acids.

Furthermore, in order to suppress the corrosion of the metal base material caused by these cleaning solutions, corrosion inhibitors are used depending on the respective cleaning chemicals, and organic chemicals are generally used as the corrosion inhibitors.

The chemical cleaning waste generated from such chemical cleaning contains copper dissolving agents, not only when organic acids with high chemical oxygen demand (COD) are used as chemical cleaning agents, but also when inorganic acids are used as chemical cleaning agents. It contains COD components originating from organic additives such as and corrosion inhibitors, and further contains large amounts of heavy metal ions originating from metal scale removed by chemical cleaning.

To prevent pollution, this chemical washing waste liquid cannot be discharged as it is, and many attempts have been made to remove COD components and heavy metal ions.

Examples of methods for removing COD components from chemical washing waste include a method in which calcium salt is added to a waste solution containing citric acid to precipitate and separate it as insoluble calcium citrate;
Add acid to the waste liquid containing ethylenediaminetetraacetate to obtain ethylenediaminetetraacetic acid (EDTA・4H), which has low solubility.
However, although these methods can reduce COD caused by citric acid and ethylenediaminetetraacetic acid to some extent, COD caused by corrosion inhibitors and other cleaning aids have been tried. Removal of COD components cannot be expected at all.

Additionally, a method of oxidizing and decomposing COD components using an oxidizing agent such as hydrogen peroxide has been studied, but this method has not yet been fully satisfactory as a waste liquid treatment method that can meet the recent tightening of wastewater regulations.

In addition, as a method for removing heavy metal ions from chemical washing waste, methods such as neutralization and coagulation have been attempted, but the sludge obtained by this method has poor concentration and dewatering properties, and also Since it contains a large amount of COD components, some kind of treatment is also required for the desorbed liquid.

In view of these circumstances, the present invention provides a new and efficient method for treating chemical washing waste liquid.

The present invention involves bringing chemical cleaning waste containing organic substances and heavy metals into contact with an oxidizing agent, and then electrolytically treating the organic substances using an insoluble electrode as an anode to decompose the heavy metals produced. This is a method for treating chemical cleaning waste liquid characterized by removing flocs.

In other words, when treating chemical washing wastewater that contains a large amount of COD components along with heavy metal ions eluted by dissolving scale components, an oxidizing agent is added to the wastewater in advance to oxidize and decompose relatively easily decomposable COD components, and then the heavy metals are removed. If necessary, an electrolyte such as sodium chloride is added to the same treatment solution, and the remaining COD components are decomposed by electrolytic treatment using an insoluble electrode as an anode, and heavy metal ions are converted into flocs with good separability. Separate and remove.

The contact treatment with the above oxidizing agent is performed using heavy metals (mostly Fe2
This is done by adding an oxidizing agent directly to the waste liquid containing (+ ions).

Any oxidizing agent may be used as long as it can oxidize and decompose the organic matter in the waste liquid (eg, hypochlorite, perchlorate, persulfate, hydrogen peroxide, etc.).

The pH at which the oxidation reaction occurs most efficiently varies depending on the type of oxidizing agent, so the pH of the waste liquid depends on the oxidizing agent used.
It is desirable to add the oxidizing agent after adjusting the H to the desired pH in advance.

For example, with hydrogen peroxide, an excellent oxidizing effect can be obtained in the pH range of 2 to 5 in the presence of Fe2+ ions, as known from the Fenton reaction.

The amount of the oxidizing agent to be added may be determined by measuring the COD value of the waste liquid, and may be added in an amount equivalent to or slightly in excess of the COD value.

In this oxidation treatment process, heavy metal ions such as iron ions and copper ions act as catalysts, and some of the organic components in the waste liquid are completely oxidized and decomposed into water and carbon dioxide through Fenton reaction and other oxidation reactions. The components remain in the waste liquid undecomposed or only after their physical form has changed due to an oxidation reaction.

Since there are substances in the oxidized liquid that are not completely oxidized, treatment with an oxidizing agent alone is not sufficient as a COD treatment method for waste liquid.

The present invention uses a treatment liquid (primary treatment liquid) treated with an oxidizing agent,
Furthermore, remaining organic components are completely decomposed and removed by electrolytic treatment using an insoluble electrode as an anode.
At this time, electrolysis is performed without separating heavy metals from the subsequent treatment solution.

Electrolytic treatment is performed by applying direct current from a rectifier to both negative and negative electrodes placed in the primary treatment solution.

The anode is an insoluble electrode made of platinum, graphite, ferrite, or the like, and the cathode is made of iron, stainless steel, or the like.

The electrolysis conditions are determined by the total amount of waste liquid, the type and amount of COD components in the waste liquid, and the allowable time until the electrolytic treatment is completed, but usually the voltage is 3 to 100 V and the current density is 1 to 30 A.
/cLrr? , the current concentration may be set in the range of 0.1 to 20 A#.

The size and number of electrode plates, the capacity of the electrolytic cell, etc. can be arbitrarily set within the range of the above electrolysis conditions.

In this case, it is possible to electrolyze the primary treatment liquid as it is; especially in the case of waste liquid containing hydrochloric acid or sulfuric acid, there is no problem in electrolyzing it as it is because the primary treatment liquid contains a large amount of electrolyte. However, in the case of cleaning waste liquid containing organic acids such as citric acid or glycolic acid as a main ingredient, since the electrolyte concentration in the primary treatment liquid is low, it is more convenient to add new electrolyte and then perform electrolytic treatment.

As the electrolyte, sodium chloride, sodium sulfate, sodium nitrate, etc. can be used, and chlorides are particularly useful.

Generally, during electrolysis, heavy metals are removed by pretreatment to prevent scale formation on the electrodes before electrolysis.

In the electrolytic treatment of the present invention, the heavy metals (mainly Fe) contained in the primary treatment liquid are electrolytically treated without being separated and removed, thereby efficiently electrolyzing and converting the heavy metals into flocs with good dehydration properties. be able to.

If the heavy metals contained in the primary treatment liquid are removed by neutralization and coagulation, a sludge with poor concentration and dewatering properties will be obtained, and the removed liquid will also contain a large amount of COD components.
The desorbed liquid also requires some further treatment.

By electrolytically treating a medium containing heavy metals, all COD components in the primary treatment liquid undergo complete oxidative decomposition due to the catalytic action of the heavy metals, and chelate compounds such as citric acid and ethylenediaminetetraacetic acid remaining in the primary treatment liquid are removed. Heavy metal ions that form and are difficult to remove through neutralization are converted into flocs with good sedimentation and dehydration properties.

Separation of heavy metal flocs produced by electrolysis is carried out by ordinary solid-liquid separation means such as sedimentation separation and filtration.

By applying the method of the present invention, COD components and heavy metal ions are almost completely removed from chemical washing waste liquid that contained COD components and heavy metal ions, and the generated treated water can be discharged as is. .

Chemical washing waste liquid containing COD components and heavy metal ions is
There is also a method of direct electrolysis without oxidation treatment, and it is also possible to remove COD components and heavy metal ions by this direct electrolysis method.

However, the method of the present invention has the advantage of higher treatment efficiency (and shorter treatment time) than the direct electrolytic treatment method.

Some organic substances in the waste liquid are poorly oxidized by electrolysis (e.g., polymeric organic substances), but in the present invention, by bringing them into contact with an oxidizing agent in advance, these substances are converted into a form that can be easily decomposed by electrolysis (e.g. (low molecular weight),
The treatment efficiency of electrolysis is higher than that of direct electrolysis.

In addition, in the two-stage treatment of the present invention, a considerable amount of COD components in the waste liquid is removed in the first treatment, and the COD components involved in electrolysis are removed.
Since the OD load is small, the electrolytic treatment is completed in a relatively short time.

On the other hand, in the method of direct electrolytic treatment, the COD load is extremely high, so the electrolysis takes a long time, heat generation and gas generation amount are large, and the efficiency is poor.

Contact treatment using an oxidizing agent in the two-stage treatment method can be completed in a relatively short time because no matter how large the amount of waste liquid is, it can be treated all at once, but electrolytic treatment has a limited treatment capacity due to equipment, so it is necessary to treat all waste liquid in one go. It takes a long time to process.

As detailed above, the present invention makes it possible to efficiently remove COD components and heavy metal ions through oxidation treatment that utilizes the catalytic action of heavy metal ions contained in chemical washing waste liquid, followed by electrolytic treatment. It is.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 0.5% citric acid, 0.5% glycolic acid, 0.1% Ivit (manufactured by Sumitomo Chemical Co., Ltd.) as an inhibitor
, 0.3% thiourea and 2000 ppm iron ions.
Contains chemical washing waste liquid (COD: 8500ppm, TOC
4,800 ppm, pH=3.5) was added with 2% (as pure hydrogen peroxide) hydrogen peroxide as an oxidizing agent and subjected to contact treatment for 2 hours.

As a result, the COD of the treatment liquid was 1 liter, 100 ppm, and the TOC was 1,500 ppm.

Take this treatment solution at 250°C, add 3% sodium chloride, and then use a platinum plate as a ν electrode with a DC current density of 8A/c.
am″ and a current concentration of 8 A# for 5 hours.

As a result, the COD and TOC of the processing solution were both 10pp.
It decreased to below m.

In addition, the iron flocs produced during electrolysis had good sedimentation properties and could be easily separated into solid and liquid.

1 Next, a dehydration test was conducted using the hydrogen peroxide treatment solution and the electrolytic treatment solution, and the dehydration properties of the sludge were compared.

The hydrogen peroxide treatment solution was adjusted to pH around 11 with NaOH,
In addition, the electrolytic treatment solution (pH = 9) was left as it was; after leaving it for a day and night, the settled sludge was collected and the S of both
A leaf test was conducted after adjusting the S concentration to 2.4%.

As a result of a test using Ca(OH)2 as a filter aid, the 1 overrate of the electrolytically treated liquid sludge was about 1.5 to 2 times that of the hydrogen peroxide treated liquid sludge.

Furthermore, when comparing the r liquid, the former was dark reddish brown to almost colorless, whereas the latter was dark reddish brown.

Moreover, the peelability of the cake was also much better in the former case.

This is because the heavy metal flocs are modified during the electrolytic treatment process, producing a sludge with extremely high r-permeability.

Example 2 Glycolic acid 1%, formic acid 0.5%, Ivit 0.15%
and chemical washing waste liquid containing 3000 ppm of iron ions (C
OD: 6400 ppm, TOC: 5100 pPm) and 3% sodium hypochlorite (NaOCl) as an oxidizing agent.
) and was subjected to contact treatment for 2 hours.

As a result, the COD decreased to 1l100pp.

Take 250 ml of the treatment solution and use it under the same conditions as in Example 1.
As a result of time electrolytic treatment, COD.

The TOC was 10 or less in all cases.

The iron flocs produced during electrolytic PM had good sedimentation properties and could be easily separated into solid and liquid.

Claims (1)

[Claims] 1. After contacting a chemical cleaning waste liquid containing organic substances and heavy metals with an oxidizing agent, without separating the heavy metals,
A method for treating chemical cleaning waste liquid characterized by electrolytically treating an insoluble electrode as an anode to decompose organic substances and remove generated heavy metal flocs. 2. The method according to claim 1, wherein the oxidizing agent is hydrogen peroxide or hypochlorous acid. 3. A method for treating chemical cleaning waste liquid according to claim 1 or 2, in which an electrolyte is added and electrolyzed during the electrolytic treatment.