WO2003105991A1

WO2003105991A1 - Porous ceramic filter and method of producing the same

Info

Publication number: WO2003105991A1
Application number: PCT/KR2003/001132
Authority: WO
Inventors: Soyoung Shin
Original assignee: Soyoung Shin
Priority date: 2002-06-12
Filing date: 2003-06-09
Publication date: 2003-12-24
Also published as: AU2003237041A1; KR20020054300A; KR100483512B1

Abstract

This invention includes a porous ceramic filter and its manufacturing method. A porous ceramic filter consists of only nanometer scale sized silica particles or which consists of nanometer scale sized silica particles such as fumed silica or colloidal silica and organic/inorganic additives. The manufacturing method includes the dispersion step making slurry with fumed silica or colloidal silica, the forming step creating a body from the slurry by molding, the drying step drying the body as well as separating the body from a mold, and the calcination step giving the dried body a heat treatment at 500~1100 °C. This invention provides an efficient method manufacturing a porous ceramic filter, in which the diameter of pores is less than 0.1 micrometer, with high filterability.

Description

Porous ceramic filter and method of producing the same

L Field of the invention and Technical backgrounds

This h vention describes the porous ceramic filter and its manufacturing process. Generally, different from synthetic organic membrane filters, ceramic filters have heat resistance, corrosion resistance, chemical resistance, acid resistance, and durability. They are used in general filtering system. Usage of ceramic filter includes water, oil and organic solvent filtering processes. Because their pore size is up to 0.1-1 tm they are filtering out impurities and are not filtering out the minerals which are good to human health. As shown in the previous example, ceramic filters are used in both aqueous and non-aqueous systems, and also their usage includes gaseous filtering process. There are no usage limitations in the kind of treated materials.

Generally, ceramic filter manufacturing process is comprised of dispersion, forming, drying, and calcination just like the general ceramic manufacturing process. The quality of filter is determined by the material composition used for manufacturing. Ceramic materials can be anything such as silica, alumina, zirconia, clay, brown coal, and chamotte, etc. Also, the kinds and properties of the materials make body with various characteristics. Dispersion step is to mix the ceramic materials with water or organic solvent such as alcohol, at times with dispersant, binders or pore making agents. Dispersant is used to prevent particles from agglomerating in the water or organic solvent and to disperse them. Binder is used to improve the binding force among the particles. Pore making agent is used to make a porous body. Above materials are homogeneously mixed with agitators, blenders, or ultrasonic machine, and also sometimes are underwent centrifugal separation processes to eliminate impurities in ceramic slurry. Casting is a step to make casting bodies with prepared slurry. The methods are molding, extrusion, centrifugal casting, pressure forming, and injection molding. Gelation agent is added just before the casting process and the product is separated from the mold to dry after casting. Drying step is for getting rid of the remaining water or organic solvent from the body. Natural drying or controlled temperature-humidity drying method is used. Calcination stage is to improve the rigidity of the dried body with heat treatment at 900-1500 ^°C so that the binding force between particles became stronger by viscous flow mechanism. If organic additives are used, the organic materials elimination process (120 - 600 °C) is added. Ceramic filter's pore must be small and of similar sizes. Ceramic filter must be porous and rigid to endure certain pressure. The high porosity of a filter means high filtering percentage, so it's desirable to make a filter as porous as possible.

Ceramic filter's pore size and distribution are important because they determine that filter's filtering ability. Existing ceramic filter has 0.1 - 1 n pores, its porosity is less than 50% and pore distribution is not so uniform. These problems can be overcome with small ceramic particles (less than 0.1 ) but it has difficulty in production processes. Generally, the smaller the particle size becomes (less than 0.1 m) i ceramic, the more problems it makes in that no homogeneous dispersion, high capillary pressure in drying process, and high shrinkage rate.

2. Aim of the invention

This invention's aim is to provide the ceramic filter and the manufacturing method, which can make a porous ceramic filter with high porosity and uniform distribution of pores from nanometer scale (sub- micron) sized silica particles.

3. Detailed description of the invention

To make the aim possible, this invention provide a manufacturing method of porous ceramic filter with more than 50% of porosity and with uniform distribution of pores from fumed silica or colloidal silica.

This invention, the manufacturing method of a porous ceramic filter, is composed of dispersion, casting, drying, and calcination step just like general ceramic filter manufacturing processes. To make a high porosity with less than 0.1 n, it is desirable to use ceramic particles with less than 0.1 β⁽\. Therefore, this invention uses fumed silica and colloidal silica.

Fumed silica is produced with Silicon tetrachloride (SiCl₄), Oxygen, and

Hydrogen by flame hydrolysis. Commercial products are sold with size of several nm up to 40 nm. (Products: Degussa, Cabot, and Wacker, etc.)

Whereas colloidal silica is colloidal slurry called silica sol which is made from alkoxy silanes such as sodium silicate, potassium silicate, or tetramethyl orthosilicate, tetraethyl orthosilicate, and catalysts (alkali or acid) and has various sizes of silica particles. It is now in market with several ran up to several n in diameter and with content of 5~50% in total mass.

Dispersion step for fumed silica is to mix it with water and make homogeneous slurry with proper content and viscosity to casting. For the strength of a wet body and the low shrinkage, high content of fumed silica is good and more than 40% we can get good drying yield. So, in case of high content, dispersant is added to make the particles repel each other among them by induced electric surface charges. And, to make slurry's pH more than 10, alkalis such as sodium hydroxide, ammonia water (NH4OH), and teframemylammonium hydroxide are used. Organic polymers such as polyvinyl alcohol, poly (2-ethyl-2-oxazoline), and polyethylenimine can be used as binder to improve the strength of a wet body and the speed of drying process

Above binders not only improve the binding force among particles but also shorten drying time by dropping hydrophilic property of particle surfaces. Besides, plasticizer such as glycerine is used to improve softness of wet body. Above materials are selectively mixed and dispersed by blenders or ultrasonic dispersion machines to their manufacturing purpose. If need arises, centrifugal separation process to eliminate high density impurities or agglomerates can be used.

Colloidal silica is sold in slurry form with 5~50% in total mass because of the characteristics of the manufacturing process. So above additives are selectively added. Either concentration or addition of fumed silica is used to achieve high silica content. Slurry manufactured by above methods undergoes casting stage. Molding, extrusion, and centrifugal casting are used in casting step and which is good method is judged by components, the size, and shape of casting body.

Gelation agents are used to induce casting. Esters such as ethyl lactate, methyl lactate are used in slurry with above pH 10 in order to lower pH. Fluorine compound such as ammonium fluoride(NH4F) is used regardless of slurry's pH. Casting body is manufactured by molding after agitating silica slurry and gelation agents.

Products after casting are separated from a mold to dry after some time interval to improve the strength of a wet body. Wet body separated from a mold undergoes natural drying or controlled chamber drying processes. Drying condition varies temperature 10-70 °C, relative humidity 20-90% with respect to the composition and size of the wet body. Drying process must continue to the point when wet body's weight loss is zero. So, it takes several hours to more than 10 days with respect to the size of the wet body.

Casting body after drying process undergoes heat treatment to eliminate remaming water and organic materials, to improve the strength of the body.

Remaining water can be eliminated at the temperature of 120-150 °C and remaining organic materials at 300-600 °C . Lastly, calcination process to improve the strength of the body takes place in electric furnaces at the temperature of 500-1100 °C and depends on the size of material sizes. It is desirable that the calcination process be done until the porosity is not excessively decreased by the too much heat treatment.

This invention can be concretely described by following examples.

Example 1)

We have made silica slurry by mixing Aerosil OX-50 (fumed silica, products: Degussa, diameter: 40nm) 0.3kg and deionized water 0.3kg with the help of high performance blender. Then, ammonium fluoride (NHjF) solution (50 weight percent concentration) lOcc was added to the slurry and agitated. During agitating, the viscosity of the slurry was lower than 50cps. However, after agitating, the viscosity was increased as times go. At last, a wet body was created without fluidity. Silica slurry in a fluid state were put into a mold and formed. In order to create tube- type filter, a mold was composed of an acrylic flat (diameter 5cm), an acrylic tube (inner diameter 5cm) installed on the flat, and a SUS rod (diameter 3cm) set up in the center of the flat. A silica body separated from a mold underwent natural drying during 4 days. Remaining water was eliminated at 150 °C for 3 hours and a body underwent calcination process at 900 °C for 1 hour. The manufactured tube-type porous ceramic filter has a length of 27cm, a thickness of 0.9cm, an average pore size of 38nm, and a porosity of 65%.

Example 2)

We have made silica slurry by mixing Aerosil OX-50 1.1kg and deionized water 1kg as described in Example 1. After 12 hours, ethyl lactate 80cc was added to silica slurry (pH=11.90). Silica slurry was agitated, put into a tube-type mold, and formed. After 2 hour, a SUS rod was eliminated and a body was separated from the mold. Drying and eliminating remaining water were done as explained in Example 1. Remaining organic materials were eliminated at the temperature of 300-600 °C during 3 hours. Calcination process was done as described in Example 1. The manufactured tube-type porous ceramic filter has an average pore size of 38nm and a porosity of 65% .

Example 3)

Mixing and dispersing process were done as described in Example 2. A tube-type ceramic filter was formed by putting the silica slurry into a mold for centrifugal casting, adding ethyl lactate, and rotating it at 1500rpm during 20 minutes. A manufactured wet body underwent drying, eliinmating remaining water and organic solvents, and calcination process just like Example 2. The manufactured tube-type porous ceramic filter has an average pore size of 34nm and a porosity of 67%. Example 4)

We have made silica slurry by mixing Aerosil 90 (fumed silica, products: Degussa, diameter 20nm) 0.3kg and deionized water 0.4kg as described in Example 2. After 12 hours, silica slurry were formed as described in Example 3. A wet body separated from the mold underwent controlled chamber drying processes at the temperate of 20 ^°C and the humidity of 80% during 5 days. Eliminating remaining water and organic materials were done as same as in Example 2. Calcination process was done at 700 ^°C for 1 hour. T e manufactured tube-type porous ceramic filter has an average pore size of 18nm and a porosity of 70%.

Example 5)

We have made silica slurry by mixing OX-K50 (colloidal silica, products: Dupont, 49.8% in total mass, diameter 40nm) 0.6kg and tetramemylammonium hydroxide 15cc as described in Example 2. (pH=11.8) After ethyl lactate 25cc was added to the slurry, they were agitated, formed in a mold just like in Example 1, and dried. Calcination process was done at 850 °C for 1 hour. The manufactured tube-type porous ceramic filter has an average pore size of 35nm and a porosity of 65%.

Example 6)

We have made silica slurry by mixing Nyacol 2040 (colloidal silica, products: Nyacol Product, Inc., 40% in total mass, diameter 20nm) 0.6kg and tetramethylamrnonium. hydroxide 15cc as described in Example 2. (pH=11.8) After ethyl lactate 20cc was added to the slurry, they were agitated, formed in a mold just like in Example 3, and dried. Calcination process was done at 650 ^°C for 1 hour. The manufactured tube-type porous ceramic filter has an average pore size of 15nm and a porosity of 70%.

Example 7)

We have made silica slurry by mixing Aerosil OX-500.9kg, Aerosil 90

0.1kg, deionized water 1kg, and tetramethylammoriium hydroxide 0.12kg as described in Example 2. After 12 hours, ethyl lactate '80cc was added to silica slurry (pH=11.60). Silica slurry was agitated, put into a tube-type mold, and formed. After 2 hour forming, a SUS rod was eliminated and a body was separated from the mold. Drying and elimmating remaining water were done as explained in Example 2. Remaining organic materials were eliminated at 600 °C during 3 hours. Calcination process was done as described in Example 2. The manufactured tube-type porous ceramic filter has a pore distribution which has two peaks at lOnm and 38nm. Porosity of filter is 60%.

4. Effects of the invention

We have invented a porous ceramic filter, which consists of nanometer scale (sub-micron) sized silica particles such as fumed silica or colloidal silica. This invention makes it possible to manufacture a ceramic filter, in which pores are uniformly distributed and their diameter is less than 0.1 micrometer, with high porosity. Therefore, nanometer scale (sub-micron) sized pores guarantee high filterability, which means that a porous ceramic filter can filter out matter larger than nanometer scale (sub- micron) without regard to the state of matter (liquefied/ vaporized). Also, the use of nanometer scale (sub-micron) sized particle can reduce the cost because they require low calcination temperature.

Claims

1. A porous ceramic filter made up of only nanometer scale (sub-micron) sized silica particle or nanometer scale (sub-micron) sized silica particle and organic/ inorganic additives.

2. The porous ceramic filter of claim 1 in which nanometer scale (sub- micron) sized silica is fumed silica or colloidal silica.

3. Method of making porous ceramic filter comprising;

a) making slurry with fumed silica or colloidal silica;

b) forming a body with the slurry;

c) drying the body after separating it from a mold;

d) calcining at 500-1100 °C .