Method for surface-treating the inner wall of a furnace, which is lined with ceramic fibers

Abstract: A method for treating the surface of the inner wall of a furnace, said inner wall being lined with ceramic fibers, involves placing a source generating an alkali component in a furnace for firing a material into a product, in which at least the inner wall is lined with a ceramic fiber body, and heating the interior of the furnace to vaporize the alkali component, thereby impregnating the surface layer portion of the ceramic fiber body with the thus vaporized alkali component.

Description: The present invention relates to a method for treating the surface of the inner wall of furnace such as a kiln or other industrial furnace, the inner wall being lined with at least one ceramic fiber body. Hitherto, kilns or furnaces lined with ceramic fibers for the purpose of improving fuel cost-effectiveness have been put into practice for firing materials into products. Such industrial furnaces lined with ceramic fibers, for instance, have their inner walls so constructed from boards or blocks comprising ceramic fibers that they have smaller heat capacity and better thermal insulating properties than those made of refractory bricks, thus enabling rapid firing and improving fuel cost-effectiveness. However, a problem with such conventional industrial furnaces, at least the inner walls of which are lined with ceramic fibers, is that a number of small particles ca. 100 to 500 µm in length fly off from the surfaces of the ceramic fiber bodies dine to the thermal expansion or contractions of ceramic fibers caused by repeated in-furnace temperature rise and drop, the in-furnace circulation of combustion gases caused by a burner and for other reasons. Such debris of ceramic fibers are likely to be deposited onto the surface of the material to be fired. In consequence, the resulting product is deteriorated severely or, to put it another way, loses surface smoothness and good appearance which are otherwise achievable by glaze, etc. Such deterioration is often referred to as fiber dirt in the art. Thus, it is strongly demanded to ensure to prevent the occurrence of the fiber dirt, when products desired to be entirely free from the fiber dirt, especially sanitary wares and tableware are made by firing. It is accordingly an object of this invention to meet such a demand by the provision of a method for treating the surface of the inner wall of a furnace, said inner wall being lined with ceramic fibers, which prevents or reduces the occurrence of debris from the surface of a ceramic fiber body, thereby avoiding the deterioration of the product and improving the performance of the furnace. EP-A-19819 describes how some vapours generated in a kiln attack the surface of ceramic fiber material, causing flaking. Various solutions are mentioned, such as coating with protective substances or forming a film penetrating into holes provided in the wall. According to this invention, there is provided a method as set out in claim 1. In one form of this invention, the source generating an alkali component is glaze applied on a material to be fired. The present invention will now be explained specifically but not exclusively with reference to the accompanying drawings, in which:- Figure 1 is a perspective view of a kiln used in one embodiment of this invention, Figure 2 is a graphical view for illustrating a preferable range of temperatures prevailing in the kiln during the surface treatment, and Figure 3 is photographs showing the morphologies of ceramic fibers on the surface layer portions of the ceramic fiber bodies, which were treated according to this invention or received no treatment at all for the purpose of comparison. In the present disclosure, the term "furnace" shall refer to a kiln or other type of industrial furnace available in the art. Although not critical, each of the ceramic fibers, for instance, may be composed of 50 % by weight of Al₂O₃, 35 % by weight of SiO₂ and 15 % by weight of ZrO₂. An industrial furnace, in which at least the inner wall is lined with ceramic fibers, is for example constructed of one or more of fiber boards, fiber blocks and fiber blankets, all formed of ceramic fibers. Typically, the alkali component is an oxide of an alkali metal, e.g. K or Na, or alkaline earth metal, e.g. Ca or Mg. As the source generating the alkali component, it may be preferable to use a solution of a compound containing an alkali metal such as K or Na or an alkaline earth metal such as Ca, Mg. Typical examples are KOH, KCl, K₂SO₄, K₂CO₃, NaOH, NaCl, Na₂SO₄, Na₂CO₃, Ca(OH)₂, CaSO₄ and CaCO₃. Solutions of these compounds may be adjusted to a concentration of 0.1 to 10 % by weight, optionally with the addition of a water-soluble binder. The alkali component generating source may be a substance such as an alkali salt, glaze, or other rigidizer (a surface hardening material) or the like. For instance, these substances may be applied on the material to be fired. When the alkali component generating source is glaze, the material to be fired may be formed of any desired feed material. This invention is effectively applicable to products on which glaze for sanitary wares and tableware are put. This is because such tableware or sanitary wares are usually produced by applying glaze on the surface of the material to be fired, during which the alkali component contained in it is vaporized and the surfaces of ceramic fibers are impregnated with the resulting vapor. Thus, the glaze does not only serve to improve the resistance of the products to water or chemicals and the mechanical strength, surface gloss and appearance of the products, but is also effective to prevent the occurrence of the fiber dirt, thereby preventing the deterioration of the products. It was also confirmed that placing in a kiln a container with only the alkali component put in it is effective. The glaze used in the invention may be based on those used for tableware and sanitary ware. The principal ingredients of the glaze may be typically basic (alkali metal oxide such as Na₂O and K₂O, and alkali earth metal oxide such as CaO and Mgo), alumina and silica. As the other ingredients, various metal oxides may be added for the purpose of adjustment of the melting point and of obtaining clear coloring. Two methods of making the glaze can be mentioned. One method is to grind the raw material such as feldspar and lime into the condition of slurry or slip. The glaze made by this method is used as the "raw glaze". Another is to melt the raw material into the state of glass, to quench it into a frit and to grind it again. Preferably, the alkali component should be impregnated in an amount of 0.01 to 0.2 kg/m² with respect to the internal surface area of the furnace or in an amount of 0.1 to 1.0 kg/m³ with respect to the internal volume of the furnace. Preferably, the alkali component generating source should have a surface area in the range of 0.1 to 20 m²/m² with respect to the internal surface area of the furnace. The vaporization of the alkali component may be achieved by heating the furnace. For instance, a heating schedule such as the rate of the temperature rise, heating time, and the rate of the temperature drop and cycle of heating may optionally be determined. More specifically, a slow heating rate of 50 to 150 °C/hour after the furnace temperature has reached 600 °C is preferably applied so as to accelerate the vaporization of the alkali component. Preferably, the alkali component should be vaporized once to about ten times. It is also desired that the impregnation of the surface layer portion of the ceramic fiber body with the vaporized alkali component be applied to a fresh kiln. When the ceramic fibers of a furnace in normal operation are impregnated with alkali, attention should be fixed to soaking time and temperature rise rate. Especially when no alkali component is fed as experienced with a furnace used for refiring or any heating carried out not to vaporize the alkali component, it is required to feed the alkali component generating source into the furnace at certain intervals of time. Moreover, firing of glazed material is more effective and advantageous, since the vaporized alkali component is deposited onto the surface layer portion of the ceramic fiber body. The present invention will now be explained in greater detail with reference to the following example and comparative example. Example and Comparative Example Experimentation was carried out with such a kiln as illustrated in Fig. 1. The kiln 1 was lined with a ceramic fiber block. The fiber block in this case is a block of a rectangular parallelepiped shape, and is made of fiber blankets which are made of ceramic fibers. The material 3 to be fired was placed on a truck 2, which was then inserted into the kiln 1. A given amount of glaze was applied on the surface of the material 3. This glaze had the composition, in weight%:- SiO₂ 77.0 - 77.5 Al₂O₃ 17.0 - 17.5 CaO 0.30 MgO 0.20 K₂O 3.75 - 3.80 Na₂O 0.65 - 0.70 As shown in Figure 2, the interior of the (single) kiln 1 was maintained at a temperature ranging from 650°C to 950°C shown by an arrow A for about 2 hours. The lower temperature limit was placed at 650°C for the reason that at below 650°C, the vapor pressure of the alkali component in the glaze was so reduced in the kiln that the surface layer portion of the ceramic fiber block could not sufficiently be impregnated with the alkali vapor. The upper temperature limit, on the other hand, was placed at 950 °C for the reason that at higher than 950 °C, the glaze on the surface of the material 3 was so rapidly densified, as shown in Fig. 2, that the glaze could not play such roles as already mentioned. Set out below are the results of the experiments. In the example according to this invention, the alkali component in the glaze applied on the above material was vaporized to impregnate the surface layer portion of the ceramic fiber block with the thus vaporized alkali component. In the comparative example, on the other hand, the surface layer portion of the ceramic fiber block received no treatment at all. Figure 3 shows the microstructures of the ceramic fibers of the ceramic fiber blocks which were treated by the vapor of the alkali component in the glaze (the example) and received no treatment at all (the comparative example). In Fig. 3, (A) and (C) refer to the comparative example and (B) and (D) to the example according to this invention. A comparison of the example with the comparative example reveals that the ceramic fibers are discrete in the fiber structure of the comparative example, whereas they are organically interconnected with one another in the example according to this invention. The results of experimentation indicates that the ceramic fibers of the comparative example produce a number of small debris particles, which are in turn deposited onto the surface of the material to be fired, causing the fiber dirt. The ceramic fibers of the example according to this invention, however, produce no or little small debris. In other words, the occurrence of the fiber dirt caused by the deposition of small ceramic fiber debris particles onto the surface of the material to be fired is about 1/100 of that in the comparative example. The results of chemical analysis of the surface layer portions of the ceramic fiber blocks are set out in Table 1. From the results of analysis shown in Table 1, it is understood that the proportions of K₂O and Na₂O, inter alia , Na₂O are much higher in the example according to this invention than in the comparative example. This appears to be because the surface layer portion of the ceramic fiber block is so impregnated with the vapor of the alkali component in the glaze that the ceramic fibers can be interconnected and crosslinked with one another, thereby hardening the surface layer portion of the ceramic fiber body. The fiber debris is thus unlikely to fly apart in the kiln. According to the inventive method for treating the surface of a ceramic fiber body, as detailed above, the alkali component generating source is first placed in a furnace. When the furnace is then heated, the alkali component is vaporized to impregnate the surface layer portion of the ceramic fiber body with the thus vaporized alkali component, whereby the fibers are interconnected with each other to harden the surface layer portion of the ceramic body. Thus, the present invention ensures that the fiber dirt is prevented from appearing on the product due to no occurrence of fiber debris, thereby improving the performance of the furnace. Especially when the present invention is applied to products such as sanitary wares and tableware, which are likely to be decreased in their commercial value by fiber debris, it is possible to prevent the deterioration of the products, carry out rapid firing and reduce fuel consumption.

Claims: A method for treating the surface of the inner wall of a furnace, said inner wall being lined with at least one ceramic fiber body, comprising placing a source generating a vapour in the furnace and heating the interior of the furnace to produce the vapour, so that the vapour contacts the surface of the fiber body, characterized in that said source is an alkali component generating source comprising a compound of an alkali metal or alkaline earth metal, and said vaporized alkali component impregnates the surface layer portion of the ceramic fiber body the alkali component being selected so that under the conditions of the treatment the fibers are interconnected with each other to harden the surface layer portion of the ceramic body. A method as claimed in claim 1, wherein said ceramic fiber body is in the form of a ceramic fiber board, block or blanket. A method as claimed in claim 1 or 2, wherein the ceramic fibers are composed of 50 % by weight of Al₂O₃, 35 % by weight of SiO₂ and 15 % by weight of ZrO₂. A method as claimed in any one of claims 1 to 3, wherein said source generating an alkali component is a substance including at least one of an alkali salt and a glaze. A method as claimed in any one of claims 1 to 4, wherein said source generating an alkali component is glaze applied to a material to be fired. A method as claimed in any one of claims 1 to 4, wherein said source generating an alkali component is a solution of compound of at least one metal selected from alkali metals and alkaline earth metals. A method as claimed in claim 6, wherein said solution has a concentration of about 0.1 to 10 % by weight, optionally with the addition of a water-soluble binder. A method as claimed in any one of claims 1 to 7, wherein said alkali component is impregnated in an amount of 0.01 to 0.2 kg/m² with respect to the internal surface area of the furnace. A method as claimed in any one of claims 1 to 8, wherein said alkali component is used in an amount of 0.1 to 1.0 kg/m³ with respect to the internal volume of the furnace. A method as claimed in any one of claims 1 to 9, wherein said source generating an alkali component has a surface area in the range of 0.1 to 20 m²/m² with respect to the internal surface area of the furnace. A method as claimed in any one of claims 1 to 10, wherein said alkali component is vaporized by heating the furnace. A method as claimed in any one of claims 1 to 11, wherein the furnace heating rate after the furnace temperature has reached 600°C is regulated to the range of 50 to 150°C/hour so as to accelerate the vaporization of the alkali component in the furnace. A method as claimed in any one of claims 1 to 12, wherein heating is carried out once to ten times to vaporize the alkali component.

Inventors: TANIGUCHI SATOSHI, YAMADA KOJI, TANIGUCHI, SATOSHI, YAMADA, KOJI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: F27D1
IPC codes: C04B41, F27D1, F27B17