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1. WO1991015433 - FOUR DE FUSION

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

Melting furnace

The present invention concerns a melting furnace for melting of raw material for mineral fibre production, which furnace on its side walls carries an optionally multi- layered refractory lining comprising materials of different thermal conductivity.

Mineral fibres are produced e.g. by melting the raw material, possibly rock mineral or slag from metal production, in an electrical melting furnace, withdrawing continuously the melt from the furnace and converting it to fibres, e.g. by transferring the melt to a spinning unit, which can consist of a number of rotating wheels, wherefrom the mineral melt is centrifuged for the formation of fibres. The thus formed fibres are collected onto a conveyor in the form of a fibre felt. The fibre felt is thereafter subjected to additional processing steps, it is e.g. impregnated with a suitable binder, such as a resin, which is activated e.g. in a curing oven, whereby the fibres are fixed to each other into a form stable felt of desired density and thickness.

The melting furnace which is used for melting of the raw material consists of a casing of steel which on its inner side wall carries π refractory lining, the type and thickness of which is chosen according to the material to be melted. Onto the bottom of the furnace a sump of iron, so called bottom iron, is formed during the operation as a result of iron oxides included in the raw material being reduced to elementary iron. The lining in the whole furnace is subject to strong chemical and thermal stresses as a consequence of which the lining wears down and gradually disappears and has to be renewed at regular intervals. It has been observed that the attacks are especially severe at boundary surfaces between phases, i.e. at the free melt surface and the boundary surface between the melt and bottom iron.

As the stresses on the lining are proportional to the temperature in the furnace, attempts have been made at reducing the wear on the lining e.g. by cooling the side walls in different ways. Thus it has been suggested to cool the furnace wall with water, either by cooling the outer wall (e.g. SE 7314539), or by using cooling elements built into the wall, e.g. cooling coils, through which water is led (DE 2626211). However, the use of water is associated with a risk for explosion in case water, e.g. as a result of structural cracks, comes into contact with the melt in the furnace. It has also been suggested to cool the outside of the furnace wall by using a cooling system in the form of outer cooling panels containing a non-explosive liquid coolig agent, e.g. oil (SE 8600514).

As an alternative to the above mentioned solutions it has also been suggested to use copper plates built into the furnace wall and the furnace roof, for the general withdrawal of surplus heat in an electric arc furnace
(DE 2443662).

Excess withdrawal of heat from the furnace is, however, not desirable for the reason that this means an increase of the energy requirement of the furnace and consequently poor heating economics in the furnace. It would thus be of importance to be able to optimize the heat withdrawal from the furnace by providing for a local increased withdrawal of heat at those locations in the furnace where the lining is especially exposed to attack from the melt, thus preserving the lining at these exposed locations. This object is achieved by means of the furnace according to the invention which is characterized in that the lining comprises at least one layer including a zone with a restricted dimension in the direction of the height of the side wall, and arranged at a level corresponding to the melt surface and/or the interface between melt and bottom iron and made of a material of a thermal conductivity which is higher than that of the material otherwise in this layer.

Thus the furnace lining comprises in at least one layer an annular zone extending around the furnace and having a restricted dimension in the vertical direction, which is made of a refractory material having a thermal conductivity higher than that of the material otherwise in the same layer. The term "layer" means in this connection not only physically separate layers, but refers also to any such layer of the lining which has a certain thickness dimension and which is situated at the same distance from the side wall of the furnace when viewed in the vertical direction of the furnace. The material with the higher thermal conductivity and the material with the lower thermal conductivity in the said layer are preferably of similar type, such as a ceramic material, in order to form as uniform and well integrated lining as possible with as small as possible heat strains in the boundary surfaces between the materials.

The material with the higher thermal conductivity thus forms a thermal bridge in the lining in the form of an annular zone, which is situated at such a distance from the furnace bottom that it in its vertical direction coincides with the free melt surface in the furnace, and/or with the liquid boundary line between the melt and the bottom iron. By incorporating a thermal bridge of the said kind in the lining it is possible to leave out further cooling systems, if desired. It is possible to obtain a sufficiently high heat withdrawal through the thermal bridge in order for frozen melt to form on the inside of the lining towards the melt, which in turn promotes the further protection of the lining against the stresses in the furnace.

According to a preferred embodiment, the thermal bridge is comprised of a mortar or bricks of graphite, C-SiC, or SiC, especially SiC. The material in the said layer can otherwise suitably be a mortar or bricks of a Al203-, MgO-, MgO/Cr203- or Al203/Cr203-based material, especially if this layer comes into contact with the melt. If the layer forms an intermediate layer, the material otherwise in the layer may be chosen more freely provided its thermal conductivity characteristics are adapted according to the material of the thermal bridge. In the above mentioned embodiment, the thermal conductivity of the thermal bridge is many times greater than in the layer otherwise. In order to achieve good results it is advantageous that the thermal conductivity of the thermal bridge is more than twice as high, and advantageously 2 to 5 times as high as the material otherwise in the layer.

The layer in question can be the only lining layer in the furnace or it can be a so-called base lining, which suitably is coated with a layer forming the actual lining towards the melt and which may be an aluminium oxide/chromium oxide lining with a high chromium content.

The invention is described in the following with reference to the appended drawing, wherein

Fig. 1 shows schematically a furnace in cross-section, and

Fig. 2 shows a section of an embodiment of the furnace wall.

The furnace as a whole is denoted with the reference numeral 1, and it comprises a bottom part 2, side walls 3 and a furnace cover 4, through which a plurality of electrodes 5 extend into the melt 6 in the furnace. The supply inlet openings for the raw material are not shown nor the outlet opening for the melt. On the surface of the melt there is a layer of unmelted raw material, and on the bottom of the furnace a sump 8 of iron is collected. On the outer side of the furnace cooling panels 9 for forced cooling are schematically shown and in which oil is used as a cooling medium. These cooling elements may be of any design, and they may extend over the whole height of the wall, or only over parts thereof.

Figure 1 shows schematically two areas in the side wall, which are exposed to a greater degree of stress than the remaining parts of the side walls, that is the contact surface between the side wall and the melt surface, and the boundary surface between the iron sump and the melt, respectively. These areas have been marked with the numerals 10 and 11 in the Figure 1. According to the invention these areas in the side wall lining are made of a material which has a higher thermal conductivity than the remaining areas of the lining in the side wall 3.

One embodiment of the invention is shown in the Figure 2. The side wall is generally denoted with the reference numeral 3. It comprises outermost the actual furnace casing 12 of steel. Next to the steel casing there is an elastic contact layer 13 of a carbon mass, the purpose of which is to improve the contact between the casing 12 and the base lining (14). As is evident from the Figure 2, the base lining comprises zones of two different kinds, that is a conventional lining 14" of aluminium oxide bricks and separate xones 14' of SiC-bricks. The zones 14' are arranged in the furnace wall at a height level which corresponds to the free melt surface and the boundary surface between the iron sump and melt, respectively. According to the embodiment of the Figure 2, the base lining 14 carries an additional layer, i.e. is the lining 15 which faces the melt itself and which suitably is an aluminium oxide-chromium oxide mortar of a high chromium content, preferably with a chromium oxide content of more than approximately 5%. Due to the intensified heat removal at the level of the melt surface and the iron sump, respectively, the temperature of the lining will at these locations be lower than the corresponding lining without intensified heat removal, whereby a crust of frozen melt can form on the inside of the side wall, which in turn promotes the durability of the linings at these areas.

In the embodiment according to the Figure 2, the oil cooling panels can be divided into suitable segments, e.g. so that the areas where the heat transfer through the wall is intensified, i.e. at the level of the melt surface and the iron sump, are provided with separate segments. These elements or panels can be provided with suitable temperature and flow meters which respond to an increased heat transfer through the furnace wall and can therefore suitably be used for monitoring the state of wear of the linings.