INORGANIC, PARTICULAR AND POROUS MATERIAL, BASED ON A LEAD VANADATE OR PHOSPHOVANADATE, USEFUL FOR CAPTURING GASY IODINE, ITS PREPARATION PROCESS AND ITS USES

Abstract

The invention relates to an inorganic material in the form of porous particles, which has a high capacity to capture iodine in the gaseous state when brought into contact with such iodine and which, once saturated with iodine, can be transformed, by heat treatment, into a ceramic of the apatite family in which the iodine is confined. Each particle of this material comprises: 1) a lead vanadate or phosphovanadate of formula (1): Pb3-xXx(VO4)2-2 y(PO4)2 y (1) in which: x = 0 or x > 0 but ≤ 0.33; y = 0 or y > 0 but < 1; X = Ba2+, Ca2+, Sr2+ or Cd2+; and 2) metallic lead or a lead salt. The invention also relates to a process for preparing the material and to the uses of said material. Applications: conditioning and storage in a matrix of radioactive iodine present in gaseous effluents produced during the processing of spent nuclear fuels and, in particular, of iodine-129.

Description

Title of the invention: INORGANIC, PARTICULATE AND POROUS MATERIAL, BASED ON A LEAD VANADATE OR PHOSPHOVANADATE, USEFUL FOR CAPTURING GAS IODINE, ITS PREPARATION PROCESS AND ITS USES Technical field

[0001] The invention relates to the field of treatment of gaseous effluents comprising iodine and, in particular, radioactive iodine.

[0002] More specifically, it relates to an inorganic material in the form of porous particles, which has a high capacity to chemisorb (i.e. to irreversibly adsorb) iodine in the gaseous state when it is brought into contact with this iodine and which, once loaded with iodine, can be transformed, by heat treatment, into a ceramic of the apatite family in which the iodine is confined.

[0003] It also relates to a process for preparing this material as well as to its uses, in particular for the conditioning of iodine present in a gaseous effluent.

[0004] The invention finds application in particular in the field of the treatment of spent nuclear fuels where it is capable of being used to condition and store in a matrix the radioactive iodine present in the gaseous effluents produced during this treatment and, in particular, iodine-129, for example in the case where the management of this iodine by isotopic dilution in marine iodine is not possible. State of the art

[0005] Iodine-129 is a long-lived, intermediate-level fission product (half-life of 15.7 million years) that is generated during the fission reactions of uranium and plutonium in nuclear power plants and is therefore present in spent nuclear fuel.

[0006] In France, the management of iodine-129 is essentially based on isotopic dilution in marine iodine, i.e. by discharge into the sea.

[0007] With a view to developing spent nuclear fuel processing plants without the possibility of maritime access or to changing regulations that would prohibit the management of iodine-129 by dumping at sea, research is being carried out on alternative methods of managing iodine-129 and, in particular, on its conditioning in a sustainable matrix, in particular because it has a re resistance to dissemination by vectors such as water, with a view to its storage in a deep geological layer. This research is part of the French project for a deep storage center for radioactive waste CIGEO led by the French National Agency for the Management of Radioactive Waste (or ANDRA).

[0008] It is known to trap, or capture, iodine present in a gaseous effluent by means of solid adsorbents, or filters, which are generally based on alumina, silica or aluminosilicates and which are typically impregnated with silver nitrate. Iodine in molecular form, or diiodine I2, reacts with silver nitrate to form silver iodide, Agi, and is retained in this form in the solid adsorbent. Adsorbents of this type are currently used to capture iodine present in gaseous effluents produced by spent nuclear fuel processing plants.

[0009] However, these adsorbents, which by nature have high exchange surfaces, are unsuitable for conditioning iodine for storage in deep geological layers.

[0010] It is also known, in particular from WO-A-96 / 18196 - hereinafter reference [1] - and from WO-A-2009 / 047246 - hereinafter reference [2] -, to condition iodine in ceramics of the apatite family which represents a mineralogical family with excellent chemical and physical durability performances. To achieve this type of conditioning, the iodine is mixed, in the form of a solid iodine compound such as lead iodide, Pbl2, or silver iodide, Agi, with a lead vanadate or phospho-vanadate, then the mixture is subjected to reactive sintering, for example by the flash sintering technique, to lead to a dense ceramic, called io-doapatite.

[0011] Such an iodoapatite cannot be directly produced from a solid adsorbent loaded with iodine. Indeed, the production of an iodoapatite from such an adsorbent requires extracting the iodine from this adsorbent and converting it into a solid chemical form compatible with its integration into the apatitic structure.

[0012] The inventors therefore decided to rethink the nature of solid adsorbents dedicated to the capture of gaseous iodine so that these adsorbents can, once loaded with iodine, be directly consolidated into high-durability packaging matrices.

[0013] They further desired that these adsorbents could be prepared by a process which was simple to implement and that their consolidation into conditioning matrices could be carried out at a temperature which minimized the risks of iodine volatilization, i.e. typically a temperature at most equal to 650°C. Statement of the invention

[0014] The invention aims precisely to propose an inorganic material which is capable of chemisorbing iodine very efficiently in gaseous form and which, once loaded with iodine, can be directly consolidated into an iodoapatite by heat treatment.

[0015] This material is in the form of open-porosity particles, each of which comprises: 1) a lead vanadate or phosphovanadate of formula (1) below: Pb5xivcu2yœo4)2y (1) in which: x is equal to 0 or x is greater than 0 but at most equal to 0.33; y is equal to 0 or y is greater than 0 but less than 1; X is a divalent cation selected from Ba2+, Ca2+, Sr2+ and Cd2+; and 2) metallic lead or a lead salt, this lead or this lead salt serving as an agent for capturing gaseous iodine.

[0016] In the foregoing and the following, we mean: * by inorganic material, a material which does not contain carbon or hydrogen; * by open porosity, a porosity consisting of pores which not only open onto the surface of the particles - and which therefore communicate with the exterior of these particles - but also communicate with other pores, themselves communicating or not with the exterior of the particles; and * by metallic lead, lead in oxidation state 0, noted Pb°.

[0017] According to the invention, the gaseous iodine capture agent may, firstly, be intimately mixed with lead vanadate or phosphovanadate, in which case this agent may be either metallic lead or a lead salt of the lead(II) sulfide, lead(II) sulfate, lead(II) carbonate, lead(II) bromide, lead(II) chloride, lead(II) fluoride, lead(II) hydroxide, lead(II) selenide, or lead(II) tellurium type, preference being given to lead(II) sulfide.

[0018] Alternatively, the gaseous iodine capture agent may be present in the pores of the particles, in which case this agent may in particular be a lead salt of the lead(II) nitrate, lead(II) chlorate or lead(II) perchlorate trihydrate type, preference being given to lead(II) nitrate.

[0019] According to the invention, the lead vanadate or phosphovanadate preferably corresponds to the formula (2) below: Pb3(VO4)22y(PO4)2y(2) in which y is equal to 0 or y is greater than 0 but less than 1.

[0020] When y is equal to 0, then the particles comprise a lead vanadate of formula Pb3(VO4)2.

[0021] When y is greater than 0 but less than 1, then y is preferably between 0.1 and 0.75 and, more preferably, between 0.1 and 0.3.

[0022] Thus, for example, the particles may comprise a lead phosphovanadate of formula Pb3(V04)ij6(P04)o,4.

[0023] Preferably, the particles have dimensions between 300 pm and 3 mm as determined, for example, by laser granulometry.

[0024] The material of the invention may in particular be prepared by a process which comprises at least the steps of: a) preparation of a mixture M comprising at least one precursor of formula (3) or formula (4) below: Pb2xXxV22yP2yO7(3) Pb3_xXx(VO4)2_2y(PO4)2y(4) in which: x is equal to 0 or x is greater than 0 but at most equal to 0.33; y is equal to 0 or y is greater than 0 but less than 1; X is a divalent cation selected from Ba2+, Ca2+, Sr2+ and Cd2+; and an alginate of an alkali metal in water; b) extrusion in a divided form of the mixture M into an aqueous solution A comprising a lead salt and, optionally, a barium salt, a calcium salt, a strontium salt or a cadmium salt, whereby beads formed from a hydrogel are obtained; (c) drying the balls; and d) calcination of the beads, whereby open-porosity particles are obtained; and which further includes: - either an addition of metallic lead or a lead salt which is insoluble or sparingly soluble in water to the precursor of formula (3) or (4) and to the alkali metal alginate in the mixture M; - or a deposit of a lead salt in the pores of the particles obtained in step d).

[0025] By lead salt insoluble or sparingly soluble in water, we mean a lead salt which has a solubility in water of at most 10 g / L at 20°C and under a pressure of one atmosphere.

[0026] In accordance with the invention, the precursor of formula (3) or (4) preferably represents from 2.5% to 5% by mass and, better still, 3% by mass of the mixture M while the alkali metal alginate preferably represents from 0.5% to 3% by mass and, better still, 2% by mass of the mixture M, all these percentages being related to the mass of the mixture M (the difference to 100% being constituted by water).

[0027] If a lead salt insoluble or sparingly soluble in water, such as lead(II) sulfide, lead(II) sulfate, lead(II) carbonate, lead(II) bromide, lead(II) chloride, lead(II) fluoride, lead(II) hydroxide, lead(II) selenide or lead(II) tellurium, is added to the mixture M, then this salt preferably represents from 0.2% to 0.5% by mass and, even better, 0.3% by mass of this mixture.

[0028] The alkali metal alginate is advantageously a sodium or potassium alginate, preference being given to a sodium alginate.

[0029] This sodium alginate is, for example, the alginate having the CAS number 9005-38-3 and which is available from the company Sigma-Aldrich under the name “Alginic acid sodium salt”.

[0030] In step b), the composition of the aqueous solution A used for the extrusion is chosen as a function of the precursor used and the composition that it is desired to confer on the particles of the material of the invention and, thereby, of the iodoapatite that will be obtained after consolidation of this material.

[0031] Thus, if the precursor of formula (3) or (4) is free of any cation X (i.e. x = 0 in this formula) and if it is desired that the particles comprise a lead vanadate or phosphovanadate of formula (1) in which a cation X is present (i.e. x > 0), then the aqueous solution A comprises, as salts, both a lead salt, for example lead(II) nitrate, and a barium salt, a calcium salt, a strontium salt or a cadmium salt, for example a nitrate.

[0032] In other cases, then the aqueous solution A may comprise, as salt, only a lead salt, for example lead(II) nitrate.

[0033] In any event, the aqueous solution A advantageously has a molar content of salt(s) of between 0.1 mol / L and 0.6 mol / L, it being understood that: - for a precursor of formula (3), this content will preferably be greater than 0.2 mol / L, while - for a precursor of formula (4), this content will preferably be at most equal to 0.2 mol / L.

[0034] Step b) can be carried out by any device which makes it possible to deliver the mixture M in a divided form and, in particular in the form of drops, in the aqueous solution A such as a syringe equipped with a needle or a conduit which comprises, at one of its ends, a needle and the other end of which is connected to a pump, for example peristaltic.

[0035] In accordance with the invention, the preparation process advantageously further comprises, between step b) and step c), a replacement of all or part of the water in the hydrogel with an organic solvent having a standard boiling point (i.e. a boiling point at a pressure of 1 atmosphere) lower than the standard boiling point of water or, in other words, lower than 100°C, in order to facilitate the drying of the beads in step c).

[0036] The organic solvent is, for example, an alcohol such as methanol or ethanol, preference being given to ethanol.

[0037] As for the replacement of all or part of the water in the hydrogel with an organic solvent, this can be achieved by draining the beads obtained in step b), rinsing them with water and then immersing them in a series of baths comprising either only the organic solvent or a water / organic solvent mixture of increasing volume concentration of organic solvent.

[0038] The drying of the beads obtained in step b) or after replacement of all or part of the water in the hydrogel with an organic solvent is, for example, carried out by placing these beads in an oven set at a temperature of 60°C, for several hours.

[0039] As for the calcination of the balls thus dried, it is advantageously carried out at a temperature ranging from 200°C to 600°C and, better still, from 350°C to 550°C, for example for 1 to 24 hours, it being understood that: - for a precursor of formula (3), the calcination temperature is preferably greater than 400°C, while - for a precursor of formula (4), the calcination temperature is preferably at most equal to 400°C.

[0040] If an addition of a lead salt is made in the pores of the particles obtained in step d), then this addition is preferably made by immersing the particles in an aqueous solution comprising this lead salt, for example at a level of 0.005 mol / L to 1.6 mol / L, then drying the particles, for example in an oven set at a temperature between 60°C and 110°C, for several hours, for example 24 hours. In which case, the lead salt is a water-soluble salt of the lead(II) nitrate, lead(II) chlorate or lead(II) perchlorate trihydrate type.

[0041] Alternatively, however, the deposition of a lead salt in the pores of the particles obtained in step d) can also be carried out by other routes such as, for example, chemical vapor deposition (or CVD) of a lead salt that is insoluble or poorly soluble in water of the lead(II) sulfide type, following a protocol such as that described by R. Yousefi et al. in Current Applied Physics 2014, 14(8), 1031-1035, hereinafter reference [3].

[0042] A material is thus obtained which has a capacity for capturing gaseous iodine which varies between 40 mg and 200 mg per g of material, i.e. a capacity which is at the top of the range of capture capacities presented by solid adsorbents which are currently used to capture the iodine present in gaseous effluents in spent nuclear fuel processing plants.

[0043] Also, the invention also relates to the use of a material as previously defined to capture the iodine present in a gaseous effluent by bringing this material into contact with the gaseous effluent.

[0044] The invention also relates to a method for treating a gaseous effluent comprising iodine, which comprises the steps of: (i) capturing iodine by bringing the gaseous effluent into contact with a material as previously defined, whereby a material comprising lead iodide is obtained; then ii) heat treatment (or reactive sintering) of the material comprising lead iodide, whereby the material is transformed into an iodoapatite by a chemical evolution which can be described by the following reaction: 3Pb3.^4VO4)ï.Ï¥(PO4^ Pbb-----> X, x and y having the same meanings as before.

[0045] According to the invention, the heat treatment preferably comprises heating the material to a temperature ranging from 400°C to 650°C, for example for 30 minutes to 2 hours. This heating may or may not be assisted by external pressure.

[0046] Preferably, this heat treatment is carried out by flash sintering (or SPS for “Spark Plasma Sintering”), for example at a temperature of 400°C for 30 minutes, with a temperature rise ramp, for example, of 50°C / min and under a uniaxial pressure, for example, of 40 MPa.

[0047] The invention finally relates to a method for conditioning the iodine present in a gaseous effluent, which comprises the implementation of the method for treating a gaseous effluent comprising iodine as previously defined.

[0048] According to the invention, the iodine present in the gaseous effluent is, preferably, radioactive iodine and, in particular, iodine-129.

[0049] Other characteristics and advantages of the invention will appear on reading the additional description which follows and which refers to the appended figures.

[0050] It goes without saying that this additional description is given only as an illustration of the subject of the invention and in no way constitutes a limitation of this subject. Brief description of the figures

[0051] [Fig-1] is an image taken by scanning electron microscopy (or SEM), in backscattered electron mode and at a magnification of x70, of a particle of a material of the invention before filling the pores of this particle with a lead salt.

[0052] [Fig.2] is an image similar to that of [Fig.l] but taken at a magnification of xl 100.

[0053] [Fig.3] illustrates the microstructure, as obtained by SEM and at a magnification of xl 000, of a particle of a material of the invention after exposure of this material to iodine vapors.

[0054] [Fig.4] illustrates the elemental mapping of iodine, as obtained by spec- energy-dispersive X-ray spectroscopy (or EDS) and at a magnification of xl 000, for a particle of a material of the invention after exposure of this material to iodine vapors. Example of an embodiment of the invention I - Preparation of a material of the invention:

[0055] A material of the invention consisting of porous particles of lead vanadate, Pb3(VO4)2, and the pores of which comprise lead(II) nitrate, Pb(NO3)2, is prepared by following the following operating protocol: 1) an aqueous solution of sodium alginate (Sigma-Aldrich) is added, with magnetic stirring, to an aqueous suspension of lead pyrovanadate, Pb2V2O7, to obtain an aqueous mixture comprising 2% by mass of sodium alginate and 3% by mass of Pb2V2O7 (i.e. 2 g of sodium alginate and 3 g of Pb2V2O7 per 100 g of water) which is maintained under gentle magnetic stirring for 4 hours; 2) the mixture obtained in step a) is extruded in the form of drops into an aqueous solution comprising 0.27 mol / L of lead(II) nitrate; this extrusion consists of circulating the mixture in a conduit which comprises, at one of its ends, a needle and the other end of which is connected to a peristaltic pump, and of dropping dropwise, via the opening of the tip of the needle, the mixture obtained in step a) into the aqueous solution of lead(II) nitrate, whereby gelatinous beads are obtained, with dimensions typically between 0.5 mm and 10 mm; these beads are formed from a mixture of Pb2V2O7 and lead alginate, the divalent lead ions having, in fact, replaced the monovalent sodium ions of the alginate during the extrusion; 3) the gelatinous beads obtained at the end of step 2) are subjected to a water / ethanol exchange, which allows the hydrogel which constitutes them to be transformed into an alcohol gel; to do this, the beads are drained, rinsed with water and immersed in an ethanol bath for 10 minutes; this immersion is repeated three times; 4) the alcoholgel beads obtained in step 3) are subjected to drying at 60°C overnight; 5) the dried alcoholgel beads are subjected to calcination at 500°C for 1 hour 30 seconds in air (with a temperature rise ramp of 5°C / min); then 6) the particles obtained in step 5) are immersed in an aqueous solution comprising 0.1 mol / L of lead(II) nitrate for one hour, then dried at 90°C for 24 hours.

[0056] During step 5), the alginate present in the alcoholgel beads is decomposed and these beads are transformed into particles, with dimensions typically between 300 μm and 3 mm, and which comprise an inorganic skeleton composed of vanadate of lead. The initial presence of organic matter, represented by alginate, in the beads allows porosity to be generated. At the same time, Pb2V2O7 is converted into Pb3(VO4) 2 due to its reaction with the lead contained in the lead alginate formed in step 2). This reaction can be written schematically as follows: PbïVzO? + PbO —> Pb^VÇMz.

[0057] As for step 6), it allows the pores of the lead(II) nitrate particles to be filled, capable of forming lead iodide, Pbl2, by reaction with gaseous iodine.

[0058] Figures 1 and 2 correspond to images taken with the SEM, in backscattered electron mode, of a particle of the material as obtained in step 5 above. [Fig. 1] corresponds to a magnification of x70 while [Fig. 2] corresponds to a magnification of x100.

[0059] [Fig.l] shows an overview of the particle while [Fig.2] shows the porous nature of this particle, the porosity corresponding to the small black dots of pseudo-spherical shape.

[0060] II - Exposure of the material of the invention to gaseous iodine:

[0061] The material, as obtained in point I above, is exposed to vapors of iodine, I2, at 60°C for 16 hours.

[0062] Figures 3 and 4 illustrate, for the first, the microstructure of a particle of the material as observed at the end of this exposure by SEM and, for the second, the X-ray mapping of iodine as observed for this particle by EDS. Figures 3 and 4 correspond to a magnification of xl 000.

[0063] In Figures 3 and 4, the darkest areas in gray level, one of which is materialized by the arrow numbered 1, correspond to areas rich in lead iodide, Pbl2, while the lightest areas in gray level, one of which is materialized by the arrow numbered 2, correspond to areas rich in lead vanadate, Pb3(VO4)2.

[0064] III - Consolidation of the material of the invention into an iodoapatite:

[0065] The iodine-loaded material, as obtained in point II above, is subjected to a heat treatment corresponding to reactive sintering by SPS at 400°C for 30 minutes, with a temperature rise ramp of 50°C / min and under a uniaxial pressure of 40 MPa.

[0066] References cited [1] WO-A-96 / 18196 [2] WO-A-2009 / 047246 [3] R. Yousefi et al., Current Applied Physics 2014, 14(8), 1031-1035

Claims

Claims

1. An inorganic material in the form of open-porosity particles, characterized in that each of the particles comprises: 1) a lead vanadate or phosphovanadate of formula (1): Pb3xXx(VO4)22y(PO4)2y (1) in which: x is equal to 0 or x is greater than 0 but at most equal to 0.33; y is equal to 0 or y is greater than 0 but less than 1; X is a divalent cation selected from Ba2+, Ca2+, Sr2+ and Cd2+; and 2) metallic lead or a lead salt.

2. The material of claim 1, wherein the lead metal or lead salt is intimately mixed with the lead vanadate or phosphovanadate.

3. The material of claim 2, wherein the lead salt is lead(II) sulfide, lead(II) sulfate, lead(II) carbonate, lead(II) bromide, lead(II) chloride, lead(II) fluoride, lead(II) hydroxide, lead(II) selenide, or lead(II) tellurium, preferably lead(II) sulfide.

4. The material of claim 1, wherein the lead salt is present in the pores of the particles.

5. The material of claim 4, wherein the lead salt is lead(II) nitrate, lead(II) chlorate or lead(II) perchlorate trihydrate, preferably lead(II) nitrate.

6. A material according to any one of claims 1 to 5, wherein the lead vanadate or phosphovanadate is of formula (2): Pb3(VO4)22y(PO4)2y(2) wherein y is 0 or y is greater than 0 but less than 1.

7. The material of claim 6, wherein the particles comprise lead vanadate of formula Pb3(VO4)2.

8. Material according to any one of claims 1 to 7, the particles of which have dimensions between 300 pm and 3 mm.

9. A process for preparing a material according to any one of claims 1 to 8, which comprises at least the steps of: a) preparing a mixture M comprising at least one precursor of formula (3) or formula (4) below: Pb2xXxV22yP2yO7(3) Pb5 SXS(VO4)2 2y(PO4)2y (4) wherein: x is equal to 0 or x is greater than 0 but at most equal to 0.33; y is equal to 0 or y is greater than 0 but less than 1; X is a divalent cation selected from Ba2+, Ca2+, Sr2+ and Cd2+; and an alginate of an alkali metal in water; b) extruding in a divided form the mixture M into an aqueous solution A comprising a lead salt and, optionally, a barium salt, a calcium salt, a strontium salt or a cadmium salt, whereby beads formed from a hydrogel are obtained; c) drying the beads; and d) calcining the beads, whereby open-porosity particles are obtained; and which further comprises: - either an addition of metallic lead or a lead salt which is insoluble or poorly soluble in water to the precursor of formula (3) or (4) and to the alkali metal alginate in the mixture M; - or a deposit of a lead salt in the pores of the particles obtained in step d).

10. A method according to claim 9, which further comprises, between step b) and step c), replacing all or part of the water in the hydrogel with an organic solvent having a standard boiling point below 100°C, preferably with an alcohol.

11. Use of a material according to any one of claims 1 to 8, for capturing iodine present in a gaseous effluent by bringing the material into contact with the gaseous effluent.

12. Use according to claim 11, wherein the iodine present in the gaseous effluent is radioactive iodine, preferably iodine-129.

13. A method of treating a gaseous effluent comprising iodine, which comprises the steps of: i) capturing the iodine by contacting the gaseous effluent with a material of any one of claims 1 to 8, whereby a material comprising lead iodide is obtained; then ii) heat treatment of the material obtained in step i), whereby the material is transformed into an iodoapatite.

14. A treatment method according to claim 13, wherein step ii) comprises heating the material to a temperature ranging from 400°C to 650°C.

15. A method of conditioning iodine present in a gaseous effluent, which comprises carrying out a method of treating a gaseous effluent comprising iodine according to claim 13 or claim 14.

16. A method according to any one of claims 13 to 15, wherein the iodine present in the gaseous effluent is radioactive iodine, preferably iodine-129.

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