US3528799A

US3528799A - Process for continuously refining cast iron into steel

Info

Publication number: US3528799A
Authority: US
Inventors: Aldo Ramacciotti
Original assignee: Centro Sperimentale Metallurgico SpA
Current assignee: Centro Sperimentale Metallurgico SpA
Priority date: 1967-03-18
Filing date: 1969-03-19
Publication date: 1970-09-15
Anticipated expiration: 1987-09-15
Also published as: LU55578A1; DE1608309B2; NL6803844A; GB1187942A; DE1608309A1

Description

Se t. 15, .1970 A. RAMACCIOTTI 1 I PROCESS FOR CONTINUOUSLY REFINING CAST IRON INTO STEEL Filed March 19, 1969 2 Sheets-Sheet 4 WEIGHING 1 CELL '5 FIG. 4 6d P 1970 A. RAMACClOTTl 3,528,799

PROCESS FOR CONTINUOUSLY REFINING CAST IRON INTO STEEL Filed Marchl9, 1969 2 Sheets-Sheet 1 United States Patent Ofice 3,528,799 Patented Sept. 15, 1970 Int. Cl. czlc 7/02, 7/04 U.S. Cl. 75-52 Claims ABSTRACT OF THE DISCLOSURE Cast iron is transformed into steel by pouring liquid cast iron on to a flux in a vertical shaft reactor so that the cast iron descends in drops scattered through the flux. Oxygen which agitates the flux-metal emulsion is introduced through the walls of the reactor by means of nozzles with substantially horizontal axes. The slag and gaseous material rise as a thick foam in the vertical shaft in a flow which is countercurrent to the flow of the descending metal. Chemical reactions among the metal, the flux and the gaseous material, which result in the refinement of the cast iron take place during the countercurrent flow of materials; the flux and gaseous materials discharge from the upper part of the reactor and the refined cast iron decants from the emulsion and is withdrawn from the tapping hole provided in the reactor.

This application is a continuation-in-part of our US. application Ser. No. 713,623, filed Mar. 18, 1968 and now abandoned.

BACKGROUND OF THE INVENTION Several processes for refining cast iron into steel by using gaseous oxygen of high purity are already known. In such processes usually the gaseous oxygen is blown above the cast iron bath through water-cooled piping. The oxygen is absorbed by the surface of the metal and slag and reacts with their components to refine the iron. Many variations of this process are known involving the use of stationary or rotating reactors, wherein powdered lime is blown in together with oxygen, and wherein oxygen is blown simultaneously above and under the bath, and so on. All of these processes however, are not continuous but are carried out as batch processes; that is, they require the successive steps of charging the reactor, refining, and discharging the reactor when the refining is over.

Non-continuous processes have many disadvantages compared to a continuous process. In a continuous proc ess every transformation from the raw materials to the finished product is in permanent operation, and the material flows constantly through the reactor, coming out thereof in a continuous manner with the characteristics required of the finished product. The advantages generally attributed to continuous processes in comparison with similar non-continuous processes are: a greater uniformity of the product, possibility of using more refined adjustment and control techniques, smaller size of the reactor per unit of product, and finally, the possibility of direct connection of. the process under consideration with processes upstream and downstream which have already been made continuous.

There would surely be remarkable advantages from the discovery of a continuous process which in addition to being continuous has all of the desirable features of current processes involving the use of high purity oxygen. In

fact, the conditions already exist for the advantageous utilization of such a continuous process. Under present circumstances, the blast furnace as an upstream process can be made continuous without excessive difliculties; while downstream, the known continuous leakage process can utilize directly a permanent flow of liquid steel coming from a continuous refining process.

Further, modern control systems involving the use of a computer can also yield suitable solutions for control with a continuous process. In view of the advantages, a series of different continuous processes of refining, which are still in the experimental phase have been proposed in the last few years. Processes are known, in which cast iron is pulverized with oxygen, i.e. spray refining from the top of a reactor. In this process the cast iron, reduced in the form of droplets reacts with the surrounding gases and collects on the bottom of the reactor, refined or prerefined.

Other processes provide a continuous flow of cast iron inside a conduit in the form of a channel, above which oxygen is blown by means of water-cooled lanes, analogously to present non-continuous processes. At the end of the reactor, the cast iron comes out transformed into steel. In another process the cast iron is introduced into a cylindric reactor and the oxygen with other components is blown from the top by means of cooled piping, so as to create an emulsion between the slag and the metal, which is blown from the bottom upwards, due to decarburizing reactions and the consequent production of gas. The emulsion obtained overflows into a second container where the separation of the metal from the slag occurs through decantation.

SUMMARY OF THE INVENTION The continuous process for transforming cast iron into steel, according to the invention is carried out by pouring liquid metal, i.e. liquid cast iron, from above onto slag in a reaction zone, so that the liquid metal descends and is scattered through the slag in the form of small drops. An oxygen stream is introduced into the slag-metal emulsion which agitates the entire mixture vigorously and also enters into reaction therewith. The mixture of slag and gaseous material rises in the form of a thick foam moving countercurrent to the descending liquid metal in the reaction zone. Thus, the refining of cast iron according to this invention is characterized by the counter-current flow of the liquid metal and the thick foam mixture of slag and gaseous material; during the countercurrent flow, the chemical reactions among the slag, gaseous materials and metal take place. The refined steel decants from the emulsion, and the mixture of slag and gaseous material discharges from the top of the reaction zone. The oxygen stream is introduced into the slag-metal mixture by means of substantially horizontal jets, i.e. at substantially right angles to the countercurrent flows of liquid metal and of slag.

The device of the present invention for the execution of the method is consituted by a vertical shaft covered inside with refractory material, and provided with a tapping hole for the refined steel. In the upper part of the device there is provided a feed conduit for the cast iron to be refined, an overflow conduit for the slag, discharge means for the gases and/ or vapors developed during the refining reaction and at least a load port for introduction of additional compounds which may, when desired take part in the refining process. Nozzles for blowing oxygen into the mass of the slag-metal emulsion, having substantially horizontal axes are positioned in the shaft walls and end inside it above the level of the pool of refined steel which collects once the operation has started.

Other objects, advantages and characteristics of the invention will be apparent from the following description which refers to embodiment forms which have been chosen by way of example only.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical section of a reactor of the present invention.

FIG. 2 is a vertical axial section of another reactor of the present invention and FIG. 3 is a view of the reactor of FIG. 2 from above.

FIG. 4 illustrates diagrammatically a reactor of this invention provided with regulating means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the process of this invention, additional materials which are commonly used in refining cast iron, such as powdered lime, ferrous minerals and solid, liquid or gaseous fuels may be blown into the slag mixture with the oxygen. Also, iron scraps, lime in pieces or chunks and ferrous minerals may be introduced into the reaction zone from above the slag.

The reactor used to carry out the transformation of cast iron into steel according to this invention may include checking or regulating means which, as a function of the shaft weight and the material contained therein, adjust the inflow into the shaft of the liquid cast iron, the oxygen and the other elements which take part in the refining reaction, so as to maintain the level of the refined steel inside the shaft substantially constant. Preferably the refractory coating inside the shaft extends only to the bottom and the lower part of the walls of the shaft; the refractory coating extends upwards, at least over the level of the refined steel; and the remaining innner walls of shaft of plate, are cooled by means of a water veil or by means of a water circulation jacket.

The apparatus of the present invention will be further described with reference to the accompanying drawings. In FIG. 1, a shaft reactor 1, of cylindric form is coated in side with refractory material based on magnesia or dolomite, and this reactor substantially constitutes the whole equipment. A bath 2 of refined steel is collected on the bottom of the shaft 1; the spout 3 at the bottom of shaft 1 acts as a tapping hole for the continuous withdrawal of the steel casting. A conduit 4, at the top of shaft 1 is provided for feeding the smelt cast iron which is poured from the skip 5 which contains the liquid cast iron coming from the blast furnace. Nozzles, 6, for blowing 0 into the mixture contained in the shaft are provided in the walls of shaft 1. An opening 7 in the upper part of shaft 1 allows for the insertion of additional materials such as scraps, ferrous minerals, etc. An outflow conduit 8 for the slag is also provided in the upper section of shaft 1. A hood 9 is placed over the upper end of shaft 1 for collecting and conveying the gases developed inside the shaft.

FIGS. 2 and 3 illustrate a reactor of this invention which is provided with water cooling means and which is partially lined with refractory material. With reference to FIGS. 2 and 3, the reactor is formed by a plate cylinder 11, the lower part of which is lined by a very thick coat of refractory material 12. A conduit 14 feeds cooling Water to an annular collector 15 which surrounds the upper edge of the reactor. Nozzles 16 which extend from the annular collector 15 provide a continuous coat of cooling water 17 which flows on to the external face of the cylinder 11, thus effecting cooling of the reactor. As a result of this cooling, the slag contained in the internal face of the cylinder 11 solidifies forming a coat 18 which behaves as an insulating layer and protects the plate of cylinder 11.

FIG. 4 illustrates a continuous automatic reactor of this invention wherein the level of the liquid steel at the bottom of the reactor is maintained automatically at a predetermined level by automatic adjustment of the rate of inflow of liquid steel, oxygen and possible of the materials such as lime and minerals which are used in the refining process, and/ or other fuel. With reference to FIG. 4, the reactor 1 rests on weighing cell 20 which Weighs the reactor and its contents and transmits this information through line 23 to the weighing cell 22 on which the ship 5 and its contents rest and which regulates the flow of liquid cast iron therefrom through line 24 depending on the information reviewed from the weighing cell 20. Weighing cell 20 also transmits information through lines 25, to the valves 6a of nozzles 6 to control the flow of oxygen and any other material blown through the nozzles into the reactor.

The process of the invention takes place in the central part 10 of the reactor 1, shown in any of the figures in the following manner After the process has started, steel bath 2 is collected at the bottom of the reactor. The collected steel bath 2 has the characteristics required of the finished product and discharges continuously through the casting hole, of the spout 3, into a collecting container or it is sent directly to the continuous leakage equipment for the production of slabs or billets. From the upper part of the reactor through the feed conduit 4, the cast iron to be refined is introduced from a suitable overstanding container 5. The whole central part 10 of the reactor is occupied by a foamy slag which is maintained in agitation by a series of jets of gaseous oxygen, pure in a high degree. The jets of oxygen are blown through water-cooled side piping 6. The oxygen reacts with the slag and the gas maintained in the vesicular state in the foam, and, finally, with the cast iron drops obtained by the crushed blast coming from the feed conduit 4. The cast iron drops, on moving downwards, react also with the slag so that, when collected in the bath 2, they are wholly refined and transformed into steel with the desired characteristics. Through the oxygen pipings 6, lime powders, minerals etc. may be also blown into the reactor, in order to reinstate or modify the slag composition; gaseous, liquid or solids fuels may also be introduced therein. Other additions, such as steel scraps, ferrous minerals, lime, limestone, fiuidizing materials, may be effected continuously or not, through the upper opening 7 of the reactor.

Finally, the slag in excess is discharged automatically through the outflow conduit 8 into a suitable container. The gases originated from the reactions and coming from the foamy slag are conveyed by means of an overstanding hood 9, cooled and purified by the usual means and discharged into the atmosphere or utilized as combustible gases.

The reactions occurring in the central part of the reactor are the following:

(a) Between gaseous oxygen and gas contained in the foam:

/2o +co=co (1) (b) Between gaseous oxygen and liquid slag:

%O +FeO= /2Fe O (2) (c) Between gaseous oxygen and the drops of liquid cast iron passing through the slag:

The reactions between oxygen and gas and those between oxygen and slag are all exothermic and cause a high heating of the slag, which allows for the kinetics of all of the reactions. The cast iron drops get warmer indirectly through the contact with the slag and directly by reaction with the gases and slag. Both the reactor 1 and the feeding container 5 of the cast iron rest on weighing cells and 22, respectively, in communication with each other through line 23, which determine and also adjust the steel level on the reactor bottom and the delivery of the cast iron introduced to the proper values.

In the first case, in fact, the weight P of the reactor is given by:

As the density 6 of the slag and that 6 of the steel are known and very different from each other, the heights lz of the steel column and hslag of the slag column, are bound from the following ratio:

stee1+ slag= tota1 Finally, P weight of the empty reactor, may be easily obtained.

At the beginning of the operation, P is obtained as follows:

The delivery of the cast iron introduced is obtained by deriving, with respect to the time, the weight of the container.

The process is started in the following way: after preheating the reactor at a temperature of 900-1000 C. by means of auxiliary burners not shown in the figure, but usually employed in pre-heating of steel works, a suitable quantity of slag casting at a low temperature is introduced on the reactor bottom, said slag being preferably formed by calcium ferrite added to 3-4% of fluorine. The tapping hole must be closed with a combustible cap or refractory mass easily removable mechanically or by use of oxygen; then the liquid cast iron is poured in, which, in this first phase, is preferably at high temperature. Generally the liquid cast iron is introduced into the reaction vessel at a temperature of about 1250 to 1450 C. The slag, in contact with the cast iron, smelts and begins to swell owing to the reactions of carbon oxidation (reaction 8). When the slag level reaches the oxygen blowing nozzles, the oxygen blowing is started together with the introduction of liquid cast iron. When the metal on the reactor bottom has reached the established level, defined as indicated above, which for safety reasons must be a little under the first set of nozzles, the tapping hole is opened and the metal is permitted to flow out. From this moment on, an automatic or manual system of regulation maintains the metal on the reactor bottom of a constant level, by reducing or increasing the cast iron feeding.

At frequent intervals, steel samples are taken at the outlet of the conduit 3, the temperature of which is also measured. This last measurement may be continuous. If the content of carbon in the steel is not the desired one, the ratio of blown oxygen-introduced liquid cast iron is modified until the steel produced has the desired com position. Corrective additions may be effected in the steel collecting container placed under the reactor, where it will also be possible to deoxidize the steel. The steel collected at the bottom of the vessel is at a temperature of about 1550 to 1700 C.

Temperature regulation of the reactor is obtained by introducing more or less iron scraps from above, more or less mineral from above or through the piping in the sides or by modifying the amount of the fuel which may be charged through the piping if desired.

In general, the amounts of oxygen, lime, iron, ore and steel scrap used in the present process, per ton of cast iron treated, are in the following ranges; 50410 Nm. of oxygen, 50-100 kg. of lime, 0-100 kg. of iron ore and 200 to 400 kg. of steel scrap.

steel The slag discharge occurs spontaneously through the conduit 8 in the upper section of the reactor but it is also possible to activate the removal of the slag by blowing suitable lime through the pipings or by adding tensioactive additives from above.

When the collection ladle of the steel is completely filled, the blast of liquid steel may be deviated by means of an Y conduit or other known means into an empty ladle which is provided for the purpose. Then the discharge is completed, the feeder of the liquid cast iron is replaced by a full one, by similar means. In such a way the process may continue indefinitely, and it is necessary to stop the operation only for the periodic restoration of the refractory coating.

If the wear of the refractory coating in the zone of the slag is much too fast, the refractory coating in this zone may also be completely removed by simply introducing water veil cooling outside the clamping sheet. A coating of solidified slag will quickly and automatically form on the inner surface of the sheet, with excellent characteristics of thermal non-conducting material, as a consequence of its high porosity. The refractory coating, of course, must be maintained in the lower zone occupied by the steel.

The process described has all the advantages, previously pointed out, of continuous processes, particularly, decreased size of the apparatus with equal capacity, simpler equipment for the smoke treatment, greater regularity in the development of said smokes, possibility of controlling the process in phase, etc.

The following is an example which further illustrates the present method and in particular exemplifies best mode contemplated for carrying out the present invention. In view of the fact that this is a continuous process involving large quantities of materials, it is understood that the data in the following example represents the average temperatures and average amounts of materials consumed and" produced over a period of time and the accuracy thereof is about ':20% for any given value at a particular time.

EXAMPLE 1 A cylindrical reactor of the type described with reference to FIGS. 2 and 3 is used, having the following dimensions:

Diameter--m. 1.5 Heightm. 3 Production-30 tons/hour CHARGE MATERIALS Liquid pig iron Consumption20 tons/hr. Mean composition by weightC='4%, Si=0.6%,

Mn=0.7%, S=0.030%, P=0.050% Cast iron feeding temperature-13 00 C.

Scrap iron Consumption-10 tons/hr.

Lime powder Consumption-1.5 tons/hr. Grading l mm.

Mineral powder Variety-ferrous ore Consumption-1.5 tons/hr. Grading- 1 mm.

Oxygen Delivery-NmE/hr. 1500 The steel obtained has the following characteristics: production 30 tons/hr.; mean composition by weight C=0.05%, Mn=0.20%, S=0.01(l%, P=0.005%; temperature of the steel bath on the bottom of the reactor- 1650 C.; slag--temperature inside the reactor-2300 C.

In comparison with the continuous processes which have been disclosed, this process has the following advantages:

(a) It is carried out within a single, very simple reactor with a low refractory consumption.

(b) The cast iron/slag contact occurs in countercurrent, resulting in better utilization of the desulphurizing and dephospherizing capacities of the slag and a lower amount of iron in the outgoing slag.

(c) The reaction surface in contact between cast iron and slag, is very large and, in comparison with the process of the type called spray refining, the contact time is remarkably higher, owing to the slower movement of the metal drops in a zone of high viscosity produced by the slag.

(d) The decarburizing reactions occur in a scattered phase and with a higher damping capacity than in a metallic bath, whereby the sprinkles and the projections are reduced. Further, the production of iron oxide smoke is strongly reduced due to the reduced direct contact between oxygen and metal.

(e) The metallic bath collecting on the bottom is in chemical balance with the overstanding slag, so that further reactions do not occur and the slag decantation is made easier.

Although for reasons of describing the present invention, certain specific embodiments have been described and illustrated, by way of example only, many modifications and variations may be made in embodying the invention, all of them, however, being to be considered as based on the following.

What I claim and desire to secure by Letters Patent is:

1. A process for continuously transforming iron into steel which comprises pouring liquid cast iron over slag in a reaction Zone so that said liquid cast iron descends through said slag in the form of small drops, and introducing an oxygen stream into said slag which agitates the mixture of said slag and iron whereby the slag and gaseous material rise in the form of a thick foam moving countercurrent to the flow of said liquid iron, and the chemical reactions among said slag, said oxygen and said cast iron which transform said cast iron into steel take place in said reaction zone between the materials in countercurrent flow, and whereby the slag and gaseous material discharge near the top of the reaction zone and the thus-refined steel decants from the mixture.

2. A process for continuously transforming cast iron into steel according to claim 1, wherein said oxygen stream is introduced into said slag by means of a plurality of jets positioned substantially at right angles to said reaction zone.

3. A process for continuously transforming cast iron into steel according to claim 1, wherein at least one material selected from the group consisting of lime powder, ferrous mineral and solid, liquid and gaseous fuel is blown into the reaction zone with said oxygen.

4. A process for continuously transforming cast iron into steel according to claim 1, in which scrap iron, pieces of lime and ferrous materials are introduced into said reaction zone from a point near the top of said zone.

5. A process for continuously transforming cast iron into steel according to claim 1, wherein said liquid cast iron is continuously poured over said slag, said oxygen is continuously introduced in a stream which vigorously agitates the mixture of slag and iron by means of a plurality of jets positioned substantially at right angles to said reaction zone and wherein slag and gaseous material are continuously discharged near the top of the reaction zone and the refined steel is continuously withdrawn from a point near the bottom of said reaction zone.

References Cited UNITED STATES PATENTS 2,572,489 10/1951 Jordan 75-61 X 2,819,160 l/1958 Bannister et al 75-52 Q,969,282 1/1961 Churcher 75-52 X 3,356,490 12/1967 Muller et a1. 756() HYLAND BIZOT, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 75-46, 60