Decontamination of metal

Details Decontamination of metal Decontamination of metal

1999-06-28

2001-01-02

Mai, Ngoclan (Department: 1742)

Hazardous or toxic waste destruction or containment

Destruction or containment of radioactive waste

Chemical conversion to a stable solid for disposal

C423S018000, C423S020000, C134S003000

Utility Patent

active

06169221

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the decontamination of radioactive metal surfaces making use of aqueous solutions containing organic acids.
BACKGROUND OF THE INVENTION
Many methods are known for the decontamination of radioactive metal surfaces. Some of these known methods make use of aqueous solutions containing organic acids.
U.S. Pat. No. 4,508,641 proposes a method using formic acid and/or acetic acid as a decontamination agent in the presence of at least one reducing agent, such as formaldehyde and/or acetaldehyde. The addition of a reducing agent causes the iron ions to remain stable in the solution, the iron compounds only being separated from the decontamination solution in a second step of the overall process.
U.S. Pat. No. 5,386,078 discloses a process for decontamination of radioactively contaminated metallic objects in which the objects are contacted with an aqueous solution containing formic acid. The concentration of formic acid is from 0.05% to 5.0% by volume. The contact between the solution and the metal object is maintained until the formic acid is nearly completely stoichiometrically depleted. This procedure is repeated until the radioactively contaminated metal object has a residual radioactivity level below a permissible threshold level. A radioactive sediment is then formed by sedimenting out metallic oxides and metallic hydroxides from the aqueous solution.
GB-A-2284702 discloses a process for the decontamination of a metallic material in which the material is contacted with a solution comprising an organic acid and the resultant metal organic compound is oxidised to form a precipitate with which the contaminants are associated. The organic acid may be formic acid, acetic acid, trifluoroacetic, citric acid or oxalic acid.
The oxidation may take place at the same time as the contaminated metal dissolution to assist the kinetics of the process and may be effected by use of a chemical oxidising agent, for instance, potassium permanganate or a peroxide such as hydrogen peroxide or by an electrochemical process. The process could be carried out with a weak organic acid solution and in the presence of a low concentration of the oxidising agent.
As described in GB-A-2284702, organic acid is allowed to react completely with the metal object.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a process for the decontamination of radioactively contaminated metal which comprises contacting the metal with a decontamination reagent solution containing an organic acid and an oxidising agent, allowing said solution to react with the contaminated metal at a pH of up to 4.5, treating the resultant solution to cause substantially complete precipitation of dissolved metal together with radionuclides and separating precipitated material, containing radioactive contaminants, from said solution.
By precipitating substantially all the dissolved metal, a high proportion of radionuclides will also enter the solid phase, either by co-precipitation or adsorption or both. Adsorption will take place into the precipitated metal which, in the case where the contaminated metal is iron or steel, will be mainly in the form of ferric hydroxide.
In a method according to the present invention, the pH of the solution is carefully controlled during the decontamination process, in particular so as not to allow the pH to rise above 4.5, preferably no greater than 3. As a result, the formation of unwanted by-products (such as soluble hydroxides and mixed ternary complexes which may interfere with subsequent process stages) is reduced. A rapid and controllable decontamination reaction is promoted by the reservoir of substantially unreacted acid. Removing the contaminated substrate at a low pH and allowing the solution to reach equilibrium results in a large percentage of the total organic acid not being bound to any metal ion in solution. Contrary to the approach taken in U.S. Pat. No. 5386078, mentioned above, decontamination is terminated at a point when the acid is very far from exhaustion or stoichiometric depletion. In general, reaction between the solution and the contaminated metal is allowed to take place up to a pH at which the metal ions approach their limit of solubility. This is often found to be in the region of pH 3, particularly with metals such as iron and lead. For other metals, the appropriate termination point might be as high as pH 4.5.
A further advantage of the large remaining fraction of organic acid is that it is available to complex any unexpected increase in the metal ions in solution and thus will prevent them catalysing the destruction of the oxidising agent.
Preferably the reaction between the solution and the metal is ceased at a pH between 2.8 and 3.0.
Preferably the reaction is ceased by separating the metal from the solution.
The organic acid may be, for example, formic acid, acetic acid, trifluoroacetic acid, citric acid or oxalic acid or a mixture thereof. A preferred acid is formic acid. Preferably, the organic acid is used in an initial concentration of up to 7.5%, more preferably from 2.5% to 5.0%. It is typically present in an aqueous solution. The solution may include another solvent.
The oxidising agent may be present in the solution from the start of the reaction with the metal but is preferably added continuously or incrementally during the reaction process. The oxidising agent may be, for example, potassium permanganate or a peroxide such as hydrogen peroxide. A preferred oxidising agent is hydrogen peroxide. Preferably, the oxidising agent is present in the solution at up to 1% of the said solution, more preferably about 0.5%.
After the reaction between the solution and the metal has been caused to cease. the precipitation of substantially all of the dissolved metal is effected by any suitable process. For example, a mineral acid may be added which will cause metal precipitation and organic acid regeneration. Alternatively, the pH may be raised by any suitable means. For instance, hydrogen peroxide can be added to the solution to destroy remaining organic acid.
In the process of the present invention, there is typically produced a polyelectrolyte metal hydroxide floc at a low pH. This floc may be formed after ceasing the reaction between the solution and the metal during the raising of the pH. Alternatively, the floc may at least begin to form during the reaction between the solution and the metal substrate.
By having at least some floc present in the solution from a relatively low pH up to and including the final pH, it is possible to remove a range of radionuclides from the solution by surface adsorption and/or co-precipitation. Different radionuclides are differently adsorbed and/or co-precipitated at different pH values. By way of examples, ruthenium achieves its highest percentage removal at a pH of approximately 4.7 and manganese at a pH of approximately 7.5.
By ceasing reaction between the solution and the metal at a pH no greater than 3, only about 20% of the stoichiometric capacity of the organic acid for metal ions is utilised. Since the organic acid is preferably used with a low initial acid concentration of less than 5% wt/vol, typically 2.5% wt/vol, the acid wastage is not costly in the context of the process as a whole. In the case where formic acid is used, no liquid effluents are generated and the only waste produced is a metal hydroxide solid together with the associated radionuclides. Accordingly, there is no prohibitive cost burden associated with utilisation of only 20% of the stoichiometric capacity of the acid.
Where the oxidising agent is added during the reaction between the solution and the metal, it is preferred that it is added in a low concentration. Where an aqueous solution of hydrogen peroxide is added, the concentration is typically up to 1% by volume and preferably about 0.5% by volume. Competing reactions take place in the solution. On the one hand, formyl radicals are formed by interaction between the formic acid and the hydrogen peroxide. The formyl radicals then c

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