EP0009433B1 - 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 powder of a metal of one of its alloys or of one of its compounds by condensation, in solid particles, of the vapor of a bath of a metallic material in fusion in a closed treatment enclosure, by means of a cooling fluid brought into contact with said vapor.

The term “metallic material” denotes either a metal proper, or an alloy of at least two metals.

The metal powders thus obtained consist of solid particles either of a single metal such as iron, zinc, magnesium, calcium, cadmium, etc., or of a metal alloy, for example a magnesium alloy - zinc, or of a metallic compound, for example zinc oxide or magnesium nitride. These powders find wide applications in various industrial branches, for the manufacture of paints, the treatment of rubbers, in the metallurgical (sintered materials), chemical (catalysts), ceramic, pharmaceutical, etc. industries.

US-A-2,934,331 discloses a method for manufacturing metal powder which uses, as cooling fluid, a hydrocarbon sprayed, through nozzles, onto a stream of metallic vapor flowing from an oven containing the metal molten, for example magnesium. The metal particles collected in collectors have an average particle size of 1 or 2 microns and varying from 0.25 microns to approximately 55 microns.

DE-A-903,777 discloses a process using a cooling liquid, for example a saline solution, at a temperature below 0 ° C. This process makes it possible to obtain neither a fine particle size nor a chemically pure product.

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 of as regular shape as possible and having a minimum particle size dispersion, that is to say located in a range between 0.02 and 0.15 microns 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 using a liquefied gas as cooling fluid, in pouring it directly, in the liquid phase, onto the surface of the bath, brought to a temperature such that its voltage of steam or at least 1.36.10 ² pascals, to evacuate it out of the enclosure while it contains, in suspension, the solid particles formed by condensation, then to separate and collect said particles to obtain the powder cited above.

Experiments carried out on various metallic materials (pure metals or alloys) have shown that the above-mentioned vapor pressure can advantageously be included in a range between 1.36.10 ² and 6.80.10 ⁴ pascals.

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 liquefied gas 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 liquefied gas results in a constant renewal of the phenomenon of condensation of the vapors above the bath.

- It follows that the solid particles which are thus formed from an emerging metallic vapor suddenly cooled have a regular shape and dimensions not exceeding a few hundred angstroms.

According to another characteristic of the invention, the liquefied gas is introduced into said enclosure and is continuously evacuated therefrom.

Continuous circulation of the liquefied gas allows continuous production of powder at an optimal particle formation regime.

According to another characteristic of the invention, the liquefied gas is removed from the enclosure in the liquid phase.

According to another characteristic of the invention, the liquefied gas is removed from the enclosure in the gas phase.

According to yet another characteristic of the invention, the liquefied gas consists of a chemically inert body or a mixture of bodies chemically inert with respect to said material.

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

According to yet another characteristic of the invention, the liquefied gas consists of a chemically active body or a mixture of bodies chemically active with respect to said material.

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

Still according to the invention, the liquefied gas consists of a mixture of chemically inert bodies and bodies chemically active with respect to said material.

The use of such a gas 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 liquefied gas directly onto the surface of the bath, means for transferring, from said enclosure, a current of fluid carrying solid metallic particles in suspension, and a closed separation chamber, connected to said transfer means and receiving the aforementioned stream of fluid, said separation chamber being provided with means for collecting the aforementioned solid particles and means for evacuating said fluid stream freed from 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 gaz liquéfié 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 gaz liquéfié 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 liquefied gas 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 liquefied gas 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 spares, above the bath, a closed enclosure 3, therefore isolated from the ambient atmosphere, in which the metallic vapor is released.

Inside the enclosure 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 dipping slightly in the metal bath M and at inside 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 liquefied gas, for example liquefied nitrogen at -196 ° C, stored in a storage device (not shown), in the reactor 4 by means of a funnel 6 housed in said reactor and opening out in the vicinity of the surface of the metal bath so that said liquefied gas 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 conduit 7.

The reactor 4 is supplied with liquefied gas with a sufficient flow rate to permanently maintain, above the metal bath M, a thick layer of liquid phase which exceeds the level of connection of the conduit 7 to the reactor. The solid particles which form in the reactor 4 as a result of the condensation of the metallic vapors thus remain in suspension in the liquid phase which is transferred, by drawing off by means of the conduit 7, into the separation chamber 8. The liquid phase then passes in 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 as a result of the reduction in the volume of the powder following the evaporation of the liquid phase. 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 numerals designate the same elements as in FIG. 1, the liquid phase 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 through. The liquid thus filtered is brought, by a first insulated pipe 14, to a recovery tank 15 and therefore it is returned, by a recycling pump 16 and a second insulated 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 liquefied gas with an insufficient flow rate, to maintain a liquid layer at- above the metal bath. In this case, the vapor is condensed at the point of impact of the liquefied gas with the surface of the bath and the metallic particles are entrained out of the enclosure 2 by 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, AI, AI, 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 the constituents (which have different melting temperatures), and of the proportions of these constituents, makes 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. Similarly, 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 liquefied gas can be constituted by inert liquefied bodies (N ₂ , Ar, He, etc ...) or active (O2, H ₂ , NH ₃ , etc ...) vis-à-vis the metallic material, by liquefied compounds such as hydrocarbons or a mixture formed of inert liquefied bodies and active liquefied bodies or else of inert liquefied bodies and liquefied compounds. In the case of such mixtures, the choice of the percentage of the active body 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 body 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.

Zn vapor pressure: 486 mm of mercury = 6,61.10 ⁴ pascals.

Cu vapor pressure: 10- ⁴ mm of mercury = 1.36.10 ⁴ pascals.

Molar fraction of zinc: 0.3.

Zinc activity in the alloy: 0.16.

Coefficient of activity of zinc in the alloy: 0.54.

Liquefied gas: nitrogen (-196 ° C).

The powder obtained after separation of the liquefied gas consists of zinc particles with a size between 0.03 and 0.10 microns, and has a specific surface (BET) of 40 m ² 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 of manufacturing a powder of a metal, of one of its alloys or of one of its compounds by condensation into solid particles of the vapour of a bath of a metallic material in fusion in a closed processing chamber, by means of a cooling fluid put into contact with the said vapour, characterised in that it consists in utilising a liquified gas as the cooling fluid, in pouring it out directly, in the liquid phase, onto the surface of the bath which has been brought to a temperature such that the pressure of the vapour is at least 1.36.10² pascals, in evacuating it from the chamber whilst it contains in suspension the solid particles formed by condensation, then in separating and collecting the said particles to obtain said powder.

2. Method according to claim 1, characterised in that the liquified gas is introduced into the said chamber and is evacuated from it, in continuous manner.

3. Method according to claim 2, characterised in that the said liquified gas is evacuated from the container in the liquid phase.

4. Method according to claim 3, characterised in that said liquid phase is evacuated by drawing off.

5. Method according to claim 2, characterised in that said liquified gas is evacuated from the chamber in the gaseous phase.

6. Method according to claim 1, characterised in that the liquified gas having been submitted to vapourisation, the separation of the particles of the gaseous phase and their collection is effected by the action of gravity.

7. Method according to claim 3, characterised in that the separation of the particles and their collection is made by filtration of the liquid phase.

8. Method according to claim 1, characterised in that said material is a pure or substantially pure metal.

9. Method according to claim 1, characterised in that said material is constituted by an alloy of two or more metals.

10. Method according to claim 1, characterised in that the liquified gas is constituted by a chemically inert body or a mixture of bodies which are chemically inert as regards said material.

11. Method according to claim 1, characterised in that the liquified gas is constituted by a chemically active body or a mixture of bodies which are chemically active as regards said material.

12. Method according to claim 1, characterised in that said liquified gas is constituted by a mixture of bodies which are chemically inert and bodies which are chemically active as regards said material.

13. Method according to claim 1, characterised in that the metallic material is constituted by a Cu-Zn alloy having 30 % Zn, in that the said alloy is brought to a temperature of 1 065 °C and in that the liquified gas is constituted by nitrogen.

14. Installation for manufacturing a powder of a metal, of one of its alloys or of one of its compounds by condensation into solid particles of the vapour of a bath of a metallic material in fusion in a closed processing chamber by means of a cooling fluid, characterised in that it comprises means (4, 5, 6) for pouring out, in continuous manner, a liquified gas directly onto the surface of the bath (M), means (7) for transferring, outside said chamber (3), a current of fluid transporting the solid metallic particles in suspension and a closed separation chamber (8) connected to said transfer means (7) and receiving said current of fluid, said separation chamber being urnished with means (11, 12) for collecting the said solid particles and means (9) for evacuating said current of fluid, which is freed from said particles, said means being necessary for carrying out of the method which is the object of the preceding claims.

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