Abstract:
A plasma torch for vaporizing a molten salt containing a volatile component and a refractory component injects the molten salt into a device that includes a cylindrical shaped outer member and a cylindrical shaped inner member coaxially positioned inside the outer member to surround a chamber. An induction coil positioned between the inner and outer members generates r.f. power which is initially used to vaporize the volatile component of the molten salt to create a carrier gas having an elevated temperature. The carrier gas then heats the refractory component, under an increased vapor pressure from the carrier gas. This action, in turn, breaks down the refractory component of the molten salt into fine droplets. These fine droplets are maintained in the chamber until they also vaporize. In one embodiment, the plasma torch includes a nozzle for spraying droplets of the molten salt into said chamber. In another embodiment, a jet is positioned at the entrance of the chamber to direct the molten salt tangentially onto the inner wall. This creates a film of the molten salt which partially evaporates in the chamber. For this embodiment a diverter is positioned at the exit of the chamber to redirect unevaporated molten salt back to the jet for recycling.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to high frequency Inductively Coupled Plasma (ICP) torches. More specifically, the present invention pertains to ICP torches which minimize the gas feed that is required to initiate and maintain the atomization or vaporization of molten salt materials. The present invention is particularly, but not exclusively, useful as an ICP torch for a molten salt, such as a multi-component nuclear waste slurry, which includes both a volatile component and a refractory component. 
     BACKGROUND OF THE INVENTION 
     Various types of ICP torches which can produce high temperature gaseous plasmas for such purposes as plasma etching, evaporation of refractory materials, spectroscopy, sintering waste incineration and mitigation are well known. In large part, the wide range of applications for which ICP torches can be used is due to the fact that these torches are generally capable of producing heat loads in excess of 100 MW/m 2  on the surface of small particles or droplets injected in the plasma. Another application, among several, which is attracting new attention is the creation of plasmas for the purposes of remediating the refractory components of nuclear waste. Importantly, it is also well known that even the more sturdy refractory materials, such as are found in nuclear waste, will vaporize under heat loads around 100 MW/m 2 . The challenge in this case is to attain and maintain such heatloads. 
     In a typical operation, an ICP torch will produce a plasma by ionizing a gaseous substance with a high frequency RF electromagnetic field (i.e. RF. power). For such operations, the gaseous substance in this case is usually referred to as a carrier gas, and the electromagnetic field is typically produced by an induction coil at frequencies in a general range of 0.4-30 MHz. In any case, the result is a high temperature gas flow having temperatures that reach upward to about 10,000-20,000° K. It happens, however, that the power density that can be generated in an ICP torch is limited by the heating of the side wall of the plasma torch chamber. Thus, the side wall of the torch chamber should have a high heat conductivity to keep the wall temperature at a sufficiently low operational temperature (e.g. significantly below the range of 10,000-20,000° K). At the same time, the side wall should also have a high electrical resistivity to allow for the penetration of an AC electromagnetic field into the plasma chamber. 
     While the ionization, atomization or vaporization of volatile components can be accomplished using heat loads that are generated at relatively low temperatures (e.g. below 100 MW/m 2  and well below the range of 10,000-20,000° K), this is not the case for refractory components. In fact, the vaporization of a refractory component will often require heat loads that are in excess of the 100 MW/m 2  mentioned above. Consequently, very high temperatures must be accommodated if refractory components are to be vaporized. 
     One solution to the high temperature problem has been to cool the wall of a plasma torch with a gas vortex that is created by injecting gas tangentially onto the wall. Although such a procedure may be efficacious for the purpose of cooling the chamber wall, it will also contribute to the throughput of the torch. Further, the total throughput will be increased if a carrier gas is used in the ICP torch along with the cooling gas vortex. In some applications, however, these consequences may present a significant disadvantage. For example, in applications where refractory components need to be vaporized, it may be desirable to minimize the amount of gas in the throughput. Specifically, when refractory components are to be vaporized in a plasma torch, it may be necessary that the resultant plasma be transferred to a vacuum chamber for subsequent processing. In such applications, the efficacy of the subsequent processing and the efficiency of vacuum pumps can only be increased by decreasing the amount of gaseous throughput. 
     In light of the above, it is an object of the present invention to provide an ICP torch and a method for vaporizing a molten salt that contains both a volatile component and a refractory component wherein the volatile component is initially vaporized to create a carrier gas that will heat the refractory component, which will then be vaporized. Another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which reduces the gas-to-waste feed ratio to minimize the gas throughput. Yet another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which will control the deposit of condensed vapors inside the chamber of the torch. Still another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which is relatively simple to manufacture, is easy to use and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, an inductively coupled plasma torch atomizes a molten salt that contains both a volatile component and a refractory component. More specifically, the molten salt is atomized in the plasma chamber of the plasma torch, in stages. Initially, the plasma torch vaporizes the volatile component of the molten salt to create a carrier gas in the chamber. The torch then uses the heat and pressure that are generated by the carrier gas to promote a subsequent vaporization of the refractory component. The result is a lower gas throughput for the plasma torch. Additionally, the plasma torch is constructed to prevent, or at least minimize, the condensation of molten salt vapors in the chamber that would otherwise adversely affect the operation of the plasma torch. 
     In the general aspects of its construction, the plasma torch of the present invention includes a cylindrical shaped outer member and a cylindrical shaped inner member that is coaxially positioned inside the outer member. With this configuration, a space is established between the two members. The purpose of this space is actually twofold. First, it is the location for the induction coil which is used to generate r.f. power for the plasma torch. Second, the space also holds a fluid coolant which cools the induction coil, as well as the torch itself. Additionally, the inner member defines an axially elongated chamber. 
     Depending on the particular mechanism that is used for injecting the molten salt into the chamber, the inner member will be constructed with different configurations. For one embodiment, the inner member will be configured to accommodate a cleaning gas which will enter the chamber and remain near the wall of the inner member. In another embodiment, the inner member is configured to support and carry a film of the molten salt. In either case, the inner wall is constructed to help minimize the gas-to-waste feed ratio and to control deposits on the wall. 
     In one embodiment of the present invention, the mechanism for injecting the molten salt into the chamber is a nozzle or multiple nozzles. Specifically, the nozzle is designed to spray droplets of the molten salt into the chamber that have diameters which are approximately less than one hundred microns (&lt;100 μm). Additionally, the injection mechanism for this embodiment can include various passageways for directing a cleaning gas, such as sodium vapor or water vapor, over the inner wall. The main purpose of this cleaning gas is to inhibit, or prevent, the condensation of molten salt vapors on the inner wall. 
     When a nozzle is used to inject a molten salt into the chamber of the plasma torch, the cylindrical inner wall will include a plurality of elongated, preferably copper, segments. Specifically, each segment is aligned substantially parallel to the axis of the chamber, each segment is juxtaposed between two other segments, and each segment is formed with an axially aligned liquid coolant channel. Further, a spacing plate, that is made of an electrically insulating material, is positioned between each pair of juxtaposed segments. Additionally, each segment is provided with a ceramic shield which is mounted on the segment to interface with the chamber. Preferably, the ceramic shield is made of a refractory material which has a low electrical resistivity and a high thermal shock resistance. 
     For another embodiment of the present invention, the mechanism for injecting a molten salt into the plasma chamber includes a jet which is positioned at one end of the chamber to direct the molten salt tangentially onto the inner wall. In general, the molten salt is injected onto the inner wall with a tangential velocity (v θ ) that is in a range from about one-half to about two meters per second (v θ =0.5 to 2 m/sec). Specifically, this is done to create a film of molten salt which will swirl through the chamber on the inner wall. As the molten salt evaporates from the surface of this film, droplets of the molten salt will react with r.f. power from the induction coil in a manner similar to the nozzle version of the present invention as previously described. Unlike the nozzle version, however, the inner wall for this embodiment will need to be relatively smooth in order to reduce turbulence in the flow of the film. Further, for this embodiment, a diverter is positioned at the opposite end of the chamber from the jet to receive the unevaporated molten salt film from the inner wall. The diverter will then remove unevaporated molten salt from the chamber and redirect the unevaporated molten salt back to the jet for recycling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is a perspective view of one embodiment of the ICP torch of the present invention, with the torch shown schematically in combination with peripheral components; 
     FIG. 2 is a cross sectional view of the ICP torch as seen along the line  2 — 2  in FIG. 1; 
     FIG. 3 is a cross sectional view of the ICP torch as seen along the line  3 — 3  in FIG. 1; 
     FIG. 4 is a perspective view of another embodiment of the ICP torch of the present invention, with the torch shown schematically in combination with peripheral components; and 
     FIG. 5 is a cross sectional view of the ICP torch as seen along the line  5 — 5  in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, one embodiment for an ICP torch in accordance with the present invention is shown and is generally designated  10 . A source  12  of the material which is to be vaporized by the torch  10  (e.g. molten salt) is also shown in FIG.  1 . As intended for the present invention, the material that is held in the source  12  includes both a volatile component, such as sodium oxide or sodium hydroxide, and a refractory component, such as the refractory metal oxides Al 2 O 3  or UO 3 . The present invention, however, is not limited to only the materials mentioned herein. Instead, the present invention contemplates the treatment of many different types of waste streams, including nuclear waste. 
     In FIG. 1 it will be seen that the torch  10  has a generally cylindrical shaped body member  14  and a nozzle  16  which is mounted at one end of the body member  14 . As shown, the nozzle  16  is connected in fluid communication with the molten salt source  12  via the feed line  18 . Additionally, FIG. 1 shows that a source  20  of a cleaning (shielding) gas and an RF generator  22  are provided as peripheral equipment. For the purposes of the present invention, the typical operational parameters for the RF generator  22  will be a frequency of about 3 MHz, a power of about 150 kW and a loop voltage of about 3 kV. 
     In detail, the construction of the torch  10  is perhaps best appreciated by cross referencing FIG. 1 with FIG.  2 . When doing so, it can be seen that the body member  14  includes several specific components which are positioned between an upper end plate assembly  24  and a lower end plate assembly  26 . These components include a cylindrical outer member  28  and a cylindrical inner member  30  which is coaxially positioned inside the outer member  28 . A space  32  is thus established between the outer member  28  and the inner member  30  which serves as a water jacket for holding a fluid coolant. More specifically, the water jacket is used to cool an induction coil  34  which is positioned in the space  32 . With this particular construction, it is preferable that the inner member  30  be made of quartz, or of some other electrically non-conductive material, so that the electromagnetic field which is generated by the induction coil  32  can radiate into the chamber  36 . 
     Between the inner member  30  and the chamber  36  of the torch  10  there is a segmented wall  38  which, in effect, is an extension of the inner member  30 . Preferably, the segmented wall  38  is made of copper and, as shown by cross referencing FIG. 2 with FIG. 3, the wall  38  includes a plurality of elongated segments  40  which surround the chamber  36 . Further, as shown in FIG. 3, there is a spacing plate  42  which is positioned between every pair of juxtaposed segments  40  (e.g. segments  40   a  and  40   b  in FIG.  3 ). Preferably, this spacing plate  42  is made of an electrically insulating material which will allow the electromagnetic field that is generated by the induction coil  34  to enter the chamber  36 . Further, as seen in FIG. 3, each of the segments  40  is provided with an axial water channel  44  which will allow water to be pumped through the segment  40  for the purpose of helping to cool the segments  40  and also the inner member  30 . 
     Still referring to FIG.  2  and FIG. 3, it will be seen that the segmented wall  38  includes a plurality of armor heat shields  46  which also help to cool the body member  14 . Preferably the shields  46  are ceramic and made of a refractory material such as SiC, Al 2 O 3 , SiN, BN, or some other suitable material which can operate at high surface temperatures with minimal thermal stress. As intended for the present invention, the shields  46  can be mechanically attached to the copper segments  40  in any manner well known in the art, such as by brazing. Further, the geometry of the shields  46  is a matter of design preference and may include stress reliefs to reduce thermal stress. As contemplated by the present invention, during the operation of the torch  10 , the surface temperature of the shields  46  will be around 1100° C. 
     Within the structure of the inner member  30  and the segmented wall  38  of the inner member  30 , FIGS. 2 and 3 indicate there is a system of various fluid passageways which will transfer a cleaning gas from the source  20  into the chamber  36 . Specifically, a feed  48  is provided to transfer a cleaning gas, such as a sodium vapor or a dry water vapor, from the source of cleaning gas  20  to the body member  14 . As best seen in FIG. 2, the cleaning (shielding) gas first enters a primary fluid passageway  50  that is located inside the inner member  30  and next to the segmented wall  38 . A plurality of cross fluid passageways  52  then pass the cleaning gas through the spacing plates  42 , and between the segments  40 , to the injection fluid passageways  54 . The cleaning gas then enter the chamber  36  from the injection fluid passageways  54  and is directed from there to cover the inner wall  56  of the chamber  36 . Additionally, it can be seen in FIG. 2 that the feed line  48  is also in fluid communication with the chamber  36  of torch  10  via a fluid passageway  58  which surrounds the nozzle  16 . In each case, the purpose of the feed line  48  and the fluid passageways  50 ,  52 ,  54  and  58  is to provide a cleaning (shielding) gas which will cover the inner wall  56 , and help protect the inner wall  56  from an unwanted build-up of deposits during the operation of the torch  10 . For the purposes of the present invention, the flow rate of the cleaning (shielding) gas will be about seven liters per minute. 
     In the operation of the torch  10  of the present invention, a gas such as Argon is first used to initiate the reaction in chamber  36 . Specifically, in order to initiate operation of the torch  10 , RF power is generated inside the chamber  36 . This is done by the generator  22 , through its connections  60   a  and  60   b  with the induction coil  34 . The Argon (or any other recycling gas) is then fed through the nozzle  16  into the chamber  36 . The result is that the RF power atomizes the Argon gas in the chamber  36  to, thereby, heat the inner wall  56  of the chamber  36  to some nominal value. Next, the cleaning (shielding) gas is introduced into the chamber. Once the cleaning gas is being introduced into the chamber  36 , the molten salt from source  12  can begin to be gradually fed into the chamber  36 . The flow rate of molten salt through the nozzle  16  and into the chamber  36  continues to be gradually increased until it reaches an operational throughput flow of about one gallon per second. While the molten salt throughput is being increased, the injection of the Argon is gradually decreased until it is no longer necessary to inject the Argon. The injection of the cleaning (shielding) gas, however, is not changed and it continues throughout the operation of the torch  10 . 
     As indicated earlier, when the molten salt is injected into the chamber  36 , it is injected as droplets  62  (see FIG. 2) which have a diameter in a range of from fifty to one hundred microns. When they are initially injected, the droplets  62  will include both the volatile component and the refractory component of the molten salt. Shortly after being injected into the chamber  36 , however, the volatile component of the droplets  62  is vaporized to create a working gas in the chamber  36 . Within the working gas which results from the vaporization of the volatile component, temperatures and pressures in the chamber  36  are dramatically increased. In turn, these temperatures and pressures break down the refractory component of the molten salt. Specifically, this break down continues until the refractory components are distilled into fine droplets  64  (see FIG. 2) of molten oxides which will have diameters that are typically less than about one micron. At this point, heat loads of approximately 100 MW/m 2  can be generated on the fine droplets  64 . As indicated above, such heat loads are sufficient for vaporization of even refractory materials. It is helpful to note that during the break down of the refractory component into the fine droplets  64 , the fine droplets  64  are accelerated to a velocity of about 100 m/s by the pressures that result from vaporization of the volatile components. With these velocities, if the chamber  36  has a length of about 5 cm, the residence time of the fine droplets  64  in the chamber  36  will be about 5×10 −4  seconds. The 5×10 −4  seconds, although short, is sufficient for full evaporation of the residual refractory component in the fine droplets  64  before they leave the torch  10  through the exit aperture  66 . 
     An alternate embodiment for the ICP torch of the present invention is shown in FIG.  4  and is generally designated  80 . Insofar as the physics involved in the operation of torch  80  is concerned, the same phenomena described above in conjunction with the torch  10  apply equally to the torch  80 . Namely, the molten salt is provided as droplets  62  which contain both a volatile component and a refractory component. The volatile component is first vaporized to create a working gas, and the working gas breaks down the refractory component into fine droplets  64 . The fine droplets  64  are then also vaporized. With this in mind, the primary differences between the torch  80  and the torch  10  is the fact that for the torch  80 , the molten salt itself is used for maintaining relatively lower operational temperatures on the structure of the torch  80 . Recall, for the torch  10  the segmented wall  38  performed this function. Additionally, for the torch  80 , the molten salt itself is used to shield the inner wall  56  from unwanted depositions. For the torch  10  this function was accomplished using the cleaning (shielding) gas. 
     As best seen in FIG. 5, the torch  80  includes an outer cylindrical member  82  and an inner cylindrical member  84 . The members  82  and  84  are coaxial and, as shown, they establish a space  86  between them for an induction coil  88 . In a manner as described above with regard to the torch  10 , the space  86  for torch  80  not only provides a position for the induction coil  88 , it also establishes a water jacket for the induction coil  88 . Further, it will be seen that the inner member  84  establishes an inner wall  90  that effectively defines the chamber  92  of torch  80 . 
     Instead of using a nozzle  16  for injecting molten salt into the chamber  92 , as is done for the chamber  36  of torch  10 , the torch  80  uses a plurality of feed lines  94   a-d . As indicated in FIG. 5, the feed lines  94 , of which the feed line  94   a  is exemplary, will introduce the molten salt as a slurry  96  which is directed tangentially against the inner wall  90 . Specifically, the slurry  96  is directed onto the inner wall with a tangential velocity that is in a range from about one-half to two meters per second (V θ =0.5 to 2 m/sec). The molten salt then swirls through the chamber  92  on the inner wall  90  as a thin film. 
     The operation of the torch  80  is initiated in a manner similar to that disclosed above for torch  10 . Specifically, a noble gas, such as Argon, is used to heat the chamber  92 . Once the chamber is heated, the throughput of molten salt is gradually increased until the Argon gas is no longer required. During the transit of the film of molten salt through the chamber  92 , the heat that is inside the chamber  92  will cause the molten salt to boil off as droplets  62 . As before, the volatile component of the droplets  62  then vaporize to break down the refractory component into fine droplets  64 . The resultant fine droplets  64  of the refractory component, in turn, also vaporize. All of this occurs in the manner described above for the operation of torch  10 . In the operation of the torch  80 , however, there will be some residual molten salt film which, after passing through the chamber  92 , will need to be recycled. This recycling is accomplished by using diverters (catchers)  98  which are positioned near the exit aperture  100  of the torch  80  to remove the molten salt film from the chamber  92 . The molten salt is then transferred from the diverters (catchers)  100  via the return lines  102  and the conduit  104  to the source  12  of molten salt for recycling. 
     While the particular Induction Plasma Torch Liquid Waste Injector as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.