EP0214212B1 - Method and device for the dispersion of ultra-fine powders - Google Patents

Abstract

Method and device for setting ultrafine powders in suspension, particularly with a view to the dispersion thereof in the atmosphere. According to the method of the invention, a gas is passed in a space delimited by the free surface of the powder (9) in order to erode said surface, and the shape and dimensions of said space are maintained approximately constant by means of a piston (4) which is displaced under the action of a gas maintained permanently under pressure. The method is applicable to the dispersion of previously compacted powders in the form of blocks.

Description

The present invention relates to a method and a device for suspending in a gas ultra-fine powders, that is to say particles whose size is less than a micrometer, in particular for dispersing these powders in the atmosphere.

Numerous types of apparatus are currently known which make it possible to disperse particles of a few tens of micrometers, they are for example of the type of powder fire extinguishers which are generally based on a process for pneumatic fluidification of the product within a reservoir of storage during implementation. However, the finest powders composed of particles whose size is close to the micrometer are often difficult to fluidize due to the cohesive properties of the particles, which prevent their flow by gravity.

During the use of such powders, which will be described hereinafter as ultra-fine, it has already been proposed to use vibrations and / or shocks, for example in vibrating hoppers, to facilitate their fluidization. These solutions result in complicated, heavy and costly mechanisms, moreover they are ill suited to the case where, to facilitate handling, the powder is in the form of blocks compacted beforehand.

US-A-3,854,634, in the name of Robert J. Hart, describes a device for suspending a powder, and which comprises a container, in which the powder is placed, and a plate which can slide in the container and rest on the free surface of the powder. Through a telescopic tube, a gas under pressure is blown through the plate, which lifts it above the powder. The gas escapes laterally along the upper surface of the powder and entraining part of it. The plate descends as a result of the departure of the powder, since the interval between the free surface of the powder and the underside of the plate remains constant. The extraction rate of the powder can be controlled by sending part of the gas from the telescopic tube directly to the upper face of the plate, thus reducing the pressure difference between the two faces of the plate which reduces the interval between the plate and the powder.

This simple device has a drawback which results from the fact that the main force which makes the plate move is the weight of the latter. The orientation of the device must be such that the plate descends substantially vertically. On the other hand, the device must be protected from strong tremors and accelerations, which obviously would disturb its operation: an acceleration upwards will cause a displacement of the plate in the same direction, and a stop of the suspension of the powder. ; similarly, significant downward acceleration can operate the machine as a compactor. As a result, in particular, the apparatus of this patent cannot operate on board a land or air vehicle.

Document CA-A-874 283 describes a device similar to the previous one, but in which the direction of gas flow is reversed so that a pressure difference, due to the pressure drop in the gas, creates a force which adds to the weight of the plate.

Document US-A-4,023,778 describes a device for suspending a chemical in compacted form in water, this device comprising a reservoir in which the product to be dispersed is enclosed between a movable piston and a antagonistic part which has a concave shape towards the inside of the container, for example frustoconical, and comprises an outlet nozzle, a conduit for bringing the water under pressure to the face of the piston opposite to the mass of the product, under suitable pressure moving the piston and the mass of product towards the opposing part, and conduits for making the water follow a path comprising an inlet passage in the reservoir, the outlet nozzle, and an intermediate space in which the water moves along the free surface of the product to erode it and entrain it towards the nozzle. This intermediate space has an almost constant shape and dimensions thanks to the fact that, as in the devices mentioned above, there is a relative displacement of the mass of product and the opposing part while the product is gradually being eroded.

The object of the invention is to provide a process analogous to that which employs this device, and which can be used for the suspension of ultrafine powders in a gas, independently of the conditions of orientation, vibrations and external accelerations. .

• - on enferme le produit à disperser dans un réservoir cylindrique ou prismatique entre un piston mobile axialement et une pièce antagoniste, cette pièce comportant une buse axiale de sortie,
• - on exerce sur la face du piston, opposée à la masse du produit, une pression de fluide apte à déplacer ledit piston et la masse du produit en direction de la pièce antagoniste,
• - on fait passer le fluide de mise en suspension le long d'un trajet comprenant un passage d'entrée, la buse de sortie et un espace intermédiaire dans lequel le fluide se déplace du passage d'entrée vers la buse de sortie en longeant la surface libre du produit pour l'éroder et entraîner ledit produit, cet espace intermédiaire ayant une forme et des dimensions sensiblement constantes grâce au fait que la surface de la masse du produit se déplace par rapport à la pièce antagoniste pour maintenir une distance à peu près constante entre la surface libre de ladite masse et la surface de la pièce antagoniste pendant que le produit est progressivement entraîné vers le passage de sortie grâce à l'action du piston, caractérisé en ce que:
• - le produit est constitué d'une poudre ultra-fine compactée et le fluide de mise en suspension est un gaz,
• - la surface de la pièce antagoniste est dans l'ensemble perpendiculaire à l'axe du réservoir et à peu près parallèle à la surface libre de la poudre,
• - la pression de fluide exercée sur le piston est adaptée en fonction de l'orientation et des accélérations auxquelles est soumis le réservoir, de façon à maintenir une distance à peu près constante entre la surface libre de la poudre et la surface antagoniste,
• - on maintient le débit du gaz qui vient lécher et éroder la masse de poudre à une valeur supérieure à une valeur plancher telle qu'il n'apparaît pas de chemins préférentiels dans cette masse entraînant une baisse importante du débit de poudre.

To obtain this result, the invention provides a method for suspending a product in a fluid, in which:

• the product to be dispersed is enclosed in a cylindrical or prismatic tank between an axially movable piston and an opposing part, this part comprising an axial outlet nozzle,
• - a fluid pressure capable of moving said piston and the mass of the product in the direction of the opposing part is exerted on the face of the piston, opposite the mass of the product;
• - the suspending fluid is passed along a path comprising an inlet passage, the outlet nozzle and an intermediate space in which the fluid moves from the inlet passage to the outlet nozzle along the free surface of the product to erode and entrain said product, this intermediate space having a substantially constant shape and dimensions thanks to the fact that the surface of the mass of the product moves relative to the opposing part to maintain a distance approximately constant between the free surface of said mass and the surface of the opposing part while the product is gradually being drawn towards the outlet passage thanks to the action of the piston, characterized in that:
• - the product consists of an ultra-fine compacted powder and the suspending fluid is a gas,
• - the surface of the opposing part is generally perpendicular to the axis of the reservoir and approximately parallel to the free surface of the powder,
• the fluid pressure exerted on the piston is adapted according to the orientation and the accelerations to which the reservoir is subjected, so as to maintain an approximately constant distance between the free surface of the powder and the opposing surface,
• - Maintaining the flow of gas which comes to lick and erode the mass of powder to a value greater than a floor value such that there do not appear preferential paths in this mass resulting in a significant drop in the powder flow.

The fact that the force which moves the piston results from a fluid pressure has the consequence that the process can be carried out for any orientation of the tank, and whatever the acceleration to which it is subjected.

Such a result could, in theory, be obtained with mechanical drive of the piston, but it would then be very difficult to obtain regular suspension of the powder. Indeed, any variation in the pressure in the intermediate space of the gas intended to entrain the powder must cause a variation in the speed of displacement of the piston. Such regulation would require, in the case of mechanical drive, a complicated and costly installation, while with the arrangement of the invention, there is self-regulation, since an increase in the pressure in the intermediate space automatically results in a decrease in the force acting on the piston, this force resulting from the pressure difference between the two faces of the piston.

According to a particularly simple embodiment, a reservoir is used, in which the opposing surface is fixed, the powder to be dispersed is placed against one face of the piston, the free surface of the powder being opposite the piston and screws. with respect to said antagonistic surface and a pressure is exerted on the face of the piston which is opposite to the powder, which differs from that which is applied to the free surface by the amount necessary to effect said displacement of the powder with respect to on the antagonistic surface.

This embodiment corresponds to a device which has only one moving part, namely the piston.

Other features and advantages of the invention will appear on reading the description which follows, and which relates to a practical example of embodiment, illustrated with the aid of the figure which represents a schematic longitudinal section view of the device. suspension according to the present invention in a first embodiment.

As can be seen in the figure, the suspension device according to the invention, according to a first embodiment, comprises a cylindrical body 1 closed at one end by a cover 21 provided with a first intake manifold 2 which is connected to a source of compressed air 22, as well as a second intake manifold 3 which is connected to a second source of compressed air 23. It should be noted that a single source may alternatively be provided compressed air 22 or 23, connected to the two pipes 2 and 3 by a pipe 24, shown in dashes.

A piston 4 is slidably mounted inside the cylinder 1 on the side of the axial tube 2. It has the shape of a cylindrical cup open towards this tube, while O-rings 5 and 6 are interposed between the side wall of cylinder 4 and the inside of cylinder 1.

On the side of the second intake manifold 3, is mounted an axial nozzle 7 which is intended for the evacuation of the dispersed product and which passes through the planar front wall of the cylinder 1. Inside the latter, the nozzle 7 is extends outwards into a disc 8 which is perpendicular to it and the periphery of which is located a short distance from the internal face of the cylinder 1.

The compacted pulverulent product 9 to be dispersed being housed between the piston 4 and the disc 8, the compressed air which enters through the tube 3 in the cylinder 1 flows between the internal face of the cylindrical wall of the latter and the periphery of the disc 8 and, along the path indicated by the arrows F, it comes to lick and erode the free surface of the compacted powder bread 9. This air thus charged with extremely fine powder particles passes through the nozzle 7 and disperses in the atmosphere.

Simultaneously, the compressed air introduced into the cylinder 1 through the tubing 2 pushes the piston 4 in the direction of the arrow F ', which displaces the compacted powder roll, as the powder is evacuated. disperse, towards the outlet nozzle 7.

The operating mode of the device which has just been described is obvious and will not be discussed in more detail. Note, however, that several methods of recharging the tank 1 can be envisaged. Thus, the device can consume the product 9 in the form of bread of powdered products previously compacted and introduced by removing the cover 21 from the storage tank 1, this recharging can be carried out by a single operator in a few seconds.

It will also be noted that during a distribution sequence, the speed of erosion of the powder 9 by the entraining gas must be greater than a floor value, the pressure drop in the peripheral passages having to be adjusted as a function the flow rate and the nature of the product to be dispersed. For this, the circulation of the powder and of the gas inside the reservoir 1 and around the discharge orifice 7 must be such that the speed of erosion of the powder remains permanently above this floor value. Indeed, if this condition is not satisfied, preferential paths appear in the mass of the product and, in order to avoid that the major part of the gas under driving pressure does not flow freely by these paths, which would entail a significant drop in the product flow, it is necessary to permanently destroy these paths and at the same time destroy any agglomerates of particles, in particular by shearing and erosion.

In practice, the distance between the opposing surface formed by the disc 8 or by the front face of the piston 4 and the free surface of the powder is of the order of 0.2 to 0.04 mm, and the speed of the gas. charged with powder measured in the discharge orifice 7, is between 35 and 300 m / s. The powder flow rate itself can vary to a very large extent by adjusting the pressure and / or adjusting the discharge means. It is thus possible to vary this flow rate between a few tens of grams per second and several kilograms per second.

The suspension formed can then be dispersed in the atmosphere at a high speed, for example 100 to 340 m / s, for air at 20 ° C.

Claims (1)

1. Process for setting a product in suspension in a fluid, wherein:

   - the product (9) to be dispersed is enclosed in a cylindrical or prismatic reservoir (1) between an axially movable piston (4) and an opposing piece (8), the latter piece comprising an axial exit nozzle (7),

   - upon the face of the piston (4) opposite to the mass of the product (19) there is exerted a fluid pressure adapted to displace the said piston and the mass of the product (9) in the direction of the opposing piece (8),

   - the suspension fluid is passed along a path comprising an entry passage (3), the exit nozzle (7) and an intermediate space in which the fluid travels from the entry passage (3) towards the exit nozzle (7) keeping along the free surface of the product in order to erode it and entrain the said product, this intermediate space having substantially constant form and dimensions by virtue of the fact that the surface of the mass of the product is displaced in relation to the opposing piece (8) to maintain an approximately constant distance between the free surface of the said mass and the surface of the opposing piece while the product is entrained progressively towards the exit passage thanks to the action of the piston (4), characterised in that

   - the product is constituted by a compacted ultra-fine powder and the suspension fluid is a gas,

   - the surface of the opposing piece (8) is as a whole perpendicular to the axis of the reservoir (1) and approximately parallel to the free surface of the powder,

   - the fluid pressure exerted upon the piston 14) is adapted as a function of the orientation and the accelerations to which the reservoir (1) is subjected, in such manner as to maintain an approximately constant distance between the free surface of the powder and the opposing surface (8),

   - the flow rate of the gas, which comes to wash and erode the mass of powder, is maintained at a value higher than a floor value such that no preferential paths appear in this mass causing a significant drop in the powder flow rate.

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