FR3130648A1 - DEVICE AND METHOD FOR GRINDING AND MIXING POWDER COMPRISING CONTRAROTATIVE GRINDING AND MIXING MOTORS - Google Patents

Abstract

The main object of the invention is a device for grinding and mixing (1) powders (P), characterized in that it comprises: a grinding vessel (2), comprising the load of powders (P) to mill in liquid phase; at least a first grinding and mixing wheel (4a) and a second grinding and mixing wheel (4b) arranged inside the grinding tank (2); a motorization system (M) for rotating said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b); a power transmission system (3) connecting the motorization system (M) to said at least one first grinding and mixing mobile (4a) and a second grinding and mixing mobile (4b), said at least one first mobile grinding and mixing (4a) and a second mobile grinding and mixing (4b) being driven in rotation in a counter-rotating manner. Figure for abstract: Figure 1

Description

DEVICE AND METHOD FOR GRINDING AND MIXING POWDER COMPRISING CONTRAROTATIVE GRINDING AND MIXING MOTORS

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

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

The invention thus proposes a device for grinding and mixing, preferably cryogenic, of powders comprising grinding and mixing mobiles driven in a counter-rotating manner, as well as a method of grinding and associated mixing, preferably cryogenic.

PRIOR ART

Grinding operations are relatively common in industry in many areas. Depending on the applications, crushers are used which can be very different depending on the charges to be crushed and their capacity for fragmentation, such as, for example, knife, flail, hammer, roller, ball, jet crushers. air, among others.

These various devices exploit four main mechanisms inducing the fragmentation of the charge, which is the origin of the reduction in size of the constituent particles of the charge to be ground, namely: impaction; shear; the compression ; and attrition.

For example, mixing and grinding systems using a mobile tank are known. They correspond to systems consisting of a tank containing the granular medium to be mixed/grinded and which undergoes this operation due to the movement of the tank. This setting in motion can be more or less rapid and according to more or less monotonous directional modes. Mention may in particular be made of mixer-type systems, V-type mixer-type systems, vibro-oscillating mixers/mills, ball mixers/mills, planetary mixers/mills, among others.

Internal mobile mixing and grinding systems are also known. They correspond to systems made up of mobiles most often subjected to rotary movements to set the granular medium in motion during the periodic movement of these mobiles. These are mobiles that can have several natures, such as quilt effect mobiles, Archimedes screw type mobiles, attrition blades, rotors/turbines, propellers, among others.

The main defect of the first category of mixers/grinders is the need by very 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 mass mixed and/or ground, this category of equipment is penalised. Moreover, this effectively limits large-scale extrapolations or at the cost of high energy costs most of the time.

Furthermore, the main drawbacks of the second category of mixer/grinder mentioned above have their origins: either the low level of energy that can be transmitted to the medium to be mixed/grinded (quilt effect or Archimedean screw-type spindles), which will limit the performance of the devices in terms, for example, of processing time and/or performance of achievable particle sizes; or the fact that they suffer from an energy level limit applicable to the material to be shredded, in particular by the centrifugal forces induced during the rotation of the rotors (attrition blades, rotors/turbines, propellers).

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

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

In order to solve this problem, it has been proposed to use a liquefied gas as the liquid which, after volatilization at room temperature and atmospheric pressure, does not require the liquid phase to be treated to recover the ground material. In addition, the use of liquefied gas, because of 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 makes it possible to limit the energy to be used to grind a given mass of materials.

Inventions have therefore been proposed exploiting this way of carrying out the grinding operation but they always suffer, whatever the liquid phase used, from a limitation of the energy applicable to the charge to be ground. Indeed, beyond a so-called “critical speed” speed threshold of rotation of the stirring/grinding mobiles, a hydro-dynamic regime takes place that no longer allows optimum mixing/grinding to be obtained. However, the level of energy per unit volume of a mill characterizes its performance and its efficiency and this energy is a function of the stirring speed, this energy being even 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 such as aforementioned. This contradiction explains the absence in the state of the art of a device which would make it possible to carry out grinding in the liquid phase while exceeding the limit of injectable energy due to the aforementioned constraints. Knowing also that there is a limit speed of rotation of the stirring/grinding mobiles within the grinders beyond which the hydrodynamic regime is disturbed and/or the mechanical strength of the rotation shafts can be called into question makes induced couples.

To improve the grinding efficiency of certain materials deemed difficult to grind, solutions of direct cryogenic grinders 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" limit curvilinear speed of the peripheral end of the grinding wheels set in rotation. to supply the grinding energy.

This limit is based on several constraints detailed below. First of all, constructive constraints because beyond a certain speed of stirring wheel subject to the friction of the viscous fluid to be stirred/grinded (consisting of the suspension between the liquid of the grinding medium and the material to be ground associated to the grinding media), the torque applied to the stirring shaft of the grinding wheel 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 (plating 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 limit speed of movement of a solid body in any liquid.

The known grinding devices suffer very mainly, and in a recurring way, from the pollution of the load by the abrasion or wear induced by the grinding media and in 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.

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

A solution has already been proposed in patent application FR 3 072 308 A1 of the Applicant, implementing dry ice (solid CO ₂ ) as the material for sealing the grinding vessel or as the material for the grinding media. This way of operating the grinding is relevant for limiting the pollution of the materials to be ground, but it does not make it possible to introduce an optimized level of mechanical energy. 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 aspect ratio ( length of the stirring/grinding rotor over the diameter of the grinding/mixing tank) less than 0.9. This low speed of the grinding media induces a drop in grinding efficiency. Furthermore, by increasing the aspect ratio, the speed is improved but the wear at the right of the wall is exacerbated. This double observation illustrates a problem that is not solved by the state of the art.

In addition, it should be noted that the micronization of powders to be ground is often complicated to optimize for targets with a particle size of less than one micron and for materials known to be difficult to grind. The micronization means are inefficient, implementing speeds limited to the critical speed, which leads to the materials being treated for several hours, or even several days, to achieve the desired particle sizes. In addition, the working volume of micronizers is often small and submicron-targeted mills are little or not extrapolable to industrial scales.

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 treatment time, minimization of the treatment time for a given particle size target and/or increase in the useful volume of submicron micronizers (thus treatment capacity).

A means is desired for efficiently applying energy to a powder in order to be able to grind it finely, this powder being preferentially suspended in a liquefied gas. By forming a liquid/solid suspension, the grinding medium is subjected to centrifugal forces when the grinding wheel is rotated above a so-called “critical” speed. Grinding is therefore limited by this critical speed which can be quickly reached for large grinders (industrial aim). There is therefore a need to overcome this critical speed threshold and thereby increase the grinding efficiency as well as the useful grinding volume.

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, it aims to remedy the limits of known systems, and to propose a mobile grinding/mixing system maximizing the quantity of energy per unit of time (power) and per unit of volume (power density) of the mill to be operated. , and without impact on the integrity of the grinding/mixing spindle.

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

The grinding and mixing device according to the invention may also comprise one or more of the following characteristics taken in isolation or in any possible technical combination.

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

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

Furthermore, said at least one first grinding and mixing wheel set and a second grinding and mixing wheel set may be of the axial, radial and/or hybrid type.

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

The distance between the bottom of the grinding tank and the grinding and mixing wheel set closest to the bottom of the grinding tank may be less than twice the diameter of the grinding and mixing wheel set. The distance between two superposed grinding and mixing wheels can be between one and five times the diameter of the grinding and mixing wheel.

According to a second embodiment, the power transmission system can be a power transmission system by planetary gear train.

The power transmission system may comprise at least two planetary gear trains, in particular as many planetary gear trains as grinding and mixing mobiles.

In addition, the planetary gear train(es) can constitute a thermal cover for said at least one first grinding and mixing wheel set and a second grinding and mixing wheel set.

Furthermore, said at least one first grinding and mixing mobile and a second grinding and mixing mobile can advantageously be chiral mobile.

Furthermore, the invention also relates, according to another of its aspects, to a process 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.

The process can be implemented 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.

The method may include the step of setting said at least one first grinding and mixing rotor and a second grinding and mixing rotor in counter-rotating rotation.

In addition, the method may comprise the step of rotating said at least one first grinding and mixing rotor and a second grinding and mixing rotor at a speed of between 10% and 150% of the cavitation speed of the fluid used in the grinding vessel.

The device and the grinding and mixing process according to the invention may include any of the characteristics set out in the description, taken in isolation or according to any technically possible combination with other characteristics.

The invention can be better understood on reading the detailed description which follows, of non-limiting examples of implementation thereof, as well as on examining the figures, schematic and partial, of the appended drawing, on which :

schematically illustrates, in a sectional view, an example of a grinding and mixing device in accordance with the invention with a first driving principle for the grinding and mixing mobiles (bevel gears),

schematically illustrates, in a cross-sectional view, another example of a grinding and mixing device in accordance with the invention with a second principle of driving the grinding and mixing mobiles (epicyclic drive),

,

And

represent, according to perspective views, examples of the type of grinding and mixing mobiles,

,

And

illustrate, according to views in partial section, the use respectively of the grinding and mixing mobiles of FIGS. 3A, 4A and 5A in a grinding vessel of a grinding and mixing device according to the invention,

illustrates, in a cross-sectional view, the current lines induced by two counter-rotating grinding and mixing wheels of the axial type for a grinding and mixing device according to the invention,

is a top view of the ,

illustrates, in a sectional view, the current lines induced by two counter-rotating grinding and mixing wheels of the radial type for a grinding and mixing device according to the invention,

is a top view of the ,

And

are two side views illustrating the principle of power transmission by bevel gear to the grinding and mixing wheels of a grinding and mixing device according to the invention,

is a partial sectional view of Figures 8A and 8B, and

And

are partial perspective views illustrating the planetary gear trains of the transmission system of a grinding and mixing device 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 shown on a uniform scale, to make the figures more readable.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to the , there is shown an example of a grinding and mixing device 1 in accordance with the invention with a first mode of driving the grinding and mixing mobiles 4a, 4b. There represents for its part a grinding and mixing device 1 with a second mode of driving the mobile grinding and mixing 4a, 4b.

The grinding and mixing device 1 is preferably a grinding and cryogenic mixing device. It advantageously allows the grinding and micronization of powders with the aid of the use of first 4a and second 4b mobiles for counter-rotating grinding and mixing.

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 superficial velocities that can be reached between the powder P to be ground and the grinding wheels and mixture 4a, 4b. In addition, this principle can make it possible to multiply by a factor that can be close to four the energy applied to the system to be ground.

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 (vacuum at the less primary) in the inter-wall volume formed by the double envelope. It provides thermal insulation.

The function of the grinding vessel 2 is to receive the load of solid powders P to be ground and mixed in the liquid phase, in particular liquefied gas, for example liquid nitrogen, as well as the grinding media Mb, for example balls , cannonballs, among others, visible in figures 1 and 2.

The grinding vessel 2 is globally of cylindrical symmetry. Its height is preferably between 0.5 and 5 times its diameter. It can optionally comprise a drain at the low point which can be used to evacuate the load and/or to recycle it within the tank 2.

Furthermore, the grinding and mixing device 1 comprises a first grinding and mixing and stirring wheel set 4a and a second counter-rotating grinding and mixing and stirring wheel set 4b arranged inside the grinding vessel 2.

The grinding and mixing mobiles 4a, 4b can be of different natures, for example of the turbine type, blades inclined or not, attrition mobiles or propellers.

In the case of propellers or inclined blades, chiral configurations may be preferred, namely that one of the mobiles is dextrorotatory and the other is levorotatory.

They conventionally have a diameter such that the ratio between the diameter of the vessel to the diameter of the grinding and mixing rotor is between 0.2 and 0.9.

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 , is less than three times their diameter in the case of identical diameters, or even the smallest diameter in the case of different diameters.

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 velocity gradients, and increased impact frequencies.

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 of the mixing and grinding. In this sense, the grinding and mixing mobiles 4a, 4b are preferentially coaxial and moved by counterclockwise rotation.

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

In FIGS. 3A and 3B, an axial-flow grinding and mixing rotor 4a, 4b is shown, of the marine propeller type. The fluid flow lines LC are ascending at the periphery of the wall and descending close to the axis of rotation of the mobile 4a, 4b.

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

In FIGS. 5A and 5B, a grinding and mixing rotor 4a, 4b with hybrid or mixed flow is represented, of the inclined straight blade type. In this case, the fluid LC flow lines are a combination of the two previous cases.

Furthermore, the is a sectional view illustrating the LC current lines induced by two counter-rotating grinding and mixing mobiles 4a, 4b of the axial type, here in the form of coaxial anti-rotational marine propellers. There is a top view of these mobiles 4a, 4b.

In this configuration, it is sought to generate confluence zones of current lines LC which collide in the close vicinity of the wall Pa located between the two grinding and mixing mobiles 4a, 4b. Zones of velocity gradient Zgv shown in FIGS. 6A and 6B are thus obtained.

Moreover, the illustrates the value ratios between the pumping rate Qp, the circulation rate Qc and the flow rate Qe which is equal to the difference between circulation rate Qc and pumping rate Qp. Furthermore, r _c represents the circulation radius.

Furthermore, the is a sectional view illustrating the lines of current LC induced by two counter-rotating grinding and mixing mobiles 4a, 4b of the radial type, here in the form of a turbine with anti-rotational coaxial blades. There is a top view of these mobiles 4a, 4b.

In this configuration, it is sought to generate confluence zones of current lines LC which collide in the close vicinity of the wall Pa located between the two grinding and mixing mobiles 4a, 4b. Zones of velocity gradient Zgv shown in FIGS. 7A and 7B are thus obtained.

Moreover, the illustrates the height h of the grinding turbine element and the extent e of the flow area representing the difference between the circulation rate Qc and the pumping rate Qp.

In general, beyond the examples of Figures 6A, 6B and 7A, 7B, in order to increase the mixing and grinding performance, it is sought to create by the rotation of the grinding and mixing mobiles 4a, 4b zones LC current lines that telescope as much as possible. The grinding and mixing wheels 4a, 4b being rotated counterclockwise, this generates flows with opposite flows which allow the impact of the grinding media Mb which split the powders in their contact or impact zone. P to grind. These favorable zones are visible in particular on the and 7B.

Moreover, still in order to increase the mixing and grinding performance, it is sought to apply strong agitation to the fluid contained in the grinding vessel 2. To do this, the grinding and mixing mobiles 4a, 4b are put into very fast spinning. However, as mentioned above, there is a "critical" speed of rotation beyond which the agitation and grinding effect is no longer optimal.

This critical speed, or limit 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.

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

In the case of ball mills, the number of critical rotations per second (Nc) can be expressed as a function of the internal diameter of the mill (D):

Knowing moreover that by definition the curvilinear speed of the mobile grinding and mixing 4a, 4b can be given according to the number of revolutions per unit of time and the distance to the axis of rotation (r = D / 2) by the following expression:
Vc = 2πr.Nc
i.e. for D=6 cm and Nc = 174 rpm, Vc = 0.5 m/s.

In actual fact, it is possible to very substantially exceed this value in a grinder of the attritor type 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 mill (D). Therefore we can estimate the order of magnitude of the depth of the vortex according to the Froude number (Fr):

with Fr = (N ² D/g).

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

In reality again, the Mb grinding media load disturbs the vortex and overall overall hydraulic behavior. The limit is therefore often linked to other considerations such as the mechanical strength limit of the rotating shaft of the grinding and mixing mobile 4a, 4b.

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

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

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

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 in line with the grinding wheel, p _A and p _B being the pressures at points A and B, and V _A and V _B being the speeds at points A and B.

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 under critical conditions. For Cpmin < - σ, cavitation is developed.

For a liquid like water, the cavitation speed is about 20 m/s. For liquid nitrogen, this speed can be lower, which accordingly limits the speed applicable to the grinding and mixing mobile 4a, 4b in the 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).

Advantageously, the invention makes it possible to stall at a limit speed at the periphery of the grinding and mixing mobiles 4a, 4b (of that induced by the cavitation phenomena) while applying a strictly anti-clockwise speed between these mobiles 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 mobiles and this without limitation due to cavitation.

Furthermore, the strictly anti-rotational rotation limits the digging of vortices and the impact of potentially negative centrifugal forces during the grinding of solids because the particles will tend to remain pressed against the walls, areas which are not necessarily the most optimal for grinding.

In addition, as mentioned previously, the illustrates an embodiment with a first mode of power transmission to the mobile grinding and mixing 4a, 4b.

Thus, a motorization system M allows 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 motorization system M. Here, the Power transmission is by bevel gear.

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

As seen on the , the two grinding and mixing mobiles 4a, 4b are arranged such that the lowest mobile 4b is a height h from the bottom of the tank 2 and the distance between the two mobiles 4a and 4b is denoted h1 . Furthermore, the references T and D designate respectively the diameter of the mobiles 4a, 4b and the diameter of the tank 2.

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.

Figures 8A, 8B and 8C illustrate an embodiment of a bevel gear power transmission.

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. Moreover, in this configuration, the motor shaft is orthogonal to the rotation shaft of the grinding and mixing mobiles 4a, 4b.

Thus, in these figures 8A, 8B and 8C are represented the stirring shaft A1 of the first mobile grinding and mixing 4a, the stirring shaft A2 of the second mobile grinding and mixing 4b, a support ring CS, ball bearings RB and the bevel gear EC linked to the motor shaft.

Furthermore, the illustrates a second mode of power transmission by planetary gear train.

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

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

Epicyclic gear sets do not conventionally allow counter-clockwise rotation to be reproduced with a strictly equivalent angular speed of the motor shaft. Indeed, an epicyclic gear train is often multiplicative, even reducing, but does not strictly restore the speed of rotation which moves it. In the present invention, a strictly opposite speed of the grinding and mixing mobiles 4a, 4b can advantageously be targeted since this optimizes the force of impact and friction in the close vicinity of the common zones of the two mobiles, as described with reference to the Figures 6A to 7B. If an epicyclic gear train is used, it is preferable to have two of them, one ensuring a multiplicative function of the rotational speed of the motor shaft and the other a reducing function so that the combination of these two -sets ensure strict transmission of the rotational speed of the motor shaft, with an overall restitution factor of 1.

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

For the first stage, an illustration of the description of the planetary gear train TE to be used as an example is provided in the following table 1:

Number of teeth (Z) Diameter (d) in mm Planetary (21) 20 20 Satellite (22) 20 20 Crown (23) 60 60

Table 1: main dimensions of the first stage of the planetary gear train

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

The basic reason noted λ is such that λ = =

The number of teeth Z is such that Z = 10 for n=4, n being the number of satellites.

For the second stage, the second planetary gear train TE is described by the data in the following table 2:

Number of teeth (Z) Diameter (d) in mm Planetary (21') 20 20 Satellite (22') 10 10 Crown (23’) 60 60

Table 2: main dimensions of the second stage of the planetary gear train

The centers 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' = 25mm.

The basic reason noted λ is such that λ = = .

It should be noted that in the case of an epicyclic train, the casing containing the train can advantageously be used to provide thermal insulation (plug) in the top of the vessel.

FIGS. 9 and 10 are partial perspective views illustrating the planetary gear sets TE of the transmission system 3 of the grinding and mixing device 1 according to the invention. Particularly, the is a cutaway view of the double planetary gear train and the is a bottom view of it for a factor of 1 transmission in counterclockwise rotation.

In these figures 9 and 10, the transmission by gearing of the torque of 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 of the 22 or 22' satellites is made to the crown 23 or 23'.

In addition, the 24 or 24' satellite carrier allows the attachment of the 22 or 22' satellites. The 25 or 25' landing gear provides protection to prevent access and forms a thermal barrier.

In addition, the M drive system consists of a drive shaft with a motor. The motor is able to generate at the level of the motor shaft a rotational speed of between 100 revolutions and 15,000 revolutions/min for a torque of between 0.1 and 10 Nm.

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

Claims (14)

Device for grinding and mixing (1) powders (P), characterized in that it comprises:
- a grinding tank (2), comprising the load of powders (P) to be ground in the liquid phase, and grinding media (Mb),
- at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b) arranged inside the grinding tank (2),
- a motorization system (M) for rotating said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b),
- a power transmission system (3) connecting the motorization system (M) to said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b),
said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b) being rotated counter-rotatingly and being coaxial. Device according to Claim 1, characterized in that it is a grinding and cryogenic mixing device, the grinding vessel (2) comprising a cryogenic fluid, in particular liquid nitrogen (N ₂ ), and being heat-insulated. Device according to Claim 1 or 2, characterized in that the distance (h ₁ ) between the said at least one first grinding and mixing wheel set (4a) and the said at least one second grinding and mixing wheel set (4b) is less three times the smallest diameter of said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b). Device according to one of the preceding claims, characterized in that the said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b) are of the axial, radial and/or hybrid type. 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. Device according to any one of claims 1 to 4, characterized in that the power transmission system (3) is a planetary gear train (TE) power transmission system. Device according to Claim 5 or 6, characterized in that the distance (h) between the bottom of the grinding vessel (2) and the grinding and mixing wheel set (4b) closest to the bottom of the grinding vessel ( 2) is less than twice the diameter (T) of the grinding and mixing rotor (4b), and in that the distance (h1) between two superimposed grinding and mixing rotors (4a, 4b) is between a times and five times the diameter (T) of the mobile grinding and mixing (4b). Device according to Claim 7, characterized in that the power transmission system (3) comprises at least two planetary gears (TE), in particular as many planetary gears (TE) as grinding and mixing wheels (4a, 4b) . Device according to Claim 7 or 8, characterized in that the planetary gear train or trains (TE) constitute a thermal cover for the said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set (4b ). Device according to any one of the preceding claims, characterized in that the said at least one first grinding and mixing spindle (4a) and a second grinding and mixing spindle (4b) are chiral spindles. Process 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. Grinding and mixing process according to claim 11, characterized in that it is implemented by means of a grinding and cryogenic mixing device using a cryogenic fluid in the grinding vessel (2), in particular liquid nitrogen, in direct contact with the powders (P) to be ground. Grinding and mixing process according to claim 11 or 12, characterized in that it comprises the step of setting said at least one first grinding and mixing wheel set (4a) and a second grinding and mixing wheel set in counter-rotation mixture (4b). 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 wheel set (4a) and a second grinding wheel set and mixing (4b) at a rate between 10% and 150% of the cavitation rate of the fluid used in the grinding vessel (2).