Abstract:
A crucible for melting material includes at least one wall at least partially enclosing a volume and including an orifice through a portion thereof, and a chill plate supported for movement between at least a first position covering the orifice and a second position. The chill plate is configured for removal of heat from the at least one wall when in the first position. Some crucibles include a substantially flat bottom plate, a plurality of hollow tubes above the substantially flat bottom plate proximate a periphery thereof, and a plurality of O-rings. Each hollow tube is in contact with at least one O-ring, and each O-ring is in contact with the substantially flat bottom plate. A method includes moving the chill plate, initiating flow of molten material, terminating flow of molten material, and replacing the chill plate. Flow of material may be controlled by providing a vacuum in the crucible.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/055,359, filed Sep. 25, 2014, the disclosure of which is hereby incorporated herein in its entirety by this reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under Contract Number DE-AC07-051D14517 awarded by the United States Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD 
       [0003]    Embodiments of the present disclosure relate generally to cold crucibles that may be used for induction melting or other high-temperature processing of materials. 
       BACKGROUND 
       [0004]    Induction melting can be used to melt and heat electrically conductive materials in a crucible or furnace, having cooled walls and a cooled floor, by applying an inductive field to the crucible. Because the walls and/or floor are maintained at a relatively low temperature, such process may be referred to in the art as cold-crucible induction melting (CCIM). CCIM has the potential to simplify and reduce the cost for stabilizing high-level radioactive waste by melting the waste. The molten waste can then be solidified into a glass, glass-ceramic, or pure ceramic form for simplified handling and storage. CCIM can also be used to process other materials, such as high-purity and corrosive products. CCIM is typically an energy-intensive process. 
         [0005]      FIG. 1  illustrates a simplified cross-sectional view of a crucible  100  that may be used in CCIM. The crucible  100  has walls  102  and a floor  104 . A cooling fluid or gas  105  (typically water) passes through the walls  102  and/or the floor  104  to maintain mechanical integrity of the crucible  100 . The walls  102  define a generally cylindrical interior, in which a material  108  to be melted (e.g., glass frit) is placed. An induction coil  106 , connected to a suitable power supply, induces a field on the material  108  in the crucible  100 , which heats the material  108 , either directly or through heat conduction from an initiator (e.g., a metal ring), and the crucible  100 . The cooling fluid  105  maintains the walls  102  and the floor  104  at a lower temperature than the interior of the crucible  100 . Therefore, a skull  110  or solid portion of material may form adjacent the walls  102  and floor  104 . The skull  110  protects the walls  102  and floor  104  while the material  108  is heated to high temperatures, and may seal gaps in the walls  102 . Because of temperature gradients within the material  108 , currents  114  may mix the material  108  during the heating process. 
         [0006]    To remove the material  108  from the crucible  100 , the crucible  100  may be tipped (i.e., rotated about a horizontal axis) to pour material out. Such a process is typically performed in batch mode, wherein substantially all of the molten material  108  is removed at once. In a batch process, the crucible  100  is filled with material  108  in solid form, the material  108  is melted, then the material  108  is poured from the crucible  100 . Some crucibles  100  for CCIM include a mechanism to remove material  108  from the bottom or side of the crucible  100 . For example, as shown in  FIG. 1 , a crucible  100  may include a tap  112  through the floor  104  of the crucible  100 . A tap  112 , which may be heated to prevent freezing of the material  108  thereon, allows molten material  108  to be removed without tipping the crucible  100 , and may allow for finer flow control than simply tipping the crucible  100 . 
         [0007]    A tap  112  in the floor  104  of the crucible  100  may typically be used only once or a limited number of times. For example, a hole may be drilled or punched into the floor  104 , after which all the material  108  (other than the solidified material that forms the skull  110 ) flows out through the tap  112 . In such an operation, there is typically little control over the flow rate of the material  108 , and it is typically difficult or impossible to stop flow before substantially all the molten material  108  leaves the crucible  100 . A slide gate, pin, or cover may be used to close the tap  112 . However, such mechanisms typically become encrusted with portions of the material  108  that solidifies thereon, making repeated use of the tap  112  difficult. Auxiliary heaters near the tap  112  may lessen this problem to an extent, but may interfere with the induction coil  106  and/or the cooling fluid  105 . 
       BRIEF SUMMARY 
       [0008]    In some embodiments, a crucible for melting material includes at least one wall at least partially enclosing a volume and including an orifice through a portion thereof, and a chill plate supported for movement between at least a first position and a second position. In the first position, the chill plate is in contact with the portion of the at least one wall and covers the orifice. In the second position, the chill plate is removed from contact with the at least one wall. The chill plate is configured for removal of heat from the portion of the at least one wall when the chill plate is in the first position. 
         [0009]    In some embodiments, a cold crucible for melting material includes a substantially flat bottom plate, a plurality of hollow tubes above the substantially flat bottom plate proximate a periphery thereof, and a plurality of O-rings. Each hollow tube of the plurality of hollow tubes is in contact with at least one O-ring of the plurality of O-rings. Each O-ring of the plurality of O-rings is in contact with the substantially flat bottom plate. 
         [0010]    A method of transferring molten material from a crucible includes moving a chill plate from a first position to a second position, initiating flow of the molten material from the crucible through an opening through a floor of the crucible, terminating the flow of molten material from the crucible through the opening while a portion of the molten material remains in the crucible, and moving the chill plate back to the first position. In the first position, the chill plate is in contact with the floor of the crucible and covering the opening. In the second position, the chill plate is removed from the opening. 
         [0011]    Some methods of controlling flow of material from a crucible include melting a portion of the material adjacent an opening in a wall of the crucible to initiate flow of molten material through the opening and providing a vacuum over the material in the crucible from a location opposite the opening. Flow of the molten material may be controlled (e.g., slowed or terminated) by applying a vacuum over the surface of the molten material within the crucible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    While the specification concludes with claims particularly pointing out and distinctly claiming what are regarded as embodiments of the present disclosure, various features and advantages of embodiments of the disclosure may be more readily ascertained from the following description of example embodiments of the disclosure when read in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a simplified cross-sectional side view illustrating a crucible for CCIM as known in the art; 
           [0014]      FIG. 2A  is a simplified cross-sectional side view illustrating a crucible according to the present disclosure; 
           [0015]      FIG. 2B  is a simplified cross-sectional side view illustrating a portion of the crucible of  FIG. 2A  in more detail; 
           [0016]      FIG. 3  is a simplified cross-sectional side view illustrating a crucible having a chill plate covering an orifice; 
           [0017]      FIG. 4  is a simplified cross-sectional side view illustrating the crucible of  FIG. 3  with the chill plate removed from the orifice; 
           [0018]      FIG. 5  is a simplified enlarged view of a portion of the crucible of  FIG. 3  when molten material is flowing therefrom; and 
           [0019]      FIG. 6  is a simplified enlarged view of the portion of the crucible of  FIG. 3  shown in  FIG. 5  when molten material has stopped flowing therefrom, and after the chill plate has been replaced. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The illustrations presented herein are not actual views of any particular crucible, but are merely idealized representations that are employed to describe example embodiments of the present disclosure. Additionally, elements common between figures may retain the same numerical designation. 
         [0021]      FIG. 2A  illustrates an embodiment of a crucible  200  for CCIM. The crucible  200  includes at least one wall defining an enclosed volume  201  and an orifice  202  (or tap hole). In some embodiments, and as shown in  FIG. 2A , the at least one wall includes a side wall  204  and a bottom plate assembly  206  (e.g., a flat plate). In other embodiments, the at least one wall may be constructed of a single wall extending around and under the enclosed volume  201 . The side wall  204  may include a plurality of hollow tubes  208 , which may be configured to pass a cooling fluid therethrough.  FIG. 2B  is an expanded detail of a portion B in  FIG. 2A , but showing only one tube  208  for simplicity. 
         [0022]    The hollow tubes  208  may be constructed of a metal or alloy, such as steel, stainless steel, copper, etc., or a ceramic, glass, or quartz material. By forming the side wall  204  of replaceable hollow tubes  208 , construction of the crucible  200  may be faster and cheaper than conventional methods (e.g., brazing, welding, etc.), which typically involve high temperatures. In particular, the hollow tubes  208  may be commercially available stainless-steel pipe or tubing, and may be assembled without the use of high-temperature processes. The bottom plate assembly  206  may be constructed of a metal or alloy, such as steel, stainless steel, or copper, and/or of a ceramic, graphite, quartz, or a combination thereof. The bottom plate assembly  206  may include the same material as the hollow tubes  208 , or may include a different material. For example, the hollow tubes  208  may be formed of stainless steel, and the bottom plate assembly  206  may be formed of copper. 
         [0023]    The construction of the crucible  200  may include mechanical connections and O-ring seals rather than conventional welding or brazing. This allows for flexibility in reconfiguration of the crucible assembly. The configuration and material of construction for the hollow tubes  208  and the bottom plate assembly  206  can be changed without remanufacturing or replacing the crucible  200 . This may simplify maintenance if damage to components occurs. 
         [0024]    The hollow tubes  208  may each be connected to the bottom plate assembly  206  by a bolt  218  and washer  220 . One or more O-rings  210  may form a fluid seal between each hollow tube  208  and the bottom plate assembly  206 . The O-rings  210  may be installed into grooves  212  machined into the hollow tubes  208  and/or the bottom plate assembly  206 . 
         [0025]    In some embodiments, the bottom plate assembly  206  may also include one or more channels  214 . For example, a channel  214  may provide a fluid connection between two or more hollow tubes  208 , such as through orifices  216  in the walls of the tubes  208 . When assembled, the hollow tubes  208  and bottom plate assembly  206  may together form one or more continuous flow paths for the cooling fluid via the orifices  216  and channels  214 . For example, cooling fluid may flow downward through one hollow tube  208 , through an orifice  216  into a channel  214  in the bottom plate assembly  206 , then through an orifice  216  in an adjacent hollow tube  208 . The cooling fluid may typically only pass through two hollow tubes  208  before recirculating to an external heat sink; however, in some embodiments, each flow path may include more than two of the hollow tubes  208  in series or in parallel. The bottom plate assembly  206  may also include channels disconnected from the hollow tubes  208 , such as for additional cooling. 
         [0026]    In some embodiments, the hollow tubes  208  may not touch the adjacent hollow tubes  208 . In such embodiments, when material is melted within the crucible  200 , some molten material may flow toward a gap between the hollow tubes  208  and solidify (due to the flow of cooling fluid in the hollow tubes  208 ), forming a skull  110  (see  FIG. 1 ) and sealing the molten material within the crucible  200 . 
         [0027]    The crucible  200  may also include one or more upper retaining members  222  (e.g., rings), which may also be connected to the hollow tubes  208  by O-rings  210 . Appropriate bolts, washers, etc., may be used to connect the hollow tubes  208  to the upper retaining members  222 . In some embodiments, the hollow tubes  208  may be connected to the upper retaining members  222  by an interference fit, rather than by bolts. The upper retaining members  222  may include fittings to connect to a source and sink of the cooling fluid. The upper retaining members  222  may also include appropriate fluid diverters to direct cooling fluid from a fluid source to the hollow tubes  208  and then to a heat sink. 
         [0028]    As shown in  FIG. 2A , the bottom plate assembly  206  may include a conductive material  224  surrounding the orifice  202 . The conductive material  224  may be electrically and/or thermally conductive. The conductive material  224  may have a melting point above the expected operating temperature of the crucible  200 , such that the material within the crucible  200  (which may be molten or critically softened) does not melt the conductive material  224  when flowing through the orifice  202 . A change in the composition of the conductive material  224  may correspond to a change in the amount of heat removed from a region of the crucible  200  near the orifice  202  (tap hole). In some instances, the conductive material  224  may have a relatively low thermal conductivity (i.e., the conductive material  224  may be an insulator) to allow for higher temperatures around the orifice  202  when the chill plate  340  is removed. In other embodiments, the conductive material  224  may have a relatively higher thermal conductivity to allow for lower temperatures around the orifice  202  when the chill plate  340  is removed. The composition of the conductive material  224  may be selected to adjust the temperature of material near the orifice  202 . For example, the conductive material  224  may include metals such as platinum, tungsten, titanium, copper, etc., and/or graphite or ceramics such as boron nitride. The conductive material  224  may be isolated from the flow of cooling fluid (i.e., the cooling fluid may not pass through the conductive material  224 ). 
         [0029]      FIG. 3  illustrates another embodiment of a crucible  300  for CCIM, which may have similar design features to those illustrated in  FIG. 2  (e.g., hollow tubes  208 , O-rings  210 , etc.). In some embodiments, the crucible  300  may include conventionally formed (e.g., welded) side walls  204  and bottom plate assembly  206 . The crucible  300  may include an inductive source, shown in  FIG. 3  as helical coil  330 . The crucible  300  may be connected to fluid lines  332  to transfer cooling fluid to and from the crucible  300 . A cover  334  may provide an air-tight seal between an interior of the crucible  300  and the environment exterior to the crucible. Cover  334  may include selectively occludable ports (not shown), such as for adding material to be melted. Fluid lines  336  may connect to the interior of the crucible  300  to provide gases or a vacuum in the crucible  300  during operation. 
         [0030]    A chill plate  340  may be disposed adjacent to and in contact with the conductive material  224  and/or the bottom plate assembly  206  surrounding the orifice  202  of the crucible  300 . The chill plate  340  may be connected to an articulating arm  342 , such that the chill plate  340  may be moved during operation of the crucible  300 .  FIG. 4  shows the chill plate  340  in a position in which the chill plate  340  does not contact the conductive material  224 . The articulating arm  342  may move the chill plate  340  out of a flow path of molten material from the crucible  300 . The articulating arm  342  may connect to hinges, motors, and/or any other means for moving the chill plate  340 . 
         [0031]    The chill plate  340  may be configured to cool the conductive material  224  and/or the bottom plate assembly  206  when the chill plate  340  is in contact with the conductive material  224 . The chill plate  340  may keep the conductive material  224  and/or the bottom plate assembly  206  cool such that the orifice  202  may remain closed when the chill plate  340  is in contact with the conductive material  224 . The chill plate  340  may include a means for transferring heat from the conductive material  224  and/or the bottom plate assembly  206 . For example, the chill plate  340  may include one or more fluid passageways and fittings configured to be connected to a source of cooling fluid (e.g., water), which may be the same or a different fluid than the fluid used to cool the walls of the crucible  300 . In some embodiments, the chill plate  340  may include a thermoelectric device, such as a device configured to remove heat from the conductive material  224  and/or the bottom plate assembly  206  and transfer waste heat to the surroundings. The chill plate  340  may also be formed of, or include, a conductive material (e.g., a conductive metal, a conductive grease or sealant, etc.), such that heat may be efficiently transferred from the conductive material  224  and/or the bottom plate assembly  206  to the chill plate  340 . 
         [0032]    The chill plate  340  may be perforated to supply gases into the mixture from the bottom of the crucible  300  in order to effect change to the atmosphere and/or melt during the initiation and melt mixing phases. Gases could include, but are not limited to, inert-atmosphere gases such as nitrogen or argon, or chemical processing gases such as oxygen or propane. 
         [0033]    The crucible  300  may be used to receive material, such as in the form of solid powders, pellets, sludge, frit, or material bearing the form of liquids or slurries into the interior thereof. The material within crucible  300  is heated with the inductive source (e.g., the helical coil  330 ), causing the material to melt and mix in the crucible  300  via currents. During heating, the chill plate  340  may be held in contact with the conductive material  224 , such that a portion of the material adjacent to and covering the orifice  202  solidifies (i.e., freezes) or remains solid. Once the material in the crucible  300  has been substantially heated and mixed, the molten material may be removed for use in a subsequent process (e.g., the material may be cast into a mold). To remove the molten material from the crucible  300 , the articulating aim  342  may move the chill plate  340 , as shown in  FIG. 4 . The conductive material  224  and the solid material over the conductive material  224  may then begin to heat, melting the solid material over the orifice  202 . In some embodiments, an auxiliary heat source (e.g., heated air, electric resistance heat, etc.) may be provided to speed the heating process once the chill plate  340  has been moved.  FIG. 5  illustrates an expanded detail view of a portion C of the crucible  300  with molten material  350  flowing through the orifice  202 . 
         [0034]    Flow of the molten material  350  may be terminated by applying a vacuum (i.e., a pressure less than atmospheric pressure) over the surface of the molten material within the crucible  300 . For example, gases may be removed from the crucible  300  via fluid lines  336  ( FIG. 4 ). When the vacuum above the molten material  350  reaches a value at which the force exerted on the molten material  350  by atmospheric pressure through the orifice  202  equals the force exerted on the molten material  350  by gravity and the pressure within the crucible  300 , the flow of the molten material  350  slows and terminates. Additional vacuum may pull the molten material  350  back into the crucible  300 , and may cause a meniscus  352  to form, as illustrated in  FIG. 6 . Vacuum may be provided by, for example, an induced draft fan controlled by a variable frequency drive. 
         [0035]    Once the flow of molten material  350  has stopped, the chill plate  340  may be moved back into contact with the conductive material  224 , and heat may be transferred from the conductive material  224  to the chill plate  340  (e.g., by conduction). This may cause some of the molten material  350  adjacent the orifice  202  to freeze again into a solid material  354  ( FIG. 6 ). However, if the molten material  350  has been pulled back into the crucible  300  before moving the chill plate  340 , the molten material  350  may not freeze to the chill plate  340 , and therefore, the chill plate  340  may be easily removed again without diminishing the ability of the system to stop flow. That is, the chill plate  340  need not directly contact the molten material  350  because flow may be controlled by the vacuum until the solid material  354  forms. Once the chill plate  340  is in contact with the conductive material  224  and the solid material  354  has been formed, the pressure in the crucible  300  may be raised (e.g., to a pressure below, equal to, or above atmospheric pressure) without restarting flow of the molten material  350 . In particular, the solid material  354  may prevent the molten material  350  from flowing through the orifice  202  so long as the chill plate  340  remains operational (e.g., cooling or removing heat) and in contact with conductive material  224 . 
         [0036]    Some of the molten material  350  may remain in the crucible  300  after a portion of the molten material  350  has been removed. Additional material to be melted may be added through a port in the top of the crucible  300  to make up for the molten material  350  removed. The crucible  300  may therefore be operated in a semi-continuous or hybrid mode having properties of both continuous and batch modes. Removal of molten material  350  from the crucible  300  may be initiated, controlled, and terminated multiple times while the crucible  300  is in operation (e.g., without substantial cool-down, cleaning, etc.). In particular, by leaving a portion of the molten material  350  within the crucible  300 , time and energy typically spent in start-up and shut-down during batch operations can be limited or avoided. The time between discharges of the molten material  350  can be reduced, allowing for more steady operations (e.g., casting operations). Furthermore, quality control may be simplified because the material within the crucible  300  need not be drained all at once; therefore, changes in composition due to variations in feed stock may have relatively smaller influence on the composition of the molten material  350 . Any such variations may be detected by testing the molten material  350  (e.g., testing a sample drawn from the orifice  202  before subsequent addition cycles) so that corrections can be made. 
         [0037]    A vacuum applied above the molten material  350  may also be used, and varied, to control the flow rate of the molten material  350  at various levels. For example, flow of the molten material  350  may be increased or decreased as necessary, without stopping flow entirely. In some embodiments, additional material to be melted may be added to the crucible  300  while the molten material  350  is flowing from the crucible  300 . In other embodiments, the material to be melted may be added during periods in which the molten material  350  is not flowing from the crucible  300  (e.g., when the chill plate  340  is in contact with the conductive material  224 . 
         [0038]    Material within the crucible  300  may be heated to any selected temperature. For example, material may be heated to a temperature of at least about 1000° C., at least about 1100° C., at least about 1200° C., at least about 1400° C., at least about 1600° C., or even at least about 2000° C. Because the walls may be cooled, a temperature profile may exist within the crucible  300 , wherein material toward the centerline of the crucible  300  is at a higher temperature than material near the side wall  204 . Similarly, a temperature profile may exist along a vertical line within the crucible  300 . Temperature gradients may cause convection of the molten material  350  within the crucible  300 , and may contribute to the formation of a molten material  350  of approximately uniform chemical composition. 
         [0039]    The methods and devices disclosed herein may be used to process a variety of materials, such as radioactive waste, ceramics, alumina, zirconia, ruby-glasses, specialty glasses, cathode ray tube (CRT) glass recycling, high-temperature high-strength fiberglass, etc. The methods disclosed may be useful for limiting contamination in molten material by avoiding the use of refractory materials. The material being melted forms a skull along the wall, which may eliminate the need for refractory materials that may contaminate a high-purity melt. High-purity and crystalline phase retention may also be maintained by avoiding reheating of solidified material between cycles of material extraction. Thus, the methods may be particularly beneficial when high-purity products are desired. Because CCIM methods typically have high operating costs due to the necessity of using cooled walls, the methods disclosed may typically be relatively more economical for higher value-added processes (i.e., processes in which the material produced is much more valuable than the input material). For example, the methods may be economical for materials having melting points above about 1200° C. 
         [0040]    Embodiments disclosed herein may allow withdrawal of material from the bottom of crucibles on multiple occasions without lowering the temperature, providing secondary heating around a tap hole, or mechanically opening a tap hole (e.g., by drilling, chipping, etc.) Therefore, a crucible may be maintained substantially continuously at an operating temperature, may be able to  melt materials faster than conventional crucibles, and may be less likely to sustain damage in opening a tap hole. Embodiments may be particularly suitable for high-temperature melting processes in which intermittent bottom taps are desirable. 
         [0041]    While the present disclosure has been described herein with respect to certain illustrated embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions, and modifications to the illustrated embodiments may be made without departing from the scope of the invention as hereinafter claimed, including legal equivalents thereof. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention. Further, disclosed embodiments have utility with different and various crucible types and configurations, including vessels using other types of heat sources (e.g., electric arc furnaces, resistive heaters, etc.).