EP0723038B1 - Process and apparatus for regenerating a used etching solution for zirconium alloys - Google Patents

Description

The invention relates to a regeneration method. of a used acid pickling solution used for pickling of elements in zirconium alloy.

Fuel assemblies for nuclear reactors and in particular the fuel assemblies for pressurized water nuclear reactors are generally consisting of a bundle of fuel rods parallel to each other and held by a frame comprising in particular longitudinal guide tubes and transverse spacer grids.

Fuel rods can be made by a zirconium alloy sheath in which pellets of combustible material are introduced.

The frame guide tubes can be also made up of zirconium tubes.

The manufacture of zirconium tubes from of drafts requires several successive passes of cold rolling which are each followed by a treatment thermal annealing.

Between each of the cold rolling passes and the subsequent heat treatment, degreasing is carried out and a chemical pickling of the laminated tube.

The chemical pickling of zirconium alloy tubes such as Zircaloy 4 is carried out using a solution of hydrofluoric acid HF containing a certain proportion of nitric acid HNO ₃ serving as catalyst for the attack of zirconium by hydrofluoric acid, according to the chemical reaction Zr + 4HF → ZrF ₄ + 2H ₂ .

The used pickling solution which is recovered in a storage tank after pickling mainly contains hydrofluoric acid, water and nitric acid as well as zirconium fluoride ZrF ₄ formed during pickling.

Zirconium alloy pickling baths containing pollutants can be treated to separate most of the water from the solution of pollutants which must be destroyed, in approved treatment centers.

Zirconium alloy tube production units must therefore bear significant costs relating to separation and destruction polluting products in approved centers and the purchase of new acid to reconstitute the pickling solution.

In addition, the destruction of used acid solutions produces nitrates which are released to the environment.

Transport of the used solution to the treatment center is also accompanied by risks which are those of road transport of Hazardous Material.

A process for the regeneration of alloy pickling solutions zirconium can therefore have both economic advantages and ecologically.

We know from US-A-5,076,884 and EP-A-0.331.231 processes for the regeneration of pickling solutions for metals such as zirconium and hafnium. These methods use a precipitation reagent which is a sodium compound.

In US-A-3,540,513, a device for processing solutions is described pickling plants containing sulfuric acid and sulphates ferrous. The device comprises first and second evaporators under vacuum arranged in series and a crystallizer as well as vapor condensers produced by evaporators. An acid solution obtained by separation at the outlet of the crystallizer is incorporated into the distillate obtained in one condensers, then the solution obtained is heated and then stored in a container of regenerated solution.

In US-A-2,993,757, a method for recovering acid solutions used for pickling stainless steel sheets or titanium. The solution is treated in first and second evaporators vacuum in series. The vapors obtained are condensed and materials polluting solids are discharged from the second evaporator in the form of a sludge to be separated by filtration and eliminated. The distillate obtained in the condenser at the outlet of the second evaporator is reused for pickling.

However, hitherto no regeneration process has been known. without addition of reagent from a fluonitric pickling solution used for pickling elements of zirconium alloy such as tubes cladding or guide tubes for reactor fuel assemblies nuclear.

The object of the invention is therefore to propose a regeneration process a used pickling solution consisting mainly of hydrofluoric acid, nitric acid and water, after use for pickling of elements in zirconium alloy, this process making it possible to limit the operating costs of production lines for alloy elements of zirconium, the release of pollutants into the environment and the risks associated with the transport of hazardous materials.

• on évapore sous vide puis on condense une partie de l'eau contenue dans la solution usagée de manière à obtenir de l'eau légèrement acide et une solution acide concentrée polluée par du fluorure de zirconium ZrF₄, représentant à peu près 30 % en volume de la solution usagée,
• on traite par évaporation sous vide dans un cristalliseur, la solution acide concentrée polluée, de manière à obtenir des cristaux de fluorure de zirconium ZrF₄, et une solution acide concentrée épurée, et
• on mélange l'eau légèrement acide et la solution acide concentrée épurée en proportions voulues pour obtenir une solution de décapage régénérée.

For this purpose, according to the method of the invention:

• evaporated in vacuo and then a part of the water contained in the used solution is condensed so as to obtain slightly acidic water and a concentrated acid solution polluted with zirconium fluoride ZrF ₄ , representing approximately 30% by volume used solution,
• the polluted concentrated acid solution is treated by vacuum evaporation in a crystallizer, so as to obtain crystals of zirconium fluoride ZrF ₄ , and a purified concentrated acid solution, and
• the slightly acidic water and the purified concentrated acid solution are mixed in desired proportions to obtain a regenerated pickling solution.

In order to clearly understand the invention, we will now describe, with reference to the attached figure, an embodiment of the process regeneration according to the invention.

The single figure is a block diagram showing the different successive operations implemented to perform the regeneration a used fluonitric acid solution, used for pickling tubes made of zirconium alloy.

In the figure, there is shown, inside the frame 1, the operations usually carried out as part of a pickling process using an acid solution and, inside box 2, the operations for regenerating a used pickling solution after use as part of the usual process of pickling alloy tubes zirconium.

The usual pickling process consists in preparing a solution of new pickling, mixing in a container 3, hydrofluoric acid, nitric acid and water in sufficient quantity to obtain good pickling efficiency. The pickling solution is used in a pickling unit 4 in which the zirconium alloy tubes are brought into contact with the pickling solution.

As part of a manufacturing process for cladding tubes or guide tubes for assemblies of fuel for nuclear reactor, we carry out successively, on tubular blanks, three operations each followed by a heat treatment of annealed. Between each of the rolling operations and the consecutive annealing, degreasing and pickling of tubular blanks or tubes by the fluonitric mixture.

We check the characteristics pickling solution and when the solution no longer has the desired characteristics for an effective implementation of the pickling process, the used pickling solution is discharged into a tank storage 5.

In the context of industrial implementation of the pickling process according to the prior art, the solutions pickling waste stored in a tank were destroyed in an approved center as represented by step 6 which is not implemented works as part of the following regeneration process the invention.

Destruction of fluonitric acid solutions used in an approved center has the disadvantage of increasing manufacturing line operating costs of zirconium alloy tubes, due to the cost of transportation and processing for the destruction of the used solution and the obligation to prepare in the storage tank 3 a fresh acid solution to from commercial products.

In addition, the destruction process in the center approved is accompanied by a production of nitrates which must be released to the environment.

The regeneration process according to the invention as shown inside frame 2 is implemented by carrying out a separation treatment on the solution of used pickling stored in the tank 5.

The solution stored in tank 5 mainly contains water, hydrofluoric acid and nitric acid as well as zirconium fluoride ZrF4 resulting from the attack of the zirconium alloy tubes by hydrofluoric acid, catalyzed with nitric acid HNO ₃ .

The separation operations necessary for the regeneration of the pickling solution by the process according to the invention are carried out successively in a vacuum evaporator 7 and in a vacuum-crystallizer evaporator 8.

To carry out the industrial implementation of the regeneration process, ZIRCOTUBE used a vacuum evaporator marketed under the name WTSE 1000 by LED ITALIA and an evaporator WTSE 150 from the same company modified to withstand aggression by concentrated acids and to constitute a crystallizer.

The vacuum evaporators used include an evaporation chamber in which a low pressure, for example of the order of 5260 Pa (40 mm of mercury).

The solution to be treated in the evaporation chamber is introduced and maintained at a moderate temperature, for example of the order of 35 ° C.

The heating of the solution to be treated is obtained through a heat pump.

Reduced chamber pressure allows to separate at least one of the constituents of the solution which is being treated, in the form of a vapor, at a moderate temperature.

The vacuum evaporator has a stage of condensation in which the condensation of the vapor separated from the solution to be treated.

The vacuum evaporator 7 and the crystallizer evaporator 8 make it possible to obtain a distillate which is condensed in a condensing stage and a concentrate remaining in the evaporation chamber.

The vacuum evaporator 7 which can be constituted by a WTSE 1000 evaporator from LED ITALIA allows evaporation and condensation of a much of the water in the pickling solution used which is introduced into the evaporation chamber evaporator 7, from the storage tank 5.

At the outlet of the condensation stage, from the vacuum evaporator, very slightly water acid and very slightly polluted which represents 70% in volume of the initial solution treated. Inside of the evaporation chamber, after evaporation of water, a concentrated acid solution representing roughly nearly 30% by volume of the initial pickling solution introduced into the evaporation chamber.

The concentrated acid solution contains hydrofluoric acid HF, nitric acid HNO ₃ , a small amount of water and a pollutant consisting of zirconium fluoride ZrF ₄ originating from the acid attack of the tubes of zirconium alloy, during stripping.

The concentrated acid solution could be partially or totally sent for destruction but, according to the method of the invention, it is preferred to carry out a second treatment of this concentrated solution in the crystallizer 8.

The crystallizer 8 which can be a vacuum evaporator operating according to the same principle as the vacuum evaporator 7 comprises, inside its condensation chamber, a container coated with a material resistant to concentrated acids and making it possible to collect the pollutant ZrF ₄ in the form of solid crystals.

The crystallizer 8 preferably comprises a plastic coating to allow it to resist the action of acids in concentrated form.

It is carried out inside the evaporator crystallizer, in the same way as in evaporator 7, evaporation and separation of a concentrated solution acid containing in substance, hydrofluoric acid, nitric acid and a very small amount of water. Concentrated acid purified by evaporation then condensed represents approximately 80% by volume of the acid polluted concentrate introduced into the crystallizer 8.

The purified concentrated acid solution recovered in the condensing stage of the crystallizer 8 contains substantially all of the nitric acid from the initial pickling solution, because the acid nitric only plays a catalytic role during pickling of the zirconium alloy.

At the end of the evaporation of the purified concentrated acid solution, there remain in the bottom of the crystallizer, ZrF ₄ crystals constituting a dry extract representing approximately 20% by volume of the initial polluted concentrated acid solution.

The purified concentrated acid obtained at the outlet of the condensing stage of the crystallizer 8 is mixed with the slightly acidic water obtained at the outlet of the condensing stage of the vacuum evaporator 7, to obtain a regenerated pickling solution which can be returned to the solution storage tank 3 pickling.

The title of the pickling solution is set at level of a processing unit 9 in which we do penetrate the mixture of concentrated acid and water supplied in outlet of the crystallizer 8 and the evaporator 7.

The discount under the solution can be made for example by reintroducing pure water or the new acid in the mixture, at the unit of treatment 9.

The title of the pickling solution is checked by a measurement unit 10 arranged on the recycling line 11 of the pickling solution regenerated inside storage tank 3.

The regenerated acid reaching the reservoir storage 3 is rid of the pollutants formed when pickling zirconium alloy tubes. Of more, the regeneration treatment according to the invention allows to recover hydrofluoric acid from the solution pickling and almost all of the acid of the fluonitric mixture. This avoids the use large quantities of expensive new materials.

However, it remains necessary to avoid concentration phenomena in the regenerated solution and recycled, performing purges of the regenerated solution, on a regular basis, and compensate for these purges by additions of a new fluonitric acid solution.

The ZrF ₄ crystals constituting a dry extract in the crystallizer can be easily extracted, because they are soluble in water.

Pure water is therefore introduced into the crystallizer, which makes it possible to dissolve the ZrF ₄ crystals. The liquid obtained is removed, so as to empty the crystallizer. An acid solution of ZrF4 is obtained in which the ZrF ₄ can precipitate as soon as the pH of the solution reaches a value of approximately 5. It is therefore possible to recover the ZrF ₄ from the aqueous solution, by neutralizing this solution.

ZrF ₄ zirconium fluoride obtained as a by-product in the regeneration process can be used as a base product in some manufacturing.

In the case where it is not possible to use the ZrF ₄ produced by the regeneration process according to the invention, it can be separated from the aqueous solution on a filter such as a filter press and conditioned, by example, for its disposal in an underground storage center.

The method according to the invention makes it possible to limit operating costs of element production units zirconium alloy such as tubes cladding and to avoid the transport of pickling solutions used containing pollutants and treatment expensive of these solutions used in specialized treatment.

The invention is not limited to the embodiment that has been described.

This is how we can use, to perform separation of the products from the used solution, vacuum evaporators of a different type from those which have been described above.

We can perform a total recycling or only partial of the used treatment solution and we can provide any type of installation to adjust the composition of the regenerated solution and to reintroduce the regenerated solution on the pickling line of the elements made of zirconium alloy.

The invention applies to the regeneration of any fluonitric acid solution used for pickling elements made of zirconium alloy.

Claims (5)

1. A process for regeneration of a used etching solution comprising mainly hydrofluoric acid, nitric acid and water, following its use in the etching of zirconium alloy elements, characterised in that:

   some of the water contained in the used solution is vacuum evaporated then condensed so as to obtain slightly acid water and a concentrated acid solution contaminated with zirconium fluoride ZrF₄, representing approximately 30% by volume of the used solution,

   the concentrated contaminated acid solution is vacuum evaporated in a crystalliser (8) so as to obtain zirconium ZrF₄ crystals and a purified concentrated acid solution, and

   the slightly acid water and the purified concentrated acid solution is mixed in the desired proportions to obtain a regenerated etching solution.
2. A process according to claim 1, characterised in that the titre of the regenerated solution obtained by mixing the water and the purified concentrated acid solution is restored, before being re-used for etching.
3. A process according to either of claims 1 or 2 characterised in that zirconium fluoride ZrF₄ crystals obtained in the crystalliser (8) are dissolved in water in order to remove them from the crystalliser (8).
4. A process according to any of claims 1 to 3 characterised in that the vacuum evaporation of the water contained in the used solution and of the concentrated contaminated acid solution in the crystalliser (8) is performed at a residual pressure of the order of 5 260 Pa (40 mm of mercury).
5. A regeneration device for a used etching solution comprising mainly hydrofluoric acid, nitric acid and water, following its use in the etching of zirconium alloy elements, comprising:

   a vacuum evaporator (7) and a crystallising vacuum evaporator (8) arranged in series so that the crystallising vacuum evaporator (8) collects the concentrate from the crystallising evaporator (8), with each of the vacuum evaporator (7) and crystallising vacuum evaporator (8) comprising a distillate condensation stage,

   means for making a proportioned mixture of the distillates produced in the vacuum evaporator (7) and in the crystallising vacuum evaporator (8),

   a unit (9) for mixing and restoring the titre in the regenerated etching solution,

   a line (11) for recycling the regenerated etching solution in an etching solution storage tank (3), and, positioned on the recycling line (11) for the regenerated etching solution, a device (10) for measuring the titre of the regenerated solution.

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