US3330900A

US3330900A - Molten metal stirring and vacuum degassing

Info

Publication number: US3330900A
Application number: US396670A
Authority: US
Inventors: Kendrick C Taylor; Theodore R Kennedy
Original assignee: Pennwalt Corp
Current assignee: Pennwalt Corp
Priority date: 1964-09-15
Filing date: 1964-09-15
Publication date: 1967-07-11
Anticipated expiration: 1984-07-11
Also published as: GB1111674A; DE1296754B

Description

yi 1967 v KQC. TAYLOR 3,330,900

MOLTENMETAL STIRRING AND VACUUMDEGASSING Filed Sept. 15. 1964 42/ I r 52 L 7 INVENTORS V THEODORE IRKE/V/VED) RENO/PICK C. MVLOR ATTOR/VEVJ United rates Patent @hfice 3,330,900 Patented July 11, 1967 3,330,900 MOL'IEN METAL STIRRING AND VACUUM DEGASSING Kendrick C. Taylor, Oreland, Pa, and Theodore R.

Kennedy, Burlington, N.J., assignors to Pennsalt Chemical Corporation, Philadelphia, Pa, a corporation of Pennsylvania Filed Sept. 15, 1964, Ser. No. 396,670 15 Claims. (Cl. 1328) In'general, this invention relates to a new and improved method and apparatus for molten metal stirring. More particularly, it is directed to the use of polyphase stirring of molten metal in a ladle to produce unidirectional stirring of molten metal therein.

The benefits of stirring molten metal for alloying vacuum, deoxidizing, degassing and the like have been known for many years. For various reasons, it is desirable that this stirring should take place in the teeming ladle.

Since practically all ladles used in a modern steel plant are essentially a substantial steel shell with a suitable refractory liner, the application of stirring forces by electromagnetic means was discouraged by the shielding action of the steel ladle shell. Further, it was generally considered hazardous and impractical to place effective induction winding within the refractory liner of the ladle shell.

Further, there is a need for a molten metal stirrer which can be utilized in a modern steel mill without the necessity of redesigning equipment presently in service. The molten metal stirring apparatus must further be easily transportable and capable of being utilized on various types of equipment,

In view of the foregoing, it is the general object of this invention to provide a new and improved molten metal stirrer.

A further object of this invention .is the provision of a new and better molten'metal stirrer which can be utilized with any teeming ladle.

Still another object of this invention is to provide a new and better molten metal stirrer utilizing electromagnetic forces.

A still further object of this invention is the provision of a new and better electromagnetic molten metal stirrer wherein minimum heating of the stirrer is achieved with maximum utilization of the polyphase power supplied.

A further object of this invention is the provision of a new and better molten metal stirrer capable of achieving stirring forces having a significant vertical component, which component is reversible.

Yet another object of the present invention is to provide a novel vacuum degassing and molten metal stirrer.

Other objects will appear hereinafter.

For the purpose of illustrating the invention, there are shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangement and instrumentalities shown.

FIGURE 1 is a cross sectional View of a vacuum degassing furnace utilizing the molten metal stirrer of the present invention.

FIGURE 2 is a crosssectional view of a second type of molten metal stirrer utilizing the principles of the present invention used to stir molten metal in a teeming ladle.

Y FIGURE 3 is a phase time diagram of the four phase electrical power system of FIGURES 1 and 2.

In FIGURE 1, there is shown apparatus incorporating the principles of the present invention.

The apparatus 10 includes a vacuum chamber 12 having a cup-shaped .bottom portion 14 and a cover or lid 16. The cover 16 has a peripheral flange 18 cooperating with a peripheral flange 20 on the upper edges of the bottom portion 14 to form a vacuum seal. The vacuum chamber 12 has a steel outer lining 22 and a refractory inner liner 24. A vacuum pump 26 has its inlet conduit 28 connected to a suitable vacuum conduit 30 at the base of the bottom portion 14.

Within cup-shaped bottom portion 14 there is provided a suitable metal ladle receiving stand 32 having a ladle recess 34 for receiving a suitable ladle 36. The ladle 36 has a steel outer shell 38 and a refractory inner liner 40. Molten metal 42 is placed in the ladle 36 for degassing purposes. The ladle 36 is supported on the stand 32. by

suitable trunnions

44 and 46 which rest on the upper surface of the stand 32. The apparatus hereinabove described is for degassing molten metal. That is, the vacuum pump 26 withdraws the atmosphere from the chamber 12 so as to surface degas the molten metal 42 in the ladle 36, In order to better aid the area degassing, electromagnetic stirring has heretofore been provided by the provision of coils mounted on the stand 32 which were energized by alternating current. However, such apparatus was inefficient in that the steel shell 38 would absorb the electromagnetic energy prior to its passage into the molten metal thus limiting the efliciency of the apparatus.

For this reason, the magnetic stirrer 48 of the present invention has been provided. The magnetic stirrer 48 is mounted on the inner surface 24 of the lid 16 by a plurality of supports 50. The magnetic stirrer 48 is fingershaped and comprises a replaceable outer cup-shaped shell 52. The replaceable cup-shaped shell 52 acts as a protective sleeve for the magnetic stirrer 48 and must be manufactured of a material which is thermal shock resistant, a poor heat conductor, and a good electrical insulator. One such material would be cast sintered silica. A graphite based silica carbon would also be a desirable material for the sleeve. In view of the extremely high temperature conditions which the protective sleeve or shell 52 must operate under, the shell should be inexpensive enough to be replaced after a few uses.

A preferred construction for the sleeve 52 comprises a ceramic sleeve with a ceramic plug at the bottom. This may be of the tar bonded type which will give it added strength and prevent cracks from developing, through which the molten metal could run.

The sleeve 52 may also be made of graphite with a closed end. Such a sleeve could be machined in one solid piece and then covered with a wash of alumina or similar material to prevent the graphite from being attached by the molten metal. A conventional refractory may be inside the graphite sleeve to thermally insulate the coil.

The shell 52 is cased about a suitable protective liner 54 which further acts as a former for the inductive coils mounted within the finger 48.

Four

coils

56, 58, 60 and 62 are mounted within the liner 54 concentric with the vertical axis of the protective liner 54. The four

coils

56, 58, 60 and 62 are connected to a polyphase low frequency source generally designated by the numeral 64. The

coils

56, 58, 60 and 62 are so connected by their passage through the lid 16 to bushings (not shown) mounted on the outer surface 22 of the cover 16. The low frequency source 64 is denoted schematically in FIGURE 1. By a low frequency source is meant a source operating within the frequency range of .1 cycle to 60 cycles per second. At the higher range of these he quencies, heating is effected of the molten metal. At the lower frequencies, maximum force is obtained for better stirring. However, it should be understood that the frequency utilized by the source 64 will be within the current, voltage and power ranges suitable for the practical stirring application.

However, in designing a polyphase stirring system to 3 7 meet the requirements discussed previously there are conditions to be met. Initially, the frequency of the alternating magnetic field achieved by the

coils

56, 58, 60' and 62 must be chosen so that the depth of current peneration will be less than the thickness of the molten metal bath to be stirred. The depth of current penetration d is defined by the well known relationship d (inches) =1.985\/L where 1- is the resistivity of the molten metal 42 in microhm centimeters, is the magnetic permeability of the molten metal 42 and f is the frequency of the polyphase low frequency source 64. The factor 1' is of the order of two hundred microhm centimeters for molten steel and ,u is essentially unity for molten metals. The dimensional value of current penetration d represents current peneration into the thickness of the molten metal 42 surrounding the shell 52. That is, the thickness of the metal 42 is equal to the inside radius of refractory liner 40 minus the outside radius of refractory shell 52. The ratio of d to the radius of the inner surface of the ladle should vary bewhere K is a ratio, d is the depth of peneration for the liner 5,4 material, r is the mean radius of the liner 54, and i is the thickness of the liner 54. It has been found by tests that a value of approximately 10 for K in Equation 2 is a reasaonable and workable balance between the heat produced in the liner which is considered wasted, and the stirring effects produced in the molten metal as discussed above. The value of K greater than 10 is advantageous and a value as low as 5 may be tolerable.

Thus, Equation 2 can be rewritten as follows:

d 2 t (inches) or preferably 0.47, t, Hm (inches) (4) where 'r is the resistivity of the liner 54 in microhm centimeters, and ,u is the magnetic permeability of the steel of the liner 54.

Under the conditions of Equation 4 the energy wasted in the liner 54 might be half the power inducedin the molten metal 42, but such conditions are operative.

The proportions of the inducing coils 56, 58, 60 and 62 likewise require proper selection. Aside from providing the proper number of turns, insulation, etc., adequate for the input conditions, it'is advantageous to arrive at a means for determining the number of coils to be used together with the number of phases and the order of connection. Certain considerations would normally indicate that a large number of coils and phases is desired. However, in practice, it is more economical to use coils appropriate to two phase or three phase supply systems. It is an advantage under certain conditions to use the normal complementary four phase or six phase system so that the inducing coils as a complete unit generate no sig nificant currents in the metallic structures that enclose the stirring assembly. For example, it is desirable to prevent the generation of any significant currents in the steel shell 22 of the vacuum chamber 12. It is herein under-' stood that the four phase system means that there is one mangetically reversed coil for each directly connected phase coil for a two phase supply and a six phase system coil progression. A complete set of coils may add up to 7 an integral multiple of three hundred and sixty electrical degrees. The distance representing three hundred and sixty electrical degrees or 1 cycle indicates the linear phase velocity. For example, if the frequency were 1 cycle per second and the

coils

56, 58, 6t) and 62 necessary to make up the three hundred and sixty degrees were spaced over a one foot distance, then the phase velocity would be one foot per second.

Although complete sets of coils adding up to integral multiples of three hundred and sixty electrical degrees are desired, a three phase system with phase coils spaced sixty electrical degrees apart could be used. The sixty degrees spacing would be achieved by the utilization of a reversed middle coil. This three phase system would add up to only electrical degrees and thus would induce an instantaneous magnetic field which might elfect surrounding structures or cause undesired heating of the molten metal in the ladle, but would still be elfective to achieve the desired stirring.

It has been found that for producing linear motion of the molten metal 42 in the ladle 36, relatively short phase coils are most effective. Analysis of this result indicates that adjacent phase coils, short in axial length relative to their diameter, have an increased degree of mutual inductance. The magnetic field mutually shared by successive coils is a measure of the translational energy appearing as a unidirectional force on the molten metal 42. This is certainly desirable. Hence it has been found that coils should be no longer axially than about the radius thereof and may decrease to lower lengths within practical limits.

If the phase coils become too short (the axial length thereof decreaseslwithout increasing the number of electrical phases, there comes a point where the complementary or reversed phase coil reduces the net flux intercepting the molten metal to an impractical value. The net flux intercepting the molten metal then only exists as shallow whorls progressing along the outer surface of the cup-shaped shell 52 of the magnetic stirrer 48 in a systematic manner, but without sufficient coupling within the molten metal 42. 7

Thus, spacing between oppositely polarized coils of the same phase should, therefore, be such that the distance between the axial centers of the said polarized coils (56, 60 or 58, 62), should not be less than twice the sum of the radial distance between the outside of the coils and the outside of the shell 52 plus one-fifth the depth of current penetration d into the molten metal as per Equation 1. For assemblies using the same number of coils as the electrical phase number, the axial coil length may range between the value of the radius of the coil down to a value equal to the radial distance between the coil and the molten metal. In this last assembly, it is assumed that the coils are connected for consistent progression in electrical degrees but there are no oppositely polarized coils.

However, as stated previously, the preferred embodiment ofthe present invention utilizes oppositely polarized coils so as to prevent the generation of significant currents in the metallic structures that might enclose the stirring assembly. 7

In operation, the lid 16 supporting stirrer48 is placed on bottom portion 14 in vacuum sealing relation. The vacuum chamber 12 is evacuated by pump 26 and stirrer 48 is energized. As the molten metal 42 is stirred gas entrained therein rises to the surface and is drawn off by the pump 26. The stirring action increases the rate at which the molten metal 42 may be degassed, and also increases the effectiveness of the degassing.

In FIGURE 2, there is shown a second embodiment of the present invention in which a magnetic stirrer 48 is utilized with a teeming ladle 36 to stir molten metal 42'. The teeming ladle 36' rests on a suitable ladle receiving stand 32'. The ladle 36 is not in a vacuum chamber, but is still desirable to stir the molten metal 42 for reasons such as the adding of alloying materials or the like. The molten metal stirrer 48' is carried by a hook 66 connected to an overhead crane in a steel mill. Thus the magnetic stirrer 48 can be moved about a steel mill to wherever it is needed. The hook 66 is connected to suitable cables 68 secured to the outer edges of a spider 70 spanning the top of the molten metal stirrer 48'. The stirrer 48 has a replaceable outer cup-shaped shell 52' and an inner protective liner 54 similar to the shell 52 and liner 54 of FIGURE 1.

Coils

56', 58, 60' and 62 are mounted within the liner 54 and connected to a suitable source of polyphase low frequency power in the manner of

coils

56, 58, 60 and 62 of FIGURE 1. Additionally, the spider 70 supports a ferromagnetic core 72 which extends axially through the coils 56', 58, 60' and 62'. The core 72 improves the efficiency of the magnetic stirrer by substantially increasing the flux generated by the polyphase power supply.

The design of the coils 56', 58', 60' and 62' and the protective liner 54' fall wtihin the design criteria discussed with respect to the embodiment of FIGURE 1. In both embodiments, all coils have been considered to be substantially the same axial length, but it will be understood that they could vary diametrically to accommodate a taper of the magnetic stirrer 48 or 48' or a taper of the ladles 36 and 36'.

Thus, better molten metal stirring apparatus has been achieved which can be easily transported to any part of a steel mill. Additionally, the coils could be water cooled by providing suitable Water conduits to the magnetic stirrer. The molten metal stirrer of the present invention achieves better stirring by utilizing a polyphase supply which eliminates extraneous magnetic fluxes and mixes the molten metal in one direction uniformly from top to bottom of the ladle. By merely reversing the energization of the phase coils, the direction of mixing can be reversed. Further, heating of the protective liner 54 has been minimized by designing the thickness of the liner in accordance with predetermined specifications.

Further, efficient operation of the polyphase mixing has been enhanced by setting ratio of the depth of current penetration in the molten metal to the radius of the inside surface of the ladle betwen .3 and .9 and preferably between .5 and .8. Oppositely polarized coils have been spaced from each other a distance not less than twice the sum of the radial distance between the outside of the coil and the outside of the refractory liner of the magnetic stirrer was one-fifth the depth of current penetration into the molten metal. Still further, the coils have been designed with an axial length equal to or less than the radius of the coils.

Thus, molten metal mixing has been achieved in an economical and practical manner by staying within the guidelines set forth.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. A magnetic stirrer comprising a plurality of spaced electrical coils mounted within a protective sleeve, said sleeve enveloping said coils so that matter to be stirred cannot flow through coils, said sleeve having a longitudinal axis, mounting means, said mounting means mounting said sleeve with its longitudinal axis disposed vertically, said stirrer having a circular cross section, said spaced electrical coils being mounted within said sleeve concentric with and spaced along said sleeve longitudinal axis, said protective sleeve being cup-shaped, the outer surface of said cup-shaped sleeve being made of a thermal shock resistant heat and electric insulating material, said sleeve including a liner on the inner surface thereof, said liner acting as a former for said coils, said liner having a thickness less than d /5r in inches where d is the depth of current penetration for the liner material and r is the mean radius of the liner.

2. The magnetic stirrer of claim 1 wherein the thickness of the liner is equal to or less than oA-T /l-b l' f in inches where T is the resistivity of the liner in microhm centimeters, ,u is the magnetic permeability of the liner material and f is the frequency of the polyphase current in the coils.

3. A magnetic stirrer comprising a plurality of spaced electrical coils mounted within a protective sleeve, said sleeve enveloping said coils so that matter cannot flow through the coils, said sleeve having a longitudinal axis, mounting means, said mounting means mounting said sleeve with its longitudinal axis disposed vertically, said stirrer having a circular horizontal cross section, said spaced coils being mounted within said sleeve concentric with and spaced along said sleeve longitudinal axis, said electrical coils including two sets of coils, each coil in one set of coils being provided with a respective magnetically reversed coil in the second set of coils, associated coils in said first and second set being spaced axially one from another with at least one non-associated phase coil being placed between each pair of associated coils in consistent phase progression, said coils adding up to integral multiples of one cycle of the applied electric power.

4. The magnetic stirrer of claim 3 wherein each of said coils has an axial length less than the radius of the coil.

5. The magnetic st-irrer of claim 4 wherein the distance between axial centers of associated coils is equal to or greater than twice the sum of the radial distance between the outside of the coils and the outside surface of the protective sleeve plus one-fifth the depth of current penetration int-o molten metal to be stirred, the depth of current penetration in inches being equal to where 1- is the resistivity of the molten metal in microhm centimeters, and ,u. is the magnetic permeability of the molten metal.

6. A magnetic stirrer comprising a plurality of spaced electrical coils mounted within a protective sleeve, said sleeve enveloping said coils so that matter cannot flow through said coils, said sleeve having a longitudinal axis, mounting means, said mounting means mounting said sleeve with its longitudinal axis disposed vertically, said stirrer having a circular horizontal cross section, said spaced electrical coils being mounted within said sleeve concentric with and spaced along said sleeve longitudinal axis, each of said coils having an axial coil length equal to a value between the coil radius and a value equal to the radial distance between the coil and the outside surface of the protective sleeve, the number of coils being equal to the phase number of an applied low frequency polyphase current.

7. Apparatus comprising a container adapted to contain molten metal, said container having an opening therein, a magnetic stirrer extending into said container to stir molten metal in said container, said magnetic stirrer comprising a plurality of spaced electrical coils, a protective sleeve, said protective sleeve enveloping said coils so that molten metal within said container cannot injure said coils, and a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in phase angles, means for supporting said stirrer in said container spaced from the bottom of said container, said means supporting said stirrer with the longitudinal axis of said stirrer disposed vertically, said polyphase supply system frequency being chosen so that the depth of current penetration of molten metal between the container and the sleeve is less than the thickness of the molten metal to be stirred, said depth of current penetration being equal to a (inches) 1.9s I af where 1- is the resisitivity of the molten metal in microhm centimeters, ,u. is the magnetic permeability of the molten metal, and f is the frequency of the polyphase supply system.

8.The molten metal stirring apparatus of claim 7 wherein the ratio of the depth of current penetration of the molten metal to the radius of the inner surface of the container varies between the limits of .3 and .9.

9. Molten metal stirring apparatus of claim 8 wherein the ratio of the depth of current penetration of the molten metal to the radius of the inner surface of the container varies between the limits of .5 and .8.

10. Apparatus in accordance with claim 7 wherein said container is mounted in a vacuum chamber, said chamber including vacuum pump means to evacuate said chamber.

11. Apparatus in accordance with claim 7 wherein said supporting means comprises a vacuum chamber enclosing said container and stirring apparatus, and pump means for evacuating said vacuum chamber.

12. Apparatus comprising a container adapted to con 8 tain molten metal, a magnetic stirrer extending into said container to stir molten metal in said container, said magnetic stirrer comprising a plurality of co-axial spaced electrical coils, a protective sleeve, said spaced electrical coils being mounted within said sleeve concentric with and spaced along the sleeve longitudinal axis, said sleeve having a circular transverse cross section and being closed at one end thereof, a protective liner mounted within said sleeve co-axial of said longitudinal axis, said protective liner forming said coils, and a low frequency polyphase supply system supplying electrical power to said coils, said coils progressively varying in electrical phase angle.

13. Apparatus in accordance with claim 12 wherein said sleeve is made of a thermal shock resistant heat and electrical insulating ceramic material.

14. Apparatus in accordance with claim 13 wherein a ceramic plug is mounted in the closed end of said sleeve.

15. Apparatus in accordance with claim 13 wherein said sleeve is made of a graphite material with an inner sleeve of refractory material.

References Cited UNITED STATES PATENTS 2,826,666 3/1958 Cater 219l0.65 X

2,972,652 2/1961 Seemann et al. 1331 X 3,053,921 9/1962 Tagliaferri 1328 3,251,921 5/1966 Hartley 1 328 FOREIGN PATENTS 1,269,156 7/ 1961 France.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

Claims

12. APPARATUS COMPRISING A CONTAINER ADAPTED TO CONTAIN MOLTEN METAL, MAGNETIC STIRRER EXTENDING INTO SAID CONTAINER TO STIR MOLTEN METAL IN SAID CONTAINER, SAID MAGNETIC STIRRER COMPRISING A PLURALITY OF A CO-AXIAL SPACED ELECTRICAL COILS, A PROTECTIVE SLEEVE, SAID SPACED ELECTRICAL COILS BEING MOUNTED WITHIN SAID SLEEVE CONCENTRIC WITH AND SPACED ALONG THE SLEEVE LONGITUDINAL AXIS, SAID SLEEVE HAVING A CIRCULAR TRANSVERSE CROSS SECTION AND BEING CLOSED AT ONE END THEREOF, A PROTECTIVE LINER MOUNTED WITHIN SAID SLEEVE CO-AXIAL OF SAID LONGITUDINAL AXIS, SAID PROTECTIVE LINER FORMING SAID COILS, AND A LOW FREQUENCY POLYPHASE SUPPLY SYSTEM SUPPLYING ELECTRICAL POWER TO SAID COILS, SAID COILS PROGRESSIVELY VARYING IN ELECTRICAL PHASE ANGLE.