FR3072307B1 - CRYOGENIC MILLING DEVICE AND METHOD WITH CONFLUENT JETS - Google Patents

Abstract

The main object of the invention is a device for crushing (1) cryogenic powder, characterized in that it comprises: a grinding tank (2) insulated, comprising n jets of distribution of suspensions (JS) powders liquid gas arranged in such a manner as to be confluent in an area close to the center of the grinding tank (2), a laser particle size analyzer (3) associated with a counter lighting device (4); a generation and feed system (5) in suspension of powders / liquefied gas; a data processing and control system (6) of the grinding device (1), the laser granulometric analysis device (3) being connected to said processing and control system (6).

Description

DEVICE AND METHOD FOR CRYOGENIC GRINDING WITH CONFLUENT JETS

DESCRIPTION

TECHNICAL AREA

The present invention relates to the field of cryogenic grinding of powders, in particular actinide powders comprising in particular uranium oxide powders (UO 2) and / or plutonium oxide powders (PuO 2), for obtaining controlled particle size powders and / or mixtures with controlled particle size and increased homogeneity compared to granular media obtained by grinding and / or dry mixing. The invention is preferably used for the manufacture of nuclear ceramic parts, namely the manufacture of nuclear fuel, in particular nuclear fuel pellets. The invention thus proposes a device for cryogenic crushing of confluent jetted powders formed by the mixture of liquefied gas / powder to be ground and an associated grinding process.

STATE OF THE PRIOR ART

Grinding operations are relatively common in the industry in many areas. Depending on the applications, it is possible to use grinders that can be very different depending on the loads to be milled and their fragmentation ability, such as, for example, knife mills, flail mills, hammers, rollers, ball mills, jet mills, between other.

These different devices exploit four major mechanisms inducing the fragmentation of the charge at the origin of the size reduction of the particles constituting the charge to be ground, namely: impaction; shearing; the compression ; and attrition.

In the context of the manufacture of ceramic parts such as nuclear fuels, ball mills are conventionally and industrially used for co-grinding powders 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.

It should nevertheless be noted that these devices suffer from several drawbacks, and in particular: the pollution of the load by the abrasion induced at the right of the balls and the crushing tank (to minimize this effect, it is potentially used uranium balls for the application using actinide powders); difficulty in controlling the grinding process in a continuous manner to avoid phenomena such as over-grinding or reagglomeration; relatively long grinding kinetics inducing times of operation which are binding (several hours of grinding necessary for a few hundred grams to tens of kilograms to be treated in the case of nuclear fuel operations) otherwise the crushed load may not not be sufficiently homogeneous and / or crushed; and the difficulty of being able to work continuously without the risk of accumulation and clogging of the solid charge, especially when it is not very flowable, which is common in the case of uranium oxide and plutonium powders.

Evolutions have therefore been proposed with, in particular, US Pat. No. 4,715,547 A implementing grinding in the liquid phase. It should be noted that this type of device does not answer either the problem posed because the liquids envisaged are aqueous or organic phases generating risks (criticality, radiolysis, ...) and very constraining effluents for certain loads to be crushed (powders fissile material, reacting with water or some organic, ...).

Knowing that to minimize the energy to be used during grinding, it is also possible to make the material fragile by cooling it to relatively low temperatures, that is to say classically below -50 ° C., certain known grinding devices implement a cryogenic cooling of the feedstock to be milled before treating it, as described in the application WO 2008/110517 A1. It should nevertheless be noted that this type of cryogenic mill does not make it possible to have a fast grinding as is attainable in liquid way. Other devices such as those described in US 3,734,412 A, EP 2,535,114 A1, GB 1,508,941 A1 or EP 1405,927 A1 implement suspensions consisting of a liquefied gas and a feedstock to be milled favoring efficiency. grinding and mixing. Nevertheless, these devices use grinding media that induce a risk of increased pollution. Moreover, they do not include any element allowing to optimize their piloting and to ensure a true continuous driving.

STATEMENT OF THE INVENTION

Accordingly, there is thus a need to provide a new type of device and method cryogenic crushing powders, especially actinide powders. In particular, there is a need for a grinding device using a mixture of liquefied gas / powder for suspending the granular filler to grind, or to grind and mix.

Beyond the reduction in the particle size of the charge, there is more precisely a need to be able to concomitantly: - to minimize the grinding energy to be applied for this operation, - to improve the homogeneity of the powder mixtures to be ground, - to minimize the quantities of liquefied gas to be used, - limit the pollution of the powders to be ground, - limit the heating of the powders, - avoid the generation of liquid effluents (aqueous or organic), - limit the risks of criticality when placing use of fissile material, - increase the crushing kinetics, - ensure a continuous and controlled operation of the grinding of the load. 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, in particular those mentioned above. According to one of its aspects, the subject of the invention is a device for cryogenic grinding of powders, in particular actinide powders, characterized in that it comprises: a heat-insulated and pressurized grinding vat, in particular between 1 bar and 40 bars absolute, in order to be able in particular to limit thermal losses, comprising: - n distribution nozzles of suspensions powders / liquefied gas arranged so as to be confluent in an area close to the center of the grinding chamber to allow the impaction of the suspensions to be ground by minimizing or even eliminating the impact on the walls of the grinding vessel, - a laser particle size analyzer associated with a counter-lighting device, - a system for generating and powder / liquefied gas supply feed system, - a data processing system and control of the grinding device, the laser particle size analysis device being connected to said treater system and driving.

Advantageously, the present invention is based on several elements of the grinding device, the latter having no support to the grinding media to prevent abrasion pollution between the powders to be ground and the body of the device. This is made possible by the use of confluent jets transporting by suspending the load to grind and making it collide at speeds, and therefore energies, controlled to the right of the jets meeting area.

It should be noted that, in the prior art, there are jet mills for cryogenic part but these are in most cases air jet mills which require the implementation of relatively large gas flow rates for allow the application of a sufficient amount of movement to the load to be crushed.

Among the technical effects exploited advantageously in the present invention, mention may be made of grinding without pollution of the powders using a jet milling configuration. The confluence of the jets is advantageously adjusted so that the angle between two adjacent nozzles is equal to 2π / η radian, and the momentum (qm) of each of the jets is 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 same injection speed.

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.

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

The n jet distribution nozzles may be arranged in one plane and the angle formed by the axes of two adjacent nozzles may be equal to 2 n / n radian.

The grinding device may further comprise a relief valve to allow to limit the pressure within the grinding vessel.

In addition, the system for generating and feeding suspensions powders / liquefied gas, can include: - a doser of powders and liquefied gas, - a mixer of powders and liquefied gas to form the suspensions to be introduced into the tank of grinding, - a dispensing pump, or booster, to introduce the suspensions powders / liquefied gas to the right of the n jets distribution jets, - a recirculation loop suspensions after introduction into the grinding tank through a valve recirculation pump and a pump for reinjecting the suspensions in the mixer of the generation system and supply.

The grinding device may also include a device for electrostatically charging the powders to be grinded.

The grinding device may further comprise a piezoelectric device and / or a sonotrode for the application of mechanical and / or acoustic vibrations when necessary.

The data processing and control system can rely on the recovery and processing of the following parameters: - flow rate and / or injection pressure imposed by the dispensing pump, - flow rates imposed by the powder and gas dosers liquefied, in other words solid and liquid filler, - granulometry of the suspensions.

In addition, the data processing and control system can make it possible to adjust these parameters by feedback to guarantee, in particular, maintaining the grinding vessel under pressure, obtaining the target particle size of the solid charge to be crushed by increasing or decreasing. the injection rate, the number of recirculation cycles per unit time, the grinding time.

In addition, according to another of its aspects, the subject of the invention is also a process for the cryogenic grinding of powders, characterized in that it is implemented by means of a device as defined above, and in that it comprises the following steps: - introduction of powder suspensions / liquefied gas through the n distribution nozzles at a rate distribute uniformly through each distribution nozzle, - flow control and recirculation suspensions according to expected efficiency evaluated through the data processing and control system incorporating particle size monitoring through the laser particle size analyzer.

The process may optionally include the step of electrostatically charging powders before the grinding operation.

In addition, the process may comprise a step of determining the granular solid charge to be ground and the quantity of liquefied gas. It may also include the step of mixing the solid charge and the liquefied gas.

The device and the grinding 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 drawings, in which: - Figure 1 schematically illustrates an example of grinding tank for a grinding device according to the invention, - Figure 2 schematically illustrates an example of grinding device according to the invention using the bowl. FIG. 3 illustrates the main shear behavior 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. viscosity of a suspension of powders in liquid nitrogen as a function of the volume fraction of powder for two values of the maximum stack volume fraction, e 5 illustrates the establishment of the reduced sedimentation rate, i.e., rendered adimensional, as a function of time; FIG. 6 illustrates the sedimentation velocities, particle alone or in suspension as a function of the particle radius, for two types of liquefied gas, FIG. 7 illustrates the projections of the vectors of the amounts of movement in the reference constituted by the plane formed by the confluent jets; FIG. 8 represents the evolution of the particle size distribution of powders as a function of time; method of grinding in a ball mill, precisely the evolution of the volume percentage with respect to the size, - Figures 9A and 9B illustrate, respectively before grinding and after grinding, changes in particle size, and - Figure 10 is a diagram which allows the comparison of the grinding paths (reference path simple mixing ball milling path dry phase and grinder N2L pathway 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 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 an example of grinding tank 2 for a grinding device 1 according to the invention.

The grinding tank 2 is a confluent jet mill to limit the pollution of the powders. In FIG. 1, the reference 11 denotes a guillotine drain valve, or else a draw-off or recirculation valve, the reference 10 designates a discharge valve for adjusting the operating pressure, the reference JS denotes the powder / gas suspension jets. liquefied, the reference ZI designates the impact zone, 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 to left of Figure 1 of the box on the right. The angle a is equal to 2ti / 3 radian because here there are 3 jets.

More specifically, the pollution-free grinding of the powders P using a jet milling configuration is based on the confluence of the jets set so that: the angle between two nozzles of the adjacent jets is equal to 2π / η radian; the amounts of movement (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.

Referring to Figure 2, there is shown schematically an example of grinding device 1 according to the invention.

Thus, the grinding device 1 comprises a thermally insulated grinding tank 2 as described with reference to FIG. 1, which furthermore comprises a laser particle size analyzer device 3 associated with a counter-lighting device 4, a system for generating and feedstock 5 in suspensions powders / liquefied gas, and a data processing and control system 6 of the grinding device 1, the laser particle size analyzer 3 being connected to this processing and control system 6.

More specifically, the system for generating and feeding 5 suspensions powders / liquefied gas, comprises: a proportioner 20 of powders P and liquefied gas GL, a mixer 21 of powders P and liquefied gas GL to form the suspensions to be introduced in the grinding tank 2, a distribution pump 22 for introducing the powder / liquefied gas suspensions to the right of the n jets JS distribution, and a recirculation loop 24 suspensions after introduction into the grinding tank 2 through of the recirculation valve 11 and a pump 26, or circulator, for reinjecting the suspensions into the mixer 21 of the generation and supply system 5.

In addition, in FIG. 2, the reference CP designates a pressure sensor and the reference V designates the emptying. In addition, although not shown, the grinding device 2 may comprise a device for electrostatic charging of the powders (P) to be ground, and also a piezoelectric device and / or a sonotrode for the application of mechanical and / or acoustic vibrations. case of need.

In order to size the cryogenic grinding device or mill 1, 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 Quemada model: η = ηί x (1 - (φ / φηι)) 2 with: φιτι: volume fraction of maximum stack, ηί: viscosity of the interstitial fluid .

Depending on the powder concentration, the suspension can then have several types of behavior. FIG. 3 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. 4 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. (ρρ-ρί) / (9.μ), with: r: particle radius; pp: density of the particles;

Pf: density of the liquefied gas.

FIG. 5 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. 6 illustrates the sedimentation velocities vp, particle alone or in suspension (hypothesis retained: 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 gas. 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.

EXAMPLE OF IMPLEMENTATION In general, grinding in the liquid phase may be considered more effective than in the dry phase since it can promote deagglomeration of the powders and makes it possible to keep the fines in suspension, which induces a targeted grinding. large particles. Moreover, 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 grinded, 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.

Furthermore, in a general manner also, in order to limit the evaporation of the liquefied gas GL, the grinding device 1 will be designed with a concern for thermal insulation (dewar tank, specific heat insulation, ...) and P powders grinding may 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 with spreading ", AL HIMBERT BIANCE, Thesis of doctorate of Paris VI, 2004.

Energy of the grinding device

The grinding device 1, or mill, is first defined by its grinding energy. Unlike the media mills, the mill 1 of the present invention is configured such that the energy injected is preferentially applied to the load to be milled. The energy E applied to the load to be milled can be evaluated according to the following expression: E = Vi mv2, where: m represents a given mass of suspension to be ground; v represents the speed of JS jets at the point of impact.

By introducing the notions of the density of the suspension (p), the unit distribution rate per nozzle (Q), and the grinding time (t), E can then be expressed by the following expression: E = 2. p. Q. t. v2.

FIG. 7 illustrates the projections of the amounts of movement in the reference mark formed by the plane formed by the JS confluent jets.

The component of the energy E applied to the load according to the component x, denoted Ex, can be expressed as follows:

with: p: density of the suspension; d: diameter of the jet; v: speed of the jet; Δΐ: time interval considered to evaluate the energy applied; n: number of jets.

With regard to the y component of energy, denoted Ey, we obtain:

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 grinders where the energy actually applied to the materials to grind represents only a few percentages of the total energy applied to the mill. Note also that the mill according to the invention is rather relevant for very abrasive powders and / or to undergo the least possible pollution since it is not used grinding media. This in fact reduces the risk of pollution.

For a given flow rate v, a length L of pipe of diameter D, the pressure loss induced by a circulating liquid of density p is expressed as follows: Δρ = λ. (Ρν2 / 2). (Ί / ϋ) with: X = 64 / Re and Re = pv.D / μ.

Considering a single-pass grinding, then N passes, the grinding energy can be evaluated as in Table 1 below:

Table 1

Typically, the granulometries of the feedstock to be milled vary as a function of the grinding time. For example, in the context of milling using ball mills, the curves of evolution of the particle size distribution can be given in FIG. 8 which represents the evolution of the particle size distribution of powders as a function of the milling time in a mill. balls, or precisely the evolution of the volume percentage% vol relative to the size Ta, the curves tl, t2 and t3 respectively representing a time t = lO.xh, a time t = xh and a time t = 0. To note in the liquid phase, the energy transmitted to the load to be milled is greater than that transmitted in the dry phase. Energy saving can reach nearly 30%.

For applied energies of the order of 100 Wh / g, the mill 1 according to the invention makes it possible to obtain the following granulometries for a granular crushing system defined by Table 2 below:

Table 2

Moreover, FIGS. 9A and 9B illustrate, respectively before grinding and after grinding, changes in particle size.

It should also be noted that the homogeneity in terms of distribution of uranium and cerium is better than with other known grinding / mixing means.

FIG. 10 allows the comparison of the grinding paths (reference path of simple mixing, ball milling method in the dry phase and N2L path mill according to the invention), and in particular makes it possible to locate the high homogeneity of the ground granular mixture by the present invention. and this in a reduced grinding time compared to the reference grinding mode (ie via dry ball mill).

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 (7)

1. Crushing device (1) cryogenic powder (P), characterized in that it comprises: - a grinding tank (2) heat-insulated and pressurized, comprising: - N nozzles distribution of jets suspensions (JS) powders / liquefied gas arranged so as to be confluent in a zone close to the center of the grinding chamber (2), - a laser particle size analyzer (3) associated with a counter-lighting device (4), - a system for generating and supplying (5) powder suspensions / liquefied gas, - a data processing and control system (6) for the grinding device (1), the laser particle size analysis device (3) being connected to said processing and control system (6). 2. Device according to claim 1, characterized in that the n jet distribution nozzles (JS) are arranged in a plane and in that the angle (a) formed by the axes of two adjacent nozzles (JS) is equal at 2π / η radian. 3. Device according to claim 1 or 2, characterized in that it further comprises a discharge valve (10). 4. Device according to one of the preceding claims, characterized in that the generation system and feed (5) in suspensions powders / liquefied gas, comprises: - a doser (20) of powders (P) and liquefied gas (GL), - a mixer (21) of powders (P) and liquefied gas (GL) to form the suspensions to be introduced into the grinding vessel (2), - a distribution pump (22) for introducing the powder suspensions gas / liquefied gas at the n jets distribution nozzles (JS), - a recirculation loop (24) suspensions after introduction into the grinding tank (2) through a recirculation valve (11) and a pump (26) for reinjecting the suspensions into the mixer (21) of the generation and supply system (5). 5. Device according to any one of the preceding claims, characterized in that the grinding device (2) further comprises a device for electrostatic charging of the powders (P) to be ground. 6. Device according to any one of the preceding claims, characterized in that the grinding device (2) further comprises a piezoelectric device and / or a sonotrode for the application of mechanical and / or acoustic vibrations. 7. Process for the cryogenic grinding of powders (P), characterized in that it is implemented by means of a device according to any one of the preceding claims, and in that it comprises the following steps: - introduction of powder suspensions (P) / liquefied gas (GL) through n distribution nozzles (JS) at a rate to distribute uniformly through each distribution nozzle (JS), - flow control and recirculation of suspensions in a function of the expected efficiency evaluated by means of the data processing and control system (6) integrating the monitoring of the particle size distribution by means of the laser particle size analysis device (3).