Device and method for grinding and mixing powders, comprising counter-rotating grinding and mixing moving members

Abstract

The invention primarily relates to a device (1) for grinding and mixing powders (P), characterized in that it comprises: a grinding tank (2) containing the load of powders (P) to be ground in liquid phase; at least a first grinding and mixing moving member (4a) and a second grinding and mixing moving member (4b) disposed inside the grinding tank (2); a motorization system (M) for rotating said at least a first grinding and mixing moving member (4a) and a second grinding and mixing moving member (4b); a power transmission system (3) connecting the motorization system (M) to said at least a first grinding and mixing moving member (4a) and a second grinding and mixing moving member (4b), said at least a first grinding and mixing moving member (4a) and a second grinding and mixing moving member (4b) being driven in a counter-rotating manner.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of mixing and grinding of powders, in particular the grinding and cryogenic mixing of powders, in particular in the liquid phase, in particular in the presence of a cryogenic fluid, to obtain submicron or even nanometric particles.

[0002] The invention preferably finds its application for any process and for any factory or industry implementing operations of mixing and / or grinding powders, in particular micronization of granular media with the aim in particular of obtaining improved performances in terms of specific energy applied and / or mixing or grinding time and / or in terms of grinding capacity of materials that are difficult to grind. It allows for example the manufacture of nanopowders that are difficult to synthesize chemically or the micronization of medicines or cosmetic materials for example.

[0003] The invention thus proposes a device for grinding and mixing, preferably cryogenic, powders comprising grinding and mixing rotors driven in a counter-rotating manner, as well as an associated grinding and mixing method, preferably cryogenic. STATE OF THE PRIOR ART

[0004] Grinding operations are relatively common in industry and in many fields. Depending on the applications, grinders are used that can vary greatly depending on the loads to be ground and their fragmentation capacity, such as knife, flail, hammer, roller, ball, air jet, and other mills.

[0005] These different devices exploit four main mechanisms inducing the fragmentation of the charge at the origin of the reduction in size of the particles constituting the charge to be ground, namely: impaction; shearing; compression; and attrition.

[0006] For example, we know of mobile tank mixing and grinding systems. They correspond to systems composed of a tank containing the granular medium to be mixed / ground and which undergoes this operation due to the movement of the tank. This movement can be more or less rapid and according to more or less monotonous directional modes. We can notably cite mixer-type systems, V-mixer-type systems, vibro-oscillating mixers / mills, ball mixers / mills, planetary mixers / mills, among others.

[0007] We also know internal mobile mixing and grinding systems. They correspond to systems composed of mobiles most often subjected to rotary movements to set the granular medium in motion during the periodic movement of these mobiles. These mobiles can have several natures, such as for example mobiles with a quilt effect, Archimedes screw type mobiles, attrition blades, rotors / turbines, propellers, among others.

[0008] The main drawback of the first category of mixers / grinders is the need, by definition of this category, to set in motion the entire mass of the granular medium relative to the terrestrial reference frame, as well as the mass of the tank itself containing the granular medium to be ground. However, this tank is sometimes much more massive than the granular medium it contains. In terms of energy expended per unit of mixed and / or ground mass, this category of equipment is penalized. Furthermore, this limits large-scale extrapolations, or at the cost of significant energy costs most of the time.

[0009] Furthermore, the main disadvantages of the second category of mixer / grinder mentioned above have their origins in: either the weakness of the energy transmissible to the medium to be mixed / grinded (comforter effect or Archimedes screw type mobiles), which will limit the performance of the devices in terms, for example, of processing time and / or achievable particle size performance; or the fact that they suffer from a limit on the energy level applicable to the material to be ground, in particular due to the centrifugal forces induced when the mobiles (attrition blades, rotors / turbines, propellers) are set into rotation.

[0010] It should also be noted that for certain types of materials to be ground, it is necessary to implement a liquid phase within these mixers / grinders to promote the distribution of the material within the grinding media and to facilitate the deagglomeration of the powder particles during processing.

[0011] Therefore, a counterpart induced by the use of this liquid phase is that it is necessary to separate this phase from the ground solid load and / or that this liquid phase must be treated after the grinding operation. Separation by filtration is not effective when the production of nanoparticles is targeted (particles too small compared to the mesh of the filtration media) and separation by evaporation is often not very profitable in terms of energy consumed per unit mass of material to be mixed / ground. Furthermore, certain liquid phases can also interact chemically with the solid load to be ground, which induces pollution and / or modifications of the solids to be ground which can be prohibitive for certain applications.

[0012] In order to solve this problem, it has been proposed to use a liquefied gas as a liquid which, after volatilization at room temperature and atmospheric pressure, makes it possible to avoid having to treat the liquid phase to recover the ground material. In addition, the use of liquefied gas, due to its very low induced temperature (of the order of - 200°C for liquid nitrogen at atmospheric pressure), also makes it possible to weaken the materials to be ground and therefore limits the energy used to grind a given mass of material.

[0013] Inventions have therefore been proposed that exploit this way of carrying out the grinding operation, but they always suffer, regardless of the liquid phase used, from a limitation of the energy applicable to the charge to be ground. Indeed, beyond a speed threshold called the "critical speed" of rotation of the stirring / grinding rotors, a hydrodynamic regime occurs that no longer allows optimal mixing / grinding to be obtained. However, the energy level per unit volume of a grinder characterizes its performance and efficiency, and this energy is a function of the stirring speed, this energy even being close to being proportional to the square of this speed. There is therefore a contradiction to be resolved in order to benefit from direct cryogenic grinding, namely grinding where the solid charge to be ground is directly suspended in a liquefied gas, while not undergoing any applicable energy limitation as mentioned above.This contradiction explains the absence in the state of the art of a device that would allow grinding in the liquid phase while exceeding the injectable energy limit due to the aforementioned constraints. Knowing also that there is a limit rotation speed of the stirring / grinding mobiles within the grinders beyond which the hydrodynamic regime is disturbed and / or the mechanical strength of the rotating shafts can be called into question due to the induced torques.

[0014] To improve the grinding efficiency of certain materials known to be difficult to grind, solutions of direct cryogenic mills with grinding mobiles have been proposed in the prior art, such as for example by applications WO 2019 / 073172 A1 and JP 2021-041404 A. These devices are relatively robust. However, as mentioned above, it is not possible for these devices to be able to apply to the material to be ground a quantity of energy greater than a so-called "critical" curvilinear limit speed of the peripheral end of the grinding mobiles set in rotation to provide the grinding energy.

[0015] This limit is based on several constraints detailed below. First of all, constructive constraints because beyond a certain speed of the stirring mobile subjected to the friction of the viscous fluid to be stirred / ground (consisting of the suspension between the liquid of the grinding medium and the material to be ground associated with the grinding media), the torque applied to the stirring shaft of the grinding mobile induces a breakage of the shaft. Also hydrodynamic constraints due to the generation of centrifugal forces at a level limiting the degrees of freedom of the fluid to be ground / mixed (pressing of the fluid and the media against the walls of the mill). But also thermodynamic constraints due to the occurrence of cavitation phenomena beyond a limiting speed of movement of a solid body in any liquid.

[0016] Known grinding devices suffer very largely, and recurrently, from pollution of the load by abrasion or wear induced by the grinding media and at the right of the grinding tank. This is due to the fact that the grinding media and / or the material to be ground have a hardness which may be greater than or equal to that of the material constituting the mill tank.

[0017] To avoid these phenomena, which can be very detrimental, or even prohibitive for certain applications such as pharmaceuticals and food, for example, it is traditionally proposed to use very hard materials, compared to the materials to be ground, to develop the grinder. Unfortunately, this strategy remains expensive and sometimes incompatible with applications that do not accept any pollution.

[0018] A solution has already been proposed in the Applicant's patent application FR 3 072 308 A1, using dry ice (solid CO 2 ) as a material for rinsing the grinding tank or as a material for the grinding media. This way of operating the grinding is relevant for limiting pollution of the materials to be ground but it does not allow an optimized level of mechanical energy to be introduced. Indeed, in an attritor type grinder as preferentially targeted by the invention, the inventors have demonstrated a significant loss of speed of the grinding media in the close vicinity of the walls of the grinder in the case of an aspect ratio (length of the stirring / grinding rotor to the diameter of the grinding / mixing tank) of less than 0.9. This low speed of the grinding media leads to a reduction in grinding efficiency. Furthermore, by increasing the aspect ratio, the speed is improved but the wear at the wall is exacerbated.This double observation illustrates a problem that is not resolved by the state of the art.

[0019] Furthermore, it should be noted that the micronization of grinding powders is often complicated to optimize for particle size targets below one micron and for materials known to be difficult to grind. Micronization methods are not very efficient, using speeds limited to the critical speed, which means that the materials have to be processed for several hours, or even several days, to achieve the desired particle sizes. In addition, the useful volume of micronizers is often small and submicron-targeted grinders are difficult or impossible to extrapolate to industrial scales.

[0020] There is thus a need to increase the grinding efficiency of mills operating in the liquid phase. There is also a need to make powder grinding devices more efficient, particularly in terms of particle size performance for a given processing time, minimizing the processing time for a given particle size target and / or increasing the useful volume of submicron micronizers (and therefore the processing capacity).

[0021] A means is desired to efficiently apply energy to a powder in order to finely grind it, this powder being preferably suspended in a liquefied gas. By forming a liquid / solid suspension, the grinding medium is subjected to centrifugal forces when the grinding rotor is rotated beyond a so-called "critical" speed. Grinding is therefore limited by this critical speed which can be quickly reached for large grinders (industrial purpose). There is therefore a need to overcome this critical speed threshold and thereby increase grinding efficiency as well as the useful grinding volume. STATEMENT OF THE INVENTION

[0022] The invention aims to at least partially remedy the needs mentioned above and the drawbacks relating to the achievements of the prior art.

[0023] In particular, it aims to address the limitations of known systems, and to propose a mobile grinding / mixing system maximizing the amount of energy per unit of time (power) and per unit of volume (volumic power) of the mill to be operated, and without impacting the integrity of the mobile grinding / mixing.

[0024] The subject of the invention, according to one of its aspects, is a device for grinding and mixing, in particular cryogenic grinding and mixing, powders, characterized in that it comprises: a grinding tank, comprising the charge of powders to be ground in liquid phase, in particular in the presence of a cryogenic fluid, for example liquid nitrogen, and grinding media, at least one first grinding and mixing mobile and one second grinding and mixing mobile arranged inside the grinding tank, a motorization system for driving in rotation said at least one first grinding and mixing mobile and one second grinding and mixing mobile, a power transmission system connecting the motorization system to said at least one first grinding and mixing mobile and one second grinding and mixing mobile, said at least one first grinding and mixing mobile and one second grinding and mixing mobile being driven in rotation in a counter-rotating manner and being coaxial.

[0025] The grinding and mixing device according to the invention may further comprise one or more of the following characteristics taken individually or in any possible technical combination.

[0026] The grinding and mixing device is preferably a cryogenic grinding and mixing device, the grinding tank comprising in particular a cryogenic fluid, in particular liquid nitrogen, and being advantageously heat-insulated.

[0027] The distance between said at least one first grinding and mixing mobile and said at least one second grinding and mixing mobile may be less than three times the smallest diameter of said at least one first grinding and mixing mobile and one second grinding and mixing mobile.

[0028] Furthermore, said at least one first grinding and mixing mobile and one second grinding and mixing mobile may be of axial, radial and / or hybrid type.

[0029] According to a first embodiment, the power transmission system may be a bevel gear power transmission system.

[0030] The distance between the bottom of the grinding tank and the grinding and mixing rotor closest to the bottom of the grinding tank may be less than twice the diameter of the grinding and mixing rotor. The distance between two superimposed grinding and mixing rotors may be between one and five times the diameter of the grinding and mixing rotor.

[0031] According to a second embodiment, the power transmission system may be an epicyclic gear train power transmission system.

[0032] The power transmission system may comprise at least two epicyclic gear trains, in particular as many epicyclic gear trains as there are grinding and mixing mobiles.

[0033] In addition, the epicyclic gear train(s) may constitute a thermal cover for said at least one first grinding and mixing mobile and a second grinding and mixing mobile.

[0034] Furthermore, said at least one first grinding and mixing mobile and one second grinding and mixing mobile may advantageously be chiral mobiles.

[0035] Furthermore, the invention also relates, according to another of its aspects, to a method for grinding and mixing powders, in particular in the presence of a cryogenic fluid, for example liquid nitrogen, characterized in that it is implemented by means of a device as defined above.

[0036] The method can be carried out by means of a cryogenic grinding and mixing device using a cryogenic fluid in the grinding tank, in particular liquid nitrogen, in direct contact with the powders to be ground.

[0037] The method may comprise the step of counter-rotating said at least one first grinding and mixing mobile and a second grinding and mixing mobile.

[0038] Furthermore, the method may comprise the step of rotating said at least one first grinding and mixing rotor and one second grinding and mixing rotor at a speed of between 10% and 150% of the cavitation speed of the fluid used in the grinding tank.

[0039] The grinding and mixing device and method according to the invention may include any of the characteristics set out in the description, taken in isolation or in any technically possible combination with other characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, as well as by examining the schematic and partial figures of the attached drawing, in which: there figure 1 schematically illustrates, in a sectional view, an example of a grinding and mixing device according to the invention with a first principle of driving the grinding and mixing mobiles (bevel gears), the figure 2 schematically illustrates, in a sectional view, another example of a grinding and mixing device according to the invention with a second principle of driving the grinding and mixing mobiles (epicyclic drive), the Figures 3A, 4A and 5A represent, in perspective views, examples of types of grinding and mixing mobiles, the Figures 3B, 4B and 5Billustrate, according to partial sectional views, the use respectively of the grinding and mixing mobiles of the Figures 3A, 4A and 5A in a grinding tank of a grinding and mixing device according to the invention, the Figure 6A illustrates, in a sectional view, the current lines induced by two counter-rotating axial-type grinding and mixing mobiles for a grinding and mixing device according to the invention, the Figure 6B is a top view of the Figure 6A , there Figure 7A illustrates, in a sectional view, the current lines induced by two counter-rotating radial-type grinding and mixing mobiles for a grinding and mixing device according to the invention, the Figure 7B is a top view of the Figure 7A , THE Figures 8A and 8Bare two side views illustrating the principle of power transmission by bevel gear to the grinding and mixing mobiles of a grinding and mixing device according to the invention, the Figure 8C is a partial sectional view of the Figures 8A and 8B , and the figures 9 and 10 are partial perspective views illustrating the epicyclic gear trains of the transmission system of a grinding and mixing device according to the invention.

[0041] Throughout these figures, like references may designate identical or similar elements.

[0042] Furthermore, the different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more readable. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0043] In reference to the figure 1, an example of a grinding and mixing device 1 according to the invention is shown with a first mode of driving the grinding and mixing mobiles 4a, 4b. The figure 2 represents a grinding and mixing device 1 with a second mode of driving the grinding and mixing mobiles 4a, 4b.

[0044] The grinding and mixing device 1 is preferably a cryogenic grinding and mixing device. It advantageously allows the grinding and micronization of powders using first 4a and second 4b counter-rotating grinding and mixing mobiles.

[0045] The use of a counter-rotating grinding and mixing system to agitate the granular suspension to be micronized can advantageously make it possible to locally cancel the centrifugation forces and to double the surface speeds achievable between the powder P to be ground and the grinding and mixing rotors 4a, 4b. In addition, this principle can make it possible to multiply by a factor which can be close to four the energy applied to the system to be ground.

[0046] Thus, the grinding and mixing device 1 firstly comprises a grinding and mixing tank 2. The grinding tank 2 is in the form of a double jacket for maintaining a low partial pressure (at least primary vacuum) in the inter-wall volume formed by the double jacket. It thus ensures thermal insulation.

[0047] The grinding tank 2 has the function of receiving the load of solid powders P to be ground and mixed in liquid phase, in particular liquefied gas, for example liquid nitrogen, as well as the grinding media Mb, for example balls, pellets, among others, visible on the Figures 1 and 2 .

[0048] The grinding tank 2 is generally cylindrical in shape. Its height is preferably between 0.5 and 5 times its diameter. It may optionally include a low point drain that can be used to evacuate the load and / or recycle it within the tank 2.

[0049] Furthermore, the grinding and mixing device 1 comprises a first grinding, mixing and stirring device 4a and a second counter-rotating grinding, mixing and stirring device 4b arranged inside the grinding tank 2.

[0050] The grinding and mixing mobiles 4a, 4b can be of different types, for example turbine type, inclined or non-inclined blades, attrition mobiles or propellers.

[0051] In the case of propellers or inclined blades, chiral configurations can be preferred, namely one of the rotors is dextrorotatory and the other is levorotatory.

[0052] They typically have a diameter such that the ratio between the diameter of the tank and the diameter of the grinding and mixing unit is between 0.2 and 0.9.

[0053] The mobiles 4a and 4b may or may not have the same diameter. Advantageously, the distance h 1 between the two mobiles 4a and 4b, visible on the figure 1 , is less than three times their diameter in the case of identical diameters, or even the smallest diameter in the case of different diameters.

[0054] Advantageously, the integration of two grinding and mixing mobiles 4a, 4b rotated in reverse movements makes it possible to obtain several advantages, and in particular an increase in the extent of the mixing and grinding zones, an increase in the speed gradients, and an increase in the impact frequencies.

[0055] In order to maximize these advantages, the grinding and mixing mobiles 4a, 4b are preferably configured to have opposite speeds at all points and at all times during the mixing and grinding. In this sense, the grinding and mixing mobiles 4a, 4b are preferably coaxial and driven by a counterclockwise rotation.

[0056] The grinding and mixing rotors 4a, 4b can be formed in various ways depending on the specificities, in particular the viscosity and the density of the medium to be ground and mixed. In particular, the grinding and mixing rotors 4a, 4b can be classified into three families, namely axial, radial and / or hybrid type. These configurations are described more precisely with reference to Figures 3A to 5B .

[0057] On the Figures 3A and 3B , a grinding and mixing mobile 4a, 4b with axial flow is shown, of the marine propeller type. The fluid flow lines LC are ascending at the periphery of the wall and descending in the close vicinity of the axis of rotation of the mobile 4a, 4b.

[0058] On the Figures 4A and 4B, a radial flow grinding and mixing mobile 4a, 4b is shown, of the six-blade turbine type. The fluid flow lines LC are then partitioned into two zones: one below the mobile and the other above the mobile.

[0059] On the Figures 5A and 5B , a grinding and mixing mobile 4a, 4b with hybrid or mixed flow is shown, of the inclined straight blade type. In this case, the fluid flow lines LC are a combination of the two previous cases.

[0060] Furthermore, the Figure 6A is a sectional view illustrating the LC streamlines induced by two counter-rotating axial-type grinding and mixing rotors 4a, 4b, here in the form of coaxial anti-rotational marine propellers. The Figure 6B is a top view of these mobiles 4a, 4b.

[0061] In this configuration, we seek to generate confluence zones of LC streamlines which telescope in the close vicinity of the wall Pa located between the two grinding and mixing mobiles 4a, 4b. We thus obtain zones of velocity gradient Zgv represented on the Figures 6A and 6B .

[0062] In addition, the Figure 6A illustrates the value ratios between the pumping flow rate Qp, the circulation flow rate Qc and the flow rate Qe which is equal to the difference between circulation flow rate Qc and pumping flow rate Qp. In addition, rc represents the circulation radius.

[0063] Furthermore, the Figure 7A is a sectional view illustrating the LC streamlines induced by two counter-rotating radial-type grinding and mixing rotors 4a, 4b, here in the form of a turbine with anti-rotating coaxial blades. The Figure 7B is a top view of these mobiles 4a, 4b.

[0064] In this configuration, we seek to generate confluence zones of LC streamlines which telescope in the close vicinity of the wall Pa located between the two grinding and mixing mobiles 4a, 4b. We thus obtain zones of velocity gradient Zgv represented on the Figures 7A and 7B .

[0065] In addition, the Figure 7A illustrates the height h of the grinding turbine element and the extent e of the flow area representing the difference between the circulation flow rate Qc and the pumping flow rate Qp.

[0066] Generally speaking, beyond the examples of Figures 6A, 6B And 7A, 7B, in order to increase the mixing and grinding performance, the aim is to create, by rotating the grinding and mixing rotors 4a, 4b, areas of streamlines LC which telescope as much as possible. The grinding and mixing rotors 4a, 4b being rotated in an anticlockwise manner, this generates opposite flow streams which allow the impact of the grinding media Mb which fractionate the powders P to be ground in their contact or impact zone. These favorable areas are visible in particular on the Figure 6B And 7B .

[0067] Furthermore, always in order to increase the mixing and grinding performance, we seek to apply strong agitation of the fluid contained in the grinding tank 2. To do this, the grinding and mixing rotors 4a, 4b are set in very rapid rotation. However, as mentioned previously, there is a “critical” rotation speed beyond which the agitation and grinding effect is no longer optimal.

[0068] This critical speed, or limiting speed, can be estimated in several ways described below, and in particular by analogy with the critical speed of rotary calender mills and by calculating the speed threshold.

[0069] First, we examine the calculation of the critical or limiting speed by analogy with the critical speed of rotary calender mills. In this case, the critical speed can be considered as the speed corresponding to the conditions where the centrifugal force becomes greater than the force of gravity applied to the fluid in tank 2.

[0070] In the case of ball mills, the number of critical revolutions per second (Nc) can be expressed as a function of the internal diameter of the mill (D): Nc = 42 , 7 D

[0071] Knowing further that by definition the curvilinear speed of the grinding and mixing mobile 4a, 4b can be given as a function of the number of revolutions per unit of time and the distance from the axis of rotation (r = D / 2) by the following expression: Vc = 2 πr . Nc or for D=6 cm and Nc = 174 rpm, Vc = 0.5 m / s.

[0072] In reality, it is possible to significantly exceed this value in an attritor type crusher as preferentially targeted by the invention. Indeed, the centrifugal forces generate a vortex which is acceptable as long as the depth of the vortex (ΔH) is of the order of magnitude of the diameter of the crusher (D). From then on, we can estimate the order of magnitude of the depth of the vortex as a function of the Froude number (Fr): Δ H D ≈ 2 Fr with Fr = (N 2< D / g).

[0073] If the vortex depth is equivalent to the diameter of the crusher, then Fr ~ ½, and therefore Vc ~ 1.7 m / s.

[0074] In reality, the load of the grinding media Mb disrupts the vortex and generally the overall hydraulic behavior. The limit is therefore often linked to other considerations such as the mechanical strength limit of the rotation shaft of the grinding and mixing mobile 4a, 4b.

[0075] The critical speed corresponding to this mechanical limit is a function in particular of the viscosity and density of the fluid in the direction of the grinding tank 2 but in general, it is considered that it is not admissible to exceed peripheral speeds at the ends of the grinding and mixing mobiles 4a, 4b greater than approximately 10 or 15 m / s.

[0076] Furthermore, we now examine the calculation of the critical or limiting speed by calculating the speed threshold beyond which cavitation phenomena are significant and induce significant wear at the level of the grinding and mixing mobiles 4a, 4b.

[0077] Cavitation, namely the appearance of vapor within the liquid, appears as soon as the pressure in the liquid undergoing the displacement of a surface reaches the saturated vapor pressure (Pvs) of the liquid due to the displacement of this surface which generates a pressure gradient by its movement.

[0078] To assess the risk of cavitation, it is particularly interesting to look at two coefficients, the pressure coefficient (Cp) and the cavitation number (σ), the definitions of which are given below: where A and B are two points located at the right of the grinding mobile, p A and p B being the pressures at points A and B, and VA and VB being the speeds at points A and B.

[0079] It should be noted that when σ is lower than the minimum value of the absolute value of Cp (| Cpmin |), there is a zone on the profile of the mobile for which the pressure becomes lower than the vapor pressure. In this case, the phenomenon of cavitation can occur, which induces deleterious wear on the grinding and mixing mobiles 4a, 4b. Precisely, for Cpmin > - σ, there is no cavitation. For Cpmin = - σ, cavitation is possible with critical conditions. For Cpmin < - σ, cavitation is developed.

[0080] For a liquid such as water, the cavitation speed is of the order of 20 m / s. For liquid nitrogen, this speed can be lower, which limits the speed applicable to the grinding and mixing mobile 4a, 4b in cryogenic phase grinders (case of direct contact of liquid nitrogen with the material to be ground as described in applications WO 2019 / 73172 A1 and JP 2021-041404 A).

[0081] Advantageously, the invention makes it possible to set a limiting speed at the periphery of the grinding and mixing rotors 4a, 4b (that induced by the cavitation phenomena) while applying a strictly counter-clockwise speed between these rotors in order to obtain, at the level of the impacts between grinding balls, speeds which can approach a value close to twice the peripheral speed of the end of the grinding and mixing rotors and this without limitation due to cavitation.

[0082] Furthermore, strictly anti-rotational rotation limits the creation of vortices and the potentially negative impact of centrifugal forces when grinding solids because the particles will tend to remain stuck to the walls, areas which are not necessarily the most optimal for grinding.

[0083] Furthermore, as previously stated, the figure 1 illustrates an embodiment with a first mode of power transmission to the grinding and mixing mobiles 4a, 4b.

[0084] Thus, a drive system M enables the counter-rotating drive of the grinding and mixing mobiles 4a, 4b and a power transmission system 3 connects the grinding and mixing mobiles 4a, 4b to the drive system M. Here, the power transmission is done by bevel gear.

[0085] This type of transmission ensures strictly counterclockwise, counter-rotating rotation with a single motor, which is advantageous in terms of cost and investment.

[0086] As visible on the figure 1 , the two grinding and mixing mobiles 4a, 4b are arranged such that the lowest mobile 4b is distant by a height h from the bottom of the tank 2 and the distance between the two mobiles 4a and 4b is noted h1. Furthermore, the references T and D respectively designate the diameter of the mobiles 4a, 4b and the diameter of the tank 2.

[0087] For example, h is less than 2T, and h1 is between T and 5T. Of course, these orders of magnitude are in no way restrictive.

[0088] THE Figures 8A, 8B and 8C illustrate an example of the implementation of a power transmission by bevel gear.

[0089] In this example, the grinding and mixing mobile 4a is for example of the ship's propeller type, and the grinding and mixing mobile 4b is for example of the inclined blade type. In addition, in this configuration, the motor shaft is orthogonal to the rotation shaft of the grinding and mixing mobiles 4a, 4b.

[0090] So, on these Figures 8A, 8B and 8C the stirring shaft A1 of the first grinding and mixing unit 4a, the stirring shaft A2 of the second grinding and mixing unit 4b, a support ring CS, ball bearings RB and the bevel gear EC connected to the motor shaft are shown.

[0091] Furthermore, the figure 2 illustrates a second mode of power transmission by epicyclic gear train.

[0092] It should be noted that whatever the type of transmission system 3 used, it must allow synchronous rotation in opposite directions of the two grinding and mixing mobiles 4a, 4b.

[0093] In this second mode, the use of an epicyclic gear train constructively makes it possible to ensure not only counterclockwise rotation of the grinding and mixing mobiles 4a, 4b but also to ensure, within the framework of the present invention, a strictly opposite speed of these mobiles for the reasons mentioned previously.

[0094] Epicyclic gear trains do not conventionally allow for the reproduction of counterclockwise rotation with a strictly equivalent angular speed of the drive shaft. Indeed, an epicyclic gear train is often multiplicative, or even reducing, but does not strictly reproduce the rotational speed which drives it. In the present invention, a strictly opposite speed of the grinding and mixing mobiles 4a, 4b can be advantageously targeted since this optimizes the impact and friction force in the close vicinity of the common areas of the two mobiles, as described with reference to Figures 6A to 7B. If an epicyclic gear train is used, it is preferable to have two, one providing a multiplicative function for the rotation speed of the motor shaft and the other a reduction function so that the combination of these two sub-assemblies ensures strict transmission of the rotation speed of the motor shaft, with a factor of 1 of overall restitution.

[0095] In the example given below, we target a reductive-multiplicative factor of 3, also called the basic ratio of the train λ.

[0096] For the first stage, an illustration of the description of the epicyclic gear train TE to be used as an example is provided in the following Table 1: Table 1: Main dimensions of the first stage of the epicyclic gear train Number of teeth (Z) Diameter (d) in mm Planetary (21) 20 20 Satellite (22) 20 20 Crown (23) 60 60

[0097] The center distance between the sun gear 21 and the satellites 22 is written a. It is such that a = R1 + R2 = R3 - R2 = 20 mm.

[0098] The basic reason noted λ is such that λ = − z 1 23 = − 1 3

[0099] The number of teeth Z is such that Z = Z 1 + Z 2 n = 10 for n = 4, n being the number of satellites.

[0100] For the second stage, the second epicyclic gear train TE is described by the data in the following table 2: Table 2: Main dimensions of the second stage of the epicyclic gear train Number of teeth (Z) Diameter (d) in mm Planetary (21') 20 20 Satellite (22') 10 10 Crown (23') 60 60

[0101] The center distances between the sun gear 21' and the satellites 22' are written a' and a" and are such that: a' = R1 + R2 = 15 mm, and a" = R1 + D2 + R2' = 25 mm.

[0102] The basic reason noted λ is such that λ = − z 3 Z 1 = − 3 1 .

[0103] Note that in the case of an epicyclic gear train, the housing enclosing the train can be used advantageously to provide thermal insulation (plug) in the tank crown.

[0104] THE figures 9 and 10are partial perspective views illustrating the epicyclic gear trains TE of the transmission system 3 of the grinding and mixing device 1 according to the invention. In particular, the figure 9 is a cutaway view of the double epicyclic gear train and the figure 10 is a view from below of it for a transmission factor of 1 in the counterclockwise direction of rotation.

[0105] On these figures 9 and 10 , the gear transmission of the torque from the motor shaft is made to the sun gear 21 or 21'. The gear transmission of the torque from the sun gear 21 or 21' is made to the satellites 22 or 22'. The gear transmission from the satellites 22 or 22' is made to the crown 23 or 23'.

[0106] In addition, the 24 or 24' satellite carrier allows the 22 or 22' satellites to be secured. The 25 or 25' landing gear cover provides protection to prevent access and forms a thermal barrier.

[0107] Furthermore, the M drive system consists of a drive shaft with a motor. The motor is capable of generating a rotation speed of between 100 and 15,000 rpm at the drive shaft for a torque of between 0.1 and 10 Nm.

[0108] Of course, the invention is not limited to the embodiments which have just been described. Various modifications can be made to it by those skilled in the art.

Claims

1. A cryogenic device (1) for grinding and mixing powders (P), comprising: - a grinding tank (2), comprising the load of powders (P) to be ground in liquid phase, and grinding media (Mb), the grinding tank (2) comprising a cryogenic fluid and being heat-insulated, - at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) disposed inside the grinding tank (2), - a motorization system (M) for driving said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) in rotation, - a power transmission system (3) connecting the motorization system (M) to said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b), characterized in that said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) are driven in rotation in a counter-rotating manner and are coaxial.

2. The device according to claim 1, characterized in that the grinding tank (2) comprises liquid nitrogen (N2).

3. The device according to claim 1 or 2, characterized in that the distance (h1) between said at least one first grinding and mixing moving member (4a) and said at least one second grinding and mixing moving member (4b) is smaller than three times the smallest diameter of said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b).

4. The device according to one of the preceding claims, characterized in that said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) are of the axial, radial and / or hybrid type.

5. The device according to any one of the preceding claims, characterized in that the power transmission system (3) is a bevel gear (EC) power transmission system.

6. The device according to any one of claims 1 to 4, characterized in that the power transmission system (3) is an epicyclic gear train (TE) power transmission system.

7. The device according to claim 5 or 6, characterized in that the distance (h) between the bottom of the grinding tank (2) and the grinding and mixing moving member (4b) the closest to the bottom of the grinding tank (2) is smaller than twice the diameter (T) of the grinding and mixing moving member (4b), and in that the distance (h1) between two superimposed grinding and mixing moving members (4a, 4b) is between one and five times the diameter (T) of the grinding and mixing moving member (4b).

8. The device according to claim 7, characterized in that the power transmission system (3) comprises at least two epicyclic gear trains (TE), in particular as many epicyclic gear trains (TE) as grinding and mixing moving members (4a, 4b).

9. The device according to claim 7 or 8, characterized in that the epicyclic gear train(s) (TE) form(s) a thermal cover for said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b).

10. The device according to any one of the preceding claims, characterized in that said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) are chiral moving members.

11. A method for grinding and mixing powders (P), characterized in that it is implemented by means of a device according to any one of the preceding claims.

12. The grinding and mixing method according to claim 11, characterized in that it is implemented by means of a cryogenic grinding and mixing device using a cryogenic fluid in the grinding tank (2), in particular liquid nitrogen, in direct contact with the powders (P) to be ground.

13. The grinding and mixing method according to claim 11 or 12, characterized in that it comprises the step of driving said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) in a counter-rotating manner.

14. The grinding and mixing method according to one of claims 11 to 13, characterized in that it comprises the step of rotating said at least one first grinding and mixing moving member (4a) and one second grinding and mixing moving member (4b) at a speed between 10% and 150% of the cavitation speed of the fluid used in the grinding tank (2).

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