KR100419060B1 - A method of preparing silica sleeve for hearth roll by sol-gel process - Google Patents

Abstract

The method for producing a siliceous sleeve of the present invention comprises the steps of: ball milling the granulated fused silica powder to distilled water so that the average particle size is 1 µm or less; Pickling and washing the ball milled silica powder slurry to prepare fused silica from which impurities are removed; Preparing a slurry by adding the fused silica to a silica sol in which ultrafine fumed silica powder is dispersed in water; Adding a salt containing F ⁻ ions as a gelling promoter to the slurry; Pouring the slurry into a mold of a desired shape to gelate the slurry; And drying the gelled molded body, followed by calcination and firing.

Description

A manufacturing method of silicate sleeve for hearth roll using the sol-gel method {A METHOD OF PREPARING SILICA SLEEVE FOR HEARTH ROLL BY SOL-GEL PROCESS}

Field of invention

The present invention relates to a method for producing a siliceous sleeve used in a steel roll for transferring steel sheet, and more particularly, to a siliceous sleeve for continuously transferring a strip of steel sheet by a sol-gel method. It relates to a manufacturing method.

Prior art

Continuous heat treatment of the silicon steel strip causes galvanizing lines and the like to build up metal iron or iron oxide on the hearth roll surface at a high temperature of 950 ° C. or higher. Attached metal iron or iron oxide causes damage such as marking or scratching on the surface of the strip. Conventional steel rolls have a high incidence of such adhesion phenomenon especially in the high temperature region, but carbon rolls or asbestos rolls have been used instead, or coating treatments have been performed on the roll surfaces, but there is no satisfactory method yet. Asbestos rolls used in the annealing-pickling process of stainless steel (A · P line) have a disadvantage in that dehydration occurs at 700 ° C. or higher to lower the strength, shorten the life, and is very expensive including the maintenance cost. Asbestos also has problems with work environment hygiene.

In order to solve this problem, a ceramic sleeve made of high-purity fused silica having a low incidence of adhesion phenomenon at a high temperature of 950 ° C. or higher is used in a hearth roll for continuous processing in place of a conventional roll. The ceramic hearth roll made of fused silicate sleeve has a very low incidence of adhesion phenomenon and excellent heat resistance, so it can be used even at a high temperature of 1100 ° C. It is possible to have low thermal conductivity, thermal insulation effect, large diameter, long shape product, and can be used in any of oxidizing and reducing atmospheres.

Conventional methods for producing a siliceous sleeve for ceramic hearth rolls include a melt casting powder blended to an appropriate particle size, dispersed in water to form a slurry, and then injected into the gypsum mold to form a slurry. There is this. However, slip-cast silicate sleeves have superior characteristics compared to the conventional hearth roll sleeves, but have a disadvantage of having a lot of internal pores due to the low density microstructure by low-temperature firing during firing after molding. These internal pores provide an easy diffusion path for impurities that can cause adhesion phenomenon on the surface of the hearth roll and cause crystallization of silica during the yarn operation. Internal pores of the siliceous sleeve are essentially produced by low temperature firing upon firing after molding. The reason why low temperature firing is performed despite the formation of internal pores is that when firing at a high temperature of 1150 to 1200 ° C., impurities present in the fused silica cause crystallization of the fused silica and are destroyed due to mismatch of the coefficient of thermal expansion during cooling after firing. to be. In case of high temperature firing using only commercially available fused silica, there is a problem in that a large portion of the roll surface must be processed to remove these crystallized surfaces due to severe crystallization on the surface of the final molded product.

In addition, in the case of slip casting, a gypsum mold is generally used, and impurities are introduced into the molded body from the gypsum by direct contact between the surface of the gypsum and the molded body. Such impurities may cause crystallization of the silica molded body upon firing after drying of the molded body. In the manufacture of the molded body by the gypsum mold, as the thickness of the layer of the molded body increases, the diffusion path of the water becomes longer, and thus, a long process time is required when the thick body is manufactured. On the other hand, since the surface of the gypsum mold is not smooth and precise control of the thickness of the ground layer is difficult, there is a disadvantage in that the molded body or the sintered body after firing has to undergo mechanical processing to have a desired dimension.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a siliceous sleeve having improved gel molding density and sinterability of a molded article by using a silica slurry in which granulated fused silica and ultrafine fumed silica are mixed.

Another object of the present invention is to provide a method for producing a siliceous sleeve which can delay crystallization of a silica gel molded body by ball milling the fused silica powder to increase its specific surface area and remove impurities through pickling and washing processes.

It is still another object of the present invention to provide a method for producing a siliceous sleeve which minimizes post-processing of a molded or sintered body by eliminating impurities by using a precision mold capable of mechanically mirror-processing the surface.

In order to achieve the above object, the present invention comprises the steps of ball milling the granulated fused silica powder to distilled water and the average particle size of less than 1㎛; Pickling and washing the ball milled silica powder slurry to prepare fused silica from which impurities are removed; Preparing a slurry by adding the fused silica to a silica sol in which ultrafine fumed silica powder is dispersed in water; Adding a salt containing F ⁻ ions as a gelling promoter to the slurry; Pouring the slurry into a mold of a desired shape to gelate the slurry; And drying the gelled molded body, and then calcining and calcining.

Hereinafter, the present invention will be described in more detail.

The method of manufacturing a siliceous sleeve by the sol-gel method is largely composed of 1) preparation of silica sol, 2) gelation of sol, 3) drying of gel, and 4) heat treatment (calcination, firing) of dry gel.

In order to obtain the molded body strength suitable for the sol-gel method, fine particles are needed as much as possible, but there are some limitations in reducing the particle size of the commercially available fused silica powder, so that the gelation reaction does not occur easily with the fused silica powder alone. In order to compensate for this, ultrafine fumed silica powder having a particle size much smaller than that of commercially available amorphous fused silica powder is added to have a particle size composition suitable for gelation reaction, thereby providing moldability to the silica slurry and adding fine particles. Improve sinterability

The silica sol used in the gelling reaction in the present invention is a mixed slurry of fused silica and ultrafine fumed silica having an appropriate particle size. First, fused silica powder and water are mixed at a mass ratio of 1: 100 to 200: 100, and then ball milled to obtain a fused silica slurry. The optimum dispersion by the optimal ball milling is determined by measuring the average particle size change and the particle size distribution of the pulverized product according to the ball milling time. In general, the average particle size is 1 μm or less, preferably 0.5 μm or less and the particle size distribution is in radians. It is preferable to become. In the present invention, fused silica having a powder purity of at least 99.5%, preferably at least 99.7% is used.

The obtained slurry is subjected to pickling and washing steps after removing water to remove impurities such as Fe ions introduced during ball milling or alkali metal elements (Na, K, Ca, etc.) present in the first fused silica powder. The pickling process generally uses an aqueous hydrochloric acid solution and the washing process is continuously performed until the hydrochloric acid ion is not detected. After the pickling and washing process, the water is completely removed using a dryer to obtain a lump of fused silica. The fused silica produced through such a process reduces the amount of impurities to less than half, because elution of the iron component by the pickling process and alkali metal ions by the washing process are removed. Reduction of the particle size by the ball mill process increases the specific surface area, that is, exposes a lot of new surfaces to enhance the removal effect of alkali metal ions. These impurities can increase the firing temperature due to their removal because they cause crystallization of the silica during the high temperature firing of the silica molded body.

In the ball mill process of the fused silica particles, it is difficult to reduce the average particle size to 0.1 μm or less due to the characteristics of the grinding process. Therefore, there is a limit to higher density only by raising the firing temperature by removing impurities through the pickling and washing processes after the ball mill process. In this invention, the sinterability of a molded object is improved using the mixed slurry of fused silica and ultrafine fumed silica. The ultrafine fumed silica is excellent in surface activity because it is easily dispersed in water and has a high specific surface area. In the present invention, an ultrafine powder having a specific surface area of 100 m ² / g or less and an average particle size of ⁷ to 100 nm can be used, and a specific surface area of 40 to 60 m ² / g and an average particle size of 30 to 50. nm is preferably used. One such ultrafine fumed silica is Aero-sil OX-50, available from Degussa, Germany.

The ultrafine fumed silica powder is added to distilled water to disperse to prepare a silica sol, and then mixed with the fused silica mass from which impurities are removed. Fused silica: ultrafine fumed silica is mixed in a mass ratio of 99: 1 to 1:99. If the amount of fumed silica is larger than the above, the sintering property is relatively excellent, but the molding density is lowered at the time of gel molding, so that the low forming density attenuates the sinterability synergistic effect by the ultra-fine particles, so that a high density sintered compact cannot be obtained. On the other hand, if the amount of fumed silica is less than the above, there is a disadvantage in that the strength of the wet molded product or the dried molded product is very weak even if gelation does not occur easily or gelates due to the decrease in the surface activity of the silica. The mass ratio of the fused silica: ultrafine fumed silica is preferably in the range of 70:30 to 90:10. The blending ratio of distilled water to silica is 5 to 80% by weight. The optimum solids content in the slurry is preferably 40 to 60% by weight.

In order to enhance dispersibility in the mixture of the fused silica and the ultrafine fumed silica sol, an ammonia hydroxide organic substance dispersant is added in an amount of 0.1 to 5.0 wt% based on the silica. If the mixture is mixed in a high speed blender and mixed, it has considerable fluidity, and ball milling to increase the mixing yields a slurry having a very low viscosity. At this time, when the amount of silica to distilled water is too small, the amount of water is large, the strength of the gel is lowered. On the contrary, when the amount of silica is too large, the ball viscosity is not high because the viscosity is high.

A salt containing 0.01 to 2% by weight of F ⁻ ions is added to the silica slurry to which the dispersant is added as a gelling accelerator with respect to distilled water. Preferred gelling promoters are NH ₄ F. The gelation proceeds within 5 to 24 hours due to the addition of the salt. In order to remove bubbles of the silica slurry, a defoaming operation is performed in a vacuum vessel and then injected into a mold. In the present invention, it is possible to minimize the post-processing of the molded body or the sintered body by preventing the introduction of impurities by using a precision mold capable of mechanically mirror surface processing. After mold injection, seal the mold to prevent evaporation of moisture. The time until completion of the gelling reaction is influenced by the amount of solids and the amount of gelling accelerator, but is usually preferably within 3 hours.

After the gelation reaction is completed, after aging for 48 hours or more, the prepared silica gel is removed from the mold and dried at room temperature. Wet molded products are different depending on the thickness of the sleeve and the component ratio of the silica, but more than 98% of the moisture is removed if kept at room temperature for more than two days. The dried gel is dried again using a dryer to remove adsorbed water on the surface of the silica particles. Since the structural water contained in the silica acts as a crystallization factor of the silica when the residual OH group is calcined at a high temperature, the secondary dried gel is calcined to completely remove the structural water of the silica. Relative density of the dried gel after the drying and calcining process as described above is different depending on the mixing ratio, but a relative density of 75% or more is obtained. Firing is carried out at a temperature of 1100 ~ 1200 ℃ in the air, and when firing at a temperature of 1200 ℃ or more so as not to exceed 30 minutes as possible. After the sintering is completed, the obtained sintered body has a smooth surface corresponding to the processing state of the mold surface, and when the shrinkage ratio of the sintered body is controlled in advance, a precise sintered body which hardly requires post-processing of the molded body or the sintered body is obtained.

The following presents a preferred embodiment to aid the understanding of the present invention. However, the following examples are provided only to more easily understand the present invention, and the present invention is not limited to the following examples.

Examples and Comparative Examples

Example 1

Molten silica powder with purity of 99.5% or more (Boramil-F product made by Boram Kemetal): Distilled water is mixed in a ratio of 10:90, and the average particle size is 0.5 μm using a jar and a ball made of SUS. Ball milled the distribution to be in radians. The obtained fused silica slurry was sufficiently dehydrated, redispersed in an aqueous 10 vol% hydrochloric acid solution, stirred for 24 hours or more to dissolve Fe ions mixed in the ball mill, and then placed in a filter with a vacuum device to sufficiently remove the liquid phase. Secondary distilled water was further added to the fused silica slurry to carry out a continuous washing process. The washing process was continued until no chlorine ions were detected in the distilled water passed through the fused silica cake (bed). Through this washing process, alkali metal elements such as Na, K, and Ca present in the first fused silica powder were removed. The washed fused silica mass was dried in a drier for at least 24 hours to completely remove moisture.

Degussa Aero-sil OX-50 was used as a fumed silica powder having a specific surface area of 50 m ² / g and an average particle size of 40 nm. 500 g of fumed silica was added to 1000 gdp of distilled water, and then dispersed well using a blender to prepare a silica sol. The fused silica lump prepared in the previous step was added thereto. 1000 g of silica mixed with fused silica: fumed silica to 80:20 was added to 1000 g of distilled water, followed by mixing for 10 minutes in a high speed blender to obtain an aqueous silica slurry having considerable fluidity.

0.3% by weight of ammonium hydroxide-based dispersant (TMAH) was added to the silica slurry in this sol state, and then mixed in a high-speed blender for 10 minutes to increase the fluidity of the mixture. Very low silica slurry was obtained. 0.1 wt.% NH ₄ F was added to the distilled water. The prepared silica slurry sol was degassed in a vacuum vessel to remove bubbles. The silica slurry was then poured into a sleeve shaped mold and sealed to prevent evaporation of moisture. After 60 minutes, gelation was completed and aged for 48 hours, the prepared silica gel was removed from the mold and placed on a soft sponge, and dried at room temperature. The first dried gel was further heated to 110 ° C. at a temperature increase rate of 5 ° C./h using a drier and maintained at that temperature for 48 hours to remove adsorbed water on the surface of silica particles. The secondary dried gel was heated up to 900 ° C. at 100 ° C./h and maintained at that temperature for 2 hours to completely remove the structural water of silica. Then, the mixture was sintered at 1150 ° C. for 2 hours, and then slowly cooled to room temperature. The manufactured siliceous sleeve sintered body had a smooth surface and had a precise structure requiring little post processing. XRD analysis of the sintered body showed that the crystal phase was less than 1% and the relative density of the fired ceramic sleeve was 93% or more.

Comparative Example 1

Silica sleeves were prepared in the same manner as in Example 1 except that only the molten silica powder ball milled to 0.5 μm and no fumed silica powder were used in the preparation of the silica slurry sol. The crystal phase was about 1% and the relative density was 85%.

The method for producing a siliceous sleeve of the present invention improves the molding density of the gel and the sinterability of the molded body by gelling a mixed silica slurry sol of fused silica and ultrafine fumed silica of granulation, and increases the specific surface area by ball milling the fused silica powder. The crystallization of the silica gel molded body may be delayed by removing impurities through the pickling and washing processes, and the post-processing of the molded body or the sintered body may be minimized by eliminating the inflow of impurities using a precision mold capable of mirror surface machining.

Claims (5)

a) adding granulated fused silica powder to distilled water and then ball milling the average particle size to 1 μm or less; b) pickling and washing the ball milled silica powder slurry to prepare fused silica from which impurities are removed; c) preparing a slurry by adding the fused silica to a silica sol in which the ultrafine fumed silica powder is dispersed in water so that the mass ratio of fused silica powder: ultrafine fumed silica powder is 70:30 to 90:10; d) adding NH ₄ F as a gelling promoter to the slurry; e) pouring the slurry into a mold of desired shape to gelate; And f) drying the gelled molded body, followed by calcination and firing Silica sleeve manufacturing method comprising a. The method of claim 1 wherein the fused silica powder has a purity of at least 99.5% and has an average particle size in the range of 0.5 μm. The method for producing a siliceous sleeve according to claim 1, wherein the specific surface area of the ultrafine fumed silica powder is 100 m ² / g or less. delete The method of claim 1, wherein the amount of NH ₄ F added is 0.05-3% by weight relative to silica.