Patent Publication Number: US-7910048-B2

Title: Apparatus for plasma synthesis of rhenium nano and micro powders

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 11/041,870, filed Jan. 25, 2005, now U.S. Pat. No. 7,494,527, and which claims the benefit of U.S. Provisional Application No. 60/538,459, filed Jan. 26, 2004, all of which is are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to rhenium synthesis. More specifically, the present invention is concerned with a process and apparatus for plasma synthesis of rhenium nano and micro powders, and for coating and near net shape deposits thereof. 
     BACKGROUND OF THE INVENTION 
     A conventional process for the production of metallic rhenium powders is described in both 1) Tribalat S. Rhénium et technétium, Gauthier-Villars, Paris, 1957, and in 2) Davenport W. H., Spelman J. W., Vaeth H. J. Rhenium Chemicals, Cleveland Refractory Metals, 1969. This conventional process is based on the hydrogen reduction of ammonium perrhenate according to the following reaction:
 
2NH 4 ReO 4 +7H 2 →2Re+2NH 3 ↑+8H 2 O↑  (1)
 
     This reaction is carried out in two consecutive steps; the first involving the thermal decomposition of ammonium perrhenate at 300° C. into gaseous ammonia and rhenium oxide (IV);
 
NH 4 ReO 4 +3/2H 2 →ReO 2 +NH 3 ↑+2H 2 O↑.  (2)
 
     The second step involves the reduction of the formed rhenium oxide, at 1000° C., to produce metallic rhenium according to the following reaction:
 
ReO 2 +2H 2 →Re+2H 2 O↑  (3)
 
     A drawback of this conventional process is that it is relatively slow, and has to be interrupted after 2 hours, in the event that the product is required in powder form. In this case, the formed sintered porous metal oxide/metallic intermediate product has to be ground to the required particle size, followed by the further hydrogen reduction of the powder for a few more hours. 
     Other processes of bulk rhenium production from the prior art include electrolyse, thermal decomposition of rhenium carbonyl or rhenium tri-chloride, or reduction of rhenium hexa-fluoride. These processes from the prior art are described in Chaudron G., Dimitrov O. Monographies sur les métaux de haute pureté, Chapitre 10 Rhénium, p. 235-242, MASSON, Paris, 1972. 
     Drawbacks from these other processes from the prior art include:
         the yielding of sponge like products difficult to handle and requiring post treatment processing, and   toxicity of the by-products (environmentally hostile).       

     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to provide improved process and apparatus for synthesis of rhenium nano and micro powders. 
     Another object of the present invention is to provide improved process and apparatus for coatings and near net shape deposits of rhenium nano and micro powders. 
     SUMMARY OF THE INVENTION 
     More specifically, in accordance with a first aspect of the present invention, there is provided a process for the synthesis of rhenium powders comprising: injecting ammonium perrhenate powder through a carrier gas in a plasma torch of a plasma reactor operated using a mixture including hydrogen as the plasma gas, yielding metallic rhenium under the following chemical reaction: 2NH 4 ReO 4 +4H 2 →2Re+N 2 ↑+8H 2 O↑; and quenching the metallic rhenium, yielding rhenium powders. 
     According to a second aspect of the present invention, there is provided an apparatus for the synthesis of rhenium powders from ammonium perrhenate, comprising: a plasma torch including a plasma chamber, a reactant feeder for injecting ammonium perrhenate powder in the plasma chamber through a carrier gas including hydrogen; and a reaction chamber mounted to the plasma torch downstream therefrom so as to be in fluid communication with the plasma torch for receiving metallic rhenium from the plasma torch; the reaction chamber being provided with quench means for rapidly cooling the metallic rhenium, yielding rhenium powders. 
     The process and apparatus according to the present invention allows for the plasma synthesis of rhenium nano and micro powders through high reaction rate due to high temperature of the plasma and the fact that the reduced substance (Re 2 O 7 ) is in the vapour state (the overall reaction is in the gaseous phase). The reaction conditions ease the formation of sub-micron and nano-sized metallic products. 
     Forming metallic rhenium powder according to a process from the present invention involves a single step, is simple, and can easily be integrated into a continuous process. 
     A process for plasma synthesis of nano and micro powders according to the present invention involves the thermal decomposition of ammonia, which forms atomic hydrogen, and such in statu nascendi formed, very reactive atomic hydrogen reduces easily the remaining rhenium oxide. 
     The decomposition of the ammonia to elemental nitrogen and hydrogen lowers the overall consumption of gaseous hydrogen to 2 moles of H 2  per mole of ammonium perrhenate as to compare to 3.5 moles of H 2  per mole of the ammonium perrhenate in the conventional process, which amount to almost 43% savings in hydrogen consumption. The remaining nitrogen is environmentally friendly. 
     The process for plasma synthesis of rhenium nano and micro powders according to the present invention yields very pure rhenium products which are limited only by the purity of the raw materials used, since high frequency electrode-less plasma discharges are known not to introduce external sources of reaction product contamination. The process may be carried out for the synthesis of rhenium powders, or for the deposits of rhenium as coatings or near net shaped part. In the latter case, all the reduction steps are accomplished during the in-flight treatment period prior to the formation of the rhenium deposit through successive impacts of the formed rhenium molten droplets on the substrate placed underneath the plasma plume. 
     Other objects, advantages and features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the appended drawings: 
         FIG. 1  is a cross-section of an apparatus for plasma synthesis of rhenium powder according to an illustrative embodiment of a first aspect of the present invention; 
         FIGS. 2A-2B  are electron micrographs of the rhenium powders obtained at the reactor bottom and filter of the apparatus from  FIG. 1  following experiments performed using a process for plasma synthesis of rhenium nano and micro powder from the present invention; and 
         FIG. 3  is are X-Ray diffraction graphs of the rhenium powders obtained at the reactor bottom and filter of the apparatus from  FIG. 1  following the experiments mentioned with reference to  FIGS. 2A-2B . 
     
    
    
     DETAILED DESCRIPTION 
     An apparatus  10  for plasma synthesis of rhenium nano and micro powders according to an illustrative embodiment of the present invention will now be described with reference to  FIG. 1 . 
     The apparatus  10  comprises a plasma reactor  12 , including a generally cylindrical reaction chamber  14  having opposite top and bottom longitudinal end apertures  16 - 18 , a plasma torch  20  mounted on top of the reaction chamber  14  so as to be in fluid communication therewith through said top end aperture  16 , and a first collector in the form of a reactor bottom collector  22  mounted to the reaction chamber  14  through the bottom end aperture  18  via a funnel  24  so as to be in fluid communication therewith and downstream thereof. 
     The plasma torch  20  is in the form of a an induction plasma torch model PL-50 from Tekna Plasma Inc. and includes a generally cylindrical plasma chamber  26 , a reactant feeder  28  for injecting ammonium perrhenate powder in the plasma chamber  26  through a carrier gas, and an input aperture  30  for feeding the plasma chamber  26  with sheath gas. Another stream of gas—so called central gas is fed tangentially into plasma chamber through separate input. The induction plasma torch  20  is powered by a radio frequency generator  32 , which is a 3 MHz generator in the case of the PL-50 model. 
     Of course the plasma torch may be of another type and have another configuration than the illustrated plasma torch  20 . 
     The reaction chamber  14  is in the form of a water-cooled stainless steel chamber, which may be of any form providing enough time to the reaction to occur. The reaction chamber  14  is provided with quench means  34  for rapidly cooling reaction products coming from the plasma torch  20 . 
     According to the illustrated embodiment of  FIG. 1 , the quench means  34  is in the form of a quench gas feeder integral to the reaction chamber  14  and located adjacent the plasma torch  20 , where the distance is controlled by the time required to complete the desired reaction and vary with processing parameters. For given processing parameters this distance was 120 mm. 
     The quench means  34  may also be in the form of a cold finger realized by inserting a water-cooled cylindrical/flat surface insert against plasma jet providing rapid cooling of the off gas, or the cold solid surface in the form of particulate matters in the form of fluid bed or an evaporating liquid injected through fine spraying nozzle thus forming either flat or hollow cone mist barrier against which the plasma gas has to go through. 
     Typical dimensions for the reaction chamber  14  are as follows: 
     length: 1.4 m; 
     diameter: 0.26 m; and 
     diameter of the top longitudinal end aperture  16 : 0.05 m. 
     The first collector  22  comprises a receptacle  36  connected to the funnel  24  so as to be in fluid communication therewith and configured and mounted to the reactor chamber  14  so as to allow collection of rhenium powder by gravity, following the thrust of the plasma jet and/or or by suction as provided by the vacuum system  37  located downstream from the first collector. As will become more apparent upon reading the following description, the vacuum  37  is coupled with the reactor bottom collector  22  so as to be in fluid communication therewith. 
     The apparatus  10  further comprises second collector  38  in the form of a cyclone collector having its inlet  40  connected to the reactor bottom collector  22  via a conduit  42  so as to be in fluid communication therewith and so as to be located downstream therefrom. 
     The apparatus  10  may also comprise a third powder collector  44  in the form of a filter collector, including porous metal filters, having its inlet  48  connected to the outlet  46  of the cyclone collector  38 . 
     Since cyclone and filter collectors and vacuum systems are believed to be well known in the art and for concision purposes they will not be described herein in more detail. 
     Of course, other configurations of reactor collectors may also be provided allowing collecting rhenium powder produced in the plasma reactor  12 . 
     A process for plasma synthesis of rhenium nano and micro powders will now be described according to an illustrative embodiment of a second aspect of the present invention. 
     The single step process is based on the flash heating, decomposition and reduction of ammonium perrhenate. The chemical reactions involved can be represented by the following transformations;
 
2NH 4 ReO 4 →Re 2 O 7 +2NH 3 ↑+H 2 O↑  (4)
 
2NH 3 →N 2 +3H 2   (5)
 
Re 2 O 7 →2Re+7/2O 2 ↑  (6)
 
Re 2 O 7 +7H 2 →2Re+7H 2 O↑  (7)
 
where the in statu nascendi formed rhenium oxide vapour (the sublimation point of Re 2 O 7  is 200° C.) is instantaneously reduced by the in statu nascendi formed, very reactive hydrogen released through the reaction of decomposition of the ammonia. This reaction may be catalytically enhanced by the metallic rhenium coming from possible thermal decomposition of Re 2 O 7  to metallic rhenium and oxygen at 800° C. The supplementary hydrogen required for the completion of the reduction process according to equation 7 is supplied from a plasma gas mixture of Ar and H 2 .
 
     Alternatively to the above-described chemical route, another chemical route for the formation of metallic rhenium according to the illustrative embodiment of the second aspect of the present invention is provided including the thermal decomposition of ammonium perrhenate to Re 2 O 7 , followed by the subsequent thermal decomposition of the formed Re 2 O 7  to metallic rhenium and oxygen at 800° C., according to reactions 4 and 6 respectively. 
     According to this second route, the liberated free oxygen, is then consumed by the strongly exothermic combustion of ammonia and free hydrogen which forms part of the plasma gas according to equations 8 and 9:
 
2NH 3 +3/2O 2 →N 2 +3H 2 O−H r =−633kJ/mol  (8)
 
2H 2 +O 2 →2H 2 O  (9)
 
     The overall reaction, independent of the reaction route, could then be represented by the following chemical transformation:
 
2NH 4 ReO 4 +4H 2 →2Re+N 2 ↑+8H 2 O↑  (10)
 
     Experimental Results 
     Rhenium metal in the form of an ultrafine powder was synthesized using the process and apparatus for plasma synthesis of rhenium nano and micro powders according to the present invention. More specifically, the plasma decomposition/reduction of ammonium perrhenate powder has been achieved using an inductively coupled radio frequency (rf) plasma reactor. The apparatus used is as illustrated in  FIG. 1  and is composed of an induction plasma torch model PL-50 by Tekna Plasma Inc. placed on the top of a water-cooled stainless steel chamber. 
     The ammonium perrhenate powder was axially injected into the center of the plasma torch  20  using argon as the carrier gas. The plasma torch was operated at near atmospheric pressure using an argon/hydrogen mixture as the plasma gas consisting of 10% volume of hydrogen. The ammonium perrhenate feed rate was varied in the range of 7.5-14.3 g/min for a plasma plate power of 60 to 65 kW. 
     As the individual ammonium perrhenate powder particles come in contact with the plasma gas, they are heated rapidly, evaporated and dissociated as per the chemical transformations (10) above. The reaction is completed in the plume of the plasma flow with the reaction products, mixed with the plasma gases, enters the quench section of the reactor  12 . At this point the reaction products are cooled rapidly through their mixture with the quench gas which can be either Argon or recycled Argon/Hydrogen mixture. The cooling of the reaction products gives rise to the homogeneous condensation of the rhenium metal in the form of an ultrafine aerosol with particle size in the nanometer to micron range depending on the cooling rate to which the reaction products were exposed. The formed rhenium powder is collected either on the cold walls of the main reaction chamber, in a downstream cyclone or in a sintered metal filter. It is common practice to expect different properties and particle size distributions of the powder collected at the different location of the reactor and powder collection system 
     Products collected from different locations were analyzed and characterized separately. 
     The powders collected from the reactor walls, reactor bottom and cyclone were micrometric in size formed of agglomerates of much finer particles (80 nm&lt;dp&lt;260 nm). Those collected in the filter (20-30% weight of the total recovered) were nanometric (30 nm&lt;dp&lt;60 nm). 
     Typical electron micrographs of the rhenium powders obtained at the reactor bottom and filter are shown in  FIGS. 2A-2B . The range of particle sizes of the powder was confirmed by a measurement of its specific surface area in m 2 /g using an adsorption isotherm (Brunauer, Emmet, Teller—BET) method. The overall conversion was near 100% based on X-Ray Diffraction (XRD) analysis of the resulting products, as shown in  FIG. 3 , which did not show any presence of residual ammonium perrhenate. The purity of the product was confirmed through residual oxygen analysis (performed using a LECO model RO500C device) which showed values less than 1000 ppm of residual oxygen in the collected rhenium powders. 
     In the event that the formed rhenium particles are impacted on the surface of a substrate before completely solidifying, they would spread on that surface forming tiny lamella that are the building blocks of a fine grained coating and/or near net shaped deposit. In the latter case, the process is continued until the required part dimensions are reached followed by the removal of the substrate using mechanical or chemical means such as respectively machining and etching. The reaction chamber to be used in this case would be similar to the reaction chamber illustrated in  FIG. 1  with the addition of an access port on the upper end  16  of the reactor  12  through which the substrate is introduced at a relatively short distance from the nozzle exit of the plasma torch  20 . Typical spraying distances used in this case can be in the range of fifteen (15) to twenty five (25) centimeters. The position of the quench gas injection is determined so as to allow the condensation of the reaction product in the form of molten rhenium droplets without freezing them, in-flight, which would prevent their deposition on the substrate surface. 
     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.