FR3072378B1 - DEVICE AND METHOD FOR MANUFACTURING CRYOGENIC CERAMIC PIECES - Google Patents

Abstract

The main object of the invention is a device for manufacturing ceramic parts (PC) obtained from powders (P), characterized in that it comprises: a device for feeding and introducing powders (P ); a device for mixing and cryogenic grinding (1) of the powders (P); optionally, a draining and pouring device (2) for introducing and filling the suspension into one or more shaping molds; a device for shaping by uniaxial pressing (3) of the mixture after volatilization of the liquefied gas; optionally, a device for shaping (4) the suspension comprising porous molds allowing the evacuation of the liquefied gas (GL) through the pores and a device for putting under a vacuum (5) and / or heating the faces external molds to promote the withdrawal of the liquid constituent of the suspension; a sintering device (6).

Description

DEVICE AND METHOD FOR MANUFACTURING CERAMIC PIECES BY WAY

CRYOGENIC

DESCRIPTION

TECHNICAL AREA

The present invention relates to the field of the manufacture of dense ceramic parts, obtained from powders of the element or elements of interest to be implemented in the form of parts.

More particularly, the invention relates to the use of these powders for producing ceramic parts with specified properties, based in particular on the following three stages: preparation of a suspension from mixing and grinding liquefied gas and powders; shaping by pressing of the powder resulting from the suspension or molding of parts resulting from the suspension; densification by sintering of the parts resulting from the shaping by adapted heat treatment.

In a preferred manner, it applies to powders of high density and / or cohesive, such as, for example, actinide powders comprising in particular uranium oxide powders (UO 2) and / or powders of oxides of plutonium (PuO 2). Also, the invention is preferably used for the manufacture of nuclear ceramic parts, namely the manufacture of nuclear fuel, including nuclear fuel pellets. The invention thus proposes a device for manufacturing ceramic parts by cryogenic means, as well as an associated manufacturing method.

STATE OF THE PRIOR ART

Due to their different production methods, actinide powders, such as uranium oxide powders (UO 2) and plutonium oxide powders (PuO 2), have different characteristics and behavior in the mixture and grinding; and in a more general way from a rheological point of view.

Thus, the fuel manufactured from a uranium oxide powder associated with plutonium oxide in the so-called MIMAS process, as described in the publication "MOx Fuel Fabrication and in-reactor performance", D. Haas , M.Lippens, Proc, of the Internat. Conference on Future Nuclear Systems, Global 97, may lead punctually to pellets with an inability to nitrate dissolution necessary for the reprocessing of the fuel. This behavior is explained by a poor homogeneity of plutonium distribution in the milled ball jar mixture characterized by the presence of islands rich in plutonium size too large to be resorbed by diffusion during sintering. These plutonium zones with a concentration potentially greater than 45% are definitely insoluble. This difficulty for grinding in the dry process would be limited, or even eliminated, with the implementation of grinding in a liquid process.

Indeed, wet grinding is a grinding method commonly used in the industrial world (excluding nuclear), because of its high efficiency and lower energy consumption. In the literature, in general, the main conclusion that emerges is the following: grinding in a liquid medium is more effective than dry grinding, however the interpretation of the generated phenomena remains difficult. For Padden and Reed, in American Ceramic Society Bulletin, SA. Padden, JS. Reed, 1993, wet milling is more energy efficient because impacts are not amortized. For El-Shall, in "Physico-Chemical Aspects of Grinding: A Review of Use of Additives," Powder Technology, Volume 38, July 1984, pp. 275-293, the effectiveness of the liquid pathway stems from the mode of transport of the material. through the mill, depending on the physical properties of the pulp or powder, its state of dispersion and its density.

The manufacture of ceramic parts is conventionally carried out by grinding in the liquid phase (aqueous), then shaping of the slip, that is to say an aqueous suspension of different mineral materials, for industrial applications such as those linked for example in the development of tableware or sanitary products. This ancestral process is industrially robust but induces several disadvantages such as slow drying of the parts and consequently delayed release or the short life of the molds. Numerous variations of this type of process are to be noted, as for example described in the applications GB 1170 277 A, CN 105601279 A, EP 0 191 409 A1, WO 2005/012205 A2 or WO 96/06811 A1. adaptations of this process have been proposed, in particular to limit the demolding times. High pressure molding can therefore be noted for this purpose, as for example described in EP 1575 745 A1, US 5,427,722 A or EP 0 463 179 A1.

In the context of the manufacture of ceramics, such as nuclear fuels, ball mills are conventionally used to co-dry pulverize oxides of uranium and plutonium oxides. Documents such as the "Process engineering of size reduction: lease milling" monograph, Austin LG et al., AIME, 1984 or GB 189626501A, US 6,473,336A, US 2,235,985A and US 2,041,287A describe several versions of these mills. balls that can be used for this type of application. The grinding in the liquid phase is not carried out conventionally because of the introduction of risks and constraints often unacceptable in the nuclear industry, such as risk of criticalities, radiolysis, generation of contaminated aqueous effluents.

Pressure casting to obtain green parts (before thermal densification by sintering) is also described in some publications as "Organizations for the Production of Rapid Cycle Sanitary Equipment", M. Vouillemet et al., Interceram, Vol. 39, No. 1, 1990, pages 17-23. It should be noted that commercially available die casting equipment and the technology for developing porous molds for these uses are available from many manufacturers such as NETZCH Inc.

Whether these processes for producing ceramics by casting are carried out under pressure or not, they are based on the aqueous solution-based slip formulation and additives of stabilizing, dispersing, rheological or flocculant type as required. In all cases these constituents, water and / or additives, are detrimental for specific uses such as those related to the development of nuclear fuels since these compounds cause the generation of contaminated waste / effluents difficult to treat and / or risks of explosion by radiolysis, or even risks of types of criticality accidents.

Previous solutions of the prior art are therefore not easily applicable for the development of specific ceramics such as for example those that constitute nuclear fuels. It should be noted that certain solutions providing a slurry composed of an organic liquid do not make it possible to respond to the problem of treating liquid effluents and / or controlling the risk of radiolysis.

In addition, certain methods or devices of the prior art using not aqueous solutions or even organic liquids but more innovatively liquefied gases. These latter solutions are no more effective for the purposes to be fulfilled by the present invention. Indeed, for example, the processes of the type of those described in GB 1 508 941 A, EP 1 405 927 A1 or EP 2 535 114 A1 implement quantities of liquefied gas too large with an uncontrolled risk of rise in pressure machines. Moreover, they do not include elements to control them in order to optimize their use. Finally, the pollution induced by abrasion between the powder and the constituent materials of the internal can not be neglected, which can be unacceptable for certain applications such as those sought by the present invention.

STATEMENT OF THE INVENTION

There is thus a need to propose a new type of device and method for manufacturing ceramic parts with specified properties from powders of the element or elements of interest to be used in the form of parts, in particular actinide powders.

In particular, there is a need to be able to concomitantly: - develop a slurry / suspension of powders without liquid generating liquid effluents contaminated by the powder or powders to be used, - develop this slip whatever the particle size of the powder or powders constituting this slurry, - develop this slurry without inducing pollution of the final product from this slip due to the use of liquefied gas inert to the powders to be implemented, - potentially develop a slurry without stabilizing additives that may pollute the green draft or undergo radiolysis that can generate flammable / explosive gases, - perform the evacuation of the liquid very quickly (ie to allow industrial rates) without damaging the blank and without possibly damaging the mold, - make green parts before sintering in a continuous way, which is particularly not obvious for the nuclear fuel application, the operation of grinding powders being currently performed in batch industrially, and - overall result in dense parts meeting the specifications after sintering. The invention aims to at least partially meet the needs mentioned above and to overcome the disadvantages relating to the achievements of the prior art. The object of the invention is, according to one of its aspects, a device for producing ceramic parts obtained from powders, in particular nuclear fuel pellets obtained from actinide powders, characterized in that comprises: - a device for supplying and introducing powders for mixing and grinding, - a device for mixing and cryogenic grinding of powders comprising an insulated tank, in which the powders and liquefied gas are introduced and put directly into operation. contact with a volume ratio between the volume of liquefied gas and the total volume of the powders, ie the ratio of the volume of liquefied gas to the total volume of the suspension to be mixed and / or milled, between 40 and 70%, and possibly a relief valve for adjusting the grinding pressure so as to minimize losses of liquefied gas, - optionally, a draining and pouring device for the introduction and filling of the suspension resulting from the grinding in one or more shaping molds, - a shaping device by uniaxial pressing of the mixture after volatilization of the liquefied gas, comprising a pressing die and a compacting press, - optionally, in case casting apparatus, a suspension shaping device comprising porous molds for discharging the liquefied gas through the pores and a vacuum device, with respect to the pressure within the molds, and / or heating of the outer faces of the molds to promote the withdrawal of the liquid constituting the suspension, this drain possibly being assisted by vibration to promote if necessary the densification of the green blank of the workpiece, - a sintering device of the molded parts raw, including a sintering furnace that can operate at about 1700 ° C in a controlled atmosphere.

Thanks to the invention, it is possible to obtain ceramic parts with high added value, in particular free of pollutants, hydrophobic and sensitive to warming.

The present invention is based on several steps: suspending in a cryogenic fluid powders to form the green blank of the piece to be sintered; grinding of the suspension; shaping, for example by pressing or pouring the suspension; in case of casting, evacuation of the cryogenic fluid by aspiration / evaporation directed through the porous mold and / or heating of the part; densification by sintering.

Advantageously, the invention thus exploits several technical effects to meet the aforementioned needs. More specifically, it is in particular to ensure the production of homogeneous parts in terms of distribution of the constituent elements of the parts to be manufactured. For MOx fuels homogenization of the UO 2 / PuO 2 distribution at the micron scale is desirable. In addition, beyond these "product" specifications, it is necessary to take into account process constraints, including criticality risk management, contaminated effluent management, radiolysis management, cadence, etc. The present invention makes it possible to meet these expectations and requirements.

Among the technical effects advantageously exploited in the present invention, mention may be made in particular of cryogenic liquid crushing which makes it possible to optimize the homogenization of the constituents of interest while minimizing the time necessary to achieve this, and minimizing the pollution of the constituents of interest. powders compared to grinding in the dry process, knowing that the fine particles in suspension remain sequestered within the suspension which avoids the dispersion of the material and which thus makes it possible to limit the risks of contamination and radiological impact for the process operators; the rapid evaporation of the cryogenic fluid constituting the suspension in view of the boiling point of the liquefied gases in question; the possibility of fluidizing and thus of homogenizing the mixtures of actinide powders which are known to be difficult to pour in a so-called dry process.

The present invention thus proposes to take into account the constraints mentioned above, the liquefied gas medium used in the present invention not being subject to radiolysis and also allowing to manage by definition any liquid effluent problem after evaporation (natural or forced) without risk of recondensation at room temperature. Finally, the use of non-hydrogenated liquefied gas does not generate a significantly increased risk of criticality compared with prior art dry processes.

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, to grind and to deagglomerate.

The device for manufacturing ceramic parts according to the invention may further comprise one or more of the following characteristics taken separately or in any possible technical combination.

The cryogenic grinding device may comprise conventional grinding media, such as UTi or ZrO2 alloy rollers for the application using actinide powders.

The cryogenic grinding device may be in the form of a ball mill or planetary type, including grinding media consist of solidified gas, loaded or not powders to grind to minimize pollution and / or increase their density .

Thus, the grinding media may be in the form of a solidified cryogenic gas and the heat-insulated tank may contain a liquefied gas whose quantity is sufficient to fill the open porosity of the bed of powder to be ground, generally of the order of 50%, for allow fluidization / mixing of the bed of powder. The icic objective is to allow the fluidification of the granular medium even when it is composed of very powders, see no, pourable while minimizing the amount of liquefied gas, to minimize the risk of overpressure.

The cryogenic grinding device may also be in the form of a confluent jet mill, in particular composed of n jets confluent, n being an integer strictly greater than 1, whose angles between two adjacent jets are advantageously equal, especially equal at 2π / η. This device can therefore in particular be equipped with devices for dispensing cryogenic fluid jets, containing in suspension the powder to be milled, in liquid form, the jets being confluent and advantageously iso-distributed (in terms of confluence angle) in the plane. constituting these jets.

The draining and pouring device can be configured to allow adjustment of the liquefied gas / solid powders fraction as needed.

In addition, the draining device and casting may comprise means for stabilizing the suspension by vibrational and / or acoustic solicitations, in particular by means of a piezoelectric and / or a sonotrode.

The device for feeding and introducing grinding powders may comprise an electrostatic charging system.

In addition, the device for supplying and introducing the powders to be milled may comprise an electrostatically chargeable binder introduction system with a charge opposite to the introduced powders, the binders being in particular organic compounds of the elastomer, resins or adhesives.

In addition, the invention further relates, according to another of its aspects, to a process for producing ceramic parts obtained from powders, characterized in that it is implemented by means of a device such as previously defined.

The device and the manufacturing method 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 detailed description which follows, non-limiting examples of implementation thereof, as well as the examination of the figures, schematic and partial, of the accompanying drawing, in which: - Figure 1 shows a block diagram illustrating a principle of the device and the method of manufacturing ceramic parts according to the invention - Figures 2 to 4 schematically illustrate different types of grinding devices for FIG. 5 illustrates an example of a draining and pouring device for a device and a method in accordance with the invention, FIGS. 6A and 6B respectively illustrate the barrier filtration. in the use of a porous mold and the filtration in depth, - Figure 7 illustrates a schematic diagram of mounting device for grinding / suspending and filling a mold p For a device according to the invention, FIG. 8 illustrates the arrangement between binder grains and powder grains in the suspension and in the green part after separation and heating, FIG. 9 illustrates the evolution of solubility. naphthalene in carbon dioxide as a function of its density for different temperatures, - Figure 10 illustrates the main behavior under shear of the suspension of powders that can be encountered, the curves respectively corresponding to a threshold fluid, a shear thinning fluid and a Newtonian fluid; FIG. 11 illustrates the viscosity of a suspension of powders in liquid nitrogen as a function of the volume fraction of powder for two values of the maximum stacking volume fraction; FIG. the reduced sedimentation rate, that is to say rendered adimensional, as a function of time, - FIG. 13 illustrates sediment velocities imentation, particle alone or in suspension as a function of particle radius, for two types of liquefied gas, FIGS. 14A and 14B illustrate two images taken under a scanning electron microscope (SEM) according to two scales of visualization of the powder, FIGS. 15A and 15B illustrate two images taken at the SEM in two viewing scales of the powder, FIGS. 16A and 16B illustrate the microstructure of an UPuO 2 fuel produced by a manufacturing method according to the invention; FIG. sintering cycle applied to the UPuO 2 pellet made by a process according to the invention; - figure 18 illustrates particle domains to be crushed by the evolution of the energy as a function of the size, - figure 19 represents the force centrifuge and the force of gravity applied to a ball, - Figure 20 shows the evolution of the density of the dry ice balls as a function of the volume fraction of solid (in the case of plutonium oxide) in the balls, - FIG. 21 represents the evolution of the particle size distribution of powders as a function of the milling time in a ball mill, ie precisely the evolution of the volume percentage with respect to the size, FIG. 22 allows the comparison of the grinding paths (single-mix reference path, dry-phase ball mill path and N2L-path mill according to the invention, and FIG. 23 illustrates the impact of the grinding of the the present invention, compared to other reference routes, on the specific surface of the charge to be milled and its particle size.

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 embodiments described hereinafter, the powders under consideration are actinide powders for the manufacture of nuclear fuel pellets. In addition, the cryogenic fluid considered here is liquefied nitrogen. However, the invention is not limited to these choices.

Referring to Figure 1, there is shown a block diagram illustrating a principle of the device and the method of manufacturing ceramic parts according to the invention.

Firstly, the device comprises a cryogenic mixing and grinding device 1 in which the powders P, the liquefied gas GL and optionally AD additives are introduced.

In addition, the device comprises a draining and pouring device 2 for the introduction and filling of the mold by the powder suspension resulting from the grinding obtained by the grinding device 1 in one or more shaping molds.

In addition, a device for shaping by uniaxial pressing 3 of the mixture after volatilization of the liquefied gas, comprising a pressing die and a compaction press, is also provided. The device 1 further comprises a shaping device 4 of the suspension comprising porous molds permitting the evacuation of the liquefied gas GL through the pores and a vacuum device 5, with respect to the pressure within the molds, and / or heating of the outer faces of the molds to promote the removal of the constituent liquid from the suspension.

Finally, the device 1 comprises a sintering device 6 for green molded parts, in particular a sintering furnace that can operate at about 1700 ° C. under a controlled atmosphere, making it possible to obtain PC ceramic parts.

These constituent elements of the device according to the invention and the steps of the manufacturing method according to the invention will be described in more detail below. Cryogenic grinding device

In order to be able to obtain a stable suspension and / or a degree of homogeneity of powder mixing P sufficient, the grinding device 1 makes it possible to grind the powders which would not be fine enough to limit agglomeration, and heterogeneities.

The suspension and grinding are performed by cryogenic grinding, that is to say operated in liquefied gas phase. To do this, several types of mill can be considered to limit the risk of pollution of the load. By way of non-exclusive illustration, the cryogenic grinding device 1 may be in the form of a ball mill, as illustrated in FIG. 2 with a vertical rotation plane, or in the form of a grinder of the type planetary, as shown in Figure 3 with a horizontal plane of rotation, they are nevertheless designed so that they can contain a liquefied gas.

To do this, they are advantageously insulated and potentially equipped with a relief valve 10 to adjust / limit the operating pressure and reduce the effects of evaporation. In addition, optionally, in order to limit the pollution of powders P to be milled, the grinding media (balls, balls, etc.) consist of solidified gas GS, for example carbon dioxide (CO 2), whose density is greater than the density of the liquefied gas GL used in the mill.

In FIGS. 2 and 3, R represents the direction of rotation, B balls composed of dry ice, which can contain part of the powder P to grind / agglomerate, GS represents the solidified gas, GL represents the liquefied gas, GL + P represents the cryogenic suspension or slip containing the liquefied gas bath GL and the powder bed P, and gP represents the powder grains.

Another alternative is to be able to implement a confluent jet mill to also limit any pollution, as shown in FIG.

Thus, in FIG. 4, the reference 11 denotes a guillotine drain valve, the reference 12 designates a sonotrode, the reference JS denotes the liquefied gas / powder (s) suspension jets, the reference ZI designates the zone of impact, the references Jn, Jn + 1 and Jn + 2 respectively denote the jets n, n + 1 and n + 2 of the loaded suspension, and the reference Z denotes an enlarged portion on the left of FIG. 4 of the zone on the right. The angle a is equal to 2π / 3 because there are 3 jets here.

More precisely, the grinding without pollution of the powders P using a jet milling configuration can in particular be based on the confluence of the jets set so that: the angle between two nozzles of the adjacent jets is equal to 2π / η, the movement amounts (qm) of each of the jets are equal.

This is made possible by the imposition of a pressure equivalent to the right of each nozzle, the latter having the same internal distribution diameter to ensure the same flow rate and the same injection speed.

Draining and casting device

An example of such a draining and pouring device 2 is illustrated in FIG. 5.

This device 2 comprises a gravity draining and / or pressure assisted capacity. This capacity is itself optionally equipped with a vibration / acoustic loading device such as a piezoelectric or a sonotrode in order to keep the suspension stable, that is to say that it does not decay in time. use of the device. If necessary, it is therefore possible to apply mechanical or acoustic vibrations by the use of piezoelectric or sonotrode. It should further be noted that in FIG. 5, BG denotes a cylinder of gas while reference numeral 20 designates the insulated capacity of liquefied gas.

In addition, depending on the ratio powders P / liquefied gas GL, it is possible to achieve in this device a setting of this ratio by adding or removing a proportion of liquefied gas.

Shaping device

Optionally, in the case of forcing casting, the previously formulated suspension is then introduced into a porous mold MP prefiguring the final form of the PC part to be produced by the method according to the invention. The MP molds may be made of different porous materials such as those of the poral metal barrier type or porous resins, as shown in FIGS. 6A and 6B, showing the barrier filtration sought in the use of the porous mold MP, FIG. 6A, in comparison with the depth filtration, FIG. 6B.

Figure 7 illustrates the schematic diagram of the assembly grinder 1 / suspension and filling of the mold MP.

In order to be able to increase the rates, the liquefied gas GL may be evacuated by evacuation / depression of the outer face of the mold MP and / or by adjusting the temperature to promote the vaporization of the liquefied gas GL. It should be noted that an excessive rise in the temperature of the mold MP is still not sought otherwise the evaporation of the liquefied gas GL, too fast, would cause bubbling and dedensification of the green part.

Similarly, the suspension can be put under strong pressure to also increase the rates and / or control of the geometry of the parts by improving the mechanical strength of the green molded parts, that is to say having not yet undergoes sintering. Note that in Figure 7, the references 15, 16 and 17 respectively represent a pump, a heater and the evacuation of gases.

Moreover, in order to increase the resistance of the green part, it is also possible to add a binder AD within the liquefied gas suspension GL / powders P. The binder can be chosen so that it is liquid to room temperature and solid at the temperature of the suspension. This auxiliary binder can advantageously be loaded in powder form. This binder may also have a particle size smaller than those of the powders to be used to promote contact with the powder grains and thus improve the resistance of the green parts once the liquefied gas has been separated and the part has been brought to ambient temperature. Thus, Figure 8 illustrates the arrangement between binder grains gAD and pellet grains gP in the slurry and in the green part after separation and warming. The arrows Z represent enlarged portions from the left to the right of the figure.

It should also be noted that the binder may optionally be selected so that it is soluble in the liquefied gas phase. This can be envisaged for organic compounds such as naphthalene in carbon dioxide (CO 2), as illustrated in FIG. 9, which represents the evolution of the solubility of naphthalene in CO 2, that is the molar fraction fm of naphthalene (× 10), according to its density μ in g / L, for different temperatures. In a more conventional manner, the shaping can be based on a shaping device by uniaxial pressing of the mixture after volatilization of the liquefied gas, comprising a pressing die and a compacting press.

Sintering device

The sintering device 6 can for example be composed of a batch oven or continuous depending on the case. For the sintering of actinide powders, the sintering temperatures will be around 1700 ° C. and the sintering atmospheres will be at controlled oxygen partial pressure.

Examples of realization

In order to size the cryogenic mixing and grinding device 1 and / or the emptying device 2 or suspension system, it is first necessary to estimate the viscosity of the suspension.

The presence of powder in the suspension induces a perturbation of the velocity fields with respect to an uncharged fluid of particles.

As a first approach, it is possible to indicate that the viscosity can be considered as proportional to the concentration of solid particles.

A certain number of models can be used to express the viscosity η as a function of φ (volume fraction of solid considering the suspension to be ground). Among these, it is possible to retain in first approach the model of Quemada: η = ηί x (1 - (φ / φιτι)) "2 with: φιτι: volume fraction of maximum stacking, ηί: viscosity of the interstitial fluid .

Depending on the powder concentration, the suspension can then have several types of behavior. FIG. 10 illustrates the principal behaviors under shear of the suspension of powders that can be encountered, the curves A1, A2 and A3 respectively corresponding to a threshold fluid, a shear thinning fluid and a Newtonian fluid.

More precisely, the majority of fluids, like dispersions, behave like so-called non-Newtonian fluids depending on the shear rate. The two most common behaviors are: - Rheofluidifier: the viscosity decreases when the shear increases which is often the case for suspensions. Particles are organized under the effect of flow and shear by hydrodynamic forces that can cause breakage of aggregates; - rheo-thickener: the viscosity increases when the shear increases. This character is less marked for suspensions and appears mainly only for very concentrated dispersions. In this case, the increase in shear results in a change of order in the dispersion and a reorganization causing the increase in viscosity.

In the present invention, it is desired a suspension behavior as much as possible stable and / or allowing its implementation from a rheological point of view. It is therefore necessary to find a compromise between viscosity and charge rate. FIG. 11 illustrates the viscosity vi, in Pa.s, of a suspension of powders in liquid nitrogen as a function of the volume fraction fv of powder for two values of the maximum stacking volume fraction, φιτι = 0.74 (maximum rate for monomodal spherical particles) and φιτι = 0.64 for a random approach.

The behavior of the suspensions resulting from cryogenic grinding in the liquid phase is also a function of sedimentation phenomena, phenomena making it possible to specify the stability of these suspensions. The sedimentation rate of the particles (vP) can be described by the following expression: vP = vps. (1-c) 4'8, with vps: sedimentation rate of an individual particle, and c: volume concentration of particles.

Knowing that vPS can be described by the following equation for sedimentation conditions driven by the conditions of Stockes: vPS = 2.r2. g. (pp-pf) / (9.p), with: r: particle radius; pp: density of the particles;

Pf: density of the liquefied gas.

Figure 12 illustrates the establishment of the sedimentation velocity vp reduced, that is to say rendered adimensional, in m / s, as a function of time t, in s.

In addition, FIG. 13 illustrates the settling velocities vp, particle alone or in suspension (concentration of 40% by volume of monodisperse particles), in m / s, as a function of the radius r of the particles, in m, and for two types of liquefied gases. More precisely, Cl, C2, C3 and C4 represent respectively the sedimentation rates for liquified N2 with suspension, liquefied CO2 with suspension, liquefied N2 without suspension and liquefied CO2 without suspension.

It may be noted that it is more advantageous from a suspension stability point of view, all other things being equal, to use nitrogen rather than CO 2, in particular because of the lower viscosity of CO 2. It should nevertheless be noted that a certain number of organic compounds are soluble in liquid CO 2, which can make it possible to use stabilizers or binders directly in the suspension. In general, in order to limit the evaporation of the liquefied gas GL, the devices, for example the grinding device 1, will be designed with a concern for thermal insulation (dewar tank, specific heat insulation, etc.) and the P powders to grind can advantageously and before contacting with the liquid nitrogen be cooled. In addition, this can also be done to avoid calefaction phenomena. To do this, ideally the temperature of the powders P would be decreased below the Leidenfrost temperature of the liquefied gas used, ie of the order of -73 ° C for liquid nitrogen, as described in "Inertial drops: from the staggering "; A.L. HIMBERT BIANCE, Doctoral Thesis Paris VI, 2004.

Example 1

FIGS. 16A and 16B illustrate the microstructure of an UPuO 2 fuel produced by a manufacturing method according to the invention, ie the microstructure of the final material produced from the process according to the invention, a cryogenic process in gentle grinding for 1 min with a mass loading of 50% of the suspension, for a minimum energy applied to the PuO 2 and UO 2 powder used to produce a nuclear fuel.

The sintering was carried out with the following conditions in connection with FIG. 17 representing the sintering cycle applied to the UPuO 2 pellet made by the process according to the invention, T denoting the temperature in ° C, and t the time in hours.

The applied pressure was provided by a uniaxial press rather than casting / die casting. The pressure applied was between 200 and 300 MPa. The mass proportion of liquid nitrogen compared to the powder was about 50% by weight.

The powders used for this manufacture were as follows:

Uranium source: U02 powders from a so-called dry process and 8% by weight of crude U308 (specific surface area 1.8 m2g_1). FIGS. 14A and 14B illustrate two snapshots taken under a scanning electron microscope (SEM) according to two viewing scales of this powder.

Plutonium source: plutonium oxide powders; characteristics: specific surface area of 5.7 m2 / g. FIGS. 15A and 15B illustrate two images taken at the SEM in two viewing scales of this powder.

More generally, the law to be retained connecting the grinding energy and the size reduction of the material to be milled is a function of the nature of the materials but also of the area of the particles to be milled (average diameter at the beginning and end of grinding operation) .

FIG. 18 illustrates these granulometric domains of particles to be milled by the evolution of the energy E in kWh / t as a function of the size in μm, each time associating a specific law whose general form is dE = -K .dx / xn, the exponent n taking the values 1, 3/2 and 2 respectively for the laws of Kick (III), Bond (II) and Von Rittinger (I).

For the adjustment of the operating parameters of the ball mill 1, the speed of rotation of the mill is conventionally set between 60 and 80% of the critical speed (Vc). This speed Vc corresponds to the equilibrium of the centrifugal forces and gravity applied to the grinding balls, as illustrated by FIG. 19, which represents the centrifugal force Fc and the force of gravity applied to a ball B.

In fact, this speed is given by the following expression:

with:

Di: inner diameter of the mill 1.

In the case of a mill incorporating media with dry ice containing itself the solid to grind, to increase the relative density compared to the liquefied gas used as grinding medium, the density of the balls pB is directly related to the rate of incorporation of the solid ε:

FIG. 20 shows the evolution of the density pB of carbide ice balls B as a function of the volume fraction of solid ε (case of plutonium oxide) in the balls. In general, the grinding in the liquid phase can be considered more effective than in the dry phase insofar as it can promote the disagglomeration of powders P and allows to keep the fines in suspension, which induces a targeted grinding of large particles. Furthermore, in the case of the present invention, the use of liquefied gas GL mechanically fragile (because of the strong cooling imposed) the materials to be milled, which makes the grinding operation even more efficient.

In fact, knowing moreover that a suspension generates turbulence and a mixing entropy higher than what can be obtained in the dry phase for the same energy transmitted to the mill, it is possible to estimate that the grinding time required in phase liquid is less than that required to apply in the dry phase.

In the context of grinding via ball mill or planetary mill, the curves of evolution of the particle size distribution can be given in FIG. 21 which represents the evolution of the particle size distribution of powders as a function of the grinding time in a ball mill. , or precisely the evolution of the volume percentage% vol with respect to the size Ta, the curves tl, t2 and t3 respectively representing a time t = 10.0xh, a time t = xh and a time t = 0. Note that In the liquid phase, the energy transmitted to the load to be ground is greater than that transmitted in the dry phase. Energy saving can reach nearly 30%.

In the case where the grinding is oriented towards a jet technology, the operation is oriented towards the disagglomeration and the following dimensioning elements: angle of the adjacent jets: α = 2 / η.π energy according to the component x noted Ex :

with: p: density of the suspension d: diameter of the jet v: jet velocity Δΐ: time interval considered to evaluate the applied energy n: number of energy jets according to the component y noted Ey:

The interest here with this type of mill is to apply energy directly to the medium to grind / disintegrate.

Energy saving is therefore very important compared to other mills where the energy actually applied to the materials to be milled represents only a few percentages of the total energy applied to the mill. Note also that this mill is rather relevant for very abrasive powders since it is not used grinding media. This in fact reduces the risk of pollution.

Example 2

A second embodiment of materials through the method of the present invention is given below. It should be noted that a focus is given on the first stages of the process (grinding and shaping) in the liquefied gas phase.

A co-grinding of two types of powders was carried out by cryogenic grinding as described here, by jet mill or ball mill that can give the same type of desired result and described below. These powders are ceria and uranium oxide in proportion 70% / 30%. The grinding was carried out for 30 minutes and this was compared to a grinding stage carried out in the solid phase or to a simple mixing step by turbulence. FIG. 22 allows the comparison of the grinding paths (reference simple mixing path, ball milling method in dry phase and N2L path mill according to the invention).

After compacting, it has been demonstrated a significant improvement in the homogeneity of distribution between uranium and cerium in the raw parts for the cryogenic crushing case compared to the other two grinding / mixing above.

Specifically, comparative tests of dry mixing in turbulat, dry grinding with a ball mill for 2 hours, and ball milling under liquid N 2 for 30 minutes lead to a distribution of U / D mapping at the U / M scale. This distinct good according to the tests. The most homogeneous distribution is obtained with the test with the ball mill under N2 liquid.

The densities of the crus, all other things being equal (pressing cycle, ...), are respectively 5.8, 6.1 and 6.2 g.cm2 for turbulence tests, and mill in liquid N2 channel ball and ball mill (dry process). The evolution of the specific surfaces and the particle size distribution are furthermore given in FIG. 23 which illustrates the impact of the grinding of the present invention, compared with other reference routes, on the specific surface of the feed to be ground and its granulometry.

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 (11)

1. Device for producing ceramic parts (PC) obtained from powders (P), characterized in that it comprises: - a device for feeding and introducing powders (P) to be mixed and grinded, a cryogenic mixing and grinding device (1) for the powders (P) comprising an insulated tank, in which the powders (P) and liquefied gas (GL) are introduced and brought into direct contact with a volume ratio between the volume; of liquefied gas (GL) and the total volume of the powders (P) of between 40 and 70%; - a device for shaping by uniaxial pressing (3) of the mixture after volatilization of the liquefied gas, comprising a pressing matrix and a compaction press, - a sintering device (6) for raw molded parts. 2. Device according to claim 1, characterized in that it comprises a draining device and casting (2) for the introduction and filling of the suspension resulting from grinding in one or more shaping molds. 3. Device according to claim 1 or 2, characterized in that it comprises a shaping device (4) of the suspension comprising porous molds for the evacuation of liquefied gas (GL) through the pores and a device depressurizing (5), with respect to the pressure within the molds, and / or heating of the outer faces of the molds to promote the removal of the liquid constituting the suspension. 4. Device according to one of the preceding claims, characterized in that the powders (P) are actinide powders. 5. Device according to any one of the preceding claims, characterized in that the grinding device (1) comprises a discharge valve (10) for adjusting the grinding pressure. 6. Device according to any one of the preceding claims, characterized in that the grinding device (1) is in the form of a ball mill or planetary type, including grinding media consisting of solidified gas ( GS), charged or not with the powders (P) to be ground. 7. Device according to one of claims 1 to 5, characterized in that the grinding device (1) is in the form of a confluent jet mill, in particular composed of n jets confluent, n being an integer strictly greater than 1, whose angles (a) between two adjacent jets are equal, in particular equal to 2π / η. 8. Device according to any one of claims 2 to 7, characterized in that the draining device and casting (2) comprises means for stabilizing the suspension by vibrational and / or acoustic stresses, in particular by means of a piezoelectric and / or sonotrode. 9. Device according to any one of the preceding claims, characterized in that the device for supplying and introducing the powders (P) to be ground comprises an electrostatic charging system. 10. Device according to any one of the preceding claims, characterized in that the device for feeding and introducing the powders to be ground comprises a system for introducing electrostatically charged binders (AD) and with a charge opposite to the powders (P ), the binders being in particular organic compounds of the elastomer type, resins or glues. 11. A method of manufacturing ceramic parts (PC) obtained from powders (P), characterized in that it is implemented by means of a device according to any one of the preceding claims.