US2968685A - Apparatus for electro-magnetic stirring - Google Patents

Description

Jan. 17, 1961 J. TosTMANN APPARATUS RoR ELECTRO-MAGNETIC STIRRING Filed Feb T0 POWER(29 lUnited States Patent O APPARATUS FOR ELECTRO-MAGNETIC STIRRIN G Johannes Tostmann, Duisburg, Germany, assignor to Demag-Elektrometallurgie G.m.b.H., Duisburg, Germany, a corporation of Germany Filed Feb. 6, 1959, Ser. No. 791,702

9 Claims. (Cl. 13-26) This invention relates to a method and apparatus for stirring of molten metal by electromagnetic induction.

In metallurgical processes such as refining and alloying; the base metal is reduced to a molten mass and reaction products are added while the molten mass is stirred to facilitate and increase the speed of the reaction process.

Maximum surface contact between the molten metal and the added materials is desirable. Maximumm contact is most readily obtained by the use of a relatively vigorous and distributed stirring of the metal to establish circulating flow of the metal in vertical planes. The vertical ow of the molten metal promotes movement of the reaction products, which tend to accumulate on the upper surface of the molten metal, downwardly into the molten metal.

The metal is reduced to a molten mass in any suitable manner such as by arc furnaces, resistance furnaces, or coreless induction furnaces. The stirring of the metal in an arc furnace is particularly desirable because the stirring not only facilitates the desired reaction of the metallic charge but also prevents excessive overheating of the molten charge immediately adjacent the arcs.

A particularly practical method of effecting the movement of a molten mass is to provide induction stirring coils adjacent the furnace. The coils are inductively related to the molten mass and establish flux ilow within the molten mass which induces eddy currents in the mass. The reaction between the main magnetic field and the induced current establishes forces within the molten mass which cause the mass to move in a predetermined manner. The degree of metal movement is directly proportional to the value of current in the nduction coils and inversely proportional to the frequency of the current which preferably is relatively low to provide suicient penetration of the flux flow into the molten mass. High current values are necessary to obtain vigorous stirring and the coils are normally formed of a tubular copper and water cooled to prevent destruction of the coils by the heat which results from ythe large current.

Generally, two different mounting arrangements of induction stirring coils have been employed. In both constructions, the furnace generally comprises a supporting refractory material supported between a steel jacket and an inner Crucible lining.

In one construction, the coils are embedded within the refractory material between the furnace liner and outer jacket. This arrangement provides close coupling to the molten mass. However, the coils cannot be replaced without relining the furnace. Further, if the lining fractures for any reason, the molten mass may work through the refractory material and come in direct contact with the water-cooled stirring coils. The high temperature may puncture the metal and cause a severe explosion. Thus, if the heating coil is punctured, the cooling liquid employed to maintain the coil at a relatively low temperature contacts the molten metal'. The high 2,963,685 Patented Jan. 17, 1961 temperature of the metal would rapidly vaporize the liquid causing a dangerous explosion.

An alternative construction which generally avoids the disadvantage of the above construction disposes the stirring coils beneath the furnace. In this construction, it has been found necessary to construct the furnace bottom of a suitable non-magnetic material to allow penetration of the flux into the molten mass. Such materials are very expensive and the furnace made in accordance with this construction is correspondingly expensive. Flu'- ther, the magnetic coupling between the coils and the molten charge is low and known systems of this variety are relatively inefficient.

In accordance with the present invention, a plurality of induction coils are individually disposed within suitable exterior recesses formed in the bottom of the mixing chamber to establish relatively close coupling to the molten mass. The outer surface of the mixing chamber is a suitable low-cost steel shell. The recesses, however, are provided with a suitable non-magnetic cap or jacket which is preferably formed of a highly temperature resistant material. The non-magnetic jackets permit penetration of the magnetic flux into the molten mass and require only a relatively small amount of non-magnetic material. Consequently, the cost of the furnace having exteriorly mounted stirring coils is substantially reduced without reducing the eliciency of the furnace. Further, the temperature resistant jackets protect the induction stirring coils from the heat of the molten mass and substantially eliminate or greatly reduce the necessity for water cooling of the coils. The lined recesses also protect the coils from overflowing molten metal and the like and therefore substantially reduce the danger of explosion due to contact between the coils and the molten metal. The jackets may be made with spaced walls to establish cooling chambers for circulation of a suitable cooling uid therethrough rather than through the heating coils.

The induction heating coils are suitably mounted upon magnetic core members to allow ready removal and repair of the induction coils. The maintenance, repair and exchange cost of the coil structure is thus maintained relatively low.

A primary object of the present invention is to provide an induction stirring system having 4active coils closely coupled to the molten mass in a relatively inexpensive furnace receptacle.

In accordance with another aspect of the present invention, a plurality of individual coils are distributed over substantially the entire bottom of the chamber. The coils are selectively connected to a power source through any suitable switching means to establish various sequences of energization or differences in the phase energization of the coils. This construction allows establishment of a great many' different directional movements of the molten metal mass and consequently allows rapid and complete mixing of the molten charge and the reaction products.

The drawing furnished herewith illustrates the best method presently contemplated for carrying out the invention.

In the drawing:

Figure l is a cross-sectional view of a cylindrical mixing furnace;

Fig. 2 is a bottom view taken on line 2 2 of Figure 1, with parts broken away to more clearly show the construction and location of the stirring coils;

Fig. 3 is `an enlarged fragmentary view of one coil section;

Fig. 4 is a schematic circuit for the multicoil induction stirring arrangement shown in Figs. 1 and 2; and

Fig. 5 is an enlarged fragmentary view similar to Fig. 3 illustrating an alternative stirring coil section.

Referring particularly to Figs. 1 and 2, a lining 1 of granular refractory material is supported within a cuplike shell 2 to form a furnace crucible adapted to receive and contain a molten charge 3.

The shell 2 is mounted within any suitable supporting means, not shown, and is formed of any suitabe inexpensive material such as sheet steel which has sufficient strength to support the refractory lining 1.

The refractory lining 1 is disposed within the shell 2 in a granular state and rammed into a compact mass. The molten metal charge 3 sinters the adjacent surface of the refractory material to form a relatively thin, hard inner coating 4. The outer portion of the lining 1 remains granular and serves as a thermal insulation and support.

The molten charge 3 may be formed within the con tainer by suitable resistor, arc or induction heating means, not shown, or may be disposed in the crucible as a previously formed molten mass for subsequent mixing with suitable reaction products, not shown.

Twelve induction stirring coils 5 are supported in distributed relation adjacent the bottom of the furnace. Referring particularly to Fig. 2, the coils 5 are cruciformly arranged with each cruciform arm 6 and '7 including two rows of coils 5 to substantially cover the entire bottom ofthe furnace.

An inter-connecting cruciform core structure for the coils 5 is supported beneath the `furnace by supporting legs 9.

The core structure 8 includes four parallel laminated yokes 10 which are shaped .to extend parallel to the curved bottom configuration of the furnace immediately beneath the parallel arranged coils 5. Magnetic poles 11 are integrally formed with the yoke members 1@ and project at right angles therefrom toward the furnace. The coils 5 are each wound on one of the poles 11 to reduce the magnetic reluctance of the magnetic circuit externally of the molten charge 3. Laminated crossyokes 12 are secured between the parallel yokes 1n of the core structure as by welding or a suitable clamping means, not shown, to reduce the reluctance to magnetic ow between the yokes 10.

Suitable energization of the induction coils 5 generates a changing or traveling magnetic flux owing from one pole 11 into the molten charge and from the charge back to the same pole through another of the poles 11 and the corresponding yokes 10. The varying flux in the molten charge induces circulating eddy currents, not shown, which co-act with the varying flux to establish forces acting on the molten charge. The forces are generally acting in vertical planes and consequently cause the molten mass to move in vertical planes to continuously carry the reaction products, not shown, from the upper surface into the charge 3.

Stirring coils 13, shown as four, are equicircumferentially spaced about the side o-f the furnace to improve the movement of the molten mass immediately adjacent the furnace Walls. The coils are wound on horizontally extending poles 14 which are secured to extensions of the cruciform core structure 8 to locate the coils 13 about the circumference of the furnace.

A plurality of recesses 15 are provided in the bottom and side of the crucible to receive correspondingly located induction heating coils 5 and 13 and poles 11 and 1d and thereby reduce the `spacing of the coils from the molten charge. In the illustrated embodiment of the invention, the poles 11 and 14 and corresponding coils 5 and 13 are partially disposed or arranged in the recesses 15.

Referring particularly to Figs. 2 and 3, a double-wall liner or jacket 16 is secured within each of the recesses 15 to protect the induction coils 5 and 13. rl`he jackets 16 are preferably of a highly temperature-resistant material such as a suitable ceramic material or an austenitic steel selected from austenitic chromium-nickel steel, austem'tic chromium-manganese steel, austenitic nickel steel and austenitic manganese steel. Such alloys are nonmagnetic and have a high specific resistance. Consequently, the magnetic field established by the induction stirring coils 5 and 13 readily penetrates through the jacket into the molten charge 3 and establishes the desired stirring action.

A pair of opposed flanges 17 and 13 are integrally formed on the exterior wall of each jacket 16 and apertured to receive a cap screw 19. The adjacent iron shell 2 is apertured and suitably threaded to receive the cap screws and rigidly clamp the jacket within the recesses 15. The flanges 17 and 18 are centrally located to support the jackets 16 with the outer edge of the jacket engaging the adjacent surface of the core structure and consequently to substantially encompass each coil within a protective shell.

ln the embodiment of the invention illustrated in Figs. l-4, the jackets 16 include a pair of spaced co-extensive walls forming a water cooling chamber Ztlyto carry away the heat from adjacent the coils 5 and 13.

nter-jacket conduits 21, shown most clearly in Fig. 2, serially connect the jackets 16 in eachof parallel coil groups on the core structure 8. An arcuate input conduit 22 encircles one end of the core structure 8 within the outer perimeter of the bottom of the shell 2 and is connected to each of the adjacent jackets to provide individual input to the several groups. A similar arcuate output conduit 23 is similarly mounted at the opposite end of the core structure to receive the Water from the several groups of jackets and to discharge or recirculate the water, in any desired manner.

As shown in Fig. 2, one pair of opposed side mounted jackets 16 adjacent the stirring coils 13 is serially connected to the neighboring jackets 16 by suitable connecting conduits 24. The opposite side mounted jackets 16 are connected to the adjacent jackets by connecting conduits 25 and to a water inlet 26 and a water outlet 27, respectively.

During the stirring operation, when relatively high currents pass through one or more of the coils 5, the cooling Water flows through the jackets 16 and serves to cool and protect the jackets 16 and the coils 5 and 13 against damage by the heat which is generated' as a result of the current flow in the coils and the heat of the molten charge. The cooling of the jackets 16 increases the current capacity of the coils 5 and 13 and may eliminate the necessity for water cooled coils.

The jackets 16 and water cooling conduits Ztl-25 may readily be made of any suitable material to withstand the temperature of the molten metal which may contact these components. Consequently, the danger of an explosion from direct contact of molten metal and the cooling lluid is essentially eliminated.

The individual heating coils 5 and 13 are adapted to be selectively energized and polarized to establish a plurality of various stirring actions. Referring particularly to Fig. 4, a schematic circuit is illustrated including a control panel 28 having twelve control buttons 29 corresponding to the twelve individual coils 5 and four control buttons 30 for coils 13. The control panel 28 is provided with a plurality of output terminals 31 which are individually serially connected to the individual coils 5 and 13 and to the corresponding control buttons 5 and 13. -A pair of input terminals 32 on the control panels are connected to any suitable low frequency or pulsating D.C. source, not shown. The frequency is generally of the order of one to two cycles per second to establish maximum stirring efficiency.

Each of the control buttons 29 and 36 is thus adapted to selectively energize one and only one of the stirring coils S and 13. The stirring pattern or flow may be changed by operating different combinations of the control buttons 29 and 30 to control the energizaton of the corresponding coils. The ux pattern is preferably established to cause the metal to roll to increase the contact of the metal and the reaction products such as slag and provide a homogeneous mixture of the whole bath. The metal can be caused to ow away from the walls of the furnace and thereby reduces the high rate of erosion which is normally evident with a stirring action which directs the alloying and reaction material to the wall and then downwardly into the mass. Further, with suitable automatic switching means the ow of the molten mass may be caused to be changed during the refining or metallurgical process.

The operation of the illustrated embodiment of the invention of Figs. 1-4, is summarized as follows:

The molten mass 3 is either pre-formed or reduced after being disposed within the furnace as solid metal. The induction coils 5 and 13 are then energized by a low frequency, high amplitude, alternating current or a pulsating D.C. current. For maximum stirring raction, the frequency of the current is normally of the order of one to two cycles per second. The magnetic ux established by the coils 5 and 13 establishes a ow pattern upwardly and horizontally into the molten mass 3 and then downwardly through an adjacent coil 5 in the connecting yokes and cross braces 12. The amplitude of the flux varies in accordance with the changing current and induces eddy currents in the molten mass. The eddy currents and the magnetic field react to establish forces within the metal which cause vertically circulating metal currents.

The protective jackets`16 are formed of a non-magnetic material having a permeability of one and a relatively high specific resistance. The jackets are therefore effectively a small air gap Which does not materially effect the passage of flux. Consequently, it does not materially interfere with the penetration of the ux into the molten mass.

Further, the low frequencies of the supplied current and the thinness and high specific resistance of the metal in the jackets permit the flux to readily pass into the molten mass. No appreciable eddy currents are established in the recess jackets 16 and the short-circuited secondary winding effect of the jackets and shell is inconsequential. This is important to avoid distortion and destruction of the magnetic ield with respect tothe molten mass.

Fig. 5 is a View similar to Fig. 3 and discloses an alternative construction for the recess jackets. Corresponding elements in Figs. 3 and 5 are similarly numbered.

Referring to Fig. 5, the refractory lining 1 is formed with recesses to receive induction stirring coils 5, as in Fig. 3. A thin solid-Wall jacket 33 lines the recesses 15 and extends outwardly over the coil 5 to shield the coil 5. Jacket 33 is formed from a suitable temperature resistant material such as the previously described austenitic steel. The coil 5 is formed of a tubular copper and adjacent coils are connected by suitable connecting conduits 34 which pass through suitable openings in the jackets 33.

The construction of Fig. 5, reduces the thickness of jacket 33 to a minimum and thus further reduces the possible interference with the passage of ux into the molten mass.

The recesses 15 in the embodiments of both Figs. 3 and 5 serve to protect the coils 5 and increase the coupling of the coils to the metallic charge. The jackets reduce the use of expensive shell material. invention provides lan efficient, low-cost furnace construction.

The exterior mounting of the coils 5 and and the connecting magnetic core structure 8 allow ready replacement and supervision of the coils without expensive furnace reconstruction.

The present invention provides an electromagnetic stirring construction and method for metallurgical furnaces and the like which promotes a high degree of efficiency in the stirring operations. The disposition and mounting of the coils allow ready maintenance and replacement The present 6 and also effectively protects the coils from hot molten spatter and from any molten metal which may tend to creep through a vein in the refractory lining.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. In a furnace having a heat insulating lining constructed and arranged to hold a molten charge of metal, an exterior recess formed in the lower surface of the refractory lining, an induction coil supported within said recess in inductive coupling to said molten charge, and a non-magnetic jacket lining said recess to support the lining and shield the coil from the molten charge without interfering with the inductive coupling of the coil to the molten charge.

2. In a processing furnace having a refractory lining supported within a metallic shell constructed and arranged to hold a molten charge of metal, a plurality of exterior recesses formed in the lower surface of the shell and refractory lining, induction coils removably supported within said recesses, and non-magnetic jackets of heat resistant material lining said recesses to support the lining and shield the coils from the molten charge without interference with the magnetic flux iiow into said molten charge.

3. In a metallurgical processing furnace having a refractory lining supported within a metallic shell to hold a molten charge of metal, a plurality of exterior recesses formed in the bottom of the shell and refractory lining, core means having projecting pole members disposed within said recesses, induction stirring coils wound on said pole members and extending into said recesses, and a heat resistant metallic jacket of non-magnetic metal lining said recesses to shield the coils from the heat of the molten charge and to prevent contact of the molten metal with said coils incident to fracturing of said refractory lining.

4. In a metallurgical processing furnace having a refractory lining supported within a metallic shell to hold a molten charge of metal, a plurality of exterior recesses formed in the lower surface of the shell and refractory lining, induction stirring coils removeably supported Within said recesses, and a metallic material lining said recesses, said metalic material being temperature resistant and non-magnetic and having a high specific electrical resistance.

5. In a metallurgical processing round cup-shaped furnace having a refractory lining supported within a metallic shell to hold a molten charge of metal, a plurality of exterior recesses formed in the lower surface of the shell and refractory lining, said recesses being cruciformaly arranged to essentially cover the entire lower surface area of the furnace induction stirring coils removably supported within said recesses, and jackets lining said recesses, said jackets being formed of a temperature resistant and non-magnetic material having a high specific electrical resistance.

6. In a metallurgical processing furnace having a refractory lining supported within a metallic shell and constructed and arranged to hold a molten charge of metal, a plurality of exterior recesses formed in the bottom of the shell and refractory lining, core means having projecting pole members disposed within said recesses, induction stirring coils wound on said pole members and extending into said recesses, double-wall jackets lining said recesses and having the walls spaced to form cooling chambers, and means to circulate a cooling uid through said jackets.

7. In a metallurgical processing chamber having a rcfractory lining within a cup-shaped supporting shell constructed and arranged to hold a molten charge of metal, a plurality of small exterior recesses in the bottom of the shell and adjacent refractory lining, said recesses being distributed about the entire lower surface, induction stirring coils disposed within said recesses, a magnetic structure having projecting pole members disposed within the recesses to reduce the reluctance of the return magnetic iiux paths between the stirring coils, and non-magnetic jackets lining said recesses and extending therefrom to substantially completely overlie the coils to shield the coils from the molten charge and permit location of the coils adjacent the peripheral portion of the shell bottom.

8. In a metallurgical processing furnace having a refractory lining supported within a metal shell constructed and arranged to hold a molten charge of metal, a plurality of exterior recesses formed in the bottom of the shell and refractory lining, core means having projecting pole members disposed Within said recesses, induction stirring coils Wound on said pole members and extending into said recesses, a double-wall jacket having a configuration corresponding to said recesses and having the walls spaced to form cooling chambers, means to secure said jackets Within said recesses, said jackets extending outwardly therefrom to substantially completely cover said induction stirring coils, and conduit means interconnecting said jackets to a source of cooling iluid to cool the jackets and the stirring coils.

9. In a metallurgical processing chamber having a refractory lining within a cup-shaped supporting shell constructed and arranged to hold a molten charge of metal, a plurality of small exterior recesses in the bottom of the shell and adjacent refractory lining, said recesses being distributed about the entire lower surface, induction stirring coils disposed within said recesses, a magnetic structure having projecting pole members disposed within the recesses to reduce the reluctance of the return magnetic iiux paths between the stirring coils, and nonmagnetic jackets lining said recesses and extending there. from to substantially completely overlie the coils to shield the coils from the molten charge and permit location of the coils adjacent the peripheral portion of the shell bottom, and control means constructed and arranged to selectively connect said coils to a source of changing current at a relatively low frequency to selectively establish different movements of the molten charge.

References Cited in the file of this patent UNITED STATES PATENTS 1,983,242 Rohn Dec. 4, 1934 2,363,582 Gerber et al. Nov. 28, 1944 2,875,261 Hauff Feb. 24, 1959 OTHER REFERENCES 432,711 Great Britain July 29, 1935 499,437 Great Britain Jan. 24, 1939

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