Abstract:
A refining hearth. The refining hearth comprises an open vessel defining a first deep zone having a predetermined depth, a second deep zone having a predetermined depth, and a shallow zone intermediate the first deep zone and the second deep zone, wherein the shallow zone has a predetermined depth less that of the first deep zone and less than that of the second deep zone. A furnace for refining metal is also disclosed which employs a similarly constructed hearth. A method of refining metal is also disclosed. The method includes depositing molten metal in a first deep pool, passing the molten metal through a shallow pool having a depth less than the depth of the first deep pool, directing an energy source at the molten metal, and passing the molten metal into a second deep pool having a depth greater than the depth of the shallow pool.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to purification hearths and, more particularly, to a hearth for refining metals such as titanium by removing high and low density inclusions therefrom. 
     2. Description of the Invention Background 
     A variety of different processes and apparatuses have been developed for obtaining relatively pure metals or alloys by separating the slag and burning off or evaporating volatile impurities from the molten metal material. One such apparatus that has been developed to accomplish those tasks is a furnace having an energy source, such as an electron beam gun or a plasma torch, directed toward the surface of the metal in the furnace. Such a furnace, in general, comprises a vacuum chamber with a hearth and crucible system on the floor of the furnace and a number of energy sources mounted above the hearth. The energy sources are used to melt metals introduced onto the hearth and, through sublimation, evaporation and dissolution, remove certain impurities from the molten metal. Additionally, currents created by thermal gradations in the molten metal stream promote inclusion removal. When electron beam sources are utilized, each electron beam can be deflected and scanned over the surfaces of the metal being melted in the hearth. Thereafter, the liquid metal flows from the hearth into the crucible. Energy sources are utilized to maintain the metal in its liquid form as it flows through the hearth to the crucible. 
     Impurities or inclusions, generally exist within metallic raw materials and can remain within the metal if they are not removed by a refinement process. Those inclusions create areas of potential failure within the metal, and are detrimental in critical applications, such as rotating parts in jet engines. It is important, therefore, when creating high quality metals, that impurities be removed from or dissolved within the metal. 
     The impurities are generally removed while the metal is in a molten state, when the impurities having varying densities may be removed by settlement or floatation mechanisms. Impurities having a greater density than the metal naturally settle out in the hearth. In a typical process, however, the lower density or neutral density inclusions can be carried into the crucible mold because the lower density or neutral density inclusions are not removed when the metal is poured from the top of a typical hearth. 
     It is desirable in certain applications for impurities or inclusions that do not settle in the hearth to be sublimated, evaporated or dissolved into the liquid metal to prevent inclusions from forming defects within the solidified metal and thereby creating points of potential failure. 
     In addition, splatter is created when heat from the energy source impinges on volatile elements within the metal. When splatter occurs, matter, including impurities in the molten stream, can be propelled upward from the surface of the molten stream and outward in all directions. Some of that splatter, therefore, is propelled toward or into the crucible, thereby bypassing at least a portion of the refining process. Thus, it is desirable to reduce or eliminate spattering of the molten stream to prevent such material from by passing the refining process. 
     Accordingly, a need exists for methods and apparatuses for breaking up inclusions in a stream of molten metal to aid in the removal of impurities from the metal and dissolution of any remaining impurities in the metal. 
     A need also exists for apparatuses and methods for removing impurities from molten metal, wherein those impurities have a density less than or approximately equal to that of the metal being processed. 
     There is a further need for apparatuses and methods for preventing matter in a molten metal stream from bypassing further steps in a refining process. 
     There is still another need for an apparatus having the above-mentioned advantages that is relatively inexpensive to manufacture and install. 
     SUMMARY OF THE INVENTION 
     In accordance with a particularly preferred form of the present invention, there is provided a refining hearth. The refining hearth comprises an open vessel defining a first deep zone having a predetermined depth, a second deep zone having a predetermined depth, and a shallow zone intermediate the first deep zone and the second deep zone. The shallow zone, furthermore, has a predetermined depth less than that of the first deep zone and less than that of the second deep zone. 
     A furnace for refining metal is also provided. The furnace comprises a refining hearth defining a first deep zone having a depth, a second deep zone having a depth and a shallow zone having a depth that is less than the depth of the first deep zone and the depth of the second deep zone and at least one energy source mounted above the hearth. 
     A method of refining metal is also disclosed. The method includes depositing molten metal in a first deep pool, passing the molten metal through a shallow pool having a depth less than the depth of the first deep pool, directing an energy source at the molten metal, and passing the molten metal into a second deep pool having a depth greater than the depth of the shallow pool, while directing an energy source at the molten metal. 
     Another method of refining metal, comprises melting raw material containing a desired metal to form a molten stream, applying energy to the surface of the molten stream, trapping impurities having a higher density than the metal, and creating turbulence in the molten stream. 
     It is a feature of the present invention to provide a series of hearths for refining and purifying metal. 
     It is another feature of the present invention to provide a hearth having sections of varying depths oriented in series. Such a multilevel structure removes undesirable inclusions by trapping certain of those inclusions in the deeper sections and by forcing other of those inclusions nearer the surface of the metal in the more shallow sections where the inclusions and impurities may be removed by sublimation, evaporation or dissolution by exposing them to high thermal energy. 
     Yet another feature of the present invention is to provide a series of pools separated by offset narrow shallow flow notches. That configuration causes the molten metal to flow along a non-linear path which circulates impurities through the molten stream, thereby exposing the impurities to high thermal energy. 
     Another feature of the present invention is the use of multiple hearths in series. The hearths are configured such that molten metal is discharged from a pour lip of the discharging hearth and cascades into the receiving hearth. Thus, the inclusions are broken up and the molten stream is mixed by the turbulence caused by the molten stream cascading from the pour lip. 
     It is another feature of the present invention that barrier walls are placed above the molten stream to prevent splattered materials from bypassing the purification system. 
     Accordingly, the present invention provides solutions to the shortcomings of prior hearths. Those of ordinary skill in the art will readily appreciate, however, that these and other details, features and advantages will become further apparent as the following detailed description of the preferred embodiments proceeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying Figures, there are shown present preferred embodiments of the invention wherein like reference numerals are employed to designate like parts and wherein: 
     FIG. 1 is a top view of a molten metal refining apparatus of the present invention; 
     FIG. 2 is a cross-sectional view of the molten metal refining apparatus of FIG. 1 containing a molten stream, taken along line II—II in FIG. 1; 
     FIG. 3 is a top view of the refining hearth of FIG. 1; 
     FIG. 4 is a top view of another embodiment of the molten metal refining apparatus of the present invention; and 
     FIG. 5 is a cross-sectional view of the molten metal refining apparatus of FIG. 4 containing a molten stream, taken along line V—V in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the Figures and descriptions of the present invention included herein illustrate and describe elements that are of particular relevance to the present invention, while eliminating, for purposes of clarity, other elements found in a typical metal manufacturing process. Because the construction and implementation of such other elements are well known in the art, and because a discussion of them would not facilitate a better understanding of the present invention, discussion of those elements is not provided herein. It is also to be understood that the embodiments of the present invention that are described herein are illustrative only and are not exhaustive of the manners of embodying the present invention. For example, it will be recognized by those skilled in the art that the present invention may be readily adapted to function with titanium processing, as well as processing other metals and materials that require refinement in a manner similar to that of titanium. It will also be recognized that the refining hearths and barriers of the present invention may be utilized alone or in various combinations with equipment discussed herein and with other equipment not discussed herein. 
     Referring now to the drawings for the purposes of illustrating the present preferred embodiments of the invention only and not for the purposes of limiting the same, FIG. 1 is a top view of a series of hearths configured to form a hearth system  20  for processing raw material into purified metal and, in particular, for creating premium grade titanium. FIG. 2 is a cross-sectional view of the hearth system  20  depicted in FIG.  1 . The apparatus of FIGS. 1 and 2 comprises an embodiment of the invention that includes a main hearth  30 , a transfer hearth  50 , a refining hearth  70 , and a crucible  150 . Those skilled in the art will recognize that each of those components  30 ,  50 ,  70 , and  150  may be used in the configuration depicted in varying combinations. In the embodiment illustrated in FIGS. 1 and 2, raw material containing titanium or another desired material, is introduced into the main hearth  30  utilizing conventional loading apparatuses and methods. The main hearth  30  includes a base  32  and side walls  34  defining a melt area and an opening  36  through which liquefied metal may pass. The raw materials are heated within the main hearth  30  by one or more energy sources such as, for example, electron beam gun  22  or plasma torches oriented above the base  32 . As the raw material is heated within the main hearth  30 , it forms a stream of molten metal  62  which flows from the main hearth  30  in the direction represented by arrow “F” in FIG.  2 . The opening  36  may be raised from the base  32  of the main hearth  30  to prevent unmelted raw material and impurities having a density greater than the metal from escaping the main hearth  30 . The opening  36  may also be narrow to minimize the amount of material escaping the main hearth  30  by way of splattering. A channel  38  may furthermore be formed at the opening  36  to direct the flow of the molten metal  62  into the transfer hearth  50 . 
     The transfer hearth  50  includes a base  52  and an upstanding wall  54  defining a pool  56 , an inlet  57 , and an outlet  59 . The transfer hearth  50  may be fabricated from copper and as illustrated in FIG. 2, may include coolant passages  64  through which a coolant, such as water, flows. It will be understood that coolant prevents the transfer hearth  50  from being damaged by the molten metal and results in the formation of a “skull” (not shown) of hardened metal on the surface  60  of the transfer hearth  50 . In operation, impurities are removed from the molten metal  62  as the metal flows through the transfer hearth  50 . Impurities having a density greater than the metal, sink to the bottom of the pool  56  and are captured at the liquid metal interface with the solidified portion of the skull. Energy sources, such as conventional electron beam guns  22  illustrated in FIG. 1, are aimed at the surface of the skull, providing a molten metal surface  62 , thereby sublimating, evaporating or dissolving impurities near the surface of the molten metallic stream  62 . 
     FIG. 3 illustrates a refining hearth  70  into which the molten metal stream  62  flows from the transfer hearth  50 . The refining hearth  70  includes a base  72  surrounded by an upstanding wall  74  defining a pool  76 . In the embodiment illustrated in FIGS. 1-3, the pool  76  is divided into a first deep zone  78 , a shallow zone  80 , and a second deep zone  82 . As can be seen in FIG. 2, the shallow zone  80  is centrally disposed between the first deep zone  78  and the second deep zone  82 . That embodiment also includes a raised lip  83  over which the refined metal  62  flows when exiting the refining hearth  70 . As illustrated in FIG. 2, the refining hearth  70  may also be fabricated from copper and may include coolant passages  79  through which a coolant, such as water, flows. The coolant prevents the refining hearth  70  from being damaged by the molten metal  62  and results in the formation of another skull (not shown) of hardened metal on the surface  81  of the refining hearth  70 . 
     As the raw materials are heated within the main hearth  30 , a stream of molten metal  62  is formed which flows into the transfer hearth  50  wherein it is further heated. Such molten stream  62  exits the transfer hearth  50  through the outlet  59  and flows over a raised lip  58  that extends up from the base  52  of the transfer hearth  50 . As may be seen in FIG. 2, as the molten stream  62  flows over the raised lip  58  of the transfer hearth  50 , it cascades into the refining hearth  70 . The refining hearth  70  is positioned such that the upper surface of the molten stream  62  in the refining hearth  70  is beneath the raised lip  58 . A drop of approximately 6″ from the raised lip  58  of the transfer hearth  50  to the base  72  of the refining hearth  70  has been found to impart a desirable amount of turbulence to the molten stream  62  as it enters the first deep zone  78  of the refining hearth  70 . As may be seen in FIG. 1, a conventional high powered electron beam gun  22   a , may be directed toward the thin molten stream  62  flowing over the raised lip  58  and cascading from the transfer hearth  50 , to remove inclusions remaining in the stream. The molten stream  62  is beneficially mixed, as it enters the refining hearth  70 , by the turbulence caused by the molten stream  62  cascading from the raised lip  58  into the refining hearth  70 , and by thermal stirring caused by the higher temperature imparted on the cascading stream by the electron beam gun  22   a . The mixing of the molten stream  62  within the refining hearth  70  breaks up inclusions and causes the dispersed impurities to move to the surface of the swirling molten stream  62  from time to time. Additional impurities may therefore be sublimated, evaporated or dissolved by a heat source such as the electron beam gun  22   a , which is aimed at the surface of the molten stream  62  where it enters the refining hearth  70 . 
     The multilevel structure of the refining hearth  70  further aids in breaking up inclusions and removing undesirable impurities in the hearth system  20 . High density inclusions and impurities that may have advanced from the transfer hearth  50  into the refining hearth  70  settle out of the stream as the turbulence subsides and become trapped in the skull (not shown) of hardened material that forms along the bottom of the refining hearth  70  due to the contact of the molten stream  62  with the cooled surface  81  of the hearth  70 . Therefore, the deep zones  78  and  82  should be of a depth sufficient to trap high density impurities, thereby preventing those impurities from passing out of the deep zones  78  and  82 . For example, it has been found that a deep zone depth of approximately 4″ (i.e., distance “A” as shown in FIG. 2) is sufficient to prevent most high density inclusions from passing out of the deep zones  78  or  82  at a flow rate of 2 fpm or less. It is also beneficial for each deep zone  78  and  82  to be of a sufficient length to allow the turbulence that exists at the upstream end  98  of the first deep zone  78  and the upstream end  94  of the second deep zone  82  to subside prior to leaving that zone  78  or  82 . That permits high density inclusions to settle to the bottom of the molten stream  62 , thereby permitting those high density inclusions to be trapped in the skull (not shown) at the surface  81  of the refining hearth  70 . For example, it has been found that a deep zone  78  having a length of from 20-30″ (represented by arrow “B” in FIG. 2) permits high density inclusions (i.e., inclusions having a density greater than the metal being refined) to settle to the bottom thereof. Likewise, a deep zone  82  having a length of from 20-30″ (represented by arrow “C” in FIG. 2) results in dissolution of inclusions having similar densities. The widths of the deep zones  78  and  82  are chosen to create the desired flow rates through the deep zones  78  and  82 . For example, it has been found that the flow rate in a deep zone having a width of 21″ and receiving molten stream  62  at a rate of 1.6 gpm, is 1 fpm. It has furthermore been discovered through experimentation that a flow rate of 1-2 fpm provides for good throughput of molten stream  62  while also providing sufficient opportunity for the removal of impurities to create acceptable quantities of high grade metal. This unique aspect of the present invention represents an improvement over prior hearth designs in that the refinement hearth reduces the molten metal dwell time required and throughout is accordingly increased. It will be appreciated, however, that deep zones of other lengths and widths may also be successfully employed without departing from the spirit and scope of the present invention and also that flow rates of lower and higher rates than indicated as examples would result in impurity removal. 
     Impurities having a density less than that of the metal rise to the surface of the molten stream  62  as the turbulence subsides in the downstream portions  87  and  102  of the deep zones  78  and  82 , respectively. Those low density impurities may, therefore, be removed from the surface of the stream by electron beam guns  22  or other energy sources directed at the surface of the stream which can result in their sublimation, evaporation or dissolution. 
     In the shallow zone  80 , the molten stream  62  forms a shallow pool (i.e., approximately 1-1.5″deep). Thus all impurities, including those having a neutral density, are forced to move to or near the surface of the metal stream  62  in the shallow zone  80 . The impurities may, therefore, be sublimated, evaporated or dissolved by an energy source such as the depicted conventional electron beam gun  22   b  which is directed at the surface of the molten stream  62 . In the embodiment illustrated in FIGS. 1-3, the shallow zone  80  extends the full width of the refining hearth  70  to minimize the increased velocity of the molten stream  62  caused by the reduction in the depth of the stream. The shallow zone  80  also extends lengthwise along the refining hearth  70  for a distance sufficient to create a large shallow area to provide a dwell time for the impurities as they pass through the shallow zone  80 , during which the turbulence induced by the energy source in the shallow zone exposes the impurities to high energy, insuring their removal by sublimation, evaporation or dissolution. For example, a shallow zone  80  that is 6-12″ long will remove a substantial quantity of impurities. In such a shallow zone  80 , The electron beam gun  22   b  is able to apply energy at a high level to the molten stream  62  for more effective impurity removal. 
     As can be seen in FIG. 2, the refining hearth  70  may include a sloping surface  88  that extends from the bottom of the deep zone  78  to the shallow zone  80  to facilitate transfer of the molten metal  62  to the shallow zone  80 . It has been found that such a sloping surface  88  creates a turbulence in the molten stream  62  passing through the shallow zone  80  which, once again, causes impurities to circulate and periodically approach the surface of the molten stream  62  as it passes through the shallow zone  80 . The sloping surface  88  is also beneficial when it comes time to clean and remove the skull from the hearth in that, when the metal solidifies, it will shrink and pull away from the refining hearth  70  and may then be easily removed without damaging the hearth  70 . 
     To facilitate transition of the molten stream  62  from the shallow zone  80  to the second deep zone  82 , a sloping surface  92  may also be provided therebetween as illustrated in FIG.  2 . The downstream sloping surface  92  creates a desirable amount of turbulence in the entering end  94  of the second deep zone  82  and facilitates easy removal of the skull as discussed above. A sloping surface (not illustrated) may also be provided on the upstream side  98  of the first deep zone  78  and a sloping surface  100  may be provided on the downstream side  102  of the second deep zone  82  to control turbulence and prevent damage to the refining hearth  70 . The second deep zone  82  is disposed downstream of the shallow zone  80  and is utilized in a manner similar to the first deep zone  78 . Additional shallow and deep zones may be formed in the refinement hearth  70  to further refine the molten stream  62  if desired. 
     The molten stream  62  flowing through the transfer hearth  70  illustrated in FIGS. 1-3 passes out of the transfer hearth  70  through the transfer hearth&#39;s raised lip  83  and into a crucible  150  or other container for further processing 
     Splatter of material in the molten stream  62  may occur for many reasons, including the impingement of an energy beam on volatile elements in the molten stream  62 . The high temperature imparted on the volatile elements by the energy beam causes those elements to evolve into a gas which propels the elements and other nearby elements out of the molten stream  62 . Splatter that is directed downstream in the hearth system  20  detrimentally bypasses part or all of the purification process, thereby reducing the quality of the refined metal. 
     To prevent splatter form being propelled downstream in the hearth system  20 , one or more barrier walls  126 ,  128  and  130  may be placed between or along the hearths  30 ,  50  and  70  as partitions. Each barrier wall  126 ,  128  and  130  may be fabricated from copper and may include coolant passages  138  through which coolant flows to prevent the barrier walls  126 ,  128  and  130  from being damaged by the high temperature of the hearth system  20  and the splattering particles. The barrier walls  126 ,  128  and  130  should extend upward from above the molten stream  62 , and should extend at least across the width of the molten stream  62 . For example, a barrier wall  126 ,  128  and  130  that extends from approximately 2″ above the surface of the stream to 132″ above the stream, and extends across the width of the hearth  50  or  70  has been found to effectively block splattering material directed downstream. However, other barrier orientations could conceivably be employed. Barrier walls  126 ,  128  and  130  may be placed anywhere along the path of the molten stream  62 . In particular, it has been found to be beneficial to place a barrier wall  126  downstream of the main hearth  30  and place other barrier walls  128  and  130  at the upper entering edge  132  of the shallow zone  80  and the upper entering edge  134  and  136  of each flow notch  106  and  108  respectively. 
     FIGS. 4 and 5 illustrate a top view and a cross-sectional view, respectively, of another furnace arrangement of the present invention. The furnace of FIGS. 4 and 5 is essentially constructed in the same manner as the furnace described above and depicted in FIGS. 1-3, except for the differences described below. The hearth system  20  of this embodiment includes a refining hearth  70  that has three deep zones  78 ,  82  and  104  interconnected by offset flow notches  106  and  108 . The flow notches  106  and  108  are formed in transverse barriers  112  and  114  that may be integrally formed in the refining hearth  70 . The flow notches  106  and  108  are shallow areas that are narrower than the width of the transfer hearth  70 . The flow notches  106  and  108  may furthermore be offset, one from another, to create non-linear flow through the deep zones  78 ,  82  and  104 . In the flow notches  106  and  108 , the molten stream  62  forms a shallow pool. Thus impurities, including those having a neutral density, are proximate to the surface of the metal stream when resident in the flow notches  106  and  108 , making them susceptible to removal by sublimation, evaporation or dissolution. Higher energies than are applied to the deep zones  78 ,  82  and  104  may be applied at flow notches  106  and  108  to enhance neutral and low density impurity removal without sacrificing the effectiveness of deep zones  78 ,  82 ,  104  for high density impurity removal. Turbulence is created at the upstream and downstream facings of the flow notches  106  and  108 , which creates beneficial mixing of the molten stream  62 . The upstream and downstream sides of the flow notches  106  and  108  may include sloping surfaces to prevent damage to the refinement hearth  70  during the removal of hardened metal. For example, the first flow notch  106  may have a sloping surface  118  on its upstream side and a sloping surface  120  on its downstream side, and the second flow notch  108  may have a sloping surface  122  on its upstream side and a sloping surface  124  on its downstream side. The non-linear flow path created by the offset flow notches  106  and  108  provides additional turbulence to the stream that aids in the dissolution of inclusions and the removal of impurities in the stream. As can also be seen from FIGS. 4 and 5, this embodiment can also employ the barrier arrangement of the present invention to control undesirable spattering of material. 
     Thus, from the foregoing discussion, it is apparent that the present hearth solves many of the problems encountered by prior hearth systems employed in furnaces for refining metal. In particular, the subject invention may be advantageously adapted to refine and purify metal in a hearth with a reduced molten dwell time, while preventing molten metal from bypassing the purification process. Those of ordinary skill in the art will, of course, appreciate that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by the skilled artisan within the principle and scope of the invention as expressed in the appended claims.