JPH0765204B2 - Method for dissolving and removing iron oxide - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for chemically dissolving and removing iron oxide on a metal surface, which is an oxide formed on the surface of a steel sheet produced by hot rolling. , An iron oxide scale formed on the inner surface of the piping equipment in contact with water, for example an iron oxide scale formed inside the jacket of a glass lining container with a jacket and piping, an oxide scale formed on the circulating water line of a high-pressure boiler, And a method for removing iron oxides and the like formed in the metal structure of a water-cooled reactor.

<Prior Art> Iron oxides formed on the metal surface at high temperature, for example, oxides formed on the surface of a steel sheet manufactured by hot rolling, oxides formed on the circulating water line of a high-pressure boiler or the inner surface of a drum. For the removal of copper, a pickling treatment has been conventionally performed.

The pickling is usually performed by immersing the sample in a solution such as sulfuric acid, hydrochloric acid or hydrofluoric acid containing an inhibitor, or by circulating these solutions through a pipe or a device which is an object to be treated. .

There is no established technique for removing the iron oxide scale in the jacket of glass-lined equipment, and the jacket is torn and physically removed, and then welding is restored. Methods are used to clean the area that the jet reaches, or to carry out acid cleaning while suppressing the corrosion of the base material.

In addition, oxides formed on the surfaces of the piping and equipment of the primary cooling water system of a nuclear power plant show strong radioactivity with the progress of operation, and decontamination of these oxides should be performed prior to the periodic inspection of these facilities. It is necessary to reduce the radiation exposure to the human body. Examples of the decontamination method include a physical method, an electrochemical method, or a chemical method as described in “Decontamination technology in nuclear facilities” [published by Techno Project Co., December 20, 1984]. Are known. As a physical method, a water jet method in which high-pressure water is jetted, a blast method in which a blast material is jetted at an ultrahigh speed, and the surface is polished by its impact force is known. Further, as an electrochemical method, an electrolytic polishing method,
The electrolytic reduction dissolution method and the like are known. The chemical method is a method that uses a processing solution mixed with an acid, an oxidizing agent, a reducing agent, a chelating agent, etc. selected in consideration of the oxide film. By circulating these processing solutions, a complicated route can be obtained. It is excellent in that it can dissolve and remove oxides on the inner surface of pipes and equipment that have A method of decontaminating a contaminated metal surface with an aqueous solution of an inorganic acid or an organic acid has already been tried. A two-stage APAC method (method using alkaline permanganate-ammonium-citrate) or V (II) ion or Cr (II) prepared by electrolytic reduction as a reducing agent
Transition gold near-ion (Low
A LOMI method using w Oxidation state Metal Ion (Japanese Patent Laid-Open No. 57-54898, etc.) is known. The NS-1 method developed by Dow Chemical Company is typical as a method of using a treatment liquid containing a chelating agent and an organic acid.

<Problems to be Solved by the Invention> Acid pickling does not dissolve the oxide scale, but the acid enters from the weak point of the oxide scale to the inside, and a local battery occurs between Fe and Fe ₃ O _4. It is believed that this action corrodes the interface between Fe and Fe ₃ O ₄ , causing the scale to fall off. Therefore, although pickling uses an inhibitor to prevent corrosion of the base material, corrosion of the base material due to acid is unavoidable, and a great deal of effort is required in selecting the inhibitor, setting pickling conditions, and managing during implementation. Need. In addition, hydrogen is generated by the normal pickling method, and this hydrogen permeates the iron material. When the iron oxide scale inside the jacket of the glass lining refiner is removed by pickling, this generated hydrogen permeates the steel material of the equipment, reaches the glass lining side and accumulates, and after accumulating a certain amount and pressure. It becomes a strong force to the glass side, peeling off the lining glass and causing so-called acid impact, which is not preferable. Therefore, a method of adding an inhibitor while suppressing the corrosion of the base material has been attempted, but it is difficult to set conditions or control at the time of implementation, and an acid impact is often caused. The method of physically removing the iron oxide scale inside the jacket of the glass-lined equipment, such as the jet cleaning method, is also structurally limited and cannot be removed sufficiently over the entire surface.

The NS-1 method uses a decontamination solution having a concentration containing about 7 wt% of a chelating agent, an organic acid, etc., and produces a large amount of contaminated waste, which has problems in storage and treatment.

Further, according to the LOMI method, V (II) or Cr (II) ions used as a reducing agent are both obtained by electrolytic reduction, and special consideration is required for their production and storage.

Furthermore, the concentration of these metal ions used in the scavenger is
It has been shown to be used at relatively high concentrations of 10 ^{-3 to 2} M.

In view of such circumstances, the present inventor has conducted extensive studies on a method for dissolving and removing iron oxide on the metal surface, and as a result, by coexisting an organic acid salt of a reducing divalent metal with a treatment liquid containing a chelating agent, the base material It has been found that the corrosion of hydrogen and the permeation of hydrogen generated therewith to the base material can be suppressed as much as possible, and that the iron oxide on the metal surface can be easily dissolved and removed with a low-concentration treatment liquid, and the present invention has been completed. .

<Means for Solving Problems> The present invention relates to a method for rapidly and efficiently dissolving and removing iron oxide on a metal surface. More specifically, in the method of dissolving and removing iron oxide on the metal surface with a treatment liquid containing a chelating agent, an organic acid salt of a reducing divalent metal is added to the treatment liquid.
TECHNICAL FIELD The present invention relates to a method for dissolving and removing iron oxide, which is characterized in that 0.05 × 10 ^{−3 to} 1.0 × 10 ⁻³ mol / coexist and the pH of a treatment liquid during the dissolving and removing operation is kept in a range of 4.0 to 5.0.

The metal surface is a metal surface on which iron oxide may be formed, for example, the inner surface of piping equipment that is generally in contact with water, the steel plate surface as a rough material produced by hot rolling, or a nuclear power plant. The inner surfaces of pipes, equipment, etc. in contact with the primary cooling water, metal structures contaminated by radioactivity, surfaces of tools, etc.

In addition, iron oxides are generated on these metal surfaces in an oxidizing atmosphere, and are generally substances containing Fe ₃ O ₄ (magnetite) as a main component, and iron oxides formed on the metal surface at high temperature. , For example, an oxide scale formed on the surface of a steel sheet manufactured by hot rolling, an iron oxide scale formed on the inner surface of piping equipment in contact with water, such as a glass lining container with a jacket and the inside of a jacket of piping Iron oxide scale, oxide scale that forms on the circulating water line of high-pressure boilers or the inner surface of drums, and in primary reactor cooling systems of reactors, corrosive oxides from structural materials adhere and accumulate on the inner surface of equipment, forming a film, etc. It is the one that is. Fe ₃ O ₄ is added to the iron oxide scale in the jacket of the glass-lined equipment with jacket.
There was an example that it was included about 7%.

In particular, the method of the present invention suppresses the generation of hydrogen as much as possible and hardly causes acid impact, so that it is preferably used for dissolving and removing the iron oxide scale in the jacket of the glass-lined equipment.

Generally, the method of the present invention is applied to a pipe in which a treatment liquid containing a chelating agent coexisting with an organic acid salt of a reduced divalent metal is formed, a pipe in which an oxide is formed, a tool in which iron oxide is formed while circulating in a device. It is performed by adjusting the pH of the treatment liquid while stirring the treatment liquid in the container.

The chelating agent used in the present invention has the role of chelating metal ions and increasing the solubility thereof to prevent precipitation, as in the case of ordinary chelating agents.

Such chelating agents include aminopolycarboxylic acids ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetetraacetic acid, polycarboxylic acids such as oxalic acid and malonic acid. Particularly, aminopolycarboxylic acids such as ethyleneamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and hydroxyethylethylenediaminetetraacetic acid (HEDTA) are preferable in that the dissolution rate and the amount of iron oxide are large. These chelating agents are usually used as sodium salts.

The concentration of the chelating agent in the treatment liquid is not particularly limited, but it is about 1 × 10 ⁻³ to 1 × as a free chelating agent.
A sufficient effect can be obtained at 10 ^-1 mol /. If the chelating agent chelate with the metal ions and becomes less, iron will not dissolve, so it is desirable to supplement or regenerate the iron to maintain the above concentration when there is a large amount of iron oxide.

As the organic acid salt of a reducing divalent metal used in the present invention,
For example, iron (II) oxalate, iron (II) fumarate, iron (II) tartrate, nickel (II) oxalate, manganese (II) oxalate, etc. can be mentioned, and iron (II) oxalate is particularly preferable. preferable.

All of these organic salts of reducing divalent metals have low solubility in water, but use of an amount within the range of the solubility has a sufficient effect. However, if it is less than about 0.05 × 10 ^-3 mol /, the effect is small, and if it is used more than about 1 × 10 ^-2 mol /, there is a risk that these metal salts will adhere to the metal surface, and further use There is no possibility that the dissolution reaction of iron oxide will be further accelerated. Therefore, the amount of the organic acid salt of a reducing divalent metal used is preferably in the range of about 0.05 × 10 ⁻³ to 1 × 10 ⁻³ mol /. By allowing these reducing divalent metal organic acid salts to coexist in the treatment liquid, the dissolution start time of the iron oxide can be shortened and the dissolution rate can be increased.

When the iron oxide is dissolved in the treatment liquid, as the iron oxide is dissolved, the pH of the solution shifts to the alkaline side, which causes the dissolution reaction to drop sharply and eventually to almost stop.

At this time, adjust the pH of the solution that shifts to the alkaline side
By maintaining the range of 4.0 to 5.0, the dissolution of iron oxide can be continuously and rapidly advanced. The pH is preferably adjusted by adding an organic acid in consideration of corrosion of the metal base material. Organic acids used include oxalic acid, formic acid,
Examples thereof include citric acid, tartaric acid, gluconic acid, glycolic acid, lactic acid and succinic acid.

Ion exchange resin is used instead of organic acid,
Can be held. As the ion exchange resin, a commonly used strongly acidic cation exchange resin is used.

The pH of the treatment liquid shifts to the alkaline side with the iron oxide solution during the dissolution operation, which causes a decrease in the dissolution rate.At this time, a part of the treatment liquid must be circulated through the cation exchange resin. The Na ion in the solution is captured by the cation exchange resin, and the pH of the treatment liquid can be adjusted and maintained in the range of about 4.0 to 5.0. This allows the dissolution of iron oxide to proceed continuously and rapidly.

The relationship between the pH of the treatment liquid of the present invention and the corrosion of the metal base material is the solution pH.
Corrosion tends to increase in the vicinity of 4.0 or below, and p
When H is around 5.0 or more, the dissolution rate of iron oxide is extremely reduced. Therefore, the pH of the processing solution should be kept in the range of about 4.0 to 5.0.

The temperature of the treatment liquid in the present invention is appropriately in the range of about 40 to 90 ° C., preferably about 60 to 90 ° C. from the viewpoint of dissolution rate and corrosion inhibition. If the temperature is lower than about 40 ° C, the dissolution rate will be slow, and if it is higher than about 90 ° C, the dissolution rate will be too fast, resulting in more corrosion and more energy for raising the temperature, which is not preferable.

The treatment time is appropriately selected in consideration of the amount of iron oxide scale and the temperature.

<Effect of the Invention> According to the method of the present invention, the organic acid salt of a reducing divalent metal used as a reducing agent is allowed to coexist at a low concentration,
Corrosion of the base material can be minimized, and iron oxide on the metal surface can be promptly dissolved and removed. Furthermore, since the method of the present invention suppresses the generation of hydrogen as much as possible, it is effective in removing the iron oxide formed in the jacket of the glass-lined equipment without causing acid shock.

<Example> Hereinafter, the present invention will be described in more detail with reference to Examples.

The pH of the solution was measured using a bench pH meter manufactured by Horiba, Ltd. The iron concentration in the solution is determined by atomic absorption method (JIS K-012
1-82).

Example 1 A flask equipped with a stirrer containing 250 ml of water was immersed in a thermostat bath at 80 ° C, and 0.84 g of disodium ethylenediaminetetraacetate (EDTA-2Na) was stirred while the water temperature reached 80 ° C.
(9 × 10 ^-3 mol /) and 0.0225 g of iron (II) oxalate (0.5
× 10 ⁻³ mol /) was added and dissolved. Next, oxalic acid is added to adjust the pH of the solution to 4.0, and hot rolling is applied to this to produce it.
Iron pieces with iron oxide film formed on the surface (40mm × 30mm
X2mmt) was put in to carry out dissolution experiments of iron oxide. The film on the surface of the iron piece was confirmed by X-ray diffraction to be mostly magnetite (Fe ₃ O ₄ ). During the experiment, the temperature was kept at 80 ° C and the solution pH was kept in the range of 4.0 to 4.5 by adding oxalic acid. As a result of the dissolution experiment, the iron oxide film was completely dissolved and removed on the surface after 15 minutes.

Comparative Example 1 An experiment was conducted in the same manner as in Example 1 except that iron (II) oxalate was not added. Even after 60 minutes, the iron oxide film on the surface was not removed.

In Example 2 the experiment was used magnetite (Fe ₃ O ₄₎ commonly known as an iron oxide.

Immerse a three-necked flask equipped with a stirrer with 250 ml of water in a constant temperature bath at 80 ° C, and when the water temperature reaches 80 ° C, add ethylenediaminetetraacetic acid disodium (EDTA-2Na) while stirring.
0.84 g (9 × 10 ⁻³ mol /) and a predetermined amount of iron (II) oxalate shown in Table 1 were added and dissolved.

When the liquid temperature reached 80 ° C, oxalic acid was added to adjust the pH of the solution to 4.
After adjusting to 0, 0.52g (9 × 10 ^-3 mo) of magnetite powder
l /) The pH of the solution, which started to shift to the alkali side as the magnetite dissolved, was constantly maintained within the range of pH 4.0-4.5 by the sequential addition of oxalic acid. After charging magnetite
After 10 minutes, 30 minutes, and 50 minutes, the solution was sampled, filtered through a G ₄ glass filter, and the filtrate was analyzed for iron concentration in the solution by an atomic absorption method.

The dissolved amount of magnetite was calculated by subtracting the amount of iron contained in the added iron (II) oxalate.

Table 1 shows the relationship between the amount of iron (II) oxalate used and the amount of magnetite dissolved.

Example 3 Iron (II) oxalate was prepared by the same apparatus and method as in Example 2.
In place of iron (II) fumarate, iron (II) tartrate, nickel (II) oxalate, manganese (II) oxalate 0.5 × 10 ^-3 m
Dissolution experiments were performed using ol /. The results are shown in Table 2.

Comparative Example 2 Example 2 was repeated except that iron (II) oxalate was not added. The results are shown in Table 3.

Comparative Example 3 The amount of iron (II) oxalate added was 0.0225 g (0.5 × 10 ⁻³ mol /
), Except that the pH was not adjusted and maintained by adding oxalic acid. The results are shown in Table 3.

Example 4 Iron oxide was dissolved in a carbon steel (mild steel) thin plate having an iron oxide film formed on the surface thereof under the conditions described in Table 4.
At the same time, the amount of hydrogen permeation was measured. Wetted area of the thin plate is 7.0c
A m ² plate with a thickness of 1.0 mm was used. The amount of hydrogen permeation was measured by an electrochemical method according to the method described in Task 3.30 of DECHEMA method corrosion corrosion protection experimental engineering (Japan Society of Materials, Corrosion Protection Division Committee). The results are shown in Table 4.

Comparative Example 4 Example 4 except that dissolution was performed using a 10% hydrochloric acid aqueous solution.
It carried out similarly to. The results are shown in Table 5.

Example 5 In place of magnetite, 5.0 g of iron oxide scale (Fe ₂ O ₃ content: 97%) that had been formed in the jacket of a glass-lined reaction kettle was added, and iron oxide was added in the same manner as in Example 2. A dissolution experiment was conducted. Experimental conditions and results are shown in Table 6.

Example 6 A strongly acidic cation exchange resin (manufactured by IR-120B Organo Co.) regenerated into H ⁺ type on a glass column having a diameter of 1.9 cm and a length of 80 cm.
Added 100 ml. The experiment was conducted using an ion-exchange resin cylinder, a three-necked flask equipped with a stirrer, having a circulation pump and a circulation pipe. Magnetite, commonly known as iron oxide, was used in this experiment.

Immerse a 3-neck flask with a stirrer containing water 1 in a thermostat bath at 80 ° C, and add ethylenediaminetetraacetic acid ・ 2 sodium (EDTA-2Na) to 3.3 while stirring when the water temperature reaches 80 ° C.
5 g (9 × 10 ^-3 mol /) and 0.09 g (0.5) iron (II) oxalate
× 10 ⁻³ mol /) was added and dissolved. When the liquid temperature reached 80 ° C., 2.08 g (9 × 10 ⁻³ mol /) of magnetite powder was added. When the pH of the solution began to shift to the alkaline side as the magnetite dissolved, part of the solution was circulated through the ion exchange resin cylinder by the circulation pump. The pH of the solution in the three-necked flask was kept in the range of 4.0 to 4.5 by adjusting the circulation flow rate.

The solution pH after 1 hour is kept in the range of 4.0 to 4.5,
At this time, the amount of magnetite dissolved is 1.33 g / and the dissolution rate is 6
It was 3.9%.

Claims (7)

[Claims] 1. A method for dissolving and removing iron oxide on a metal surface with a treatment liquid containing a chelating agent, wherein an organic acid salt of a reducing divalent metal is contained in the treatment liquid in an amount of 0.05 × 10 ^{−3 to} 1.0 × 10 ^−3. A method for dissolving and removing iron oxide, characterized in that the pH of the treatment liquid is kept in the range of 4.0 to 5.0 during the dissolving operation. 2. A chelating agent is ethylenediaminetetraacetic acid,
The iron oxide according to claim 1, which is at least one chelating agent selected from the group consisting of nitrilotriacetic acid, hydroxyethylethylenediaminetetraacetic acid, oxalic acid, malonic acid and alkali metal salts thereof. Dissolution removal method. 3. An organic acid salt of a reducing divalent metal comprises iron (II) oxalate, iron (II) fumarate, iron (II) tartrate, nickel (II) oxalate and manganese (II) oxalate. The method for dissolving and removing iron oxide according to claim 1, which is at least one organic acid salt selected from the group. 4. The method for dissolving and removing iron oxide according to claim 1, wherein the temperature of the treatment liquid is 40 to 90 ° C. 5. The method for dissolving and removing iron oxide according to claim 1, wherein an organic acid is used as a method for maintaining the pH of the treatment liquid. 6. The organic acid is at least one organic acid selected from the group consisting of oxalic acid, formic acid, citric acid, tartaric acid, gluconic acid, glycolic acid, lactic acid and succinic acid. The method for dissolving and removing iron oxide according to the item. 7. The method for dissolving and removing iron oxide according to claim 1, wherein a part of the treatment liquid is circulated through a cation exchange resin as a method for maintaining the pH of the treatment liquid. .