Device for mixing powders by cryogenic fluid

Abstract

A device for mixing powders by cryogenic fluid, characterised in that it comprises at least: a chamber for mixing powders, comprising a cryogenic fluid; a chamber for supplying powders in order to allow the powders to be introduced into the mixing chamber; means for agitation in the mixing chamber so as to allow the mixing of the powders placed in suspension in the cryogenic fluid.

Claims

1. A device for mixing powders by a cryogenic fluid, comprising: a plurality of mixing chambers, each mixing chamber comprising an agitator and a cryogenic fluid, wherein the mixing chambers are arranged successively in series one after the other; a plurality of supplying chambers configured to introduce the powders into at least a first mixing chamber; a plurality of restricting systems, each restricting system being located between two successive mixing chambers, wherein each restricting system is configured to constrain the distribution of powders from one mixing chamber to the next, and wherein each restricting system is configured to adjust the flow of the powders through each successive mixing chamber; and an electrostatic charge system for electrostatically charging the powders intended to be introduced into the mixing chambers.

2. The device as claimed in claim 1, wherein the cryogenic fluid is liquefied nitrogen.

3. A device for mixing powders by a cryogenic fluid, comprising: a plurality of mixing chambers, each mixing chamber comprising an agitator, wherein each mixing chamber is configured to accept a cryogenic fluid, wherein the mixing chambers are arranged successively in series one after the other; a plurality of supplying chambers configured to introduce the powders into at least a first mixing chamber; a plurality of restricting systems, each restricting system located between two successive mixing chambers, wherein each restricting system is configured to constrain the distribution of powders from one mixing chamber to the next, and wherein each restricting system is configured to adjust the flow of the powders through each successive mixing chambers; and an electrostatic charge system for electrostatically charging the powders intended to be introduced into the mixing chambers.

4. The device according to claim 3, wherein the cryogenic fluid comprises a liquified gas.

5. The device as claimed in claim 3, wherein each agitator comprises mobile mixing devices.

6. The device according to claim 5, wherein the mobile mixing devices comprise mobile grinding facilities.

7. The device as claimed in claim 3, wherein each agitator comprises a device capable of generating vibrations.

8. The device as claimed in claim 3, wherein the restricting systems comprise screens.

9. The device as claimed in claim 3, wherein the restricting systems comprise diaphragms.

10. The device as claimed in claim 3, wherein the restricting systems progressively restrict the flow of the powders through the plurality of mixing chambers such that a section of passage of an (n−1)th restricting system is configured to pass powder particles that are larger or at a greater rate than the powder particles passed by an nth restricting system.

11. The device as claimed in claim 3, wherein a section of the restricting systems is less than a section length necessary for the powders to agglomerate.

12. The device as claimed in claim 3, wherein the plurality of mixing chambers and the plurality of the restricting systems are arranged along the same vertical direction in such a way as to allow for a flow of powders under the effect of gravity.

13. The device according to claim 3, wherein a portion of the powders is put into contact with a portion of the electrostatic charge system in order to be positively electrostatically charged and wherein the other portion of the powders is 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.

14. The device as claimed in claim 3, wherein the supplying chambers comprise hoppers with an adjustable supply and/or metering systems.

15. The device as claimed in claim 3, wherein each agitator further comprises a gyroscopic agitator.

16. The device as claimed in claim 3, wherein each mixing chamber further comprises a second means of agitation in the form of a device capable of producing ultrasonic vibrations further comprising sonotrodes.

17. The device as claimed in claim 3, wherein each agitator is configured to create a suspension comprising the powders and the cryogenic fluid.

18. A method for mixing powders by a cryogenic fluid, employing the device of claim 3, comprising the following steps: a) introducing powders intended to be mixed into the mixing chambers through one or more of the supplying chambers, b) mixing the powders in the mixing chambers to form a suspension of the powders in a cryogenic fluid and, c) obtaining a mixture formed from the powders.

19. The method according to claim 18, further comprising during the first step a), electrostatically charging the powders with positive or negative charges in order to favor differentiated local agglomeration.

20. The method according to claim 18, comprising the step of progressively restraining the passage of the flow of the powders through the mixing chambers through the restricting systems with a decreasing section of passage according to the flow of the powders.

21. The method according to claim 18, wherein the powders to be mixed are actinide powders.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention can be better understood when reading the following detailed description, of non-limiting embodiments of the latter, as well as examining the figures, diagrammatical and partial, of the annexed drawing, wherein:

(2) FIG. 1 shows a diagram illustrating the general principle of a device for mixing powders by a cryogenic fluid according to a first embodiment of the invention,

(3) FIG. 2 diagrammatically shows the agglomeration of particles of powders charged oppositely prior to the introduction thereof into mixing chambers of a device in accordance with the principle of FIG. 1,

(4) FIGS. 3 and 4 respectively show two examples of devices in accordance with the first embodiment of the invention,

(5) FIGS. 5A, 5B and 5C diagrammatically show alternative embodiments of mobile mixing facilities of devices of FIGS. 3 and 4,

(6) FIGS. 6 and 7 graphically show examples of changes in the mixing of powders of a device in accordance with the invention as a function of time,

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

(8) FIGS. 9, 10 and 11 respectively show photographs of a first type of powders before mixing, of a second type of powders before mixing, and of the mixture obtained from the first and second types of powders after mixing through a device and a method in accordance with the invention.

(9) In all of these figures, identical references can designate identical or similar elements.

(10) In addition, the various portions shown in the figures are not necessarily shown according to a uniform scale, in order to render the figures more legible.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(11) Note that in the embodiments described hereinafter, the powders P considered are actinide powders that allow for the manufacture of pellets of nuclear fuel. In addition, the cryogenic fluid considered here is liquefied nitrogen. However, the invention is not limited to these choices.

(12) In reference to FIG. 1, a diagram is shown illustrating the general principle of a device 1 for mixing powders P by a cryogenic fluid according to a first embodiment of the invention.

(13) According to this principle, the device 1 comprises a number n of mixing chambers E1, . . . , En of the powders P, arranged successively in series one after the other according to the same vertical direction in such a way that the powders can circulate through the mixing chambers E1, . . . , En under the effect of the force of gravity. Moreover, the device 1 comprises a number n−1 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 chambers E1, . . . , En, in order to constrain the distribution of powders P from one mixing chamber E1, . . . , En to the next. Examples of such systems for restricting the passage R1, . . . , Rn−1 are shown in what follows in reference in particular to FIGS. 3 and 4.

(14) In addition, the device 1 also comprises two chambers A1 and A2 for supplying powders P, provided in particular for distributing powders of different types.

(15) The two chambers A1 and A2 for supplying powders P allows for the introduction of the powders P into the first mixing chamber E1 in contact with the cryogenic fluid FC of the first chamber E1. Then the powders P successively pass through the systems for restricting the passage R1, . . . , Rn−1 and the mixing chambers E2, . . . , En, with each mixing chamber comprising a cryogenic fluid FC.

(16) In addition, each mixing chamber E1, . . . , En comprises means for agitation 2 allowing for the mixing of powders P placed in suspension in the cryogenic fluid FC. Examples of such means of agitation 2 are provided in what follows in reference in particular to FIGS. 3 and 4.

(17) The two chamber for supplying A1 and A2 comprise for example hoppers with adjustable supply, using for example an endless screw, and/or systems of the metering type, in particular vibrating plates or tunnels.

(18) Furthermore, advantageously, the device 1 further comprises an electrostatic charge system C+, C− of the powders P introduced into the mixing chambers E1, . . . , En.

(19) In particular, the portion of the powders P contained in the first chamber for supplying A1 is put into contact with the positive portion C+ of the electrostatic charge system in order to be positively electrostatically charged, while the portion of the powders P contained in the second chamber for supplying A2 is put into contact with the negative portion C− of the electrostatic charge system in order to be negatively electrostatically charged.

(20) In this way, it is possible to allow for a differentiated local agglomeration, in other words prevent self-agglomeration. As shown in FIG. 2, which diagrammatically shows the agglomeration of particles of powders P charged oppositely prior to the introduction thereof into the mixing chambers E1, . . . , En, with the particles of the two powders P to be mixed being of an opposite electrostatic charge, a possible reagglomeration will occur mostly through the interposing of powders with a nature, and therefore charge, that are different. This as such makes it possible to favour mixing on the scale of the particles that comprise the powders P to be mixed.

(21) The invention as such makes use of various technical effects that make it possible in particular to achieve the desired level of homogenisation, such as those described hereinafter: the improved deagglomeration, at least partial, of the powders P when the latter are placed in suspension 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 to water, with the latter being used advantageously without using any additive that is difficult to eliminate, the agitation close to the regime of a perfectly agitated reactor implemented by the movement of the means for agitation, able or not able to use the placing in vibration of the suspension as described in what follows, with these vibrations then being advantageously unsteady in order to limit the heterogeneous zones.

(22) In reference now to FIGS. 3 and 4, two examples of devices 1 in accordance with the first embodiment of the invention are diagrammatically shown, of which the principle has been described hereinabove in reference to FIG. 1.

(23) In each one of these two examples, the device 1 comprises, in addition to the elements described hereinabove in reference to FIG. 1, an agitation motor 5 able to drive in rotation first means of agitation 2a having the form of mobile mixing facilities 2a in the mixing chambers E1, . . . , En.

(24) These mobile mixing facilities 2a can comprise mobile grinding facilities. These mobile mixing facilities 2a can further comprise blades, mobile facilities with a duvet effect, turbines and/or blades, with these types of mobile facilities being respectively shown in the FIGS. 5A, 5B and 5C. In the embodiments of FIGS. 3 and 4, the mobile mixing facilities 2a comprise turbines.

(25) Moreover, in each one of these two examples, the device 1 further comprises second means of agitation 2b in the form of means for generating ultrasonic vibrations comprising sonotrodes 2b.

(26) In addition, the two embodiments shown in the FIGS. 3 and 4 are differentiated by the nature of the systems for restricting the passage R1, . . . , Rn−1 used.

(27) As such, in the embodiment of FIG. 3, the systems for restricting the passage R1, . . . , Rn−1 comprise diaphragms.

(28) In the embodiment of FIG. 4, the systems for restricting the passage R1, . . . , Rn−1 comprise screens, more precisely meshes of screens.

(29) In these two examples, the systems for restricting the passage R1, . . . , Rn−1 have a section of passage that can be adjusted and as such arranged in such a way that their sections of passage are ranked from the largest to the finest in the descending direction of the flow of powders P. Advantageously also, the sections of passage of these systems for restricting the passage R1, . . . , Rn−1 are less than the section of natural flow of the powders P in order to force the deagglomeration before the passage through these sections.

(30) An example of dimensioning of a device shall now be described 1 in accordance with the invention according to the first embodiment of the invention.

(31) For the dimensioning of the mixing chambers E1, . . . , En, it is necessary to evaluate in particular: the speeds of the mobile mixing facilities 2a in order to allow for the pulling off of the particles of powders P from the bottom of each mixing chamber E1, . . . , En, the mixing time of the powders, the flow rate of the powders P, namely the quantity of powders P that can be mixed per unit of time.

(32) For this, the equation given by the Zwietering correlation can be used, namely:

(33) $N\min =\phi .Math.{\left(\frac{\mathrm{DT}}{\mathrm{DA}}\right)}^{\alpha }.Math.g^{0.45}.Math.\frac{{\left({\rho }_P-{\rho }_L\right)}^{0.45}.Math.{\mu }_L^{0.1}.Math.d_P^{0.2}.Math.{\left(\mathrm{Ws}\right)}^{0.13}}{{\mathrm{DA}}^{0.85}-{\rho }_L^{0.55}},$
wherein in particular: Nmin represents the minimum frequency of agitation to have the pulling off of the particles of powders P, DT represents the diameter of the mobile mixing facility 2a, DA represents the diameter of the mixing chamber E1, . . . , En, ρ.sub.P represents the density of the powder P, ρ.sub.L represents the density of the cryogenic fluid FC, μ.sub.L represents the viscosity of the cryogenic fluid FC, d.sub.P represents the diameter of the particles of powder P, Ws represents the mass ratio between the solid phase and the liquefied phase, in percentages.

(34) Moreover, the following equations can also be used:
Q.sub.p=0.73.Math.ND.sup.3, Q.sub.c=2.Math.Q.sub.p, tm=3.Math.tc, tc=V/Qc and P=N.sub.p.Math.ρ.Math.N.sup.3.Math.d.sup.5,
wherein in particular: Q.sub.p represents the pumping flow rate, Q.sub.c represents the circulation flow rate, N represents the speed of agitation, d represents the diameter of the mobile mixing facility, P represents the agitation power.

(35) The table 1 hereinafter as such gives the dimensioning obtained of a device 1 according to the invention in order to obtain 1 kg/h of shred.

(36) TABLE-US-00001 TABLE 1 Characteristics of the device 1 Values Volume of a mixing chamber E1, . . . , En 100 mL Diameter of a mixing chamber E1, . . . , En 10 cm Content of powder P in the suspension 10% Rotation frequency of the mobile mixing facilities 8 s.sup.−1 Diameter of a mobile mixing facility 4 cm Pumping flow rate 3.7.10.sup.−4 m.sup.3/s Circulation flow rate 7.5.10.sup.−4 m.sup.3/s Mixing time (tm) for a chamber with a 10% load (A) ~0.40 s Mixing capacity ~0.9 kg/h Number of mixing chambers 4 Agitation power 105 W/m.sup.3

(37) The device 1 obtained then has a mixing response shown by the graph of FIG. 6, showing the change X of the mixture as a function of time t, which is the curve X(t)=A.Math.[1−exp(−k.Math.t)], k being a given coefficient, A a mixing load, and tm the mixing time.

(38) Advantageously, the putting into series of n mixing chambers E1, . . . , En having a unit volume Vn such that the global volume V of the mixing chambers E1, etc., is such that V=n.Math.Vn.

(39) In this case indeed, the global mixing time t′m is less than the mixing time tm for the volume V. The difference is as great between these mixing times as n is large, as shown by the graph of FIG. 7, showing the change X of the mixture as a function of time t, similarly to FIG. 6, with the times t1 and t2 of the first and second chambers and the times t′m and tm.

(40) Also shown, in reference to FIG. 8, a diagram showing a device 1 for mixing powders P by a cryogenic fluid according to a second embodiment of the invention.

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

(42) More precisely, these means of mixing MG are according to a movement of the gyroscopic type, or close to being so, allowing for the rotation of the mixing chamber E1 according to the three axes X1, X2 and X3 of three-dimensional metrology. This type of agitation by gyroscopic movement favours the mixture of powders P when they have high densities compared to the density of the phase of the cryogenic fluid FC located in the mixing chamber E1.

(43) In addition, the mixing chamber E1 comprises means for agitation 2a, for example in the form of turbines.

(44) The effectiveness of the mixture that can be achieved through this invention can be characterised by the homogeneity of the granular medium obtained after mixing. As such, FIGS. 9, 10 and 11 respectively show photographs of a first type of powders before mixing, of a second type of powders before mixing, and of the mixture obtained from the first and second types of powders after mixing through a device 1 and a method in accordance with the invention.

(45) More precisely, FIG. 9 shows aggregates of cerium dioxide powders CeO.sub.2, FIG. 10 shows aggregates of alumina powders Al.sub.2O.sub.3, and FIG. 11 shows 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 the mixing cryogenic fluid.

(46) It is then observed, despite a short mixing time (30 s) of the aforementioned powders and implemented in an equimassic manner (equal proportion in mass of the two powders), good homogeneity of the granular medium after mixing, as shown in FIG. 11, with a size of the aggregates close to that of the powders to be mixed, here with a dimension close to 5 μm.

(47) Of course, the invention is not limited to the embodiments that have just been described. Various modifications can be made thereto by those skilled in the art.