EP3370856B1 - Device for mixing powders by cryogenic fluid and generating vibrations and process - Google Patents

Description

TECHNICAL AREA

The present invention relates to the field of the preparation of granular media, and more specifically to the mixture of powders, in particular actinide powders, and to their deagglomeration / reagglomeration to obtain a mixture of high homogeneity by means of a cryogenic fluid , also called median cryogenic.

In a preferred manner, it applies to high density and / or cohesive powders, such as actinide powders. The invention thus preferably has its application for the mixture of actinide powders for the formation of nuclear fuel, in particular nuclear fuel pellets.

The invention thus proposes a device for mixing powders by cryogenic fluid, as well as a method for mixing powders associated with it.

STATE OF THE PRIOR ART

The implementation of the various stages of preparation of a granular medium, in particular from actinide powders, to form nuclear fuel pellets after forming by pressing, is essential because it mainly determines the control of the microstructure of the substrate. final product but also the presence or absence of defects of macroscopic aspects within a fuel pellet. In particular, the mixture of actinide powders to allow the production of nuclear fuel is a key step in controlling the quality of the fuel pellet obtained, which is most often subject to compliance with stringent requirements in terms of microstructure and impurities.

The industrial, conventional and historical process of powder metallurgy applied to the development of nuclear fuel is based on mixing steps, grinding and / or granulation, all carried out in the dry process. Indeed, the implementation of liquid in the nuclear industry induces the generation of effluents that can be difficult to treat. Also, for the preparation of a granular medium for the purpose of developing nuclear fuel, it is not conventionally used for processes other than those using the dry route.

To achieve the mixing of the powders, various devices are known from the prior art, which can be decomposed according to the families described below.

First of all, there is the principle of the mixer in dry phase without internal media. It may in particular be a Turbula® type mixer from the company WAB which by more or less complex movements of the tank containing the powders to be mixed, allows more or less 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 areas, for which mixing does not occur or at least in an incorrect manner and not admissible. The kinematics of this type of mixer is generally not complex enough to induce a thorough mixture, that is to say a mixture that is satisfactory in terms of homogeneity, without focusing 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 deagglomeration to be sufficient to reach sufficient degrees of homogeneity in the case where the size of these agglomerates is too large (in particular to be compensated during the sintering step).

The principle of the media mixer is also known. According to this principle and to promote the mixing operation, one or more mobile can be used within the tank containing the powder to be mixed. These mobiles can be blades, turbines, shares, ribbons, worms, among others. To improve the mixing, the tank can itself be mobile. This type of mixer may be more efficient than the previous category but is still insufficient and suffers limitations. In fact, the stirring induces a modification of the granular medium by agglomeration or disagglomeration difficult to control, which induces a proliferation of powders and / or degradation of the flowability of the granular medium. Moreover, the use of mobile (media) for mixing causes pollution (contaminations) when it comes to mixing abrasive powders such as those to be implemented for the realization of nuclear fuel. In addition, the mobiles implemented induce retentions that generate very high dose rates in the case of the development of nuclear fuel.

There is also the principle of the grinder-type mixer. Indeed, depending on the mode of use and the type of technology of some grinders, it is possible to make 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 which reduce the size. particles of powders. This causes the generation of fine particles and a modification of the specific surface which also has an impact on the possibility of later use of the powders after their mixing (modification of the flowability, of the reactivity (possible oxidation), of the sinterability of the powders , ...). In the context of the manufacture of nuclear fuel, 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. Moreover, clogging phenomena can be induced.

After using these different types of mixer, it is often necessary to achieve agglomeration or granulation. In addition, these devices are generally discontinuous, which can be problematic in industrial processes.

In general, the aforementioned mixers are not fully satisfactory for mixing certain powders, such as actinide powders, and it is necessary to carry out a granulation step in order to obtain a flowable granular medium.

Other mixers are also known, implementing a multiphasic medium, namely fluid-solid phases. These mixers can be classified in two main categories described below.

First, there are liquid / solid type mixers. These mixers are not effective for the implementation of soluble powders with the liquid phase used in the mixer or if the powders are modified by contact with the fluid. Moreover, for powders having a high density compared to the liquid introduced into the mixer, the mixture is most often not effective or requires significant stirring speeds. Indeed, the take-off speed of a particle from the bottom of the agitator is directly related to the density difference between the particles constituting the powders and that of the liquid for suspending. In this case, it can be used viscous liquids but this induces an increased energy demand, and this in proportion to the increase in viscosity before reaching a turbulent regime to promote mixing. Moreover, in this case of liquid / solid type mixer, there is also the question of the separation of the liquid phase and the solid phase after mixing. In the case of the mixture of actinide powders, this type of mixer would induce very heavy contaminated effluents to be reprocessed, which is unacceptable. In addition, in practice, complete and homogeneous suspension can not be achieved when small particle size powders are to be mixed. More precisely, to reach an optimal homogenization, the so-called non-dimensional number of Archimedes must be greater than 10 (ie the viscosity forces are lower than the forces of gravity and inertia). Knowing that the constituent particles of the powders to be mixed have relatively small diameters, typically less than 10 microns, it is not conceivable to make homogeneous and complete suspensions with this type of device without using complementary mixing means. In this sense, technologies, such as that described in the patent application CA 2 882 302 A1 have been proposed but remain nevertheless inoperative for a mixture of actinide powders, the vibration means used not allowing sufficient homogenization in view of the objectives of homogenization to achieve and the particularities of the actinide powders. In addition, for reasons of criticality control, the volume of the mixer must be limited, to prevent any risk of double loading that could lead to exceeding the critical mass allowable. Indeed, in a conventional liquid / solid mixer, the particle density per volume of The tank can not be large unless it exceeds a stirring power that is too high, or undergoes slow mixing kinetics.

Finally, it should be noted that liquid phase powder mixers, in particular of the type described in the patent applications CA 2 882 302 A1 , WO 2006/0111266 A1 and WO 1999/010092 A1 , are not suitable for the problematic of a mixture of powders of actinide powders type, since they would require too great stirring speeds to hope to take off the powders from the bottom of the stirring tank and reach levels of consistent with those sought in the nuclear industry. In addition, once again, they would induce contaminated effluents, difficult to manage industrially but also risks of criticality or even radiolysis of the liquid phase used because of the nature of the powders to be used (beyond the fact that these can interact chemically with the liquid used).

Then there are also gas / solid mixers. This type of mixer can be operative and does not induce a risk of criticality. However, this type of mixer is only slightly effective for powders that do not have sufficient fluidization properties, typically type C powders according to the classification of D. Geldart as described in Powder Technology, Vol.7, 1973 . However, this characteristic of poor fluidization is encountered for cohesive actinide powders such as those used to manufacture nuclear fuel. Furthermore, beyond the difficulty of the fluidization, given the densities of the powders to be fluidized for mixing, the superficial gas velocity should be large and at least equal to the minimum fluidization speed. Also, this type of mixer appears only poorly suited to the mixture of cohesive powders and a fortiori high density.

US2004 / 0031754 A1 discloses a device and method for mixing powders using vibrations.

STATEMENT OF THE INVENTION

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

• désagglomérer les poudres à mélanger sans nécessairement en modifier leur surface spécifique et générer de fines particules,
• mélanger les poudres avec un niveau d'homogénéité suffisant pour obtenir un mélange de poudres répondant aux spécifications, notamment en termes d'homogénéité (i.e. permettant notamment d'obtenir un volume élémentaire représentatif (VER) au sein du milieu granulaire de l'ordre de quelques micromètres cubes à environ 10 µm³),
• ne pas induire de pollution des poudres à mélanger, ni de modification de la chimie de surface, ni générer d'effluents liquides difficiles à traiter,
• ne pas induire de risque de criticité spécifique,
• ne pas induire de risque de radiolyse spécifique,
• ne pas induire d'échauffement des poudres à mélanger,
• s'appuyer sur un mélangeur à diamètre limité pour maîtriser le risque de criticité même en cas d'erreur de chargement du mélangeur,
• réaliser l'opération de mélange en limitant autant que possible l'énergie dépensée et ce en un temps relativement court par rapport aux autres mélangeurs, soit de l'ordre de quelques minutes comparativement à quelques heures (pour d'autres systèmes de mélange comme les broyeurs à boulets), pour une même quantité de matière à mélanger,
• disposer d'un procédé de mélange continu ou quasiment continu.

In particular, there is a need to be able to concomitantly:

• deagglomerate the powders to be mixed without necessarily modifying their specific surface and generating fine particles,
• mixing the powders with a level of homogeneity sufficient to obtain a mixture of powders meeting the specifications, especially in terms of homogeneity (ie allowing in particular to obtain a representative elementary volume (VER) within the granular medium of the order of a few cubic microns to about 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 to induce a risk of specific criticality,
• do not induce specific radiolysis risk,
• do not induce heating of the powders to be mixed,
• use a mixer with a limited diameter to control the risk of criticality even in case of mixer loading error,
• perform the mixing operation by limiting as much as possible the energy expended and this in a relatively short time compared to other mixers, being 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.

The object of the invention is to at least partially remedy the needs mentioned above and the drawbacks relating to the embodiments of the prior art.

• une enceinte de mélange des poudres, comportant un fluide cryogénique, pourvue de moyens pour former un lit de poudres fluidisé,
• une enceinte d'alimentation en poudres pour permettre l'introduction des poudres dans l'enceinte de mélange,
• une enceinte d'alimentation en fluide cryogénique pour permettre l'introduction du fluide cryogénique dans l'enceinte de mélange,
• un système de génération de vibrations, notamment par ondes ultrasonores, dans le lit de poudres fluidisé,
• un système de pilotage du système de génération de vibrations.

According to one of its aspects, the subject of the invention is a device for mixing powders, in particular actinide powders, by cryogenic fluid, characterized in that it comprises:

• a powder mixing chamber, comprising a cryogenic fluid, provided with means for forming a bed of fluidized powders,
• a powder supply enclosure to allow the introduction of powders into the mixing chamber,
• a chamber for supplying cryogenic fluid to allow the introduction of the cryogenic fluid into the mixing chamber,
• a system for generating vibrations, in particular by ultrasonic waves, in the fluidized bed of powders,
• a control system of the vibration generation system.

Advantageously, within the mixing chamber, the powders are subjected to fluidization through the cryogenic fluid to obtain the fluidized bed of powders.

In addition, this bed of fluidized powders is subjected to the vibrations of the vibration generating system to preferentially obtain a major disorder in the suspension of powders and cryogenic fluid, these vibrations being controlled by means of the control system to optimize the mixed.

It should be noted that, in the usual way, a cryogenic fluid here designates a liquefied gas kept in the liquid state at low temperature. This liquefied gas is chemically inert under the conditions of implementation of the invention for the powders to be mixed and deagglomerated.

The powder mixing device according to the invention may further comprise one or more of the following characteristics taken separately or in any possible technical combinations.

The cryogenic fluid may comprise a weakly hydrogenated liquid, ie a liquid comprising at most one hydrogen atom per molecule of liquid, having a boiling point lower than that of water.

The device may further comprise an analysis system, in particular a system for measuring the solid concentration (ie powders) of the suspension of powders and of cryogenic fluid in the mixing chamber, the operation of which is in particular controlled by the system. piloting.

The mixing chamber may be configured in such a way that the introduction of cryogenic fluid into it allows fluidization of the powders to be mixed by percolating the cryogenic fluid through the thus fluidized bed of powders.

Furthermore, the mixing chamber may comprise a distribution system, in particular a sintered grid or part, cryogenic fluid through the fluidized bed of powders to allow a homogeneous distribution of the cryogenic fluid in the fluidized bed.

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

The sonotrodes can be controlled independently by the control system to induce a periodic phase shift of the phases between the sonotrodes to introduce unsteady interference improving mixing within the fluidized bed of powders.

The sonotrodes can still be configured to generate pseudo-chaotic oscillations, potentially for example by means of a Van der Pol type oscillation generator.

The mixing device may further comprise stirring means in the mixing chamber to promote the mixing of the powders placed in suspension in the cryogenic fluid, including in particular grinding means, for example of the balls, pebbles, among others .

In addition, the mixing device may also include a system for electrostatically charging the powders intended to be introduced into the mixing chamber.

Part of the powders may in particular be brought into contact with one part of the electrostatic charge system to be electrostatically charged in a positive manner and the other part of the powders may be brought into contact with the other part of the electrostatic charge system to be charged. Electrostatically negative, to allow differentiated local agglomeration. When mixing more than two types of powders, some powders may be either positively charged, or negatively charged, or without charge.

The cryogenic fluid may also be of any type, in particular being liquefied nitrogen or argon. It should be noted that the use of nitrogen is relevant because of 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 fuel operations (BET measurement, ...). The use of this type of cryogenic fluid does not therefore induce any additional particular risk in the production process.

1. a) introduction de poudres destinées à être mélangées dans l'enceinte de mélange par le biais de l'enceinte d'alimentation en poudres,
2. b) introduction de fluide cryogénique destiné à permettre la fluidisation du lit fluidisé de poudres dans l'enceinte de mélange par le biais de l'enceinte d'alimentation en fluide cryogénique,
3. c) mise en vibration de la suspension de poudres et de fluide cryogénique dans l'enceinte de mélange par le biais du système de génération de vibrations,
4. d) obtention d'un mélange formé à partir des poudres après évaporation du fluide cryogénique.

In addition, another aspect of the invention relates to a process for mixing powders, in particular actinide powders, by cryogenic fluid, characterized in that it is implemented by means of a device as defined above, and in that it comprises the following steps:

1. a) introduction of powders for mixing into the mixing chamber via the powder supply enclosure,
2. b) introduction of cryogenic fluid intended to allow fluidization of the fluidized bed of powders in the mixing chamber through the cryogenic fluid supply chamber,
3. c) vibrating the suspension of powders and cryogenic fluid in the mixing chamber through the vibration generating system,
4. d) obtaining a mixture formed from the powders after evaporation of the cryogenic fluid.

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

The method may also include the step of controlling the vibration generating system through the control system, in particular according to the particle concentration of the suspension.

The device and method for mixing powders according to the invention may comprise any of the features set forth in the description, taken alone or in any technically possible combination with other characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 représente un schéma illustrant le principe général d'un dispositif de mélange de poudres par fluide cryogénique conforme à l'invention,
• la figure 2 représente partiellement un exemple de dispositif conforme à l'invention,
• la figure 3 illustre une représentation de lignes d'interférences induites par deux sources vibratoires ayant la même fréquence de pulsation,
• les figures 4A et 4B illustrent la génération d'oscillations stables induites par un oscillateur de type Van der Pol après convergence, et les figures 5A et 5B illustrent la génération d'oscillations quasi chaotiques d'un oscillateur de type Van der Pol lorsque ses paramètres de pilotage sont adaptés, et
• les figures 6, 7 et 8 représentent respectivement des photographies d'un premier type de poudres avant mélange, d'un deuxième type de poudres avant mélange, et du mélange obtenu des premier et deuxième types de poudres après mélange par le biais d'un dispositif et d'un procédé conformes à l'invention.

The invention will be better understood on reading the following detailed description, non-limiting examples of implementation thereof, as well as on examining the schematic and partial figures of the appended drawing. on which :

• the figure 1 represents a diagram illustrating the general principle of a device for mixing powders by cryogenic fluid according to the invention,
• the figure 2 represents in part an example of a device according to the invention,
• the figure 3 illustrates a representation of interference lines induced by two vibratory sources having the same pulse frequency,
• the Figures 4A and 4B illustrate the generation of stable oscillations induced by a Van der Pol type oscillator after convergence, and the Figures 5A and 5B illustrate the generation of quasi-chaotic oscillations of a Van der Pol type oscillator when its driving parameters are adapted, and
• the Figures 6, 7 and 8 respectively represent photographs of a first type of powders before mixing, a second type of powders before mixing, and the mixture obtained of the first and second types of powders after mixing by means of a device and a method according to to the invention.

In all of these figures, identical references may designate identical or similar elements.

In addition, the different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

It is noted that in the exemplary embodiments described below, the P powders considered are actinide powders for 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.

With reference to the figure 1 , there is shown a diagram illustrating the general principle of a device 1 for mixing powders P by cryogenic fluid according to the invention.

According to this principle, the device 1 comprises a mixing chamber E1, preferably insulated, powders P provided with means for forming a bed of fluidized powders Lf, visible on the figure 2 described later.

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

The supply chamber B1 in cryogenic fluid FC may correspond to a distribution chamber or a cryogenic fluid recirculation chamber FC. This feed enclosure B1 can allow the distribution and / or recycling of cryogenic fluid FC. It can in particular partly rely on a pressurization of a liquefied gas supply tank.

Furthermore, advantageously, the device 1 also comprises a vibration generation system Vb in the fluidized bed of powders Lf, a control system Sp of this vibration generation system Vb, and a concentration analysis system Ac of the suspension of powders P and cryogenic fluid FC in the mixing chamber E1, the operation of which is controlled by the control system Sp.

The control system Sp can in particular make it possible to control the operation of the device 1 and the data processing, in particular in terms of powder supply conditions P, FC cryogenic fluid and / or in terms of amplitude of the vibrations.

Advantageously, as it will appear more clearly with reference to the figure 2 the mixing chamber E1 is configured in such a way that the introduction of cryogenic fluid FC into it makes it possible to fluidize the powders P with mixing by percolation of the cryogenic fluid FC through the bed of powders and fluidized Lf.

With reference to the figure 2 precisely, there is shown partially and schematically an example of mixing device 1 according to the invention.

This mixing device 1 comprises a mixing chamber E1 forming a reservoir of vertical main axis advantageously having a symmetry of revolution, in particular in the form of a cylinder, and being advantageously insulated to minimize thermal losses as its purpose is to receive a phase of circulating liquefied gas.

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 gate or sintered part, for distributing the cryogenic fluid FC homogeneously on the passage section of the fluidized bed Lf.

Furthermore, the mixing chamber E1 may be equipped with a diverging zone in order to disengage the smaller powder particles P and allow them to remain in the fluidized bed zone Lf.

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 including an optical sensor Co to observe the fluidized bed Lf powder P through a viewing port H. The Ac system is thus interfaced through the fluidized bed Lf.

The concentration analysis system Ac, equipped with the optical sensor Co, can make it possible to analyze the concentration of the powders P, or even to analyze the granulometry of the granular medium formed in the mixing chamber E1.

The concentration analysis system Ac may comprise an optical fiber of emitting type (light source illuminating the fluidized bed Lf) and receiver (sensor). It can still include a camera. It should 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.

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

As can be seen on the figure 2 , the vibration generation system Vb is introduced at the right of the fluidized bed Lf closest to the introduction of cryogenic fluid FC. In particular, the sonotrodes So can dive within the fluidized bed Lf.

The sonotrodes So can be controlled independently by the driving system Sp (not shown in FIG. figure 2 ) to induce a periodic phase shift of the phases 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 3 illustrates a representation of the interference lines induced by two vibratory sources S1 and S2 having the same pulse frequency.

Furthermore, advantageously, the control of the vibrations through the control system Sp can induce quasi-chaotic vibratory signals. This can be achieved by driving the sonotrodes So like so many Van der Pol type oscillators with unsteady tuning parameters. As such, Figures 4A-4B and 5A-5B illustrate the forms of the interferences within the suspension of P powders induced by two sources having the same phase of pulsation, these phases being constant. More specifically, Figures 4A and 4B illustrate the generation of stable oscillations after convergence (parameters of the chosen oscillator: a = 2.16, b = 2.28 and w ₀ = 3 for a motion equation of type x "+ ax '. (x ² / b ² - 1) + w ₀ ² .x = 0), while the Figures 5A and 5B illustrate the generation of quasi-chaotic oscillations of a Van der Pol type oscillator, of the type x "+ ax '. (x ² / b ² - 1) + w ₀ ² .x = 0, by temporal variation of the wo pulse.

It should be noted that, by varying the phases of the vibration sources, the interferences can move 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.

The application of vibrations according to complex oscillations, notably quasi- chaotic, works to an almost ideal mixing effect.

Furthermore, it should also be noted that the feed chamber A1 for the powders P (not shown in FIG. figure 2 ) can allow a gravity feed, or even a worm-type device, or even through a vibrating bed, for example.

In addition, advantageously, the powders P can be electrostatically charged with opposite charges to allow during the suspension to obtain a differentiated reagglomeration.

Table 1 below also gives an example of sizing of a device 1 according to the invention. <I><u> Table 1 </ u></i> Features of the device 1 Values Effective diameter of mixing chamber E1 15 cm Useful height of mixing chamber E1 40 cm FC Cryogenic Fluid Flow Rate 0.5 m ³ / h P powder charge 2 kg Mixing time about 5 minutes

The effectiveness of the mixture that can be achieved by means of the present invention can be characterized by the homogeneity of the granular medium obtained after mixing. Thus, Figures 6, 7 and 8 respectively represent photographs of a first type of powders before mixing, a second type of powders before mixing, and the mixture obtained of the first and second types of powders after mixing by means of a device 1 and a process according to the invention.

More specifically, the figure 6 represents aggregates of cerium dioxide powders CeO ₂ , the figure 7 represents aggregates of alumina powders Al ₂ O ₃ , and the figure 8 represents the mixture of these powders obtained with a mixing time of about 30 seconds.

There is then a good homogeneity of the granular medium after mixing (two powders used with equivalent masses). Indeed, on the figure 8 , it can be seen that for a scale of a few tens of microns, the aggregates of the two powders are present in a relatively evenly distributed manner and the size of the aggregates has little variation (similar to that of the initial powders to be mixed, namely here with a dimension close to 5 μm).

• la désagglomération, au moins partielle, améliorée des poudres P lorsque celles-ci sont mises en suspension dans le liquide cryogénique FC,
• l'amélioration de la mouillabilité des poudres P en utilisant le gaz liquéfié constitué par le fluide cryogénique FC, qui est un liquide à faible tension de surface, comparativement à l'eau, celui-ci pouvant être avantageusement employé sans utilisation d'additif difficile à éliminer,
• l'agitation proche du régime d'un réacteur parfaitement agité mise en œuvre par le mouvement des moyens d'agitation, pouvant ou non utiliser la mise en vibration de la suspension comme décrit par la suite, ces vibrations étant alors avantageusement instationnaires pour limiter les zones d'hétérogénéités.

The invention thus exploits various technical effects that make it possible in particular to reach the desired level of homogenization, such as those described below:

• the at least partial deagglomeration, improved P powders when they are suspended in the cryogenic liquid FC,
• improving the wettability of the powders P by using the liquefied gas constituted by the cryogenic fluid FC, which is a liquid with a low surface tension, compared with the water, which can be advantageously used without the use of a difficult additive to eliminate,
• the agitation close to the regime of a perfectly stirred reactor implemented by the movement of the stirring means, may or may not use the vibration of the suspension as described below, these vibrations then being advantageously unsteady to limit the areas of heterogeneity.

Of course, the invention is not limited to the embodiments which have just been described. Various modifications may be made by the skilled person.

Claims (15)

1. Device (1) for mixing powders (P) by a cryogenic fluid, in particular actinide powerds, comprising :

   - a chamber (E1) for mixing the powders (P), comprising a cryogenic fluid (FC), provided with means for forming a fluidised powder bed (Lf),

   - a chamber (A1) for supplying powders (P) in order to allow the powders (P) to be introduced into the mixing chamber (E1),

   - a chamber (B1) for supplying cryogenic fluid (FC) in order to allow the cryogenic fluid (FC) to be introduced into the mixing chamber (E1),

   - a system for generating vibrations (Vb) in the fluidised powder bed (Lf),

   - a system (Sp) for controlling the system for generating vibrations (Vb).
2. Device according to claim 1, characterised in that the cryogenic fluid (FC) comprises a slightly hydrogenated liquid, which is a liquid comprising at most one hydrogen atom per molecule of liquid, having a boiling temperature less than that of water.
3. Device according to claim 1 or 2, characterised in that it further comprises a system for analysing the concentration (Ac) of the suspension of powders (P) and of the cryogenic fluid (FC) in the mixing chamber (E1), of which the operation is in particular controlled by the controlling system (Sp).
4. Device as claimed in any preceding claim, characterised in that the mixing chamber (E1) is configured in such a way that the introduction of cryogenic fluid (FC) into the latter allows for a fluidisation of the powders (P) to be mixed by percolation of the cryogenic fluid (FC) through the powder bed fluidised as such (Lf).
5. Device as claimed in any preceding claim, characterised in that the mixing chamber (E1) comprises a distribution system (Sd), in particular a grille or a sintered part, of the cryogenic fluid (FC) through the fluidised bed (Lf) of powders (P) in order to allow for a homogeneous distribution of the cryogenic fluid (FC) in the fluidised bed (Lf).
6. Device as claimed in any preceding claim, characterised in that the system for generating vibrations (Vb) is at least partially located in the fluidised bed (Lf) of powders (P).
7. Device according to claim 6, characterised in that the system for generating vibrations (Vb) comprises sonotrodes (So) introduced into the fluidised bed (Lf) of powders (P).
8. Device according to claim 7, characterised in that the sonotrodes (So) are controlled independently by the controlling system (Sp) in order to induce a periodic phase shift of the phases between the sonotrodes (So) in order to introduce unsteady interferences that improve the mixture within the fluidised bed (Lf) of powders (P).
9. Device according to claim 6 or 7, characterised in that the sonotrodes (So) are configured to generate pseudo-chaotic oscillations of the Van der Pol type.
10. Device as claimed in any preceding claim, characterised in that it further comprises means for agitation in the mixing chamber (E1) so as to allow the mixing of the powders (P) placed in suspension in the cryogenic fluid (FC), comprising in particular means for grinding.
11. Device as claimed in any preceding claim, characterised in that it comprises a system of electrostatic charge of the powders (P) intended to be introduced into the mixing chamber (E1), .
    a portion of the powders (P) being particulary put into contact with a portion of the electrostatic charge system in order to be positively electrostatically charged and the other portion of the powders (P) being particularly put into contact with the other portion of the electrostatic charge system in order to be negatively electrostatically charged, in order to allow for a differentiated local agglomeration.
12. Device as claimed in any preceding claim, characterised in that the cryogenic fluid (FC) is liquefied nitrogen.
13. Method for mixing powders (P) by a cryogenic fluid, implemented by means of a device (1) as claimed in any preceding claim, and comprising the following steps:

    a) introduction of powders (P) intended to be mixed into the mixing chamber (E1) through the chamber (A1) for supplying powders (P),

    b) introduction of cryogenic fluid (FC) intended to allow for the fluidisation of the fluidised bed (Lf) of powders (P) into the mixing chamber (E1) through the chamber (B1) for supplying cryogenic fluid (FC),

    c) setting into vibration of the suspension of powders (P) and of cryogenic fluid (FC) in the mixing chamber (E1) through the system for generating vibrations (Vb),

    d) obtaining of a mixture formed from powders (P) after evaporation of the cryogenic fluid (FC).
14. Method according to claim 13, characterised in that during the first step a), the powders are electrostatically charged differently, in particular oppositely, in order to favour differentiated local agglomeration.
15. Method according to claim 13 or 14, characterised in that it also comprises the step of controlling the system for generating vibrations (Vb) through the controlling system (Sp).

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