APPARATUS FOR PRODUCING METAL POWDER

Abstract: A method for producing metal powder compri­ sing steps of feeding a moltenb metal into a closed vessel, atomizing of the metal by inert gas and finaly cooling of particles in an atmosphere of inert gas and atomized water is provided. An apparatus for producing metal powder comprises a closed vessel (1) proveded with atomizing nozzles (2) and means for provid­ ing inert gas, and another set of nozzles (4) for supply of atomized cooling medium located on one or several vertically dis­ placed levels along the internal perimeter of the vessel (1).

Description: The present invention relates to an apparatus for producing metal powders, and particularly aluminium powder, comprising a closed vessel , means for feeding molten metal, means for the supply of inert gas, and first and second nozzles for atomizing and cooling the metal. Several techniques for producing metal powder are previously known, e.g. reduction of oxide particles by gases, electrolysis of metal salt solutions, decomposition of gaseous metal substances by means of heating and atomization. Each of these techniques covers furthermore several methods and apparatusses for producing metal powder. With regard to the atomization technique, which the present invention relates to, the most known method is the one where the molten metal is atomized by a pressurized fluid by means of an atomizing nozzle. Air has been used previously in such nozzles as the atomizing fluid and the atomized molten metal was cooled down in an air atmosphere. However, many deficiencies are associated with the prior art devices employing air as the atomizing and cooling medium. Due to the oxygen content in air the resulting metal powder is oxygen enriched (contaminated), something which results in limited application possibilities for the powder. Furthermore, there is a danger of explosive combination hydrogen/oxygen when atomizing certain reactive alloys, and large space demanding cooling towers were required in order to cool down the atomized molten metal. Consequently, other atomizing fluids are applied today instead of the air. E.g. U.S. Patent No. 4,080,126 discloses a method and an apparatus for producing metal powder where water is applied as the atomizing fluid. The apparatus comprises a closed vessel provided with apparatuses for inlet of inert gas and an atomizing nozzle located in an upper region of the vessel for impinging a water stream on a vertically descending stream of molten metal. The atomization of the molten metal takes place in a non-oxidizing atmosphere and the use of water ensures a rapid cooling of the atomized molten metal due to a direct contact water/molten metal under the atomization process. The resulting metal powder has a low oxygen content and a homogeneous structure thanks to the rapid cooling process. However, the shape of the achieved powder particles is irregular and suitable for molding by powder metallurgy, but not quite applicable for purposes reguiring fine and uniform particles. A medium diameter of particles provided by this method is in a range from 150 to 175µm, while particles provided by the method in accordance to the present invention have a diameter in a range from 75 to 100µm. Furthermore, use of water as an atomizing fluid results in substantially larger variation in particle size and substantially higher content of water in the provided metal powder. A device for manufacture of spherical metallic powder is disclosed in the above described U.S. Patent No. 4,080,126, both with regard to the applied apparatus and the mode of operation except for the application of inert gas as the atomizing fluid. Fine particles with uniform size are achieved, but the cooling of the atomized metal in the atmosphere of inert gas gives so low cooling rate that the resulting particles are less homogeneous. Furthermore, devices applying inert gas as a cooling medium requires also, as mentioned above in connection with cooling conducted in air, large and space demanding cooling towers. Finally, GB-A-812 341 discloses a process for manufacture of metal powders wherein a stream of molten metal is subjected to the action in succession of at least two jets of dissimilar fluids, and preferably conical jets of fluids are employed. The second jet body is disposed right under (close below) the first jet body, and the object of this second jet body is to atomise still further the jet of liquid. It is an object of the present invention to provide an apparatus for producing metal powders without the above mentioned drawbacks and deficiencies, and particularly metal powder having a fine and homogenous structure, low oxygen content, small and uniform particle size. Another object of the present invention is to provide an improved apparatus having modest space requirement. In accordance with this invention there is provided an apparatus for producing metal powders in which the atomized molten metal is cooled down in an atmosphere of atomized water or a water solution of chlorides and inert gas. The apparatus for producing of metal powders comprises a closed vessel where the metal is atomized and provided with nozzles supplying atomized water or water solution of Ni and/or Cr-chlorides. The invention will be described in more details in connection with specific embodiments therof referring to the accompanying drawing, Figure 1, illustrating schematically an example of set-up of an apparatus according to the present invention. The apparatus comprises a closed vessel or tank 1. Molten metal is transferred from a reservoir 3 to an atomizing nozzle 2 provided in an upper part of the tank 1 and supplying the tank with atomized metal. The nozzle is advantageously of type USGA (Ultrasonic gas atomization) where the molten metal is atomized by inert gas, e.g. nitrogen or argon, but other types of nozzles can also be applied. The applied atomization pressure is on average 5 to 50.10⁵ N/m² or preferably in a range of 15 to 30.10⁵N/m² which results in metal powder particles having a median diameter from 75 to 100 µm. Inside of the tank there are provided atomizing cooling nozzles 4 supplied with cooling medium water by lines 5. The water atomizing nozzles are located along the perimeter of the tank 1 on several vertically distributed levels. The water atomizing jet is in this manner orientated radially towards the central part of the tank and an advantageous mixing is achieved in the tank. The water atomizing nozzles, or the nozzles located at the top level of the illustrated configuration of nozzles are intentionally located in a certain distance X from the atomizing nozzle 2 applied on the molten metal. This distance is important in order to achieve a super cooling of the molten particles before their arrival into a solidification zone with the water atomizing jets. The super cooling of the molten particles results in a special fine and homogenous structure of the metal particles due to the further reduced solidification time. According to the conducted trials a distance X from 20-30 cm gives favourable results in this respect. The further solidification and cooling of the metal particles during their axial movement through the tank occurs by collision with the atomized water particles moving across (in a radial direction) the tank. In this manner the "steamfilm" formed around the metal particles is broken down in an efficient way and a high rate of solidification and cooling is achieved. The inert gas applied for the atomization of molten metal is normally adequate to maintain the required inert atmosphere in the tank. In order to maintain a required constant pressure slightly above the atmosphere pressure the lower part of the tank 1 is provided with collecting means 9 for the "spent" inert gas, located between the water atomizing nozzles 4 and the level of metal powder or water level. The collecting means (collector) comprising an inwardly recessed skirt 11 extent along the tank periphery connected by a pipe 10 to exhausting means (not shown in the figure). The "spent" inert gas contains also water particles and steam. Some of this steam and water particles condenses and deposits in the collector 9 and is drained, e.g. by a pipe and returned to the tank. The metal powder and the water collected at the bottom of the tank is transfered by means of a pump 6 through pipe conducts 7 to a centrifugal decanter 8 which separates the metal powder from the water phase. The powder containing from 4 to 7 % water after decantation is passed over to an indirect heated drying drum in order to reduce the water content to an acceptable level. In the above described apparatus for producing of metal powders according to the present invention water was applied as the cooling medium. It is considered advantageous to apply a water solution containing nickel and/or chromium chlorides as the cooling medium in the process of manufacturing aluminium or aluminium alloy powders. The particles achieved from conducted trials are covered with a thin layer of Ni and/or Cr-substances. This "coating" has a beneficient effect on control of oxidation and pick-up of humidity. Co-extrusion of fibre reinforced materials (composites) can be mentioned as a typical application of aluminium powder where the Ni/Cr- coating of the particle surface will improve the bonding between fibres, such as SiC, SiO₂, Al₂O₃) and the matrix. Furthermore, metal powders produced by the method and apparatus according to this invention are especially useful and applicable for plasma spraying of articles. The powder can of course be advantageously applied also for other purposes, e.g. for compacting and following extrusion of articles, for forging and machine work (treatment).

Claims: Apparatus for producing metal powder comprising a closed vessel (1), means for feeding molten metal, means for the supply of inert gas, and first (2) and second nozzles (4) for atomizing and cooling the metal respectively, characterized in that the second nozzles (4) are disposed in the vessel at a distance sufficient from the first nozzle (2) to allow supercooling of the metal particles, whereby the second nozzles provide an atmosphere of atomised cooling medium in the vessel (1) to further solidify and cool the particles. Apparatus according to claim 1, characterized in that the cooling medium consists of atomized water solution of nickel chloride or nickel and chromium chlorides. Apparatus according to claim 3, characterized in that the second nozzles (4) for atomizing the cooling medium are located at a distance of 20 to 30 cm from the nozzles for atomizing of the molten metal.

Inventors: AURAN, LARS, GRAVEM, ALF EINAR, ROD, IVAR OLAV, TANGEN, OLE
Applicants: NORSK HYDRO A.S.
Application year: 1988
Main classification: B22F9
IPC codes: B22F9