EP3370855B1 - Device for mixing powders by cryogenic fluid 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 relies on mixing, grinding and / or granulation steps, 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 deagglomeration which is difficult to control, which induces a proliferation of powders and / or a degradation of the flowability of the granular medium. In addition, 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, US 4,917,834 discloses a device and a mixing method using a cryogenic fluid.

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,
• une enceinte d'alimentation en poudres pour permettre l'introduction des poudres dans l'enceinte de mélange,
• des moyens d'agitation dans l'enceinte de mélange pour permettre le mélange des poudres mises en suspension dans le fluide cryogénique.

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 at least:

• a powder mixing chamber, comprising a cryogenic fluid,
• a powder supply enclosure to allow the introduction of powders into the mixing chamber,
• stirring means in the mixing chamber to allow mixing of the powders suspended in the cryogenic fluid.

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.

According to a first embodiment of the invention, the device may comprise means for mixing the mixing chamber according to a gyroscopic movement.

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

• une pluralité d'enceintes de mélange des poudres, disposées successivement en série les unes après les autres, l'enceinte d'alimentation en poudres permettant l'introduction des poudres dans au moins la première enceinte de mélange,
• une pluralité de systèmes de restriction de passage des poudres, chaque système de restriction de passage étant situé entre deux enceintes de mélange successives, pour contraindre la distribution de poudres d'une enceinte de mélange à la suivante.

According to a second embodiment of the invention, the device may comprise:

• a plurality of powder mixing chambers, successively arranged in series one after the other, the powder supply enclosure for introducing the powders into at least the first mixing chamber,
• a plurality of powder passage restriction systems, each passage restriction system being located between two successive mixing chambers, to constrain the distribution of powders from one mixing chamber to the next.

Each mixing chamber may then comprise a cryogenic fluid, in particular being filled with a cryogenic fluid, and stirring means, in particular being equipped with stirring means, to allow mixing of the powders placed in suspension in the cryogenic fluid.

Moreover, the stirring means may comprise mixing mobiles, in particular blades, turbines and / or duvet mobiles, among others.

These mixing mobiles may comprise grinding mobiles, for example of the type balls, pebbles, among others.

In addition, the stirring means may also comprise means for generating vibrations, in particular ultrasonic vibrations, in particular sonotrodes.

In addition, passage restriction systems may include sieves. The passage restriction systems may further include diaphragms.

The passage restricting systems may be adjustable and configured so that their passage cross-section decreases as a function of the flow of powder flow through the plurality of mixing chambers, the passage section of a (n-1) The passage restriction system is thus greater than the passage section of an nth passage restriction system by following the flow flow of the powders.

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 deagglomerate when they pass from one mixing chamber to the other . Thus, the residence time of the particles to be mixed is intrinsically sufficient to allow disagglomeration.

Furthermore, the plurality of mixing chambers and the plurality of powder passage restriction systems may advantageously be arranged in the same vertical direction so as to allow the powder to flow under the effect of gravity.

In addition, the device preferably comprises a system for electrostatically charging the powders intended to be introduced into the mixing chamber or chambers.

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 the processes used for the development of the plutonium-based nuclear fuel are inert to the environment. nitrogen and that liquid nitrogen is itself used in some fuel operations (BET measurement, ...). The use of this type of cryogenic fluid does not therefore induce any additional particular risk in the production process.

The device may especially comprise at least two powder supply enclosures, and in particular as many powder supply enclosures as types of powders to mix.

The supply enclosure (s) may comprise adjustable feed hoppers and / or metering type systems, especially trays or vibrating corridors.

1. a) introduction de poudres destinées à être mélangées dans la ou les enceintes de mélange par le biais de la ou des enceintes d'alimentation,
2. b) mélange des poudres dans la ou les enceintes de mélange, mises en suspension dans un fluide cryogénique, par le biais des moyens d'agitation,
3. c) obtention d'un mélange formé à partir des poudres.

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 (s) through the feed chamber (s),
2. b) mixing the powders in the mixing chamber or chambers, suspended in a cryogenic fluid, by means of stirring means,
3. c) obtaining a mixture formed from the powders.

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.

According to a first embodiment of the method, the device may comprise a single mixing chamber, and said mixing chamber may be animated with a gyroscopic type of movement to allow mixing of the powders.

According to a second embodiment of the method, the device may comprise a plurality of powder mixing enclosures, successively arranged in series one after the other, or the powder supply enclosures for introducing the powders into the minus the first mixing chamber, and a plurality of powder passage restriction systems, each passage restriction system being located between two successive mixing chambers, to constrain the distribution of powders from one mixing chamber to the next, each mixing chamber comprising a cryogenic fluid and stirring means to allow mixing of the powders suspended in the cryogenic fluid, the process then possibly comprising the step of progressively restricting the passage of the flow of the powders through mixing enclosures through p-section passage restriction systems decreasing wettage according to the flow of the powders.

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 selon un premier mode de réalisation de l'invention,
• la figure 2 représente schématiquement l'agglomération de particules de poudres chargées de manière opposée préalablement à leur introduction dans des enceintes de mélange d'un dispositif conforme au principe de la figure 1,
• les figures 3 et 4 représentent respectivement deux exemples de dispositifs conformes au premier mode de réalisation de l'invention,
• les figures 5A, 5B et 5C représentent schématiquement des variantes de réalisation des mobiles de mélange des dispositifs des figures 3 et 4,
• les figures 6 et 7 représentent graphiquement des exemples d'évolution de mélanges de poudres d'un dispositif conforme à l'invention en fonction du temps,
• la figure 8 représente un schéma illustrant un dispositif de mélange de poudres par fluide cryogénique selon un deuxième mode de réalisation de l'invention, et
• les figures 9, 10 et 11 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 a first embodiment of the invention,
• the figure 2 schematically represents the agglomeration of oppositely charged powder particles prior to their introduction into mixing chambers of a device according to the principle of figure 1 ,
• the Figures 3 and 4 represent respectively two examples of devices according to the first embodiment of the invention,
• the FIGS. 5A, 5B and 5C schematically represent alternative embodiments of mobile devices mixing devices Figures 3 and 4 ,
• the Figures 6 and 7 graphically represent examples of evolution of powder mixtures of a device according to the invention as a function of time,
• the figure 8 represents a diagram illustrating a device for mixing powders by cryogenic fluid according to a second embodiment of the invention, and
• the Figures 9, 10 and 11 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 making it possible to produce pellets of nuclear fuel. 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 a first embodiment of the invention.

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

Furthermore, the device 1 comprises an n-1 number of passage restriction systems R1,..., Rn-1 of the powders P, each passage restriction system R1,..., Rn-1 being situated between two mixing chambers E1, ..., In successive, to constrain the distribution of powders P of a mixing chamber E1, ..., En to the following. Examples of such passage restriction systems R1,..., Rn-1 are presented hereinafter with particular reference to Figures 3 and 4 .

In addition, the device 1 also comprises two feed enclosures A1 and A2 in powders P, provided in particular for dispensing powders of different types.

The two feed enclosures A1 and A2 in powders P allow the introduction of powders P into the first mixing chamber 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 chambers E2,..., In each mixing enclosure comprising a cryogenic fluid FC.

In addition, each mixing chamber E1,..., Comprises stirring means 2 for mixing the powders P suspended in the cryogenic fluid FC. Examples of such stirring means 2 are given hereinafter with particular reference to Figures 3 and 4 .

The two feed enclosures A1 and A2 comprise for example adjustable feed hoppers, for example using a worm, and / or metering-type systems, including trays or vibrating corridors.

In addition, advantageously, the device 1 further comprises an electrostatic charging system C +, C- powders P introduced into the mixing chambers E1, ..., En.

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

In this way, it is possible to allow a differentiated local agglomeration, in other words to avoid self-agglomeration. As illustrated by figure 2 which schematically represents the agglomeration of the powder particles P loaded in an opposite manner prior to their introduction into the mixing chambers E1,..., In, the particles of the two powders P to be mixed being of opposite electrostatic charge, a possible reagglomeration will operate mainly by intercalating powders of nature, and therefore different charges. This thus makes it possible to promote the mixing at the level of the constitutive particles of the powders P to be mixed.

• 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 étant 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, the latter being advantageously employed without the use of additives that are difficult 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.

Referring now to Figures 3 and 4 two examples of devices 1 according to the first embodiment of the invention, the principle of which has been described above with reference to FIG. figure 1 .

In each of these two examples, the device 1 comprises, in addition to the elements described above with reference to the figure 1 an agitating motor 5 capable of rotating first stirring means 2a in the form of mixing wheels 2a in the mixing chambers E1, ..., En.

These mixing mobiles 2a may comprise grinding mobiles. These mixing mobiles 2a can also comprise blades, quilt mobiles, turbines and / or blades, these types of mobiles being respectively represented on the blades. FIGS. 5A, 5B and 5C . In the exemplary embodiments of Figures 3 and 4 the mixing mobiles 2a comprise turbines.

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

Moreover, the two exemplary embodiments represented on the Figures 3 and 4 are differentiated by the nature of the passage restriction systems R1,..., Rn-1 used.

Thus, in the embodiment of the figure 3 , the passage restriction systems R1,..., Rn-1 comprise diaphragms.

In the exemplary embodiment of the figure 4 , the passage restriction systems R1,..., Rn-1 comprise sieves, more specifically sieve meshes.

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 thinnest in the direction downstream of the powders stream 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 the deagglomeration before the passage through these sections.

An example of dimensioning of a device 1 according to the invention according to the first embodiment of the invention will now be described.

• les vitesses des mobiles de mélange 2a pour permettre le décollage des particules de poudres P du fond de chaque enceinte de mélange E1, ..., En,
• le temps de mélange des poudres,
• le débit de poudres P, à savoir la quantité de poudres P pouvant être mélangée par unité de temps.

For the dimensioning of the mixing chambers E1, ..., En, it is necessary to evaluate in particular:

• the speeds of the mixing mobiles 2a to allow the particles of powder P to take off from the bottom of each mixing chamber E1,.
• the mixing time of the powders,
• the flow rate of powders P, namely the quantity of powders P that can be mixed per unit of time.

dans laquelle notamment :

• Nmin représente la fréquence d'agitation minimale pour avoir le décollage des particules de poudres P,
• DT représente le diamètre du mobile de mélange 2a,
• DA représente le diamètre de l'enceinte de mélange E1, ..., En,
• ρ_P représente la masse volumique de la poudre P,
• ρ_L représente la masse volumique du fluide cryogénique FC,
• µ_L représente la viscosité du fluide cryogénique FC,
• dp représente le diamètre des particules de poudre P,
• Ws représente le ratio massique entre phase solide et phase liquéfié, en pourcentages.

For this, the equation given by the Zwietering correlation can be exploited, namely: $$\mathit{Nmin}=\phi .{\left(\frac{\mathit{DT}}{\mathit{DA}}\right)}^{\alpha }.g^{0.45}.\frac{{\left({\rho }_P-{\rho }_{\mathrm{The}}\right)}^{0.45}.{{\mu }_{\mathrm{The}}}^{0.1}.{d_P}^{0.2}.{\left(\mathit{ws}\right)}^{0.13}}{{\mathit{DA}}^{0.85}-{{\rho }_{\mathrm{The}}}^{0.55}},$$

in which, in particular:

• Nmin represents the minimum stirring frequency for taking off the P powder particles,
• DT represents the diameter of the mixing wheel 2a,
• DA represents the diameter of the mixing chamber E1, ..., In,
• ρ _P represents the density of the powder P,
• ρ _L represents the density of the cryogenic fluid FC,
• μ _L represents the viscosity of the cryogenic fluid FC,
• dp represents the diameter of the powder particles P,
• Ws represents the mass ratio between solid phase and liquefied phase, in percentages.

• Q_p représente le débit de pompage,
• Q_c représente le débit de circulation,
• N représente la vitesse d'agitation,
• d représente le diamètre du mobile de mélange,
• P représente la puissance d'agitation.

In addition, the following equations can also be exploited:
Q _p = 0.73.ND ³ , Q _c = 2.Q _p , tm = 3.tc, tc = V / Qc and P = N _p .ρ.N ³ d ⁵ , in which in particular:

• Q _p represents the pumping rate,
• Q _c represents the flow rate,
• N represents the stirring speed,
• d represents the diameter of the mixing mobile,
• P represents the stirring power.

Table 1 below thus gives the dimensioning obtained from a device 1 according to the invention for obtaining 1 kg / h of ground material. <i><u> Table 1 </ u></i> Features of the device 1 Values Volume of a mixing chamber E1, ..., in 100 mL Diameter of a mixing chamber E1, ..., in 10 cm P powder content in the suspension 10% Rotation frequency of mixing mobiles 8 s ^-1 Diameter of a mixing mobile 4cm Pump flow 3.7.10 ^-4 m ³ / s Flow rate 7.5.10 ^-4 m ³ / s Mixing time (tm) for an enclosure with a 10% charge (A) ~ 0.40 s Mixing capacity ~ 0.9 kg / h Number of mixing chamber 4 Stirring power 105 W / m ³

The device 1 obtained then has a mixture response illustrated by the graph of the figure 6 , representing the evolution X of the mixture as a function of time t, ie the curve X (t) = A. [1-exp (-kt)], k being a given coefficient, A mixing load, and tm the time mixture.

Advantageously, the series of n mixing enclosures E1, ..., Having a unit volume Vn such that the overall volume V of mixing chambers E1, ..., En such that V = n.Vn .

In this case, in fact, the overall mixing time is less than the mixing time tm for the volume V. The difference is greater between these mixing times than n is large, as shown in the graph. of the figure 7 , representing the evolution X of the mixture as a function of time t, in a manner similar to the figure 6 , with the times t1 and t2 of the first and second speakers and the times t'm and tm.

We have also shown, with reference to the figure 8 , a diagram illustrating a device 1 for mixing powders P by cryogenic fluid according to a second embodiment of the invention.

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

More specifically, these mixing means MG are in a gyroscopic type of movement, or close to being, allowing the rotation of the mixing chamber E1 along the three axes X1, X2 and X3 of the three-dimensional metrology. This type of gyro-motion stirring favors the mixing of powders P when they have high densities compared to the density of the cryogenic fluid phase FC located in the mixing chamber E1.

In addition, the mixing chamber E1 comprises stirring means 2a, for example in the form of turbines.

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 9, 10 and 11 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 9 represents aggregates of cerium dioxide powders CeO ₂ , the figure 10 represents aggregates of alumina powders Al ₂ O ₃ , and the figure 11 represents the mixture of these powders obtained with a mixing time of about 30 s and the use of a single mixing chamber containing liquid nitrogen as a cryogenic mixing fluid.

Then, despite a short time (30 s) of mixing of the aforementioned powders and implemented in an equimassic manner (equal proportion by weight of the two powders), a good homogeneity of the granular medium after mixing is observed, as illustrated on FIG. figure 11 , with an aggregate size close to that of the powders to be mixed, or here with a dimension close to 5 microns.

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 powders, comprising at least:

   - a plurality of mixing chambers (E1-En) of the powders (P), each one comprising a cryogenic fluid (FC), arranged successively in series one after the other,

   - a chamber (A1, A2) for supplying powders (P) in order to allow the powders (P) to be introduced into at least the first mixing chamber (E1),

   - a plurality of systems for restricting the passage (R1-Rn-1) of the powders (P), with each system for restricting the passage (R1-Rn-1) being located between two successive mixing chamber (E1-En), in order to constrain the distribution of powders (P) from one mixing chamber (E1-En) to the next, with each system for restricting the passage (R1-Rn-1) able to be adjusted,

   - means for agitation (2, 2a, 2b) in each one of the mixing chambers (E1-En) so as to allow the mixing of the powders (P) placed in suspension in the cryogenic fluid (FC).
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 as claimed in claim 1 or 2, characterised in that the means for agitation comprise mobile mixing devices (2a), in particular blades, turbines and/or mobile facility with a duvet effect, in particular mobile mixing facilities (2a) comprising mobile grinding facilities.
4. Device as claimed in any preceding claim, characterised in that the means for agitation comprise means for generating vibrations (2b), in particular ultrasonic vibrations, in particular sonotrodes (2b).
5. Device as claimed in any preceding claim, characterised in that the systems for restricting the passage (R1-Rn-1) comprise screens and /or diaphragms.
6. Device as claimed in any preceding claim, characterised in that the systems for restricting the passage (R1-Rn-1) are configured so that their section of passage is decreasing according to the flow of the powders (P) through the plurality of mixing chambers (E1-En), the section of passage of an (n-1)th system for restricting the passage (Rn-1) being as such greater than the section of passage of an nth system of restricting the passage (Rn) by following the flow of the powders (P).
7. Device as claimed in any preceding claim, characterised in that the section of passage of the systems for restricting the passage (R1-Rn-1) is less than the natural section of the flow of the powders (P).
8. Device as claimed in any preceding claim, characterised in that the plurality of mixing chambers (E1-En) and the plurality of the systems for restricting the passage (R1-Rn-1) of the powders (P) are arranged along the same vertical direction in such a way as to allow for a flow of powders (P) under the effect of gravity.
9. Device as claimed in any preceding claim, characterised in that it comprises a system of electrostatic charge (C+, C-) of the powders (P) intended to be introduced into the mixing chamber or chambers (E1-En), a portion of the powders (P) being particularly is put into contact with a portion of the electrostatic charge system (C+) in order to be positively electrostatically charged and the other portion of the powders (P) being particularly is put into contact with the other portion of the electrostatic charge system (C-) in order to be negatively electrostatically charged, in order to allow for a differentiated local agglomeration.
10. Device as claimed in any preceding claim, characterised in that the cryogenic fluid (FC) is liquefied nitrogen.
11. Device as claimed in any preceding claim, characterised in that it comprises at least two chambers (A1, A2) for supplying powders (P), and in particular as many chambers (A1, A2) for supplying powders (P) as there are types of powders (P) to be fixed.
12. Device as claimed in any preceding claim, characterised in that the chamber or chambers for supplying (A1, A2) comprise hoppers with an adjustable supply and/or systems of the metering type, in particular vibrating plates or tunnels.
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 or chambers (E1-En) through the chamber or chambers for supplying (A1, A2),

    b) mixing of the powders (P) in the mixing chamber or chambers (E1-En), placed in suspension in a cryogenic fluid (FC), through means for agitation (2, 2a, 2b),

    c) obtaining of a mixture formed from powders (P).
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 comprises the step consisting in progressively restraining the passage of the flow of the powders (P) through the mixing chambers (E1-En) through systems for restricting the passage (R1-Rn-1) with a decreasing section of passage according to the flow of the powders (P).

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