Abstract:
A method of controlling total jet pressure from the head of a metallurgical lance for introducing gas into a volume of metal in a vessel. The lance head has at least one ejector, which has a nozzle located in a bore of the lance head and has an annular gas passage between at least one nozzle and the wall of the bore. The method proceeds by adjusting the annular gas velocity from a first Mach number to a second mach number to effect change of the total jet pressure.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to metallurgical lances with annular gas flow control.  
       BACKGROUND OF THE INVENTION  
       [0002]     In steel making, it is known to provide a metallurgical lance above a volume of molten metal in a vessel for the supply of a jet of oxygen thereto. In the early stages of a steel making process, it may be desirable to have a softer blow so as to produce a relatively large contact area to aid in formation of slag. A blow refers to the nature of the oxygen jets in steelmaking or the period of time in which the oxygen is flowing during the conversion process. Once a slag has been formed, applying a harder blow may reduce the jet impact area, provide better jet penetration and better and more complete oxygen reaction with the metal bath. Attempts have been made to effect soft and hard blows, but there is still a need for a more satisfactory arrangement in this respect.  
       SUMMARY OF THE INVENTION  
       [0003]     The invention is based on the discovery that the performance of a metallurgical lance may be improved to provide satisfactory soft and hard blows by providing an annular gas flow around a main (central) jet of oxygen and varying the annular gas velocity.  
         [0004]     The invention provides a metallurgical lance with which soft and hard blows can be more satisfactorily effected.  
         [0005]     The invention also provides a method of controlling the total pressure exerted by the jet from the head of a metallurgical lance for introducing gas into a volume of molten metal in a vessel, the lance head having at least one ejector comprising a nozzle located in a bore in the lance head and having an annular gas passage between the nozzle and the wall of the bore, the method including changing the annular gas velocity from a first Mach number to a second Mach number so as to change the total pressure exerted by the jet.  
         [0006]     In another embodiment, this invention provides a metallurgical lance head for introducing gas into a volume of metal in a vessel. The head has at least one ejector comprising a nozzle located in a bore of the head and having an annular gas passageway between the ejector and a wall of the bore. The wall curves inwards at an intermediate region before a discharge end of the ejector. The wall near the intermediate region provides a convergent distal end portion of a secondary gas passageway.  
         [0007]     In yet another embodiment, this invention provides a metallurgical lance head for introducing gas into a volume of metal in a vessel. The head has at least one ejector comprising a nozzle located in a bore of the head and having an annular gas passageway between the ejector and a wall of the bore. The wall curves inwards at an intermediate region before a discharge end of the ejector. The wall near the intermediate region provides a convergent distal end portion of a secondary gas passageway. Further, the head has a plurality of secondary gas outlets separated from and surrounding a primary gas outlet. The secondary gas outlet is directed at an angle away from the annular gas outlet effective to form a gaseous layer around a gas discharged from the primary gas outlet.  
         [0008]     The annular gas flow advantageously protects the convergent-divergent internal profile of the ejector from damage due to exposure to the harsh converter atmosphere. Conventional lances have carefully designed and machined convergent-divergent nozzle profiles for specific oxygen flowrates and supply pressure. The nozzles are designed such that the flow issuing from the convergent-divergent nozzle is expanded to the ambient pressure in the converter proximate to the nozzle exit. A lower volume of gas flowing through the nozzle results in a lower pressure at the nozzle exit, known as an ‘underblown’ condition, and leads to the reticulation of material from the converter atmosphere into the nozzle and exacerbated nozzle wear. The flow of annular gas around the ejector ensures that not only is clean gas passed continuously around the ejector exit, but that any material recirculated into the ejector is also clean. This serves to protect the critical dimensions of the convergent nozzle from damage through normal and underblown conditions and extends nozzle life.  
         [0009]     Ejector outlets may constitute primary gas outlets, with the method also including providing a plurality of secondary gas outlets adjacent each primary gas outlet and supplying the shrouding gas also to the secondary gas outlets. The gas supplied to the ejector and the annular passage are independently selected from the group consisting of oxygen, argon, nitrogen or mixtures thereof. The primary gas is selected from the group consisting of oxygen, argon, nitrogen or mixtures thereof. The secondary gas is a shrouding gas, and is selected from the group consisting of oxygen, argon, nitrogen or mixtures thereof.  
         [0010]     In this invention, the annular gas velocity is changed from or to about mach  1 , at which velocity the total jet pressure is at a minimum. In one embodiment, the annular gas velocity is changed from about  1  mach at which velocity the total pressure is at a minimum. In another embodiment, the annular gas velocity is changed to about 1 mach at which velocity the total pressure is at a minimum. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:  
         [0012]      FIG. 1  is a sectional side view of a lance head in accordance with one embodiment of the invention,  
         [0013]      FIG. 2  is a front view of the lance head of  FIG. 1 ,  
         [0014]      FIG. 3  is a sectional side view of a lance head in accordance with another embodiment of the invention,  
         [0015]      FIG. 4  is a front view of the lance head of  FIG. 3 , and  
         [0016]      FIG. 5  is a graph showing the relationship of the total pressure of a gas jet emitted from the lance head relative to the velocity of the shroud gas. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]     Referring to  FIGS. 1 and 2 , a head  12  of a metallurgical lance has a central gas chamber  14  which supplies gas to a plurality of spaced and preferably inclined ejectors  16  or nozzles. Each of the ejectors  16  are preferably disposed radially outwardly with respect to horizontal axis “A” of the head  12 . Preferably, but only by way of example, there is a plurality of ejectors  16  per head  12 , such as four (4) ejectors  16  per head  12 , although such number can be varied depending upon the application required. Each ejector  16  comprises a passageway  18  and is mounted in a bore  20  at a tip  22  of the lance head  12 . The passageways  18  are of convergent-divergent kind, and which are able to eject oxygen at supersonic velocity. The oxygen is supplied at an elevated pressure to the central gas chamber  14 . Each ejector  16  is recessed from the tip  22  within its respective bore  20 .  
         [0018]     An annular gas passage  24  is provided between each ejector  16  and a wall  25  of the bore  20 . The wall  25  of the bore  20  curves inwards at an intermediate region  27  before a discharge end  29  of the ejector  16 . The wall  25  at the region  27  in combination with the ejector  16  provides a generally convergent distal end portion  31  of the annular shrouding gas passage  24 . Spacers  23  are disposed in the passage  24  to provide support for the ejectors  16  arranged in the bore  20 . The spacers  23  are designed to only minimally restrict the gas flow.  
         [0019]     Downstream from the distal end of the ejector  16 , the wall  25  of the bore  20  no longer converges, but rather retains an approximately constant diameter at right angles to a longitudinal axis B of the bore  20  (which is coaxial with the passageway  18 ). Shroud gas fed by an annular feed passage  26  surrounds the central gas chamber  14  and is independent therefrom. Annular passages are also provided for cooling fluid, such as an inner cooling passage  28  and an outer cooling passage  30  in communication by a connecting passage  32 .  
         [0020]     It has been found in the invention that manipulating the velocity of the shroud gas at the convergent distal end portion  31  can affect the force (or total pressure) exerted downstream of the ejectors  16  along an axis of the combined jet exhausted from passage  18 . This force or total pressure of the combined jet is important in that it affects jet penetration into the bath of molten metal and is an indicator of jet velocity decay and cross-sectional growth. The total pressure exerted by the jet is that exerted when the gas is brought to rest or stagnation, in which stagnation pressure is defined as the pressure that a fluid exerts when it is motionless.  
         [0021]      FIG. 5  shows the effect of shroud Mach number on the total pressure as measured by a stagnation probe positioned 15.5 inches from the distal end of the bore  20  along the axis of the combined jet. As the Mach number of the annular gas flow at portion  31  is increased towards unity (or Mach 1) while the primary jet velocity at passage  18  remains constant at Mach 2.1, there is a rapid drop in the total pressure of the combined jet. This is unexpected because the thrust of the combined annular jet and central jet has increased and a corresponding increase in the total pressure might have been expected. Because there is an actual reduction in jet centre-line stagnation pressure, the combined jet spread at a greater rate. A further increase in Mach number of the annular jet has the effect of increasing the combined jet pressure. Thus, it is possible to control the performance of the jet in terms of total pressure by variation of the shroud Mach number.  
         [0022]     A series of experiments were performed using a set of central nozzles and shroud pieces to investigate the effect of shroud mach on jet force. For each combined nozzle configuration the same central nozzle Mach number (2.1) and flow (652 scfm dry air) were used, and shrouds of constant flow (196 scfm dry air) and differing Mach number (0.5-2.1) were used. Thus, for all experiments discussed here a constant mass flow of 844 scfm was used. Furthermore, for each case shown, the ejector tip  16  was aligned to flush with the front face of the lance  22 , although the effect is not limited to this case. In addition, a constant nozzle tip thickness was maintained leading to constant initial central jet and shroud separation. Jet performance was determined by measuring the jet total pressure using a stagnation probe mounted on a 3-axis positioning device downstream of the jet so as to facilitate accurate traverses of the probe across the jet cross-section and location of the probe on the jet axis.  
         [0023]     As discussed above, the Mach number of the shroud may have a direct effect on the total pressure or force exerted by the jet. As can be seen in FIG ( 5 ), which shows the stagnation probe pressure at 15.5 inches, as the Mach number of the shroud is increased towards unity, a rapid drop in pressure is seen. This is surprising given that the thrust of the combined shroud and central jet has increased. A reduction in jet centerline stagnation pressure leads one to conclude that the combined jet has spread at a greater rate when considering the conservation of mass. Further increases in shroud Mach number have the effect of increasing combined jet pressure as the initial combined jet thrust is further increased. Thus, it is possible to control the performance of the jet in terms of total pressure by varying the shroud Mach number or flow rate.  
         [0024]     Thus, in a basic oxygen furnace (BOF) application, it is possible to supply additional flow through the shroud and create a softer blow by causing the Mach number of the shroud gas to rise to near unity from a lower flow rate and Mach number, or supply additional flow through the shroud and create a harder blow by causing the Mach number to rise from near unity at a lower flow rate to a higher Mach number at a higher flow rate. Thus, it is also possible to reduce the flow through the shroud from a high Mach number hard blowing value to a Mach number near unity to create a softer blow. It is also possible increase the flow through the shroud from a low Mach number soft blowing value higher than unity to create a harder blow.  
         [0025]     Another embodiment of a lance head according to the invention is shown in  FIGS. 3 and 4 . The same reference numerals will be used to illustrate elements which are the same or similar to corresponding elements in the lance head shown in  FIGS. 1 and 2 , unless otherwise indicated.  
         [0026]     It will be seen that the lance head shown in  FIGS. 3 and 4  differs from the lance head shown in  FIGS. 1 and 2  in that the lance head shown in  FIGS. 3 and 4  has a plurality of secondary oxygen port passageways  220  in communication with shroud gas passages  26 . Each of the passageways  220  is provided with a port  221  at passages  26  and a port  222  at the tip  22  of the head  12 . The gas flow at passage  26  is split to introduce into the passage  24  and the passageway  220 . The discharge of the gas at the port  222  coacts with the gas jet at the end of the bore  20  downstream from the ejector  16 . As shown in  FIG. 4 , the passageways  220  are arranged in the head  12  such that the ports  222  are in an arch or semi-circular arrangement partially surrounding a respective one of the bores  20  and located between bore  20  and a periphery  226  of the lance head  12 . Effectively, the plurality of secondary oxygen port passageway  220  is arranged circumferentially or on a circle that is concentric within the primary oxygen port passageway  18 . Each secondary oxygen port  220  is fed from the port  221  communicating with the shroud gas supply passage  26 . The angle of the secondary oxygen port passageway  220  diverges from the primary oxygen port passageway  18  (based on the longitudinal axis of each passageway near the distal end of the lance head) in the range of up to about 45°. Preferably, the range is from about 5° to about 25°. More preferably, the range is from about 10° to about 20°. The embodiment shown in  FIGS. 3 and 4  can be operated in accordance with the invention in a manner similar to that described in connection with the embodiments shown in  FIGS. 1 and 2 . The ejectors  16  may be demountable as described in co-pending application U.S. Ser. No. 10/881,489, filed 30 Jun. 2004.  
         [0027]     It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as described herein. It should be understood that embodiments described above are not only in the alternative, but combined.