Abstract:
A process for producing atomized powder, such as a metal powder, using a recirculating atomization gas, such as helium, and the apparatus used for producing the atomized powder

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a process and apparatus for producing atomized powder from a molten stream of material, such as a molten metal stream, using a recirculating gas, such as helium gas.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known that atomized powders can be produced by injecting a gas stream around a molten material stream through an atomizating nozzle. The atomized powders produced in this type of atomizating process can vary in shape and size. Generally, in some applications, it is desirable or necessary to have the powder in the shape of small spherical particles. Making small spherical particles depends on many factors such as material composition, temperature and velocity of the molten material, gas composition, and temperature and velocity of the gas. For example, using helium as the atomizing gas has been known to reduce particle size and improve production capacity. U.S. Pat. No. 4,988,464 discloses a molten material that is fed through a nozzle and gas is fed around and along the molten stream through nozzle. The gas stream forms an outwardly expanding cone that defines an outer boundary and an inner boundary of the gas cone. This divergence increases the area of contact between the annular gas stream and the ambient atmosphere within the annular gas stream. A portion of the gas injected though nozzle in the outwardly expanding cone defined by the boundaries reverses its flow direction and flows toward the nozzle in a direction axially opposite to that of the diverging annular gas stream. The atomizing gas may reverse direction from the entrance of nozzle forming droplet and then the spherical particles. It has been observed that higher velocity in the direction has produced smaller, more spherical droplets. In addition to the nozzle described above, other nozzle designs, such as a “free fall” design should give more spherical particles with the higher gas velocities available with helium. Since sonic velocity is directly proportional to gas density, helium has the higher potential velocity than other gases except hydrogen. Thus, helium is the preferred gas for applications requiring smaller and more spherical particles.  
           [0003]    U.S. Pat. No. 5,390,533 discloses a process and system for pressurizing a vessel for integrity testing with gas comprising helium, and purifying the gas comprising helium for reuse. The process for purifying the gas stream comprising helium comprises drying the gas stream; separating the dried gas stream in a membrane separator stage into a helium-enriched permeate product stream and a helium-depleted raffinate stream; recovering helium in the raffinate stream in a membrane stripper stage thereby producing a purge stream; and purging water from the dryer with the purge stream.  
           [0004]    U.S. Pat. No. 4,845,334 discloses a system and method for conditioning and recycling inert gases that are used in a plasma furnace. The method comprises the steps of receiving the gas that is output from the plasma furnace and cooling the gas to a desired temperature. Substantially all the dust is removed from the gas and then the gas is compressed to the desired pressure using an oil-flooded screw compressor. Any oil which is introduced by the compressor is removed from the gas and an alarm is provided to signal a high oil level. Substantially all water vapor in the gas is removed and the gas is filtered to remove any remaining dust and small particulates. The amounts of water vapor and oxygen in the gas are monitored. The gas is then recycled to the plasma furnace.  
           [0005]    U.S. Pat. No. 5,377,491 discloses a system and process for recovering high purity coolant gas from at least one fiber optic heat exchanger, characterized by controlling a flow of coolant gas into and out of the heat exchanger using a pressure, impurity and/or flow rate monitoring or transmitting means in conjunction with a flow adjusting or controlling device to limit air or other gas infiltration into at least one fiber optic passageway of the heat exchanger. A sealing means may also be used at at least one end of the fiber optic passageway to further reduce air or other gas infiltration into the passageway. The resulting high purity coolant gas from the outlet of the heat exchanger is delivered to the inlet of the heat exchanger. Optionally, the resulting coolant gas from the outlet of the heat exchanger may be cooled, filtered and/or purified before being delivered to the inlet of heat exchanger.  
           [0006]    U.S. Pat. No. 5,158,625 discloses a process for heat treating articles by hardening them in a recirculating gas medium which is in contact with the treated articles, the hardening gas being cooled by means of a heat exchanger, of the type in which helium is used as hardening gas, and is stored under holding pressure in a buffer container, wherein at the end of a hardening operation, a helium load is extracted from the treatment enclosure, in final phase by means of pump until a primary vacuum is obtained, the extracted helium is brought to purifying pressure by means of a compressor associated to a mechanical filter, and the helium under purifying pressure is sent to a purifier in which impurities are removed, after which it is transferred, if desired, after recompression in the buffer container.  
           [0007]    It is an object of this invention to provide a cost effective helium recovery system for atomization furnaces to produced atomization powder.  
           [0008]    It is another object of this invention to provide an atomization furnace with a helium recovery system that will remove contaminants such as O 2 , Ng, H 2 O, CO, C 2 O, metal, and metal salts from spent helium exiting from the atomization furnace.  
         SUMMARY OF THE INVENTION  
         [0009]    The invention relates to a process for producing atomization powder using recirculating atomizing gas comprising the steps:  
           [0010]    (a) feeding a stream of atomizing gas, such as helium, at the desired temperature and pressure along with a stream of molten material into at least one atomization furnace such that the atomizing gas contacts the stream of molten material to form droplets and then solidifying the droplets to form powder;  
           [0011]    (b) removing particulates from the spent atomizing gas;  
           [0012]    (c) feeding at least a portion of the particulate-free atomizing gas to a purification unit to remove selected impurities; and  
           [0013]    (d) recirculating the purified atomizing gas back into the atomization furnace.  
           [0014]    The atomizing gas is generally an inert or substantially inert gas such as argon, helium or nitrogen. Some atomization furnace systems use water or a combination of inert gases. However, manufacturers of specialty powdered metals can not use water as the atomizing gas and preferably would like to use helium because of its inertness, good thermal conductivity and high sonic velocity. The sonic velocity of helium is approximately three times greater than argon. As stated above, higher velocities result in a smaller and more spherical particle. Spent helium gas from an atomization furnace could contain one or more contaminants such as oxygen, nitrogen, water, carbon monoxide, carbon dioxide, hydrogen, metal, and/or metal salts. The subject invention is directed to a recirculating atomizing gas system that will remove one or more of the contaminants from a spent atomizing gas from an atomization furnace. An atomization furnace generally consists of several components such as a vacuum or induction furnace where a batch of metal is first melted in a tundish and then atomized in an atomizing tower or chamber. The molten metal then flows through the tundish down through a small nozzle where it is atomized by the gas. The metal droplets are cooled as they float downward in the chamber. In some cases a bath of liquefied gas is used in the bottom to provide additional cooling. The use of helium may allow for a much simpler or more flexible design. For instance, for any particular furnace, the time spend atomizing may represent a small fraction of a day such as less than an hour or two. If the gas cost is significantly reduced, then the furnace design might change to allow the continuous melting and atomizing. The resultant furnace may be significantly smaller with higher capacity  
           [0015]    The use of a helium recycle system has economic advantages over an argon recycle system since the separation of argon from other contaminants is much more difficult than helium from the same contaminants. An argon recycle system would most likely have a cryogenic column for purification. The use of a cryogenic column and supporting equipment is significantly more capital intensive than a helium recycle system. However, an atomizing application may use argon recycle with membrane, thermal swing absorbent (TSA), pressure swing adsorption (PSA) and/or copper oxide technology. The choice of purification technology will depend on the atomizer off-gas impurities and atomizer inlet gas specifications.  
           [0016]    Generally the molten material is metal such as iron, steel, copper, nickel, aluminum, magnesium, lead, tin, titanium, cobalt, vanadium, tantalum and their alloys, or it may also be used to produce non-metallic powders such as employing oxides and/or ceramic materials as the molten stream.  
           [0017]    This invention also relates to an apparatus for producing atomization powder using recirculating helium-based gas comprising at least one atomization furnace having at least one input adapted for receiving gas and molten material and at least one output adapted for discharging spent gas and powder; a particulate removal unit coupled to the at least one atomization furnace and adapted for removing particulates from the spent gas; a purification unit coupled at one end to the particulate removal unit and at an opposite end to the input of the at least one atomization furnace and operatable such that helium-based gas can be recirculated to the atomization furnace for the production of atomized powder on a continuous basis. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0018]    Other objects, features and advantages will occur to those skilled in the art from the following description of preferred embodiments and the following schematic diagram of a system for producing atomization powder using recirculating atomization gas pursuant to this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to the FIG. 1, a helium recovery system is shown with the preferred expected flows and equipment required for a metal atomization furnace that processes 2500 lbs/hr of molten metal. FIG. 1 shows a metal atomization furnace  20  that can make fine powder using helium at a flow rate such as 4000 SCFM at a pressure of approximately 1100 psia. The helium gas with a light loading of impurities and particulate (generally &lt;2PPM impurities and &lt;1.5 mg/m 3 ) leaves the atomization furnace at a low pressure (generally 14.7-19.7 psia) via duct  22  and passes through valve  23 . The impure helium gas is cooled in heat exchanger  24  and has the particulate removed in bag house or cyclone  25 . After the output gas has been cooled, it is transported to a cyclone  25  so that particulates may be removed. The cyclone  25  is sized so that the particulates which average between about 10 microns and about 200 microns in size can be removed from the bottom of the cyclone. As in a typical cyclone, the particulate output gas enters the cyclone chamber tangentially, the cleansed gases leaving through a central opening at the top of the cyclone  25 . The particulate, by virtue of their inertia, will tend to move toward the outside spearator wall from which they migrate to the bottom of the cyclone.  
         [0020]    The particulate free gas then enters low pressure receiver  27  (pressure between about 14 and about 20 psia, and preferably between about 15 and about 17 psia). The low pressure receiver allows changes in system flows to have a minimal impact on the recovery system or the metal atomization furnace. Impure helium enters a suction of compressor  29  via duct  41  for a boost to the helium recovery system operating pressure (generally between about 300 to about 1600 psia). For the recovery system shown in this example, the desired pressure and flow is generally 1250 psia at 4000 SCFM.  
         [0021]    The discharge of the compressor flows through duct  30  to ducts  31  and  32 . For the powdered metal application being discussed in this example the impurity loading in the helium leaving the furnace is light (&lt;2PPM). Duct  31  takes about 10% of the total flow or 400 SCFM for purification through cryogenic purification unit  33 . The cryogenic purification unit  33  contains molecular sieve at liquid nitrogen temperatures. The cryogenic purification unit  33  removes effectively all gaseous impurities at the PPM level and even to the PPB level except for hydrogen and neon. The pure helium leaves the cryogenic purification unit via duct  34  and rejoins the main gas flow in duct  32 . The flow through duct  30  is controlled by valve  35 . Decreasing flow through valve  35  will increase flow through duct  31 . The cryogenic purification unit  33  can operate at liquid oxygen temperatures if oxygen adsorption is not important and up to 200° K if carbon monoxide or hydrogen disulfide need to be removed. Other contaminants such as carbon dioxide and water could be effectively removed at room temperature.  
         [0022]    The high pressure purified helium  32  enters high pressure receiver  36  (pressure between about 50 and about 1600 psia, and preferably between about 100 and about 300 psia) and then enters the metal atomization furnace through duct  38 . High pressure receiver  36  will minimize changes in pressure caused by changes in flow through the helium recovery system.  
         [0023]    Helium consumption will occur during the metal atomization process, during regeneration of the purification unit  33 , when the furnace is opened for powdered metal removal or maintenance and through any leaks in the system. Helium make-up will come from helium storage  39  through duct  40  as the pressure in low pressure receiver  27  drops below set point. A typical set point for the low pressure receiver could be about 15 psia. Compressor  29  suction will cause the pressure to drop in low pressure receiver  27  as the quantity of helium in the recovery system drops.  
         [0024]    The metal atomization furnace will periodically be opened to atmosphere. After the furnace is opened, the furnace chamber will be pumped down through duct  42  by vacuum pump  43 . Before pump down, valve  44  is opened and the inlet to the furnace (not shown) and valve  23  is closed. The furnace chamber is pumped down by pump  43  to remove a majority of the air from the furnace (˜10 milliTorr). Following the pump down valve  44  is closed and the furnace is back filled with helium (duct not shown) to operating pressure (e.g. 15 psia). The inlet to the furnace  20  and valve  23  are opened after the furnace is back filled with helium. Before a melt is started, some metal atomization systems will not use vacuum pump  43  but will purge or remove contaminates from the furnace via the helium recovery system.  
         [0025]    The helium recovery system can operate when the furnace is not. Helium will bypass the furnace by flowing through duct  45  and valve  46  to duct  47 . The helium recovery system would operate in bypass when the helium quality was not in specification and needed to be purified before the start of a batch of metal or if an unexpected load of impurities entered the recovery system and needed to be removed before entering the furnace. The furnace bypass could also be used to adjust the flow to the furnace through duct  38  if metal atomization requires variable flow rates of helium. Oil from the pumps could be removed from oil removal unit  49  via duct  50  and fed through duct  48 .  
         [0026]    A Preferred Mode Of Operation  
         [0027]    Under the best mode of operation, the system would remove gas from the atomizer  20  and pass through duct  22  through the cooling shell and tube heat exchanger  24  and particulate removal cyclone  25 . The particulate free gas would pass through duct  47  and duct  45  whereas surge tank  27  is not needed. Compressor  29  could be comprised of an oil flooded screw compressor followed by a valve-in-piston compressor that increases the pressure to about 190 psig and about 1200 psig respectively. The compressed gas stream passes through oil removal filtration and becomes essentially hydrocarbon free gas in duct  30 . A flow element in duct  31  controls the opening of valve  35  suggest that 10% of the volumetric flow in duct  30  passes through duct  31  to the cryogenic adsorption unit  33 . The cryogenic adsorption unit  33  removes essentially all the impurities in the helium. The pure helium mixes in duct  34  mixes with the helium stream that passes through valve  35  in duct  32 . The gas stream in duct  32  contains contaminants at a level below that required by the gas specification. Gas fills ballast tank  36  at approximately the same rate as the gas exits the ballast tank  36  via duct  38 . Duct  38  delivers gas to the atomizer that requires gas at a minimum pressure of 1150 psig. Any excess gas can pass thorough duct  45  via valve  46  to duct  47 . The pressures in ballast tank  36  and duct  45  control valve  46 . Valve  46  will open if ballast tank  36  pressure is above 1150 psig and pressure in duct  41  below 0 psig. If ballast tank  36  pressure is below 1150 psig, then pressure in duct  41  will continue to fall below 0 psig and helium will flow through a regulator in helium manifold  51  from helium storage  39 . Once per week, when the helium recovery system is not in operation, cryogenic adsorption unit  33  is regenerated. The sieve bed inside the adsorption unit can be heated to 200° F. and dry helium as it passed through the bed to remove the impurities. Cryogenic adsorption unit  33  is then cooled to operational temperatures with liquid nitrogen and is ready for service. The helium recovery system would be placed in operation before flow through duct  38  to atomizer  20  is commenced. At helium recovery system startup, gas would flow through duct  45  and valve  46  to duct  47 . Valve  35  would follow the same control logic as described above.  
         [0028]    In another embodiment of the subject invention, the recirculated gas would comprise a mixture of two or more gases such as argon, nitrogen and helium. However, a recovery system based on argon or nitrogen may require a cryogenic separation column to remove impurities. For atomization applications where the discharge pressure of the furnace is too low, a blower can be installed in duct  22  or duct  47  to satisfy the suction requirements of the compressor and the pressure drop encountered in heat exchanger  24  and particulate removal  25 . For applications where helium of high purity is needed, then the cryogenic purification unit  33  can be removed from duct  31  and  34  and placed in duct  30 .  
         [0029]    The cryogenic purification unit  33  can be replaced with thermal swing adsorption (TSA), pressure swing adsorption (PSA) or membrane technology depending on the purity requirements in the furnace and the contaminate load from the furnace. The particulate removal  25  will vary for each application depending on the individual size, hazards and total volume of particulate from the furnace. Other choices for particulate removal  25  could consist of but not limited to cartridge filters, scrubbers, and cyclones. The heat exchanger  24  could be placed before or after particulate removal  25  depending on the application.  
         [0030]    The helium remaining in furnace  20  before the furnace is opened for powder removal or maintenance could be removed by vacuum pump  43 . The discharge of the vacuum pump would need to be oil and particulate free. Oil filtration  49  would be used to remove hydrocarbons and particulate from the discharge of vacuum pump  43 . If the discharge of vacuum pump  43  has a high temperature then the discharge should be ducted to duct  22 . If the discharge of the vacuum pump has a temperature close to ambient, then it should be ducted to duct  47 . Surge tank  27  should have the necessary volume to capture the evacuated gas. In this case, vacuum pump  43  could operate while compression equipment  29  was not operating. If the evacuation of atomizer  20  took place while the helium recovery system was operating then surge tank  27  would not be needed and ballast tank  36  would need to have the extra volume.  
         [0031]    Compressor  29  can be split into two compressors. The first compressor would use a low pressure frame design that tends to be less expensive such as an oil-flooded screw. The discharge of the oil-flooded screw would then feed the suction for the more expensive high pressure compressor. Feeding the suction of the high pressure machine with intermediate pressure gas will significantly shrink the size of the high pressure machine.  
         [0032]    Table 1 discloses the methods of purification dependent on the type and level of impurities. Item #1 uses a copper oxide getter to remove oxygen only. For a gas stream leaving the furnace with just water as the impurity, then in Item 2 only a dryer (TSA) is need to purify the helium. If only water and oxygen are present then in Item 3, a copper oxide getter and dryer may be the most economical method for purifying the helium item. Item #4 has a light loading of impurities from the atomizer and only requires purification for 10% of the total flow. Items #5, #6, #7 and #8 each increases the percentage of the total flow that must be purified for the helium to maintain specified purity. The purification unit changes from cryogenic adsorption to PSA as economics for PSA improves with increasing flow. Item #9 uses a membrane to remove oxygen and nitrogen from a steam where an excess of nitrogen is present. Item #10 uses a membrane and TSA. The TSA was added to remove water. Item #11 uses a copper oxide getter in the main stream to remove oxygen. The slipstream contains the membrane and TSA to control the nitrogen and water. In Item #11, the membrane was not used as the primary purification for oxygen since more than a 10% slipstream would have been required to maintain the helium specification. The different purity specifications may be a result of economic conditions. For example, a powder manufacture may accept an increase concentration of nitrogen to allow for a less expensive helium recovery system.  
                                                           TABLE 1                           Purification Vs Impurities in Atomization Off-Gas                        Type of   Gas Specifica-           Impurities   Slip Stream %   Purifier   tions       Item   (FIGURE 1,   (FIGURE 1,   (FIGURE 1,   (FIGURE 1,       #   #22)   #31)   #33)   #32)                    1   10 PPM to 100   100%   Copper Oxide   &lt;20 PPM O 2             PPM Oxygen       Getter   &lt;20 PPM N 2             Only           &lt;20 PPM H 2 O       2   H 2 O Only   Dependent on   TSA   &lt;20 PPM O 2                 PPM of H 2 O       &lt;20 PPM N 2                         &lt;20 PPM H 2 O       3   20 PPM H 2 O   50%   Copper Oxide   &lt;20 PPM O 2             10 PPM O 2         Getter &amp; TSA   &lt;20 PPM N 2                         &lt;20 PPM H 2 O       4   &lt;2 PPM O 2     10%   Cryogenic   &lt;20 PPM O 2             &lt;2 PPM N 2         Adsorption   &lt;20 PPM N 2             &lt;2 PPM H 2 O           &lt;20 PPM H 2 O       5   &lt;4 PPM O 2     20%   Modified   &lt;20 PPM O 2             &lt;4 PPM N 2         Cryogenic   &lt;20 PPM N 2             &lt;4 PPM H 2 O       Adsorption or   &lt;20 PPM H 2 O                   PSA       6   &lt;6 PPM O 2     30%   Modified   &lt;20 PPM O 2             &lt;6 PPM N 2         Cryogenic   &lt;20 PPM N 2             &lt;6 PPM H 2 O       Adsorption or   &lt;20 PPM H 2 O                   PSA       7   &lt;8 PPM O 2     40%   Modified   &lt;20 PPM O 2             &lt;8 PPM N 2         Cryogenic   &lt;20 PPM N 2             &lt;8 PPM H 2 O       Adsorption or   &lt;20 PPM H 2 O                   PSA       8   &lt;10 PPM O 2     50%   PSA   &lt;20 PPM O 2             &lt;10 PPM N 2             &lt;20 PPM N 2             &lt;10 PPM H 2 O           &lt;20 PPM H 2 O       9   &lt;2 PPM O 2     10%   Membrane   &lt;20 PPM O 2             10% N 2             1% N 2         10   &lt;2 PPM O 2     10%   Membrane and   &lt;20 PPM O 2             10% N 2         TSA   1% N 2             &lt;2 PPM H 2 O           &lt;2 PPM H 2 O       11   &lt;2 PPM O 2     10%   Membrane,   &lt;20 PPM O 2             10% N 2     Membrane and   TSA and   1% N 2             &lt;2 PPM H 2 O   TSA 100%   Copper Oxide   &lt;2 PPM H 2 O               CuP Getter   Getter                  
 
         [0033]    Although the invention has been described with reference to specific embodiments as examples, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims.