Device and method for the contactless grinding and mixing of powders, comprising a rotating inner casing

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 particular in liquid phase; a protective inner casing (3), disposed inside the grinding tank (2), defining an inner volume (Vi); a grinding and mixing moving member (4), disposed inside the inner volume (Vi) of the protective inner casing (3); permanent magnets (7) located on the protective inner casing (3); and drive magnets (8a) for rotating the protective inner casing (3) by interacting with the permanent magnets (7), which drive magnets are located outside the grinding tank (2) opposite the permanent magnets (7).

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of powder grinding, in particular cryogenic grinding of powders, in particular in the liquid phase, to obtain submicron or even nanometric particles, while limiting impurities.

[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 performance 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.

[0003] The invention thus proposes a device for grinding and mixing, preferably cryogenic, without contact of powders comprising an internal rotating protective casing located inside a grinding tank, 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 constituent particles of the charge to be ground, namely: impaction; shearing; compression; and attrition.

[0006] CN112337587 describes a grinding device according to the preamble of the appended claim 1.

[0007] 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.

[0008] 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.

[0009] 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 mill 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 mill 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 the grinding efficiency in this area. 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.

[0010] 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 a critical speed, which leads to processing the materials for several hours, or even several days, to achieve the desired particle sizes, thereby leading to pollution of the loads to be ground. In addition, the useful volume of micronizers is often small and submicron-targeted grinders are difficult or impossible to extrapolate to industrial scales.

[0011] There is therefore a need to optimize the grinding efficiency of mills, particularly those operating in the liquid phase, and in particular to increase the amount of energy per unit of time (power) and per unit of volume (volume power) of the mill to be operated.

[0012] Knowing, however, that to increase the power of the crusher, it is necessary to increase the rotation speed of the grinding mobiles and therefore the abrasion / wear of the grinding media and the grinding tank, a difficulty must be overcome to resolve the objective of increasing sufficient volumetric power while limiting the pollution of the load to be ground.

[0013] 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 processing time for a given particle size target and / or increasing the useful volume of submicron micronizers (and therefore the processing capacity). There is also a need to enable a requirement to limit impurities introduced during micronization.

[0014] 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. To the extent that the increase in stirring energy (enabling efficient operations in terms of particle size or processing time, for example) involves an increase in the speed of the mill's stirring rotor, there is concomitantly increased wear of the internals of the mill, at which the differential speed of the wall / grinding balls and / or powder to be ground is greatest.Knowing also that increasing the rotation speed of the stirring rotor can lead, from a certain speed, to reaching a centrifugal regime that is less effective for grinding, a means is sought to increase the surface wall / powder speeds without reaching this centrifugal regime and without inducing wear, which in turn causes the generation of pollutants within the grinder. STATEMENT OF THE INVENTION

[0015] The invention aims to at least partially remedy the needs mentioned above and the drawbacks relating to the achievements of the prior art.

[0016] In particular, it aims to address the problem of grinding efficiency without causing pollution of the load to be ground by wear of the internal wall of the grinder.

[0017] 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 particular in liquid phase, and grinding media, an internal protective casing, arranged inside the grinding tank, defining an internal volume, a grinding and mixing mobile, arranged in the internal volume of the internal protective casing, permanent magnets located at the level of the internal protective casing, drive magnets, for rotating the internal protective casing by interaction with the permanent magnets, located outside the grinding tank opposite the permanent magnets.

[0018] 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.

[0019] The grinding and mixing device according to the invention is advantageously a cryogenic grinding and mixing device, the grinding tank comprising a cryogenic fluid, in particular liquid nitrogen, and being in particular heat-insulated, for example by means of an insulator of predetermined thickness.

[0020] The grinding tank is advantageously in the form of a reservoir providing thermal insulation, notably in the form of a double vacuum jacket, and allows the internal protective jacket to be received.

[0021] The grinding tank is advantageously made of a non-magnetizable material, for example stainless steel, in particular type 316L.

[0022] The internal volume of the internal protective envelope can advantageously include the grinding media.

[0023] The internal protective casing advantageously has a geometry similar to that of the grinding tank, being smaller in size so as to fit into the grinding tank.

[0024] The grinding and mixing unit can be driven in rotation by means of a drive system with a transmission arm. The grinding and mixing unit can thus transmit the mechanical power delivered by the drive to the fluid to be ground in the form of convective movements by circular agitation.

[0025] Permanent magnets can be inserted into the inner protective casing or placed between the inner protective casing and the grinding tank.

[0026] Alternatively, the drive magnets may include fixed, non-rotating, reciprocating drive electromagnets located outside the grinding vessel.

[0027] The drive magnets may also include movable drive magnets, rotated by means of a drive motor.

[0028] In addition, the internal protective envelope may consist of or may include a coating in a material of hardness greater than that of the powders to be ground, in particular under cryogenic temperature conditions, and sublimating at room temperature and atmospheric pressure.

[0029] The inner protective envelope may in particular consist of or may include a coating of solidified carbon dioxide. The solidified carbon dioxide may be obtained by solidifying liquid carbon dioxide by cooling or by compacting dry ice or by machining.

[0030] The internal protective envelope may, for example, include raised attrition elements on its internal wall, in particular attrition pins, including solidified carbon dioxide.

[0031] The relief elements of attrition, or even grinding, can be obtained by solidification of liquid carbon dioxide by cooling or by compaction of dry ice or by machining.

[0032] The permanent magnets and drive magnets can be arranged axisymmetrically with respect to the axis of rotation of the grinding tank.

[0033] Permanent magnets can be arranged by crimping and / or forcing.

[0034] Furthermore, the invention also relates, according to another of its aspects, to a method of grinding and mixing, in particular cryogenic grinding and mixing, of powders, characterized in that it is implemented by means of a device as defined above.

[0035] The method can advantageously 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.

[0036] The method may include the step of rotating the inner protective casing by means of fixed alternating drive electromagnets powered according to the distance between permanent magnets and electromagnets.

[0037] The method may further comprise the step of rotating the internal protective casing by means of mobile drive magnets rotated by means of a drive motor.

[0038] The rotation of the movable drive magnets can advantageously be carried out so that the direction of rotation varies alternately. In this way, it may be possible to increase the grinding efficiency.

[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: [ Fig. 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 internal protective casing, [ Fig. 2 ] schematically illustrates, in a sectional view, the example of a grinding and mixing device in accordance with the invention of the figure 1 with a second principle of driving the internal protective envelope, [ Fig. 3 ] And [ Fig. 4 ] schematically illustrate, from a top view, two stages of the rotation of the internal protective envelope, [ Fig. 5] represents, in a partially sectional and perspective view, an example of a grinding and mixing device according to the invention, [ Fig. 6 ] represents, in a partial perspective view, the grinding and mixing device of the Figure 5 , And [ Fig. 7 ] represents, partially in section, the grinding and mixing device of the Figure 5 .

[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 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 internal protective casing. The figure 2represents the same grinding and mixing device 1 with a second mode of driving the internal protective casing.

[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 a contactless drive wall so as to be able to limit the introduction of impurities, in particular by abrasion and wear of the internals of the grinding and mixing device.

[0045] Thus, the grinding and mixing device 1 firstly comprises a grinding 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. It is advantageously made of a non-magnetizable material, for example stainless steel, in particular type 316L.

[0046] 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.

[0047] The grinding tank 2 is generally cylindrical in shape. Its height is advantageously 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.

[0048] The grinding tank 2 is designed to accommodate an internal protective casing 3 inserted between the grinding tank 2 and the fluid to be ground, powders and liquid.

[0049] The internal protective casing 3 comprises an internal volume Vi which may comprise grinding media Mb, for example balls, pellets, among others, visible on the Figures 1 and 2 . It advantageously has a geometry similar to that of the grinding tank 2, being of smaller size so as to fit into the grinding tank 2.

[0050] The inner protective envelope 3 consists mainly of solidified carbon dioxide CO 2 (s), i.e. CO 2 in a compact solid state, with a density at least equal to 60% of the theoretical density. This can be obtained by solidification of liquid carbon dioxide CO 2 (l) by cooling or by compaction of dry ice or by machining.

[0051] It should be noted that in contact with a liquefied gas at temperatures around -200°C at atmospheric pressure, solidified carbon dioxide becomes a relatively hard material since it goes from a hardness around 1.5 on the Mohs scale to a hardness close to 8 on the same scale.

[0052] Furthermore, to increase the attrition phenomena, the internal protective casing 3 comprises attrition pins 6 on its internal wall, which comprise solidified carbon dioxide CO 2 (s), obtained by solidification of liquid carbon dioxide CO 2 (l) by cooling or by compaction of dry ice or by machining. The length of the attrition pins 6 is advantageously of an order of magnitude less than one sixth of the diameter of the grinding and mixing mobile 4.

[0053] In addition, as always visible on the Figures 1 and 2, the grinding and mixing device 1 comprises a grinding and mixing mobile 4 arranged in the internal volume Vi of the internal protective casing 3.

[0054] The grinding and mixing mobile 4 is driven in rotation by means of a motorization system with transmission arm. The grinding and mixing mobile 4 can thus transmit the mechanical power delivered by the motorization to the fluid to be ground in the form of convective movements by circular movement agitation. The motorization can advantageously comprise an electric motor equipped with a speed variator allowing it to be adjusted between a few tens of revolutions per minute, typically of the order of 50 revolutions / min, to a few thousand revolutions per minute, potentially up to 10,000 or even 20,000 revolutions / min.

[0055] The grinding and mixing mobile 4 can be of different types, for example of the turbine type, inclined or non-inclined blades, attrition mobile or propeller. It has a diameter of the grinding and mixing mobile such that the ratio between the diameter of the tank and the diameter of the grinding and mixing mobile is between 0.2 and 0.9.

[0056] Furthermore, the grinding and mixing device 1 comprises permanent magnets 7 located at the level of the internal protective casing 3. These permanent magnets 7 are inserted into the internal protective casing 3, as shown, or alternatively placed between the internal protective casing 3 and the grinding tank 2.

[0057] Furthermore, the grinding and mixing device 1 also comprises drive magnets 8a, 8b, for rotation, arrow R on the Figures 3 and 4, of the internal protective casing 3 by interaction with the permanent magnets 7, located outside the grinding tank 2 opposite the permanent magnets 7.

[0058] In the training mode of the figure 1 , the drive magnets 8a, 8b comprise fixed, non-rotationally driven, alternating drive electromagnets 8a placed outside the grinding tank 2.

[0059] In the training mode of the figure 2 , the drive magnets 8a, 8b comprise movable drive magnets 8b, driven in rotation by means of a drive motor 9.

[0060] The permanent magnets 7 and the drive magnets 8a, 8b are arranged axisymmetrically with respect to the axis of rotation X of the grinding tank 2. The permanent magnets 7 can be arranged by crimping and / or by forcing.

[0061] The grinding and mixing device 1 according to the invention therefore allows both the use of an internal protective casing 3 based on solidified carbon dioxide (CO 2 (s)) but also the rotation of this casing 3 at angular speeds potentially equivalent to those of the grinding and mixing mobile 4.

[0062] The inner protective casing 3 is rotated in the opposite direction to the rotation of the grinding and mixing mobile 4. The rotation speed is less than or equal to the rotation speed of the grinding and mixing mobile 4.

[0063] The rotation of the internal protective casing 3 is ensured by means of the drive motor 9 and the moving magnets 8b or by means of the alternating electromagnets 8a.

[0064] In this respect, the Figures 3 and 4 illustrate the movement of the internal protective casing 3 by means of alternating electromagnets 8a.

[0065] Thus, in order to allow rotation of the internal protective casing 3, the electromagnets 8a are powered from the moment when, at a given time t0 represented on the figure 3 , the distance between the permanent magnet 7 of the internal protective casing 3 and the electromagnet 8a becomes less than the maximum distance d max of magnetic influence, shown in the figure 3 . Then, from the moment, at time t1 represented on the figure 4 , where the two normals of the magnetic surfaces (surface of the permanent magnet 7 and surface of the electromagnet 7) form a limit angle θ limit , visible on the figure 4 , tending towards zero, the power supply to the electromagnet 8a is interrupted.

[0066] Furthermore, the Figures 5, 6 and 7partially represent another example of a grinding and mixing device 1 according to the invention, using drive magnets in the form of electromagnets 8a. The previously described references are not repeated here.

[0067] In this example of realization, as visible on the figure 7 , the permanent magnets 7 can be located between the internal protective casing 3 and the grinding tank 2.

[0068] It should also be noted that, according to another embodiment, it is possible to envisage that the rotation of the internal tank 2 is ensured in such a way that there is a periodic and abrupt change in its direction of rotation. Indeed, this type of modification of the direction of rotation induces impaction phenomena which can strongly promote micronization.

[0069] 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 device (1) for grinding and mixing powders (P), comprising: - a grinding tank (2), comprising the load of powders (P) to be ground, in particular in liquid phase, and grinding medias (Mb), - a protective inner casing (3), arranged inside the grinding tank (2), defining an inner volume (Vi), - a grinding and mixing movable member (4), arranged in the inner volume (Vi) of the protective inner casing (3), characterized in that it comprises: - permanent magnets (7) located at the protective inner casing (3), - drive magnets (8a, 8b), for rotating (R) the protective inner casing (3) by interaction with the permanent magnets (7), located outside the grinding tank (2) facing the permanent magnets (7).

2. The device according to claim 1, characterized in that it is a cryogenic grinding and mixing device, the grinding tank (2) comprising a cryogenic fluid, in particular liquid nitrogen (N2), and being thermally insulated.

3. The device according to claim 1 or 2, characterized in that the protective inner casing (3) consists of or comprises a coating in a material of greater hardness than that of the powders (P) to be ground, in particular under cryogenic temperature conditions, and sublimating at ambient temperature and atmospheric pressure.

4. The device according to claim 3, characterized in that the protective inner casing (3) consists of or comprises a coating of solidified carbon dioxide (CO2(s)).

5. The device according to claim 4, characterized in that the protective inner casing (3) comprises attrition relief elements (6) on its inner wall, in particular attrition pins (6), comprising solidified carbon dioxide (CO2(s)).

6. The device according to any one of the preceding claims, characterized in that the permanent magnets (7) are inserted into the protective inner casing (3) or placed between the protective inner casing (3) and the grinding tank (2).

7. The device according to any one of the preceding claims, characterized in that the drive magnets (8a, 8b) comprise fixed alternating drive electromagnets (8a), without rotational drive, placed outside the grinding tank (2).

8. The device according to any one of the preceding claims, characterized in that the drive magnets (8a, 8b) comprise movable drive magnets (8b), driven in rotation via a drive motor (9).

9. The device according to any one of the preceding claims, characterized in that the permanent magnets (7) and the drive magnets (8a, 8b) are arranged in an axisymmetric manner with respect to the axis of rotation (X) of the grinding tank (2).

10. 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.

11. The grinding and mixing method according to claim 10, 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.

12. The grinding and mixing method according to claim 10 or 11, characterized in that it comprises the step of rotating the protective inner casing (3) by means of movable drive magnets (8b) rotated by means of a drive motor (9).

13. The grinding and mixing method according to claim 10 or 11, characterized in that the rotation of the movable drive magnets (8b) is performed such that the direction of rotation varies alternately.

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