EP0127906A1 - Process for removing heavy metallic elements suspended in a liquid - Google Patents

Abstract

1. Process for the removal of heavy metal particles in suspension in a nitric medium, characterized in that : - an appropriate quantity of a finely divided ferromagnetic adjuvant in particulate state is fixed within the magnetized packing of an electromagnetic filter ; - another appropriate quantity of the same finely divided ferromagnetic adjuvant in particulate state is added to the said nitric medium containing the heavy metal particles to be extracted ; - the nitric medium containing the finely divided ferromagnetic adjuvant in particulate state and the said heavy metal particles to be extracted is stirred, the stirring time being limited so as to avoid an excessive dissolution of the said ferromagnetic adjuvant in the said nitric medium ; and - the said nitric medium containing the heavy metal particles to be extracted and the said ferromagnetic adjuvant in suspension is filtered by means of the preblocked electromagnetic filter.

Description

It is known that irradiated fuels from the electro-nuclear industry are reprocessed in order, in particular, to recover the fissile materials contained in the irradiated elements by separating them from the fission products. To this end, these fuels are dissolved in a nitric medium. After complete dissolution and adjustment of the acidity and of the uranium concentration, the solution is subjected to several solvent extractions in order, firstly, to rid the uranium and the plutonium of the fission products associated with them, then to separate them from each other.

However, certain fission products present in irradiated fuels, such as ruthenium for example, are not very soluble in the nitric medium from the dissolver in the reprocessing chain. As ruthenium 106 is abundantly produced by the fission of uranium, deposits of ruthenium form and strongly contaminate the walls of the components of the reprocessing chain which contain the nitric solutions.

For example, in a dissolver containing a nitric fuel solution with 300 grams of uranium per liter, there is a proportion of solid products resulting from the presence of approximately 3 kg of platinoid elements, of approximately 0, 2 kg of colloids and around 2.8 kg of cladding residues per tonne of fuel. In the mixture of platinoid elements, ruthenium is by far the most abundant element. Because of these drawbacks, attempts have been made to eliminate ruthenium in suspension in nitric solutions for reprocessing nuclear fuels.

Particles up to a few microns in size can be separated from the nitric solution by filtration or centrifugation. On the other hand, the ruthenium particles whose size is of the order of or less than one micron cannot be removed by these conventional methods.

Document US-A-3 351 203 discloses a process for extracting non-magnetic particles such as particles of glass, plastic or stainless steel suspended in a liquid, for example sewage. According to this method, ferromagnetic particles are distributed inside the liquid to be filtered and a filter 24 is produced by means of an electromagnet 16 disposed around the pipe 15 in which the liquid is circulated. When the majority of the particles have been retained, the filtering operation is carried out on the particles in suspension in the liquid.

However, such a process does not apply to the filtration of heavy metal particles, such as for example ruthenium.

The problem of the formation of radioactive deposits originating in particular from ruthenium in the various devices constituting the reprocessing chain therefore arose from the construction of the first reprocessing plants and has so far found no solution.

This problem is solved according to the present invention. Indeed, it specifically relates to a process for the elimination of metal particles of this size suspended in a liquid.

This process is particularly applicable to the elimination of heavy metallic elements such as ruthenium in suspension in nitric solutions for reprocessing irradiated fuel elements in nuclear reactors. But it also applies to the filtration of other liquids charged with insoluble metal particles, such as for example water from irradiated fuel element storage pools.

The method according to the present invention is based on the use of a finely divided ferromagnetic filter aid, in order to separate, by means of a magnetic field, the insoluble heavy metal elements which could not be effectively extracted by other means. By heavy elements is meant particles whose mass is large relative to that of the particles of the ferromagnetic adjuvant used. Thus, in the case of ruthenium particles and as an adjuvant of magnetite, a large fraction of the ruthenium particles has a diameter of a few tenths of a micron, and the magnetite has a diameter distribution curve which has a maximum at around 0.2 pm; moreover, the density of ruthenium is close to 12, and due to the agglomeration of the particles of suspended magnetic adjuvant, these have an even more apparent density, in particular lower than the density of the homogeneous massive magnetite which is close to 5. For example, for an apparent density of magnetite of 3, an average diameter of magnetite particles equal to 0.2 u and a diameter of ruthenium particles equal to 0.6 µ, we find that the ratio of masses is equal to 108.

The insoluble elements are found in the solution in metallic form. In the case in particular of a nitric solution of irradiated fuels, they cannot be included in a crystalline network, such as for example a crystalline network of ferrite as that can occur with other elements such as copper, manganese in a mixture of iron-based corrosion products in water at high temperature. Furthermore, these elements cannot be the subject of an electrostatic attraction such as that which would be created between a ferromagnetic adjuvant and dielectric elements such as fibers, textiles, etc. Under these conditions, the forces of attraction between the particles of the ferromagnetic adjuvant and the metallic particles, of the force type of VAN der WAALS, cannot be sufficient to retain the latter. On the contrary, the process according to the invention is based on the fact that the particles of this adjuvant tend to spontaneously agglomerate into larger grains. The agglomeration is also due to the attraction of the tiny magnetic dipoles that the particles become adjuvant if they are affected by a magnetic field. The larger particles that result from these phenomena, the last of which is called magnetic coagulation, are very spongy in nature. They contain a large quantity of occluded liquid. One or more non-ferromagnetic particles trapped according to this process in such an agglomerate of magnetite, are then naturally transported with it under the effect of a magnetic field gradient. Independently of the entanglement in the spongy structure during the spontaneous formation of an agglomerate or during the magnetic coagulation of particles of adjuvant, it can occur, in a nonhomogeneous magnetic field, due to displacements of the particles of adjuvant in the direction positive of the field gradient, a transfer of kinetic energy by the shocks of ferromagnetic particles in large quantities on the heavy metallic particles which thus move in the direction of the former.

It is possible to determine the yield of the process according to the invention. For example, in the case where ruthenium particles contained in the nitric medium for dissolving irradiated fuel are extracted. To this end, fine ruthenium powder is placed in a 4N solution of uranyl nitrate, which is stirred and slightly heated, for example around 50 ° C., to simulate a nitric fuel solution. irradiated which spontaneously heats up under the effect of the decaying radiation of the fission products. To simplify the measurement of the amounts of ruthenium, the metal powder was previously irradiated in a reactor in order to mark it with radioactive ruthenium 103, a gamma emitter. After addition of finely divided magnetite, which was prepared separately by alkaline precipitation in a solution of ferrous iron, and whose particles initially measure from 0.1 μm to a few μm, the stirring of the liquid is continued for a few minutes. By measuring the radioactivity of ruthenium 103 in the liquid before and after the extraction of the magnetite by means of one or more magnets, the yield of the ruthenium extraction can easily be appreciated. On the other hand, the material balance of ruthenium 103 which can easily be established by measuring the activity of this radionuclide in the aliquot of metallic ruthenium before the experiment, then in the magnetite sludges after the extraction, makes it possible to monitor this performance.

Extraction yields of between 97 and 99% were thus measured.

According to the invention, the adjuvant is said to be "static", that is to say it is first fixed in the lining of an electromagnetic filter within the limit of the usable capacity thereof, the suspension of non-magnetic particles being then filtered by means of the pre-blocked electromagnetic filter. This embodiment applies more particularly to the case where it is desired to limit the duration of contact of the adjuvant with a liquid which would risk dissolving it in undesirable quantities.

• - précolmatage d'un filtre électromagnétique au moyen d'un adjuvant ferromagnétique finement divisé ;
• - agitation du liquide contenant les particules métalliques lourdes afin de les maintenir en suspension
• - filtration du liquide contenant les éléments métalliques lourds en suspension au moyen dudit filtre électromagnétique précolmaté.

The ferromagnetic adjuvant then forms a layer of mud having a large surface in which the ruthenium particles are trapped. This process is characterized by the following successive stages:

• - pre-clogging of an electromagnetic filter by means of a finely divided ferromagnetic adjuvant;
• - agitation of the liquid containing the heavy metallic particles in order to keep them in suspension
• - filtration of the liquid containing the heavy metallic elements in suspension by means of said precolated electromagnetic filter.

• - addition, dans un liquide auxiliaire, d'un adjuvant ferromagnétique ;
• - agitation de ce liquide auxiliaire, afin de réaliser une suspension de l'adjuvant ferromagnétique ;
• - filtration du liquide auxiliaire au moyen d'un filtre électromagnétique.

Preferably, the pre-clogging of the electromagnetic filter comprises the following steps:

• - addition, in an auxiliary liquid, of a ferromagnetic adjuvant;
• - Agitation of this auxiliary liquid, in order to produce a suspension of the ferromagnetic adjuvant;
• - filtration of the auxiliary liquid by means of an electromagnetic filter.

In the preferred case where the lining of the electromagnetic filter consists of steel balls, the efficiency of the process according to the invention is enhanced by the fact that the speed vector of the particles to be separated must rotate by 90 °, when these go from one layer of balls to the next. The inertia of these particles leads them to strike the surface of the balls arranged in their path and to remain fixed in the layer of ferromagnetic adjuvant.

Preferably, the ferromagnetic adjuvant used is ferrite, or alternatively magnetite.

The process for removing heavy metallic elements according to the invention leads to the production of a dense sludge, constituted by a mixture of the ferromagnetic adjuvant and heavy metallic elements.

This sludge is retained in the filter lining.

This dense sludge can be easily separated from the wash water by decantation. The metal particles can then be separated from the magnetite, either to isolate them under a small volume of waste, or with a view to the profitability of the metals which are their main constituents.

• - la figure 1 illustre les étapes de précolmatage, d'agitation et de filtration du procédé selon l'invention ;
• - la figure 2 illustre une application à la chaîne de dissolution du combustible irradié du procédé selon l'invention décrit en référence à la figure 1.

In any case, the characteristics and advantages of the invention will appear better after the following description of examples of implementation, given by way of explanation and in no way limiting, of the method according to the present invention, with reference to the appended drawings in which:

• - Figure 1 illustrates the pre-clogging, stirring and filtration steps of the process according to the invention;
• FIG. 2 illustrates an application to the dissolving chain of the spent fuel of the method according to the invention described with reference to FIG. 1.

In FIG. 1, the method according to the invention is illustrated, using a static ferromagnetic adjuvant. This process is called "static" because the ferromagnetic filter aid is fixed in the lining of the electromagnetic filter before the nitric solution is filtered. It advantageously applies when it is desired to avoid prolonged contact of the magnetite with the nitric solution.

The electromagnetic filter designated by the reference 10 schematically comprises a non-magnetic envelope 12, partially filled with a magnetizable lining 14, and disposed inside a winding 16 supplied by a voltage source 20. Preferably, the magnetizable lining 14 is made up of steel balls, steel the grade of which is chosen for its magnetic properties and its resistance to corrosion in a nitric medium.

The lining 14 is supported by a grid 15. An iron frame 17 formed of at least two stirrups constitutes with the lining 14 of the filter a magnetic circuit and limits the leakage magnetic field.

Capacity 30 contains the nitric solution 2 of spent fuel. This capacity 30 can be the dissolver or a relay capacity. According to the invention, magnetite 2 is added to the nitric solution 2 at a rate of 3 g per 250 mg fraction of insoluble platinoid elements to be separated. The nitric solution of irradiated fuel 2 heats up slightly under the effect of ionizing radiation from the fission products. It is stirred for some time by means of the agitator 31 so as to maintain in suspension in the nitric solution the magnetite and the insoluble particles including the fission products platinoids. To limit the dissolution of the magnetite, the contact time of the latter and of the solution 2 is preferably short, of the order of fifteen minutes.

According to the method of the invention, the first step consists in fixing, inside the ball lining 14 of the filter 10, an appropriate quantity of ferromagnetic adjuvant. For this purpose, there is a capacity 60, independent of the dissolution chain containing a certain volume of water inside. from which a suspension of the ferromagnetic adjuvant is produced.

The volume of water contained in the capacity 60 is preferably reduced for reasons of convenience, but it is however sufficient for the suspension to be perfectly fluid, so as to distribute the adjuvant over the entire lining 14 by saturation of layers successive. Such a suspension can contain, for example, several tens of grams of magnetite per liter; it is prepared by introducing into the container 60 the quantity of magnetite corresponding approximately to the usable capacity of the electromagnetic filter 10, and by homogenizing it by stirring.

One can also proceed simply by introducing the magnetite into the water, the pH of which has been previously adjusted by alkalization to the approximate value of the pH at the isoelectric point of the suspension. The value of this pH is around 9.5. When this value is reached, the suspension remains substantially homogeneous for a fairly long period and agitation becomes unnecessary.

The nitric solution of spent fuel 2 is contained in a capacity 30 of the dissolution chain. This capacity can be the dissolver or a relay capacity. The insoluble solid particles in suspension are kept by stirring by means of a stirrer 31.

The content of the container 60 is then filtered so as to fix the ferromagnetic adjuvant in the lining 14. The effluent from the filter 10 consists of clear water 61 which is collected in a container 62. If the magnetite suspension has been homogenized at the pH of the isoelectric point, the adjuvant fixed in the filter lining must be washed with water to extract the alkali residue.

The nitric solution of irradiated fuel 2 contained in the container 30 is then passed through the filter 10 thus pre-clogged, the quantity of this being chosen so that the approximate mass of insoluble platinoid elements which it contains corresponds to the ratio mass of ruthenium and ferromagnetic adjuvant contained in the filter allowing the best extraction yield.

After fixing insoluble solid particles in the lining 14 of the filter 10, the filter is cleaned.

This unclogging can be carried out in different ways.

The document FR - A - 2 341 347 relates to a method of unclogging an electromagnetic filter. It describes such a method applicable in the case of an electromagnetic filter placed in a pipe and operating in a closed circuit, the filter being traversed from bottom to top by the liquid which it is desired to filter.

In the case where the liquid to be filtered circulates in the opposite direction, that is to say from top to bottom, as is the case in the description made with reference to FIG. 1, any other simple method may be used. For example, we can demagnetize the lining 14, and entrain the mud which hangs on the balls by means of a current of washing water. It is also possible to drain the filter 10 from its lining of balls 14, for example by emptying the casing 12 into a washing container containing water. The clean beads are then removed to be replaced in the envelope 12.

FIG. 2 shows an application to the nitric dissolution chain of irradiated fuel of the method of implementing the method according to present invention using a static ferromagnetic adjuvant described with reference to Figure 4. In this figure, the reference 50 denotes a dissolver containing the nitric solution 2 to be filtered. In parallel with this dissolver 50, there is a capacity 90 containing the ferromagnetic adjuvant suspended in water stirred by means of an agitator 91. According to a variant, the suspension of adjuvant in water is stabilized by alkalization until 'at pH at isoelectric point.

• - on fixe l'adjuvant ferromagnétique contenu dans la capacité 90 dans le garnissage des filtres électromagnétiques 10a et 10b jusqu'à concurrence de leur capacité utilisable. L'effluent de ces filtres est de l'eau claire ou encore de l'eau moyennement alcaline. Pour éviter le danger d'un bouchonnage ultérieur par des précipitations qui pourraient se produire dans la solution nitrique, l'adjuvant dans le garnissage des filtres 10a et 10b est lavé avec de l'acide nitrique étendu au même pH que celui de la solution de combustible 2, et provenant d'une capacité 92 jumelée avec la capacité 90 ;
• - on filtre la solution nitrique du dissolveur 50 à raison, pour chaque filtre électromagnétique, d'un volume de solution 2 tel que la charge de solides insolubles qu'il contient corresponde à un rapport optimal des quantités d'adjuvant ferromagnétique et de particules insolubles.

The removal of heavy metallic elements suspended in solution 2 takes place as follows:

• - The ferromagnetic adjuvant contained in the capacity 90 is fixed in the lining of the electromagnetic filters 10a and 10b up to their usable capacity. The effluent from these filters is clear water or moderately alkaline water. To avoid the danger of subsequent plugging by precipitation which could occur in the nitric solution, the adjuvant in the lining of the filters 10a and 10b is washed with extended nitric acid at the same pH as that of the solution. fuel 2, and coming from a capacity 92 combined with capacity 90;
• the nitric solution of the dissolver 50 is filtered at the rate, for each electromagnetic filter, of a volume of solution 2 such that the charge of insoluble solids which it contains corresponds to an optimal ratio of the quantities of ferromagnetic adjuvant and insoluble particles .

The operation of filters 10a and 10b is alternative. The filter 10b takes over from the filter 10a when the latter is being unclogged.

The nitric effluents from filters 10a and 10b are a clarified nitric solution 35 which is collected in the extractor 40.

During the regeneration by unclogging of the electromagnetic filter 10a, the second electromagnetic filter 10b takes over and similarly filters another suspension charge 2. The effluent originating from the filtration of the nitric solution 2 constitutes the clarified nitric solution 35 which is directed towards the extractor 40. The unclogging sludge 36 coming from the electromagnetic filters 10a and 10b are directed towards the waste treatment device common to the two filters, represented at 41. Thus, the two filters operating alternately in filtration and in regeneration treat the nitric solution 2 added with the ferromagnetic adjuvant 32 by successive aliquots and avoids prolonged contact between the nitric medium and the adjuvant. This avoids the dissolution of the adjuvant in the nitric medium.

The water effluent (or moderately alkaline water) 37 resulting from the operation of placing the ferromagnetic adjuvant in the lining of the filters by filtration of the suspension contained in the capacity 90 is preferably collected in a capacity 42 and recycled in capacity 90.

The process for removing heavy metal particles according to the invention leads to a dense sludge easily separated from the washing water by decantation and which contains the insoluble fission products. The latter can then be separated from the ferromagnetic adjuvant, either to isolate them under a small volume of waste, or for the profitability of the platinoid metals which are the major constituents thereof.

Claims (7)

1. Method for removing particles in suspension in a liquid in which a finely divided ferromagnetic adjuvant is used in the state of particles, said particles being extracted by application of a magnetic field to the ferromagnetic particles, characterized by the following successive steps : - pre-clogging of an electromagnetic filter by means of a finely divided ferromagnetic adjuvant,; - agitation of the liquid containing the particles in order to keep them in suspension; - filtration of the liquid containing the particles to be extracted in suspension by means of said pre-clogged electromagnetic filter. 2. Method according to claim 1, characterized in that the particles to be extracted are heavy metallic particles, the ratio of the mass of the metallic particles to be extracted to the mass of the ferromagnetic adjuvant particles being greater than 100. 3. Method according to claim 2, characterized in that the metal particles consist of ruthenium. 4. Method according to any one of claims 1 to 3, characterized in that the ferromagnetic adjuvant is chosen from the group consisting of ferrite and magnetite. 5. Method according to any one of claims 1 to 4, characterized in that it further comprises, following the filtration of the liquid, a step of unclogging the electromagnetic filter. 6. Method according to any one of claims 1 to 5, characterized in that the stirring time is limited to half an hour. 7. Method according to any one of claims 1 to 6, characterized in that the ferromagnetic adjuvant is magnetite, of which a mass of between six and twelve times that of the mass of particles of heavy metallic elements is used in suspension in the quantity of liquid to be treated.

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