WO1997017475A1

WO1997017475A1 - Process for melting and refining ferrous scrap through use of oxygen injection

Info

Publication number: WO1997017475A1
Application number: PCT/US1996/017742
Authority: WO
Inventors: Scott Gregory; Daniel Ferguson; Frank Slootman; Thierry Darle; Frederic Viraize
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Air Liquide America Corp
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude; Air Liquide America Corp
Priority date: 1995-11-09
Filing date: 1996-10-30
Publication date: 1997-05-15
Anticipated expiration: 1998-05-09
Also published as: AR004147A1; ID18397A; AU7607996A

Abstract

A process of injecting oxidizing gas into a furnace for post combustion of gaseous CO during production of steel from a charge placed in the furnace includes a first period of injection and a second period of injection. During the first period of injection, an oxidizing gas is injected into the furnace in the space above the melt which has resulted from melting of the charge. The first period of injection occurs during the end of the melt-in phase of the first charge as the first charge approaches flat-bath conditions and prior to loading of a subsequent charge into the furnace. During the second period of injection, an oxidizing gas is injected by way of the injectors into the space above the melt in the furnace, with the second period of injection occurring during refining of the melt in the furnace. This method results in the production of useful heat energy through combustion of the CO and this heat energy which has been found to significantly reduce the electrical energy requirements of the electric arc furnace.

Description

PROCESS FOR MELTING AND REFINING FERROUS SCRAP THROUGH USE OF OXYGEN INJECTION

FIELD OF THE INVENTION

The present invention concerns the melting and refining of a charge in a furnace. More particularly, the present invention involves the injection of O₂ into a furnace containing ferrous scrap to facilitate melting and refining of the ferrous scrap.

BACKGROUND OF THE INVENTION

Various methods are known for melting and refining a charge such as ferrous scrap and one such method involves the use of an electric arc furnace. In this type of method, charge which can be in the form of ferrous scrap, for example, is loaded into the furnace, typically in several successive loadings or buckets. One or more electrodes which are connected to an electrical energy source extend into the furnace. These electrodes serve as a conductor of current to cause arcs to pass from the electrodes to the charge (i.e. , ferrous scrap) in the furnace to effect melting of the charge in the furnace. During loading of the charge into the furnace, carbon and fluxes (e.g., CaO and MgO) are also typically added. In addition, O₂ is injected directly into the melting metal charge by way of, for example, a lance and this injected O₂ reacts with the charge carbon to produce carbon monoxide CO. The combustion of organics and other materials during melting also results in production of H₂.

Electrical arc furnaces require a great amount of electricity to melt the charge and produce a melt whose composition and/or chemistry can then be modified or altered (i.e., refined) to produce the desired steel end product. To reduce the electrical energy requirements of these types of furnaces, efforts have been made to facilitate the melting process by utilizing the potential heat energy that is available in the gaseous H₂ and CO in the furnace. That is, the combustion or oxidation of gaseous CO and H, is known to provide significant amounts of heat energy. Indeed, the oxidation or combustion of CO, for example, releases approximately three times the amount of energy as the oxidation of C.

To harness this potential energy that exists in the off-gas (gaseous CO and H^ of the furnace, proposals have been made to inject O₂ into the space in the furnace located above the charge so that this O₂ chemically reacts with the gaseous CO and H₂ to produce heat which is then used to melt the charge. U.S. Patent Nos. 5,373,530, 5,344,122 and 5,166,950 disclose various system components for use in connection with such a post combustion O₂ injection system.

It has been found that the injection of O₂ into an electric arc furnace in the manner generally referred to above for purposes of oxidizing the CO and H₂ that is present in the furnace is quite advantageous in that it reduces the amount of electrical energy necessary to melt a given amount of charge in the furnace. That is, for a given amount of metal charge in a furnace, a particular quantity of heat energy must be input to effect melting of the charge. Thus, the heat energy that is produced through the combustion of the gaseous CO and H₂ in the furnace is available for use in melting the charge in the furnace and so the electrical energy requirements are correspondingly reduced. While this post combustion O₂ injection system has been found to be quite beneficial, a need exists for improving upon and further developing this methodology to permit realization of further advantageous results and even greater benefits.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a process of injecting oxidizing gas into a furnace for post combustion of gaseous CO during production of steel from a charge placed in the furnace includes a first period of injection and a second period of injection. During the first period of injection, an oxidizing gas is injected into the furnace in the space above the melt which has resulted from melting of the charge. The first period of injection occurs during the end of the melt-in phase of the first charge as the first charge approaches flat-bath conditions and prior to loading of a subsequent charge into the furnace. During the second period of injection, an oxidizing gas is injected by way of the injectors into the space above the melt in the furnace, with the second period of injection occurring during refining of the melt in the furnace. This method results in the production of useful heat energy through combustion of the CO and this heat energy has been found to provide significant benefits with respect to, for example, electrical energy savings.

According to another aspect of the invention, an apparatus for melting and refining ferrous scrap to produce steel includes a furnace for receiving ferrous scrap, at least one electrode extending into the furnace for producing arcs to melt ferrous scrap in the furnace, and a plurality of injectors connected to a source of oxidizing gas and positioned at an intermediate point along a height of the furnace to inject oxidizing gas into the furnace in a space above the ferrous scrap in the furnace, said injectors being oriented downwardly from a horizontal plane. The injectors are preferably angled downwardly at an angle of between about 5° and about 20°.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing and additional features of the present invention will become more apparent from the following detailed description considered in conjunction with the accompanying drawing figures in which like elements are designated by like reference numerals and wherein:

FIG. 1 is a cross-sectional side view of an electric arc furnace used in conjunction with the present invention; FIG. 2 is a top view of the electric arc furnace shown in FIG. 1 with the cover removed;

FIG. 3 is a graph generally illustrating an O₂ injection flowrate with respect to time, and depicting the injection of O₂ during the refining phase and during flat-bath conditions; and FIG. 4 is a schematic illustration of a control and analysis system that can be utilized in connection with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, the use of an electric arc furnace to produce steel from a charge (e.g., ferrous scrap) involves placing into the furnace an initial load of charge and typically also carbon and fluxes (e.g., Cao and MgO), lowering the electrodes to the appropriate position within the furnace and then initiating operation of the furnace. During operation of the furnace, electric current passes from the electrodes down through an arc to the charge, through the charge, and up through an arc to an adjacent electrode. The portion of the charge located underneath and around the electrodes begins to melt, thereby forming a pool of molten metal within the furnace. During melting of the metal charge, oxygen is injected directly into the molten metal by way of a lance. Thus, carbon monoxide (CO) and H₂ are generated during the melt cycle from the primary combustion of organics and the carbon charge, with H₂ being primarily generated during the initial portion of the melt-in phase. The CO evolution continues as foamy slag is generated and carbon boils out of the melt.

The operation of the furnace continues in this way until all of or substantially all of the charge in the furnace has been melted to produce a flat- bath condition within the furnace. Typically, one or more charges or buckets of ferrous scrap are introduced into the furnace to complete the batch and after each successive charge, the foregoing operation occurs. Once all of the charges for a given batch have been melted, the melt in the furnace is subjected to a refining operation during which the steel is superheated and the chemistry of the steel is adjusted for the desired end use. This refining period typically starts at or near the end of the melt-in phase of the last bucket and ends just prior to when the steel is tapped from the furnace.

As mentioned above, recent proposals for decreasing the electrical energy requirements of electric arc furnaces have focused on utilizing the energy present in the off-gases (i.e. , CO and H^ produced during the melt-in phase. The energy present in the CO and H₂ includes potential energy which refers to the heat released by the combustion reactions when the CO and H₂ off-gases react with O₂. To recover this potential energy, the off-gases must be oxidized to release the heat. This release of the heat energy is achieved by injecting O₂ into the furnace by way of injectors. More particularly, it has been found that through injection of O₂ into the furnace in the space above the charge, post combustion of CO and H₂ can be achieved and this results in the creation of significant additional heat energy which can be used to facilitate the melting of the charge. The heat energy produced by these chemical reactions advantageously reduces the electrical energy requirements of the overall process. It is to be understood that, generally speaking, O₂ refers to an oxidizing gas capable of producing CO, and capable of producing CO₂ with CO in the process conditions. This thus encompasses air and oxygen enriched air comprising more than 50% vol O₂, and more preferably oxygen enriched air comprising more than about 90% vol O₂.

The efficiency of the off-gas heat recovery is a function of the heat transfer efficiency which represents the measure of the amount of released energy that is actually absorbed by the charge in the furnace. Thus, it was initially thought that the heat energy released by the post combustion of CO and H₂ could be advantageously used to reduce the electrical energy requirements of the electric arc furnace only if that heat energy could be efficiently transferred to the charge material or ferrous scrap in the furnace. Thus, it was believed that once the melt-in phase of the operation was completed and all of or substantially all of the ferrous scrap or charge in the furnace was melted, the electrical energy savings benefit associated with post combustion of the off -gases CO and H₂ would disappear because of the unavailability of ferrous scrap or charge in the furnace necessary for an effective heat transfer. Consequently, the original thinking was that the injection of C^ into the furnace in the space above the charge for post combustion should cease during the refining phase in which flat- bath or substantially flat-bath conditions exist, as well as during portions of the melt-in phase in which flat-bath or substantially flat-bath conditions exist. An d injection profile along these lines is generally depicted in Fig. 3 which illustrates the O₂ flowrate over time (specific numerical values are not included as they tend to be somewhat specific to the furnace and CO generation).

As can be seen from FIG. 3, soon after the charge or ferrous scrap is loaded into the furnace, the 0₂ flowrate would increase significantly to provide the O₂ necessary for effecting post combustion of the CO and H₂. As illustrated, the O₂ injection flowrate would vary depending upon, for example, the amount of CO and H₂ available in the furnace for post combustion as determined through off-gas analysis. This off -gas analysis can be carried out through use of a system similar to that described in U.S. Patent No. 5,344, 122, the disclosure of which is incorporated herein by reference. As the melt-in phase of operation continues, the charge or ferrous scrap existing in the furnace for effecting transfer of the heat energy produced from the post combustion of the CO and H₂ decreases.

Then, as the material in the furnace approaches a flat-bath condition, the O₂ flow rate would significantly decrease. As discussed above, once all of or substantially all of the ferrous scrap in the furnace has been melted (i.e., once the melt in the furnace reaches a flat-bath or substantially flat-bath condition), it was thought that since there would be no available ferrous scrap or metal charge in the furnace to effect a transfer of the heat energy produced from the combustion of the CO and H₂₎ the injection of O₂ should cease at such point. Thought was also given to possibly injecting O₂ into the furnace during refining and other flat-bath conditions for preventive purposes to avoid plugging of the injectors. An O₂ flowrate that would be useful in this context for preventing plugging of the injectors would be a function of the furnace, but would be on the order of 1000 scf/hr to 2000 scf/hr. for each injector. This preventive level of O₂ injection is schematically illustrated by the level A in FIG. 3. To the extent such a protective flow were employed, it would not result in significant post combustion of O₂. The present invention represents a departure from the foregoing thinking and is based upon the unexpected discovery that significant benefits can be obtained by providing oxygen injection during the refining phase and other flat- bath conditions even though there exists no metal charge or ferrous scrap in the furnace at such times to effect the transfer of heat energy produced through combustion of the CO and H₂.

Figs. 1 and 2 illustrate a preferred form of the furnace utilized in conjunction with the present invention. As seen with reference to Fig. 1, the furnace 10 includes a hearth 12 and a roof 14 which also allows the insertion of three electrodes 16 which protrude through three holes in the roof. Fig. 1 illustrates the molten steel or melt 18 in the hearth 12 as well as the cover layer of slag 20. A fourth hole 30 is provided for the off-gases released during the operation of the furnace as seen in FIG. 2.

The furnace 10 is also provided with a plurality of O₂ injectors 22, only one of which is illustrated in Fig. 1. The injectors 22 are connected to an source 40. As seen with reference to Fig. 2, the illustrated furnace is preferably provided with six O₂ injectors 22 spaced around the periphery of the furnace. Although six injectors are illustrated, the number can vary slightly, for example between four and eight. The injectors 22 are oriented so that the oxygen is injected at an angle a with respect to respective axes 24 that are radially oriented relative to the vertical or central axis of the furnace. The angle a is in the range of 20°-60°, preferably 30°-40°. In this regard, the injectors 22 can be similar in construction to those disclosed in U.S. Patent No. 5,373,530, the disclosure of which is incorporated herein by reference. As described in this patent, the orientation of the injectors 22 in this manner provides a non-radial injection of O^ along a somewhat tangential path.

As seen with reference to Fig. 1 , the injectors 22 are also advantageously disposed at an intermediate point along the height of the furnace 10 and are oriented to direct the injected oxygen downwardly at an angle δ with respect to a horizontal plane 26. The angle δ is in the range of 5° -40° and preferably within the range of about 5° and 20°. The injectors can be designed to be adjustable in this regard to allow the downward orientation of the injectors 22 to be varied depending upon the operating conditions and other factors associated with a given furnace. This downward orientation of the injectors 22 is quite advantageous in several respects. When O₂ injection occurs into the furnace along a horizontal plane, the oxygen may tend to be drawn through the off-gas hole or fourth hole 30 in the cover through which the off-gases flow. Orienting the injectors 22 downwardly tends to increase the residence time of the O₂ in the furnace because the oxygen is injected away from the fourth hole 30. The result is an increased ability of the oxygen to effect post combustion. In addition, orienting the injectors 22 downwardly directs the injected oxygen down towards the slag which is believed to provide benefits with respect to advantageously affecting the characteristics of the slag. The process according to the present invention is similar to that described above and illustrated in Fig. 3, except that oxygen is injected into the space above the melt in the furnace by way of the injectors 22 during the refining phase and during other flat bath conditions to effect continued combustion of the CO and H₂ during those time periods in which it was thought that no benefit could be derived from combustion of the CO and H₂. This O₂ injection into the space above the melt in the furnace occurs at a flowrate significantly greater than the level A which was though might be necessary for preventing plugging of the injectors 22. The injection of oxygen during the refining phase and during other flat bath conditions can be performed at a flowrate that depends upon the particular conditions and operating parameters of the furnace, but is typically such that the total flowrate into the furnace through all of the injectors 22 is at least about 20,000 scf/hr and preferably greater than 25,000 scf/hr. Thus, for a furnace like that shown in FIGS. 1 and 2, the flowrate through each of the injectors will be on the order of at least about 4, 150 scf/hr. This is schematically represented by the level B of O₂ injection depicted in FIG. 3. Through use of an off-gas analysis system such as that described in the aforementioned U.S. Patent No. 5,344,122, the off-gas from the furnace can be analyzed to determine the amount of CO present in the furnace for post combustion. This information can then be used to control O₂ injection through the injectors 22 so that an appropriate amount of O₂ is injected into the furnace to effect post combustion of a significant amount of the CO in the furnace during the refining phase and other flat-bath conditions.

To control oxygen injection into the furnace in accordance with the determined amount of CO present in the furnace for post combustion, a system such as that generally illustrated in Fig. 4 can be employed. The system includes an off-gas sample conditioning and analysis unit 42 that is connected to a probe or other similar device 40 that captures a portion of the off-gas from the furnace 10. The off-gas is then conditioned and analyzed for CO content in the unit 42 (the amounts of other materials can also be analyzed). The results of that analysis are then input to a post combustion control system 44 for controlling post combustion in the furnace through oxygen injection. The post combustion control system 44 controls the operation of a valve train 46 that is connected to the injectors 22. The valve train 46 is also connected to a source of oxygen 48. Thus, depending upon the amount of CO determined to be present in the furnace through conditioning and analysis of the off-gas, the post combustion control system 44 controls the valve train 46 to inject into the furnace by way of the injectors 22 an amount of sufficient to effect, to the extent desired, combustion of all or a portion of the determined amount of CO in the furnace. A furnace control system 50 can also be employed to control various other operating characteristics of the furnace 10 and can be used to input relevant information about the operation of the furnace into the post combustion control system 44 to achieve the desired amount of O₂ injection. Further, the off-gas sample conditioning and analysis unit 42 can be connected to a monitor/data acquisition unit 52 which can be in the form of, for example, a computer terminal. This allows the off-gas analysis to be monitored and other appropriate infoπnation obtained.

The particular amount of O₂ injected into the furnace during refining and other flat bath conditions should preferably approach that which would effect combustion of a significant amount of the CO in the furnace, although this may be limited by a variety of factors. For example, if too much oxygen is injected into the furnace, the resulting post combustion might create a heat level within the furnace that is undesirable from the standpoint of the potential adverse affect on the furnace (i.e., the walls of the furnace may become too hot). Thus, various considerations must be taken into account in deterrnining the amount of O₂ that should be injected into the furnace in the space above the melt to effect the aforementioned post combustion during refining and other flat-bath conditions.

To the extent preventive O₂ injection might be utilized to prevent plugging of the injectors 22, that relatively low O₂ injection level will not produce the types of benefits that have been realized through use of the post combustion method of the present invention. In an attempt to quantify this significant difference in some manner while also taking into account variations between furnaces, one factor which affects the overall efficiency of the off-gas heat recovery is what can be termed the post combustion ratio (PCR). This ratio is represented as:

%CO.

ΨoCO + %CO.

This ratio PCR can be deterrnined through off-gas analysis. Thus, if all of the CO in the furnace is combusted with O₂, the PCR would be 1. The available post combustion ratio (APCR) is represented as: APCR = [1-PCR].

In comparing the post combustion method of the present invention (i.e. , during refining and other flat-bath conditions) with that which might result from preventive O₂ injection, the post combustion method of the present invention would be capable of reducing the APCR by 40% relative to protective O₂ injection. By way of example, if the PCR for protective O₂ flow is on the order of 20% , the APCR would be 0.8 = (1 - 0.2). Effecting post combustion during refining and other flat-bath conditions in accordance with the present invention would thus reduce this value to 0.48 = (0.8 • 0.6) which means that the post combustion ratio PCR during refining and other flat-bath conditions in accordance with the present invention would be at least 0.52 = (1 - 0.48); a ratio significantly higher than the assumed PCR associated with protective O₂ injection.

It has been found that by continuing the O₂ injection during the refining phase and other flat-bath conditions in accordance with the present invention, a further significant reduction in the electrical energy requirements can be realized. Due to the complexities of the steel making process, it is not entirely understood why this O₂ injection during the refining phase and other flat-bath conditions of the melt is able to achieve such advantageous results, but it is believed that it may be due, at least in part, to improved electrical characteristics attributable to several factors.

In one respect, the furnace is not completely sealed and does not provide an absolutely air-tight environment within the furnace. For example, an air gap is typically present around the slag door and this air gap, and others, serve as an entry point for air ingress into the furnace. During operation of the furnace, a vacuum is created within the furnace and so in the absence of significant O₂ injection during operation of the furnace, air from outside the furnace will be drawn into the furnace interior. The nitrogen in the ingress air absorbs heat from the interior of the furnace which of course means that more electrical energy is necessary. By providing O₂ injection not only during the melt-in phase, but also during the refining phase and other flat-bath conditions, the air ingress into the furnace is limited and so significantly less heat energy is absorbed from the furnace interior by the nitrogen. In fact, the present invention provides a dual benefit in this regard in that not only is heat energy not removed as a result of less air ingress, but additional heat energy is actually produced in the furnace as a result of O₂ injection and post combustion of CO.

It is also believed that O injection during refining and other flat-bath conditions produces hotter gases around the electrodes as a result of the combustion of the CO. When O₂ is injected into the furnace with ferrous scrap still present in the furnace, the heat produced by the combustion of CO is, in large part, drawn to the scrap through heat transfer. However, during the refining phase and other flat-bath conditions, the heat produced by the heat combustion of the CO is better able to reach the arcs at the electrodes, thus producing hotter gases around the electrodes. It is believed that this is beneficial in that it promotes stabilization of arc ionization.

It is possible also that the injection of O₂ towards the slag during the refining phase and other flat-bath conditions may advantageously change or alter the physical properties of the slag such as the slag composition. In one respect, the post combustion increases the temperature of the slag which, among other things, causes the slag conditioners (e.g., CaO, MgO) to dissolve more quickly. It is believed that this O₂ injection in accordance with the present invention also affects the slag basicity (CaO/SiO) in a way that positively influences slag foaming. The slag basicity is advantageously affected at least in part by the higher temperature mentioned above. Also, the oxygen improves slag foaming for a given amount of FeO by increasing the oxygen flow. It is believed that O₂ injection in accordance with the present invention may also advantageously affect the slag viscosity and the surface tension of the slag. Due to the significant and important role that slag plays in the steel making process, it is believed that these changes in the physical properties of the slag may increase the active power input. It is believed that the downward orientation of the injectors 22 directing the oxygen towards the slag may contribute to these beneficial results.

In accordance with the process of the present invention, a significantly greater total amount of O₂ is injected into the furnace between the time when the first load of ferrous scrap is loaded into the furnace and the time of tapping and this is due to the fact that 0₂ injection occurs not only during the melt-in phase, but also during the refining phase and other flat-bath conditions. Comparing a furnace operation in accordance with the present invention with two other furnace operations in which O₂ injection was utilized but was ceased during the refining phase and other flat-bath conditions, it was observed that the total amount of d injected into the furnace during a complete operating cycle (i.e. , tap-to-tap) in accordance with the present invention (816 scf/ton) was on the order of about twice that which was injected in the other two furnaces (403 scf/ton and 416 scf/ton) with almost the same amount of carbon additions in all three furnaces. To evaluate the impact of the present invention on electric arc furnace production as well as various characteristics associated therewith, the present invention was evaluated and compared with an electric arc furnace in which no O₂ injection was utilized. From an electrical energy standpoint, it was found that the use of O₂ injection resulted in an increase in active power throughout the heat cycle, particularly during late melting and flat bath conditions in which an increase of several MW (mega watts) was observed. This was also accompanied by a corresponding decrease in arc reactance and increase in power factor. It is believed that several factors contributed to these improved electrical characteristics.

In one respect, post combustion of CO and H₂ through O₂ injection transfers a significant amount of energy to the scrap as a result of reduced process times for each bucket. Also, the increased thermal energy results in faster and more consistent scrap feed into the arc flare, thereby reducing the number and magnitude of cave-ins. The faster scrap feed and melting rate was substantiated by the observed arc instability percentage. The faster scrap feed and melting rate means that the charge in the furnace approaches the flat-bath condition more quickly, thereby facilitating development of the foaming slag so as to advantageously effect the electrical energy input rate. The slag condition of the furnace during late melting and flat-bath conditions is demonstrated by the arc harmonics percentage, also known as the slag index. The arc harmonics percentage provides a useful indication of the formation and effectiveness of the foaming slag during flat-bath conditions. Arc harmonics are related to the wave shape of the phase current so that the more the wave shape approaches a sinusoidal form, the more power can be transmitted to the steel. An effective slag cover stabilizes the atmosphere around the arc, thereby reducing the voltage required for arc initiation to occur.

A linear inverse relationship exists between arc harmonics and active power so that for both conditions, the arc harmonics remain essentially constant (e.g., about 78%) until the percent arc instability decreases below 20% . The use of O₂ injection to effect post combustion of CO and H₂ decreases the arc harmonics percentage sooner. Further, in a three charge operation, it was observed that an effective slag was developed during the end of the second charge and that this effective slag was carried over to the third charge. This helps further stabilize the arc during the melting phase and permits realization of higher active powers.

Through use of O₂ injection to effect post combustion of the CO and H₂, an increased active energy input was observed during the refining phase. The reason for this is the lower arc harmonic percentages resulting from the aforementioned slag carryover. The arc harmonic percentages were on the order of 25 % lower resulting in a 2-3 MW gain in active power. This effect is due not only to the compositional difference in the slag, but also to the cumulative effect of slag development and carryover during the successive melting steps. Visual observations of the partially melted scrap pile just prior to back charging reveals that combusting the CO and H₂ through O₂ injection results in a hotter and more uniform heating distribution of unmelted scrap and a larger molten pool in the center of the furnace. With this larger molten pool and higher scrap temperatures, a greater percentage of charge lime and fluxes are mel ted-in, thereby providing the basis for the formation of an effective slag. The slag partially submerges the arc during the melting of the subsequent back charge so as to improve arc instability and active power during the melting phase. Further, since a significant amount of charge carbon remains in the slag, the slag controls the oxidation of the molten steel and reduces slag FeO concentration, which greatly facilitates the foaming action by releasing CO gas and improving slag viscosity.

As the bath becomes flat at the beginning of the refining phase, since the slag has already been developed, the refined slag is easier to maintain in its foaming state with a supersonic lance due to the higher viscosity and more manageable FeO contents. The operator need only trim the slag volume and composition with injected lime and carbon to completely submerge the arc and obtain optimal arc harmonics and power input rates.

It has also been found that by combusting the CO and H₂ through utilization of O₂ injection, a significant improvement in power-on-time and tap- to-tap time can be realized with the same carbon input. Further, a significant increase in the production rate of the furnace can be achieved.

It is to be understood that the injection of O₂ during refining and flat-bath conditions in accordance with the present invention is not necessarily continuous and is not necessarily made at a continuous flow rate. Indeed, the oxygen flow rate can be increased, decreased, maintained at a constant level, etc. depending upon the particular benefits obtained in a particular furnace.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims be embraced thereby.

Claims

CLAIMS:

1. Process for melting and refining ferrous scrap in a furnace, comprising: providing ferrous scrap in a furnace; melting the ferrous scrap at least partially by electric arc heating; injecting into the melting ferrous scrap an oxidizing gas capable of producing CO; injecting into the furnace above the melting scrap by way of injectors an oxidizing gas capable of producing CC^ with CO to generate heat which assists the melting of the ferrous scrap to produce a melt in which substantially all of said ferrous scrap has been melted; refining said melt; analyzing off-gas from the furnace to determine the amount of CO in the furnace during the refining of the melt; and injecting into the furnace an oxidizing gas capable of producing

CO₂ with CO, said oxidizing gas being injected above the melt by way of the injectors during the refining of the melt to produce heat that facilitates refining of the melt, the oxidizing gas injected into the furnace during refining being based upon the amount of CO in the furnace determined from analysis of the off-gas in the furnace.

2. Process according to Claim 1, wherein the placing of ferrous scrap in the furnace occurs in at least first and second separate charges with the ferrous scrap of the first charge being melted to achieve a substantially flat-bath condition of the melt before the second charge of ferrous scrap, said step of analyzing off -gas from the furnace including continuous analysis of off-gas, and including injecting an oxidizing gas into the furnace above the melt during said substantially flat-bath condition based on the amount of CO in the furnace determined from analysis of the off-gas in the furnace.

3. Process according to Claim 2, wherein the rate at which the oxidizing gas is injected into the furnace during the substantially flat-bath condition between the first and second loadings is at least about 20,000 scf/hr.

4. Process according to Claim 1, wherein an oxidizing gas is injected into the furnace above the melt during refining when the melt is in a flat-bath condition.

5. Process according to Claim 4, wherein the rate at which the oxidizing gas is injected into the furnace above the melt during refining is at least about 20,000 scf/hr.

6. Process of injecting an oxidizing gas into a furnace for post combustion of gaseous CO during production of steel from a charge placed in the furnace, comprising: injecting into the furnace an oxidizing gas capable of producing CO₂ with CO, said oxidizing gas being injected through injectors during melting of the charge to cause oxidation of CO and production of heat useful in facilitating the melting of the charge to produce a melt; analyzing off-gas from the furnace to determine the amount of CO in the furnace; and injecting into the furnace an oxidizing gas capable of producing CO₂ with CO O₂, said oxidizing gas being injected into the furnace above the melt during refining of the melt based on the amount of CO determined to be in the furnace from analysis of off-gas from the furnace.

7. The method according to Claim 6, wherein said step of injecting the oxidizing gas into the furnace during refining of the melt occurs when the melt is in a flat-bath condition.

8. The method according to claim 6, wherein the oxidizing gas is injected into the furnace during refining until melted steel is tapped from the furnace.

9. Process according to Claim 6, wherein the rate at which the oxidizing gas is injected into the furnace by way of said injectors during refining is at least about 20,000 scf/hr.

10. Process according to Claim 6, wherein the oxidizing gas is injected into the furnace through the injectors at an angle disposed downwardly towards the melt with respect to a horizontal plane.

11. Process according to Claim 10, wherein said angle is between about 5° and 20°.

12. Process according to Claim 6, wherein said step of injecting oxidizing gas into the furnace during melting includes injecting an oxidizing gas into the furnace after a first charge has been placed in the furnace and after a second charge has been placed in the furnace, the first charge being melted to a substantially flat-bath condition followed by placement of the second charge in the furnace, said step of injecting an oxidizing gas into the furnace after the first charge including injecting an oxidizing gas into the furnace during said flat-bath condition by way of the injectors at a rate greater than that which is necessary to prevent plugging of the injectors.

13. Process according to Claim 12, wherein the oxidizing gas is injected into the furnace during said flat-bath condition at a rate of at least about 20,000 scf/hr.

14. Process according to Claim 6, wherein the oxidizing gas is injected into the furnace during refining at a rate greater than that which is necessary to prevent plugging of the injectors

15. Process of injecting an oxidizing gas into a furnace for post combustion of gaseous CO during production of steel from a charge placed in the furnace, comprising: a first period of injection during which oxidizing gas is injected by way of injectors into a space in the furnace above melt in the furnace which has resulted from melting of a charge, said first period of injection occurring after placing one charge into the furnace and prior to placing a subsequent charge into the furnace; and a second period of injection during which oxidizing gas is injected by way of the injectors into the space above melt in the furnace, said second period of injection occurring after placing the subsequent charge in the furnace, at least one of said first period of injection and said second period of injection occurring during a substantially flat-bath condition of the melt in the furnace and occurring at a rate greater than that which is necessary to prevent plugging of the injectors.

16. Process according to Claim 15, wherein said first period of injection occurs when said melt is in a substantially flat-bath condition.

17. Process according to Claim 16, wherein said second period of injection occurs during refining.

18. Process according to Claim 15, wherein said at least one of the first and second periods of injection occurs at a rate that is at least about 20,000 scf/hr.

19. Apparatus for melting and refining ferrous scrap to produce steel, comprising: a furnace for receiving fenous scrap; at least one electrode extending into the furnace for producing arcs to melt ferrous scrap in the furnace; a plurality of injectors connected to a source of oxidizing gas and positioned at an intermediate point along a height of the furnace to inject oxidizing gas into the furnace in a space above the ferrous scrap in the furnace, said injectors being oriented downwardly from a horizontal plane.

20. Apparatus according to Claim 19, wherein said injectors are oriented downwardly at an angle between about 5° and about 20°.

21. Apparatus according to Claim 19, wherein said injectors are non- radially oriented with respect to a vertical axis of the furnace.