Patent Publication Number: US-2023139976-A1

Title: Tilting Melting Hearth System And Method For Recycling Metal

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional No. 63/273,291, filed Oct. 29, 2021, which is incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to a system and method of melting, atomizing, or casting recycled scrap material including reactive metals such as titanium, zirconium, nickel, cobalt and their alloys, and nonreactive metals such as steel, iron and their alloys. 
     This disclosure can be used in any melting process that has a requirement of knowing the volume or weight of the contents of a melting vessel contained in an atmospherically controlled chamber. 
     BACKGROUND 
     There is a need in the industry for greater process control in a tilting hearth atomization system, specifically one where the deposition of liquid metal in a gas stream or fixed container are repeatable, and precision amounts are crucial to the process parameters. 
     The current state of the art with DPA (Direct Pour Atomization) or DPI (Direct Pour Ingot) production requires an amount of molten material to be poured through a calibrated pour notch creating a uniform pour stream into a preferred location in an atomization gas stream, or into a small induction crucible for near net shape, void-free castings. In some systems, the physical window of either target has an approximate diameter of about 0.875″ and a preferred diameter of the pour stream of molten metal of about 0.375″. Any pouring outside of this window can lead to an ineffective process, a clogged gas die, or a plugged induction mold or crucible. Any variation in the pour stream diameter itself as it interacts with a stable gas stream can lead to variability in a gas to metal ratio, which in turn leads to poor atomization performance. 
     One additional consideration is that while processing scrap material as feedstock, the variable shape and weight per charge loaded into the hearth can cause differences in the operational parameters preventing formation of a precision pour stream. This in turn leads to variability in processing of the above metals. Those skilled in the art of gas atomization understand that a specific gas-to-metal ratio by weight is desirable and variations from that will create inefficiencies in production yield. 
     U.S. Pat. Nos. 9,925,591 and 10,654,106, which are incorporated herein by reference, disclose an exemplary metallurgical system that includes a tilting melting hearth system. In this tilting melting hearth system, current operation standards call for the operator to make a judgment based on visual input from the process cameras to determine the fluid level in the tilting melting hearth prior to every pour. The operator must visually judge whether or not the fluid level is correct for the process, and when to start the atomization gas stream that intersects with the molten pour stream. This operator judgment is made up to 20 times per heat. The fluid level has a narrow window of operational parameters where a lesser amount can result in a lack of fluid pressure leading to a short and unstable pouring event. Conversely, an overfill of the hearth can cause a dribble of molten material to build up on the gas die leading to a premature shut down of the atomization or ingot making process. Narrowing the parameters even more are processes in which alloys are being created in the melting hearth via magnetic stirring where melt cycles are defined by energy input per weight of material and a characterized vaporization rate of materials is determined. 
     The present disclosure is directed to a tilting melting hearth system that overcomes some of the above-described shortcomings of prior art systems. The present disclosure is also directed to a method for recycling metal using the tilting melting hearth system. 
     SUMMARY 
     A tilting melting hearth system includes a tilting melting hearth having a melting cavity and a heat source configured to melt a metal into a molten metal, and a pour notch configured to pour the molten metal from the melting cavity. The tilting melting hearth system also includes a central processing unit (CPU) for controlling a hearth tilt angle of the tilting melting hearth and a pour rate from the melting cavity. The central processing unit (CPU) includes an automated hearth tilting program configured to select a hearth tilt profile and to control the hearth tilt angle as a function of the hearth tilt profile and a weight of the molten metal in the tilting melting hearth. The central processing unit (CPU) can also be configured to control a sequence of feeding, melting, pouring and either atomizing or casting the molten metal. The tilting melting hearth system also includes an actuator in signal communication with the central processing unit (CPU) coupled to a linkage configured to support and move the tilting melting hearth to a desired hearth tilt angle. The tilting melting hearth system also includes a weight measuring device operably associated with the actuator in signal communication with the central processing unit (CPU) configured to measure the weight of the molten metal in the melting cavity. The tilting melting hearth system can also include a digital readout in signal communication with the central processing unit (CPU) configured to display data and to provide information to the central processing unit (CPU). The tilting melting hearth system can also include an atomization die in flow communication with the tilting melting hearth configured to receive a stream of molten metal from the pour notch and generate a metal powder comprised of particles having a desired particle shape and particle size. Alternately, the tilting melting hearth system can also include a casting die in flow communication with the tilting melting hearth configured to receive the stream of molten metal from the pour notch and generate a casting of the metal. 
     The tilting melting hearth system removes the decision-making burden from an operator as to when the fluid level of the molten metal in the tilting melting hearth is correct, which in turn drives a consistency of the process and economy of gas usage. By determining a weight of the molten metal in the tilting melting hearth, and utilizing the automated hearth tilting program, a selection of a hearth tilt profile, or combinations of multiple profiles, can be made. Based on the known density of the metal, and the calculated fluid amount of the molten metal, an entire sequence can be reduced to a single operation. For example, a sequence can include feeding the metal into the tilting melting hearth, melting the metal into the molten metal, and pouring the molten metal into either the atomization die or the casting die. In this example, all aspects of the sequence are controlled by the central processing unit (CPU). The tilting melting hearth system can also provide additional feeding of the metal into the tilting melting hearth, and/or additional pouring into the atomization die or the casting die, to keep the fluid level of the molten metal inside operation parameters. 
     A method for recycling metal includes the steps of providing a tilting melting hearth system comprising a tilting melting hearth having a heat source and a melting cavity configured to melt a recycled metal into a molten metal, and a pour notch configured to pour the molten metal from the melting cavity. The tilting melting hearth system also includes a central processing unit (CPU) configured to control the tilting melting hearth having a hearth tilting program configured to select a hearth tilt profile as a function of a weight of the molten metal in the tilting melting hearth. The method also includes the steps of determining the weight of the molten material in the tilting melting hearth, selecting a hearth tilt profile using the hearth tilting program, and controlling a hearth tilt angle and a pour rate from the pour notch using the hearth tilt profile and the weight of the molten metal in the tilting melting hearth. The method can also include the steps of controlling a sequence of feeding, melting, pouring and either atomizing and or casting the molten metal, utilizing the hearth tilting program. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a tilting melting hearth system; 
         FIG.  2 A  is a schematic diagram of an atomization die of the tilting melting hearth system; 
         FIG.  2 B  is a schematic diagram of a casting die of the tilting melting hearth system; 
         FIG.  3 A  is a perspective view of a metal powder fabricated using the tilting melting hearth system; 
         FIG.  3 B  is an enlarged schematic cross-sectional view of a single metal particle of the metal powder; 
         FIG.  4    is a perspective view of a metal casting fabricated using the tilting melting hearth system; 
         FIG.  5    is a perspective view illustrating different hearth tilt angles for a tilting melting hearth of the tilting melting hearth system; 
         FIG.  6 A  is a graph illustrating a representative hearth tilt profile for the tilting melting hearth system based on weight of the molten metal; 
         FIG.  6 B  is a graph illustrating a representative hearth tilt profile for the tilting melting hearth system based on a fluid level of the molten metal in the titling melting hearth; and 
         FIG.  7    is a flow chart illustrating a representative operational sequence for the tilting melting hearth system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , a tilting melting hearth system  10  is illustrated schematically. The tilting melting hearth system  10  includes a tilting melting hearth  12  having a melting cavity  62  configured to melt a metal  14  into a molten metal  16  and a pour notch  60 . A feeder  64 , such as a tube, channel, or conveyor, in close proximity to the tilting melting hearth  12 , feeds the metal  14  into the melting cavity  62 . The tiling melting hearth  12  also includes an induction coil  24  configured to heat the molten metal  16  in the melting cavity  62 . In addition, the tilting melting hearth system  10  includes an external heat source  22 , such as a plasma torch system, a plasma transferred arc system, an electric arc system, an induction system, a photon system, or an electron beam energy system in close proximity to the melting cavity  62  of the tilting melting hearth  12 , which is also configured to heat the molten metal  16 . The tilting melting hearth system  10  can be configured to form alloys with or without magnetic stirring where melt cycles are defined by energy input per weight of material and a characterized vaporization rate can be determined. Previously cited U.S. Pat. Nos. 9,925,591 and 10,654,106 describe further details of the tilting melting hearth  12  including electromagnetic stirring. 
     The tilting melting hearth system  10  also includes a central processing unit (CPU)  18  for controlling the tilting melting hearth  12 . As will be further explained, the central processing unit (CPU)  18  includes an automated hearth tilting program  20  configured to maintain one or more hearth tilt profiles  66 A ( FIG.  6 A ) or  66 B ( FIG.  6 B ). The tilt profiles can be based on a weight or a level of the molten metal  16  in the tilting melting hearth  12 , as well as other parameters. The central processing unit (CPU)  18  can also control a sequence of feeding, melting, pouring and either atomizing or casting the molten metal  16 . The central processing unit (CPU)  18  can comprise an off the shelf component purchased from a commercial manufacturer. The automated hearth tilting program  20  can include computer code having a set of instructions, or a system of rules, written in a particular programming language (e.g., a source code and an object code). The automated hearth tilting program  20  can be written using techniques that are known in the art and information on the desired hearth tilt profiles  66 A ( FIG.  6 A ) or  66 B ( FIG.  6 B ). 
     The tilting melting hearth system  10  also includes an actuator  26  in signal communication with the central processing unit (CPU)  18  having a linkage  28  configured to support and move the tilting melting hearth  12  to a desired hearth tilt angle. The actuator  26  can comprise an off the shelf component, such as a hydraulic cylinder purchased from a commercial manufacturer. The linkage  28  can be fabricated using techniques that are known in the art to perform tilting, as well as rotation about a longitudinal axis. The tilting melting hearth system  10  also includes a weight measuring device  30  operably associated with the actuator  26  in signal communication with the central processing unit (CPU)  18  configured to measure a weight of the molten metal  16  in the tilting melting hearth  12 . By way of example, the weight measuring device  30  can comprise a load cell, such as a tension and/or compression load cell, or similar device. The hearth tilting program  20  uses information from the weight measuring device  30  to maintain a desired hearth tilt profile and a uniform pour rate. For example, a hearth tilt profile  66 A ( FIG.  6 A ) can be based on the weight of the molten metal  16  in the melting cavity  62 . As another example, a hearth tilt profile  66 B ( FIG.  6 B ) can be based on the level of the molten metal  16  in the melting cavity  62  as determined by weight, density and volume calculations. These hearth tilt profiles  66 A,  66 B, as well as others, can be determined using mathematical calculations (and experimental data, if needed) to determine and maintain a uniform stream of molten metal  40  ( FIG.  2 A ) and a uniform pour rate. In addition, multiple profiles can be combined to provide an optimal profile. 
     The tilting melting hearth system  10  also includes a digital readout  32  in signal communication with the central processing unit (CPU)  18  having a display screen  34  configured to display information and a keypad  36  configured to input information to the central processing unit (CPU)  18 . The digital readout  32  can comprise an off the shelf component purchased from a commercial manufacturer. 
     The tilting melting hearth system  10  can also include an atomization die  38  in flow communication with the tilting melting hearth  12  configured to receive the stream of molten metal  40  ( FIG.  2 A ) and generate a metal powder  42  ( FIG.  3 A ) comprised of particles  44  ( FIG.  3 B ) having a desired particle shape and particle size. Alternately, the tilting melting hearth system  10  can also include a casting die  46  ( FIG.  2 B ) in flow communication with the tilting melting hearth  12  configured to receive the stream of molten metal and generate a casting  48  ( FIG.  4   ). 
     As shown in  FIG.  2 A , the atomization die  38  can include a metal body  50  having passageways for inert gas jets  52 . The atomization die  38  also includes an orifice  54  in the center, a cover  56 , and a gas inlet  58 . The inert gas jets  52 , which are arranged in a circular pattern, impinge inert gas onto the stream of molten metal  40 . The inert gas jets  52  all converge on the stream of molten metal  40  within the atomization die  38  to disintegrate the stream of molten metal  40  and generate the metal powder  42  ( FIG.  3 A ) forming the particles  44  ( FIG.  3 B ) with a desired shape (e.g., spherical) and particle size (e.g., diameter D of 1-500 μm). The particles  44  ( FIG.  3 B ) cool in free-fall until reaching the bottom of an atomization tower (not shown). The metal powder  42  ( FIG.  3 A ) is segregated into groups of similar particle size using gravity, screening, or cyclonic separation. 
     As shown in  FIG.  5   , the tilting melting hearth  12  can be tilted at different hearth tilt angles from 0 to 90 degrees, measured from a horizontal placement of the tilting melting hearth  12  to pour the molten metal  16  through the pour notch  60  into the atomization die  38  ( FIG.  2 A ) or casting die  46  ( FIG.  2 B ) with a uniform stream of molten metal  40  ( FIG.  2 A ) and a uniform pour rate. By determining a weight of the molten metal  16  in the tilting melting hearth  12 , and utilizing the automated hearth tilting program  20 , a selection of a hearth tilt profile, or combinations of multiple profiles, can be made. For example,  FIG.  6 A  illustrates the most basic hearth tilt profile  66  in which the hearth tilt angle increases as the weight of the molten metal  16  in the melting cavity  62  of the tilting melting hearth  12  decreases. In this example, the readout of the digital readout  32  can be zeroed to eliminate the weight of the tilting melting hearth  12 . In addition, the hearth tilting program  20  controls the tilting melting hearth  12  using angle and time calculations. In addition, different profiles can be used for any segment of the hearth tilt profile  66 A, and an interface with the weight measuring device  30 . This allows the amount of material in the tilting melting hearth  12  to force a selection of pre-programmed hearth tilt profiles  66 A ( FIG.  6 A ) or  66 B ( FIG.  6 B ) that best fit the operation parameters. 
     Based on the known density of the metal  14 , and the calculated fluid amount of the molten metal  16 , an entire sequence can be reduced to a single operation. For example, an exemplary sequence  68  shown in  FIG.  7   , can include feeding the metal  14  into the tilting melting hearth  12 , melting the metal  14  into the molten metal  16 , and pouring the molten metal  16  into either the atomization die  38  or the casting die  46 . In this example, all aspects of the sequence  68  are controlled by the central processing unit (CPU)  18 . The tilting melting hearth system  10  can also provide additional feeding of the metal  14  into the tilting melting hearth  12  and/or additional pouring into the atomization die  38  or the casting die  46  to keep the fluid level of the molten metal  16  and the pour rate inside operation parameters. The tilting melting hearth system  10  and the central processing unit (CPU)  18  can also be configured to control electromagnetic stirring of the tilting melting hearth  12  such that a stirring power level can be varied with the amount of molten metal  16  in the tilting melting hearth  12 . 
     The metal  14  can comprise any feedstock, including but not limited to: bars, blocks, rounds, chunks, powders, flakes, pellets or any size or shape that can be fed into a vessel. By way of example, recycled scrap metals can include reactive metals such as titanium, zirconium, nickel, cobalt and alloys thereof. As another example, recycled scrap metals can include nonreactive metals, such as steel, iron and alloys thereof. In an exemplary embodiment, scrap metals can be collected from a battlefield near a forward operating base. In another embodiment, parts can be recycled on board an aircraft carrier, oil rig, or some other remote facility. Preferably, large pieces of scrap metal are collected, analyzed by handheld XRF, and cut to pieces smaller than 6″ in diameter. Smaller fragments of scrap metals are preferably not collected due to lower yield, greater variations in alloy composition, and increased likelihood of contamination. 
     Example. The operational range of Ti 6-4 and other material of the same density can be approximately 5.5 kg to 7 kg of pre-alloyed material (e.g., metal  14  ( FIG.  1   )) in the tilting melting hearth  12 . Effective alloy producing amounts requiring stirring can be approximately 3.5 kg to 4 kg. This illustrates that in practice the volume of material required to produce alloy from raw material, or to correct any composition of an alloy, is much more narrow than processing pre-alloyed material. The operation of adjusting the composition of an alloy by either adding virgin material or elevating individual constituents requires the same process control as creating an initial alloy from raw constituents. Hence there is a need to identify a precise amount of material in the tilting melting hearth  12 . 
     While a number of exemplary aspects and embodiment have been discussed above, those of skill in the art will recognize certain modification, permutations, addition, and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, and sub-combinations as are within their true spirit and scope.