EP0381587B1 - Continuous-sintering furnace - Google Patents

Description

The invention emerges from the field of high temperature and controlled atmosphere sintering ovens. This type of oven is mainly used for cooking preformed parts by cold pressing of refractory powders, such as those used as nuclear fuels.

The present invention relates more precisely to the internal structure of sintering furnaces.

The production by sintering of pellets involves during the forming, the addition of organic binders which must be removed and collected during a thermal operation called "preheating", which precedes the actual sintering. Since the thermal operations must be carried out under a controlled atmosphere, it is necessary to take measures so that the binders or the decomposition by-products released cannot diffuse into the sintering zone and exert an influence there. In addition, any work on materials, for example such as plutonium, involves waterproof equipment to avoid alpha contamination. The other materials used to prepare such pellets can be refractory metals, such as molybdenum, silicon, tantalum, tungsten. Metallic oxides, such as uranium oxide or mixed uranium-plutonium oxides can also be used, as can borides, carbides, nitrides and metallic silicides. Finally, refractory ceramics, such as thorine and zirconia are also materials that can be used to form such pellets.

Referring to Figure 1, this type of tunnel-shaped oven is usually formed of several sections, each consisting of a core of refractory bricks. It is surrounded, on the entire periphery, by insulating bricks, not shown. The core contains a set of parts made of refractory materials, such as sole 3, edges 4, and support parts 5 and 6 forming a path for circulation of the containers containing the products to be sintered. The whole is mounted in a steel body which ensures the mechanical strength of the entire oven. Heating is obtained by means of electrical resistors 8 in the form of a solenoid arranged inside the body. They provide heating of the tunnel thus formed in the middle of the core to a temperature which is between 1600 and 1800 ° C., in the usual applications.

We see in Figure 1, how the electrical resistors 8 in the form of a solenoid are positioned relative to the circulation path constituted by the soles 3, the banks 4 and the support parts 5 and 6. This circulation path is surrounded by the turns of the solenoids, the latter coming to be embedded in the base 10 of the path of circulation of the containers.

This design has many disadvantages, the main of which are as follows. The replacement of an electrical resistance 8 is only possible after the uncoupling of two firmly coupled sections and after the removal of the refractory bricks 1 and the insulation bricks which surround the core of the furnace.

Adjusting the height and alignment of the floors 3 also requires the sections to be uncoupled. Due to the position and shape of the resistors 8, control and regulation thermocouples 9 are placed on the periphery of the internal wall of the tunnel, therefore in the immediate vicinity of the resistors 8. This results in an imprecision in the temperature of the sintering zone being further from the resistors 8. Finally, a sector of electrical resistors is located below the soles 3, which causes the existence of a zone which is not swept by the protective gases which circulate in the tunnel. There follows a risk of overheating and accelerated corrosion of the resistors 8 which can cause them to rupture, but also the melting of certain pieces of refractory materials which can cause disorders among the floors 3, the banks 4 and the pieces of support 5 and 6.

On the other hand, the document GB-A-673 532 describes a tunnel oven for the thermal treatment of vitrification of certain objects, such as grindstones. The interior of the furnace comprises heating elements placed transversely and passing through the core of the enclosure and the external enclosure by crossings. However, the replacement of these heating elements requires the dismantling of part of the oven.

The object of the invention is to remedy these drawbacks by proposing a sintering oven having a different structure.

To this end, the main object of the invention is a continuous sintering oven comprising an external enclosure inside which is placed a refractory core with horizontal axis, provided with heating means distributed inside and the along the refractory heart in passing through the refractory core and the external enclosure by crossings, the furnace consisting of several sections, fixed to each other and each having two end faces.

According to the invention, the bushings are notches placed on the end faces of the sections and the heating means are heating elements each covering part of the length of an internal side wall of a section.
This allows an element of the heating means to be changed without dismantling the section inside which this heating element is mounted.

The heating means according to consist of electrical resistors placed laterally on the internal walls of the refractory core, on either side of the soles constituting the circulation paths of the products to be sintered.

Preferably, the refractory core thus formed has a square section.

According to one aspect of the invention, the refractory core consists of refractory elements assembled by interlocking by means of tenons and mortises.

According to a technical characteristic of the invention, the sole and the edges are produced in the form of monobloc pieces in the shape of an elongated I symmetrical and reversible.

Regulation control thermocouples can be placed in the first passage orifices located in the axis of the furnace, at the top, at the level of the bricks constituting the vault of the refractory core.

When the circulation of protective gas is necessary, second gas inlet passage orifices are provided, situated in the axis of the furnace, at the top, at the level of the bricks constituting the vault of the refractory core.

To adjust the level of the hearths, the refractory core can be placed on adjustment cylinders passing through the rest of the oven.

A water cooling coil can be provided and brazed outside the metallic external enclosure.

• figure 1, une perspective cavalière mettant en évidence un détail de réalisation d'un four de frittage selon l'art antérieur;
• figure 2, une coupe transversale d'un four de frittage selon l'invention ;
• figure 3, une perspective cavalière relative au positionnement des éléments chauffants dans un four de frittage selon l'invention ;
• figure 4, une coupe longitudinale d'un tronçon d'un four de frittage selon l'invention.

The invention and its various technical characteristics will be better understood on reading the description which follows and which is annexed to the figures representing respectively:

• Figure 1, a perspective view showing a detail of an embodiment of a sintering furnace according to the prior art;
• Figure 2, a cross section of a sintering furnace according to the invention;
• Figure 3, a perspective view of the positioning of the heating elements in a sintering oven according to the invention;
• Figure 4, a longitudinal section of a section of a sintering furnace according to the invention.

Referring to Figures 2, 3 and 4, the continuous sintering furnace according to the invention is a tubular enclosure with a horizontal axis, divided into several sections. The design and structure of each section are directly related to the design of the heating means, and in particular with regard to their fixing and mounting. This design of the heating means is illustrated in FIG. 3 in which the four heating elements 18 of a section are shown.

Figure 4 is a vertical section (AA) of a section of the sintering furnace according to the invention. In correspondence with this figure and Figure 3, one can immediately notice notches 20 made on the ends 25 and 26 of the section. These notches 20 are located at mid-height of the section and are intended to leave a passage for the heating elements 18. The shape of these heating elements 18 is symbolized in this figure 4 by lines mixed and is such that these heating elements 18 cover the entire length of the internal side walls of the section. The two ends, marked 27 and 28 in FIG. 3, of the heating elements pass through the refractory core by the notches 20 placed on the ends 25 and 26 of the section. With the aid of O-rings, not shown, placed around the ends 27 and 28 of the heating elements 18, at the level of the external enclosure 17, said bushings are sealed. The heating elements 18 and their ends 27 and 28 installed in the notches 20 are embedded in a fibrous insulator.

It is found that these heating elements 18 of a section can be removed by simple horizontal translation, once the section has been separated from the adjacent sections to which it is fixed. By this mounting design, it therefore avoids dismantling the components of the section. Referring to Figure 2, the heating elements 18 can be fixed by a simple suspension by means of hooks 24 planted in the side walls of the tunnel.

The heating elements 18 are preferably made of doped molybdenum. Their electrical connections are brought back to the end faces 25 and 26 of the section, in particular at the level of the external enclosure 17.

It can be seen that these heating elements 18 no longer surround the path consisting mainly of the sole plates 13 and their edges 23. Consequently, a technical characteristic according to the invention is that the sole plates 13 and the edges 23 are formed from one-piece parts. , in the form of an elongated I, that is to say of a strictly symmetrical. This has the advantage, in the event of wear of these sole plates 13, that they are reversible and therefore usable on their two faces.

The structure of the refractory core is such that its sintering tunnel has a substantially square section, as shown in FIG. 2. The walls of this tunnel are made of refractory elements 11, assembled to each other by interlocking by means of studs 31 and mortises 32. The whole constitutes a self-locking structure. More precisely, these bricks consist of an electrofused refractory material entitled "S 5 211" supplied by the European Society of Refractory Products (SEPR). Thus constituted, these refractory bricks ensure the mechanical stability of the refractory core up to temperatures around 1,800 ° C. This makes it possible to obtain a uniform temperature in the tunnel of the refractory core and thus to form a first thermal barrier. It is on the side walls of these refractory elements 11 that hooks 24 are placed, now hanging the heating elements 18 along the walls of the tunnel.

The second thermal barrier consists of twenty bricks 12 made of insulating material, preferably a light aluminous insulator of the "IRCOR 34" type, supplied by the same company (SEPR). The technology for assembling these parts is the same as that used with the refractory parts 11, that is to say that it uses the assembly by tenons 31 and mortises 32, also contributing to the same good mechanical strength. all.

This set of two thermal barriers is positioned on two base pieces 14 made of electrofused refractory material, called "JARGAL" (registered trademark) PMS, this material also being supplied by the company called SEPR. These two basic parts 14 therefore support the refractory core. They are positioned on jacks 21, which are placed in a vertical operational position. This makes it possible to adjust the attitude of the path, by varying the height of the floors 13. The refractory core is positioned laterally with respect to the external enclosure 17 by means of metal guides 33.

This whole assembly is surrounded by a fibrous insulating material 15. The whole is enclosed in the cylindrical metallic enclosure 17, which is fixed on a support 34 of the oven.

The oven has thermocouples 19, placed at the top of the refractory core, in the refractory bricks of the vault. They are more precisely placed in first orifices 35 passing through the various upper layers of the furnace in an alumina sheath, and positioned in the axis of the furnace.

Second orifices 36 can also be used for the entry of the gas which must circulate in the refractory core.

A water cooling coil 37, made of copper tubes, is brazed outside the external enclosure 17 which is metallic.

At each end 25 and 26 of a section, four plates 38 of refractory material are placed opposite the fibrous insulation 15 to ensure its mechanical strength.

In accordance with the presence of these end refractory plates 38, and with reference to FIG. 4, the ends 27 and 28 of the heating elements 18 may have a slightly twisted shape, so as to leave the space necessary for these refractory plates end 38. The ends 27 and 28 of the heating elements 18 can also consist of electrical connection cables extending these heating elements 18.

A clearance J exists between the base plate 14 and the lateral metal guides 33 of the oven. This clearance J is intended to compensate for any variations in the size of the oven due to the thermal expansion of the elements of the oven, and to avoid disorders during transport.

The metallic external enclosure 17 makes it possible to ensure the mechanical stability of the assembly and to link the sections together, by means of flanges. It also ensures the containment of the entire oven. The different sections of the furnace are assembled by means of male and female sockets, at the level of the refractory core.

It can therefore be seen that the structure of this oven allows the heating elements 18, in this case the resistors, to be changed without removing the refractory parts. The resistors can be extracted and replaced by the end faces 25 and 26 of the section, after the latter has been uncoupled from these adjacent sections.

Likewise, adjusting the height and alignment of the sliding plates 13 does not require the sections to be uncoupled, this adjustment being provided by the jacks 21 from the outside of the metallic external enclosure 17.

The heating elements 18 benefit from good protection, since they are located entirely in the gaseous atmosphere of the sintering tunnel, therefore their corrosion is very limited.

The sintering gases are entirely confined inside the refractory core, thanks to the assemblies by tenons 31 and mortises 32 of the pieces of refractory material 11 constituting the sections, and O-rings located at the flanges allowing the connection between sections.

The location of the thermocouples for controlling and regulating temperatures 19 located in the axis of the oven, just above the plates 13 where the products to be sintered circulate, makes it possible to better control the sintering temperatures, since these thermocouples 19 do not are not in the immediate vicinity of the heating elements 18.

Thanks to the structure of the sintering oven according to the invention, a time limitation is obtained, and therefore the cost of intervention on this type of oven for replacements of defective heating parts.

If this furnace is used for sintering uranium oxide fuel, or mixed uranium-plutonium oxides, this structure makes it possible to limit the volume of radioactive waste to only the parts replaced.

Claims (10)

1. Continuous sintering furnace, comprising an outer enclosure (17), inside which is placed a refractory core (1) of horizontal axis, fitted with heating means (18) distributed inside and along the refractory core (1), passing through the refractory core (1) and the outer enclosure (17) via penetrations (20), the furnace boing constituted by several segments, fixed to one another and each having two end faces (25 and 26), characterized in that the penetrations are notches placed on the end faces (25 and 26) of the segments and the heating means (18) are heating elements each covering a part of the length of one inner side wall of one segment.
2. Sintering furnace according to Claim 1, comprising a conveyance path composed of hearth plates (13), characterized in that the heating means (18) consist of electrical resistors placed laterally on the inner walls of the refractory core (1), on either side of the hearth plates (13)
3. Furnace according to Claim 1 or 2, characterized in that the notches (20) are equipped with O-ring seals at the level of the outer enclosure (17) to constitute leak-tight penetrations.
4. Sintering furnace according to Claim 2, characterized in that the refractory core (1) is of square cross-section.
5. Sintering furnace according to any one of the preceding claims, characterized in that the refractory core (1) consists of refractory elements (11) assembled by interlocking using tenons (31) and mortices (32).
6. Sintering furnace according to Claim 2, comprising edges (23) constituting a part of the conveyance path, characterized in that the hearth plates (13) and the edges (23) are constituted by monobloc pieces in the shape of an elongate I, which are symmetrical and reversible.
7. Sintering furnace according to any one of the preceding claims, characterized in that it comprises first passage orifices (35) for thermocouples (19) situated in the axis of the furnace, high up, at the level of the refractory bricks (11) constituting the roof of the refractory core (1).
8. Sintering furnace according to any one of the preceding claims, characterized in that it comprises second passage orifices (36) for inlet of gases, situated in the axis of the furnace, high up, at the level of the refractory bricks (11) forming the roof of the refractory core (1).
9. Sintering furnace according to any one of the preceding claims, characterized in that the refractory core (1) is placed on adjusting jacks (21) passing through the remainder of the furnace, and making it possible to adjust the attitude of the hearth plates (13).
10. Sintering furnace according to any one of the preceding claims, comprising the outer enclosure (17) being metallic, characterized in that a water cooling coil (37) is brazed to the outside of the outer enclosure (17).

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