EP1485664A2

EP1485664A2 - Core-type furnace

Info

Publication number: EP1485664A2
Application number: EP03709913A
Authority: EP
Inventors: Patrice Brun; Jacques Lacombe; Christian Ladirat; Francis Bousquet
Original assignee: Commissariat a lEnergie Atomique CEA; Compagnie Generale des Matieres Nucleaires SA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA; Orano Demantelement SAS
Priority date: 2002-02-04
Filing date: 2003-01-31
Publication date: 2004-12-15
Anticipated expiration: 2023-01-31
Also published as: DE60300941D1; ES2243892T3; KR101006751B1; FR2835601A1; US6996153B2; DE60300941T2; JP4287281B2; RU2301949C2; ATE298873T1; FR2835601B1; JP2005517148A; WO2003067166A2; CN100402962C; WO2003067166A3; CN1628233A; RU2004126690A; KR20040077935A; US20050129087A1; AU2003214344A1; EP1485664B1

Abstract

Vertical segments of a side wall of a crucible for an induction furnace are assembled at an adjustable, invariable position by screws screwed into tapped holes of a flange common to all segments. A precise assembly is thus obtained producing no deformation and no internal stresses. The segments are coated with a ceramic coating for their protection and to prevent formation of electric arcs. Junction edges of faces are rounded to achieve the same effect. Water cooling boxes of the lower furnace hearth are similarly constructed. The apparatus can, as an example, be applied to vitrification techniques.

Description

INDUCTION OVEN DESCRIPTION

The subject of this invention is a crucible and a hearth of an induction furnace for the incineration and vitrification of organic matter, the vitrification of radioactive or non-radioactive waste, the vitrification of hazardous waste and the fusion of refractory bodies. The structure of such ovens essentially comprises a refractory concrete hearth comprising cooling water circuits on which is installed a side wall called the crucible, surrounded by an inductive coil where an electric current flows at frequencies above 100 kHz which is the source of the power produced inside the crucible to melt the material present therein. These ovens are mainly used for the incineration and vitrification of organic matter, the vitrification of radioactive or non-radioactive waste and the fusion of refractory bodies. The industries most likely to use them are the waste processing industry, including nuclear and hazardous, and the glass industry. The side wall of the crucible is normally made of metallic material permeable to magnetic fields. It contains a cooling circuit so that, firstly, the wall withstands the very high temperatures reached to melt refractory materials such as glass and, secondly, to compensate for the electrical power dissipated by joule effect in the structure. Such a crucible is called a "cold crucible". In addition, it is normally divided into vertical sectors, joined by their transverse faces by interposing an electrical insulating material, to limit the currents induced in the wall which would produce thermal losses and an electromagnetic coupling between the inductor and the contents of the crucible. The vertical sectors are arranged like a barrel stave. The cooling circuit ^• is usually composed of vertical pipes drilled in each of the sectors.

The sectors of the side wall of the crucible must be held together. A first means consists in surrounding the crucible with a circular bandage made of cement or glass fabric impregnated with elastomer or epoxy resin. Another means which offers greater cohesion is to weld the sectors to each other on a circular flange above the inductor where the intensity of the magnetic field is lower. A last type of mounting which is preferred for the invention consists in assembling the vertical sectors making up the shell by screws on a circular flange above the inductor. To facilitate assembly, the sectors are provided with assembly tabs on the part mounted outside the shell.

The sole supporting the ferrule is made up of metal boxes crossed by a cooling circuit placed in refractory concrete or made up of metal tubes of various sections (round, square, rectangular, and) mounted in parallel or in rafters and placed in refractory concrete. The boxes or tubes are spaced from each other by a width of refractory concrete. One of the faces is positioned so as to be perfectly vis-à-vis with the molten contents of the oven. Like the tubes, the shape of the boxes can be diverse: rectangular, triangular, etc.

The known crucibles and soles suffer from insufficiency which can be detailed as follows. To be applicable to the combustion-vitrification of organic materials on molten glass baths, or the fusion of refractory bodies in an induction furnace, much higher frequencies and heats than in other applications are necessary. Risks of electric short circuits can appear between the metallic elements constituting the cold crucible (the sectors, the flanges), constituting the hearth support of the crucible (cooled metal cans) and between the elements of the crucible and the hearth. These short circuits appear even when the electrical insulators between the crucible sectors and the bottom boxes are wide. Without being exhaustive, these electrical short circuits between the crucible sectors and the bottom boxes are possible due to the presence of carbon deposited on the internal walls during the combustion of organic matter, or by the formation of sulphate lakes on the surface of the glass baths coming into contact with different sectors and the insulators electrical in the inter-sectors, or for example due to the release of large quantities of water during the fusion of refractory oxides. These short circuits create irreparable damage to the electrical insulators placed between the elements constituting the crucible, to the refractory concrete placed between the cooling boxes of the hearth, or even pierce the metallic elements of the hearth and the crucible. electrical circuits are also detrimental to the proper use of induction energy.

In the applications mentioned, corrosive atmospheres at high temperatures are produced, which damage the metal elements of the furnace constituted by the crucible and the hearth or force them to be constructed with materials having high electrical resistivities, greatly increasing the electrical losses.

Whatever the shape of the crucible sectors (parallelepiped profiles, T-shaped, triangle shapes, etc.) and the bottom, the sharp edges of these adjacent metal elements are sources of significant electric arcs (electric spike effect). This appearance of electric arcs is mainly favored by the operating regime imposed at high frequencies higher than 100 kHz for glass and waste treatment applications on molten glass baths. These electric arcs are energetic and harmful to the resistance of the electrical insulators of the crucible and to the concrete of the hearth. It is specified that if the crucible sectors were round or ovoid, this would eliminate these peak effects but at the detriment to the tightness of the furnace shell by reducing the thickness of the electrical insulation between the sectors too much, hence the problems of material and gas leakage as soon as a slight deterioration in the appearance of the insulating material.

It is to remedy these drawbacks that a new kind of crucible and hearth of an induction furnace is proposed as the invention.

To avoid the appearance of electric arcs, the solution adopted consists in coating the metal sectors constituting the crucible and the metal boxes of the hearth on one or all of their faces with a layer of electrical ceramic insulation: at least on the internal and lateral faces of the sectors facing each other to eliminate electric arcs, or, depending on chemical and electrical attack, on all faces including the head, the foot and the face facing the outside of the oven. These ceramic coatings are in addition to the electrical insulators positioned between the crucible sectors and the bottom boxes and they allow perfect electrical protection between the various metallic elements of the furnace and even between the metallic elements and the molten coating. In addition, thus coated, the crucible sectors and the boxes of the hearths protected from chemical attack due to the glasses, gases and the various wastes fed into the crucible supported by the hearth. Refractory ceramic coatings, which are perfectly electrically insulating, are produced for example with an acetylene flame or with a plasma torch. The materials more frequently screened are based on alumina, nullity, cordierite, zircon, zirconia, zirconate and silicon carbide, with different dopants compatible with electrical stresses. Once coated on one or all of their faces, the metal boxes are placed in the floor by interposing an electrical insulator such as refractory concrete. With regard to the crucible sectors, once coated on one or all of the faces of ceramic electrical insulators, they can be mounted and screwed onto the cooled flange which can also be coated with electrical insulator. In the description of the invention, a mounting of the crucibles will be detailed by screws because it makes it possible to limit the mechanical assembly (local compressions) and thermal (if there are welds) constraints, but the invention applies perfectly to the other mounting types detailed in the state of the art.

It is found in the literature that it is preferable to make chamfers on the sharp edges to avoid embrittlement of the ceramic coating and its chipping. If a chamfer on the sharp edges of the sectors aids in the satisfactory deposition of ceramic electrical insulation on the faces of the sectors, it is by no means sufficient to counter the appearance of electric arcs for frequencies greater than 100 kHz. between the boxes of the hearth and the faces of the sectors constituting the internal part of the crucible, which for example are vis-à-vis carbonaceous dust from the combustion of organic matter on the molten glass bath or opposite the elements to be vitrified.

The sharp edges oriented on the internal face of the oven are machined with rounded radius of curvature. The removal of any sharp edges by machining in the radius of curvature concerns the edges facing the interior of the induction furnace. The presence of chamfers on the other edges, outside the crucible, may suffice without being an obligation. The dimensioning of these radii of curvature achieves the following functionalities:

. The radius of curvature should not be small (less than 1 mm) for example to avoid trapping material in the free air gap between the sectors when varying the height of the glass bath, as in some configurations described in the state of the art, it is possible to keep an electrical insulator such as mica in one intersector (mica thickness between 0.1 and 4 mm) or to mount the joined elements without additional electrical insulators than the deposition ceramic. The radius of curvature must be small (less than 5 mm) to be sure that the cooled metal sectors are close enough to prevent molten glass from coming into contact with the electrical insulation placed in the intervals, at the risk of damaging it and to make the material flee from the interior of the crucible towards the exterior. The invention differs in the specific case of incineration and vitrification applications of organic materials, vitrification of waste and fusion of refractory bodies by the low values of the heat fluxes exchanged between the material to be vitrified and the walls of the furnace. By way of example, these fluxes are weaker by an order of magnitude than in cold metal melting crucibles by the creation against the wall of the furnace of a self-crucible of glass, solid and refractory. Under these conditions, the electrical protection ceramic materials are perfectly cooled, preventing their degradation, their flaking and above all preventing pollution of the vitrified material. The invention will now be described in more detail and in all its aspects in connection with the figures:

FIG. 1 represents a crucible welded in accordance with the prior art, FIGS. 2 and 3 illustrate an embodiment of a crucible of the invention,

FIGS. 4 and 5 illustrate the method of manufacturing the crucible,

- And Figures 6 and 7 illustrate a sole of the invention.

Referring to FIG. 1, a crucible comprises a refractory concrete hearth which bears the reference 1, a side wall bearing the reference 2, its stainless steel sectors bearing the reference 3, intermediate layers of electrical insulators the reference 4, and turns of the inductor the reference 5. The details of construction and arrangement of these elements are in accordance with the preceding description. The side part 2 is only partially shown, but it is clear that it extends over a circle or a complete revolution, as for any other crucible including those of the invention. Each of the sectors 3 is hollowed out with a cooling circuit 6 extending over almost its entire height and composed here of a pair of vertical parallel conduits and joining at the bottom of the sectors 3 (only one of these conduits being visible in the sectional representation). The coolant inlet and outlet holes 7 and 8 communicate the conduits outside sectors 3 and they open into superimposed collectors 9 and 10 belonging to the same flange 11 to which the sectors 3 are welded by a circular bead 12 at their upper outer edge. Even with this weld, it is possible to add an external bandage 13 to the structure under the flange 11 to improve the cohesion of the side wall 2 and ensure gas tightness. The drawbacks mentioned previously of the two methods of assembling the side wall 2 do not however disappear if these methods are combined. The sole 1 is cooled by a circulation of water in metal cans which have not been shown here.

An embodiment of the invention will now be described, first using Figures 2 and 3. The sectors of the side wall are marked with the reference 20. They have the same external shape and are also traversed by a pair of conduits of a cooling circuit 21, the ends of which open to the outside by tubes 23a and 23b (FIG. 3). But unlike the known embodiment, the sectors 20 of the invention are not bare but covered with a ceramic coating 22 which can be chosen from compositions based on alumina, mullite, cordierite, zircons, zirconia, or zirconate, different additives that can be added according to the thermal, chemical and electrical constraints that the crucible will have to face. A single sector 20 is shown with the coating 22 in Figure 2, but all are actually provided. Similarly, the coating 22 is present on the sector 20 of FIG. 3, but has not been shown for the sake of lightening the drawing. It is recommended to cover at least the internal face 24 of the sectors 20 and their lateral faces 25 and 26, which are the faces subjected to corrosion and to the appearance of electric arcs; however, it is not bad to also cover the outer face 27, which is shown here or even the upper and lower faces. As the chemical attacks or the risks of electric short-circuits which could justify the use of the coating 22 would come from the gases staying above the molten material and the particles and flies entrained in these gases rather than from the molten material itself since a function of these cold crucibles is to maintain a solid thickness of the contents of the crucible on the side wall, the coating 22 extends to the top of the sectors 20. Its thickness is between 50 μm and 500 μm depending on the applications. An additional provision to reduce the probability of electric arcs, while allowing better adhesion of the coating 22, is to remove the sharp edges between the faces 24 to 27 of the sectors 20: here, the edges 28 and 29 internal to the oven (between the internal face 24 and the lateral faces 25 and 26) have been rounded to a radius of curvature which may be from one to five millimeters, and the other edges, such as 30 and 31 (between the external face 27 and the other lateral 25 and 26) were simply chamfered; this latter arrangement is only necessary to facilitate adhesion of the coating 22 at the junction of two coated faces. The horizontal edges of the sectors 20, at the top and at the bottom, can also be rounded or chamfered if electric arcs are to be feared with neighboring elements.

Referring more particularly to FIG. 3, it can be seen that the flange 11 has disappeared and that the cooling circuits 21 are not associated with collectors such as 9 and 10 adjacent to the crucible but are completely separate, the tubes 23a and 23b are extending outside. The sectors 20 comprise an upper tab 32, also in a sector of a circle which overhangs the external face 27. It comprises a notch 33 ′ opening to the outside. A flat flange 34 of circular shape is placed on all the legs 32 and includes threads 35. Screws 36 are engaged in the threads 35 through the notches 33 and support below the lugs 32, which they hold against the flat flange 34. Thus, the sectors 20 are held in place and form a single assembly. An external bandage 37 can be added to seal the crucible in air and make the assembly even firmer, without however being essential; it can be made of solid glass fabric impregnated with elastomer or epoxy resin. Finally, layers of electrical insulator 38, made of mica for example, can be introduced between the lateral faces 25 and 26 of neighboring sectors 20.

A ceramic coating 57 can also be placed on the flange 34, and above all on its lower face 58 which touches the tabs 32 of the sectors 20. Here also it is useful to chamfer the edges joining two ceramic coated faces.

Another arrangement, made possible by the flat flange 34, would consist in adding a cover 39 placed on it and retained by clamps 40 to the screws 41 engaged in the threads of the flat flange 34 in order to confine the continuous of the crucible and ensure a perfect seal.

It has already been mentioned that the precise and invariable adjustment of the sectors 20 offered by the screw assembly and the flat flange 34 makes it possible to coat the sectors 20 with ceramic without risk for the latter. It is now necessary to describe a method of assembling the side wall which makes it possible not to expose the ceramic to damage even at this time: this is what is done by referring to FIGS. 4 and 5. The sectors 20 , after being machined to a precision sufficient in the necessary places (in particular on the lower face, placed on the concrete floor 1, on the upper face of the legs 32 and on the lateral faces 25 and 26) and coated with ceramic by plasma deposition and abrasive polishing , are roughly placed on the flat flange after being turned over, a conical centering wedge 42 is placed on them, and clamps 43 are engaged around them and tightened to approach them until they all touch the conical flank of the corner 42. The layers of electrical insulation 38 have already been inserted. Depending on the height of the corner 42 and the tightening of the collars 43, the diameter of the side wall and its preload can be adjusted. The screws 36 are then tightened to join the tabs 32 to the flat flange 34 located under them. The assembly is then completed. The bandage 37 can be formed first by windings 371 situated between the cable ties 43, then by the complementary windings when the cable ties 43 have been removed. This laying of the bandage in two stages makes it possible not to loosen the preload of the side wall by prematurely loosening the collars 43.

Figures 6 and 7 illustrate the sole 46 of the embodiment of the invention. It comprises a main plate 47 provided with a central concavity which occupies the cooling boxes 48. Each box 48 comprises a supply pipe 49 and a water outlet pipe 50. In a similar manner to the sectors 20 of the crucible, care is taken to protect the boxes 48 metallic chemical and thermal aggressions and to guard against the opposition of electric arcs arising from them. They too are coated with ceramic, at least on the upper face (overlooking the molten bath) 51; the coating has the reference 52. And the edges 53 delimiting this upper face 51 are rounded, also at a radius of curvature of one to five millimeters; the other edges 56 (vertical and delimiting the lower face 55) can also be rounded or at least chamfered, especially if the lateral faces 54 and lower 55 which they delimit are also coated with ceramic.

Claims

1) Induction furnace including a side wall (2) essentially composed of sectors (20) vertical fused, characterized in that the sectors are coated (22) of ^ceramic at least on inner faces (24) and side faces ( 25, 26) and at least vertical edges (28, 29) joining said inner faces to said side faces are rounded.

2) Induction furnace according to claim 1, comprising a hearth on which the side wall is placed, characterized in that the hearth is provided with cooling box (48) coated (52) with ceramic at least on upper faces (51 ), and at least edges (53) delimiting said upper face (51) are rounded.

3) Induction furnace crucible according to claim 1 or 2, characterized in that the sectors are also coated with ceramic on external faces (27).

4) Induction furnace according to claim 2, characterized in that the cooling boxes are also coated with ceramic on the lower (54) and side (55) faces.

5) Induction furnace according to any one of the preceding claims, characterized in that the sectors are provided with lugs (32) for assembly to an upper flange, and in that the flange is coated with ceramic at least on one side affecting sectors. 6) Induction furnace according to any one of claims 1 to 5, characterized in that the faces of the sectors and the water boxes which are coated with ceramic are joined together by rounded or chamfered edges (28, 29, 30 , 31).

7) Induction furnace according to any one of claims 1 to 6, characterized in that the ceramic has a base chosen from mullite, alumina, cordierite, zircon, zirconia, and zirconate.