US3592629A

US3592629A - Method for refining molten metal

Info

Publication number: US3592629A
Application number: US655921A
Authority: US
Inventors: Ryo Ando; Tutomu Fukushima; Eiichi Hiraguchi; Kokichi Hagiwara
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1966-07-28
Filing date: 1967-07-25
Publication date: 1971-07-13
Anticipated expiration: 1988-07-13
Also published as: NL6710429A

Abstract

A METHOD FOR REFINING A MOLTEN METAL SUCH AS MOLTEN PIG IRON. THE MOLTEN METAL WHICH CONTAINS IMPURITIES IS POURED INTO A VESSEL SUCH AS A SUITABLE LADLE. A PURIFYING AGENT WHICH REACTS WITH THE MOLTEN METAL TO ELIMINATE AT LEAST PART OF THE IMPURITIES THEREOF IS ADDED TO THE MOLTEN METAL. THEN THE MOLTEN METAL TO WHICH THE PURIFYING AGENT HAS BEEN ADDED IS STIRRED ONLY IN THE REGION OF THE SURFACE OF THE MOLTEN METAL. THE ADDING OF THE PURIFYING AGENT TO THE MOLTEN METAL AND THE STIRRING THEREOF TAKES PLACE AFTER THE MOLTEN METAL HAS BEEN POURED INTO THE VESSEL DURING A PERIOD OF TIME WHEN PARTS OF THE BODY OF MOLTEN METAL WITHIN THE VESSSEL ARE STILL IN MOTION. IN THE CASE OF PIG IRON THIS PERIOD OF TIME HAS A DURATION OF AT LEAST ONE HOUR.

Description

July 13, 1971 mro ANDO ETAL 3,592,629

METHOD FOR REFINING MOLTEN METAL Filed July 25, 1967 5 Sheets-Sheet 1 Fig I mvrzmoa R70 urn m) FlJ/(uSH/MA, M m HM/ M ,E/lcn/ BY H/RAm/cm Am M, W

rip-n RYo ANDO ETAL 3,592,629

METHOD FOR REFINING IOLTEN METAL July 13, 1911 Filed July 25, 1967 5 Sheets-Sheet a Fig 3 Fig 4 INVENTOR.

July 13, 1971 RYO ANDO EI'AL IBTHOD FOR REPINING IOLI'EN METAL 5 Sheets-Shut I Filed July 25, 1967 Fig 5 Angle of Inclination Fig 6 INVENTOR WW) Fu/(us m y 1971 RYO moo ET L METHOD FOR REFINING ULTEN METAL 5 Sheets-Sheet 5 Filed July 25, 1967 w 5 555 271; Fun;

Time(Min.)

Fig 10 Number of Stirring Rods 3. f 1 Number of Stlrrlng Rm] l 0 0 0 m 9 w 1 m Depth of Immersion (Cm) 1 u FM'UJIIIMA, #4 K4 "FA 5 "CH! Hilde United States Patent US. C]. 75-58 7 Claims ABSTRACT OF THE DISCLOSURE A method for refining a molten metal such as molten pig iron. The molten metal which contains impurities is poured into a vessel such as a suitable ladle. A purifying agent which reacts with the molten metal to eliminate at least part of the impurities thereof is added to the molten metal. Then the molten metal to which the purifying agent has been added is stirred only in the region of the surface of the molten metal. The adding of the purifying agent to the molten metal and the stirring thereof takes place after the molten metal has been poured into the vessel during a period of time when parts of the body of molten metal within the vessel are still in motion. In the case of pig iron this period of time has a duration of at least one hour.

BACKGROUND OF THE INVENTION The present invention relates to the refining of molten metals.

In particular, the present invention is applicable to the refining of molten pig iron and is especially useful in desulfurizing and in dephosphorizing the molten pig iron.

At the present time impurities are removed from molten pig iron, or the composition of alloys, when manufacturing pig iron, steel, and the like, are adjusted in blast furnaces, converters, open-hearth furnaces, electric furnaces, cupolas, etc. However, only a limited extent of refinement can be carried out with such conventional structures.

There has thus been a long-felt want in the art for an inexpensive pre-treatment or post-treatment of the molten metal to refine the latter in a manner which can be easily practiced and which can achieve constant, predictable results.

Among the presently known methods for carrying out pretreatments, such as desulfurization, are a ladle-transfer pouring method, the Pellan process, a method wherein a powdered desulfurizing agent such as carbide and the like is blown in with a gas in which the powder is suspended, an agitation method utilizing an impeller, and similar methods. All of these known methods have the serious disadvantage of being incapable of achieving a constant rate of desulfurization, and in addition they suffer from the disadvantage of an unavoidable loss of desulfurizing agent which does not react with the molten pig iron. This latter loss is excessive due to spattering of the molten metal or insutficient agitation thereof, thus undesirably increasing the cost of the treatment. Similiar difficulties and disadvantages are encountered in an unavoidable manner with known dephosphorization treatments as well as in known treatments for adjusting the composition of alloys.

A more effective desulfurization method, according to which the ladle is vibrated, has been recently developed. With this latter method the ladle is vibrated so as to promote a more intimate contact between the molten pig iron and the purifying agent, thus increasing the refining efficiency. However, the cost of the vibrating installation and the power required to vibrate a huge ladle and its contents is prohibitive, and in addition the ratio of the volume of 3,592,629 Patented July 13, 1971 the molten pig iron to the volume of the ladle is small. Furthermore, this latter method is accompanied by the serious disadvantage of an unavoidable, excessive drop in the temperature of the molten pig iron.

Inasmuch as the refinement of molten metals such as molten irons or iron alloys is brought about by reactions between the slag and molten pig iron and reacting agents which are in a gaseous state, in order to accelerate the reactions it is essential to increase the contact area between the treated metal which is in a liquid state and the reacting agent which is in a gaseous state, and the attempt is always to bring the reaction agent which is in the gaseous state into contact with constantly changing liquid surfaces of the molten metal which is treated. These factors are well recognized by those skilled in the art. Thus, in order to effectively utilize the reaction agents it is essential to provide a constant agitation of the reaction agents so as to cause them to come into intimate contact with the molten metal such as molten pig iron. While a method such as the above ladle vibrating method achieves a certain degree of refinement, in a manner similar to other known methods, since the reaction agents are distributed over the entire surface of the molten pig iron the entire contents of the latter are stirred and it is necessary to vibrate or oscillate the huge ladle in its entirety in order to bring about the desired extent of contact between the treated metal and the purifying agent.

Thus, with the prior art techniques it has always been considered essential to provide the largest possible area of contact between the purifying agent and thus the entire mass of molten iron is stirred or otherwise agitated in order to attempt to achieve the intimate contact with the purifying agent which will give the most satisfactory results.

SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a method for effectively refining molten metals while avoiding the drawbacks of the prior art as referred to above.

Thus, it is an object of the invention to provide a method capable of effectively refining a molten metal in a simple, economical manner, utilizing a relatively small degree of agitation of the body of molten metal.

In accordance with the invention the molten metal which contains the impurities and which is to be refined is poured into a vessel such as a suitable ladle, and a suitable purifying agent is added to react with the molten metal so as to eliminate at least part of the impurities thereof. Then this molten metal to which the purifying agent has been added is stirred only in the region of the surface of the molten metal. The adding of the purifying agent to the molten metal and the stirring thereof is carried out during a period of time when parts of the body of molten metal within the vessel are still in motion.

BRIEF DESCRIPTION OF DRAWINGS The invention is illustrated by way of example in the accompanying drawings which form part of this application and in which:

FIG. 1 is a sectional elevation of one possible embodiment of an apparatus for practicing the method of the present invention;

FIG. 2 is a top plan view of the structure of FIG. 1 with the ladle cover removed;

FIG. 3 is a schematic sectional elevation of a further embodiment of an apparatus for practicing the method of the invention;

FIG. 4 is a schematic sectional elevation of yet another embodiment of an apparatus for practicing the method of the invention;

FIG. 5 is a graph illustrating the operation of the embodiment of FIG. 4;

FIG. 6 is a schematic sectional elevation of yet another embodiment of an apparatus for practicing the method of the invention;

FIG. 7 is a graphic illustration of one example of a method carried out according to the invention;

FIG. 8 is a graphic illustration of another specific example of a method according to the invention;

FIG. 9 is a graphic illustration of the time during which parts of a body of molten metal remain in motion after the molten metal is poured into a vessel; and

FIG. 10 is a graph illustrating the results achieved in tests to determine the relation between the depth of immersion of stirrers and the number thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention is based upon the concept or discovery that after a molten metal is poured into a vessel parts of the body of molten metal remain in motion over a substantial period of time. Thus, it has been found that due to natural convection, diffusion, and/or inertia of various parts of the molten metal, there are within the body of molten metal currents which persist over a substantial period of time after the metal is poured into a vessel. Thus, when a molten metal such as molten pig iron is poured into a ladle from a blast furnace, or is transferred to a vessel such as a torpedo car to be used in connection with charging an open-hearth furnace or converter, or when the molten pig iron is situated within the front part of a cupola or in a trough which forms a runner from a blast furnace, flowing movements of parts of the body of molten metal within the vessel, in the form of currents of the molten metal, persist for a considerable period of time after the metal has been poured.

In order to establish these facts, experiments were carried out. In these experiments molten pig iron was poured into a ton ladle from a blast furnace, and an isotope of gold Au 128 was placed in the bottom of the ladle. The relationship between the elapsed time and the intensity of radiation emanating from the surface of the molten pig iron was measured to obtain the results which are illustrated in FIG. 9. The curve D of FIG. 9 shows the results of measurements obtained when Au 128 was incorporated into the molten metal 16 minutes after the pouring thereof into the ladle. The curve E of FIG. 9 shows the results obtained with measurements taken in a test where Au 128 was incorporated into the ladle 39 minutes after the molten pig iron was poured. The curve F shows measurements taken when Au 128 was incorporated into the body of molten metal minutes after the pouring thereof into the ladle. From these curves it has been discovered that indeed a molten metal such as molten pig iron, after it is poured into a vessel such as a suitable ladle, continues to have parts of the body of molten metal which persist in motion over a very long period of time which in the case of pig iron has a duration of at least one hour. Furthermore, the curves demonstrate that the molten metal such as molten pig iron mixes with the additive at an extremenly high speed.

Based upon the results of these tests, it has been concluded that sufficient refining, such as desulfurization, can be carried out by taking advantage of the fact that parts of the body of molten metal, after it is poured into a vessel, persist in motion over a long period of time so that it is in fact unnecessary to stir the molten metal throughout the body thereof and instead the stirring can be limited only to a relatively shallow portion of the molten metal in the region of its surface where the molten metal is in contact with the purifying agent. It thus becomes unnecessary to carry out the stirring all the way down to the bottom of the vessel, as long as the refining operations are carried out after the pouring during the period of time when parts of the body of molten metal persist in their flowing movements.

In the case of the desulfurization of pig iron extensive experiments have demonstrated that 90% desulfurization can be achieved when molten pig iron which has been poured into the ladle is stirred with a stirring means which has stirrer rods which extend into the body of molten metal downwardly from the surface thereof to a depth which is less than V3 of the total depth of the molten metal. In this case desulfurization of 8590% was achieved with the stirrer rods extending into the body of molten metal downwardly below the surface thereof to a depth of about Vin to j of the total depth of the molten pig iron in the ladle.

FIG. 10 is a graph illustrating the results achieved during desulfurization on the one hand with a single stirrer rod inserted to a depth of l0-30% of the total depth of the molten metal and on the other hand with three stirrer rods also inserted to a depth of l0-30% of the total depth of the molten metal. Thus, the lower curve of FIG. 10 illustrates stirring with a single stirrer, while the upper curve illustrates stirring with three stirrers, and it is appaent from FIG. 10 in both cases a relatively high degree of desulfurization was achieved even though the stirring operations were limited to the region of the surface of the molten metal.

In carrying out the tests used to provide the graph shown in FIG. 10, the molten pig iron was poured into a ladle having a capacity of 30 tons up to a depth of 2000 mm. within the vessel, and CaC was added at a rate of 4 kg./THM. The stirring rods were rotated at rpm. It was found that where the stirrer rods have an irregular cross section, such as a cross section of X- shaped or Y-shaped configuration, a somewhat better desulfurization was achieved than in the case where stirrer rods of circular cross section were used, and in addition it was found that a depth of immersion of the stirrer rods less than As of the total depth of the molten pig iron was suflicient to achieve the desired results.

Thus. with the invention it is possible, by incorporating a purifying agent such as a desulfurizing agent or a dephosphorizing agent into the molten pig iron after it is poured into a ladle or similar vessel and while currents persist in the body of molten metal to bring about the desired degree of refining while stirring only a relatively shallow portion of the molten pig iron in the region of the surface thereof by way of a suitable stirring means. Thus, the desired degree of desulfurization or dephosphorization, in the case of molten pig iron, can be achieved without stirring the body of molten metal all the way down to the lowermost part thereof in the vessel. It is only necessary to stir the molten pig iron in the region of its surface, so that it becomes possible to use a stirring means of simple construction, thus reducing the cost of the installation as well as the cost of the power required to operate the stirring installation.

FIGS. 1 and 2 illustrate one possible embodiment of an apparatus for practicing the method of the invention. With this embodiment there are four stirrers in the form of elongated vertically extending rods 5 of X-shaped cross section. These rods are fixed at their top ends to the lower surface of a horizontal disc 4 which thus fixedly carries the stirrers 5. These stirrers extend perpendicularly to the surface of the body of molten metal in the vessel 1 downwardly into the molten metal only to a depth which is in the region of the surface thereof, this depth being less than one third of the total depth of the body of molten metal. Thus, there is shown in FIG. 1 a body of molten pig iron 2 which is to be refined by way of a purifying agent 3 which floats on the surface of the molten pig iron and which is added after the molten pig iron is introduced into the vessel 1, which may take the form of a ladle, although it may also be in the form of the runner of a blast furnace or the front lower part of a cupola where the molten metal is accessible, as is well known. The stirrer-carrying disc 4 is driven by an eccentric drive which forms part of the stirring means of this embodiment and which includes the bevel gear train 8. This gear train drives a disc which is eccentrically and pivotally connected to a corner portion of an eccentric driving disc which is of substantially triangular configuration, as is apparent from FIG. 2. The discs which are respectively pivotally and eccentrically connected to the corners of the plate 7 are supported for rotary motion about their axes, respectively, which extend vertically, in any suitable bearings, so that these relatively small circular discs support the eccentric plate 7 for eccentric rotary motion, and only one of these circular discs need be driven by way of the drive 8 as, illustrated. A central portion of the triangular eccentric camming plate 7 fixedly carries a shaft 6 which extends vertically down to the top of the stirrer-carrying disc 4, this shaft 6 being fixed to a central part of the disc 4 as well as to a central part of the plate 7. Thus, with this construction the stirrers S will be driven eccentrically along closed paths, bringing about stirring of the molten metal in the region of the surface thereof. Of course, these operations, as well as the addition of the purifying agent 3, are carried out after the pouring of the molten pig iron 2 into the vessel 1 while parts of the body of molten metal in the vessel 1 continue in motion.

While the stirrers 5 of FIGS. 1 and 2 are shown as having an X-shaped cross section, other cross-sectional configurations can be used such as Y-shaped, square, triangular, or even circular cross-sectional configurations, although the irregular configurations provide superior results, as pointed out above.

The vessel 1 is covered by a lid which is indicated schematically in FIG. 1 and which is omitted from FIG. 2 so as to illustrate more clearly the stirring means. This lid has a downwardly extending side wall engaging the exterior upper portion of the vessel and a top wall formed with an opening large enough to accommodate the eccentric movement of the shaft 6. In addition, suitable tubes 10 extend through the side walls of the lid to the space beneath the latter to introduce a suitable protective or atmospheric gas into the vessel Over the contents thereof. In the case of desulfurizing molten pig iron, the purifying agent is carbide, calcium nitride, soda ash, and the like, while reducing gas such as coke furnace gas, blast furnace gas, etc., are used as the atmospheric, protective gas forming the atmosphere above the contents of the vessel during the purifying reactions. In order to bring about dephosphorization of the molten pig iron, the purifying agents are a highly basic sintered ore, limestone, and the like, while the protective atmosphere introduced through the tubes 10 is oxygen, as is well known.

The apparatus for practicing the method of the invention which thus requires the plurality of elongated stirrers to be moved only in the region of the surface of the molten body of metal is far simpler in construction and requires far less power than conventional structures of this type.

The material which is used for the stirrers 5 is refractory so as to prevent erosion thereof and so as to have no influence 0n the refining reactions. For example, in the case of desulfurization of molten pig iron, reducing materials such as refractory materials of graphite are advantageously used for the stirrers, particularly since they have a high thermal resistance and will remain uninfluenced by heat shock. In the case of dephosphorization of molten pig iron, since the stirrers are in contact with a highly basic slag, the stirrers are made of a basic refractory material such as magnesia dolomite. However, inasmuch as these latter materials do not have a particularly high thermal resistance and may indeed be influenced by heat shock, it is preferred to reinforce the stirrers by surrounding them with graphite-type materials, such as those materials of the rods which are used in the case of desulfurization of the pig iron. Thus, in this case, namely in the case of dephosphorization, the stirrers which are made of a basically refractory material such as magnesia dolomite are covered with coatings of a graphite-type of material.

According to the embodiment of the apparatus for practicing the method of the invention which is illustrated in FIG. 3, the stirrer of the stirring means takes the form of a hollow cylindrical member 13 whose axis extends vertically, and a lower end portion of this cylindrical hollow stirrer is vertically immersed into the body of molten pig iron 12 only to a depth which is situated in the region of the surface thereof, this molten pig iron 12 having been poured into the vessel 11. Within the hollow stirrer 13 is situated at least part of the purifying agent 3 which floats on the upper surface of the molten pig iron 12. The cylindrical stirrer 13 is fixedly connected to a flange which is situated at the bottom end of a hollow shaft 15 which is eccentrically driven as through a drive similar to that of FIGS. 1 and 2. Thus, this drive includes the eccentric camming plate 16 which may be similar to the plate 7 and the bevel gear drive 17 which drives one of the rotary discs which is pivotally connected with a corner portion of the plate 16 at a location spaced from the center of the disc which is driven by the bevel gear drive 17.

A suitable protective gas is supplied to the interior of the cylinder 13 through a pipe 18 so as to control the gaseous phase or protective atmosphere within the cylinder 13. For example, in order to bring about desulfurization, a reducing gas such as coke furnace gas or the like, is admitted through the pipe 18, while in the case of dephosphorization oxygen is supplied at a suitable pressure.

With this embodiment the upper surface of the molten pig iron which is situated outwardly beyond the cylindrical stirrer 13 is heated by combustion of suitable gases introduced into the interior of the lid 19 through the pipes 20. This lid 19 has a construction similar to the lid of FIG. 1 and is also provided in its top wall with an opening large enough to accommodate the required extent of eccentric movement of the hollow shaft 15, and of course the pipe 18 moves with the hollow shaft 15, this pipe 18 having a flexible, hose-type of connection with the source of protective gas.

In a further embodiment of the invention which is illustrated in FIG. 4 the stirring of the body of molten metal is brought about by way of a single elongated stirrer 13a which may have an X-shaped cross section, or an eliptical cross section, if desired. With this embodiment the single stirrer is introduced through the opening in the lid into the body of molten metal on which the purifying agent floats to a depth which is limited to the region of the surface of the body of molten metal, and through a drive such as the gear transmission 21 illustrated in FIG. 4 the single elongated stirrer 13a is simply rotated about its axis, any suitable bearing structure being provided to support the stirrer for rotary movement about its axis. Thus, in this case the stirrer does not extend perpendicularly to the surface of the molten metal but is rather inclined thereto at an angle other than a right angle.

The stirring effect which is achieved with the embodiment of FIG. 4 is determined by the angle of inclination of the elongated stirrer and the speed at which it is rotated. The curves of FIG. 5 illustrate the relation between the angle of inclination of the stirrer and the number of revolutions thereof. Curve A shows the results achieved at various angles of inclination when the stirrer is rotated at a high speed, while the curve B shows the results achieved at different angles of inclination when the stirrer is rotated at an intermediate speed, and curve C shows the stirrer effects achieved at different inclinations of the stirrer when the latter is rotated at a low speed.

In connection with FIG. 5, the high, intermediate and low speeds are, respectively, 130 r.p.m., 110 r.p.m., and r.p.m. In connection with Examples 1, 3, and 4, set

forth below, the depth of immersion of the stirrers were within a range of about 200 mm. to 250 mm.

Referring now to FIG. 6, the embodiment of the apparatus for practicing the method of the invention which is illustrated therein includes a vessel such as the ladle 26 in which the body of molten metal 27, such as pig iron, is located. The vessel 26 is carried on a base member which fixedly carries the upwardly directed standards or columns and 25' which are fixed to each other at their top ends by an upper beam which carries at its underside suitable brackets on which pulleys and 30 are supported for rotary movement. A beam 29 extends horizontally between the standards 25 and 2S and is formed with openings or notches through which the standards extend, this beam 29 fixedly carrying at its underside blocks 33 and 33' which slidably engage the columns 25 and 25'. Cables, ropes, or the like 30 and 31 are fixed to the outer ends of the beam 29, beyond the columns 25 and 25' and extend respectively around the

pulleys

30 and 30, so that these cables may be actuated to raise and lower the beam 29. Instead of a cable-and-pulley system for raising and lowering the beam 29, structures such as hydraulic jacks may be used.

The beam 29 carries an electric motor 32, connected to any suitable source of current, and this motor through a suitable transmission drives the crank shaft 34 which is supported for rotation in suitable bearing.

blocks

35 and 35 which are fixed to and extend upwardly from the beam 29. At its intermediate crank portion, the crank shaft 34 is pivotally connected to a connecting rod 37 which is in turn pivotally connected at 38 with the top end of a vertically displaceable plunger 36 whose axis coincides with the central axis of the ladle 26, if desired. The beam 29 is formed with a suitable opening carrying a bushing or the like through which the plunger 36 is guided for vertical reciprocating movement. The lower end of the plunger 36 is pivotally connected at 40 and 40' with a pair of bell cranks 39 and 39', respectively, which are in turn pivotally connected at the intersections of their arms to supporting

brackets

41 and 41 fixed to and extending downwardly from the beam 29. Pivotal connections 42 and 42' are provided for the bell cranks 39 and 39 at the

brackets

41 and 41, respectively. The lower ends of the bell cranks 39 and 39' are respectively fixed to the top ends of a pair of

elongated stirrers

28 and 28 so that these stirrers, which may have an X- shaped cross section, respectively form extensions of the arms of the bell cranks which extend downwardly from the

pivots

42 and 42, respectively.

Thus, with this embodiment the stirrer means includes the crank drive which produces oscillatory swinging movement of the stirrers 28 and 28' back and forth about the pivots 42 and 42', and these stirrers 2S and 28' are symmeans acts only at the region of the surface of the molten metal such as molten pig iron, so that the cost of the apparatus and the cost for operating the same is relatively low, while at the same time a highly efiicient refining can be achieved even though the stirring is limited to the region of the surface of the body of molten metal. Moreover, the protective atmosphere in which the refining actions take place can be readily controlled because the refining reactions take place only at the region of the surface of the molten metal. In addition, full utilization is made of the purifying agent since there is substantiallv no loss thereof resulting from spattering or resulting from excessive amounts of purifying agent which do not enter into the reaction.

It is to be noted that the entire agitating action with the invention results solely from the movements within the body of molten metal which still persist plus the move ments derived from the stirring action, with this action being not only limited to the region of the surface of the molten metal but also a stirring action difierent from simple rotary movement of a stirrer about its own axis with the latter extending perpendicularly to the surface of the molten metal. Thus, in all cases except that illustrated in FIG. 4 the stirrers move bodily with respect to the vessel so that they have a motion according to which they are displaced in their entirety with respect to the vessel which contains the molten metal. In the case of FIG. 4, while the stirrer rotates about its own axis, this axis is not perpendicular to the surface of the body of molten metal so that the flat portions of the stirrer have vertical components of movement, rather than solely horizontal components of movement, resulting in a stirring action beyond that which would be achieved if the stirrer was simply rotated about its own axis with the latter axis extending perpendicularly to the surface of the molten metal.

The following are examples of desulfurization and dephosphorization of runs of molten pig iron provided in connection with the manufacture of steel. The molten pig iron is poured into a ladle in accordance with the method of the invention. The ladle has in these examples an internal diameter of 1800 mm., a depth of 2000 mm., and contains the molten pig iron which is in the vessel at a depth of 1800 mm. The charge in the ladle at each run for each example was 30 tons of molten pig iron.

Example 1 Desulfurization tests were carried outwith an apparatus having the construction shown in FIG. 1. The elongated stirrers had a length of 1000 mm., and three stirrers were used while an eccentric motion was imparted thereto. This eccentric motion had an eccentricity of mm. The results of the test runs are indicated in Table 1.

TABLE 1.RUN NO. 1

Concentration of S, percent Percent of Periods of Before After tlesullurtreatment,

Desullurizing agent (kg) treatment treatment jzing R.p.1n. min.

Carbide 8 0. 045 0. 009 80 10 Carbide 16 0, 032 0.003 01 120 10 Calcium nitride 1U 0. 028 0.003 80 15 metrically situated with respect to the axis of the vessel Example 2 26. It will be noted that in accordance with the invention the stirrers 28 also extend to only a small extent into the body of molten metal beneath the surface thereof so that the stirring is limited to the region of the surface of the body of molten metal.

Similar desulfurizing tests were made with the apparatus illustrated in FIG. 4. In this case the length of the stirrer was also 1000 mm., and of course only one stirrer was used. The depth of immersion was 200250 mm. Re-

Thus, with the method of the invention the stirring 7 sults of these tests are shown in Table 2.

C oncentration of sulfur, percent Percent of Angle of Period Before After desulfurinclination, treatment,

Desulfurizlng agent tkg.) treatment treatmentization Rpm, degrees min.

Carbide 8 0. 042 0. 010 76 95 25 10 Carbide 10 I 0. 036 0, 004 80 100 10 Calcium nitride 20 0. 034 0. 003 82 100 15 Example 3 to which the location of the entire stirrer with respect to Dephosphorization tests were carried out with the stirring apparatus shown in FIG. 3. The stirrer was rotated at 110 r.p.m., and the dephosphorizing agent was quick lime and a highly basic sintered ore (having a composition of FeO 50%, Fe O 42.7%, SiO 3.5%, CaO 41.7% and CaO/SiO 11.9%). Oxygen was ejected through a lance opening into the hollow cylindrical stirrer. The results obtained were plotted on a graph to provide the curves shown in FIG. 7.

Example 4 The results of dephosphorizing runs using the apparatus shown in FIG. 4 were plotted in a graph to provide the curves of FIG. 8. The angle of inclination of the stirrer was 20 with respect to a vertical line, and the stirrer was rotated at a speed of 130 r.p.m. while oxygen was admitted to the space beneath the lid through a suitable lance.

While in the above examples reference is made to desulfurization and dephosphorization of molten pig iron, the invention can equally well be applied to the refining of other molten metals such as iron alloys, and in addition it can be used for adjusting the composition of metal alloys such as iron alloys.

What is claimed is:

1. In a method of refining a molten metal which contains impurities, the steps of pouring the molten metal into a vessel, to provide therein a body of molten metal, parts of which are in motion due to the pouring of the molten metal, adding to the molten metal a purifying agent which reacts therewith to eliminate at least part of the impurities therefrom, and stirring the molten metal, to which the purifying agent has been added, only in the region of the surface of the molten metal, said steps of adding the purifying agent and stirring the molten metal being performed after the metal is poured into the vessel during a period of time when the parts of the body of molten metal within the vessel are still in motion, due to the pouring of the molten metal, and the entire relative movement between the purifying agent and the molten metal being derived solely from the movement of the parts of the body of molten metal and the stirring action, said stirring action being carried out with at least one stirrer extending into the molten metal only to a depth of 10-30% of the total depth thereof with the stirrer having a bodily movement with respect to the vessel according the vessel continuously changes.

2. In a method as recited in claim 1, said metal being molten pig iron and said period of time having a duration of at least one hour.

3. In a method as recited in claim 1, said metal being molten pig iron and said purifying agent being a desulfurizing agent.

4. In a method as recited in claim 1, said metal being molten pig iron and said purifying agent being a dephosphorizing agent.

5. In a method as recited in claim 1, said step of stirring the molten metal being carried out with stirrers which extend into the molten metal perpendicularly to the surface thereof and which carry out eccentric movements with respect to the vessel according to which the stirrers are displaced in their entirety along a given closed path with respect to the vessel.

6. In a method as recited in claim 5 and wherein said stirrers swing toward and away from each other respectively about horizontal axes.

7. In a method as recited in claim 1 and wherein said stirrer carries out an eccentric movement with respect to the vessel and is in the form itself of a hollow enclosure to which the purifying agent is confined.

References Cited UNITED STATES PATENTS 2,319,402 5/1943 Heller 26634 3,392,009 7/1968 Holmes -59 3,459,536 8/1969 Touzalin 75-45 3,334,993 8/1967 Nojima 75-45 2,290,961 7/1942 Heuer 7549X 2,397,737 4/1946 Heuer 75-55 3,278,295 10/1966 Ostberg 7561 FOREIGN PATENTS 684,048 12/1952 Great Britain 7593 242,012 1/1963 Australia 7561 L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE Certificate Patent No. 3,592,629 Patented July 13, 1971 Ryo Ando, Tsutomu Fukushima, Eiichi Hiraguchi, and Kokichi Hagiwara Application having been made by Ryo Ando, Tsutomu Fukushiina, Eiichi Hiraguchi, and Kokichi Hagiwara, the inventors named in the patent above identified, and Nippon Kokan Kabushiki Kaisha, Tokyo, J apen, a corporation of Japan, the assignee,

for the issuance of a certlficate under the PIOVISIOHS of Title 35, Section 256, of the United States Code, deleting the names of Kokichi Hagiwam and Eiichi Hiraguchi as joint inventors, and a, showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 23rd day of May 1972, certified that the names of the said Kokichi Hagiwara and Eiichi Hiraguchi are hereby deleted from the said FRED W. SHERLING Associate Solicitor.