Abstract:
Process for melting a metal charge in a rotary furnace equipped with at least one oxygen burner, comprising the steps of: 
     (i) adding between 1.5 and 9% of a charge of solid fuel to the metal charge to form a combined charge; and 
     (ii) injecting at least one jet of oxygen in a direction of the combined charge in the furnace.

Description:
BACKGROUND OF THE INVENTION 
     (i) Field of the Invention 
     The present invention relates to processes for melting metal charges in a rotary furnace equipped with at least one oxygen burner. 
     (ii) Description of Related Art 
     In known processes the oxygen burner, controlled in stoichiometric conditions, ensures the melting of the metal charge containing, optionally and for purely metallurgical reasons, small quantities of solid fuels, generally not exceeding 1% of the metal charge, in order to limit the formation of undesirable unburnt volatile compounds which, also where the oxygen burner is sued, limit the conditions in which the combustion is performed and, consequently, the rate of melting of the charge in the furnace. 
     A process for melting solid materials using an air or oxycombustible burner well under stoichiometric is known in DE-A-4142301, in which process oxygen is added in the oven with the aid of nozzles. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The objective of the present invention is to create an improved process enabling the rate and efficiency of melting in a given furnace to be significantly increased, while reducing the overall energy consumption. 
     To do this, according to one characteristic of the invention, the process includes the stages of adding a charge of solid fuel included between 1.5 and 9% to the metal charge to be melted and of injecting at least one jet of oxygen in the direction of the combine charge in the furnace. 
     According to other characteristics of the invention: 
     the proportion of charge of solid fuels in the metal charge is between 1.5 and 9%, advantageously between 2 and 6%; 
     the oxygen is injected at a speed close to the speed of sound or supersonic; 
     the oxygen jet is injected, as soon as the burner is brought into action, between the flame of the burner and the combined charge in the furnace. 
     the oxygen is injected at a speed which is close to the speed of sound or supersonic; 
     the jet of oxygen is injected, as soon as the burner is brought into action, between the flame of the burner and the combined charge in the furnace. 
     Another objective of the present invention is a rotary furnace for implementing such a process, including, besides an oxygen burner, at least one oxygen lance placed so as to direct at least one jet of oxygen towards the bottom of the furnace. 
     With the process according to the invention the combustion is extended into the charge itself, where the oxygen injected by the lance interacts with the solid fuel which burns in direct contact with the metal, thus extremely considerably increasing the reaction surface and thus promoting accelerated melting without affecting the temperature conditions at the furnace refractory and therefore not reducing the lifetime of the latter. Furthermore, since an appreciable proportion, exceeding 35%, of the total combustion energy is provided in the charge by the solid fuel, the power of the burner and hence its cost can be significantly reduced. 
     Other characteristics and advantages of the present invention will emerge from the following description of embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view, in lengthwise section, of an embodiment of a furnace for melting metal according to the invention; 
     FIGS. 2 and 3 are, respectively, side and sectional views of an embodiment of a multitube oxygen lance; 
     FIG. 4 is a partial view in lengthwise section of a burner with integrated lance according to the invention; 
     FIG. 5 is an end view of the burner of FIG. 4; 
     FIG. 6 is a view in lengthwise section of another embodiment of a burner with integrated lance according to the invention; 
     FIG. 7 is an end view of the burner of FIG. 6; 
     FIGS. 8 to 11 are graphs illustrating the operating parameters according to the conditions of Tables 1 to 3; 
     FIG. 12 is a graph illustrating the relationships between the rate of melting and the percentage of energy of combustion in the combined charge of the furnace. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 a rotary furnace 1 is shown, in the end door 4 of which are fitted an oxygen burner 5 pointing towards the charge and an oxygen lance 2 which can be positioned adjustably by virtue of a guiding device 3. According to the invention the lance 2 is pointed so as to direct, in the furnace 1, a high-speed, typically supersonic jet of oxygen towards a combined charge of metal, typically of steel, to be melted and of a solid fuel in proportions which are typically higher than 2% of the metal charge. This solid fuel is typically anthracite, graphite, especially electrode graphite, or other products containing carbon and hydrogen, especially solid polyolefins. Examples of operating conditions are given later in relation to Tables 1 to 3 and FIGS. 8 to 12. FIGS. 2 and 3 show a particular embodiment of an oxygen lance 2 including an upper main oxygen delivery 7 and two lower oxygen deliveries 6 enabling differentiated oxygen jets to be ejected in the direction of the charge and below the flame of the burner 5. The lance body 2 comprises a groove 8a interacting with a rib 8b of the guiding device 3 for maintaining a correct orientation of the tubes 6 and 7 when the lance 2 is being adjusted forward or backward in the furnace 1. 
     FIGS. 4 and 5 show an oxygen burner comprising a central delivery 12 of fuel gas into a shell forming a channel 9a for oxygen introduced via an entry 9, the fuel gas being ejected by the injectors 10 lying in the oxygen exit orifices in the nozzle of the burner, which are here angularly distributed around the axis of the burner. In the lower part of the latter the combined oxygen/gaseous fuel ejection orifices are replaced by at least one lance 2, as described in relation to FIGS. 2 and 3, and the upstream portion of which lies in the central fuel delivery 12. The end of a central circuit for cooling the nozzle of the burner is shown at 11. 
     FIGS. 6 and 7 show a cooled oxygen burner comprising a peripheral jacketing 11 for circulating water, introduced at 13 and discharged at 14. As in the embodiment of FIGS. 4 and 5, the burner includes a central fuel gas delivery 12 lying in an oxygen ejection channel 9a and opening outwards via a series of ejectors 10, here distributed angularly and regularly. Here, at least one, in this case two oxygen lances 2 lie in the lower portion of the main oxygen channel 9a and open out to the exterior of the burner below the ejectors 10. In this embodiment the main oxygen in the channel 9a, cooled by the jacketing 11, takes part in the cooling of the oxygen lances 2. 
     Depending on the geography of the furnace, the oxygen lance is adjusted so as to eject the jets of oxygen in the direction towards the charge at an angle of between 5 and 25° in relation to the axis of the furnace. The flow rate of the oxygen jets ejected by the lance is chosen to be between 25 and 150% of the flow rate of oxygen in the oxygen burner. 
     Depending on the dimensions of the furnace, a second oxygen lance may be provided, also directed towards the charge, in the opposite end of the furnace to the burner. 
     The oxygen being fed, both to the lance and to the oxygen burner, is advantageously oxygen with a purity of between 88 and 95%, supplied on site by a unit for separating gas from the air using adsorption, of the type known as PSA. 
     Particular operating conditions will now be described. The solid fuel, in proportions of 3.2% of the steel charge, in this case approximately 5.3 tons, is anthracite, and the oxygen injected by the lance 2 is ejected at a supersonic speed at an angle of approximately 10° in relation to the axis of the furnace. 
     The generalized combustion of the anthracite charge is obtained approximately 10 minutes after the full power of the burner is applied, in order to redistill thus the 7% of volatile compounds which the charge contains. Subsequently, when the combined charge in the furnace reaches the proper temperature, the 86.5% of carbon in the solid charge are converted to carbon monoxide while rising towards the surface of the charge. Under the flame of the burner the oxygen ejected by the lance produces an intense combustion zone which is particularly radiant and which is virtually entirely reflected towards the charge by the screening effect provided by the flame of the burner, which thus protects the walls of the furnace. 
     Thus, in accordance with the objectives of the invention, a high thermal efficiency of combustion of the unburnt residues by the injected oxygen is obtained, with a consequent increase in the energy yield per unit of time throughout the duration of the process, a reduced usage of the furnace refractory and smaller losses of the metal components of the charge. 
     In the Tables which follow, references 1 to 18 correspond to melting processes without oxygen injection with reduced anthracite charges, references 19 to 22 using an oxygen injection directed towards a metal charge containing 1.5% of anthracite, raised to 3% in references 23 to 28. 
     The values shown in Tables 1 to 3 are the following: 
     anthracite: weight in kg per one charge of metal, 
     time: respectively: melting/holding at temperature/total time, 
     temperature: ° C., 
     melting rate: ° C./minute/5.3 ton of charge total consumption: propane/oxygen, 
     specific consumption: m 3  /100° C./5.3 t (burner+lance), 
     steel analysis: Ce/C/Si. 
     
                       TABLE 1______________________________________                          Rate of                                 TotalRef. Anthracite         Time     Temperature                          melting                                 consumption______________________________________ 1   80       55/41/96 1.361   14.18  107/536 2   80       55/37/92 1.367   14.86  103/514 3   80       55/55/110                  1.321   12.00  123/614 4   80       55/42/97 1.370   14.i2  108/542 5   80       55/42/97 1.346   13.88  108/542 6   80       55/42/97 1.321   13.62  108/542 7   80       55/43/98 1.376   14.05  109/547 8   80       55/42/97 1.362   14.04  108/542 9   80       55/46/101                  1.341   13.28  113/56410   80       55/44/99 1.340   13.50  111/55311   80       55/49/104                  1.405   13.50  116/58112   80       55/42/97 1.324   13.60  108/54213   80       55/35/90 1.291   14.34  101/50314   80       55/44/99 1.324   13.37  111/55315   80       55/53/108                  1.298   12.02  121/60316   80       55/50/105                  1.379   13.30  117/58617   80       55/44/99 1.377   13.91  111/56318   80       55/43/98 1.345   13.72  109/54719   80       55/30/85 1.399   16.46   83/54220   80       55/30/85 1.364   16.05   83/54221   80       55/29/84 1.381   16.44   82/53622   80       55/30/85 1.370   16.12   83/54223   150      40/40/80 1.360   17.00   79/39724   150      40/32/72 1.360   18.90   72/35825   150      40/35/75 1.367   18.20   75/37526   150      Change27   150      40/35/75 1.436   19.15   75/37528   150      33/32/65 1.422   21.90   65/32529   170      33/27/60 1.330   22.17   60/300______________________________________ 
    
     
                       TABLE 2______________________________________                        Spec.                        consumption                        Propane/                                Oxygen                                      TotalRef. Anthracite         Time     Temp. oxyg.   lance oxygen______________________________________ 1   80       55/41/96 1.361 7.88/39.38 2   80       55/37/92 1.367 7.50/37.60 3   80       55/55/110                  1.321 9.30/46.48 4   80       55/42/97 1.370 7.90/39.56 5   80       55/42/97 1.346 8.05/40.27 6   80       55/42/97 1.321 8.20/41.03 7   80       55/43/98 1.376 7.95/39.75 8   80       55/42/97 1.362 7.95/39.75 9   80       55/46/101                  1.341 8.41/42.0610   80       55/44/99 1.340 8.25/41.2711   80       55/49/104                  1.405 8.26/41.3512   80       55/42/97 1.324 8.18/40.9413   80       5s/35/90 1.291 7.79/38.9614   80       55/44/99 1.324 8.35/41.7715   80       55/53/108                  1.298 9.29/46.4716   80       55/50/105                  1.379 8.50/42.4917   80       55/44/99 1.377 8.02/40.1618   80       55/43/98 1.345 8.13/40.6719   80       55/30/85 1.399 5.93/38.7420   80       55/30/85 1.364 6.09/39.7421   80       55/29/84 1.381 5.94/38.8122   80       55/30/85 1.370 6.06/39.5623   150      40/40/80 1.360 5.81/29.19                                233   63024   150      40/32/72 1.360 5.29/26.32                                223   58125   150      40/35/75 1.367 5.49/7.43                                230   60526   150      change27   150      40/35/75 1.436 5.22/26.11                                219   59428   150      33/32/65 1.422 4.57/22.86                                203   52829   170      33/27/60 1.330 4.51/22.41                                234   532______________________________________ 
    
     
                       TABLE 3______________________________________                         Spec.Ref. Anthracite         Time     Temp.  consumption                                 Steel analysis______________________________________ 1   80       55/41/96 1.361 2   80       55/37/92 1.367 3   80       55/55/110                  1.321 4   80       55/42/97 1.370 5   80       55/42/97 1.346 6   80       55/42/97 1.321 7   80       55/43/98 1.376 8   80       55/42/97 1.362          3.81/3.13/1.38 9   80       55/46/101                  1.341          3.59/3.09/1.1810   80       55/44/99 1.340          3.63/3.19/1.2711   80       55/49/104                  1.40512   80       55/42/97 1.324          3.64/3.09/1.8813   80       55/35/90 1.291          3.70/3.16/1.9914   80       55/44/99 1.324          3.67/3.17/1.4415   80       55/53/108                  1.298          3.52/3.09/1.3416   80       55/50/105                  1.379          3.62/3.04/1.6817   80       55/44/99 1.37718   80       55/43/98 1.34519   80       55/30/85 1.39920   80       55/30/85 1.36421   80       55/29/84 1.38122   80       55/30/85 1.370          3.85/3.23/1.8023   150      40/40/80 1.360  46.32   3.58/3.03/1.5624   150      40/32/72 1.360  42.72   3.51/3.01/1.4425   150      40/35/75 1.367  44.26   3.74/3.21/1.5126   150      change27   150      40/35/75 1.436  41.36   3.71/3.17/1.5528   150      33/32/65 1.422  37.13   3.58/3.06/1.5129   170      33/27/60 1.330  40.00______________________________________ 
    
     FIG. 8, which illustrates the rates of melting in ° C./minute for a 5.3 t charge for each of references 1 to 29 of the above Tables, shows that the rate changes from above 15 to more than 20 in the case of references 28 and 29, which enables the period of noncontinuous rotation of the furnace to be reduced from 55 minutes to 33 minutes and the interval between rotations from 5 to 3 minutes. 
     FIG. 9, which illustrates the consumption of propane (bottom curve) and of oxygen (top curve) for each of the references 1 to 29, shows that the specific consumption of propane can go down as far as 4.6 m 3  with an appreciably stable oxygen consumption. 
     FIG. 10 shows that the efficiency of melting moves from slightly more than 50% to more than 60-65%. 
     FIG. 11 shows that the energy consumption in kWh can be brought down from approximately 700 kWh to less than 600 kWh. 
     FIG. 12 shows that, according to references 1 to 29, the percentage of energy in the charge changes from less than 20 to more than 40 with a corresponding increase in the rate of melting from 15 to 22° C./minute.