Process for melting a metal charge in a rotary furnace and rotary furnace for implementing such a process

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.degree. 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.degree. 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: .degree. C., melting rate: .degree. C./minute/5.3 ton of charge total consumption: propane/oxygen, specific consumption: m.sup.3 /100.degree. 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 .degree. 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.sup.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.degree. C./minute.

Claims

1. 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% by weight based on the metal charge 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 at an angle in a range from 5 to 25 degrees in relation to the axis of the furnace.

2. Process according to claim 1, wherein the charge of solid fuel in the metal charge is present in a proportion between 1.5% and 9%.

3. Process according to claim 2, wherein the charge of solid fuel in the metal charge is present in a proportion between 2 and 6%.

4. Process according to claim 1 wherein the oxygen is injected at a supersonic speed.

5. Process according to claim 1 wherein the jet of oxygen is injected between a flame of the burner and the combined charge in the furnace.

6. Process according claim 1 wherein the oxygen is injected as soon as the burner is brought into action.

7. Process according to claim 1, further comprising supplying at least the injected oxygen from a unit for separating gas from air using adsorption.

8. Rotary furnace for melting a metal charge, comprising:

(i) at least one oxygen burner at an end of the furnace; and

(ii) at least one oxygen lance disposed at an angle in a range from 5 to 25 degrees in relation to the axis of the furnace to direct at least one jet of oxygen towards a bottom of the furnace.

9. Furnace according to claim 8, wherein the lance comprises at least two oxygen injection channels.

10. Furnace according to claim 8 wherein the lance is disposed below the burner.

11. Furnace according to claim 8 wherein the lance is disposed in the burner.

12. Furnace according to claim 8 wherein the burner further comprises a plurality of angularly distributed injectors.

13. Furnace according to claim 9, wherein the lance is disposed below the burner.

14. Furnace according to claim 9, wherein the lance is disposed in the burner.

15. Furnace according to claim 9, wherein the burner further comprises a plurality of angularly distributed ejectors.

16. Furnace according to claim 10, wherein the burner further comprises a plurality of angularly distributed injectors.

17. Furnace according to claim 11, wherein the burner further comprises a plurality of angularly distributed injectors.