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Purification processRelated Patent Categories: Specialized Metallurgical Processes, Compositions For Use Therein, Consolidated Metal Powder Compositions, And Loose Metal Particulate Mixtures, Processes, Producing Or Purifying Free Metal Powder Or Producing Or Purifying Alloys In Powder Form (i.e., Named Or Of Size Up To 1,000 Microns In Its Largest Dimension), Utilizing Electrothermic, Magnetic, Or Wave EnergyPurification process description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060130610, Purification process. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This invention relates to a method for the purification of metal particles, for example, metal powders and other finely sized metal samples, and, in particular, to ones that have been produced by the process of electrochemical reduction of metal compounds. The invention is particularly suited to the purification of titanium powder formed from the electrolytic reduction of titanium oxide TiO.sub.2. [0002] WO 99/64638 describes methods for the electrolytic reduction (or "electro-reduction") of metal compounds. Certain embodiments of those methods involve the electrolysis of metal oxides or other compounds in a cell containing a liquid (fused salt) electrolyte and an anode, the metal oxide forming or contacting the cathode. Conditions are controlled so as to bring about the selective dissolution of the oxygen of the cathode in preference to deposition of the metal cation of the fused electrolyte. The metals extracted by such methods, however, often require further purification. [0003] GB2359564 is directed to improved methods for carrying out such processes, particularly to produce powdered titanium, and some of those methods are summarised below: Production of Powder by Reduction of Sintered Metal Oxide Granules [0004] Sintered granules or powders of metal oxide can be used as the feedstock for the electrolysis described in the above referenced method, as long as appropriate conditions are present. In one example, powdered titanium dioxide in the form of granules or a powder is used, the powdered particles preferably having a size in the region of 200 .mu.m. [0005] As illustrated in FIG. 1, the granules of titanium dioxide 1 are held in a basket 2 below a carbon anode 3 located in a crucible 4 having a molten salt 5 therein. These are prevented from sintering together by maintaining particle motion by any appropriate method e.g. in a fluidised bed arrangement. Agitation is provided either by mechanical vibration or by the injection of gas underneath the basket. Mechanical vibration can, for example, be provided by ultrasonic transducers mounted on the outside of the crucible or on control rods. The key variables to be adjusted are the frequency and amplitude of the vibrations in order to get an average particle contact time which is long enough to get reduction, but short enough to prevent diffusion bonding of the particles into a solid mass. Similar principles would apply to agitation by the injection of gas, except here the flow rate of the gas and size of the gas bubbles would be the variables controlling particle contact time. Production of Powder by Deposition of M.sub.1 onto the Cathode [0006] If a metal is deposited onto a cathode (based on the electrolytic process previously described) from a second source of the metal at a more positive potential, the resulting metal deposited thereon is dendritic in structure. This is particularly so where the metal is titanium. This form of titanium is easy to break up in to a powder as individual particles are connected together only by a small surface area. This method can be used to produce titanium powder from titania. In this method, illustrated in FIG. 2, a second cathode 6 is provided which is maintained at a potential that is more negative than the first cathode 7. When the deposition of titanium on the first cathode has progressed sufficiently, the second electrode is switched on, leading to the dissolution of titanium from the first cathode and deposition 8 onto the second cathode 6 in dendritic form. The other reference numerals represent the same items as in FIG. 1. Use of Continuous Powder Feed [0007] Continuously feeding a fine powder of metal oxide into the electrochemical cell allows for a constant current and a higher reaction rate. A carbon electrode is preferred for this method. This method permits the use of cheaper feedstock as a sintering and/or forming stage is no longer needed. [0008] This method is shown in FIG. 3 which shows a conducting crucible 1 which is made the cathode containing a molten salt 2 and inserted therein is an anode 3. Titanium dioxide powder 4 is fed into the crucible where it undergoes reduction and is deposited at the base of the crucible. The thick arrow shows the increasing thickness of the reduced feedstock 5. [0009] In electro-reduction methods such as those exemplified above, the purity of the metal powder may be affected by the staring materials and processing parameters used. For example, it has been found that some variations of the method when used to produce titanium powder result in powder that contains "contaminating impurities" resulting therefrom, including light metals such as magnesium or calcium, as well as salts such as calcium chloride (the latter being a preferred electrolyte). Such impurities are known to affect the mechanical properties of alloyed metal components, for example, salt inclusions are known to affect the fatigue performance and weldability of titanium alloys. [0010] A consequence of the presence of those contaminating impurities in electrolytically reduced metal powders is that they are unsuitable for use in lower temperature powder metallurgy processes such as sintering or forging which are performed at temperatures below the alloy melting temperature. [0011] In accordance with the present invention there is provided a method for purifying metal M.sub.1 particles manufactured by an electrochemical reduction process, the method comprising the steps of: [0012] introducing the metal M.sub.1 particles into a heat source at a temperature substantially equal to or higher than the melting point of M.sub.1 so as to cause vaporisation of some or substantially all of the contaminating impurities present; [0013] removing the vaporised impurities from the vicinity of the particles; [0014] and cooling the purified metal M.sub.1 particles. [0015] The present invention thus provides a method for purifying electrochemically reduced metal particles so as to reduce or remove the aforementioned "contaminating impurities" introduced by the reduction process, so as to enable, for example, direct use of the electrolytically reduced metals in lower temperature powder metallurgy processes. [0016] It has been found that the kinds of impurities introduced by such electro-reduction processes are usually more volatile than the metal M.sub.1 and may be conveniently removed by the present process. Such impurities may comprise light metals, for example, calcium and magnesium, as well as salts from the electrolyte, such as calcium chloride. For effective removal of impurities without significant loss of metal M.sub.1, the impurities should preferably be more volatile than the metal M.sub.1 by at least a factor of ten, or preferably 20, in terms of their respective vapour pressures at the processing temperature. Preferably, the processing temperature and/or heating time should be sufficient to remove substantially all the contaminating impurities, for example, so as to reduce their total level to less than 50 ppm, or less than 10% their initial total concentration. Preferably the process is conducted so that there is little or no evaporation (for example, less than 5%) of the metal M.sub.1. [0017] Another feature of metal powders produced by electrolytic reduction of oxide or oxides is that the powder particles tend to be irregular in shape, contain internal cavities and have a rough outer surface. For certain powder metallurgy processes these features are undesirable, and a fully dense spherical particle morphology is preferred, for example in order to impart good flowability. Hence, although the particles may only be partially melted in order to remove said impurities, in most cases the particles need to be fully melted to achieve dense, spherical particles. [0018] Desirably, the heat source is arranged such that the metal particles may be allowed to free fall through the heat source. Suitable heat sources include but are not strictly limited to a hot plasma torch, a hot gas flame, a tube furnace, an induction coil, electric arcs and lasers. [0019] In one embodiment of the invention, the particles may be blown through a plasma torch into the flame and allowed to free fall into a collecting vessel. The torch is preferably arranged at a sufficient height above the vessel such that any portion of the particles that is melted by the heat source is substantially solidified before collection. This prevents distortion of the shape of the particles. [0020] In another embodiment the established method of levitation melting may be used to hold particles of powder in a surrounding heat source for a period sufficient to vaporise the impurities. In a further embodiment, zero gravity processing may be used. The common purpose of these embodiments is to suspend the particles individually in mid-air, out of contact with any surfaces, so as to enable particles to be heated and melted and resolidified individually, without contact with each other, or a container. [0021] The temperature of the heat source should be at or above the melting point of the metal M.sub.1. For titanium the desired point is about 1680.degree. C. and for titanium alloys approximately 1500-1800.degree. C. It is known that an argon or helium arc plasma torch can achieve temperatures sufficient to melt titanium powder particles entrained in the flame. Ideally, the process is conducted in a controlled atmosphere, for example, at low pressure/vacuum and/or in an inert atmosphere. In the case of Ti it is important that the powder particles are kept separate from oxygen or nitrogen while above a temperature of about 500.degree. C. This is achieved in practice either by processing in a vacuum or in an atmosphere of Ar. Continue reading about Purification process... 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