EP4288359A1 - Method and device for transporting powders - Google Patents

Abstract

The invention relates to a method for transporting non-flowable powders (P). This method comprises the following steps: mixing and suspending powders (P) and carbon dioxide in solid form, with introduction of a cryogenic fluid (FC), to give a cryogenic suspension (SC); mobilising the cryogenic suspension (SC) to allow it to be transported; controlling the mobilization of the cryogenic suspension (SC) as a function of one or more parameters linked to the first step of mixing and suspending.

Description

METHOD AND DEVICE FOR TRANSPORTING POWDERS

DESCRIPTION

TECHNICAL FIELD The invention relates to the field of transporting powders, and particularly the field of non-flowable powders which may be of any known type. For example, without being limiting, the powders can be of high density and/or cohesive.

The invention is applicable to any industrial process using powders, particularly non-flowable powders. It refers to a method for transporting powders and an associated device.

PRIOR ART

Conventionally, different ways make it possible to accomplish the function of transporting powders, described below according to four concepts. Firstly, the plane or vibrating corridor systems are composed of plane or pipe sections subjected to vibratory movements inducing an overall component directed in the direction of the desired movement. This type of solution is for example described in the article entitled "Modelling the dynamic behavior of a vibrating floor: interaction with the granular medium", Benoît GELY, Thesis from Sigma Clermont Auvergne University, September 2017. These systems also present several disadvantages. They can induce strong dispersions of powders and segregation. In addition, they are not well suited to the large change in elevation gain.

In addition, pneumatic transport systems are composed of sealed pipes placed under a partial vacuum, or more rarely under overpressure, to allow the drive, by pressure difference inducing a circulation of air direction, towards the point of routing. powder. This type of solution is for example described in the article entitled “Pneumatic handling of bulk products”, Thierry DESTOOP, Engineering techniques, Reference AG7510 v2, October 10, 2013. These systems however, have several drawbacks. On the one hand, they are effective only for powders of minimum flowability. On the other hand, they are effective only for low particle sizes and/or apparent densities. Impacts at pipe bends can cause a change in the granular medium. In addition, they can lead to a risk of clogging and require filtering the vents.

Then, the mechanical conveying systems are composed of mobiles most often subjected to rotary movements to push the granular medium at each periodic movement. These are typically endless screws or Archimedes' screws. Belt systems, or even buckets, are also possible. This type of solution is for example described in the article entitled “Continuous mechanical handling of bulk product”, Claude SAUDEMONT, Engineering techniques, Reference AG7511 vl, July 10, 2002. These systems also have several drawbacks. They can modify the granular medium by locally compacting the powder. They can induce segregation. They operate on straight sections. In addition, in the case of belt or bucket conveyors, the problem is the dispersion of material and the lack of control over the quantity of material precisely debited.

Dredging systems are finally suspension pumping systems, most often aqueous, allowing the transport of the granular medium by means of a suction pump. These systems also have drawbacks. Indeed, they can induce strong liquid entrainment compared to the granular medium to be transferred. In addition, the transport of powders is impossible in the case of powders that are soluble or sensitive to the liquid used. They can also lead to segregation of the granular medium.

It therefore appears that the four types of solutions proposed above in the face of the problem of transporting powders are therefore not entirely satisfactory, if at all for the transport of non-flowable powders.

Specifically, a need remains to accomplish the function of transport, or transfer, of a non-flowable granular medium with particularly the following requirements: quickly, continuously, precisely in terms of distributed flow rate, safe with an absence of dispersion which could induce explosive atmospheres, and energy efficient; without inducing segregation of the granular medium to be transported; without being limited by the topology of the route to be followed (ascent, descent, any change of orientation, etc.); without compaction of the granular medium; without risk of dispersion of fine particles constituting the granular medium to be transported.

EXPOSED

The invention aims to meet at least partially the needs mentioned above and to remedy the drawbacks relating to the embodiments of the prior art.

The invention specifically seeks to be able to convey the granular medium as if it were liquid but without then having to separate the powder from the carrier fluid in a costly and/or long manner. It also seeks not to induce any effluents that are difficult to treat, as well as not to induce pollution of the granular medium and to make it possible to convey all types of powders, and mainly those that cannot be poured, with a particle size ranging from a few nanometers to a few centimeters and variable densities without constraint, namely powders ranging from very sparse to very dense.

The subject of the invention, according to one of its aspects, is a method for transporting non-flowable powders, characterized in that it comprises the following steps: a) mixing and suspending powders and carbon dioxide under solid form, with the introduction of a cryogenic fluid, to obtain a cryogenic suspension, the proportions by density of the powders and of the carbon dioxide verifying the following equation (i):

(i): 40% < [powders] _Voi + [C0 ₂ (s)] _Voi < 80%, where:

[powders] _Voi is the proportion by density of the powders,

[C0 ₂ (s)] _VOi is the proportion by density of carbon dioxide in solid form, b) movement of the cryogenic suspension to allow its transport, c) controlling the movement of the cryogenic suspension as a function of one or more parameters linked to step a) of mixing and suspension.

The method according to the invention may also comprise one or more of the following characteristics taken individually or in any possible technical combination.

The method according to the invention is preferably suitable for powders qualified as “unflowable”. The notion of “flowability” corresponds to the property of a granular medium to flow naturally. It can be characterized by several methods. One of them can come from a measurement of the Carr index type. By definition, this index is determined as the ratio between the difference between the apparent volume occupied by a given quantity of powders and the packed volume of the same quantity of powders, all normalized to the apparent volume. Beyond a Carr index of 25, the granular medium is conventionally considered to be very poorly flowable. Below a Carr index of 15, the granular medium is considered relatively well flowable. Thus, within the meaning of the invention, by non-flowable powders is meant powders whose Carr index is strictly greater than 15, and preferably greater than or equal to 25.

In addition, the conditions of equation (i) of step a) of mixing and setting in motion advantageously make it possible to obtain a stable and pumpable cryogenic suspension. Also, advantageously, the cryogenic suspension is stable and pumpable.

It should be noted that the term "stable" means that a suspension is considered stable when the time required for the complete settling of the suspension is at least ten times greater than the time of the transport operation, or transfer, of that -this. Typically, in the context of the invention, the duration of transport, or transfer, of powders can be of the order of a few minutes while the duration of stability can be of the order of one hour.

Advantageously, the presence of carbon dioxide in solid form in the cryogenic suspension can make it possible to act as a steric stabilizer for the powders in order to prevent their sedimentation. It should also be noted that by "pumpable" is meant the ability of a formulation to be implemented by means of a conventional pumping system, such as a piston or rotor pump. It should however be noted that a suspension characterized as "pumpable" is not necessarily intended to be pumped but is capable of being so if necessary. This notion of "pumpable" appears for example in the presentation entitled "Formulation, homogeneity and pumpability", François DE LARRARD, BétonlabPro 3, Lesson N°13, Laboratoire Central des Ponts et Chaussées - Center de Nantes (LCPC). More intrinsically, a suspension is considered to be "pumpable" insofar as the driving force accessible by conventional pumping systems (notably piston or rotor pump) to allow its movement in a given circuit is greater than the driving force. braking induced by the viscosity of the suspension. Conventionally, a suspension having a viscosity of the order of 100,000 mPa.s is considered to be non-pumpable. A suspension having a viscosity of less than 20,000 mPa.s is considered to be pumpable.

Advantageously, the cryogenic fluid is a gas liquefied at ambient temperature and pressure. It may in particular be liquid nitrogen (N2). However, this choice is not limiting. The cryogenic fluid makes it possible to define the fluidic behavior, in particular liquid, of the cryogenic suspension and makes it possible to maintain the carbon dioxide (CO2) in solid form.

In addition, solid carbon dioxide, also called dry ice, can be in the form of granules and/or powders. This dry ice makes it possible, by its size or occupation rate in the cryogenic suspension, to stabilize the powders to be transported.

The method may comprise step a') of weighing the powders to be transported and step a'') of weighing carbon dioxide in solid form, steps a') and a'') being prior to step a) mixing and suspending.

Furthermore, step b) of setting the cryogenic suspension in motion can comprise placing the cryogenic suspension under vacuum, pumping or pressurizing. Step c) of controlling the setting in motion of the cryogenic suspension can comprise step c′) of acquisition and processing of the measurement of the stirring torque of the cryogenic suspension.

In addition, step c) of controlling the setting in motion of the cryogenic suspension can comprise step c′′) of measuring the pressure and/or opening of the pump carried out during step b) of setting in motion of the cryogenic suspension.

Also, step b) of setting the cryogenic suspension in motion can be followed by a step d) of transport, in particular followed by a step e) of phase separation for obtaining transported powders and setting implementation of a step f) for recycling the cryogenic fluid.

In addition, the mean diameter of the particle size of the carbon dioxide in solid form can be between 0.1 and 10 times that of the particle size of the powders to be transported.

It should be noted that the notion of “mean diameter” of a granular medium is used insofar as the granular medium considered is not made up of solid particles all having the same size and not generally strictly spherical. In this case, the particle size is a distribution of size, surface area, or even equivalent volume. With this static distribution, it is possible to associate a concept of average dimension also called “average diameter”. Such a notion is for example described in the article "Characterization of particle size", John DODDS, Gérard BALUAIS, Geological Sciences, bulletins and memories, 46-1-4 pages 79-104, 1993.

The carbon dioxide loading rate in solid form can be between 0.1 and 10 times that of the powders to be transported.

Furthermore, the viscosity of the cryogenic suspension as a function of the loading rate and the particle size of the constituent solid of this cryogenic suspension can be expressed by the following equation (ii):

(ii): m/m ₀ = (1 + ½ ·[N] ·F/(1-FLM ² , where: m is the viscosity of the cryogenic suspension; mo is the viscosity of the liquid phase; [N] is a constant;

F is the volume of solid in the volume of the cryogenic suspension;

O _m is the maximum volume of solid in the volume of the cryogenic suspension.

Another subject of the invention, according to another of its aspects, is a device for transporting non-flowable powders, for implementing the method as defined above, characterized in that it comprises:

- a system for mixing and suspending powders, carbon dioxide in solid form and cryogenic fluid to form the cryogenic suspension,

- a system for setting the cryogenic suspension in motion,

- a control system for setting the cryogenic suspension in motion.

The mixing and suspension system may include:

- a mixing tank,

- a mixing and stirring device, located inside the mixing tank,

- means for the controlled introduction of the powders to be transported and carbon dioxide in solid form into the mixing tank,

- a means for measuring the level of the cryogenic suspension formed, at least partly located inside the mixing tank.

The system for setting the cryogenic suspension in motion can also comprise a pressure difference transport device.

In addition, the transport device by pressure difference can comprise means for pressurizing, pumping or depressurizing the cryogenic suspension.

DESCRIPTION OF FIGURES

The invention can be better understood with the help of the detailed description of non-limiting examples of implementation and the examination of the figures, schematic and partial, on which: - Figure 1 shows a simplified flowchart of the control of a method according to the invention,

- Figure 2 shows a block diagram illustrating an example of a method for transporting non-flowable powders according to the invention,

- Figures 3 to 5 schematically illustrate three separate examples of devices for transporting non-flowable powders in accordance with the invention,

- Figure 6 graphically illustrates the evolution of the agitation torque of the cryogenic suspension as a function of the agitation time and three introductions of solid charge, and

- Figures 7, 8 and 9 represent respectively the evolution of the viscosity as a function of the shear rate for suspensions of alumina and dry ice in liquid nitrogen, the evolution of the viscosity as a function of the shear rate for different concentrations of dry ice suspensions in liquid nitrogen, and the evolution of the volume according to the size of the particles of alumina powder that can be used to make the cryogenic suspension.

In these figures, identical references can 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.

STATEMENT OF EMBODIMENTS

The cryogenic fluid FC is considered here to be liquefied nitrogen (N2) but this choice is not limiting.

FIG. 1 is a simplified flowchart of the piloting necessary for the proper conduct of a method in accordance with the invention. It makes it possible to specify the sequence of the measurements and the input and output data necessary for the control.

There is shown in Figure 2 a block diagram illustrating an example of a method for transporting non-flowable powders P according to the invention. This method thus comprises a step a′) of weighing the powders P to be transported and a step a″ of weighing the carbon dioxide in solid form C02(s). The references BR denote feedback loops.

Then, the method comprises a step a) of mixing and suspending the powders P and the carbon dioxide in solid form CÜ2(s), with the introduction of the cryogenic fluid FC, to obtain the cryogenic suspension SC .

Once the cryogenic suspension SC has been formed, a step b) is implemented to set the cryogenic suspension SC in motion to allow it to be transported, either by placing it under vacuum, or by pumping, or by pressurizing the cryogenic suspension SC .

Then, a step d) allows the transport or transfer of the suspension before a phase separation step e) allowing the transported or transferred powders Pt to be obtained. A recycling of liquid nitrogen can be provided during a step f ).

During the transport process, a step c) of controlling the setting in motion of the cryogenic suspension SC is implemented. Thus, a step c′) of acquiring and processing the measurement of the stirring torque Co of the cryogenic suspension SC makes it possible to optimize the mixing of the suspension.

In addition, a step c") of pressure measurement and/or pump opening is carried out during step b) of setting in motion. A step c'") also makes it possible to measure the flow rate during the 'step d) of transport and a step e') makes it possible to measure the temperature during step e) of phase separation.

In addition, FIGS. 3 to 5 make it possible to illustrate three examples of devices 30 for transporting non-flowable powders P in accordance with the invention.

In these three embodiments, each device 30 firstly comprises a system 40 for mixing and suspending powders P, carbon dioxide in solid form CÜ2(s) and cryogenic fluid FC to form the cryogenic suspension SC.

The mixing and suspending system 40 may in particular include at least some of the elements of the devices described in French patent applications FR 3 042 985 A1 and FR 3 042986 A1. This system 40 comprises a mixing tank 41. The mixing tank 41 is heat-insulated, thermally insulated, to allow the liquefied gas to be kept in the form of liquid nitrogen without excessive volatization. Ideally, heat losses would be around 2% per day or even less.

In addition, the system 40 comprises a mixing and stirring device 42, located inside the mixing tank 41. This mixing and stirring device 42 may in particular be a stirring wheel, for example of the blade type. , propeller, turbine, anchor, attritor or others, chosen in particular according to the viscosity of the cryogenic suspension SC envisaged. The mixing and stirring device 42 is driven in rotation to generate the agitation by means of a drive motor 45. This motor 45 incorporates a means for measuring the torque Co of the cryogenic suspension SC in order to identify whether the suspension is homogeneous and the loading rate adapted.

The system 40 also comprises means 43a, 43b for the controlled introduction of the powders P to be transported and carbon dioxide in solid form CÜ(s) into the mixing tank 41. This is in particular a first hopper d feed 43a for the introduction of the powders P to be transported and a second feed hopper 43b for the introduction of carbon dioxide in solid form C0 ₂ (s). The controlled introduction is done by weighing or dosing. To do this, the feed hoppers 43a, 43b are used in connection with respectively weighing systems 46a, 46b corresponding to suspended scales or load cells. It is thus possible to follow the mass introduced as a function of time.

Depending on the specificity of the granular medium to be transported, namely the cryogenic suspension, in particular according to its particle size and its density, the proportions of powders P, carbon dioxide in solid form CÜ (s) and liquid nitrogen can vary. However, in order to obtain a cryogenic suspension SC which is stable and pumpable within the meaning of the definitions given above, the proportions by density of the powders P and of the carbon dioxide CÜ (s) satisfy the following equation (i):

(i): 40% < [powders] _Voi + [C0 ₂ (s)] _Voi < 80%, where: [powders] _voi is the proportion by density of the powders P,

[C0 ₂ (s)] _VOi is the proportion by density of carbon dioxide in solid form C0 (s).

To obtain a cryogenic suspension SC which is pumpable and stable within the meaning of the invention, the major parameters to be determined and/or monitored are:

- the load rate of the powders P, namely the volume of solid over the total volume of the suspension SC: it will be advantageous to seek to increase this rate to the highest possible value to optimize the quantity of powders P transported for a volume of cryogenic suspension SC moved given;

- the charge rate of carbon dioxide in solid form CÜ2(s): this rate is conventionally a function of the quantity of powders P to be introduced into the cryogenic suspension SC;

- the density of the powders P to be transported: in general, the denser the powders P and the larger the particle size, the more one will seek to constitute viscous suspensions incorporating large quantities of carbon dioxide C0 (s) to limit the risks of settling powders P to be transported within the cryogenic suspension SC;

- the particle size of the carbon dioxide in solid form CÜ2(s), given in particular by the mean diameter of the particle size distribution of the carbon dioxide available to formulate the cryogenic suspension SC;

- the particle size of the powders P given in particular by the mean diameter of the particle size distribution of the granular medium to be transported.

Also, advantageously:

- the particle size of the carbon dioxide in solid form CÜ2(s) is linked to that of the powders P to be transported: more precisely, it is substantially of the same order of magnitude, the average diameter being approximately between 0.1 and 10 times the particle size powders P to be transported;

- the carbon dioxide charge rate in solid form CÜ2(s) is linked to that of the powders P to be transported: more precisely, it is substantially of the same order magnitude, the value being between approximately 0.1 and 10 times the content of powders to be transported;

- the liquid nitrogen content is as limited as possible: it is advantageously less than 70% by volume; this liquid nitrogen content must nevertheless make it possible to make the suspension flowable and cannot be less than 5% by volume;

- the particle size of the solid phase, including the powders P to be transported and the carbon dioxide in solid form CÜ2(s), is less than 10 times the diameter of the transport piping, otherwise segregation could occur and lead to loss the integrity of the granular medium to be transported.

These requirements can advantageously make it possible to optimize the transport of the powders P for a given volume of cryogenic suspension SC while guaranteeing the implementation of the suspension by pumping, pressurizing or placing under vacuum. This can thus result in a limited viscosity, of the order of 100,000 mPa.s, and the suspension formulation adapted to the transport piping.

The particle size of the dry ice can advantageously be between 500 and 900 μm.

The system 40 finally comprises a means 44 for measuring the level of the cryogenic suspension SC formed, at least partly located inside the mixing tank 41. More particularly, this measuring means 44 can take the form of a bubble cane or an ultrasonic probe.

The system 50 for setting the cryogenic suspension SC in motion also makes it possible to convey the latter towards the expected point of arrival. This system 50 comprises a transport device by pressure difference 51.

For each embodiment of Figures 3 to 5, the transport device by pressure difference 51 is different. These may be means for pressurizing, pumping or depressurizing the cryogenic suspension SC.

In FIG. 3, the transport device by pressure difference 51 corresponds to a depressurization avoiding the use of a cryogenic pump. In this figure 3, the reference Te designates the temperature. It thus comprises a pump 52 for creating a vacuum. This is a vacuum pump for placing a transport tank 58 in depression, in which the cryogenic suspension SC must be collected. This mode of transportation of the suspension makes it possible to avoid the use of a cryogenic circulation pump, which is potentially complex and costly and cannot convey objects whose average particle size would be greater than a few millimeters. Furthermore, this configuration allows the recirculation of liquid nitrogen.

The transfer of the cryogenic suspension SC to the transport tank 58 takes place by means of a heat-insulated pipe 53 which allows the transport of the formulated suspension while limiting heat losses to the transport arrival point.

The transport tank 58 is similar to a degasser. This is an enclosure equipped with a thermostatically controlled heating system and a valve and pressure control system, symbolized by the reference Pr in figure 3.

In addition, at the outlet of the pump 52, a media filter 54 adapted to the particle size of the granular medium to be transported is present, upstream of the vents Ev. This filter 54 can for example be a paper/glass fiber or ceramic candle filter.

In this figure 3, just as for figures 4 and 5 described next, a first tank 71 for supplying liquid nitrogen, insulated, and a second tank 72 for supplying nitrogen in the form of compressed gas are present. Similarly, for the three embodiments of Figures 3, 4, and 5, a mass flow meter 74 is present at the level of the mixing tank 41, for example of the Coriolis or ultrasonic type.

A recycling pot 73 is finally provided which forms a buffer tank used for the recirculation of liquid nitrogen.

FIG. 4 represents a transport device by pressure difference 51 which corresponds to pressurization of the mixing tank 41 containing the cryogenic suspension SC. Also, in this example, the mixing tank 41 includes means for measuring pressure MP. In this configuration, another mass flowmeter 59, for example of the Coriolis or ultrasonic type, is used. In addition, a valve 75 for supplying nitrogen in the form of gas is provided for the pressurization.

FIG. 5 represents an example in which the transport device by pressure difference 51 corresponds to pumping by a circulation pump 57 for drawing off the cryogenic suspension SC.

The circulation pump 57 is a suction pump. It can for example be a piston or a rotor (peristaltic), in particular with a flexible junction of the polytetrafluoroethylene (PTFE) type. The pump 57 is then cryogenic and made up of internals making it possible to mechanically resist the temperature of the suspension, generally close to -196° C., and allowing the transfer of solid matter constituting the suspension. The particle size of the cryogenic suspension SC cannot then conventionally be greater than a few millimeters.

A three-way valve 55, provided for adjusting the opening of the bypass loop, and another valve 56, provided for opening and closing the bypass, are also present in this configuration of transport device 51.

Furthermore, a control system 60 for setting the cryogenic suspension SC in motion is provided. This control system 60 allows in particular the setting in motion of the cryogenic suspension SC according to at least one parameter linked to the mixing and suspension system 40, in particular the couple Co. The control system 60 makes it possible to compiling all of the measurements taken on the device 30 for transporting the powders P, and allows the control actions or feedbacks on the controllable components, such as valves, pump, stirring motor, etc.

The control system 60 thus integrates the acquisition and processing of several data:

- the measurement of the quantity of materials of the dry ice MC02 and of the powders Mpoudre, as well as of the cryogenic fluid FC, so as to evaluate the volume and/or mass contents of the constituents of the cryogenic suspension SC; - measuring the level of the mixing tank 41 to avoid its clogging and to evaluate the density of the cryogenic suspension SC formed;

- the measurement of the agitation couple Co making it possible to evaluate the viscosity of the cryogenic suspension SC and making it possible to check that the latter is homogeneous and sufficiently agitated.

Concerning the agitation torque parameter Co, FIG. 6 graphically represents the value of the agitation torque Co as a function of time t. The references A0, Al, A2 and A3 correspond respectively to the no-load torque, to a first addition of solid load, to a second addition of solid load and to a third addition of solid load. Thus, with each introduction of material Al, A2 and A3, the torque Co increases for a given stirring speed. However, after a certain period of agitation, the couple Co tends to stabilize as shown by the stages in FIG. 6. This then makes it possible to possibly introduce an additional quantity of powders P into the cryogenic suspension SC if the flow setpoint of transport imposes it, for example.

Depending on the configuration used for the system 50 for setting the cryogenic suspension SC in motion, namely those described in the three embodiments of FIGS. 3, 4 and 5, the control system 60 also integrates the acquisition and processing of data described below.

In the case of a configuration of the type described with reference to FIG. 3, namely transport by transport tank 58 placed under vacuum, the control system 60 allows the measurement of the pressure in the transport tank 58 and measuring the temperature in line with the transport tank 58 to monitor the volatization of the liquefied gas except in the case of recycling of this liquefied gas. The separation can take place by simple difference in density and withdrawal taking into account the fact that the liquid nitrogen has a density generally lower than the granular medium to be transported, of the order of 0.8.

In the case of a configuration of the type described with reference to FIG. 4, namely with pressurization of the mixing tank 41 containing the cryogenic suspension SC, the control system 60 allows the pressure measurement to adjust the transport rate of the suspension measured by mass flow meter, for example of the Coriolis or ultrasonic type. In the case of a configuration of the type described with reference to FIG. 5, namely that by circulation pump 57 for drawing off the cryogenic suspension SC, the control system 60 allows the measurement of the opening of the valves 55 and 56 to adjust the withdrawal rate of the SC cryogenic suspension.

With reference to FIGS. 7 and 8, the rheological behavior of several cryogenic suspensions SC that can be envisaged within the scope of the invention is described. In addition, Figure 9 illustrates the particle size distribution of the alumina powder used for these SC cryogenic suspensions.

The particle size of the dry ice used is advantageously derived from a particle size cut made by sieving between 500 and 900 μm.

Specifically, figure 7 represents the evolution of the viscosity v, expressed in mPa.s, as a function of the shear rate te, expressed in s ¹ , for suspensions of alumina (Al2O3), dry ice in liquid nitrogen .

FIG. 8 represents the evolution of the viscosity v, expressed in mPa.s, as a function of the shear rate te, expressed in s ¹ , for different concentrations of suspensions of dry ice in liquid nitrogen.

Thus, FIGS. 7 and 8 make it possible to illustrate the viscosities of cryogenic suspensions and to show the influence of the dry ice content on the viscosity of the fluid to be transferred. FIG. 9 represents the evolution of the volume V, expressed in %, as a function of the size S, expressed in μm, of the particles of alumina powder which can be used to produce the cryogenic suspension SC.

Generally, the rheological behavior of SC cryogenic suspensions can be approximated by semi-empirical laws. By way of example, an expression of the viscosity of the suspension as a function of the loading rate and the particle size of the solid constituting this suspension can be given below by equation (ii):

(ii): p/po = (1 + ½ ·[N] ·F/(1-FL ² , where: p is the viscosity of the suspension; po is the viscosity of the liquid phase;

[N] is a constant; F is the volume of solid in the volume of the suspension;

O _m is the maximum volume of solid in the volume of the suspension.

Knowing the viscosity of the suspensions, it is then possible to deduce the transport flow rate Qy possible according to the pumping performance of the system implemented, given via the pressure difference between the upstream and downstream of the system. pumping rated DR. We can thus obtain the equation (iii) given below:

(iii): DR = (8 p L) -QV/(TÎ-R ⁴ ),

DR is the pressure difference between upstream and downstream of the pumping system; m is the viscosity of the suspension;

Q.v is the volume flow;

R is the radius of the fluid transport pipe;

L is the length of the transmission line.

It is possible to estimate below, in the case of some suspensions formulated for the invention, the performance of the driving force system to be implemented to induce efficient transport of the alumina powders as illustrated by the figure 9. The following table A gives some values obtained.

Table A

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

Claims

1. Method for transporting non-flowable powders (P), characterized in that it comprises the following steps: a) mixing and suspending powders (P) and carbon dioxide in solid form (CÜ2(s)) , with the introduction of a cryogenic fluid (FC), to obtain a cryogenic suspension (SC), the proportions by density of the powders (P) and of the carbon dioxide (CÜ2(s)) satisfying the equation (i) following: (i): 40% < [powders] _VOi + [C0 ₂ (s)] _VOi < 80%, where: [powders] _VOi is the proportion by density of the powders (P), [C0 ₂ (s)] _VOi is the proportion by density of carbon dioxide in solid form (C0 ₂ (s)), b) movement of the cryogenic suspension (SC) to allow its transport, c) control of setting the cryogenic suspension (SC) in motion as a function of one or more parameters (Co) linked to stage a) of mixing and suspension. 2. Method according to claim 1, characterized in that it comprises step a′) of weighing the powders (P) to be transported and step a″ of weighing the carbon dioxide in solid form (CÜ2(s )), steps a′) and a″ being prior to step a) of mixing and suspending. 3. Method according to claim 1 or 2, characterized in that step b) of setting the cryogenic suspension (SC) in motion comprises placing under vacuum, pumping or pressurizing the cryogenic suspension (SC) . 4. Method according to one of the preceding claims, characterized in that step c) of controlling the setting in motion of the cryogenic suspension (SC) comprises step c′) of acquiring and processing the measurement of the stirring torque (Co) of the cryogenic suspension (SC). 5. Method according to any one of the preceding claims, characterized in that step c) of controlling the setting in motion of the cryogenic suspension (SC) comprises step c'') of measuring pressure and/or pump opening performed during step b) of setting the cryogenic suspension (SC) in motion. 6. Method according to any one of the preceding claims, characterized in that step b) of setting the cryogenic suspension (SC) in motion is followed by a step d) of transport, in particular followed by a step e ) of phase separation for obtaining transported powders (Pt) and the implementation of a stage f) of recycling the cryogenic fluid (CF). 7. Method according to any one of the preceding claims, characterized in that the average diameter of the particle size of the carbon dioxide in solid form (CÜ2 (s)) is between 0.1 and 10 times that of the particle size of the powders (P) to carry. 8. Method according to any one of the preceding claims, characterized in that the charge rate of carbon dioxide in solid form (CÜ2 (s)) is between 0.1 and 10 times that of powders (P) to be transported . 9. Method according to any one of the preceding claims, characterized in that the viscosity (m) of the cryogenic suspension (SC) as a function of the loading rate and the particle size of the solid constituting this cryogenic suspension (SC) is expressed by the following equation (ii): (ii): m/m ₀ = (1 + ½ [N] F/(1-FLM ² , where: m is the viscosity of the cryogenic suspension (SC); mo is the viscosity of the liquid phase; [N] is a constant; F is the volume of solid in the volume of the cryogenic suspension (SC); O _m is the maximum volume of solid in the volume of the cryogenic suspension (SC). 10. Device (30) for transporting non-flowable powders (P), for implementing the method for transporting according to any one of the preceding claims, comprising: - a system (40) for mixing and suspending powders (P), carbon dioxide in solid form (CÜ2(s)) and cryogenic fluid (FC) to form the cryogenic suspension (SC), - a system (50) for setting the cryogenic suspension (SC) in motion, - a control system (60) for setting the cryogenic suspension (SC) in motion, characterized in that the mixing and suspension system (40) comprises: - a mixing tank (41), - a mixing and stirring device (42), located inside the mixing tank (41), - means for the controlled introduction (43a, 43b) of the powders (P) to be transported and of carbon dioxide in solid form (CÜ2(s)) into the mixing tank (41), - a means of measuring (44) the level of the cryogenic suspension (SC) formed, at least partly located inside the mixing tank (41). 11. Device according to claim 10, characterized in that the system (50) for setting the cryogenic suspension (SC) in motion comprises a transport device by pressure difference (51). 12. Device according to claim 11, characterized in that the transport device by pressure difference (51) comprises means for pressurizing, pumping or depressurizing the cryogenic suspension (SC).