US3536476A

US3536476A - Method for producing steel

Info

Publication number: US3536476A
Application number: US648634A
Authority: US
Inventors: Walter Hess
Original assignee: Rheinische Stahlwerke
Current assignee: Rheinische Stahlwerke
Priority date: 1967-06-26
Filing date: 1967-06-26
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27

Description

Oct. 27, 1970 w. HESS METHOD FOR PRODUCING STEEL Original Filed Aug. 10, 1965 TSheets-Sheet 1 mwll Oct. 27, 1970 muses 3,536,476

METHOD FOR PRODUCING swam. I I Original Filed Aug. 10. 1965 v sneeis-sheet 2 nxv. on

ATTORNEY Oct. 27, 1970 w, 555 3,536,476

METHOD FOR PRODUCING STEEL Original Filed Aug. 10. 1965 7 Sheets-Sheet s INVENTOR HALTER HESS BY Oct. 27, 1970 w, ess 3,536,47

METHOD FOR PRODUCING STEEL Original Filed Aug. 10. 1965 7 Sheeis-$heet FIG. I!

mvrm'ron WALTFZF mzss 0d. 1970 w. HESS ,536, 76

METHOD FOR PRODUCING STEEL Original Filed Aug. 10. 1965 7 Sheets-Sheet 5 INVENTOR WALTER HESS BY Oct. 27, 1970 Original Filed Aug. 10. 1965 Oct. 27, 1970 w. HESS I mas-31s METHOD FOR PRODUCING STEEL Original Filed Aug. 10. 1965 I 7 sheets-sheet"? F I 6. l2

/ /f 5 IG/Z7 Q 4 32 FIG. [3

INVENTOR v WALTER HESS BY ATTORNEY "United States Patent 015cc 3,536,476 Patented Oct. 2' 7, 1970 Int. Cl. C21c 5/04 [1.5. CI. 75-43 9 Claims ABSTRACT OF THE DISCLOSURE Steel-forming, at least partially solid material is converted into steel by subjecting the material to the combustion heat produced by burning a fuel with oxygen the major portion of the oxygen being supplied together with the fuel in the form of at least technically pure oxygen gas of at least 70% oxygen concentration, and a minor portion of the oxygen being supplied in the form of preheated air, so as to melt the steel forming material, subjecting the molten steel forming material to heat pro duced by burning a fuel with an amount of technical oxygen which is equal to or greater than the amount required for combustion of the fuel so that escape of carbonmonoxide gas from the molten steel-forming material is caused, and reacting the escaped carbonmonoxide gas with preheated air so as to convert the carbonmonoxide into carbondioxide.

CROSS REFERENCES TO RELATED APPLICATIONS The present application is a divisional application of my copending application Ser. No. 478,618, filed Aug. 10, 1965, now abandoned, and entitled Method and Apparatus for Producing Steel.

BACKGROUND OF THE INVENTION The present invention relates to a method for producing steel and, more particularly, the present invention is concerned with producing steel in a hearth furnace, and with a novel structure of a hearth furnace particularly suitable for this purpose.

It has been proposed to heat hearth furnaces in which steel-forming ferrous material such as scrap or pig iron is to be refined to steel, by utilizing burners which operate with technically pure oxygen in place of combustion air.

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 efficiency.

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.

However, the utilization of oxygen according to any of these suggested methods will result in only a limited increase in the yield 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. Furthermore, utilization of methods such as described above is limited because the regenerative chambers can only take up a small portion of the excess heat of the waste gases which is derived from combustion with pure oxygen.

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 efiiciency of the oxidation of iron is low because the major portion of the thus-formed oxides must be substantially 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.

SUMMARY OF THE INVENTION According to the present invention, an at least partially solid, steel-forming material selected from the group consisting of scrape iron, pig iron and mixtures thereof, is converted into steel by introducing the steel-forming material into a hearth furnace, introducing through at least one downwardly directed burner into the hearth furnace a fuel and at least technically pure oxygen gas of at least 70% concentration in an amount equal to the major portion of the oxygen required for burning the fuel, and also introducing into the hearth furnace, spaced from the burner, preheated air in an amount sutficient to supply together with the above-mentioned oxygen introduced through the burner at least 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. This is followed by introducing through the burner into the hearth furnace, and burning therein, a fuel and an amount of at least technically pure oxygen which is at least stoichiometrically sufficient for burning the last introduced fuel, so as to exposed the molten steel-forming material to the heat produced by combustion of the fuel, thereby forming carbonmonoxide gas in the molten material, the latter escaping from the molten steel-forming material, and introducing preheated air into the hearth furnace, spaced from the burner or burners, in an amount sufiicient to oxidize the carbon monoxide gas to carbon dioxide gas.

The novel features which are considered as characteristic 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 sectional view through the upper portion of a furnace according to the present invention, omitting the right hand furnace port and taken along the line I-I of FIG. 2;

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

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

FIG. 4 is a cross sectional elevational view of the left hand port portion of the furnace illustrated in FIGS. 1 and 2, including the regenerative chamber arrangement;

FIG. 5 is a somewhat schematic elevational cross sectional view of the hearth portion of another furnace according to the present invention;

FIG. 6 is a cross sectional plan view of the hearth furnace illustrated in FIG. 5;

FIG. 7 is a cross sectional elevational fragmentary view of the left hand portion of the furnace of FIGS. 5 and 6;

FIG. 8 is a schematic elevational view of the left-hand heat exchanger and regenerative chamber arrangement associated with the hearth furnace of FIGS. 5-7;

FIG. 9 is a schematic plan view of the arrangement illustrated in FIG. 8;

FIG. 10 is a somewhat schematic elevational cross sectional view of another hearth furnace arrangement according to the present invention;

FIG. 11 is a cross sectional plan view of the furnace illustrated in FIG. 10, taken along line XIXI;

FIG. 12 is a somewhat schematic cross sectional elevational view of an arrangement for reducing the cross sectional dimensions of the air passage into the left-hand port of the hearth furnace, is operative position;

FIG. 13 is a cross sectional elevational view of the arrangement illustrated in FIG. 12, in a position in which the air passage is not restricted;

FIG. 14 is a somewhat schematic cross sectional elevational view of another embodiment of the arrangement for reducing the cross sectional area available for air passage into the left hand port of the hearth furnace; and

FIG. 15 is a cross sectional plan view of the arrangement illustrated in FIG. 14, taken along line XV-XV.

4 DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention contemplates a method of con verting an at least partially solid steel-forming material into steel, comprising the steps of melting the steel-forming material by subjecting the same to the combustion heat produced by burning a fuel with oxygen the major portion of which is supplied together with the fuel in the form of at least technically pure oxygen gas of at least oxygen concentration, and a minor portion of which is supplied in the form of preheated air, subjecting the thus molten steel-forming material to heat produced by burning a fuel with an amount of technical oxygen being at least equal to the amount thereof which is stoichlometrically required for combustion of the fuel, thereby causing carbon monoxides gas to escape from the molten steel-forming material, and reacting the escaped carbon monoxide gas with preheated air so as to convert the carbon monoxide gas into carbon dioxide.

According to a preferred embodiment of the present invention the method of converting in a health furnace an at least partially solid steel-forming material, selected from the group consisting of scrap iron, pig iron and mixtures thereof, under formation of steel comprises the steps of introducing said steel-forming material into the hearth furnace, introducing through a plurality of downwardly directed burners into the hearth furnace a fuel and at least technically pure oxygen gas of at least 70% oxygen concentration in an amount equal to the major portion of the oxygen required for burning the fuel, and also introducig into the hearth furace, spaced from the burners, preheated air in an amount suflicient 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, introducing through the burner into the heart furnace and burning therein a fuel and an amout of at least technically pure oxygen 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, the latter escaping from the molten steel-forming material, introducing at a pressure of at least about 1.3 absolute atmospheres preheated air into the hearth furnaces spaced from the burners in an amount sufficient to oxidize to CO gas to CO gas, and withdrawing combustion gases from the hearth furnace at substantially atmospheric pressure.

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 formmg 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 air.

Furthermore, the present invention also contemplates to branch off a portion of the hot waste gases correspond- 1ng approximately to the amount of waste gases formed by combustion of the technically pure oxygen which has been introduced through the burners, so that the heat content of the branched off portion of the waste gases may be immediately utilized, for instance for preheating water, superheating steam or the like, while the residual waste gases are introduced into the regenerative chambers. Preferably, the branched-off waste gases after giving up their heat content and the waste gases which have passed through the regenerative chambers are then again united and jointly disposed of.

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 80% 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 would melt only very slowly and thus would retard the melting process.

(4) The heat content of the combustion gases which exceeds the amount of heat required for preheating the air which is to be introduced into the hearth furnace, is withdrawn without passing through the regenerative chambers. In other words, the stream of combustion gases is divided into two streams and the portion thereof which corresponds to the amount of combustion gases formed by utilization of the at least technically pure oxygen will not pass through the regenerative chambers but will be directly introduced into an independent heat exchange arrangement such as a superheater for steam or the like.

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 Kcal./m. an hour.

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

A Denotes the ratio between the amount of oxygen which is theoretically required for combustion of the fuel and the oxygen which is actually supplied. Thus, x1.1 denotes an oxygen excess of over the theoretically required amount, and, for instance, 07 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 2,300 standard cubic meters per hour.

A Technical oxygen plus air equals 1.15 and thus A ail equals 0.37.

Consequently, the air required equals or about 5,100 standard cubic meters per hour.

By way of comparison it is noted that a conventional open hearth furnace of similar eifect would require 15,600 standard cubic meters of air per hour. In other words, by proceeding in accordance with the present invention, 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 )078 technically pure oxygen.

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 \l.15. Since x018 is introduced into the burner in the form of technically pure oxygen, the required amount of separately introduced air equals x037.

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 l,300 l0.7 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. 0/ h. are oxidized within the molten bath to CO and the thus formed CO is then oxidized to CO with air above the level of the bath. The theoretical air requirements therefor ()\=l.0) are 3,600 standard cubic meters of air per hour. Since it is desired to operate with a 25% excess ()\=l.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 suflice.

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 conventionally 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 the 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 electromagnetic 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 sulficiently 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 2,000 kgs. per m? of hearth furnace and 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, than 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.

Referring now to the drawing, and particularly to FIGS. l-4, 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 meter. 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 meters. The distance of this circle from the refractory lining of the furnace will be about 7'5 centimeter and the distance between the two fiame 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 5.35:3 i.e., smaller than 2.

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 x:0.7 to 0.8. For instance, 33.3 kilograms of oil per minute are introduced together with 56 standard cubic meters of oxygen per minute. Thereby, within the effective area 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 metal 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 sufficiently large.

However, by proceeding in accordance with the present invention, as described above, the amount of waste gas is equal to about 1.5 times the amount of preheated air which is to be introduced into the furnace chamber. For this reason, according to a preferred embodiment of the present invention, the stream of waste gases leaving the hearth chamber is divided and the sensible heat of a portion of the waste gas stream which is not required for preheating air is directly utilized in some other heat exchange arrangement.

The entire waste gases formed by reacting the oxygen supplied through burners 4 and the oxygen supplied with the preheated air introduced in direction of arrow 7, which waste gases are indicated in FIGS. 1 and 4, as I and II, pass through slag-catcher 8 into chamber 9. From chamber 9, a portion of the waste gases corresponding to the amount of preheated air introduced in the direction of arrow 7 is passed through the checquer work of regenerative chamber 10, while an amount of waste gas which corresponds approximately to the amount of technically pure oxygen introduced through burners 4 is passed through heat exchanger 11. After passing either through chamber 10 or heat exchanger 11, the two waste gas streams I and II are again united and withdrawn through conduit 12.

Heat exchanger 11 may be cut off from chamber 9 by means of slide valve 13. Furthermore, flap valve 14 is provided for either shutting off the supply of fresh air III through conduit 15, or for guiding fresh air III from conduit 15 into regenerative chamber 10.

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 portiton 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. -9 of the drawing.

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

FIG. 6 illustrates the dimensions of the hearth and the position of the burners. The geometric proportions as shown in FIG. 6, 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. 5, the hearth furnace 1a contains the molten bath 2a of steel forming material, and includes a roof 3a through which three burners 4 extend downwardly towards the molten material 211 in the hearth portion of the furnace. Conduits 6a serve for alternatingly introducing preheated air into the hearth furnace or withdrawing combustion gases from the same.

FIG. 7 illustrates a portion of hearth furnace 1 with conduits 6a leading to slag pockets 7a. It will be seen that the main conduit 6a which leads to the regenerative chambers and through which the gas stream identified by arrow II passes increases in cross sectional dimensions in the direction towards the regenerative chambers (not shown). The second conduit 6a through which the gas stream indicated by arrow III passes is of considerably smaller cross section and only between about 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. 9, slag chambers 7a are arranged separately for gas stream II and gas stream III. FIGS. 8 and 9 show that gas stream III will pass through heat exchanger 8a into waste gas conduit 9a and gas stream II will pass through regenative chamber 10a and from there also into waste gas conduit 9a. The thus combined waste gases pass through flap valve 13a into conduit 11a which leads to a smoke stack (not shown). Slide valve 14a 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 14a, the desired optimum temperature can be obtained in the regenerative or checquer chamber 10a, 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 10a into the hearth furnace, flap valve 13a 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. 5, 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 metallugrical reaction proceed with sufficient 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 15a, shown in FIG. 5. An arrangement which has been found to give good results is the induction coil arrangement available 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. 5-9 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 4a. 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. 5-9, 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 Keal./m. h., or for the three burners a total of 30,000,000 Keal./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 A: 1.10 and during the refining period following the introduction of the Stahleisen to )\=l.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. 10 and 11. 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 preceeding example. Thus, hearth 1a, roof 3a and burners 4a are arranged as described in the preceeding 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 1 l 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. 10. Consequently, the dimensions of conduit 5a may be only about half the size as in the preceeding example, and conduit 6a and slag pocket 7a form together a single unit. As shown in FIG. 11a, the major portion of the waste gases will be passed in the direction of arrow II through checquers chamber a, 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'a into air conduit 5a. A reversal of the gas and air streams is achieved by reversing the positions of slide valves 13a. 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 5a and that therefore the dimensions of the air conduit 5a 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. 10 into the hearth chamber. Since air conduit 5a for all practical purposes is not subject to wear, the speed of the air stream emanating from conduit So will remain practically unchanged during the entire life plan 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 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 140-150 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% sulphur, 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 500,000 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 )\=0.6.

Very good results are achieved by introducing a total amount of oxygen equal to MAO-1.15, whereby between 2065 and A075 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 A065 or close A075 of oxygen. If technically pure oxygen is particularly expensive, it is more economical to introduce only between 06 and A065 oxygen through the burners, although this will somewhat reduce the efficiency 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 A065 and A070 will give the same good results as burners which are supplied with oxygen in an amount of A0.70- \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 A0.78- \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 w 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 07 simultaneous with introduction of preheated air containing an amount of oxygen equal to 70.45 will give good results.

The air conduit 5a of FIG. 10 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 rn./sec. In order to achieve optimum efliciency 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 II also serves as a waste gas conduit, the speed of flow 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 particularly 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 the preheated air into the hearth portion of the furnace at a relatively high speed and thus through a conduit opening of relatively small cross sectional area. On the other hand, for the free with withdrawal of the combustion gases which have a volume equal to between about 150% and 200% of the volume of the introduced preheated air, a considerably larger cross sectional area of the withdrawal conduit is required, particularly since it is desirable to withdraw the combustion gases at substantially atmospheric pressure while the preheated air preferably is introduced at an absolute pressure of at least 1.3 atmospheres. Thus, if the same conduit serves for alternatingly introducing preheated air and withdrawing combustion gases, a problem arises. In the case of reciprocatively heated one way hearth furnaces this difliculty does not arise. I

However, the hearth furnaces which are presently in use, generally provide for regenerative preheating of the air, and in this case the above discussed difiiculties should be considered.

According to a preferred embodiment of the present invention, these difliculties are overcome by introducing the preheated air through one port of the hearth furnace at a pressure of at least 1.3 atmospheres absolute, while the combustion gases are withdrawn through the opposite port of the hearth furnace at substantially atmospheric pressure.

This is made possible according to the present invention by incorporating in the port portions of the hearth furnace blocking arrangements which may reduce to a desired degree the cross sectional area of the conduit during introduction of preheated air therethrough.

Preferably, these blocking arrangements, when fully extended into the conduit, will block the center portion thereof, so that two lateral air conduits remain. The blocking arrangement may be located in the back walls or in the roof of the furnace ports and preferably, the blocking arrangements are encased in a separate closed housing.

Referring now again to the drawings, and particularly to FIGS. 12-15 it will be seen that, as shown in FIGS. 12 and 13, a portion 21 of the roof 22 of the air conduit is lowered by means of a conventional hydraulic device 23. The furnace remains tight due to the arrangement of seal 24 between the movable, wate-cooled box 25 and the stationary casing 26. In the position illustrated in FIG. 12 the cross sectional dimension of the conduit is reduced and thus the air passing therethrough will attain a greater speed. When waste gases are to be withdrawn through the same port, the blocking device is raised as illustrated in FIG. 13 so as to make the entire cross sectional area of the conduit available for the passage of the waste gases therethrough.

It is particularly advantageous, as illustrated in FIGS. 14 and 15, to introduce as blocking member into the conduit a wedge shaped body which will divide the air stream passing through the conduit into two about equal lateral streams which are so directed thatthe y will bypass the flame cones formed by the burners. The Wedge shaped body 7 may be moved between a blocking and a rest position by means of water-cooled rod 28, and is located during passage of waste gases through the conduit in housing 29 which is formed with a packing box 30 through which passes water cooled rod 28. In the position illustrated in FIG. 14, the front face of blocking member 27 forms part of the wall of the unrestricted conduit while, as illustrated in FIG. 15, in extended position the blocking member will close the center portion of the conduit so that the air stream 31 will be divided into two partial air streams 32 and 33 which enter the furnace hearth in outwardly flaring directions so as to avoid direct contact between the two air streams and the flame cones formed by the burners.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A method of converting an at least partially solid steel-forming material into steel in a hearth furnace, comprising the steps of introducing said steel forming material into the hearth portion of said hearth furnace; introducing into said hearth portion through at least one substantially downwardly directed burner, a fuel and at least technically pure oxygen gas of at least about 70% concentration in an amount equal to at least about 60%, but less than the total amount of oxygen stoichiometrically required for burning said fuel, and also simultaneously directing a preheated mixture of an inert gas and oxygen into said hearth portion from an inlet spaced from said burner, said mixture having an oxygen content substantially corresponding to that of air and said mixture being introduced in an amount sufficient to supply, together with said oxygen introduced through said burner, at least the stoichiometric amount of oxygen required for combustion of said fuel, and burning said fuel in said hearth furnace upon formation of a flame cone diverging towards said steel-forming material without contacting the walls of said hearth furnace, thereby melting the solid portion of said steel-forming material and forming slag thereon while protecting the walls of the furnace; thereafter introducing through said burner into said hearth portion and burning therein additional fuel and an amount of at least technically pure oxygen which is at least stoichiometrically sufficient for burning said fuel, thereby forming a flame di rected towards said slag and said molten steel-forming material refining said molten material, and, by the heat produced by said flame upon conversion of a portion of the carbon thereof into CO gas which escapes upwardly from said molten material and slag; and simultaneously with introduction of said additional fuel and oxygen introducing a second preheated mixture of inert gas and oxygen, the latter being present into a proportion substantially corresponding to the oxygen content of air, into said hearth portion from an inlet spaced from said burners so as to contact said escaping CO gas with said oxygen-containing mixture and to substantially oxidize said CO gas to C0 2. A method as defined in claim 1, wherein said steelforming material is selected from the group consisting of scrap iron, pig iron and mixtures thereof; and wherein said mixtures of inert gas and oxygen consists essentially of air.

3. A method as defined in claim 1, wherein said fuel and said at least technically pure oxygen gas are introduced into the hearth furnace through a plurality of downwardly directed burners.

4. A method as defined in claim 1, wherein said at least stoichiometrically suflicient amount of oxygen is between 10 and 30% greater than the stoichiometrically required amount thereof.

5. A method as defined in claim 1, wherein said steelforming material is introduced into said hearth furnace in an amount equal to at least 2600 kg. per square meter of hearth area.

6. A method as defined in claim 5, wherein said steelforming material is introduced into said hearth furnace in an amount equal to between about 3000 and 3500 kg. per square meter of hearth area.

7. A method as defined in claim 1, wherein said less than stoichiometrically required amount of oxygen is equal to between about 60 and of the amount of oxygen required for burning of said fuel.

8. A method as defined in claim 1, 'wherein, in an Irreversible manner, waste gases are withdrawn from one port of said furnace at substantially ambient pressure,

15 16 and at least said second preheated mixture of inert gas 3,115,405 12/1963 Boyd 75-60 and oxygen is introduced through the other port of said 3,447,920 6/1969 Bartu 75-43 furnace at a significantly higher pressure.

9. A method as defined in claim 8, wherein said signifi- FOREIGN PATENTS cantly higher pressure equals at least about 1.3 absolute 847 848 9/1960 Great Britain atmospheres. 5

References L. DEWAYNE RUTLEDGE, Primary Examiner UNITED STATES PATENTS G. K. WHITE, Assistant Examiner 1,839,927 1/1932 Neuhauss 75-60 X 2,446,511 8/1948 Kerry et a1 75-43 US. Cl. X.R.

3,113,765 12/1963 MCGOugh 75-43 10 7560