EP2132361A1

EP2132361A1 - Method for removing deposits containing magnetite and copper from containers in industrial and power plants

Info

Publication number: EP2132361A1
Application number: EP08715871A
Authority: EP
Inventors: Konrad Bitter; Ursula Hollwedel; Enkhtsetseg Batchuluun
Original assignee: Areva NP GmbH
Current assignee: Areva GmbH
Priority date: 2007-03-07
Filing date: 2008-02-20
Publication date: 2009-12-16
Also published as: CN101611172A; ZA200904772B; CA2678753A1; TW200902764A; TWI387668B; CN101611172B; US7931753B2; WO2008107072A1; DE102007023247B3; US20100012154A1; KR20090119928A; UA93451C2; JP5055388B2; JP2010520466A; CA2678753C; RU2453636C2; RU2009136990A; KR101136308B1

Abstract

The invention relates to a method for removing deposits containing magnetite and copper from a container, particularly from a steam generator of a nuclear power plant, wherein - in a first step the container is treated using an alkaline cleaning solution comprising a complexing agent forming a complex soluble with iron ions, a reducing agent, and an alkalizing agent, and - in a second step a further complexing agent forming a more stable complex with iron III ions than the complexing agent used in the first step and an oxidant are added to the cleaning solution of the first step present in the container.

Description

description

Process for removing magnetite and copper deposits from industrial and power plant tanks

The invention relates to a method for the removal of magnetite and copper-containing deposits from containers of industrial and power plants, especially from steam generators of nuclear power plants. The copper comes from components such as pumps, valves, brass-plated capacitors and the like. and is present in metallic form, but also in part as copper oxide. Most of the installations of the water-steam circuit of nuclear power plants consist of carbon steel or low-alloy steels. Some of the deposits adhere to the component surfaces as deposits, and some of them settle as sludge in containers, such as the steam generators, such as a steam generator. The deposits must be removed from time to time because, for example in the case of steam generators, they hinder the heat transfer to heat exchanging walls or cause selective corrosion. To remove the deposits, the inner surfaces of the container are contacted with a cleaning solution, usually at elevated temperature, to dissolve the deposits containing magnetite (Fe ₃ O ₄ ), copper oxide (Cu ₂ O) and metallic copper. In order to avoid a corrosive attack, which is caused by a reduction in the pH of the cleaning solution by mixing in a mixture, the material of the container, which is referred to below as the base metal, is generally in alkaline solution (pH> 7 ) worked. Metallic copper leaves dissolve only in the presence of an oxidizing agent. The dissolution of the magnetite is usually under reducing conditions to avoid oxidative dissolution of the base metal. In such methods, the magnetite is first dissolved under reducing conditions with the addition of a complexing agent. After removal of the cleaning solution and optionally rinsing of the container, the metallic copper is dissolved with an alkaline cleaning solution in the presence of an oxidizing agent and a complexing agent. As the oxidizing agent strong oxidizing agents such as oxygen and hydrogen peroxide are used, which would convert the dissolved Fe ²⁺ immediately into Fe ³⁺ . Therefore, the container must be emptied before performing the copper dissolution, which increases the amount of cleaning solution to be disposed. If the reaction solution of the magnetite dissolution were not removed and an oxidizing agent was added to this solution, this would convert the complex-dissolved iron-II into iron-III, which would react with the parent metal with the dissolution of elemental iron.

A method which is intended to remedy this situation is known from DE 198 57 342. The magnetite and copper dissolution is carried out with a single cleaning solution, which is changed after the completion of the iron dissolution so that it is suitable for the copper dissolution. First, the container is treated with an alkaline cleaning solution at a temperature of about 160 ⁰ C, which contains a reducing agent, such as hydrazine, and as a complexing agent nitrilotriacetic acid (NTA). NTA forms a soluble complex with Fe-II ions, accelerating the magnetite dissolution and keeping the iron II in solution in complexed form. Due to the reducing agent is in Magnetite containing iron-III to iron-II and the Cu-I of the above-mentioned copper oxide reduced to metallic copper. The alkalizing agents used are, for example, ammonia or morpholine. To dissolve the copper, the cleaning solution is cooled to 50 ^° C. to 160 ^° C., its pH is increased, and oxygen is introduced to adjust the oxidizing conditions or hydrogen peroxide is added. A disadvantage of this method is a relatively high base metal removal.

From US 3,627,687 a method is known in which magnetite and copper are dissolved with a cleaning solution which is from the beginning designed to simultaneously dissolve magnetite and copper. It is adjusted to a pH of 7 to 10 and contains 1% to 10% of a polyacetic acid, for example ethylenediaminetetraacetic acid (EDTA) as a complexing agent and 0.1 to 5% of a polyethyleneimine. This method is associated with a relatively high base metal removal despite the use of corrosion inhibitors. In addition, most inhibitors work at temperatures above 120 ⁰ C or decompose. Inhibitors which can be used at the temperatures mentioned contain sulfur.

The object of the invention is to propose a method of the aforementioned type, which achieves a low base metal removal without draining the cleaning solution between the magnetite and the copper dissolution.

This object is achieved according to claim 1 by a method in which, in a first step, the container is treated with an alkaline cleaning solution comprising a complexing agent which forms a soluble complex with iron (II) ions, a reducing agent and an alkalizing agent contains, and in a second step in the cleaning solution of the first step in the container, another complexing agent which forms a more stable complex with ferric ions than the complexing agent used in the first step, and an oxidizing agent are metered.

In the method according to the invention, the dissolution of the magnetite is carried out in virtually the same way as in the method of DE 198 57 342. An attack of the cleaning solution on the base metal and a corresponding material removal are relatively low in such a procedure, especially if at temperatures from 140 ⁰ C to 180 ⁰ C, as in a preferred process variant, worked. At such temperatures, the reaction between the complexing agent and the iron II originating from the magnetite proceeds much faster than the base metal attack, which also proceeds via iron II. In the second process step, it is problematic that the cleaning agent still contains the iron II complex of the first process step. If, in fact, the oxidizing agent required for the dissolution of metallic copper is metered in, it can hardly be avoided that the complexed iron (II) is oxidized and an iron (III) complex is formed. Iron (III) complexes of the complexing agents used in processes of the present type, such as EDTA and NTA, are less stable in alkaline solution than the corresponding iron (II) complexes, ie, under the conditions prevailing in the second process step, they can be destroyed, with the liberated iron -III ions with hydroxide ions present in the solution form a sparingly soluble precipitate of iron hydroxide, which would have to be removed by extensive rinsing out of the container. In addition, it would come from magnetite coatings or - Deposits liberated surfaces to a reaction between the elemental iron of the parent metal and ferric ions to form ferrous iron: Fe + 2 Fe ³⁺ > 3 Fe ²⁺ . The divalent iron is oxidized to trivalent iron due to the presence of the oxidizing agent, which in turn reacts with the iron of the parent metal. In addition to the copper dissolution, therefore, there is a corrosion of the base metal. According to the invention, this undesired reaction is at least suppressed by adding a complexing agent which, under the conditions prevailing in the second step, forms a complex with iron (III) ions which is more stable than the corresponding complex of the complexing agent of the first process step. In this way, the concentration of free iron (III) ions is reduced by, for example, immediately trapping freshly formed iron (III) ions. A Grundmetallabtrag in the second process step is thereby prevented or at least reduced.

In a preferred process variant, the further complexing agent and then the oxidizing agent are added first. As a result, any free, that is not complexed, iron II ions are bound by the further complexing agent in the cleaning solution so that, upon addition of the oxidizing agent, there are no longer any free iron II ions from which iron (III) ions can form. This effect is particularly effective if, before the addition of the oxidizing agent, thorough mixing of the cleaning solution takes place, preferably by blowing in a gas which is weakly or only slightly oxidizing, such as air or better an inert gas such as nitrogen or argon. The oxidizing agent added in the second process step has two functions. It serves on the one hand to oxidize metallic copper to Cu-II, which is complexed by the other complexing agent and optionally by excess complexing agent of the first process step. By metering in a quantity of oxidizing agent which is more than stoichiometric in comparison with the amount of copper to be dissolved, unconsumed reducing agent is neutralized in the first process step.

Conventional methods use hydrogen peroxide or oxygen to oxidize the copper. These are very strong oxidizing agents with a correspondingly strong oxidation action against free or complexed iron-II and against the mentioned iron complexes. Thus, the use of such an oxidizing agent is always accompanied by an increase in the concentration of free iron (III) ions and a corresponding base metal erosion. According to the invention, this negative effect is at least mitigated by using an oxidizing agent whose redox potential in alkaline solution is lower than that of hydrogen peroxide or oxygen. As demonstrated below experiments, hydroxylamine is particularly suitable. Its oxidizing power is sufficient to oxidize metallic copper and hydrazine unused in the first process step. However, its oxidative effect on complexed or free iron-II is weaker than is the case with the classic oxidants oxygen and hydrogen peroxide, so that a basic metal removal takes place only to a reduced extent. The addition of the polyethyleneimine is primarily for the purpose of preventing the formation of free Fe-III ions. Accordingly, in a preferred variant of the method, a quantity which is substoichiometric in relation to the amount of copper to be dissolved is added. At best, some of the copper ions are complexed by the polyethyleneimine. To bind the remaining amount of copper or in general to complex the copper, the cleaning solution is used a further complexing agent, for example a complexing agent already used in the first process step, such as EDTA or NTA. The used is polyethyleneimine at least on a part of the N-atoms of the main chain a carboxyl group, for example CH ₃ COO ^"bound.

The copper dissolution is accelerated in a preferred process variant in that the cleaning solution ammonium in the form of at least one ammonium salt, preferably ammonium carbonate is supplied. Ammonium ions catalyze the dissolution of copper in the presence of an oxidizing agent in a manner known per se. Unlike chloride or sulfate-containing ammonium salts, ammonium carbonate does not cause corrosion. Further acceleration of the copper dissolution takes place with the aid of ammonium nitrate.

The first process step is preferably carried out at a temperature of 140 ⁰ C to 180 ⁰ C. The addition of a corrosion inhibitor is not required because a corrosive attack of the complexing agent on the base metal is practically not to be feared. Namely, at such high temperatures, the complexing reaction between the complexing agent and iron II and / or iron III ions originating from the magnetite proceeds much faster than the Dissolution of the base metal by the complexing agent. However, the first process step does not necessarily have to be carried out in the stated high-temperature range. Also conceivable is a temperature below 100 ^° C., for example in the range from 80 ^° C. to 95 ^° C. However, the addition of a corrosion inhibitor is expedient, since the complexing of the magnetite-derived iron (II) and / or iron (III) Ion is slowed down and accordingly more complexing agent is available for the dissolution of the base metal.

The implementation of the second process step is generally carried out at a temperature of below 100 ⁰ C, preferably in the range of 80 ⁰ C to 95 ⁰ C. At low temperatures, the risk of hydroxylamine decomposing to NO ₂ is significantly lower than at higher temperatures. NO ₂ would decompose the complexing agents used.

The effectiveness of the proposed method has been tested by a variety of experiments. Three of these experiments are explained in more detail below:

In the case of a temperature above 100 ⁰ C Verfahrenstempe- temperature the experiments in a stainless steel autoclave (TA2) (Test no. 508) were (# 512, 507 and no.) Is performed, otherwise in an open container such as a beaker , To simulate deposits containing magnetite and copper, original deposits or sludges from a steam generator of a nuclear power plant are added to respective tanks in an amount of 15 g, containing therein 85% magnetite, 10% Cu and 5% Cu ₂ O. It should be noted that percentages generally refer to percent by weight. In order to reduce the material removal of carbon steel To test surfaces, samples of this material were hung on Teflon coated stainless steel rods in the autoclave.

Experiment No. 507:

This experiment relates to a variant of the method in which the Magnetitauflösung at a temperature of more than 100 ⁰ C, specifically at 160 ⁰ C and the copper in the unpressurized range, ie at a temperature below 100 ⁰ C temperature, namely at about 90 ⁰ C. , After heating the autoclave to 160 ⁰ C, 445 ml of deionized are filled and purged with argon to remove air or dissolved in the deionized oxygen. Then, 200 ml of an aqueous reaction solution containing 65.6 g of (NH ₄ ) ₃ -EDTA is added, which is an excess of 5% over the stoichiometric amount, that is, the amount of EDTA required to complex the amount of iron contained in the magnetite. The reaction solution also contains 22 ml of a 25 percent hydrazine hydrate solution. The metered amount of hydrazine is four times the stoichiometric amount. The excess ensures that despite a loss of hydrazine due to thermal or catalytic decomposition (due to the presence of metallic copper) is always a sufficient amount for reducing the magnetite present in the magnetite available. During the magnetite dissolution, a pH of approx. 9 is set in the cleaning solution.

After about 2 hours, the second process step is initiated by the solution is cooled to 80 ⁰ C and a complexing agent is metered, compared to the complexing agent used in the first step 1 (EDTA) Fe III ions bind more firmly, namely a polyethyleneimine available under the trade name Trilon P from BASF in the form of the 1: 3.4 diluted aqueous original BASF solution. Trilon ^ P has a molecular weight of about 50,000 and a nitrogen / carbon atom ratio in the main chain of 0.5. Above all, this complexing agent binds possibly present free iron (III) ions, which is the case, for example, when the amount of magnetite sludge present in a container was underestimated and therefore an insufficient amount of EDTA was added. In order to achieve the most complete possible binding of the free iron (III) ions by Trilon P or a complexing agent with a comparatively strong affinity for iron ions, the cleaning solution is mixed by blowing an inert gas. Now, 200 ml of an aqueous solution containing 36 ml of a 50 percent hydroxylamine solution is fed. The amount of hydroxylamine contained therein is double stoichiometric with respect to the presence of metallic copper and residual hydrazine. The excess oxidant ensures that all residual hydrazine is neutralized and sufficient oxidant is available to oxidize all copper to Cu-II. Subsequently, EDTA is fed into the autoclave in an amount which is more than stoichiometric with respect to the amount of copper present (dissolved Cu-II), for example with an excess of 7.2%, in order to bind the Cu-II formed. In order to monitor the progress of the copper dissolution, small samples of the cleaning solution are continuously withdrawn and their copper content is determined, for example by titration. At the end of the second process step, after about 6 hours, in the present case, 85% of the originally present metallic copper went into solution (see table below) and can - in the application, for example in the cleaning of a steam generator - by draining the cleaning solution from the container. Under the conditions of the described experiment, only a removal of the C-steel samples (or of the base metal in the application case) of 7 μm or a weight loss of 0.0029 g / cτn ² results (see table below). Of the available magnetite go 96% in solution.

Experiment No. 508:

In a procedure according to experiment 508, the first process step is carried out at a temperature below 100 ^° C., specifically at 92 ^° C. A container to be cleaned can be opened to the atmosphere. Accordingly, no autoclave is required for the experiment. In an open container (a beaker) are added 1000 ml deionized water and added to 400 ml of an aqueous solution after heating to 92 ⁰ C, the 68 g (NH ₄ J ₂ -EDTA, 3.8 g of hydrazine hydrate, 10 ml Korantin PM and 2 ml of Plurafac contains Korantin PM is a corrosion inhibitor, Plurafac is a surfactant Both are available from BASF A surfactant improves the adhesion of the inhibitor on the bare surfaces of the parent metal.

The amount of EDTA used corresponds to 111% of the stoichiometric amount required for complexing the amount of iron present (10.4 g). The reducing agent (hydrazine) is gem. As in the high temperature method. Experiment 507 added in excess (about 4-fold stoichiometric). During the magnetic dissolution, a pH of about 9 is maintained.

By sampling and performing analyzes, the progress of the magnetite dissolution is monitored. If there are signs net, that the magnetite dissolution approaching its end, in the present case after about 20 hours, the process step 2 is initiated by dosing 50 ml of a 1: 3.4 diluted aqueous solution of Trilon P whereby the cleaning solution is cooled to about 85 ° C. , After mixing by blowing an inert gas, 100 ml of a reaction solution containing 26 ml of a 50% aqueous hydroxylamine solution are added, which corresponds to about 20 g of hydroxy lamin. This amount is four times stoichiometric with respect to the existing metallic copper and residual hydrazine. After mixing with inert gas, finally, 100 ml of a reaction solution are added, the 15.5 g (NH ₄ ) ₂ -EDTA and to accelerate the copper dissolution and added as a buffer 20 g of ammonium carbamate and 20 g of ammonium nitrate. After about 6 hours, the copper dissolution is completed. It achieves a magnetite resolution of 96%, a copper resolution of 85% with a removal of the C-steel samples of 18 microns and a weight loss of 0.0113 g / cm ² .

Trial No. 512:

Experiment 512 essentially forms the method acc. US Pat. No. 3,627,687, in which the magnetite and the copper dissolution is carried out with one and the same alkaline cleaning solution. The cleaning solution acc. The said US patent essentially contains EDTA and as further complexing agent a polyethyleneimine, which is used in the present experiment in the form of Trilon P. In an autoclave of the type used in Experiment No. 507, the copper-containing magnetite slurry used in the other experiments and 550 ml of deionized are added. After rinsing with inert gas is heated to 160 ⁰ C and 240 ml of a added to aqueous reagent solution. This contains 61 g (NH ₄ ) ₃ - EDTA and 12 ml Trilon ^ P (original solution from BASF). At the beginning of the experiment, the cleaning solution has a pH of about 9. After about 6.5 hours, the experiment is terminated. The result is a magnetite resolution of 87%, a copper resolution of only 5.14% with a removal of the C-steel samples of 27 microns and a weight loss of 0.0213 g / cm ² .

Claims

claims

1. A method for removing magnetite and copper-containing deposit from a container, in particular from a steam generator of a nuclear power plant, in which

in a first step the container is treated with an alkaline cleaning solution which contains a complexing agent which forms a soluble complex with iron ions, a reducing agent and an alkalizing agent, and

- In a second step in the present in the container cleaning solution of the first step, another complexing agent that forms a more stable complex with ferric ions than the complexing agent used in the first step, and an oxidizing agent are added.

2. The method according to claim 1, characterized in that first the further complexing agent, and then the oxidizing agent is added.

3. The method according to claim 1 or 2, characterized in that after the addition of the further complexing agent and before the addition of the oxidizing agent, the cleaning solution is mixed by blowing a gas.

4. The method according to claim 3, characterized in that air, nitrogen and / or argon are injected.

5. The method according to any one of the preceding claims, characterized in that in comparison to the amount of copper present in the container, a superstoichiometric amount of oxidizing agent is added.

6. The method according to any one of the preceding claims, characterized in that an oxidizing agent is used whose redox potential is lower than that of hydrogen peroxide.

7. The method according to claim 6, characterized in that hydroxylamine is used as the oxidizing agent.

8. The method according to any one of the preceding claims, characterized in that in the second process step as a further complexing agent, a modified polyethyleneimine is added.

9. The method according to claim 8, characterized by the use of a sodium-free modified polyethylene lenimins.

10. The method according to claim 8 or 9, characterized by the use of a carboxyl group-containing polyethyleneimine.

11. The method according to claim 8, 9 or 10, characterized in that the polyethyleneimine is added in a substoichiometric amount with respect to the amount of copper to be dissolved and another complexing agent.

12. The method according to any one of the preceding claims, characterized in that the cleaning solution located in the container at least one ammonium salt is supplied.

13. The method according to claim 12, characterized by the addition of ammonium carbonate.

14. The method according to claim 12 or 13, characterized by the addition of ammonium nitrate.

15. The method according to any one of the preceding claims, characterized in that the first process step is carried out at a temperature of 140 ⁰ C to 180 ⁰ C.

16. The method according to any one of the preceding claims, characterized in that the second process step is carried out at a temperature of less than 100 ⁰ C.

17. The method according to claim 16, characterized by a temperature of 80 ⁰ C to 95 ⁰ C.