KR100417682B1 - A composition for molding silica sleeve and a method of preparing silica sleeve therefrom - Google Patents

Abstract

The present invention relates to a fused silica powder and ultrafine fumed silica powder of granules, a basic aqueous solution as a dispersant, polyethyleneimine (PEI), a pH reducing agent, and a composition for forming a siliceous sleeve including formamide as a drying aid and silicate therefrom. Provided is a method of making a sleeve. The method for producing a siliceous sleeve of the present invention comprises the steps of dispersing ultrafine fumed silica powder in water in a fused silica powder of granulation to prepare a water-based silica slurry; Adding a basic aqueous solution to the slurry to increase the pH of the slurry to 10 or more and redispersing; Adding polyethyleneimine (PEI), a pH reducing agent, and a drying aid to the redispersed slurry to mix uniformly; Mole of the desired shape

Pouring into gel to gel; And drying the gelled molded body, followed by calcination and firing.

Description

A composition for forming a siliceous sleeve and a method for producing a siliceous sleeve therefrom {A COMPOSITION FOR MOLDING SILICA SLEEVE AND A METHOD OF PREPARING SILICA SLEEVE THEREFROM}

Field of invention

The present invention relates to a composition for forming a siliceous sleeve for use in a steel roll for transferring steel sheet and a method for producing a siliceous sleeve from the same, and more particularly, to a non-toxic polyethyleneimine (PEI) for preparing a silica slurry sol. The present invention relates to an environment-friendly and economical composition for forming a siliceous sleeve, which is used as a binder and to which a drying aid is added to improve the drying ability of the gel, and a method for producing the siliceous sleeve therefrom.

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. 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 of various products in place of conventional steel or asbestos sleeve. 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 as compared to the conventional hearth roll sleeves, but have a disadvantage of having a lot of internal pores due to low density because they have a low density microstructure by low temperature firing during firing after molding.

As a method for compensating for the drawbacks of the slip casting, there is a direct coagulation casting using a sol-gel process. The manufacturing process of the siliceous sleeve by the coagulation method is generally based on the production of silica sol, gelation of the sol, drying of the gel, and heat treatment (calcination, firing) of the drying gel. In the coagulation method, the surface charge of the solid particles in the oxide slurry is dispersed by mutual repulsive force, and then, by adding an acid or a base, the pH of the slurry is moved to an intrinsic isoelectric point region of the oxide to remove the mutual repulsive force by the electrostatic force. It is a method of applying the phenomenon that the particles are strongly bonded to each other and solidified by interparticle van der Waals. In fact, such coagulation generally occurs at the isoelectric point, but in the case of silica, the same coagulation occurs in a region other than the pH 2-2.5, i.e., pH 7.5-9.5.

The silica sol is prepared by adding commercially available granulated fused silica powder and ultra fine fumed silica powder to water, mixing the same, adding a dispersing agent and a binder, and then dispersing it. As the dispersant, tetramethylammonium hydroxide and tetraethylammonium hydroxide, which are quaternary ammonium hydroxides exhibiting high pH characteristics, are used. These dispersants induce electrostatic dispersion and bring the pH of the finally obtained silica sol to at least 10. As the binder, polyethyloxazoline, polymethyloxazoline, polyacrylamide-based polymer and the like are used. Such a binder increases the bonding strength of the particles in the gel due to its adsorption properties, thereby preventing cracks in the molded body when the wet gel is dried, and increasing the strength of the dried gel. As the dispersant, tetramethylammonium hydroxide and tetraethylammonium hydroxide, which are quaternary ammonium hydroxides exhibiting high pH characteristics, are used. These dispersants induce electrostatic dispersion and bring the pH of the finally obtained silica sol to at least 10.

After sealing the silica sol in which the fused silica, dispersant and binder are dispersed in water, the sol is stabilized by standing at room temperature for at least 24 hours. To the stabilized sol a pH reducing agent, which is a water soluble aliphatic ester of an acid, such as ethyl lactate, methyl lactate, ethyl formate, methyl formate, is added to gel the sol. Before the sol mixture thus prepared is gelled, a shaped body of the desired form can be obtained and poured into a mold for gelation (coagulation).

Once gelling is complete and of a certain strength, it is removed from the mold to separate the gel and then dried. The dried gel is first heat treated to remove organic matter in the gel, and the gel molded body from which the organic matter is removed is sintered to prepare a siliceous sleeve.

Through the solidification process, it is possible to easily produce a large silica siliceous sleeve of several tens of kg or more. However, the polymers used as the binders may cause environmental pollution, so that a strong toxic gas is generated in the first organic material removal step of the manufacturing process. In addition, there is a disadvantage that the price of the pH reducing agent used when the pH of the slurry is moved to a high pH region and then to a low pH region where a coagulation reaction occurs.

An object of the present invention is to form a siliceous sleeve which is less toxic and does not cause environmental pollution by using a binder capable of shifting the isoelectric point of silica to a high pH range, as well as reducing the use amount of expensive pH reducing agents. To provide a composition for and a method for producing a siliceous sleeve therefrom.

Another object of the present invention is to prepare a composition for forming a siliceous sleeve and a siliceous sleeve therefrom, which can greatly reduce the probability of drying failure in the production of large silicidic sleeve by using a drying aid that can improve the drying ability of the gel. It is to provide a method.

In order to achieve the above object, the present invention includes an aqueous silica slurry of granulated fused silica powder and ultrafine fumed silica powder, a basic aqueous solution as a dispersant, polyethyleneimine (PEI), a pH reducing agent, and formamide as a drying aid. It provides a composition for forming a siliceous sleeve.

In addition, the method for producing a siliceous sleeve of the present invention comprises the steps of dispersing the ultrafine fumed silica powder in water in the fused silica powder of granulation to prepare a water-based silica slurry; Redispersing by adding a basic aqueous solution to the slurry; Adding polyethyleneimine (PEI), a pH reducing agent, and a drying aid to the redispersed slurry to mix uniformly; 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.

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

Aqueous silica slurry is prepared by mixing a commercially available molten silica powder having a suitable average particle size and a high purity ultrafine fumed silica powder in a suitable ratio and dispersed in water. In the present invention, it is preferable to use fused silica having a purity of 99.5% or more, and more preferably having a purity of 99.7% or more. First, the fused silica powder and water are mixed at a mass ratio of 1: 2 to 4: 1, and then ball milled so that the average particle size is in the range of 1 to 40 μm and the particle size distribution is in the radian type to obtain a fused silica slurry. The preferable range of the said average particle size is 15-25 micrometers. The slurry is pickled and washed to remove impurities such as Fe ions introduced during ball milling and 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.

In the present invention, since it is difficult to induce a solidification reaction with only the fused silica particles of the granulated particles, ultrafine fumed silica powder is added to induce a solidification reaction and improve sinterability by addition of fine particles. 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 20 to 300 m ² / g 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 may be preferably used. One such ultrafine fumed silica is Aero-sil OX-50, available from Degussa, Germany. Ultrafine fumed silica powder is added to distilled water and dispersed to prepare a silica sol.

To the prepared silica sol is added fused silica from which impurities are removed in the previous step. Fused silica: ultrafine fumed silica is mixed in a mass ratio of 40:60 to 95: 5. If the amount of fumed silica is higher than the above, the solidification reaction is easily progressed to increase the strength of the molded body, but the wear resistance property of the roll decreases and the probability of cracking in the molded body due to the large number of micropores present in the molded body during drying. This is big. On the other hand, when 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 solidification does not easily occur or solidifies due to the decrease in surface activity of silica. It is preferable that the mass ratio of the said fused silica: ultrafine fumed silica exists in the range of 70: 30-80: 20. The amount of silica in the total slurry is at least 50% by weight and is generally contained at 50 to 80% by weight. The amount of silica in the slurry is preferably present in as large an amount as possible.

A basic aqueous solution is added to the silica slurry in the sol state prepared in the above manner to increase the pH to 11 or more to uniformly redisperse the silica sol. As the basic aqueous solution, tetramethylammonium hydroxide (TMAH) or tetraethylammonium hydroxide (TEAH) containing NH ₄ ^- ions can be preferably used. The basic aqueous solution is preferably added in an amount of 2.8% by weight relative to the silica, wherein the pH of the slurry increases to 11 or more. In order to increase the mixing degree of the slurry, the silica slurry can be obtained by performing ball milling using quartz and balls. After the slurry is aged at room temperature for 24 hours, defoaming is performed in a vacuum vessel to remove bubbles.

Water-soluble polyethyleneimine is added to the slurry to which the basic solution is added. The average molecular weight of polyethyleneimine is preferably 1,000,000 or less, and this polymer should be completely removed in the calcination step of the molded body so that it does not remain as a substance causing crystallization upon firing of silica, which is a subsequent process. Polyethyleneimine can serve as a binder at the interface of silica particles and thus can be used in place of conventional toxic binders such as polyethyloxazoline. Polyethyleneimine has the property of shifting the isoelectric point of silica from pH 2.5 to 3.5 to pH 10.6. Therefore, polyethyleneimine may serve to move a slurry having a high pH to a low pH region by addition of a basic solution, thereby replacing the role of a pH reducing agent (coagulant) that is used in the past. Therefore, in the present invention, by adding polyethyleneimine having a low toxicity to the slurry, it is possible to minimize environmental pollution and to reduce the amount of expensive pH reducing agent, thereby reducing the manufacturing cost of the siliceous sleeve.

In the present invention, the polyethyleneimine is used as the main coagulant and the existing pH reducing agent is used as the coagulant. Polystyreneimines and pH-reducing agents, used as coagulants, are added in amounts that drop the slurry to near 10.6. The pH reducing agent is preferably added in an equivalence ratio of 0.1 to 0.3 with respect to the basic aqueous solution. As the pH reducing agent, methyl formate or ethyl lactate may be preferably used.

Formamide is added to the mixed slurry in an amount of 2 to 150% by weight based on the basic aqueous solution as a drying aid. If the amount of formamide exceeds 150% by weight, there is a problem in that the liquid phase increases in the total gel, and when used in an amount less than 2% by weight, the effect of improving the drying performance is not preferable, which is not preferable. The formamide acts as a drying aid to significantly improve the drying ability of the gel, and according to the improvement of the drying ability, it is possible to greatly reduce the probability of dry breakage in the production of large silica-like sleeves. It also acts as a coagulant because it can be dropped. Thus, the use of formamide may also describe an additional economic effect of reducing the amount of expensive pH reducing agents (coagulants).

The sufficiently stirred silica slurry is poured into a metal mold capable of precision machining in the form of a sleeve. After mold injection, seal the mold to prevent evaporation of moisture. Over time, the pH of the silica slurry injected into the mold gradually decreases and solidifies rapidly when it reaches 10.6. The time to completion of the coagulation reaction is influenced by the amount of solids and the amount of coagulant, but is usually preferably within 20 to 40 minutes.

After solidification is completed, the prepared silica gel is aged from at least 24 hours and then dried at room temperature. Wet molded products are different depending on the thickness of the sleeve and the component ratio of silica, but at least one week at room temperature to remove more than 90% of the moisture. 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. Firing is carried out at a temperature of 1000 to 1400 ° C., preferably 1150 to 1250 ° C. in air, and when firing at a temperature of 1200 ° C. or higher, the firing time should not exceed 30 minutes. 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 merely provided 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 a purity of 99.5% or more (product of Boramil-F manufactured by Boram Kemetal): Distilled water is mixed at a ratio of 10:90 and the ball is made to have an average particle size of 20 μm by using a jar and a ball made of SUS. Milled. 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). The washed fused silica mass was dried in a drier for at least 24 hours to completely remove moisture.

As the fumed silica powder for inducing coagulation reaction, Aero-sil OX-50 manufactured by Degussa having a specific surface area of 50 m ² / g and an average particle size of 40 nm was used. 1000 g of the mixed silica so as to have a mass ratio of fused silica: fumed silica prepared in the previous step 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.

Tetramethylammonium hydroxide was added to the silica slurry in an amount of 2.8% by weight relative to the silica, followed by stirring to raise the pH of the silica sol to 11 or more. In order to increase the mixing degree of the slurry, the silica slurry was obtained by ball milling for 8 hours using a quartz jar and a ball. The slurry was aged at room temperature for 24 hours and then degassed in a vacuum vessel to remove bubbles.

0.25 wt% of polyethyleneimine having an average molecular weight of 750,000 was added to the silica slurry.

To the silica slurry was added 25% by weight of ethyl lactate relative to tetramethylammonium hydroxide and 100% by weight of formamide relative to tetramethylammonium hydroxide with slow stirring. The sufficiently stirred silica slurry was poured into a sleeve-shaped mold and sealed to prevent evaporation of moisture. Over time, the silica sol solidified with a decrease in pH. After coagulation was completed, aged for 24 hours, the prepared silica gel was removed from the mold and placed on a soft bottom to dry 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 compact showed that the crystal phase was less than 1% and the rest was composed of amorphous material. The relative density of the fired ceramic sleeve was within 85 to 95%.

Comparative Example 1

Polyethyloxazoline as a binder was added in an amount of 0.25% by weight with respect to silica, and a pH reducing agent, ethyl lactate was added in an amount of 25% by weight with respect to tetramethylammonium hydroxide, and no formamide was used. Silica sleeve was prepared in the same manner as in Example 1. The prepared siliceous sleeve had a crystal phase of less than 1% as a result of XRD analysis and a relative density was in the range of 83 to 93%. However, in this case, the dry fracture frequently occurs during the drying of the wet molded body of the sleeve, and also a lot of toxic gases are generated during calcination.

The present invention uses polyethyleneimine which is not toxic when preparing a silica slurry sol as a binder, and improves the drying ability of the gel by using a drying aid. The use of an imine and a drying aid has the effect of reducing the amount of expensive pH reducing agent added to provide an environment-friendly and economical composition for forming a siliceous sleeve and a method for producing a siliceous sleeve therefrom.

Claims (11)

Aqueous silica slurry of fused silica powder having an average particle size in the range of 1 to 40 μm and ultrafine fumed silica powder having an average particle size in the range of 7 to 100 nm; Basic aqueous solution as a dispersant; Polyethyleneimine (PEI); pH reducing agents; And formamide as a drying aid, wherein the formamide is added in an amount of 5 to 150% by weight based on the basic aqueous solution. The composition for forming a siliceous sleeve according to claim 1, wherein the fused silica powder has a purity of 99.5% or more and has an average particle size in a range of 15 to 25 μm. The composition for forming a siliceous sleeve according to claim 1, wherein the ultra-fine fumed silica powder has a specific surface area of 20 to 300 m ² / g. The composition for forming a siliceous sleeve according to claim 1, wherein the ultrafine fumed silica powder has a specific surface area of 40 to 60 m ² / g and an average particle size of 30 to 50 nm. The composition for forming a siliceous sleeve according to claim 1, wherein the fused silica powder: ultrafine fumed silica powder has a mass ratio of 40:60 to 95: 5. The composition for forming a siliceous sleeve according to claim 1, wherein the basic aqueous solution is tetramethylammonium hydroxide (TMAH) or tetraethylammonium hydroxide (TEAH) and is added so that the pH of the slurry is 10 or more. The composition of claim 1, wherein the polyethyleneimine has an average molecular weight of 1,000,000 or less. The composition of claim 1, wherein the pH reducing agent is methyl formate or ethyl lactate. delete Preparing a water-based silica slurry by dispersing ultrafine fumed silica powder having an average particle size in the range of 7 to 100 nm in water to a fused silica powder having an average particle size in the range of 1 to 40 μm; Redispersing by adding a basic aqueous solution to the slurry; Adding formamide to the redispersed slurry with polyethyleneimine (PEI), a pH reducing agent and a drying aid to uniformly mix (the amount of formamide is 5 to 150% by weight relative to the basic aqueous solution); Pouring the slurry into a mold of a desired shape to gelate the slurry; And Drying the gelled molded body, and then calcining and firing Silica sleeve manufacturing method comprising a. The method of claim 10, wherein the firing step is carried out at a temperature of 1000 ~ 1400 ℃.