Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
1998-08-18
2002-04-30
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S010190
Reexamination Certificate
active
06379419
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for the production of fine and ultrafine powders of various materials such as metals, alloys, ceramics, composites and the like with controlled physical properties. To carry out the method, a novel and flexible transferred arc plasma system providing the ability to control powder properties with a high production rate has been developed. The transferred arc plasma system comprises a transferred arc plasma reactor and a separate quench system within which powder condensation occurs.
BACKGROUND OF THE INVENTION
Fine powders of metals, alloys, ceramics, composites and the like have a wide variety of applications in various fields such as aeronautics, electronics, microelectronics, ceramics and medicine. Currently, generation of fine powder, i.e., powders having an average particle size between 0.1 and 10 &mgr;m, is mainly accomplished via 3 different techniques: 1) hydrometallurgy, 2) spray pyrolysis and 3) milling. Among the disadvantages of the above techniques are high operating costs, production of non-spherical particles and generation of toxic or difficult to handle by-products.
The benefits obtained with ultrafine powders, i.e., powders with an average particle size lower than 100 nm, are mainly due to their small particle size, which results in a higher surface area/volume ratio. Consequently, ultrafine powders may have advantages over fine powders when used in the above fields.
The preferred methods for the production of fine powders are hydrometallurgy and spray pyrolysis. However, these methods have several major drawbacks including preparation and handling of the feed materials like chlorides and nitrates, which are very often toxic and difficult to handle, environmental emission control requirements for gaseous and liquid effluents, and a difficulty to produce average particle sizes below 100 nm.
Thermal plasma based vapor condensation methods have demonstrated their ability to generate average particle sizes below 100 nm without the handling and environmental problems associated with hydrometallurgical and spray pyrolysis methods. These problems are avoided because the feed materials are generally inert. Examples of such materials include pure metals, alloys, oxides, carbonates etc. Such plasma methods are able to vaporize or decompose these feed materials because of the high-energy input that can be achieved.
Thermal plasma generation is typically accomplished via 2 methods, i.e., high intensity DC arcs which uses currents higher than 50 A and pressures higher than 10 kPa, or high frequency discharges such as an RF plasma. Because of their high-energy efficiency, DC arcs are generally preferred. DC arcs are classified as transferred when one of the electrodes is a material being processed, and non-transferred when the electrodes are non-consumable. Since transferred arc systems pass electrical current directly through a material being processed, their energy efficiency is higher than non-transferred transferred arc systems. Because of the extremely high heat input into the material acting as the electrode, vaporization or decomposition occurs, thus producing a vapor phase that is then cooled to induce the formation of the powder. The powder product is then typically recovered in a filtration unit.
Thermal plasma based vapor condensation methods which utilize a transferred arc have not been successful up to now to generate fine or ultrafine powders of materials like metals, alloys, ceramics or composites on a commercial scale because of their low energy efficiency, low production rate, poor yield, and rudimentary control of powder properties such as particle size and distribution, shape, and crystallinity. In addition, this method is typically used for the production of powders with an average particle size lower than 0.1 &mgr;m, which has also contributed to its lack of success on an industrial scale because today's market requires powders with larger particle sizes.
In addition to producing fine and ultrafine powders of various pure materials, transferred arc plasma systems can also be used for the production of fine and ultrafine powders resulting from the interaction of two or more components (chemical reaction) or elements (alloying).
Although transferred arc plasma systems can operate batchwise, it is preferred that they be operated in a continuous manner. The material to be vaporized or decomposed can be fed continuously in the reactor in several manners. For example, it can be fed into a crucible either from the top thereof by a side tube in the reactor wall. The material can also be pushed upward underneath the plasma in a continuous manner, or fed directly into the plasma torch. Depending on the powder to be produced, the operator will select the appropriate method. Generally, the preferred feeding method is through one or more tubes located in the upper portion of the reactor. The feed materials can be in solid (wire, rod, bar, chunks, shots etc.) or liquid form. When in liquid form, the feed material can also be pumped into the reactor.
U.S. Pat. No. 4,376,740 discloses a method for producing fine metal powders which involves reacting a molten metal or alloy with hydrogen using an arc or plasma discharge, or an infrared radiation which dissolves the hydrogen in the metal. When the dissolved hydrogen is released from the molten metal, fine metal powders are generated. Using this method, a low production rate and yield is attained because of the use of a cold-walled reactor and a water-cooled copper mold which is used to support the material being processed. The maximum production rate reported is less than 240 g/hr. Further, there is no mention or suggestion of control of powder properties.
A critical aspect of transferred arc plasma systems is that they consume a lot of energy. It is therefore imperative to maximize its efficiency to have a viable commercial method. This means that the temperature within the reactor must be maintained as high as possible to prevent condensation of the vaporized or decomposed materials therein, either on the plasma chamber walls, outside surface of the plasma torch or the mold, which is very often a crucible. Such maximization would obviously result in higher production yields of powders. Because of the extreme conditions prevailing in the transferred arc reactor, many elements are generally water-cooled to extend their operating life. Obviously, such cooling has the effect of reducing the energy efficiency of the method. It has been proposed in Ageorges et al. in
Plasma Chem. and Plasma Processing
, 1993, 13 (4) 613-632 to modify the interior of a transferred arc reactor by covering its internal surfaces with a graphite lining to retain as much heat as possible inside the reactor.
Ageorges et al. supra, also disclose the production of ultrafine aluminium nitride (AlN) powder using a transferred arc thermal plasma based vapor condensation method. Vaporizing aluminium and reacting it with nitrogen and ammonia in an insulated plasma chamber produces the desired aluminium nitride product. Aluminium is vaporized by using it as the anode material in a transferred arc configuration that employs a thoriated tungsten tip cathode. The aluminium being vaporized is in the form of an ingot placed in a graphite crucible surrounded by a water-cooled stainless steel support. Because of the presence of that water-cooled jacket, the energy efficiency of vaporization is reduced. A disadvantage of this process is due to the fact that the formation of powder occurs in the plasma chamber because of the injection of reactive gases in the plasma chamber, i.e., nitrogen and ammonia. Ageorges et al. specifically state that the plasma chamber is “filled with fume products which recirculate in the furnace”. As a result, powder property control is very crude because of the difficulty in properly controlling nucleation and growth of the powder product in the plasma chamber. The particles produced are reported to have a nominal particle size of 135 nm based on specific surface are
Addona Tony
Boulos Maher I.
Celik Cesur
Chen Gangqiang
Davis H. John
Combs-Morillo Janelle
Noranda Inc.
Wyszomierski George
Zavis Katten Muchin
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