US3461215A - Electric induction furnace - Google Patents

Description

2, 1969 J. REBOUX ELECTRIC INDUCTION FURNACE Filed March 27, 1967 United States Patent 3,461,215 ELECTRIC INDUCTION FURNACE Jean Reboux, 91 Savigny-sur-Orge, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Mar. 27, 1967, Ser. No. 626,291 Claims priority, application France, Apr. 5, 1966,

Int. Cl. nosb 5/12, 9/02 US. CI. 13-27 5 Claims ABSTRACT OF THE DISCLOSURE The invention relates to electric furnaces of the highfrequency induction type primarily employed for the fabrication of refractory material by fusion followed by solidification (the so-called fusion-cast refractories)and more especially fusion-cast refractory oxides which are fed into the furnace in the powdered state-by virtue of a displacement of the inductor coil in a direction parallel to its axis along the charge of refractory material.

It is already known to prepare fusion-cast refractories in a high-frequency induction furnace. For this purpose, use has been made in particular of an electric furnace constituted by a slotted double metallic wall cooled by a circulation of fluid, which serves both as a single-coil inductor and as a melting crucible.

Since refractory material has extremely high electrical resistivity in the cold state, it is necessary to preheat the furnace charge at the beginning of the operation up to a temperature which is such that the induced currents can flow through the charge.

A furnace of this type can be employed for the purpose of melting refractory materials having a starting temperature (or inductivity temperature) which practically coincides with their melting point (alumina, magnesia, silica, for example). Thus, the refractory material which is located in the vicinity of the wall of the singleturn inductor which forms a crucible has very poor electrical conductivity, thereby forestalliug any danger of arcing between the edges of the slot of the single-turn induetor. On the other hand, a furnace of this type becomes virtually unserviceable for the purpose of melting the large number of refractory materials which have a starting temperature (1000 to 1500 C., for example) which is much lower than their melting point. This is the case of many known refractory oxides such as zirconia and uranium dioxide whose melting point is not below 2600 C. In point of fact, if the known device comprising a double metallic wall were employed, the material would rapidly become conductive at certain points in the vicinity of the slot of the single-turn inductor, thus shortcircuiting the inductor and causing damage to this latter while at the same time stopping both the induction heating and the melting of the refractory charge.

In order to protect the inductor, there can be placed between said inductor and the charge of refractory material a tube of quartz or of silica having a double wall which is cooled by a circulation of fluid. Unfortunately, this tube, which is relatively costly, becomes damaged in contact with the charge. In fact, if the cooling liquid is maintained, for example, at 50 C. and the refractory material has poor heat conductivity, the temperature gradient within the material is very steep and is also subject to irregularities. As a result, the quartz is subjected to very high stresses, in particular on the external tube-wall; these stresses exceed the elastic limit and even the yield point of the material, thereby resulting in surface cracking. The tube is thus unfit for further use after a single melting of a refractory charge. Such stresses can even result in the failure of the tube wall during the first heat and make it impossible to carry the process to completion.

The primary object of the invention is to circumvent both the disadvantages of the known method referred-to above as well as those which are attached to the expedient last mentioned. With this object in mind, the invention proposes an electric induction furnace for melting a charge of refractory material and comprising: an electric inductor coil having a number of turns; a sheath which is coaxial with said coil and which is in direct contact with the material to be melted, said sheath being made up of a plurality of identical longitudinal conducting elements of tubular shape which are cooled by a circulation of fluid and separated by electrically insulating refractory material; a device for supplying the furnace with material to be melted which is in a divided state; and means for supplying the inductor coil with radio-frequency current.

The thin-walled metallic elements which are separated longitudinally by insulating material constitute a heat sink which is interposed between the inductor and the refractory charge. By providing these elements with a very thin wall, surface currents induced therein can be reduced to a very small value and losses can thus be minimized. The electric voltage developed between the edges of two adjacent elements which are separated by an insulator is accordingly divided with respect to the total induced voltage by the number of elements which are insulated from each other, thereby providing a reliable means of preventing arc formation between these elements.

A better understanding of the invention will be gained from the following description of a furnace which constitutes one embodiment of the invention and which is given solely by way of example without implied limitation. The description refers to the accompanying drawings, in which FIG. 1 is an isometric view of the furnace and FIG. 2 is a perspective view in transverse cross-sec tion along the line IIII of FIG. 1, part of the sheath being shown in plan view for more clarity.

The furnace which is illustrated comprises an inductor coil 1 which is supplied from a high-frequency generator which surrounds a sheath 2 for receiving a charge 3 of refractory material to be melted.

The inductor coil 1 consists of a number of turns of copper tubing cooled by a circulation of water. The inductor coil is capable of moving parallel to its axis with respect to the sheath 2 both in the direction of the arrow 4 and in the opposite direction; a regulated device (not shown) which is known per se makes it possible to carry out this displacement at a variable speed.

The sheath 2 which is coaxial with the inductor coil 1 and not in contact therewith is constituted by an openended tube formed of a plurality of electrically and thermally conducting elements 5 (of which there are fourteen in the form of construction illustrated), said elements being identical and separated by insulating strips 6 the lower portions of which are shown in FIG. 1 which may be formed of ceramic material such as, in particular,

either quartz or alumina. Each element 5 is constituted by a tubular casing of very thin copper sheet, a flow of cooling liquid (usually water) being admitted therein through a tube 7 which extends to the bottom of the casing and discharges through tubes 8 which withdraw the liquid from the top of the casing. The thickness of the elements 5 must be as small as possible in order to minimize losses. Thus, a thickness of 7 mm. can be adopted in the case of tubes 5 mm. in diameter. It is apparent from FIG. 2 that the insulating strips project inwards from the conducting elements 5 to a slight extent (of the order of one millimeter )so as to extend the insulation distance and also to prevent the appearance of arcing.

The interassembly of conducting elements 5 and strips 6 is carried out in the mode of execution which is illustrated by winding a band of refractory insulating fabric 17 having high heat resistance (glass or alumina fabric, for example), as shown in chain-dotted lines. In FIG. 1, for clarity, the fabric band has been shown around the lower portion only of the sheath.

Other solutions can be contemplated. In particular, the conducting elements can consist of thin copper tubes of circular or profiled sectional configuration which can be insulated by spraying alumina onto each tube with a spray-gun, then interassembled by means of top and bottom manifolds located outside the field of the inductor coil. This solution is advantageous when provision is made for a large number of conducting elements, namely between ten and twenty-four (this last-mentioned figure being virtually a maximum in the case of diameters commonly adopted). On the contrary, the solution which is shown in FIGS. 1 and 2 is preferable in the case of a number of elements up to eighteen. It would appear that the number of four elements constitutes a minimum.

The sheath 2 as thus constructed is practically transparent to the radio-frequency radiation of the inductor coil 11.

A distributor 10 (such as a hopper, for example) serves to feed the charge 3 of refractory material at a variable flow rate into the sheath 2 in a divided form (powder, granular particles and the like) in order that the charge can be evenly distributed within the sheath, which would not be permitted, for example, by a feed in the form of elongated flakes.

The sheath 2 is closed at the bottom by means of a removable shutter 11 which serves to prevent the divided material from escaping as it is being introduced. Said shutter consists of a ceramic block or, better still, by a base-plate provided with a system for circulating cooling water therein.

If the material to be processed has to be protected against the action of air during the operation, which is the case of a large number of refractory compounds, the furnace can be placed in a protective atmosphere by interposing between the inductor coil 1 and the sheath 2 an impervious casing 16 of refractory insulating material (such as quartz, for example) which is shown in chaindotted lines in FIG. 1. This casing does not reduce the electrical efficiency to any significant extent and is not subject to any degradation since it becomes heated only to a limited extent. The casing is fitted with tubes for the purpose of establishing a protective atmosphere therein.

By way of example, it can be mentioned that a furnace of the type discussed in the foregoing which is intended for the treatment of zirconia has been constructed by making use of elements having a radial dimension of 5 mm. and delimiting a zone for the reception of zirconia which is 40 mm. in diameter. The length of the sheath can attain 5 to 6 times that of the inductor coil.

The operation of the furnace for the preparation of fusion-cast refractory material will now be described in reference to FIG. 1 which illustrates an intermediate stage of operation after starting of the fusion process.

On account of the very low conductance of the refractory material in the cold state, starting of the fusion can be carried out as a rule only by adopting the following expedients which are carried into effect when the sheath 2 is closed by the shutter 11, the inductor coil 1 is in the bottom position and a thin layer of material in the divided state is placed over the shutter:

If the operation can be performed in contact with air, there are accordingly placed at the center and on the layer of powdered material thin chips or flakes of the metal whose oxide constitutes the furnace charge 3 (aluminum for starting the meltdown of an alumina charge). Under the action of induced currents, the metal oxidizes in the presence of air according to a strongly exothermic reaction which heats the contiguous charge; thus, the charge itself becomes conductive with respect to the induced currents and fusion then takes place;

If the operation has to be performed out of contact with air, there is placed on the layer of material a tungsten filament in which induced currents are generated. Once the melting point is reached, this filament drops onto the bottom and can be removed by cutting away from the product;

In all cases, preheating can be carried out by means of a plasma torch or auxiliary arc torch, subject to the penalty of contamination.

In all cases, heating, conductivity and fusion propagate from point to point within the charge until the cold wall effect in the vicinity of the sheath 2 limits this heating to a point below the threshold value at Which resistivity drops sharply. Provided that this resistivity drop does not occur, the induced currents cannot circulate and the annular zone 12 which is in contact with the sheath remains in the powdered condition or in a more or less sintered state.

Once the entire layer has melted with the exception of the portion contained in the annular zone 12, the sheath 2 is fed with material to be treated. At the same time, the inductor coil 1 is displaced in the direction of the arrow 4 at a speed which is regulated so that its fusion progresses within the charge 3 at the same rate as the rise in the level of the charge. The portion which was previously melted re-solidifies behind the inductor coil 1 and produces a compact mass. Thus, in FIG. 1, the entire fraction 13 of the furnace charge which is located within the annular zone has successively undergone fusion and solidification. Above the portion 14 which is in process of melting, there remains a layer 15 of material which is still in the divided state.

FIG. 2, which constitutes a cross-section through the portion 14, shows a molten central portion 14 contained within a thin vitrified lining 14" which constitutes an auto-crucible (which will form a gangue after solidification) and which is surrounded, between the vitrified lining and the sheath 2, by the annular zone 12 which has usually remained in the powdered state but which can be in a more or less sintered state. This zone of powdered material constitutes a thermal barrier which protects the sheath 2.

Once the inductor coil has reached the top of the sheath 2, all feeding of material is cut off; by virtue of the presence of the annular zone 12 of material which has not been melted between the rod of fusion-cast material and the sheath 2, they can readily be separated. The sheath 2 can usually be recovered and its cost price is in any case lower than that of a quartz heat sink; in any case, the conducting elements can be recovered and only the insulating seals need to be repaired after a few heats.

It is then necessary to strip the bar of fusion-cast mate rial of parts containing impurities or which have an inhomogeneous structure, especially by cutting-off the ends and by machining the lateral surface.

The process can be utilized in particular:

For the preparation of fusion-cast U0 in either a neutral or reducing atmosphere, the frequency of the current supply to the inductor being comprised between 500 kc./s. (in the case of large diameters) and a few mc./s.; in the majority of cases, the rise time of the inductor is of the order of 10 cm./hours. The annular zone of powdered material has a thickness of 1 to 2 mm. The feed must be carried out with powdered U the grains of which are sufficiently uniform to ensure suitable distribution within the sheath. In this connection, it should be borne in mind that fusion-cast U0 is very difiicult to produce in a single-turn induction furnace which forms a container and that, in any case, the electrical efficiency (and consequently the economic yield of the operation) is considerably higher with a multiple-turn inductor coil and a sheath which is transparent to electromagnetic induction.

For the preparation of fusion-cast refractory oxides which have low conductivity (for example: A1 0 MgO under a high pressure of oxygen).

For the preparation of fusion-cast refractory oxides such as zirconia: in this case, the powdered zone has a thickness which frequently attains 5 mm.

The above list is evidently not limitative: in all cases, a very homogeneous melt is obtained.

The sheath of the furnace hereinabove described has a circular cross-section. However, it is understood that 7 there is nothing to prevent the adoption of a square or star-shaped cross-section, for example, in order to obtain a rod of similar shape.

What we claim is:

1. An electric induction furnace for melting refractory material comprising an inductor coil fed by a radiofrequency generator; a sheath coaxial with and located inside said inductor coil for receiving said material, said sheath consisting of a plurality of longitudinal tubes of thermally conducting material insulated from the inductor coil, electrically insulating material separating said tubes whereby said sheath is substantially transparent to radio-frequency radiation from said inductor coil, means for circulating a cooling fluid through said tubes; an means for feeding said material in divided conditio into said sheath for direct contact with said sheath an an electrically insulating band tightly applied against sai sheath to hold said tubes and said insulating together 2. An electric furnace in accordance with claim 1 including means for producing relative displacement o the inductor coil and the sheath parallel to their commo: axis.

3. An electric furnace in accordance with claim 1 wherein said insulating material consists of ceramic pack ing strips placed between said elements.

4. An electric furnace in accordance with claim 1 wherein said sheath has the shape of a cylindrical tub having a transverse cross-section similar in shape to th desired cross-section of the rod of refractory materia produced in said sheath after fusion and re-solidificatioi of said material.

5. An electric furnace in accordance with claim 1 said means for circulating a cooling fluid including a conduit for delivering said fluid to the bottom portio1 of said tube and another conduit for removing said fluit from the upper portion of said tube.

References Cited UNITED STATES PATENTS 1,801,791 4/1931 Breisky et al. 13--2.1 1,975,438 10/1934 Sorrel 219-10.4S 3,223,519 12/1965 Schippereit 1321 JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner US. Cl. X.R. 21910

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