JP2015009224A - Treatment method for chemical cleaning waste liquid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel treatment method which enables compact equipment to efficiently treat chemical cleaning waste liquid at normal temperature without generating ammonia gas as much as possible and without generating secondary waste organic materials such as chelating agents, so that the treated chemical cleaning waste liquid satisfies COD (chemical oxygen demand) and total nitrogen emission standards that are emission evaluation items to the environment.SOLUTION: A treatment method for a chemical cleaning waste liquid is a method for treating a chemical cleaning waste liquid generated by chemically cleaning articles with metal compounds deposited thereon by a chelating agent-containing cleaning liquid, the treatment method comprising: a first step of adding chloride ions of 5 g/L or more to the chemical cleaning waste liquid, subjecting the chemical cleaning waste liquid to an electrolytic treatment with a diamond electrode as an anode, decomposing COD components, and oxidizing organic nitrogen and ammonia nitrogen to nitrogen gas and nitrate nitrogen; and a second step of bringing the chemical cleaning waste liquid containing the nitrate nitrogen obtained by the first step into contact with a reduction catalyst for reduction to nitrogen.

Description

  The present invention is a treatment of chemical cleaning waste liquid discharged when chemically cleaning metal compounds adhering to the inner surface of a heat transfer tube such as a steam generator of a nuclear power generation facility, a steam generation tube or a heat transfer tube of a power generation boiler. More particularly, the present invention relates to a method for treating a chemical cleaning waste liquid containing iron ions and copper ions discharged when cleaning deposits such as iron oxide and copper oxide.

  Heat transfer tubes such as steam generators of nuclear power generation facilities, and heat transfer tubes of steam generators or generator boilers, such as iron oxide and copper oxide, are brought into contact with water and steam due to heat generated during power generation. Metal compounds represented by metal oxides adhere. Since the thermal efficiency of the heat transfer tube is lowered due to the adhesion of these metal compounds, chemical cleaning is periodically performed to remove the metal compound adhering to the inner surface of the heat transfer tube or the like. For chemical cleaning, a chelating agent (EDTA-ammonium salt) is often used, and the chemical cleaning waste liquid contains ammonia-derived ammonium and ammonium compounds in addition to chelate compounds such as iron and copper.

As a method of treating these chemical cleaning waste liquids, there is a method of reducing the volume by heating. However, since the ammonia gas is generated from the ammonium compound in the chemical cleaning waste liquid by the heat treatment, the working environment is deteriorated. Therefore, a method has been proposed in which sulfuric acid is added in advance to adjust the pH of the chemical cleaning waste liquid to 7 or less, and ammonia is fixed as ammonium sulfate to suppress generation of ammonia gas (Patent Document 1). However, in the method described in Patent Document 1, in order to process a large amount of chemical cleaning waste liquid of 50 to 400 m ³ in an actual plant, the heat treatment equipment becomes large and the processing speed is as low as 10 to 40 kg / m ² · h. There are problems such as a long heat treatment time. In addition, since sulfuric acid and ammonia are accompanied to some extent by the water vapor that evaporates during heating, they cannot be released into the atmosphere as they are, and a recovery facility is required.

  In order to solve the problems of the treatment method disclosed in Patent Document 1, it is concentrated by RO (reverse osmosis) membrane treatment under the condition of pH 6-8, then heated and dried, and the generated ammonia is brought into contact with the oxidation catalyst. And a method of decomposing it into nitrogen and water has been proposed (Patent Document 2). However, under the conditions of pH 6-8, ammonia molecules remain in the liquid, and thus nitrogen treatment of the permeated liquid that has passed through the RO membrane is required. There is also a risk of leakage of ammonia gas generated when the concentrated solution concentrated by the RO membrane is heated and dried.

JP 2002-346544 A Japanese Patent No. 5050447

  The present invention eliminates the above-mentioned problems of the prior art, does not generate ammonia gas, and does not generate secondary waste of organic substances such as chelating agents in a small device, and efficiently generates chemical cleaning waste liquid at room temperature. It is an object of the present invention to provide a new treatment method that satisfies the emission standards for COD (chemical oxygen demand) and total nitrogen, which are the evaluation items for emission into the environment.

  ADVANTAGE OF THE INVENTION According to this invention, the processing method of the chemical cleaning waste liquid produced | generated by carrying out the chemical cleaning of the articles | goods to which the metal compound adhered to the chelating agent containing cleaning liquid is provided. In this treatment method, 5 g / L or more of chloride ions is added to the chemical cleaning waste liquid, and subjected to an electrolysis treatment using a diamond electrode as an anode to decompose a COD component, thereby converting organic nitrogen and ammonia nitrogen into nitrogen. Including a first step of oxidizing to gas and nitrate nitrogen, and a second step of bringing the chemical cleaning waste liquid containing nitrate nitrogen obtained in the first step into contact with a reduction catalyst and reducing to nitrogen. Features.

The chloride ion is preferably added as one or more selected from potassium chloride, sodium chloride, potassium hypochlorite, and sodium hypochlorite.
In the first step, it is preferable to maintain the pH of the chemical cleaning waste liquid at 8 or more and 9 or less.

In the second step, it is preferable to add a reducing agent to the chemical cleaning waste liquid.
The reducing agent is preferably at least one selected from sodium borohydride, sodium hypophosphite, and hydrazine.

In the second step, the reduction catalyst preferably contains palladium and copper, more preferably palladium and copper supported on titanium dioxide.
The second step is preferably performed at a temperature of 80 ° C. or lower.

  In the treatment method of the present invention, a chemical cleaning waste liquid generated by chemically cleaning an article to which a metal compound is adhered with a cleaning liquid containing a chelating agent can be used to reduce the amount of an organic substance such as a chelating agent without generating ammonia gas with a small apparatus. It does not generate secondary waste and can be detoxified efficiently at room temperature, and it can satisfy COD (Chemical Oxygen Demand) and total nitrogen emission standards, which are evaluation items for the release into the environment.

It is explanatory drawing which shows the outline of the test apparatus used in the Example.

Preferred embodiment

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.
As a chemical cleaning waste liquid to which the treatment method of the present invention can be applied, a metal compound adhering to a heat transfer tube such as a steam generator of a nuclear power generation facility, a heat transfer tube of a steam generation or power generation boiler, or the like is cleaned with a chelating agent. A chemical cleaning waste liquid discharged when chemical cleaning is performed using an agent can be preferably mentioned. In particular, it is effective for treating a chemical cleaning waste liquid containing a chelating agent and ammonium ions. As a typical chemical cleaning waste liquid to be treated in the first step of the present invention, there is a waste liquid containing about 10 g / L EDTA, about 4000 mg / L ammonium ion, about 100 mg / L hydrazine and having a pH of 9.4. EDTA 5-50 g / L, ammonium ion 3000-6000 mg / L, hydrazine 50-300 mg / L may be in the range of pH 8.0-10.0, EDTA 10-30 g / L, ammonium ion 4000- It is preferable that it is 5000 mg / L and pH 8.0-9.0.

  In the treatment method of the present invention, the chemical cleaning waste liquid is subjected to an electrolysis treatment using a diamond electrode as an anode to decompose a COD component and oxidize organic nitrogen and ammonia nitrogen to nitrogen gas and nitrate nitrogen. And a second step of bringing the chemical cleaning waste liquid containing nitrate nitrogen obtained in the first step into contact with a reduction catalyst to reduce it to nitrogen.

  The diamond electrode is an electrode in which Ni, Ta, Ti, Mo, W, Zr, Nb, or Si is used as a substrate, and conductive diamond is coated on the substrate surface in a thin film by a hot filament CVD method. A diamond electrode has a large potential difference between oxygen generation and hydrogen generation in water electrolysis (a large thermodynamic window), and hardly generates oxygen. Since oxygen is difficult to generate, electrolytic reactions other than electrolysis of water are likely to proceed, and OH radicals, which are strong oxidants, are generated. This OH radical oxidizes organic nitrogen and ammonia nitrogen derived from the chelating agent and ammonium ions in the chemical cleaning waste liquid.

Electrolysis with a diamond electrode is preferably performed with a diamond electrode for both electrodes. The current density of the diamond electrode is preferably 100 to 150 mA / cm ² , and the circulation flow rate is preferably in the range of 500 to 2000 ml / min per liter of the treatment liquid from the first step.

  In electrolysis in the first step, it is preferable to add 5 g / L or more of sodium chloride to the chemical cleaning waste liquid. As the chloride ion, potassium chloride, sodium chloride, potassium hypochlorite, sodium hypochlorite and the like can be added in one or more combinations. Chloride ions are electrolytes, and hypochlorite ions generated by oxidation with OH radicals oxidize ammonium ions to nitrogen gas. The pH of the liquid may be adjusted as long as it is an alkaline agent that does not contain a nitrogen component, but versatile sodium hydroxide is preferred. Since the chemical cleaning waste liquid contains a chelating agent, an intermediate product containing a carboxylic acid is generated by the decomposition of the chelating agent at the beginning of electrolysis. When carboxylic acid is generated, the pH of the chemical cleaning waste liquor is changed to an acidic range, and peroxidation of organic nitrogen and ammonia nitrogen (N oxidation number-III) derived from ammonium ions is likely to occur, and nitrate nitrogen (N A large amount of oxidation number + V), and the total amount of nitrogen increases. Therefore, it is preferable that the pH of the chemical cleaning waste liquid in the first step is maintained in a weak alkaline region, preferably 8 or more, and the residual amount of nitrate nitrogen (N oxidation number + V) is about 10% of the total nitrogen. . Further, the upper limit of the pH is preferably 9 or less. Since the acid dissociation constant pKa of ammonia is 9.25, by controlling it to 9 or less, the form of ammonia in the liquid can be retained more as ammonium ions than ammonia molecules, and the generation of ammonia gas is suppressed. Most of the first step is an oxidation reaction of ammonium ions. If the presence of ammonium ions in the liquid is reduced by this reaction, ammonium ions are generated from ammonia molecules according to Le Chatelier's law. Therefore, the amount of ammonia molecules that cause odors also decreases, so that diffusion of ammonia gas can be controlled.

The amount of chloride ions added to the first step is preferably in the range of 5 g / L to 30 g / L, and is preferably about 10 g / L.
In the second step, the chemical cleaning waste liquid containing the nitrogen gas and nitrate nitrogen oxidized in the first step is brought into contact with a reduction catalyst to reduce nitrate nitrogen to nitrogen gas. As the reduction catalyst, the reduction catalyst preferably contains palladium and copper, and more preferably a reduction catalyst obtained by supporting palladium and copper on titanium dioxide. By contacting with a reduction catalyst containing palladium and copper, nitrate nitrogen (N oxidation number + V) becomes nitrite nitrogen (N oxidation number + III), oxidized nitrogen (N oxidation number + II), suboxidation state. It is reduced to nitrogen gas via nitrogen (N oxidation number + I). A reduction catalyst comprising palladium and copper supported on activated carbon or titanium dioxide suppresses overreduction from nitrate nitrogen to ammonia nitrogen.

The reduction catalyst can be prepared by any of the following methods.
(1) A dispersion of titanium dioxide fine particles or activated carbon fine particles is prepared. A metal salt aqueous solution containing a predetermined amount of palladium and copper is added thereto, the metal salt aqueous solution is absorbed into titanium dioxide fine particles or activated carbon fine particles, then dried, and then reduced to a reducing gas such as hydrogen or ammonia at a temperature of 200 to 800 ° C. The reduction catalyst fine particles can be obtained by reducing treatment for about 0.5 to 6 hours under an atmosphere. As the metal salt, palladium and copper salts such as palladium nitrate, palladium chloride, palladium acetate, tetraammine palladium, copper chloride and the like that are soluble in water can be used. As a drying method, conventionally known methods such as freeze drying, spray drying, stationary drying, and rotary evaporator can be employed. In the above, when the reduction temperature is less than 200 ° C., the reduction of the metal salt becomes insufficient and the generation of metal fine particles becomes insufficient. When the reduction temperature exceeds 800 ° C., the metal fine particles grow too much, the catalyst fine particles are strongly aggregated and the dispersibility is lowered, and may not be separated by ultrafiltration or the like. Stable treatment of water containing nitrate nitrogen becomes difficult. A preferable reduction temperature is in the range of 250 ° C to 600 ° C. The method for measuring the particle diameter of the metal fine particles in the catalyst particles of the present invention can be measured by FE-TEM (STEM-HAADF method: field emission transmission electron microscope (high angle scattering annular dark field scanning transmission microscope)).

  (2) A dispersion of titanium dioxide fine particles or activated carbon fine particles is prepared. A metal salt aqueous solution containing a predetermined amount of palladium and copper is added to this, and then a reducing agent such as sodium borohydride, sodium hypophosphite, hydrazine is added, and palladium and copper are deposited on titanium dioxide fine particles or activated carbon fine particles. Let Then, if necessary, by performing autoclaving at 100 to 300 ° C., palladium and copper fine particles deposited in the solution can be deposited on the carrier particles. Subsequently, the catalyst is separated by filtration, dried in the same manner as in (1), and then subjected to heat treatment (preferably heat treatment in a reducing atmosphere) to obtain reduced catalyst particles.

  (3) To a mixed aqueous solution of palladium nitrate and copper nitrate, a solution in which ferrous sulfate was dissolved as a reducing agent in an aqueous citric acid solution was added to prepare a palladium-copper alloy fine particle liquid. The obtained dispersion was washed with water by centrifugation to remove impurities such as iron ions, and then dispersed in water to obtain a dispersion of alloy fine particles having a concentration of 1% by weight. Titanium dioxide fine particles were mixed with this palladium-copper alloy fine particle dispersion to prepare catalyst particles having palladium-copper alloy fine particles supported on a titanium dioxide fine particle support. Then, the catalyst is separated by filtration, dried in the same manner as in (1), and then heat-treated (preferably heat-treated in an inert gas or reducing gas atmosphere) to obtain reduced catalyst fine particles.

  Next, it is preferable to add a reducing agent as a pretreatment at the beginning of the second step. Examples of the reducing agent include sodium borohydride, sodium hypophosphite, hydrazine and the like. The pretreatment means an operation for reducing an oxidizing substance such as hypochlorite ion and chlorate ion generated when sodium chloride or the like is added in the first step. For example, the amount of hydrazine added for pretreatment is initially equivalent to hypochlorite nitrate ions and 1 to 2 equivalents of chlorate ions. Since the neutralization reaction between hydrazine and hypochlorite ions proceeds rapidly even at room temperature, an equivalent amount is sufficient. The neutralization reaction between chlorate ions and hydrazine proceeds slowly at the time of heating up to 80 ° C., so that an excessive amount can be added. By these reactions, damage to the catalyst due to oxidation is prevented by eliminating oxidizing substances in the liquid.

  As the main reaction in the second step, reduction of nitrate ions is performed. Examples of the reducing agent used for reducing nitrate ions include sodium borohydride, sodium hypophosphite, hydrazine, and the like. Since these have a reducing action, nitrate nitrogen (the oxidation number of N + V) can be reduced to nitrogen gas in the presence of the reduction catalyst. The amount of reducing agent required for this reaction is twice the equivalent of nitrate ions. As a method for adding the reducing agent, a fixed amount is slowly added over 2 hours which is a standard of the reaction time. The reduction reaction of nitrate ions is performed while purging nitrogen gas in the atmosphere of the separable flask.

  The second step is preferably performed at a temperature of 80 ° C. or lower, preferably room temperature to 60 ° C. The higher the temperature in the second step, the higher the reactivity. However, in consideration of operational safety and environmental load, 80 ° C. or lower is preferable. By setting the temperature of the second step to room temperature to 80 ° C., the neutralization reaction of the oxidizing substance and the reduction reaction of nitrate nitrogen generated in the first step can be promoted.

Hereinafter, the present invention will be described specifically by way of examples. In Examples, unless otherwise specified, “%” means mass%.
The test apparatus shown in FIG. 1 was used. The first step is performed in an electrolysis tank 10 having an electrode portion comprising an anode 12 and a cathode 14 which are diamond electrodes. In the second step, the circulating water from the electrolysis tank 10 is received in a separable flask 20 installed on a stirrer 22 with a heater.

(First Step) A 1000 ml separable flask was charged with the chemical cleaning waste liquid. While stirring with a stirrer and a rotor at room temperature, it was introduced into an electrolysis tank using a diamond electrode from a separable flask using a pump, electrolyzed, returned to the separable flask, and circulated at a circulation flow rate of 710 ml / min. I let you. A diamond electrode having an electrode effective area of 73 cm ² was used as an anode and a cathode, and the current density was set to 100 or 150 mA / cm ² . To the electrolysis tank, 10 g / L of sodium chloride was added.

  (Second step) The 90% decomposed solution in the first step was used as a simulated waste solution. The simulated waste liquid contains EDTA 1 g / L, iron ions 100 mg / L, sodium chloride 10 g / L, nitrite ions 500 mg / L, and nitrate ions 1000 mg / L. The simulated waste liquid was placed in a separable flask, and an activated carbon-supported palladium-copper reduction catalyst was added and heated to 80 ° C. Thereto, hydrazine in an amount of 2 mol times the total of nitrite ions and nitrate ions was added dropwise over a period of 2 hours to allow the reaction to proceed.

<COD removal rate>
COD was measured by the Mn method, and was consumed by measuring the amount of potassium permanganate oxidized when the 5 mmol / L potassium permanganate solution mixed with test water was heated with boiling water for 30 minutes. Calculate the amount of oxygen. Silver nitrate is used to prevent the effects of chloride ions.

The removal rate (%) is expressed by the following equation.
[Formula] COD removal rate (%) = COD after treatment / COD before treatment × 100
<Total nitrogen concentration>
For total nitrogen, add sodium persulfate to the sample and oxidize at 140 ° C for 30 minutes in an autoclave to make all nitrate ions. Then, measure the nitrate ion concentration with a UV photometer, I went and asked.

<Total nitrogen removal rate>
The total nitrogen removal rate (%) can be expressed by the following formula.
[Formula] Total nitrogen removal rate (%) = total nitrogen concentration after treatment / total nitrogen concentration before treatment × 100
<Ammonium ion removal rate>
Ammonium ions were colored blue by the indophenol method, and ammonium ions were measured with a visible photometer. The ammonium ion removal rate (%) can be expressed by the following formula.
[Formula] Ammonium ion removal rate (%) = Ammonium ion concentration after treatment / Ammonium ion concentration before treatment × 100
<Nitrate ion>
The nitrate ions were measured in the same manner as the total nitrogen measurement method, and nitrate ions were measured with a UV photometer.

[Preparation of catalyst]
In addition to 0.01 g of pure water in an amount of 0.01 part by weight per 1 part by weight of alloy fine particles from which trisodium citrate can be obtained in advance, the concentration in terms of metal is 10% by weight. Copper nitrate and palladium nitrate were added so that the weight ratio of the mixture was 20:80, and an aqueous solution of ferrous sulfate equivalent to the total number of moles of copper nitrate and palladium nitrate in the liquid was added. And stirred for 1 hour to obtain a dispersion of alloy fine particles. The average particle size of the alloy fine particles was 8 nm.

The obtained dispersion was washed with water by centrifugation to remove impurities, and then dispersed in water to obtain a dispersion of alloy fine particles having a concentration of 1% by weight. 100 g of activated carbon (manufactured by Nippon Enviro Chemicals Co., Ltd .: Shirasagi P, average particle size 50 μm) was mixed with 100 g of this dispersion and stirred for 2 hours.
Next, after lyophilization, the catalyst was subjected to heat treatment at 250 ° C. for 2 hours in a hydrogen-nitrogen mixed gas atmosphere to prepare a reduced catalyst. The palladium content of the catalyst was 0.8% by weight and the copper content was 0.2% by weight.

A titanium dioxide-supported palladium-copper catalyst was also prepared in the same manner as described above.
[Catalyst performance test]
While putting 200 g of simulated waste liquid (nitrate ion 1000 mg / L, nitrite ion 500 mg / L, iron ion 100 mg / L, EDTA 10 g / L, sodium chloride 10 g / L, total nitrogen 380 mg / L) into a separable flask while heating Then, 9.1 g of activated carbon-supported palladium-copper catalyst was added (0.5 g metal / L simulated waste liquid). When the temperature reached 80 ° C., 33.2 g of hydrazine (4 mol times of nitrate ion + nitrite ion) was added dropwise over 2 hours while nitrogen was bubbled, and the reaction was performed.

  As a result, the total nitrogen was less than 5 mg / L in both tests using the activated carbon supported catalyst and the titanium dioxide supported catalyst. As a result of testing again with a new simulated waste liquid using this catalyst, the total nitrogen again became less than 5 mg / L, and the possibility of repeated use was found.

[Example 1]
As a chemical cleaning waste liquid, prepare a test liquid containing EDTA 10 g / L, ammonium ion 4000 mg / L, hydrazine 100 mg / L, iron ion 2000 mg / L, copper ion concentration 20 mg / L at pH 9.4, and adding sodium chloride. It was made to change and it processed using the test apparatus shown in FIG. 1, and the activated carbon carrying | support reduction catalyst prepared above, and measured the COD removal rate, the total nitrogen removal rate, and the ammonium ion removal rate. The results are shown in Table 1.

  In the case of no addition of sodium chloride, the ammonium ion removal rate was small and 26.8%. However, the addition of 10 g / L of sodium chloride improved the ammonium ion removal rate to 99.7%. It can be said that the COD removal rate and the total nitrogen removal rate do not originate from the concentration of sodium chloride because the decomposition of EDTA proceeds by OH radicals generated from the electrodes.

[Example 2]
As the chemical cleaning waste liquid, the following test liquids 1 to 3 were processed using the test apparatus shown in FIG. 1, and the nitrate ion concentration, the total nitrogen concentration, the total nitrogen removal rate, and the ammonium ion removal rate were measured. The results are shown in Table 2. Since pH fluctuated with the lapse of processing time, it is indicated by a range of values between an initial stage and an end stage.
<Test solution 1>
At pH 9.3, EDTA 10 g / L, ammonium ion 4000 mg / L, hydrazine 100 mg / L, iron ion 2000 mg / L, total nitrogen 4160 mg / L and no nitrate ion.
<Test solution 2>
Sulfuric acid was added to test solution 1 to adjust the pH to 7.8 <Test solution 3>
Sodium hydroxide was added to the test solution 1 to adjust the pH to 10.2.

  In the test solution 1 that was not adjusted to pH and the test solution 2 that was adjusted to pH 7.8, the pH of the test solution shifted to an acidic range with the lapse of treatment time, and the total nitrogen concentration and nitrate ion concentration decreased so much. Absent. In particular, in the test solution 2 adjusted to pH 7.8, it can be seen that the total nitrogen concentration and the nitrate ion concentration are higher than before the treatment, and the peroxidation proceeds. On the other hand, in the test solution 3 in which the pH is adjusted to 10.2, the pH finally becomes 8.2, and both the total nitrogen concentration and nitrate ion are sufficiently reduced.

Claims (8)

A method for treating a chemical cleaning waste liquid generated by chemically cleaning an article having a metal compound attached thereto with a chelating agent-containing cleaning liquid,
Add 5g / L or more of chloride ions to the chemical cleaning waste liquid, and subject it to electrolysis using a diamond electrode as the anode to decompose COD components, and convert organic nitrogen and ammonia nitrogen into nitrogen gas and nitrate nitrogen. A first step of oxidizing to
A chemical cleaning waste liquid containing nitrate nitrogen obtained in the first step is brought into contact with a reduction catalyst to reduce to nitrogen;
A method for treating a chemical cleaning waste liquid, comprising:   The addition of the chloride ion is performed by adding at least one selected from potassium chloride, sodium chloride, potassium hypochlorite, and sodium hypochlorite. Chemical cleaning waste liquid treatment method.   The chemical cleaning waste liquid treatment method according to claim 1 or 2, wherein in the first step, the pH of the chemical cleaning waste liquid is maintained at 8 or more and 9 or less.   The method for treating a chemical cleaning waste liquid according to any one of claims 1 to 3, wherein a reducing agent is added to the chemical cleaning waste liquid in the second step.   The chemical cleaning waste liquid treatment method according to claim 4, wherein the reducing agent is at least one selected from sodium borohydride, sodium hypophosphite, and hydrazine.   The method for treating a chemical cleaning waste liquid according to any one of claims 1 to 5, wherein in the second step, the reduction catalyst contains palladium and copper.   6. The chemical cleaning waste liquid treatment method according to claim 5, wherein the reduction catalyst comprises palladium and copper supported on activated carbon or titanium dioxide.   The method for treating a chemical cleaning waste liquid according to any one of claims 1 to 7, wherein the second step is performed at a temperature of 80 ° C or lower.