US2871008A

US2871008A - Apparatus for gas flushing of molten metal

Info

Publication number: US2871008A
Application number: US480542A
Authority: US
Inventors: Spire Etienne
Original assignee: Air Liquide SA
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1950-11-02
Filing date: 1955-01-07
Publication date: 1959-01-27
Anticipated expiration: 1976-01-27

Description

Jan. 27, 1959 E. SPIRE Original Filed Nov. 2, 1950 $1 v 5 A v d J 8 ,4 M

INVENTOR ETIENNE SPIRE BY A 14.

ATTORNEYS APPARATUS FOR GAS FLUSHING OF MOLTEN METAL 2 Sheets-Sheet 1 E. SPIRE 2,871,008

Jan. 27, 1959 APPARATUS FOR GAS FLUSHING OF MOLTEN METAL 2 Sheets-Sheet 2 Original Filed Nov.'2, 1950 INVENTORY ETIENNE SPIRE avd m, flvdew ms Unite APPARATUS FOR GAS FLUSHING OF MOLTEN METAL Etienne Spire, Montreal, Canada, assignor to lAir Liquide, Societe pour lEtude et lExploitation des Procedes Georges Claude Original application November 2, 1950, Serial No. 193,581, now abandoned. Divided and this application January 7, 1955, Serial No. 480,542

Claims priority, application France November 3, 1949 Claims. (Cl. 26639) of a porous material, through which a gasmay be blown into the liquid contents of such a vessel, particularly with the purpose of bringing about intimate contact between the insufiiated gas and the contents of the vessel, and also with the purpose of agitating a metallurgical fusion bath so as to produce intimate contact between the constituents of this bath as well as between these constituents and a second phase of the charge, for instance a slag.

A first object of the invention is to provide an apparatus of the above type, capable of satisfying the severe requirements to be met when such apparatus is intended for regular use in the metallurgical industries, particularly in the steel making industry, where the apparatus must usefulness out of a given volume of gas, particularly when u this gas isa comparatively costly one, such as for .instance argon or helium.

. Another object is to provide, for the stirring of a metal.

lurgical fusion bath, an apparatus which is simpler and less expensive to operate than competitive devices such as rotating furnaces and electromagnetic stirring devices, while achieving comparable or even better efficiency.

The apparatus according to the present invention comprises a vessel having an outer shell and a refractory lining, a porous refractory plug inserted in this lining, in the bot-tom of the vessel, and a gas chamber with a gas inlet across the outer shell of the vessel.

The porous plug should be permeable to gases but impervious to the molten metal which is to be treated;

A satisfactory range of porosity is from about .1 to about 5 litres per minute, per square centimeter of surface, per centimeter of thickness, per centimeter mereury of pressure. A preferred range is from about 0.1 to about 1.5 litres per minute per square centimeter of surface, per centimeter of thickness, per centimeter mercury of pressure. These calibrations are as measured at room temperature with air as the gas.

When the apparatus is to be used for stirring a fusion bath, the applicant has found that the flow ratemust be suflicient to agitate the molten metal so that there is at least a mild surging action, as for example when a kettle containing Water starts to boil, to a violent boiling action which should not be suflicient to splash the metal out of the container. The applicant has found that a satisfactory rate of flow is from about 0.2 litre to about 2 atent on e resist considerable heat, as well as thermal shocks and 2,87hhh3 2 litres per minute, per square centimeter of the plug area, for a plug havinga surface area of about square centimeters, in the case of a charge from about 25 to about 30 centimeters in depth.

. The refractory medium is non-reactive with the metal or gas and is homogeneous in the sense that the pores are substantially equally distributed over its surface and sufliciently spaced apart that separate bubbles are formed in the metal by the gas emerging from the refractory. The bubbles are so spaced that they act as discrete bubbles. The refractory has substantially no crust which, if present, would tend to give unevenness in porosity. The factors described are important in causing a large surface area of the gas to be presented to the metal as opposed, for example, to the relatively small surface area presented by the large bubbles emerging from the end. of a tube or lance, as taught by the prior art.

This apparatus is also useful for flushing a molten metal with inert gas, for example argon, for removing dissolved undesirable gases, as well as for accelerating the dissolution of an alloy addition, or for homogenizing the composition or the temperature of the charge, or both. Flushing may also be advantageously carried out by means of the apparatus of the invention for removing solid impurities from a molten metal, the process being somewhat similar to the process of froth flotation in the treatment of metallic ore As regards the nature of the porous refractory, alumina, silicon carbide, graphite,magnesia and fire clay base materials give satisfactory results when using inert gas. Silicon carbide is a preferred material.

The vessel is preferably a portable ladle, in which the metal may be subjected to gas insufliation while the ladle is being transferred from the furnace to the mold. A

container is preferred whose depth is at least twice its surface which is exposed for direct contact with the liquid contents of the vessel. should preferably beflush with the surface of the adjacent lining. The side faces of the plug are in contact with the adjacent lining, to which they may be sealed by an appropriate cement such for example as a chromium-oxide base cement containing a small amount of bentonite and sodium silicate. A gas chamber is pro vided upstream of the rear face of the plug, that is the one opposed to the face which comes into contact with the. liquid contents of the vessel. This gas chamber is provided with a gas inlet across the outer shell of the vessel.

The apparatus mayalso comprisesupporting means for the porous plug.

A preferred embodiment of the invention is one in which the porous plug is frusto-conical in shape, as this shape helps to maintain a tight joint between the plug and the adjacent material. Preferably, the plug is self-sustaining, and should retain this feature even after being in contact with molten metal for a long time, for instance half an hour or more.

Another embodiment ofthe invention is one in which the side wall of the porous plug is impermeable to gases, so as to prevent the gas from escaping through the side wall of the plug into the adjacent lining.

A preferred embodiment is one in which the side wall of the plug is enveloped with a steel shell having the same contour as the wall and sealed to it with an appropriate refractory cement. The thickness of this shell is preferably of about A or less.

The invention will be more fully understood by referring to the appended drawing, in which Figure I ha cross-section along the vertical axis of a ladle embodying a refractory porous element at its base.

This face of the plug Figure 2 is a similar view of ladle wherein the gas chamber is of smaller size. i

Figure 3 shows a plug having its side wall fitted with a steel shell.

Figure 4 shows a plug having only the lowerpart of its side wall fitted with a steel shell.

Figure 5 is a vertical axial cross-section of a ladle in which the porous plug isfitted with a steel shell and held in place by a screw applying pressure at the base of the plug.

Figure 6 is a vertical axial cross-section of a ladle wherein the'porous plug is encased in a steel shell having a corrugated side wall.

Figure 7 is a vertical axial cross-section of a ladle in which a socket receiving the porous plug is removably secured to the bottom of the ladle.

In Figure l, a ladle 1 comprises a metallic outer shell 2 and a refractory lining 3, made for instance of fire clay. The base of the ladle is provided with a refractory porous plug 4 which has an upper face Ssubstantially llush with the adjacent inner wall of the lining. The side walls 6 of the plug are in close contact with the adjacent lining, to which they are preferably sealed by means of an appropriate refractory, cement, and the lower or outside face '7' of the plug is enclosed within the gas chamber 8. This gas chamber 8 is provided with a nipple 9 through which gas is admitted under pressure. This gas diffuses through the plug l and comes into contact with the molten metal at the'upperpartS of the plug. The gas is finely divided into small bubbles and flushes through the molten metal. The porous plug t is held in place by extensions lid of a lid 12 which closes the bottom of the gas chamber 8, to which it is removably attached by bolts 13.

In the modified embodiment of Figure 2, the gas chamher 8 is of smaller size and, instead of having a side Wall integral with the outer shell of the ladle, it is removably secured to the bottom of the ladle by means of bolts 3.4.

It is provided wtih an inner shouldered flange 15' for supporting the lower or upstream face of the porous plug.

As can be seen from Figures 1 and 2, the porous plug is frustrum shaped, and it is fitted in the ladle with the smaller base upwards. as shown in Figure 6, in which the larger-base is in contact with the molten metal.

It has been found that when operating with an apparatus such as shown in Figures 1 and 2, some of the insufflated gas has a tendency topa'ss between the lateral surface of contact 6 of the porous plug 4 with the ladle It and then to diffuse through the lining 3 and in the r intervals between the lining 3 and the outer shell 2 of the ladle, without having any flushing action in the molten metal 16. This can be avoided by reducing greatly the permeability of the side wall surface 6 of the porous plug 4 as shown in Figures 3 and 4. Figure 3 shows the porous plug l having its side surface coated with an impermeable coating 16 which has been prepared, for example, by glazing or by preparing a porous element having an outer layer less porous than the core.

The porous plug may be either prefired or in the chemically bonded stage, and hardened 'by drying in a moderate heat. An alternate method for scaling up the pores of the side wall. comprises impregnating this Wall with a refractory cement. The elimination of gas leakage through the side wall of the plug may also be obtained by enveloping this wall with a steel shell 16 which has the same contour as the wall and is scaled to it with a refractory cement.

For using the apparatus of the invention with an inert gas, the steel shell around the porous plug may have a thickness from about .008? up to about This shell may extend from one base of the frustrum to the other. If however the insuiliated gas is an oxidizing one, the steel shell around the plug maybe made to extend This position may be reversed from the lower plug base only up to about /2" from the top surface which is in contact with the molten metal, as shown Figure 4.

There are many advantages in employing the plug encased in a steel shell. it completely eliminates gas leakage from the side wall of the plug into the ladle lining as mentioned previously; it eliminates the necessity of having a dense layer or an impregnated surface on the wall of the plug to suppress leakage; it protects the plug during shipment, handling and assembly; it facilitates mounting of the plug in its holding means and protects the plug during the operation of ramming up the space between the plug and ladle lining. in the chemically bonded 'plug, it is possible to pack the ramming material directly into the steel shell. Thus, aside from the elimination of the firing operation, the operation of sealing the plug to the shell may be eliminated.

Further embodiments of the invention are shown in Figures 5, 6 and 7, in which the porous plug is held in its housing by'pressure. In Figures 5 and 7, a perforated metallic plate 17 is used for distributing the pressure uniformly across the section of the lower plug base 7. In Figure 5, pressure is exerted through a spider 19 held in place with a bayonet fit 20 extending out from the side wall 21 of the gas chamber 8. The pressure is applied by tightening-bolt 22 on the female thread 23 in the spider l9.

In using the larger base of the plug 4 in contact with the molten metal, such as in Fi ure 6, the pressure of the column of molten metal is used for holding down the plug 4. The steel shell 16 to prevent gas leakage may be in'the form of a cup with a perforated bottom 24 to allow passage of gas. This shell may also be provided with corrugations 25 along the side as illustrated in Figure 6, which help in retaining the porous material tightly in the shell.

When the plug is to be fitted within the ladle with its smaller base in contact with the molten metal, it is advantageous to provide means for allowing the plug to be inserted into and removed from the housing 21 through the gas chamber 8. Thus, in Figure 5, the bottom of the gas chamber consists of removable plate 12, held by bolts 13. In Figure 7, an arm 26 is pivoted and can revolve around a spacer 27 attached to the bottom of the vessel 2 and is held there by a bolt 2% and a nut 29. The arm 26 carries a bolt 30 which is swung into the position shown in Figure 7 when the assembly is in working position. Bolt 30 is tightened, pressing onto the flange 31 of the plug housing 21 for holding this housing in place. The plug housing 21 is similar to that shown in Figure 5, and supports the plug 4 by means of bayonet lugs 20, spider 19 and bolt 22.

The housing 21 is provided by a bottom lid 12 which is held in place by a second spider 32 and a bolt 33, the spider32 being removably supported by extension arms 34 attached to the housing 21. For protecting the portion of the plug assembly extending below the bottom of the ladle 1 and to allow this ladle to stand upright, the ladle may be provided with legs 35.

The apparatus according to the present invention has been found highly suitable for carrying into practice various metallurgical operations in which a gas must be insufflated into a molten bath of metal.

In addition to the flushing and homogenizing applications mentioned in the foregoing, the apparatus of the invention is specially suitable for agitating molten metal in contact with a second phase of the charge, for example slag, to remove undesirable constituents, especially sulphur or phosphorous in steel or iron. The amount of gas to be used is in relation to the weight of the molten metal. For example, 5 liters of inert gas per minute would be passed for kilograms of molten metal and 30 liters per minute would be passed for 1000 kilograms of molten metal during 5 to 20 minutes.

The pressure of the flushing gas is in relation with the height and density of the molten metal in the container and the permeability of the porous plug. It has been found that the flushing gases enter the molten metal at a temperature which is substantially that of the molten metal. This is due to the fact that the porou refractory medium absorbs some of the heat dissipated by the molten metal, thereby heating the flushing gas while it diffuses through this gas-porous refractory medium. One advantage of the apparatus of the invention is that the cooling rate of the treated metal is appreciably less than when the gas is introduced through a tube or lance, as in the prior art.

Another advantage is that the desiliconizing of pig iron can be readily done in a ladle, instead of doing so in the open hearth, thereby saving one operation.

It has also been found that the quality of the castings derived, for example, from a 3515 stainless steel which was flushed with argon was improved to a point where the castings were substantially free from porosities.

It is most desirable to effect the degassing treatment of the metal just prior to teeming and, for any metal, the shorter the lapse of time between completion of degassing and teeming, the more elfective will be the treatment.

In ferrous metals and alloys, the apparatus of the invention is especially useful for accelerating reactions of the molten metal with a second phase in contact with the metal, the rate of which is usually dependent upon diffusion. For instance, sulphur can be removed efiiciently and rapidly from pig iron and from steel, using an inert gas, such as for instance nitrogen, at least in the case of pig iron, and a desulphurizing slag. Sulphur removal in a melting furnace is a costly and lengthy operation since it depends on diffusion of the sulphur from the metal up to the surface in contact with the slag. By agitating the molten bath with gas insufilated by means of applicants apparatus, metal is continually brought into contact with the slag, thus eliminating to a great extent the factor of diffusion.

With the possibility of removing both sulphur and phosphorus in the specially designed ladle, it may be economical to line melting furnaces completely with acid refractory material instead of basic material to stand up to the highly basic slags which are necessary to reduce the sulphur and phosphorus impurities in steel.

Cast iron, similar to pig iron, may be treated with the apparatus of the invention to lower sulphur and phosphorus. The recent development of nodular cast iron may employ this apparatus with an appropriate slag to lower the sulphur content before inoculating with the nodularizing alloy since a large amount of the alloy added is used up in desulphurizing the metal. The introduction of an inert gas may also act as a blanket to protect the nodularizing alloy from reacting with the oxygen in the air. Recovery of such alloy, with present practice, is not greater than 15%; thus, protection of this alloy with an inert gas may greatly increase the recovery. Aside from probable increase in recovery, the

agitation mixes the bath uniformly and removes undesirable inclusions.

I claim:

1. Apparatus for stirring, in a metallurgical fusion bath, a molten metal phase in contact with a slag phase by insutllating a gas into said bath, said apparatus comprising, in combination, a refractory lined vessel having a depth at least equal to its width, an homogeneous self'sustaining gas-permeable ceramic refractory block impervious to molten metals, sealed in an aperture in the central portion of the bottom of said vessel, said block having an upper face substantially level with the vessel lining, the area of said face being less than about one eighth of the surface of the vessel bottom, the side wall of said block being provided with a gas-impervious peripheral coating, and means for insufilating a gas into said vessel through said permeable block.

2. Apparatus according to claim 1, in which the vessel depth is at least twice the width of said vessel.

3. Apparatus according to claim 1, in which the ratio between the inner cross-sectional area of said vessel at its normal filling level and the area of the exposed face of the porous block in the bottom of said vessel is at least equal to 36.

4. Apparatus according to claim 1, in which the porous block is removably sealed to the vessel lining, and the coating over the side wall of the porous block is a shell of thin steel sheet, permanently cemented to said block. I t

5. As a new article of manufacture, a self-sustaining homogeneous ceramic refractory porous plug impervious to molten metals and adapted for being removably inserted in an aperture in the Wall of a metallurgical vessel for insufilating a gas into the contents thereof, said plug having a tapering side wall permanently cemented to a close-fitting steel shell.

References Cited in the file of this patent UNITED STATES PATENTS 23,123 Stewart et al. Mar. 1, 1859 51,401 Bessemer Dec. 5, 1865 728,261 Mark May 19, 1903 1,390,694 Frowley Sept. 13, 1921 1,763,248 Moore June 10, 1930 1,836,196 Snelling Dec. 15, 1931 2,265,511 Brassert Dec. 9, 1941 2,510,932 Poland June 6, 1950,

FOREIGN PATENTS 2,432 Great Britain Sept. 18, 1857 498,983 Belgium Nov. 14, 1950 684,048 Great Britain Dec. 10, 1952 OTHER REFERENCES Pages 817-820 inclusive of Metals Process, Nitrogen Degassing of Non-Ferrous Metals, June 1949.