Abstract:
A system wherein oxygen is provided to a shaft furnace in addition to the blast air, and a flame shroud is formed around the oxygen stream enabling deep penetration of the oxygen into the furnace for not only combusting carbonaceous material for generating heat but also for cutting and oxidizing metal within the furnace.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to the operation of a shaft furnace and, more particularly, to the operation of a shaft furnace using oxygen enrichment.  
         BACKGROUND ART  
         [0002]    A shaft furnace, such as a cupola or blast furnace, is a vertical, generally cylindrical furnace wherein metal is melted. The furnace is typically charged with alternating layers of coke and metal along with limestone or other fluxing material. During operation metal is heated and then melted as it descends downward through the shaft and collects in the hearth or crucible at the bottom of the furnace as a molten metal pool. The fluxing material is also heated and melted, and the resultant lighter molten slag accumulates as a layer on top of the molten metal. The molten metal and slag is tapped from the furnace into a runner through a tapping spout located at the base of the furnace and the molten slag is subsequently removed by skimming in the runner system.  
           [0003]    The coke combusts with incoming air to form carbon dioxide in an exothermic reaction which generates heat which is employed to melt the metal. The carbon dioxide rising within the furnace also reacts with coke to form carbon monoxide. While this is a heat consuming reaction, it is important as the produced carbon monoxide and the partial reduction of carbon dioxide serve to hold down the rate of metallic oxidation within the furnace.  
           [0004]    In order to improve the operation of a shaft furnace, there has long been practiced the provision of oxygen to the furnace in addition to the primary air. One of the most successful commercial oxygen enrichment cupola practices is the supersonic direct injection process disclosed in U.S. Pat. No. 4,324,583 wherein oxygen is provided to a cupola at a supersonic velocity.  
           [0005]    The operation of shaft furnaces such as cupola furnaces for melting metal has significant economic importance and thus any improvement would be highly desirable.  
           [0006]    Accordingly, it is an object of this invention to provide a system for melting metal in a shaft furnace which enables an improvement in operation over heretofore available systems for melting metal in a shaft furnace.  
         SUMMARY OF THE INVENTION  
         [0007]    The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention one aspect of which is:  
           [0008]    A method for melting metal in a shaft furnace comprising:  
           [0009]    (A) passing blast air into a shaft furnace containing a charge comprising metal, carbonaceous material, and flux material;  
           [0010]    (B) providing a stream of oxygen from a lance, and surrounding the stream of oxygen with combusting fuel and oxidant forming a flame shroud around the stream of oxygen;  
           [0011]    (C) passing the stream of oxygen into the shaft furnace; and  
           [0012]    (D) combusting carbonaceous material and cutting and oxidizing metal with the stream of oxygen within the shaft furnace and generating heat for melting metal within the shaft furnace.  
           [0013]    Another aspect of the invention is:  
           [0014]    A shaft furnace for melting metal comprising a refractory lined furnace wall, a blast air tuyere positioned in the furnace wall for passing blast air into the shaft furnace, and an oxygen lance positioned within the blast air tuyere, said oxygen lance having a tip which is recessed from the end of the blast air tuyere, having means for passing oxygen in an oxygen stream out from the lance at the tip, and having means for providing fuel and oxidant out from the lance to form a flame shroud around the oxygen stream.  
           [0015]    As used herein the term “oxygen” means a fluid having a molecular oxygen concentration of at least 70 mole percent.  
           [0016]    As used herein the term “blast air” means a fluid comprising primarily molecular oxygen and molecular nitrogen, such as ambient air. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a cross-sectional view of a lance positioned for providing an oxygen stream into a shaft furnace which, in this embodiment, is a cupola furnace.  
         [0018]    [0018]FIG. 2 is a head on view of one embodiment of a lance tip which may be employed for providing an oxygen stream in the practice of this invention.  
         [0019]    [0019]FIG. 3 is a cross-sectional view of the lance shown in FIG. 2 in operation showing the oxygen stream and the flame shroud.  
         [0020]    [0020]FIG. 4 is a cross-sectional plan view of a cupola furnace showing an arrangement which was used to test the method of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The invention will be described in detail with reference to the Drawings and in reference to the operation of a cupola furnace. In the Drawings common or similar elements have the same numeral.  
         [0022]    Referring now to FIG. 1, tuyere  50  is positioned in sidewall  51  of a cupola furnace for providing blast air from blast air source  52  into the cupola furnace. Typically a shaft furnace such as a cupola furnace will have from 4 to 10 blast air tuyeres positioned around its perimeter so as to provide blast air in a relatively even distribution pattern into the shaft furnace.  
         [0023]    The cupola furnace contains carbonaceous material such as coke and also contains solid metal to be melted. Typically the metal is ferrous metal such as pig iron, scrap iron or scrap steel. The solid metal and carbonaceous material charge in the cupola furnace is in alternating layers of metal and carbonaceous material. The charge also typically contains flux material to facilitate the refining of the molten metal resulting from the operation of the cupola furnace and to protect the refractory lining from excessive wear.  
         [0024]    Oxygen lance  1  is used to provide oxygen to the cupola furnace. The embodiment of the invention illustrated in FIG. 1 is a preferred embodiment wherein the oxygen lance is positioned within the blast air tuyere. The oxygen lance may also be positioned outside the blast air tuyere so as to provide the oxygen to the cupola furnace separately from the blast air. Oxygen is provided to oxygen lance  1  as shown by input arrow  53 . Preferably the oxygen is commercially pure oxygen having an oxygen concentration of at least 90 mole percent. Fuel, such as methane, propane, natural gas and the like, is provided to oxygen lance  1  as shown by flow arrow  54 , and oxidant, which is preferably commercially pure oxygen, is provided to oxygen lance  1  as shown by flow arrow  55 . The oxygen, fuel and oxidant flow through oxygen lance  1  and are ejected from the tip or face  5  of oxygen lance  1 . Preferably, as shown in FIG. 1, tip  5  of oxygen lance  1  is recessed, typically from 2 to 6 inches, from the tip or end  56  of blast air tuyere  50 .  
         [0025]    [0025]FIG. 2 illustrates one embodiment of an arrangement of the apertures on the lance face which may be used in the practice of this invention, and FIG. 3 illustrates that embodiment of the lance in cross-section. Referring now to FIGS. 2 and 3, oxygen passes through central passage  2  of lance  1  and is ejected from opening  11  on lance face  5  to form oxygen stream  20 . Preferably, as shown in FIG. 3, central passage  2  communicates with converging/diverging nozzle  57  which serves to deliver the oxygen from central passage  2  to opening  11  on face  5  and then out of lance  1  in stream  20 . The converging/diverging nozzle imparts a supersonic velocity to oxygen stream  20 . Preferably the velocity of oxygen stream  20  is within the range of from 700 to 2100 feet per second.  
         [0026]    Gaseous fuel is passed through inner annular passage  3  of oxygen lance  1 . Near the tip of oxygen lance  1  inner annular passage  3  communicates with a plurality of individual passages  7  which come out on lance face  5  as inner ring of holes  9 . The gaseous fuel, e.g. natural gas, passes out from the tip of lance  1  through these inner holes  9 . Oxidant is passed through outer annular passage  4  of oxygen lance  1 . Near the tip of oxygen lance  1  outer annular passage  4  communicates with a plurality of individual passages  8  which come out on lance face  5  as outer ring of holes  10 . The oxidant, e.g. commercially pure oxygen, passes out from the tip of lance  1  through these outer rings of holes  10 .  
         [0027]    The gaseous fuel ejected from lance  1  through inner holes  9  and the oxidant ejected from lance  1  through outer holes  10  mix and combust to form flame shroud  23  around and along the length of oxygen stream  20 . Flame shroud  23  serves to shield oxygen stream  20  from ambient gases which would otherwise aspirate into a high velocity gas stream such as a supersonic gas jet. This flow or aspiration of ambient gas into a gas stream expands the gas stream and reduces its velocity. In contrast, with the use of the flame shroud of this invention, the diameter of the oxygen stream remains essentially constant for a distance of at least 20 d or until the oxygen stream impacts the furnace contents, whichever event occurs sooner, where d is the exit diameter of opening  11 , and, in addition, the velocity of the oxygen stream remains essentially constant for the same distance, after the oxygen stream is ejected from the tip  5  of oxygen lance  1 . This has the effect of maintaining the momentum of the oxygen stream concentrated within the relatively small resulting cross-sectional area of the oxygen stream  20  and not dissipated such as is the case with conventional supersonic injection practice in shaft furnaces. That is, the flame shroud serves to maintain the oxygen stream coherent from its ejection from the lance to impact with the cupola charge. Thus the oxygen stream ejected from oxygen lance  1  impacts the charge within the cupola furnace with greater force than is possible with conventional practice and this enables the oxygen to penetrate deeper into the charge within the cupola furnace than would otherwise be possible. This deeper penetration enhances the evenness of the combustion of the carbonaceous material within the cupola furnace which in turn improves the efficiency and thus the productivity and raw material consumption of the cupola furnace operation. In addition, the deeper penetration enables cutting and oxidizing metal within the furnace. In pilot scale testing of the invention in a box filled with foundry coke, the coherent oxygen jet of this invention was able to penetrate into the coke bed for distances of from 25 to 48 inches. In contrast, using the same conditions but employing conventional supersonic injection of oxygen, the oxygen jet was able to penetrate into the coke bed for distances of only from 12 to 16 inches.  
         [0028]    In some situations it may be preferred to provide the fuel and oxidant for the flame shroud around the stream of oxygen from one ring of holes rather than the two rings of holes of the embodiment illustrated in FIG. 2. In yet another variation which may be preferred in some situations, the oxidant for the flame shroud is the same fluid as the oxygen for the oxygen stream and, most preferably, that oxidant is taken from the main or central oxygen passage using a bleed line to provide the oxidant fluid to the flame shroud oxidant provision means.  
         [0029]    [0029]FIG. 4 illustrates in top cross-sectional view a commercial cupola furnace which was altered to test the invention. The cupola furnace  60  had an inside diameter of  102  inches and was refractory lined and water cooled. The cupola was normally operated with  10  blast air tuyeres  50  arranged as shown in FIG. 4 which supplied blast air at a rate of about 15000 cubic feet per minute. Oxygen was also supplied through each of tuyeres  50  using the conventional supersonic direct injection practice disclosed in U.S. Pat. No. 4,324,583.  
         [0030]    To demonstrate the advantages attainable with the practice of this invention two of the tuyeres  50  were altered by inserting therein oxygen lance  1  of the invention, as is shown in FIG. 4, and the cupola furnace operation was carried out. In these demonstration tests, the oxygen flow was 16000 standard cubic feet per minute (Scfh), the gaseous fuel flow was 2700 Scfh and the oxidant flow was 2025 Scfh. The productivity improvement for the cupola furnace achieved in these tests with only two of the 10 tuyeres converted to use the invention ranged from 1.61 to 2.13 percent wherein productivity is defined as the maximum tons of iron and steel scrap per hour that can be consumed and melted by the cupola furnace.  
         [0031]    These examples and comparative examples serve to demonstrate the significant advantages attainable with the practice of this invention over the heretofore most advanced commercial cupola furnace practice.  
         [0032]    Although the invention has been described in detail with reference to a certain preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.