EP0009433A1 - Process and apparatus for producing metallic powder starting from a molten metal or alloy - Google Patents

Abstract

A system for producing metal powder in which a cryogenic fluid in the liquid phase is poured over a metal bath having a vapor pressure of at least 1 mm Hg, and the solid particles suspended in the fluid are separated therefrom and collected. The particles find application in the manufacture of paints, in the treatment of rubber, and in the metallurgical, chemical pharmaceutical and ceramic industries.

Description

The present invention essentially relates to a process for manufacturing a metallic powder by lowering the temperature of the vapor of a molten metallic material in a closed treatment enclosure, leading to the transformation of said vapor into solid particles.

The term "metallic material" designates either a metal proper, or an alloy of at least two metals.

By "metallic powders" is meant powders which consist of solid particles either of a single metal such as iron, zinc, magnesium, calcium, cadmium, etc., or of an alloy metallic, for example a magnesium-zinc alloy, or even a metallic compound, for example zinc oxide or magnesium nitride. Such powders find wide applications in various industrial branches, in particular for the manufacture of paints, the treatment of rubbers, in the metallurgical (sintered materials), chemical (catalysts), ceramic, pharmaceutical, etc. industries.

There is already known a method for manufacturing metal powders from a molten metal which consists in sweeping the vapor of the molten metal by means of an inert gas previously cooled to cause the condensation of said vapor. However, this process achieves only a very low refrigeration supply and does not make it possible to obtain large quantities of powder. In addition, the powder obtained is formed of particles of irregular shape and having a large particle size dispersion.

A problem currently posed in the powdered metal technique is the obtaining, in industrial quantities, of extremely divided powders having an average particle size of the order of 0.08 microns, consisting of particles having a shape as regular as possible and having a minimum particle size dispersion, that is to say located in a particle size range between 0.02 and 0.13 micron

and finally having a high chemical purity.

These objects are achieved with the method according to the invention by the fact that it consists in pouring onto the bath, brought to a temperature such that its vapor pressure is at least 1 mm of mercury, a cryogenic fluid in the liquid phase, to evacuate outside the enclosure the cryogenic fluid which contains, in suspension, the solid particles, to separate the latter from said fluid and collecting them to obtain the aforementioned powder.

Experiments on various metallic materials (pure metals or alloys) have shown that the aforementioned vapor pressure can advantageously be in a range between 1 and 500 millimeters of mercury.

A high vapor pressure causes accelerated evaporation of the metal bath and therefore makes the process applicable on an industrial scale. The use of a cryogenic fluid in the liquid phase causes very rapid cooling, therefore an energetic quenching, of the metallic vapor and allows the direct passage from the gaseous state to the solid state. This change of state and the evacuation of solid particles concomitant with that of the cryogenic fluid results in a constant renewal of the phenomenon of condensation of the vapors above the bath.

As a result, the solid particles which are thus formed from an incipient suddenly cooled metallic vapor have a regular shape and dimensions not exceeding a few hundred angstroms.

According to another characteristic of the invention, the cryogenic fluid is introduced into said enclosure and is continuously removed therefrom.

A continuous circulation of cryogenic fluid allows a continuous production of powder at an optimal particle formation regime.

According to another characteristic of the invention, the cryogenic fluid is discharged in the liquid phase.

According to another characteristic of the invention, the cryogenic fluid is discharged in the gas phase.

According to yet another characteristic of the invention, the cryogenic fluid consists of a chemically inert element or a mixture of chemically inert elements.

The use of such a cryogenic fluid makes it possible to obtain metallic powders formed from chemically pure metals.

According to yet another characteristic of the invention, the cryogenic fluid consists of a chemically active element or a mixture of chemically active elements.

The use of such a cryogenic fluid allows the formation of specific chemical compounds, for example oxides, nitrides or metal hydrides.

Still according to the invention, the cryogenic fluid consists of a mixture of chemically inert elements and chemically active elements.

The use of such a fluid makes it possible to control the formation of the chemical compounds which it is desired to obtain.

The invention also relates to an installation for implementing the aforementioned method, this installation comprising means for continuously discharging a cryogenic fluid in the liquid phase inside a closed enclosure, means for transferring, out of from said enclosure, a stream of fluid carrying solid metallic particles in suspension, and a closed separation chamber; connected to said transfer means and receiving the above-mentioned fluid stream, said separation chamber being provided with means for collecting the aforementioned solid particles and means for discharging said stream of fluid free of said particles.

Other characteristics and advantages of the invention will emerge during the description which follows.

• - La figure 1 montre de façon schématique, une installation pour la mise en oeuvre du procédé selon l'invention, dans laquelle le fluide cryogénique est évacué en phase liquide, la collection des particules se faisant par gravité ;
• - La figure 2 montre une variante de l'installation de la figure 1, dans laquelle la collection des particules se fait par filtration.
• - La figure 3 montre, de façon schématique et partielle, une installation dans laquelle le fluide cryogénique est évacué en phase gazeuse.

In the attached drawing, given by way of nonlimiting example:

• - Figure 1 shows schematically, an installation for implementing the method according to the invention, in which the cryogenic fluid is discharged in the liquid phase, the collection of particles being by gravity;
• - Figure 2 shows a variant of the installation of Figure 1, in which the collection of particles is by filtration.
• - Figure 3 shows, schematically and partially, an installation in which the cryogenic fluid is discharged in the gas phase.

According to the embodiment represented in FIG. 1, the installation comprises a melting device 1, for example an induction furnace or a heating crucible, which contains the metallic material M in the liquid state, and is closed by a cover. 2 which thus provides, above the bath, a closed enclosure 3, therefore isolated from the ambient atmosphere in which the metallic vapor is released.

Inside the enclosure 3 is housed a reactor 4, constituted by a tubular sleeve of section slightly smaller than that of the enclosure, open at its two ends, the lower end plunging slightly into the metal bath M and at the interior of which is concentrated most of the vapor phase of the metallic material.

The furnace or the like 1 and the reactor 4 are made of any refractory material, of the type usually used in metallurgy, the furnace being provided with heating means (not shown) which make it possible to maintain the molten metal at the temperature necessary to obtain the desired vapor pressure. A pipe 5 pours out a cryogenic fluid, for example liquefied nitrogen at -196 ° C, stored in a storage device (not shown), in the reactor 4 via a funnel 6 housed in said reactor and opening out in the vicinity of the surface of the metal bath so that said fluid arrives just above the latter. The reactor 4 is connected, by a heat-insulated conduit 7, to a closed separation chamber 8 which communicates with the outside only by a one-way pressure limiting valve 9. In the chamber 8 are housed containers 11 for collecting the particles , these containers being mounted on a rotary support 10 which makes it possible to take them in turn below the duct 7.

The reactor 4 is supplied with cryogenic liquid with a sufficient flow rate to permanently maintain, above the metal bath M, a thick layer of cryogenic liquid which exceeds the level of connection of the conduit 7 to the reactor. The solid particles which form in reactor 4 as a result of the condensation of metallic vapors thus remain in suspension in the cryogenic liquid which is transferred, by drawing off by means of the conduit 7, into the separation chamber 8. The cryogenic liquid then passes to the gaseous state, creating and maintaining in the chamber 8 a neutral atmosphere and the solid particles separate by gravity and fall into the containers 11 where they are collected to give a powder. The filling of these containers must be carried out in several stages owing to the reduction in the volume of the powder following the evaporation of the cryogenic liquid. The rotary support allows these successive filling steps to be carried out.

According to the embodiment represented in FIG. 2, in which the same reference numbers designate the same elements as in FIG. 1, the cryogenic liquid charged with particles in suspension and brought into the separation chamber 8 by the conduit 7 is received in separation vessels 12 provided with a filtering wall 13 which retains the particles and lets the liquid pass. The liquid thus filtered is brought, by a first lagged pipe 14, to a recovery tank 15 and from there it is returned, by a recycling pump 16 and a second lagged pipe 17, to reactor 4.

According to the embodiment of FIG. 3, in which the same reference numerals also designate the same elements as in FIGS. 1 and 2, the reactor 4 is supplied with cryogenic liquid with an insufficient flow rate to maintain a liquid layer above the metal bath. In this case, the vapor is condensed at the point of impact of the cryogenic liquid with the surface of the bath and the metallic particles are entrained out of the enclosure 2 by the fluid in the vapor phase. The recovery of these particles can be done by gravity.

The metallic material can consist of a metal (Fe, Cu, Zn, Mg, Al, etc.) or an alloy (brass, bronze, etc.).

It should be noted that, in the latter case, the choice of the composition of this alloy, that is to say the choice of constituents (which have different melting temperatures), and the proportions of these constituents, make it possible to adjust the kinetics of evaporation. Thus, for example, an alloy having a high proportion of a metal with a low melting point, such as Mg, makes it possible to obtain a metallic vapor formed almost exclusively of said metal with low melting point. Likewise, by using an alloy of copper (low volatile metal) and zinc (very volatile metal), the composition of this alloy can be determined so as to obtain, for a chosen temperature of the metal bath, a high vapor pressure of the zinc. The solid particles obtained are then formed exclusively of zinc.

The cryogenic fluid can consist of inert liquefied elements (N2, Ar, He, etc ...) or active (02, H2, NH3 etc ...) by liquefied compounds such as hydrocarbons or a mixture formed of inert liquefied elements and active liquefied elements or else inert liquefied elements and liquefied compounds. In the case of such mixtures, the choice of the percentage of the active element or of the compound makes it possible to adjust the kinetics of the reaction of the combination of the metal with the metalloid which constitutes said element or resulting from the decomposition of said compound.

An example of the production of a zinc powder from a Cu-Zn alloy will be given below, according to the embodiment of FIG. 1.

Metallic material: UZ 30 alloy (AFNOR standard): Cu = 70% - Zn = 30% Temperature of the metal bath: 1065 ° C Vapor pressure of Zn: 486 mm of mercury Vapor pressure of Cu: 10 ^-4 mm of mercury Molar fraction of zinc; 0.3 Zinc activity in the alloy: 0.16 Zinc activity coefficient in the alloy: 0.54 Cryogenic fluid: liquid nitrogen (-196 ° C)

The powder obtained after separation of the cryogenic fluid. consists of zinc particles of dimension between 0.03 and 0.10 micron, and has a specific surface (BET) of 40 m2 per gram.

The fact of using a Cu-Zn alloy makes it possible to overheat the Zn, therefore to obtain a high zinc vapor pressure compared to the vapor pressure of copper and consequently to obtain metallic particles formed only of zinc.

Many variants could be made to the process described above without departing from the scope of the invention. Thus, the heating of the melting device 1 could be obtained for example by induction, or by means of radiation, for example concentrated solar radiation by means of an optical system or radiation produced by a laser, or also by means of an arc or an electrical resistance so as to obtain a melting and a point or overall overheating of the material to be vaporized.

We could even use plasma heating. Similarly one could, instead of nitrogen, use another inert gas like argon.

Claims (14)

1. - Method for manufacturing a metallic powder by lowering the temperature of the vapor of a molten metallic material in a closed treatment enclosure, causing said vapor to be transformed into solid particles, characterized in that it consists in pouring onto said bath brought to a temperature such that its vapor pressure is at least 1 mm of mercury, a cryogenic fluid in the liquid phase, to evacuate from the enclosure, the cryogenic fluid which contains, in suspension , the solid particles, to separate the latter from said fluid and to collect them to obtain the aforementioned powder. 2. - Method according to claim 1, characterized in that the cryogenic fluid is introduced into said enclosure and is evacuated continuously. 3. - Method according to claim 2, characterized in that said cryogenic fluid is discharged from the enclosure in the liquid phase. 4. - Method according to claim 3, characterized in that the aforementioned liquid phase is removed by drawing off. 5. - Method according to claim 2, characterized in that said cryogenic fluid is discharged from the enclosure in the gas phase. 6. - Method according to claim 1, characterized in that the separation of the particles and their collection is done by gravitational action. 7. - Method according to claim 3, characterized in that the separation of the particles and their collection is done by filtration. 8. - Method according to claim 1, characterized in that the aforementioned material is a pure or substantially pure metal. 9. Method according to claim 1, characterized in that the aforementioned material consists of an alloy of two or more metals. 10. - Method according to claim 1, characterized in that the cryogenic fluid consists of a chemical element only inert or a mixture of chemically inert elements. 11. - Method according to claim 1, characterized in that the above cryogenic fluid consists of a chemically active element or a mixture of chemically active elements. 12. - Method according to claim 1, characterized in that the above cryogenic fluid consists of a mixture of chemically inert elements and chemically active elements. 13. - Method according to claim 1, characterized in that the metallic material consists of a Cu-Zn alloy containing 30% Zn, in that said alloy is brought to a temperature of 1065 ° C and in that the fluid cryogenic consists of nitrogen. 14. - Installation for the manufacture of a metallic powder by solidification of the vapor of a bath of a molten metallic material in a closed enclosure for the implementation of the method according to one of claims 1 to 13, characterized in that it comprises means 5 for continuously discharging a cryogenic fluid in the liquid phase inside the aforementioned enclosure 3, means 7 for transferring, out of said enclosure, a stream of fluid carrying solid metal particles in suspension and a closed separation chamber 8 connected to said transfer means 7 and receiving the above-mentioned fluid stream, said separation chamber being provided with means 11 for collecting the aforementioned solid particles and means 9 for discharging said stream of fluid freed from said particles.

Images