DEVICE FOR GRANULATING POWDERS BY CRYOGENS NUSTARDING

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of the preparation of granular media, and more precisely to the granulation of powders, in particular actinide powders, to obtain granules by cryogenic atomization.

[0002] In a preferred manner, it applies to the atomization of powders which must not be brought into contact with a liquid, in particular water, in particular for reasons of contamination of these powders by the liquid, for reasons of instability of the powders, or even for reasons of risk of criticality in the case of the use of radioactive powders.

[0003] The invention preferably finds its application for the granulation of actinide powders allowing the formation of nuclear fuel, in particular nuclear fuel pellets.

[0004] The invention thus proposes a device for granulating powders by cryogenic atomization, as well as an associated powder granulation method. STATE OF THE PRIOR ART

[0005] The implementation of the various stages of preparation of a granular medium, in particular from actinide powders to form nuclear fuel pellets after shaping by pressing, is essential because it mainly determines the control of the microstructure of the final product but also the presence or absence of macroscopic appearance defects within a fuel pellet. In particular, the mixing of actinide powders to enable the production of nuclear fuel constitutes a key stage in the control of the quality of the fuel pellet obtained, which most often is subject to compliance with strict requirements in terms of microstructure and impurities.

[0006] The classic and historical industrial process of powder metallurgy applied to the production of nuclear fuel is based on mixing, grinding and / or granulation steps, all carried out using the dry process. Indeed, the use of liquid in the nuclear industry leads to the generation of effluents that can be difficult to treat. Also, for the preparation of a granular medium with a view to producing nuclear fuel, processes other than those using the dry process are not traditionally used.

[0007] To achieve the mixing of powders, various devices are known from the prior art, which can be broken down into the families described below.

[0008] First of all, there is the principle of the dry phase mixer without internal media. This can notably be a Turbula ®< type mixer from the WAB company which, through more or less complex movements of the tank containing the powders to be mixed, allows a more or less significant homogenization of the granular medium. Generally, the efficiency of this type of mixer is limited. Indeed, depending on the type of powders to be mixed, there may remain heterogeneous zones, for which the mixing does not take place or at least in an incorrect and unacceptable manner. The kinematics of this type of mixer are generally not complex enough to induce a thorough mixing, that is to say a satisfactory mixture in terms of homogeneity, without advanced development itself or a penalizing mixing time at the industrial level.Furthermore, the energy transmitted to the granular medium in this type of mixer does not allow sufficient deagglomeration to be achieved to achieve sufficient degrees of homogeneity in the case where the size of these agglomerates is too large (in particular to be compensated during the sintering stage).

[0009] The principle of the media mixer is also known. According to this principle and in order to facilitate the mixing operation, one or more mobiles can be used within the tank containing the powder to be mixed. These mobiles can be blades, turbines, ploughshares, ribbons, endless screws, among others. To improve the mixing, the tank itself can be mobile. This type of mixer can be more efficient than the previous category but is still insufficient and suffers from limitations. Indeed, mixing induces a modification of the granular medium by agglomeration or a deagglomeration that is difficult to control, which leads to an overflow of the powders and / or a degradation of the flowability of the granular medium. Furthermore, the use of mobiles (media) for mixing leads to pollution (contamination) when it comes to mixing abrasive powders such as those to be used for the production of nuclear fuel.In addition, the mobiles used induce retentions which generate dose rates which have a very significant impact in the case of nuclear fuel production.

[0010] There is also the principle of the grinder-type mixer. Indeed, depending on the mode of use and the type of technology of certain grinders, it is possible to produce powder mixtures by co-grinding. This type of operation makes it possible to obtain a satisfactory mixture, from a homogeneity point of view, but requires a relatively long grinding time, typically several hours, and also induces grinding phenomena that reduce the size of the powder particles. This causes the generation of fine particles and a modification of the specific surface area which also has an impact on the possibility of subsequently using the powders after their mixing (modification of the flowability, the reactivity (possible oxidation), the sinterability of the powders, etc.).In the context of nuclear fuel manufacturing, the co-grinding operation, by generating fine particles, causes a significant radiological impact, due to the retention and propensity of the fine particles to disperse. In addition, clogging phenomena can be induced.

[0011] After using these different types of mixers, it is often necessary to carry out agglomeration or granulation. In addition, these devices are generally discontinuous, which can be problematic in industrial processes.

[0012] Other mixers are also known, using a multiphase medium, namely fluid-solid phases. Liquid / solid type mixers, for example, are not suitable for the use of soluble powders with the liquid phase used in the mixer or if the powders are modified by contact with the fluid. Furthermore, for powders having a high density compared to the liquid introduced into the mixer, the mixing is most often not efficient or requires high stirring speeds. Indeed, the speed at which a particle detaches from the bottom of the stirrer is directly linked to the difference in density between the particles constituting the powders and that of the liquid allowing suspension.

[0013] Generally speaking, the aforementioned mixers are not fully satisfactory for mixing certain powders, such as actinide powders, and for allowing industrial implementation for, for example, easy filling of press molds, for shaping operations. It should be noted that liquid phase powder mixers, in particular of the type described in patent applications CA 2 882 302 A1, WO 2006 / 0111266 A1 and WO 1999 / 010092 A1, are not suitable for the problem of mixing powders of the actinide powder type, because they would require stirring speeds that are too high to hope to detach the powders from the bottom of the stirring tank and achieve levels of homogeneity consistent with those sought in the nuclear industry.Furthermore, once again, they would induce contaminated effluents, difficult to manage industrially but also risks of criticality, or even radiolysis of the liquid phase used due to the nature of the powders to be used (beyond the fact that the latter can interact chemically with the liquid used).

[0014] It is therefore generally necessary to follow this with a granulation step in order to obtain a flowable granular medium.

[0015] Granulation can be done in a conventional manner using the methods described below.

[0016] First, by mechanical granulation, the mixed powders are compacted, then granulated by grinding / crushing. This step generates fine particles, which is penalizing from a contamination risk point of view when the powders are radioactive powders, such as actinide powders.

[0017] Granulation can also be done by sieving or forcing. In this case, the powders are forced to pass through a sieve whose mesh size is chosen to control the size of the agglomerates.

[0018] Granulation can finally be carried out by atomization. In this case, the powders are suspended in a liquid phase such as an aqueous solution with additives, for example. These additives, generally organic compounds, as mentioned in patent application EP 1 137 597 A1, can be used to control the viscosity and surface tension of the suspensions to enable atomization control. The use of organic compounds is however likely to generate pollution of the powders to be atomized. Furthermore, the use of liquid for atomization requires a drying step, the adjustment of which is not trivial and requires at least the presence of heating elements to carry out the drying, as required for example in patent application EP 1 137 597 A1.

[0019] Furthermore, when it comes to carrying out the operations of mixing the powders and then granulating them, several difficulties arise. Thus, there may be a risk of deterioration of the mixture obtained when transferring the powders from one stage to another, namely from the mixing stage to the granulation stage. In addition, there may be a difficulty in transferring the powders after mixing, as mixing very often causes the powders to overflow, negatively impacting their flowability. Finally, this can generate fine particles, which are detrimental to the use of radioactive powders, such as actinide powders.

[0020] Furthermore, US 3,321,560 A describes a method for producing fuel for a nuclear reactor, and US 2014 / 0000297 A1 describes a method for producing particles using cryogenic fluids. STATEMENT OF THE INVENTION

[0021] There is thus a need to propose a new type of powder granulation device for the preparation of granular media, and in particular for the granulation of actinide powders.

[0022] In particular, there is a need to be able to simultaneously: deagglomerate the powders to be mixed without necessarily modifying their specific surface area and generating fine particles, mix the powders with a sufficient level of homogeneity to obtain a mixture of powders meeting the specifications, in particular in terms of homogeneity (i.e. making it possible in particular to obtain a representative elementary volume (REV) within the granular medium of the order of a few cubic micrometers to approximately 10 µm 3< ), not induce pollution of the powders to be mixed, nor modification of the surface chemistry, nor generate liquid effluents that are difficult to treat, not induce a risk of specific criticality, not induce a risk of specific radiolysis, not induce heating of the powders to be mixed, rely on a mixer with a limited diameter to control the risk of criticality even in the event of an error in loading the mixer, carry out the mixing operation by limiting as much as possible the energy expended and this in a relatively short time compared to other mixers, i.e. of the order of a few minutes compared to a few hours (for other mixing systems such as ball mills), for the same quantity of material to be mixed, have a continuous or almost continuous mixing process.

[0023] The invention aims to at least partially remedy the needs mentioned above and the drawbacks relating to the achievements of the prior art.

[0024] The subject of the invention, according to one of its aspects, is a device for granulating powders, in particular actinide powders, by cryogenic atomization, characterized in that it comprises: a device for mixing powders using cryogenic fluid, comprising at least one powder mixing chamber, comprising a cryogenic fluid, a device for atomizing a suspension of powders mixed by the powder mixing device to enable granulation of the powders, comprising: means for fractionating the powder suspension enabling adjustment of the size of the droplets of powder suspension to be atomized, means for adjusting the humidity of the mixed powders and / or the humidity of the atomization atmosphere.

[0025] It should be noted that, usually, a cryogenic fluid refers to a liquefied gas kept in a liquid state at low temperature.

[0026] Thanks to the invention, it is possible to couple an operation of mixing powders in the liquefied gas phase with atomization of the suspension of mixed powders. The mixture using a cryogenic fluid (liquefied gas), the separation of the liquid constituting the suspension of powders itself can thus not rely on drying but on returning the suspension to ambient temperature, which greatly facilitates the operation of separating the liquid constituting the suspension of the powders thus granulated.

[0027] The powder granulation device according to the invention may also include one or more of the following characteristics taken in isolation or in any possible technical combination.

[0028] The cryogenic fluid may include a weakly hydrogenated liquid, i.e. a liquid containing at most one hydrogen atom per liquid molecule, with a boiling point lower than that of water.

[0029] The powder suspension fractionating means may be configured to allow adjustment of the diameter of the powder suspension droplets to be atomized, such that the diameter of the powder suspension droplets is defined according to the following relationship: d o D = G fD ν 1 We 1 Re A D with We = ρv 2< .(do ) / σ and Re = ρd ov / µ, where: f represents the vibration frequency of the atomizing device, v represents the speed of the powder suspension, ρ represents the density of the powder suspension to be fractionated, µ represents the viscosity of the powder suspension to be fractionated, σ represents the surface tension of the powder suspension to be fractionated, A represents the oscillation amplitude of an atomizing nozzle of the atomizing device, do represents the droplet diameter, and D represents the diameter of an atomizing nozzle of the atomizing device.

[0030] Advantageously, the means for fractionating the powder suspension can be implemented by means of at least one atomizing nozzle of the atomizing device, in particular a vibrating nozzle, for example with a vibration frequency of between 100 Hz and 10,000 Hz, or a piezoelectric nozzle, for which in particular it may be possible to adjust the frequency f and also the amplitude A in order to be able to easily adjust the particle size of the powder suspension droplets according to the relationship given above.

[0031] Furthermore, the means for fractionating the powder suspension can be configured to allow the adjustment of the diameter of the powder suspension droplets to be atomized by modulation according to a diameter reduction factor R between the diameter of the powder suspension droplets to be atomized and the diameter of the granules, or agglomerates, obtained after atomization of the suspension of mixed powders and evaporation of the cryogenic fluid, so that: d 0 d s = R = U f U i 3 Or : d 0 represents the diameter of the droplets, ds represents the diameter of the granules, [U] f represents the volume occupancy rate of the powders in the agglomerate of granules formed after granulation, and [U] i represents the powder concentration of the suspension of powders to be atomized.

[0032] The means for adjusting the humidity of the mixed powders and / or the humidity of the atomization atmosphere may include means for baking the powders. These means for baking the powders may allow the humidity to be adjusted, in particular before atomization, by baking the powders to a greater or lesser extent.

[0033] The means for adjusting the humidity of the mixed powders and / or the humidity of the atomization atmosphere may also include a humidity generator and a dehumidifier. Humidity adjustment can thus be achieved by means of this humidity generator associated with the dehumidifier, allowing a wide range of water contents to be obtained, to ensure a more or less humid atomization atmosphere.

[0034] In addition, the mixing device may also include: a powder feed chamber to allow the introduction of the powders into the mixing chamber, stirring means in the mixing chamber to allow the mixing of the powders suspended in the cryogenic fluid.

[0035] According to one embodiment, the mixing device may comprise means for mixing the mixing enclosure according to a gyroscopic movement.

[0036] In particular, the means of mixing according to a gyroscopic movement can allow the rotation of the mixing chamber according to the three axes of three-dimensional metrology. This type of agitation by gyroscopic movement can in particular make it possible to promote the mixing of powders when they have high densities compared to the density of the fluidic phase of the cryogenic fluid located in the mixing chamber.

[0037] According to another embodiment, the mixing device may comprise: a plurality of powder mixing enclosures, arranged successively in series one after the other, the powder supply enclosure allowing the introduction of the powders into at least the first mixing enclosure, a plurality of powder passage restriction systems, each passage restriction system being located between two successive mixing enclosures, to constrain the distribution of powders from one mixing enclosure to the next.

[0038] Each mixing chamber can then include a cryogenic fluid and stirring means to allow the mixing of the powders suspended in the cryogenic fluid.

[0039] Furthermore, the stirring means may include mixing devices, including blades, turbines and / or quilt effect devices, among others.

[0040] These mixing mobiles can include grinding mobiles.

[0041] In addition, the stirring means may also include means for generating vibrations, in particular ultrasonic vibrations, in particular sonotrodes.

[0042] In addition, flow restriction systems may include screens. Flow restriction systems may also include diaphragms.

[0043] The passage restriction systems can be adjustable and configured so that their passage section decreases as a function of the flow rate of the powders through the plurality of mixing enclosures, the passage section of an (n-1)th passage restriction system thus being greater than the passage section of an nth passage restriction system following the flow rate of the powders.

[0044] In addition, the passage section of the first passage restriction system may be less than the natural flow section of the powders so as to generate a passage restriction from the first passage restriction system.

[0045] Furthermore, the plurality of mixing enclosures and the plurality of powder passage restriction systems can advantageously be arranged in the same vertical direction so as to allow the powders to flow under the effect of gravity.

[0046] The mixing device may comprise at least two powder supply enclosures, and in particular as many powder supply enclosures as there are types of powders to be mixed.

[0047] The feed enclosure(s) may include adjustable feed hoppers and / or dosing-type systems, including vibrating trays or chutes.

[0048] The mixing device may also include: a powder mixing enclosure, comprising a cryogenic fluid, provided with means for forming a fluidized powder bed, a powder supply enclosure to allow the introduction of the powders into the mixing enclosure, a cryogenic fluid supply enclosure to allow the introduction of the cryogenic fluid into the mixing enclosure, a system for generating vibrations in the fluidized powder bed, a system for controlling the vibration generation system.

[0049] The mixing device may also include a system for analyzing the concentration of the suspension of powders and cryogenic fluid in the mixing chamber, the operation of which is notably controlled by the control system.

[0050] The mixing chamber can be configured such that the introduction of cryogenic fluid into it allows fluidization of the powders to be mixed by percolation of the cryogenic fluid through the bed of powders thus fluidized.

[0051] The mixing chamber may include a distribution system, in particular a grid or a sintered part, for the cryogenic fluid through the fluidized bed of powders to allow homogeneous distribution of the cryogenic fluid in the fluidized bed.

[0052] The vibration generation system may be at least partially located in the fluidized bed of powders. In particular, the vibration generation system may comprise sonotrodes introduced into the fluidized bed of powders.

[0053] The sonotrodes can be driven independently by the drive system to induce periodic phase shifting between the sonotrodes to introduce unsteady interference improving mixing within the fluidized bed of powders.

[0054] Sonotrodes can be configured to generate pseudo-chaotic Van der Pol type oscillations.

[0055] The mixing device may further comprise stirring means in the mixing chamber to enable the powders suspended in the cryogenic fluid to be mixed, in particular comprising grinding means, for example of the ball, roller, etc. type.

[0056] In addition, the device preferably includes a system for electrostatically charging the powders intended to be introduced into the mixing chamber(s).

[0057] In particular, a portion of the powders may be brought into contact with one portion of the electrostatic charging system to be electrostatically charged positively, and the other portion of the powders may be brought into contact with the other portion of the electrostatic charging system to be electrostatically charged negatively, in order to enable differentiated local agglomeration. When mixing more than two types of powders, some powders may be either positively charged, negatively charged, or uncharged.

[0058] The cryogenic fluid can also be of any type, including liquefied nitrogen or argon. It should be noted that the use of nitrogen is relevant due to its low price but also because the glove boxes and processes used for the development of plutonium-based nuclear fuel are inerted with nitrogen and liquid nitrogen is itself used in certain operations on the fuel (BET measurement, etc.). The use of this type of cryogenic fluid therefore does not introduce any particular additional risk into the development process.

[0059] Furthermore, the invention also relates, according to another of its aspects, to a method for granulating powders, in particular actinide powders, by cryogenic atomization, characterized in that it is carried out by means of a device as defined above, and in that it comprises the following steps: a) introducing powders and cryogenic fluid into at least one mixing chamber of the powder mixing device by cryogenic fluid to obtain a suspension of powders and cryogenic fluid, b) atomizing the suspension of powders and cryogenic fluid by means of the atomizing device to allow granulation of the powders, c) obtaining granules formed from the powders.

[0060] Obtaining granules according to step c) can be carried out by sublimation of the cryogenic fluid. In this way, it may be possible to obtain spheroidal granules making it possible to form a flowable granular medium.

[0061] During the first step a), the powders can advantageously be electrostatically charged differently, in particular in an opposite manner in the presence of at least two types of powders, to promote differentiated local agglomeration.

[0062] The device and method for granulating powders according to the invention may include any of the characteristics set out in the description, taken in isolation or in any technically possible combination with other characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, as well as by examining the schematic and partial figures of the attached drawing, in which: there figure 1 represents a diagram illustrating the general principle of a device for granulating powders by cryogenic atomization in accordance with the invention, the figure 2 schematically illustrates the phases undergone by the suspension of atomized powders to obtain powder granules, the figure 3represents a diagram illustrating the general principle of an example of a powder mixing device of a powder granulation device by cryogenic atomization in accordance with the invention, the figure 4 schematically represents the agglomeration of oppositely charged powder particles prior to their introduction into mixing chambers of a powder mixing device conforming to the principle of figure 3 , THE Figures 5 and 6 respectively represent two examples of mixing devices conforming to the general principle of the figure 3 for a granulation device according to the invention, the Figures 7A, 7B and 7C schematically represent variants of the mixing mobiles of the mixing devices of the Figures 5 and 6 , there figure 8 represents a diagram illustrating another example of a powder mixing device of a powder granulation device by cryogenic atomization in accordance with the invention, the figure 9 represents a diagram illustrating the general principle of another device for mixing powders by cryogenic fluid for a device for granulating powders by cryogenic atomization in accordance with the invention, the figure 10 partially represents another example of a mixing device for a device for granulating powders by cryogenic atomization in accordance with the invention, the figure 11 illustrates a representation of interference lines induced by two vibration sources having the same pulsation frequency, and the Figures 12A and 12B illustrate the generation of stable oscillations after convergence, and the Figures 13A and 13B illustrate the generation of quasi-chaotic oscillations of a Van der Pol type oscillator.

[0064] Throughout these figures, like references may designate identical or similar elements.

[0065] Furthermore, the different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more readable. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0066] It is noted that in the embodiments described below, the powders P considered are actinide powders allowing the manufacture of nuclear fuel pellets. In addition, the cryogenic fluid considered here is liquefied nitrogen. However, the invention is not limited to these choices.

[0067] In reference to the figure 1 , a diagram illustrating the general principle of a device 20 for granulating powders P by cryogenic atomization in accordance with the invention is shown.

[0068] According to this principle, the device 20 for granulating powders P by cryogenic atomization comprises a device 1 for mixing powders P by cryogenic fluid FC and a device 10 for atomizing a suspension of powders P mixed by the device 1 for mixing powders P to allow granulation of the powders P.

[0069] The device 1 for mixing powders P comprises a mixing chamber E1 for the powders P, into which the cryogenic fluid FC and the powders P coming from a supply device A1 are introduced.

[0070] The atomization device 10 is coupled to the device 1 for mixing the powders P, and comprises a nozzle 11 for atomizing droplets Go of powders P. Advantageously, the atomization device 10 comprises a sonotrode.

[0071] Advantageously, the device 10 for atomizing the suspension of powders P mixed by the device 1 for mixing powders P comprises means for fractionating the suspension of powders P allowing the adjustment of the size of the droplets Go of powders P to be atomized. In addition, this device 10 also comprises means for adjusting the humidity of the mixed powders P and / or the humidity of the atomization atmosphere.

[0072] Adjusting the humidity of the powders P to be mixed and atomized, or even adjusting the humidity of the atmosphere in which the atomization of the powders takes place, can make it possible to adjust the cohesion of the agglomerates, or granules Gs, resulting from the atomization, by creating liquid bridges between aggregates Gs, as described further on with reference to the figure 2 .

[0073] The control of the droplet size Go of the suspension of mixed powders P can be achieved through various relationships as described below.

[0074] Indeed, the means for fractionating the suspension of powders P are advantageously configured to allow the adjustment of the diameter d 0 of the droplets Go of powders P to be atomized, so that the diameter d 0 of the droplets Go of powders P is defined according to the following relationship: d o D = G fD ν 1 We 1 Re A D with We = ρv 2< .(do ) / σ and Re = ρd ov / µ, where: f represents the vibration frequency of the atomizing device 10, v represents the speed of the powder suspension P, ρ represents the density of the powder suspension P to be fractionated, µ represents the viscosity of the powder suspension P to be fractionated, σ represents the surface tension of the powder suspension P to be fractionated, A represents the oscillation amplitude of the atomizing nozzle 11 of the atomizing device 10, do represents the diameter of the droplets Go, and D represents the diameter of the atomizing nozzle 11 of the atomizing device 10.

[0075] Furthermore, the powder content in the suspension of mixed powders P to be atomized can advantageously be modulated to control the diameter reduction factor R between the diameter do of the droplets Go of powders P to be atomized and the diameter ds of the granules Gs, or agglomerates, obtained after atomization of the suspension of mixed powders P and evaporation of the cryogenic fluid FG.

[0076] Thus, the reduction factor R can be approximated by the following formula: d 0 d s = R = U f U i 3 Or : do represents the diameter of the droplets Go, ds represents the diameter of the granules Gs, [U] f represents the volume occupancy rate of the powders P in the agglomerate of granules Gs formed after granulation, and [U] i represents the concentration of powders P in the suspension of powders P to be atomized.

[0077] Beyond controlling the diameter do of the droplets Go of the powder suspension P by means of one or more of the aforementioned parameters, adjusting the humidity of the powders P makes it possible to provide increased cohesion of the granules Gs, or agglomerates. This humidity adjustment can be carried out when the powders P are introduced into the mixing chamber E1 with the liquefied gas FG, or even when the liquefied gas FG evaporates at the outlet of the atomizing nozzle 11, as illustrated in figure 2 described below.

[0078] So, with reference to the figure 2 , the phases undergone by the suspension of atomized powders P to obtain powder granules Gs are schematically illustrated.

[0079] In phase a, we find the droplets Go of powders P resulting from the atomization of the suspension of powders P. These droplets Go comprise the liquefied gas FG and the powders P.

[0080] During phase b, the liquefied gas FG evaporates. The humidity level R Hu can be adjusted at this level, as shown.

[0081] Then, in phase c, the agglomeration of the powders P is obtained to obtain spherical granules Gs formed of particles of powders P between which there are liquid bridges of non-evaporated liquefied gas FG.

[0082] Referring now to the figure 3 , a diagram is shown illustrating the general principle of an example of a device 1 for mixing powders P by cryogenic fluid for a device 20 for granulating powders P by cryogenic atomization in accordance with the invention, such as for example described previously with reference to the figure 1 .

[0083] According to this principle, the mixing device 1 comprises a number n of mixing chambers E1, ..., En of the powders P, arranged successively in series one after the other in the same vertical direction so that the powders can circulate through the mixing chambers E1, ..., En under the effect of the force of gravity.

[0084] Furthermore, the device 1 comprises a number n-1 of passage restriction systems R1, ..., Rn-1 of the powders P, each passage restriction system R1, ..., Rn-1 being located between two successive mixing enclosures E1, ..., En, to constrain the distribution of powders P from one mixing enclosure E1, ..., En to the next. Examples of such passage restriction systems R1, ..., Rn-1 are presented below with reference in particular to Figures 5 and 6 .

[0085] In addition, flow restriction systems may include screens. Flow restriction systems may also include diaphragms.

[0086] The passage restriction systems can be adjustable and configured so that their passage section decreases as a function of the flow rate of the powders through the plurality of mixing enclosures, the passage section of an (n-1)th passage restriction system thus being greater than the passage section of an nth passage restriction system following the flow rate of the powders.

[0087] In addition, the passage section of the passage restriction systems may be smaller than the natural flow section of the powders so that these powders are necessarily deagglomerated when they pass from one mixing enclosure to another. Thus, the residence time of the particles to be mixed is intrinsically sufficient to allow deagglomeration.

[0088] In addition, the device 1 also comprises two supply enclosures A1 and A2 for powders P, designed in particular to distribute powders of different types.

[0089] The two supply enclosures A1 and A2 for powders P allow the introduction of powders P into the first mixing enclosure E1 in contact with the cryogenic fluid FC of the first enclosure E1. Then, the powders P successively pass through the passage restriction systems R1, ..., Rn-1 and the mixing enclosures E2, ..., En, each mixing enclosure comprising a cryogenic fluid FC.

[0090] In addition, each mixing chamber E1, ..., En comprises stirring means 2 allowing the mixing of the powders P suspended in the cryogenic fluid FC. Examples of such stirring means 2 are given below with reference in particular to Figures 5 and 6 .

[0091] The two feeding enclosures A1 and A2 include, for example, hoppers with adjustable feeding, using for example a worm screw, and / or dosing type systems, in particular vibrating trays or corridors.

[0092] Furthermore, advantageously, the device 1 also comprises an electrostatic charging system C+, C- of the powders P introduced into the mixing chambers E1, ..., En.

[0093] In particular, the part of the powders P contained in the first supply enclosure A1 is brought into contact with the positive part C+ of the electrostatic charging system to be electrostatically charged in a positive manner, while the part of the powders P contained in the second supply enclosure A2 is brought into contact with the negative part C- of the electrostatic charging system to be electrostatically charged in a negative manner.

[0094] In this way, it is possible to enable differentiated local agglomeration, in other words to avoid self-agglomeration. As illustrated by figure 4, which schematically represents the agglomeration of oppositely charged powder particles P prior to their introduction into the mixing chambers E1, ..., En, the particles of the two powders P to be mixed being of opposite electrostatic charge, any reagglomeration will mainly take place by intercalation of powders of different nature, and therefore charge. This thus makes it possible to promote mixing at the scale of the constituent particles of the powders P to be mixed.

[0095] The invention thus exploits different technical effects making it possible in particular to achieve the desired level of homogenization, such as those described below: the improved deagglomeration, at least partial, of the powders P when these are suspended in the cryogenic liquid FC, the improvement of the wettability of the powders P by using the liquefied gas constituted by the cryogenic fluid FC, which is a liquid with low surface tension, compared to water, this being advantageously used without the use of an additive which is difficult to eliminate, the agitation close to the regime of a perfectly stirred reactor implemented by the movement of the agitation means, which may or may not use the vibration of the suspension as described below, these vibrations then being advantageously unsteady to limit the zones of heterogeneity.

[0096] Now referring to the Figures 5 and 6, two examples of mixing devices 1 are shown schematically for a device 20 for granulating powders P by cryogenic atomization in accordance with the invention, the principles of which have been described previously with reference to the figure 3 .

[0097] In each of these two examples, the mixing device 1 comprises, in addition to the elements described previously with reference to the figure 3 , a stirring motor 5 capable of rotating first stirring means 2a in the form of mixing rotors 2a in the mixing chambers E1, ..., En.

[0098] These mixing mobiles 2a may comprise grinding mobiles. These mixing mobiles 2a may also comprise blades, quilt effect mobiles, turbines and / or blades, these types of mobiles being respectively represented on the Figures 7A, 7B and 7C . In the examples of realization of the Figures 5 and 6, the mixing mobiles 2a comprise turbines.

[0099] Furthermore, in each of these two examples, the device 1 also comprises second stirring means 2b in the form of means for generating ultrasonic vibrations comprising sonotrodes 2b.

[0100] In addition, the two examples of realization represented on the Figures 5 and 6 differ in the nature of the passage restriction systems R1, ..., Rn-1 used.

[0101] Thus, in the example of realization of the Figure 5 , the passage restriction systems R1, ..., Rn-1 include diaphragms.

[0102] In the example of realization of the figure 6 , the passage restriction systems R1, ..., Rn-1 comprise sieves, more precisely sieve meshes.

[0103] In these two examples, the passage restriction systems R1, ..., Rn-1 have an adjustable passage section and are thus arranged in such a way that their passage sections are classified from the largest to the finest in the downward direction of the flow of powders P. Advantageously also, the passage sections of these passage restriction systems R1, ..., Rn-1 are smaller than the natural flow section of the powders P in order to force deagglomeration before passage through these sections.

[0104] It has further been represented, with reference to the figure 8 , a diagram illustrating another example of a device 1 for mixing powders P for a device 20 for granulating powders P by cryogenic atomization in accordance with the invention.

[0105] In this example, the device 1 comprises a single mixing chamber E1 and mixing means MG of this mixing chamber E1 according to a gyroscopic movement.

[0106] More precisely, these MG mixing means according to a gyroscopic movement allow the rotation of the mixing chamber E1 according to the three axes X1, X2 and X3 of the three-dimensional metrology. This type of agitation by gyroscopic movement promotes the mixing of the powders P when they have high densities compared to the density of the fluidic phase of the cryogenic fluid FC located in the mixing chamber E1.

[0107] In addition, the mixing enclosure E1 comprises stirring means 2a, for example in the form of turbines.

[0108] Referring now to the figure 9 , a diagram is shown illustrating the general principle of another example of a device 1 for mixing powders P by cryogenic fluid for a granulation device 20 according to the invention.

[0109] According to this principle, the mixing device 1 comprises a heat-insulated mixing enclosure E1 for the powders P provided with means for forming a fluidized bed of powders Lf, visible on the figure 10 described later.

[0110] In addition, the mixing device 1 comprises a supply enclosure A1 for powders P to allow the introduction of powders P into the mixing enclosure E1, and a supply enclosure B1 for cryogenic fluid FC to allow the introduction of the cryogenic fluid FC into the mixing enclosure E1. In this way, it is possible to obtain a suspension of powders P and cryogenic fluid FC in the mixing enclosure E1 forming a fluidized bed Lf.

[0111] The FC cryogenic fluid supply enclosure B1 may correspond to a distribution enclosure or a FC cryogenic fluid recirculation enclosure. This supply enclosure B1 may allow the distribution and / or recycling of FC cryogenic fluid. In particular, it may partly rely on pressurizing a liquefied gas supply tank.

[0112] Furthermore, advantageously, the mixing device 1 also comprises a system for generating vibrations Vb in the fluidized powder bed Lf, a control system Sp for this vibration generation system Vb, and a system for analyzing the concentration Ac of the suspension of powders P and cryogenic fluid FC in the mixing enclosure E1, the operation of which is controlled by the control system Sp.

[0113] The control system Sp can in particular allow the control of the operation of the device 1 and the processing of data, in particular in terms of supply conditions for powders P, cryogenic fluid FC and / or in terms of vibration amplitude.

[0114] Advantageously, as will appear more clearly with reference to the figure 10 , the mixing enclosure E1 is configured such that the introduction of cryogenic fluid FC into it allows fluidization of the powders P to be mixed by percolation of the cryogenic fluid FC through the bed of powders thus fluidized Lf.

[0115] In reference to the figure 10 precisely, an example of a mixing device 1 for a granulation device 20 according to the invention has been partially and schematically represented.

[0116] This mixing device 1 comprises a mixing enclosure E1 forming a tank with a vertical main axis having a symmetry of revolution, in particular in the form of a cylinder, and being advantageously heat-insulated to minimize thermal losses as its purpose is to receive a circulating liquefied gas phase.

[0117] Advantageously, the cryogenic fluid FC (liquefied gas) is introduced into the lower part of the mixing chamber E1, at the inlet of the fluidized bed Lf of powders P, by means of a distribution system Sd, in particular in the form of a grid or sintered part, making it possible to distribute the cryogenic fluid FC homogeneously over the passage section of the fluidized bed Lf.

[0118] Furthermore, the mixing chamber E1 can be equipped with a diverging zone in order to disengage the smallest powder particles P and allow them to remain in the fluidized bed zone Lf.

[0119] In addition, a system for analyzing the concentration Ac of the suspension of powders P and cryogenic fluid FC in the mixing chamber E1 is also provided, this system Ac comprising in particular an optical sensor Co making it possible to observe the fluidized bed Lf of powders P through a viewing window H. The system Ac is thus interfaced through the fluidized bed Lf.

[0120] The Ac concentration analysis system, equipped with the Co optical sensor, can be used to analyze the concentration of P powders, and also to analyze the particle size of the granular medium formed in the E1 mixing chamber.

[0121] The Ac concentration analysis system can include an optical fiber of the emitting type (light source illuminating the fluidized bed Lf) and receiving type (sensor). It can also include a camera. It should then be noted that the concentration of the particles is a function of the distance between the emitting fiber and the receiving fiber, the particle size distribution, the refractive index of the granular medium, and the wavelength of the incident beam in the dispersion medium.

[0122] Furthermore, the device 1 comprises the vibration generation system Vb. This system advantageously comprises sonotrodes So.

[0123] As can be seen on the figure 10 , the vibration generation system Vb is introduced at the right of the fluidized bed Lf as close as possible to the introduction of cryogenic fluid FC. In particular, the sonotrodes So can immerse within the fluidized bed Lf.

[0124] The So sonotrodes can be controlled independently by the Sp control system (not shown in the figure 10 ) to induce a periodic phase shift between the vibration sources in order to introduce unsteady interference, so as to improve the mixing within the fluidized bed Lf of powders P. As such, the figure 11 illustrates a representation of the interference lines induced by two vibration sources S1 and S2 having the same pulsation frequency.

[0125] Furthermore, advantageously, vibration control by means of the Sp control system can induce chaotic vibration signals. This can be achieved by controlling the So sonotrodes as Van der Pol type oscillators. In this respect, the Figures 12A-12B and 13A-13Billustrate the forms of interference within the suspension of powders P induced by two sources having the same pulsation phase, these phases being constant. More precisely, the Figures 12A and 12B illustrate the generation of stable oscillations after convergence (a = 2.16, b = 2.28 and w 0 = 3), while the Figures 13A and 13B illustrate the generation of quasi-chaotic oscillations of a Van der Pol type oscillator, with equation of the type x" + ax'.(x 2

[0126] It should be noted that, by varying the phases of the vibration sources, the interferences can move by a distance equivalent to the order of magnitude of the wavelength of the vibrations injected into the fluidized bed Lf. This then allows an additional degree of mixing.

[0127] The application of vibrations in complex, particularly quasi-chaotic, oscillations produces an almost perfect mixing effect.

[0128] Furthermore, it should also be noted that the A1 supply enclosure for the P powders (not shown on the figure 10 ) can allow feeding by gravity, or even by a screw-type device, or even by means of a vibrating bed, for example.

[0129] Furthermore, advantageously, the P powders can be electrostatically charged with opposite charges to allow differentiated reagglomeration to be obtained during suspension.

[0130] Of course, the invention is not limited to the embodiments which have just been described. Various modifications can be made to it by those skilled in the art.

Claims

1. A device (20) for granulating powders (P), in particular actinide powders, by cryogenic atomisation, characterised in that it includes: - a device (1) for mixing powders (P) with cryogenic fluid (FC), including at least one chamber (E1-En) for mixing the powders (P), including a cryogenic fluid (FC), - a device (10) for atomising a suspension of powders (P) mixed by the device (1) for mixing powders (P) to allow granulation of the powders (P), including: - means for fractionating the suspension of powders (P) allowing the adjustment of the size of the droplets (Go) of powders (P) to be atomised, - means for adjusting the humidity of the mixed powders (P) and / or the humidity of the atomisation atmosphere.

2. The device according to claim 1, characterised in that the mixing device (1) further includes: - a powder (P) supply chamber (A1, A2) to allow the introduction of the powders (P) into the mixing chamber (E1-En), - means (2, 2a, 2b) for stirring in the mixing chamber (E1-En) to allow mixing of the powders (P) suspended in the cryogenic fluid (FC).

3. The device according to claim 2, characterised in that the mixing device (1) includes means (MG) for mixing the mixing chamber (E1-En) according to a gyroscopic movement.

4. The device according to claim 2, characterised in that the mixing device (1) includes: - a plurality of powder (P) mixing chambers (E1-En), disposed successively in series one after the other, the powder (P) supply chamber (A1, A2) allowing the introduction of the powders (P) into at least the first mixing chamber (E1), - a plurality of powder (P) passage restriction systems (R1-Rn-1), each passage restriction system (R1-Rn-1) being located between two successive mixing chambers (E1-En), to constrain the distribution of powders (P) from one mixing chamber (E1-En) to the next, each mixing chamber (E1-En) including a cryogenic fluid (FC) and stirring means (2, 2a, 2b) to allow mixing of the powders (P) suspended in the cryogenic fluid (FC).

5. The device according to one of claims 2 to 4, characterised in that the stirring means include mixing rotors (2a), in particular blades, turbines, and / or quilt effect rotors.

6. The device according to any one of claims 2 to 5, characterised in that the stirring means include means for generating vibrations (2b), in particular ultrasonic vibrations, in particular sonotrodes (2b).

7. The device according to any one of claims 4 to 6, characterised in that the passage restriction systems (R1-Rn-1) include sieves and / or diaphragms.

8. The device according to claim 1, characterised in that the mixing device (1) further includes: - a powder (P) mixing chamber (E1), including a cryogenic fluid (FC), provided with means for forming a fluidised powder bed (Lf), - a powder (P) supply chamber (A1) to allow the introduction of the powders (P) into the mixing chamber (E1), - a cryogenic fluid (FC) supply chamber (B1) to allow the introduction of the cryogenic fluid (FC) into the mixing chamber (E1), - a system (Vb) for generating vibrations in the fluidised powder bed (Lf), - a system (Sp) for controlling the vibration generation system (Vb), - possibly, a system (Ac) for analysing the concentration of the suspension of powders (P) and cryogenic fluid (FC) in the mixing chamber (E1), the operation of which is in particular controlled by the control system (Sp).

9. The device according to claim 8, characterised in that the mixing chamber (E1) includes a system (Sd), in particular a grid or a sintered part, for distributing the cryogenic fluid (FC) through the powder (P) fluidised bed (Lf) to allow a homogeneous distribution of the cryogenic fluid (FC) in the fluidised bed (Lf).

10. The device according to claim 8 or 9, characterised in that the vibration generation system (Vb) is at least partially located in the powder (P) fluidised bed (Lf), the vibration generation system (Vb) including in particular sonotrodes (So) introduced into the powder (P) fluidised bed (Lf), the sonotrodes (So) being in particular controlled independently by the control system (Sp) to induce a periodic phase shift between the sonotrodes (So) in order to introduce unsteady interference improving the mixing within the powder (P) fluidised bed (Lf), the sonotrodes (So) being in particular configured to generate pseudo-chaotic oscillations of the Van der Pol type.

11. The device according to any one of claims 2 to 10, characterised in that the mixing device (1) includes an electrostatic charging system (C+, C-) for the powders (P) intended to be introduced into the mixing chamber(s) (E1-En), a portion of the powders (P) being in particular contacted with a portion of the electrostatic charging system (C+) to be electrostatically positively charged and the other portion of the powders (P) being in particular contacted with the other portion of the electrostatic charging system (C-) to be electrostatically negatively charged, in order to allow differentiated local agglomeration.

12. A method for granulating powders (P) by cryogenic atomisation, characterised in that it is carried out by means of a device (20) according to any one of the preceding claims, and in that it includes the following steps: a) introducing powders (P) and cryogenic fluid (FC) into at least one mixing chamber (E1-En) of the device (1) for mixing powders (P) with cryogenic fluid (FC) to obtain a suspension of powders (P) and cryogenic fluid (FC), b) atomising the suspension of powders (P) and cryogenic fluid (FC) by means of the atomisation device (10) to allow granulation of the powders (P), c) obtaining granules formed from the powders (P).

13. The method according to claim 12, characterised in that during the first step a), the powders (P) are electrostatically charged in an opposite manner to promote differentiated local agglomeration.