Patent Publication Number: US-2007108673-A1

Title: Molten metal processing

Description:
This invention concerns improvements in and relating to metal processing, particularly to apparatus and methods for introducing gases and/or solids into molten metal.  
      At various stages during the melting, treatment, purification and distribution of molten metals it is desirable to introduce external materials into the molten metal. These may be gases to extract undesired components from the molten metal into an accompanying slag, powders to introduce desired components into the molten metal or other materials.  
      Existing techniques for achieving this aim include the use of lances which are dipped into the molten metal and through which the materials are introduced. The lances are separate components from the chamber containing the molten metal and are removed from the molten metal when not being used. Existing techniques face a number of limitations, particularly in relation to the amount of material which can successfully be delivered with time and the degree of contact provided between the material and the molten metal, given that the material is introduced at a discrete location and needs to be spread throughout the molten metal.  
      The present invention has amongst its aims the provision of apparatus and methods for introducing materials into molten metal. The present invention has amongst its aims the provision of apparatus and methods for introducing materials into molten metal so as to provide greater contact between the material added and the molten metal as a whole. The present invention has amongst its aims the provision of apparatus and methods for introducing materials which allow a greater range of materials to be successfully introduced using the same apparatus or method.  
      According to a first aspect of the invention we provide processing apparatus for molten metal, the apparatus including a chamber for the molten metal, a molten metal mover, an inlet leading from the chamber to the molten metal mover and an outlet leading from the molten metal mover to the chamber, a tube leading into the apparatus between the start of the inlet and the end of the outlet.  
      The processing apparatus may be for melting metal or metal containing materials and/or for processing molten metal and/or for purifying molten metal and/or for distributing molten metal at a location, the location being between 10 cm before the start of the inlet and 10 cm after the end of the outlet, preferably being between the start of the inlet and the end of the inlet. The processing apparatus may be for introducing just a gas into a volume of already molten metal. The chamber may be a charge well, particularly in such a case. The apparatus may further include a furnace, ideally separate from the charge well. Preferably the outlet from the charge well leads to the furnace. Preferably the inlet to the charge well leads from the furnace.  
      The chamber may be a furnace itself.  
      The molten metal mover may be a pump. The pump may be a mechanical pump, but is preferably an electromagnetic pump.  
      The tube preferably leads into the apparatus other than within the molten metal mover, particularly the part thereof which imparts motion to the molten metal. Preferably the tube leads into the apparatus between the end of the molten metal mover and the end of the outlet.  
      The tube may be provided in a wall of the chamber. The tube preferably leads into the apparatus at the end of the outlet.  
      The tube may be provided in a component around the outlet. The tube preferably leads into the apparatus at the beginning of the outlet. The tube preferably leads into the apparatus at the outlet from the molten metal mover.  
      The tube may lead to a location at which molten metal is provided in use, a gas permeable element being provided between the end of the tube at the location at which molten metal is present in use.  
      According to a second aspect of the invention we provide processing apparatus for molten metal, the apparatus comprising a tube, the tube leading to a location at which molten metal is provided in use, a gas permeable element being provided between the end of the tube at the location at which molten metal is present in use.  
      Preferably the apparatus includes a chamber for the molten metal. Preferably the apparatus includes a molten metal mover. Preferably the apparatus includes an inlet leading from the chamber to the molten metal mover. Preferably the apparatus includes an outlet leading from the molten metal mover to the chamber. Preferably the tube leads into the apparatus at a location, the location being between 10 cm before the start of the inlet and 10 cm after the end of the outlet, preferably being between the start of the inlet and the end of the inlet.  
      The tube preferably leads into the apparatus other than within the molten metal mover, particularly the part thereof which imparts motion to the molten metal. Preferably the tube leads into the apparatus between the end of the molten metal mover and the end of the outlet.  
      The tube may be provided in a component around the outlet The tube preferably leads into the outlet at a location removed from the charge well. The tube preferably leads into the apparatus at the beginning of the outlet. The tube preferably leads into the apparatus at the outlet from the molten metal mover.  
      According to a third aspect of the invention we provide processing apparatus for introducing a gas to a volume of molten metal, the apparatus including a charge well into which solid metal to be melted is introduced, an electromagnetic pump, an inlet leading from the charge well to the pump, an outlet leading from the pump to the charge well, a tube leading into the outlet from the pump, the tube leading into the outlet at a location removed from the charge well.  
      Preferably the apparatus includes a furnace. Preferably the outlet form the charge well leads to the furnace and the metal then passes on to the pump. Preferably the inlet to the charge well leads from the furnace, via the pump.  
      The tube may lead to a location at which molten metal is provided in use, a gas permeable element being provided between the end of the tube at the location at which molten metal is present in use.  
      According to a fourth aspect of the invention we provide processing apparatus for introducing solid metal to a volume of molten metal, the apparatus including a charge well into which solid metal to be melted is introduced, an electromagnetic pump, an inlet leading from the charge well to the pump and an outlet leading from the pump to the charge well, a tube leading into the outlet from the pump, the tube being provided in a wall of the charge well.  
      Preferably the apparatus includes a furnace, ideally separate from the charge well. Preferably the outlet from the charge well leads to the furnace. Preferably the inlet to the charge well leads from the furnace.  
      According to a fifth aspect of the invention we provide processing apparatus for molten metal, the apparatus including a chamber for the molten metal, a molten metal mover, an inlet leading from the chamber to the molten metal mover and an outlet leading from the molten metal mover to the chamber, a conduit leading into the apparatus between the inlet and the outlet, the conduit being provided with a gas permeable element at its end, in contact with the inside of the chamber.  
      The aspects of the invention may include one or more of the following features, options or possibilities.  
      The metal may be aluminum. The solid metal may be scrap metal, for instance used beverage cans.  
      The chamber, particularly a charge well, is refractory lined.  
      The electromagnetic pump preferably includes a through passage enclosed within one or more conducting coils. A silicon carbide passage is preferred. Preferably a multilayered coil is provided. Preferably the polarity across the coils can be reversed. Preferably the voltage applied to the coils can be varied.  
      The inlet and/or outlet are preferably refractory lined. The inlet may lead directly to the chamber or charge well. The inlet may lead to the chamber or charge well via one or more other components, for instance an intervening chamber. The outlet may lead indirectly to the chamber or charge well, but preferably leads there directly.  
      In one embodiment, the invention may particularly provide from the following. The tube may be substantially vertically provided, for instance within 5° of vertical. Preferably the tube is in contact with the chamber/charge well along at least a part of its length. Preferably the contact extends the entire length of the tube, at least below the level of the molten metal in use. Preferably the tube is received within the wall in a passage. Preferably the passage has an opening to the chamber/charge well along at least part of its length. Preferably the opening extends along the entire length of the passage, at least below the level of the molten metal in use. Preferably the passage is of circular cross-section with a portion cut away. The cut away portion is preferably determined by the interception of the passage&#39;s circular cross-section with the chamber/charge well. Preferably the tube is of circular cross-section. Preferably the tube is a snug fit within the passage. Preferably the tube is of metal. Preferably the end of the tube is flush with the adjoining portion of the inlet or outlet Preferably the end of the tube is flush with the adjoining portion of the refractory lining. Preferably the end of the tube is provided in the outlet. Preferably the end of the tube is provided at the end of the outlet nearest the chamber/charge well.  
      In another embodiment, the invention may particularly provide from the following. The tube may be substantially vertically provided, for instance within 5° of the vertical. Preferably the tube is separate from the chamber/charge well along its length. The tube may be provided within the refractory surround for the chamber/charge well. The tube may be provided in a separate component, particularly one mounted on the inlet to the chamber and/or outlet from the molten metal mover. Preferably the tube is of circular cross-section. One or more tubes may be provided. Preferably the tubes approach the inlet to the chamber and/or outlet from the molten metal mover tangentially. Preferably the tube is of metal and/or the material being introduced through a gas permeable element provided on the end of a tube. The end of the tube contacts or enters a gas permeable element, such as a porous block.  
      Where a gas permeable element is provided, the element is preferably permeable to gas without being permeable to molten metal, particularly aluminum. The element may be provided with a gas tight seal around one or more surfaces. The seal may be provided by a part of the element or a component which surrounds those parts of the element. The element preferably has at least one surface leading into the apparatus. The one or more surfaces may be configured to match one or more adjoining parts, for instance the adjoining parts of the outlet. The one or more surfaces are preferably flush with the surrounding parts of the outlet.  
      Preferably the tube is connected to a source of material to be added to the molten metal. The source may be a pressurized source of a gas or more than one gas. The source may be a source of solid material, for instance one or more powders. The source may provide a mixture of materials.  
      The method may particularly provide that gas leaves the gas permeable element and enters the molten metal, without molten metal entering the gas permeable element.  
      Preferably the gas reacts with the metal or one or more constituent parts thereof, for instance magnesium in aluminum. Preferably the reaction has completed before the gas reaches the end of the outlet and/or enters the chamber. Preferably the gas is not longer present as bubbles by the time it reaches the end of the outlet and/or enters the chamber. Preferably the gas flow rate into the outlet and/or the distance between the location at which gas enters the outlet and the end of the outlet are controlled so as to ensure the reaction is completed and/or no gas is left and/or no gas bubbles are left.  
      Particularly in another embodiment, the invention may particularly provide from the following. The gas permeable element may be provided in the side wall. The element may be a truncated cone. The element may have a porosity of between 5 and 20%, more preferably 12%+/−2%. One or more tubes may be provided in the element. The tubes may be connected to a chamber supplied with gas. Three tubes may be provided. The tubes may be filled apart from a series of slots. Four slots may be provided. The slots may be parallel to one another. The element may be provided in the side wall adjacent the outlet, for instance within 20 degrees of the start thereof. The element may be provided in the part of the chamber wall followed by the metal as it moves from the inlet towards the outlet. Preferably the element is provided within 10% of the same height, and ideally at the same height, as the outlet.  
      According to a fifth aspect of the invention we provide a method of introducing material into a molten metal, the method including providing apparatus including a chamber, providing molten metal within the chamber and circulating the molten metal through an outlet from the chamber to a molten metal mover and from the molten metal mover through an outlet to the chamber, the material being introduced through a tube which enters the apparatus between the start of the inlet and the end of the outlet.  
      The material may be introduced as a solid. The material may be introduced as a gas. The material may include one or more materials. The material may include one or more solids and one or more gases. Different materials may be introduced at different times. The material may be introduced to remove a component of the molten metal. The material may be introduced to add a component to the molten metal. 
    
    
      Various embodiments of the invention will now be described, by way of example only, in which:  
       FIG. 1  illustrates a charge well and electromagnetic pump according to one embodiment of the invention;  
       FIG. 2  illustrates a plan view of the charge well showing the material charging tube;  
       FIG. 3  illustrates a partial sectioned side view showing the interrelationship between the end of the material charging tube and the pump outlet;  
       FIG. 4  illustrates a cross-section through a pump outlet and end section of a material charging tube according to another embodiment of the invention; and  
       FIG. 5  illustrates a partial sectioned side view showing the interrelationship between the material charging tube and the pump outlet. 
    
    
       FIG. 1  illustrates one possible embodiment of the invention, in this case in relation to solid metal melting apparatus. The apparatus includes a charge well  1  into which solid metal  3  is introduced so as to intimately contact it with and melt it into molten metal  5 . The charge well  1  has an internal profile which in combination with rapid molten metal flow causes a vortex on the molten metal surface which promotes the blending of the solid metal  3  into the molten metal  5  rapidly and efficiently. The rapid molten metal flow is generated by an electromagnetic pump unit  7  which causes molten metal  5  to leave the charge well  1  via exit  9  pass along pipe  11  through the furnace, not shown, and back to pump unit  7  along pipe  12  and then back to the charge well  1  along pipe  13  to inlet  15 . Electromagnetic pumps work on the linear motor principal in which a conductor is magnetically repulsed by a magnetic field generated by the surrounding coil. The pipe  13  is aligned with the charge well  1  in a substantially tangential manner to promote the vortex formation in the charge well  1 . Further details of the design, its principals and operation are to be found in GB-B-2269889 the contents of which, particularly in relation to features of the charge well configuration, electromagnetic pump principles and operation and system configuration, are incorporated herein by reference.  
      This embodiment represents just one situation in which the present invention might be used, other possibilities include molten metal circulation systems, furnaces or other solid metal charging installations where metal flow occurs.  
      Whilst the configuration described above is useful for introducing solid metal  3  into the molten metal  5  merely introducing a substance to the molten metal surface is not sufficient in the case of lighter materials or materials in finer form. If powders are introduced to the surface then a significant proportion thereof will remain on the surface and not contact the molten metal  5 . The problem is clearly greater still if the material to be added is a gas.  
      To introduce gases into molten metal lances are frequently used in the prior art. These are hollow tubes formed of refractory material which are maneuvered over the molten metal and dipped in when it is desired to add material. The material, gaseous, is then blown down the lance and into the molten metal. A significant portion of the gas escapes straight to the surface and its function is lost as a result There are also limits on the rate of gas introduction, as too high a gas flow rate leads to substantial bubbles forming in the molten metal which is undesirable. As a consequence gas introduction using lances is time consuming and relatively wasteful. There are also safety issues where for instance the material being introduced is hazardous, for instance chlorine.  
      The embodiment of the invention illustrated in  FIG. 1  involves a material charging tube  20  which is fed material X from a feeder  22 . Where the material is solid then a separate gas feeder  24  may be provided to assist the charging of the material into and along tube  20  or act as a supply of alternative material Y.  
      As can be seen in the plan view of  FIG. 2 , the material charging tube  22  is retained within a passage  26  in the refractory lining  28  of the charge well  1 . The passage  26  is open to the inside  30  of the charge well  1 , and as a consequence molten metal  5  contacts the outside of the tube  22  in use. The passage  26  encloses the tube  22  to a sufficient extent, however, to fully retain it in position; the gap  32  is of lesser dimension than the diameter D of the tube  22 . The contact between the molten metal  5  and the tube  22  means that the tube is kept hot in use and this avoids any problems with the material X holding up in the tube  22  or molten metal  5  siphoning out of the charge well  1 .  
      To maximise the contact between the material X and the molten metal  5  the end  34  of the tube  22  is positioned in the part  36  of the refractory lining  28  which leads from the outlet pipe  11  into the charge well  1 . At this point the molten metal has a very high flow rate as the molten metal  5  is passing through a limited cross-section and the pump unit  7  outputs a significant flow. As the material X leaves the end  34  of the tube  22  it is quickly swept by the molten metal  5  away from the end  34  and into the molten metal  5 . The turbulent flow characteristics and high speed of the molten metal mean that the material X is quickly and widely distributed within the molten metal  5  as a whole. Additionally as the material X is introduced a long way from the molten metal surface in the charge well take up of the material X by the time any of it reaches the surface is almost total (92% compared with 40 to 50% from lances). Due to the flow rate present high flow rates of material X into the molten metal  5  can also be provided with out risk of large bubble formation (300% the flow rate of lances).  
      The material X, particularly in the case of gaseous material, is desirably added at the exit from the pump unit  7 , rather than at the inlet, so as to avoid the presence of gas voids within the pump unit  7  which impairs its pumping capacity. Solid materials, could be introduced into the inlet, where the flow rate is also high, without such problems, if desired.  
      The passage  26  can be drilled into existing refractory linings  28  to allow retro-fitting of the invention if desired. Alternatively the refractory ling  28  can be produced with the passage  26  present from the start. The tube  22  is inserted from above.  
      The invention is suitable for use in introducing a wide variety of materials into the molten metal, including but not limited to: chlorine, nitrogen, alloying materials (metal and/or non-metal), fluxes, silicon, etc.  
      The materials can be introduced individually or in combinations. For instance, nitrogen gas can be used to convey a powder into the molten metal whilst also providing the purifying action of nitrogen at the same time. Different materials may be added at different times using the invention, for instance chlorine followed by nitrogen etc.  
      An embodiment of the invention, particularly suited to the introduction of gases only, is shown in  FIGS. 4 and 5 . Such an embodiment is particularly suited to introducing chlorine into molten aluminum to remove magnesium, for instance.  
      The gas  40  to be introduced is fed down a material charging tube  42 . A single tube or multiple tubes may be used. The tube  42  is surrounded by refractory material  44  and ends at porous block  46 . The porous block  46  is surrounded by refractory material  44  to provide a gas tight seal around it, other than where the porous block  46  is exposed at surface  48  to the inside of the outlet pipe  50  which returns the molten metal from the pump, not shown. the porous block  46  occupies a position around part of the side of the outlet pipe  50  and hence the gas flowing through the porous block  46  and into the outlet pipe  50  tends to enter the outlet pipe tangentially.  
      The porous material is designed to have small pores. As a result, small gas bubbles are formed at its surface and are chopped into the molten metal by the metals flow past the surface of the porous block These then disperse quickly in the metal. Furthermore, the small size of the bubbles means that they have a higher surface area to volume ratio and so react quicker with the metal or constituents of it.  
      The porous material block is such that even when gas pressure is removed, no molten metal flows into the porous block. As a consequence, there is no risk of molten metal cooling in the porous block and solidifying there. The problem with molten metal syphoning up the material charging tube is also avoided. Because no metal ever enters the material charging tube there is no need to keep the tube hot This allows it to be positioned in a variety of locations. It is believed that a slow decrease in the gas pressure causes a very small amount of metal to enter the skin of the block and freeze there. When a high gas pressure is applied at the start of the next gas phase then this metal is blown out and gas flow recommences.  
      The porous material block may have an apparent porosity of around 12%, density of around 3.06 g/cm3. It may be an alumina spinel.  
      In one advantageous position, as shown in  FIG. 4 , the material charging tube is provided at the outlet from the pump unit, with the length of the return pipe between there and the charge well, furnace or other large volume of metal. As a result of this position it is possible to set the gas flow so that the reaction between the gas and the metal has completed before the metal leaves the confines of the return pipe. This has the advantage of eliminating gas loss out of the metal by bubbles reaching the surface. Positioning the material charging tube at this position still has the benefits of the gas being introduced at a high flow location, due to the relative constriction of the return pipe, which gives turbulent conditions and leads to quick and widespread mixing of the gas with the metal. The gas still does not impair the pumping function because gas voids in the pump itself are avoided.  
      An still further alternative is shown in  FIG. 6  in terms of a porous material block  100  which is mounted in the side wall  102  of a charge well  104  or the like. Molten metal is pumped into the charge well through inlet  106  and leaves through outlet  108 . This action causes a vortex to form in the charge well. Gas entering through the porous material block  100  enters the metal and is at least in part swept into the outlet and on into the furnace. By the time the metal returns to the open charge well it has fully dissolved.  
      More detail of the porous material block  100  is shown in  FIG. 6 . A feed tube  110  connects to a void  112  which in turn feeds gas to three tubes  114  (only one of which is shown). The tubes  114  have a series of longitudinally extending slits in them. The slits are sized according to the gas bubble size desired, but generally ensure small and hence quickly dissolved bubbles. The tubes  114  are evenly dispersed within the alumina spinal material  116 .