US3567203A

US3567203A - Method and apparatus for producing steel

Info

Publication number: US3567203A
Application number: US756342A
Authority: US
Inventors: Walter Hess
Original assignee: Rheinische Stahlwerke
Current assignee: Rheinische Stahlwerke
Priority date: 1968-08-13
Filing date: 1968-08-13
Publication date: 1971-03-02
Anticipated expiration: 1988-03-02

Abstract

A HEARTH FURNACE IN WHICH BURNER MEANS EXTENDING INTO THE FURNACE CHAMBER PRODUCE FLAME CONES BY BURNING A MIXTURE OF FUEL AND OXYGEN FOR MELTING AND REFINING STEELFORMING MATERIAL LOCATED IN A HEARTH PORTION OF THE FURNACE CHAMBER, AND IN WHICH HEATED AIR PASSED FROM REGENERATIVE CHAMBERS INTO THE FURNACE CHAMBER IS GUIDED ABOUT THE FLAME CONES AND ONTO THE MATERIAL MELTING IN THE HEARTH PORTION.

Description

March 2, 1971 w. HESS 3,567,203

METHOD AND APPARATUS FOR PRODUCING STEEL Filed Aug. 15. 1968 7 Sheets-Sh eet 1 W. HESS March 2, 1971 7 7 METHOD AND APPARATUS FOR PRODUCING STEEL Filed Aug. 15, 1968 7 Sheets-Sheet 2 FIG. 7

INVENTOR.

w. HESS METHOD AND APPARATUS FOR PRODUCING STEEL Filed Aug. 15, 1968 March 2-, 1971 7 Sheets-Sheet 5 q H L Q Q 4% [M M a 2 B I017 O m 5 OQMN IIOQMN G w s 2 lmvlw a m 2 m March 2, 1971 w. HESS METHOD AND APPARATUS FOR PRODUCING STEEL Filed Aug. 13, 1968 FIG. 10

7 Sheets-Sheet 4 March 2, 1971 w, HESS 3,567,203

METHOD AND APPARATUS FOR PRODUCING 'STEEL Filed Aug. 13. 1968 7 Sheets-Sheet 5 ;\;\J m r I a wj i a x 6 N r m //////////J\/ Q I R I" IIINVENTOR.

March 2, '1971 w. HESS 3,567,203

METHOD AND APPARATUS FOR PRODUCING STEEL 7- Sheets-Sheet 6 Filed Aug. 13, 1968 INVENTOR mm Hess mane/1341.

ATTORNEY March 2, 1971 w. HESS 3,567,203

METHOD AND APPARATUS FOR PRCDUCING STEEL Filed Aug. 15, 1968 7 Sheets-Sheet 7 k J'ETE FIG. /4

INVENTOR M41512 fies! BY f-j/ul ATT RNEY United States Patent Ofice 3,567,203 METHOD AND APPARATUS FOR PRODUCHNG STEEL Walter Hess, Essen, Germany, assignor to Rheinische Stahlwerke, Essen, Germany Continuation-impart of application Ser. No. 478,618,

Aug. 10, 1965. This application Aug. 13, 1968, Ser.

US. Cl. 266-34 Int. Cl. CZlc 7/00 ABSTRACT OF THE DISCLOSURE A hearth furnace in which burner means extending into the furnace chamber produce flame cones by burning a mixture of fuel and oxygen for melting and refining steelforming material located in a hearth portion of the furnace chamber, and in which heated air passed from regenerative chambers into the furnace chamber is guided about the flame cones and onto the material melting in the hearth portion.

CROSS REFERENCES TO RELATED APPLICATION The present application is a continuation-in-part application of the co-pending application, Ser. No. 478,618, filed Aug. 10, 1965, now abandoned.

BACKGROUND OF THE INVENTION As compared with using preheated combustion air, the advantage of operating the burners with technically pure oxygen will be found in the reduction of the amount of gas which has to be introduced into the burners in order to supply the oxygen required for combustion of the fuel. By utilizing technically pure oxygen it is possible to obtain high flame temperatures, for instance, when operating with fuel oil, temperatures of up to 2800 C. The high temperature differential achieved thereby, combined with a relatively small loss of sensible heat in the waste gases, will result in a relatively high thermal efiiciency.

It has been indicated as a particular advantage of the above-mentioned method that the hearth furnace can be of relatively simple structure, since no regenerative chambers are required. However, practical experiments based on the foregoing considerations have not been successful, particularly because the refractory lining of the furnace does not withstand exposure to these operating conditions so as to permit an economical operation.

It has also been attempted to burn in existing open hearth furnaces technically pure oxygen in addition to preheated air, by enriching the combustion air, generally prior to passing through the regenerative chambers, with technically pure oxygen, or by introducing such oxygen directly into the furnace chambers within the vicinity of the end burners.

More recently, experiments have been reported according to which in conventional open hearth furnaces, in addition to the conventional heating by means of end burners, heating was provided by means of oxygen burners which were built into the arched roof of the furnace chamber.

9 Claims 3,567,203 Patented Mar. 2, 1971 However, the utilization of oxygen according to any of these suggested methods will result in only a limited increase in the production capacity and a limited reduction in the fuel consumption of the hearth furnace.

For instance, the reduction in the fuel requirements of the furnace does not suflice to compensate for the electric energy required for producing the technically pure oxygen gas.

In addition, combustion with technically pure oxygen as proposed and attempted up to now, is connected with the disadvantage that at flame temperatures of between 2500 and 2800 C. about 50% of the CO formed during the combustion is again dissociated. The oxygen which is freed by dissociation of CO will react with the metallic charge which is at red heat, primarily under formation of FeO. This is connected with the following disadvantages:

(1) The oxidation of iron withdraws oxygen which thus is not available for combustion of the fuel, so that the gas which flows towards the cooler zones of the furnace will not be subjected to further combustion and the latent heat of the fuel will not be completely utilized.

(2) The thermal efliciency of the oxidation of iron is low because the major portion of the thus-formed oxides must be subsequently reduced and, consequently, no economic advantage is obtained which would offset the costs of producing the portion of the oxygen gas which will be used for forming oxides with the ferrous material.

(3) The FeO-containing slag which is formed thereby will cause an early heat insulation of the charge and thus will retard heat transmission.

(4) The FeO-slag severely attacks the refractory lining of the furnace.

It is therefore an object of the present invention to overcome the above-discussed difficulties and disadvantages connected with the utilization of at least technically pure oxygen for at least the partial combustion of the fuel in a hearth furnace.

It is a further object of the present invention to provide an apparatus which will permit in a particularly economical and effective manner to utilize at least technically pure oxygen for the refining of steel-forming material in a hearth furnace.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

SUMMARY OF THE INVENTION With these and other objects in view, the present invention relates to a hearth furnace mainly comprising a furnace chamber having a front and a rear wall and including between the walls a hearth portion adapted for receiving and refining therein steel-forming material, and a furnace roof superimposed and spaced from the hearth portion, the furnace chamber terminating at opposite ends thereof in port portions adapted for the passage of preheated air therethrough, burner means extending downwardly into the furnace chamber through a center portion of the roof and terminating in the vicinity of but upwardly spaced from the hearth portion for burning a mixture of fuel and at least 70% pure oxygen so as to form downwardly directed flame cones having a base whose periphery is spaced from the front and rear wall, regenerative chamber means located adjacent at least one of the port portions of the furnace chamber, and a pair of conduit means respectively connecting the regenerative chamber means with the port portions so that hot gas from the furnace chamber may be passed into the regenerative chamber means and heated air from the latter be guided into the furnace chamber about the flame cones, at least that port portion through which heated air passes into the furnace chamber being inclined towards 3 the hearth portions so that the heated air passing therethrough will pass over the surface of the material melting in the hearth portion.

The furnace according to the present invention includes preferably also means cooperating with that port portions through which heated air is passed from the regenerative chamber means into the furnace chamber for guiding the heated air passing through the port portion in two streams respectively to opposite sides past the flame cones produced by the burner means.

The means cooperating with the port portions through which heated air is passed from the regenerative chamber means into the furnace chamber may comprise means located in the aforementioned port portions for dividing the stream of heated air, while passing through the port portion, into two narrow streams respectively passing at opposite sides of a longitudinal extending vertical plane of symmetry of the furnace so that the two streams of heated air will pass through the furnace chamber to opposite sides of and closely adjacent the flame cones produced by the burner means and over the surface of the material melting in the hearth portion.

The regenerative chamber means may comprise a pair of regenerative chambers respectively located adjacent the two opposite port portions, and means in the conduit means for selectively directing hot gas from the furnace chamber through one of the port portions into one of the regenerative chambers, and heated air from the other of the regenerative chambers through the other port portion into the furnace chamber, and the means for dividing the stream of heated air passing from the respective regenerative chamber into the furnace chamber may comprise adjustable blocking means associated with the port portions and including a blocking member for each port portion movable between an active position inserted into the respective port portions for dividing the same into narrow channels respectively located at opposite sides of a longitudinal vertical plane of symmetry of the furnace and an inoperative position withdrawn from the port portion, and the blocking member associated with the port portions through which heated air is passed from the respective regenerative chamber to the furnace chamber is placed in said active position so that the stream of heated air is divided into two streams respectively passing to opposite sides of the flame cones formed by the burner means.

On the other hand, the means for directing the heated air passing from the regenerative chamber into the furnace chamber in two streams and to opposite sides past the flame cones may comprise two pairs of nozzle means respectively located in the furnace chamber adjacent the port portions for blowing jets of compressed air into the furnace chamber, in which each pair of nozzle means is provided with a common shut-off device for selectively activating and deactivating the pair of nozzle means. The nozzle means of each pair are respectively located to opposite sides and equally spaced from a longitudinally extending vertical plane of symmetry of the furnace chamber with the axes of the nozzle means extending substantially parallel to the aforementioned plane of symmetry and downwardly inclined toward the hearth portion. In this arrangement the shut-off device for that pair of nozzle means which is located adjacent the port portion through which heated air is passed from the respective regenerative chamber into the furnace chamber is opened, while the shut-off device of the other pair of nozzle means is closed so that the heated air passing through the port portion into the furnace chamber is deflected by the injector action produced by the jets of compressed air to pass with increased speed to opposite sides of the flame cones produced by the burner means and onto the material which is melted in the hearth portion. This arrangement includes preferably further means for heating the compressed air fed through the nozzle means to a temperature of about 600 C.

According to the present invention at least partially solid steel forming material, selected from the group consisting of scrap iron, pig iron and mixtures thereof, is converted in the hearth furnace into steel by introducing through one or a plurality of downwardly directed burners to the hearth furnace a fuel and at least technically pure oxygen of at least 70% of oxygen concentration in an amount equal to the major portion of the oxygen required for burning the fuel while at the same time preheated air is introduced into the hearth furnace spaced from the burners to supply together with the oxygen introduced through the burners the amount of oxygen required for combustion of the fuel, so as to burn the fuel in the hearth furnace and thereby to melt the steel-forming material, after which at least technically pure oxygen is introduced through the burners into the hearth furnace in an amount which is between 10 and 30% greater than the amount of oxygen stoichiometrically required for burning the fuel, so as to expose the molten steel-forming material to the heat produced by combustion of the fuel, thereby forming CO gas in the molten material which escapes therefrom, while preheated air of at least 1.3 absolute atmosphere is introduced into the hearth furnace spaced from the burners in an amount suflicient to oxidize the CO gas to CO gas, and the combustion gases are withdrawn from the hearth furnace at substantially atmospheric pressure.

The preheated air is thereby introduced into the furnace to pass about the flame cones produced by the burner means to assure thereby that the CO gas emanating from the molten material is completely oxidized directly above the bath of molten material in the hearth portion of the furnace.

Thus, according to the present invention, steel is produced in open hearth furnaces utilizing burners which are directed downwardly towards the surface of the steel forming material, which burners are operated with technically pure oxygen of at least 70% oxygen concentration in such a manner that during the melting period the oxygen required for fuel combustion is introduced to at least 60% through the burners and only to a smaller proportion of up to at most 40% in the form of preheated air, the latter being introduced into the furnace chambers separately and spaced from the burners. The entire fuel is introduced through the burners. During the refining period which follows the melting down of the charge, oxygen is introduced through the burners at least in a stoichiometric proportion relative to the fuel and preferably in an excess of between 10 and 30% over the stoichiometrically required oxygen amount, while the CO gas which during the refining escapes from the molten steel-forming bath is subjected to combustion with a corresponding amount of separately introduced preheated a1r.

Thus, according to the present invention the following takes place:

1) During the melting period, the burners through which fuel and at least technically pure oxygen gas pass into the hearth chamber are supplied with a proportion of at least technically pure oxygen gas which is below that which will cause a substantial dissociation of CO and of H 0. This is achieved by supplying through the burners only the major portion, at least 60% preferably not more than of the total oxygen amount which is required for combustion of the fuel, while the remainder of the required oxygen is separately supplied in the form of preheated air.

(2) The part of the fuel which is thus not completely oxidized by the oxygen supplied in the form of at least technically pure oxygen, which partly oxidized fuel will consist nearly exclusively of CO, is then subjected to further combustion with preheated air. The preheating of the air required for this purpose is preferably carried out in regenerative chambers, which, however, may be considerably smaller than those required in connection with the operation of conventional open-hearth furnaces.

(3) The after-burning of the incompletely combusted fuel with the oxygen of the preheated air will take place outside of the flame cone formed beneath the burners and will thus serve to heat the marginal portions of the solid charge which, on the one hand, are required as a protective layer for the refractory lining, and which, on the other hand, in the case of combustion of the fuel exclusively with at least technically pure oxygen without preheated air would melt only very slowly and thus would retard the melting process.

Example I below will serve for illustrating a manner of carrying out the method of the present invention. It should be noted that all examples herein are given as illustrative only and without limiting the invention to the specific details of the examples.

EXAMPLE I Steel is produced in an open hearth furnace having a hearth area of 14.6 m The molten charge weighs 3,000 kg./m or a total of about 45,000 kilograms.

The maximum amount of combustion heat supplied equals 850,000 Kcl/m. an hour.

A oxygen supplied as technically pure oxygen during the melting period equals 0.78, and A technically pure oxygen supplied during refining equals 1.18.

A denotes the ratio between the amount of oxygen which is theoreticaly required for combustion of the fuel and the oxygen which is actually supplied. Thus, A 1.1 denotes an oxygen excess of over the theoretically required amount, and, for instance, A 0.7 denotes oxygen supply equal to 70% of the theoretically required amount.

During melting down of the charge the fuel oil consumption equals 1,300 kg./h. Thus, the oxygen requirement will be or about 5,100 standard cubic meters per hour.

By way of comparison it is noted that a conventional open hearth furnace of similar effect would require 15,600 standard cubic meters of air per hour. In other words, by proceeding in accordance with the present in vention, the horizontal cross section of the regenerative chamber may be reduced to /3 while the height of the regenerative chamber remains unchanged.

The calculations above are carried out on the basis that per kilogram of fuel oil 2.25 standard cubic meters of oxygen are required. Since it is intended to provide the burner with A 0.78 technically pure oxygen, 1,300 2.25 x078, or 2,300 standard cubic meters of technically pure oxygen are required per hour.

For complete combustion an excess of oxygen or air is required. This is accomplished under the intended conditions by providing a total oxygen supply composed of the oxygen supplied as technically pure oxygen and the oxygen content of the air which equals A 1.15. Since 0.78 is introduced into the burner in the form of technically pure oxygen, the required amount of separately introduced air equals A 0.37.

For each kilogram of oil, upon stoichiometric combustion, 10.7 standard cubic meters of air are required and it follows therefrom that the total air requirements equal 1,300 10.7 0.37, or 5,100 standard cubic meters per hour.

The refining of the molten charge is then carried out at a maximum refining speed of 1.8% C/h. 810 kg. C/h.

are oxidized within the molten bath to CO and the thus formed 00 is then oxidized to CO with air above the level of the bath. The theoretical air requirements therefor (A=1.0) are 3,600 standard cubic meters of air per hour. Since it is desired to operate with a 25% excess (A=1.25) 4,500 standard cubic meters of air per hour Will be used. The size of the regenerative chamber determined in connection with the melting of the charge will sufiice.

The output per unit of hearth area can be increased due to the fact that the heat supplied per unit of hearth area is more than double the conventional supplied amount of heat. This will result in a considerable reduction of initial costs and in a relative reduction of heat radiation losses. The output of molten steel per unit area of the hearth in German type furnaces varies between 1,400 and 2,400 kg./m. and increases with increasing size of the furnace. In the case of US. type hearth furnaces the output per unit area of hearth is constant at 2,500 kg./m. In accordance with the method of the present invention, an output of between 2,600 and 3,900 preferably between 3,000 and 3,500 kg./m. and by application of conventional electro-magnetic stirring by means such as an induction coil, an output of about 5,000 kg./m. can be easily achieved.

It follows that a furnace including two burners and having, for instance, a hearth area of 15 m. can be operated with a molten charge weighing between 40,000 and 50,000 kg., and when electromagnetically stirring the molten charge, the weight of the same may be increased to about 75,000 kg.

In a furnace including two burners and two furnace doors and alternatingly introducing scrap through these doors into the furnace, it is possible to supply to the scrap the required basic heat so that drops of metal which quickly start to form and to drop from the surface of the charge will not freeze when such molten metal drops reach the lower portions of the scrap layer in the furnace. The great amount of heat which is supplied in localized areas by the burners will permit a faster introduction of the charge than would be possible by utilizing a charging box arrangement. For this reason, it is proposed according to the present invention to introduce the charge by means of tip chutes in portions of between 5,000 and 10,000 kgs. which are placed on the scrap cone formed in the hearth furnace directly underneath the burners.

For effective utilization of the heat it is required that the scrap is poured underneath the burners in portions. This can be done by conventional box charging, provided that the capacity of the charging boxes is sufficiently great, such as between 3,000 and 4,000 kgs. or by means of tip chutes of the type used in the LD steel making process, and, for instance, illustrated in FIG. 3 of the present drawing. Thereby, the total scrap charge should be at least 1,500 kgs. per m? of hearth area per hour, preferably between 2,000 and 2,500 kgs., however, not more than 3,000 kgs. per In. of hearth surface an hour.

Furthermore, according to the present invention, the doors of the furnace will have a vertical cross section corresponding to an arc of a circle and each door will be divided horizontally into an upper and a lower portion so that the door can be raised along water cooled skid rails in such a manner that either the upper portion of the door alone is raised and opened, or that both the upper as well as the lower portion of the door are jointly raised and opened. Opening of only the upper portion of the door will serve for charging of the furnace by means of chutes, while opening of the entire door will be required for taking samples and also for patching or repairing the hearth lining.

For producing 100,000 kgs. of molten steel in a hearth furnace which operates in combination with a continuous casting arrangement, and when it is desired to operate without electromagnetic stirring, it will be necessary to double the number of the burners. In such case, preferably a furnace will be utilized which is provided with four doors and four burners and the hearth surface of which has a length of about 10 meters and a width of about 3.5 meters.

The method of the present invention has the following advantages as compared with conventional hearth steel smelting processes:

(1) The charge is very flexible since it may consist of any desired proportions of scrap and pig iron.

(2) If, more than 50% of the charge consists of pig iron, then cooling with ore is required and in the case of large amounts of ore, it is advantageous to introduce the same in a continuous manner.

(3) A highly effective utilization of heat.

(4) Partial replacement of technically pure oxygen with preheated air.

(5) The characteristics of the waste gases will be such that their purification is relatively simple and thus can be carried out in a simple and highly economical manner.

(6) A simple arrangement, since for the melting of the charge as well as for refining of the same only the two (or more) burners are required.

(7) In comparison with conventional open hearth furnaces, smaller dimensions of the furnace arrangement due to reduction of the hearth area and of the volume of the regenerative chambers.

The novel features which are considered as characteristics for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a somewhat schematic elevational cross sec tional view through the upper portion of a furnace according to the present invention, omitting the right hand furnace port and taken along the line II of FIG. 2.

FIG. 2 is a cross sectional plan view of the furnace illustrated in FIG. 1, taken along line 11-11 of FIG. 1;

FIG. 3 is an elevational cross sectional view taken along line III-III of FIG. 2;

FIG. 4 is a somewhat schematic elevational cross sectional view of the hearth portion of another furnace according to the present invention;

FIG. 5 is a cross sectional plan view of the hearth furnace illustrated in FIG. 4;

FIG. 6 is a cross sectional elevational fragmentary view of the left-hand portion of the furnace of FIGS. 4 and 5.

FIG. 7 is a schematic elevational view of the lefthand heat exchanger and regenerative chamber arrangement associated with the hearth furnace of FIGS. 4-6.

FIG. 8 is a schematic plan view of the arrangement illustrated in FIG. 7;

FIG. 9 is a somewhat schematic elevational cross sectional view of another hearth furnace arrangement according to the present invention;

FIG. 10 is a schematic plan section of the hearth furnace arrangement shown in FIG. 9, taken along line XX of FIG. 9.

FIG. 11 is a schematic cross sectional elevational view of an arrangement for dividing the hot air passing through one of the port portions into the furnace chamber into two air streams;

FIG. 12 is a cross sectional plan view of the arrangement illustrated in FIG. 11 taken along the line XIIXII;

FIG. 13 is a schematic elevational cross sectional view of the hearth portion of another furnace according to the present invention;

FIG. 14 is a partial cross sectional plan view taken along the line XIVXIV of FIG. 13; and

FIG. 15 is a cross sectional view taken along the line XVXV Of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, and particularly to FIGS. 1-3, it may be assumed that the hearth furnace has a capacity of 45,000 kilograms of molten steel and, as illustrated in the left-hand portion of FIG. 1, the bath of molten material will have a depth of 0.9 m. However, it may also be assumed that by utilizing an electromagnetic stirring device, the weight of the molten steel to be formed per charge will be increased to about 75,000 kilograms and, as illustrated in the right hand portion of FIG. 1 the depth of the molten bath will then be about 1.30 meters.

In the center portion of the hearth furnace, substantially in planes extending perpendicular to the longitudinal axis'of hearth furnace 1 and passing through the center of doors 2, respectively, oil burners 4 are arranged extending downwardly through the furnace roof 3. Oil burners 2 are operated with fuel oil and technically pure oxygen having an oxygen content of and will produce flame cones indicated by reference numeral 5.

The immediate effect of each of the burners will extend in a circle having a diameter of about 1.5 meters. In other words, the flame cone base in contact with the surface of the molten charge will be of substantially circular shape and will have a diameter of about 1.5 meter. The distance of this circle from the refractory lining of the furnace will be about 75 centimeters and the distance between the two flame circles about 1 meter. This will result in a relationship between length of the hearth portion and width of the hearth portion which is equal to 5.53:3 i.e., smaller than 2. The oil burners 4 are adjustable in vertical direction by means not shown in the drawing.

Through the furnace port portion at the right hand of FIG. 1 (not completely illustrated), air which had been preheated in the right hand regenerative chamber (not shown) will be introduced into the furnace chamber in the direction of arrow 7.

During charging of the furnace, the burners 4 are operated with an amount of oxygen which is less than the stoichiometrically required amount, preferably A 07 to 0.8. For instance. 33.3 kilograms of oil per minute are introduced together with 56 standard cubic meters of oxygen supplied through the burners plus the oxygen of the burners, combustion gases are produced which contain about 50% CO. For complete combustion of the carbon monoxide, preheated air is introduced into the furnace chamber in the direction of arrow 7. The amount of preheated air which is introduced in the direction of arrow 7, will be such that the total amount of oxygen available within the hearth furnace, i.e., the technically pure oxygen supplied through the burners plus the oxygen of the preheated air, will amount to )\=1.1 to 1.2. In accordance with the present example, for instance standard cubic meters of preheated air per minute will be introduced.

After melting down the charge during the refining period, the amount of oxygen in the form of technically pure oxygen which is introduced through burners 4 will be at least equal to the stoichiometrically required amount for combustion of the fuel, preferably somewhat more, such as \=1.11.2. In addition, the amount of preheated air which will be introduced in the direction of arrow 7 will be such that the CO gas which escapes from the molten gas during the refining process will be completely burned to CO In conventional open hearth furnaces the amount of waste gas or combustion gas is approximately equal to the amount of preheated air which is introduced into the furnace, and thus the heat taken up by the regenerative chamber will be substantially equal to the amount of heat given off by the chamber, provided that the dimensions of the regenerative chamber are sufiiciently large.

Referring now to FIG. 3, it will be seen that the doors 2 of the furnace chamber consist of two halves 16 and 17 which divide the respective door substantially horizontally into an upper and a lower door portion, in such a manner that the arc-shaped upper and lower door portions 16 and 17 may be jointly moved upwardly along correspondingly arc shaped rails, or only the upper door portion 16 may be moved upwardly for the purpose of introducing charging chute 18 into the thus half-opened door.

The following example will describe the process of the present invention with reference to FIGS. 4-8 of the drawing.

EXAMPLE II In this case, the charge consists of 50% liquid and 50% solid constituents.

FIG. illustrates the dimensions of the hearth and the position of the burners. The geometric proportions as shown in FIG. 5, are important in this case in order to achieve the desired good heat transfer and high durability or long service life of the furnace, particularly of the refractory lining of the same. It will be seen that the three burners are arranged in a vertical plane along the longitudinal axis of the hearth, at a distance of 2.5 meters from the front and rear wall of the furnace, and that the two outer burners are arranged at a distance of two meters from the furnace ports.

As illustrated in FIG. 4, the hearth furnace 1 contains the molten bath 2 of steel forming material, and includes a roof 3 through which three burners 4- extend downwardly towards the molten material 2 in the hearth portion of the furnace. Conduits 6 serve for alternatingly introducing preheated air into the hearth furnace or withdrawing combustion gases from the same.

FIG. 6 illustrates a portion of hearth furnace 1 with conduits 6 leading toslag pockets 7. It will be seen that the main conduit 6 which leads to the regenerative chambers and through which the gas stream identified by arrow 11 passes increases in cross sectional dimensions in the direction towards the regenerative chambers (not shown). The second conduit 6 through which the gas stream indicated by arrow III passes is of considerably smaller cross section and only between about /6 and /3 of the total amount of combustion gas forms gas stream III, while the major portion of the combustion gas forms gas stream II.

As shown in FIG. 8, slag chambers 7 are arranged separately for gas stream II and gas stream III. FIGS. 7 and 9 show that gas stream III will pass through heat exchanger 8 into waste gas conduit 9 and gas stream II will pass through regenerative chamber 10 and from there also into waste gas conduit 9. The thus combined waste gases pass through fiap valve 13 into conduit 11 which leads to a smoke stack (not shown). Slide valve 14 is opened during withdrawal of combustion gas from the hearth furnace and serves for throttling gas stream III. By controlling the position of slide valve 14, the desired optimum temperature can be obtained in the regenerative or checquer chamber 10', since by controlling the volume of gas stream III automatically also the volume of gas stream II is controlled. When preheated air is to be introduced from regenerative chamber 10 into the hearth furnace, flap valve 13 is turned into the position indicated by dash lines and slide valve 14 is closed.

The average depth of the molten bath in the hearth furnace of FIG. 4, is 0.9 meters and thus at the given dimensions the weight of molten steel produced in one charge equals 250,000 kg. In order to have the metallurgical reaction proceed with sufiicient speed, it is necessary at such depth of the molten bath to utilize electromagnetic stirring of the bath. This can be accomplished, for instance, by means of induction coil 15, shown in FIG. 4. An arrangement which has been found to give good results in the induction coil arrangement available 10 from the firm Asea, Sweden, and generally used for stirring the steel bath in electric arc furnaces.

Steel production in the arrangement illustrated in FIGS. 48 is then carried out in the following manner:

tons of scrap iron are introduced into the hearth furnace, through the doors thereof, by means of charging boxes each having a volume of 4 m so that the scrap iron is deposited underneath burners 4. With commercial scrap iron qualities, the optimum charging will be 2,250 kg. of scrap per m. of hearth area an hour. In the arrangement illustrated in FIGS. 4-8, this will amount to the charging of 92,000 kg. of scrap per hour and thus the total time required for charging 135,000 kg. of scrap will be 1 and a half-hours. After allowing the thus introduced scrap to be heated for one-half hour after completion of charging of the same, 135,000 kg. of liquid pig iron for steel making purposes,so called Stahleisen-are introduced into the hearth furnace. The first sample may be drawn between and minutes after charging of the furnace has been started, and one hour later the furnace may be tapped. The maximum heat supplied to the hearth equals 735,000 KcaL/h. or for the three burners a total of 30,000,000 KcaL/ h. With commercial scrap qualities, the average heat requirement will be 580,000 Kcal./m. h., which corresponds to a total heat consumption of about 350,000 Kcal./t., at an oxygen consumption of between 60 and 65 standard cubic meters per ton. The term ton or the abbreviation t is meant to denote 1,000 kg.

Until the liquid Stahleisen is introduced into the hearth, the burners are supplied with between 65 and 70% of the oxygen theoretically required for combustion of the fuel. Preheated air is introduced in such an amount that the total oxygen available in the hearth furnace equals 1.10 times the theoretically required amount of oxygen. After introduction of the steel iron, the oxygen supply through the burners is first adjusted to )\1.10 and during the refining period following the introduction of the Stahleisen to \=1.3. The amount of air which is simultaneously introduced generally remains constant, i.e., the amount of oxygen contained in the introduced preheated air equals about 45-50% of the oxygen amount required for complete combustion of the fuel which is simultaneously introduced through the burners.

EXAMPLE III According to the present example, a charge consisting of 75% scrap iron and 25% liquid Stahleisen is to be converted into steel. A furnace arrangement which is particularly suitable for these conditions is illustrated in FIGS. 9 and 10. The plan view of the hearth furnace portion of the arrangement utilized according to the present example is similar to that of the hearth furnace of the preceding example. Thus, hearth 1, roof 3 and burners 4 are arranged as described in the preceding example, however, the bath of molten steel forming material has only an average depth of 0.5 meters and the tapping weight is about 150,000 kg.

According to the presently described embodiment, the waste gases are conducted in a novel and particularly advantageous manner which will be utilized especially if the hearth furnace arrangement has been newly constructed, while the waste gases will flow, as described in the preceding example primarily when an existing hearth furnace arrangement is to be converted with as little cost as possible so as to be suitable for the process as described in Example II.

According to the present example, the waste gases II are withdrawn only at one side of the hearth chamber, namely through the left hand port as illustrated in FIG. 9. Consequently, the dimensions of conduit 5 may be only about half the size as in the preceding example, and conduit 6 and slag pocket 7 form together a single unit. As shown in FIG. 10, the major portion of the waste gases will be passed in the direction of arrow II through checquers chamber 10, While a minor portion of the waste gases, indicated by arrow III will pass through the heat exchanger.

The air stream I passes through checquer chamber 10' into air conduit 5. A reversal of the gas and air streams is achieved by reversing the positions of slide valves 13. Another system of slide valves or the like for reversing the flow of gas and air which is conventional in open hearth furnaces and therefore not illustrated in the drawing, is arranged between the checquer chambers and the smoke stack on the one hand, and the air fan and the checquer chambers on the other hand.

The presently described arrangement has the particular advantage that only air will pass through air conduit and that therefore the dimensions of the air conduit 5 can be relatively small, thereby causing passage of the preheated air stream therethrough at a relatively high speed which is adjusted to the desired outlet speed of the heated air from the right-hand port portion of the hearth furnace of FIG. 9 into the hearth chamber. Since air conduit 5 for all practical purposes is not subject to Wear, the speed of the air stream emanating from the conduit 5 will remain practically unchanged during the entire life span of the hearth furnace arrangement.

The optimum charging speed, according to the present example, is 2.25 t./m. h., or for the 120 tons of scrap iron which are to be charged in the furnace, one hour and 20 minutes will be required. After completion of the charging of the scrap iron, the scrap 'will be heated in the hearth furnace for 30 minutes and thereafter the liquid Stahleisen will be introduced. The best moment for introduction of the liquid Stahleisen depends on the type of the scrap and will be so chosen that on isolated portions of the scrap heap isolated liquid sumps have been formed, however, as yet no larger communicating liquid portions have been formed, however, as yet no larger communicating liquid portions have been formed of the scrap. In the case of primarily heavy scrap, this point, i.e., the point at which Stahleisen is to be introduced will be reached only between 40 and 45 minutes after completion of the charging of the scrap. In the first mentioned case, i.e., when the liquid Stahleisen is introduced 30 minutes after completion of the scrap, the first sample may be drawn 140l50 minutes after charging of the hearth furnace has commenced. For producing commercial qualities of steel, the latter may be tapped after a further refining time of between 40 and 50 minutes. If special steel qualities are to be produced, particularly steel containing less than 0.028% sul hur, it is necessary to remove the slag several times and the refining time may be extended to one hour or even one hour and 20 minutes. However, for commercial steel qualities the total time from commencement of the charging until tapping of the steel generally will be 3 hours and 10 minutes. Thereby, at an average heat supply of 590,000 Kcal./m. h., for each melt about 75,000,000 Kcal. or 590,000 Kcal./t. are consumed. The oxygen requirements under these conditions equal about 80-85 standard cubic meters per ton of steel.

It has been described further above that during melting of the charge at least 60% of the required amount of oxygen are to be introduced through the burners in the form of at least technically pure oxygen, and that at most 40% of the required oxygen is to be introduced in the form of preheated air. These percentage figures relate to the theoretical oxygen requirement, and are to be understood so that the burner must be supplied with oxygen in an amount which is not to be less than A-0.6.

Very good results are achieved by introducing a total amount of oxygen equal to A 1.10-1.15, whereby between A 0.65 and A 0.75 are introduced through the burners in the form of technically pure oxygen and the balance in the form of preheated air. It is mainly an economical question, depending primarily on the cost of the technically pure oxygen, whether the burners are to be supplied with close to A 0.65 or close to A 0.75 of oxygen. If technically pure oxygen is particularly expensive, it is more economical to introduce only between A 0.6 and A 0.65 oxygen through the burners, although this will somewhat reduce the efliciency of the hearth furnace operation. A further, important feature for adjusting the burners is the accurate control of the introduction of preheated air. The better the introduction of preheated air is controlled, the smaller can be the proportion of oxygen which is introduced through the burners, since with properly controlled air supply, burners which are fed with oxygen in amounts equal to between A 0.65 and A 0.70 will give the same good results as burners which are supplied with oxygen in an amount of A 0.70-A 0.75 but with inferior control of the introduction of the preheated air. If it is not possible to introduce the preheated air at an unchanging rate and at relatively high speed, for instance because an existing hearth furnace arrangement is to be used with as little reconstruction as possible, it might become desirable to introduce through the burners technically pure oxygen in an amount corresponding to A 0.78-A 0.80, provided that the cost of oxygen are relatively low. In all cases, it is important in order to achieve a good heat economy that total A-between about 1.10 and 1.15. As a standard operational procedure or average value, generally introduction of technically pure oxygen through the burners in an amount equal to A 0.7 simultaneous with introduction of preheated air containing an amount of oxygen equal to A 0.45 will give good results.

The air conduit 5 of FIG. 9 should be so dimensioned that the air stream which is introduced therethrough will have, at a temperature of about 1200 C., at its point of entry into the hearth furnace a speed of not less than 40 m./sec. In order to achieve optimum efficiency of the hearth furnace, it is desirable to increase this speed to between about 60 and m./sec.

With respect to the speed of entry of the air stream into the hearth furnace, the length of the hearth has to be considered and it is quite obvious that, for instance, in a furnace having a hearth length of between 6 and 8 meters, good combustion may still be achieved at an air speed of 40 m./sec., while furnaces with a hearth length of between 10 and 12 meters will require for the same results an air entry speed of about 60 m./sec. However, in order to achieve even higher air speeds, such as for instance 70 m./sec., certain structural requirements must be met as will be more fully described further below.

According to Example II, it will be necessary to be satisfied with an air entry speed of about 40 m./sec., since by reduction of the cross-sectional dimensions of the air conduit which in the embodiment described in connection with Example H also serves as a Waste gas conduit, the speed of fiow of the waste gases through the conduit would become too high and would result in considerable wear of the walls of the air and waste gas conduit so that frequent repairs would be required.

It will be understood from the embodiment of the present invention described in Example III, that for achieving best results with respect to complete combustion of the incompletely reacted combustion gases which are formed during melting down of the charge, as well as in order to complete combustion of CO emanating from the molten charge during refining of the same, it is desirable to introduce preheated air into the hearth portion of the furnace at relatively high speed in such a manner that the preheated air is guided in two streams past the flame cones produced by the burner means so that after burning will take place about the flame cones, which during melting down of the charge will result in complete combustion of the incompletely reacted combustion gases and which during refining of the charge will result in complete combustion of CO emanating from the molten charge.

To accomplish the desired result of introducing the preheated air in two lateral streams with increased speed into the hearth portion of the furnace while permitting free withdrawal of the combustion gases which have a volume equal to between ISO-200% of the volume of the introduced preheated air which is preferably withdrawn at substantially atmospheric pressure while the preheated air is preferably introduced at an absolute pressure of at least 1.3 atmospheres, the furnace according to the present invention may be provided with blocking arrangements incorporated into the port portions of the hearth furnace which may reduce to a desired degree the cross sectional area of the conduit during introduc tion of preheated air therethrough and which divides the preheated air passing through the respective port portion into two lateral streams.

Such an arrangement is shown in FIGS. 11 and 12 in which the means for dividing the heated air passing from the respective regenerative chamber into the furnace chamber comprises a blocking member in form of a Wedge-shaped body 27 movable in and out of the port portion through which heated air is passed into the furnace chamber. The wedge-shaped body 27 may be moved between an inactive position, as shown in full lines in FIG. 11 in 'Which it is located in a packing box 30 and withdrawn from the port portion so that combustion gases from the furnace chamber may pass through the fullyopened port portion with the respective regenerative chamber, and an active position as shown in dotted lines in FIG. 11 and in full lines in FIG. 12 so that the air stream '31 passing through the port portion will be divided into two partial air streams 32 and 33 which enter the furnace chamber in outwardly flaring direction to pass through the spaces between the flame cones and the front and rear wall of the furnace chamber. The wedge-shaped body 27 is moved between the positions thereof by watercooled rod 28 which passes substantially fluid tightly sealed through an end wall of the box 30.

Another arrangement for guiding the heated air passing through the respective port portion into the furnace chamber in two streams past the flame cones produced by the burner means and onto the material melting in the hearth portion of the furnace is shown in FIGS. 13-15.

In this arrangement two pairs of nozzle means 34 and 35 are respectively arranged symmetrically with respect to a longitudinal vertical plane of symmetry of the furnace chamber in the latter and respectively adjacent the two opposite port portions near the roof of the furnace chamber. The axis of each nozzle means is arranged in such a manner that the substantially conical jet of compressed air 10 emanating therefrom is directed onto the upper surface of the material melting in the hearth portion of the furnace and so that the outer boundary line 37 of the conical jet includes with the longitudinal axis of the furnace an angle 38 of substantially degrees as shown in FIG. 14, so that the air jets respectively emanating from the nozzles will pass in the space between the flame cones 5 and the front and rear wall of the furnace chamber. Each pair of nozzle means is supplied from a common conduit in which a shut-off device, for instance shut-off valve 39 is located, as schematically illustrated in FIG. 14 for the pair of nozzle means 35, so that the nozzle means may be controlled in such a manner that compressed air is passed through that part of nozzle means which is located adjacent the port portion through which heated air is passed into the furnace chamber while the other pair of nozzle means located at the port portion through which hot combustion gases are passed from the furnace chamber into the respective regenerative chamber is deactivated. The compressed air furnished to the nozzle means is preferably preheated before entering the nozzle means, for instance by preheating means 40, as schematically indicated in FIG. 14, which preheating means may be heated in any way not shown in the drawing and not forming part of the present invention, for instance by the hot combustion gases discharged from the furnace chamber. The compressed air is preferably preheated to a temperature of about 600 C. and it is supplied to the nozzle means with an overpressure of about 6 atmospheres. Compressed air is blown through the nozzle means into the furnace chamber in an amount of 4% to 16% of the preheated air fed in the furnace chamber from the regenerative chambers so that the air jets 36 emanating from the nozzle means will deflect by injector action the preheated air passing from the respective regenerative chamber into the furnace chamber in two streams past the flame cones produced by the burner means.

The nozzle means are formed from heat-resistance steel with a refractory coating and the shut-off pair of nozzle means is cooled with cold compressed air in a manner not illustrated in the drawing.

The arrangement shown in FIGS. 13-15 has the additional advantage that it can be built in existing Siemens- Martin ovens respectively basic open heaters with relatively small expenditures and that the service life of the lining of the furnace will be greatly improved due to the complete combustion of the gases in the furnace chamber which results that the fire-proof lining of the furnace chamber will not be contacted with CO gas.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of furnaces for steel production differing from the types described above.

While the invention has been illustrated and described as embodied in a hearth furnace for steel production in which preheated air is guided in two streams past flame cones directed from above onto a central portion of steelforming material in a hearth portion of the furnace, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

What is claimed as new and desired to be secured by Letters Patent is:

1. A hearth furnace comprising, in combination, a furnace chamber having a front and a rear wall and including between said walls a hearth portion adapted for receiving and refining therein steel-forming material, and a furnace roof superimposed and spaced from said hearth portion, said furnace chamber terminating at opposite ends thereof in port portions adapted for the passage of preheated air therethrough; burner means extending downwardly into said furnace chamber through a center portion of said roof and terminating in the vicinity of but upwardly spaced from said hearth portion for burning a mixture of fuel and at least 70% pure oxygen so as to form downwardly directed flame cones having a base whose periphery is spaced from said front and rear wall of said furnace chamber; regenerative chamber means located adjacent at least one of said port portions of said furnace chamber; a pair of conduit means respectively connecting said regenerative chamber means with said port portions so that hot gas from said furnace chamber may be passed into said regenerative chamber means and heated air from the latter be guided into said furnace chamber and closely about said flame cones, at least that port portion through which heated air passes into the furnace chamber being inclined toward said hearth portion so that the heated air passing therethrough will pass over the surface of the material melting in said hearth portion; and means cooperating with that port portion through which heated air is passed from said regenerative chamber means into said furnace chamber for guiding the heated air passing through said port portion in two streams respectively to opposite sides and closely adjacent past the flame cones produced by said burner means.

2. A hearth furnace as defined in claim ll, wherein said burner means are adjustable in vertical direction so that the distance between said burner means and the material 15 in said hearth portion may be adjusted to a distance most favorable for melting and refining the material in said hearth portion.

3. A hearth furnace as defined in claim 1, wherein said means cooperating with said port portion through which heated air is passed from said regenerative chamber means into said furnace chamber comprises means located in said port portion for dividing the stream of heated air while passing through said port portions into two narrow streams respectively passing at opposite sides of a longitudinal extending vertical plane of symmetry of said furnace so that said two streams of heated air will pass through said furnace chamber to opposite sides of and closely adjacent the flame cones produced by said burner means and over the surface of the material melting in said hearth portion.

4. A hearth furnace as defined in claim 1, wherein said means for guiding the heated air passing into the furnace chamber into two streams respectively to opposite sides past the flame cones comprises adjustable blocking means associated with said port portions and including a blocking member for each port portion movable between an active position inserted in the respective port portion for dividing the same into two narrow channels respectively located at opposite sides of a longitudinal vertical plane of symmetry of said furnace, and an inoperative position withdrawn from said port portion, the blocking member associated with the port portion through which heated air is passed into the furnace chamber is placed in said active position so that the stream of heated air is divided into two streams respectively passing to opposite sides of and closely adjacent the flame cones formed by said burner means.

5. A hearth furnace as defined in claim 4, wherein said regenerative chamber means comprises a pair of regenerative chambers respectively located adjacent said opposite port portions, and means in said conduit means for selectively directing hot gas from said furnace chamber through one of the port portions into one of said regenerative chambers, and heated air from the other of the regenerative chambers through the other port portion into said furnace chamber, the blocking member associated with said other port portion is placed in said active position and the blocking member located in the one port portion is placed in said inactive position.

6. A hearth furnace as defined in claim 1, wherein said means for guiding the heated air passing into said furnace chamber into two streams respectively to opposite sides past the flame cones comprises a pair of nozzle means located in said furnace chamber adjacent and to opposite sides of the port portion through which heated air from the regenerative chamber means is passed into the furnace chamber for blowing jets of compressed air respectively to opposite sides of the flame cones produced by said burner means and downwardly toward said hearth portion to thus deflect the heated air passing through said port portion in the direction of said jets.

7. A hearth furnace as defined in claim 1, wherein said regenerative chamber means comprises a pair of 16 regenerative chambers respectively located adjacent said port portions and means in said conduit means for selectively directing hot gas from said furnace chamber through one of said port portions into one of said regenerative chambers and heated air from the other of said regenerative chamber through the other port portion into the furnace chamber, and wherein said means for guiding the heated air passing into the furnace chamber in two streams respectively to opposite sides past the flame cones produced by said burner means comprises two pairs of nozzle means respectively to opposite sides past the flame cones produced by said burner means comprises two pairs of nozzle means respectively located in said furnace chamber adjacent said port portions for blowing jets of compressed air into said furnace chamber, each pair of nozzle means being provided withv a common shut-off device for selectively activating and deactivating said pair of nozzle means and the nozzle means of each pair being respectively located to opposite sides and equally spaced from a longitudinal vertical plane of symmetry of said furnace chamber with the axes of said nozzle means downwardly inclined towards the hearth portion and so that the outer boundary line of the conical jet emanating from the respective nozzle means includes in a horizontal plane with the longitudinal axis of said furnace chamber an angle of substantially 5 degrees, the shut-off device for that pair of nozzle means which is located adjacent that port portion through which heated air is passed from the respective regenerative chamber into the furnace chamber is opened while the shut-01f device of the other pair of nozzle means is closed so that the heated air passing through said port portion in the furnace chamber is deflected by the injector action produced by said jets of compressed air to pass with increased speed to opposite sides of and closely adjacent the flame cones produced by said burner means and onto the upper surface of the material which is melted in said hearth portion.

8. A hearth furnace as defined in claim 7, and including means for preheating the compressed air fed through said nozzle means to a temperature of about 600 C.

9. A hearth furnace as defined in claim 8, wherein the compressed air fed through the nozzle means is about 416% of the heated air passing through the respective port portions into said furnace chamber.

References Cited UNITED STATES PATENTS 3,129,930 4/1964 Labat-Camy 26633 3,132,854 5/1964 Hilliard et a1. 26343 3,165,301 1/1965 Riviere 263- 15 3,194,650 7/1965 Kurzinski 7543 GERALD A. DOST, Primary Examiner US. Cl. X.R.