EP3370855A1

EP3370855A1 - Device for mixing powders by cryogenic fluid

Info

Publication number: EP3370855A1
Application number: EP16791565.1A
Authority: EP
Inventors: Méryl BROTHIER; Stéphane VAUDEZ
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2015-11-04
Filing date: 2016-11-03
Publication date: 2018-09-12
Anticipated expiration: 2036-11-03
Also published as: RU2018120089A3; FR3042985A1; US10981126B2; RU2018120089A; CN108348874A; WO2017076944A1; JP2018538526A; RU2718716C2; JP6804530B2; US20180318778A1; EP3370855B1; CN108348874B

Abstract

The invention relates mainly to a device (1) for mixing powders (P) by cryogenic fluid, characterised in that it comprises at least: a chamber (E1-En) for mixing powders (P), comprising a cryogenic fluid (FC); a chamber (A1, A2) for supplying powders (P) in order to allow the powders (P) to be introduced into the mixing chamber (E1-En); means (2) for agitation in the mixing chamber (E1-En) so as to allow the mixing of the powders (P) placed in suspension in the cryogenic fluid (FC).

Description

DEVICE FOR MIXING CRYOGENIC FLUID POWDERS

DESCRIPTION

TECHNICAL FIELD The present invention relates to the field of the preparation of granular media, and more specifically to the mixing of powders, in particular of 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 uniform 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 μιη, 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 patent application CA 2 882 302 A1, have been proposed but remain nonetheless ineffective for a mixture of actinide powders, the vibration means used not allowing sufficient homogenization in view of the homogenization objectives to be achieved and the peculiarities 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 patent applications CA 2,882,302 A1, WO 2006/0111266 A1 and WO 1999/010092 A1, are not suitable for the problematics of a mixture of powders of actinide powders type, since they would require stirring speeds too high to hope to take off the powders from the bottom of the stirring tank and achieve homogeneity levels consistent with those sought in the industry nuclear. 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 D. Geldart's classification as described in the publication Powder Technology, Vol. , 1973. 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.

SUMMARY OF THE INVENTION There is thus a need to provide a new type of powder mixing device for the preparation of granular media, and especially for the mixture of actinide powders.

In particular, there is a need to be able to concomitantly: deagglomerate the powders to be mixed without necessarily modifying their specific surface area 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 from a few cubic micrometers to about 10 μιη ³ ),

- Do not induce pollution of the powders to be mixed, nor modification of the surface chemistry, nor generate liquid effluents difficult to treat,

- not to induce a risk of specific criticality,

- not to induce any risk of specific radiolysis,

not to induce heating of the powders to be mixed,

- rely on a mixer with a limited diameter to control the risk of criticality even in case of error loading the mixer,

- Perform the mixing operation by limiting as much as possible the energy spent and 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 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.

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 mixing chamber for powders, comprising a cryogenic fluid,

- A powder supply chamber to allow the introduction of powders into the mixing chamber, stirring means in the mixing chamber to allow mixing of the powders placed in suspension 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.

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 allowing the introduction of 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 restriction systems may be adjustable and configured so that their passage section decreases as a function of the flow of powder flow through the plurality of mixing chambers, the passage section of a 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.

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:

a) introduction of powders for mixing into the mixing chamber (s) through the feed chamber (s), b) mixing the powders in the mixing chamber or chambers, suspended in a cryogenic fluid, by means of stirring means,

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

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 :

FIG. 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,

FIG. 2 schematically represents the agglomeration of particles of powders loaded in opposite manner prior to their introduction into mixing chambers of a device according to the principle of FIG. 1,

FIGS. 3 and 4 respectively represent two examples of devices according to the first embodiment of the invention,

FIGS. 5A, 5B and 5C schematically represent variant embodiments of the mixing mobiles of the devices of FIGS. 3 and 4,

FIGS. 6 and 7 show graphically examples of evolution of powder mixtures of a device according to the invention as a function of time,

FIG. 8 represents a diagram illustrating a device for mixing powders by cryogenic fluid according to a second embodiment of the invention, and

FIGS. 9, 10 and 11 respectively represent photographs of a first type of powders before mixing, of a second type of powders before mixing, and of the mixture obtained of the first and second types of powders after mixing by means of a device and a method according 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.

Referring to 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 flow through the mixing chambers. El, 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 located between two mixing chambers E1, in succession, to constrain the distribution of powders P of a mixing chamber El, En to the following. Examples of such passage restriction systems RI, Rn-1 are presented hereinafter with reference in particular to FIGS. 3 and 4.

In addition, the device 1 also comprises two supply 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 the 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 restriction systems of FIG. passage RI, Rn-1 and mixing chambers E2, En, each mixing chamber comprising a cryogenic fluid FC.

In addition, each mixing chamber El, En 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 reference in particular to FIGS. 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 charge system C +, C- powders P introduced into the mixing chambers El En.

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 charge system to be electrostatically charged in a positive manner, whereas 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 in FIG. 2, which shows schematically the agglomeration of the particles of powders P loaded in opposite manner prior to their introduction into the mixing chambers E1, E1, the particles of the two powders P to be mixed being of opposite electrostatic charge, a possible reagglomeration will occur 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.

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:

at least partial disaggregation of the powders P when they 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 a low surface tension, compared with the water, the latter being advantageously employed without the use of a difficult additive to eliminate,

- 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 areas of heterogeneity. Referring now to FIGS. 3 and 4, two examples of devices 1 according to the first embodiment of the invention, the principle of which has been described previously with reference to FIG. 1, are diagrammatically represented.

In each of these two examples, the device 1 comprises, in addition to the elements previously described with reference to FIG. 1, an agitation motor 5 capable of driving in rotation first stirring means 2a in the form of mobile mixing 2a in the mixing chambers El, En.

These mixing mobiles 2a may comprise grinding mobiles. These mixing mobiles 2a may also comprise blades, duvet mobiles, turbines and / or blades, these types of mobiles being respectively represented in FIGS. 5A, 5B and 5C. In the exemplary embodiments of FIGS. 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.

In addition, the two exemplary embodiments shown in FIGS. 3 and 4 are distinguished by the nature of the passage restriction systems R 1, R 1-1 used.

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

In the embodiment of Figure 4, the passage restriction systems RI, Rn-1 include sieves, more precisely mesh sieves.

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 downward direction of the flow of flow. 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 disagglomeration before passing 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. For the dimensioning of the mixing chambers El, En, it is necessary to evaluate in particular:

the speeds of the mixing mobiles 2a to enable the particles of powder P to take off from the bottom of each mixing chamber El, En,

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.

For this, the equation given by the Zwietering correlation can be exploited, namely:

in which, in particular:

- Nmin represents the minimum stirring frequency to have the takeoff of powder particles P,

DT represents the diameter of the mixing wheel 2a,

DA represents the diameter of the mixing chamber El, En,

pp represents the density of the powder P,

PL represents the density of the cryogenic fluid FC,

μί 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.

In addition, the following equations can also be exploited:

Q _p = 0.73. ND ³ , Q _c = 2.Q _P , tm = 3.tc, te = V / Qc and P = N _p .pN 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.

Table 1

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

Advantageously, the series of n mixing enclosures El, having a unit volume Vn such that the overall volume V of the mixing chambers El, En is 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 FIG. 7, representing the evolution X of the mixture as a function of time t, in a manner similar to FIG. 6, with the times t1 and t2 of the first and second enclosures and the times t'm and tm.

FIG. 8 also shows a diagram illustrating a device 1 for mixing powders P with a 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 El along the three axes XI, X2 and X3 of the three-dimensional metrology. This type of gyroscopic movement stirring promotes 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 El.

In addition, the mixing chamber El 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, FIGS. 9, 10 and 11 respectively represent photographs of a first type of powders before mixing, of a second type of powders before mixing, and of the mixture obtained of the first and second types of powders after mixing through a device 1 and a method according to the invention.

More precisely, FIG. 9 represents aggregates of cerium dioxide powders C0 ₂ , FIG. 10 represents aggregates of alumina powders AI ₂ O ₃ , and FIG. 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 is observed after mixing, as illustrated in FIG. 11, with an aggregate size close to that of the powders to be blended, ie here with a dimension close to 5 μιη.

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

1. Device (1) for mixing powders (P) with a cryogenic fluid, characterized in that it comprises at least:

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

a feed enclosure (Al, A2) in powders (P) to allow the introduction of the powders (P) into at least the first mixing chamber (El),

a plurality of passage restriction systems (R1-Rn-1) for the powders (P), each passage restriction system (R1-Rn-1) being located between two successive mixing chambers (El-En), for forcing the distribution of powders (P) from one mixing chamber (El-En) to the next, each restriction of passage system (R1-Rn-1) being adjustable,

- stirring means (2, 2a, 2b) in each of the mixing chambers (El-En) to allow mixing of the powders (P) suspended in the cryogenic fluid (FC).

2. Device according to claim 1, characterized in that the powders (P) to be mixed are actinide powders.

3. Device according to claim 1 or 2, characterized in that the cryogenic fluid (FC) comprises a weakly hydrogenated liquid, a liquid having at most one hydrogen atom per molecule of liquid, having a boiling temperature below that of water.

4. Device according to any one of the preceding claims, characterized in that the stirring means comprise mixing mobiles (2a), in particular blades, turbines and / or mobile effect duvet.

5. Device according to claim 4, characterized in that the mixing mobiles (2a) comprise grinding wheels.

6. Device according to any one of the preceding claims, characterized in that the stirring means comprise means for generating vibrations (2b), in particular ultrasonic vibrations, in particular sonotrodes (2b).

7. Device according to any one of the preceding claims, characterized in that the passage restriction systems (R1-Rn-1) comprise sieves.

8. Device according to any one of the preceding claims, characterized in that the passage restriction systems (R1-Rn-1) comprise diaphragms.

9. Device according to any one of the preceding claims, characterized in that the passage restriction systems (R1-Rn-1) are configured so that their passage section is decreasing according to the flow of the powder (P-1). ) through the plurality of mixing chamber (El-En), the passage section of a (nl) th passage restriction system (Rn-1) being thus greater than the passage section of an nth passage restriction system (Rn) following the flow flow of the powders (P).

10. Device according to any one of the preceding claims, characterized in that the passage section of the passage restriction systems (R1-Rn-1) is smaller than the natural flow section of the powders (P).

11. Device according to any one of the preceding claims, characterized in that the plurality of mixing chambers (El-En) and the plurality of passage restriction systems (Rl-Rn-1) of the powders (P) are disposed in the same vertical direction so as to allow a flow of powders (P) under the effect of gravity.

12. Device according to any one of the preceding claims, characterized in that it comprises an electrostatic charging system (C +, C-) powders (P) intended to be introduced into the mixing chamber (s) (El-En). ).

13. Device according to claim 12, characterized in that a part of the powders (P) is brought into contact with a part of the electrostatic charge system (C +) to be electrostatically charged in a positive manner and in that the other part powder (P) is brought into contact with the other part of the electrostatic charge system (C-) to be electrostatically charged in a negative manner to allow differentiated local agglomeration.

14. Device according to any one of the preceding claims, characterized in that the cryogenic fluid (FC) is liquefied nitrogen.

15. Device according to any one of the preceding claims, characterized in that it comprises at least two feed enclosures (Al, A2) powder (P), and in particular as many feed enclosures (Al, A2). ) in powders (P) than types of powders (P) to be mixed.

16. Device according to any one of the preceding claims, characterized in that the supply enclosure (s) (A1, A2) comprise hoppers with adjustable feed and / or metering type systems, including trays or vibrating corridors.

17. Process for mixing powders (P) with a cryogenic fluid, characterized in that it is implemented by means of a device (1) according to any one of the preceding claims, and in that it comprises the following steps :

a) introduction of powders (P) to be mixed in the mixing chamber (s) (El-En) through the supply chamber (s) (Al, A2), b) mixing of the powders (P) in the mixing chamber (s) (El-En), suspended in a cryogenic fluid (FC), by means of the stirring means (2, 2a, 2b),

c) obtaining a mixture formed from the powders (P).

18. The method of claim 17, characterized in that during the first step a), the powders are electrostatically charged in a different manner, in particular in an opposite manner, to promote differentiated local agglomeration.

19. The method of claim 17 or 18, characterized in that it comprises the step of progressively restricting the passage of the flow of the powders (P) through the mixing chambers (El-En) through the passage restriction systems (Rl-Rn-1) of decreasing passage cross-section according to the flow flow of the powders (P).